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INTERNET-DRAFT                                          Clifford Neuman
                                                                USC-ISI
                                                                 Tom Yu
                                                            Sam Hartman
                                                            Ken Raeburn
                                                                    MIT
                                                          March 2, 2003
                                              Expires 2 September, 2003

            The Kerberos Network Authentication Service (V5)
            draft-ietf-krb-wg-kerberos-clarifications-03.txt

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

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   http://www.ietf.org/shadow.html.

   To learn the current status of any Internet-Draft, please check the
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   Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe),
   ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

   The distribution of this memo is unlimited. It is filed as draft-
   ietf-krb-wg-kerberos-clarifications-03.txt, and expires 2 September
   2003.  Please send comments to: ietf-krb-wg@anl.gov

ABSTRACT

   This document provides an overview and specification of Version 5 of
   the Kerberos protocol, and updates RFC1510 to clarify aspects of the
   protocol and its intended use that require more detailed or clearer
   explanation than was provided in RFC1510. This document is intended
   to provide a detailed description of the protocol, suitable for
   implementation, together with descriptions of the appropriate use of
   protocol messages and fields within those messages.



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   This document contains a subset of the changes considered and
   discussed in the Kerberos working group and is intended as an interim
   description of Kerberos. Additional changes to the Kerberos protocol
   have been proposed and will appear in a subsequent extensions
   document.

   This document is not intended to describe Kerberos to the end user,
   system administrator, or application developer. Higher level papers
   describing Version 5 of the Kerberos system [NT94] and documenting
   version 4 [SNS88], are available elsewhere.

OVERVIEW

   This INTERNET-DRAFT describes the concepts and model upon which the
   Kerberos network authentication system is based. It also specifies
   Version 5 of the Kerberos protocol.

   The motivations, goals, assumptions, and rationale behind most design
   decisions are treated cursorily; they are more fully described in a
   paper available in IEEE communications [NT94] and earlier in the
   Kerberos portion of the Athena Technical Plan [MNSS87]. The protocols
   have been a proposed standard and are being considered for
   advancement for draft standard through the IETF standard process.
   Comments are encouraged on the presentation, but only minor
   refinements to the protocol as implemented or extensions that fit
   within current protocol framework will be considered at this time.

   Requests for addition to an electronic mailing list for discussion of
   Kerberos, kerberos@MIT.EDU, may be addressed to kerberos-
   request@MIT.EDU.  This mailing list is gatewayed onto the Usenet as
   the group comp.protocols.kerberos. Requests for further information,
   including documents and code availability, may be sent to info-
   kerberos@MIT.EDU.

BACKGROUND

   The Kerberos model is based in part on Needham and Schroeder's
   trusted third-party authentication protocol [NS78] and on
   modifications suggested by Denning and Sacco [DS81]. The original
   design and implementation of Kerberos Versions 1 through 4 was the
   work of two former Project Athena staff members, Steve Miller of
   Digital Equipment Corporation and Clifford Neuman (now at the
   Information Sciences Institute of the University of Southern
   California), along with Jerome Saltzer, Technical Director of Project
   Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other
   members of Project Athena have also contributed to the work on
   Kerberos.




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   Version 5 of the Kerberos protocol (described in this document) has
   evolved from Version 4 based on new requirements and desires for
   features not available in Version 4. The design of Version 5 of the
   Kerberos protocol was led by Clifford Neuman and John Kohl with much
   input from the community. The development of the MIT reference
   implementation was led at MIT by John Kohl and Theodore Ts'o, with
   help and contributed code from many others. Since RFC1510 was issued,
   extensions and revisions to the protocol have been proposed by many
   individuals. Some of these proposals are reflected in this document.
   Where such changes involved significant effort, the document cites
   the contribution of the proposer.

   Reference implementations of both version 4 and version 5 of Kerberos
   are publicly available and commercial implementations have been
   developed and are widely used. Details on the differences between
   Kerberos Versions 4 and 5 can be found in [KNT94].



































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                           TTaabbllee ooff CCoonntteennttss


1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
1.1. Cross-realm operation . . . . . . . . . . . . . . . . . . . . .   9
1.2. Choosing a principal with which to communicate  . . . . . . . .  10
1.3. Authorization . . . . . . . . . . . . . . . . . . . . . . . . .  11
1.4. Extending Kerberos Without Breaking Interoperability  . . . . .  11
1.4.1. Compatibility with RFC 1510 . . . . . . . . . . . . . . . . .  12
1.4.2. Sending Extensible Messages . . . . . . . . . . . . . . . . .  13
1.5. Environmental assumptions . . . . . . . . . . . . . . . . . . .  13
1.6. Glossary of terms . . . . . . . . . . . . . . . . . . . . . . .  14
2. Ticket flag uses and requests . . . . . . . . . . . . . . . . . .  16
2.1. Initial, pre-authenticated, and hardware authenticated
      tickets  . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
2.2. Invalid tickets . . . . . . . . . . . . . . . . . . . . . . . .  17
2.3. Renewable tickets . . . . . . . . . . . . . . . . . . . . . . .  18
2.4. Postdated tickets . . . . . . . . . . . . . . . . . . . . . . .  18
2.5. Proxiable and proxy tickets . . . . . . . . . . . . . . . . . .  19
2.6. Forwardable tickets . . . . . . . . . . . . . . . . . . . . . .  20
2.7. Transited Policy Checking . . . . . . . . . . . . . . . . . . .  21
2.8. OK as Delegate  . . . . . . . . . . . . . . . . . . . . . . . .  21
2.9. Other KDC options . . . . . . . . . . . . . . . . . . . . . . .  22
2.9.1. Renewable-OK  . . . . . . . . . . . . . . . . . . . . . . . .  22
2.9.2. ENC-TKT-IN-SKEY . . . . . . . . . . . . . . . . . . . . . . .  22
2.9.3. Passwordless Hardware Authentication  . . . . . . . . . . . .  22
3. Message Exchanges . . . . . . . . . . . . . . . . . . . . . . . .  23
3.1. The Authentication Service Exchange . . . . . . . . . . . . . .  23
3.1.1. Generation of KRB_AS_REQ message  . . . . . . . . . . . . . .  24
3.1.2. Receipt of KRB_AS_REQ message . . . . . . . . . . . . . . . .  24
3.1.3. Generation of KRB_AS_REP message  . . . . . . . . . . . . . .  25
3.1.4. Generation of KRB_ERROR message . . . . . . . . . . . . . . .  27
3.1.5. Receipt of KRB_AS_REP message . . . . . . . . . . . . . . . .  28
3.1.6. Receipt of KRB_ERROR message  . . . . . . . . . . . . . . . .  29
3.2. The Client/Server Authentication Exchange . . . . . . . . . . .  29
3.2.1. The KRB_AP_REQ message  . . . . . . . . . . . . . . . . . . .  29
3.2.2. Generation of a KRB_AP_REQ message  . . . . . . . . . . . . .  29
3.2.3. Receipt of KRB_AP_REQ message . . . . . . . . . . . . . . . .  30
3.2.4. Generation of a KRB_AP_REP message  . . . . . . . . . . . . .  32
3.2.5. Receipt of KRB_AP_REP message . . . . . . . . . . . . . . . .  33
3.2.6. Using the encryption key  . . . . . . . . . . . . . . . . . .  33
3.3. The Ticket-Granting Service (TGS) Exchange  . . . . . . . . . .  34
3.3.1. Generation of KRB_TGS_REQ message . . . . . . . . . . . . . .  35
3.3.2. Receipt of KRB_TGS_REQ message  . . . . . . . . . . . . . . .  37
3.3.3. Generation of KRB_TGS_REP message . . . . . . . . . . . . . .  37
3.3.3.1. Checking for revoked tickets  . . . . . . . . . . . . . . .  40
3.3.3.2. Encoding the transited field  . . . . . . . . . . . . . . .  40
3.3.4. Receipt of KRB_TGS_REP message  . . . . . . . . . . . . . . .  42



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3.4. The KRB_SAFE Exchange . . . . . . . . . . . . . . . . . . . . .  42
3.4.1. Generation of a KRB_SAFE message  . . . . . . . . . . . . . .  42
3.4.2. Receipt of KRB_SAFE message . . . . . . . . . . . . . . . . .  43
3.5. The KRB_PRIV Exchange . . . . . . . . . . . . . . . . . . . . .  44
3.5.1. Generation of a KRB_PRIV message  . . . . . . . . . . . . . .  44
3.5.2. Receipt of KRB_PRIV message . . . . . . . . . . . . . . . . .  44
3.6. The KRB_CRED Exchange . . . . . . . . . . . . . . . . . . . . .  45
3.6.1. Generation of a KRB_CRED message  . . . . . . . . . . . . . .  45
3.6.2. Receipt of KRB_CRED message . . . . . . . . . . . . . . . . .  46
3.7. User to User Authentication Exchanges . . . . . . . . . . . . .  46
4. Encryption and Checksum Specifications  . . . . . . . . . . . . .  48
5. Message Specifications  . . . . . . . . . . . . . . . . . . . . .  49
5.1. Specific Compatibility Notes on ASN.1 . . . . . . . . . . . . .  51
5.1.1. ASN.1 Distinguished Encoding Rules  . . . . . . . . . . . . .  51
5.1.2. Optional Integer Fields . . . . . . . . . . . . . . . . . . .  51
5.1.3. Empty SEQUENCE OF Types . . . . . . . . . . . . . . . . . . .  51
5.1.4. Unrecognized Tag Numbers  . . . . . . . . . . . . . . . . . .  52
5.1.5. Tag Numbers Greater Than 30 . . . . . . . . . . . . . . . . .  52
5.2. Basic Kerberos Types  . . . . . . . . . . . . . . . . . . . . .  52
5.2.1. KerberosString  . . . . . . . . . . . . . . . . . . . . . . .  52
5.2.2. Realm and PrincipalName . . . . . . . . . . . . . . . . . . .  54
5.2.3. KerberosTime  . . . . . . . . . . . . . . . . . . . . . . . .  54
5.2.4. Constrained Integer types . . . . . . . . . . . . . . . . . .  55
5.2.5. HostAddress and HostAddresses . . . . . . . . . . . . . . . .  55
5.2.6. AuthorizationData . . . . . . . . . . . . . . . . . . . . . .  56
5.2.6.1. IF-RELEVANT . . . . . . . . . . . . . . . . . . . . . . . .  57
5.2.6.2. KDCIssued . . . . . . . . . . . . . . . . . . . . . . . . .  57
5.2.6.3. AND-OR  . . . . . . . . . . . . . . . . . . . . . . . . . .  59
5.2.6.4. MANDATORY-FOR-KDC . . . . . . . . . . . . . . . . . . . . .  59
5.2.7. PA-DATA . . . . . . . . . . . . . . . . . . . . . . . . . . .  59
5.2.7.1. PA-TGS-REQ  . . . . . . . . . . . . . . . . . . . . . . . .  60
5.2.7.2. Encrypted Timestamp Pre-authentication  . . . . . . . . . .  60
5.2.7.3. PA-PW-SALT  . . . . . . . . . . . . . . . . . . . . . . . .  61
5.2.7.4. PA-ETYPE-INFO . . . . . . . . . . . . . . . . . . . . . . .  61
5.2.7.5. PA-ETYPE-INFO2  . . . . . . . . . . . . . . . . . . . . . .  62
5.2.8. KerberosFlags . . . . . . . . . . . . . . . . . . . . . . . .  63
5.2.9. Cryptosystem-related Types  . . . . . . . . . . . . . . . . .  64
5.3. Tickets . . . . . . . . . . . . . . . . . . . . . . . . . . . .  65
5.4. Specifications for the AS and TGS exchanges . . . . . . . . . .  73
5.4.1. KRB_KDC_REQ definition  . . . . . . . . . . . . . . . . . . .  73
5.4.2. KRB_KDC_REP definition  . . . . . . . . . . . . . . . . . . .  80
5.5. Client/Server (CS) message specifications . . . . . . . . . . .  84
5.5.1. KRB_AP_REQ definition . . . . . . . . . . . . . . . . . . . .  84
5.5.2. KRB_AP_REP definition . . . . . . . . . . . . . . . . . . . .  87
5.5.3. Error message reply . . . . . . . . . . . . . . . . . . . . .  88
5.6. KRB_SAFE message specification  . . . . . . . . . . . . . . . .  88
5.6.1. KRB_SAFE definition . . . . . . . . . . . . . . . . . . . . .  88
5.7. KRB_PRIV message specification  . . . . . . . . . . . . . . . .  90



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5.7.1. KRB_PRIV definition . . . . . . . . . . . . . . . . . . . . .  90
5.8. KRB_CRED message specification  . . . . . . . . . . . . . . . .  91
5.8.1. KRB_CRED definition . . . . . . . . . . . . . . . . . . . . .  91
5.9. Error message specification . . . . . . . . . . . . . . . . . .  93
5.9.1. KRB_ERROR definition  . . . . . . . . . . . . . . . . . . . .  93
5.10. Application Tag Numbers  . . . . . . . . . . . . . . . . . . .  95
6. Naming Constraints  . . . . . . . . . . . . . . . . . . . . . . .  96
6.1. Realm Names . . . . . . . . . . . . . . . . . . . . . . . . . .  96
6.2. Principal Names . . . . . . . . . . . . . . . . . . . . . . . .  98
6.2.1. Name of server principals . . . . . . . . . . . . . . . . . .  99
7. Constants and other defined values  . . . . . . . . . . . . . . . 100
7.1. Host address types  . . . . . . . . . . . . . . . . . . . . . . 100
7.2. KDC messaging - IP Transports . . . . . . . . . . . . . . . . . 101
7.2.1. UDP/IP transport  . . . . . . . . . . . . . . . . . . . . . . 101
7.2.2. TCP/IP transport  . . . . . . . . . . . . . . . . . . . . . . 101
7.2.3. KDC Discovery on IP Networks  . . . . . . . . . . . . . . . . 103
7.2.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names  . . . . 103
7.2.3.2. Specifying KDC Location information with DNS SRV
      records  . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
7.2.3.3. KDC Discovery for Domain Style Realm Names on IP
      Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.3. Name of the TGS . . . . . . . . . . . . . . . . . . . . . . . . 104
7.4. OID arc for KerberosV5  . . . . . . . . . . . . . . . . . . . . 104
7.5. Protocol constants and associated values  . . . . . . . . . . . 104
7.5.1. Key usage numbers . . . . . . . . . . . . . . . . . . . . . . 105
7.5.2. PreAuthentication Data Types  . . . . . . . . . . . . . . . . 106
7.5.3. Address Types . . . . . . . . . . . . . . . . . . . . . . . . 107
7.5.4. Authorization Data Types  . . . . . . . . . . . . . . . . . . 107
7.5.5. Transited Encoding Types  . . . . . . . . . . . . . . . . . . 107
7.5.6. Protocol Version Number . . . . . . . . . . . . . . . . . . . 107
7.5.7. Kerberos Message Types  . . . . . . . . . . . . . . . . . . . 108
7.5.8. Name Types  . . . . . . . . . . . . . . . . . . . . . . . . . 108
7.5.9. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . 108
8. Interoperability requirements . . . . . . . . . . . . . . . . . . 110
8.1. Specification 2 . . . . . . . . . . . . . . . . . . . . . . . . 110
8.2. Recommended KDC values  . . . . . . . . . . . . . . . . . . . . 113
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . . . 113
10. Security Considerations  . . . . . . . . . . . . . . . . . . . . 113
11. Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . 117
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 117
13. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
A. ASN.1 module  . . . . . . . . . . . . . . . . . . . . . . . . . . 120
B. Changes since RFC-1510  . . . . . . . . . . . . . . . . . . . . . 129
END NOTES  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131







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1. Introduction

   Kerberos provides a means of verifying the identities of principals,
   (e.g. a workstation user or a network server) on an open
   (unprotected) network. This is accomplished without relying on
   assertions by the host operating system, without basing trust on host
   addresses, without requiring physical security of all the hosts on
   the network, and under the assumption that packets traveling along
   the network can be read, modified, and inserted at will[1]. Kerberos
   performs authentication under these conditions as a trusted third-
   party authentication service by using conventional (shared secret key
   [2]) cryptography. Kerberos extensions (outside the scope of this
   document) can provide for the use of public key cryptography during
   certain phases of the authentication protocol [@RFCE: if PKINIT
   advances concurrently include reference to the RFC here]. Such
   extensions support Kerberos authentication for users registered with
   public key certification authorities and provide certain benefits of
   public key cryptography in situations where they are needed.

   The basic Kerberos authentication process proceeds as follows: A
   client sends a request to the authentication server (AS) requesting
   "credentials" for a given server. The AS responds with these
   credentials, encrypted in the client's key. The credentials consist
   of a "ticket" for the server and a temporary encryption key (often
   called a "session key"). The client transmits the ticket (which
   contains the client's identity and a copy of the session key, all
   encrypted in the server's key) to the server. The session key (now
   shared by the client and server) is used to authenticate the client,
   and may optionally be used to authenticate the server. It may also be
   used to encrypt further communication between the two parties or to
   exchange a separate sub-session key to be used to encrypt further
   communication.

   Implementation of the basic protocol consists of one or more
   authentication servers running on physically secure hosts. The
   authentication servers maintain a database of principals (i.e., users
   and servers) and their secret keys. Code libraries provide encryption
   and implement the Kerberos protocol.  In order to add authentication
   to its transactions, a typical network application adds one or two
   calls to the Kerberos library directly or through the Generic
   Security Services Application Programming Interface, GSSAPI,
   described in separate document [ref to GSSAPI RFC]. These calls
   result in the transmission of the necessary messages to achieve
   authentication.

   The Kerberos protocol consists of several sub-protocols (or
   exchanges).  There are two basic methods by which a client can ask a
   Kerberos server for credentials. In the first approach, the client



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   sends a cleartext request for a ticket for the desired server to the
   AS. The reply is sent encrypted in the client's secret key. Usually
   this request is for a ticket-granting ticket (TGT) which can later be
   used with the ticket-granting server (TGS).  In the second method,
   the client sends a request to the TGS. The client uses the TGT to
   authenticate itself to the TGS in the same manner as if it were
   contacting any other application server that requires Kerberos
   authentication. The reply is encrypted in the session key from the
   TGT.  Though the protocol specification describes the AS and the TGS
   as separate servers, they are implemented in practice as different
   protocol entry points within a single Kerberos server.

   Once obtained, credentials may be used to verify the identity of the
   principals in a transaction, to ensure the integrity of messages
   exchanged between them, or to preserve privacy of the messages. The
   application is free to choose whatever protection may be necessary.

   To verify the identities of the principals in a transaction, the
   client transmits the ticket to the application server. Since the
   ticket is sent "in the clear" (parts of it are encrypted, but this
   encryption doesn't thwart replay) and might be intercepted and reused
   by an attacker, additional information is sent to prove that the
   message originated with the principal to whom the ticket was issued.
   This information (called the authenticator) is encrypted in the
   session key, and includes a timestamp. The timestamp proves that the
   message was recently generated and is not a replay.  Encrypting the
   authenticator in the session key proves that it was generated by a
   party possessing the session key. Since no one except the requesting
   principal and the server know the session key (it is never sent over
   the network in the clear) this guarantees the identity of the client.

   The integrity of the messages exchanged between principals can also
   be guaranteed using the session key (passed in the ticket and
   contained in the credentials). This approach provides detection of
   both replay attacks and message stream modification attacks. It is
   accomplished by generating and transmitting a collision-proof
   checksum (elsewhere called a hash or digest function) of the client's
   message, keyed with the session key. Privacy and integrity of the
   messages exchanged between principals can be secured by encrypting
   the data to be passed using the session key contained in the ticket
   or the sub-session key found in the authenticator.

   The authentication exchanges mentioned above require read-only access
   to the Kerberos database. Sometimes, however, the entries in the
   database must be modified, such as when adding new principals or
   changing a principal's key.  This is done using a protocol between a
   client and a third Kerberos server, the Kerberos Administration
   Server (KADM). There is also a protocol for maintaining multiple



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   copies of the Kerberos database. Neither of these protocols are
   described in this document.

1.1. Cross-realm operation

   The Kerberos protocol is designed to operate across organizational
   boundaries. A client in one organization can be authenticated to a
   server in another. Each organization wishing to run a Kerberos server
   establishes its own "realm". The name of the realm in which a client
   is registered is part of the client's name, and can be used by the
   end-service to decide whether to honor a request.

   By establishing "inter-realm" keys, the administrators of two realms
   can allow a client authenticated in the local realm to prove its
   identity to servers in other realms[3]. The exchange of inter-realm
   keys (a separate key may be used for each direction) registers the
   ticket-granting service of each realm as a principal in the other
   realm. A client is then able to obtain a ticket-granting ticket for
   the remote realm's ticket-granting service from its local realm. When
   that ticket-granting ticket is used, the remote ticket-granting
   service uses the inter-realm key (which usually differs from its own
   normal TGS key) to decrypt the ticket-granting ticket, and is thus
   certain that it was issued by the client's own TGS. Tickets issued by
   the remote ticket-granting service will indicate to the end-service
   that the client was authenticated from another realm.

   A realm is said to communicate with another realm if the two realms
   share an inter-realm key, or if the local realm shares an inter-realm
   key with an intermediate realm that communicates with the remote
   realm. An authentication path is the sequence of intermediate realms
   that are transited in communicating from one realm to another.

   Realms may be organized hierarchically. Each realm shares a key with
   its parent and a different key with each child. If an inter-realm key
   is not directly shared by two realms, the hierarchical organization
   allows an authentication path to be easily constructed. If a
   hierarchical organization is not used, it may be necessary to consult
   a database in order to construct an authentication path between
   realms.

   Although realms are typically hierarchical, intermediate realms may
   be bypassed to achieve cross-realm authentication through alternate
   authentication paths (these might be established to make
   communication between two realms more efficient). It is important for
   the end-service to know which realms were transited when deciding how
   much faith to place in the authentication process. To facilitate this
   decision, a field in each ticket contains the names of the realms
   that were involved in authenticating the client.



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   The application server is ultimately responsible for accepting or
   rejecting authentication and SHOULD check the transited field. The
   application server may choose to rely on the KDC for the application
   server's realm to check the transited field. The application server's
   KDC will set the TRANSITED-POLICY-CHECKED flag in this case. The KDCs
   for intermediate realms may also check the transited field as they
   issue ticket-granting tickets for other realms, but they are
   encouraged not to do so. A client may request that the KDCs not check
   the transited field by setting the DISABLE-TRANSITED-CHECK flag. KDCs
   are encouraged but not required to honor this flag.

1.2. Choosing a principal with which to communicate

   The Kerberos protocol provides the means for verifying (subject to
   the assumptions in 1.5) that the entity with which one communicates
   is the same entity that was registered with the KDC using the claimed
   identity (principal name). It is still necessary to determine whether
   that identity corresponds to the entity with which one intends to
   communicate.

   When appropriate data has been exchanged in advance, this
   determination may be performed syntactically by the application based
   on the application protocol specification, information provided by
   the user, and configuration files. For example, the server principal
   name (including realm) for a telnet server might be derived from the
   user specified host name (from the telnet command line), the "host/"
   prefix specified in the application protocol specification, and a
   mapping to a Kerberos realm derived syntactically from the domain
   part of the specified hostname and information from the local
   Kerberos realms database.

   One can also rely on trusted third parties to make this
   determination, but only when the data obtained from the third party
   is suitably integrity protected while resident on the third party
   server and when transmitted.  Thus, for example, one should not rely
   on an unprotected domain name system record to map a host alias to
   the primary name of a server, accepting the primary name as the party
   one intends to contact, since an attacker can modify the mapping and
   impersonate the party with which one intended to communicate.

   Implementations of Kerberos and protocols based on Kerberos MUST NOT
   use insecure DNS queries to canonicalize the hostname components of
   the service principal names.  In an environment without secure name
   service, application authors MAY append a statically configured
   domain name to unqualified hostnames before passing the name to the
   security mechanisms, but should do no more than that.  Secure name
   service facilities, if available, might be trusted for hostname
   canonicalization, but such canonicalization by the client SHOULD NOT



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   be required by an KDC implementation.

   Implementation note: Many current implementations do some degree of
   canonicalization of the provided service name, often using DNS even
   though it creates security problems. However there is no consistency
   among implementations about whether the service name is case folded
   to lower case or whether reverse resolution is used. To maximize
   interoperability and security, applications SHOULD provide security
   mechanisms with names which result from folding the user-entered name
   to lower case, without performing any other modifications or
   canonicalization.

1.3. Authorization

   As an authentication service, Kerberos provides a means of verifying
   the identity of principals on a network. Authentication is usually
   useful primarily as a first step in the process of authorization,
   determining whether a client may use a service, which objects the
   client is allowed to access, and the type of access allowed for each.
   Kerberos does not, by itself, provide authorization. Possession of a
   client ticket for a service provides only for authentication of the
   client to that service, and in the absence of a separate
   authorization procedure, it should not be considered by an
   application as authorizing the use of that service.

   Such separate authorization methods MAY be implemented as application
   specific access control functions and may utilize files on the
   application server, or on separately issued authorization credentials
   such as those based on proxies [Neu93], or on other authorization
   services. Separately authenticated authorization credentials MAY be
   embedded in a ticket's authorization data when encapsulated by the
   KDC-issued authorization data element.

   Applications should not accept the mere issuance of a service ticket
   by the Kerberos server (even by a modified Kerberos server) as
   granting authority to use the service, since such applications may
   become vulnerable to the bypass of this authorization check in an
   environment if they interoperate with other KDCs or where other
   options for application authentication (e.g.  the PKTAPP proposal)
   are provided.

1.4. Extending Kerberos Without Breaking Interoperability

   As the deployed base of Kerberos implementations grows, extending
   Kerberos becomes more important. Unfortunately some extensions to the
   existing Kerberos protocol create interoperability issues because of
   uncertainty regarding the treatment of certain extensibility options
   by some implementations. This section includes guidelines that will



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   enable future implementations to maintain interoperability.

   Kerberos provides a general mechanism for protocol extensibility.
   Some protocol messages contain typed holes -- sub-messages that
   contain an octet-string along with an integer that defines how to
   interpret the octet-string. The integer types are registered
   centrally, but can be used both for vendor extensions and for
   extensions standardized through the IETF.

1.4.1. Compatibility with RFC 1510

   It is important to note that existing Kerberos message formats can
   not be readily extended by adding fields to the ASN.1 types. Sending
   additional fields often results in the entire message being discarded
   without an error indication. Future versions of this specification
   will provide guidelines to ensure that ASN.1 fields can be added
   without creating an interoperability problem.

   In the meantime, all new or modified implementations of Kerberos that
   receive an unknown message extension SHOULD preserve the encoding of
   the extension but otherwise ignore the presence of the extension.
   Recipients MUST NOT decline a request simply because an extension is
   present.

   There is one exception to this rule. If an unknown authorization data
   element type is received by a server other than the ticket granting
   service either in an AP-REQ or in a ticket contained in an AP-REQ,
   then authentication MUST fail. One of the primary uses of
   authorization data is to restrict the use of the ticket. If the
   service cannot determine whether the restriction applies to that
   service then a security weakness may result if the ticket can be used
   for that service. Authorization elements that are optional SHOULD be
   enclosed in the AD-IF-RELEVANT element.

   The ticket granting service MUST ignore but propagate to derivative
   tickets any unknown authorization data types, unless those data types
   are embedded in a MANDATORY-FOR-KDC element, in which case the
   request will be rejected.  This behavior is appropriate because
   requiring that the ticket granting service understand unknown
   authorization data types would require that KDC software be upgraded
   to understand new application-level restrictions before applications
   used these restrictions, decreasing the utility of authorization data
   as a mechanism for restricting the use of tickets. No security
   problem is created because services to which the tickets are issued
   will verify the authorization data.

   Implementation note: Many RFC 1510 implementations ignore unknown
   authorization data elements. Depending on these implementations to



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   honor authorization data restrictions may create a security weakness.

1.4.2. Sending Extensible Messages

   Care must be taken to ensure that old implementations can understand
   messages sent to them even if they do not understand an extension
   that is used. Unless the sender knows an extension is supported, the
   extension cannot change the semantics of the core message or
   previously defined extensions.

   For example, an extension including key information necessary to
   decrypt the encrypted part of a KDC-REP could only be used in
   situations where the recipient was known to support the extension.
   Thus when designing such extensions it is important to provide a way
   for the recipient to notify the sender of support for the extension.
   For example in the case of an extension that changes the KDC-REP
   reply key, the client could indicate support for the extension by
   including a padata element in the AS-REQ sequence. The KDC should
   only use the extension if this padata element is present in the AS-
   REQ. Even if policy requires the use of the extension, it is better
   to return an error indicating that the extension is required than to
   use the extension when the recipient may not support it; debugging
   why implementations do not interoperate is easier when errors are
   returned.

1.5. Environmental assumptions

   Kerberos imposes a few assumptions on the environment in which it can
   properly function:

   *  "Denial of service" attacks are not solved with Kerberos. There
      are places in the protocols where an intruder can prevent an
      application from participating in the proper authentication steps.
      Detection and solution of such attacks (some of which can appear
      to be not-uncommon "normal" failure modes for the system) is
      usually best left to the human administrators and users.

   *  Principals MUST keep their secret keys secret. If an intruder
      somehow steals a principal's key, it will be able to masquerade as
      that principal or impersonate any server to the legitimate
      principal.

   *  "Password guessing" attacks are not solved by Kerberos. If a user
      chooses a poor password, it is possible for an attacker to
      successfully mount an offline dictionary attack by repeatedly
      attempting to decrypt, with successive entries from a dictionary,
      messages obtained which are encrypted under a key derived from the
      user's password.



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   *  Each host on the network MUST have a clock which is "loosely
      synchronized" to the time of the other hosts; this synchronization
      is used to reduce the bookkeeping needs of application servers
      when they do replay detection. The degree of "looseness" can be
      configured on a per-server basis, but is typically on the order of
      5 minutes. If the clocks are synchronized over the network, the
      clock synchronization protocol MUST itself be secured from network
      attackers.

   *  Principal identifiers are not recycled on a short-term basis. A
      typical mode of access control will use access control lists
      (ACLs) to grant permissions to particular principals. If a stale
      ACL entry remains for a deleted principal and the principal
      identifier is reused, the new principal will inherit rights
      specified in the stale ACL entry. By not re-using principal
      identifiers, the danger of inadvertent access is removed.

1.6. Glossary of terms

      Below is a list of terms used throughout this document.

   Authentication
      Verifying the claimed identity of a principal.

   Authentication header
      A record containing a Ticket and an Authenticator to be presented
      to a server as part of the authentication process.

   Authentication path
      A sequence of intermediate realms transited in the authentication
      process when communicating from one realm to another.

   Authenticator
      A record containing information that can be shown to have been
      recently generated using the session key known only by the client
      and server.

   Authorization
      The process of determining whether a client may use a service,
      which objects the client is allowed to access, and the type of
      access allowed for each.

   Capability
      A token that grants the bearer permission to access an object or
      service. In Kerberos, this might be a ticket whose use is
      restricted by the contents of the authorization data field, but
      which lists no network addresses, together with the session key
      necessary to use the ticket.



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   Ciphertext
      The output of an encryption function. Encryption transforms
      plaintext into ciphertext.

   Client
      A process that makes use of a network service on behalf of a user.
      Note that in some cases a Server may itself be a client of some
      other server (e.g. a print server may be a client of a file
      server).

   Credentials
      A ticket plus the secret session key necessary to successfully use
      that ticket in an authentication exchange.

   Encryption Type (etype)
      When associated with encrypted data, an encryption type identifies
      the algorithm used to encrypt the data and is used to select the
      appropriate algorithm for decrypting the data.  Encryption type
      tags are communicated in other messages to enumerate algorithms
      that are desired, supported, preferred, or allowed to be used for
      encryption of data between parties.  This preference is combined
      with local information and policy to select an algorithm to be
      used.

   KDC
      Key Distribution Center, a network service that supplies tickets
      and temporary session keys; or an instance of that service or the
      host on which it runs. The KDC services both initial ticket and
      ticket-granting ticket requests. The initial ticket portion is
      sometimes referred to as the Authentication Server (or service).
      The ticket-granting ticket portion is sometimes referred to as the
      ticket-granting server (or service).

   Kerberos
      The name given to the Project Athena's authentication service, the
      protocol used by that service, or the code used to implement the
      authentication service.  The name is adopted from the three-headed
      dog which guards Hades.

   Key Version Number (kvno)
      A tag associated with encrypted data identifies which key was used
      for encryption when a long lived key associated with a principal
      changes over time.  It is used during the transition to a new key
      so that the party decrypting a message can tell whether the data
      was encrypted using the old or the new key.

   Plaintext
      The input to an encryption function or the output of a decryption



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      function. Decryption transforms ciphertext into plaintext.

   Principal
      A named client or server entity that participates in a network
      communication, with one name that is considered canonical.

   Principal identifier
      The canonical name used to uniquely identify each different
      principal.

   Seal
      To encipher a record containing several fields in such a way that
      the fields cannot be individually replaced without either
      knowledge of the encryption key or leaving evidence of tampering.

   Secret key
      An encryption key shared by a principal and the KDC, distributed
      outside the bounds of the system, with a long lifetime. In the
      case of a human user's principal, the secret key MAY be derived
      from a password.

   Server
      A particular Principal which provides a resource to network
      clients.  The server is sometimes referred to as the Application
      Server.

   Service
      A resource provided to network clients; often provided by more
      than one server (for example, remote file service).

   Session key
      A temporary encryption key used between two principals, with a
      lifetime limited to the duration of a single login "session".

   Sub-session key
      A temporary encryption key used between two principals, selected
      and exchanged by the principals using the session key, and with a
      lifetime limited to the duration of a single association.

   Ticket
      A record that helps a client authenticate itself to a server; it
      contains the client's identity, a session key, a timestamp, and
      other information, all sealed using the server's secret key. It
      only serves to authenticate a client when presented along with a
      fresh Authenticator.


2. Ticket flag uses and requests



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   Each Kerberos ticket contains a set of flags which are used to
   indicate attributes of that ticket. Most flags may be requested by a
   client when the ticket is obtained; some are automatically turned on
   and off by a Kerberos server as required. The following sections
   explain what the various flags mean and give examples of reasons to
   use them. With the exception of the INVALID flag clients MUST ignore
   ticket flags that are not recognized. KDCs MUST ignore KDC options
   that are not recognized. Some implementations of RFC 1510 are known
   to reject unknown KDC options, so clients may need to resend a
   request without KDC new options absent if the request was rejected
   when sent with option added since RFC 1510. Since new KDCs will
   ignore unknown options, clients MUST confirm that the ticket returned
   by the KDC meets their needs.

   Note that it is not, in general, possible to determine whether an
   option was not honored because it was not understood or because it
   was rejected either through configuration or policy. When adding a
   new option to the Kerberos protocol, designers should consider
   whether the distinction is important for their option. In cases where
   it is, a mechanism for the KDC to return an indication that the
   option was understood but rejected needs to be provided in the
   specification of the option. Often in such cases, the mechanism needs
   to be broad enough to permit an error or reason to be returned.

2.1. Initial, pre-authenticated, and hardware authenticated tickets

   The INITIAL flag indicates that a ticket was issued using the AS
   protocol, rather than issued based on a ticket-granting ticket.
   Application servers that want to require the demonstrated knowledge
   of a client's secret key (e.g. a password-changing program) can
   insist that this flag be set in any tickets they accept, and thus be
   assured that the client's key was recently presented to the
   application client.

   The PRE-AUTHENT and HW-AUTHENT flags provide additional information
   about the initial authentication, regardless of whether the current
   ticket was issued directly (in which case INITIAL will also be set)
   or issued on the basis of a ticket-granting ticket (in which case the
   INITIAL flag is clear, but the PRE-AUTHENT and HW-AUTHENT flags are
   carried forward from the ticket-granting ticket).

2.2. Invalid tickets

   The INVALID flag indicates that a ticket is invalid. Application
   servers MUST reject tickets which have this flag set. A postdated
   ticket will be issued in this form. Invalid tickets MUST be validated
   by the KDC before use, by presenting them to the KDC in a TGS request
   with the VALIDATE option specified. The KDC will only validate



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   tickets after their starttime has passed. The validation is required
   so that postdated tickets which have been stolen before their
   starttime can be rendered permanently invalid (through a hot-list
   mechanism) (see section 3.3.3.1).

2.3. Renewable tickets

   Applications may desire to hold tickets which can be valid for long
   periods of time. However, this can expose their credentials to
   potential theft for equally long periods, and those stolen
   credentials would be valid until the expiration time of the
   ticket(s). Simply using short-lived tickets and obtaining new ones
   periodically would require the client to have long-term access to its
   secret key, an even greater risk. Renewable tickets can be used to
   mitigate the consequences of theft. Renewable tickets have two
   "expiration times": the first is when the current instance of the
   ticket expires, and the second is the latest permissible value for an
   individual expiration time. An application client must periodically
   (i.e. before it expires) present a renewable ticket to the KDC, with
   the RENEW option set in the KDC request. The KDC will issue a new
   ticket with a new session key and a later expiration time. All other
   fields of the ticket are left unmodified by the renewal process. When
   the latest permissible expiration time arrives, the ticket expires
   permanently. At each renewal, the KDC MAY consult a hot-list to
   determine if the ticket had been reported stolen since its last
   renewal; it will refuse to renew such stolen tickets, and thus the
   usable lifetime of stolen tickets is reduced.

   The RENEWABLE flag in a ticket is normally only interpreted by the
   ticket-granting service (discussed below in section 3.3). It can
   usually be ignored by application servers. However, some particularly
   careful application servers MAY disallow renewable tickets.

   If a renewable ticket is not renewed by its expiration time, the KDC
   will not renew the ticket. The RENEWABLE flag is reset by default,
   but a client MAY request it be set by setting the RENEWABLE option in
   the KRB_AS_REQ message. If it is set, then the renew-till field in
   the ticket contains the time after which the ticket may not be
   renewed.

2.4. Postdated tickets

   Applications may occasionally need to obtain tickets for use much
   later, e.g. a batch submission system would need tickets to be valid
   at the time the batch job is serviced. However, it is dangerous to
   hold valid tickets in a batch queue, since they will be on-line
   longer and more prone to theft.  Postdated tickets provide a way to
   obtain these tickets from the KDC at job submission time, but to



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   leave them "dormant" until they are activated and validated by a
   further request of the KDC. If a ticket theft were reported in the
   interim, the KDC would refuse to validate the ticket, and the thief
   would be foiled.

   The MAY-POSTDATE flag in a ticket is normally only interpreted by the
   ticket-granting service. It can be ignored by application servers.
   This flag MUST be set in a ticket-granting ticket in order to issue a
   postdated ticket based on the presented ticket. It is reset by
   default; it MAY be requested by a client by setting the ALLOW-
   POSTDATE option in the KRB_AS_REQ message.  This flag does not allow
   a client to obtain a postdated ticket-granting ticket; postdated
   ticket-granting tickets can only by obtained by requesting the
   postdating in the KRB_AS_REQ message. The life (endtime-starttime) of
   a postdated ticket will be the remaining life of the ticket-granting
   ticket at the time of the request, unless the RENEWABLE option is
   also set, in which case it can be the full life (endtime-starttime)
   of the ticket-granting ticket. The KDC MAY limit how far in the
   future a ticket may be postdated.

   The POSTDATED flag indicates that a ticket has been postdated. The
   application server can check the authtime field in the ticket to see
   when the original authentication occurred. Some services MAY choose
   to reject postdated tickets, or they may only accept them within a
   certain period after the original authentication. When the KDC issues
   a POSTDATED ticket, it will also be marked as INVALID, so that the
   application client MUST present the ticket to the KDC to be validated
   before use.

2.5. Proxiable and proxy tickets

   At times it may be necessary for a principal to allow a service to
   perform an operation on its behalf. The service must be able to take
   on the identity of the client, but only for a particular purpose. A
   principal can allow a service to take on the principal's identity for
   a particular purpose by granting it a proxy.

   The process of granting a proxy using the proxy and proxiable flags
   is used to provide credentials for use with specific services. Though
   conceptually also a proxy, users wishing to delegate their identity
   in a form usable for all purpose MUST use the ticket forwarding
   mechanism described in the next section to forward a ticket-granting
   ticket.

   The PROXIABLE flag in a ticket is normally only interpreted by the
   ticket-granting service. It can be ignored by application servers.
   When set, this flag tells the ticket-granting server that it is OK to
   issue a new ticket (but not a ticket-granting ticket) with a



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   different network address based on this ticket. This flag is set if
   requested by the client on initial authentication. By default, the
   client will request that it be set when requesting a ticket-granting
   ticket, and reset when requesting any other ticket.

   This flag allows a client to pass a proxy to a server to perform a
   remote request on its behalf (e.g. a print service client can give
   the print server a proxy to access the client's files on a particular
   file server in order to satisfy a print request).

   In order to complicate the use of stolen credentials, Kerberos
   tickets are usually valid from only those network addresses
   specifically included in the ticket[4]. When granting a proxy, the
   client MUST specify the new network address from which the proxy is
   to be used, or indicate that the proxy is to be issued for use from
   any address.

   The PROXY flag is set in a ticket by the TGS when it issues a proxy
   ticket.  Application servers MAY check this flag and at their option
   they MAY require additional authentication from the agent presenting
   the proxy in order to provide an audit trail.

2.6. Forwardable tickets

   Authentication forwarding is an instance of a proxy where the service
   granted is complete use of the client's identity. An example where it
   might be used is when a user logs in to a remote system and wants
   authentication to work from that system as if the login were local.

   The FORWARDABLE flag in a ticket is normally only interpreted by the
   ticket-granting service. It can be ignored by application servers.
   The FORWARDABLE flag has an interpretation similar to that of the
   PROXIABLE flag, except ticket-granting tickets may also be issued
   with different network addresses. This flag is reset by default, but
   users MAY request that it be set by setting the FORWARDABLE option in
   the AS request when they request their initial ticket-granting
   ticket.

   This flag allows for authentication forwarding without requiring the
   user to enter a password again. If the flag is not set, then
   authentication forwarding is not permitted, but the same result can
   still be achieved if the user engages in the AS exchange specifying
   the requested network addresses and supplies a password.

   The FORWARDED flag is set by the TGS when a client presents a ticket
   with the FORWARDABLE flag set and requests a forwarded ticket by
   specifying the FORWARDED KDC option and supplying a set of addresses
   for the new ticket. It is also set in all tickets issued based on



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   tickets with the FORWARDED flag set. Application servers may choose
   to process FORWARDED tickets differently than non-FORWARDED tickets.

   If addressless tickets are forwarded from one system to another,
   clients SHOULD still use this option to obtain a new TGT in order to
   have different session keys on the different systems.

2.7. Transited Policy Checking

   In Kerberos, the application server is ultimately responsible for
   accepting or rejecting authentication and SHOULD check that only
   suitably trusted KDCs are relied upon to authenticate a principal.
   The transited field in the ticket identifies which realms (and thus
   which KDCs) were involved in the authentication process and an
   application server would normally check this field. If any of these
   are untrusted to authenticate the indicated client principal
   (probably determined by a realm-based policy), the authentication
   attempt MUST be rejected. The presence of trusted KDCs in this list
   does not provide any guarantee; an untrusted KDC may have fabricated
   the list.

   While the end server ultimately decides whether authentication is
   valid, the KDC for the end server's realm MAY apply a realm specific
   policy for validating the transited field and accepting credentials
   for cross-realm authentication. When the KDC applies such checks and
   accepts such cross-realm authentication it will set the TRANSITED-
   POLICY-CHECKED flag in the service tickets it issues based on the
   cross-realm TGT. A client MAY request that the KDCs not check the
   transited field by setting the DISABLE-TRANSITED-CHECK flag. KDCs are
   encouraged but not required to honor this flag.

   Application servers MUST either do the transited-realm checks
   themselves, or reject cross-realm tickets without TRANSITED-POLICY-
   CHECKED set.

2.8. OK as Delegate

   For some applications a client may need to delegate authority to a
   server to act on its behalf in contacting other services.  This
   requires that the client forward credentials to an intermediate
   server.  The ability for a client to obtain a service ticket to a
   server conveys no information to the client about whether the server
   should be trusted to accept delegated credentials.  The OK-AS-
   DELEGATE provides a way for a KDC to communicate local realm policy
   to a client regarding whether an intermediate server is trusted to
   accept such credentials.

   The OK-AS-DELEGATE flag from the copy of the ticket flags in the



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   encrypted part of the KDC reply indicates to the client that the
   server (not the client) specified in the ticket has been determined
   by policy of the realm to be a suitable recipient of delegation.  A
   client can use the presence of this flag to help it make a decision
   whether to delegate credentials (either grant a proxy or a forwarded
   ticket-granting ticket) to this server.  Ignore the value of this
   flag. When setting this flag, an administrator should consider the
   Security and placement of the server on which the service will run,
   as well as whether the service requires the use of delegated
   credentials.

2.9. Other KDC options

   There are three additional options which MAY be set in a client's
   request of the KDC.

2.9.1. Renewable-OK

   The RENEWABLE-OK option indicates that the client will accept a
   renewable ticket if a ticket with the requested life cannot otherwise
   be provided. If a ticket with the requested life cannot be provided,
   then the KDC MAY issue a renewable ticket with a renew-till equal to
   the requested endtime. The value of the renew-till field MAY still be
   adjusted by site-determined limits or limits imposed by the
   individual principal or server.

2.9.2. ENC-TKT-IN-SKEY

   In its basic form the Kerberos protocol supports authentication in a
   client-server
    setting and is not well suited to authentication in a peer-to-peer
   environment because the long term key of the user does not remain on
   the workstation after initial login. Authentication of such peers may
   be supported by Kerberos in its user-to-user variant. The ENC-TKT-IN-
   SKEY option supports user-to-user authentication by allowing the KDC
   to issue a service ticket encrypted using the session key from
   another ticket-granting ticket issued to another user. The ENC-TKT-
   IN-SKEY option is honored only by the ticket-granting service. It
   indicates that the ticket to be issued for the end server is to be
   encrypted in the session key from the additional second ticket-
   granting ticket provided with the request. See section 3.3.3 for
   specific details.

2.9.3. Passwordless Hardware Authentication

   The OPT-HARDWARE-AUTH option indicates that the client wishes to use
   some form of hardware authentication instead of or in addition to the
   client's password or other long-lived encryption key. OPT-HARDWARE-



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   AUTH is honored only by the authentication service. If supported and
   allowed by policy, the KDC will return an errorcode
   KDC_ERR_PREAUTH_REQUIRED and include the required METHOD-DATA to
   perform such authentication.

3. Message Exchanges

   The following sections describe the interactions between network
   clients and servers and the messages involved in those exchanges.

3.1. The Authentication Service Exchange

                             Summary

         Message direction       Message type    Section
         1. Client to Kerberos   KRB_AS_REQ      5.4.1
         2. Kerberos to client   KRB_AS_REP or   5.4.2
                                 KRB_ERROR       5.9.1

   The Authentication Service (AS) Exchange between the client and the
   Kerberos Authentication Server is initiated by a client when it
   wishes to obtain authentication credentials for a given server but
   currently holds no credentials. In its basic form, the client's
   secret key is used for encryption and decryption. This exchange is
   typically used at the initiation of a login session to obtain
   credentials for a Ticket-Granting Server which will subsequently be
   used to obtain credentials for other servers (see section 3.3)
   without requiring further use of the client's secret key. This
   exchange is also used to request credentials for services which must
   not be mediated through the Ticket-Granting Service, but rather
   require a principal's secret key, such as the password-changing
   service[5]. This exchange does not by itself provide any assurance of
   the identity of the user[6].

   The exchange consists of two messages: KRB_AS_REQ from the client to
   Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these
   messages are described in sections 5.4.1, 5.4.2, and 5.9.1.

   In the request, the client sends (in cleartext) its own identity and
   the identity of the server for which it is requesting credentials,
   other information about the credentials it is requesting, and a
   randomly generated nonce which can be used to detect replays, and to
   associate replies with the matching requests. This nonce MUST be
   generated randomly by the client and remembered for checking against
   the nonce in the expected reply. The response, KRB_AS_REP, contains a
   ticket for the client to present to the server, and a session key
   that will be shared by the client and the server.  The session key
   and additional information are encrypted in the client's secret key.



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   The encrypted part of the KRB_AS_REP message also contains the nonce
   which MUST be matched with the nonce from the KRB_AS_REQ message.

   Without pre-authentication, the authentication server does not know
   whether the client is actually the principal named in the request. It
   simply sends a reply without knowing or caring whether they are the
   same. This is acceptable because nobody but the principal whose
   identity was given in the request will be able to use the reply. Its
   critical information is encrypted in that principal's key. However,
   an attacker can send a KRB_AS_REQ message to get known plaintext in
   order to attack the principal's key. Especially if the key is based
   on a password, this may create a security exposure. So, the initial
   request supports an optional field that can be used to pass
   additional information that might be needed for the initial exchange.
   This field SHOULD be used for pre-authentication as described in
   sections 3.1.1 and 5.2.7.

   Various errors can occur; these are indicated by an error response
   (KRB_ERROR) instead of the KRB_AS_REP response. The error message is
   not encrypted. The KRB_ERROR message contains information which can
   be used to associate it with the message to which it replies. The
   contents of the KRB_ERROR message are not integrity-protected. As
   such, the client cannot detect replays, fabrications or
   modifications. A solution to this problem will be included in a
   future version of the protocol.

3.1.1. Generation of KRB_AS_REQ message

   The client may specify a number of options in the initial request.
   Among these options are whether pre-authentication is to be
   performed; whether the requested ticket is to be renewable,
   proxiable, or forwardable; whether it should be postdated or allow
   postdating of derivative tickets; and whether a renewable ticket will
   be accepted in lieu of a non-renewable ticket if the requested ticket
   expiration date cannot be satisfied by a non-renewable ticket (due to
   configuration constraints).

   The client prepares the KRB_AS_REQ message and sends it to the KDC.

3.1.2. Receipt of KRB_AS_REQ message

   If all goes well, processing the KRB_AS_REQ message will result in
   the creation of a ticket for the client to present to the server. The
   format for the ticket is described in section 5.3. The contents of
   the ticket are determined as follows.

   Because Kerberos can run over unreliable transports such as UDP, the
   KDC MUST be prepared to retransmit responses in case they are lost.



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   If a KDC receives a request identical to one it has recently
   successfully processed, the KDC MUST respond with a KRB_AS_REP
   message rather than a replay error.  In order to reduce ciphertext
   given to a potential attacker, KDCs MAY send the same response
   generated when the request was first handled. KDCs MUST obey this
   replay behavior even if the actual transport in use is reliable.

3.1.3. Generation of KRB_AS_REP message

   The authentication server looks up the client and server principals
   named in the KRB_AS_REQ in its database, extracting their respective
   keys. If the requested client principal named in the request is not
   known because it doesn't exist in the KDC's principal database, then
   an error message with a KDC_ERR_C_PRINCIPAL_UNKNOWN is returned.

   If required, the server pre-authenticates the request, and if the
   pre-authentication check fails, an error message with the code
   KDC_ERR_PREAUTH_FAILED is returned. If pre-authentication is
   required, but was not present in the request, an error message with
   the code KDC_ERR_PREAUTH_REQUIRED is returned and a METHOD-DATA
   object will be stored in the e-data field of the KRB-ERROR message to
   specify which pre-authentication mechanisms are acceptable.  Usually
   this will include PA-ETYPE-INFO and/or PA-ETYPE-INFO2 elements as
   described below. If the server cannot accommodate any encryption type
   requested by the client, an error message with code
   KDC_ERR_ETYPE_NOSUPP is returned. Otherwise the KDC generates a
   'random' session key[7].

   When responding to an AS request, if there are multiple encryption
   keys registered for a client in the Kerberos database, then the etype
   field from the AS request is used by the KDC to select the encryption
   method to be used to protect the encrypted part of the KRB_AS_REP
   message which is sent to the client. If there is more than one
   supported strong encryption type in the etype list, the KDC SHOULD
   use the first valid strong etype for which an encryption key is
   available.

   When the user's key is generated from a password or pass phrase, the
   string-to-key function for the particular encryption key type is
   used, as specified in [@KCRYPTO]. The salt value and additional
   parameters for the string-to-key function have default values
   (specified by section 4 and by the encryption mechanism
   specification, respectively) that may be overridden by pre-
   authentication data (PA-PW-SALT, PA-AFS3-SALT, PA-ETYPE-INFO, PA-
   ETYPE-INFO2, etc). Since the KDC is presumed to store a copy of the
   resulting key only, these values should not be changed for password-
   based keys except when changing the principal's key.




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   When the AS server is to include pre-authentication data in a KRB-
   ERROR or in an AS-REP, it MUST use PA-ETYPE-INFO2, not PA-ETYPE-INFO,
   if the etype field of the client's AS-REQ lists at least one "newer"
   encryption type.  Otherwise (when the etype field of the client's AS-
   REQ does not list any "newer" encryption types) it MUST send both,
   PA-ETYPE-INFO2 and PA-ETYPE-INFO (both with an entry for each
   enctype).  A "newer" enctype is any enctype first officially
   specified concurrently with or subsequent to the issue of this RFC.
   The enctypes DES, 3DES or RC4 and any defined in [RFC1510] are not
   newer enctypes.

   It is not possible to reliably generate a user's key given a pass
   phrase without contacting the KDC, since it will not be known whether
   alternate salt or parameter values are required.

   The KDC will attempt to assign the type of the random session key
   from the list of methods in the etype field. The KDC will select the
   appropriate type using the list of methods provided together with
   information from the Kerberos database indicating acceptable
   encryption methods for the application server. The KDC will not issue
   tickets with a weak session key encryption type.

   If the requested start time is absent, indicates a time in the past,
   or is within the window of acceptable clock skew for the KDC and the
   POSTDATE option has not been specified, then the start time of the
   ticket is set to the authentication server's current time. If it
   indicates a time in the future beyond the acceptable clock skew, but
   the POSTDATED option has not been specified then the error
   KDC_ERR_CANNOT_POSTDATE is returned. Otherwise the requested start
   time is checked against the policy of the local realm (the
   administrator might decide to prohibit certain types or ranges of
   postdated tickets), and if acceptable, the ticket's start time is set
   as requested and the INVALID flag is set in the new ticket. The
   postdated ticket MUST be validated before use by presenting it to the
   KDC after the start time has been reached.

   The expiration time of the ticket will be set to the earlier of the
   requested endtime and a time determined by local policy, possibly
   determined using realm or principal specific factors. For example,
   the expiration time MAY be set to the earliest of the following:

      *  The expiration time (endtime) requested in the KRB_AS_REQ
         message.

      *  The ticket's start time plus the maximum allowable lifetime
         associated with the client principal from the authentication
         server's database.




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      *  The ticket's start time plus the maximum allowable lifetime
         associated with the server principal.

      *  The ticket's start time plus the maximum lifetime set by the
         policy of the local realm.

   If the requested expiration time minus the start time (as determined
   above) is less than a site-determined minimum lifetime, an error
   message with code KDC_ERR_NEVER_VALID is returned. If the requested
   expiration time for the ticket exceeds what was determined as above,
   and if the 'RENEWABLE-OK' option was requested, then the 'RENEWABLE'
   flag is set in the new ticket, and the renew-till value is set as if
   the 'RENEWABLE' option were requested (the field and option names are
   described fully in section 5.4.1).

   If the RENEWABLE option has been requested or if the RENEWABLE-OK
   option has been set and a renewable ticket is to be issued, then the
   renew-till field MAY be set to the earliest of:

      *  Its requested value.

      *  The start time of the ticket plus the minimum of the two
         maximum renewable lifetimes associated with the principals'
         database entries.

      *  The start time of the ticket plus the maximum renewable
         lifetime set by the policy of the local realm.

   The flags field of the new ticket will have the following options set
   if they have been requested and if the policy of the local realm
   allows: FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE.
   If the new ticket is postdated (the start time is in the future), its
   INVALID flag will also be set.

   If all of the above succeed, the server will encrypt the ciphertext
   part of the ticket using the encryption key extracted from the server
   principal's record in the Kerberos database using the encryption type
   associated with the server principal's key (this choice is NOT
   affected by the etype field in the request). It then formats a
   KRB_AS_REP message (see section 5.4.2), copying the addresses in the
   request into the caddr of the response, placing any required pre-
   authentication data into the padata of the response, and encrypts the
   ciphertext part in the client's key using an acceptable encryption
   method requested in the etype field of the request, or in some key
   specified by pre-authentication mechanisms being used.

3.1.4. Generation of KRB_ERROR message




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   Several errors can occur, and the Authentication Server responds by
   returning an error message, KRB_ERROR, to the client, with the error-
   code and e-text fields set to appropriate values. The error message
   contents and details are described in Section 5.9.1.

3.1.5. Receipt of KRB_AS_REP message

   If the reply message type is KRB_AS_REP, then the client verifies
   that the cname and crealm fields in the cleartext portion of the
   reply match what it requested. If any padata fields are present, they
   may be used to derive the proper secret key to decrypt the message.
   The client decrypts the encrypted part of the response using its
   secret key, verifies that the nonce in the encrypted part matches the
   nonce it supplied in its request (to detect replays). It also
   verifies that the sname and srealm in the response match those in the
   request (or are otherwise expected values), and that the host address
   field is also correct. It then stores the ticket, session key, start
   and expiration times, and other information for later use. The last-
   req field (and the deprecated key-expiration field) from the
   encrypted part of the response MAY be checked to notify the user of
   impending key expiration. This enables the client program to suggest
   remedial action, such as a password change.

   Upon validation of the KRB_AS_REP message (by checking the returned
   nonce against that sent in the KRB_AS_REQ message) the client knows
   that the current time on the KDC is that read from the authtime field
   of the encrypted part of the reply. The client can optionally use
   this value for clock synchronization in subsequent messages by
   recording with the ticket the difference (offset) between the
   authtime value and the local clock. This offset can then be used by
   the same user to adjust the time read from the system clock when
   generating messages [DGT96].

   This technique MUST be used when adjusting for clock skew instead of
   directly changing the system clock because the KDC reply is only
   authenticated to the user whose secret key was used, but not to the
   system or workstation. If the clock were adjusted, an attacker
   colluding with a user logging into a workstation could agree on a
   password, resulting in a KDC reply that would be correctly validated
   even though it did not originate from a KDC trusted by the
   workstation.

   Proper decryption of the KRB_AS_REP message is not sufficient for the
   host to verify the identity of the user; the user and an attacker
   could cooperate to generate a KRB_AS_REP format message which
   decrypts properly but is not from the proper KDC. If the host wishes
   to verify the identity of the user, it MUST require the user to
   present application credentials which can be verified using a



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   securely-stored secret key for the host. If those credentials can be
   verified, then the identity of the user can be assured.

3.1.6. Receipt of KRB_ERROR message

   If the reply message type is KRB_ERROR, then the client interprets it
   as an error and performs whatever application-specific tasks are
   necessary to recover.

3.2. The Client/Server Authentication Exchange

                                Summary
   Message direction                         Message type    Section
   Client to Application server              KRB_AP_REQ      5.5.1
   [optional] Application server to client   KRB_AP_REP or   5.5.2
                                             KRB_ERROR       5.9.1

   The client/server authentication (CS) exchange is used by network
   applications to authenticate the client to the server and vice versa.
   The client MUST have already acquired credentials for the server
   using the AS or TGS exchange.

3.2.1. The KRB_AP_REQ message

   The KRB_AP_REQ contains authentication information which SHOULD be
   part of the first message in an authenticated transaction. It
   contains a ticket, an authenticator, and some additional bookkeeping
   information (see section 5.5.1 for the exact format). The ticket by
   itself is insufficient to authenticate a client, since tickets are
   passed across the network in cleartext[8], so the authenticator is
   used to prevent invalid replay of tickets by proving to the server
   that the client knows the session key of the ticket and thus is
   entitled to use the ticket. The KRB_AP_REQ message is referred to
   elsewhere as the 'authentication header.'

3.2.2. Generation of a KRB_AP_REQ message

   When a client wishes to initiate authentication to a server, it
   obtains (either through a credentials cache, the AS exchange, or the
   TGS exchange) a ticket and session key for the desired service. The
   client MAY re-use any tickets it holds until they expire. To use a
   ticket the client constructs a new Authenticator from the system
   time, its name, and optionally an application specific checksum, an
   initial sequence number to be used in KRB_SAFE or KRB_PRIV messages,
   and/or a session subkey to be used in negotiations for a session key
   unique to this particular session.  Authenticators MAY NOT be re-used
   and will be rejected if replayed to a server[9]. If a sequence number
   is to be included, it SHOULD be randomly chosen so that even after



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   many messages have been exchanged it is not likely to collide with
   other sequence numbers in use.

   The client MAY indicate a requirement of mutual authentication or the
   use of a session-key based ticket (for user to user authentication -
   see section 3.7) by setting the appropriate flag(s) in the ap-options
   field of the message.

   The Authenticator is encrypted in the session key and combined with
   the ticket to form the KRB_AP_REQ message which is then sent to the
   end server along with any additional application-specific
   information.

3.2.3. Receipt of KRB_AP_REQ message

   Authentication is based on the server's current time of day (clocks
   MUST be loosely synchronized), the authenticator, and the ticket.
   Several errors are possible. If an error occurs, the server is
   expected to reply to the client with a KRB_ERROR message. This
   message MAY be encapsulated in the application protocol if its 'raw'
   form is not acceptable to the protocol.  The format of error messages
   is described in section 5.9.1.

   The algorithm for verifying authentication information is as follows.
   If the message type is not KRB_AP_REQ, the server returns the
   KRB_AP_ERR_MSG_TYPE error. If the key version indicated by the Ticket
   in the KRB_AP_REQ is not one the server can use (e.g., it indicates
   an old key, and the server no longer possesses a copy of the old
   key), the KRB_AP_ERR_BADKEYVER error is returned. If the USE-SESSION-
   KEY flag is set in the ap-options field, it indicates to the server
   that user-to-user authentication is in use, and that the ticket is
   encrypted in the session key from the server's ticket-granting ticket
   rather than in the server's secret key. See section 3.7 for a more
   complete description of the affect of user to user authentication on
   all messages in the Kerberos protocol.

   Since it is possible for the server to be registered in multiple
   realms, with different keys in each, the srealm field in the
   unencrypted portion of the ticket in the KRB_AP_REQ is used to
   specify which secret key the server should use to decrypt that
   ticket. The KRB_AP_ERR_NOKEY error code is returned if the server
   doesn't have the proper key to decipher the ticket.

   The ticket is decrypted using the version of the server's key
   specified by the ticket. If the decryption routines detect a
   modification of the ticket (each encryption system MUST provide
   safeguards to detect modified ciphertext; see section 6), the
   KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that



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   different keys were used to encrypt and decrypt).

   The authenticator is decrypted using the session key extracted from
   the decrypted ticket. If decryption shows it to have been modified,
   the KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm of
   the client from the ticket are compared against the same fields in
   the authenticator.  If they don't match, the KRB_AP_ERR_BADMATCH
   error is returned; this normally is caused by a client error or
   attempted attack. The addresses in the ticket (if any) are then
   searched for an address matching the operating-system reported
   address of the client. If no match is found or the server insists on
   ticket addresses but none are present in the ticket, the
   KRB_AP_ERR_BADADDR error is returned. If the local (server) time and
   the client time in the authenticator differ by more than the
   allowable clock skew (e.g., 5 minutes), the KRB_AP_ERR_SKEW error is
   returned.

   Unless the application server provides its own suitable means to
   protect against replay (for example, a challenge-response sequence
   initiated by the server after authentication, or use of a server-
   generated encryption subkey), the server MUST utilize a replay cache
   to remember any authenticator presented within the allowable clock
   skew. Careful analysis of the application protocol and implementation
   is recommended before eliminating this cache. The replay cache will
   store at least the server name, along with the client name, time and
   microsecond fields from the recently-seen authenticators and if a
   matching tuple is found, the KRB_AP_ERR_REPEAT error is returned
   [10]. If a server loses track of authenticators presented within the
   allowable clock skew, it MUST reject all requests until the clock
   skew interval has passed, providing assurance that any lost or
   replayed authenticators will fall outside the allowable clock skew
   and can no longer be successfully replayed [11].

   Implementation note: If a client generates multiple requests to the
   KDC with the same timestamp, including the microsecond field, all but
   the first of the requests received will be rejected as replays. This
   might happen, for example, if the resolution of the client's clock is
   too coarse.  Implementations SHOULD ensure that the timestamps are
   not reused, possibly by incrementing the microseconds field in the
   time stamp when the clock returns the same time for multiple
   requests.

   If multiple servers (for example, different services on one machine,
   or a single service implemented on multiple machines) share a service
   principal (a practice we do not recommend in general, but acknowledge
   will be used in some cases), they should also share this replay
   cache, or the application protocol should be designed so as to
   eliminate the need for it. Note that this applies to all of the



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   services, if any of the application protocols does not have replay
   protection built in; an authenticator used with such a service could
   later be replayed to a different service with the same service
   principal but no replay protection, if the former doesn't record the
   authenticator information in the common replay cache.

   If a sequence number is provided in the authenticator, the server
   saves it for later use in processing KRB_SAFE and/or KRB_PRIV
   messages. If a subkey is present, the server either saves it for
   later use or uses it to help generate its own choice for a subkey to
   be returned in a KRB_AP_REP message.

   The server computes the age of the ticket: local (server) time minus
   the start time inside the Ticket. If the start time is later than the
   current time by more than the allowable clock skew or if the INVALID
   flag is set in the ticket, the KRB_AP_ERR_TKT_NYV error is returned.
   Otherwise, if the current time is later than end time by more than
   the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED error is
   returned.

   If all these checks succeed without an error, the server is assured
   that the client possesses the credentials of the principal named in
   the ticket and thus, the client has been authenticated to the server.

   Passing these checks provides only authentication of the named
   principal; it does not imply authorization to use the named service.
   Applications MUST make a separate authorization decisions based upon
   the authenticated name of the user, the requested operation, local
   access control information such as that contained in a .k5login or
   .k5users file, and possibly a separate distributed authorization
   service.

3.2.4. Generation of a KRB_AP_REP message

   Typically, a client's request will include both the authentication
   information and its initial request in the same message, and the
   server need not explicitly reply to the KRB_AP_REQ. However, if
   mutual authentication (not only authenticating the client to the
   server, but also the server to the client) is being performed, the
   KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options
   field, and a KRB_AP_REP message is required in response. As with the
   error message, this message MAY be encapsulated in the application
   protocol if its "raw" form is not acceptable to the application's
   protocol. The timestamp and microsecond field used in the reply MUST
   be the client's timestamp and microsecond field (as provided in the
   authenticator) [12]. If a sequence number is to be included, it
   SHOULD be randomly chosen as described above for the authenticator. A
   subkey MAY be included if the server desires to negotiate a different



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   subkey. The KRB_AP_REP message is encrypted in the session key
   extracted from the ticket.

3.2.5. Receipt of KRB_AP_REP message

   If a KRB_AP_REP message is returned, the client uses the session key
   from the credentials obtained for the server [13] to decrypt the
   message, and verifies that the timestamp and microsecond fields match
   those in the Authenticator it sent to the server. If they match, then
   the client is assured that the server is genuine. The sequence number
   and subkey (if present) are retained for later use.

3.2.6. Using the encryption key

   After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and
   server share an encryption key which can be used by the application.
   In some cases, the use of this session key will be implicit in the
   protocol; in others the method of use must be chosen from several
   alternatives. The 'true session key' to be used for KRB_PRIV,
   KRB_SAFE, or other application-specific uses MAY be chosen by the
   application based on the session key from the ticket and subkeys in
   the KRB_AP_REP message and the authenticator [14]. To mitigate the
   effect of failures in random number generation on the client it is
   strongly encouraged that any key derived by an application for
   subsequent use include the full key entropy derived from the KDC
   generated session key carried in the ticket. We leave the protocol
   negotiations of how to use the key (e.g. selecting an encryption or
   checksum type) to the application programmer; the Kerberos protocol
   does not constrain the implementation options, but an example of how
   this might be done follows.

   One way that an application may choose to negotiate a key to be used
   for subsequent integrity and privacy protection is for the client to
   propose a key in the subkey field of the authenticator. The server
   can then choose a key using the proposed key from the client as
   input, returning the new subkey in the subkey field of the
   application reply. This key could then be used for subsequent
   communication.

   To make this example more concrete, if the communication patterns of
   an application dictates the use of encryption modes of operation
   incompatible with the encryption system used for the authenticator,
   then a key compatible with the required encryption system may be
   generated by either the client, the server, or collaboratively by
   both and exchanged using the subkey field.  This generation might
   involve the use of a random number as a pre-key, initially generated
   by either party, which could then be encrypted using the session key
   from the ticket, and the result exchanged and used for subsequent



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   encryption. By encrypting the pre-key with the session key from the
   ticket, randomness from the KDC generated key is assured of being
   present in the negotiated key. Application developers must be careful
   however, to use a means of introducing this entropy that does not
   allow an attacker to learn the session key from the ticket if it
   learns the key generated and used for subsequent communication. The
   reader should note that this is only an example, and that an analysis
   of the particular cryptosystem to be used, must be made before
   deciding how to generate values for the subkey fields, and the key to
   be used for subsequent communication.

   With both the one-way and mutual authentication exchanges, the peers
   should take care not to send sensitive information to each other
   without proper assurances. In particular, applications that require
   privacy or integrity SHOULD use the KRB_AP_REP response from the
   server to client to assure both client and server of their peer's
   identity. If an application protocol requires privacy of its
   messages, it can use the KRB_PRIV message (section 3.5). The KRB_SAFE
   message (section 3.4) can be used to assure integrity.

3.3. The Ticket-Granting Service (TGS) Exchange

                             Summary
         Message direction       Message type     Section
         1. Client to Kerberos   KRB_TGS_REQ      5.4.1
         2. Kerberos to client   KRB_TGS_REP or   5.4.2
                                 KRB_ERROR        5.9.1

   The TGS exchange between a client and the Kerberos Ticket-Granting
   Server is initiated by a client when it wishes to obtain
   authentication credentials for a given server (which might be
   registered in a remote realm), when it wishes to renew or validate an
   existing ticket, or when it wishes to obtain a proxy ticket. In the
   first case, the client must already have acquired a ticket for the
   Ticket-Granting Service using the AS exchange (the ticket-granting
   ticket is usually obtained when a client initially authenticates to
   the system, such as when a user logs in). The message format for the
   TGS exchange is almost identical to that for the AS exchange.  The
   primary difference is that encryption and decryption in the TGS
   exchange does not take place under the client's key. Instead, the
   session key from the ticket-granting ticket or renewable ticket, or
   sub-session key from an Authenticator is used. As is the case for all
   application servers, expired tickets are not accepted by the TGS, so
   once a renewable or ticket-granting ticket expires, the client must
   use a separate exchange to obtain valid tickets.

   The TGS exchange consists of two messages: A request (KRB_TGS_REQ)
   from the client to the Kerberos Ticket-Granting Server, and a reply



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   (KRB_TGS_REP or KRB_ERROR). The KRB_TGS_REQ message includes
   information authenticating the client plus a request for credentials.
   The authentication information consists of the authentication header
   (KRB_AP_REQ) which includes the client's previously obtained ticket-
   granting, renewable, or invalid ticket.  In the ticket-granting
   ticket and proxy cases, the request MAY include one or more of: a
   list of network addresses, a collection of typed authorization data
   to be sealed in the ticket for authorization use by the application
   server, or additional tickets (the use of which are described later).
   The TGS reply (KRB_TGS_REP) contains the requested credentials,
   encrypted in the session key from the ticket-granting ticket or
   renewable ticket, or if present, in the sub-session key from the
   Authenticator (part of the authentication header). The KRB_ERROR
   message contains an error code and text explaining what went wrong.
   The KRB_ERROR message is not encrypted. The KRB_TGS_REP message
   contains information which can be used to detect replays, and to
   associate it with the message to which it replies. The KRB_ERROR
   message also contains information which can be used to associate it
   with the message to which it replies. The same comments about
   integrity protection of KRB_ERROR messages mentioned in section 3.1
   apply to the TGS exchange.

3.3.1. Generation of KRB_TGS_REQ message

   Before sending a request to the ticket-granting service, the client
   MUST determine in which realm the application server is believed to
   be registered [15]. If the client knows the service principal name
   and realm and it does not already possess a ticket-granting ticket
   for the appropriate realm, then one must be obtained. This is first
   attempted by requesting a ticket-granting ticket for the destination
   realm from a Kerberos server for which the client possesses a ticket-
   granting ticket (using the KRB_TGS_REQ message recursively). The
   Kerberos server MAY return a TGT for the desired realm in which case
   one can proceed. Alternatively, the Kerberos server MAY return a TGT
   for a realm which is 'closer' to the desired realm (further along the
   standard hierarchical path between the client's realm and the
   requested realm server's realm). It should be noted in this case that
   misconfiguration of the Kerberos servers may cause loops in the
   resulting authentication path, which the client should be careful to
   detect and avoid.

   If the Kerberos server returns a TGT for a 'closer' realm other than
   the desired realm, the client MAY use local policy configuration to
   verify that the authentication path used is an acceptable one.
   Alternatively, a client MAY choose its own authentication path,
   rather than relying on the Kerberos server to select one. In either
   case, any policy or configuration information used to choose or
   validate authentication paths, whether by the Kerberos server or



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   client, MUST be obtained from a trusted source.

   When a client obtains a ticket-granting ticket that is 'closer' to
   the destination realm, the client MAY cache this ticket and reuse it
   in future KRB-TGS exchanges with services in the 'closer' realm.
   However, if the client were to obtain a ticket-granting ticket for
   the 'closer' realm by starting at the initial KDC rather than as part
   of obtaining another ticket, then a shorter path to the 'closer'
   realm might be used. This shorter path may be desirable because fewer
   intermediate KDCs would know the session key of the ticket involved.
   For this reason, clients SHOULD evaluate whether they trust the
   realms transited in obtaining the 'closer' ticket when making a
   decision to use the ticket in future.

   Once the client obtains a ticket-granting ticket for the appropriate
   realm, it determines which Kerberos servers serve that realm, and
   contacts one. The list might be obtained through a configuration file
   or network service or it MAY be generated from the name of the realm;
   as long as the secret keys exchanged by realms are kept secret, only
   denial of service results from using a false Kerberos server.

   (This paragraph changed) As in the AS exchange, the client MAY
   specify a number of options in the KRB_TGS_REQ message. One of these
   options is the ENC-TKT-IN-SKEY option used for user-to-user
   authentication. An overview of user to user authentication can be
   found in section 3.7. When generating the KRB_TGS_REQ message, this
   option indicates that the client is including a ticket-granting
   ticket obtained from the application server in the additional tickets
   field of the request and that the KDC SHOULD encrypt the ticket for
   the application server using the session key from this additional
   ticket, instead of using a server key from the principal database.

   The client prepares the KRB_TGS_REQ message, providing an
   authentication header as an element of the padata field, and
   including the same fields as used in the KRB_AS_REQ message along
   with several optional fields: the enc-authorizatfion-data field for
   application server use and additional tickets required by some
   options.

   In preparing the authentication header, the client can select a sub-
   session key under which the response from the Kerberos server will be
   encrypted [16]. If the sub-session key is not specified, the session
   key from the ticket-granting ticket will be used. If the enc-
   authorization-data is present, it MUST be encrypted in the sub-
   session key, if present, from the authenticator portion of the
   authentication header, or if not present, using the session key from
   the ticket-granting ticket.




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   Once prepared, the message is sent to a Kerberos server for the
   destination realm.

3.3.2. Receipt of KRB_TGS_REQ message

   The KRB_TGS_REQ message is processed in a manner similar to the
   KRB_AS_REQ message, but there are many additional checks to be
   performed. First, the Kerberos server MUST determine which server the
   accompanying ticket is for and it MUST select the appropriate key to
   decrypt it. For a normal KRB_TGS_REQ message, it will be for the
   ticket granting service, and the TGS's key will be used. If the TGT
   was issued by another realm, then the appropriate inter-realm key
   MUST be used. If the accompanying ticket is not a ticket-granting
   ticket for the current realm, but is for an application server in the
   current realm, the RENEW, VALIDATE, or PROXY options are specified in
   the request, and the server for which a ticket is requested is the
   server named in the accompanying ticket, then the KDC will decrypt
   the ticket in the authentication header using the key of the server
   for which it was issued. If no ticket can be found in the padata
   field, the KDC_ERR_PADATA_TYPE_NOSUPP error is returned.

   Once the accompanying ticket has been decrypted, the user-supplied
   checksum in the Authenticator MUST be verified against the contents
   of the request, and the message rejected if the checksums do not
   match (with an error code of KRB_AP_ERR_MODIFIED) or if the checksum
   is not keyed or not collision-proof (with an error code of
   KRB_AP_ERR_INAPP_CKSUM). If the checksum type is not supported, the
   KDC_ERR_SUMTYPE_NOSUPP error is returned. If the authorization-data
   are present, they are decrypted using the sub-session key from the
   Authenticator.

   If any of the decryptions indicate failed integrity checks, the
   KRB_AP_ERR_BAD_INTEGRITY error is returned.

   As discussed in section 3.1.2, the KDC MUST send a valid KRB_TGS_REP
   message if it receives a KRB_TGS_REQ message identical to one it has
   recently processed. However, if the authenticator is a replay, but
   the rest of the request is not identical, then the KDC SHOULD return
   KRB_AP_ERR_REPEAT.

3.3.3. Generation of KRB_TGS_REP message

   The KRB_TGS_REP message shares its format with the KRB_AS_REP
   (KRB_KDC_REP), but with its type field set to KRB_TGS_REP. The
   detailed specification is in section 5.4.2.

   The response will include a ticket for the requested server or for a
   ticket granting server of an intermediate KDC to be contacted to



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   obtain the requested ticket. The Kerberos database is queried to
   retrieve the record for the appropriate server (including the key
   with which the ticket will be encrypted). If the request is for a
   ticket-granting ticket for a remote realm, and if no key is shared
   with the requested realm, then the Kerberos server will select the
   realm 'closest' to the requested realm with which it does share a
   key, and use that realm instead. If the requested server cannot be
   found in the TGS database, then a TGT for another trusted realm MAY
   be returned instead of a ticket for the service. This TGT is a
   referral mechanism to cause the client to retry the request to the
   realm of the TGT.  These are the only cases where the response for
   the KDC will be for a different server than that requested by the
   client.

   By default, the address field, the client's name and realm, the list
   of transited realms, the time of initial authentication, the
   expiration time, and the authorization data of the newly-issued
   ticket will be copied from the ticket-granting ticket (TGT) or
   renewable ticket. If the transited field needs to be updated, but the
   transited type is not supported, the KDC_ERR_TRTYPE_NOSUPP error is
   returned.

   If the request specifies an endtime, then the endtime of the new
   ticket is set to the minimum of (a) that request, (b) the endtime
   from the TGT, and (c) the starttime of the TGT plus the minimum of
   the maximum life for the application server and the maximum life for
   the local realm (the maximum life for the requesting principal was
   already applied when the TGT was issued). If the new ticket is to be
   a renewal, then the endtime above is replaced by the minimum of (a)
   the value of the renew_till field of the ticket and (b) the starttime
   for the new ticket plus the life (endtime-starttime) of the old
   ticket.

   If the FORWARDED option has been requested, then the resulting ticket
   will contain the addresses specified by the client. This option will
   only be honored if the FORWARDABLE flag is set in the TGT. The PROXY
   option is similar; the resulting ticket will contain the addresses
   specified by the client. It will be honored only if the PROXIABLE
   flag in the TGT is set. The PROXY option will not be honored on
   requests for additional ticket-granting tickets.

   If the requested start time is absent, indicates a time in the past,
   or is within the window of acceptable clock skew for the KDC and the
   POSTDATE option has not been specified, then the start time of the
   ticket is set to the authentication server's current time. If it
   indicates a time in the future beyond the acceptable clock skew, but
   the POSTDATED option has not been specified or the MAY-POSTDATE flag
   is not set in the TGT, then the error KDC_ERR_CANNOT_POSTDATE is



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   returned. Otherwise, if the ticket-granting ticket has the MAY-
   POSTDATE flag set, then the resulting ticket will be postdated and
   the requested starttime is checked against the policy of the local
   realm. If acceptable, the ticket's start time is set as requested,
   and the INVALID flag is set. The postdated ticket MUST be validated
   before use by presenting it to the KDC after the starttime has been
   reached. However, in no case may the starttime, endtime, or renew-
   till time of a newly-issued postdated ticket extend beyond the renew-
   till time of the ticket-granting ticket.

   If the ENC-TKT-IN-SKEY option has been specified and an additional
   ticket has been included in the request, it indicates that the client
   is using user- to-user authentication to prove its identity to a
   server that does not have access to a persistent key. Section 3.7
   describes the affect of this option on the entire Kerberos protocol.
   When generating the KRB_TGS_REP message, this option in the
   KRB_TGS_REQ message tells the KDC to decrypt the additional ticket
   using the key for the server to which the additional ticket was
   issued and verify that it is a ticket-granting ticket. If the name of
   the requested server is missing from the request, the name of the
   client in the additional ticket will be used. Otherwise the name of
   the requested server will be compared to the name of the client in
   the additional ticket and if different, the request will be rejected.
   If the request succeeds, the session key from the additional ticket
   will be used to encrypt the new ticket that is issued instead of
   using the key of the server for which the new ticket will be used.

   If the name of the server in the ticket that is presented to the KDC
   as part of the authentication header is not that of the ticket-
   granting server itself, the server is registered in the realm of the
   KDC, and the RENEW option is requested, then the KDC will verify that
   the RENEWABLE flag is set in the ticket, that the INVALID flag is not
   set in the ticket, and that the renew_till time is still in the
   future. If the VALIDATE option is requested, the KDC will check that
   the starttime has passed and the INVALID flag is set. If the PROXY
   option is requested, then the KDC will check that the PROXIABLE flag
   is set in the ticket. If the tests succeed, and the ticket passes the
   hotlist check described in the next section, the KDC will issue the
   appropriate new ticket.

   The ciphertext part of the response in the KRB_TGS_REP message is
   encrypted in the sub-session key from the Authenticator, if present,
   or the session key from the ticket-granting ticket. It is not
   encrypted using the client's secret key. Furthermore, the client's
   key's expiration date and the key version number fields are left out
   since these values are stored along with the client's database
   record, and that record is not needed to satisfy a request based on a
   ticket-granting ticket.



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3.3.3.1. Checking for revoked tickets

   Whenever a request is made to the ticket-granting server, the
   presented ticket(s) is(are) checked against a hot-list of tickets
   which have been canceled. This hot-list might be implemented by
   storing a range of issue timestamps for 'suspect tickets'; if a
   presented ticket had an authtime in that range, it would be rejected.
   In this way, a stolen ticket-granting ticket or renewable ticket
   cannot be used to gain additional tickets (renewals or otherwise)
   once the theft has been reported to the KDC for the realm in which
   the server resides. Any normal ticket obtained before it was reported
   stolen will still be valid (because they require no interaction with
   the KDC), but only until their normal expiration time. If TGT's have
   been issued for cross-realm authentication, use of the cross-realm
   TGT will not be affected unless the hot-list is propagated to the
   KDCs for the realms for which such cross-realm tickets were issued.

3.3.3.2. Encoding the transited field

   If the identity of the server in the TGT that is presented to the KDC
   as part of the authentication header is that of the ticket-granting
   service, but the TGT was issued from another realm, the KDC will look
   up the inter-realm key shared with that realm and use that key to
   decrypt the ticket. If the ticket is valid, then the KDC will honor
   the request, subject to the constraints outlined above in the section
   describing the AS exchange.  The realm part of the client's identity
   will be taken from the ticket-granting ticket. The name of the realm
   that issued the ticket-granting ticket, if it is not the realm of the
   client principal, will be added to the transited field of the ticket
   to be issued. This is accomplished by reading the transited field
   from the ticket-granting ticket (which is treated as an unordered set
   of realm names), adding the new realm to the set, then constructing
   and writing out its encoded (shorthand) form (this may involve a
   rearrangement of the existing encoding).

   Note that the ticket-granting service does not add the name of its
   own realm. Instead, its responsibility is to add the name of the
   previous realm.  This prevents a malicious Kerberos server from
   intentionally leaving out its own name (it could, however, omit other
   realms' names).

   The names of neither the local realm nor the principal's realm are to
   be included in the transited field. They appear elsewhere in the
   ticket and both are known to have taken part in authenticating the
   principal. Since the endpoints are not included, both local and
   single-hop inter-realm authentication result in a transited field
   that is empty.




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   Because the name of each realm transited is added to this field, it
   might potentially be very long. To decrease the length of this field,
   its contents are encoded. The initially supported encoding is
   optimized for the normal case of inter-realm communication: a
   hierarchical arrangement of realms using either domain or X.500 style
   realm names. This encoding (called DOMAIN-X500-COMPRESS) is now
   described.

   Realm names in the transited field are separated by a ",". The ",",
   "\", trailing "."s, and leading spaces (" ") are special characters,
   and if they are part of a realm name, they MUST be quoted in the
   transited field by preceding them with a "\".

   A realm name ending with a "." is interpreted as being prepended to
   the previous realm. For example, we can encode traversal of EDU,
   MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:

      "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".

   Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points,
   that they would not be included in this field, and we would have:

      "EDU,MIT.,WASHINGTON.EDU"

   A realm name beginning with a "/" is interpreted as being appended to
   the previous realm.  For the purpose of appending, the realm
   preceding the first listed realm is considered to be the null realm
   ("").  If a realm name beginning with a "/" is to stand by itself,
   then it SHOULD be preceded by a space (" "). For example, we can
   encode traversal of /COM/HP/APOLLO, /COM/HP, /COM, and /COM/DEC as:

      "/COM,/HP,/APOLLO, /COM/DEC".

   Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints,
   they would not be included in this field, and we would have:

      "/COM,/HP"

   A null subfield preceding or following a "," indicates that all
   realms between the previous realm and the next realm have been
   traversed.  For the purpose of interpreting null subfields, the
   client's realm is considered to precede those in the transited field,
   and the server's realm is considered to follow them.  Thus, "," means
   that all realms along the path between the client and the server have
   been traversed. ",EDU, /COM," means that all realms from the client's
   realm up to EDU (in a domain style hierarchy) have been traversed,
   and that everything from /COM down to the server's realm in an X.500
   style has also been traversed. This could occur if the EDU realm in



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   one hierarchy shares an inter-realm key directly with the /COM realm
   in another hierarchy.

3.3.4. Receipt of KRB_TGS_REP message

   When the KRB_TGS_REP is received by the client, it is processed in
   the same manner as the KRB_AS_REP processing described above. The
   primary difference is that the ciphertext part of the response must
   be decrypted using the sub-session key from the Authenticator, if it
   was specified in the request, or the session key from the ticket-
   granting ticket, rather than the client's secret key. The server name
   returned in the reply is the true principal name of the service.

3.4. The KRB_SAFE Exchange

   The KRB_SAFE message MAY be used by clients requiring the ability to
   detect modifications of messages they exchange. It achieves this by
   including a keyed collision-proof checksum of the user data and some
   control information. The checksum is keyed with an encryption key
   (usually the last key negotiated via subkeys, or the session key if
   no negotiation has occurred).

3.4.1. Generation of a KRB_SAFE message

   When an application wishes to send a KRB_SAFE message, it collects
   its data and the appropriate control information and computes a
   checksum over them.  The checksum algorithm should be the keyed
   checksum mandated to be implemented along with the crypto system used
   for the sub-session or session key. The checksum is generated using
   the sub-session key if present, and the session key. Some
   implementations use a different checksum algorithm for the KRB_SAFE
   messages but doing so in a interoperable manner is not always
   possible.

   Implementations SHOULD accept any checksum algorithm they implement
   that both have adequate security and that have keys compatible with
   the sub-session or session key. Unkeyed or non-collision-proof
   checksums are not suitable for this use.

   The control information for the KRB_SAFE message includes both a
   timestamp and a sequence number. The designer of an application using
   the KRB_SAFE message MUST choose at least one of the two mechanisms.
   This choice SHOULD be based on the needs of the application protocol.

   Sequence numbers are useful when all messages sent will be received
   by one's peer. Connection state is presently required to maintain the
   session key, so maintaining the next sequence number should not
   present an additional problem.



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   If the application protocol is expected to tolerate lost messages
   without them being resent, the use of the timestamp is the
   appropriate replay detection mechanism. Using timestamps is also the
   appropriate mechanism for multi-cast protocols where all of one's
   peers share a common sub-session key, but some messages will be sent
   to a subset of one's peers.

   After computing the checksum, the client then transmits the
   information and checksum to the recipient in the message format
   specified in section 5.6.1.

3.4.2. Receipt of KRB_SAFE message

   When an application receives a KRB_SAFE message, it verifies it as
   follows.  If any error occurs, an error code is reported for use by
   the application.

   The message is first checked by verifying that the protocol version
   and type fields match the current version and KRB_SAFE, respectively.
   A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
   error. The application verifies that the checksum used is a
   collision-proof keyed checksum that uses keys compatible with the
   sub-session or session key as appropriate (or with the application
   key derived from the session or sub-session keys), and if it is not,
   a KRB_AP_ERR_INAPP_CKSUM error is generated.  The sender's address
   MUST be included in the control information; the recipient verifies
   that the operating system's report of the sender's address matches
   the sender's address in the message, and (if a recipient address is
   specified or the recipient requires an address) that one of the
   recipient's addresses appears as the recipient's address in the
   message. To work with network address translation, senders MAY use
   the directional address type specified in section 8.1 for the sender
   address and not include recipient addresses. A failed match for
   either case generates a KRB_AP_ERR_BADADDR error. Then the timestamp
   and usec and/or the sequence number fields are checked. If timestamp
   and usec are expected and not present, or they are present but not
   current, the KRB_AP_ERR_SKEW error is generated. If the server name,
   along with the client name, time and microsecond fields from the
   Authenticator match any recently-seen (sent or received) such tuples,
   the KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence
   number is included, or a sequence number is expected but not present,
   the KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp
   and usec or a sequence number is present, a KRB_AP_ERR_MODIFIED error
   is generated. Finally, the checksum is computed over the data and
   control information, and if it doesn't match the received checksum, a
   KRB_AP_ERR_MODIFIED error is generated.

   If all the checks succeed, the application is assured that the



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   message was generated by its peer and was not modified in transit.

3.5. The KRB_PRIV Exchange

   The KRB_PRIV message MAY be used by clients requiring confidentiality
   and the ability to detect modifications of exchanged messages. It
   achieves this by encrypting the messages and adding control
   information.

3.5.1. Generation of a KRB_PRIV message

   When an application wishes to send a KRB_PRIV message, it collects
   its data and the appropriate control information (specified in
   section 5.7.1) and encrypts them under an encryption key (usually the
   last key negotiated via subkeys, or the session key if no negotiation
   has occurred). As part of the control information, the client MUST
   choose to use either a timestamp or a sequence number (or both); see
   the discussion in section 3.4.1 for guidelines on which to use. After
   the user data and control information are encrypted, the client
   transmits the ciphertext and some 'envelope' information to the
   recipient.

3.5.2. Receipt of KRB_PRIV message

   When an application receives a KRB_PRIV message, it verifies it as
   follows.  If any error occurs, an error code is reported for use by
   the application.

   The message is first checked by verifying that the protocol version
   and type fields match the current version and KRB_PRIV, respectively.
   A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
   error. The application then decrypts the ciphertext and processes the
   resultant plaintext. If decryption shows the data to have been
   modified, a KRB_AP_ERR_BAD_INTEGRITY error is generated.

   The sender's address MUST be included in the control information; the
   recipient verifies that the operating system's report of the sender's
   address matches the sender's address in the message.  If a recipient
   address is specified or the recipient requires an address then one of
   the recipient's addresses MUST also appear as the recipient's address
   in the message.  Where a sender's or receiver's address might not
   otherwise match the address in a message because of network address
   translation, an application MAY be written to use addresses of the
   directional address type in place of the actual network address.

   A failed match for either case generates a KRB_AP_ERR_BADADDR error.
   To work with network address translation, implementations MAY use the
   directional address type defined in section 7.1 for the sender



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   address and include no recipient address. Then the timestamp and usec
   and/or the sequence number fields are checked. If timestamp and usec
   are expected and not present, or they are present but not current,
   the KRB_AP_ERR_SKEW error is generated. If the server name, along
   with the client name, time and microsecond fields from the
   Authenticator match any recently-seen such tuples, the
   KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence number
   is included, or a sequence number is expected but not present, the
   KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp and
   usec or a sequence number is present, a KRB_AP_ERR_MODIFIED error is
   generated.

   If all the checks succeed, the application can assume the message was
   generated by its peer, and was securely transmitted (without
   intruders able to see the unencrypted contents).

3.6. The KRB_CRED Exchange

   The KRB_CRED message MAY be used by clients requiring the ability to
   send Kerberos credentials from one host to another. It achieves this
   by sending the tickets together with encrypted data containing the
   session keys and other information associated with the tickets.

3.6.1. Generation of a KRB_CRED message

   When an application wishes to send a KRB_CRED message it first (using
   the KRB_TGS exchange) obtains credentials to be sent to the remote
   host. It then constructs a KRB_CRED message using the ticket or
   tickets so obtained, placing the session key needed to use each
   ticket in the key field of the corresponding KrbCredInfo sequence of
   the encrypted part of the KRB_CRED message.

   Other information associated with each ticket and obtained during the
   KRB_TGS exchange is also placed in the corresponding KrbCredInfo
   sequence in the encrypted part of the KRB_CRED message. The current
   time and, if specifically required by the application (and
   communicated from the recipient to the sender by application specific
   means) the nonce, s-address, and r-address fields, are placed in the
   encrypted part of the KRB_CRED message which is then encrypted under
   an encryption key previously exchanged in the KRB_AP exchange
   (usually the last key negotiated via subkeys, or the session key if
   no negotiation has occurred).

   Implementation note: When constructing a KRB_CRED message for
   inclusion in a GSSAPI initial context token, the MIT implementation
   of Kerberos will not encrypt the KRB_CRED message if the session key
   is a DES or triple DES key.  For interoperability with MIT, the
   Microsoft implementation will not encrypt the KRB_CRED in a GSSAPI



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   token if it is using a DES session key. Starting at version 1.2.5,
   MIT Kerberos can receive and decode either encrypted or unencrypted
   KRB_CRED tokens in the GSSAPI exchange. The Heimdal implementation of
   Kerberos can also accept either encrypted or unencrypted KRB_CRED
   messages. Since the KRB_CRED message in a GSSAPI token is encrypted
   in the authenticator, the MIT behavior does not present a security
   problem, although it is a violation of the Kerberos specification.

3.6.2. Receipt of KRB_CRED message

   When an application receives a KRB_CRED message, it verifies it. If
   any error occurs, an error code is reported for use by the
   application. The message is verified by checking that the protocol
   version and type fields match the current version and KRB_CRED,
   respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or
   KRB_AP_ERR_MSG_TYPE error. The application then decrypts the
   ciphertext and processes the resultant plaintext. If decryption shows
   the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY error is
   generated.

   If present or required, the recipient MAY verify that the operating
   system's report of the sender's address matches the sender's address
   in the message, and that one of the recipient's addresses appears as
   the recipient's address in the message. The address check does not
   provide any added security, since the address if present has already
   been checked in the KRB_AP_REQ message and there is not any benefit
   to be gained by an attacker in reflecting a KRB_CRED message back to
   its originator. Thus, the recipient MAY ignore the address even if
   present in order to work better in NAT environments. A failed match
   for either case generates a KRB_AP_ERR_BADADDR error. Recipients MAY
   skip the address check as the KRB_CRED message cannot generally be
   reflected back to the originator.  The timestamp and usec fields (and
   the nonce field if required) are checked next. If the timestamp and
   usec are not present, or they are present but not current, the
   KRB_AP_ERR_SKEW error is generated.

   If all the checks succeed, the application stores each of the new
   tickets in its credentials cache together with the session key and
   other information in the corresponding KrbCredInfo sequence from the
   encrypted part of the KRB_CRED message.

3.7. User to User Authentication Exchanges

   User to User authentication provides a method to perform
   authentication when the verifier does not have a access to long term
   service key. This might be the case when running a server (for
   example a window server) as a user on a workstation. In such cases,
   the server may have access to the ticket-granting ticket obtained



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   when the user logged in to the workstation, but because the server is
   running as an unprivileged user it might not have access to system
   keys. Similar situations may arise when running peer-to-peer
   applications.

                             Summary
       Message direction                    Message type     Sections
       0. Message from application server   Not Specified
       1. Client to Kerberos                KRB_TGS_REQ      3.3 + 5.4.1
       2. Kerberos to client                KRB_TGS_REP or   3.3 + 5.4.2
                                            KRB_ERROR        5.9.1
       3. Client to Application server      KRB_AP_REQ       3.2 + 5.5.1

   To address this problem, the Kerberos protocol allows the client to
   request that the ticket issued by the KDC be encrypted using a
   session key from a ticket-granting ticket issued to the party that
   will verify the authentication.  This ticket-granting ticket must be
   obtained from the verifier by means of an exchange external to the
   Kerberos protocol, usually as part of the application protocol. This
   message is shown in the summary above as message 0. Note that because
   the ticket-granting ticket is encrypted in the KDC's secret key, it
   can not be used for authentication without posession of the
   corresponding secret key.  Furthermore, because the verifier does not
   reveal the corresponding secret key, providing a copy of the
   verifier's ticket-granting ticket does not allow impersonation of the
   verifier.

   Message 0 in the table above represents an application specific
   negotation between the client and server, at the end of which both
   have determined that they will use user to user authentication and
   the client has obtained the server's TGT.

   Next, the client includes the server's TGT as an additional ticket in
   its KRB_TGS_REQ request to the KDC (message 1 in the table above) and
   specifyies the ENC-TKT-IN-SKEY option in its request.

   If validated according to the instructions in 3.3.3, the application
   ticket returned to the client (message 2 in the table above) will be
   encrypted using the session key from the additional ticket and the
   client will note this when it uses or stores the application ticket.

   When contacting the server using a ticket obtained for user to user
   authentication (message 3 in the table above), the client MUST
   specify the USE-SESSION-KEY flag in the ap-options field. This tells
   the application server to use the session key associated with its
   ticket-granting ticket to decrypt the server ticket provided in the
   application request.




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4. Encryption and Checksum Specifications

   The Kerberos protocols described in this document are designed to
   encrypt messages of arbitrary sizes, using stream or block encryption
   ciphers.  Encryption is used to prove the identities of the network
   entities participating in message exchanges. The Key Distribution
   Center for each realm is trusted by all principals registered in that
   realm to store a secret key in confidence. Proof of knowledge of this
   secret key is used to verify the authenticity of a principal.

   The KDC uses the principal's secret key (in the AS exchange) or a
   shared session key (in the TGS exchange) to encrypt responses to
   ticket requests; the ability to obtain the secret key or session key
   implies the knowledge of the appropriate keys and the identity of the
   KDC. The ability of a principal to decrypt the KDC response and
   present a Ticket and a properly formed Authenticator (generated with
   the session key from the KDC response) to a service verifies the
   identity of the principal; likewise the ability of the service to
   extract the session key from the Ticket and prove its knowledge
   thereof in a response verifies the identity of the service.

   [@KCRYPTO] defines a framework for defining encryption and checksum
   mechanisms for use with Kerberos. It also defines several such
   mechanisms, and more may be added in future updates to that document.

   The string-to-key operation provided by [@KCRYPTO] is used to produce
   a long-term key for a principal (generally for a user). The default
   salt string, if none is provided via pre-authentication data, is the
   concatenation of the principal's realm and name components, in order,
   with no separators.  Unless otherwise indicated, the default string-
   to-key opaque parameter set as defined in [@KCRYPTO] is used.

   Encrypted data, keys and checksums are transmitted using the
   EncryptedData, EncryptionKey and Checksum data objects defined in
   section 5.2.9. The encryption, decryption, and checksum operations
   described in this document use the corresponding encryption,
   decryption, and get_mic operations described in [@KCRYPTO], with
   implicit "specific key" generation using the "key usage" values
   specified in the description of each EncryptedData or Checksum object
   to vary the key for each operation. Note that in some cases, the
   value to be used is dependent on the method of choosing the key or
   the context of the message.

   Key usages are unsigned 32 bit integers; zero is not permitted. The
   key usage values for encrypting or checksumming Kerberos messages are
   indicated in section 5 along with the message definitions. Key usage
   values 512-1023 are reserved for uses internal to a Kerberos
   implementation. (For example, seeding a pseudo-random number



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   generator with a value produced by encrypting something with a
   session key and a key usage value not used for any other purpose.)
   Key usage values between 1024 and 2047 (inclusive) are reserved for
   application use; applications SHOULD use even values for encryption
   and odd values for checksums within this range. Key usage values are
   also summarized in a table in section 7.5.1.

   There might exist other documents which define protocols in terms of
   the RFC1510 encryption types or checksum types. Such documents would
   not know about key usages. In order that these specifications
   continue to be meaningful until they are updated, if not key usage
   values are specified then key usages 1024 and 1025 must be used to
   derive keys for encryption and checksums, respectively (this does not
   apply to protocols that do their own encryption independent of this
   framework, directly using the key resulting from the Kerberos
   authentication exchange.) New protocols defined in terms of the
   Kerberos encryption and checksum types SHOULD use their own key usage
   values.

   Unless otherwise indicated, no cipher state chaining is done from one
   encryption operation to another.

   Implementation note: While not recommended, some application
   protocols will continue to use the key data directly, even if only in
   currently existing protocol specifications. An implementation
   intended to support general Kerberos applications may therefore need
   to make key data available, as well as the attributes and operations
   described in [@KCRYPTO].  One of the more common reasons for directly
   performing encryption is direct control over negotiation and
   selection of a "sufficiently strong" encryption algorithm (in the
   context of a given application). While Kerberos does not directly
   provide a facility for negotiating encryption types between the
   application client and server, there are approaches for using
   Kerberos to facilitate this negotiation - for example, a client may
   request only "sufficiently strong" session key types from the KDC and
   expect that any type returned by the KDC will be understood and
   supported by the application server.

5. Message Specifications

   NOTE: The ASN.1 collected here should be identical to the contents of
   Appendix A. In case of conflict, the contents of Appendix A shall
   take precedence.

   The Kerberos protocol is defined here in terms of Abstract Syntax
   Notation One (ASN.1) [X680], which provides a syntax for specifying
   both the abstract layout of protocol messages as well as their
   encodings. Implementors not utilizing an existing ASN.1 compiler or



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   support library are cautioned to thoroughly understand the actual
   ASN.1 specification to ensure correct implementation behavior, as
   there is more complexity in the notation than is immediately obvious,
   and some tutorials and guides to ASN.1 are misleading or erroneous.

   Note that in several places, there have been changes here from RFC
   1510 that change the abstract types. This is in part to address
   widespread assumptions that various implementors have made, in some
   cases resulting in unintentional violations of the ASN.1 standard.
   These are clearly flagged where they occur. The differences between
   the abstract types in RFC 1510 and abstract types in this document
   can cause incompatible encodings to be emitted when certain encoding
   rules, e.g. the Packed Encoding Rules (PER), are used. This
   theoretical incompatibility should not be relevant for Kerberos,
   since Kerberos explicitly specifies the use of the Distinguished
   Encoding Rules (DER). It might be an issue for protocols wishing to
   use Kerberos types with other encoding rules. (This practice is not
   recommended.) With very few exceptions (most notably the usages of
   BIT STRING), the encodings resulting from using the DER remain
   identical between the types defined in RFC 1510 and the types defined
   in this document.

   The type definitions in this section assume an ASN.1 module
   definition of the following form:

   KerberosV5Spec2 {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) kerberosV5(2) modules(4) krb5spec2(2)
   } DEFINITIONS EXPLICIT TAGS ::= BEGIN

   -- rest of definitions here

   END

   This specifies that the tagging context for the module will be
   explicit and non-automatic.

   Note that in some other publications [RFC1510] [RFC1964], the "dod"
   portion of the object identifier is erroneously specified as having
   the value "5".  In the case of RFC 1964, use of the "correct" OID
   value would result in a change in the wire protocol; therefore, it
   remains unchanged for now.

   Note that elsewhere in this document, nomenclature for various
   message types is inconsistent, but seems to largely follow C language
   conventions, including use of underscore (_) characters and all-caps
   spelling of names intended to be numeric constants. Also, in some
   places, identifiers (especially ones refering to constants) are



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   written in all-caps in order to distinguish them from surrounding
   explanatory text.

   The ASN.1 notation does not permit underscores in identifiers, so in
   actual ASN.1 definitions, underscores are replaced with hyphens (-).
   Additionally, structure member names and defined values in ASN.1 MUST
   begin with a lowercase letter, while type names MUST begin with an
   uppercase letter.

5.1. Specific Compatibility Notes on ASN.1

   For compatibility purposes, implementors should heed the following
   specific notes regarding the use of ASN.1 in Kerberos. These notes do
   not describe deviations from standard usage of ASN.1. The purpose of
   these notes is to instead describe some historical quirks and non-
   compliance of various implementations, as well as historical
   ambiguities, which, while being valid ASN.1, can lead to confusion
   during implementation.

5.1.1. ASN.1 Distinguished Encoding Rules

   The encoding of Kerberos protocol messages shall obey the
   Distinguished Encoding Rules (DER) of ASN.1 as described in [X690].
   Some implementations (believed to be primarly ones derived from DCE
   1.1 and earlier) are known to use the more general Basic Encoding
   Rules (BER); in particular, these implementations send indefinite
   encodings of lengths. Implementations MAY accept such encodings in
   the interests of backwards compatibility, though implementors are
   warned that decoding fully-general BER is fraught with peril.

5.1.2. Optional Integer Fields

   Some implementations do not internally distinguish between an omitted
   optional integer value and a transmitted value of zero. The places in
   the protocol where this is relevant include various microseconds
   fields, nonces, and sequence numbers. Implementations SHOULD treat
   omitted optional integer values as having been transmitted with a
   value of zero, if the application is expecting this.

5.1.3. Empty SEQUENCE OF Types

   There are places in the protocol where a message contains a SEQUENCE
   OF type as an optional member. This can result in an encoding that
   contains an empty SEQUENCE OF encoding. The Kerberos protocol does
   not semantically distinguish between an absent optional SEQUENCE OF
   type and a present optional but empty SEQUENCE OF type.
   Implementations SHOULD NOT send empty SEQUENCE OF encodings that are
   marked OPTIONAL, but SHOULD accept them as being equivalent to an



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   omitted OPTIONAL type. In the ASN.1 syntax describing Kerberos
   messages, instances of these problematic optional SEQUENCE OF types
   are indicated with a comment.

5.1.4. Unrecognized Tag Numbers

   Future revisions to this protocol may include new message types with
   different APPLICATION class tag numbers. Such revisions should
   protect older implementations by only sending the message types to
   parties that are known to understand them, e.g. by means of a flag
   bit set by the receiver in a preceding request. In the interest of
   robust error handling, implementations SHOULD gracefully handle
   receiving a message with an unrecognized tag anyway, and return an
   error message if appropriate.

5.1.5. Tag Numbers Greater Than 30

   A naive implementation of a DER ASN.1 decoder may experience problems
   with ASN.1 tag numbers greater than 30, due to such tag numbers being
   encoded using more than one byte. Future revisions of this protocol
   may utilize tag numbers greater than 30, and implementations SHOULD
   be prepared to gracefully return an error, if appropriate, if they do
   not recognize the tag.

5.2. Basic Kerberos Types

   This section defines a number of basic types that are potentially
   used in multiple Kerberos protocol messages.

5.2.1. KerberosString

   The original specification of the Kerberos protocol in RFC 1510 uses
   GeneralString in numerous places for human-readable string data.
   Historical implementations of Kerberos cannot utilize the full power
   of GeneralString.  This ASN.1 type requires the use of designation
   and invocation escape sequences as specified in ISO-2022/ECMA-35
   [ISO-2022/ECMA-35] to switch character sets, and the default
   character set that is designated as G0 is the ISO-646/ECMA-6
   [ISO-646,ECMA-6] International Reference Version (IRV) (aka U.S.
   ASCII), which mostly works.

   ISO-2022/ECMA-35 defines four character-set code elements (G0..G3)
   and two Control-function code elements (C0..C1). DER prohibits the
   designation of character sets as any but the G0 and C0 sets.
   Unfortunately, this seems to have the side effect of prohibiting the
   use of ISO-8859 (ISO Latin) [ISO-8859] character-sets or any other
   character-sets that utilize a 96-character set, since it is
   prohibited by ISO-2022/ECMA-35 to designate them as the G0 code



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   element. This side effect is being investigated in the ASN.1
   standards community.

   In practice, many implementations treat GeneralStrings as if they
   were 8-bit strings of whichever character set the implementation
   defaults to, without regard for correct usage of character-set
   designation escape sequences. The default character set is often
   determined by the current user's operating system dependent locale.
   At least one major implementation places unescaped UTF-8 encoded
   Unicode characters in the GeneralString. This failure to adhere to
   the GeneralString specifications results in interoperability issues
   when conflicting character encodings are utilized by the Kerberos
   clients, services, and KDC.

   This unfortunate situation is the result of improper documentation of
   the restrictions of the ASN.1 GeneralString type in prior Kerberos
   specifications.

   The new (post-RFC 1510) type KerberosString, defined below, is a
   GeneralString that is constrained to only contain characters in
   IA5String

      KerberosString  ::= GeneralString (IA5String)

   US-ASCII control characters should in general not be used in
   KerberosString, except for cases such as newlines in lengthy error
   messages. Control characters SHOULD NOT be used in principal names or
   realm names.

   For compatibility, implementations MAY choose to accept GeneralString
   values that contain characters other than those permitted by
   IA5String, but they should be aware that character set designation
   codes will likely be absent, and that the encoding should probably be
   treated as locale-specific in almost every way. Implementations MAY
   also choose to emit GeneralString values that are beyond those
   permitted by IA5String, but should be aware that doing so is
   extraordinarily risky from an interoperability perspective.

   Some existing implementations use GeneralString to encode unescaped
   locale-specific characters. This is a violation of the ASN.1
   standard. Most of these implementations encode US-ASCII in the left-
   hand half, so as long the implementation transmits only US-ASCII, the
   ASN.1 standard is not violated in this regard. As soon as such an
   implementation encodes unescaped locale-specific characters with the
   high bit set, it violates the ASN.1 standard.

   Other implementations have been known to use GeneralString to contain
   a UTF-8 encoding. This also violates the ASN.1 standard, since UTF-8



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   is a different encoding, not a 94 or 96 character "G" set as defined
   by ISO 2022.  It is believed that these implementations do not even
   use the ISO 2022 escape sequence to change the character encoding.
   Even if implementations were to announce the change of encoding by
   using that escape sequence, the ASN.1 standard prohibits the use of
   any escape sequences other than those used to designate/invoke "G" or
   "C" sets allowed by GeneralString.

   Future revisions to this protocol will almost certainly allow for a
   more interoperable representation of principal names, probably
   including UTF8String.

   Note that applying a new constraint to a previously unconstrained
   type constitutes creation of a new ASN.1 type. In this particular
   case, the change does not result in a changed encoding under DER.

5.2.2. Realm and PrincipalName

   Realm           ::= KerberosString

   PrincipalName   ::= SEQUENCE {
           name-type       [0] Int32,
           name-string     [1] SEQUENCE OF KerberosString
   }

   Kerberos realm names are encoded as KerberosStrings. Realms shall not
   contain a character with the code 0 (the US-ASCII NUL). Most realms
   will usually consist of several components separated by periods (.),
   in the style of Internet Domain Names, or separated by slashes (/) in
   the style of X.500 names. Acceptable forms for realm names are
   specified in section 6.1.. A PrincipalName is a typed sequence of
   components consisting of the following sub-fields:

   name-type
      This field specifies the type of name that follows. Pre-defined
      values for this field are specified in section 6.2. The name-type
      SHOULD be treated as a hint. Ignoring the name type, no two names
      can be the same (i.e. at least one of the components, or the
      realm, must be different).

   name-string
      This field encodes a sequence of components that form a name, each
      component encoded as a KerberosString. Taken together, a
      PrincipalName and a Realm form a principal identifier. Most
      PrincipalNames will have only a few components (typically one or
      two).

5.2.3. KerberosTime



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   KerberosTime    ::= GeneralizedTime -- with no fractional seconds

   The timestamps used in Kerberos are encoded as GeneralizedTimes. A
   KerberosTime value shall not include any fractional portions of the
   seconds.  As required by the DER, it further shall not include any
   separators, and it shall specify the UTC time zone (Z). Example: The
   only valid format for UTC time 6 minutes, 27 seconds after 9 pm on 6
   November 1985 is 19851106210627Z.

5.2.4. Constrained Integer types

   Some integer members of types SHOULD be constrained to values
   representable in 32 bits, for compatibility with reasonable
   implementation limits.

   Int32           ::= INTEGER (-2147483648..2147483647)
                       -- signed values representable in 32 bits

   UInt32          ::= INTEGER (0..4294967295)
                       -- unsigned 32 bit values

   Microseconds    ::= INTEGER (0..999999)
                       -- microseconds

   While this results in changes to the abstract types from the RFC 1510
   version, the encoding in DER should be unaltered. Historical
   implementations were typically limited to 32-bit integer values
   anyway, and assigned numbers SHOULD fall in the space of integer
   values representable in 32 bits in order to promote interoperability
   anyway.

   There are several integer fields in messages that are constrained to
   fixed values.

   pvno
      also TKT-VNO or AUTHENTICATOR-VNO, this recurring field is always
      the constant integer 5. There is no easy way to make this field
      into a useful protocol version number, so its value is fixed.

   msg-type
      this integer field is usually identical to the application tag
      number of the containing message type.

5.2.5. HostAddress and HostAddresses

   HostAddress     ::= SEQUENCE  {
           addr-type       [0] Int32,
           address         [1] OCTET STRING



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   }

   -- NOTE: HostAddresses is always used as an OPTIONAL field and
   -- should not be empty.
   HostAddresses   -- NOTE: subtly different from rfc1510,
                   -- but has a value mapping and encodes the same
           ::= SEQUENCE OF HostAddress

   The host address encodings consists of two fields:

   addr-type
      This field specifies the type of address that follows. Pre-defined
      values for this field are specified in section 7.5.3.

   address
      This field encodes a single address of type addr-type.

5.2.6. AuthorizationData

      -- NOTE: AuthorizationData is always used as an OPTIONAL field and
      -- should not be empty.
      AuthorizationData       ::= SEQUENCE OF SEQUENCE {
              ad-type         [0] Int32,
              ad-data         [1] OCTET STRING
      }

   ad-data
      This field contains authorization data to be interpreted according
      to the value of the corresponding ad-type field.

   ad-type
      This field specifies the format for the ad-data subfield. All
      negative values are reserved for local use. Non-negative values
      are reserved for registered use.

   Each sequence of type and data is referred to as an authorization
   element.  Elements MAY be application specific, however, there is a
   common set of recursive elements that should be understood by all
   implementations. These elements contain other elements embedded
   within them, and the interpretation of the encapsulating element
   determines which of the embedded elements must be interpreted, and
   which may be ignored.

   These common authorization data elements are recursively defined,
   meaning the ad-data for these types will itself contain a sequence of
   authorization data whose interpretation is affected by the
   encapsulating element. Depending on the meaning of the encapsulating
   element, the encapsulated elements may be ignored, might be



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   interpreted as issued directly by the KDC, or they might be stored in
   a separate plaintext part of the ticket. The types of the
   encapsulating elements are specified as part of the Kerberos
   specification because the behavior based on these values should be
   understood across implementations whereas other elements need only be
   understood by the applications which they affect.

   Authorization data elements are considered critical if present in a
   ticket or authenticator. Unless encapsulated in a known authorization
   data element amending the criticality of the elements it contains, if
   an unknown authorization data element type is received by a server
   either in an AP-REQ or in a ticket contained in an AP-REQ, then
   authentication MUST fail.  Authorization data is intended to restrict
   the use of a ticket. If the service cannot determine whether the
   restriction applies to that service then a security weakness may
   result if the ticket can be used for that service. Authorization
   elements that are optional can be enclosed in AD-IF-RELEVANT element.

   In the definitions that follow, the value of the ad-type for the
   element will be specified as the least significant part of the
   subsection number, and the value of the ad-data will be as shown in
   the ASN.1 structure that follows the subsection heading.

             contents of ad-data          ad-type

    DER encoding of AD-IF-RELEVANT        1

    DER encoding of AD-KDCIssued          4

    DER encoding of AD-AND-OR             5

    DER encoding of AD-MANDATORY-FOR-KDC  8

5.2.6.1. IF-RELEVANT

   AD-IF-RELEVANT          ::= AuthorizationData

   AD elements encapsulated within the if-relevant element are intended
   for interpretation only by application servers that understand the
   particular ad-type of the embedded element. Application servers that
   do not understand the type of an element embedded within the if-
   relevant element MAY ignore the uninterpretable element. This element
   promotes interoperability across implementations which may have local
   extensions for authorization.  The ad-type for AD-IF-RELEVANT is (1).

5.2.6.2. KDCIssued

   AD-KDCIssued            ::= SEQUENCE {



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           ad-checksum     [0] Checksum,
           i-realm         [1] Realm OPTIONAL,
           i-sname         [2] PrincipalName OPTIONAL,
           elements        [3] AuthorizationData
   }

   ad-checksum
      A checksum over the elements field using a cryptographic checksum
      method that is identical to the checksum used to protect the
      ticket itself (i.e. using the same hash function and the same
      encryption algorithm used to encrypt the ticket) using the key
      used to protect the ticket, and a key usage value of 19.

   i-realm, i-sname
      The name of the issuing principal if different from the KDC
      itself.  This field would be used when the KDC can verify the
      authenticity of elements signed by the issuing principal and it
      allows this KDC to notify the application server of the validity
      of those elements.

   elements
      A sequence of authorization data elements issued by the KDC.

   The KDC-issued ad-data field is intended to provide a means for
   Kerberos principal credentials to embed within themselves privilege
   attributes and other mechanisms for positive authorization,
   amplifying the privileges of the principal beyond what can be done
   using a credentials without such an a-data element.

   This can not be provided without this element because the definition
   of the authorization-data field allows elements to be added at will
   by the bearer of a TGT at the time that they request service tickets
   and elements may also be added to a delegated ticket by inclusion in
   the authenticator.

   For KDC-issued elements this is prevented because the elements are
   signed by the KDC by including a checksum encrypted using the
   server's key (the same key used to encrypt the ticket - or a key
   derived from that key). Elements encapsulated with in the KDC-issued
   element will be ignored by the application server if this "signature"
   is not present. Further, elements encapsulated within this element
   from a ticket-granting ticket MAY be interpreted by the KDC, and used
   as a basis according to policy for including new signed elements
   within derivative tickets, but they will not be copied to a
   derivative ticket directly. If they are copied directly to a
   derivative ticket by a KDC that is not aware of this element, the
   signature will not be correct for the application ticket elements,
   and the field will be ignored by the application server.



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   This element and the elements it encapulates MAY be safely ignored by
   applications, application servers, and KDCs that do not implement
   this element.

   The ad-type for AD-KDC-ISSUED is (4).

5.2.6.3. AND-OR

   AD-AND-OR               ::= SEQUENCE {
           condition-count [0] INTEGER,
           elements        [1] AuthorizationData
   }


   When restrictive AD elements are encapsulated within the and-or
   element, the and-or element is considered satisfied if and only if at
   least the number of encapsulated elements specified in condition-
   count are satisifed.  Therefore, this element MAY be used to
   implement an "or" operation by setting the condition-count field to
   1, and it MAY specify an "and" operation by setting the condition
   count to the number of embedded elements. Application servers that do
   not implement this element MUST reject tickets that contain
   authorization data elements of this type.

   The ad-type for AD-AND-OR is (5).

5.2.6.4. MANDATORY-FOR-KDC

   AD-MANDATORY-FOR-KDC    ::= AuthorizationData

   AD elements encapsulated within the mandatory-for-kdc element are to
   be interpreted by the KDC. KDCs that do not understand the type of an
   element embedded within the mandatory-for-kdc element MUST reject the
   request.

   The ad-type for AD-MANDATORY-FOR-KDC is (8).

5.2.7. PA-DATA

   Historically, PA-DATA have been known as "pre-authentication data",
   meaning that they were used to augment the initial authentication
   with the KDC.  Since that time, they have also been used as a typed
   hole with which to extend protocol exchanges with the KDC.

   PA-DATA         ::= SEQUENCE {
           -- NOTE: first tag is [1], not [0]
           padata-type     [1] Int32,
           padata-value    [2] OCTET STRING -- might be encoded AP-REQ



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   }

   padata-type
      indicates the way that the padata-value element is to be
      interpreted.  Negative values of padata-type are reserved for
      unregistered use; non-negative values are used for a registered
      interpretation of the element type.

   padata-value
      Usually contains the DER encoding of another type; the padata-type
      field identifies which type is encoded here.

       padata-type        name           contents of padata-value

       1            pa-tgs-req       DER encoding of AP-REQ

       2            pa-enc-timestamp DER encoding of PA-ENC-TIMESTAMP

       3            pa-pw-salt       salt (not ASN.1 encoded)

       11           pa-etype-info    DER encoding of ETYPE-INFO

       19           pa-etype-info2   DER encoding of ETYPE-INFO2

      This field MAY also contain information needed by certain
      extensions to the Kerberos protocol. For example, it might be used
      to initially verify the identity of a client before any response
      is returned.

      The padata field can also contain information needed to help the
      KDC or the client select the key needed for generating or
      decrypting the response. This form of the padata is useful for
      supporting the use of certain token cards with Kerberos. The
      details of such extensions are specified in separate documents.
      See [Pat92] for additional uses of this field.

5.2.7.1. PA-TGS-REQ

   In the case of requests for additional tickets (KRB_TGS_REQ), padata-
   value will contain an encoded AP-REQ. The checksum in the
   authenticator (which MUST be collision-proof) is to be computed over
   the KDC-REQ-BODY encoding.

5.2.7.2. Encrypted Timestamp Pre-authentication

   There are pre-authentication types that may be used to pre-
   authenticate a client by means of an encrypted timestamp.




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   PA-ENC-TIMESTAMP        ::= EncryptedData -- PA-ENC-TS-ENC

   PA-ENC-TS-ENC           ::= SEQUENCE {
           patimestamp     [0] KerberosTime -- client's time --,
           pausec          [1] Microseconds OPTIONAL
   }

   Patimestamp contains the client's time, and pausec contains the
   microseconds, which MAY be omitted if a client will not generate more
   than one request per second. The ciphertext (padata-value) consists
   of the PA-ENC-TS-ENC encoding, encrypted using the client's secret
   key and a key usage value of 1.

   This pre-authentication type was not present in RFC 1510, but many
   implementations support it.

5.2.7.3. PA-PW-SALT

   The padata-value for this pre-authentication type contains the salt
   for the string-to-key to be used by the client to obtain the key for
   decrypting the encrypted part of an AS-REP message. Unfortunately,
   for historical reasons, the character set to be used is unspecified
   and probably locale-specific.

   This pre-authentication type was not present in RFC 1510, but many
   implementations support it. It is necessary in any case where the
   salt for the string-to-key algorithm is not the default.

   In the trivial example, a zero-length salt string is very commonplace
   for realms that have converted their principal databases from
   Kerberos 4.

   A KDC SHOULD NOT send PA-PW-SALT when issuing a KRB-ERROR message
   that requests additional pre-authentication. Implementation note:
   some KDC implementations issue an erroneous PA-PW-SALT when issuing a
   KRB-ERROR message that requests additional pre-authentication.
   Therefore, clients SHOULD ignore a PA-PW-SALT accompanying a KRB-
   ERROR message that requests additional pre-authentication.

5.2.7.4. PA-ETYPE-INFO

   The ETYPE-INFO pre-authentication type is sent by the KDC in a KRB-
   ERROR indicating a requirement for additional pre-authentication. It
   is usually used to notify a client of which key to use for the
   encryption of an encrypted timestamp for the purposes of sending a
   PA-ENC-TIMESTAMP pre-authentication value. It MAY also be sent in an
   AS-REP to provide information to the client about which key salt to
   use for the string-to-key to be used by the client to obtain the key



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   for decrypting the encrypted part the AS-REP.

   ETYPE-INFO-ENTRY        ::= SEQUENCE {
           etype           [0] Int32,
           salt            [1] OCTET STRING OPTIONAL
   }

   ETYPE-INFO              ::= SEQUENCE OF ETYPE-INFO-ENTRY

   The salt, like that of PA-PW-SALT, is also completely unspecified
   with respect to character set and is probably locale-specific.

   If ETYPE-INFO is sent in an AS-REP, there shall be exactly one ETYPE-
   INFO-ENTRY, and its etype shall match that of the enc-part in the AS-
   REP.

   This pre-authentication type was not present in RFC 1510, but many
   implementations that support encrypted timestamps for pre-
   authentication need to support ETYPE-INFO as well.

5.2.7.5. PA-ETYPE-INFO2

   The ETYPE-INFO2 pre-authentication type is sent by the KDC in a KRB-
   ERROR indicating a requirement for additional pre-authentication. It
   is usually used to notify a client of which key to use for the
   encryption of an encrypted timestamp for the purposes of sending a
   PA-ENC-TIMESTAMP pre-authentication value. It MAY also be sent in an
   AS-REP to provide information to the client about which key salt to
   use for the string-to-key to be used by the client to obtain the key
   for decrypting the encrypted part the AS-REP.

   ETYPE-INFO2-ENTRY       ::= SEQUENCE {
           etype           [0] Int32,
           salt            [1] KerberosString OPTIONAL,
           s2kparams       [2] OCTET STRING OPTIONAL
   }

   ETYPE-INFO2              ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO-ENTRY

   The type of the salt is KerberosString, but existing installations
   might have locale-specific characters stored in salt strings, and
   implementors MAY choose to handle them.

   The interpretation of s2kparams is specified in the cryptosystem
   description associated with the etype. Each cryptosystem has a
   default interpretation of s2kparams that will hold if that element is
   omitted from the encoding of ETYPE-INFO2-ENTRY.




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   If ETYPE-INFO2 is sent in an AS-REP, there shall be exactly one
   ETYPE-INFO2-ENTRY, and its etype shall match that of the enc-part in
   the AS-REP.

   The preferred ordering of pre-authentication data that modify client
   key selection is: ETYPE-INFO2, followed by ETYPE-INFO, followed by
   PW-SALT. A KDC shall send all of these pre-authentication data that
   it supports, in the preferred ordering, when issuing an AS-REP or
   when issuing a KRB-ERROR requesting additional pre-authentication.

   The ETYPE-INFO2 pre-authentication type was not present in RFC 1510.

5.2.8. KerberosFlags

   For several message types, a specific constrained bit string type,
   KerberosFlags, is used.

   KerberosFlags   ::= BIT STRING (SIZE (32..MAX)) -- minimum number of bits
                       -- shall be sent, but no fewer than 32

   Compatibility note: the following paragraphs describe a change from
   the RFC1510 description of bit strings that would result in
   incompatility in the case of an implementation that strictly
   conformed to ASN.1 DER and RFC1510.

   ASN.1 bit strings have multiple uses. The simplest use of a bit
   string is to contain a vector of bits, with no particular meaning
   attached to individual bits. This vector of bits is not necessarily a
   multiple of eight bits long.  The use in Kerberos of a bit string as
   a compact boolean vector wherein each element has a distinct meaning
   poses some problems. The natural notation for a compact boolean
   vector is the ASN.1 "NamedBit" notation, and the DER require that
   encodings of a bit string using "NamedBit" notation exclude any
   trailing zero bits. This truncation is easy to neglect, especially
   given C language implementations that naturally choose to store
   boolean vectors as 32 bit integers.

   For example, if the notation for KDCOptions were to include the
   "NamedBit" notation, as in RFC 1510, and a KDCOptions value to be
   encoded had only the "forwardable" (bit number one) bit set, the DER
   encoding MUST include only two bits: the first reserved bit
   ("reserved", bit number zero, value zero) and the one-valued bit (bit
   number one) for "forwardable".

   Most existing implementations of Kerberos unconditionally send 32
   bits on the wire when encoding bit strings used as boolean vectors.
   This behavior violates the ASN.1 syntax used for flag values in RFC
   1510, but occurs on such a widely installed base that the protocol



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   description is being modified to accomodate it.

   Consequently, this document removes the "NamedBit" notations for
   individual bits, relegating them to comments. The size constraint on
   the KerberosFlags type requires that at least 32 bits be encoded at
   all times, though a lenient implementation MAY choose to accept fewer
   than 32 bits and to treat the missing bits as set to zero.

   Currently, no uses of KerberosFlags specify more than 32 bits worth
   of flags, although future revisions of this document may do so. When
   more than 32 bits are to be transmitted in a KerberosFlags value,
   future revisions to this document will likely specify that the
   smallest number of bits needed to encode the highest-numbered one-
   valued bit should be sent. This is somewhat similar to the DER
   encoding of a bit string that is declared with the "NamedBit"
   notation.

5.2.9. Cryptosystem-related Types

   Many Kerberos protocol messages contain an EncryptedData as a
   container for arbitrary encrypted data, which is often the encrypted
   encoding of another data type. Fields within EncryptedData assist the
   recipient in selecting a key with which to decrypt the enclosed data.

   EncryptedData   ::= SEQUENCE {
           etype   [0] Int32 -- EncryptionType --,
           kvno    [1] UInt32 OPTIONAL,
           cipher  [2] OCTET STRING -- ciphertext
   }

   etype
      This field identifies which encryption algorithm was used to
      encipher the cipher.

   kvno
      This field contains the version number of the key under which data
      is encrypted. It is only present in messages encrypted under long
      lasting keys, such as principals' secret keys.

   cipher
      This field contains the enciphered text, encoded as an OCTET
      STRING.  (Note that the encryption mechanisms defined in
      [@KCRYPTO] MUST incorporate integrity protection as well, so no
      additional checksum is required.)

   The EncryptionKey type is the means by which cryptographic keys used
   for encryption are transfered.




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   EncryptionKey   ::= SEQUENCE {
           keytype         [0] Int32 -- actually encryption type --,
           keyvalue        [1] OCTET STRING
   }

   keytype
      This field specifies the encryption type of the encryption key
      that follows in the keyvalue field. While its name is "keytype",
      it actually specifies an encryption type. Previously, multiple
      cryptosystems that performed encryption differently but were
      capable of using keys with the same characteristics were permitted
      to share an assigned number to designate the type of key; this
      usage is now deprecated.

   keyvalue
      This field contains the key itself, encoded as an octet string.

   Messages containing cleartext data to be authenticated will usually
   do so by using a member of type Checksum. Most instances of Checksum
   use a keyed hash, though exceptions will be noted.

   Checksum        ::= SEQUENCE {
           cksumtype       [0] Int32,
           checksum        [1] OCTET STRING
   }

   cksumtype
      This field indicates the algorithm used to generate the
      accompanying checksum.

   checksum
      This field contains the checksum itself, encoded as an octet
      string.

   See section 4 for a brief description of the use of encryption and
   checksums in Kerberos.

5.3. Tickets

   This section describes the format and encryption parameters for
   tickets and authenticators. When a ticket or authenticator is
   included in a protocol message it is treated as an opaque object. A
   ticket is a record that helps a client authenticate to a service. A
   Ticket contains the following information:

   Ticket          ::= [APPLICATION 1] SEQUENCE {
           tkt-vno         [0] INTEGER (5),
           realm           [1] Realm,



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           sname           [2] PrincipalName,
           enc-part        [3] EncryptedData -- EncTicketPart
   }

   -- Encrypted part of ticket
   EncTicketPart   ::= [APPLICATION 3] SEQUENCE {
           flags                   [0] TicketFlags,
           key                     [1] EncryptionKey,
           crealm                  [2] Realm,
           cname                   [3] PrincipalName,
           transited               [4] TransitedEncoding,
           authtime                [5] KerberosTime,
           starttime               [6] KerberosTime OPTIONAL,
           endtime                 [7] KerberosTime,
           renew-till              [8] KerberosTime OPTIONAL,
           caddr                   [9] HostAddresses OPTIONAL,
           authorization-data      [10] AuthorizationData OPTIONAL
   }

   -- encoded Transited field
   TransitedEncoding       ::= SEQUENCE {
           tr-type         [0] Int32 -- must be registered --,
           contents        [1] OCTET STRING
   }

   TicketFlags     ::= KerberosFlags
           -- reserved(0),
           -- forwardable(1),
           -- forwarded(2),
           -- proxiable(3),
           -- proxy(4),
           -- may-postdate(5),
           -- postdated(6),
           -- invalid(7),
           -- renewable(8),
           -- initial(9),
           -- pre-authent(10),
           -- hw-authent(11),
   -- the following are new since 1510
           -- transited-policy-checked(12),
           -- ok-as-delegate(13)

   tkt-vno
      This field specifies the version number for the ticket format.
      This document describes version number 5.

   realm
      This field specifies the realm that issued a ticket. It also



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      serves to identify the realm part of the server's principal
      identifier. Since a Kerberos server can only issue tickets for
      servers within its realm, the two will always be identical.

   sname
      This field specifies all components of the name part of the
      server's identity, including those parts that identify a specific
      instance of a service.

   enc-part
      This field holds the encrypted encoding of the EncTicketPart
      sequence.  It is encrypted in the key shared by Kerberos and the
      end server (the server's secret key), using a key usage value of
      2.

   flags
      This field indicates which of various options were used or
      requested when the ticket was issued. The meanings of the flags
      are:

         Bit(s)  Name                   Description

         0       reserved               Reserved for future expansion of this
                                        field.

                                        The FORWARDABLE flag is normally only
                                        interpreted by the TGS, and can be
                                        ignored by end servers. When set, this
         1       forwardable            flag tells the ticket-granting server
                                        that it is OK to issue a new
                                        ticket-granting ticket with a
                                        different network address based on the
                                        presented ticket.

                                        When set, this flag indicates that the
                                        ticket has either been forwarded or
         2       forwarded              was issued based on authentication
                                        involving a forwarded ticket-granting
                                        ticket.

                                        The PROXIABLE flag is normally only
                                        interpreted by the TGS, and can be
                                        ignored by end servers. The PROXIABLE
                                        flag has an interpretation identical
         3       proxiable              to that of the FORWARDABLE flag,
                                        except that the PROXIABLE flag tells
                                        the ticket-granting server that only
                                        non-ticket-granting tickets may be



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                                        issued with different network
                                        addresses.

         4       proxy                  When set, this flag indicates that a
                                        ticket is a proxy.

                                        The MAY-POSTDATE flag is normally only
                                        interpreted by the TGS, and can be
         5       may-postdate           ignored by end servers. This flag
                                        tells the ticket-granting server that
                                        a post-dated ticket MAY be issued
                                        based on this ticket-granting ticket.

                                        This flag indicates that this ticket
                                        has been postdated. The end-service
         6       postdated              can check the authtime field to see
                                        when the original authentication
                                        occurred.

                                        This flag indicates that a ticket is
                                        invalid, and it must be validated by
         7       invalid                the KDC before use. Application
                                        servers must reject tickets which have
                                        this flag set.

                                        The RENEWABLE flag is normally only
                                        interpreted by the TGS, and can
                                        usually be ignored by end servers
         8       renewable              (some particularly careful servers MAY
                                        disallow renewable tickets). A
                                        renewable ticket can be used to obtain
                                        a replacement ticket that expires at a
                                        later date.

                                        This flag indicates that this ticket
         9       initial                was issued using the AS protocol, and
                                        not issued based on a ticket-granting
                                        ticket.

                                        This flag indicates that during
                                        initial authentication, the client was
                                        authenticated by the KDC before a
         10      pre-authent            ticket was issued. The strength of the
                                        pre-authentication method is not
                                        indicated, but is acceptable to the
                                        KDC.

                                        This flag indicates that the protocol



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                                        employed for initial authentication
                                        required the use of hardware expected
         11      hw-authent             to be possessed solely by the named
                                        client. The hardware authentication
                                        method is selected by the KDC and the
                                        strength of the method is not
                                        indicated.

                                        This flag indicates that the KDC for
                                        the realm has checked the transited
                                        field against a realm defined policy
                                        for trusted certifiers. If this flag
                                        is reset (0), then the application
                                        server must check the transited field
                                        itself, and if unable to do so it must
                                        reject the authentication. If the flag
         12      transited-             is set (1) then the application server
                 policy-checked         MAY skip its own validation of the
                                        transited field, relying on the
                                        validation performed by the KDC. At
                                        its option the application server MAY
                                        still apply its own validation based
                                        on a separate policy for acceptance.

                                        This flag is new since RFC 1510.

                                        This flag indicates that the server
                                        (not the client) specified in the
                                        ticket has been determined by policy
                                        of the realm to be a suitable
                                        recipient of delegation. A client can
                                        use the presence of this flag to help
                                        it make a decision whether to delegate
                                        credentials (either grant a proxy or a
                                        forwarded ticket-granting ticket) to
         13      ok-as-delegate         this server. The client is free to
                                        ignore the value of this flag. When
                                        setting this flag, an administrator
                                        should consider the Security and
                                        placement of the server on which the
                                        service will run, as well as whether
                                        the service requires the use of
                                        delegated credentials.

                                        This flag is new since RFC 1510.

         14-31   reserved               Reserved for future use.




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   key
      This field exists in the ticket and the KDC response and is used
      to pass the session key from Kerberos to the application server
      and the client.

   crealm
      This field contains the name of the realm in which the client is
      registered and in which initial authentication took place.

   cname
      This field contains the name part of the client's principal
      identifier.

   transited
      This field lists the names of the Kerberos realms that took part
      in authenticating the user to whom this ticket was issued. It does
      not specify the order in which the realms were transited. See
      section 3.3.3.2 for details on how this field encodes the
      traversed realms.  When the names of CA's are to be embedded in
      the transited field (as specified for some extensions to the
      protocol), the X.500 names of the CA's SHOULD be mapped into items
      in the transited field using the mapping defined by RFC2253.

   authtime
      This field indicates the time of initial authentication for the
      named principal. It is the time of issue for the original ticket
      on which this ticket is based. It is included in the ticket to
      provide additional information to the end service, and to provide
      the necessary information for implementation of a `hot list'
      service at the KDC. An end service that is particularly paranoid
      could refuse to accept tickets for which the initial
      authentication occurred "too far" in the past. This field is also
      returned as part of the response from the KDC.  When returned as
      part of the response to initial authentication (KRB_AS_REP), this
      is the current time on the Kerberos server.  It is NOT recommended
      that this time value be used to adjust the workstation's clock
      since the workstation cannot reliably determine that such a
      KRB_AS_REP actually came from the proper KDC in a timely manner.


   starttime

      This field in the ticket specifies the time after which the ticket
      is valid. Together with endtime, this field specifies the life of
      the ticket. If the starttime field is absent from the ticket, then
      the authtime field SHOULD be used in its place to determine the
      life of the ticket.




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   endtime
      This field contains the time after which the ticket will not be
      honored (its expiration time). Note that individual services MAY
      place their own limits on the life of a ticket and MAY reject
      tickets which have not yet expired. As such, this is really an
      upper bound on the expiration time for the ticket.

   renew-till
      This field is only present in tickets that have the RENEWABLE flag
      set in the flags field. It indicates the maximum endtime that may
      be included in a renewal. It can be thought of as the absolute
      expiration time for the ticket, including all renewals.

   caddr
      This field in a ticket contains zero (if omitted) or more (if
      present) host addresses. These are the addresses from which the
      ticket can be used. If there are no addresses, the ticket can be
      used from any location. The decision by the KDC to issue or by the
      end server to accept addressless tickets is a policy decision and
      is left to the Kerberos and end-service administrators; they MAY
      refuse to issue or accept such tickets. Because of the wide
      deployment of network address translation, it is recommended that
      policy allow the issue and acceptance of such tickets.

      Network addresses are included in the ticket to make it harder for
      an attacker to use stolen credentials. Because the session key is
      not sent over the network in cleartext, credentials can't be
      stolen simply by listening to the network; an attacker has to gain
      access to the session key (perhaps through operating system
      security breaches or a careless user's unattended session) to make
      use of stolen tickets.

      It is important to note that the network address from which a
      connection is received cannot be reliably determined. Even if it
      could be, an attacker who has compromised the client's workstation
      could use the credentials from there. Including the network
      addresses only makes it more difficult, not impossible, for an
      attacker to walk off with stolen credentials and then use them
      from a "safe" location.

   authorization-data
      The authorization-data field is used to pass authorization data
      from the principal on whose behalf a ticket was issued to the
      application service. If no authorization data is included, this
      field will be left out. Experience has shown that the name of this
      field is confusing, and that a better name for this field would be
      restrictions. Unfortunately, it is not possible to change the name
      of this field at this time.



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      This field contains restrictions on any authority obtained on the
      basis of authentication using the ticket. It is possible for any
      principal in posession of credentials to add entries to the
      authorization data field since these entries further restrict what
      can be done with the ticket.  Such additions can be made by
      specifying the additional entries when a new ticket is obtained
      during the TGS exchange, or they MAY be added during chained
      delegation using the authorization data field of the
      authenticator.

      Because entries may be added to this field by the holder of
      credentials, except when an entry is separately authenticated by
      encapsulation in the KDC-issued element, it is not allowable for
      the presence of an entry in the authorization data field of a
      ticket to amplify the privileges one would obtain from using a
      ticket.

      The data in this field may be specific to the end service; the
      field will contain the names of service specific objects, and the
      rights to those objects. The format for this field is described in
      section 5.2.6.  Although Kerberos is not concerned with the format
      of the contents of the sub-fields, it does carry type information
      (ad-type).

      By using the authorization_data field, a principal is able to
      issue a proxy that is valid for a specific purpose. For example, a
      client wishing to print a file can obtain a file server proxy to
      be passed to the print server. By specifying the name of the file
      in the authorization_data field, the file server knows that the
      print server can only use the client's rights when accessing the
      particular file to be printed.

      A separate service providing authorization or certifying group
      membership may be built using the authorization-data field. In
      this case, the entity granting authorization (not the authorized
      entity), may obtain a ticket in its own name (e.g. the ticket is
      issued in the name of a privilege server), and this entity adds
      restrictions on its own authority and delegates the restricted
      authority through a proxy to the client. The client would then
      present this authorization credential to the application server
      separately from the authentication exchange.  Alternatively, such
      authorization credentials MAY be embedded in the ticket
      authenticating the authorized entity, when the authorization is
      separately authenticated using the KDC-issued authorization data
      element (see 5.2.6.2).

      Similarly, if one specifies the authorization-data field of a
      proxy and leaves the host addresses blank, the resulting ticket



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      and session key can be treated as a capability. See [Neu93] for
      some suggested uses of this field.

      The authorization-data field is optional and does not have to be
      included in a ticket.

5.4. Specifications for the AS and TGS exchanges

   This section specifies the format of the messages used in the
   exchange between the client and the Kerberos server. The format of
   possible error messages appears in section 5.9.1.

5.4.1. KRB_KDC_REQ definition

   The KRB_KDC_REQ message has no application tag number of its own.
   Instead, it is incorporated into one of KRB_AS_REQ or KRB_TGS_REQ,
   which each have an application tag, depending on whether the request
   is for an initial ticket or an additional ticket. In either case, the
   message is sent from the client to the KDC to request credentials for
   a service.

   The message fields are:

   AS-REQ          ::= [APPLICATION 10] KDC-REQ

   TGS-REQ         ::= [APPLICATION 12] KDC-REQ

   KDC-REQ         ::= SEQUENCE {
           -- NOTE: first tag is [1], not [0]
           pvno            [1] INTEGER (5) ,
           msg-type        [2] INTEGER (10 -- AS -- | 12 -- TGS --),
           padata          [3] SEQUENCE OF PA-DATA OPTIONAL
                               -- NOTE: not empty --,
           req-body        [4] KDC-REQ-BODY
   }

   KDC-REQ-BODY    ::= SEQUENCE {
           kdc-options             [0] KDCOptions,
           cname                   [1] PrincipalName OPTIONAL
                                       -- Used only in AS-REQ --,
           realm                   [2] Realm
                                       -- Server's realm
                                       -- Also client's in AS-REQ --,
           sname                   [3] PrincipalName OPTIONAL,
           from                    [4] KerberosTime OPTIONAL,
           till                    [5] KerberosTime,
           rtime                   [6] KerberosTime OPTIONAL,
           nonce                   [7] UInt32,



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           etype                   [8] SEQUENCE OF Int32 -- EncryptionType
                                       -- in preference order --,
           addresses               [9] HostAddresses OPTIONAL,
           enc-authorization-data  [10] EncryptedData -- AuthorizationData --,
           additional-tickets      [11] SEQUENCE OF Ticket OPTIONAL
                                           -- NOTE: not empty
   }

   KDCOptions      ::= KerberosFlags
           -- reserved(0),
           -- forwardable(1),
           -- forwarded(2),
           -- proxiable(3),
           -- proxy(4),
           -- allow-postdate(5),
           -- postdated(6),
           -- unused7(7),
           -- renewable(8),
           -- unused9(9),
           -- unused10(10),
           -- opt-hardware-auth(11),
           -- unused12(12),
           -- unused13(13),
   -- 15 is reserved for canonicalize
           -- unused15(15),
   -- 26 was unused in 1510
           -- disable-transited-check(26),
   --
           -- renewable-ok(27),
           -- enc-tkt-in-skey(28),
           -- renew(30),
           -- validate(31)

   The fields in this message are:

   pvno
      This field is included in each message, and specifies the protocol
      version number. This document specifies protocol version 5.

   msg-type
      This field indicates the type of a protocol message. It will
      almost always be the same as the application identifier associated
      with a message. It is included to make the identifier more readily
      accessible to the application. For the KDC-REQ message, this type
      will be KRB_AS_REQ or KRB_TGS_REQ.

   padata
      Contains pre-authentication data. Requests for additional tickets



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      (KRB_TGS_REQ) MUST contain a padata of PA-TGS-REQ.

      The padata (pre-authentication data) field contains a sequence of
      authentication information which may be needed before credentials
      can be issued or decrypted.

   req-body
      This field is a placeholder delimiting the extent of the remaining
      fields. If a checksum is to be calculated over the request, it is
      calculated over an encoding of the KDC-REQ-BODY sequence which is
      enclosed within the req-body field.

   kdc-options
      This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to
      the KDC and indicates the flags that the client wants set on the
      tickets as well as other information that is to modify the
      behavior of the KDC.  Where appropriate, the name of an option may
      be the same as the flag that is set by that option. Although in
      most case, the bit in the options field will be the same as that
      in the flags field, this is not guaranteed, so it is not
      acceptable to simply copy the options field to the flags field.
      There are various checks that must be made before honoring an
      option anyway.

      The kdc_options field is a bit-field, where the selected options
      are indicated by the bit being set (1), and the unselected options
      and reserved fields being reset (0). The encoding of the bits is
      specified in section 5.2. The options are described in more detail
      above in section 2. The meanings of the options are:

         Bits    Name                     Description

         0       RESERVED                 Reserved for future expansion of
                                          this field.

                                          The FORWARDABLE option indicates
                                          that the ticket to be issued is to
                                          have its forwardable flag set. It
         1       FORWARDABLE              may only be set on the initial
                                          request, or in a subsequent request
                                          if the ticket-granting ticket on
                                          which it is based is also
                                          forwardable.

                                          The FORWARDED option is only
                                          specified in a request to the
                                          ticket-granting server and will only
                                          be honored if the ticket-granting



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                                          ticket in the request has its
         2       FORWARDED                FORWARDABLE bit set. This option
                                          indicates that this is a request for
                                          forwarding. The address(es) of the
                                          host from which the resulting ticket
                                          is to be valid are included in the
                                          addresses field of the request.

                                          The PROXIABLE option indicates that
                                          the ticket to be issued is to have
                                          its proxiable flag set. It may only
         3       PROXIABLE                be set on the initial request, or in
                                          a subsequent request if the
                                          ticket-granting ticket on which it
                                          is based is also proxiable.

                                          The PROXY option indicates that this
                                          is a request for a proxy. This
                                          option will only be honored if the
                                          ticket-granting ticket in the
         4       PROXY                    request has its PROXIABLE bit set.
                                          The address(es) of the host from
                                          which the resulting ticket is to be
                                          valid are included in the addresses
                                          field of the request.

                                          The ALLOW-POSTDATE option indicates
                                          that the ticket to be issued is to
                                          have its MAY-POSTDATE flag set. It
         5       ALLOW-POSTDATE           may only be set on the initial
                                          request, or in a subsequent request
                                          if the ticket-granting ticket on
                                          which it is based also has its
                                          MAY-POSTDATE flag set.

                                          The POSTDATED option indicates that
                                          this is a request for a postdated
                                          ticket. This option will only be
                                          honored if the ticket-granting
                                          ticket on which it is based has its
         6       POSTDATED                MAY-POSTDATE flag set. The resulting
                                          ticket will also have its INVALID
                                          flag set, and that flag may be reset
                                          by a subsequent request to the KDC
                                          after the starttime in the ticket
                                          has been reached.

         7       RESERVED                 This option is presently unused.



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                                          The RENEWABLE option indicates that
                                          the ticket to be issued is to have
                                          its RENEWABLE flag set. It may only
                                          be set on the initial request, or
                                          when the ticket-granting ticket on
         8       RENEWABLE                which the request is based is also
                                          renewable. If this option is
                                          requested, then the rtime field in
                                          the request contains the desired
                                          absolute expiration time for the
                                          ticket.

         9       RESERVED                 Reserved for PK-Cross

         10      RESERVED                 Reserved for future use.

         11      RESERVED                 Reserved for opt-hardware-auth.

         12-25   RESERVED                 Reserved for future use.

                                          By default the KDC will check the
                                          transited field of a
                                          ticket-granting-ticket against the
                                          policy of the local realm before it
                                          will issue derivative tickets based
                                          on the ticket-granting ticket. If
                                          this flag is set in the request,
                                          checking of the transited field is
                                          disabled. Tickets issued without the
         26      DISABLE-TRANSITED-CHECK  performance of this check will be
                                          noted by the reset (0) value of the
                                          TRANSITED-POLICY-CHECKED flag,
                                          indicating to the application server
                                          that the tranisted field must be
                                          checked locally. KDCs are
                                          encouraged but not required to honor
                                          the DISABLE-TRANSITED-CHECK option.

                                          This flag is new since RFC 1510

                                          The RENEWABLE-OK option indicates
                                          that a renewable ticket will be
                                          acceptable if a ticket with the
                                          requested life cannot otherwise be
                                          provided. If a ticket with the
                                          requested life cannot be provided,
         27      RENEWABLE-OK             then a renewable ticket may be
                                          issued with a renew-till equal to



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                                          the requested endtime. The value
                                          of the renew-till field may still be
                                          limited by local limits, or limits
                                          selected by the individual principal
                                          or server.

                                          This option is used only by the
                                          ticket-granting service. The
                                          ENC-TKT-IN-SKEY option indicates
         28      ENC-TKT-IN-SKEY          that the ticket for the end server
                                          is to be encrypted in the session
                                          key from the additional
                                          ticket-granting ticket provided.

         29      RESERVED                 Reserved for future use.

                                          This option is used only by the
                                          ticket-granting service. The RENEW
                                          option indicates that the present
                                          request is for a renewal. The ticket
                                          provided is encrypted in the secret
                                          key for the server on which it is
         30      RENEW                    valid. This option will only be
                                          honored if the ticket to be renewed
                                          has its RENEWABLE flag set and if
                                          the time in its renew-till field has
                                          not passed. The ticket to be renewed
                                          is passed in the padata field as
                                          part of the authentication header.

                                          This option is used only by the
                                          ticket-granting service. The
                                          VALIDATE option indicates that the
                                          request is to validate a postdated
                                          ticket. It will only be honored if
                                          the ticket presented is postdated,
                                          presently has its INVALID flag set,
         31      VALIDATE                 and would be otherwise usable at
                                          this time. A ticket cannot be
                                          validated before its starttime. The
                                          ticket presented for validation is
                                          encrypted in the key of the server
                                          for which it is valid and is passed
                                          in the padata field as part of the
                                          authentication header.
   cname and sname
      These fields are the same as those described for the ticket in
      section 5.3. The sname may only be absent when the ENC-TKT-IN-SKEY



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      option is specified. If absent, the name of the server is taken
      from the name of the client in the ticket passed as additional-
      tickets.

   enc-authorization-data
      The enc-authorization-data, if present (and it can only be present
      in the TGS_REQ form), is an encoding of the desired authorization-
      data encrypted under the sub-session key if present in the
      Authenticator, or alternatively from the session key in the
      ticket-granting ticket (both the Authenticator and ticket-granting
      ticket come from the padata field in the KRB_TGS_REQ). The key
      usage value used when encrypting is 5 if a sub-session key is
      used, or 4 if the session key is used.

   realm
      This field specifies the realm part of the server's principal
      identifier. In the AS exchange, this is also the realm part of the
      client's principal identifier.

   from
      This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket
      requests when the requested ticket is to be postdated. It
      specifies the desired start time for the requested ticket. If this
      field is omitted then the KDC SHOULD use the current time instead.

   till
      This field contains the expiration date requested by the client in
      a ticket request. It is not optional, but if the requested endtime
      is "19700101000000Z", the requested ticket is to have the maximum
      endtime permitted according to KDC policy. Implementation note:
      This special timestamp corresponds to a UNIX time_t value of zero
      on most systems.

   rtime
      This field is the requested renew-till time sent from a client to
      the KDC in a ticket request. It is optional.

   nonce
      This field is part of the KDC request and response. It is intended
      to hold a random number generated by the client. If the same
      number is included in the encrypted response from the KDC, it
      provides evidence that the response is fresh and has not been
      replayed by an attacker.  Nonces MUST NEVER be reused.

   etype
      This field specifies the desired encryption algorithm to be used
      in the response.




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   addresses
      This field is included in the initial request for tickets, and
      optionally included in requests for additional tickets from the
      ticket-granting server. It specifies the addresses from which the
      requested ticket is to be valid. Normally it includes the
      addresses for the client's host. If a proxy is requested, this
      field will contain other addresses. The contents of this field are
      usually copied by the KDC into the caddr field of the resulting
      ticket.

   additional-tickets
      Additional tickets MAY be optionally included in a request to the
      ticket-granting server. If the ENC-TKT-IN-SKEY option has been
      specified, then the session key from the additional ticket will be
      used in place of the server's key to encrypt the new ticket. When
      the ENC-TKT-IN-SKEY option is used for user-to-user
      authentication, this addional ticket MAY be a TGT issued by the
      local realm or an inter-realm TGT issued for the current KDC's
      realm by a remote KDC. If more than one option which requires
      additional tickets has been specified, then the additional tickets
      are used in the order specified by the ordering of the options
      bits (see kdc-options, above).

   The application tag number will be either ten (10) or twelve (12)
   depending on whether the request is for an initial ticket (AS-REQ) or
   for an additional ticket (TGS-REQ).

   The optional fields (addresses, authorization-data and additional-
   tickets) are only included if necessary to perform the operation
   specified in the kdc-options field.

   It should be noted that in KRB_TGS_REQ, the protocol version number
   appears twice and two different message types appear: the KRB_TGS_REQ
   message contains these fields as does the authentication header
   (KRB_AP_REQ) that is passed in the padata field.

5.4.2. KRB_KDC_REP definition

   The KRB_KDC_REP message format is used for the reply from the KDC for
   either an initial (AS) request or a subsequent (TGS) request. There
   is no message type for KRB_KDC_REP. Instead, the type will be either
   KRB_AS_REP or KRB_TGS_REP. The key used to encrypt the ciphertext
   part of the reply depends on the message type. For KRB_AS_REP, the
   ciphertext is encrypted in the client's secret key, and the client's
   key version number is included in the key version number for the
   encrypted data. For KRB_TGS_REP, the ciphertext is encrypted in the
   sub-session key from the Authenticator, or if absent, the session key
   from the ticket-granting ticket used in the request.  In that case,



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   no version number will be present in the EncryptedData sequence.

   The KRB_KDC_REP message contains the following fields:

   AS-REP          ::= [APPLICATION 11] KDC-REP

   TGS-REP         ::= [APPLICATION 13] KDC-REP

   KDC-REP         ::= SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (11 -- AS -- | 13 -- TGS --),
           padata          [2] SEQUENCE OF PA-DATA OPTIONAL
                                   -- NOTE: not empty --,
           crealm          [3] Realm,
           cname           [4] PrincipalName,
           ticket          [5] Ticket,
           enc-part        [6] EncryptedData
                                   -- EncASRepPart or EncTGSRepPart,
                                   -- as appropriate
   }

   EncASRepPart    ::= [APPLICATION 25] EncKDCRepPart

   EncTGSRepPart   ::= [APPLICATION 26] EncKDCRepPart

   EncKDCRepPart   ::= SEQUENCE {
           key             [0] EncryptionKey,
           last-req        [1] LastReq,
           nonce           [2] UInt32,
           key-expiration  [3] KerberosTime OPTIONAL,
           flags           [4] TicketFlags,
           authtime        [5] KerberosTime,
           starttime       [6] KerberosTime OPTIONAL,
           endtime         [7] KerberosTime,
           renew-till      [8] KerberosTime OPTIONAL,
           srealm          [9] Realm,
           sname           [10] PrincipalName,
           caddr           [11] HostAddresses OPTIONAL
   }

   LastReq         ::=     SEQUENCE OF SEQUENCE {
           lr-type         [0] Int32,
           lr-value        [1] KerberosTime
   }

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      either KRB_AS_REP or KRB_TGS_REP.



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   padata
      This field is described in detail in section 5.4.1. One possible
      use for this field is to encode an alternate "salt" string to be
      used with a string-to-key algorithm. This ability is useful to
      ease transitions if a realm name needs to change (e.g. when a
      company is acquired); in such a case all existing password-derived
      entries in the KDC database would be flagged as needing a special
      salt string until the next password change.

   crealm, cname, srealm and sname
      These fields are the same as those described for the ticket in
      section 5.3.

   ticket
      The newly-issued ticket, from section 5.3.

   enc-part
      This field is a place holder for the ciphertext and related
      information that forms the encrypted part of a message. The
      description of the encrypted part of the message follows each
      appearance of this field.

      The key usage value for encrypting this field is 3 in an AS-REP
      message, using the client's long-term key or another key selected
      via pre-authentication mechanisms. In a TGS-REP message, the key
      usage value is 8 if the TGS session key is used, or 9 if a TGS
      authenticator subkey is used.

      Compatibility note: Some implementations unconditionally send an
      encrypted EncTGSRepPart (application tag number 26) in this field
      regardless of whether the reply is a AS-REP or a TGS-REP. In the
      interests of compatibility, implementors MAY relax the check on
      the tag number of the decrypted ENC-PART.

   key
      This field is the same as described for the ticket in section 5.3.

   last-req
      This field is returned by the KDC and specifies the time(s) of the
      last request by a principal. Depending on what information is
      available, this might be the last time that a request for a
      ticket-granting ticket was made, or the last time that a request
      based on a ticket-granting ticket was successful. It also might
      cover all servers for a realm, or just the particular server. Some
      implementations MAY display this information to the user to aid in
      discovering unauthorized use of one's identity. It is similar in
      spirit to the last login time displayed when logging into
      timesharing systems.



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      lr-type
         This field indicates how the following lr-value field is to be
         interpreted. Negative values indicate that the information
         pertains only to the responding server. Non-negative values
         pertain to all servers for the realm.

         If the lr-type field is zero (0), then no information is
         conveyed by the lr-value subfield. If the absolute value of the
         lr-type field is one (1), then the lr-value subfield is the
         time of last initial request for a TGT. If it is two (2), then
         the lr-value subfield is the time of last initial request. If
         it is three (3), then the lr-value subfield is the time of
         issue for the newest ticket-granting ticket used. If it is four
         (4), then the lr-value subfield is the time of the last
         renewal. If it is five (5), then the lr-value subfield is the
         time of last request (of any type).  If it is (6), then the lr-
         value subfield is the time when the password will expire.  If
         it is (7), then the lr-value subfield is the time when the
         account will expire.

      lr-value
         This field contains the time of the last request. The time MUST
         be interpreted according to the contents of the accompanying
         lr-type subfield.

   nonce
      This field is described above in section 5.4.1.

   key-expiration
      The key-expiration field is part of the response from the KDC and
      specifies the time that the client's secret key is due to expire.
      The expiration might be the result of password aging or an account
      expiration. If present, it SHOULD be set to the earliest of the
      user's key expiration and account expiration.  The use of this
      field is deprecated and the last-req field SHOULD be used to
      convey this information instead.  This field will usually be left
      out of the TGS reply since the response to the TGS request is
      encrypted in a session key and no client information need be
      retrieved from the KDC database. It is up to the application
      client (usually the login program) to take appropriate action
      (such as notifying the user) if the expiration time is imminent.

   flags, authtime, starttime, endtime, renew-till and caddr
      These fields are duplicates of those found in the encrypted
      portion of the attached ticket (see section 5.3), provided so the
      client MAY verify they match the intended request and to assist in
      proper ticket caching. If the message is of type KRB_TGS_REP, the
      caddr field will only be filled in if the request was for a proxy



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      or forwarded ticket, or if the user is substituting a subset of
      the addresses from the ticket-granting ticket. If the client-
      requested addresses are not present or not used, then the
      addresses contained in the ticket will be the same as those
      included in the ticket-granting ticket.

5.5. Client/Server (CS) message specifications

   This section specifies the format of the messages used for the
   authentication of the client to the application server.

5.5.1. KRB_AP_REQ definition

   The KRB_AP_REQ message contains the Kerberos protocol version number,
   the message type KRB_AP_REQ, an options field to indicate any options
   in use, and the ticket and authenticator themselves. The KRB_AP_REQ
   message is often referred to as the 'authentication header'.

   AP-REQ          ::= [APPLICATION 14] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (14),
           ap-options      [2] APOptions,
           ticket          [3] Ticket,
           authenticator   [4] EncryptedData -- Authenticator
   }

   APOptions       ::= KerberosFlags
           -- reserved(0),
           -- use-session-key(1),
           -- mutual-required(2)

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_AP_REQ.

   ap-options
      This field appears in the application request (KRB_AP_REQ) and
      affects the way the request is processed. It is a bit-field, where
      the selected options are indicated by the bit being set (1), and
      the unselected options and reserved fields being reset (0). The
      encoding of the bits is specified in section 5.2. The meanings of
      the options are:

         Bit(s)  Name            Description

         0       reserved        Reserved for future expansion of this field.

                                 The USE-SESSION-KEY option indicates that the



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                                 ticket the client is presenting to a server
         1       use-session-key is encrypted in the session key from the
                                 server's ticket-granting ticket. When this
                                 option is not specified, the ticket is
                                 encrypted in the server's secret key.

                                 The MUTUAL-REQUIRED option tells the server
         2       mutual-required that the client requires mutual
                                 authentication, and that it must respond with
                                 a KRB_AP_REP message.

         3-31    reserved        Reserved for future use.

   ticket
      This field is a ticket authenticating the client to the server.

   authenticator
      This contains the encrypted authenticator, which includes the
      client's choice of a subkey.

   The encrypted authenticator is included in the AP-REQ; it certifies
   to a server that the sender has recent knowledge of the encryption
   key in the accompanying ticket, to help the server detect replays. It
   also assists in the selection of a "true session key" to use with the
   particular session.  The DER encoding of the following is encrypted
   in the ticket's session key, with a key usage value of 11 in normal
   application exchanges, or 7 when used as the PA-TGS-REQ PA-DATA field
   of a TGS-REQ exchange (see section 5.4.1):

   -- Unencrypted authenticator
   Authenticator   ::= [APPLICATION 2] SEQUENCE  {
           authenticator-vno       [0] INTEGER (5),
           crealm                  [1] Realm,
           cname                   [2] PrincipalName,
           cksum                   [3] Checksum OPTIONAL,
           cusec                   [4] Microseconds,
           ctime                   [5] KerberosTime,
           subkey                  [6] EncryptionKey OPTIONAL,
           seq-number              [7] UInt32 OPTIONAL,
           authorization-data      [8] AuthorizationData OPTIONAL
   }

   authenticator-vno
      This field specifies the version number for the format of the
      authenticator. This document specifies version 5.

   crealm and cname
      These fields are the same as those described for the ticket in



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      section 5.3.

   cksum
      This field contains a checksum of the application data that
      accompanies the KRB_AP_REQ, computed using a key usage value of 10
      in normal application exchanges, or 6 when used in the TGS-REQ PA-
      TGS-REQ AP-DATA field.

   cusec
      This field contains the microsecond part of the client's
      timestamp. Its value (before encryption) ranges from 0 to 999999.
      It often appears along with ctime. The two fields are used
      together to specify a reasonably accurate timestamp.

   ctime
      This field contains the current time on the client's host.

   subkey
      This field contains the client's choice for an encryption key
      which is to be used to protect this specific application session.
      Unless an application specifies otherwise, if this field is left
      out the session key from the ticket will be used.

   seq-number
      This optional field includes the initial sequence number to be
      used by the KRB_PRIV or KRB_SAFE messages when sequence numbers
      are used to detect replays (It may also be used by application
      specific messages).  When included in the authenticator this field
      specifies the initial sequence number for messages from the client
      to the server. When included in the AP-REP message, the initial
      sequence number is that for messages from the server to the
      client. When used in KRB_PRIV or KRB_SAFE messages, it is
      incremented by one after each message is sent.  Sequence numbers
      fall in the range of 0 through 2^32 - 1 and wrap to zero following
      the value 2^32 - 1.

      For sequence numbers to adequately support the detection of
      replays they SHOULD be non-repeating, even across connection
      boundaries. The initial sequence number SHOULD be random and
      uniformly distributed across the full space of possible sequence
      numbers, so that it cannot be guessed by an attacker and so that
      it and the successive sequence numbers do not repeat other
      sequences.

      Implmentation note: historically, some implementations transmit
      signed twos-complement numbers for sequence numbers. In the
      interests of compatibility, implementations MAY accept the
      equivalent negative number where a positive number greater than



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      2^31 - 1 is expected.

      Implementation note: as noted before, some implementations omit
      the optional sequence number when its value would be zero.
      Implementations MAY accept an omitted sequence number when
      expecting a value of zero, and SHOULD NOT transmit an
      Authenticator with a initial sequence number of zero.

   authorization-data
      This field is the same as described for the ticket in section 5.3.
      It is optional and will only appear when additional restrictions
      are to be placed on the use of a ticket, beyond those carried in
      the ticket itself.

5.5.2. KRB_AP_REP definition

   The KRB_AP_REP message contains the Kerberos protocol version number,
   the message type, and an encrypted time-stamp. The message is sent in
   response to an application request (KRB_AP_REQ) where the mutual
   authentication option has been selected in the ap-options field.

   AP-REP          ::= [APPLICATION 15] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (15),
           enc-part        [2] EncryptedData -- EncAPRepPart
   }

   EncAPRepPart    ::= [APPLICATION 27] SEQUENCE {
           ctime           [0] KerberosTime,
           cusec           [1] Microseconds,
           subkey          [2] EncryptionKey OPTIONAL,
           seq-number      [3] UInt32 OPTIONAL
   }

   The encoded EncAPRepPart is encrypted in the shared session key of
   the ticket. The optional subkey field can be used in an application-
   arranged negotiation to choose a per association session key.

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_AP_REP.

   enc-part
      This field is described above in section 5.4.2. It is computed
      with a key usage value of 12.

   ctime
      This field contains the current time on the client's host.



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   cusec
      This field contains the microsecond part of the client's
      timestamp.

   subkey
      This field contains an encryption key which is to be used to
      protect this specific application session. See section 3.2.6 for
      specifics on how this field is used to negotiate a key. Unless an
      application specifies otherwise, if this field is left out, the
      sub-session key from the authenticator, or if also left out, the
      session key from the ticket will be used.

   seq-number
      This field is described above in section 5.3.2.

5.5.3. Error message reply

   If an error occurs while processing the application request, the
   KRB_ERROR message will be sent in response. See section 5.9.1 for the
   format of the error message. The cname and crealm fields MAY be left
   out if the server cannot determine their appropriate values from the
   corresponding KRB_AP_REQ message. If the authenticator was
   decipherable, the ctime and cusec fields will contain the values from
   it.

5.6. KRB_SAFE message specification

   This section specifies the format of a message that can be used by
   either side (client or server) of an application to send a tamper-
   proof message to its peer. It presumes that a session key has
   previously been exchanged (for example, by using the
   KRB_AP_REQ/KRB_AP_REP messages).

5.6.1. KRB_SAFE definition

   The KRB_SAFE message contains user data along with a collision-proof
   checksum keyed with the last encryption key negotiated via subkeys,
   or the session key if no negotiation has occurred. The message fields
   are:

   KRB-SAFE        ::= [APPLICATION 20] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (20),
           safe-body       [2] KRB-SAFE-BODY,
           cksum           [3] Checksum
   }

   KRB-SAFE-BODY   ::= SEQUENCE {



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           user-data       [0] OCTET STRING,
           timestamp       [1] KerberosTime OPTIONAL,
           usec            [2] Microseconds OPTIONAL,
           seq-number      [3] UInt32 OPTIONAL,
           s-address       [4] HostAddress,
           r-address       [5] HostAddress OPTIONAL
   }

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_SAFE.

   safe-body
      This field is a placeholder for the body of the KRB-SAFE message.

   cksum
      This field contains the checksum of the application data, computed
      with a key usage value of 15.

      The checksum is computed over the encoding of the KRB-SAFE
      sequence.  First, the cksum is set to a type zero, zero-length
      value and the checksum is computed over the encoding of the KRB-
      SAFE sequence, then the checksum is set to the result of that
      computation, and finally the KRB-SAFE sequence is encoded again.
      This method, while different than the one specified in RFC 1510,
      corresponds to existing practice.

   user-data
      This field is part of the KRB_SAFE and KRB_PRIV messages and
      contain the application specific data that is being passed from
      the sender to the recipient.

   timestamp
      This field is part of the KRB_SAFE and KRB_PRIV messages. Its
      contents are the current time as known by the sender of the
      message. By checking the timestamp, the recipient of the message
      is able to make sure that it was recently generated, and is not a
      replay.

   usec
      This field is part of the KRB_SAFE and KRB_PRIV headers. It
      contains the microsecond part of the timestamp.

   seq-number
      This field is described above in section 5.3.2.

   s-address
      Sender's address.



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      This field specifies the address in use by the sender of the
      message.  It MAY be omitted if not required by the application
      protocol.

   r-address
      This field specifies the address in use by the recipient of the
      message. It MAY be omitted for some uses (such as broadcast
      protocols), but the recipient MAY arbitrarily reject such
      messages. This field, along with s-address, can be used to help
      detect messages which have been incorrectly or maliciously
      delivered to the wrong recipient.

5.7. KRB_PRIV message specification

   This section specifies the format of a message that can be used by
   either side (client or server) of an application to securely and
   privately send a message to its peer. It presumes that a session key
   has previously been exchanged (for example, by using the
   KRB_AP_REQ/KRB_AP_REP messages).

5.7.1. KRB_PRIV definition

   The KRB_PRIV message contains user data encrypted in the Session Key.
   The message fields are:

   KRB-PRIV        ::= [APPLICATION 21] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (21),
                           -- NOTE: there is no [2] tag
           enc-part        [3] EncryptedData -- EncKrbPrivPart
   }

   EncKrbPrivPart  ::= [APPLICATION 28] SEQUENCE {
           user-data       [0] OCTET STRING,
           timestamp       [1] KerberosTime OPTIONAL,
           usec            [2] Microseconds OPTIONAL,
           seq-number      [3] UInt32 OPTIONAL,
           s-address       [4] HostAddress -- sender's addr --,
           r-address       [5] HostAddress OPTIONAL -- recip's addr
   }

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_PRIV.

   enc-part
      This field holds an encoding of the EncKrbPrivPart sequence
      encrypted under the session key, with a key usage value of 13.



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      This encrypted encoding is used for the enc-part field of the KRB-
      PRIV message.

   user-data, timestamp, usec, s-address and r-address
      These fields are described above in section 5.6.1.

   seq-number
      This field is described above in section 5.3.2.

5.8. KRB_CRED message specification

   This section specifies the format of a message that can be used to
   send Kerberos credentials from one principal to another. It is
   presented here to encourage a common mechanism to be used by
   applications when forwarding tickets or providing proxies to
   subordinate servers. It presumes that a session key has already been
   exchanged perhaps by using the KRB_AP_REQ/KRB_AP_REP messages.

5.8.1. KRB_CRED definition

   The KRB_CRED message contains a sequence of tickets to be sent and
   information needed to use the tickets, including the session key from
   each.  The information needed to use the tickets is encrypted under
   an encryption key previously exchanged or transferred alongside the
   KRB_CRED message. The message fields are:

   KRB-CRED        ::= [APPLICATION 22] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (22),
           tickets         [2] SEQUENCE OF Ticket,
           enc-part        [3] EncryptedData -- EncKrbCredPart
   }

   EncKrbCredPart  ::= [APPLICATION 29] SEQUENCE {
           ticket-info     [0] SEQUENCE OF KrbCredInfo,
           nonce           [1] UInt32 OPTIONAL,
           timestamp       [2] KerberosTime OPTIONAL,
           usec            [3] Microseconds OPTIONAL,
           s-address       [4] HostAddress OPTIONAL,
           r-address       [5] HostAddress OPTIONAL
   }

   KrbCredInfo     ::= SEQUENCE {
           key             [0] EncryptionKey,
           prealm          [1] Realm OPTIONAL,
           pname           [2] PrincipalName OPTIONAL,
           flags           [3] TicketFlags OPTIONAL,
           authtime        [4] KerberosTime OPTIONAL,



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           starttime       [5] KerberosTime OPTIONAL,
           endtime         [6] KerberosTime OPTIONAL,
           renew-till      [7] KerberosTime OPTIONAL,
           srealm          [8] Realm OPTIONAL,
           sname           [9] PrincipalName OPTIONAL,
           caddr           [10] HostAddresses OPTIONAL
   }

   pvno and msg-type
      These fields are described above in section 5.4.1. msg-type is
      KRB_CRED.

   tickets
      These are the tickets obtained from the KDC specifically for use
      by the intended recipient. Successive tickets are paired with the
      corresponding KrbCredInfo sequence from the enc-part of the KRB-
      CRED message.

   enc-part
      This field holds an encoding of the EncKrbCredPart sequence
      encrypted under the session key shared between the sender and the
      intended recipient, with a key usage value of 14. This encrypted
      encoding is used for the enc-part field of the KRB-CRED message.

      Implementation note: implementations of certain applications, most
      notably certain implementations of the Kerberos GSS-API mechanism,
      do not separately encrypt the contents of the EncKrbCredPart of
      the KRB-CRED message when sending it.  In the case of those GSS-
      API mechanisms, this is not a security vulnerability, as the
      entire KRB-CRED message is itself embedded in an encrypted
      message.

   nonce
      If practical, an application MAY require the inclusion of a nonce
      generated by the recipient of the message. If the same value is
      included as the nonce in the message, it provides evidence that
      the message is fresh and has not been replayed by an attacker. A
      nonce MUST NEVER be reused; it SHOULD be generated randomly by the
      recipient of the message and provided to the sender of the message
      in an application specific manner.

   timestamp and usec
      These fields specify the time that the KRB-CRED message was
      generated.  The time is used to provide assurance that the message
      is fresh.

   s-address and r-address
      These fields are described above in section 5.6.1. They are used



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      optionally to provide additional assurance of the integrity of the
      KRB-CRED message.

   key
      This field exists in the corresponding ticket passed by the KRB-
      CRED message and is used to pass the session key from the sender
      to the intended recipient. The field's encoding is described in
      section 5.2.9.

   The following fields are optional. If present, they can be associated
   with the credentials in the remote ticket file. If left out, then it
   is assumed that the recipient of the credentials already knows their
   value.

   prealm and pname
      The name and realm of the delegated principal identity.

   flags, authtime, starttime, endtime, renew-till, srealm, sname, and
      caddr
      These fields contain the values of the corresponding fields from
      the ticket found in the ticket field. Descriptions of the fields
      are identical to the descriptions in the KDC-REP message.

5.9. Error message specification

   This section specifies the format for the KRB_ERROR message. The
   fields included in the message are intended to return as much
   information as possible about an error. It is not expected that all
   the information required by the fields will be available for all
   types of errors. If the appropriate information is not available when
   the message is composed, the corresponding field will be left out of
   the message.

   Note that since the KRB_ERROR message is not integrity protected, it
   is quite possible for an intruder to synthesize or modify such a
   message. In particular, this means that the client SHOULD NOT use any
   fields in this message for security-critical purposes, such as
   setting a system clock or generating a fresh authenticator. The
   message can be useful, however, for advising a user on the reason for
   some failure.

5.9.1. KRB_ERROR definition

   The KRB_ERROR message consists of the following fields:

   KRB-ERROR       ::= [APPLICATION 30] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (30),



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           ctime           [2] KerberosTime OPTIONAL,
           cusec           [3] Microseconds OPTIONAL,
           stime           [4] KerberosTime,
           susec           [5] Microseconds,
           error-code      [6] Int32,
           crealm          [7] Realm OPTIONAL,
           cname           [8] PrincipalName OPTIONAL,
           realm           [9] Realm -- service realm --,
           sname           [10] PrincipalName -- service name --,
           e-text          [11] KerberosString OPTIONAL,
           e-data          [12] OCTET STRING OPTIONAL
   }

   pvno and msg-type
      These fields are described above in section 5.4.1. +A msg-type is
      KRB_ERROR.

   ctime
      This field is described above in section 5.4.1.

   cusec
      This field is described above in section 5.5.2.

   stime
      This field contains the current time on the server. It is of type
      KerberosTime.

   susec
      This field contains the microsecond part of the server's
      timestamp. Its value ranges from 0 to 999999. It appears along
      with stime. The two fields are used in conjunction to specify a
      reasonably accurate timestamp.

   error-code
      This field contains the error code returned by Kerberos or the
      server when a request fails. To interpret the value of this field
      see the list of error codes in section 7.5.9. Implementations are
      encouraged to provide for national language support in the display
      of error messages.

   crealm, cname, srealm and sname
      These fields are described above in section 5.3.

   e-text
      This field contains additional text to help explain the error code
      associated with the failed request (for example, it might include
      a principal name which was unknown).




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   e-data
      This field contains additional data about the error for use by the
      application to help it recover from or handle the error. If the
      errorcode is KDC_ERR_PREAUTH_REQUIRED, then the e-data field will
      contain an encoding of a sequence of padata fields, each
      corresponding to an acceptable pre-authentication method and
      optionally containing data for the method:

      METHOD-DATA     ::= SEQUENCE OF PA-DATA

   For error codes defined in this document other than
   KDC_ERR_PREAUTH_REQUIRED, the format and contents of the e-data field
   are implementation-defined. Similarly, for future error codes, the
   format and contents of the e-data field are implementation-defined
   unless specified. Whether defined by the implementation or in a
   future document, the e-data field MAY take the form of TYPED-DATA:

   TYPED-DATA      ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
           data-type       [0] INTEGER,
           data-value      [1] OCTET STRING OPTIONAL
   }

5.10. Application Tag Numbers

   The following table lists the application class tag numbers used by
   various data types defined in this section.

    Tag Number(s)    Type Name    Comments

    0                             unused

    1              Ticket         PDU

    2              Authenticator  non-PDU

    3              EncTicketPart  non-PDU

    4-9                           unused

    10             AS-REQ         PDU

    11             AS-REP         PDU

    12             TGS-REQ        PDU

    13             TGS-REP        PDU

    14             AP-REQ         PDU



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    15             AP-REP         PDU

    16             RESERVED16     TGT-REQ (for user-to-user)

    17             RESERVED17     TGT-REP (for user-to-user)

    18-19                         unused

    20             KRB-SAFE       PDU

    21             KRB-PRIV       PDU

    22             KRB-CRED       PDU

    23-24                         unused

    25             EncASRepPart   non-PDU

    26             EncTGSRepPart  non-PDU

    27             EncApRepPart   non-PDU

    28             EncKrbPrivPart non-PDU

    29             EncKrbCredPart non-PDU

    30             KRB-ERROR      PDU

   The ASN.1 types marked as "PDU" (Protocol Data Unit) in the above are
   the only ASN.1 types intended as top-level types of the Kerberos
   protcol, and are the only types that may be used as elements in
   another protocol that makes use of Kerberos.

6. Naming Constraints

6.1. Realm Names

   Although realm names are encoded as GeneralStrings and although a
   realm can technically select any name it chooses, interoperability
   across realm boundaries requires agreement on how realm names are to
   be assigned, and what information they imply.

   To enforce these conventions, each realm MUST conform to the
   conventions itself, and it MUST require that any realms with which
   inter-realm keys are shared also conform to the conventions and
   require the same from its neighbors.

   Kerberos realm names are case sensitive. Realm names that differ only



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   in the case of the characters are not equivalent. There are presently
   three styles of realm names: domain, X500, and other. Examples of
   each style follow:

        domain:   ATHENA.MIT.EDU
          X500:   C=US/O=OSF
         other:   NAMETYPE:rest/of.name=without-restrictions

   Domain syle realm names MUST look like domain names: they consist of
   components separated by periods (.) and they contain neither colons
   (:) nor slashes (/). Though domain names themselves are case
   insensitive, in order for realms to match, the case must match as
   well. When establishing a new realm name based on an internet domain
   name it is recommended by convention that the characters be converted
   to upper case.

   X.500 names contain an equal (=) and cannot contain a colon (:)
   before the equal. The realm names for X.500 names will be string
   representations of the names with components separated by slashes.
   Leading and trailing slashes will not be included. Note that the
   slash separator is consistent with Kerberos implementations based on
   RFC1510, but it is different from the separator recommended in
   RFC2253.

   Names that fall into the other category MUST begin with a prefix that
   contains no equal (=) or period (.) and the prefix MUST be followed
   by a colon (:) and the rest of the name. All prefixes must be
   assigned before they may be used. Presently none are assigned.

   The reserved category includes strings which do not fall into the
   first three categories. All names in this category are reserved. It
   is unlikely that names will be assigned to this category unless there
   is a very strong argument for not using the 'other' category.

   These rules guarantee that there will be no conflicts between the
   various name styles. The following additional constraints apply to
   the assignment of realm names in the domain and X.500 categories: the
   name of a realm for the domain or X.500 formats must either be used
   by the organization owning (to whom it was assigned) an Internet
   domain name or X.500 name, or in the case that no such names are
   registered, authority to use a realm name MAY be derived from the
   authority of the parent realm. For example, if there is no domain
   name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can
   authorize the creation of a realm with that name.

   This is acceptable because the organization to which the parent is
   assigned is presumably the organization authorized to assign names to
   its children in the X.500 and domain name systems as well. If the



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   parent assigns a realm name without also registering it in the domain
   name or X.500 hierarchy, it is the parent's responsibility to make
   sure that there will not in the future exist a name identical to the
   realm name of the child unless it is assigned to the same entity as
   the realm name.

6.2. Principal Names

   As was the case for realm names, conventions are needed to ensure
   that all agree on what information is implied by a principal name.
   The name-type field that is part of the principal name indicates the
   kind of information implied by the name. The name-type SHOULD be
   treated only as a hint to interpreting the meaning of a name. It is
   not significant when checking for equivalence. Principal names that
   differ only in the name-type identify the same principal. The name
   type does not partition the name space. Ignoring the name type, no
   two names can be the same (i.e. at least one of the components, or
   the realm, MUST be different). The following name types are defined:

   name-type      value   meaning

   name types

   NT-UNKNOWN        0  Name type not known
   NT-PRINCIPAL      1  Just the name of the principal as in DCE, or for users
   NT-SRV-INST       2  Service and other unique instance (krbtgt)
   NT-SRV-HST        3  Service with host name as instance (telnet, rcommands)
   NT-SRV-XHST       4  Service with host as remaining components
   NT-UID            5  Unique ID
   NT-X500-PRINCIPAL 6  Encoded X.509 Distingished name [RFC 2253]
   NT-SMTP-NAME      7  Name in form of SMTP email name (e.g. user@foo.com)
   NT-ENTERPRISE    10   Enterprise name - may be mapped to principal name

   When a name implies no information other than its uniqueness at a
   particular time the name type PRINCIPAL SHOULD be used. The principal
   name type SHOULD be used for users, and it might also be used for a
   unique server. If the name is a unique machine generated ID that is
   guaranteed never to be reassigned then the name type of UID SHOULD be
   used (note that it is generally a bad idea to reassign names of any
   type since stale entries might remain in access control lists).

   If the first component of a name identifies a service and the
   remaining components identify an instance of the service in a server
   specified manner, then the name type of SRV-INST SHOULD be used. An
   example of this name type is the Kerberos ticket-granting service
   whose name has a first component of krbtgt and a second component
   identifying the realm for which the ticket is valid.




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   If the first component of a name identifies a service and there is a
   single component following the service name identifying the instance
   as the host on which the server is running, then the name type SRV-
   HST SHOULD be used. This type is typically used for Internet services
   such as telnet and the Berkeley R commands. If the separate
   components of the host name appear as successive components following
   the name of the service, then the name type SRV-XHST SHOULD be used.
   This type might be used to identify servers on hosts with X.500 names
   where the slash (/) might otherwise be ambiguous.

   A name type of NT-X500-PRINCIPAL SHOULD be used when a name from an
   X.509 certificate is translated into a Kerberos name. The encoding of
   the X.509 name as a Kerberos principal shall conform to the encoding
   rules specified in RFC 2253.

   A name type of SMTP allows a name to be of a form that resembles a
   SMTP email name. This name, including an "@" and a domain name, is
   used as the one component of the principal name.

   A name type of UNKNOWN SHOULD be used when the form of the name is
   not known. When comparing names, a name of type UNKNOWN will match
   principals authenticated with names of any type. A principal
   authenticated with a name of type UNKNOWN, however, will only match
   other names of type UNKNOWN.

   Names of any type with an initial component of 'krbtgt' are reserved
   for the Kerberos ticket granting service. See section 7.5.8 for the
   form of such names.

6.2.1. Name of server principals

   The principal identifier for a server on a host will generally be
   composed of two parts: (1) the realm of the KDC with which the server
   is registered, and (2) a two-component name of type NT-SRV-HST if the
   host name is an Internet domain name or a multi-component name of
   type NT-SRV-XHST if the name of the host is of a form such as X.500
   that allows slash (/) separators. The first component of the two- or
   multi-component name will identify the service and the latter
   components will identify the host. Where the name of the host is not
   case sensitive (for example, with Internet domain names) the name of
   the host MUST be lower case. If specified by the application protocol
   for services such as telnet and the Berkeley R commands which run
   with system privileges, the first component MAY be the string 'host'
   instead of a service specific identifier. When a host has an official
   name and one or more aliases and the official name can be reliably
   determined, the official name of the host SHOULD be used when
   constructing the name of the server principal.




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7. Constants and other defined values

7.1. Host address types

   All negative values for the host address type are reserved for local
   use.  All non-negative values are reserved for officially assigned
   type fields and interpretations.

   Internet (IPv4) Addresses

      Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded
      in MSB order. The IPv4 loopback address SHOULD NOT appear in a
      Kerberos packet. The type of IPv4 addresses is two (2).

   Internet (IPv6) Addresses

      IPv6 addresses [RFC2373] are 128-bit (16-octet) quantities,
      encoded in MSB order. The type of IPv6 addresses is twenty-four
      (24). The following addresses MUST NOT appear in any Kerberos
      packet:

         *  the Unspecified Address
         *  the Loopback Address
         *  Link-Local addresses

      IPv4-mapped IPv6 addresses MUST be represented as addresses of
      type 2.

   DECnet Phase IV addresses

      DECnet Phase IV addresses are 16-bit addresses, encoded in LSB
      order. The type of DECnet Phase IV addresses is twelve (12).

   Netbios addresses

      Netbios addresses are 16-octet addresses typically composed of 1
      to 15 alphanumeric characters and padded with the US-ASCII SPC
      character (code 32).  The 16th octet MUST be the US-ASCII NUL
      character (code 0).  The type of Netbios addresses is twenty (20).

   Directional Addresses

      In many environments, including the sender address in KRB_SAFE and
      KRB_PRIV messages is undesirable because the addresses may be
      changed in transport by network address translators. However, if
      these addresses are removed, the messages may be subject to a
      reflection attack in which a message is reflected back to its
      originator. The directional address type provides a way to avoid



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      transport addresses and reflection attacks. Directional addresses
      are encoded as four byte unsigned integers in network byte order.
      If the message is originated by the party sending the original
      KRB_AP_REQ message, then an address of 0 SHOULD be used. If the
      message is originated by the party to whom that KRB_AP_REQ was
      sent, then the address 1 SHOULD be used. Applications involving
      multiple parties can specify the use of other addresses.

      Directional addresses MUST only be used for the sender address
      field in the KRB_SAFE or KRB_PRIV messages. They MUST NOT be used
      as a ticket address or in a KRB_AP_REQ message. This address type
      SHOULD only be used in situations where the sending party knows
      that the receiving party supports the address type. This generally
      means that directional addresses may only be used when the
      application protocol requires their support. Directional addresses
      are type (3).

7.2. KDC messaging - IP Transports

   Kerberos defines two IP transport mechanisms for communication
   between clients and servers: UDP/IP and TCP/IP.

7.2.1. UDP/IP transport

   Kerberos servers (KDCs) supporting IP transports MUST accept UDP
   requests and SHOULD listen for such requests on port 88 (decimal)
   unless specifically configured to listen on an alternative UDP port.
   Alternate ports MAY be used when running multiple KDCs for multiple
   realms on the same host.

   Kerberos clients supporting IP transports SHOULD support the sending
   of UDP requests. Clients SHOULD use KDC discovery [7.2.3] to identify
   the IP address and port to which they will send their request.

   When contacting a KDC for a KRB_KDC_REQ request using UDP/IP
   transport, the client shall send a UDP datagram containing only an
   encoding of the request to the KDC. The KDC will respond with a reply
   datagram containing only an encoding of the reply message (either a
   KRB_ERROR or a KRB_KDC_REP) to the sending port at the sender's IP
   address. The response to a request made through UDP/IP transport MUST
   also use UDP/IP transport. If the response can not be handled using
   UDP (for example because it is too large), the KDC MUST return
   KRB_ERR_RESPONSE_TOO_BIG, forcing the client to retry the request
   using the TCP transport.

7.2.2. TCP/IP transport

   Kerberos servers (KDCs) supporting IP transports MUST accept TCP



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   requests and SHOULD listen for such requests on port 88 (decimal)
   unless specifically configured to listen on an alternate TCP port.
   Alternate ports MAY be used when running multiple KDCs for multiple
   realms on the same host.

   Clients MUST support the sending of TCP requests, but MAY choose to
   intially try a request using the UDP transport. Clients SHOULD use
   KDC discovery [7.2.3] to identify the IP address and port to which
   they will send their request.

   Implementation note: Some extensions to the Kerberos protocol will
   not succeed if any client or KDC not supporting the TCP transport is
   involved.  Implementations of RFC 1510 were not required to support
   TCP/IP transports.

   When the KRB_KDC_REQ message is sent to the KDC over a TCP stream,
   the response (KRB_KDC_REP or KRB_ERROR message) MUST be returned to
   the client on the same TCP stream that was established for the
   request. The KDC MAY close the TCP stream after sending a response,
   but MAY leave the stream open for a reasonable period of time if it
   expects a followup. Care must be taken in managing TCP/IP connections
   on the KDC to prevent denial of service attacks based on the number
   of open TCP/IP connections.

   The client MUST be prepared to have the stream closed by the KDC at
   anytime after the receipt of a response. A stream closure SHOULD NOT
   be treated as a fatal error. Instead, if multiple exchanges are
   required (e.g., certain forms of pre-authentication) the client may
   need to establish a new connection when it is ready to send
   subsequent messages. A client MAY close the stream after receiving a
   response, and SHOULD close the stream if it does not expect to send
   followup messages.

   A client MAY send multiple requests before receiving responses,
   though it must be prepared to handle the connection being closed
   after the first response.

   Each request (KRB_KDC_REQ) and response (KRB_KDC_REP or KRB_ERROR)
   sent over the TCP stream is preceded by the length of the request as
   4 octets in network byte order. The high bit of the length is
   reserved for future expansion and MUST currently be set to zero.

   If multiple requests are sent over a single TCP connection, and the
   KDC sends multiple responses, the KDC is not required to send the
   responses in the order of the corresponding requests. This may permit
   some implementations to send each response as soon as it is ready
   even if earlier requests are still being processed (for example,
   waiting for a response from an external device or database).



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7.2.3. KDC Discovery on IP Networks

   Kerberos client implementations MUST provide a means for the client
   to determine the location of the Kerberos Key Distribution Centers
   (KDCs).  Traditionally, Kerberos implementations have stored such
   configuration information in a file on each client machine.
   Experience has shown this method of storing configuration information
   presents problems with out-of-date information and scaling problems,
   especially when using cross-realm authentication. This section
   describes a method for using the Domain Name System [RFC 1035] for
   storing KDC location information.

7.2.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names

   In Kerberos, realm names are case sensitive. While it is strongly
   encouraged that all realm names be all upper case this recommendation
   has not been adopted by all sites. Some sites use all lower case
   names and other use mixed case. DNS on the other hand is case
   insensitive for queries. Since "MYREALM", "myrealm", and "MyRealm"
   are all different it is necessary that only one of the possible
   combinations of upper and lower case characters be used. This
   restriction may be lifted in the future as the DNS naming scheme is
   expanded to support non-US-ASCII names.

7.2.3.2. Specifying KDC Location information with DNS SRV records

   KDC location information is to be stored using the DNS SRV RR [RFC
   2052].  The format of this RR is as follows:

      Service.Proto.Realm TTL Class SRV Priority Weight Port Target

   The Service name for Kerberos is always "_kerberos".

   The Proto can be one of "_udp", "_tcp". If these SRV records are to
   be used, both "_udp" and "_tcp" records MUST be specified for all KDC
   deployments.

   The Realm is the Kerberos realm that this record corresponds to.

   TTL, Class, SRV, Priority, Weight, and Target have the standard
   meaning as defined in RFC 2052.

   As per RFC 2052 the Port number used for "_udp" and "_tcp" SRV
   records SHOULD be the value assigned to "kerberos" by the Internet
   Assigned Number Authority: 88 (decimal) unless the KDC is configured
   to listen on an alternate TCP port.

   Implementation note: Many existing client implementations do not



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   support KDC Discovery and are configured to send requests to the IANA
   assigned port (88 decimal), so it is strongly recommended that KDCs
   be configured to listen on that port.

7.2.3.3. KDC Discovery for Domain Style Realm Names on IP Networks

   These are DNS records for a Kerberos realm EXAMPLE.COM. It has two
   Kerberos servers, kdc1.example.com and kdc2.example.com. Queries
   should be directed to kdc1.example.com first as per the specified
   priority. Weights are not used in these sample records.

     _kerberos._udp.EXAMPLE.COM.     IN   SRV   0 0 88 kdc1.example.com.
     _kerberos._udp.EXAMPLE.COM.     IN   SRV   1 0 88 kdc2.example.com.
     _kerberos._tcp.EXAMPLE.COM.     IN   SRV   0 0 88 kdc1.example.com.
     _kerberos._tcp.EXAMPLE.COM.     IN   SRV   1 0 88 kdc2.example.com.

7.3. Name of the TGS

   The principal identifier of the ticket-granting service shall be
   composed of three parts: (1) the realm of the KDC issuing the TGS
   ticket (2) a two-part name of type NT-SRV-INST, with the first part
   "krbtgt" and the second part the name of the realm which will accept
   the ticket-granting ticket. For example, a ticket-granting ticket
   issued by the ATHENA.MIT.EDU realm to be used to get tickets from the
   ATHENA.MIT.EDU KDC has a principal identifier of "ATHENA.MIT.EDU"
   (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A ticket-granting
   ticket issued by the ATHENA.MIT.EDU realm to be used to get tickets
   from the MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU"
   (realm), ("krbtgt", "MIT.EDU") (name).

7.4. OID arc for KerberosV5

   This OID MAY be used to identify Kerberos protocol messages
   encapsulated in other protocols. It also designates the OID arc for
   KerberosV5-related OIDs assigned by future IETF action.
   Implementation note:: RFC 1510 had an incorrect value (5) for "dod"
   in its OID.

   id-krb5         OBJECT IDENTIFIER ::= {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) kerberosV5(2)
   }

   Assignment of OIDs beneath the id-krb5 arc must be obtained by
   contacting krb5-oid-registrar@mit.edu.

7.5. Protocol constants and associated values




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   The following tables list constants used in the protocol and define
   their meanings. Ranges are specified in the "specification" section
   that limit the values of constants for which values are defined here.
   This allows implementations to make assumptions about the maximum
   values that will be received for these constants. Implementation
   receiving values outside the range specified in the "specification"
   section MAY reject the request, but they MUST recover cleanly.

7.5.1. Key usage numbers

   The encryption and checksum specifications in [@KCRYPTO] require as
   input a "key usage number", to alter the encryption key used in any
   specific message, to make certain types of cryptographic attack more
   difficult. These are the key usage values assigned in this document:

           1.          AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted
                       with the client key (section 5.2.7.2)
           2.          AS-REP Ticket and TGS-REP Ticket (includes TGS session
                       key or application session key), encrypted with the
                       service key (section 5.3)
           3.          AS-REP encrypted part (includes TGS session key or
                       application session key), encrypted with the client key
                       (section 5.4.2)
           4.          TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
                       the TGS session key (section 5.4.1)
           5.          TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
                       the TGS authenticator subkey (section 5.4.1)
           6.          TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum,
                       keyed with the TGS session key (sections 5.5.1)
           7.          TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator
                       (includes TGS authenticator subkey), encrypted with the
                       TGS session key (section 5.5.1)
           8.          TGS-REP encrypted part (includes application session
                       key), encrypted with the TGS session key (section
                       5.4.2)
           9.          TGS-REP encrypted part (includes application session
                       key), encrypted with the TGS authenticator subkey
                       (section 5.4.2)
           10.         AP-REQ Authenticator cksum, keyed with the application
                       session key (section 5.5.1)
           11.         AP-REQ Authenticator (includes application
                       authenticator subkey), encrypted with the application
                       session key (section 5.5.1)
           12.         AP-REP encrypted part (includes application session
                       subkey), encrypted with the application session key
                       (section 5.5.2)
           13.         KRB-PRIV encrypted part, encrypted with a key chosen by
                       the application (section 5.7.1)



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           14.         KRB-CRED encrypted part, encrypted with a key chosen by
                       the application (section 5.8.1)
           15.         KRB-SAFE cksum, keyed with a key chosen by the
                       application (section 5.6.1)
           19.         AD-KDC-ISSUED checksum (ad-checksum in 5.2.6.4)
         22-24.        Reserved for use in GSSAPI mechanisms derived from RFC
                       1964. (raeburn/MIT)
    16-18,20-21,25-511. Reserved for future use in Kerberos and related
                       protocols.
        512-1023.      Reserved for uses internal to a Kerberos
                       implementation.
         1024.         Encryption for application use in protocols that
                       do not specify key usage values
         1025.         Checksums for application use in protocols that
                       do not specify key usage values
       1026-2047.      Reserved for application use.


7.5.2. PreAuthentication Data Types

   padata and data types           padata-type value  comment

   PA-TGS-REQ                      1
   PA-ENC-TIMESTAMP                2
   PA-PW-SALT                      3
   [reserved]                      4
   PA-ENC-UNIX-TIME                5        (deprecated)
   PA-SANDIA-SECUREID              6
   PA-SESAME                       7
   PA-OSF-DCE                      8
   PA-CYBERSAFE-SECUREID           9
   PA-AFS3-SALT                    10
   PA-ETYPE-INFO                   11
   PA-SAM-CHALLENGE                12       (sam/otp)
   PA-SAM-RESPONSE                 13       (sam/otp)
   PA-PK-AS-REQ                    14       (pkinit)
   PA-PK-AS-REP                    15       (pkinit)
   PA-ETYPE-INFO2                  19       (replaces pa-etype-info)
   PA-USE-SPECIFIED-KVNO           20
   PA-SAM-REDIRECT                 21       (sam/otp)
   PA-GET-FROM-TYPED-DATA          22       (embedded in typed data)
   TD-PADATA                       22       (embeds padata)
   PA-SAM-ETYPE-INFO               23       (sam/otp)
   PA-ALT-PRINC                    24       (crawdad@fnal.gov)
   PA-SAM-CHALLENGE2               30       (kenh@pobox.com)
   PA-SAM-RESPONSE2                31       (kenh@pobox.com)
   PA-EXTRA-TGT                    41       Reserved extra TGT
   TD-PKINIT-CMS-CERTIFICATES      101      CertificateSet from CMS



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   TD-KRB-PRINCIPAL                102      PrincipalName
   TD-KRB-REALM                    103      Realm
   TD-TRUSTED-CERTIFIERS           104      from PKINIT
   TD-CERTIFICATE-INDEX            105      from PKINIT
   TD-APP-DEFINED-ERROR            106      application specific
   TD-REQ-NONCE                    107      INTEGER
   TD-REQ-SEQ                      108      INTEGER
   PA-PAC-REQUEST                  128      (jbrezak@exchange.microsoft.com)

7.5.3. Address Types

   Address type                   value

   IPv4                             2
   Directional                      3
   ChaosNet                         5
   XNS                              6
   ISO                              7
   DECNET Phase IV                 12
   AppleTalk DDP                   16
   NetBios                         20
   IPv6                            24

7.5.4. Authorization Data Types

   authorization data type         ad-type value
   AD-IF-RELEVANT                     1
   AD-INTENDED-FOR-SERVER             2
   AD-INTENDED-FOR-APPLICATION-CLASS  3
   AD-KDC-ISSUED                      4
   AD-AND-OR                          5
   AD-MANDATORY-TICKET-EXTENSIONS     6
   AD-IN-TICKET-EXTENSIONS            7
   AD-MANDATORY-FOR-KDC               8
   reserved values                    9-63
   OSF-DCE                            64
   SESAME                             65
   AD-OSF-DCE-PKI-CERTID              66         (hemsath@us.ibm.com)
   AD-WIN2K-PAC                      128         (jbrezak@exchange.microsoft.com)

7.5.5. Transited Encoding Types

   transited encoding type         tr-type value
   DOMAIN-X500-COMPRESS            1
   reserved values                 all others

7.5.6. Protocol Version Number




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   Label               Value   Meaning or MIT code

   pvno                    5   current Kerberos protocol version number

7.5.7. Kerberos Message Types

   message types

   KRB_AS_REQ             10   Request for initial authentication
   KRB_AS_REP             11   Response to KRB_AS_REQ request
   KRB_TGS_REQ            12   Request for authentication based on TGT
   KRB_TGS_REP            13   Response to KRB_TGS_REQ request
   KRB_AP_REQ             14   application request to server
   KRB_AP_REP             15   Response to KRB_AP_REQ_MUTUAL
   KRB_RESERVED16         16   Reserved for user-to-user krb_tgt_request
   KRB_RESERVED17         17   Reserved for user-to-user krb_tgt_reply
   KRB_SAFE               20   Safe (checksummed) application message
   KRB_PRIV               21   Private (encrypted) application message
   KRB_CRED               22   Private (encrypted) message to forward credentials
   KRB_ERROR              30   Error response

7.5.8. Name Types

   name types

   KRB_NT_UNKNOWN        0  Name type not known
   KRB_NT_PRINCIPAL      1  Just the name of the principal as in DCE, or for users
   KRB_NT_SRV_INST       2  Service and other unique instance (krbtgt)
   KRB_NT_SRV_HST        3  Service with host name as instance (telnet, rcommands)
   KRB_NT_SRV_XHST       4  Service with host as remaining components
   KRB_NT_UID            5  Unique ID
   KRB_NT_X500_PRINCIPAL 6  Encoded X.509 Distingished name [RFC 2253]
   KRB_NT_SMTP_NAME      7  Name in form of SMTP email name (e.g. user@foo.com)
   KRB_NT_ENTERPRISE    10   Enterprise name - may be mapped to principal name

7.5.9. Error Codes

   error codes

   KDC_ERR_NONE                    0   No error
   KDC_ERR_NAME_EXP                1   Client's entry in database has expired
   KDC_ERR_SERVICE_EXP             2   Server's entry in database has expired
   KDC_ERR_BAD_PVNO                3   Requested protocol version number
                                          not supported
   KDC_ERR_C_OLD_MAST_KVNO         4   Client's key encrypted in old master key
   KDC_ERR_S_OLD_MAST_KVNO         5   Server's key encrypted in old master key
   KDC_ERR_C_PRINCIPAL_UNKNOWN     6   Client not found in Kerberos database
   KDC_ERR_S_PRINCIPAL_UNKNOWN     7   Server not found in Kerberos database



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   KDC_ERR_PRINCIPAL_NOT_UNIQUE    8   Multiple principal entries in database
   KDC_ERR_NULL_KEY                9   The client or server has a null key
   KDC_ERR_CANNOT_POSTDATE        10   Ticket not eligible for postdating
   KDC_ERR_NEVER_VALID            11   Requested start time is later than end time
   KDC_ERR_POLICY                 12   KDC policy rejects request
   KDC_ERR_BADOPTION              13   KDC cannot accommodate requested option
   KDC_ERR_ETYPE_NOSUPP           14   KDC has no support for encryption type
   KDC_ERR_SUMTYPE_NOSUPP         15   KDC has no support for checksum type
   KDC_ERR_PADATA_TYPE_NOSUPP     16   KDC has no support for padata type
   KDC_ERR_TRTYPE_NOSUPP          17   KDC has no support for transited type
   KDC_ERR_CLIENT_REVOKED         18   Clients credentials have been revoked
   KDC_ERR_SERVICE_REVOKED        19   Credentials for server have been revoked
   KDC_ERR_TGT_REVOKED            20   TGT has been revoked
   KDC_ERR_CLIENT_NOTYET          21   Client not yet valid - try again later
   KDC_ERR_SERVICE_NOTYET         22   Server not yet valid - try again later
   KDC_ERR_KEY_EXPIRED            23   Password has expired
                                             - change password to reset
   KDC_ERR_PREAUTH_FAILED         24   Pre-authentication information was invalid
   KDC_ERR_PREAUTH_REQUIRED       25   Additional pre-authenticationrequired
   KDC_ERR_SERVER_NOMATCH         26   Requested server and ticket don't match
   KDC_ERR_MUST_USE_USER2USER     27   Server principal valid for user2user only
   KDC_ERR_PATH_NOT_ACCPETED      28   KDC Policy rejects transited path
   KDC_ERR_SVC_UNAVAILABLE        29   A service is not available
   KRB_AP_ERR_BAD_INTEGRITY       31   Integrity check on decrypted field failed
   KRB_AP_ERR_TKT_EXPIRED         32   Ticket expired
   KRB_AP_ERR_TKT_NYV             33   Ticket not yet valid
   KRB_AP_ERR_REPEAT              34   Request is a replay
   KRB_AP_ERR_NOT_US              35   The ticket isn't for us
   KRB_AP_ERR_BADMATCH            36   Ticket and authenticator don't match
   KRB_AP_ERR_SKEW                37   Clock skew too great
   KRB_AP_ERR_BADADDR             38   Incorrect net address
   KRB_AP_ERR_BADVERSION          39   Protocol version mismatch
   KRB_AP_ERR_MSG_TYPE            40   Invalid msg type
   KRB_AP_ERR_MODIFIED            41   Message stream modified
   KRB_AP_ERR_BADORDER            42   Message out of order
   KRB_AP_ERR_BADKEYVER           44   Specified version of key is not available
   KRB_AP_ERR_NOKEY               45   Service key not available
   KRB_AP_ERR_MUT_FAIL            46   Mutual authentication failed
   KRB_AP_ERR_BADDIRECTION        47   Incorrect message direction
   KRB_AP_ERR_METHOD              48   Alternative authentication method required
   KRB_AP_ERR_BADSEQ              49   Incorrect sequence number in message
   KRB_AP_ERR_INAPP_CKSUM         50   Inappropriate type of checksum in message
   KRB_AP_PATH_NOT_ACCEPTED       51   Policy rejects transited path
   KRB_ERR_RESPONSE_TOO_BIG       52   Response too big for UDP, retry with TCP
   KRB_ERR_GENERIC                60   Generic error (description in e-text)
   KRB_ERR_FIELD_TOOLONG          61   Field is too long for this implementation
   KDC_ERROR_CLIENT_NOT_TRUSTED      62 Reserved for PKINIT
   KDC_ERROR_KDC_NOT_TRUSTED         63 Reserved for PKINIT



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   KDC_ERROR_INVALID_SIG             64 Reserved for PKINIT
   KDC_ERR_KEY_TOO_WEAK              65 Reserved for PKINIT
   KDC_ERR_CERTIFICATE_MISMATCH      66 Reserved for PKINIT
   KRB_AP_ERR_NO_TGT                 67 No TGT available to validate USER-TO-USER
   KDC_ERR_WRONG_REALM               68 USER-TO-USER TGT issued different KDC
   KRB_AP_ERR_USER_TO_USER_REQUIRED  69 Ticket must be for USER-TO-USER
   KDC_ERR_CANT_VERIFY_CERTIFICATE   70 Reserved for PKINIT
   KDC_ERR_INVALID_CERTIFICATE             71 Reserved for PKINIT
   KDC_ERR_REVOKED_CERTIFICATE             72 Reserved for PKINIT
   KDC_ERR_REVOCATION_STATUS_UNKNOWN       73 Reserved for PKINIT
   KDC_ERR_REVOCATION_STATUS_UNAVAILABLE   74 Reserved for PKINIT
   KDC_ERR_CLIENT_NAME_MISMATCH            75 Reserved for PKINIT
   KDC_ERR_KDC_NAME_MISMATCH               76 Reserved for PKINIT

8. Interoperability requirements

   Version 5 of the Kerberos protocol supports a myriad of options.
   Among these are multiple encryption and checksum types, alternative
   encoding schemes for the transited field, optional mechanisms for
   pre-authentication, the handling of tickets with no addresses,
   options for mutual authentication, user to user authentication,
   support for proxies, forwarding, postdating, and renewing tickets,
   the format of realm names, and the handling of authorization data.

   In order to ensure the interoperability of realms, it is necessary to
   define a minimal configuration which must be supported by all
   implementations. This minimal configuration is subject to change as
   technology does. For example, if at some later date it is discovered
   that one of the required encryption or checksum algorithms is not
   secure, it will be replaced.

8.1. Specification 2

   This section defines the second specification of these options.
   Implementations which are configured in this way can be said to
   support Kerberos Version 5 Specification 2 (5.2). Specification 1
   (deprecated) may be found in RFC1510.

   Transport

      TCP/IP and UDP/IP transport MUST be supported by clients and KDCs
      claiming conformance to specification 2.

   Encryption and checksum methods

      The following encryption and checksum mechanisms MUST be
      supported.




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      Encryption: AES256-CTS-HMAC-SHA1-96
      Checksums: HMAC-SHA1-96-AES256

      Implementations SHOULD support other mechanisms as well, but the
      additional mechanisms may only be used when communicating with
      principals known to also support them. The mechanisms that SHOULD
      be supported are:

      Encryption:  DES-CBC-MD5, DES3-CBC-SHA1-KD
      Checksums:   DES-MD5, HMAC-SHA1-DES3-KD

      Implementations MAY support other mechanisms as well, but the
      additional mechanisms may only be used when communicating with
      principals known to also support them.

      Implementation note: earlier implementations of Kerberos generate
      messages using the CRC-32, RSA-MD5 checksum methods. For
      interoperability with these earlier releases implementors MAY
      consider supporting these checksum methods but should carefully
      analyze the security impplications to limit the situations within
      which these methods are accepted.

   Realm Names

      All implementations MUST understand hierarchical realms in both
      the Internet Domain and the X.500 style. When a ticket-granting
      ticket for an unknown realm is requested, the KDC MUST be able to
      determine the names of the intermediate realms between the KDCs
      realm and the requested realm.

   Transited field encoding

      DOMAIN-X500-COMPRESS (described in section 3.3.3.2) MUST be
      supported.  Alternative encodings MAY be supported, but they may
      be used only when that encoding is supported by ALL intermediate
      realms.

   Pre-authentication methods

      The TGS-REQ method MUST be supported. The TGS-REQ method is not
      used on the initial request. The PA-ENC-TIMESTAMP method MUST be
      supported by clients but whether it is enabled by default MAY be
      determined on a realm by realm basis. If not used in the initial
      request and the error KDC_ERR_PREAUTH_REQUIRED is returned
      specifying PA-ENC-TIMESTAMP as an acceptable method, the client
      SHOULD retry the initial request using the PA-ENC-TIMESTAMP pre-
      authentication method. Servers need not support the PA-ENC-
      TIMESTAMP method, but if not supported the server SHOULD ignore



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      the presence of PA-ENC-TIMESTAMP pre-authentication in a request.

      The ETYPE-INFO2 method MUST be supported; this method is used to
      communicate the set of supported encryption types, and
      corresponding salt and string to key paramters. The ETYPE-INFO
      method SHOULD be supported for interoperability with older
      implementation.

   Mutual authentication

      Mutual authentication (via the KRB_AP_REP message) MUST be
      supported.

   Ticket addresses and flags

      All KDCs MUST pass through tickets that carry no addresses (i.e.
      if a TGT contains no addresses, the KDC will return derivative
      tickets).  Implementations SHOULD default to requesting
      addressless tickets as this significantly increases
      interoperability with network address translation.  In some cases
      realms or application servers MAY require that tickets have an
      address.

      Implementations SHOULD accept directional address type for the
      KRB_SAFE and KRB_PRIV message and SHOULD include directional
      addresses in these messages when other address types are not
      available.

      Proxies and forwarded tickets MUST be supported. Individual realms
      and application servers can set their own policy on when such
      tickets will be accepted.

      All implementations MUST recognize renewable and postdated
      tickets, but need not actually implement them. If these options
      are not supported, the starttime and endtime in the ticket shall
      specify a ticket's entire useful life. When a postdated ticket is
      decoded by a server, all implementations shall make the presence
      of the postdated flag visible to the calling server.

   User-to-user authentication

      Support for user to user authentication (via the ENC-TKT-IN-SKEY
      KDC option) MUST be provided by implementations, but individual
      realms MAY decide as a matter of policy to reject such requests on
      a per-principal or realm-wide basis.

   Authorization data




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      Implementations MUST pass all authorization data subfields from
      ticket-granting tickets to any derivative tickets unless directed
      to suppress a subfield as part of the definition of that
      registered subfield type (it is never incorrect to pass on a
      subfield, and no registered subfield types presently specify
      suppression at the KDC).

      Implementations MUST make the contents of any authorization data
      subfields available to the server when a ticket is used.
      Implementations are not required to allow clients to specify the
      contents of the authorization data fields.

   Constant ranges

      All protocol constants are constrained to 32 bit (signed) values
      unless further constrained by the protocol definition. This limit
      is provided to allow implementations to make assumptions about the
      maximum values that will be received for these constants.
      Implementation receiving values outside this range MAY reject the
      request, but they MUST recover cleanly.

8.2. Recommended KDC values

   Following is a list of recommended values for a KDC configuration.

   minimum lifetime              5 minutes
   maximum renewable lifetime    1 week
   maximum ticket lifetime       1 day
   acceptable clock skew         5 minutes
   empty addresses               Allowed.
   proxiable, etc.               Allowed.

9. IANA considerations

   Section 7 of this document specifies protocol constants and other
   defined values required for the interoperability of multiple
   implementations. Until otherwise specified in a subsequent RFC,
   allocations of additional protocol constants and other defined values
   required for extensions to the Kerberos protocol will be administered
   by the Kerberos Working Group.

10. Security Considerations

   As an authentication service, Kerberos provides a means of verifying
   the identity of principals on a network. Kerberos does not, by
   itself, provide authorization. Applications should not accept the
   issuance of a service ticket by the Kerberos server as granting
   authority to use the service, since such applications may become



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   vulnerable to the bypass of this authorization check in an
   environment if they inter-operate with other KDCs or where other
   options for application authentication are provided.

   Denial of service attacks are not solved with Kerberos. There are
   places in the protocols where an intruder can prevent an application
   from participating in the proper authentication steps. Because
   authentication is a required step for the use of many services,
   successful denial of service attacks on a Kerberos server might
   result in the denial of other network services that rely on Kerberos
   for authentication. Kerberos is vulnerable to many kinds of denial of
   service attacks: denial of service attacks on the network which would
   prevent clients from contacting the KDC; denial of service attacks on
   the domain name system which could prevent a client from finding the
   IP address of the Kerberos server; and denial of service attack by
   overloading the Kerberos KDC itself with repeated requests.

   Interoperability conflicts caused by incompatible character-set usage
   (see 5.2.1) can result in denial of service for clients that utilize
   character-sets in Kerberos strings other than those stored in the KDC
   database.

   Authentication servers maintain a database of principals (i.e., users
   and servers) and their secret keys. The security of the
   authentication server machines is critical. The breach of security of
   an authentication server will compromise the security of all servers
   that rely upon the compromised KDC, and will compromise the
   authentication of any principals registered in the realm of the
   compromised KDC.

   Principals must keep their secret keys secret. If an intruder somehow
   steals a principal's key, it will be able to masquerade as that
   principal or impersonate any server to the legitimate principal.

   Password guessing attacks are not solved by Kerberos. If a user
   chooses a poor password, it is possible for an attacker to
   successfully mount an off-line dictionary attack by repeatedly
   attempting to decrypt, with successive entries from a dictionary,
   messages obtained which are encrypted under a key derived from the
   user's password.

   Unless pre-authentication options are required by the policy of a
   realm, the KDC will not know whether a request for authentication
   succeeds. An attacker can request a reply with credentials for any
   principal. These credentials will likely not be of much use to the
   attacker unless it knows the client's secret key, but the
   availability of the response encrypted in the client's secret key
   provides the attacker with ciphertext that may be used to mount brute



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   force or dictionary attacks to decrypt the credentials, by guessing
   the user's password. For this reason it is strongly encouraged that
   Kerberos realms require the use of pre-authentication. Even with pre-
   authentication, attackers may try brute force or dictionary attacks
   against credentials that are observed by eavesdropping on the
   network.

   Because a client can request a ticket for any server principal and
   can attempt a brute force or dictionary attack against the server
   principal's key using that ticket, it is strongly encouraged that
   keys be randomly generated (rather than generated from passwords) for
   any principals that are usable as the target principal for a
   KRB_TGS_REQ or KRB_AS_REQ messages.

   Each host on the network must have a clock which is loosely
   synchronized to the time of the other hosts; this synchronization is
   used to reduce the bookkeeping needs of application servers when they
   do replay detection. The degree of "looseness" can be configured on a
   per-server basis, but is typically on the order of 5 minutes. If the
   clocks are synchronized over the network, the clock synchronization
   protocol must itself be secured from network attackers.

   Principal identifiers must not recycled on a short-term basis. A
   typical mode of access control will use access control lists (ACLs)
   to grant permissions to particular principals. If a stale ACL entry
   remains for a deleted principal and the principal identifier is
   reused, the new principal will inherit rights specified in the stale
   ACL entry. By not reusing principal identifiers, the danger of
   inadvertent access is removed.

   Proper decryption of an KRB_AS_REP message from the KDC is not
   sufficient for the host to verify the identity of the user; the user
   and an attacker could cooperate to generate a KRB_AS_REP format
   message which decrypts properly but is not from the proper KDC. To
   authenticate a user logging on to a local system, the credentials
   obtained in the AS exchange may first be used in a TGS exchange to
   obtain credentials for a local server. Those credentials must then be
   verified by a local server through successful completion of the
   Client/Server exchange.

   Kerberos credentials contain clear-text information identifying the
   principals to which they apply. If privacy of this information is
   needed, this exchange should itself be encapsulated in a protocol
   providing for confidentiality on the exchange of these credentials.

   Applications must take care to protect communications subsequent to
   authentication either by using the KRB_PRIV or KRB_SAFE messages as
   appropriate, or by applying their own confidentiality or integrity



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   mechanisms on such communications. Completion of the KRB_AP_REQ and
   KRB_AP_REP exchange without subsequent use of confidentiality and
   integrity mechanisms provides only for authentication of the parties
   to the communication and not confidentiality and integrity of the
   subsequent communication. Application applying confidentiality and
   protections mechanisms other than KRB_PRIV and KRB_SAFE must make
   sure that the authentication step is appropriately linked with the
   protected communication channel that is established by the
   application.

   Unless the application server provides its own suitable means to
   protect against replay (for example, a challenge-response sequence
   initiated by the server after authentication, or use of a server-
   generated encryption subkey), the server must utilize a replay cache
   to remember any authenticator presented within the allowable clock
   skew. All services sharing a key need to use the same replay cache.
   If separate replay caches are used, then and authenticator used with
   one such service could later be replayed to a different service with
   the same service principal.

   If a server loses track of authenticators presented within the
   allowable clock skew, it must reject all requests until the clock
   skew interval has passed, providing assurance that any lost or
   replayed authenticators will fall outside the allowable clock skew
   and can no longer be successfully replayed.

   Implementations of Kerberos should not use untrusted directory
   servers to determine the realm of a host. To allow such would allow
   the compromise of the directory server to enable an attacker to
   direct the client to accept authentication with the wrong principal
   (i.e. one with a similar name, but in a realm with which the
   legitimate host was not registered).

   Implementations of Kerberos must not use DNS to canonicalize the host
   components of service principal names. To allow such canonicalization
   would allow a compromise of the DNS to result in a client obtaining
   credentials and correctly authenticating to the wrong principal.
   Though the client will know who it is communicating with, it will not
   be the principal with which it intended to communicate.

   If the Kerberos server returns a TGT for a 'closer' realm other than
   the desired realm, the client may use local policy configuration to
   verify that the authentication path used is an acceptable one.
   Alternatively, a client may choose its own authentication path,
   rather than relying on the Kerberos server to select one. In either
   case, any policy or configuration information used to choose or
   validate authentication paths, whether by the Kerberos server or
   client, must be obtained from a trusted source.



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   The Kerberos protocol in its basic form does not provide perfect
   forward secrecy for communications. If traffic has been recorded by
   an eavesdropper, then messages encrypted using the KRB_PRIV message,
   or messages encrypted using application specific encryption under
   keys exchanged using Kerberos can be decrypted if any of the user's,
   application server's, or KDC's key is subsequently discovered. This
   is because the session key use to encrypt such messages is
   transmitted over the network encrypted in the key of the application
   server, and also encrypted under the session key from the user's
   ticket-granting ticket when returned to the user in the KRB_TGS_REP
   message. The session key from the ticket-granting ticket was sent to
   the user in the KRB_AS_REP message encrypted in the user's secret
   key, and embedded in the ticket-granting ticket, which was encrypted
   in the key of the KDC. Application requiring perfect forward secrecy
   must exchange keys through mechanisms that provide such assurance,
   but may use Kerberos for authentication of the encrypted channel
   established through such other means.

11. Author's Addresses


       Clifford Neuman
       Information Sciences Institute
       University of Southern California
       4676 Admiralty Way
       Marina del Rey, CA 90292, USA
       Email: bcn@isi.edu

       Tom Yu
       Massachusetts Institute of Technology
       77 Massachusetts Avenue
       Cambridge, MA 02139, USA
       Email: tlyu@mit.edu

       Sam Hartman
       Massachusetts Institute of Technology
       77 Massachusetts Avenue
       Cambridge, MA 02139, USA
       Email: hartmans@mit.edu

       Kenneth Raeburn
       Massachusetts Institute of Technology
       77 Massachusetts Avenue
       Cambridge, MA 02139, USA
       Email: raeburn@MIT.EDU


12. Acknowledgements



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   This document is a revision to RFC1510 which was co-authored with
   John Kohl.  The specification of the Kerberos protocol described in
   this document is the result of many years of effort.  Over this
   period many individuals have contributed to the definition of the
   protocol and to the writing of the specification. Unfortunately it is
   not possible to list all contributors as authors of this document,
   though there are many not listed who are authors in spirit, because
   they contributed text for parts of some sections, because they
   contributed to the design of parts of the protocol, or because they
   contributed significantly to the discussion of the protocol in the
   IETF common authentication technology (CAT) and Kerberos working
   groups.

   Among those contributing to the development and specification of
   Kerberos were Jeffrey Altman, John Brezak, Marc Colan, Johan
   Danielsson, Don Davis, Doug Engert, Dan Geer, Paul Hill, John Kohl,
   Marc Horowitz, Matt Hur, Jeffrey Hutzelman, Paul Leach, John Linn,
   Ari Medvinsky, Sasha Medvinsky, Steve Miller, Jon Rochlis, Jerome
   Saltzer, Jeffrey Schiller, Jennifer Steiner, Ralph Swick, Mike Swift,
   Jonathan Trostle, Theodore Ts'o, Brian Tung, Jacques Vidrine, Assar
   Westerlund, and Nicolas Williams. Many other members of MIT Project
   Athena, the MIT networking group, and the Kerberos and CAT working
   groups of the IETF contributed but are not listed.

13. REFERENCES

   [@KRYPTO]
      RFC-Editor: To be replaced by RFC number for draft-ietf-krb-wg-
      crypto.

   [@AES]
      RFC-Editor: To be replaced by RFC number for draft-raeburn0krb-
      rijndael-krb.

   [DGT96]
      Don Davis, Daniel Geer, and Theodore Ts'0, "Kerberos With Clocks
      Adrift: History, Protocols, and Implementation", USENIX Computing
      Systems 9:1 (Januart 1996).

   [DS81]
      Dorothy E. Denning and Giovanni Maria Sacco, "Time-stamps in Key
      Distribution Protocols," Communications of the ACM, Vol. 24(8),
      pp.  533-536 (August 1981).

   [ISO-646/ECMA-6]
      7-bit Coded Character Set

   [ISO-2022/ECMA-35]



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      Character Code Structure and Extension Techniques

   [ISO-4873/ECMA-43]
      8-bit Coded Character Set Structure and Rules

   [KNT94]

      John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o, "The
      Evolution of the Kerberos Authentication System". In Distributed
      Open Systems, pages 78-94. IEEE Computer Society Press, 1994.

   [MNSS87]
      S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. Saltzer,
      Section E.2.1: Kerberos Authentication and Authorization System,
      M.I.T. Project Athena, Cambridge, Massachusetts (December 21,
      1987).

   [Neu93]
      B. Clifford Neuman, "Proxy-Based Authorization and Accounting for
      Distributed Systems," in Proceedings of the 13th International
      Conference on Distributed Computing Systems, Pittsburgh, PA (May,
      1993).

   [NS78]
      Roger M. Needham and Michael D. Schroeder, "Using Encryption for
      Authentication in Large Networks of Computers," Communications of
      the ACM, Vol. 21(12), pp. 993-999 (December, 1978).

   [NT94]
      B. Clifford Neuman and Theodore Y. Ts'o, "An Authentication
      Service for Computer Networks," IEEE Communications Magazine, Vol.
      32(9), pp.  33-38 (September 1994).

   [Pat92].
      J. Pato, Using Pre-Authentication to Avoid Password Guessing
      Attacks, Open Software Foundation DCE Request for Comments 26
      (December 1992).

   [RFC1035]
      P.V. Mockapetris, RFC1035: "Domain Names - Implementations and
      Specification," November 1, 1987, Obsoletes - RFC973, RFC882,
      RFC883. Updated by RFC1101, RFC1183, RFC1348, RFCRFC1876, RFC1982,
      RFC1995, RFC1996, RFC2065, RFC2136, RFC2137, RFC2181, RFC2308,
      RFC2535, RFC2845, and RFC3425. Status: Standard.

   [RFC1510]
      J. Kohl and  B. C. Neuman, RFC1510: "The Kerberos Network
      Authentication Service (v5)," September 1993, Status: Proposed



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      Standard.

   [RFC2026]
      S. Bradner, RFC2026:  "The Internet Standard Process - Revision
      3," October 1996, Obsoletes - RFC 1602, Status: Best Current
      Practice.

   [RFC2052]
      A. Gulbrandsen and P. Vixie, RFC2052: "A DNS RR for Specifying the
      Location of Services (DNS SRV)," October 1996, Obseleted by
      RFC2782, Status: Experimental

   [RFC2253]
      M. Wahl, S. Killie, and T. Howes, RFC2253: "Lightweight Directory
      Access Protocol (v3): UTF-8 String Representation or Distinguished
      Names," December 1997, Obsoletes - RFC1779, Updated by RFC3377,
      Status: Proposed Standard.

   [RFC2273]
      D. Levi, P. Meyer, and B. Stewart, RFC2273: "SNMPv3 Applications,"
      January 1998, Obsoletes - RFC2263, Obsoleted by RFC2573, Status:
      Proposed Standard.

   [RFC2373]
      R. Hinden, S. Deering, RFC2373: "IP Version 6 Addressing
      Architecture," July 1998, Status: Proposed Standard.

   [SNS88]
      J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Kerberos: An
      Authentication Service for Open Network Systems," pp. 191-202 in
      Usenix Conference Proceedings, Dallas, Texas (February, 1988).

   [X680]
      Abstract Syntax Notation One (ASN.1): Specification of Basic
      Notation, ITU-T Recommendation X.680 (1997) | ISO/IEC
      International Standard 8824-1:1998.

   [X690]
      ASN.1 encoding rules: Specification of Basic Encoding Rules (BER),
      Canonical Encoding Rules (CER) and Distinguished Encoding Rules
      (DER), ITU-T Recommendation X.690 (1997)| ISO/IEC International
      Standard 8825-1:1998.

A. ASN.1 module

   KerberosV5Spec2 {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) kerberosV5(2) modules(4) krb5spec2(2)



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   } DEFINITIONS EXPLICIT TAGS ::= BEGIN

   -- OID arc for KerberosV5
   --
   -- This OID may be used to identify Kerberos protocol messages
   -- encapsulated in other protocols.
   --
   -- This OID also designates the OID arc for KerberosV5-related OIDs.
   --
   -- NOTE: RFC 1510 had an incorrect value (5) for "dod" in its OID.
   id-krb5         OBJECT IDENTIFIER ::= {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) kerberosV5(2)
   }

   Int32           ::= INTEGER (-2147483648..2147483647)
                       -- signed values representable in 32 bits

   UInt32          ::= INTEGER (0..4294967295)
                       -- unsigned 32 bit values

   Microseconds    ::= INTEGER (0..999999)
                       -- microseconds

   KerberosString  ::= GeneralString (IA5String)

   Realm           ::= KerberosString

   PrincipalName   ::= SEQUENCE {
           name-type       [0] Int32,
           name-string     [1] SEQUENCE OF KerberosString
   }

   KerberosTime    ::= GeneralizedTime -- with no fractional seconds

   HostAddress     ::= SEQUENCE  {
           addr-type       [0] Int32,
           address         [1] OCTET STRING
   }

   -- NOTE: HostAddresses is always used as an OPTIONAL field and
   -- should not be empty.
   HostAddresses   -- NOTE: subtly different from rfc1510,
                   -- but has a value mapping and encodes the same
           ::= SEQUENCE OF HostAddress

   -- NOTE: AuthorizationData is always used as an OPTIONAL field and
   -- should not be empty.



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   AuthorizationData       ::= SEQUENCE OF SEQUENCE {
           ad-type         [0] Int32,
           ad-data         [1] OCTET STRING
   }

   PA-DATA         ::= SEQUENCE {
           -- NOTE: first tag is [1], not [0]
           padata-type     [1] Int32,
           padata-value    [2] OCTET STRING -- might be encoded AP-REQ
   }

   KerberosFlags   ::= BIT STRING (SIZE (32..MAX)) -- minimum number of bits
                       -- shall be sent, but no fewer than 32

   EncryptedData   ::= SEQUENCE {
           etype   [0] Int32 -- EncryptionType --,
           kvno    [1] UInt32 OPTIONAL,
           cipher  [2] OCTET STRING -- ciphertext
   }

   EncryptionKey   ::= SEQUENCE {
           keytype         [0] Int32 -- actually encryption type --,
           keyvalue        [1] OCTET STRING
   }

   Checksum        ::= SEQUENCE {
           cksumtype       [0] Int32,
           checksum        [1] OCTET STRING
   }

   Ticket          ::= [APPLICATION 1] SEQUENCE {
           tkt-vno         [0] INTEGER (5),
           realm           [1] Realm,
           sname           [2] PrincipalName,
           enc-part        [3] EncryptedData -- EncTicketPart
   }

   -- Encrypted part of ticket
   EncTicketPart   ::= [APPLICATION 3] SEQUENCE {
           flags                   [0] TicketFlags,
           key                     [1] EncryptionKey,
           crealm                  [2] Realm,
           cname                   [3] PrincipalName,
           transited               [4] TransitedEncoding,
           authtime                [5] KerberosTime,
           starttime               [6] KerberosTime OPTIONAL,
           endtime                 [7] KerberosTime,
           renew-till              [8] KerberosTime OPTIONAL,



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           caddr                   [9] HostAddresses OPTIONAL,
           authorization-data      [10] AuthorizationData OPTIONAL
   }

   -- encoded Transited field
   TransitedEncoding       ::= SEQUENCE {
           tr-type         [0] Int32 -- must be registered --,
           contents        [1] OCTET STRING
   }

   TicketFlags     ::= KerberosFlags
           -- reserved(0),
           -- forwardable(1),
           -- forwarded(2),
           -- proxiable(3),
           -- proxy(4),
           -- may-postdate(5),
           -- postdated(6),
           -- invalid(7),
           -- renewable(8),
           -- initial(9),
           -- pre-authent(10),
           -- hw-authent(11),
   -- the following are new since 1510
           -- transited-policy-checked(12),
           -- ok-as-delegate(13)

   AS-REQ          ::= [APPLICATION 10] KDC-REQ

   TGS-REQ         ::= [APPLICATION 12] KDC-REQ

   KDC-REQ         ::= SEQUENCE {
           -- NOTE: first tag is [1], not [0]
           pvno            [1] INTEGER (5) ,
           msg-type        [2] INTEGER (10 -- AS -- | 12 -- TGS --),
           padata          [3] SEQUENCE OF PA-DATA OPTIONAL
                               -- NOTE: not empty --,
           req-body        [4] KDC-REQ-BODY
   }

   KDC-REQ-BODY    ::= SEQUENCE {
           kdc-options             [0] KDCOptions,
           cname                   [1] PrincipalName OPTIONAL
                                       -- Used only in AS-REQ --,
           realm                   [2] Realm
                                       -- Server's realm
                                       -- Also client's in AS-REQ --,
           sname                   [3] PrincipalName OPTIONAL,



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           from                    [4] KerberosTime OPTIONAL,
           till                    [5] KerberosTime,
           rtime                   [6] KerberosTime OPTIONAL,
           nonce                   [7] UInt32,
           etype                   [8] SEQUENCE OF Int32 -- EncryptionType
                                       -- in preference order --,
           addresses               [9] HostAddresses OPTIONAL,
           enc-authorization-data  [10] EncryptedData -- AuthorizationData --,
           additional-tickets      [11] SEQUENCE OF Ticket OPTIONAL
                                           -- NOTE: not empty
   }

   KDCOptions      ::= KerberosFlags
           -- reserved(0),
           -- forwardable(1),
           -- forwarded(2),
           -- proxiable(3),
           -- proxy(4),
           -- allow-postdate(5),
           -- postdated(6),
           -- unused7(7),
           -- renewable(8),
           -- unused9(9),
           -- unused10(10),
           -- opt-hardware-auth(11),
           -- unused12(12),
           -- unused13(13),
   -- 15 is reserved for canonicalize
           -- unused15(15),
   -- 26 was unused in 1510
           -- disable-transited-check(26),
   --
           -- renewable-ok(27),
           -- enc-tkt-in-skey(28),
           -- renew(30),
           -- validate(31)

   AS-REP          ::= [APPLICATION 11] KDC-REP

   TGS-REP         ::= [APPLICATION 13] KDC-REP

   KDC-REP         ::= SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (11 -- AS -- | 13 -- TGS --),
           padata          [2] SEQUENCE OF PA-DATA OPTIONAL
                                   -- NOTE: not empty --,
           crealm          [3] Realm,
           cname           [4] PrincipalName,



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           ticket          [5] Ticket,
           enc-part        [6] EncryptedData
                                   -- EncASRepPart or EncTGSRepPart,
                                   -- as appropriate
   }

   EncASRepPart    ::= [APPLICATION 25] EncKDCRepPart

   EncTGSRepPart   ::= [APPLICATION 26] EncKDCRepPart

   EncKDCRepPart   ::= SEQUENCE {
           key             [0] EncryptionKey,
           last-req        [1] LastReq,
           nonce           [2] UInt32,
           key-expiration  [3] KerberosTime OPTIONAL,
           flags           [4] TicketFlags,
           authtime        [5] KerberosTime,
           starttime       [6] KerberosTime OPTIONAL,
           endtime         [7] KerberosTime,
           renew-till      [8] KerberosTime OPTIONAL,
           srealm          [9] Realm,
           sname           [10] PrincipalName,
           caddr           [11] HostAddresses OPTIONAL
   }

   LastReq         ::=     SEQUENCE OF SEQUENCE {
           lr-type         [0] Int32,
           lr-value        [1] KerberosTime
   }

   AP-REQ          ::= [APPLICATION 14] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (14),
           ap-options      [2] APOptions,
           ticket          [3] Ticket,
           authenticator   [4] EncryptedData -- Authenticator
   }

   APOptions       ::= KerberosFlags
           -- reserved(0),
           -- use-session-key(1),
           -- mutual-required(2)

   -- Unencrypted authenticator
   Authenticator   ::= [APPLICATION 2] SEQUENCE  {
           authenticator-vno       [0] INTEGER (5),
           crealm                  [1] Realm,
           cname                   [2] PrincipalName,



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           cksum                   [3] Checksum OPTIONAL,
           cusec                   [4] Microseconds,
           ctime                   [5] KerberosTime,
           subkey                  [6] EncryptionKey OPTIONAL,
           seq-number              [7] UInt32 OPTIONAL,
           authorization-data      [8] AuthorizationData OPTIONAL
   }

   AP-REP          ::= [APPLICATION 15] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (15),
           enc-part        [2] EncryptedData -- EncAPRepPart
   }

   EncAPRepPart    ::= [APPLICATION 27] SEQUENCE {
           ctime           [0] KerberosTime,
           cusec           [1] Microseconds,
           subkey          [2] EncryptionKey OPTIONAL,
           seq-number      [3] UInt32 OPTIONAL
   }

   KRB-SAFE        ::= [APPLICATION 20] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (20),
           safe-body       [2] KRB-SAFE-BODY,
           cksum           [3] Checksum
   }

   KRB-SAFE-BODY   ::= SEQUENCE {
           user-data       [0] OCTET STRING,
           timestamp       [1] KerberosTime OPTIONAL,
           usec            [2] Microseconds OPTIONAL,
           seq-number      [3] UInt32 OPTIONAL,
           s-address       [4] HostAddress,
           r-address       [5] HostAddress OPTIONAL
   }

   KRB-PRIV        ::= [APPLICATION 21] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (21),
                           -- NOTE: there is no [2] tag
           enc-part        [3] EncryptedData -- EncKrbPrivPart
   }

   EncKrbPrivPart  ::= [APPLICATION 28] SEQUENCE {
           user-data       [0] OCTET STRING,
           timestamp       [1] KerberosTime OPTIONAL,
           usec            [2] Microseconds OPTIONAL,



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           seq-number      [3] UInt32 OPTIONAL,
           s-address       [4] HostAddress -- sender's addr --,
           r-address       [5] HostAddress OPTIONAL -- recip's addr
   }

   KRB-CRED        ::= [APPLICATION 22] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (22),
           tickets         [2] SEQUENCE OF Ticket,
           enc-part        [3] EncryptedData -- EncKrbCredPart
   }

   EncKrbCredPart  ::= [APPLICATION 29] SEQUENCE {
           ticket-info     [0] SEQUENCE OF KrbCredInfo,
           nonce           [1] UInt32 OPTIONAL,
           timestamp       [2] KerberosTime OPTIONAL,
           usec            [3] Microseconds OPTIONAL,
           s-address       [4] HostAddress OPTIONAL,
           r-address       [5] HostAddress OPTIONAL
   }

   KrbCredInfo     ::= SEQUENCE {
           key             [0] EncryptionKey,
           prealm          [1] Realm OPTIONAL,
           pname           [2] PrincipalName OPTIONAL,
           flags           [3] TicketFlags OPTIONAL,
           authtime        [4] KerberosTime OPTIONAL,
           starttime       [5] KerberosTime OPTIONAL,
           endtime         [6] KerberosTime OPTIONAL,
           renew-till      [7] KerberosTime OPTIONAL,
           srealm          [8] Realm OPTIONAL,
           sname           [9] PrincipalName OPTIONAL,
           caddr           [10] HostAddresses OPTIONAL
   }

   KRB-ERROR       ::= [APPLICATION 30] SEQUENCE {
           pvno            [0] INTEGER (5),
           msg-type        [1] INTEGER (30),
           ctime           [2] KerberosTime OPTIONAL,
           cusec           [3] Microseconds OPTIONAL,
           stime           [4] KerberosTime,
           susec           [5] Microseconds,
           error-code      [6] Int32,
           crealm          [7] Realm OPTIONAL,
           cname           [8] PrincipalName OPTIONAL,
           realm           [9] Realm -- service realm --,
           sname           [10] PrincipalName -- service name --,
           e-text          [11] KerberosString OPTIONAL,



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           e-data          [12] OCTET STRING OPTIONAL
   }

   METHOD-DATA     ::= SEQUENCE OF PA-DATA

   TYPED-DATA      ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
           data-type       [0] INTEGER,
           data-value      [1] OCTET STRING OPTIONAL
   }

   -- preauth stuff follows

   PA-ENC-TIMESTAMP        ::= EncryptedData -- PA-ENC-TS-ENC

   PA-ENC-TS-ENC           ::= SEQUENCE {
           patimestamp     [0] KerberosTime -- client's time --,
           pausec          [1] Microseconds OPTIONAL
   }

   ETYPE-INFO-ENTRY        ::= SEQUENCE {
           etype           [0] Int32,
           salt            [1] OCTET STRING OPTIONAL
   }

   ETYPE-INFO              ::= SEQUENCE OF ETYPE-INFO-ENTRY

   ETYPE-INFO2-ENTRY       ::= SEQUENCE {
           etype           [0] Int32,
           salt            [1] KerberosString OPTIONAL,
           s2kparams       [2] OCTET STRING OPTIONAL
   }

   ETYPE-INFO2             ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO-ENTRY

   AD-IF-RELEVANT          ::= AuthorizationData

   AD-KDCIssued            ::= SEQUENCE {
           ad-checksum     [0] Checksum,
           i-realm         [1] Realm OPTIONAL,
           i-sname         [2] PrincipalName OPTIONAL,
           elements        [3] AuthorizationData
   }

   AD-AND-OR               ::= SEQUENCE {
           condition-count [0] INTEGER,
           elements        [1] AuthorizationData
   }




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   AD-MANDATORY-FOR-KDC    ::= AuthorizationData

   END

B. Changes since RFC-1510

   This document replaces RFC-1510 and clarifies specification of items
   that were not completely specified. Where changes to recommended
   implementation choices were made, or where new options were added,
   those changes are described within the document and listed in this
   section. More significantly, "Specification 2" in section 8 changes
   the required encryption and checksum methods to bring them in line
   with the best current practices and to deprecate methods that are no
   longer considered sufficiently strong.

   Discussion was added to section 1 regarding the ability to rely on
   the KDC to check the transited field, and on the inclusion of a flag
   in a ticket indicating that this check has occurred. This is a new
   capability not present in RFC1510. Pre-existing implementations may
   ignore or not set this flag without negative security implications.

   The definition of the secret key says that in the case of a user the
   key may be derived from a password. In 1510, it said that the key was
   derived from the password. This change was made to accommodate
   situations where the user key might be stored on a smart-card, or
   otherwise obtained independent of a password.

   The introduction mentions the use of public key cryptography for
   initial authentication in Kerberos by reference. RFC1510 did not
   include such a reference.

   Section 1.2 was added to explain that while Kerberos provides
   authentication of a named principal, it is still the responsibility
   of the application to ensure that the authenticated name is the
   entity with which the application wishes to communicate.

   Discussion of extensibility has been added to the introduction.

   Discussion of how extensibility affects ticket flags and KDC options
   was added to the introduction of section 2. No changes were made to
   existing options and flags specified in RFC1510, though some of the
   sections in the specification were renumbered, and text was revised
   to make the description and intent of existing options clearer,
   especially with respect to the ENC-TKT-IN-SKEY option (now section
   2.9.2) which is used for user-to-user authentication.  The new option
   and ticket flag transited policy checking (section 2.7) was added.

   A warning regarding generation of session keys for application use



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   was added to section 3, urging the inclusion of key entropy from the
   KDC generated session key in the ticket. An example regarding use of
   the sub-session key was added to section 3.2.6. Descriptions of the
   pa-etype-info, pa-etype-info2, and pa-pw-salt pre-authentication data
   items were added. The recommendation for use of pre-authentication
   was changed from "may" to "should" and a note was added regarding
   known plaintext attacks.

   In RFC 1510, section 4 described the database in the KDC. This
   discussion was not necessary for interoperability and unnecessarily
   constrained implementation. The old section 4 was removed.

   The current section 4 was formerly section 6 on encryption and
   checksum specifications. The major part of this section was brought
   up to date to support new encryption methods, and move to a separate
   document. Those few remaining aspects of the encryption and checksum
   specification specific to Kerberos are now specified in section 4.

   Significant changes were made to the layout of section 5 to clarify
   the correct behavior for optional fields. Many of these changes were
   made necessary because of improper ASN.1 description in the original
   Kerberos specification which left the correct behavior
   underspecified. Additionally, the wording in this section was
   tightened wherever possible to ensure that implementations conforming
   to this specification will be extensible with the addition of new
   fields in future specifications.

   Text was added describing time_t=0 issues in the ASN.1. Text was also
   added, clarifying issues with implementations treating omitted
   optional integers as zero. Text was added clarifying behavior for
   optional SEQUENCE or SEQUENCE OF that may be empty. Discussion was
   added regarding sequence numbers and behavior of some
   implementations, including "zero" behavior and negative numbers. A
   compatibility note was added regarding the unconditional sending of
   EncTGSRepPart regardless of the enclosing reply type. Minor changes
   were made to the description of the HostAddresses type. Integer types
   were constrained. KerberosString was defined as a (significantly)
   constrained GeneralString. KerberosFlags was defined to reflect
   existing implementation behavior that departs from the definition in
   RFC 1510. The transited-policy-checked(12) and the ok-as-delegate(13)
   ticket flags were added. The disable-transited-check(26) KDC option
   was added.

   Descriptions of commonly implemented PA-DATA were added to section 5.
   The description of KRB-SAFE has been updated to note the existing
   implementation behavior of double-encoding.

   There were two definitions of METHOD-DATA in RFC 1510. The second



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   one, intended for use with KRB_AP_ERR_METHOD was removed leaving the
   SEQUENCE OF PA-DATA definition.

   Section 7, naming constraints, from RFC1510 was moved to section 6.

   Words were added describing the convention that domain based realm
   names for newly created realms should be specified as upper case.
   This recommendation does not make lower case realm names illegal.
   Words were added highlighting that the slash separated components in
   the X500 style of realm names is consistent with existing RFC1510
   based implementations, but that it conflicts with the general
   recommendation of X.500 name representation specified in RFC2253.

   Section 8, network transport, constants and defined values, from
   RFC1510 was moved to section 7.  Since RFC1510, the definition of the
   TCP transport for Kerberos messages was added, and the encryption and
   checksum number assignments have been moved into a separate document.

   "Specification 2" in section 8 of the current document changes the
   required encryption and checksum methods to bring them in line with
   the best current practices and to deprecate methods that are no
   longer considered sufficiently strong.

   Two new sections, on IANA considerations and security considerations
   were added.

   The pseudo-code has been removed from the appendix. The pseudo-code
   was sometimes misinterpreted to limit implementation choices and in
   RFC 1510, it was not always consistent with the words in the
   specification. Effort was made to clear up any ambiguities in the
   specification, rather than to rely on the pseudo-code.

   An appendix was added containing the complete ASN.1 module drawn from
   the discussion in section 5 of the current document.

   An appendix was added defining those authorization data elements that
   must be understood by all Kerberos implementations.

END NOTES

   [TM] Project Athena, Athena, and Kerberos are trademarks of the
   Massachusetts Institute of Technology (MIT). No commercial use of
   these trademarks may be made without prior written permission of MIT.

   [1] Note, however, that many applications use Kerberos' functions
   only upon the initiation of a stream-based network connection. Unless
   an application subsequently provides integrity protection for the
   data stream, the identity verification applies only to the initiation



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   of the connection, and does not guarantee that subsequent messages on
   the connection originate from the same principal.

   [2] Secret and private are often used interchangeably in the
   literature.  In our usage, it takes two (or more) to share a secret,
   thus a shared DES key is a secret key. Something is only private when
   no one but its owner knows it. Thus, in public key cryptosystems, one
   has a public and a private key.

   [3] Of course, with appropriate permission the client could arrange
   registration of a separately-named principal in a remote realm, and
   engage in normal exchanges with that realm's services. However, for
   even small numbers of clients this becomes cumbersome, and more
   automatic methods as described here are necessary.

   [4] Though it is permissible to request or issue tickets with no
   network addresses specified.

   [5] The password-changing request must not be honored unless the
   requester can provide the old password (the user's current secret
   key). Otherwise, it would be possible for someone to walk up to an
   unattended session and change another user's password.

   [6] To authenticate a user logging on to a local system, the
   credentials obtained in the AS exchange may first be used in a TGS
   exchange to obtain credentials for a local server. Those credentials
   must then be verified by a local server through successful completion
   of the Client/Server exchange.

   [7] "Random" means that, among other things, it should be impossible
   to guess the next session key based on knowledge of past session
   keys. This can only be achieved in a pseudo-random number generator
   if it is based on cryptographic principles. It is more desirable to
   use a truly random number generator, such as one based on
   measurements of random physical phenomena.

   [8] Tickets contain both an encrypted and unencrypted portion, so
   cleartext here refers to the entire unit, which can be copied from
   one message and replayed in another without any cryptographic skill.

   [9] Note that this can make applications based on unreliable
   transports difficult to code correctly. If the transport might
   deliver duplicated messages, either a new authenticator must be
   generated for each retry, or the application server must match
   requests and replies and replay the first reply in response to a
   detected duplicate.

   [10] Note also that the rejection here is restricted to



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   authenticators from the same principal to the same server. Other
   client principals communicating with the same server principal should
   not be have their authenticators rejected if the time and microsecond
   fields happen to match some other client's authenticator.

   [11] If this is not done, an attacker could subvert the
   authentication by recording the ticket and authenticator sent over
   the network to a server and replaying them following an event that
   caused the server to lose track of recently seen authenticators.

   [12] In the Kerberos version 4 protocol, the timestamp in the reply
   was the client's timestamp plus one. This is not necessary in version
   5 because version 5 messages are formatted in such a way that it is
   not possible to create the reply by judicious message surgery (even
   in encrypted form) without knowledge of the appropriate encryption
   keys.

   [13] Note that for encrypting the KRB_AP_REP message, the sub-session
   key is not used, even if present in the Authenticator.

   [14] Implementations of the protocol may provide routines to choose
   subkeys based on session keys and random numbers and to generate a
   negotiated key to be returned in the KRB_AP_REP message.

   [15]This can be accomplished in several ways. It might be known
   beforehand (since the realm is part of the principal identifier), it
   might be stored in a nameserver, or it might be obtained from a
   configuration file. If the realm to be used is obtained from a
   nameserver, there is a danger of being spoofed if the nameservice
   providing the realm name is not authenticated.  This might result in
   the use of a realm which has been compromised, and would result in an
   attacker's ability to compromise the authentication of the
   application server to the client.

   [16] If the client selects a sub-session key, care must be taken to
   ensure the randomness of the selected sub-session key. One approach
   would be to generate a random number and XOR it with the session key
   from the ticket-granting ticket.













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