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INTERNET-DRAFT                                          Clifford Neuman
                                                              John Kohl
                                                          Theodore Ts'o
                                                                 Tom Yu
                                                            Sam Hartman
                                                            Ken Raeburn
                                                         Jeffrey Altman
                                                       November 1, 2002
                                                    Expires 1 May, 2003

The Kerberos Network Authentication Service (V5)
draft-ietf-krb-wg-kerberos-clarifications-02

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
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The distribution of this memo is unlimited. It is filed as
draft-ietf-krb-wg-kerberos-clarifications-02.txt, and expires 1 May 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.

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.

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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.

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 T'so, 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 [KNT92].

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

Table of Contents

1.      Introduction
1.1.    Cross-realm operation
1.2.    Choosing a principal with which to communicate
1.3.    Authorization
1.5.    Environmental assumptions
2.      Ticket flag uses and requests
2.1.    Initial, pre-authenticated, and hardware authenticated tickets
2.2.    Invalid tickets
2.3.    Renewable tickets
2.4.    Postdated tickets
2.5.    Proxiable and proxy tickets
2.6.    Forwardable tickets
2.8.    Other KDC options
3.      Message Exchanges
3.1.    The Authentication Service Exchange
3.1.1.  Generation of KRB_AS_REQ message
3.1.2.  Receipt of KRB_AS_REQ message
3.1.3.  Generation of KRB_AS_REP message
3.1.4.  Generation of KRB_ERROR message
3.1.5.  Receipt of KRB_AS_REP message
3.1.6.  Receipt of KRB_ERROR message
3.2.    The Client/Server Authentication Exchange
3.2.1.  The KRB_AP_REQ message
3.2.2.  Generation of a KRB_AP_REQ message
3.2.3.  Receipt of KRB_AP_REQ message
3.2.4.  Generation of a KRB_AP_REP message
3.2.5.  Receipt of KRB_AP_REP message
3.2.6.  Using the encryption key
3.3.    The Ticket-Granting Service (TGS) Exchange
3.3.1.  Generation of KRB_TGS_REQ message
3.3.2.  Receipt of KRB_TGS_REQ message
3.3.3.  Generation of KRB_TGS_REP message
3.3.3.1.  Checking for revoked tickets
3.3.3.2.  Encoding the transited field
3.3.4.  Receipt of KRB_TGS_REP message
3.4.    The KRB_SAFE Exchange
3.4.1.  Generation of a KRB_SAFE message
3.4.2.  Receipt of KRB_SAFE message
3.5.    The KRB_PRIV Exchange
3.5.1.  Generation of a KRB_PRIV message
3.5.2.  Receipt of KRB_PRIV message
3.6.    The KRB_CRED Exchange
3.6.1.  Generation of a KRB_CRED message
3.6.2.  Receipt of KRB_CRED message
3.7.    User to User Authentication Exchanges
4.      Encryption and Checksum Specifications

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5.      Message Specifications
5.1.    Specific Compatibility Notes on ASN.1
5.1.1.  ASN.1 Distinguished Encoding Rules
5.1.2.  Optional Integer Fields
5.1.3.  Empty SEQUENCE OF Types
5.1.4.  Unrecognized Tag Numbers
5.1.5.  Tag Numbers Greater Than 30
5.2.    Basic Kerberos Types
5.2.1.  KerberosString
5.2.2.  Realm and PrincipalName
5.2.3.  KerberosTime
5.2.4.  Constrained Integer types
5.2.5.  HostAddress and HostAddresses
5.2.6.  AuthorizationData
5.2.6.1. IF-RELEVANT
5.2.6.4. KDCIssued
5.2.6.5. AND-OR
5.2.6.8. MANDATORY-FOR-KDC
5.2.7. PA-DATA
5.2.7.1. PA-TGS-REQ
5.2.7.2. Encrypted Timestamp Pre-authentication
5.2.7.5. PA-ETYPE-INFO2
5.2.8. KerberosFlags
5.2.9. Cryptosystem-related Types
5.3.   Tickets
5.4.   Specifications for the AS and TGS exchanges
5.4.1. KRB_KDC_REQ definition
5.4.2. KRB_KDC_REP definition
5.5.   Client/Server (CS) message specifications
5.5.1. KRB_AP_REQ definition
5.5.2. KRB_AP_REP definition
5.5.3. Error message reply
5.6.   KRB_SAFE message specification
5.7.   KRB_PRIV message specification
5.8.   KRB_CRED message specification
5.9.   Error message specification
5.10.  Application Tag Numbers
6.     Naming Constraints
6.1.   Realm Names
6.2.   Principal Names
6.2.1. Name of server principals


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7.     Constants and other defined values
7.1.   Host address types
7.2.   KDC messaging
7.2.1  IP Transports
7.2.1.1. UDP/IP transport
7.2.1.2. TCP/IP transport
7.2.1.3  KDC Discovery on IP Networks
7.2.1.3.1. DNS vs. Kerberos - Case Sensitivity of Realm Names
7.2.1.3.2.  Specifying KDC Location information with DNS SRV records
7.2.1.3.3. KDC Discovery for Domain Style Realm Names on IP Networks
7.3.   Name of the TGS
7.4.   OID arc for KerberosV5
7.5.   Protocol constants and associated values
7.5.1. Key usage numbers
7.5.2. PreAuthentication Data Types
7.5.3. Address Types
7.5.4. Authorization Data Types
7.5.5. Transited Encoding Types
7.5.6. Protocol Version Number
7.5.7. Kerberos Message Types
7.5.8. Name Types
7.5.9. Error Codes
8.     Interoperability requirements
8.1.   Specification 2
8.2.   Recommended KDC values
9.     IANA considerations
10.    Security Considerations
11.    Acknowledgement and References
A.     ASN.1 module
B.     Changes since RFC-1510

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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[1.1]. Kerberos performs authentication under these conditions as a
trusted third-party authentication service by using conventional (shared
secret key [1.2]) 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 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.

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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 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[1.3]. 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.
draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003


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.

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 KDC's 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 KDC's not check the transited field by
setting the DISABLE-TRANSITED-CHECK flag. KDC's 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.

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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 wile 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, GSSAPI, and SASL [RFC 2222] MUST NOT use DNS to
canonicalize the host components of service principal names. Application
authors MAY wish to append a statically configured domain name to
unqualified hosts before passing the name to security mechanisms.

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 wether 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 tickets
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.

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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 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 SHOULD 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 honor
authorization data restrictions may create a security weakness.

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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.
   * 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.

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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.
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.
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
     Aside from the 3-headed dog guarding Hades, the name given to Project
     Athena's authentication service, the protocol used by that service, or
     the code used to implement the authentication service.
Plaintext
     The input to an encryption function or the output of a decryption
     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.

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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.

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2. Ticket flag uses and requests

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
and not 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
usually 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 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).

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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 wish to 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 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.

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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, user's wishing to delegate their identity for ANY 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 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[2.1]. 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.

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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 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 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.

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2.8. Other KDC options

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

2.8.1 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 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.

2.8.2 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.

2.8.3 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-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.

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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[3.1]. This
exchange does not by itself provide any assurance of the the identity of the
user[3.2].

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. 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 section 3.1.1.

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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; whether the
client requests an anonymous ticket; 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. 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 wish
to 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 the PA-ETYPE-INFO
pre-authentication field will be included in the KRB-ERROR message. If the
server cannot accommodate an 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[3.3].

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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 first valid strong etype for which an encryption key is
available is used.

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 6 and by
the encryption mechanism specification, respectively) that may be overridden
by preauthentication data (PA-PW-SALT, PA-AFS3-SALT, PA-ETYPE-INFO,
PA-S2K-PARAMS, 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.

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.

When the etype field is present in a KDC request, whether an AS or TGS
request, 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 minimum 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.
   * 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.

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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
is set to the minimum 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, ANONYMOUS. If
the new ticket is post-dated (the start time is in the future), its INVALID
flag will also be set.

If all of the above succeed, the server will encrypt 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

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
key-expiration field from the encrypted part of the response may be checked
to notify the user of impending key expiration (the client program could
then suggest remedial action, such as a password change).

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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 [cite Davis and Geer].

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 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[3.4], 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.'

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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 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[3.5]. If a sequence number is to be included, it should be randomly
chosen so that even after 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 authentciation - 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 different keys were used to encrypt and decrypt).

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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 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 [3.7]. 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 re-played
authenticators will fall outside the allowable clock skew and can no longer
be successfully replayed[3.8].

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 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.

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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)[3.9]. 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 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[3.10] 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
draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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[3.11]. 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 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.

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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 (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[3.12]. 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
draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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 wish to use local policy configuration to
verify that the authentication path used is an acceptable one.
Alternatively, a client may wish to 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.

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 ssesion 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.

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In preparing the authentication header, the client can select a sub-session
key under which the response from the Kerberos server will be
encrypted[3.13]. 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.

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 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
draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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 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.

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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 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.

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 KDC's for the
realms for which such cross-realm tickets were issued.

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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.

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"

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

A realm name beginning with a "/" is interpreted as being appended to the
previous realm[18]. If it 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
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[19]. Thus,
"," means that all realms along the path between the client and the server
have been traversed. ",EDU, /COM," means that 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 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 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, or the
session key. Some implementations use a different checksum algorithm for
KRB_SAFE messages but doing so in a interoperable manner is impossible.
Implementations should accept any checksum algorithm they implement that
both has adequate security and that has keys compatible with the sub-session
or session key. Unkeyed or non-collision-proof checksums are not suitable
for this use.

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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.

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, 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 wish to 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[20] ) 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 message was
generated by its peer and was not modified in transit.

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

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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 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 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 tripple DES key.
For interoperability with MIT, the Microsoft implementation will not encrypt
the KRB_CRED in a GSSAPI 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. v

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 wish to
draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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 wish to skip the address check as the KRB_CRED message
cannot generally be reflected back toThe 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 ticket 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

(This entire subsection is new) 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 when the user
logged in to the workstation, but because the server is running as an
unprivleged 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
    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 a ticket
granting ticket issued to the party that will verify the authentication.
This ticket granting ticket must be obtained from the verifier by a means
exchange external to the Kerberos protococl, 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, can not be used for authentication without posession of the
corresponding secret key, and because the verifier does not give out the
corresponding secret key, providing a copy of the verifiers ticket granting
ticket does not allow impersonation of the verifier.

Once the verifier's ticket granting ticket has been obtained by the client,
by specifying the ENC-TKT-IN-SKEY option to the KDC, the client can include
the ticket as an additional ticket in its KRB_TGS_REQ frequest to the KDC
(message 1 in the table above).

If validated according to the instructions in 3.3.3, the application ticket
returned to the client (message 3 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.

[RFCEDITOR: Insert citation for 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 preauthentication 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 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 8.4.

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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, key usages 1024 and 1025 must be used to
derive keys for encryption and checksums, respectively. (Of course, 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 we don't recommend it, undoubtedly some
application protocols will continue to use the key data directly, even if
only in some of the currently existing protocol specifications. [4.1]. An
implementation intended to support general Kerberos applications may
therefore need to make the key data available, as well as the attributes and
operations described in [KCRYPTO].

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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 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 will be clearly
flagged when 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 written in all-caps in order to
distinguish them from surrounding explanatory text.

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

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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 to switch character sets, and the
default character set that is designated as G0 is the 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)
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
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.

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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 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 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 7. 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 7.2. 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).
     This constraint may be eliminated in the future.
draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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

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 usually is identical to the application tag number
     of the containing message type.

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5.2.5. HostAddress and HostAddresses

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

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 8.1.
address
     This field encodes a single address of type addr-type.

The two forms differ slightly. HostAddress contains exactly one address;
HostAddresses contains a sequence of possibly many addresses.

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 recursivly 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 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
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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 SHOULD 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-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.

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5.2.6.4. KDCIssued

AD-KDCIssued            ::= SEQUENCE {
        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 priveleges 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.

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

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5.2.6.5. AND-OR

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

When restrictive AD elements are encapsulated within the and-or element are
encountered, only the number specified in condition-count of the
encapsulated conditions must be met in order to satisfy this element. 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.

5.2.6.8. 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 if-relevant element must reject the request.

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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
}

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 PA-ETYPE-INFO

 19           pa-etype-info2   DER encoding of PA-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.

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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.

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 preauthentication type was not present in RFC 1510, but many
implementations support it.

5.2.7.3. PA-PW-SALT

The padata-value for this preauthentication 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 preauthentication 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 preauthentication. Implementation note: some KDC
implementations issue an erroneous PA-PW-SALT when issuing a KRB-ERROR
message that requests additional preauthentication. Therefore, clients
should ignore a PA-PW-SALT accompanying a KRB-ERROR message that requests
additional preauthentication.

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5.2.7.4. PA-ETYPE-INFO

The ETYPE-INFO preauthentication type is sent by the KDC in a KRB-ERROR
indicating a requirement for additional preauthentication. 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
preauthentication 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-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 preauthentication type was not present in RFC 1510, but many
implementations that support encrypted timestamps for preauthentication need
to support ETYPE-INFO as well.

5.2.7.5. PA-ETYPE-INFO2

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

The ETYPE-INFO2 preauthentication 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 may 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 only include
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 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.

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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.

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.

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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,
        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
}

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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 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.

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                                     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
                                     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
                                     wish to 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.

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                                     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
                                     preauthentication method is not
                                     indicated, but is acceptable to the
                                     KDC.

                                     This flag indicates that the protocol
                                     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[24].
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 it is absent from the ticket, its value should be treated as
     that of the authtime field.
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.
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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 zero-address tickets is a policy decision and is left to the
     Kerberos and end-service administrators; they may refuse to issue or
     accept such tickets. The suggested and default policy, however, is that
     such tickets will only be issued or accepted when additional
     information that can be used to restrict the use of the ticket is
     included in the authorization_data field. Such a ticket is a
     capability.

     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.

     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.
     Although Kerberos is not concerned with the format of the contents of
     the sub-fields, it does carry type information (ad-type).

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     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 B.4).

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

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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,
        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
}

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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
     (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. In most requests for initial authentication
     (KRB_AS_REQ) and most replies (KDC-REP), the padata field will be left
     out.
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.
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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
                                       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.

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                                       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       UNUSED                   This option is presently unused.

                                       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

                                       These options are presently unused.
      10-13   UNUSED                   Option 11 is reserved for future is
                                       as opt-hardware-auth.

      14-25   RESERVED                 Reserved for future use.
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                                       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. KDC's 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
                                       the 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.
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                                       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. sname may only be absent when the ENC-TKT-IN-SKEY 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
     from the padata field in the KRB_AP_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 for. 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 it 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 re-used.
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etype
     This field specifies the desired encryption algorithm to be used in the
     response.
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, no version number will be present in the EncryptedData
sequence.

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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.
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.
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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
     preauthentication 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 wish to 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.
     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.
     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.
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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. 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 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.
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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
                              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.
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crealm and cname
     These fields are the same as those described for the ticket in section
     5.3.
cksum
     This field contains a checksum of the 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 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 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.

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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.
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.

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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 {
        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.
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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.

     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[32], with a key usage value of 13. 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.

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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,
        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.
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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 of the Kerberos GSS-API mechanism, do not encrypt the KRB-CRED
     message when sending it. In the case of the GSS-API mechanism, this is
     not a security vulnerability, as the KRB-CRED message itself is itself
     encrypted inside an Authenticator.
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 re-used; 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
     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.

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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),
        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. 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 8. 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
     }

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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

 4-10                          unused

 10             AS-REQ         PDU

 11             AS-REP         PDU

 12             TGS-REQ        PDU

 13             TGS-REP        PDU

 14             AP-REQ         PDU

 15             AP-REP         PDU

 16             RESERVED16     TGT-REQ

 17             RESERVED17     TGT-REP

 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.

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

     domain:   ATHENA.MIT.EDU (example)
       X500:   C=US/O=OSF (example)
      other:   NAMETYPE:rest/of.name=without-restrictions (example)
   reserved:   reserved, but will not conflict with above

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
draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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 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

   NT-UNKNOWN        0   Name type not known
   NT-PRINCIPAL      1   General principal name (e.g. username,
                                                 or DCE principal)
   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 slash-separated host name components
   NT-UID            5   Unique ID
   NT-X500-PRINCIPAL 6   Encoded X.509 Distingished name [RFC 1779]
   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 instance is a single component following the service name and the
instance identifies 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 8.2.4 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.

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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.

The values of the types for the following addresses are chosen to match the
defined address family constants in the Berkeley Standard Distributions of
Unix. They can be found in with symbolic names AF_xxx (where xxx is an
abbreviation of the address family name).

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 [Westerlund]

IPv6 addresses are 128-bit (16-octet) quantities, encoded in MSB order. The
type of IPv6 addresses is twenty-four (24). [RFC2373]. The following
addresses (see [RFC1884]) 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
characters, trailing blank (ascii char 20) filled, with a 16th octet of 0x0.
The type of Netbios addresses is 20 (0x14).

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 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.

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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 XX (0xXX).

7.2. KDC messaging

7.2.1 IP Transports

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

7.2.1.1. 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 [7.2.1.3] 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 an error forcing the client to retry the request using the TCP
transport.

7.2.1.2. TCP/IP transport [Westerlund,Danielsson]

Kerberos servers (KDCs) supporting IP transports MUST accept TCP 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
[7.2.1.3] 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
draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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 preauthentication) 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).

7.2.1.3 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.

7.2.1.3.1. 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-ASCII names.

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7.2.1.3.2. 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 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.

7.2.1.3.3. 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).

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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

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.

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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)
        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 5.8.1)
        19.         AD-KDCIssued checksum (ad-checksum in appendix B.4)
      22-24.        Reserved for use in GSSAPI mechanisms derived from RFC
                    1964. (raeburn/MIT)
 18, 20-21, 25-511. Reserved for future use in Kerberos and related
                    protocols.
     512-1023.      Reserved for uses internal to a Kerberos
                    implementation.

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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        (depricated)
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)
TD-PKINIT-CMS-CERTIFICATES      101      CertificateSet from CMS
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
ChaosNet                         5
XNS                              6
ISO                              7
DECNET Phase IV                 12
AppleTalk DDP                   16
NetBios                         20
IPV6                            24

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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-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

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]

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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
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 [40]
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
draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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 (pkinit)
KDC_ERROR_KDC_NOT_TRUSTED               63 (pkinit)
KDC_ERROR_INVALID_SIG                   64 (pkinit)
KDC_ERR_KEY_TOO_WEAK                    65 (pkinit)
KDC_ERR_CERTIFICATE_MISMATCH            66 (pkinit)
KRB_AP_ERR_NO_TGT                       67 (user-to-user)
KDC_ERR_WRONG_REALM                     68 (user-to-user)
KRB_AP_ERR_USER_TO_USER_REQUIRED        69 (user-to-user)
KDC_ERR_CANT_VERIFY_CERTIFICATE         70 (pkinit)
KDC_ERR_INVALID_CERTIFICATE             71 (pkinit)
KDC_ERR_REVOKED_CERTIFICATE             72 (pkinit)
KDC_ERR_REVOCATION_STATUS_UNKNOWN       73 (pkinit)
KDC_ERR_REVOCATION_STATUS_UNAVAILABLE   74 (pkinit)
KDC_ERR_CLIENT_NAME_MISMATCH            75 (pkinit)
KDC_ERR_KDC_NAME_MISMATCH               76 (pkinit)

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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.

Encyrption: 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.

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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 preauthentication method. Servers need not support the
PA-ENC-TIMESTAMP method, but if not supported the server should ignore 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 KDC's 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.

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.

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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

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
empty addresses               only when suitable  restrictions  appear
                              in authorization data
proxiable, etc.               Allowed.

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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.

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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 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 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 preauthentication,
attackers may try brute force or dictionary attacks against credentials that
are observed by eavesdropping on the network.

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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 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 re-using
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
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.

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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 wish to use local policy configuration to
verify that the authentication path used is an acceptable one.
Alternatively, a client may wish to 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.

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 the 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.

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11. Acknowledgement and References

11.1 ACKNOWLEDGEMENTS

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 John Brezac, Marc Colan, Don Davis, Doug Engert, Dan Geer, Paul Hill,
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, Brian Tung, and
Assar Westerlund. 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.

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11.1. REFERENCES

[Blumenthal96]
     Blumenthal, U., "A Better Key Schedule for DES-Like Ciphers",
     Proceedings of PRAGOCRYPT '96, 1996.
[Bellare98]
     Bellare, M., Desai, A., Pointcheval, D., Rogaway, P., "Relations Among
     Notions of Security for Public-Key Encryption Schemes". Extended
     abstract published in Advances in Cryptology- Crypto 98 Proceedings,
     Lecture Notes in Computer Science Vol. 1462, H. Krawcyzk ed.,
     Springer-Verlag, 1998.
[DES77]
     National Bureau of Standards, U.S. Department of Commerce, "Data
     Encryption Standard," Federal Information Processing Standards
     Publication 46, Washington, DC (1977).
[DESM80]
     National Bureau of Standards, U.S. Department of Com- merce, "DES Modes
     of Operation," Federal Information Processing Standards Publication 81,
     Springfield, VA (December 1980).
[Dolev91]
     Dolev, D., Dwork, C., Naor, M., "Non-malleable cryptography",
     Proceedings of the 23rd Annual Symposium on Theory of Computing, ACM,
     1991.
[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).
[DS90]
     Don Davis and Ralph Swick, "Workstation Services and Kerberos
     Authentication at Project Athena," Technical Memorandum TM-424, MIT
     Laboratory for Computer Science (February 1990).
[Horowitz96]
     Horowitz, M., "Key Derivation for Authentication, Integrity, and
     Privacy", draft-horowitz-key-derivation-02.txt, August 1998.
[HorowitzB96]
     Horowitz, M., "Key Derivation for Kerberos V5", draft-
     horowitz-kerb-key-derivation-01.txt, September 1998.
[IS3309]
     International Organization for Standardization, "ISO Information
     Processing Systems - Data Communication - High-Level Data Link Control
     Procedure - Frame Struc- ture," IS 3309 (October 1984). 3rd Edition.
[ISO-646/ECMA-6]
     7-bit Coded Character Set
[ISO-1022/ECMA-35]
     Character Code Structure and Extension Techniques
[ISO-4873/ECMA-43]
     8-bit Coded Character Set Structure and Rules
[KBC96]
     H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: Keyed- Hashing for
     Message Authentication," Working Draft
     draft-ietf-ipsec-hmac-md5-01.txt, (August 1996).
[KNT92]
     John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o, "The Evolution
     of the Kerberos Authentication Service," in an IEEE Computer Society
     Text soon to be published (June 1992).

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

[Krawczyk96]
     Krawczyk, H., Bellare, and M., Canetti, R., "HMAC: Keyed-Hashing for
     Message Authentication", draft-ietf-ipsec-hmac- md5-01.txt, August,
     1996.
[LGDSR87]
     P. J. Levine, M. R. Gretzinger, J. M. Diaz, W. E. Som- merfeld, and K.
     Raeburn, Section E.1: Service Manage- ment System, M.I.T. Project
     Athena, Cambridge, Mas- sachusetts (1987).
[MD4-92]
     R. Rivest, "The MD4 Message Digest Algorithm," RFC 1320, MIT Laboratory
     for Computer Science (April 1992).
[MD5-92]
     R. Rivest, "The MD5 Message Digest Algorithm," RFC 1321, MIT Laboratory
     for Computer Science (April 1992).
[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 Com- puting Systems, Pittsburgh, PA (May,
     1993).
[NS78]
     Roger M. Needham and Michael D. Schroeder, "Using Encryption for
     Authentication in Large Networks of Com- puters," Communications of the
     ACM, Vol. 21(12), pp. 993-999 (December, 1978).
[NT94]
     B. Clifford Neuman and Theodore Y. Ts'o, "An Authenti- cation Service
     for Computer Networks," IEEE Communica- tions 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).
     [RFC-2279]
     UTF-8, a transformation format of ISO-10646
[SG92]
     Stuart G. Stubblebine and Virgil D. Gligor, "On Message Integrity in
     Cryptographic Protocols," in Proceedings of the IEEE Symposium on
     Research in Security and Privacy, Oakland, California (May 1992).
[SNS88]
     J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Ker- beros: An
     Authentication Service for Open Network Sys- tems," pp. 191-202 in
     Usenix Conference Proceedings, Dallas, Texas (February, 1988).
[X509-88]
     CCITT, Recommendation X.509: The Directory Authentica- tion Framework,
     December 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.

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

A. ASN.1 module

KerberosV5Spec2 {
        iso(1) identified-organization(3) dod(6) internet(1)
        security(5) kerberosV5(2) modules(4) krb5spec2(2)
} 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.
AuthorizationData       ::= SEQUENCE OF SEQUENCE {
        ad-type         [0] Int32,
        ad-data         [1] OCTET STRING
}

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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,
        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
}

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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,
        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
}

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

EncASRepPart    ::= [APPLICATION 25] EncKDCRepPart

EncTGSRepPart   ::= [APPLICATION 26] EncKDCRepPart

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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,
        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
}

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

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,
        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
}

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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,
        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

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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
}

AD-MANDATORY-FOR-KDC    ::= AuthorizationData

END

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003


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.

B.1 Changes to Section 1

Discussion was added to section on 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 also mentions the use of public key 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.

B.1 Changes to Section 2

Discussion of how extensibility affects ticket flags and KDC options was
added to the introduction. 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 2.8.2) which is used for user-to-user authentication.
The new option and ticket flag transited policy checking (section 2.7) was
added.

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

B.2 Changes to Section 3

A warning regarding generation of session keys for application use was
added, 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, and pa-pw-salt
preauthentication data items were added. The recommendation for use of
preauthentication was changed from "may" to "should" and a note was added
regarding known plaintext attacks.

B.3 Changes to Section 4

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.

B.5 Changes to Section 5

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 of double-encoding.

There were two definitions of METHOD-DATA in RFC 1510. The second one,
intended for use with KRB_AP_ERR_METHOD was removed leaving the SEQUENCE OF
PA-DATA definition.

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

B.6 Changes to Section 6

The old section 6 on encryption and checksums was moved to section 4, and
changes were already described. The new section 6, naming constraints was
formerly section 7.

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.

B.7 Changes to Section 7

The current section 7 was formerly section 8, on network transport,
constants and defined values. 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.

B.8 Changes to Section 8

"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.

B.9 New section 9 and 10 added

Two new section, on IANA considerations and security considerations were
added.

B.11 Change to Appendices

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.

An appendix was added defining those authorization data elements that must
be understood by all Kerberos implementations.

draft-ietf-krb-wg-kerberos-clarifications-02         Expires 1 May 2003

FOOTNOTES

[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.

[1.1] 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 of the connection,
and does not guarantee that subsequent messages on the connection originate
from the same principal.

[1.2] 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.

[1.3] 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.

[2.1] Though it is permissible to request or issue tickets with no network
addresses specified.

[3.1] 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.

[3.2] 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.

[3.3] "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.

[3.4] 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.

[3.5] 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.

  ------------------------------------------------------------------------
  ------------------------------------------------------------------------
[3.7]Note also that the rejection here is restricted to 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.

[3.8] 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.

[3.9] 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.

[3.10] Note that for encrypting the KRB_AP_REP message, the sub-session key
is not used, even if present in the Authenticator.

[3.11] Implementations of the protocol may wish to 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.

[3.12]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.

[3.13] 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.

[4.1] Perhaps one of the more common reasons for directly performing
encryption is direct control over the negotiation and to select a
"sufficiently strong" encryption algorithm (whatever that means in the
context of a given application). While Kerberos directly provides no
facility for negotiating encryption types between the application client and
server, there are other means for accomplishing similar goals. For example,
requesting only "sufficiently strong" session key types from the KDC, and
assuming that any type returned by the KDC will be understood and supported
by the application server.


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