</
 draft-ietf-cat-kerberos-01.txt   rfc1510.txt 
INTERNET-DRAFT John Kohl Network Working Group J. Kohl
B. Clifford Neuman Request for Comments: 1510 Digital Equipment Corporation
1 September 1992 C. Neuman
ISI
The Kerberos Network Authentication Service (V5) September 1993
_S_T_A_T_U_S _O_F _T_H_I_S _M_E_M_O
This document is an Internet Draft. Internet Drafts
are working documents of the Internet Engineering Task Force
(IETF), its Areas, and its Working Groups. Note that other
groups may also distribute working documents as Internet
Drafts.
Internet Drafts are draft documents valid for a maximum
of six months. Internet Drafts may be updated, replaced,
or obsoleted by other documents at any time. It is not
appropriate to use Internet Drafts as reference material or
to cite them other than as a "working draft" or "work in
progress."
Please check the I-D abstract listing contained in each
Internet Draft directory to learn the current status of this
or any other Internet Draft. Distribution of this memo is
unlimited. Please send comments to "krb-protocol@MIT.EDU."
_A_B_S_T_R_A_C_T
This document gives an overview and specification of
Version 5 of the protocol for the Kerberos network authenti-
cation system. Version 4, described elsewhere [1,2], is
presently in production use at MIT's Project Athena, and at
other Internet sites.
_O_V_E_R_V_I_E_W
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 proto-
col.
The motivations, goals, assumptions, and rationale
behind most design decisions are treated cursorily; for Ver-
sion 4 they are fully described in the Kerberos portion of
__________________________
Project Athena, Athena, Athena MUSE, Discuss, Hesiod,
Kerberos, Moira, and Zephyr are trademarks of the Mas-
sachusetts Institute of Technology (MIT). No commer-
cial use of these trademarks may be made without prior
written permission of MIT.
Overview - 1 - Expires 28 February 1993
Version 5 - Revision 5.1
the Athena Technical Plan [1]. The protocols are under
review, and are not being submitted for consideration as an
Internet standard at this time. Comments are encouraged.
Requests for addition to an electronic mailing list for dis-
cussion 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.
8
_B_A_C_K_G_R_O_U_N_D
The Kerberos model is based in part on Needham and
Schroeder's trusted third-party authentication protocol [3]
and on modifications suggested by Denning and Sacco [4].
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 Ker-
beros. Version 4 is publicly available, and has seen wide
use across the Internet.
Version 5 (described in this document) has evolved from
Version 4 based on new requirements and desires for features
not available in Version 4. Details on the differences
between Kerberos Versions 4 and 5 can be found in [5].
_1. _I_n_t_r_o_d_u_c_t_i_o_n
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 authentication by the host operating sys-
tem, without basing trust on host addresses, without requir-
ing physical security of all the hosts on the network, and
under the assumption that packets traveling along the net-
work can be read, modified, and inserted at will[1]. Ker-
beros performs authentication under these conditions as a
trusted third-party authentication service by using conven-
tional (shared secret key[2]) cryptography.
__________________________
9[1] Note, however, that many applications use Kerberos'
functions only upon the initiation of a stream-based
network connection, and assume the absence of any ``hi-
jackers'' who might subvert such a connection. Such
use implicitly trusts the host addresses involved.
9[2] _S_e_c_r_e_t and _p_r_i_v_a_t_e 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
_s_e_c_r_e_t key. Something is only private when no one but
9Section 1. - 2 - Expires 28 February 1993
Version 5 - Revision 5.1
The 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 1) a "ticket" for the
server and 2) a temporary encryption key (often called a
"session key"). The client transmits the ticket (which con-
tains the client's identity and a copy of the session key,
all encrypted in the server's key) to the server. The ses-
sion 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.
The implementation consists of one or more authentica-
tion 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 pro-
tocol. In order to add authentication to its transactions,
a typical network application adds one or two calls to the
Kerberos library, which results in the transmission of the
necessary messages to achieve authentication.
The Kerberos protocol consists of several sub-protocols
(or exchanges). There are two 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
sends the TGT to the TGS in the same manner as if it were
contacting any other application server which requires Ker-
beros credentials. The reply is encrypted in the session
key from the TGT.
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 tran-
saction, the client transmits the ticket to the server.
Since the ticket is sent "in the clear" (parts of it are
encrypted, but this encryption doesn't thwart replay) and
__________________________
its owner knows it. Thus, in public key cryptosystems,
one has a public and a _p_r_i_v_a_t_e key.
Section 1. - 3 - Expires 28 February 1993
Version 5 - Revision 5.1
might be intercepted and reused by an attacker, additional
information is sent to prove that the message was originated
by the principal to whom the ticket was issued. This infor-
mation (called the _a_u_t_h_e_n_t_i_c_a_t_o_r) 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 princi-
pals 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 passed in the
ticket, and contained in the credentials.
The authentication exchanges mentioned above require
read-only access to the Kerberos database. Sometimes, how-
ever, 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).
The administration protocol is not described in this docu-
ment. There is also a protocol for maintaining multiple
copies of the Kerberos database, but this can be considered
an implementation detail and may vary to support different
database technologies.
_1._1. _C_r_o_s_s-_R_e_a_l_m _O_p_e_r_a_t_i_o_n
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 use its authentication remotely[3]. The exchange
__________________________
9[3] Of course, with appropriate permission the client
could arrange registration of a separately-named prin-
cipal in a remote realm, and engage in normal exchanges
with that realm's services. However, for even small
9Section 1.1. - 4 - Expires 28 February 1993
Version 5 - Revision 5.1
of inter-realm keys (a separate key may be used for each
direction) registers the ticket-granting service of each
realm as a principal in the other realm. A client is then
able to obtain a ticket-granting ticket for the remote
realm's ticket-granting service from its local realm. When
that ticket-granting ticket is used, the remote ticket-
granting service uses the inter-realm key (which usually
differs from its own normal TGS key) to decrypt the ticket-
granting ticket, and is thus certain that it was issued by
the client's own TGS. Tickets issued by the remote ticket-
granting service will indicate to the end-service that the
client was authenticated from another realm.
A realm is said to _c_o_m_m_u_n_i_c_a_t_e 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 _a_u_t_h_e_n_t_i_c_a_t_i_o_n
_p_a_t_h is the sequence of intermediate realms that are tran-
sited in communicating from one realm to another.
Realms are typically 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 authen-
tication path to be easily constructed. If a hierarchical
organization is not used, it may be necessary to consult
some database in order to construct an authentication path
between realms.
Although realms are typically hierarchical, intermedi-
ate realms may be bypassed to achieve cross-realm authenti-
cation 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.
8
_1._2. _E_n_v_i_r_o_n_m_e_n_t_a_l _a_s_s_u_m_p_t_i_o_n_s
Kerberos imposes a few assumptions on the environment in
which it can properly function:
o+ "Denial of service" attacks are not solved with Ker-
beros. There are places in these 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
__________________________
numbers of clients this becomes cumbersome, and more
automatic methods as described here are necessary.
9
Section 1.2. - 5 - Expires 28 February 1993
Version 5 - Revision 5.1
users.
o+ 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.
o+ 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 detec-
tion. The degree of "looseness" can be configured on a
per-server basis. If the clocks are synchronized over
the network, the clock synchronization protocol must
itself be secured from network attackers.
o+ Principal identifiers are not recycled on a short-term
basis. A typical mode of access control will use
access control lists (ACLs) to grant permissions to
particular principals. If a stale ACL entry remains
for a deleted principal and the principal identifier is
reused, the new principal will inherit rights specified
in the stale ACL entry. By not re-using principal
identifiers, the danger of inadvertent access is
removed.
_1._3. _G_l_o_s_s_a_r_y _o_f _t_e_r_m_s
Below is a list of terms used throughout this document.
Authentication Verifying the claimed identity of a
principal.
Authentication headerA 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 tran-
sited 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
Section 1.3. - 6 - Expires 28 February 1993
Version 5 - Revision 5.1
the client is allowed to access, and the
type of access allowed for each.
Capability A token that grants the bearer permis-
sion 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 ser-
vice 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 ser-
vice).
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 authentica-
tion service.
Section 1.3. - 7 - Expires 28 February 1993
Version 5 - Revision 5.1
Plaintext The input to an encryption function or
the output of a decryption function.
Decryption transforms ciphertext into
plaintext.
Principal A uniquely named client or server
instance that participates in a network
communication.
Principal identifierThe name used to uniquely identify each
different principal.
Seal To encipher a record containing several
fields in such a way that the fields
cannot be individually replaced without
either knowledge of the encryption key
or leaving evidence of tampering.
Secret key An encryption key shared by a principal
and the KDC, distributed outside the
bounds of the system, with a long life-
time. In the case of a human user's
principal, the secret key is derived
from a password.
Server A particular Principal which provides a
resource to network clients.
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 "ses-
sion".
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 dura-
tion of a single association.
Ticket A record that helps a client authenti-
cate itself to a server; it contains the
Section 1.3. - 8 - Expires 28 February 1993 The Kerberos Network Authentication Service (V5)
Version 5 - Revision 5.1 Status of this Memo
client's identity, a session key, a This RFC specifies an Internet standards track protocol for the
timestamp, and other information, all Internet community, and requests discussion and suggestions for
sealed using the server's secret key. improvements. Please refer to the current edition of the "Internet
It only serves to authenticate a client Official Protocol Standards" for the standardization state and status
when presented along with a fresh of this protocol. Distribution of this memo is unlimited.
Authenticator.
_2. _T_i_c_k_e_t _f_l_a_g _u_s_e_s _a_n_d _r_e_q_u_e_s_t_s Abstract
Each Kerberos ticket contains a set of flags which are used This document gives an overview and specification of Version 5 of the
to indicate various attributes of that ticket. Most flags protocol for the Kerberos network authentication system. Version 4,
may be requested by a client when the ticket is obtained; described elsewhere [1,2], is presently in production use at MIT's
some are automatically turned on and off by a Kerberos Project Athena, and at other Internet sites.
server as required. The following sections explain what the
various flags mean, and gives examples of reasons to use
such a flag.
_2._1. _I_n_i_t_i_a_l _a_n_d _p_r_e-_a_u_t_h_e_n_t_i_c_a_t_e_d _t_i_c_k_e_t_s Overview
The INITIAL flag indicates that a ticket was issued Project Athena, Athena, Athena MUSE, Discuss, Hesiod, Kerberos,
using the AS protocol and not issued based on a ticket- Moira, and Zephyr are trademarks of the Massachusetts Institute of
granting ticket. Application servers that want to require Technology (MIT). No commercial use of these trademarks may be made
the knowledge of a client's secret key (e.g. a password- without prior written permission of MIT.
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 addition This RFC describes the concepts and model upon which the Kerberos
information about the initial authentication, regardless of network authentication system is based. It also specifies Version 5
whether the current ticket was issued directly (in which of the Kerberos protocol.
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. _I_n_v_a_l_i_d _t_i_c_k_e_t_s The motivations, goals, assumptions, and rationale behind most design
decisions are treated cursorily; for Version 4 they are fully
described in the Kerberos portion of the Athena Technical Plan [1].
The protocols are under review, and are not being submitted for
consideration as an Internet standard at this time. Comments are
encouraged. 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.
The INVALID flag indicates that a ticket is invalid. Background
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 tick-
ets 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).
_2._3. _R_e_n_e_w_a_b_l_e _t_i_c_k_e_t_s The Kerberos model is based in part on Needham and Schroeder's
trusted third-party authentication protocol [3] and on modifications
suggested by Denning and Sacco [4]. 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 4 is publicly available, and has seen wide use across the
Internet.
Applications may desire to hold tickets which can be Version 5 (described in this document) has evolved from Version 4
valid for long periods of time. However, this can expose based on new requirements and desires for features not available in
their credentials to potential theft for equally long Version 4. Details on the differences between Kerberos Versions 4
periods, and those stolen credentials would be valid until and 5 can be found in [5].
Section 2.3. - 9 - Expires 28 February 1993 Table of Contents
Version 5 - Revision 5.1 1. Introduction ....................................... 5
1.1. Cross-Realm Operation ............................ 7
1.2. Environmental assumptions ........................ 8
1.3. Glossary of terms ................................ 9
2. Ticket flag uses and requests ...................... 12
2.1. Initial and pre-authenticated tickets ............ 12
2.2. Invalid tickets .................................. 12
2.3. Renewable tickets ................................ 12
2.4. Postdated tickets ................................ 13
2.5. Proxiable and proxy tickets ...................... 14
2.6. Forwardable tickets .............................. 15
2.7. Other KDC options ................................ 15
3. Message Exchanges .................................. 16
3.1. The Authentication Service Exchange .............. 16
3.1.1. Generation of KRB_AS_REQ message ............... 17
3.1.2. Receipt of KRB_AS_REQ message .................. 17
3.1.3. Generation of KRB_AS_REP message ............... 17
3.1.4. Generation of KRB_ERROR message ................ 19
3.1.5. Receipt of KRB_AS_REP message .................. 19
3.1.6. Receipt of KRB_ERROR message ................... 20
3.2. The Client/Server Authentication Exchange ........ 20
3.2.1. The KRB_AP_REQ message ......................... 20
3.2.2. Generation of a KRB_AP_REQ message ............. 20
3.2.3. Receipt of KRB_AP_REQ message .................. 21
3.2.4. Generation of a KRB_AP_REP message ............. 23
3.2.5. Receipt of KRB_AP_REP message .................. 23
3.2.6. Using the encryption key ....................... 24
3.3. The Ticket-Granting Service (TGS) Exchange ....... 24
3.3.1. Generation of KRB_TGS_REQ message .............. 25
3.3.2. Receipt of KRB_TGS_REQ message ................. 26
3.3.3. Generation of KRB_TGS_REP message .............. 27
3.3.3.1. Encoding the transited field ................. 29
3.3.4. Receipt of KRB_TGS_REP message ................. 31
3.4. The KRB_SAFE Exchange ............................ 31
3.4.1. Generation of a KRB_SAFE message ............... 31
3.4.2. Receipt of KRB_SAFE message .................... 32
3.5. The KRB_PRIV Exchange ............................ 33
3.5.1. Generation of a KRB_PRIV message ............... 33
3.5.2. Receipt of KRB_PRIV message .................... 33
3.6. The KRB_CRED Exchange ............................ 34
3.6.1. Generation of a KRB_CRED message ............... 34
3.6.2. Receipt of KRB_CRED message .................... 34
4. The Kerberos Database .............................. 35
4.1. Database contents ................................ 35
4.2. Additional fields ................................ 36
4.3. Frequently Changing Fields ....................... 37
4.4. Site Constants ................................... 37
5. Message Specifications ............................. 38
5.1. ASN.1 Distinguished Encoding Representation ...... 38
5.2. ASN.1 Base Definitions ........................... 38
5.3. Tickets and Authenticators ....................... 42
5.3.1. Tickets ........................................ 42
5.3.2. Authenticators ................................. 47
5.4. Specifications for the AS and TGS exchanges ...... 49
5.4.1. KRB_KDC_REQ definition ......................... 49
5.4.2. KRB_KDC_REP definition ......................... 56
5.5. Client/Server (CS) message specifications ........ 58
5.5.1. KRB_AP_REQ definition .......................... 58
5.5.2. KRB_AP_REP definition .......................... 60
5.5.3. Error message reply ............................ 61
5.6. KRB_SAFE message specification ................... 61
5.6.1. KRB_SAFE definition ............................ 61
5.7. KRB_PRIV message specification ................... 62
5.7.1. KRB_PRIV definition ............................ 62
5.8. KRB_CRED message specification ................... 63
5.8.1. KRB_CRED definition ............................ 63
5.9. Error message specification ...................... 65
5.9.1. KRB_ERROR definition ........................... 66
6. Encryption and Checksum Specifications ............. 67
6.1. Encryption Specifications ........................ 68
6.2. Encryption Keys .................................. 71
6.3. Encryption Systems ............................... 71
6.3.1. The NULL Encryption System (null) .............. 71
6.3.2. DES in CBC mode with a CRC-32 checksum (descbc-crc)71
6.3.3. DES in CBC mode with an MD4 checksum (descbc-md4) 72
6.3.4. DES in CBC mode with an MD5 checksum (descbc-md5) 72
6.4. Checksums ........................................ 74
6.4.1. The CRC-32 Checksum (crc32) .................... 74
6.4.2. The RSA MD4 Checksum (rsa-md4) ................. 75
6.4.3. RSA MD4 Cryptographic Checksum Using DES
(rsa-md4-des) ......................................... 75
6.4.4. The RSA MD5 Checksum (rsa-md5) ................. 76
6.4.5. RSA MD5 Cryptographic Checksum Using DES
(rsa-md5-des) ......................................... 76
6.4.6. DES cipher-block chained checksum (des-mac)
6.4.7. RSA MD4 Cryptographic Checksum Using DES
alternative (rsa-md4-des-k) ........................... 77
6.4.8. DES cipher-block chained checksum alternative
(des-mac-k) ........................................... 77
7. Naming Constraints ................................. 78
7.1. Realm Names ...................................... 77
7.2. Principal Names .................................. 79
7.2.1. Name of server principals ...................... 80
8. Constants and other defined values ................. 80
8.1. Host address types ............................... 80
8.2. KDC messages ..................................... 81
8.2.1. IP transport ................................... 81
8.2.2. OSI transport .................................. 82
8.2.3. Name of the TGS ................................ 82
8.3. Protocol constants and associated values ......... 82
9. Interoperability requirements ...................... 86
9.1. Specification 1 .................................. 86
9.2. Recommended KDC values ........................... 88
10. Acknowledgments ................................... 88
11. References ........................................ 89
12. Security Considerations ........................... 90
13. Authors' Addresses ................................ 90
A. Pseudo-code for protocol processing ................ 91
A.1. KRB_AS_REQ generation ............................ 91
A.2. KRB_AS_REQ verification and KRB_AS_REP generation 92
A.3. KRB_AS_REP verification .......................... 95
A.4. KRB_AS_REP and KRB_TGS_REP common checks ......... 96
A.5. KRB_TGS_REQ generation ........................... 97
A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation 98
A.7. KRB_TGS_REP verification ......................... 104
A.8. Authenticator generation ......................... 104
A.9. KRB_AP_REQ generation ............................ 105
A.10. KRB_AP_REQ verification ......................... 105
A.11. KRB_AP_REP generation ........................... 106
A.12. KRB_AP_REP verification ......................... 107
A.13. KRB_SAFE generation ............................. 107
A.14. KRB_SAFE verification ........................... 108
A.15. KRB_SAFE and KRB_PRIV common checks ............. 108
A.16. KRB_PRIV generation ............................. 109
A.17. KRB_PRIV verification ........................... 110
A.18. KRB_CRED generation ............................. 110
A.19. KRB_CRED verification ........................... 111
A.20. KRB_ERROR generation ............................ 112
the expiration time of the ticket(s). Simply using short- 1. Introduction
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 inter- Kerberos provides a means of verifying the identities of principals,
preted by the ticket-granting service (discussed below in (e.g., a workstation user or a network server) on an open
section 3.3). It can usually be ignored by application (unprotected) network. This is accomplished without relying on
servers. However, some particularly careful application authentication by the host operating system, without basing trust on
servers may wish to disallow renewable tickets. 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. (Note,
however, that many applications use Kerberos' functions only upon the
initiation of a stream-based network connection, and assume the
absence of any "hijackers" who might subvert such a connection. Such
use implicitly trusts the host addresses involved.) Kerberos
performs authentication under these conditions as a trusted third-
party authentication service by using conventional cryptography,
i.e., shared secret key. (shared secret key - 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.)
If a renewable ticket is not renewed by its expiration The authentication process proceeds as follows: A client sends a
time, the KDC will not renew the ticket. The RENEWABLE flag request to the authentication server (AS) requesting "credentials"
is reset by default, but a client may request it be set by for a given server. The AS responds with these credentials,
setting the RENEWABLE option in the KRB_AS_REQ message. If encrypted in the client's key. The credentials consist of 1) a
it is set, then the renew-till field in the ticket contains "ticket" for the server and 2) a temporary encryption key (often
the time after which the ticket may not be renewed. 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.
_2._4. _P_o_s_t_d_a_t_e_d _t_i_c_k_e_t_s The implementation 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, which results in the transmission of the
necessary messages to achieve authentication.
Applications may occasionally need to obtain tickets The Kerberos protocol consists of several sub-protocols (or
for use much later, e.g. a batch submission system would exchanges). There are two methods by which a client can ask a
need tickets to be valid at the time the batch job is ser- Kerberos server for credentials. In the first approach, the client
viced. However, it is dangerous to hold valid tickets in a sends a cleartext request for a ticket for the desired server to the
batch queue, since they will be on-line longer and more AS. The reply is sent encrypted in the client's secret key. Usually
prone to theft. Postdated tickets provide a way to obtain this request is for a ticket-granting ticket (TGT) which can later be
these tickets from the KDC at job submission time, but to used with the ticket-granting server (TGS). In the second method,
leave them "dormant" until they are activated and validated the client sends a request to the TGS. The client sends the TGT to
by a further request of the KDC. If a ticket theft were the TGS in the same manner as if it were contacting any other
reported in the interim, the KDC would refuse to validate application server which requires Kerberos credentials. The reply is
the ticket, and the thief would be foiled. encrypted in the session key from the TGT.
The MAY-POSTDATE flag in a ticket is normally only Once obtained, credentials may be used to verify the identity of the
interpreted by the ticket-granting service. It can be principals in a transaction, to ensure the integrity of messages
ignored by application servers. This flag must be set in a exchanged between them, or to preserve privacy of the messages. The
ticket-granting ticket in order to issue a postdated ticket application is free to choose whatever protection may be necessary.
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
Section 2.4. - 10 - Expires 28 February 1993 To verify the identities of the principals in a transaction, the
client transmits the ticket to the 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
was originated by 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.
Version 5 - Revision 5.1 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 passed in the ticket, and
contained in the credentials.
a client to obtain a postdated ticket-granting ticket; post- The authentication exchanges mentioned above require read-only access
dated ticket-granting tickets can only by obtained by to the Kerberos database. Sometimes, however, the entries in the
requesting the postdating in the KRB_AS_REQ message. The database must be modified, such as when adding new principals or
life (endtime-starttime) of a postdated ticket will be the changing a principal's key. This is done using a protocol between a
remaining life of the ticket-granting ticket at the time of client and a third Kerberos server, the Kerberos Administration
the request, unless the RENEWABLE option is also set, in Server (KADM). The administration protocol is not described in this
which case it can be the full life (endtime-starttime) of document. There is also a protocol for maintaining multiple copies of
the ticket-granting ticket. The KDC may limit how far in the Kerberos database, but this can be considered an implementation
the future a ticket may be postdated. detail and may vary to support different database technologies.
The POSTDATED flag indicates that a ticket has been 1.1. Cross-Realm Operation
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. _P_r_o_x_i_a_b_l_e _a_n_d _p_r_o_x_y _t_i_c_k_e_t_s 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.
At times it may be necessary for a principal to allow a By establishing "inter-realm" keys, the administrators of two realms
service to perform an operation on its behalf. The service can allow a client authenticated in the local realm to use its
must be able to take on the identity of the client, but only authentication remotely (Of course, with appropriate permission the
for a particular purpose. A principal can allow a service client could arrange registration of a separately-named principal in
to take on the principal's identity for a particular purpose a remote realm, and engage in normal exchanges with that realm's
by granting it a proxy. services. However, for even small numbers of clients this becomes
cumbersome, and more automatic methods as described here are
necessary). 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.
The PROXIABLE flag in a ticket is normally only inter- A realm is said to communicate with another realm if the two realms
preted by the ticket-granting service. It can be ignored by share an inter-realm key, or if the local realm shares an inter-realm
application servers. When set, this flag tells the ticket- key with an intermediate realm that communicates with the remote
granting server that it is OK to issue a new ticket (but not realm. An authentication path is the sequence of intermediate realms
a ticket-granting ticket) with a different network address that are transited in communicating from one realm to another.
based on this ticket. This flag is set by default.
This flag allows a client to pass a proxy to a server Realms are typically organized hierarchically. Each realm shares a
to perform a remote request on its behalf, e.g. a print ser- key with its parent and a different key with each child. If an
vice client can give the print server a proxy to access the inter-realm key is not directly shared by two realms, the
client's files on a particular file server in order to hierarchical organization allows an authentication path to be easily
satisfy a print request. constructed. If a hierarchical organization is not used, it may be
necessary to consult some database in order to construct an
authentication path between realms.
In order to complicate the use of stolen credentials, Although realms are typically hierarchical, intermediate realms may
Kerberos tickets are usually valid from only those network be bypassed to achieve cross-realm authentication through alternate
addresses specifically included in the ticket[4]. For this authentication paths (these might be established to make
reason, a client wishing to grant a proxy must request a new communication between two realms more efficient). It is important
ticket valid for the network address of the service to be for the end-service to know which realms were transited when deciding
granted the proxy. how much faith to place in the authentication process. To facilitate
9__________________________ this decision, a field in each ticket contains the names of the
9[4] It is permissible to request or issue tickets with realms that were involved in authenticating the client.
no network addresses specified, but we do not recommend
it.
Section 2.5. - 11 - Expires 28 February 1993 1.2. Environmental assumptions
Version 5 - Revision 5.1 Kerberos imposes a few assumptions on the environment in which it can
properly function:
The PROXY flag is set in a ticket by the TGS when it + "Denial of service" attacks are not solved with Kerberos. There
issues a proxy ticket. Application servers may check this are places in these protocols where an intruder intruder can
flag and require additional authentication from the agent prevent an application from participating in the proper
presenting the proxy in order to provide an audit trail. 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.
_2._6. _F_o_r_w_a_r_d_a_b_l_e _t_i_c_k_e_t_s + 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.
Authentication forwarding is an instance of the proxy + "Password guessing" attacks are not solved by Kerberos. If a
case where the service is granted complete use of the user chooses a poor password, it is possible for an attacker to
client's identity. An example where it might be used is successfully mount an offline dictionary attack by repeatedly
when a user logs in to a remote system and wants authentica- attempting to decrypt, with successive entries from a
tion to work from that system as if the login were local. dictionary, messages obtained which are encrypted under a key
derived from the user's password.
The FORWARDABLE flag in a ticket is normally only + Each host on the network must have a clock which is "loosely
interpreted by the ticket-granting service. It can be synchronized" to the time of the other hosts; this
ignored by application servers. The FORWARDABLE flag has an synchronization is used to reduce the bookkeeping needs of
interpretation similar to that of the PROXIABLE flag, except application servers when they do replay detection. The degree
ticket-granting tickets may also be issued with different of "looseness" can be configured on a per-server basis. If the
network addresses. This flag is reset by default, but users clocks are synchronized over the network, the clock
may request that it be set by setting the FORWARDABLE option synchronization protocol must itself be secured from network
in the AS request when they request their initial ticket- attackers.
granting ticket.
This flag allows for authentication forwarding without + Principal identifiers are not recycled on a short-term basis. A
requiring the user to enter a password again. If the flag typical mode of access control will use access control lists
is not set, then authentication forwarding is not permitted, (ACLs) to grant permissions to particular principals. If a
but the same end result can still be achieved if the user stale ACL entry remains for a deleted principal and the
engages in the AS exchange with the requested network principal identifier is reused, the new principal will inherit
addresses and supplies a password. rights specified in the stale ACL entry. By not re-using
principal identifiers, the danger of inadvertent access is
removed.
The FORWARDED flag is set by the TGS when a client 1.3. Glossary of terms
presents a ticket with the FORWARDABLE flag set and requests
it be set by specifying the FORWARDED KDC option and supply-
ing 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 wish to process FORWARDED
tickets differently than non-FORWARDED tickets.
_2._7. _O_t_h_e_r _K_D_C _o_p_t_i_o_n_s Below is a list of terms used throughout this document.
There are two additional options which may be set in a Authentication Verifying the claimed identity of a
client's request of the KDC. principal.
The RENEWABLE-OK option indicates that the client will Authentication header A record containing a Ticket and an
accept a renewable ticket if a ticket with the requested Authenticator to be presented to a
life cannot otherwise be provided. If a ticket with the server as part of the authentication
requested life cannot be provided, then the KDC may issue a process.
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.
The ENC-TKT-IN-SKEY option is honored only by the Authentication path A sequence of intermediate realms transited
ticket-granting service. It indicates that the to-be-issued in the authentication process when
communicating from one realm to another.
Section 2.7. - 12 - Expires 28 February 1993 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.
Version 5 - Revision 5.1 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.
ticket for the end server is to be encrypted in the session Capability A token that grants the bearer permission
key from the additional ticket-granting ticket provided with to access an object or service. In
the request. See section 3.3.3 for specific details. 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.
_3. _M_e_s_s_a_g_e _E_x_c_h_a_n_g_e_s Ciphertext The output of an encryption function.
Encryption transforms plaintext into
ciphertext.
The following sections describe the interactions between Client A process that makes use of a network
network clients and servers and the messages involved in service on behalf of a user. Note that
those exchanges. 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).
_3._1. _T_h_e _A_u_t_h_e_n_t_i_c_a_t_i_o_n _S_e_r_v_i_c_e _E_x_c_h_a_n_g_e Credentials A ticket plus the secret session key
necessary to successfully use that
ticket in an authentication exchange.
Summary KDC Key Distribution Center, a network service
_M_e_s_s_a_g_e _d_i_r_e_c_t_i_o_n _M_e_s_s_a_g_e _t_y_p_e _S_e_c_t_i_o_n that supplies tickets and temporary
1. Client to Kerberos KRB_AS_REQ 5.4.1 session keys; or an instance of that
2. Kerberos to client KRB_AS_REP or 5.4.2 service or the host on which it runs.
KRB_ERROR 5.8.1 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).
The Authentication Service (AS) Exchange between the Kerberos Aside from the 3-headed dog guarding
client and the Kerberos Authentication Server is usually in- Hades, the name given to Project
itiated by a client when it wishes to obtain authentication Athena's authentication service, the
credentials for a given server but currently holds no protocol used by that service, or the
credentials. The client's secret key is used for encryption code used to implement the authentication
and decryption. This exchange is typically used at the ini- service.
tiation 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 ser-
vices which must not be mediated through the Ticket-Granting
Service, but rather require a principal's secret key, such
as the password-changing service[5].
The exchange consists of two messages: KRB_AS_REQ from Plaintext The input to an encryption function or
the client to Kerberos, and KRB_AS_REP or KRB_ERROR in the output of a decryption function.
reply. The formats for these messages are described in sec- Decryption transforms ciphertext into
tions 5.4.1, 5.4.2, and 5.8.1. plaintext.
In the request, the client sends (in cleartext) its own Principal A uniquely named client or server
identity and the identity of the server for which it is instance that participates in a network
requesting credentials. The response, KRB_AS_REP, contains communication.
a ticket for the client to present to the server, and a ses-
sion 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 KRB_AS_REP message contains
information which can be used to detect replays, and to
__________________________
9[5] The password-changing request must not be honored
unless the requester can provide the old password (the
user's current secret key). Otherwise, it would be
possible for someone to walk up to an unattended ses-
sion and change another user's password.
9
Section 3.1. - 13 - Expires 28 February 1993 Principal identifier The name used to uniquely identify each
different principal.
Version 5 - Revision 5.1 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.
associate it with the message to which it replies. Various Secret key An encryption key shared by a principal
errors can occur; these are indicated by an error response and the KDC, distributed outside the
(KRB_ERROR) instead of the KRB_AS_REP response. The error bounds of the system, with a long lifetime.
message is not encrypted. The KRB_ERROR message also con- In the case of a human user's
tains information which can be used to associate it with the principal, the secret key is derived
message to which it replies. The lack of encryption in the from a password.
KRB_ERROR message precludes the ability to detect replays or
fabrications of such messages.
In the normal case the authentication server does not Server A particular Principal which provides a
know whether the client is actually the principal named in resource to network clients.
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. The ini-
tial request supports an optional field that can be used to
pass additional information that might be needed for the
initial exchange. This field may be used for pre-
authentication if desired, but the mechanism is not
currently specified.
_3._1._1. _G_e_n_e_r_a_t_i_o_n _o_f _K_R_B__A_S__R_E_Q _m_e_s_s_a_g_e Service A resource provided to network clients;
often provided by more than one server
(for example, remote file service).
The client may specify a number of options in the ini- Session key A temporary encryption key used between
tial request. Among these options are whether the requested two principals, with a lifetime limited
ticket is to be renewable, proxiable, or forwardable; to the duration of a single login "session".
whether it should be postdated or allow postdating of
derivative tickets; and whether a renewable ticket will be
accepted in lieu of a non-renewable ticket if the requested
ticket expiration date cannot be satisfied by a non-
renewable ticket (due to configuration constraints; see sec-
tion 4). See section A.1 for pseudocode.
The client prepares the KRB_AS_REQ message and sends it Sub-session key A temporary encryption key used between
to the KDC. two principals, selected and exchanged
by the principals using the session key,
and with a lifetime limited to the duration
of a single association.
_3._1._2. _R_e_c_e_i_p_t _o_f _K_R_B__A_S__R_E_Q _m_e_s_s_a_g_e 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.
If all goes well, processing the KRB_AS_REQ message 2. Ticket flag uses and requests
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 5.3.1. The contents of the ticket are
determined as follows.
_3._1._3. _G_e_n_e_r_a_t_i_o_n _o_f _K_R_B__A_S__R_E_P _m_e_s_s_a_g_e Each Kerberos ticket contains a set of flags which are used to
indicate various 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 gives
examples of reasons to use such a flag.
The authentication server looks up the client and 2.1. Initial and pre-authenticated tickets
server principals named in the KRB_AS_REQ in its database,
extracting their respective keys. If the server cannot
accommodate the requested encryption type, an error message
with code KDC_ERR_ETYPE_NOSUPP is returned. Otherwise it
generates a "random" session key[6].
__________________________
Section 3.1.3. - 14 - Expires 28 February 1993 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 knowledge of a client's
secret key (e.g., a passwordchanging 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.
Version 5 - Revision 5.1 The PRE-AUTHENT and HW-AUTHENT flags provide addition 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).
If the requested start time is absent or indicates a 2.2. Invalid tickets
time in the past, then the start time of the ticket is set
to the authentication server's current time. If it indicates
a time in the future, 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 post-
dated 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 minimum The INVALID flag indicates that a ticket is invalid. Application
of the following: 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).
o+The expiration time (endtime) requested in the KRB_AS_REQ 2.3. Renewable tickets
message.
o+The ticket's start time plus the maximum allowable lifetime Applications may desire to hold tickets which can be valid for long
associated with the client principal (the authentication periods of time. However, this can expose their credentials to
server's database includes a maximum ticket lifetime field potential theft for equally long periods, and those stolen
in each principal's record; see section 4). credentials would be valid until the expiration time of the
ticket(s). Simply using shortlived 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.
o+The ticket's start time plus the maximum allowable lifetime The RENEWABLE flag in a ticket is normally only interpreted by the
associated with the server principal. 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.
o+The ticket's start time plus the maximum lifetime set by If a renewable ticket is not renewed by its expiration time, the KDC
the policy of the local realm. 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.
If the requested expiration time minus the start time 2.4. Postdated tickets
(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).
__________________________ Applications may occasionally need to obtain tickets for use much
[6] "Random" means that, among other things, it should later, e.g., a batch submission system would need tickets to be valid
be impossible to guess the next session key based on at the time the batch job is serviced. However, it is dangerous to
knowledge of past session keys. This can only be hold valid tickets in a batch queue, since they will be on-line
achieved in a pseudo-random number generator if it is longer and more prone to theft. Postdated tickets provide a way to
based on cryptographic principles. It would be more obtain these tickets from the KDC at job submission time, but to
desirable to use a truly random number generator, such leave them "dormant" until they are activated and validated by a
as one based on measurements of random physical further request of the KDC. If a ticket theft were reported in the
phenomena. interim, the KDC would refuse to validate the ticket, and the thief
would be foiled.
Section 3.1.3. - 15 - Expires 28 February 1993 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.
Version 5 - Revision 5.1 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.
If the RENEWABLE option has been requested or if the 2.5. Proxiable and proxy tickets
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:
o+Its requested value. 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.
o+The start time of the ticket plus the minimum of the two The PROXIABLE flag in a ticket is normally only interpreted by the
maximum renewable lifetimes associated with the principals' ticket-granting service. It can be ignored by application servers.
database entries. 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 by
default.
o+The start time of the ticket plus the maximum renewable This flag allows a client to pass a proxy to a server to perform a
lifetime set by the policy of the local realm. 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.
The flags field of the new ticket will have the follow- In order to complicate the use of stolen credentials, Kerberos
ing options set if they have been requested and if the pol- tickets are usually valid from only those network addresses
icy of the local realm allows: FORWARDABLE, MAY-POSTDATE, specifically included in the ticket (It is permissible to request or
POSTDATED, PROXIABLE, RENEWABLE. If the new ticket is post- issue tickets with no network addresses specified, but we do not
dated (the start time is in the future), its INVALID flag recommend it). For this reason, a client wishing to grant a proxy
will also be set. must request a new ticket valid for the network address of the
service to be granted the proxy.
If all of the above succeed, the server formats a The PROXY flag is set in a ticket by the TGS when it issues a
KRB_AS_REP message (see section 5.4.2), copying the proxy ticket. Application servers may check this flag and require
addresses in the request into the caddr of the response, additional authentication from the agent presenting the proxy in
placing any required pre-authentication data into the padata order to provide an audit trail.
of the response, and encrypts the ciphertext part in the
client's key using the requested encryption method, and
sends it to the client. See section A.2 for pseudocode.
_3._1._4. _G_e_n_e_r_a_t_i_o_n _o_f _K_R_B__E_R_R_O_R _m_e_s_s_a_g_e 2.6. Forwardable tickets
Several errors can occur, and the Authentication Server Authentication forwarding is an instance of the proxy case where the
responds by returning an error message, KRB_ERROR, to the service is granted complete use of the client's identity. An example
client, with the error-code and e-text fields set to where it might be used is when a user logs in to a remote system and
appropriate values. The error message contents and details wants authentication to work from that system as if the login were
are described in Section 5.8.1. local.
_3._1._5. _R_e_c_e_i_p_t _o_f _K_R_B__A_S__R_E_P _m_e_s_s_a_g_e 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.
If the reply message type is KRB_AS_REP, then the This flag allows for authentication forwarding without requiring the
client verifies that the cname and crealm fields in the user to enter a password again. If the flag is not set, then
cleartext portion of the reply match what it requested. If authentication forwarding is not permitted, but the same end result
any padata fields are present, they may be used to derive can still be achieved if the user engages in the AS exchange with the
the proper secret key to decrypt the message. The client requested network addresses and supplies a password.
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, 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
Section 3.1.5. - 16 - Expires 28 February 1993 The FORWARDED flag is set by the TGS when a client presents a ticket
with the FORWARDABLE flag set and requests it be set 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 wish to process
FORWARDED tickets differently than non-FORWARDED tickets.
Version 5 - Revision 5.1 2.7. Other KDC options
remedial action, such as a password change). See section There are two additional options which may be set in a client's
A.3 for pseudocode. request of the KDC. 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.
Proper decryption of the KRB_AS_REP message is _n_o_t suf- The ENC-TKT-IN-SKEY option is honored only by the ticket-granting
ficient to verify the identity of the user; the user and an service. It indicates that the to-be-issued ticket for the end
attacker could cooperate to generate a KRB_AS_REP format server is to be encrypted in the session key from the additional
message which decrypts properly but is not from the proper ticket-granting ticket provided with the request. See section 3.3.3
KDC. If the host wishes to verify the identity of the user, for specific details.
it must require the user to present application credentials
which can be verified using a securely-stored secret key.
If those credentials can be verified, then the identity of
the user can be assured.
_3._1._6. _R_e_c_e_i_p_t _o_f _K_R_B__E_R_R_O_R _m_e_s_s_a_g_e 3. Message Exchanges
If the reply message type is KRB_ERROR, then the client The following sections describe the interactions between network
interprets it as an error and performs whatever clients and servers and the messages involved in those exchanges.
application-specific tasks are necessary to recover.
_3._2. _T_h_e _C_l_i_e_n_t/_S_e_r_v_e_r _A_u_t_h_e_n_t_i_c_a_t_i_o_n _E_x_c_h_a_n_g_e 3.1. The Authentication Service Exchange
Summary Summary
_M_e_s_s_a_g_e _d_i_r_e_c_t_i_o_n _M_e_s_s_a_g_e _t_y_p_e _S_e_c_t_i_o_n
Client to Application server KRB_AP_REQ 5.5.1
[optional] Application server to client KRB_AP_REP or 5.5.2
KRB_ERROR 5.8.1
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. _T_h_e _K_R_B__A_P__R_E_Q _m_e_s_s_a_g_e
The KRB_AP_REQ contains authentication information
which should be part of the first message in an authenti-
cated 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 insuf-
ficient to authenticate a client, since tickets are passed
across the network in cleartext[7], 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 it. The KRB_AP_REQ message is
referred to elsewhere as the "authentication header."
9__________________________
9[7] 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.
Section 3.2.1. - 17 - Expires 28 February 1993
Version 5 - Revision 5.1
_3._2._2. _G_e_n_e_r_a_t_i_o_n _o_f _a _K_R_B__A_P__R_E_Q _m_e_s_s_a_g_e
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. The client then 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. Authentica-
tors may not be re-used and will be rejected if replayed to
a server[8]. 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 authen-
tication or the use of a session-key based ticket by setting
the appropriate flag(s) in the ap-options field of the mes-
sage.
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 addi-
tional application-specific information. See section A.9
for pseudocode.
_3._2._3. _R_e_c_e_i_p_t _o_f _K_R_B__A_P__R_E_Q _m_e_s_s_a_g_e
Authentication is based on the server's current time of
day (clocks must be loosely synchronized), the authentica-
tor, 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 accept-
able to the protocol. The format of error messages is
described in section 5.8.1.
The algorithm for verifying authentication information Message direction Message type Section
is as follows. If the message type is not KRB_AP_REQ, the 1. Client to Kerberos KRB_AS_REQ 5.4.1
server returns the KRB_AP_ERR_MSG_TYPE error. If the key 2. Kerberos to client KRB_AS_REP or 5.4.2
version indicated by the Ticket in the KRB_AP_REQ is not one KRB_ERROR 5.9.1
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-
__________________________
9[8] Note that this can make applications based on un-
reliable transports difficult to code correctly, if the
transport might deliver duplicated messages. In such
cases, a new authenticator must be generated for each
retry.
9
Section 3.2.3. - 18 - Expires 28 February 1993
Version 5 - Revision 5.1
SESSION-KEY flag is set in the ap-options field, it indi-
cates to the server that the ticket is encrypted in the ses-
sion key from the server's ticket-granting ticket rather
than its secret key[9]. 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 The Authentication Service (AS) Exchange between the client and the
server's key specified by the ticket. If the decryption Kerberos Authentication Server is usually initiated by a client when
routines detect a modification of the ticket (each encryp- it wishes to obtain authentication credentials for a given server but
tion system must provide safeguards to detect modified currently holds no credentials. The client's secret key is used for
ciphertext; see section 6), the KRB_AP_ERR_BAD_INTEGRITY encryption and decryption. This exchange is typically used at the
error is returned (chances are good that different keys were initiation of a login session, to obtain credentials for a Ticket-
used to encrypt and decrypt). 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. (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.) This exchange does not by itself
provide any assurance of the the identity of the user. (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 the local server through successful completion of the
Client/Server exchange.)
The authenticator is decrypted using the session key The exchange consists of two messages: KRB_AS_REQ from the client to
extracted from the decrypted ticket. If decryption shows it Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these
to have been modified, the KRB_AP_ERR_BAD_INTEGRITY error is messages are described in sections 5.4.1, 5.4.2, and 5.9.1.
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 (they might not match, for example, if the wrong
session key was used to encrypt the authenticator). 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 In the request, the client sends (in cleartext) its own identity and
authenticator differ by more than the allowable clock skew the identity of the server for which it is requesting credentials.
(e.g., 5 minutes), the KRB_AP_ERR_SKEW error is returned. The response, KRB_AS_REP, contains a ticket for the client to present
If the server name, along with the client name, time and to the server, and a session key that will be shared by the client
microsecond fields from the Authenticator match any and the server. The session key and additional information are
recently-seen such tuples, the KRB_AP_ERR_REPEAT error is encrypted in the client's secret key. The KRB_AS_REP message
returned[10]. The server must remember any authenticator contains information which can be used to detect replays, and to
presented within the allowable clock skew, so that a replay associate it with the message to which it replies. Various errors
attempt is guaranteed to fail. If a server loses track of can occur; these are indicated by an error response (KRB_ERROR)
any authenticator presented within the allowable clock skew, instead of the KRB_AS_REP response. The error message is not
__________________________ encrypted. The KRB_ERROR message also contains information which can
9[9] This is used for user-to-user authentication as be used to associate it with the message to which it replies. The
described in [6]. lack of encryption in the KRB_ERROR message precludes the ability to
9[10] Note that the rejection here is restricted to au- detect replays or fabrications of such messages.
thenticators from the same principal to the same
server. Other client principals communicating with the
same server principal should not be have their authen-
ticators rejected if the time and microsecond fields
happen to match some other client's authenticator.
Section 3.2.3. - 19 - Expires 28 February 1993 In the normal case 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. 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 may be used for preauthentication if desired, but the
mechanism is not currently specified.
Version 5 - Revision 5.1 3.1.1. Generation of KRB_AS_REQ message
it must reject all requests until the clock skew interval The client may specify a number of options in the initial request.
has passed. This assures that any lost or re-played authen- Among these options are whether preauthentication is to be performed;
ticators will fall outside the allowable clock skew and can whether the requested ticket is to be renewable, proxiable, or
no longer be successfully replayed (If this is not done, an forwardable; whether it should be postdated or allow postdating of
attacker could conceivably record the ticket and authentica- derivative tickets; and whether a renewable ticket will be accepted
tor sent over the network to a server, then disable the in lieu of a non-renewable ticket if the requested ticket expiration
client's host, pose as the disabled host, and replay the date cannot be satisfied by a nonrenewable ticket (due to
ticket and authenticator to subvert the authentication.). configuration constraints; see section 4). See section A.1 for
If a sequence number is provided in the authenticator, the pseudocode.
server saves it for later use in processing KRB_SAFE and/or
KRB_PRIV messages. If a subkey is present, the server
either saves it for later use or uses it to help generate
its own choice for a subkey to be returned in a KRB_AP_REP
message.
The server computes the age of the ticket: local The client prepares the KRB_AS_REQ message and sends it to the KDC.
(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. Oth-
erwise, 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 3.1.2. Receipt of KRB_AS_REQ message
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. See section A.10 for
pseudocode.
_3._2._4. _G_e_n_e_r_a_t_i_o_n _o_f _a _K_R_B__A_P__R_E_P _m_e_s_s_a_g_e 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 5.3.1. The
contents of the ticket are determined as follows.
Typically, a client's request will include both the 3.1.3. Generation of KRB_AS_REP message
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)[11].
__________________________
9[11] 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 mes-
sages are formatted in such a way that it is not possi-
ble to create the reply by judicious message surgery
(even in encrypted form) without knowledge of the ap-
propriate encryption keys.
9
Section 3.2.4. - 20 - Expires 28 February 1993 The authentication server looks up the client and server principals
named in the KRB_AS_REQ in its database, extracting their respective
keys. 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 the server cannot accommodate
the requested encryption type, an error message with code
KDC_ERR_ETYPE_NOSUPP is returned. Otherwise it generates a "random"
session key ("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 would be
more desirable to use a truly random number generator, such as one
based on measurements of random physical phenomena.).
Version 5 - Revision 5.1 If the requested start time is absent or indicates a time in the
past, then the start time of the ticket is set to the authentication
server's current time. If it indicates a time in the future, 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.
If a sequence number is to be included, it should be ran- The expiration time of the ticket will be set to the minimum of the
domly chosen as described above for the authenticator. A following:
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. See section A.11
for pseudocode.
_3._2._5. _R_e_c_e_i_p_t _o_f _K_R_B__A_P__R_E_P _m_e_s_s_a_g_e +The expiration time (endtime) requested in the KRB_AS_REQ
message.
If a KRB_AP_REP message is returned, the client uses +The ticket's start time plus the maximum allowable lifetime
the session key from the credentials obtained for the associated with the client principal (the authentication
server[12] to decrypt the message, and verifies that the server's database includes a maximum ticket lifetime field
timestamp and microsecond fields match those in the Authen- in each principal's record; see section 4).
ticator 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.
See section A.12 for pseudocode.
_3._2._6. _U_s_i_n_g _t_h_e _e_n_c_r_y_p_t_i_o_n _k_e_y +The ticket's start time plus the maximum allowable lifetime
associated with the server principal.
After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, +The ticket's start time plus the maximum lifetime set by
the client and server share an encryption key which can be the policy of the local realm.
used by the application. The "true session key" to be used
for KRB_PRIV, KRB_SAFE, or other application-specific uses
may be chosen by the application based on the subkeys in the
KRB_AP_REP message and the authenticator[13]. 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 a
several alternatives. We leave the protocol negotiations of
how to use the key (e.g. selecting an encryption or check-
sum type) to the application programmer; the Kerberos proto-
col does not constrain the implementation options.
With both the one-way and mutual authentication If the requested expiration time minus the start time (as determined
exchanges, the peers should take care not to send sensitive above) is less than a site-determined minimum lifetime, an error
information to each other without proper assurances. In message with code KDC_ERR_NEVER_VALID is returned. If the requested
particular, applications that require privacy or integrity expiration time for the ticket exceeds what was determined as above,
should use the KRB_AP_REP or KRB_ERROR responses from the and if the "RENEWABLE-OK" option was requested, then the "RENEWABLE"
server to client to assure both client and server of their flag is set in the new ticket, and the renew-till value is set as if
peer's identity. If an application protocol requires the "RENEWABLE" option were requested (the field and option names are
privacy of its messages, it can use the KRB_PRIV message described fully in section 5.4.1). If the RENEWABLE option has been
(section 3.5). The KRB_SAFE message (section 3.4) can be 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
9[12] Note that for encrypting the KRB_AP_REP message, minimum of:
the sub-session key is not used, even if present in the
Authenticator.
9[13] Implementations of the protocol may wish to pro-
vide routines to choose subkeys based on session keys
and random numbers and to orchestrate a negotiated key
to be returned in the KRB_AP_REP message.
Section 3.2.6. - 21 - Expires 28 February 1993 +Its requested value.
Version 5 - Revision 5.1 +The start time of the ticket plus the minimum of the two
maximum renewable lifetimes associated with the principals'
database entries.
used to assure integrity. +The start time of the ticket plus the maximum renewable
lifetime set by the policy of the local realm.
_3._3. _T_h_e _T_i_c_k_e_t-_G_r_a_n_t_i_n_g _S_e_r_v_i_c_e (_T_G_S) _E_x_c_h_a_n_g_e The flags field of the new ticket will have the following options set
if they have been requested and if the policy of the local realm
allows: FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE.
If the new ticket is postdated (the start time is in the future), its
INVALID flag will also be set.
Summary If all of the above succeed, the server formats a KRB_AS_REP message
_M_e_s_s_a_g_e _d_i_r_e_c_t_i_o_n _M_e_s_s_a_g_e _t_y_p_e _S_e_c_t_i_o_n (see section 5.4.2), copying the addresses in the request into the
1. Client to Kerberos KRB_TGS_REQ 5.4.1 caddr of the response, placing any required pre-authentication data
2. Kerberos to client KRB_TGS_REP or 5.4.2 into the padata of the response, and encrypts the ciphertext part in
KRB_ERROR 5.8.1 the client's key using the requested encryption method, and sends it
to the client. See section A.2 for pseudocode.
The TGS exchange between a client and the Kerberos 3.1.4. Generation of KRB_ERROR message
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 authenti-
cates to the system, such as when a user logs in). The mes-
sage format for the TGS exchange is almost identical to that
for the AS exchange. The primary difference is that encryp-
tion 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 Several errors can occur, and the Authentication Server responds by
(KRB_TGS_REQ) from the client to the Kerberos Ticket- returning an error message, KRB_ERROR, to the client, with the
Granting Server, and a reply (KRB_TGS_REP or KRB_ERROR). error-code and e-text fields set to appropriate values. The error
The KRB_TGS_REQ message includes information authenticating message contents and details are described in Section 5.9.1.
the client plus a request for credentials. The authentica-
tion 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 col-
lection 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 creden-
tials, 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 authentica-
tion 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 informa-
tion which can be used to detect replays, and to associate
Section 3.3. - 22 - Expires 28 February 1993 3.1.5. Receipt of KRB_AS_REP message
Version 5 - Revision 5.1 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, 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). See section A.3 for
pseudocode.
it with the message to which it replies. The KRB_ERROR mes- Proper decryption of the KRB_AS_REP message is not sufficient to
sage also contains information which can be used to associ- verify the identity of the user; the user and an attacker could
ate it with the message to which it replies, but the lack of cooperate to generate a KRB_AS_REP format message which decrypts
encryption in the KRB_ERROR message precludes the ability to properly but is not from the proper KDC. If the host wishes to
detect replays or fabrications of such messages. 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. If those credentials can be verified, then the identity
of the user can be assured.
_3._3._1. _G_e_n_e_r_a_t_i_o_n _o_f _K_R_B__T_G_S__R_E_Q _m_e_s_s_a_g_e 3.1.6. Receipt of KRB_ERROR message
Before sending a request to the ticket-granting ser- If the reply message type is KRB_ERROR, then the client interprets it
vice, the client must determine in which realm the applica- as an error and performs whatever application-specific tasks are
tion server is registered[14]. If the client does not necessary to recover.
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 the local Kerberos server (using the KRB_TGS_REQ
message recursively). The Kerberos server may return a TGT
for the desired realm in which case one can proceed. Alter-
natively, the Kerberos server may return a TGT for a realm
which is "closer" to the desired realm (further along the
standard hierarchical path), in which case this step must be
repeated with a Kerberos server in the realm specified in
the returned TGT. If neither are returned, then the request
must be retried with a Kerberos server for a realm higher in
the hierarchy. This request will itself require a ticket-
granting ticket for the higher realm which must be obtained
by recursively applying these directions.
Once the client obtains a ticket-granting ticket for 3.2. The Client/Server Authentication Exchange
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; as
long as the secret keys exchanged by realms are kept secret,
only denial of service results from a false Kerberos server.
As in the AS exchange, the client may specify a number Summary
of options in the KRB_TGS_REQ message. 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-authorization-data field for
__________________________
9[14] This can be accomplished in several ways. It
might be known beforehand (since the realm is part of
the principal identifier), or it might be stored in a
nameserver. Presently, however, this information is
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 au-
thentication of the application server to the client.
9
Section 3.3.1. - 23 - Expires 28 February 1993 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
Version 5 - Revision 5.1 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.
application server use and additional tickets required by 3.2.1. The KRB_AP_REQ message
some options.
In preparing the authentication header, the client can The KRB_AP_REQ contains authentication information which should be
select a sub-session key under which the response from the part of the first message in an authenticated transaction. It
Kerberos server will be encrypted[15]. If the sub-session contains a ticket, an authenticator, and some additional bookkeeping
key is not specified, the session key from the ticket- information (see section 5.5.1 for the exact format). The ticket by
granting ticket will be used. If the enc-authorization-data itself is insufficient to authenticate a client, since tickets are
is present, it must be encrypted in the sub-session key, if passed across the network in cleartext(Tickets contain both an
present, from the authenticator portion of the authentica- encrypted and unencrypted portion, so cleartext here refers to the
tion header, or if not present in the session key from the entire unit, which can be copied from one message and replayed in
ticket-granting ticket. another without any cryptographic skill.), 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 it. The KRB_AP_REQ message is referred to elsewhere
as the "authentication header."
Once prepared, the message is sent to a Kerberos server 3.2.2. Generation of a KRB_AP_REQ message
for the destination realm. See section A.5 for pseudocode.
_3._3._2. _R_e_c_e_i_p_t _o_f _K_R_B__T_G_S__R_E_Q _m_e_s_s_a_g_e 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. The client
then 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 (Note that this can make
applications based on unreliable transports difficult to code
correctly, if the transport might deliver duplicated messages. In
such cases, a new authenticator must be generated for each retry.).
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 KRB_TGS_REQ message is processed in a manner simi- The client may indicate a requirement of mutual authentication or the
lar to the KRB_AS_REQ message, but there are many additional use of a session-key based ticket by setting the appropriate flag(s)
checks to be performed. First, the Kerberos server must in the ap-options field of the message.
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 ser-
vice, 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 grant-
ing 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 The Authenticator is encrypted in the session key and combined with
user-supplied checksum in the Authenticator must be verified the ticket to form the KRB_AP_REQ message which is then sent to the
against the contents of the request, and the message end server along with any additional application-specific
rejected if the checksums do not match (with an error code information. See section A.9 for pseudocode.
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 sup-
ported, 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.
__________________________
9[15] 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 ran-
dom number and XOR it with the session key from the
ticket-granting ticket.
9
Section 3.3.2. - 24 - Expires 28 February 1993 3.2.3. Receipt of KRB_AP_REQ message
Version 5 - Revision 5.1 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.
If any of the decryptions indicate failed integrity The algorithm for verifying authentication information is as follows.
checks, the KRB_AP_ERR_BAD_INTEGRITY error is returned. 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 the ticket is encrypted in the session key from the
server's ticket-granting ticket rather than its secret key (This is
used for user-to-user authentication as described in [6]). 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.
_3._3._3. _G_e_n_e_r_a_t_i_o_n _o_f _K_R_B__T_G_S__R_E_P _m_e_s_s_a_g_e 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).
The KRB_TGS_REP message shares its format with the The authenticator is decrypted using the session key extracted from
KRB_AS_REP (KRB_KDC_REP), but with its type field set to the decrypted ticket. If decryption shows it to have been modified,
KRB_TGS_REP. The detailed specification is in section the KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm
5.4.2. 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 (they might not match, for example, if the wrong
session key was used to encrypt the authenticator). 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.
The response will include a ticket for the requested If the local (server) time and the client time in the authenticator
server. The Kerberos database is queried to retrieve the differ by more than the allowable clock skew (e.g., 5 minutes), the
record for the requested server (including the key with KRB_AP_ERR_SKEW error is returned. If the server name, along with
which the ticket will be encrypted). If the request is for the client name, time and microsecond fields from the Authenticator
a ticket granting ticket for a remote realm, and if no key match any recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
is shared with the requested realm, then the Kerberos server returned (Note that the rejection here is restricted to
will select the realm "closest" to the requested realm with authenticators from the same principal to the same server. Other
which it does share a key, and use that realm instead. This client principals communicating with the same server principal should
is the only case where the response from the KDC will be for not be have their authenticators rejected if the time and microsecond
a different server than that requested by the client. fields happen to match some other client's authenticator.). The
server must remember any authenticator presented within the allowable
clock skew, so that a replay attempt is guaranteed to fail. If a
server loses track of any authenticator presented within the
allowable clock skew, it must reject all requests until the clock
skew interval has passed. This assures that any lost or re-played
authenticators will fall outside the allowable clock skew and can no
longer be successfully replayed (If this is not done, an attacker
could conceivably record the ticket and authenticator sent over the
network to a server, then disable the client's host, pose as the
disabled host, and replay the ticket and authenticator to subvert the
authentication.). 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.
By default, the address field, the client's name and The server computes the age of the ticket: local (server) time minus
realm, the list of transited realms, the time of initial the start time inside the Ticket. If the start time is later than
authentication, the expiration time, and the authorization the current time by more than the allowable clock skew or if the
data of the newly-issued ticket will be copied from the INVALID flag is set in the ticket, the KRB_AP_ERR_TKT_NYV error is
ticket-granting ticket (TGT) or renewable ticket. If the returned. Otherwise, if the current time is later than end time by
transited field needs to be updated, but the transited type more than the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED error
is not supported, the KDC_ERR_TRTYPE_NOSUPP error is is returned.
returned.
If the request specifies an endtime, then the endtime If all these checks succeed without an error, the server is assured
of the new ticket is set to the minimum of (a) that request, that the client possesses the credentials of the principal named in
(b) the endtime from the TGT, and (c) the starttime of the the ticket and thus, the client has been authenticated to the server.
TGT plus the minimum of the maximum life for the application See section A.10 for pseudocode.
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 3.2.4. Generation of a KRB_AP_REP message
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 or indicates a Typically, a client's request will include both the authentication
time in the past, then the start time of the ticket is set information and its initial request in the same message, and the
to the authentication server's current time. If it 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). [Note: 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.] 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. See section A.11 for
pseudocode.
Section 3.3.3. - 25 - Expires 28 February 1993 3.2.5. Receipt of KRB_AP_REP message
Version 5 - Revision 5.1 If a KRB_AP_REP message is returned, the client uses the session key
from the credentials obtained for the server (Note that for
encrypting the KRB_AP_REP message, the sub-session key is not used,
even if present in the Authenticator.) 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. See section A.12 for
pseudocode.
indicates a time in the future, but the POSTDATED option has 3.2.6. Using the encryption key
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 post-
dated 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. How-
ever, in no case may the starttime, endtime, or renew-till
time of a newly-issued postdated ticket extend beyond the
renew-till time of the ticket-granting ticket.
If the ENC-TKT-IN-SKEY option has been specified, and After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and
if an additional ticket has been included in the request, server share an encryption key which can be used by the application.
then the KDC will verify that the principal identifier of The "true session key" to be used for KRB_PRIV, KRB_SAFE, or other
the server in the ticket matches the requested server in the application-specific uses may be chosen by the application based on
KDC request (to make sure someone doesn't insert a different the subkeys in the KRB_AP_REP message and the authenticator
ticket in the request), decrypt the additional ticket using (Implementations of the protocol may wish to provide routines to
the key for the server to which it was issued, verify that choose subkeys based on session keys and random numbers and to
it is a ticket-granting ticket, and use the session key from orchestrate a negotiated key to be returned in the KRB_AP_REP
the additional ticket to encrypt the new ticket it will message.). In some cases, the use of this session key will be
issue instead of encrypting the new ticket in the key of the implicit in the protocol; in others the method of use must be chosen
server for which it is to be issued[16]. from a several alternatives. 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.
If the name of the server in the ticket that is With both the one-way and mutual authentication exchanges, the peers
presented to the KDC as part of the authentication header is should take care not to send sensitive information to each other
not that of the ticket-granting server itself, and the without proper assurances. In particular, applications that require
server is registered in the realm of the KDC, If the RENEW privacy or integrity should use the KRB_AP_REP or KRB_ERROR responses
option is requested, then the KDC will verify that the from the server to client to assure both client and server of their
RENEWABLE flag is set in the ticket and that the renew_till peer's identity. If an application protocol requires privacy of its
time is still in the future. If the VALIDATE option is messages, it can use the KRB_PRIV message (section 3.5). The KRB_SAFE
rqeuested, the KDC will check that the starttime has passed message (section 3.4) can be used to assure integrity.
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, the KDC will
issue the appropriate new ticket.
Whenever a request is made to the ticket-granting 3.3. The Ticket-Granting Service (TGS) Exchange
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 dates 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
__________________________
9[16] This allows easy implementation of user-to-user
authentication [6], which uses ticket-granting ticket
session keys in lieu of secret server keys in situa-
tions where such secret keys could be easily comprom-
ised.
9
Section 3.3.3. - 26 - Expires 28 February 1993 Summary
Version 5 - Revision 5.1 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
theft has been reported. Any normal ticket obtained before The TGS exchange between a client and the Kerberos Ticket-Granting
it was reported stolen will still be valid (because they Server is initiated by a client when it wishes to obtain
require no interaction with the KDC), but only until their authentication credentials for a given server (which might be
normal expiration time. 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 ciphertext part of the response in the KRB_TGS_REP The TGS exchange consists of two messages: A request (KRB_TGS_REQ)
message is encrypted in the sub-session key from the Authen- from the client to the Kerberos Ticket-Granting Server, and a reply
ticator, if present, or the session key key from the (KRB_TGS_REP or KRB_ERROR). The KRB_TGS_REQ message includes
ticket-granting ticket. It is not encrypted using the information authenticating the client plus a request for credentials.
client's secret key. Furthermore, the client's key's The authentication information consists of the authentication header
expiration date and the key version number fields are left (KRB_AP_REQ) which includes the client's previously obtained ticket-
out since these values are stored along with the client's granting, renewable, or invalid ticket. In the ticket-granting
database record, and that record is not needed to satisfy a ticket and proxy cases, the request may include one or more of: a
request based on a ticket-granting ticket. See section A.6 list of network addresses, a collection of typed authorization data
for pseudocode. 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 subsession 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, but the lack of encryption in
the KRB_ERROR message precludes the ability to detect replays or
fabrications of such messages.
_3._3._3._1. _E_n_c_o_d_i_n_g _t_h_e _t_r_a_n_s_i_t_e_d _f_i_e_l_d 3.3.1. Generation of KRB_TGS_REQ message
If the identity of the server in the TGT that is Before sending a request to the ticket-granting service, the client
presented to the KDC as part of the authentication header is must determine in which realm the application server is registered
that of the ticket-granting service, but the TGT was issued [Note: This can be accomplished in several ways. It might be known
from another realm, the KDC will look up the inter-realm key beforehand (since the realm is part of the principal identifier), or
shared with that realm and use that key to decrypt the it might be stored in a nameserver. Presently, however, this
ticket. If the ticket is valid, then the KDC will honor the information is obtained from a configuration file. If the realm to
request, subject to the constraints outlined above in the be used is obtained from a nameserver, there is a danger of being
section describing the AS exchange. The realm part of the spoofed if the nameservice providing the realm name is not
client's identity will be taken from the ticket-granting authenticated. This might result in the use of a realm which has
ticket. The name of the realm that issued the ticket- been compromised, and would result in an attacker's ability to
granting ticket will be added to the transited field of the compromise the authentication of the application server to the
ticket to be issued. This is accomplished by reading the client.]. If the client does not already possess a ticket-granting
transited field from the ticket-granting ticket, adding the ticket for the appropriate realm, then one must be obtained. This is
new realm, then constructing and writing out its encoded first attempted by requesting a ticket-granting ticket for the
(shorthand) form (this may involve a rearrangement of the destination realm from the local Kerberos server (using the
existing encoding). 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), in which case this step must be repeated with a
Kerberos server in the realm specified in the returned TGT. If
neither are returned, then the request must be retried with a
Kerberos server for a realm higher in the hierarchy. This request
will itself require a ticket-granting ticket for the higher realm
which must be obtained by recursively applying these directions.
Note that the ticket-granting service does not add the Once the client obtains a ticket-granting ticket for the appropriate
name of its own realm. Instead, its responsibility is to realm, it determines which Kerberos servers serve that realm, and
add the name of the previous realm. This prevents a mali- contacts one. The list might be obtained through a configuration file
cious Kerberos server from intentionally leaving out its own or network service; as long as the secret keys exchanged by realms
name (it could, however, omit other realms' names). are kept secret, only denial of service results from a false Kerberos
server.
The names of neither the local realm nor the As in the AS exchange, the client may specify a number of options in
principal's realm are to be included in the transited field. the KRB_TGS_REQ message. The client prepares the KRB_TGS_REQ
They appear elsewhere in the ticket and both are known to message, providing an authentication header as an element of the
have taken part in authenticating the principal. Since the padata field, and including the same fields as used in the KRB_AS_REQ
endpoints are not included, both local and single-hop message along with several optional fields: the enc-authorization-
inter-realm authentication result in a transited field that data field for application server use and additional tickets required
is empty. by some options.
Because the name of each realm transited is added to In preparing the authentication header, the client can select a sub-
this field, it might potentially be very long. To decrease session key under which the response from the Kerberos server will be
the length of this field, its contents are encoded. The encrypted (If the client selects a sub-session key, care must be
initially supported encoding is optimized for the normal taken to ensure the randomness of the selected subsession key. One
case of inter-realm communication: a hierarchical approach would be to generate a random number and XOR it with the
session key from the ticket-granting ticket.). 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 in the
session key from the ticket-granting ticket.
Section 3.3.3.1. - 27 - Expires 28 February 1993 Once prepared, the message is sent to a Kerberos server for the
destination realm. See section A.5 for pseudocode.
Version 5 - Revision 5.1 3.3.2. Receipt of KRB_TGS_REQ message
arrangement of realms using either domain or X.500 style The KRB_TGS_REQ message is processed in a manner similar to the
realm names. This encoding (called DOMAIN-X500-COMPRESS) is KRB_AS_REQ message, but there are many additional checks to be
now described. 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.
Realm names in the transited field are separated by a Once the accompanying ticket has been decrypted, the user-supplied
",". The ",", "\", trailing "."s, and leading spaces (" ") checksum in the Authenticator must be verified against the contents
are special characters, and if they are part of a realm of the request, and the message rejected if the checksums do not
name, they must be quoted in the transited field by preced- match (with an error code of KRB_AP_ERR_MODIFIED) or if the checksum
ing them with a "\". 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.
A realm name ending with a "." is interpreted as being If any of the decryptions indicate failed integrity checks, the
prepended to the previous realm. For example, we can encode KRB_AP_ERR_BAD_INTEGRITY error is returned.
traversal of EDU, MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU,
and CS.WASHINGTON.EDU as:
"EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.". 3.3.3. Generation of KRB_TGS_REP message
Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end- The KRB_TGS_REP message shares its format with the KRB_AS_REP
points, that they would not be included in this field, and (KRB_KDC_REP), but with its type field set to KRB_TGS_REP. The
we would have: detailed specification is in section 5.4.2.
"EDU,MIT.,WASHINGTON.EDU" The response will include a ticket for the requested server. The
Kerberos database is queried to retrieve the record for the requested
server (including the key with which the ticket will be encrypted).
If the request is for a ticket granting ticket for a remote realm,
and if no key is shared with the requested realm, then the Kerberos
server will select the realm "closest" to the requested realm with
which it does share a key, and use that realm instead. This is the
only case where the response from the KDC will be for a different
server than that requested by the client.
A realm name beginning with a "/" is interpreted as being By default, the address field, the client's name and realm, the list
appended to the previous realm[17]. If it is to stand by of transited realms, the time of initial authentication, the
itself, then it should be preceded by a space (" "). For expiration time, and the authorization data of the newly-issued
example, we can encode traversal of /COM/HP/APOLLO, /COM/HP, ticket will be copied from the ticket-granting ticket (TGT) or
/COM, and /COM/DEC as: 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.
"/COM,/HP,/APOLLO, /COM/DEC". 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 (endtimestarttime) of the old
ticket.
Like the example above, if /COM/HP/APOLLO and /COM/DEC are If the FORWARDED option has been requested, then the resulting ticket
endpoints, they they would not be included in this field, will contain the addresses specified by the client. This option will
and we would have: 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.
"/COM,/HP" If the requested start time is absent or indicates a time in the
past, then the start time of the ticket is set to the authentication
server's current time. If it indicates a time in the future, 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
MAYPOSTDATE 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.
A null subfield preceding or following a "," indicates If the ENC-TKT-IN-SKEY option has been specified and an additional
that all realms between the previous realm and the next ticket has been included in the request, the KDC will decrypt the
realm have been traversed[18]. Thus, "," means that all additional ticket using the key for the server to which the
realms along the path between the client and the server have additional ticket was issued and verify that it is a ticket-granting
been traversed. ",EDU, /COM," means that that all realms ticket. If the name of the requested server is missing from the
from the client's realm up to EDU (in a domain style request, the name of the client in the additional ticket will be
__________________________ used. Otherwise the name of the requested server will be compared to
9[17] For the purpose of appending, the realm preceding the name of the client in the additional ticket and if different, the
the first listed realm is considered to be the null request will be rejected. If the request succeeds, the session key
realm (""). from the additional ticket will be used to encrypt the new ticket
9[18] For the purpose of interpreting null subfields, that is issued instead of using the key of the server for which the
the client's realm is considered to precede those in new ticket will be used (This allows easy implementation of user-to-
the transited field, and the server's realm is con- user authentication [6], which uses ticket-granting ticket session
sidered to follow them. keys in lieu of secret server keys in situations where such secret
keys could be easily compromised.).
Section 3.3.3.1. - 28 - Expires 28 February 1993 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, and the server is registered in the realm of
the KDC, If the RENEW option is requested, then the KDC will verify
that the RENEWABLE flag is set in the ticket and that the renew_till
time is still in the future. If the VALIDATE option is rqeuested,
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,
the KDC will issue the appropriate new ticket.
Version 5 - Revision 5.1 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 dates 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. 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.
hierarchy) have been traversed, and that everything from The ciphertext part of the response in the KRB_TGS_REP message is
/COM down to the server's realm in an X.500 style has also encrypted in the sub-session key from the Authenticator, if present,
been traversed. This could occur if the EDU realm in one or the session key key from the ticket-granting ticket. It is not
hierarchy shares an inter-realm key directly with the /COM encrypted using the client's secret key. Furthermore, the client's
realm in another hierarchy. 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. See section A.6 for pseudocode.
_3._3._4. _R_e_c_e_i_p_t _o_f _K_R_B__T_G_S__R_E_P _m_e_s_s_a_g_e 3.3.3.1. Encoding the transited field
When the KRB_TGS_REP is received by the client, it is pro- If the identity of the server in the TGT that is presented to the KDC
cessed in the same manner as the KRB_AS_REP processing as part of the authentication header is that of the ticket-granting
described above. The primary difference is that the cipher- service, but the TGT was issued from another realm, the KDC will look
text part of the response must be decrypted using the ses- up the inter-realm key shared with that realm and use that key to
sion key from the ticket-granting ticket rather than the decrypt the ticket. If the ticket is valid, then the KDC will honor
client's secret key. See section A.7 for pseudocode. 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 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).
_3._4. _T_h_e _K_R_B__S_A_F_E _E_x_c_h_a_n_g_e 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 KRB_SAFE message may be used by clients requiring The names of neither the local realm nor the principal's realm are to
the ability to detect modifications of messages they be included in the transited field. They appear elsewhere in the
exchange. It achieves this by including a keyed collision- ticket and both are known to have taken part in authenticating the
proof checksum of the user data and some control informa- principal. Since the endpoints are not included, both local and
tion. The checksum is keyed with an encryption key (usually single-hop inter-realm authentication result in a transited field
the last key negotiated via subkeys, or the session key if that is empty.
no negotiation has occured).
_3._4._1. _G_e_n_e_r_a_t_i_o_n _o_f _a _K_R_B__S_A_F_E _m_e_s_s_a_g_e 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.
When an application wishes to send a KRB_SAFE message, it Realm names in the transited field are separated by a ",". The ",",
collects its data and the appropriate control information "\", trailing "."s, and leading spaces (" ") are special characters,
and computes a checksum over them. The checksum algorithm and if they are part of a realm name, they must be quoted in the
should be some sort of keyed one-way hash function (such as transited field by preceding them with a "\".
the RSA-MD5-DES checksum algorithm specified in section
6.4.5, or the DES MAC), generated using the sub-session key
if present, or the session key. Different algorithms may be
selected by changing the checksum type in the message.
Unkeyed or non-collision-proof checksums are not suitable
for this use.
The control information for the KRB_SAFE message A realm name ending with a "." is interpreted as being prepended to
includes both a timestamp and a sequence number. The the previous realm. For example, we can encode traversal of EDU,
designer of an application using the KRB_SAFE message must MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:
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 "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".
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 prob-
lem.
If the application protocol is expected to tolerate Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were endpoints,
lost messages without them being resent, the use of the that they would not be included in this field, and we would have:
timestamp is the appropriate replay detection mechanism.
Using timestamps is also the appropriate mechanism for
Section 3.4.1. - 29 - Expires 28 February 1993 "EDU,MIT.,WASHINGTON.EDU"
Version 5 - Revision 5.1 A realm name beginning with a "/" is interpreted as being appended to
the previous realm (For the purpose of appending, the realm preceding
the first listed realm is considered to be the null realm ("")). If
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:
multi-cast protocols where all of one's peers share a common "/COM,/HP,/APOLLO, /COM/DEC".
sub-session key, but some messages will be sent to a subset
of one's peers.
After computing the checksum, the client then transmits Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints,
the information and checksum to the recipient in the message they they would not be included in this field, and we would have:
format specified in section 5.6.1.
_3._4._2. _R_e_c_e_i_p_t _o_f _K_R_B__S_A_F_E _m_e_s_s_a_g_e "/COM,/HP"
When an application receives a KRB_SAFE message, it verifies A null subfield preceding or following a "," indicates that all
it as follows. If any error occurs, an error code is realms between the previous realm and the next realm have been
reported for use by the application. traversed (For the purpose of interpreting null subfields, the
client's realm is considered to precede those in the transited field,
and the server's realm is considered to follow them.). Thus, ","
means that all realms along the path between the client and the
server have been traversed. ",EDU, /COM," means that 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.
The message is first checked by verifying that the pro- 3.3.4. Receipt of KRB_TGS_REP message
tocol 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, and if it is not, a
KRB_AP_ERR_INAPP_CKSUM error is generated. 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. 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 timestamp 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 When the KRB_TGS_REP is received by the client, it is processed in
that the message was generated by its peer and was not modi- the same manner as the KRB_AS_REP processing described above. The
fied in transit. primary difference is that the ciphertext part of the response must
be decrypted using the session key from the ticket-granting ticket
rather than the client's secret key. See section A.7 for pseudocode.
_3._5. _T_h_e _K_R_B__P_R_I_V _E_x_c_h_a_n_g_e 3.4. The KRB_SAFE Exchange
The KRB_PRIV message may be used by clients requiring The KRB_SAFE message may be used by clients requiring the ability to
confidentiality and the ability to detect modifications of detect modifications of messages they exchange. It achieves this by
exchanged messages. It achieves this by encrypting the mes- including a keyed collisionproof checksum of the user data and some
sages and adding control information. 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 occured).
Section 3.5. - 30 - Expires 28 February 1993 3.4.1. Generation of a KRB_SAFE message
Version 5 - Revision 5.1 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 some sort of
keyed one-way hash function (such as the RSA-MD5-DES checksum
algorithm specified in section 6.4.5, or the DES MAC), generated
using the sub-session key if present, or the session key. Different
algorithms may be selected by changing the checksum type in the
message. Unkeyed or non-collision-proof checksums are not suitable
for this use.
_3._5._1. _G_e_n_e_r_a_t_i_o_n _o_f _a _K_R_B__P_R_I_V _m_e_s_s_a_g_e 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.
When an application wishes to send a KRB_PRIV message, it Sequence numbers are useful when all messages sent will be received
collects its data and the appropriate control information by one's peer. Connection state is presently required to maintain
(specified in section 5.7.1) and encrypts them under an the session key, so maintaining the next sequence number should not
encryption key (usually the last key negotiated via subkeys, present an additional problem.
or the session key if no negotiation has occured). 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. _R_e_c_e_i_p_t _o_f _K_R_B__P_R_I_V _m_e_s_s_a_g_e 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.
When an application receives a KRB_PRIV message, it verifies After computing the checksum, the client then transmits the
it as follows. If any error occurs, an error code is information and checksum to the recipient in the message format
reported for use by the application. specified in section 5.6.1.
The message is first checked by verifying that the pro- 3.4.2. Receipt of KRB_SAFE message
tocol 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 gen-
erated. 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. 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. Finally, the check-
sum 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 can assume When an application receives a KRB_SAFE message, it verifies it as
the message was generated by its peer, and was securely follows. If any error occurs, an error code is reported for use by
transmitted (without intruders able to see the unencrypted the application.
contents).
Section 3.5.2. - 31 - Expires 28 February 1993 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
collisionproof keyed checksum, and if it is not, a
KRB_AP_ERR_INAPP_CKSUM error is generated. 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. 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 timestamp and usec or a sequence
number is present, a KRB_AP_ERR_MODIFIED error is generated.
Version 5 - Revision 5.1 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.
_4. _T_h_e _K_e_r_b_e_r_o_s _D_a_t_a_b_a_s_e If all the checks succeed, the application is assured that the
message was generated by its peer and was not modified in transit.
The Kerberos server must have access to a database contain- 3.5. The KRB_PRIV Exchange
ing the principal identifiers and secret keys of principals
to be authenticated[19].
_4._1. _D_a_t_a_b_a_s_e _c_o_n_t_e_n_t_s 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.
A database entry should contain at least the following 3.5.1. Generation of a KRB_PRIV message
fields:
_F_i_e_l_d _V_a_l_u_e 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 occured). 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.
name Principal's identif- 3.5.2. Receipt of KRB_PRIV message
ier
key Principal's secret key
p_kvno Principal's key version
max_life Maximum lifetime for Tickets
max_renewable_life Maximum total lifetime for renewable Tickets
The name field is an encoding of the principal's identifier. When an application receives a KRB_PRIV message, it verifies it as
The key field contains an encryption key. This key is the follows. If any error occurs, an error code is reported for use by
principal's secret key. (The key can be encrypted before the application.
storage under a Kerberos "master key" to protect it in case
the database is compromised but the master key is not. In
that case, an extra field must be added to indicate the mas-
ter key version used, see below.) The p_kvno field is the
key version number of the principal's secret key. The
max_life field contains the maximum allowable lifetime (end-
time - starttime) for any Ticket issued for this principal.
The max_renewable_life field contains the maximum allowable
total lifetime for any renewable Ticket issued for this
principal. (See section 3.1 for a description of how these
lifetimes are used in determining the lifetime of a given
Ticket.)
A server may provide KDC service to several realms, as The message is first checked by verifying that the protocol version
long as the database representation provides a mechanism to and type fields match the current version and KRB_PRIV, respectively.
distinguish between principal records with identifiers which A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
differ only in the realm name. 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
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. 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 timestamp and usec or a sequence
number is present, a KRB_AP_ERR_MODIFIED error is generated.
When an application server's key changes, if the change If all the checks succeed, the application can assume the message was
is routine (i.e. not the result of disclosure of the old generated by its peer, and was securely transmitted (without
key), the old key should be retained by the server until all intruders able to see the unencrypted contents).
__________________________
9[19] The implementation of the Kerberos server need not
combine the database and the server on the same
machine; it is feasible to store the principal database
in, say, a network name service, as long as the entries
stored therein are protected from disclosure to and
modification by unauthorized parties. However, we
recommend against such strategies, as they can make
system management and threat analysis quite complex.
9
Section 4.1. - 32 - Expires 28 February 1993 3.6. The KRB_CRED Exchange
Version 5 - Revision 5.1 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.
tickets that had been issued using that key have expired. 3.6.1. Generation of a KRB_CRED message
Because of this, it is possible for several keys to be
active for a single principal. Ciphertext encrypted in a
principal's key is always tagged with the version of the key
that was used for encryption, to help the recipient find the
proper key for decryption.
When more than one key is active for a particular prin- When an application wishes to send a KRB_CRED message it first (using
cipal, the principal will have more than one record in the the KRB_TGS exchange) obtains credentials to be sent to the remote
Kerberos database. The keys and key version numbers will host. It then constructs a KRB_CRED message using the ticket or
differ between the records (the rest of the fields may or tickets so obtained, placing the session key needed to use each
may not be the same). Whenever Kerberos issues a ticket, or ticket in the key field of the corresponding KrbCredInfo sequence of
responds to a request for initial authentication, the most the encrypted part of the the KRB_CRED message.
recent key (known by the Kerberos server) will be used for
encryption. This is the key with the highest key version
number.
_4._2. _A_d_d_i_t_i_o_n_a_l _f_i_e_l_d_s 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 raddress fields, are placed in the encrypted part of the
KRB_CRED message which is then encrypted under an encryption key
previosuly exchanged in the KRB_AP exchange (usually the last key
negotiated via subkeys, or the session key if no negotiation has
occured).
Project Athena's KDC implementation uses additional fields 3.6.2. Receipt of KRB_CRED message
in its database:
_F_i_e_l_d _V_a_l_u_e 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.
K_kvno Kerberos' key version If present or required, the recipient verifies that the operating
expiration Expiration date for entry system's report of the sender's address matches the sender's address
attributes Bit field of attributes in the message, and that one of the recipient's addresses appears as
mod_date Timestamp of last modification the recipient's address in the message. A failed match for either
mod_name Modifying principal's identifier case generates a KRB_AP_ERR_BADADDR error. The timestamp and usec
fields (and the nonce field if required) are checked next. If the
timestamp and usec are not present, or they are present but not
current, the KRB_AP_ERR_SKEW error is generated.
The K_kvno field indicates the key version of the Kerberos If all the checks succeed, the application stores each of the new
master key under which the principal's secret key is tickets in its ticket cache together with the session key and other
encrypted. information in the corresponding KrbCredInfo sequence from the
encrypted part of the KRB_CRED message.
After an entry's expiration date has passed, the KDC 4. The Kerberos Database
will return an error to any client attempting to gain tick-
ets as or for the principal. (A database may want to main-
tain two expiration dates: one for the principal, and one
for the principal's current key. This allows password aging
to work independently of the principal's expiration date.
However, due to the limited space in the responses, the KDC
must combine the key expiration and principal expiration
date into a single value called "key_exp", which is used as
a hint to the user to take administrative action.)
The attributes field is a bitfield used to govern the The Kerberos server must have access to a database containing the
operations involving the principal. This field might be principal identifiers and secret keys of principals to be
useful in conjunction with user registration procedures, for authenticated (The implementation of the Kerberos server need not
site-specific policy implementations (Project Athena combine the database and the server on the same machine; it is
currently uses it for their user registration process con- feasible to store the principal database in, say, a network name
trolled by the system-wide database service, Moira [7]),. service, as long as the entries stored therein are protected from
or to identify the "string to key" conversion algorithm used disclosure to and modification by unauthorized parties. However, we
for a principal's key[20]. Other bits are used to indicate recommend against such strategies, as they can make system management
__________________________ and threat analysis quite complex.).
9[20] See the discussion of the padata field in section
9 4.1. Database contents
Version 5 - Revision 5.1 A database entry should contain at least the following fields:
that certain ticket options should not be allowed in tickets Field Value
encrypted under a principal's key (one bit each): Disallow
issuing postdated tickets, disallow issuing forwardable
tickets, disallow issuing tickets based on TGT authentica-
tion, disallow issuing renewable tickets, disallow issuing
proxiable tickets, and disallow issuing tickets for which
the principal is the server.
The mod_date field contains the time of last modifica- name Principal's identifier
tion of the entry, and the mod_name field contains the name key Principal's secret key
of the principal which last modified the entry. p_kvno Principal's key version
max_life Maximum lifetime for Tickets
max_renewable_life Maximum total lifetime for renewable
Tickets
_4._3. _F_r_e_q_u_e_n_t_l_y _C_h_a_n_g_i_n_g _F_i_e_l_d_s The name field is an encoding of the principal's identifier. The key
field contains an encryption key. This key is the principal's secret
key. (The key can be encrypted before storage under a Kerberos
"master key" to protect it in case the database is compromised but
the master key is not. In that case, an extra field must be added to
indicate the master key version used, see below.) The p_kvno field is
the key version number of the principal's secret key. The max_life
field contains the maximum allowable lifetime (endtime - starttime)
for any Ticket issued for this principal. The max_renewable_life
field contains the maximum allowable total lifetime for any renewable
Ticket issued for this principal. (See section 3.1 for a description
of how these lifetimes are used in determining the lifetime of a
given Ticket.)
Some KDC implementations may wish to maintain the last A server may provide KDC service to several realms, as long as the
time that a request was made by a particular principal. database representation provides a mechanism to distinguish between
Information that might be maintained includes the time of principal records with identifiers which differ only in the realm
the last request, the time of the last request for a name.
ticket-granting ticket, the time of the last use of a
ticket-granting ticket, or other times. This information
can then be returned to the user in the last-req field (see
section 5.2).
Other frequently changing information that can be main- When an application server's key changes, if the change is routine
tained is the latest expiration time for any tickets that (i.e., not the result of disclosure of the old key), the old key
have been issued using each key. This field would be used should be retained by the server until all tickets that had been
to indicate how long old keys must remain valid to allow the issued using that key have expired. Because of this, it is possible
continued use of outstanding tickets. for several keys to be active for a single principal. Ciphertext
encrypted in a principal's key is always tagged with the version of
the key that was used for encryption, to help the recipient find the
proper key for decryption.
_4._4. _S_i_t_e _C_o_n_s_t_a_n_t_s When more than one key is active for a particular principal, the
principal will have more than one record in the Kerberos database.
The keys and key version numbers will differ between the records (the
rest of the fields may or may not be the same). Whenever Kerberos
issues a ticket, or responds to a request for initial authentication,
the most recent key (known by the Kerberos server) will be used for
encryption. This is the key with the highest key version number.
The KDC implementation should have the following confi- 4.2. Additional fields
gurable constants or options, to allow an administrator to
make and enforce policy decisions:
o+ The minimum supported lifetime (used to determine whether Project Athena's KDC implementation uses additional fields in its
the KDC_ERR_NEVER_VALID error should be returned). This database:
constant should reflect reasonable expectations of
round-trip time to the KDC, encryption/decryption time,
and processing time by the client and target server, and
it should allow for a minimum "useful" lifetime.
o+ The maximum allowable total (renewable) lifetime of a Field Value
ticket (renew_till - starttime).
o+ The maximum allowable lifetime of a ticket (endtime - K_kvno Kerberos' key version
starttime). expiration Expiration date for entry
attributes Bit field of attributes
mod_date Timestamp of last modification
mod_name Modifying principal's identifier
o+ Whether to allow the issue of tickets with empty address The K_kvno field indicates the key version of the Kerberos master key
fields (including the ability to specify that such tick- under which the principal's secret key is encrypted.
ets may only be issued if the request specifies some
__________________________
5.4.2 for details on why this can be useful.
Section 4.4. - 34 - Expires 28 February 1993 After an entry's expiration date has passed, the KDC will return an
error to any client attempting to gain tickets as or for the
principal. (A database may want to maintain two expiration dates:
one for the principal, and one for the principal's current key. This
allows password aging to work independently of the principal's
expiration date. However, due to the limited space in the responses,
the KDC must combine the key expiration and principal expiration date
into a single value called "key_exp", which is used as a hint to the
user to take administrative action.)
Version 5 - Revision 5.1 The attributes field is a bitfield used to govern the operations
involving the principal. This field might be useful in conjunction
with user registration procedures, for site-specific policy
implementations (Project Athena currently uses it for their user
registration process controlled by the system-wide database service,
Moira [7]), or to identify the "string to key" conversion algorithm
used for a principal's key. (See the discussion of the padata field
in section 5.4.2 for details on why this can be useful.) Other bits
are used to indicate that certain ticket options should not be
allowed in tickets encrypted under a principal's key (one bit each):
Disallow issuing postdated tickets, disallow issuing forwardable
tickets, disallow issuing tickets based on TGT authentication,
disallow issuing renewable tickets, disallow issuing proxiable
tickets, and disallow issuing tickets for which the principal is the
server.
authorization_data). The mod_date field contains the time of last modification of the
entry, and the mod_name field contains the name of the principal
which last modified the entry.
o+ Whether proxiable, forwardable, renewable or post-datable 4.3. Frequently Changing Fields
tickets are to be issued.
_5. _M_e_s_s_a_g_e _S_p_e_c_i_f_i_c_a_t_i_o_n_s Some KDC implementations may wish to maintain the last time that a
request was made by a particular principal. Information that might
be maintained includes the time of the last request, the time of the
last request for a ticket-granting ticket, the time of the last use
of a ticket-granting ticket, or other times. This information can
then be returned to the user in the last-req field (see section 5.2).
The following sections describe the exact contents and Other frequently changing information that can be maintained is the
encoding of protocol messages and objects. The ASN.1 base latest expiration time for any tickets that have been issued using
definitions are presented in the first subsection. The each key. This field would be used to indicate how long old keys
remaining subsections specify the protocol objects (tickets must remain valid to allow the continued use of outstanding tickets.
and authenticators) and messages. Specification of encryp-
tion and checksum techniques, and the fields related to
them, appear in section 6.
_5._1. _A_S_N._1 _D_i_s_t_i_n_g_u_i_s_h_e_d _E_n_c_o_d_i_n_g _R_e_p_r_e_s_e_n_t_a_t_i_o_n 4.4. Site Constants
All uses of ASN.1 in Kerberos shall use the Dis- The KDC implementation should have the following configurable
tinguished Encoding Representation of the data elements as constants or options, to allow an administrator to make and enforce
described in the X.509 specification, section 8.7 [8]. policy decisions:
_5._2. _A_S_N._1 _B_a_s_e _D_e_f_i_n_i_t_i_o_n_s + The minimum supported lifetime (used to determine whether the
KDC_ERR_NEVER_VALID error should be returned). This constant
should reflect reasonable expectations of round-trip time to the
KDC, encryption/decryption time, and processing time by the client
and target server, and it should allow for a minimum "useful"
lifetime.
The following ASN.1 base definitions are used in the + The maximum allowable total (renewable) lifetime of a ticket
rest of this section. Note that since the underscore char- (renew_till - starttime).
acter (_) is not permitted in ASN.1 names, the hyphen (-) is
used in its place for the purposes of ASN.1 names.
Realm ::= GeneralString + The maximum allowable lifetime of a ticket (endtime - starttime).
PrincipalName ::= SEQUENCE {
name-type[0] INTEGER,
name-string[1] SEQUENCE OF GeneralString
}
Kerberos realms are encoded as GeneralStrings. Realms shall + Whether to allow the issue of tickets with empty address fields
not contain a character with the code 0 (the ASCII NUL). (including the ability to specify that such tickets may only be
Most realms will usually consist of several components issued if the request specifies some authorization_data).
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 com-
ponents consisting of the following sub-fields:
name-type This field specifies the type of name that fol- + Whether proxiable, forwardable, renewable or post-datable tickets
lows. Pre-defined values for this field are are to be issued.
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.
name-stringThis field encodes a sequence of components that 5. Message Specifications
Section 5.2. - 35 - Expires 28 February 1993 The following sections describe the exact contents and encoding of
protocol messages and objects. The ASN.1 base definitions are
presented in the first subsection. The remaining subsections specify
the protocol objects (tickets and authenticators) and messages.
Specification of encryption and checksum techniques, and the fields
related to them, appear in section 6.
Version 5 - Revision 5.1 5.1. ASN.1 Distinguished Encoding Representation
form a name, each component encoded as a General- All uses of ASN.1 in Kerberos shall use the Distinguished Encoding
String. Taken together, a PrincipalName and a Representation of the data elements as described in the X.509
Realm form a principal identifier. Most Princi- specification, section 8.7 [8].
palNames will have only a few components (typi-
cally one or two).
KerberosTime ::= GeneralizedTime 5.2. ASN.1 Base Definitions
-- Specifying UTC time zone (Z)
The timestamps used in Kerberos are encoded as General- The following ASN.1 base definitions are used in the rest of this
izedTimes. An encoding shall specify the UTC time zone (Z) section. Note that since the underscore character (_) is not
and shall not include any fractional portions of the permitted in ASN.1 names, the hyphen (-) is used in its place for the
seconds. It further shall not include any separators. purposes of ASN.1 names.
Example: The only valid format for UTC time 6 minutes, 27
seconds after 9 pm on 6 November 1985 is 19851106210627Z.
HostAddress ::= SEQUENCE { Realm ::= GeneralString
addr-type[0] INTEGER, PrincipalName ::= SEQUENCE {
address[1] OCTET STRING name-type[0] INTEGER,
} name-string[1] SEQUENCE OF GeneralString
}
HostAddresses ::= SEQUENCE OF SEQUENCE { Kerberos realms are encoded as GeneralStrings. Realms shall not
addr-type[0] INTEGER, contain a character with the code 0 (the ASCII NUL). Most realms
address[1] OCTET STRING 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:
The host adddress encodings consists of two 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.
addr-type This field specifies the type of address that name-string This field encodes a sequence of components that
follows. Pre-defined values for this field are form a name, each component encoded as a General
specified in section 8.1. String. Taken together, a PrincipalName and a Realm
form a principal identifier. Most PrincipalNames
will have only a few components (typically one or two).
8 KerberosTime ::= GeneralizedTime
address This field encodes a single address of type addr- -- Specifying UTC time zone (Z)
type.
The two forms differ slightly. HostAddress contains exactly The timestamps used in Kerberos are encoded as GeneralizedTimes. An
one address; HostAddresses contains a sequence of possibly encoding shall specify the UTC time zone (Z) and shall not include
many addresses. any fractional portions of the seconds. It further shall not include
any separators. Example: The only valid format for UTC time 6
minutes, 27 seconds after 9 pm on 6 November 1985 is 19851106210627Z.
AuthorizationData ::= SEQUENCE OF SEQUENCE { HostAddress ::= SEQUENCE {
ad-type[0] INTEGER, addr-type[0] INTEGER,
ad-data[1] OCTET STRING address[1] OCTET STRING
} }
ad-data This field contains authorization data to be HostAddresses ::= SEQUENCE OF SEQUENCE {
interpreted according to the value of the addr-type[0] INTEGER,
corresponding ad-type field. address[1] OCTET STRING
}
ad-type This field specifies the format for the ad-data The host adddress encodings consists of two fields:
9Section 5.2. - 36 - Expires 28 February 1993 addr-type This field specifies the type of address that
follows. Pre-defined values for this field are
specified in section 8.1.
Version 5 - Revision 5.1 address This field encodes a single address of type addr-type.
subfield. All negative values are reserved for The two forms differ slightly. HostAddress contains exactly one
local use. Non-negative values are reserved for address; HostAddresses contains a sequence of possibly many
registered use. addresses.
APOptions ::= BIT STRING { AuthorizationData ::= SEQUENCE OF SEQUENCE {
reserved(0), ad-type[0] INTEGER,
use-session-key(1), ad-data[1] OCTET STRING
mutual-required(2) }
}
8 ad-data This field contains authorization data to be
TicketFlags ::= BIT STRING { interpreted according to the value of the
reserved(0), corresponding ad-type field.
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)
}
8 ad-type This field specifies the format for the ad-data
KDCOptions ::= BIT STRING { subfield. All negative values are reserved for
reserved(0), local use. Non-negative values are reserved for
forwardable(1), registered use.
forwarded(2),
proxiable(3),
proxy(4),
allow-postdate(5),
postdated(6),
unused7(7),
renewable(8),
unused9(9),
unused10(10),
unused11(11),
renewable-ok(27),
enc-tkt-in-skey(28),
renew(30),
validate(31)
}
LastReq ::= SEQUENCE OF SEQUENCE { APOptions ::= BIT STRING {
lr-type[0] INTEGER, reserved(0),
lr-value[1] KerberosTime use-session-key(1),
} mutual-required(2)
}
lr-type This field indicates how the following lr-value TicketFlags ::= BIT STRING {
field is to be interpreted. Negative values 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)
}
Section 5.2. - 37 - Expires 28 February 1993 KDCOptions ::= BIT STRING {
reserved(0),
forwardable(1),
forwarded(2),
proxiable(3),
proxy(4),
allow-postdate(5),
postdated(6),
unused7(7),
renewable(8),
unused9(9),
unused10(10),
unused11(11),
renewable-ok(27),
enc-tkt-in-skey(28),
renew(30),
validate(31)
}
Version 5 - Revision 5.1 LastReq ::= SEQUENCE OF SEQUENCE {
lr-type[0] INTEGER,
lr-value[1] KerberosTime
}
indicate that the information pertains only to the lr-type This field indicates how the following lr-value
responding server. Non-negative values pertain to field is to be interpreted. Negative values indicate
all servers for the realm. 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 informa- If the lr-type field is zero (0), then no information
tion is conveyed by the lr-value subfield. If the is conveyed by the lr-value subfield. If the
absolute value of the lr-type field is one (1), absolute value of the lr-type field is one (1),
then the lr-value subfield is the time of last then the lr-value subfield is the time of last
initial request for a TGT. If it is two (2), then initial request for a TGT. If it is two (2), then
the lr-value subfield is the time of last initial the lr-value subfield is the time of last initial
request. If it is three (3), then the lr-value request. If it is three (3), then the lr-value
subfield is the time of issue for the newest subfield is the time of issue for the newest
ticket-granting ticket used. If it is four (4), ticket-granting ticket used. If it is four (4),
then the lr-value subfield is the time of the last then the lr-value subfield is the time of the last
renewal. If it is five (5), then the lr-value renewal. If it is five (5), then the lr-value
subfield is the time of last request (of any subfield is the time of last request (of any
type). type).
lr-value This field contains the time of the last request. lr-value This field contains the time of the last request.
The time must be interpreted according to the con- The time must be interpreted according to the contents
tents of the accompanying lr-type subfield. of the accompanying lr-type subfield.
See section 6 for the definitions of Checksum, Check- See section 6 for the definitions of Checksum, ChecksumType,
sumType, EncryptedData, EncryptionKey, EncryptionType, and EncryptedData, EncryptionKey, EncryptionType, and KeyType.
KeyType.
_5._3. _T_i_c_k_e_t_s _a_n_d _A_u_t_h_e_n_t_i_c_a_t_o_r_s 5.3. Tickets and Authenticators
This section describes the format and encryption param- This section describes the format and encryption parameters for
eters for tickets and authenticators. When a ticket or tickets and authenticators. When a ticket or authenticator is
authenticator is included in a protocol message it is included in a protocol message it is treated as an opaque object.
treated as an opaque object.
_5._3._1. _T_i_c_k_e_t_s 5.3.1. Tickets
A ticket is a record that helps a client authenticate A ticket is a record that helps a client authenticate to a service.
to a service. A Ticket contains the following information: A Ticket contains the following information:
Ticket ::= [APPLICATION 1] SEQUENCE { Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno[0] INTEGER, tkt-vno[0] INTEGER,
realm[1] Realm, realm[1] Realm,
sname[2] PrincipalName, sname[2] PrincipalName,
enc-part[3] EncryptedData enc-part[3] EncryptedData
} }
-- Encrypted part of ticket -- Encrypted part of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE { EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags[0] TicketFlags, flags[0] TicketFlags,
key[1] EncryptionKey, key[1] EncryptionKey,
crealm[2] Realm, crealm[2] Realm,
cname[3] PrincipalName, cname[3] PrincipalName,
transited[4] TransitedEncoding, transited[4] TransitedEncoding,
authtime[5] KerberosTime, authtime[5] KerberosTime,
starttime[6] KerberosTime OPTIONAL, starttime[6] KerberosTime OPTIONAL,
endtime[7] KerberosTime,
Section 5.3.1. - 38 - Expires 28 February 1993 renew-till[8] KerberosTime OPTIONAL,
caddr[9] HostAddresses OPTIONAL,
Version 5 - Revision 5.1 authorization-data[10] AuthorizationData OPTIONAL
endtime[7] KerberosTime,
renew-till[8] KerberosTime OPTIONAL,
caddr[9] HostAddresses OPTIONAL,
authorization-data[10] AuthorizationData OPTIONAL
} }
-- encoded Transited field -- encoded Transited field
TransitedEncoding ::= SEQUENCE { TransitedEncoding ::= SEQUENCE {
tr-type[0] INTEGER, -- must be registered tr-type[0] INTEGER, -- must be registered
contents[1] OCTET STRING contents[1] OCTET STRING
} }
The encoding of EncTicketPart is encrypted in the key shared The encoding of EncTicketPart is encrypted in the key shared by
by Kerberos and the end server (the server's secret key). Kerberos and the end server (the server's secret key). See section 6
See section 6 for the format of the ciphertext. for the format of the ciphertext.
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 identi-
cal.
sname This field specifies the name part of the server's
identity.
enc-part This field holds the encrypted encoding of the
EncTicketPart sequence.
flags This field indicates which of various options were
used or requested when the ticket was issued. 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). Bit 0 is the most significant bit. The
encoding of the bits is specified in section 5.2.
The flags are described in more detail above in
section 2. The meanings of the flags are:
_B_i_t(_s) _N_a_m_e _D_e_s_c_r_i_p_t_i_o_n
0 RESERVED
7 Reserved for future expansion of this
field.
Section 5.3.1. - 39 - Expires 28 February 1993
Version 5 - Revision 5.1
1 FORWARDABLE
7 The FORWARDABLE flag is normally only
interpreted by the TGS, and can be
ignored by end servers. When set, this
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.
2 FORWARDED
7 When set, this flag indicates that the
ticket has either been forwarded or was
issued based on authentication involving
a forwarded ticket-granting ticket.
3 PROXIABLE
7 The PROXIABLE flag is normally only
interpreted by the TGS, and can be
ignored by end servers. The PROXIABLE
flag has an interpretation identical 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
7 When set, this flag indicates that a
ticket is a proxy.
5 MAY-POSTDATE tkt-vno This field specifies the version number for the ticket
7 The MAY-POSTDATE flag is normally only format. This document describes version number 5.
interpreted by the TGS, and can be
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.
6 POSTDATED realm This field specifies the realm that issued a ticket. It
7 This flag indicates that this ticket has also serves to identify the realm part of the server's
been postdated. The end-service can principal identifier. Since a Kerberos server can only
check the authtime field to see when the issue tickets for servers within its realm, the two will
original authentication occurred. always be identical.
7 INVALID sname This field specifies the name part of the server's
7 This flag indicates that a ticket is identity.
invalid, and it must be validated by the
KDC before use. Application servers
must reject tickets which have this flag
set.
8 RENEWABLE enc-part This field holds the encrypted encoding of the
7 The RENEWABLE flag is normally only EncTicketPart sequence.
interpreted by the TGS, and can usually
be ignored by end servers (some particu-
larly careful servers may wish to disal-
low renewable tickets). A renewable
ticket can be used to obtain a replace-
ment ticket that expires at a later
date.
Section 5.3.1. - 40 - Expires 28 February 1993 flags This field indicates which of various options were used or
requested when the ticket was issued. 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). Bit 0 is the most significant bit. The
encoding of the bits is specified in section 5.2. The
flags are described in more detail above in section 2. The
meanings of the flags are:
Version 5 - Revision 5.1 Bit(s) Name Description
9 INITIAL 0 RESERVED Reserved for future expansion of this
7 This flag indicates that this ticket was field.
issued using the AS protocol, and not
issued based on a ticket-granting
ticket.
10 PRE-AUTHENT 1 FORWARDABLE The FORWARDABLE flag is normally only
7 This flag indicates that during initial interpreted by the TGS, and can be
authentication, the client was authenti- ignored by end servers. When set,
cated by the KDC before a ticket was this flag tells the ticket-granting
issued. The strength of the pre- server that it is OK to issue a new
authentication method is not indicated, ticket- granting ticket with a
but is acceptable to the KDC. different network address based on
the presented ticket.
11 HW-AUTHENT 2 FORWARDED When set, this flag indicates that
7 This flag indicates that the protocol the ticket has either been forwarded
employed for initial authentication or was issued based on authentication
required the use of hardware expected to involving a forwarded ticket-granting
be possessed solely by the named client. ticket.
The hardware authentication method is
selected by the KDC and the strength of
the method is not indicated.
12-31 RESERVED 3 PROXIABLE The PROXIABLE flag is normally only
7 Reserved for future use. interpreted by the TGS, and can be
ignored by end servers. The PROXIABLE
flag has an interpretation identical
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.
key This field exists in the ticket and the KDC 4 PROXY When set, this flag indicates that a
response and is used to pass the session key from ticket is a proxy.
Kerberos to the application server and the client.
The field's encoding is described in section 6.1.
crealm This field contains the name of the realm in which 5 MAY-POSTDATE The MAY-POSTDATE flag is normally
the client is registered and in which initial only interpreted by the TGS, and can
authentication took place. be 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.
cname This field contains the name part of the client's 6 POSTDATED This flag indicates that this ticket
principal identifier. has been postdated. The end-service
can check the authtime field to see
when the original authentication
occurred.
transited This field lists the names of the Kerberos realms 7 INVALID This flag indicates that a ticket is
that took part in authenticating the user to whom invalid, and it must be validated by
this ticket was issued. It does not specify the the KDC before use. Application
order in which the realms were transited. See servers must reject tickets which
section 3.3.3.1 for details on how this field have this flag set.
encodes the traversed realms.
authtime This field indicates the time of initial authenti- 8 RENEWABLE The RENEWABLE flag is normally only
cation for the named principal. It is the time of interpreted by the TGS, and can
issue for the original ticket on which this ticket usually be ignored by end servers
is based. It is included in the ticket to provide (some particularly careful servers
additional information to the end service, and to may wish to disallow renewable
provide the necessary information for implementa- tickets). A renewable ticket can be
tion of a `hot list' service at the KDC. An end used to obtain a replacement ticket
service that is particularly paranoid could refuse that expires at a later date.
Section 5.3.1. - 41 - Expires 28 February 1993 9 INITIAL This flag indicates that this ticket
was issued using the AS protocol, and
not issued based on a ticket-granting
ticket.
Version 5 - Revision 5.1 10 PRE-AUTHENT This flag indicates that during
initial authentication, the client
was authenticated by the KDC before a
ticket was issued. The strength of
the preauthentication method is not
indicated, but is acceptable to the
KDC.
to accept tickets for which the initial authenti- 11 HW-AUTHENT This flag indicates that the protocol
cation occurred "too far" in the past. employed for initial authentication
required the use of hardware expected
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 field is also returned as part of the 12-31 RESERVED Reserved for future use.
response from the KDC. When returned as part of
the response to initial authentication
(KRB_AS_REP), this is the current time on the Ker-
beros server[21].
starttime This field in the ticket specifies the time after key This field exists in the ticket and the KDC response and is
which the ticket is valid. Together with endtime, used to pass the session key from Kerberos to the
this field specifies the life of the ticket. If application server and the client. The field's encoding is
it is absent from the ticket, its value should be described in section 6.2.
treated as that of the authtime field.
endtime This field contains the time after which the crealm This field contains the name of the realm in which the
ticket will not be honored (its expiration time). client is registered and in which initial authentication
Note that individual services may place their own took place.
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-tillThis field is only present in tickets that have cname This field contains the name part of the client's principal
the RENEWABLE flag set in the flags field. It identifier.
indicates the maximum endtime that may be included
in a renewal. It can be thought of as the abso-
lute expiration time for the ticket, including all
renewals.
caddr This field in a ticket contains zero (if omitted) transited This field lists the names of the Kerberos realms that took
or more (if present) host addresses. These are part in authenticating the user to whom this ticket was
the addresses from which the ticket can be used. issued. It does not specify the order in which the realms
If there are no addresses, the ticket can be used were transited. See section 3.3.3.1 for details on how
from any location. The decision by the KDC to this field encodes the traversed realms.
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 sug-
gested and default policy, however, is that such
tickets will only be issued or accepted when
__________________________
9[21] This time value might be used (at the host's op-
tion) to adjust the workstation's clock. HOWEVER, this
is not recommended, since the client cannot determine
that such a KRB_AS_REP actually came from the proper
KDC in a timely manner unless the enclosed ticket can
be used in communication with a server whose secrets
are uncompromised.
9
Section 5.3.1. - 42 - Expires 28 February 1993 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.
Version 5 - Revision 5.1 This field is also returned as part of the response from
the KDC. When returned as part of the response to initial
authentication (KRB_AS_REP), this is the current time on
the Kerberos server (It is NOT recommended that this time
value be used to adjust the workstation's clock since the
workstation cannot reliably determine that such a
KRB_AS_REP actually came from the proper KDC in a timely
manner.).
additional information that can be used to res- starttime This field in the ticket specifies the time after which the
trict the use of the ticket is included in the ticket is valid. Together with endtime, this field
authorization_data field. Such a ticket is a specifies the life of the ticket. If it is absent from
capability. the ticket, its value should be treated as that of the
authtime field.
Network addresses are included in the ticket to endtime This field contains the time after which the ticket will
make it harder for an attacker to use stolen not be honored (its expiration time). Note that individual
credentials. Because the session key is not sent services may place their own limits on the life of a ticket
over the network in cleartext, credentials can't and may reject tickets which have not yet expired. As
be stolen simply by listening to the network; an such, this is really an upper bound on the expiration time
attacker has to gain access to the session key for the ticket.
(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 renew-till This field is only present in tickets that have the
from which a connection is received cannot be RENEWABLE flag set in the flags field. It indicates the
reliably determined. Even if it could be, an maximum endtime that may be included in a renewal. It can
attacker who has compromised the client's worksta- be thought of as the absolute expiration time for the
tion could use the credentials from there. ticket, including all renewals.
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 caddr This field in a ticket contains zero (if omitted) or more
The authorization-data field is used to pass (if present) host addresses. These are the addresses from
authorization data from the principal on whose which the ticket can be used. If there are no addresses,
behalf a ticket was issued to the application ser- the ticket can be used from any location. The decision
vice. If no authorization data is included, this by the KDC to issue or by the end server to accept zero-
field will be left out. The data in this field address tickets is a policy decision and is left to the
are specific to the end service. It is expected Kerberos and end-service administrators; they may refuse to
that the field will contain the names of service issue or accept such tickets. The suggested and default
specific objects, and the rights to those objects. policy, however, is that such tickets will only be issued
The format for this field is described in section or accepted when additional information that can be used to
5.2. Although Kerberos is not concerned with the restrict the use of the ticket is included in the
format of the contents of the subfields, it does authorization_data field. Such a ticket is a capability.
carry type information (ad-type).
By using the authorization_data field, a principal Network addresses are included in the ticket to make it
is able to issue a proxy that is valid for a harder for an attacker to use stolen credentials. Because
specific purpose. For example, a client wishing the session key is not sent over the network in cleartext,
to print a file can obtain a file server proxy to credentials can't be stolen simply by listening to the
be passed to the print server. By specifying the network; an attacker has to gain access to the session key
name of the file in the authorization_data field, (perhaps through operating system security breaches or a
the file server knows that the print server can careless user's unattended session) to make use of stolen
only use the client's rights when accessing the tickets.
particular file to be printed.
It is interesting to note that if one specifies It is important to note that the network address from which
the authorization-data field of a proxy and leaves a connection is received cannot be reliably determined.
the host addresses blank, the resulting ticket and Even if it could be, an attacker who has compromised the
session key can be treated as a capability. See 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.
Section 5.3.1. - 43 - Expires 28 February 1993 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. The data in this field are specific to the end
service. It is expected that 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 subfields, it does carry type
information (ad-type).
Version 5 - Revision 5.1 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.
[9] for some suggested uses of this field. It is interesting to note that 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 [9] for some suggested
uses of this field.
The authorization-data field is optional and does The authorization-data field is optional and does not have
not have to be included in a ticket. to be included in a ticket.
_5._3._2. _A_u_t_h_e_n_t_i_c_a_t_o_r_s 5.3.2. Authenticators
An authenticator is a record sent with a ticket to a An authenticator is a record sent with a ticket to a server to
server to certify the client's knowledge of the encryption certify the client's knowledge of the encryption key in the ticket,
key in the ticket, to help the server detect replays, and to to help the server detect replays, and to help choose a "true session
help choose a "true session key" to use with the particular key" to use with the particular session. The encoding is encrypted
session. The encoding is encrypted in the ticket's session in the ticket's session key shared by the client and the server:
key shared by the client and the server:
-- Unencrypted authenticator -- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE { Authenticator ::= [APPLICATION 2] SEQUENCE {
authenticator-vno[0] INTEGER, authenticator-vno[0] INTEGER,
crealm[1] Realm, crealm[1] Realm,
cname[2] PrincipalName, cname[2] PrincipalName,
cksum[3] Checksum OPTIONAL, cksum[3] Checksum OPTIONAL,
cusec[4] INTEGER, cusec[4] INTEGER,
ctime[5] KerberosTime, ctime[5] KerberosTime,
subkey[6] EncryptionKey OPTIONAL, subkey[6] EncryptionKey OPTIONAL,
seq-number[7] INTEGER OPTIONAL, seq-number[7] INTEGER OPTIONAL,
authorization-data[8] AuthorizationData OPTIONAL authorization-data[8] AuthorizationData OPTIONAL
} }
authenticator-vno
This field specifies the version number for the
format of the authenticator. This document speci-
fies version 5.
crealm and cname
These fields are the same as those described for
the ticket in section 5.3.1.
cksum This field contains a checksum of the the applica- authenticator-vno This field specifies the version number for the
tion data that accompanies the KRB_AP_REQ. format of the authenticator. This document specifies
version 5.
cusec This field contains the microsecond part of the crealm and cname These fields are the same as those described for the
client's timestamp. Its value (before encryption) ticket in section 5.3.1.
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 cksum This field contains a checksum of the the application data
client's host. that accompanies the KRB_AP_REQ.
Section 5.3.2. - 44 - Expires 28 February 1993 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.
Version 5 - Revision 5.1 ctime This field contains the current time on the client's host.
subkey This field contains the client's choice for an subkey This field contains the client's choice for an encryption
encryption key which is to be used to protect this key which is to be used to protect this specific
specific application session. Unless an applica- application session. Unless an application specifies
tion specifies otherwise, if this field is left otherwise, if this field is left out the session key from
out the session key from the ticket will be used. the ticket will be used.
seq-numberThis optional field includes the initial sequence seq-number This optional field includes the initial sequence number
number to be used by the KRB_PRIV or KRB_SAFE mes- to be used by the KRB_PRIV or KRB_SAFE messages when
sages when sequence numbers are used to detect sequence numbers are used to detect replays (It may also be
replays (It may also be used by application used by application specific messages). When included in
specific messages). When included in the authen- the authenticator this field specifies the initial sequence
ticator this field specifies the initial sequence number for messages from the client to the server. When
number for messages from the client to the server. included in the AP-REP message, the initial sequence number
When included in the AP-REP message, the initial is that for messages from the server to the client. When
sequence number is that for messages from the used in KRB_PRIV or KRB_SAFE messages, it is incremented by
server to the client. When used in KRB_PRIV or one after each message is sent.
KRB_SAFE messages, it is incremented by one after
each message is sent.
For sequence numbers to adequately support the For sequence numbers to adequately support the detection of
detection of replays they should be non-repeating, replays they should be non-repeating, even across
even across connection boundaries. The initial connection boundaries. The initial sequence number should
sequence number should be random and uniformly be random and uniformly distributed across the full space
distributed across the full space of possible of possible sequence numbers, so that it cannot be guessed
sequence numbers, so that it cannot be guessed by by an attacker and so that it and the successive sequence
an attacker and so that it and the successive numbers do not repeat other sequences.
sequence numbers do not repeat other sequences.
authorization-data authorization-data This field is the same as described for the ticket
This field is the same as described for the ticket in section 5.3.1. It is optional and will only appear when
in section 5.3.1. It is optional and will only additional restrictions are to be placed on the use of a
appear when additional restrictions are to be ticket, beyond those carried in the ticket itself.
placed on the use of a ticket, beyond those car-
ried in the ticket itself.
_5._4. _S_p_e_c_i_f_i_c_a_t_i_o_n_s _f_o_r _t_h_e _A_S _a_n_d _T_G_S _e_x_c_h_a_n_g_e_s 5.4. Specifications for the AS and TGS exchanges
This section specifies the format of the messages used This section specifies the format of the messages used in exchange
in exchange between the client and the Kerberos server. The between the client and the Kerberos server. The format of possible
format of possible error messages appears in section 5.8.1. error messages appears in section 5.9.1.
_5._4._1. _K_R_B__K_D_C__R_E_Q _d_e_f_i_n_i_t_i_o_n 5.4.1. KRB_KDC_REQ definition
The KRB_KDC_REQ message has no type of its own. The KRB_KDC_REQ message has no type of its own. Instead, its type is
Instead, its type is one of KRB_AS_REQ or KRB_TGS_REQ one of KRB_AS_REQ or KRB_TGS_REQ depending on whether the request is
depending on whether the request is for an initial ticket or for an initial ticket or an additional ticket. In either case, the
an additional ticket. In either case, the message is sent message is sent from the client to the Authentication Server to
from the client to the Authentication Server to request request credentials for a service.
credentials for a service.
The message fields are: The message fields are:
AS-REQ ::= [APPLICATION 10] KDC-REQ AS-REQ ::= [APPLICATION 10] KDC-REQ
Section 5.4.1. - 45 - Expires 28 February 1993
Version 5 - Revision 5.1
TGS-REQ ::= [APPLICATION 12] KDC-REQ TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ ::= SEQUENCE { KDC-REQ ::= SEQUENCE {
pvno[1] INTEGER, pvno[1] INTEGER,
msg-type[2] INTEGER, msg-type[2] INTEGER,
padata[3] SEQUENCE OF PA-DATA OPTIONAL, padata[3] SEQUENCE OF PA-DATA OPTIONAL,
req-body[4] KDC-REQ-BODY req-body[4] KDC-REQ-BODY
} }
PA-DATA ::= SEQUENCE { PA-DATA ::= SEQUENCE {
padata-type[1] INTEGER, padata-type[1] INTEGER,
padata-value[2] OCTET STRING, padata-value[2] OCTET STRING,
-- might be encoded AP-REQ -- might be encoded AP-REQ
} }
KDC-REQ-BODY ::= SEQUENCE { KDC-REQ-BODY ::= SEQUENCE {
kdc-options[0] KDCOptions, kdc-options[0] KDCOptions,
cname[1] PrincipalName OPTIONAL, cname[1] PrincipalName OPTIONAL,
-- Used only in AS-REQ -- Used only in AS-REQ
realm[2] Realm, -- Server's realm realm[2] Realm, -- Server's realm
-- Also client's in AS-REQ -- Also client's in AS-REQ
sname[3] PrincipalName, sname[3] PrincipalName OPTIONAL,
from[4] KerberosTime OPTIONAL, from[4] KerberosTime OPTIONAL,
till[5] KerberosTime, till[5] KerberosTime,
rtime[6] KerberosTime OPTIONAL, rtime[6] KerberosTime OPTIONAL,
nonce[7] INTEGER, nonce[7] INTEGER,
etype[8] SEQUENCE OF INTEGER, -- EncryptionType, etype[8] SEQUENCE OF INTEGER, -- EncryptionType,
-- in preference order -- in preference order
addresses[9] HostAddresses OPTIONAL, addresses[9] HostAddresses OPTIONAL,
enc-authorization-data[10] EncryptedData OPTIONAL, enc-authorization-data[10] EncryptedData OPTIONAL,
-- Encrypted AuthorizationData encoding -- Encrypted AuthorizationData encoding
additional-tickets[11] SEQUENCE OF Ticket OPTIONAL additional-tickets[11] SEQUENCE OF Ticket OPTIONAL
} }
The fields in this message are: The fields in this message are:
pvno This field is included in each message, and speci-
fies the protocol version number. This document
specifies protocol version 5.
msg-type This field indicates the type of a protocol mes-
sage. 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 The padata (pre-authentication data) field con-
tains a sequence of authentication information
which may be needed before credentials can be
Section 5.4.1. - 46 - Expires 28 February 1993
Version 5 - Revision 5.1
issued or decrypted. In the case of requests for
additional tickets (KRB_TGS_REQ), this field will
include an element with padata-type of PA-TGS-REQ
and data of an authentication header (ticket-
granting ticket and authenticator). The checksum
in the authenticator (which must be collision-
proof) is to be computed over the KDC-REQ-BODY
encoding. In most requests for initial authenti-
cation (KRB_AS_REQ) and most replies (KDC-REP),
the padata field will be left out. This field may
also contain information needed by certain exten-
sions to the Kerberos protocol. For example, it
might be used to initially verify the identity of
a client before any response is returned, or it
might contain information needed to help the KDC
or the client select the key needed for generating
or decrypting the response. The latter cases
would be useful for supporting the use of certain
"smartcards" with Kerberos. The details of such
extensions are not presently specified.
padata-type
The padata-type element of the padata field indi-
cates 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.
req-body This field is a placeholder delimiting the extent
of the remaining fields. If a checksum is to be
calculated over the request, it is calculated over
an encoding of the KDC-REQ-BODY sequence which is
enclosed within the req-body field.
kdc-options
This field appears in the KRB_AS_REQ and
KRB_TGS_REQ requests to the KDC and indicates the
flags that the client wants set on the tickets as
well as other information that is to modify the
behavior of the KDC. Where appropriate, the name
of an option may be the same as the flag that is
set by that option. Although in most case, the
bit in the options field will be the same as that
in the flags field, this is not guaranteed, so it
is not acceptable to simply copy the options field
to the flags field. There are various checks that
must be made before honoring an option anyway.
The kdc_options field is a bit-field, where the
selected options are indicated by the bit being
Section 5.4.1. - 47 - Expires 28 February 1993
Version 5 - Revision 5.1
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:
_B_i_t(_s)_N_a_m_e _D_e_s_c_r_i_p_t_i_o_n
0 RESERVED
7 Reserved for future expansion of this
field.
1 FORWARDABLE pvno This field is included in each message, and specifies the
7 The FORWARDABLE option indicates that protocol version number. This document specifies protocol
the ticket to be issued is to have its version 5.
forwardable flag set. It may only be
set on the initial request, or in a sub-
sequent request if the ticket-granting
ticket on which it is based is also for-
wardable.
2 FORWARDED msg-type This field indicates the type of a protocol message. It
7 The FORWARDED option is only specified will almost always be the same as the application
in a request to the ticket-granting identifier associated with a message. It is included to
server and will only be honored if the make the identifier more readily accessible to the
ticket-granting ticket in the request application. For the KDC-REQ message, this type will be
has its FORWARDABLE bit set. This KRB_AS_REQ or KRB_TGS_REQ.
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.
3 PROXIABLE padata The padata (pre-authentication data) field contains a of
7 The PROXIABLE option indicates that the authentication information which may be needed before
ticket to be issued is to have its prox- credentials can be issued or decrypted. In the case of
iable flag set. It may only be set on requests for additional tickets (KRB_TGS_REQ), this field
the initial request, or in a subsequent will include an element with padata-type of PA-TGS-REQ and
request if the ticket-granting ticket on data of an authentication header (ticket-granting ticket
which it is based is also proxiable. and authenticator). The checksum in the authenticator
(which must be collisionproof) is to be computed over the
KDC-REQ-BODY encoding. In most requests for initial
authentication (KRB_AS_REQ) and most replies (KDC-REP), the
padata field will be left out.
4 PROXY This field may also contain information needed by certain
7 The PROXY option indicates that this is extensions to the Kerberos protocol. For example, it might
a request for a proxy. This option will be used to initially verify the identity of a client before
only be honored if the ticket-granting any response is returned. This is accomplished with a
ticket in the request has its PROXIABLE padata field with padata-type equal to PA-ENC-TIMESTAMP and
bit set. The address(es) of the host padata-value defined as follows:
from which the resulting ticket is to be
valid are included in the addresses
field of the request.
5 ALLOW-POSTDATE padata-type ::= PA-ENC-TIMESTAMP
7 The ALLOW-POSTDATE option indicates that padata-value ::= EncryptedData -- PA-ENC-TS-ENC
the ticket to be issued is to have its
MAY-POSTDATE flag set. It may only be
set on the initial request, or in a sub-
sequent request if the ticket-granting
ticket on which it is based also has its
MAY-POSTDATE flag set.
Section 5.4.1. - 48 - Expires 28 February 1993 PA-ENC-TS-ENC ::= SEQUENCE {
patimestamp[0] KerberosTime, -- client's time
pausec[1] INTEGER OPTIONAL
}
with patimestamp containing the client's time and pausec
containing 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
sequence, encrypted using the client's secret key.
Version 5 - Revision 5.1 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
"smartcards" with Kerberos. The details of such extensions
are beyond the scope of this specification. See [10] for
additional uses of this field.
6 POSTDATED padata-type The padata-type element of the padata field indicates the
7 The POSTDATED option indicates that this way that the padata-value element is to be interpreted.
is a request for a postdated ticket. Negative values of padata-type are reserved for
This option will only be honored if the unregistered use; non-negative values are used for a
ticket-granting ticket on which it is registered interpretation of the element type.
based has its 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 req-body This field is a placeholder delimiting the extent of the
7 This option is presently unused. 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.
8 RENEWABLE kdc-options This field appears in the KRB_AS_REQ and KRB_TGS_REQ
7 The RENEWABLE option indicates that the requests to the KDC and indicates the flags that the client
ticket to be issued is to have its wants set on the tickets as well as other information that
RENEWABLE flag set. It may only be set is to modify the behavior of the KDC. Where appropriate,
on the initial request, or when the the name of an option may be the same as the flag that is
ticket-granting ticket on which the set by that option. Although in most case, the bit in the
request is based is also renewable. If options field will be the same as that in the flags field,
this option is requested, then the rtime this is not guaranteed, so it is not acceptable to simply
field in the request contains the copy the options field to the flags field. There are
desired absolute expiration time for the various checks that must be made before honoring an option
ticket. anyway.
9-26 RESERVED The kdc_options field is a bit-field, where the selected
7 Reserved for future use. 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:
27 RENEWABLE-OK Bit(s) Name Description
7 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,
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.
28 ENC-TKT-IN-SKEY 0 RESERVED Reserved for future expansion of this
7 This option is used only by the ticket- field.
granting service. The ENC-TKT-IN-SKEY
option indicates that the ticket for the
end server is to be encrypted in the
session key from the additional ticket-
granting ticket provided.
29 RESERVED 1 FORWARDABLE The FORWARDABLE option indicates that
7 Reserved for future use. the ticket to be issued is to have its
forwardable flag set. It 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.
Section 5.4.1. - 49 - Expires 28 February 1993 2 FORWARDED 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 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.
Version 5 - Revision 5.1 3 PROXIABLE The PROXIABLE option indicates that
the ticket to be issued is to have its
proxiable flag set. It 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 proxiable.
30 RENEW 4 PROXY The PROXY option indicates that this
7 This option is used only by the ticket- is a request for a proxy. This option
granting service. The RENEW option will only be honored if the ticket-
indicates that the present request is granting ticket in the request has its
for a renewal. The ticket provided is PROXIABLE bit set. The address(es) of
encrypted in the secret key for the the host from which the resulting
server on which it is valid. This ticket is to be valid are included in
option will only be honored if the the addresses field of the request.
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.
31 VALIDATE 5 ALLOW-POSTDATE The ALLOW-POSTDATE option indicates
7 This option is used only by the ticket- that the ticket to be issued is to
granting service. The VALIDATE option have its MAY-POSTDATE flag set. It
indicates that the request is to vali- may only be set on the initial
date a postdated ticket. It will only request, or in a subsequent request if
be honored if the ticket presented is the ticket-granting ticket on which it
postdated, presently has its INVALID is based also has its MAY-POSTDATE
flag set, and would be otherwise usable flag set.
at this time. A ticket cannot be vali-
dated 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 6 POSTDATED The POSTDATED option indicates that
These fields are the same as those described for this is a request for a postdated
the ticket in section 5.3.1. ticket. This option will only be
honored if the ticket-granting ticket
on which it is based has its 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.
enc-authorization-data 7 UNUSED This option is presently unused.
The enc-authorization-data, if present (and it can
only be present in the TGS_REQ form), is an encod-
ing 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.
realm This field specifies the realm part of the 8 RENEWABLE The RENEWABLE option indicates that
server's principal identifier. In the AS the ticket to be issued is to have its
exchange, this is also the realm part of the RENEWABLE flag set. It may only be
client's principal identifier. set on the initial request, or when
the ticket-granting ticket on 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.
from This field is included in the KRB_AS_REQ and 9-26 RESERVED Reserved for future use.
KRB_TGS_REQ ticket requests when the requested
ticket is to be postdated. It specifies the
Section 5.4.1. - 50 - Expires 28 February 1993 27 RENEWABLE-OK 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, 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.
Version 5 - Revision 5.1 28 ENC-TKT-IN-SKEY This option is used only by the
ticket-granting service. The ENC-
TKT-IN-SKEY option indicates that the
ticket for the end server is to be
encrypted in the session key from the
additional ticket-granting ticket
provided.
desired start time for the requested ticket. 29 RESERVED Reserved for future use.
till This field contains the expiration date requested 30 RENEW This option is used only by the
by the client in a ticket request. 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
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.
rtime This field is the requested renew-till time sent 31 VALIDATE This option is used only by the
from a client to the KDC in a ticket request. It ticket-granting service. The VALIDATE
is optional. 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, 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.
nonce This field is part of the KDC request and cname and sname These fields are the same as those described for the
response. It it intended to hold a random number ticket in section 5.3.1. sname may only be absent when the
generated by the client. If the same number is ENC-TKT-IN-SKEY option is specified. If absent, the name
included in the encrypted response from the KDC, of the server is taken from the name of the client in the
it provides evidence that the response is fresh ticket passed as additional-tickets.
and has not been replayed by an attacker. Nonces
must never be re-used. Ideally, it should be gen-
erated randomly, but if the correct time is known,
it may suffice[22].
etype This field specifies the desired encryption algo- enc-authorization-data The enc-authorization-data, if present (and it
rithm to be used in the response. 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.
addresses This field is included in the initial request for realm This field specifies the realm part of the server's
tickets, and optionally included in requests for principal identifier. In the AS exchange, this is also the
additional tickets from the ticket-granting realm part of the client's principal identifier.
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 from This field is included in the KRB_AS_REQ and KRB_TGS_REQ
Additional tickets may be optionally included in a ticket requests when the requested ticket is to be
request to the ticket-granting server. If the postdated. It specifies the desired start time for the
ENC-TKT-IN-SKEY option has been specified, then requested ticket.
the session key from the additional ticket will be
used in place of the server's key to encrypt the
new ticket. If more than one option which
__________________________
9[22] Note, however, that if the time is used as the
nonce, one must make sure that the workstation time is
monotonically increasing. If the time is ever reset
backwards, there is a small, but finite, probability
that a nonce will be reused.
9
Section 5.4.1. - 51 - Expires 28 February 1993 till This field contains the expiration date requested by the
client in a ticket request.
Version 5 - Revision 5.1 rtime This field is the requested renew-till time sent from a