RFC1510 - The Kerberos Network Authentication Service (V5)

Network Working Group J. Kohl
Request for Comments: 1510 Digital Equipment Corporation
C. Neuman
ISI
September 1993
The Kerberos Network Authentication Service (V5)
Status of this Memo
This RFCspecifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.
Abstract
This document gives an overview and specification of Version 5 of the
protocol for the Kerberos network authentication system. Version 4,
described elsewhere [1,2], is presently in prodUCtion use at MIT"s
Project Athena, and at other Internet sites.
Overview
Project Athena, Athena, Athena MUSE, Discuss, Hesiod, Kerberos,
Moira, and Zephyr are trademarks of the Massachusetts Institute of
Technology (MIT). No commercial use of these trademarks may be made
without prior written permission of MIT.
This RFCdescribes the concepts and model upon which the Kerberos
network authentication system is based. It also specifies Version 5
of the Kerberos protocol.
The motivations, goals, assumptions, and rationale behind most design
decisions are treated cursorily; 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.
Background
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.
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].
Table of Contents
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
1. Introduction
Kerberos provides a means of verifying the identities of principals,
(e.g., a workstation user or a network server) on an open
(unprotected) network. This is accomplished without relying on
authentication by the host operating system, without basing trust on
host addresses, without requiring physical security of all the hosts
on the network, and under the assumption that packets traveling along
the network can be read, modified, and inserted at will. (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.)
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
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.
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.
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 Kerberos 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 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.
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.
The authentication exchanges mentioned above require read-only Access
to the Kerberos database. Sometimes, however, the entries in the
database must be modified, such as when adding new principals or
changing a principal"s key. This is done using a protocol between a
client and a third Kerberos server, the Kerberos Administration
Server (KADM). The administration protocol is not described in this
document. 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. Cross-Realm Operation
The Kerberos protocol is designed to operate across organizational
boundaries. A client in one organization can be authenticated to a
server in another. Each organization wishing to run a Kerberos
server establishes its own "realm". The name of the realm in which a
client is registered is part of the client"s name, and can be used by
the end-service to decide whether to honor a request.
By establishing "inter-realm" keys, the administrators of two realms
can allow a client authenticated in the local realm to use its
authentication remotely (Of course, with appropriate permission the
client could arrange registration of a separately-named principal in
a remote realm, and engage in normal exchanges with that realm"s
services. However, for even small numbers of clients this becomes
cumbersome, and more automatic methods as described here are
necessary). The exchange of inter-realm keys (a separate key may be
used for each direction) registers the ticket-granting service of
each realm as a principal in the other realm. A client is then able
to obtain a ticket-granting ticket for the remote realm"s ticket-
granting service from its local realm. When that ticket-granting
ticket is used, the remote ticket-granting service uses the inter-
realm key (which usually differs from its own normal TGS key) to
decrypt the ticket-granting ticket, and is thus certain that it was
issued by the client"s own TGS. Tickets issued by the remote ticket-
granting service will indicate to the end-service that the client was
authenticated from another realm.
A realm is said to communicate with another realm if the two realms
share an inter-realm key, or if the local realm shares an inter-realm
key with an intermediate realm that communicates with the remote
realm. An authentication path is the sequence of intermediate realms
that are transited in communicating from one realm to another.
Realms 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 authentication 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, intermediate realms may
be bypassed to achieve cross-realm authentication through alternate
authentication paths (these might be established to make
communication between two realms more efficient). It is important
for the end-service to know which realms were transited when deciding
how much faith to place in the authentication process. To facilitate
this decision, a field in each ticket contains the names of the
realms that were involved in authenticating the client.
1.2. Environmental assumptions
Kerberos imposes a few assumptions on the environment in which it can
properly function:
+ "Denial of service" attacks are not solved with Kerberos. There
are places in these protocols where an intruder intruder can
prevent an application from participating in the proper
authentication steps. Detection and solution of such attacks
(some of which can appear to be not-uncommon "normal" failure
modes for the system) is usually best left to the human
administrators and users.
+ Principals must keep their secret keys secret. If an intruder
somehow steals a principal"s key, it will be able to masquerade
as that principal or impersonate any server to the legitimate
principal.
+ "PassWord guessing" attacks are not solved by Kerberos. If a
user chooses a poor password, it is possible for an attacker to
successfully mount an offline dictionary attack by repeatedly
attempting to decrypt, with successive entries from a
dictionary, messages obtained which are encrypted under a key
derived from the user"s password.
+ Each host on the network must have a clock which is "loosely
synchronized" to the time of the other hosts; this
synchronization is used to reduce the bookkeeping needs of
application servers when they do replay detection. The degree
of "looseness" can be configured on a per-server basis. If the
clocks are synchronized over the network, the clock
synchronization protocol must itself be secured from network
attackers.
+ Principal identifiers are not recycled on a short-term basis. A
typical mode of access control will use access control lists
(ACLs) to grant permissions to particular principals. If a
stale ACL entry remains for a deleted principal and the
principal identifier is reused, the new principal will inherit
rights specified in the stale ACL entry. By not re-using
principal identifiers, the danger of inadvertent access is
removed.
1.3. Glossary of terms
Below is a list of terms used throughout this document.
Authentication Verifying the claimed identity of a
principal.
Authentication header A record containing a Ticket and an
Authenticator to be presented to a
server as part of the authentication
process.
Authentication path A sequence of intermediate realms transited
in the authentication process when
communicating from one realm to another.
Authenticator A record containing information that can
be shown to have been recently generated
using the session key known only by the
client and server.
Authorization The process of determining whether a
client may use a service, which objects
the client is allowed to access, and the
type of access allowed for each.
Capability A token that grants the bearer permission
to access an object or service. In
Kerberos, this might be a ticket whose
use is restricted by the contents of the
authorization data field, but which
lists no network addresses, together
with the session key necessary to use
the ticket.
Ciphertext The output of an encryption function.
Encryption transforms plaintext into
ciphertext.
Client A process that makes use of a network
service on behalf of a user. Note that
in some cases a Server may itself be a
client of some other server (e.g., a
print server may be a client of a file
server).
Credentials A ticket plus the secret session key
necessary to successfully use that
ticket in an authentication exchange.
KDC Key Distribution Center, a network service
that supplies tickets and temporary
session keys; or an instance of that
service or the host on which it runs.
The KDC services both initial ticket and
ticket-granting ticket requests. The
initial ticket portion is sometimes
referred to as the Authentication Server
(or service). The ticket-granting
ticket portion is sometimes referred to
as the ticket-granting server (or service).
Kerberos Aside from the 3-headed dog guarding
Hades, the name given to Project
Athena"s authentication service, the
protocol used by that service, or the
code used to implement the authentication
service.
Plaintext The input to an encryption function or
the output of a decryption function.
Decryption transforms ciphertext into
plaintext.
Principal A uniquely named client or server
instance that participates in a network
communication.
Principal identifier The name used to uniquely identify each
different principal.
Seal To encipher a record containing several
fields in such a way that the fields
cannot be individually replaced without
either knowledge of the encryption key
or leaving evidence of tampering.
Secret key An encryption key shared by a principal
and the KDC, distributed outside the
bounds of the system, with a long lifetime.
In the case of a human user"s
principal, the secret key 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 "session".
Sub-session key A temporary encryption key used between
two principals, selected and exchanged
by the principals using the session key,
and with a lifetime limited to the duration
of a single association.
Ticket A record that helps a client authenticate
itself to a server; it contains the
client"s identity, a session key, a
timestamp, and other information, all
sealed using the server"s secret key.
It only serves to authenticate a client
when presented along with a fresh
Authenticator.
2. Ticket flag uses and requests
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.
2.1. Initial and pre-authenticated tickets
The INITIAL flag indicates that a ticket was issued using the AS
protocol and not issued based on a ticket-granting ticket.
Application servers that want to require the 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.
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).
2.2. Invalid tickets
The INVALID flag indicates that a ticket is invalid. Application
servers must reject tickets which have this flag set. A postdated
ticket will usually be issued in this form. Invalid tickets must be
validated by the KDC before use, by presenting them to the KDC in a
TGS request with the VALIDATE option specified. The KDC will only
validate tickets after their starttime has passed. The validation is
required so that postdated tickets which have been stolen before
their starttime can be rendered permanently invalid (through a hot-
list mechanism).
2.3. Renewable tickets
Applications may desire to hold tickets which can be valid for long
periods of time. However, this can expose their credentials to
potential theft for equally long periods, and those stolen
credentials would be valid until the expiration time of the
ticket(s). Simply using 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.
The RENEWABLE flag in a ticket is normally only interpreted by the
ticket-granting service (discussed below in section 3.3). It can
usually be ignored by application servers. However, some
particularly careful application servers may wish to disallow
renewable tickets.
If a renewable ticket is not renewed by its expiration time, the KDC
will not renew the ticket. The RENEWABLE flag is reset by default,
but a client may request it be set by setting the RENEWABLE option
in the KRB_AS_REQ message. If it is set, then the renew-till field
in the ticket contains the time after which the ticket may not be
renewed.
2.4. Postdated tickets
Applications may occasionally need to obtain tickets for use much
later, e.g., a batch submission system would need tickets to be valid
at the time the batch job is serviced. However, it is dangerous to
hold valid tickets in a batch queue, since they will be on-line
longer and more prone to theft. Postdated tickets provide a way to
obtain these tickets from the KDC at job submission time, but to
leave them "dormant" until they are activated and validated by a
further request of the KDC. If a ticket theft were reported in the
interim, the KDC would refuse to validate the ticket, and the thief
would be foiled.
The MAY-POSTDATE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
This flag must be set in a ticket-granting ticket in order to issue a
postdated ticket based on the presented ticket. It is reset by
default; it may be requested by a client by setting the ALLOW-
POSTDATE option in the KRB_AS_REQ message. This flag does not allow
a client to obtain a postdated ticket-granting ticket; postdated
ticket-granting tickets can only by obtained by requesting the
postdating in the KRB_AS_REQ message. The life (endtime-starttime)
of a postdated ticket will be the remaining life of the ticket-
granting ticket at the time of the request, unless the RENEWABLE
option is also set, in which case it can be the full life (endtime-
starttime) of the ticket-granting ticket. The KDC may limit how far
in the future a ticket may be postdated.
The POSTDATED flag indicates that a ticket has been postdated. The
application server can check the authtime field in the ticket to see
when the original authentication occurred. Some services may choose
to reject postdated tickets, or they may only accept them within a
certain period after the original authentication. When the KDC issues
a POSTDATED ticket, it will also be marked as INVALID, so that the
application client must present the ticket to the KDC to be validated
before use.
2.5. Proxiable and proxy tickets
At times it may be necessary for a principal to allow a service to
perform an operation on its behalf. The service must be able to take
on the identity of the client, but only for a particular purpose. A
principal can allow a service to take on the principal"s identity for
a particular purpose by granting it a proxy.
The PROXIABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
When set, this flag tells the ticket-granting server that it is OK to
issue a new ticket (but not a ticket-granting ticket) with a
different network address based on this ticket. This flag is set by
default.
This flag allows a client to pass a proxy to a server to perform a
remote request on its behalf, e.g., a print service client can give
the print server a proxy to access the client"s files on a particular
file server in order to satisfy a print request.
In order to complicate the use of stolen credentials, Kerberos
tickets are usually valid from only those network addresses
specifically included in the ticket (It is permissible to request or
issue tickets with no network addresses specified, but we do not
recommend it). For this reason, a client wishing to grant a proxy
must request a new ticket valid for the network address of the
service to be granted the proxy.
The PROXY flag is set in a ticket by the TGS when it issues a
proxy ticket. Application servers may check this flag and require
additional authentication from the agent presenting the proxy in
order to provide an audit trail.
2.6. Forwardable tickets
Authentication forwarding is an instance of the proxy case where the
service is granted complete use of the client"s identity. An example
where it might be used is when a user logs in to a remote system and
wants authentication to work from that system as if the login were
local.
The FORWARDABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
The FORWARDABLE flag has an interpretation similar to that of the
PROXIABLE flag, except ticket-granting tickets may also be issued
with different network addresses. This flag is reset by default, but
users may request that it be set by setting the FORWARDABLE option in
the AS request when they request their initial ticket-granting
ticket.
This flag allows for authentication forwarding without requiring the
user to enter a password again. If the flag is not set, then
authentication forwarding is not permitted, but the same end result
can still be achieved if the user engages in the AS exchange with the
requested network addresses and supplies a password.
The FORWARDED flag is set by the TGS when a client presents a ticket
with the FORWARDABLE flag set and requests 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.
2.7. Other KDC options
There are two additional options which may be set in a client"s
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.
The ENC-TKT-IN-SKEY option is honored only by the ticket-granting
service. It indicates that the to-be-issued ticket for the end
server is to be encrypted in the session key from the additional
ticket-granting ticket provided with the request. See section 3.3.3
for specific details.
3. Message Exchanges
The following sections describe the interactions between network
clients and servers and the messages involved in those exchanges.
3.1. The Authentication Service Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_AS_REQ 5.4.1
2. Kerberos to client KRB_AS_REP or 5.4.2
KRB_ERROR 5.9.1
The Authentication Service (AS) Exchange between the client and the
Kerberos Authentication Server is usually initiated by a client when
it wishes to obtain authentication credentials for a given server but
currently holds no credentials. The client"s secret key is used for
encryption and decryption. This exchange is typically used at the
initiation of a login session, to obtain credentials for a Ticket-
Granting Server, which will subsequently be used to obtain
credentials for other servers (see section 3.3) without requiring
further use of the client"s secret key. This exchange is also used
to request credentials for services which must not be mediated
through the Ticket-Granting Service, but rather require a principal"s
secret key, such as the password-changing service. (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 exchange consists of two messages: KRB_AS_REQ from the client to
Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these
messages are described in sections 5.4.1, 5.4.2, and 5.9.1.
In the request, the client sends (in cleartext) its own identity and
the identity of the server for which it is requesting credentials.
The response, KRB_AS_REP, contains a ticket for the client to present
to the server, and a session key that will be shared by the client
and the server. The session key and additional information are
encrypted in the client"s secret key. The KRB_AS_REP message
contains information which can be used to detect replays, and to
associate it with the message to which it replies. Various errors
can occur; these are indicated by an error response (KRB_ERROR)
instead of the KRB_AS_REP response. The error message is not
encrypted. The KRB_ERROR message also contains information which can
be used to associate it with the message to which it replies. The
lack of encryption in the KRB_ERROR message precludes the ability to
detect replays or fabrications of such messages.
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.
3.1.1. Generation of KRB_AS_REQ message
The client may specify a number of options in the initial request.
Among these options are whether preauthentication is to be performed;
whether the requested ticket is to be renewable, proxiable, or
forwardable; whether it should be postdated or allow postdating of
derivative tickets; and whether a renewable ticket will be accepted
in lieu of a non-renewable ticket if the requested ticket expiration
date cannot be satisfied by a nonrenewable ticket (due to
configuration constraints; see section 4). See section A.1 for
pseudocode.
The client prepares the KRB_AS_REQ message and sends it to the KDC.
3.1.2. Receipt of KRB_AS_REQ message
If all goes well, processing the KRB_AS_REQ message will result in
the creation of a ticket for the client to present to the server.
The format for the ticket is described in section 5.3.1. The
contents of the ticket are determined as follows.
3.1.3. Generation of KRB_AS_REP message
The authentication server looks up the client and server principals
named in the KRB_AS_REQ in its database, extracting their respective
keys. If 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.).
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.
The expiration time of the ticket will be set to the minimum of the
following:
+The expiration time (endtime) requested in the KRB_AS_REQ
message.
+The ticket"s start time plus the maximum allowable lifetime
associated with the client principal (the authentication
server"s database includes a maximum ticket lifetime field
in each principal"s record; see section 4).
+The ticket"s start time plus the maximum allowable lifetime
associated with the server principal.
+The ticket"s start time plus the maximum lifetime set by
the policy of the local realm.
If the requested expiration time minus the start time (as determined
above) is less than a site-determined minimum lifetime, an error
message with code KDC_ERR_NEVER_VALID is returned. If the requested
expiration time for the ticket exceeds what was determined as above,
and if the "RENEWABLE-OK" option was requested, then the "RENEWABLE"
flag is set in the new ticket, and the renew-till value is set as if
the "RENEWABLE" option were requested (the field and option names are
described fully in section 5.4.1). If the RENEWABLE option has been
requested or if the RENEWABLE-OK option has been set and a renewable
ticket is to be issued, then the renew-till field is set to the
minimum of:
+Its requested value.
+The start time of the ticket plus the minimum of the two
maximum renewable lifetimes associated with the principals"
database entries.
+The start time of the ticket plus the maximum renewable
lifetime set by the policy of the local realm.
The flags field of the new ticket will have the following options set
if they have been requested and if the policy of the local realm
allows: FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE.
If the new ticket is postdated (the start time is in the future), its
INVALID flag will also be set.
If all of the above succeed, the server formats a KRB_AS_REP message
(see section 5.4.2), copying the addresses in the request into the
caddr of the response, placing any required pre-authentication data
into the padata of the response, and encrypts the ciphertext part in
the client"s key using the requested encryption method, and sends it
to the client. See section A.2 for pseudocode.
3.1.4. Generation of KRB_ERROR message
Several errors can occur, and the Authentication Server responds by
returning an error message, KRB_ERROR, to the client, with the
error-code and e-text fields set to appropriate values. The error
message contents and details are described in Section 5.9.1.
3.1.5. Receipt of KRB_AS_REP message
If the reply message type is KRB_AS_REP, then the client verifies
that the cname and crealm fields in the cleartext portion of the
reply match what it requested. If any padata fields are present,
they may be used to derive the proper secret key to decrypt the
message. The client decrypts the encrypted part of the response
using its secret key, verifies that the nonce in the encrypted part
matches the nonce it supplied in its request (to detect replays). It
also verifies that the sname and srealm in the response match those
in the request, 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.
Proper decryption of the KRB_AS_REP message is not sufficient to
verify the identity of the user; the user and an attacker could
cooperate to generate a KRB_AS_REP format message which decrypts
properly but is not from the proper KDC. If the host wishes to
verify the identity of the user, it must require the user to present
application credentials which can be verified using a securely-stored
secret key. If those credentials can be verified, then the identity
of the user can be assured.
3.1.6. Receipt of KRB_ERROR message
If the reply message type is KRB_ERROR, then the client interprets it
as an error and performs whatever application-specific tasks are
necessary to recover.
3.2. The Client/Server Authentication Exchange
Summary
Message direction Message type Section
Client to Application server KRB_AP_REQ 5.5.1
[optional] Application server to client KRB_AP_REP or 5.5.2
KRB_ERROR 5.9.1
The client/server authentication (CS) exchange is used by network
applications to authenticate the client to the server and vice versa.
The client must have already acquired credentials for the server
using the AS or TGS exchange.
3.2.1. The KRB_AP_REQ message
The KRB_AP_REQ contains authentication information which should be
part of the first message in an authenticated transaction. It
contains a ticket, an authenticator, and some additional bookkeeping
information (see section 5.5.1 for the exact format). The ticket by
itself is insufficient to authenticate a client, since tickets are
passed across the network in cleartext(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.), 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."
3.2.2. Generation of a KRB_AP_REQ message
When a client wishes to initiate authentication to a server, it
obtains (either through a credentials cache, the AS exchange, or the
TGS exchange) a ticket and session key for the desired service. The
client may re-use any tickets it holds until they expire. 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 client may indicate a requirement of mutual authentication or the
use of a session-key based ticket by setting the appropriate flag(s)
in the ap-options field of the message.
The Authenticator is encrypted in the session key and combined with
the ticket to form the KRB_AP_REQ message which is then sent to the
end server along with any additional application-specific
information. See section A.9 for pseudocode.
3.2.3. Receipt of KRB_AP_REQ message
Authentication is based on the server"s current time of day (clocks
must be loosely synchronized), the authenticator, and the ticket.
Several errors are possible. If an error occurs, the server is
expected to reply to the client with a KRB_ERROR message. This
message may be encapsulated in the application protocol if its "raw"
form is not acceptable to the protocol. The format of error messages
is described in section 5.9.1.
The algorithm for verifying authentication information is as follows.
If the message type is not KRB_AP_REQ, the server returns the
KRB_AP_ERR_MSG_TYPE error. If the key version indicated by the Ticket
in the KRB_AP_REQ is not one the server can use (e.g., it indicates
an old key, and the server no longer possesses a copy of the old
key), the KRB_AP_ERR_BADKEYVER error is returned. If the USE-
SESSION-KEY flag is set in the ap-options field, it indicates to the
server that 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.
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 authenticator is decrypted using the session key extracted from
the decrypted ticket. If decryption shows it to have been modified,
the KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm
of the client from the ticket are compared against the same fields in
the authenticator. If they don"t match, the KRB_AP_ERR_BADMATCH
error is returned (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 authenticator
differ by more than the allowable clock skew (e.g., 5 minutes), the
KRB_AP_ERR_SKEW error is returned. 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
returned (Note that the rejection here is restricted to
authenticators from the same principal to the same server. Other
client principals communicating with the same server principal should
not be have their authenticators rejected if the time and microsecond
fields happen to match some other client"s authenticator.). 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.
The server computes the age of the ticket: local (server) time minus
the start time inside the Ticket. If the start time is later than
the current time by more than the allowable clock skew or if the
INVALID flag is set in the ticket, the KRB_AP_ERR_TKT_NYV error is
returned. Otherwise, if the current time is later than end time by
more than the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED error
is returned.
If all these checks succeed without an error, the server is assured
that the client possesses the credentials of the principal named in
the ticket and thus, the client has been authenticated to the server.
See section A.10 for pseudocode.
3.2.4. Generation of a KRB_AP_REP message
Typically, a client"s request will include both the authentication
information and its initial request in the same message, and the
server need not explicitly reply to the KRB_AP_REQ. However, if
mutual authentication (not only authenticating the client to the
server, but also the server to the client) is being performed, the
KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options
field, and a KRB_AP_REP message is required in response. As with the
error message, this message may be encapsulated in the application
protocol if its "raw" form is not acceptable to the application"s
protocol. The timestamp and microsecond field used in the reply must
be the client"s timestamp and microsecond field (as provided in the
authenticator). [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.
3.2.5. Receipt of KRB_AP_REP message
If a KRB_AP_REP message is returned, the client uses the session key
from the credentials obtained for the server (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.
3.2.6. Using the encryption key
After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and
server share an encryption key which can be used by the application.
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
(Implementations of the protocol may wish to provide 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.). 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 checksum type)
to the application programmer; the Kerberos protocol does not
constrain the implementation options.
With both the one-way and mutual authentication exchanges, the peers
should take care not to send sensitive information to each other
without proper assurances. In particular, applications that require
privacy or integrity should use the KRB_AP_REP or KRB_ERROR responses
from the server to client to assure both client and server of their
peer"s identity. If an application protocol requires privacy of its
messages, it can use the KRB_PRIV message (section 3.5). The KRB_SAFE
message (section 3.4) can be used to assure integrity.
3.3. The Ticket-Granting Service (TGS) Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_TGS_REQ 5.4.1
2. Kerberos to client KRB_TGS_REP or 5.4.2
KRB_ERROR 5.9.1
The TGS exchange between a client and the Kerberos Ticket-Granting
Server is initiated by a client when it wishes to obtain
authentication credentials for a given server (which might be
registered in a remote realm), when it wishes to renew or validate an
existing ticket, or when it wishes to obtain a proxy ticket. In the
first case, the client must already have acquired a ticket for the
Ticket-Granting Service using the AS exchange (the ticket-granting
ticket is usually obtained when a client initially authenticates to
the system, such as when a user logs in). The message format for the
TGS exchange is almost identical to that for the AS exchange. The
primary difference is that encryption and decryption in the TGS
exchange does not take place under the client"s key. Instead, the
session key from the ticket-granting ticket or renewable ticket, or
sub-session key from an Authenticator is used. As is the case for
all application servers, expired tickets are not accepted by the TGS,
so once a renewable or ticket-granting ticket expires, the client
must use a separate exchange to obtain valid tickets.
The TGS exchange consists of two messages: A request (KRB_TGS_REQ)
from the client to the Kerberos Ticket-Granting Server, and a reply
(KRB_TGS_REP or KRB_ERROR). The KRB_TGS_REQ message includes
information authenticating the client plus a request for credentials.
The authentication information consists of the authentication header
(KRB_AP_REQ) which includes the client"s previously obtained ticket-
granting, renewable, or invalid ticket. In the ticket-granting
ticket and proxy cases, the request may include one or more of: a
list of network addresses, a collection of typed authorization data
to be sealed in the ticket for authorization use by the application
server, or additional tickets (the use of which are described later).
The TGS reply (KRB_TGS_REP) contains the requested credentials,
encrypted in the session key from the ticket-granting ticket or
renewable ticket, or if present, in the 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.
3.3.1. Generation of KRB_TGS_REQ message
Before sending a request to the ticket-granting service, the client
must determine in which realm the application server is registered
[Note: 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 authentication of the application server to the
client.]. If the client does not already possess a ticket-granting
ticket for the appropriate realm, then one must be obtained. This is
first attempted by requesting a ticket-granting ticket for the
destination realm from 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.
Alternatively, the Kerberos server may return a TGT for