INTERNET-DRAFT Clifford Neuman
John Kohl
Theodore Ts'o
November 18th, 1998
The Kerberos Network Authentication Service (V5)
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Please send comments to: krb-protocol@MIT.EDU
ABSTRACT
This document provides an overview and specification of Version 5 of the
Kerberos protocol, and updates RFC1510 to clarify aspects of the protocol
and its intended use that require more detailed or clearer explanation than
was provided in RFC1510. This document is intended to provide a detailed
description of the protocol, suitable for implementation, together with
descriptions of the appropriate use of protocol messages and fields within
those messages.
This document is not intended to describe Kerberos to the end user, system
administrator, or application developer. Higher level papers describing
Version 5 of the Kerberos system [NT94] and documenting version 4 [SNS88],
are available elsewhere.
OVERVIEW
This INTERNET-DRAFT describes the concepts and model upon which the
Kerberos network authentication system is based. It also specifies Version
5 of the Kerberos protocol.
The motivations, goals, assumptions, and rationale behind most design
decisions are treated cursorily; they are more fully described in a paper
available in IEEE communications [NT94] and earlier in the Kerberos portion
of the Athena Technical Plan [MNSS87]. The protocols have been a proposed
standard and are being considered for advancement for draft standard
through the IETF standard process. Comments are encouraged on the
presentation, but only minor refinements to the protocol as implemented or
extensions that fit within current protocol framework will be considered at
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this time.
Requests for addition to an electronic mailing list for discussion of
Kerberos, kerberos@MIT.EDU, may be addressed to kerberos-request@MIT.EDU.
This mailing list is gatewayed onto the Usenet as the group
comp.protocols.kerberos. Requests for further information, including
documents and code availability, may be sent to info-kerberos@MIT.EDU.
BACKGROUND
The Kerberos model is based in part on Needham and Schroeder's trusted
third-party authentication protocol [NS78] and on modifications suggested
by Denning and Sacco [DS81]. The original design and implementation of
Kerberos Versions 1 through 4 was the work of two former Project Athena
staff members, Steve Miller of Digital Equipment Corporation and Clifford
Neuman (now at the Information Sciences Institute of the University of
Southern California), along with Jerome Saltzer, Technical Director of
Project Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many
other members of Project Athena have also contributed to the work on
Kerberos.
Version 5 of the Kerberos protocol (described in this document) has evolved
from Version 4 based on new requirements and desires for features not
available in Version 4. The design of Version 5 of the Kerberos protocol
was led by Clifford Neuman and John Kohl with much input from the
community. The development of the MIT reference implementation was led at
MIT by John Kohl and Theodore T'so, with help and contributed code from
many others. Since RFC1510 was issued, extensions and revisions to the
protocol have been proposed by many individuals. Some of these proposals
are reflected in this document. Where such changes involved significant
effort, the document cites the contribution of the proposer.
Reference implementations of both version 4 and version 5 of Kerberos are
publicly available and commercial implementations have been developed and
are widely used. Details on the differences between Kerberos Versions 4 and
5 can be found in [KNT92].
1. Introduction
Kerberos provides a means of verifying the identities of principals, (e.g.
a workstation user or a network server) on an open (unprotected) network.
This is accomplished without relying on assertions by the host operating
system, without basing trust on host addresses, without requiring physical
security of all the hosts on the network, and under the assumption that
packets traveling along the network can be read, modified, and inserted at
will[1]. Kerberos performs authentication under these conditions as a
trusted third-party authentication service by using conventional (shared
secret key [2] cryptography. Kerberos extensions have been proposed and
implemented that provide for the use of public key cryptography during
certain phases of the authentication protocol. These extensions provide for
authentication of users registered with public key certification
authorities, and allow the system to provide certain benefits of public key
cryptography in situations where they are needed.
The basic Kerberos authentication process proceeds as follows: A client
sends a request to the authentication server (AS) requesting 'credentials'
for a given server. The AS responds with these credentials, encrypted in
the client's key. The credentials consist of 1) a 'ticket' for the server
and 2) a temporary encryption key (often called a "session key"). The
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client transmits the ticket (which contains the client's identity and a
copy of the session key, all encrypted in the server's key) to the server.
The session key (now shared by the client and server) is used to
authenticate the client, and may optionally be used to authenticate the
server. It may also be used to encrypt further communication between the
two parties or to exchange a separate sub-session key to be used to encrypt
further communication.
Implementation of the basic protocol consists of one or more authentication
servers running on physically secure hosts. The authentication servers
maintain a database of principals (i.e., users and servers) and their
secret keys. Code libraries provide encryption and implement the Kerberos
protocol. In order to add authentication to its transactions, a typical
network application adds one or two calls to the Kerberos library directly
or through the Generic Security Services Application Programming Interface,
GSSAPI, described in separate document. These calls result in the
transmission of the necessary messages to achieve authentication.
The Kerberos protocol consists of several sub-protocols (or exchanges).
There are two basic methods by which a client can ask a Kerberos server for
credentials. In the first approach, the client sends a cleartext request
for a ticket for the desired server to the AS. The reply is sent encrypted
in the client's secret key. Usually this request is for a ticket-granting
ticket (TGT) which can later be used with the ticket-granting server (TGS).
In the second method, the client sends a request to the TGS. The client
uses the TGT to authenticate itself to the TGS in the same manner as if it
were contacting any other application server that requires Kerberos
authentication. The reply is encrypted in the session key from the TGT.
Though the protocol specification describes the AS and the TGS as separate
servers, they are implemented in practice as different protocol entry
points within a single Kerberos server.
Once obtained, credentials may be used to verify the identity of the
principals in a transaction, to ensure the integrity of messages exchanged
between them, or to preserve privacy of the messages. The application is
free to choose whatever protection may be necessary.
To verify the identities of the principals in a transaction, the client
transmits the ticket to the application server. Since the ticket is sent
"in the clear" (parts of it are encrypted, but this encryption doesn't
thwart replay) and might be intercepted and reused by an attacker,
additional information is sent to prove that the message originated with
the principal to whom the ticket was issued. This information (called the
authenticator) is encrypted in the session key, and includes a timestamp.
The timestamp proves that the message was recently generated and is not a
replay. Encrypting the authenticator in the session key proves that it was
generated by a party possessing the session key. Since no one except the
requesting principal and the server know the session key (it is never sent
over the network in the clear) this guarantees the identity of the client.
The integrity of the messages exchanged between principals can also be
guaranteed using the session key (passed in the ticket and contained in the
credentials). This approach provides detection of both replay attacks and
message stream modification attacks. It is accomplished by generating and
transmitting a collision-proof checksum (elsewhere called a hash or digest
function) of the client's message, keyed with the session key. Privacy and
integrity of the messages exchanged between principals can be secured by
encrypting the data to be passed using the session key contained in the
ticket or the subsession key found in the authenticator.
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The authentication exchanges mentioned above require read-only access to
the Kerberos database. Sometimes, however, the entries in the database must
be modified, such as when adding new principals or changing a principal's
key. This is done using a protocol between a client and a third Kerberos
server, the Kerberos Administration Server (KADM). There is also a protocol
for maintaining multiple copies of the Kerberos database. Neither of these
protocols are described in this document.
1.1. Cross-Realm Operation
The Kerberos protocol is designed to operate across organizational
boundaries. A client in one organization can be authenticated to a server
in another. Each organization wishing to run a Kerberos server establishes
its own 'realm'. The name of the realm in which a client is registered is
part of the client's name, and can be used by the end-service to decide
whether to honor a request.
By establishing 'inter-realm' keys, the administrators of two realms can
allow a client authenticated in the local realm to prove its identity to
servers in other realms[3]. The exchange of inter-realm keys (a separate
key may be used for each direction) registers the ticket-granting service
of each realm as a principal in the other realm. A client is then able to
obtain a ticket-granting ticket for the remote realm's ticket-granting
service from its local realm. When that ticket-granting ticket is used, the
remote ticket-granting service uses the inter-realm key (which usually
differs from its own normal TGS key) to decrypt the ticket-granting ticket,
and is thus certain that it was issued by the client's own TGS. Tickets
issued by the remote ticket-granting service will indicate to the
end-service that the client was authenticated from another realm.
A realm is said to communicate with another realm if the two realms share
an inter-realm key, or if the local realm shares an inter-realm key with an
intermediate realm that communicates with the remote realm. An
authentication path is the sequence of intermediate realms that are
transited in communicating from one realm to another.
Realms 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 a database in
order to construct an authentication path between realms.
Although realms are typically hierarchical, intermediate realms may be
bypassed to achieve cross-realm authentication through alternate
authentication paths (these might be established to make communication
between two realms more efficient). It is important for the end-service to
know which realms were transited when deciding how much faith to place in
the authentication process. To facilitate this decision, a field in each
ticket contains the names of the realms that were involved in
authenticating the client.
The application server is ultimately responsible for accepting or rejecting
authentication and should check the transited field. The application server
may choose to rely on the KDC for the application server's realm to check
the transited field. The application server's KDC will set the
TRANSITED-POLICY-CHECKED flag in this case. The KDC's for intermediate
realms may also check the transited field as they issue
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ticket-granting-tickets for other realms, but they are encouraged not to do
so. A client may request that the KDC's not check the transited field by
setting the DISABLE-TRANSITED-CHECK flag. KDC's are encouraged but not
required to honor this flag.
1.2. Authorization
As an authentication service, Kerberos provides a means of verifying the
identity of principals on a network. Authentication is usually useful
primarily as a first step in the process of authorization, determining
whether a client may use a service, which objects the client is allowed to
access, and the type of access allowed for each. Kerberos does not, by
itself, provide authorization. Possession of a client ticket for a service
provides only for authentication of the client to that service, and in the
absence of a separate authorization procedure, it should not be considered
by an application as authorizing the use of that service.
Such separate authorization methods may be implemented as application
specific access control functions and may be based on files such as the
application server, or on separately issued authorization credentials such
as those based on proxies [Neu93] , or on other authorization services.
Applications should not be modified to accept the issuance of a service
ticket by the Kerberos server (even by an modified Kerberos server) as
granting authority to use the service, since such applications may become
vulnerable to the bypass of this authorization check in an environment if
they interoperate with other KDCs or where other options for application
authentication (e.g. the PKTAPP proposal) are provided.
1.3. 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 can prevent an application
from participating in the proper authentication steps. Detection and
solution of such attacks (some of which can appear to be nnot-uncommon
'normal' failure modes for the system) is usually best left to the
human administrators and users.
* Principals must keep their secret keys secret. If an intruder somehow
steals a principal's key, it will be able to masquerade as that
principal or impersonate any server to the legitimate principal.
* 'Password guessing' attacks are not solved by Kerberos. If a user
chooses a poor password, it is possible for an attacker to
successfully mount an offline dictionary attack by repeatedly
attempting to decrypt, with successive entries from a dictionary,
messages obtained which are encrypted under a key derived from the
user's password.
* Each host on the network must have a clock which is 'loosely
synchronized' to the time of the other hosts; this synchronization is
used to reduce the bookkeeping needs of application servers when they
do replay detection. The degree of "looseness" can be configured on a
per-server basis, but is typically on the order of 5 minutes. If the
clocks are synchronized over the network, the clock synchronization
protocol must itself be secured from network attackers.
* Principal identifiers are not recycled on a short-term basis. A
typical mode of access control will use access control lists (ACLs) to
grant permissions to particular principals. If a stale ACL entry
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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.
1.4. 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.
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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.
The server is sometimes refered to as the Application Server.
Service
A resource provided to network clients; often provided by more than
one server (for example, remote file service).
Session key
A temporary encryption key used between two principals, with a
lifetime limited to the duration of a single login "session".
Sub-session key
A temporary encryption key used between two principals, selected and
exchanged by the principals using the session key, and with a lifetime
limited to the duration of a single association.
Ticket
A record that helps a client authenticate itself to a server; it
contains the client's identity, a session key, a timestamp, and other
information, all sealed using the server's secret key. It only serves
to authenticate a client when presented along with a fresh
Authenticator.
2. Ticket flag uses and requests
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 demonstrated knowledge of a client's secret key (e.g. a
password-changing program) can insist that this flag be set in any tickets
they accept, and thus be assured that the client's key was recently
presented to the application client.
The PRE-AUTHENT and HW-AUTHENT flags provide 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
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KDC before use, by presenting them to the KDC in a TGS request with the
VALIDATE option specified. The KDC will only validate tickets after their
starttime has passed. The validation is required so that postdated tickets
which have been stolen before their starttime can be rendered permanently
invalid (through a hot-list mechanism) (see section 3.3.3.1).
2.3. Renewable tickets
Applications may desire to hold tickets which can be valid for long periods
of time. However, this can expose their credentials to potential theft for
equally long periods, and those stolen credentials would be valid until the
expiration time of the ticket(s). Simply using short-lived tickets and
obtaining new ones periodically would require the client to have long-term
access to its secret key, an even greater risk. Renewable tickets can be
used to mitigate the consequences of theft. Renewable tickets have two
"expiration times": the first is when the current instance of the ticket
expires, and the second is the latest permissible value for an individual
expiration time. An application client must periodically (i.e. before it
expires) present a renewable ticket to the KDC, with the RENEW option set
in the KDC request. The KDC will issue a new ticket with a new session key
and a later expiration time. All other fields of the ticket are left
unmodified by the renewal process. When the latest permissible expiration
time arrives, the ticket expires permanently. At each renewal, the KDC may
consult a hot-list to determine if the ticket had been reported stolen
since its last renewal; it will refuse to renew such stolen tickets, and
thus the usable lifetime of stolen tickets is reduced.
The RENEWABLE flag in a ticket is normally only interpreted by the
ticket-granting service (discussed below in section 3.3). It can usually be
ignored by application servers. However, some particularly careful
application servers may wish to disallow renewable tickets.
If a renewable ticket is not renewed by its expiration time, the KDC will
not renew the ticket. The RENEWABLE flag is reset by default, but a client
may request it be set by setting the RENEWABLE option in the KRB_AS_REQ
message. If it is set, then the renew-till field in the ticket contains the
time after which the ticket may not be renewed.
2.4. Postdated tickets
Applications may occasionally need to obtain tickets for use much later,
e.g. a batch submission system would need tickets to be valid at the time
the batch job is serviced. However, it is dangerous to hold valid tickets
in a batch queue, since they will be on-line longer and more prone to
theft. Postdated tickets provide a way to obtain these tickets from the KDC
at job submission time, but to leave them "dormant" until they are
activated and validated by a further request of the KDC. If a ticket theft
were reported in the interim, the KDC would refuse to validate the ticket,
and the thief would be foiled.
The MAY-POSTDATE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers. This
flag must be set in a ticket-granting ticket in order to issue a postdated
ticket based on the presented ticket. It is reset by default; it may be
requested by a client by setting the ALLOW-POSTDATE option in the
KRB_AS_REQ message. This flag does not allow a client to obtain a postdated
ticket-granting ticket; postdated ticket-granting tickets can only by
obtained by requesting the postdating in the KRB_AS_REQ message. The life
(endtime-starttime) of a postdated ticket will be the remaining life of the
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ticket-granting ticket at the time of the request, unless the RENEWABLE
option is also set, in which case it can be the full life
(endtime-starttime) of the ticket-granting ticket. The KDC may limit how
far in the future a ticket may be postdated.
The POSTDATED flag indicates that a ticket has been postdated. The
application server can check the authtime field in the ticket to see when
the original authentication occurred. Some services may choose to reject
postdated tickets, or they may only accept them within a certain period
after the original authentication. When the KDC issues a POSTDATED ticket,
it will also be marked as INVALID, so that the application client must
present the ticket to the KDC to be validated before use.
2.5. Proxiable and proxy tickets
At times it may be necessary for a principal to allow a service to perform
an operation on its behalf. The service must be able to take on the
identity of the client, but only for a particular purpose. A principal can
allow a service to take on the principal's identity for a particular
purpose by granting it a proxy.
The process of granting a proxy using the proxy and proxiable flags is used
to provide credentials for use with specific services. Though conceptually
also a proxy, user's wishing to delegate their identity for ANY purpose
must use the ticket forwarding mechanism described in the next section to
forward a ticket granting ticket.
The PROXIABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers. When
set, this flag tells the ticket-granting server that it is OK to issue a
new ticket (but not a ticket-granting ticket) with a different network
address based on this ticket. This flag is set if requested by the client
on initial authentication. By default, the client will request that it be
set when requesting a ticket granting ticket, and reset when requesting any
other ticket.
This flag allows a client to pass a proxy to a server to perform a remote
request on its behalf, e.g. a print service client can give the print
server a proxy to access the client's files on a particular file server in
order to satisfy a print request.
In order to complicate the use of stolen credentials, Kerberos tickets are
usually valid from only those network addresses specifically included in
the ticket[4]. When granting a proxy, the client must specify the new
network address from which the proxy is to be used, or indicate that the
proxy is to be issued for use from any address.
The PROXY flag is set in a ticket by the TGS when it issues a proxy ticket.
Application servers may check this flag and at their option they may
require additional authentication from the agent presenting the proxy in
order to provide an audit trail.
2.6. Forwardable tickets
Authentication forwarding is an instance of a proxy where the service 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.
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The FORWARDABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers. The
FORWARDABLE flag has an interpretation similar to that of the PROXIABLE
flag, except ticket-granting tickets may also be issued with different
network addresses. This flag is reset by default, but users may request
that it be set by setting the FORWARDABLE option in the AS request when
they request their initial ticket- granting ticket.
This flag allows for authentication forwarding without requiring the user
to enter a password again. If the flag is not set, then authentication
forwarding is not permitted, but the same result can still be achieved if
the user engages in the AS exchange specifying the requested network
addresses and supplies a password.
The FORWARDED flag is set by the TGS when a client presents a ticket with
the FORWARDABLE flag set and requests a forwarded ticket by specifying the
FORWARDED KDC option and supplying a set of addresses for the new ticket.
It is also set in all tickets issued based on tickets with the FORWARDED
flag set. Application servers may choose to process FORWARDED tickets
differently than non-FORWARDED tickets.
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 ticket to be issued for the end server is to be
encrypted in the session key from the a additional second 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 initiated by a client when it wishes to
obtain authentication credentials for a given server but currently holds no
credentials. In its basic form, the client's secret key is used for
encryption and decryption. This exchange is typically used at the
initiation of a login session to obtain credentials for a Ticket-Granting
Server which will subsequently be used to obtain credentials for other
servers (see section 3.3) without requiring further use of the client's
secret key. This exchange is also used to request credentials for services
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which must not be mediated through the Ticket-Granting Service, but rather
require a principal's secret key, such as the password-changing service[5].
This exchange does not by itself provide any assurance of the the identity
of the user[6].
The exchange consists of two messages: KRB_AS_REQ from the client to
Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these
messages are described in sections 5.4.1, 5.4.2, and 5.9.1.
In the request, the client sends (in cleartext) its own identity and the
identity of the server for which it is requesting credentials. 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 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,
fabrications, or modifications of such messages.
Without preautentication, 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 as
described in section [hl<>].
3.1.1. Generation of KRB_AS_REQ message
The client may specify a number of options in the initial request. Among
these options are whether pre-authentication is to be performed; whether
the requested ticket is to be renewable, proxiable, or forwardable; whether
it should be postdated or allow postdating of derivative tickets; and
whether a renewable ticket will be accepted in lieu of a non-renewable
ticket if the requested ticket expiration date cannot be satisfied by a
non-renewable ticket (due to configuration constraints; 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
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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[7].
If there are multiple encryption keys registered for a client in the
Kerberos database (or if the key registered supports multiple encryption
types; e.g. DES-CBC-CRC and DES-CBC-MD5), then the etype field from the AS
request is used by the KDC to select the encryption method to be used for
encrypting the response to the client. If there is more than one supported,
strong encryption type in the etype list, the first valid etype for which
an encryption key is available is used. The encryption method used to
respond to a TGS request is taken from the keytype of the session key found
in the ticket granting ticket.
When the etype field is present in a KDC request, whether an AS or TGS
request, the KDC will attempt to assign the type of the random session key
from the list of methods in the etype field. The KDC will select the
appropriate type using the list of methods provided together with
information from the Kerberos database indicating acceptable encryption
methods for the application server. The KDC will not issue tickets with a
weak session key encryption type.
If the requested start time is absent, indicates a time in the past, or is
within the window of acceptable clock skew for the KDC and the POSTDATE
option has not been specified, then the start time of the ticket is set to
the authentication server's current time. If it indicates a time in the
future beyond the acceptable clock skew, but the POSTDATED option has not
been specified then the error KDC_ERR_CANNOT_POSTDATE is returned.
Otherwise the requested start time is checked against the policy of the
local realm (the administrator might decide to prohibit certain types or
ranges of postdated tickets), and if acceptable, the ticket's start time is
set as requested and the INVALID flag is set in the new ticket. The
postdated ticket must be validated before use by presenting it to the KDC
after the start time has been reached.
The expiration time of the ticket will be set to the 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:
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* 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 post-dated (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 (or are otherwise expected values), and that the host
address field is also correct. It then stores the ticket, session key,
start and expiration times, and other information for later use. The
key-expiration field from the encrypted part of the response may be checked
to notify the user of impending key expiration (the client program could
then suggest remedial action, such as a password change). 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 for the host. If those credentials can
be verified, then the identity of the user can be assured.
3.1.6. Receipt of KRB_ERROR message
If the reply message type is KRB_ERROR, then the client interprets it as an
error and performs whatever application-specific tasks are necessary to
recover.
3.2. The Client/Server Authentication Exchange
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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[DS90], so the authenticator is used to prevent invalid replay of
tickets by proving to the server that the client knows the session key of
the ticket and thus is entitled to use the ticket. The KRB_AP_REQ message
is referred to elsewhere as the 'authentication header.'
3.2.2. Generation of a KRB_AP_REQ message
When a client wishes to initiate authentication to a server, it obtains
(either through a credentials cache, the AS exchange, or the TGS exchange)
a ticket and session key for the desired service. The client may re-use any
tickets it holds until they expire. To use a ticket the client constructs a
new Authenticator from the 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[LGDSR87]. 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
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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[10]. 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[11]. 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
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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.
Passing these checks provides only authentication of the named principal;
it does not imply authorization to use the named service. Applications must
make a separate authorization decisions based upon the authenticated name
of the user, the requested operation, local acces control information such
as that contained in a .k5login or .k5users file, and possibly a separate
distributed authorization service.
3.2.4. Generation of a KRB_AP_REP message
Typically, a client's request will include both the authentication
information and its initial request in the same message, and the server
need not explicitly reply to the KRB_AP_REQ. However, if mutual
authentication (not only authenticating the client to the server, but also
the server to the client) is being performed, the KRB_AP_REQ message will
have MUTUAL-REQUIRED set in its ap-options field, and a KRB_AP_REP message
is required in response. As with the error message, this message may be
encapsulated in the application protocol if its "raw" form is not
acceptable to the application's protocol. The timestamp and microsecond
field used in the reply must be the client's timestamp and microsecond
field (as provided in the authenticator)[12]. If a sequence number is to be
included, it should be randomly chosen as described above for the
authenticator. A subkey may be included if the server desires to negotiate
a different 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[13] to decrypt the message, and
verifies that the timestamp and microsecond fields match those in the
Authenticator it sent to the server. If they match, then the client is
assured that the server is genuine. The sequence number and subkey (if
present) are retained for later use. 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[14]. In some cases,
the use of this session key will be implicit in the protocol; in others the
method of use must be chosen from several alternatives. We leave the
protocol negotiations of how to use the key (e.g. selecting an encryption
or checksum type) to the application programmer; the Kerberos protocol does
not constrain the implementation options, but an example of how this might
be done follows.
One way that an application may choose to negotiate a key to be used for
subequent integrity and privacy protection is for the client to propose a
key in the subkey field of the authenticator. The server can then choose a
key using the proposed key from the client as input, returning the new
subkey in the subkey field of the application reply. This key could then be
used for subsequent communication. To make this example more concrete, if
the encryption method in use required a 56 bit key, and for whatever
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reason, one of the parties was prevented from using a key with more than 40
unknown bits, this method would allow the the party which is prevented from
using more than 40 bits to either propose (if the client) an initial key
with a known quantity for 16 of those bits, or to mask 16 of the bits (if
the server) with the known quantity. The application implementor is warned,
however, that this is only an example, and that an analysis of the
particular crytosystem to be used, and the reasons for limiting the key
length, must be made before deciding whether it is acceptable to mask bits
of the key.
With both the one-way and mutual authentication exchanges, the peers should
take care not to send sensitive information to each other without proper
assurances. In particular, applications that require privacy or integrity
should use the KRB_AP_REP response from the server to client to assure both
client and server of their peer's identity. If an application protocol
requires privacy of its messages, it can use the KRB_PRIV message (section
3.5). The KRB_SAFE message (section 3.4) can be used to assure integrity.
3.3. The Ticket-Granting Service (TGS) Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_TGS_REQ 5.4.1
2. Kerberos to client KRB_TGS_REP or 5.4.2
KRB_ERROR 5.9.1
The TGS exchange between a client and the Kerberos Ticket-Granting Server
is initiated by a client when it wishes to obtain authentication
credentials for a given server (which might be registered in a remote
realm), when it wishes to renew or validate an existing ticket, or when it
wishes to obtain a proxy ticket. In the first case, the client must already
have acquired a ticket for the Ticket-Granting Service using the AS
exchange (the ticket-granting ticket is usually obtained when a client
initially authenticates to the system, such as when a user logs in). The
message format for the TGS exchange is almost identical to that for the AS
exchange. The primary difference is that encryption and decryption in the
TGS exchange does not take place under the client's key. Instead, the
session key from the ticket-granting ticket or renewable ticket, or
sub-session key from an Authenticator is used. As is the case for all
application servers, expired tickets are not accepted by the TGS, so once a
renewable or ticket-granting ticket expires, the client must use a separate
exchange to obtain valid tickets.
The TGS exchange consists of two messages: A request (KRB_TGS_REQ) from the
client to the Kerberos Ticket-Granting Server, and a reply (KRB_TGS_REP or
KRB_ERROR). The KRB_TGS_REQ message includes information authenticating the
client plus a request for credentials. The authentication information
consists of the authentication header (KRB_AP_REQ) which includes the
client's previously obtained ticket-granting, renewable, or invalid ticket.
In the ticket-granting ticket and proxy cases, the request may include one
or more of: a list of network addresses, a collection of typed
authorization data to be sealed in the ticket for authorization use by the
application server, or additional tickets (the use of which are described
later). The TGS reply (KRB_TGS_REP) contains the requested credentials,
encrypted in the session key from the ticket-granting ticket or renewable
ticket, or if present, in the sub-session key from the Authenticator (part
of the authentication header). The KRB_ERROR message contains an error code
and text explaining what went wrong. The KRB_ERROR message is not
encrypted. The KRB_TGS_REP message contains information which can be used
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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[15]. 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 a
Kerberos server for which the client does posess a ticket-granting ticket
(using the KRB_TGS_REQ message recursively). The Kerberos server may return
a TGT for the desired realm in which case one can proceed. Alternatively,
the Kerberos server may return a TGT for a realm which is 'closer' to the
desired realm (further along the standard hierarchical path), 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 the appropriate realm,
it determines which Kerberos servers serve that realm, and contacts one.
The list might be obtained through a configuration file or network service
or it may be generated from the name of the realm; as long as the secret
keys exchanged by realms are kept secret, only denial of service results
from using a false Kerberos server.
As in the AS exchange, the client may specify a number 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 application server
use and additional tickets required by some options.
In preparing the authentication header, the client can select a sub-session
key under which the response from the Kerberos server will be
encrypted[16]. If the sub-session key is not specified, the session key
from the ticket-granting ticket will be used. If the enc-authorization-data
is present, it must be encrypted in the sub-session key, if present, from
the authenticator portion of the authentication header, or if not present,
using the session key from the ticket-granting ticket.
Once prepared, the message is sent to a Kerberos server for the destination
realm. See section A.5 for pseudocode.
3.3.2. Receipt of KRB_TGS_REQ message
The KRB_TGS_REQ message is processed in a manner similar to the KRB_AS_REQ
message, but there are many additional checks to be performed. First, the
Kerberos server must determine which server the accompanying ticket is for
and it must select the appropriate key to decrypt it. For a normal
KRB_TGS_REQ message, it will be for the ticket granting service, and the
TGS's key will be used. If the TGT was issued by another realm, then the
appropriate inter-realm key must be used. If the accompanying ticket is not
a ticket granting ticket for the current realm, but is for an application
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server in the current realm, the RENEW, VALIDATE, or PROXY options are
specified in the request, and the server for which a ticket is requested is
the server named in the accompanying ticket, then the KDC will decrypt the
ticket in the authentication header using the key of the server for which
it was issued. If no ticket can be found in the padata field, the
KDC_ERR_PADATA_TYPE_NOSUPP error is returned.
Once the accompanying ticket has been decrypted, the user-supplied checksum
in the Authenticator must be verified against the contents of the request,
and the message rejected if the checksums do not match (with an error code
of KRB_AP_ERR_MODIFIED) or if the checksum is not keyed or not
collision-proof (with an error code of KRB_AP_ERR_INAPP_CKSUM). If the
checksum type is not supported, the KDC_ERR_SUMTYPE_NOSUPP error is
returned. If the authorization-data are present, they are decrypted using
the sub-session key from the Authenticator.
If any of the decryptions indicate failed integrity checks, the
KRB_AP_ERR_BAD_INTEGRITY error is returned.
3.3.3. Generation of KRB_TGS_REP message
The KRB_TGS_REP message shares its format with the KRB_AS_REP
(KRB_KDC_REP), but with its type field set to KRB_TGS_REP. The detailed
specification is in section 5.4.2.
The response will include a ticket for the requested server. 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.
By default, the address field, the client's name and realm, the list of
transited realms, the time of initial authentication, the expiration time,
and the authorization data of the newly-issued ticket will be copied from
the ticket-granting ticket (TGT) or renewable ticket. If the transited
field needs to be updated, but the transited type is not supported, the
KDC_ERR_TRTYPE_NOSUPP error is returned.
If the request specifies an endtime, then the endtime of the new ticket is
set to the minimum of (a) that request, (b) the endtime from the TGT, and
(c) the starttime of the TGT plus the minimum of the maximum life for the
application server and the maximum life for the local realm (the maximum
life for the requesting principal was already applied when the TGT was
issued). If the new ticket is to be a renewal, then the endtime above is
replaced by the minimum of (a) the value of the renew_till field of the
ticket and (b) the starttime for the new ticket plus the life
(endtime-starttime) of the old ticket.
If the FORWARDED option has been requested, then the resulting ticket will
contain the addresses specified by the client. This option will only be
honored if the FORWARDABLE flag is set in the TGT. The PROXY option is
similar; the resulting ticket will contain the addresses specified by the
client. It will be honored only if the PROXIABLE flag in the TGT is set.
The PROXY option will not be honored on requests for additional
ticket-granting tickets.
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If the requested start time is absent, indicates a time in the past, or is
within the window of acceptable clock skew for the KDC and the POSTDATE
option has not been specified, then the start time of the ticket is set to
the authentication server's current time. If it indicates a time in the
future beyond the acceptable clock skew, but the POSTDATED option has not
been specified or the MAY-POSTDATE flag is not set in the TGT, then the
error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the
ticket-granting ticket has the MAY-POSTDATE flag set, then the resulting
ticket will be postdated and the requested starttime is checked against the
policy of the local realm. If acceptable, the ticket's start time is set as
requested, and the INVALID flag is set. The postdated ticket must be
validated before use by presenting it to the KDC after the starttime has
been reached. However, in no case may the starttime, endtime, or renew-till
time of a newly-issued postdated ticket extend beyond the renew-till time
of the ticket-granting ticket.
If the ENC-TKT-IN-SKEY option has been specified and an additional ticket
has been included in the request, the KDC will decrypt the additional
ticket using the key for the server to which the additional ticket was
issued and verify that it is a ticket-granting ticket. If the name of the
requested server is missing from the request, the name of the client in the
additional ticket will be used. Otherwise the name of the requested server
will be compared to the name of the client in the additional ticket and if
different, the request will be rejected. If the request succeeds, the
session key from the additional ticket will be used to encrypt the new
ticket that is issued instead of using the key of the server for which the
new ticket will be used[17].
If the name of the server in the ticket that is presented to the KDC as
part of the authentication header is not that of the ticket-granting server
itself, the server is registered in the realm of the KDC, and the RENEW
option is requested, then the KDC will verify that the RENEWABLE flag is
set in the ticket, that the INVALID flag is not set in the ticket, and that
the renew_till time is still in the future. If the VALIDATE option is
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, and the
ticket passes the hotlist check described in the next paragraph, the KDC
will issue the appropriate new ticket.
3.3.3.1. Checking for revoked tickets
Whenever a request is made to the ticket-granting server, the presented
ticket(s) is(are) checked against a hot-list of tickets which have been
canceled. This hot-list might be implemented by storing a range of issue
timestamps for 'suspect tickets'; if a presented ticket had an authtime in
that range, it would be rejected. In this way, a stolen ticket-granting
ticket or renewable ticket cannot be used to gain additional tickets
(renewals or otherwise) once the theft has been reported. 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.
The ciphertext part of the response in the KRB_TGS_REP message is encrypted
in the sub-session key from the Authenticator, if present, or the session
key key from the ticket-granting ticket. It is not encrypted using the
client's secret key. Furthermore, the client's key's expiration date and
the key version number fields are left out since these values are stored
along with the client's database record, and that record is not needed to
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satisfy a request based on a ticket-granting ticket. See section A.6 for
pseudocode.
3.3.3.2. Encoding the transited field
If the identity of the server in the TGT that is presented to the KDC as
part of the authentication header is that of the ticket-granting service,
but the TGT was issued from another realm, the KDC will look up the
inter-realm key shared with that realm and use that key to decrypt the
ticket. If the ticket is valid, then the KDC will honor the request,
subject to the constraints outlined above in the section describing the AS
exchange. The realm part of the client's identity will be taken from the
ticket-granting ticket. The name of the realm that issued the
ticket-granting ticket will be added to the transited field of the ticket
to be issued. This is accomplished by reading the transited field from the
ticket-granting ticket (which is treated as an unordered set of realm
names), adding the new realm to the set, then constructing and writing out
its encoded (shorthand) form (this may involve a rearrangement of the
existing encoding).
Note that the ticket-granting service does not add the name of its own
realm. Instead, its responsibility is to add the name of the previous
realm. This prevents a malicious Kerberos server from intentionally leaving
out its own name (it could, however, omit other realms' names).
The names of neither the local realm nor the principal's realm are to be
included in the transited field. They appear elsewhere in the ticket and
both are known to have taken part in authenticating the principal. Since
the endpoints are not included, both local and single-hop inter-realm
authentication result in a transited field that is empty.
Because the name of each realm transited is added to this field, it might
potentially be very long. To decrease the length of this field, its
contents are encoded. The initially supported encoding is optimized for the
normal case of inter-realm communication: a hierarchical arrangement of
realms using either domain or X.500 style realm names. This encoding
(called DOMAIN-X500-COMPRESS) is now described.
Realm names in the transited field are separated by a ",". The ",", "\",
trailing "."s, and leading spaces (" ") are special characters, and if they
are part of a realm name, they must be quoted in the transited field by
preced- ing them with a "\".
A realm name ending with a "." is interpreted as being prepended to the
previous realm. For example, we can encode traversal of EDU, MIT.EDU,
ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:
"EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".
Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points, that
they would not be included in this field, and we would have:
"EDU,MIT.,WASHINGTON.EDU"
A realm name beginning with a "/" is interpreted as being appended to the
previous realm[18]. If it is to stand by itself, then it should be preceded
by a space (" "). For example, we can encode traversal of /COM/HP/APOLLO,
/COM/HP, /COM, and /COM/DEC as:
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"/COM,/HP,/APOLLO, /COM/DEC".
Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints, they
they would not be included in this field, and we would have:
"/COM,/HP"
A null subfield preceding or following a "," indicates that all realms
between the previous realm and the next realm have been traversed[19].
Thus, "," means that all realms along the path between the client and the
server have been traversed. ",EDU, /COM," means that that all realms from
the client's realm up to EDU (in a domain style hierarchy) have been
traversed, and that everything from /COM down to the server's realm in an
X.500 style has also been traversed. This could occur if the EDU realm in
one hierarchy shares an inter-realm key directly with the /COM realm in
another hierarchy.
3.3.4. Receipt of KRB_TGS_REP message
When the KRB_TGS_REP is received by the client, it is processed in the same
manner as the KRB_AS_REP processing described above. The primary difference
is that the ciphertext part of the response must be decrypted using the
session key from the ticket-granting ticket rather than the client's secret
key. See section A.7 for pseudocode.
3.4. The KRB_SAFE Exchange
The KRB_SAFE message may be used by clients requiring the ability to detect
modifications of messages they exchange. It achieves this by including a
keyed collision-proof checksum of the user data and some control
information. The checksum is keyed with an encryption key (usually the last
key negotiated via subkeys, or the session key if no negotiation has
occured).
3.4.1. Generation of a KRB_SAFE message
When an application wishes to send a KRB_SAFE message, it collects its data
and the appropriate control information and computes a checksum over them.
The checksum algorithm should be a 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.
The control information for the KRB_SAFE message includes both a timestamp
and a sequence number. The designer of an application using the KRB_SAFE
message must choose at least one of the two mechanisms. This choice should
be based on the needs of the application protocol.
Sequence numbers are useful when all messages sent will be received by
one's peer. Connection state is presently required to maintain the session
key, so maintaining the next sequence number should not present an
additional problem.
If the application protocol is expected to tolerate lost messages without
them being resent, the use of the timestamp is the appropriate replay
detection mechanism. Using timestamps is also the appropriate mechanism for
multi-cast protocols where all of one's peers share a common sub-session
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key, but some messages will be sent to a subset of one's peers.
After computing the checksum, the client then transmits the information and
checksum to the recipient in the message format specified in section 5.6.1.
3.4.2. Receipt of KRB_SAFE message
When an application receives a KRB_SAFE message, it verifies it as follows.
If any error occurs, an error code is reported for use by the application.
The message is first checked by verifying that the protocol version and
type fields match the current version and KRB_SAFE, respectively. A
mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error.
The application verifies that the checksum used is a collision-proof keyed
checksum, 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 (sent or received[20] ) such tuples,
the KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence number
is included, or a sequence number is expected but not present, the
KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp and usec or
a sequence number is present, a KRB_AP_ERR_MODIFIED error is generated.
Finally, the checksum is computed over the data and control information,
and if it doesn't match the received checksum, a KRB_AP_ERR_MODIFIED error
is generated.
If all the checks succeed, the application is assured that the message was
generated by its peer and was not modi- fied in transit.
3.5. The KRB_PRIV Exchange
The KRB_PRIV message may be used by clients requiring confidentiality and
the ability to detect modifications of exchanged messages. It achieves this
by encrypting the messages and adding control information.
3.5.1. Generation of a KRB_PRIV message
When an application wishes to send a KRB_PRIV message, it collects its data
and the appropriate control information (specified in section 5.7.1) and
encrypts them under an encryption key (usually the last key negotiated via
subkeys, or the session key if no negotiation has 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. Receipt of KRB_PRIV message
When an application receives a KRB_PRIV message, it verifies it as follows.
If any error occurs, an error code is reported for use by the application.
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The message is first checked by verifying that the protocol version and
type fields match the current version and KRB_PRIV, respectively. A
mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error.
The application then decrypts the ciphertext and processes the resultant
plaintext. If decryption shows the data to have been modified, a
KRB_AP_ERR_BAD_INTEGRITY error is generated. The 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.
If all the checks succeed, the application can assume the message was
generated by its peer, and was securely transmitted (without intruders able
to see the unencrypted contents).
3.6. The KRB_CRED Exchange
The KRB_CRED message may be used by clients requiring the ability to send
Kerberos credentials from one host to another. It achieves this by sending
the tickets together with encrypted data containing the session keys and
other information associated with the tickets.
3.6.1. Generation of a KRB_CRED message
When an application wishes to send a KRB_CRED message it first (using the
KRB_TGS exchange) obtains credentials to be sent to the remote host. It
then constructs a KRB_CRED message using the ticket or tickets so obtained,
placing the session key needed to use each ticket in the key field of the
corresponding KrbCredInfo sequence of the encrypted part of the the
KRB_CRED message.
Other information associated with each ticket and obtained during the
KRB_TGS exchange is also placed in the corresponding KrbCredInfo sequence
in the encrypted part of the KRB_CRED message. The current time and, if
specifically required by the application the nonce, s-address, and
r-address fields, are placed in the encrypted part of the KRB_CRED message
which is then encrypted under an encryption key previosuly exchanged in the
KRB_AP exchange (usually the last key negotiated via subkeys, or the
session key if no negotiation has occured).
3.6.2. Receipt of KRB_CRED message
When an application receives a KRB_CRED message, it verifies it. If any
error occurs, an error code is reported for use by the application. The
message is verified by checking that the protocol version and type fields
match the current version and KRB_CRED, respectively. A mismatch generates
a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application then
decrypts the ciphertext and processes the resultant plaintext. If
decryption shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY
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error is generated.
If present or required, the recipient verifies that the operating system's
report of the sender's address matches the sender's address in the message,
and that one of the recipient's addresses appears as the recipient's
address in the message. A failed match for either 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.
If all the checks succeed, the application stores each of the new tickets
in its ticket cache together with the session key and other information in
the corresponding KrbCredInfo sequence from the encrypted part of the
KRB_CRED message.
4. The Kerberos Database
The Kerberos server must have access to a database contain- ing the
principal identifiers and secret keys of principals to be
authenticated[21].
4.1. Database contents
A database entry should contain at least the following fields:
Field Value
name Principal's identifier
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. 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.)
A server may provide KDC service to several realms, as long as the database
representation provides a mechanism to distinguish between principal
records with identifiers which differ only in the realm name.
When an application server's key changes, if the change is routine (i.e.
not the result of disclosure of the old key), the old key should be
retained by the server until all tickets that had been issued using that
key have expired. 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.
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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.
4.2. Additional fields
Project Athena's KDC implementation uses additional fields in its database:
Field Value
K_kvno Kerberos' key version
expiration Expiration date for entry
attributes Bit field of attributes
mod_date Timestamp of last modification
mod_name Modifying principal's identifier
The K_kvno field indicates the key version of the Kerberos master key under
which the principal's secret key is encrypted.
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.)
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 [LGDSR87]), to identify whether a
principal can play the role of a client or server or both, to note whether
a server is appropriate trusted to recieve credentials delegated by a
client, or to identify the 'string to key' conversion algorithm used for a
principal's key[22]. 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.
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.
4.3. Frequently Changing Fields
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).
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Other frequently changing information that can be maintained is the latest
expiration time for any tickets that have been issued using each key. This
field would be used to indicate how long old keys must remain valid to
allow the continued use of outstanding tickets.
4.4. Site Constants
The KDC implementation should have the following configurable constants or
options, to allow an administrator to make and enforce policy decisions:
* 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 maximum allowable total (renewable) lifetime of a ticket
(renew_till - starttime).
* The maximum allowable lifetime of a ticket (endtime - starttime).
* Whether to allow the issue of tickets with empty address fields
(including the ability to specify that such tickets may only be issued
if the request specifies some authorization_data).
* Whether proxiable, forwardable, renewable or post-datable tickets are
to be issued.
5. Message Specifications
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.
Optional field in ASN.1 sequences
For optional integer value and date fields in ASN.1 sequences where a
default value has been specified, certain default values will not be
allowed in the encoding because these values will always be represented
through defaulting by the absence of the optional field. For example, one
will not send a microsecond zero value because one must make sure that
there is only one way to encode this value.
Additional fields in ASN.1 sequences
Implementations receiving Kerberos messages with additional fields present
in ASN.1 sequences should carry the those fields through unmodified when
the message is forwarded. Implementation should drop such fields if the
sequence is reencoded.
5.1. ASN.1 Distinguished Encoding Representation
All uses of ASN.1 in Kerberos shall use the Distinguished Encoding
Representation of the data elements as described in the X.509
specification, section 8.7 [X509-88].
5.3. ASN.1 Base Definitions
The following ASN.1 base definitions are used in the rest of this section.
Note that since the underscore character (_) is not permitted in ASN.1
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names, the hyphen (-) is used in its place for the purposes of ASN.1 names.
Realm ::= GeneralString
PrincipalName ::= SEQUENCE {
name-type[0] INTEGER,
name-string[1] SEQUENCE OF GeneralString
}
Kerberos realms are encoded as GeneralStrings. Realms shall not contain a
character with the code 0 (the ASCII NUL). Most realms will usually consist
of several components separated by periods (.), in the style of Internet
Domain Names, or separated by slashes (/) in the style of X.500 names.
Acceptable forms for realm names are specified in section 7. A
PrincipalName is a typed sequence of components consisting of the following
sub-fields:
name-type
This field specifies the type of name that follows. Pre-defined values
for this field are specified in section 7.2. The name-type should be
treated as a hint. Ignoring the name type, no two names can be the
same (i.e. at least one of the components, or the realm, must be
different). This constraint may be eliminated in the future.
name-string
This field encodes a sequence of components that form a name, each
component encoded as a GeneralString. Taken together, a PrincipalName
and a Realm form a principal identifier. Most PrincipalNames will have
only a few components (typically one or two).
KerberosTime ::= GeneralizedTime
-- Specifying UTC time zone (Z)
The timestamps used in Kerberos are encoded as GeneralizedTimes. An
encoding shall specify the UTC time zone (Z) and shall not include 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.
HostAddress ::= SEQUENCE {
addr-type[0] INTEGER,
address[1] OCTET STRING
}
HostAddresses ::= SEQUENCE OF HostAddress
The host adddress encodings consists of two fields:
addr-type
This field specifies the type of address that follows. Pre-defined
values for this field are specified in section 8.1.
address
This field encodes a single address of type addr-type.
The two forms differ slightly. HostAddress contains exactly one address;
HostAddresses contains a sequence of possibly many addresses.
AuthorizationData ::= SEQUENCE OF SEQUENCE {
ad-type[0] INTEGER,
ad-data[1] OCTET STRING
}
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ad-data
This field contains authorization data to be interpreted according to
the value of the corresponding ad-type field.
ad-type
This field specifies the format for the ad-data subfield. All negative
values are reserved for local use. Non-negative values are reserved
for registered use.
Each sequence of type and data is refered to as an authorization element.
Elements may be application specific, however, there is a common set of
recursive elements that should be understood by all implementations. These
elements contain other elements embedded within them, and the
interpretation of the encapsulating element determines which of the
embedded elements must be interpreted, and which may be ignored.
Definitions for these common elements may be found in Appendix B.
TicketExtensions ::= SEQUENCE OF SEQUENCE {
te-type[0] INTEGER,
te-data[1] OCTET STRING
}
te-data
This field contains opaque data that must be caried with the ticket to
support extensions to the Kerberos protocol including but not limited
to some forms of inter-realm key exchange and plaintext authorization
data. See appendix C for some common uses of this field.
te-type
This field specifies the format for the te-data subfield. All negative
values are reserved for local use. Non-negative values are reserved
for registered use.
APOptions ::= BIT STRING
-- reserved(0),
-- use-session-key(1),
-- mutual-required(2)
TicketFlags ::= BIT STRING
-- 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),
-- transited-policy-checked(12),
-- ok-as-delegate(13)
KDCOptions ::= BIT STRING
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
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-- proxiable(3),
-- proxy(4),
-- allow-postdate(5),
-- postdated(6),
-- unused7(7),
-- renewable(8),
-- unused9(9),
-- unused10(10),
-- unused11(11),
-- unused12(12),
-- unused13(13),
-- disable-transited-check(26),
-- renewable-ok(27),
-- enc-tkt-in-skey(28),
-- renew(30),
-- validate(31)
ASN.1 Bit strings have a length and a value. When used in Kerberos for the
APOptions, TicketFlags, and KDCOptions, the length of the bit string on
generated values should be the smallest number of bits needed to include
the highest order bit that is set (1), but in no case less than 32 bits.
The ASN.1 representation of the bit strings uses unnamed bits, with the
meaning of the individual bits defined by the comments in the specification
above. Implementations should accept values of bit strings of any length
and treat the value of flags corresponding to bits beyond the end of the
bit string as if the bit were reset (0). Comparison of bit strings of
different length should treat the smaller string as if it were padded with
zeros beyond the high order bits to the length of the longer string[23].
LastReq ::= SEQUENCE OF SEQUENCE {
lr-type[0] INTEGER,
lr-value[1] KerberosTime
}
lr-type
This field indicates how the following lr-value field is to be
interpreted. Negative values indicate that the information pertains
only to the responding server. Non-negative values pertain to all
servers for the realm. If the lr-type field is zero (0), then no
information is conveyed by the lr-value subfield. If the absolute
value of the lr-type field is one (1), then the lr-value subfield is
the time of last initial request for a TGT. If it is two (2), then the
lr-value subfield is the time of last initial request. If it is three
(3), then the lr-value subfield is the time of issue for the newest
ticket-granting ticket used. If it is four (4), then the lr-value
subfield is the time of the last renewal. If it is five (5), then the
lr-value subfield is the time of last request (of any type). If it is
(6), then the lr-value subfield is the time when the password will
expire.
lr-value
This field contains the time of the last request. the time must be
interpreted according to the contents of the accompanying lr-type
subfield.
See section 6 for the definitions of Checksum, ChecksumType, EncryptedData,
EncryptionKey, EncryptionType, and KeyType.
5.3. Tickets and Authenticators
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This section describes the format and encryption parameters for tickets and
authenticators. When a ticket or authenticator is included in a protocol
message it is treated as an opaque object.
5.3.1. Tickets
A ticket is a record that helps a client authenticate to a service. A
Ticket contains the following information:
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno[0] INTEGER,
realm[1] Realm,
sname[2] PrincipalName,
enc-part[3] EncryptedData,
extensions[4] TicketExtensions OPTIONAL
}
-- Encrypted part of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags[0] TicketFlags,
key[1] EncryptionKey,
crealm[2] Realm,
cname[3] PrincipalName,
transited[4] TransitedEncoding,
authtime[5] KerberosTime,
starttime[6] KerberosTime OPTIONAL,
endtime[7] KerberosTime,
renew-till[8] KerberosTime OPTIONAL,
caddr[9] HostAddresses OPTIONAL,
authorization-data[10] AuthorizationData OPTIONAL
}
-- encoded Transited field
TransitedEncoding ::= SEQUENCE {
tr-type[0] INTEGER, -- must be
registered
contents[1] OCTET STRING
}
The encoding of EncTicketPart is encrypted in the key shared by Kerberos
and the end server (the server's secret key). See section 6 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 identical.
sname
This field specifies the name part of the server's identity.
enc-part
This field holds the encrypted encoding of the EncTicketPart sequence.
extensions
This optional field contains a sequence of extentions that may be used
to carry information that must be carried with the ticket to support
several extensions, including but not limited to plaintext
authorization data, tokens for exchanging inter-realm keys, and other
information that must be associated with a ticket for use by the
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application server. See Appendix C for definitions of some common
extensions.
Note that some older versions of Kerberos did not support this field.
Because this is an optional field it will not break older clients, but
older clients might strip this field from the ticket before sending it
to the application server. This limits the usefulness of this ticket
field to environments where the ticket will not be parsed and
reconstructed by these older Kerberos clients.
If it is known that the client will strip this field from the ticket,
as an interim measure the KDC may append this field to the end of the
enc-part of the ticket and append a traler indicating the lenght of
the appended extensions field. (this paragraph is open for discussion,
including the form of the traler).
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:
Bit(s) Name Description
0 RESERVED
Reserved for future expansion of this
field.
1 FORWARDABLE
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
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
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
When set, this flag indicates that a
ticket is a proxy.
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5 MAY-POSTDATE
The MAY-POSTDATE flag is normally only
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
This flag indicates that this ticket has
been postdated. The end-service can
check the authtime field to see when the
original authentication occurred.
7 INVALID
This flag indicates that a ticket is
invalid, and it must be validated by the
KDC before use. Application servers
must reject tickets which have this flag
set.
8 RENEWABLE
The RENEWABLE flag is normally only
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.
9 INITIAL
This flag indicates that this ticket was
issued using the AS protocol, and not
issued based on a ticket-granting
ticket.
10 PRE-AUTHENT
This flag indicates that during initial
authentication, the client was authenti-
cated by the KDC before a ticket was
issued. The strength of the pre-
authentication method is not indicated,
but is acceptable to the KDC.
11 HW-AUTHENT
This flag indicates that the protocol
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.
12 TRANSITED This flag indicates that the KDC for the
POLICY-CHECKED realm has checked the transited field
against a realm defined policy for
trusted certifiers. If this flag is
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reset (0), then the application server
must check the transited field itself,
and if unable to do so it must reject
the authentication. If the flag is set
(1) then the application server may skip
its own validation of the transited
field, relying on the validation
performed by the KDC. At its option the
application server may still apply its
own validation based on a separate
policy for acceptance.
13 OK-AS-DELEGATE This flag indicates that the server (not
the client) specified in the ticket has
been determined by policy of the realm
to be a suitable recipient of
delegation. A client can use the
presence of this flag to help it make a
decision whether to delegate credentials
(either grant a proxy or a forwarded
ticket granting ticket) to this server.
The client is free to ignore the value
of this flag. When setting this flag,
an administrator should consider the
Security and placement of the server on
which the service will run, as well as
whether the service requires the use of
delegated credentials.
14 ANONYMOUS
This flag indicates that the principal
named in the ticket is a generic princi-
pal for the realm and does not identify
the individual using the ticket. The
purpose of the ticket is only to
securely distribute a session key, and
not to identify the user. Subsequent
requests using the same ticket and ses-
sion may be considered as originating
from the same user, but requests with
the same username but a different ticket
are likely to originate from different
users.
15-31 RESERVED
Reserved for future use.
key
This field exists in the ticket and the KDC response and is used to
pass the session key from Kerberos to the application server and the
client. The field's encoding is described in section 6.2.
crealm
This field contains the name of the realm in which the client is
registered and in which initial authentication took place.
cname
This field contains the name part of the client's principal
identifier.
transited
This field lists the names of the Kerberos realms that took part in
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authenticating the user to whom this ticket was issued. It does not
specify the order in which the realms were transited. See section
3.3.3.2 for details on how this field encodes the traversed realms.
When the names of CA's are to be embedded inthe transited field (as
specified for some extentions to the protocol), the X.500 names of the
CA's should be mapped into items in the transited field using the
mapping defined by RFC2253.
authtime
This field indicates the time of initial authentication for the named
principal. It is the time of issue for the original ticket on which
this ticket is based. It is included in the ticket to provide
additional information to the end service, and to provide the
necessary information for implementation of a `hot list' service at
the KDC. An end service that is particularly paranoid could refuse to
accept tickets for which the initial authentication occurred "too far"
in the past. This field is also returned as part of the response from
the KDC. When returned as part of the response to initial
authentication (KRB_AS_REP), this is the current time on the Ker-
beros server[24].
starttime
This field in the ticket specifies the time after which the ticket is
valid. Together with endtime, this field specifies the life of the
ticket. If it is absent from the ticket, its value should be treated
as that of the authtime field.
endtime
This field contains the time after which the ticket will not be
honored (its expiration time). Note that individual services may place
their own limits on the life of a ticket and may reject tickets which
have not yet expired. As such, this is really an upper bound on the
expiration time for the ticket.
renew-till
This field is only present in tickets that have the RENEWABLE flag set
in the flags field. It indicates the maximum endtime that may be
included in a renewal. It can be thought of as the absolute expiration
time for the ticket, including all renewals.
caddr
This field in a ticket contains zero (if omitted) or more (if present)
host addresses. These are the addresses from which the ticket can be
used. If there are no addresses, the ticket can be used from any
location. The decision by the KDC to issue or by the end server to
accept zero-address tickets is a policy decision and is left to the
Kerberos and end-service administrators; they may refuse to issue or
accept such tickets. The suggested and default policy, however, is
that such tickets will only be issued or accepted when additional
information that can be used to restrict the use of the ticket is
included in the authorization_data field. Such a ticket is a
capability.
Network addresses are included in the ticket to make it harder for an
attacker to use stolen credentials. Because the session key is not
sent over the network in cleartext, credentials can't be stolen simply
by listening to the network; an attacker has to gain access to the
session key (perhaps through operating system security breaches or a
careless user's unattended session) to make use of stolen tickets.
It is important to note that the network address from which a
connection is received cannot be reliably determined. Even if it could
be, an attacker who has compromised the client's worksta- tion could
use the credentials from there. Including the network addresses only
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makes it more difficult, not impossible, for an attacker to walk off
with stolen credentials and then use them from a "safe" location.
authorization-data
The authorization-data field is used to pass authorization data from
the principal on whose behalf a ticket was issued to the application
service. If no authorization data is included, this field will be left
out. Experience has shown that the name of this field is confusing,
and that a better name for this field would be restrictions.
Unfortunately, it is not possible to change the name of this field at
this time.
This field contains restrictions on any authority obtained on the
basis of authentication using the ticket. It is possible for any
principal in posession of credentials to add entries to the
authorization data field since these entries further restrict what can
be done with the ticket. Such additions can be made by specifying the
additional entries when a new ticket is obtained during the TGS
exchange, or they may be added during chained delegation using the
authorization data field of the authenticator.
Because entries may be added to this field by the holder of
credentials, it is not allowable for the presence of an entry in the
authorization data field of a ticket to amplify the priveleges one
would obtain from using a ticket.
The data in this field may be specific to the end service; the field
will contain the names of service specific objects, and the rights to
those objects. The format for this field is described in section 5.2.
Although Kerberos is not concerned with the format of the contents of
the sub-fields, it does carry type information (ad-type).
By using the authorization_data field, a principal is able to issue a
proxy that is valid for a specific purpose. For example, a client
wishing to print a file can obtain a file server proxy to be passed to
the print server. By specifying the name of the file in the
authorization_data field, the file server knows that the print server
can only use the client's rights when accessing the particular file to
be printed.
A separate service providing authorization or certifying group
membership may be built using the authorization-data field. In this
case, the entity granting authorization (not the authorized entity),
obtains a ticket in its own name (e.g. the ticket is issued in the
name of a privelege server), and this entity adds restrictions on its
own authority and delegates the restricted authority through a proxy
to the client. The client would then present this authorization
credential to the application server separately from the
authentication exchange.
Similarly, if one specifies the authorization-data field of a proxy
and leaves the host addresses blank, the resulting ticket and session
key can be treated as a capability. See [Neu93] for some suggested
uses of this field.
The authorization-data field is optional and does not have to be
included in a ticket.
5.3.2. Authenticators
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An authenticator is a record sent with a ticket to a server to certify the
client's knowledge of the encryption key in the ticket, to help the server
detect replays, and to help choose a "true session key" to use with the
particular session. The encoding is encrypted in the ticket's session key
shared by the client and the server:
-- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE {
authenticator-vno[0] INTEGER,
crealm[1] Realm,
cname[2] PrincipalName,
cksum[3] Checksum OPTIONAL,
cusec[4] INTEGER,
ctime[5] KerberosTime,
subkey[6] EncryptionKey OPTIONAL,
seq-number[7] INTEGER OPTIONAL,
authorization-data[8] AuthorizationData OPTIONAL
}
authenticator-vno
This field specifies the version number for the format of the
authenticator. This document specifies version 5.
crealm and cname
These fields are the same as those described for the ticket in section
5.3.1.
cksum
This field contains a checksum of the the applica- tion data that
accompanies the KRB_AP_REQ.
cusec
This field contains the microsecond part of the client's timestamp.
Its value (before encryption) ranges from 0 to 999999. It often
appears along with ctime. The two fields are used together to specify
a reasonably accurate timestamp.
ctime
This field contains the current time on the client's host.
subkey
This field contains the client's choice for an encryption key which is
to be used to protect this specific application session. Unless an
application specifies otherwise, if this field is left out the session
key from the ticket will be used.
seq-number
This optional field includes the initial sequence number to be used by
the KRB_PRIV or KRB_SAFE messages when sequence numbers are used to
detect replays (It may also be used by application specific messages).
When included in the authenticator this field specifies the initial
sequence number for messages from the client to the server. When
included in the AP-REP message, the initial sequence number is that
for messages from the server to the client. When used in KRB_PRIV or
KRB_SAFE messages, it is incremented by one after each message is
sent. Sequence numbers fall in the range of 0 through 2^32 - 1 and
wrap to zero following the value 2^32 - 1.
For sequence numbers to adequately support the detection of replays
they should be non-repeating, even across connection boundaries. The
initial sequence number should be random and uniformly distributed
across the full space of possible sequence numbers, so that it cannot
be guessed by an attacker and so that it and the successive sequence
numbers do not repeat other sequences.
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authorization-data
This field is the same as described for the ticket in section 5.3.1.
It is optional and will only appear when additional restrictions are
to be placed on the use of a ticket, beyond those carried in the
ticket itself.
5.4. Specifications for the AS and TGS exchanges
This section specifies the format of the messages used in the exchange
between the client and the Kerberos server. The format of possible error
messages appears in section 5.9.1.
5.4.1. KRB_KDC_REQ definition
The KRB_KDC_REQ message has no type of its own. Instead, its type is one of
KRB_AS_REQ or KRB_TGS_REQ depending on whether the request is for an
initial ticket or an additional ticket. In either case, the message is sent
from the client to the Authentication Server to request credentials for a
service.
The message fields are:
AS-REQ ::= [APPLICATION 10] KDC-REQ
TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ ::= SEQUENCE {
pvno[1] INTEGER,
msg-type[2] INTEGER,
padata[3] SEQUENCE OF PA-DATA OPTIONAL,
req-body[4] KDC-REQ-BODY
}
PA-DATA ::= SEQUENCE {
padata-type[1] INTEGER,
padata-value[2] OCTET STRING,
-- might be encoded AP-REQ
}
KDC-REQ-BODY ::= SEQUENCE {
kdc-options[0] KDCOptions,
cname[1] PrincipalName OPTIONAL,
-- Used only in AS-REQ
realm[2] Realm, -- Server's realm
-- Also client's in AS-REQ
sname[3] PrincipalName OPTIONAL,
from[4] KerberosTime OPTIONAL,
till[5] KerberosTime OPTIONAL,
rtime[6] KerberosTime OPTIONAL,
nonce[7] INTEGER,
etype[8] SEQUENCE OF INTEGER,
-- EncryptionType,
-- in preference order
addresses[9] HostAddresses OPTIONAL,
enc-authorization-data[10] EncryptedData OPTIONAL,
-- Encrypted AuthorizationData
-- encoding
additional-tickets[11] SEQUENCE OF Ticket OPTIONAL
}
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The fields in this message are:
pvno
This field is included in each message, and specifies the protocol
version number. This document specifies protocol version 5.
msg-type
This field indicates the type of a protocol message. It will almost
always be the same as the application identifier associated with a
message. It is included to make the identifier more readily accessible
to the application. For the KDC-REQ message, this type will be
KRB_AS_REQ or KRB_TGS_REQ.
padata
The padata (pre-authentication data) field contains a sequence of
authentication information which may be needed before credentials can
be 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 authentication (KRB_AS_REQ) and
most replies (KDC-REP), the padata field will be left out.
This field may also contain information needed by certain extensions
to the Kerberos protocol. For example, it might be used to initially
verify the identity of a client before any response is returned. This
is accomplished with a padata field with padata-type equal to
PA-ENC-TIMESTAMP and padata-value defined as follows:
padata-type ::= PA-ENC-TIMESTAMP
padata-value ::= EncryptedData -- PA-ENC-TS-ENC
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.
[use-specified-kvno item is here for discussion and may be removed] It
may also be used by the client to specify the version of a key that is
being used for accompanying preauthentication, and/or which should be
used to encrypt the reply from the KDC.
PA-USE-SPECIFIED-KVNO ::= Integer
The KDC should only accept and abide by the value of the
use-specified-kvno preauthentication data field when the specified key
is still valid and until use of a new key is confirmed. This situation
is likely to occur primarily during the period during which an updated
key is propagating to other KDC's in a realm.
The padata field can also contain information needed to help the KDC
or the client select the key needed for generating or decrypting the
response. This form of the padata is useful for supporting the use of
certain token cards with Kerberos. The details of such extensions are
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specified in separate documents. See [Pat92] for additional uses of
this field.
padata-type
The padata-type element of the padata field indicates the way that the
padata-value element is to be interpreted. Negative values of
padata-type are reserved for unregistered use; non-negative values are
used for a registered interpretation of the element type.
req-body
This field is a placeholder delimiting the extent of the remaining
fields. If a checksum is to be calculated over the request, it is
calculated over an encoding of the KDC-REQ-BODY sequence which is
enclosed within the req-body field.
kdc-options
This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to the
KDC and indicates the flags that the client wants set on the tickets
as well as other information that is to modify the behavior of the
KDC. Where appropriate, the name of an option may be the same as the
flag that is set by that option. Although in most case, the bit in the
options field will be the same as that in the flags field, this is not
guaranteed, so it is not acceptable to simply copy the options field
to the flags field. There are various checks that must be made before
honoring an option anyway.
The kdc_options field is a bit-field, where the selected options are
indicated by the bit being set (1), and the unselected options and
reserved fields being reset (0). The encoding of the bits is specified
in section 5.2. The options are described in more detail above in
section 2. The meanings of the options are:
Bit(s) Name Description
0 RESERVED
Reserved for future expansion of
this
field.
1 FORWARDABLE
The FORWARDABLE option indicates
that
the ticket to be issued is to have
its
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
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.
3 PROXIABLE
The PROXIABLE option indicates that
the
ticket to be issued is to have its
prox-
iable flag set. It may only be set
on
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the initial request, or in a
subsequent
request if the ticket-granting ticket
on
which it is based is also proxiable.
4 PROXY
The PROXY option indicates that this
is
a request for a proxy. This option
will
only be honored if the
ticket-granting
ticket in the request has its
PROXIABLE
bit set. The address(es) of the
host
from which the resulting ticket is to
be
valid are included in the
addresses
field of the request.
5 ALLOW-POSTDATE
The ALLOW-POSTDATE option indicates
that
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.
6 POSTDATED
The POSTDATED option indicates that
this
is a request for a postdated
ticket.
This option will only be honored if
the
ticket-granting ticket on which
it is based has its MAY-POSTDATE
flag set.
The resulting ticket will also have
its
INVALID flag set, and that flag may
be
reset by a subsequent request to the
KDC
after the starttime in the ticket
has
been reached.
7 UNUSED
This option is presently unused.
8 RENEWABLE
The RENEWABLE option indicates that
the
ticket to be issued is to have
its
RENEWABLE flag set. It may only be
set
on the initial request, or when
the
ticket-granting ticket on which
the
request is based is also renewable.
If
this option is requested, then the
rtime
field in the request contains
the
desired absolute expiration time for
the
ticket.
9-13 UNUSED
These options are presently unused.
14 REQUEST-ANONYMOUS
The REQUEST-ANONYMOUS option
indicates
that the ticket to be issued is not
to
identify the user to which it
was
issued. Instead, the principal
identif-
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ier is to be generic, as specified
by
the policy of the realm (e.g.
usually
anonymous@realm). The purpose of
the
ticket is only to securely distribute
a
session key, and not to identify
the
user. The ANONYMOUS flag on the
ticket
to be returned should be set. If
the
local realms policy does not
permit
anonymous credentials, the request is
to
be rejected.
15-25 RESERVED
Reserved for future use.
26 DISABLE-TRANSITED-CHECK
By default the KDC will check the
transited field of a ticket-granting-
ticket against the policy of the local
realm before it will issue derivative
tickets based on the ticket granting
ticket. If this flag is set in the
request, checking of the transited
field
is disabled. Tickets issued without
the
performance of this check will be
noted
by the reset (0) value of the
TRANSITED-POLICY-CHECKED flag,
indicating to the application server
that the tranisted field must be
checked
locally. KDC's are encouraged but not
required to honor the
DISABLE-TRANSITED-CHECK option.
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.
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.
29 RESERVED
Reserved for future use.
30 RENEW
This option is used only by the
ticket-
granting service. The RENEW
option
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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.
31 VALIDATE
This option is used only by the
ticket-
granting service. The VALIDATE
option
indicates that the request is to
vali-
date 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
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
These fields are the same as those described for the ticket in section
5.3.1. sname may only be absent when the ENC-TKT-IN-SKEY option is
specified. If absent, the name of the server is taken from the name of
the client in the ticket passed as additional-tickets.
enc-authorization-data
The enc-authorization-data, if present (and it can only be present in
the TGS_REQ form), is an encoding of the desired authorization-data
encrypted under the sub-session key if present in the Authenticator,
or alternatively from the session key in the ticket-granting ticket,
both from the padata field in the KRB_AP_REQ.
realm
This field specifies the realm part of the server's principal
identifier. In the AS exchange, this is also the realm part of the
client's principal identifier.
from
This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket
requests when the requested ticket is to be postdated. It specifies
the desired start time for the requested ticket. If this field is
omitted then the KDC should use the current time instead.
till
This field contains the expiration date requested by the client in a
ticket request. It is optional and if omitted the requested ticket is
to have the maximum endtime permitted according to KDC policy for the
parties to the authentication exchange as limited by expiration date
of the ticket granting ticket or other preauthentication credentials.
rtime
This field is the requested renew-till time sent from a client to the
KDC in a ticket request. It is optional.
nonce
This field is part of the KDC request and response. It it intended to
hold a random number generated by the client. If the same number is
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included in the encrypted response from the KDC, it provides evidence
that the response is fresh and has not been replayed by an attacker.
Nonces must never be re-used. Ideally, it should be generated
randomly, but if the correct time is known, it may suffice[25].
etype
This field specifies the desired encryption algorithm to be used in
the response.
addresses
This field is included in the initial request for tickets, and
optionally included in requests for additional tickets from the
ticket-granting server. It specifies the addresses from which the
requested ticket is to be valid. Normally it includes the addresses
for the client's host. If a proxy is requested, this field will
contain other addresses. The contents of this field are usually copied
by the KDC into the caddr field of the resulting ticket.
additional-tickets
Additional tickets may be optionally included in a request to the
ticket-granting server. If the ENC-TKT-IN-SKEY option has been
specified, then the session key from the additional ticket will be
used in place of the server's key to encrypt the new ticket. If more
than one option which requires additional tickets has been specified,
then the additional tickets are used in the order specified by the
ordering of the options bits (see kdc-options, above).
The application code will be either ten (10) or twelve (12) depending on
whether the request is for an initial ticket (AS-REQ) or for an additional
ticket (TGS-REQ).
The optional fields (addresses, authorization-data and additional-tickets)
are only included if necessary to perform the operation specified in the
kdc-options field.
It should be noted that in KRB_TGS_REQ, the protocol version number appears
twice and two different message types appear: the KRB_TGS_REQ message
contains these fields as does the authentication header (KRB_AP_REQ) that
is passed in the padata field.
5.4.2. KRB_KDC_REP definition
The KRB_KDC_REP message format is used for the reply from the KDC for
either an initial (AS) request or a subsequent (TGS) request. There is no
message type for KRB_KDC_REP. Instead, the type will be either KRB_AS_REP
or KRB_TGS_REP. The key used to encrypt the ciphertext part of the reply
depends on the message type. For KRB_AS_REP, the ciphertext is encrypted in
the client's secret key, and the client's key version number is included in
the key version number for the encrypted data. For KRB_TGS_REP, the
ciphertext is encrypted in the sub-session key from the Authenticator, or
if absent, the session key from the ticket-granting ticket used in the
request. In that case, no version number will be present in the
EncryptedData sequence.
The KRB_KDC_REP message contains the following fields:
AS-REP ::= [APPLICATION 11] KDC-REP
TGS-REP ::= [APPLICATION 13] KDC-REP
KDC-REP ::= SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
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padata[2] SEQUENCE OF PA-DATA OPTIONAL,
crealm[3] Realm,
cname[4] PrincipalName,
ticket[5] Ticket,
enc-part[6] EncryptedData
}
EncASRepPart ::= [APPLICATION 25[27]] EncKDCRepPart
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
EncKDCRepPart ::= SEQUENCE {
key[0] EncryptionKey,
last-req[1] LastReq,
nonce[2] INTEGER,
key-expiration[3] KerberosTime OPTIONAL,
flags[4] TicketFlags,
authtime[5] KerberosTime,
starttime[6] KerberosTime OPTIONAL,
endtime[7] KerberosTime,
renew-till[8] KerberosTime OPTIONAL,
srealm[9] Realm,
sname[10] PrincipalName,
caddr[11] HostAddresses OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is either
KRB_AS_REP or KRB_TGS_REP.
padata
This field is described in detail in section 5.4.1. One possible use
for this field is to encode an alternate "mix-in" string to be used
with a string-to-key algorithm (such as is described in section
6.3.2). This ability is useful to ease transitions if a realm name
needs to change (e.g. when a company is acquired); in such a case all
existing password-derived entries in the KDC database would be flagged
as needing a special mix-in string until the next password change.
crealm, cname, srealm and sname
These fields are the same as those described for the ticket in section
5.3.1.
ticket
The newly-issued ticket, from section 5.3.1.
enc-part
This field is a place holder for the ciphertext and related
information that forms the encrypted part of a message. The
description of the encrypted part of the message follows each
appearance of this field. The encrypted part is encoded as described
in section 6.1.
key
This field is the same as described for the ticket in section 5.3.1.
last-req
This field is returned by the KDC and specifies the time(s) of the
last request by a principal. Depending on what information is
available, this might be the last time that a request for a
ticket-granting ticket was made, or the last time that a request based
on a ticket-granting ticket was successful. It also might cover all
servers for a realm, or just the particular server. Some
implementations may display this information to the user to aid in
discovering unauthorized use of one's identity. It is similar in
spirit to the last login time displayed when logging into timesharing
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systems.
nonce
This field is described above in section 5.4.1.
key-expiration
The key-expiration field is part of the response from the KDC and
specifies the time that the client's secret key is due to expire. The
expiration might be the result of password aging or an account
expiration. This field will usually be left out of the TGS reply since
the response to the TGS request is encrypted in a session key and no
client information need be retrieved from the KDC database. It is up
to the application client (usually the login program) to take
appropriate action (such as notifying the user) if the expiration time
is imminent.
flags, authtime, starttime, endtime, renew-till and caddr
These fields are duplicates of those found in the encrypted portion of
the attached ticket (see section 5.3.1), provided so the client may
verify they match the intended request and to assist in proper ticket
caching. If the message is of type KRB_TGS_REP, the caddr field will
only be filled in if the request was for a proxy or forwarded ticket,
or if the user is substituting a subset of the addresses from the
ticket granting ticket. If the client-requested addresses are not
present or not used, then the addresses contained in the ticket will
be the same as those included in the ticket-granting ticket.
5.5. Client/Server (CS) message specifications
This section specifies the format of the messages used for the
authentication of the client to the application server.
5.5.1. KRB_AP_REQ definition
The KRB_AP_REQ message contains the Kerberos protocol version number, the
message type KRB_AP_REQ, an options field to indicate any options in use,
and the ticket and authenticator themselves. The KRB_AP_REQ message is
often referred to as the 'authentication header'.
AP-REQ ::= [APPLICATION 14] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
ap-options[2] APOptions,
ticket[3] Ticket,
authenticator[4] EncryptedData
}
APOptions ::= BIT STRING {
reserved(0),
use-session-key(1),
mutual-required(2)
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_AP_REQ.
ap-options
This field appears in the application request (KRB_AP_REQ) and affects
the way the request is processed. It is a bit-field, where the
selected options are indicated by the bit being set (1), and the
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unselected options and reserved fields being reset (0). The encoding
of the bits is specified in section 5.2. The meanings of the options
are:
Bit(s) Name Description
0 RESERVED
Reserved for future expansion of
this
field.
1 USE-SESSION-KEY
The USE-SESSION-KEY option
indicates
that the ticket the client is
presenting
to a server is encrypted in the
session
key from the server's
ticket-granting
ticket. When this option is not
speci-
fied, the ticket is encrypted in
the
server's secret key.
2 MUTUAL-REQUIRED
The MUTUAL-REQUIRED option tells
the
server that the client requires
mutual
authentication, and that it must
respond
with a KRB_AP_REP message.
3-31 RESERVED
Reserved for future use.
ticket
This field is a ticket authenticating the client to the server.
authenticator
This contains the authenticator, which includes the client's choice of
a subkey. Its encoding is described in section 5.3.2.
5.5.2. KRB_AP_REP definition
The KRB_AP_REP message contains the Kerberos protocol version number, the
message type, and an encrypted time- stamp. The message is sent in in
response to an application request (KRB_AP_REQ) where the mutual
authentication option has been selected in the ap-options field.
AP-REP ::= [APPLICATION 15] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
enc-part[2] EncryptedData
}
EncAPRepPart ::= [APPLICATION 27[29]] SEQUENCE {
ctime[0] KerberosTime,
cusec[1] INTEGER,
subkey[2] EncryptionKey OPTIONAL,
seq-number[3] INTEGER OPTIONAL
}
The encoded EncAPRepPart is encrypted in the shared session key of the
ticket. The optional subkey field can be used in an application-arranged
negotiation to choose a per association session key.
pvno and msg-type
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These fields are described above in section 5.4.1. msg-type is
KRB_AP_REP.
enc-part
This field is described above in section 5.4.2.
ctime
This field contains the current time on the client's host.
cusec
This field contains the microsecond part of the client's timestamp.
subkey
This field contains an encryption key which is to be used to protect
this specific application session. See section 3.2.6 for specifics on
how this field is used to negotiate a key. Unless an application
specifies otherwise, if this field is left out, the sub-session key
from the authenticator, or if also left out, the session key from the
ticket will be used.
5.5.3. Error message reply
If an error occurs while processing the application request, the KRB_ERROR
message will be sent in response. See section 5.9.1 for the format of the
error message. The cname and crealm fields may be left out if the server
cannot determine their appropriate values from the corresponding KRB_AP_REQ
message. If the authenticator was decipherable, the ctime and cusec fields
will contain the values from it.
5.6. KRB_SAFE message specification
This section specifies the format of a message that can be used by either
side (client or server) of an application to send a tamper-proof message to
its peer. It presumes that a session key has previously been exchanged (for
example, by using the KRB_AP_REQ/KRB_AP_REP messages).
5.6.1. KRB_SAFE definition
The KRB_SAFE message contains user data along with a collision-proof
checksum keyed with the last encryption key negotiated via subkeys, or the
session key if no negotiation has occured. The message fields are:
KRB-SAFE ::= [APPLICATION 20] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
safe-body[2] KRB-SAFE-BODY,
cksum[3] Checksum
}
KRB-SAFE-BODY ::= SEQUENCE {
user-data[0] OCTET STRING,
timestamp[1] KerberosTime OPTIONAL,
usec[2] INTEGER OPTIONAL,
seq-number[3] INTEGER OPTIONAL,
s-address[4] HostAddress OPTIONAL,
r-address[5] HostAddress OPTIONAL
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_SAFE.
safe-body
This field is a placeholder for the body of the KRB-SAFE message.
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cksum
This field contains the checksum of the application data. Checksum
details are described in section 6.4. The checksum is computed over
the encoding of the KRB-SAFE sequence. First, the cksum is zeroed and
the checksum is computed over the encoding of the KRB-SAFE sequence,
then the checksum is set to the result of that computation, and
finally the KRB-SAFE sequence is encoded again.
user-data
This field is part of the KRB_SAFE and KRB_PRIV messages and contain
the application specific data that is being passed from the sender to
the recipient.
timestamp
This field is part of the KRB_SAFE and KRB_PRIV messages. Its contents
are the current time as known by the sender of the message. By
checking the timestamp, the recipient of the message is able to make
sure that it was recently generated, and is not a replay.
usec
This field is part of the KRB_SAFE and KRB_PRIV headers. It contains
the microsecond part of the timestamp.
seq-number
This field is described above in section 5.3.2.
s-address
This field specifies the address in use by the sender of the message.
r-address
This field specifies the address in use by the recipient of the
message. It may be omitted for some uses (such as broadcast
protocols), but the recipient may arbitrarily reject such messages.
This field along with s-address can be used to help detect messages
which have been incorrectly or maliciously delivered to the wrong
recipient.
5.7. KRB_PRIV message specification
This section specifies the format of a message that can be used by either
side (client or server) of an application to securely and privately send a
message to its peer. It presumes that a session key has previously been
exchanged (for example, by using the KRB_AP_REQ/KRB_AP_REP messages).
5.7.1. KRB_PRIV definition
The KRB_PRIV message contains user data encrypted in the Session Key. The
message fields are:
KRB-PRIV ::= [APPLICATION 21] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
enc-part[3] EncryptedData
}
EncKrbPrivPart ::= [APPLICATION 28[31]] SEQUENCE {
user-data[0] OCTET STRING,
timestamp[1] KerberosTime OPTIONAL,
usec[2] INTEGER OPTIONAL,
seq-number[3] INTEGER OPTIONAL,
s-address[4] HostAddress OPTIONAL, -- sender's
addr
r-address[5] HostAddress OPTIONAL -- recip's
addr
}
pvno and msg-type
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These fields are described above in section 5.4.1. msg-type is
KRB_PRIV.
enc-part
This field holds an encoding of the EncKrbPrivPart sequence encrypted
under the session key[32]. This encrypted encoding is used for the
enc-part field of the KRB-PRIV message. See section 6 for the format
of the ciphertext.
user-data, timestamp, usec, s-address and r-address
These fields are described above in section 5.6.1.
seq-number
This field is described above in section 5.3.2.
5.8. KRB_CRED message specification
This section specifies the format of a message that can be used to send
Kerberos credentials from one principal to another. It is presented here to
encourage a common mechanism to be used by applications when forwarding
tickets or providing proxies to subordinate servers. It presumes that a
session key has already been exchanged perhaps by using the
KRB_AP_REQ/KRB_AP_REP messages.
5.8.1. KRB_CRED definition
The KRB_CRED message contains a sequence of tickets to be sent and
information needed to use the tickets, including the session key from each.
The information needed to use the tickets is encrypted under an encryption
key previously exchanged or transferred alongside the KRB_CRED message. The
message fields are:
KRB-CRED ::= [APPLICATION 22] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER, -- KRB_CRED
tickets[2] SEQUENCE OF Ticket,
enc-part[3] EncryptedData
}
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {
ticket-info[0] SEQUENCE OF KrbCredInfo,
nonce[1] INTEGER OPTIONAL,
timestamp[2] KerberosTime OPTIONAL,
usec[3] INTEGER OPTIONAL,
s-address[4] HostAddress OPTIONAL,
r-address[5] HostAddress OPTIONAL
}
KrbCredInfo ::= SEQUENCE {
key[0] EncryptionKey,
prealm[1] Realm OPTIONAL,
pname[2] PrincipalName OPTIONAL,
flags[3] TicketFlags OPTIONAL,
authtime[4] KerberosTime OPTIONAL,
starttime[5] KerberosTime OPTIONAL,
endtime[6] KerberosTime OPTIONAL
renew-till[7] KerberosTime OPTIONAL,
srealm[8] Realm OPTIONAL,
sname[9] PrincipalName OPTIONAL,
caddr[10] HostAddresses OPTIONAL
}
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pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_CRED.
tickets
These are the tickets obtained from the KDC specifically for use by
the intended recipient. Successive tickets are paired with the
corresponding KrbCredInfo sequence from the enc-part of the KRB-CRED
message.
enc-part
This field holds an encoding of the EncKrbCredPart sequence encrypted
under the session key shared between the sender and the intended
recipient. This encrypted encoding is used for the enc-part field of
the KRB-CRED message. See section 6 for the format of the ciphertext.
nonce
If practical, an application may require the inclusion of a nonce
generated by the recipient of the message. If the same value is
included as the nonce in the message, it provides evidence that the
message is fresh and has not been replayed by an attacker. A nonce
must never be re-used; it should be generated randomly by the
recipient of the message and provided to the sender of the message in
an application specific manner.
timestamp and usec
These fields specify the time that the KRB-CRED message was generated.
The time is used to provide assurance that the message is fresh.
s-address and r-address
These fields are described above in section 5.6.1. They are used
optionally to provide additional assurance of the integrity of the
KRB-CRED message.
key
This field exists in the corresponding ticket passed by the KRB-CRED
message and is used to pass the session key from the sender to the
intended recipient. The field's encoding is described in section 6.2.
The following fields are optional. If present, they can be associated with
the credentials in the remote ticket file. If left out, then it is assumed
that the recipient of the credentials already knows their value.
prealm and pname
The name and realm of the delegated principal identity.
flags, authtime, starttime, endtime, renew-till, srealm, sname, and caddr
These fields contain the values of the correspond- ing fields from the
ticket found in the ticket field. Descriptions of the fields are
identical to the descriptions in the KDC-REP message.
5.9. Error message specification
This section specifies the format for the KRB_ERROR message. The fields
included in the message are intended to return as much information as
possible about an error. It is not expected that all the information
required by the fields will be available for all types of errors. If the
appropriate information is not available when the message is composed, the
corresponding field will be left out of the message.
Note that since the KRB_ERROR message is not protected by any encryption,
it is quite possible for an intruder to synthesize or modify such a
message. In particular, this means that the client should not use any
fields in this message for security-critical purposes, such as setting a
system clock or generating a fresh authenticator. The message can be
useful, however, for advising a user on the reason for some failure.
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5.9.1. KRB_ERROR definition
The KRB_ERROR message consists of the following fields:
KRB-ERROR ::= [APPLICATION 30] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
ctime[2] KerberosTime OPTIONAL,
cusec[3] INTEGER OPTIONAL,
stime[4] KerberosTime,
susec[5] INTEGER,
error-code[6] INTEGER,
crealm[7] Realm OPTIONAL,
cname[8] PrincipalName OPTIONAL,
realm[9] Realm, -- Correct realm
sname[10] PrincipalName, -- Correct name
e-text[11] GeneralString OPTIONAL,
e-data[12] OCTET STRING OPTIONAL,
e-cksum[13] Checksum OPTIONAL,
e-typed-data[14] SEQUENCE of ETypedData
OPTIONAL
}
ETypedData ::= SEQUENCE {
e-data-type [1] INTEGER,
e-data-value [2] OCTET STRING,
}
pvno and msg-type
These fields are described above in section 5.4.1. msg-type is
KRB_ERROR.
ctime
This field is described above in section 5.4.1.
cusec
This field is described above in section 5.5.2.
stime
This field contains the current time on the server. It is of type
KerberosTime.
susec
This field contains the microsecond part of the server's timestamp.
Its value ranges from 0 to 999999. It appears along with stime. The
two fields are used in conjunction to specify a reasonably accurate
timestamp.
error-code
This field contains the error code returned by Kerberos or the server
when a request fails. To interpret the value of this field see the
list of error codes in section 8. Implementations are encouraged to
provide for national language support in the display of error
messages.
crealm, cname, srealm and sname
These fields are described above in section 5.3.1.
e-text
This field contains additional text to help explain the error code
associated with the failed request (for example, it might include a
principal name which was unknown).
e-data
This field contains additional data about the error for use by the
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application to help it recover from or handle the error. If the
errorcode is KDC_ERR_PREAUTH_REQUIRED, then the e-data field will
contain an encoding of a sequence of padata fields, each corresponding
to an acceptable pre-authentication method and optionally containing
data for the method:
METHOD-DATA ::= SEQUENCE of PA-DATA
If the error-code is KRB_AP_ERR_METHOD, then the e-data field will
contain an encoding of the following sequence:
METHOD-DATA ::= SEQUENCE {
method-type[0] INTEGER,
method-data[1] OCTET STRING OPTIONAL
}
method-type will indicate the required alternate method; method-data
will contain any required additional information.
e-cksum
This field contains an optional checksum for the KRB-ERROR message.
The checksum is calculated over the Kerberos ASN.1 encoding of the
KRB-ERROR message with the checksum absent. The checksum is then added
to the KRB-ERROR structure and the message is re-encoded. The Checksum
should be calculated using the session key from the ticket granting
ticket or service ticket, where available. If the error is in response
to a TGS or AP request, the checksum should be calculated uing the the
session key from the client's ticket. If the error is in response to
an AS request, then the checksum should be calulated using the
client's secret key ONLY if there has been suitable preauthentication
to prove knowledge of the secret key by the client[33]. If a checksum
can not be computed because the key to be used is not available, no
checksum will be included.
e-typed-data
[This field for discussion, may be deleted from final spec] This field
contains optional data that may be used to help the client recover
from the indicated error. [This could contain the METHOD-DATA
specified since I don't think anyone actually uses it yet. It could
also contain the PA-DATA sequence for the preauth required error if we
had a clear way to transition to the use of this field from the use of
the untype e-data field.] For example, this field may specify the key
version of the key used to verify preauthentication:
e-data-type := 20 -- Key version number
e-data-value := Integer -- Key version number used to verify
preauthentication
6. Encryption and Checksum Specifications
The Kerberos protocols described in this document are designed to use
stream encryption ciphers, which can be simulated using commonly available
block encryption ciphers, such as the Data Encryption Standard, [DES77] in
conjunction with block chaining and checksum methods [DESM80]. Encryption
is used to prove the identities of the network entities participating in
message exchanges. The Key Distribution Center for each realm is trusted by
all principals registered in that realm to store a secret key in
confidence. Proof of knowledge of this secret key is used to verify the
authenticity of a principal.
The KDC uses the principal's secret key (in the AS exchange) or a shared
session key (in the TGS exchange) to encrypt responses to ticket requests;
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the ability to obtain the secret key or session key implies the knowledge
of the appropriate keys and the identity of the KDC. The ability of a
principal to decrypt the KDC response and present a Ticket and a properly
formed Authenticator (generated with the session key from the KDC response)
to a service verifies the identity of the principal; likewise the ability
of the service to extract the session key from the Ticket and prove its
knowledge thereof in a response verifies the identity of the service.
The Kerberos protocols generally assume that the encryption used is secure
from cryptanalysis; however, in some cases, the order of fields in the
encrypted portions of messages are arranged to minimize the effects of
poorly chosen keys. It is still important to choose good keys. If keys are
derived from user-typed passwords, those passwords need to be well chosen
to make brute force attacks more difficult. Poorly chosen keys still make
easy targets for intruders.
The following sections specify the encryption and checksum mechanisms
currently defined for Kerberos. The encodings, chaining, and padding
requirements for each are described. For encryption methods, it is often
desirable to place random information (often referred to as a confounder)
at the start of the message. The requirements for a confounder are
specified with each encryption mechanism.
Some encryption systems use a block-chaining method to improve the the
security characteristics of the ciphertext. However, these chaining methods
often don't provide an integrity check upon decryption. Such systems (such
as DES in CBC mode) must be augmented with a checksum of the plain-text
which can be verified at decryption and used to detect any tampering or
damage. Such checksums should be good at detecting burst errors in the
input. If any damage is detected, the decryption routine is expected to
return an error indicating the failure of an integrity check. Each
encryption type is expected to provide and verify an appropriate checksum.
The specification of each encryption method sets out its checksum
requirements.
Finally, where a key is to be derived from a user's password, an algorithm
for converting the password to a key of the appropriate type is included.
It is desirable for the string to key function to be one-way, and for the
mapping to be different in different realms. This is important because
users who are registered in more than one realm will often use the same
password in each, and it is desirable that an attacker compromising the
Kerberos server in one realm not obtain or derive the user's key in
another.
For an discussion of the integrity characteristics of the candidate
encryption and checksum methods considered for Kerberos, the the reader is
referred to [SG92].
6.1. Encryption Specifications
The following ASN.1 definition describes all encrypted messages. The
enc-part field which appears in the unencrypted part of messages in section
5 is a sequence consisting of an encryption type, an optional key version
number, and the ciphertext.
EncryptedData ::= SEQUENCE {
etype[0] INTEGER, -- EncryptionType
kvno[1] INTEGER OPTIONAL,
cipher[2] OCTET STRING -- ciphertext
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}
etype
This field identifies which encryption algorithm was used to encipher
the cipher. Detailed specifications for selected encryption types
appear later in this section.
kvno
This field contains the version number of the key under which data is
encrypted. It is only present in messages encrypted under long lasting
keys, such as principals' secret keys.
cipher
This field contains the enciphered text, encoded as an OCTET STRING.
The cipher field is generated by applying the specified encryption
algorithm to data composed of the message and algorithm-specific inputs.
Encryption mechanisms defined for use with Kerberos must take sufficient
measures to guarantee the integrity of the plaintext, and we recommend they
also take measures to protect against precomputed dictionary attacks. If
the encryption algorithm is not itself capable of doing so, the protections
can often be enhanced by adding a checksum and a confounder.
The suggested format for the data to be encrypted includes a confounder, a
checksum, the encoded plaintext, and any necessary padding. The msg-seq
field contains the part of the protocol message described in section 5
which is to be encrypted. The confounder, checksum, and padding are all
untagged and untyped, and their length is exactly sufficient to hold the
appropriate item. The type and length is implicit and specified by the
particular encryption type being used (etype). The format for the data to
be encrypted is described in the following diagram:
+-----------+----------+-------------+-----+
|confounder | check | msg-seq | pad |
+-----------+----------+-------------+-----+
The format cannot be described in ASN.1, but for those who prefer an
ASN.1-like notation:
CipherText ::= ENCRYPTED SEQUENCE {
confounder[0] UNTAGGED[35] OCTET STRING(conf_length) OPTIONAL,
check[1] UNTAGGED OCTET STRING(checksum_length) OPTIONAL,
msg-seq[2] MsgSequence,
pad UNTAGGED OCTET STRING(pad_length) OPTIONAL
}
One generates a random confounder of the appropriate length, placing it in
confounder; zeroes out check; calculates the appropriate checksum over
confounder, check, and msg-seq, placing the result in check; adds the
necessary padding; then encrypts using the specified encryption type and
the appropriate key.
Unless otherwise specified, a definition of an encryption algorithm that
specifies a checksum, a length for the confounder field, or an octet
boundary for padding uses this ciphertext format[36]. Those fields which
are not specified will be omitted.
In the interest of allowing all implementations using a particular
encryption type to communicate with all others using that type, the
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specification of an encryption type defines any checksum that is needed as
part of the encryption process. If an alternative checksum is to be used, a
new encryption type must be defined.
Some cryptosystems require additional information beyond the key and the
data to be encrypted. For example, DES, when used in cipher-block-chaining
mode, requires an initialization vector. If required, the description for
each encryption type must specify the source of such additional
information. 6.2. Encryption Keys
The sequence below shows the encoding of an encryption key:
EncryptionKey ::= SEQUENCE {
keytype[0] INTEGER,
keyvalue[1] OCTET STRING
}
keytype
This field specifies the type of encryption key that follows in the
keyvalue field. It will almost always correspond to the encryption
algorithm used to generate the EncryptedData, though more than one
algorithm may use the same type of key (the mapping is many to one).
This might happen, for example, if the encryption algorithm uses an
alternate checksum algorithm for an integrity check, or a different
chaining mechanism.
keyvalue
This field contains the key itself, encoded as an octet string.
All negative values for the encryption key type are reserved for local use.
All non-negative values are reserved for officially assigned type fields
and interpreta- tions.
6.3. Encryption Systems
6.3.1. The NULL Encryption System (null)
If no encryption is in use, the encryption system is said to be the NULL
encryption system. In the NULL encryption system there is no checksum,
confounder or padding. The ciphertext is simply the plaintext. The NULL Key
is used by the null encryption system and is zero octets in length, with
keytype zero (0).
6.3.2. DES in CBC mode with a CRC-32 checksum (des-cbc-crc)
The des-cbc-crc encryption mode encrypts information under the Data
Encryption Standard [DES77] using the cipher block chaining mode [DESM80].
A CRC-32 checksum (described in ISO 3309 [ISO3309]) is applied to the
confounder and message sequence (msg-seq) and placed in the cksum field.
DES blocks are 8 bytes. As a result, the data to be encrypted (the
concatenation of confounder, checksum, and message) must be padded to an 8
byte boundary before encryption. The details of the encryption of this data
are identical to those for the des-cbc-md5 encryption mode.
Note that, since the CRC-32 checksum is not collision-proof, an attacker
could use a probabilistic chosen-plaintext attack to generate a valid
message even if a confounder is used [SG92]. The use of collision-proof
checksums is recommended for environments where such attacks represent a
significant threat. The use of the CRC-32 as the checksum for ticket or
authenticator is no longer mandated as an interoperability requirement for
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Kerberos Version 5 Specification 1 (See section 9.1 for specific details).
6.3.3. DES in CBC mode with an MD4 checksum (des-cbc-md4)
The des-cbc-md4 encryption mode encrypts information under the Data
Encryption Standard [DES77] using the cipher block chaining mode [DESM80].
An MD4 checksum (described in [MD492]) is applied to the confounder and
message sequence (msg-seq) and placed in the cksum field. DES blocks are 8
bytes. As a result, the data to be encrypted (the concatenation of
confounder, checksum, and message) must be padded to an 8 byte boundary
before encryption. The details of the encryption of this data are identical
to those for the des-cbc-md5 encryption mode.
6.3.4. DES in CBC mode with an MD5 checksum (des-cbc-md5)
The des-cbc-md5 encryption mode encrypts information under the Data
Encryption Standard [DES77] using the cipher block chaining mode [DESM80].
An MD5 checksum (described in [MD5-92].) is applied to the confounder and
message sequence (msg-seq) and placed in the cksum field. DES blocks are 8
bytes. As a result, the data to be encrypted (the concatenation of
confounder, checksum, and message) must be padded to an 8 byte boundary
before encryption.
Plaintext and DES ciphtertext are encoded as blocks of 8 octets which are
concatenated to make the 64-bit inputs for the DES algorithms. The first
octet supplies the 8 most significant bits (with the octet's MSbit used as
the DES input block's MSbit, etc.), the second octet the next 8 bits, ...,
and the eighth octet supplies the 8 least significant bits.
Encryption under DES using cipher block chaining requires an additional
input in the form of an initialization vector. Unless otherwise specified,
zero should be used as the initialization vector. Kerberos' use of DES
requires an 8 octet confounder.
The DES specifications identify some 'weak' and 'semi-weak' keys; those
keys shall not be used for encrypting messages for use in Kerberos.
Additionally, because of the way that keys are derived for the encryption
of checksums, keys shall not be used that yield 'weak' or 'semi-weak' keys
when eXclusive-ORed with the hexadecimal constant F0F0F0F0F0F0F0F0.
A DES key is 8 octets of data, with keytype one (1). This consists of 56
bits of key, and 8 parity bits (one per octet). The key is encoded as a
series of 8 octets written in MSB-first order. The bits within the key are
also encoded in MSB order. For example, if the encryption key is
(B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8) where
B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 are the
parity bits, the first octet of the key would be B1,B2,...,B7,P1 (with B1
as the MSbit). [See the FIPS 81 introduction for reference.]
String to key transformation
To generate a DES key from a text string (password), a "salt" is
concatenated to the text string, and then padded with ASCII nulls to an 8
byte boundary. This "salt" is normally the realm and each component of the
principal's name appended. However, sometimes different salts are used ---
for example, when a realm is renamed, or if a user changes her username, or
for compatibility with Kerberos V4 (whose string-to-key algorithm uses a
null string for the salt). This string is then fan-folded and
eXclusive-ORed with itself to form an 8 byte DES key. Before
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eXclusive-ORing a block, every byte is shifted one bit to the left to leave
the lowest bit zero. The key is the "corrected" by correcting the parity on
the key, and if the key matches a 'weak' or 'semi-weak' key as described in
the DES specification, it is eXclusive-ORed with the constant
00000000000000F0. This key is then used to generate a DES CBC checksum on
the initial string (with the salt appended). The result of the CBC checksum
is the "corrected" as described above to form the result which is return as
the key. Pseudocode follows:
name_to_default_salt(realm, name) {
s = realm
for(each component in name) {
s = s + component;
}
return s;
}
key_correction(key) {
fixparity(key);
if (is_weak_key_key(key))
key = key XOR 0xF0;
return(key);
}
string_to_key(string,salt) {
odd = 1;
s = string + salt;
tempkey = NULL;
pad(s); /* with nulls to 8 byte boundary */
for(8byteblock in s) {
if(odd == 0) {
odd = 1;
reverse(8byteblock)
}
else odd = 0;
left shift every byte in 8byteblock one bit;
tempkey = tempkey XOR 8byteblock;
}
tempkey = key_correction(tempkey);
key = key_correction(DES-CBC-check(s,tempkey));
return(key);
}
6.3.5. Triple DES with HMAC-SHA1 Kerberos Encryption Type with Key
Derivation [Horowitz]
NOTE: This description currently refers to documents, the contents of which
might be bettered included by value in this spec. The description below was
provided by Marc Horowitz, and the form in which it will finally appear is
yet to be determined. This description is included in this version of the
draft because it does describe the implemenation ready for use with the MIT
implementation. Note also that the encryption identifier has been left
unspecified here because the value from Marc Horowitz's spec conflicted
with some other impmenentations implemented based on perevious versions of
the specification.
This encryption type is based on the Triple DES cryptosystem, the HMAC-SHA1
[Krawczyk96] message authentication algorithm, and key derivation for
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Kerberos V5 [HorowitzB96].
The des3-cbc-hmac-sha1 encryption type has been assigned the value ??. The
hmac-sha1-des3 checksum type has been assigned the value 12.
Encryption Type des3-cbc-hmac-sha1
EncryptedData using this type must be generated as described in
[Horowitz96]. The encryption algorithm is Triple DES in Outer-CBC mode. The
keyed hash algorithm is HMAC-SHA1. Unless otherwise specified, a zero IV
must be used. If the length of the input data is not a multiple of the
block size, zero octets must be used to pad the plaintext to the next
eight-octet boundary. The counfounder must be eight random octets (one
block).
Checksum Type hmac-sha1-des3
Checksums using this type must be generated as described in [Horowitz96].
The keyed hash algorithm is HMAC-SHA1.
Common Requirements
The EncryptionKey value is 24 octets long. The 7 most significant bits of
each octet contain key bits, and the least significant bit is the inverse
of the xor of the key bits.
For the purposes of key derivation, the block size is 64 bits, and the key
size is 168 bits. The 168 bits output by key derivation are converted to an
EncryptionKey value as follows. First, the 168 bits are divided into three
groups of 56 bits, which are expanded individually into 64 bits as follows:
1 2 3 4 5 6 7 p
9 10 11 12 13 14 15 p
17 18 19 20 21 22 23 p
25 26 27 28 29 30 31 p
33 34 35 36 37 38 39 p
41 42 43 44 45 46 47 p
49 50 51 52 53 54 55 p
56 48 40 32 24 16 8 p
The "p" bits are parity bits computed over the data bits. The output of the
three expansions are concatenated to form the EncryptionKey value.
When the HMAC-SHA1 of a string is computed, the key is used in the
EncryptedKey form.
Key Derivation
In the Kerberos protocol, cryptographic keys are used in a number of
places. In order to minimize the effect of compromising a key, it is
desirable to use a different key for each of these places. Key derivation
[Horowitz96] can be used to construct different keys for each operation
from the keys transported on the network. For this to be possible, a small
change to the specification is necessary.
This section specifies a profile for the use of key derivation [Horowitz96]
with Kerberos. For each place where a key is used, a ``key usage'' must is
specified for that purpose. The key, key usage, and encryption/checksum
type together describe the transformation from plaintext to ciphertext, or
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plaintext to checksum.
Key Usage Values
This is a complete list of places keys are used in the kerberos protocol,
with key usage values and RFC 1510 section numbers:
1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the
client key (section 5.4.1)
2. AS-REP Ticket and TGS-REP Ticket (includes tgs session key or
application session key), encrypted with the service key
(section 5.4.2)
3. AS-REP encrypted part (includes tgs session key or application
session key), encrypted with the client key (section 5.4.2)
4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs
session key (section 5.4.1)
5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the tgs
authenticator subkey (section 5.4.1)
6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed
with the tgs session key (sections 5.3.2, 5.4.1)
7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes tgs
authenticator subkey), encrypted with the tgs session key
(section 5.3.2)
8. TGS-REP encrypted part (includes application session key),
encrypted with the tgs session key (section 5.4.2)
9. TGS-REP encrypted part (includes application session key),
encrypted with the tgs authenticator subkey (section 5.4.2)
10. AP-REQ Authenticator cksum, keyed with the application session
key (section 5.3.2)
11. AP-REQ Authenticator (includes application authenticator
subkey), encrypted with the application session key (section
5.3.2)
12. AP-REP encrypted part (includes application session subkey),
encrypted with the application session key (section 5.5.2)
13. KRB-PRIV encrypted part, encrypted with a key chosen by the
application (section 5.7.1)
14. KRB-CRED encrypted part, encrypted with a key chosen by the
application (section 5.6.1)
15. KRB-SAVE cksum, keyed with a key chosen by the application
(section 5.8.1)
18. KRB-ERROR checksum (e-cksum in section 5.9.1)
19. AD-KDCIssued checksum (ad-checksum in appendix B.1)
20. Checksum for Mandatory Ticket Extensions (appendix B.6)
21. Checksum in Authorization Data in Ticket Extensions (appendix B.7)
Key usage values between 1024 and 2047 (inclusive) are reserved for
application use. Applications should use even values for encryption and odd
values for checksums within this range.
A few of these key usages need a little clarification. A service which
receives an AP-REQ has no way to know if the enclosed Ticket was part of an
AS-REP or TGS-REP. Therefore, key usage 2 must always be used for
generating a Ticket, whether it is in response to an AS- REQ or TGS-REQ.
There might exist other documents which define protocols in terms of the
RFC1510 encryption types or checksum types. Such documents would not know
about key usages. In order that these documents continue to be meaningful
until they are updated, key usages 1024 and 1025 must be used to derive
keys for encryption and checksums, respectively. New protocols defined in
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terms of the Kerberos encryption and checksum types should use their own
key usages. Key usages may be registered with IANA to avoid conflicts. Key
usages must be unsigned 32 bit integers. Zero is not permitted.
Defining Cryptosystems Using Key Derivation
Kerberos requires that the ciphertext component of EncryptedData be
tamper-resistant as well as confidential. This implies encryption and
integrity functions, which must each use their own separate keys. So, for
each key usage, two keys must be generated, one for encryption (Ke), and
one for integrity (Ki):
Ke = DK(protocol key, key usage | 0xAA)
Ki = DK(protocol key, key usage | 0x55)
where the protocol key is from the EncryptionKey from the wire protocol,
and the key usage is represented as a 32 bit integer in network byte order.
The ciphertest must be generated from the plaintext as follows:
ciphertext = E(Ke, confounder | plaintext | padding) |
H(Ki, confounder | plaintext | padding)
The confounder and padding are specific to the encryption algorithm E.
When generating a checksum only, there is no need for a confounder or
padding. Again, a new key (Kc) must be used. Checksums must be generated
from the plaintext as follows:
Kc = DK(protocol key, key usage | 0x99)
MAC = H(Kc, plaintext)
Note that each enctype is described by an encryption algorithm E and a
keyed hash algorithm H, and each checksum type is described by a keyed hash
algorithm H. HMAC, with an appropriate hash, is recommended for use as H.
Key Derivation from Passwords
The well-known constant for password key derivation must be the byte string
{0x6b 0x65 0x72 0x62 0x65 0x72 0x6f 0x73}. These values correspond to the
ASCII encoding for the string "kerberos".
6.4. Checksums
The following is the ASN.1 definition used for a checksum:
Checksum ::= SEQUENCE {
cksumtype[0] INTEGER,
checksum[1] OCTET STRING
}
cksumtype
This field indicates the algorithm used to generate the accompanying
checksum.
checksum
This field contains the checksum itself, encoded as an octet string.
Detailed specification of selected checksum types appear later in this
section. Negative values for the checksum type are reserved for local use.
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All non-negative values are reserved for officially assigned type fields
and interpretations.
Checksums used by Kerberos can be classified by two properties: whether
they are collision-proof, and whether they are keyed. It is infeasible to
find two plaintexts which generate the same checksum value for a
collision-proof checksum. A key is required to perturb or initialize the
algorithm in a keyed checksum. To prevent message-stream modification by an
active attacker, unkeyed checksums should only be used when the checksum
and message will be subsequently encrypted (e.g. the checksums defined as
part of the encryption algorithms covered earlier in this section).
Collision-proof checksums can be made tamper-proof if the checksum value is
encrypted before inclusion in a message. In such cases, the composition of
the checksum and the encryption algorithm must be considered a separate
checksum algorithm (e.g. RSA-MD5 encrypted using DES is a new checksum
algorithm of type RSA-MD5-DES). For most keyed checksums, as well as for
the encrypted forms of unkeyed collision-proof checksums, Kerberos prepends
a confounder before the checksum is calculated.
6.4.1. The CRC-32 Checksum (crc32)
The CRC-32 checksum calculates a checksum based on a cyclic redundancy
check as described in ISO 3309 [ISO3309]. The resulting checksum is four
(4) octets in length. The CRC-32 is neither keyed nor collision-proof. The
use of this checksum is not recommended. An attacker using a probabilistic
chosen-plaintext attack as described in [SG92] might be able to generate an
alternative message that satisfies the checksum. The use of collision-proof
checksums is recommended for environments where such attacks represent a
significant threat.
6.4.2. The RSA MD4 Checksum (rsa-md4)
The RSA-MD4 checksum calculates a checksum using the RSA MD4 algorithm
[MD4-92]. The algorithm takes as input an input message of arbitrary length
and produces as output a 128-bit (16 octet) checksum. RSA-MD4 is believed
to be collision-proof.
6.4.3. RSA MD4 Cryptographic Checksum Using DES (rsa-md4-des)
The RSA-MD4-DES checksum calculates a keyed collision-proof checksum by
prepending an 8 octet confounder before the text, applying the RSA MD4
checksum algorithm, and encrypting the confounder and the checksum using
DES in cipher-block-chaining (CBC) mode using a variant of the key, where
the variant is computed by eXclusive-ORing the key with the constant
F0F0F0F0F0F0F0F0[39]. The initialization vector should be zero. The
resulting checksum is 24 octets long (8 octets of which are redundant).
This checksum is tamper-proof and believed to be collision-proof.
The DES specifications identify some weak keys' and 'semi-weak keys'; those
keys shall not be used for generating RSA-MD4 checksums for use in
Kerberos.
The format for the checksum is described in the follow- ing diagram:
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| des-cbc(confounder + rsa-md4(confounder+msg),key=var(key),iv=0) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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The format cannot be described in ASN.1, but for those who prefer an
ASN.1-like notation:
rsa-md4-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE {
confounder[0] UNTAGGED OCTET STRING(8),
check[1] UNTAGGED OCTET STRING(16)
}
6.4.4. The RSA MD5 Checksum (rsa-md5)
The RSA-MD5 checksum calculates a checksum using the RSA MD5 algorithm.
[MD5-92]. The algorithm takes as input an input message of arbitrary length
and produces as output a 128-bit (16 octet) checksum. RSA-MD5 is believed
to be collision-proof.
6.4.5. RSA MD5 Cryptographic Checksum Using DES (rsa-md5-des)
The RSA-MD5-DES checksum calculates a keyed collision-proof checksum by
prepending an 8 octet confounder before the text, applying the RSA MD5
checksum algorithm, and encrypting the confounder and the checksum using
DES in cipher-block-chaining (CBC) mode using a variant of the key, where
the variant is computed by eXclusive-ORing the key with the hexadecimal
constant F0F0F0F0F0F0F0F0. The initialization vector should be zero. The
resulting checksum is 24 octets long (8 octets of which are redundant).
This checksum is tamper-proof and believed to be collision-proof.
The DES specifications identify some 'weak keys' and 'semi-weak keys';
those keys shall not be used for encrypting RSA-MD5 checksums for use in
Kerberos.
The format for the checksum is described in the following diagram:
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| des-cbc(confounder + rsa-md5(confounder+msg),key=var(key),iv=0) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
The format cannot be described in ASN.1, but for those who prefer an
ASN.1-like notation:
rsa-md5-des-checksum ::= ENCRYPTED UNTAGGED SEQUENCE {
confounder[0] UNTAGGED OCTET STRING(8),
check[1] UNTAGGED OCTET STRING(16)
}
6.4.6. DES cipher-block chained checksum (des-mac)
The DES-MAC checksum is computed by prepending an 8 octet confounder to the
plaintext, performing a DES CBC-mode encryption on the result using the key
and an initialization vector of zero, taking the last block of the
ciphertext, prepending the same confounder and encrypting the pair using
DES in cipher-block-chaining (CBC) mode using a a variant of the key, where
the variant is computed by eXclusive-ORing the key with the hexadecimal
constant F0F0F0F0F0F0F0F0. The initialization vector should be zero. The
resulting checksum is 128 bits (16 octets) long, 64 bits of which are
redundant. This checksum is tamper-proof and collision-proof.
The format for the checksum is described in the following diagram:
+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+
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| des-cbc(confounder + des-mac(conf+msg,iv=0,key),key=var(key),iv=0) |
+--+--+--+--+--+--+--+--+-----+-----+-----+-----+-----+-----+-----+-----+
The format cannot be described in ASN.1, but for those who prefer an
ASN.1-like notation:
des-mac-checksum ::= ENCRYPTED UNTAGGED SEQUENCE {
confounder[0] UNTAGGED OCTET STRING(8),
check[1] UNTAGGED OCTET STRING(8)
}
The DES specifications identify some 'weak' and 'semi-weak' keys; those
keys shall not be used for generating DES-MAC checksums for use in
Kerberos, nor shall a key be used whose variant is 'weak' or 'semi-weak'.
6.4.7. RSA MD4 Cryptographic Checksum Using DES alternative (rsa-md4-des-k)
The RSA-MD4-DES-K checksum calculates a keyed collision-proof checksum by
applying the RSA MD4 checksum algorithm and encrypting the results using
DES in cipher-block-chaining (CBC) mode using a DES key as both key and
initialization vector. The resulting checksum is 16 octets long. This
checksum is tamper-proof and believed to be collision-proof. Note that this
checksum type is the old method for encoding the RSA-MD4-DES checksum and
it is no longer recommended.
6.4.8. DES cipher-block chained checksum alternative (des-mac-k)
The DES-MAC-K checksum is computed by performing a DES CBC-mode encryption
of the plaintext, and using the last block of the ciphertext as the
checksum value. It is keyed with an encryption key and an initialization
vector; any uses which do not specify an additional initialization vector
will use the key as both key and initialization vector. The resulting
checksum is 64 bits (8 octets) long. This checksum is tamper-proof and
collision-proof. Note that this checksum type is the old method for
encoding the DES-MAC checksum and it is no longer recommended. The DES
specifications identify some 'weak keys' and 'semi-weak keys'; those keys
shall not be used for generating DES-MAC checksums for use in Kerberos.
7. Naming Constraints
7.1. Realm Names
Although realm names are encoded as GeneralStrings and although a realm can
technically select any name it chooses, interoperability across realm
boundaries requires agreement on how realm names are to be assigned, and
what information they imply.
To enforce these conventions, each realm must conform to the conventions
itself, and it must require that any realms with which inter-realm keys are
shared also conform to the conventions and require the same from its
neighbors.
Kerberos realm names are case sensitive. Realm names that differ only in
the case of the characters are not equivalent. There are presently four
styles of realm names: domain, X500, other, and reserved. Examples of each
style follow:
domain: ATHENA.MIT.EDU (example)
X500: C=US/O=OSF (example)
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other: NAMETYPE:rest/of.name=without-restrictions (example)
reserved: reserved, but will not conflict with above
Domain names must look like domain names: they consist of components
separated by periods (.) and they contain neither colons (:) nor slashes
(/). Domain names must be converted to upper case when used as realm names.
X.500 names contain an equal (=) and cannot contain a colon (:) before the
equal. The realm names for X.500 names will be string representations of
the names with components separated by slashes. Leading and trailing
slashes will not be included.
Names that fall into the other category must begin with a prefix that
contains no equal (=) or period (.) and the prefix must be followed by a
colon (:) and the rest of the name. All prefixes must be assigned before
they may be used. Presently none are assigned.
The reserved category includes strings which do not fall into the first
three categories. All names in this category are reserved. It is unlikely
that names will be assigned to this category unless there is a very strong
argument for not using the 'other' category.
These rules guarantee that there will be no conflicts between the various
name styles. The following additional constraints apply to the assignment
of realm names in the domain and X.500 categories: the name of a realm for
the domain or X.500 formats must either be used by the organization owning
(to whom it was assigned) an Internet domain name or X.500 name, or in the
case that no such names are registered, authority to use a realm name may
be derived from the authority of the parent realm. For example, if there is
no domain name for E40.MIT.EDU, then the administrator of the MIT.EDU realm
can authorize the creation of a realm with that name.
This is acceptable because the organization to which the parent is assigned
is presumably the organization authorized to assign names to its children
in the X.500 and domain name systems as well. If the parent assigns a realm
name without also registering it in the domain name or X.500 hierarchy, it
is the parent's responsibility to make sure that there will not in the
future exists a name identical to the realm name of the child unless it is
assigned to the same entity as the realm name.
7.2. Principal Names
As was the case for realm names, conventions are needed to ensure that all
agree on what information is implied by a principal name. The name-type
field that is part of the principal name indicates the kind of information
implied by the name. The name-type should be treated 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). The following name types are
defined:
name-type value meaning
NT-UNKNOWN 0 Name type not known
NT-PRINCIPAL 1 General principal name (e.g. username, or DCE
principal)
NT-SRV-INST 2 Service and other unique instance (krbtgt)
NT-SRV-HST 3 Service with host name as instance (telnet,
rcommands)
NT-SRV-XHST 4 Service with slash-separated host name components
NT-UID 5 Unique ID
NT-X500-PRINCIPAL 6 Encoded X.509 Distingished name [RFC 1779]
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When a name implies no information other than its uniqueness at a
particular time the name type PRINCIPAL should be used. The principal name
type should be used for users, and it might also be used for a unique
server. If the name is a unique machine generated ID that is guaranteed
never to be reassigned then the name type of UID should be used (note that
it is generally a bad idea to reassign names of any type since stale
entries might remain in access control lists).
If the first component of a name identifies a service and the remaining
components identify an instance of the service in a server specified
manner, then the name type of SRV-INST should be used. An example of this
name type is the Kerberos ticket-granting service whose name has a first
component of krbtgt and a second component identifying the realm for which
the ticket is valid.
If instance is a single component following the service name and the
instance identifies the host on which the server is running, then the name
type SRV-HST should be used. This type is typically used for Internet
services such as telnet and the Berkeley R commands. If the separate
components of the host name appear as successive components following the
name of the service, then the name type SRV-XHST should be used. This type
might be used to identify servers on hosts with X.500 names where the slash
(/) might otherwise be ambiguous.
A name type of NT-X500-PRINCIPAL should be used when a name from an X.509
certificiate is translated into a Kerberos name. The encoding of the X.509
name as a Kerberos principal shall conform to the encoding rules specified
in RFC 2253.
A name type of UNKNOWN should be used when the form of the name is not
known. When comparing names, a name of type UNKNOWN will match principals
authenticated with names of any type. A principal authenticated with a name
of type UNKNOWN, however, will only match other names of type UNKNOWN.
Names of any type with an initial component of 'krbtgt' are reserved for
the Kerberos ticket granting service. See section 8.2.3 for the form of
such names.
7.2.1. Name of server principals
The principal identifier for a server on a host will generally be composed
of two parts: (1) the realm of the KDC with which the server is registered,
and (2) a two-component name of type NT-SRV-HST if the host name is an
Internet domain name or a multi-component name of type NT-SRV-XHST if the
name of the host is of a form such as X.500 that allows slash (/)
separators. The first component of the two- or multi-component name will
identify the service and the latter components will identify the host.
Where the name of the host is not case sensitive (for example, with
Internet domain names) the name of the host must be lower case. If
specified by the application protocol for services such as telnet and the
Berkeley R commands which run with system privileges, the first component
may be the string 'host' instead of a service specific identifier. When a
host has an official name and one or more aliases, the official name of the
host must be used when constructing the name of the server principal.
8. Constants and other defined values
8.1. Host address types
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All negative values for the host address type are reserved for local use.
All non-negative values are reserved for officially assigned type fields
and interpretations.
The values of the types for the following addresses are chosen to match the
defined address family constants in the Berkeley Standard Distributions of
Unix. They can be found in with symbolic names AF_xxx (where xxx is an
abbreviation of the address family name).
Internet (IPv4) Addresses
Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded in MSB
order. The type of IPv4 addresses is two (2).
Internet (IPv6) Addresses [Westerlund]
IPv6 addresses are 128-bit (16-octet) quantities, encoded in MSB order. The
type of IPv6 addresses is twenty-four (24). [RFC1883] [RFC1884]. The
following addresses (see [RFC1884]) MUST not appear in any Kerberos packet:
* the Unspecified Address
* the Loopback Address
* Link-Local addresses
IPv4-mapped IPv6 addresses MUST be represented as addresses of type 2.
CHAOSnet addresses
CHAOSnet addresses are 16-bit (2-octet) quantities, encoded in MSB order.
The type of CHAOSnet addresses is five (5).
ISO addresses
ISO addresses are variable-length. The type of ISO addresses is seven (7).
Xerox Network Services (XNS) addresses
XNS addresses are 48-bit (6-octet) quantities, encoded in MSB order. The
type of XNS addresses is six (6).
AppleTalk Datagram Delivery Protocol (DDP) addresses
AppleTalk DDP addresses consist of an 8-bit node number and a 16-bit
network number. The first octet of the address is the node number; the
remaining two octets encode the network number in MSB order. The type of
AppleTalk DDP addresses is sixteen (16).
DECnet Phase IV addresses
DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order. The
type of DECnet Phase IV addresses is twelve (12).
Netbios addresses
Netbios addresses are 16-octet addresses typically composed of 1 to 15
characters, trailing blank (ascii char 20) filled, with a 16th octet of
0x0. The type of Netbios addresses is 20 (0x14).
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8.2. KDC messages
8.2.1. UDP/IP transport
When contacting a Kerberos server (KDC) for a KRB_KDC_REQ request using UDP
IP transport, the client shall send a UDP datagram containing only an
encoding of the request to port 88 (decimal) at the KDC's IP address; the
KDC will respond with a reply datagram containing only an encoding of the
reply message (either a KRB_ERROR or a KRB_KDC_REP) to the sending port at
the sender's IP address. Kerberos servers supporting IP transport must
accept UDP requests on port 88 (decimal). The response to a request made
through UDP/IP transport must also use UDP/IP transport.
8.2.2. TCP/IP transport [Westerlund,Danielsson]
Kerberos servers (KDC's) should accept TCP requests on port 88 (decimal)
and clients should support the sending of TCP requests on port 88
(decimal). When the KRB_KDC_REQ message is sent to the KDC over a TCP
stream, a new connection will be established for each authentication
exchange (request and response). The KRB_KDC_REP or KRB_ERROR message will
be returned to the client on the same TCP stream that was established for
the request. The response to a request made through TCP/IP transport must
also use TCP/IP transport. Implementors should note that some extentions to
the Kerberos protocol will not work if any implementation not supporting
the TCP transport is involved (client or KDC). Implementors are strongly
urged to support the TCP transport on both the client and server and are
advised that the current notation of "should" support will likely change in
the future to must support. The KDC may close the TCP stream after sending
a response, but may leave the stream open if it expects a followup - in
which case it may close the stream at any time if resource constratints or
other factors make it desirable to do so. Care must be taken in managing
TCP/IP connections with the KDC to prevent denial of service attacks based
on the number of TCP/IP connections with the KDC that remain open. If
multiple exchanges with the KDC are needed for certain forms of
preauthentication, multiple TCP connections may be required. A client may
close the stream after receiving response, and should close the stream if
it does not expect to send followup messages. The client must be prepared
to have the stream closed by the KDC at anytime, in which case it must
simply connect again when it is ready to send subsequent messages.
The first four octets of the TCP stream used to transmit the request
request will encode in network byte order the length of the request
(KRB_KDC_REQ), and the length will be followed by the request itself. The
response will similarly be preceeded by a 4 octet encoding in network byte
order of the length of the KRB_KDC_REP or the KRB_ERROR message and will be
followed by the KRB_KDC_REP or the KRB_ERROR response. If the sign bit is
set on integer represented by the first 4 octets, then the next 4 octets
will be read, extending the length of the field by another 4 octets (less 1
bit).
8.2.3. OSI transport
During authentication of an OSI client to an OSI server, the mutual
authentication of an OSI server to an OSI client, the transfer of
credentials from an OSI client to an OSI server, or during exchange of
private or integrity checked messages, Kerberos protocol messages may be
treated as opaque objects and the type of the authentication mechanism will
be:
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OBJECT IDENTIFIER ::= {iso (1), org(3), dod(6),internet(1),
security(5),kerberosv5(2)}
Depending on the situation, the opaque object will be an authentication
header (KRB_AP_REQ), an authentication reply (KRB_AP_REP), a safe message
(KRB_SAFE), a private message (KRB_PRIV), or a credentials message
(KRB_CRED). The opaque data contains an application code as specified in
the ASN.1 description for each message. The application code may be used by
Kerberos to determine the message type.
8.2.3. Name of the TGS
The principal identifier of the ticket-granting service shall be composed
of three parts: (1) the realm of the KDC issuing the TGS ticket (2) a
two-part name of type NT-SRV-INST, with the first part "krbtgt" and the
second part the name of the realm which will accept the ticket-granting
ticket. For example, a ticket-granting ticket issued by the ATHENA.MIT.EDU
realm to be used to get tickets from the ATHENA.MIT.EDU KDC has a principal
identifier of "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU")
(name). A ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be
used to get tickets from the MIT.EDU realm has a principal identifier of
"ATHENA.MIT.EDU" (realm), ("krbtgt", "MIT.EDU") (name).
8.3. Protocol constants and associated values
The following tables list constants used in the protocol and defines their
meanings. Ranges are specified in the "specification" section that limit
the values of constants for which values are defined here. This allows
implementations to make assumptions about the maximum values that will be
received for these constants. Implementation receiving values outside the
range specified in the "specification" section may reject the request, but
they must recover cleanly.
Encryption type etype value block size minimum pad size confounder
size
NULL 0 1 0 0
des-cbc-crc 1 8 4 8
des-cbc-md4 2 8 0 8
des-cbc-md5 3 8 0 8
4
des3-cbc-md5 5 8 0 8
6
des3-cbc-sha1 7 8 0 8
sign-dsa-generate 8 (pkinit)
encrypt-rsa-priv 9 (pkinit)
encrypt-rsa-pub 10 (pkinit)
rsa-pub-md5 11 (pkinit)
rsa-pub-sha1 12 (pkinit)
des3kd-cbc-sha1 ?? 8 0 8
ENCTYPE_PK_CROSS 48 (reserved for pkcross)
0x8003
Checksum type sumtype value checksum size
CRC32 1 4
rsa-md4 2 16
rsa-md4-des 3 24
des-mac 4 16
des-mac-k 5 8
rsa-md4-des-k 6 16
rsa-md5 7 16
rsa-md5-des 8 24
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rsa-md5-des3 9 24
hmac-sha1-des3 12 20 (I had this as 10, is it
12)
padata type padata-type value
PA-TGS-REQ 1
PA-ENC-TIMESTAMP 2
PA-PW-SALT 3
4
PA-ENC-UNIX-TIME 5
PA-SANDIA-SECUREID 6
PA-SESAME 7
PA-OSF-DCE 8
PA-CYBERSAFE-SECUREID 9
PA-AFS3-SALT 10
PA-ETYPE-INFO 11
SAM-CHALLENGE 12 (sam/otp)
SAM-RESPONSE 13 (sam/otp)
PA-PK-AS-REQ 14 (pkinit)
PA-PK-AS-REP 15 (pkinit)
PA-PK-AS-SIGN 16 (pkinit)
PA-PK-KEY-REQ 17 (pkinit)
PA-PK-KEY-REP 18 (pkinit)
PA-USE-SPECIFIED-KVNO 20
authorization data type ad-type value
AD-KDC-ISSUED 1
AD-INTENDED-FOR-SERVER 2
AD-INTENDED-FOR-APPLICATION-CLASS 3
AD-IF-RELEVANT 4
AD-OR 5
AD-MANDATORY-TICKET-EXTENSIONS 6
AD-IN-TICKET-EXTENSIONS 7
reserved values 8-63
OSF-DCE 64
SESAME 65
Ticket Extension Types
TE-TYPE-NULL 0 Null ticket extension
TE-TYPE-EXTERNAL-ADATA 1 Integrity protected authorization data
2 TE-TYPE-PKCROSS-KDC (I have reservations)
TE-TYPE-PKCROSS-CLIENT 3 PKCROSS cross realm key ticket
TE-TYPE-CYBERSAFE-EXT 4 Assigned to CyberSafe Corp
5 TE-TYPE-DEST-HOST (I have reservations)
alternate authentication type method-type value
reserved values 0-63
ATT-CHALLENGE-RESPONSE 64
transited encoding type tr-type value
DOMAIN-X500-COMPRESS 1
reserved values all others
Label Value Meaning or MIT code
pvno 5 current Kerberos protocol version number
message types
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KRB_AS_REQ 10 Request for initial authentication
KRB_AS_REP 11 Response to KRB_AS_REQ request
KRB_TGS_REQ 12 Request for authentication based on TGT
KRB_TGS_REP 13 Response to KRB_TGS_REQ request
KRB_AP_REQ 14 application request to server
KRB_AP_REP 15 Response to KRB_AP_REQ_MUTUAL
KRB_SAFE 20 Safe (checksummed) application message
KRB_PRIV 21 Private (encrypted) application message
KRB_CRED 22 Private (encrypted) message to forward
credentials
KRB_ERROR 30 Error response
name types
KRB_NT_UNKNOWN 0 Name type not known
KRB_NT_PRINCIPAL 1 Just the name of the principal as in DCE, or for
users
KRB_NT_SRV_INST 2 Service and other unique instance (krbtgt)
KRB_NT_SRV_HST 3 Service with host name as instance (telnet,
rcommands)
KRB_NT_SRV_XHST 4 Service with host as remaining components
KRB_NT_UID 5 Unique ID
KRB_NT_X500_PRINCIPAL 6 Encoded X.509 Distingished name [RFC 2253]
error codes
KDC_ERR_NONE 0 No error
KDC_ERR_NAME_EXP 1 Client's entry in database has expired
KDC_ERR_SERVICE_EXP 2 Server's entry in database has expired
KDC_ERR_BAD_PVNO 3 Requested protocol version number not
supported
KDC_ERR_C_OLD_MAST_KVNO 4 Client's key encrypted in old master key
KDC_ERR_S_OLD_MAST_KVNO 5 Server's key encrypted in old master key
KDC_ERR_C_PRINCIPAL_UNKNOWN 6 Client not found in Kerberos database
KDC_ERR_S_PRINCIPAL_UNKNOWN 7 Server not found in Kerberos database
KDC_ERR_PRINCIPAL_NOT_UNIQUE 8 Multiple principal entries in database
KDC_ERR_NULL_KEY 9 The client or server has a null key
KDC_ERR_CANNOT_POSTDATE 10 Ticket not eligible for postdating
KDC_ERR_NEVER_VALID 11 Requested start time is later than end
time
KDC_ERR_POLICY 12 KDC policy rejects request
KDC_ERR_BADOPTION 13 KDC cannot accommodate requested option
KDC_ERR_ETYPE_NOSUPP 14 KDC has no support for encryption type
KDC_ERR_SUMTYPE_NOSUPP 15 KDC has no support for checksum type
KDC_ERR_PADATA_TYPE_NOSUPP 16 KDC has no support for padata type
KDC_ERR_TRTYPE_NOSUPP 17 KDC has no support for transited type
KDC_ERR_CLIENT_REVOKED 18 Clients credentials have been revoked
KDC_ERR_SERVICE_REVOKED 19 Credentials for server have been revoked
KDC_ERR_TGT_REVOKED 20 TGT has been revoked
KDC_ERR_CLIENT_NOTYET 21 Client not yet valid - try again later
KDC_ERR_SERVICE_NOTYET 22 Server not yet valid - try again later
KDC_ERR_KEY_EXPIRED 23 Password has expired - change password
to reset
KDC_ERR_PREAUTH_FAILED 24 Pre-authentication information was
invalid
KDC_ERR_PREAUTH_REQUIRED 25 Additional pre-authenticationrequired
[40]
KDC_ERR_SERVER_NOMATCH 26 Requested server and ticket don't match
KDC_ERR_MUST_USE_USER2USER 27 Server principal valid for user2user
only
KDC_ERR_PATH_NOT_ACCPETED 28 KDC Policy rejects transited path
KRB_AP_ERR_BAD_INTEGRITY 31 Integrity check on decrypted field
failed
KRB_AP_ERR_TKT_EXPIRED 32 Ticket expired
KRB_AP_ERR_TKT_NYV 33 Ticket not yet valid
KRB_AP_ERR_REPEAT 34 Request is a replay
KRB_AP_ERR_NOT_US 35 The ticket isn't for us
KRB_AP_ERR_BADMATCH 36 Ticket and authenticator don't match
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KRB_AP_ERR_SKEW 37 Clock skew too great
KRB_AP_ERR_BADADDR 38 Incorrect net address
KRB_AP_ERR_BADVERSION 39 Protocol version mismatch
KRB_AP_ERR_MSG_TYPE 40 Invalid msg type
KRB_AP_ERR_MODIFIED 41 Message stream modified
KRB_AP_ERR_BADORDER 42 Message out of order
KRB_AP_ERR_BADKEYVER 44 Specified version of key is not
available
KRB_AP_ERR_NOKEY 45 Service key not available
KRB_AP_ERR_MUT_FAIL 46 Mutual authentication failed
KRB_AP_ERR_BADDIRECTION 47 Incorrect message direction
KRB_AP_ERR_METHOD 48 Alternative authentication method
required
KRB_AP_ERR_BADSEQ 49 Incorrect sequence number in message
KRB_AP_ERR_INAPP_CKSUM 50 Inappropriate type of checksum in
message
KRB_AP_PATH_NOT_ACCEPTED 51 Policy rejects transited path
KRB_ERR_RESPONSE_TOO_BIG 52 Response too big for UDP, retry with TCP
KRB_ERR_GENERIC 60 Generic error (description in e-text)
KRB_ERR_FIELD_TOOLONG 61 Field is too long for this
implementation
KDC_ERROR_CLIENT_NOT_TRUSTED 62 (pkinit)
KDC_ERROR_KDC_NOT_TRUSTED 63 (pkinit)
KDC_ERROR_INVALID_SIG 64 (pkinit)
KDC_ERR_KEY_TOO_WEAK 65 (pkinit)
KDC_ERR_CERTIFICATE_MISMATCH 66 (pkinit)
9. Interoperability requirements
Version 5 of the Kerberos protocol supports a myriad of options. Among
these are multiple encryption and checksum types, alternative encoding
schemes for the transited field, optional mechanisms for
pre-authentication, the handling of tickets with no addresses, options for
mutual authentication, user to user authentication, support for proxies,
forwarding, postdating, and renewing tickets, the format of realm names,
and the handling of authorization data.
In order to ensure the interoperability of realms, it is necessary to
define a minimal configuration which must be supported by all
implementations. This minimal configuration is subject to change as
technology does. For example, if at some later date it is discovered that
one of the required encryption or checksum algorithms is not secure, it
will be replaced.
9.1. Specification 2
This section defines the second specification of these options.
Implementations which are configured in this way can be said to support
Kerberos Version 5 Specification 2 (5.1). Specification 1 (depricated) may
be found in RFC1510.
Transport
TCP/IP and UDP/IP transport must be supported by KDCs claiming conformance
to specification 2. Kerberos clients claiming conformance to specification
2 must support UDP/IP transport for messages with the KDC and should
support TCP/IP transport.
Encryption and checksum methods
The following encryption and checksum mechanisms must be supported.
Implementations may support other mechanisms as well, but the additional
mechanisms may only be used when communicating with principals known to
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also support them: This list is to be determined.
Encryption: DES-CBC-MD5
Checksums: CRC-32, DES-MAC, DES-MAC-K, and DES-MD5
Realm Names
All implementations must understand hierarchical realms in both the
Internet Domain and the X.500 style. When a ticket granting ticket for an
unknown realm is requested, the KDC must be able to determine the names of
the intermediate realms between the KDCs realm and the requested realm.
Transited field encoding
DOMAIN-X500-COMPRESS (described in section 3.3.3.2) must be supported.
Alternative encodings may be supported, but they may be used only when that
encoding is supported by ALL intermediate realms.
Pre-authentication methods
The TGS-REQ method must be supported. The TGS-REQ method is not used on the
initial request. The PA-ENC-TIMESTAMP method must be supported by clients
but whether it is enabled by default may be determined on a realm by realm
basis. If not used in the initial request and the error
KDC_ERR_PREAUTH_REQUIRED is returned specifying PA-ENC-TIMESTAMP as an
acceptable method, the client should retry the initial request using the
PA-ENC-TIMESTAMP preauthentication method. Servers need not support the
PA-ENC-TIMESTAMP method, but if not supported the server should ignore the
presence of PA-ENC-TIMESTAMP pre-authentication in a request.
Mutual authentication
Mutual authentication (via the KRB_AP_REP message) must be supported.
Ticket addresses and flags
All KDC's must pass on tickets that carry no addresses (i.e. if a TGT
contains no addresses, the KDC will return derivative tickets), but each
realm may set its own policy for issuing such tickets, and each application
server will set its own policy with respect to accepting them.
Proxies and forwarded tickets must be supported. Individual realms and
application servers can set their own policy on when such tickets will be
accepted.
All implementations must recognize renewable and postdated tickets, but
need not actually implement them. If these options are not supported, the
starttime and endtime in the ticket shall specify a ticket's entire useful
life. When a postdated ticket is decoded by a server, all implementations
shall make the presence of the postdated flag visible to the calling
server.
User-to-user authentication
Support for user to user authentication (via the ENC-TKT-IN-SKEY KDC
option) must be provided by implementations, but individual realms may
decide as a matter of policy to reject such requests on a per-principal or
realm-wide basis.
Neuman, Ts'o, Kohl Expires: 18 May 1999
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Authorization data
Implementations must pass all authorization data subfields from
ticket-granting tickets to any derivative tickets unless directed to
suppress a subfield as part of the definition of that registered subfield
type (it is never incorrect to pass on a subfield, and no registered
subfield types presently specify suppression at the KDC).
Implementations must make the contents of any authorization data subfields
available to the server when a ticket is used. Implementations are not
required to allow clients to specify the contents of the authorization data
fields.
Constant ranges
All protocol constants are constrained to 32 bit (signed) values unless
further constrained by the protocol definition. This limit is provided to
allow implementations to make assumptions about the maximum values that
will be received for these constants. Implementation receiving values
outside this range may reject the request, but they must recover cleanly.
9.2. Recommended KDC values
Following is a list of recommended values for a KDC implementation, based
on the list of suggested configuration constants (see section 4.4).
minimum lifetime 5 minutes
maximum renewable lifetime 1 week
maximum ticket lifetime 1 day
empty addresses only when suitable restrictions appear
in authorization data
proxiable, etc. Allowed.
10. REFERENCES
[NT94] B. Clifford Neuman and Theodore Y. Ts'o, "An Authenti-
cation Service for Computer Networks," IEEE Communica-
tions Magazine, Vol. 32(9), pp. 33-38 (September 1994).
[MNSS87] S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H.
Saltzer, Section E.2.1: Kerberos Authentication and
Authorization System, M.I.T. Project Athena, Cambridge,
Massachusetts (December 21, 1987).
[SNS88] J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Ker-
beros: An Authentication Service for Open Network Sys-
tems," pp. 191-202 in Usenix Conference Proceedings,
Dallas, Texas (February, 1988).
[NS78] Roger M. Needham and Michael D. Schroeder, "Using
Encryption for Authentication in Large Networks of Com-
puters," Communications of the ACM, Vol. 21(12),
pp. 993-999 (December, 1978).
[DS81] Dorothy E. Denning and Giovanni Maria Sacco, "Time-
stamps in Key Distribution Protocols," Communications
of the ACM, Vol. 24(8), pp. 533-536 (August 1981).
[KNT92] John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o,
Neuman, Ts'o, Kohl Expires: 18 May 1999
INTERNET-DRAFT draft-ietf-cat-kerberos-r-03 November 18 1998
"The Evolution of the Kerberos Authentication Service,"
in an IEEE Computer Society Text soon to be published
(June 1992).
[Neu93] B. Clifford Neuman, "Proxy-Based Authorization and
Accounting for Distributed Systems," in Proceedings of
the 13th International Conference on Distributed Com-
puting Systems, Pittsburgh, PA (May, 1993).
[DS90] Don Davis and Ralph Swick, "Workstation Services and
Kerberos Authentication at Project Athena," Technical
Memorandum TM-424, MIT Laboratory for Computer Science
(February 1990).
[LGDSR87] P. J. Levine, M. R. Gretzinger, J. M. Diaz, W. E. Som-
merfeld, and K. Raeburn, Section E.1: Service Manage-
ment System, M.I.T. Project Athena, Cambridge, Mas-
sachusetts (1987).
[X509-88] CCITT, Recommendation X.509: The Directory Authentica-
tion Framework, December 1988.
[Pat92]. J. Pato, Using Pre-Authentication to Avoid Password
Guessing Attacks, Open Software Foundation DCE Request
for Comments 26 (December 1992).
[DES77] National Bureau of Standards, U.S. Department of Com-
merce, "Data Encryption Standard," Federal Information
Processing Standards Publication 46, Washington, DC
(1977).
[DESM80] National Bureau of Standards, U.S. Department of Com-
merce, "DES Modes of Operation," Federal Information
Processing Standards Publication 81, Springfield, VA
(December 1980).
[SG92] Stuart G. Stubblebine and Virgil D. Gligor, "On Message
Integrity in Cryptographic Protocols," in Proceedings
of the IEEE Symposium on Research in Security and
Privacy, Oakland, California (May 1992).
[IS3309] International Organization for Standardization, "ISO
Information Processing Systems - Data Communication -
High-Level Data Link Control Procedure - Frame Struc-
ture," IS 3309 (October 1984). 3rd Edition.
[MD4-92] R. Rivest, "The MD4 Message Digest Algorithm," RFC
1320, MIT Laboratory for Computer Science (April
1992).
[MD5-92] R. Rivest, "The MD5 Message Digest Algorithm," RFC
1321, MIT Laboratory for Computer Science (April
1992).
[KBC96] H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication," Working Draft
draft-ietf-ipsec-hmac-md5-01.txt, (August 1996).
[Horowitz96] Horowitz, M., "Key Derivation for Authentication,
Neuman, Ts'o, Kohl Expires: 18 May 1999
INTERNET-DRAFT draft-ietf-cat-kerberos-r-03 November 18 1998
Integrity, and Privacy", draft-horowitz-key-derivation-02.txt,
August 1998.
[HorowitzB96] Horowitz, M., "Key Derivation for Kerberos V5", draft-
horowitz-kerb-key-derivation-01.txt, September 1998.
[Krawczyk96] Krawczyk, H., Bellare, and M., Canetti, R., "HMAC:
Keyed-Hashing for Message Authentication", draft-ietf-ipsec-hmac-
md5-01.txt, August, 1996.
A. Pseudo-code for protocol processing
This appendix provides pseudo-code describing how the messages are to be
constructed and interpreted by clients and servers.
A.1. KRB_AS_REQ generation
request.pvno := protocol version; /* pvno = 5 */
request.msg-type := message type; /* type = KRB_AS_REQ */
if(pa_enc_timestamp_required) then
request.padata.padata-type = PA-ENC-TIMESTAMP;
get system_time;
padata-body.patimestamp,pausec = system_time;
encrypt padata-body into request.padata.padata-value
using client.key; /* derived from password */
endif
body.kdc-options := users's preferences;
body.cname := user's name;
body.realm := user's realm;
body.sname := service's name; /* usually "krbtgt", "localrealm" */
if (body.kdc-options.POSTDATED is set) then
body.from := requested starting time;
else
omit body.from;
endif
body.till := requested end time;
if (body.kdc-options.RENEWABLE is set) then
body.rtime := requested final renewal time;
endif
body.nonce := random_nonce();
body.etype := requested etypes;
if (user supplied addresses) then
body.addresses := user's addresses;
else
omit body.addresses;
endif
omit body.enc-authorization-data;
request.req-body := body;
kerberos := lookup(name of local kerberos server (or servers));
send(packet,kerberos);
wait(for response);
if (timed_out) then
retry or use alternate server;
endif
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A.2. KRB_AS_REQ verification and KRB_AS_REP generation
decode message into req;
client := lookup(req.cname,req.realm);
server := lookup(req.sname,req.realm);
get system_time;
kdc_time := system_time.seconds;
if (!client) then
/* no client in Database */
error_out(KDC_ERR_C_PRINCIPAL_UNKNOWN);
endif
if (!server) then
/* no server in Database */
error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
endif
if(client.pa_enc_timestamp_required and
pa_enc_timestamp not present) then
error_out(KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP));
endif
if(pa_enc_timestamp present) then
decrypt req.padata-value into decrypted_enc_timestamp
using client.key;
using auth_hdr.authenticator.subkey;
if (decrypt_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
if(decrypted_enc_timestamp is not within allowable skew)
then
error_out(KDC_ERR_PREAUTH_FAILED);
endif
if(decrypted_enc_timestamp and usec is replay)
error_out(KDC_ERR_PREAUTH_FAILED);
endif
add decrypted_enc_timestamp and usec to replay cache;
endif
use_etype := first supported etype in req.etypes;
if (no support for req.etypes) then
error_out(KDC_ERR_ETYPE_NOSUPP);
endif
new_tkt.vno := ticket version; /* = 5 */
new_tkt.sname := req.sname;
new_tkt.srealm := req.srealm;
reset all flags in new_tkt.flags;
/* It should be noted that local policy may affect the */
/* processing of any of these flags. For example, some */
/* realms may refuse to issue renewable tickets */
if (req.kdc-options.FORWARDABLE is set) then
set new_tkt.flags.FORWARDABLE;
endif
if (req.kdc-options.PROXIABLE is set) then
set new_tkt.flags.PROXIABLE;
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endif
if (req.kdc-options.ALLOW-POSTDATE is set) then
set new_tkt.flags.MAY-POSTDATE;
endif
if ((req.kdc-options.RENEW is set) or
(req.kdc-options.VALIDATE is set) or
(req.kdc-options.PROXY is set) or
(req.kdc-options.FORWARDED is set) or
(req.kdc-options.ENC-TKT-IN-SKEY is set)) then
error_out(KDC_ERR_BADOPTION);
endif
new_tkt.session := random_session_key();
new_tkt.cname := req.cname;
new_tkt.crealm := req.crealm;
new_tkt.transited := empty_transited_field();
new_tkt.authtime := kdc_time;
if (req.kdc-options.POSTDATED is set) then
if (against_postdate_policy(req.from)) then
error_out(KDC_ERR_POLICY);
endif
set new_tkt.flags.POSTDATED;
set new_tkt.flags.INVALID;
new_tkt.starttime := req.from;
else
omit new_tkt.starttime; /* treated as authtime when omitted */
endif
if (req.till = 0) then
till := infinity;
else
till := req.till;
endif
new_tkt.endtime := min(till,
new_tkt.starttime+client.max_life,
new_tkt.starttime+server.max_life,
new_tkt.starttime+max_life_for_realm);
if ((req.kdc-options.RENEWABLE-OK is set) and
(new_tkt.endtime < req.till)) then
/* we set the RENEWABLE option for later processing */
set req.kdc-options.RENEWABLE;
req.rtime := req.till;
endif
if (req.rtime = 0) then
rtime := infinity;
else
rtime := req.rtime;
endif
if (req.kdc-options.RENEWABLE is set) then
set new_tkt.flags.RENEWABLE;
new_tkt.renew-till := min(rtime,
new_tkt.starttime+client.max_rlife,
new_tkt.starttime+server.max_rlife,
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new_tkt.starttime+max_rlife_for_realm);
else
omit new_tkt.renew-till; /* only present if RENEWABLE */
endif
if (req.addresses) then
new_tkt.caddr := req.addresses;
else
omit new_tkt.caddr;
endif
new_tkt.authorization_data := empty_authorization_data();
encode to-be-encrypted part of ticket into OCTET STRING;
new_tkt.enc-part := encrypt OCTET STRING
using etype_for_key(server.key), server.key, server.p_kvno;
/* Start processing the response */
resp.pvno := 5;
resp.msg-type := KRB_AS_REP;
resp.cname := req.cname;
resp.crealm := req.realm;
resp.ticket := new_tkt;
resp.key := new_tkt.session;
resp.last-req := fetch_last_request_info(client);
resp.nonce := req.nonce;
resp.key-expiration := client.expiration;
resp.flags := new_tkt.flags;
resp.authtime := new_tkt.authtime;
resp.starttime := new_tkt.starttime;
resp.endtime := new_tkt.endtime;
if (new_tkt.flags.RENEWABLE) then
resp.renew-till := new_tkt.renew-till;
endif
resp.realm := new_tkt.realm;
resp.sname := new_tkt.sname;
resp.caddr := new_tkt.caddr;
encode body of reply into OCTET STRING;
resp.enc-part := encrypt OCTET STRING
using use_etype, client.key, client.p_kvno;
send(resp);
A.3. KRB_AS_REP verification
decode response into resp;
if (resp.msg-type = KRB_ERROR) then
if(error = KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP)) then
set pa_enc_timestamp_required;
goto KRB_AS_REQ;
endif
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process_error(resp);
return;
endif
/* On error, discard the response, and zero the session key */
/* from the response immediately */
key = get_decryption_key(resp.enc-part.kvno, resp.enc-part.etype,
resp.padata);
unencrypted part of resp := decode of decrypt of resp.enc-part
using resp.enc-part.etype and key;
zero(key);
if (common_as_rep_tgs_rep_checks fail) then
destroy resp.key;
return error;
endif
if near(resp.princ_exp) then
print(warning message);
endif
save_for_later(ticket,session,client,server,times,flags);
A.4. KRB_AS_REP and KRB_TGS_REP common checks
if (decryption_error() or
(req.cname != resp.cname) or
(req.realm != resp.crealm) or
(req.sname != resp.sname) or
(req.realm != resp.realm) or
(req.nonce != resp.nonce) or
(req.addresses != resp.caddr)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
/* make sure no flags are set that shouldn't be, and that all that
*/
/* should be are set
*/
if (!check_flags_for_compatability(req.kdc-options,resp.flags)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
if ((req.from = 0) and
(resp.starttime is not within allowable skew)) then
destroy resp.key;
return KRB_AP_ERR_SKEW;
endif
if ((req.from != 0) and (req.from != resp.starttime)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
if ((req.till != 0) and (resp.endtime > req.till)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
if ((req.kdc-options.RENEWABLE is set) and
(req.rtime != 0) and (resp.renew-till > req.rtime)) then
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destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
if ((req.kdc-options.RENEWABLE-OK is set) and
(resp.flags.RENEWABLE) and
(req.till != 0) and
(resp.renew-till > req.till)) then
destroy resp.key;
return KRB_AP_ERR_MODIFIED;
endif
A.5. KRB_TGS_REQ generation
/* Note that make_application_request might have to recursivly
*/
/* call this routine to get the appropriate ticket-granting ticket
*/
request.pvno := protocol version; /* pvno = 5 */
request.msg-type := message type; /* type = KRB_TGS_REQ */
body.kdc-options := users's preferences;
/* If the TGT is not for the realm of the end-server */
/* then the sname will be for a TGT for the end-realm */
/* and the realm of the requested ticket (body.realm) */
/* will be that of the TGS to which the TGT we are */
/* sending applies */
body.sname := service's name;
body.realm := service's realm;
if (body.kdc-options.POSTDATED is set) then
body.from := requested starting time;
else
omit body.from;
endif
body.till := requested end time;
if (body.kdc-options.RENEWABLE is set) then
body.rtime := requested final renewal time;
endif
body.nonce := random_nonce();
body.etype := requested etypes;
if (user supplied addresses) then
body.addresses := user's addresses;
else
omit body.addresses;
endif
body.enc-authorization-data := user-supplied data;
if (body.kdc-options.ENC-TKT-IN-SKEY) then
body.additional-tickets_ticket := second TGT;
endif
request.req-body := body;
check := generate_checksum (req.body,checksumtype);
request.padata[0].padata-type := PA-TGS-REQ;
request.padata[0].padata-value := create a KRB_AP_REQ using
the TGT and checksum
/* add in any other padata as required/supplied */
kerberos := lookup(name of local kerberose server (or servers));
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send(packet,kerberos);
wait(for response);
if (timed_out) then
retry or use alternate server;
endif
A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation
/* note that reading the application request requires first
determining the server for which a ticket was issued, and choosing
the
correct key for decryption. The name of the server appears in the
plaintext part of the ticket. */
if (no KRB_AP_REQ in req.padata) then
error_out(KDC_ERR_PADATA_TYPE_NOSUPP);
endif
verify KRB_AP_REQ in req.padata;
/* Note that the realm in which the Kerberos server is operating is
determined by the instance from the ticket-granting ticket. The
realm
in the ticket-granting ticket is the realm under which the ticket
granting ticket was issued. It is possible for a single Kerberos
server to support more than one realm. */
auth_hdr := KRB_AP_REQ;
tgt := auth_hdr.ticket;
if (tgt.sname is not a TGT for local realm and is not req.sname)
then
error_out(KRB_AP_ERR_NOT_US);
realm := realm_tgt_is_for(tgt);
decode remainder of request;
if (auth_hdr.authenticator.cksum is missing) then
error_out(KRB_AP_ERR_INAPP_CKSUM);
endif
if (auth_hdr.authenticator.cksum type is not supported) then
error_out(KDC_ERR_SUMTYPE_NOSUPP);
endif
if (auth_hdr.authenticator.cksum is not both collision-proof and
keyed) then
error_out(KRB_AP_ERR_INAPP_CKSUM);
endif
set computed_checksum := checksum(req);
if (computed_checksum != auth_hdr.authenticatory.cksum) then
error_out(KRB_AP_ERR_MODIFIED);
endif
server := lookup(req.sname,realm);
if (!server) then
if (is_foreign_tgt_name(req.sname)) then
server := best_intermediate_tgs(req.sname);
else
/* no server in Database */
error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
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endif
endif
session := generate_random_session_key();
use_etype := first supported etype in req.etypes;
if (no support for req.etypes) then
error_out(KDC_ERR_ETYPE_NOSUPP);
endif
new_tkt.vno := ticket version; /* = 5 */
new_tkt.sname := req.sname;
new_tkt.srealm := realm;
reset all flags in new_tkt.flags;
/* It should be noted that local policy may affect the */
/* processing of any of these flags. For example, some */
/* realms may refuse to issue renewable tickets */
new_tkt.caddr := tgt.caddr;
resp.caddr := NULL; /* We only include this if they change */
if (req.kdc-options.FORWARDABLE is set) then
if (tgt.flags.FORWARDABLE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.FORWARDABLE;
endif
if (req.kdc-options.FORWARDED is set) then
if (tgt.flags.FORWARDABLE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.FORWARDED;
new_tkt.caddr := req.addresses;
resp.caddr := req.addresses;
endif
if (tgt.flags.FORWARDED is set) then
set new_tkt.flags.FORWARDED;
endif
if (req.kdc-options.PROXIABLE is set) then
if (tgt.flags.PROXIABLE is reset)
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.PROXIABLE;
endif
if (req.kdc-options.PROXY is set) then
if (tgt.flags.PROXIABLE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.PROXY;
new_tkt.caddr := req.addresses;
resp.caddr := req.addresses;
endif
if (req.kdc-options.ALLOW-POSTDATE is set) then
if (tgt.flags.MAY-POSTDATE is reset)
error_out(KDC_ERR_BADOPTION);
endif
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set new_tkt.flags.MAY-POSTDATE;
endif
if (req.kdc-options.POSTDATED is set) then
if (tgt.flags.MAY-POSTDATE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.POSTDATED;
set new_tkt.flags.INVALID;
if (against_postdate_policy(req.from)) then
error_out(KDC_ERR_POLICY);
endif
new_tkt.starttime := req.from;
endif
if (req.kdc-options.VALIDATE is set) then
if (tgt.flags.INVALID is reset) then
error_out(KDC_ERR_POLICY);
endif
if (tgt.starttime > kdc_time) then
error_out(KRB_AP_ERR_NYV);
endif
if (check_hot_list(tgt)) then
error_out(KRB_AP_ERR_REPEAT);
endif
tkt := tgt;
reset new_tkt.flags.INVALID;
endif
if (req.kdc-options.(any flag except ENC-TKT-IN-SKEY, RENEW,
and those already processed) is set) then
error_out(KDC_ERR_BADOPTION);
endif
new_tkt.authtime := tgt.authtime;
if (req.kdc-options.RENEW is set) then
/* Note that if the endtime has already passed, the ticket would
*/
/* have been rejected in the initial authentication stage, so
*/
/* there is no need to check again here
*/
if (tgt.flags.RENEWABLE is reset) then
error_out(KDC_ERR_BADOPTION);
endif
if (tgt.renew-till < kdc_time) then
error_out(KRB_AP_ERR_TKT_EXPIRED);
endif
tkt := tgt;
new_tkt.starttime := kdc_time;
old_life := tgt.endttime - tgt.starttime;
new_tkt.endtime := min(tgt.renew-till,
new_tkt.starttime + old_life);
else
new_tkt.starttime := kdc_time;
if (req.till = 0) then
till := infinity;
else
till := req.till;
endif
new_tkt.endtime := min(till,
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new_tkt.starttime+client.max_life,
new_tkt.starttime+server.max_life,
new_tkt.starttime+max_life_for_realm,
tgt.endtime);
if ((req.kdc-options.RENEWABLE-OK is set) and
(new_tkt.endtime < req.till) and
(tgt.flags.RENEWABLE is set) then
/* we set the RENEWABLE option for later processing
*/
set req.kdc-options.RENEWABLE;
req.rtime := min(req.till, tgt.renew-till);
endif
endif
if (req.rtime = 0) then
rtime := infinity;
else
rtime := req.rtime;
endif
if ((req.kdc-options.RENEWABLE is set) and
(tgt.flags.RENEWABLE is set)) then
set new_tkt.flags.RENEWABLE;
new_tkt.renew-till := min(rtime,
new_tkt.starttime+client.max_rlife,
new_tkt.starttime+server.max_rlife,
new_tkt.starttime+max_rlife_for_realm,
tgt.renew-till);
else
new_tkt.renew-till := OMIT; /* leave the renew-till field
out */
endif
if (req.enc-authorization-data is present) then
decrypt req.enc-authorization-data into
decrypted_authorization_data
using auth_hdr.authenticator.subkey;
if (decrypt_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
endif
new_tkt.authorization_data := req.auth_hdr.ticket.authorization_data
+
decrypted_authorization_data;
new_tkt.key := session;
new_tkt.crealm := tgt.crealm;
new_tkt.cname := req.auth_hdr.ticket.cname;
if (realm_tgt_is_for(tgt) := tgt.realm) then
/* tgt issued by local realm */
new_tkt.transited := tgt.transited;
else
/* was issued for this realm by some other realm */
if (tgt.transited.tr-type not supported) then
error_out(KDC_ERR_TRTYPE_NOSUPP);
endif
new_tkt.transited := compress_transited(tgt.transited +
tgt.realm)
/* Don't check tranited field if TGT for foreign realm,
* or requested not to check */
if (is_not_foreign_tgt_name(new_tkt.server)
&& req.kdc-options.DISABLE-TRANSITED-CHECK not set) then
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/* Check it, so end-server does not have to
* but don't fail, end-server may still accept it */
if (check_transited_field(new_tkt.transited) == OK)
set new_tkt.flags.TRANSITED-POLICY-CHECKED;
endif
endif
endif
encode encrypted part of new_tkt into OCTET STRING;
if (req.kdc-options.ENC-TKT-IN-SKEY is set) then
if (server not specified) then
server = req.second_ticket.client;
endif
if ((req.second_ticket is not a TGT) or
(req.second_ticket.client != server)) then
error_out(KDC_ERR_POLICY);
endif
new_tkt.enc-part := encrypt OCTET STRING using
using etype_for_key(second-ticket.key),
second-ticket.key;
else
new_tkt.enc-part := encrypt OCTET STRING
using etype_for_key(server.key), server.key,
server.p_kvno;
endif
resp.pvno := 5;
resp.msg-type := KRB_TGS_REP;
resp.crealm := tgt.crealm;
resp.cname := tgt.cname;
resp.ticket := new_tkt;
resp.key := session;
resp.nonce := req.nonce;
resp.last-req := fetch_last_request_info(client);
resp.flags := new_tkt.flags;
resp.authtime := new_tkt.authtime;
resp.starttime := new_tkt.starttime;
resp.endtime := new_tkt.endtime;
omit resp.key-expiration;
resp.sname := new_tkt.sname;
resp.realm := new_tkt.realm;
if (new_tkt.flags.RENEWABLE) then
resp.renew-till := new_tkt.renew-till;
endif
encode body of reply into OCTET STRING;
if (req.padata.authenticator.subkey)
resp.enc-part := encrypt OCTET STRING using use_etype,
req.padata.authenticator.subkey;
else resp.enc-part := encrypt OCTET STRING using use_etype, tgt.key;
send(resp);
A.7. KRB_TGS_REP verification
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decode response into resp;
if (resp.msg-type = KRB_ERROR) then
process_error(resp);
return;
endif
/* On error, discard the response, and zero the session key from
the response immediately */
if (req.padata.authenticator.subkey)
unencrypted part of resp := decode of decrypt of
resp.enc-part
using resp.enc-part.etype and subkey;
else unencrypted part of resp := decode of decrypt of resp.enc-part
using resp.enc-part.etype and tgt's session
key;
if (common_as_rep_tgs_rep_checks fail) then
destroy resp.key;
return error;
endif
check authorization_data as necessary;
save_for_later(ticket,session,client,server,times,flags);
A.8. Authenticator generation
body.authenticator-vno := authenticator vno; /* = 5 */
body.cname, body.crealm := client name;
if (supplying checksum) then
body.cksum := checksum;
endif
get system_time;
body.ctime, body.cusec := system_time;
if (selecting sub-session key) then
select sub-session key;
body.subkey := sub-session key;
endif
if (using sequence numbers) then
select initial sequence number;
body.seq-number := initial sequence;
endif
A.9. KRB_AP_REQ generation
obtain ticket and session_key from cache;
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_AP_REQ */
if (desired(MUTUAL_AUTHENTICATION)) then
set packet.ap-options.MUTUAL-REQUIRED;
else
reset packet.ap-options.MUTUAL-REQUIRED;
endif
if (using session key for ticket) then
set packet.ap-options.USE-SESSION-KEY;
else
reset packet.ap-options.USE-SESSION-KEY;
endif
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packet.ticket := ticket; /* ticket */
generate authenticator;
encode authenticator into OCTET STRING;
encrypt OCTET STRING into packet.authenticator using session_key;
A.10. KRB_AP_REQ verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_AP_REQ) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
if (packet.ticket.tkt_vno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.ap_options.USE-SESSION-KEY is set) then
retrieve session key from ticket-granting ticket for
packet.ticket.{sname,srealm,enc-part.etype};
else
retrieve service key for
packet.ticket.{sname,srealm,enc-part.etype,enc-part.skvno};
endif
if (no_key_available) then
if (cannot_find_specified_skvno) then
error_out(KRB_AP_ERR_BADKEYVER);
else
error_out(KRB_AP_ERR_NOKEY);
endif
endif
decrypt packet.ticket.enc-part into decr_ticket using retrieved key;
if (decryption_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
decrypt packet.authenticator into decr_authenticator
using decr_ticket.key;
if (decryption_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
if (decr_authenticator.{cname,crealm} !=
decr_ticket.{cname,crealm}) then
error_out(KRB_AP_ERR_BADMATCH);
endif
if (decr_ticket.caddr is present) then
if (sender_address(packet) is not in decr_ticket.caddr) then
error_out(KRB_AP_ERR_BADADDR);
endif
elseif (application requires addresses) then
error_out(KRB_AP_ERR_BADADDR);
endif
if (not in_clock_skew(decr_authenticator.ctime,
decr_authenticator.cusec)) then
error_out(KRB_AP_ERR_SKEW);
endif
if (repeated(decr_authenticator.{ctime,cusec,cname,crealm})) then
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error_out(KRB_AP_ERR_REPEAT);
endif
save_identifier(decr_authenticator.{ctime,cusec,cname,crealm});
get system_time;
if ((decr_ticket.starttime-system_time > CLOCK_SKEW) or
(decr_ticket.flags.INVALID is set)) then
/* it hasn't yet become valid */
error_out(KRB_AP_ERR_TKT_NYV);
endif
if (system_time-decr_ticket.endtime > CLOCK_SKEW) then
error_out(KRB_AP_ERR_TKT_EXPIRED);
endif
if (decr_ticket.transited) then
/* caller may ignore the TRANSITED-POLICY-CHECKED and do
* check anyway */
if (decr_ticket.flags.TRANSITED-POLICY-CHECKED not set) then
if (check_transited_field(decr_ticket.transited) then
error_out(KDC_AP_PATH_NOT_ACCPETED);
endif
endif
endif
/* caller must check decr_ticket.flags for any pertinent details */
return(OK, decr_ticket, packet.ap_options.MUTUAL-REQUIRED);
A.11. KRB_AP_REP generation
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_AP_REP */
body.ctime := packet.ctime;
body.cusec := packet.cusec;
if (selecting sub-session key) then
select sub-session key;
body.subkey := sub-session key;
endif
if (using sequence numbers) then
select initial sequence number;
body.seq-number := initial sequence;
endif
encode body into OCTET STRING;
select encryption type;
encrypt OCTET STRING into packet.enc-part;
A.12. KRB_AP_REP verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_AP_REP) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
cleartext := decrypt(packet.enc-part) using ticket's session key;
if (decryption_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
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if (cleartext.ctime != authenticator.ctime) then
error_out(KRB_AP_ERR_MUT_FAIL);
endif
if (cleartext.cusec != authenticator.cusec) then
error_out(KRB_AP_ERR_MUT_FAIL);
endif
if (cleartext.subkey is present) then
save cleartext.subkey for future use;
endif
if (cleartext.seq-number is present) then
save cleartext.seq-number for future verifications;
endif
return(AUTHENTICATION_SUCCEEDED);
A.13. KRB_SAFE generation
collect user data in buffer;
/* assemble packet: */
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_SAFE */
body.user-data := buffer; /* DATA */
if (using timestamp) then
get system_time;
body.timestamp, body.usec := system_time;
endif
if (using sequence numbers) then
body.seq-number := sequence number;
endif
body.s-address := sender host addresses;
if (only one recipient) then
body.r-address := recipient host address;
endif
checksum.cksumtype := checksum type;
compute checksum over body;
checksum.checksum := checksum value; /* checksum.checksum */
packet.cksum := checksum;
packet.safe-body := body;
A.14. KRB_SAFE verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_SAFE) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
if (packet.checksum.cksumtype is not both collision-proof and keyed)
then
error_out(KRB_AP_ERR_INAPP_CKSUM);
endif
if (safe_priv_common_checks_ok(packet)) then
set computed_checksum := checksum(packet.body);
if (computed_checksum != packet.checksum) then
error_out(KRB_AP_ERR_MODIFIED);
endif
return (packet, PACKET_IS_GENUINE);
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else
return common_checks_error;
endif
A.15. KRB_SAFE and KRB_PRIV common checks
if (packet.s-address != O/S_sender(packet)) then
/* O/S report of sender not who claims to have sent it */
error_out(KRB_AP_ERR_BADADDR);
endif
if ((packet.r-address is present) and
(packet.r-address != local_host_address)) then
/* was not sent to proper place */
error_out(KRB_AP_ERR_BADADDR);
endif
if (((packet.timestamp is present) and
(not in_clock_skew(packet.timestamp,packet.usec))) or
(packet.timestamp is not present and timestamp expected)) then
error_out(KRB_AP_ERR_SKEW);
endif
if (repeated(packet.timestamp,packet.usec,packet.s-address)) then
error_out(KRB_AP_ERR_REPEAT);
endif
if (((packet.seq-number is present) and
((not in_sequence(packet.seq-number)))) or
(packet.seq-number is not present and sequence expected)) then
error_out(KRB_AP_ERR_BADORDER);
endif
if (packet.timestamp not present and packet.seq-number not present)
then
error_out(KRB_AP_ERR_MODIFIED);
endif
save_identifier(packet.{timestamp,usec,s-address},
sender_principal(packet));
return PACKET_IS_OK;
A.16. KRB_PRIV generation
collect user data in buffer;
/* assemble packet: */
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_PRIV */
packet.enc-part.etype := encryption type;
body.user-data := buffer;
if (using timestamp) then
get system_time;
body.timestamp, body.usec := system_time;
endif
if (using sequence numbers) then
body.seq-number := sequence number;
endif
body.s-address := sender host addresses;
if (only one recipient) then
body.r-address := recipient host address;
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endif
encode body into OCTET STRING;
select encryption type;
encrypt OCTET STRING into packet.enc-part.cipher;
A.17. KRB_PRIV verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_PRIV) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
cleartext := decrypt(packet.enc-part) using negotiated key;
if (decryption_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
if (safe_priv_common_checks_ok(cleartext)) then
return(cleartext.DATA, PACKET_IS_GENUINE_AND_UNMODIFIED);
else
return common_checks_error;
endif
A.18. KRB_CRED generation
invoke KRB_TGS; /* obtain tickets to be provided to peer */
/* assemble packet: */
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_CRED */
for (tickets[n] in tickets to be forwarded) do
packet.tickets[n] = tickets[n].ticket;
done
packet.enc-part.etype := encryption type;
for (ticket[n] in tickets to be forwarded) do
body.ticket-info[n].key = tickets[n].session;
body.ticket-info[n].prealm = tickets[n].crealm;
body.ticket-info[n].pname = tickets[n].cname;
body.ticket-info[n].flags = tickets[n].flags;
body.ticket-info[n].authtime = tickets[n].authtime;
body.ticket-info[n].starttime = tickets[n].starttime;
body.ticket-info[n].endtime = tickets[n].endtime;
body.ticket-info[n].renew-till = tickets[n].renew-till;
body.ticket-info[n].srealm = tickets[n].srealm;
body.ticket-info[n].sname = tickets[n].sname;
body.ticket-info[n].caddr = tickets[n].caddr;
done
get system_time;
body.timestamp, body.usec := system_time;
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if (using nonce) then
body.nonce := nonce;
endif
if (using s-address) then
body.s-address := sender host addresses;
endif
if (limited recipients) then
body.r-address := recipient host address;
endif
encode body into OCTET STRING;
select encryption type;
encrypt OCTET STRING into packet.enc-part.cipher
using negotiated encryption key;
A.19. KRB_CRED verification
receive packet;
if (packet.pvno != 5) then
either process using other protocol spec
or error_out(KRB_AP_ERR_BADVERSION);
endif
if (packet.msg-type != KRB_CRED) then
error_out(KRB_AP_ERR_MSG_TYPE);
endif
cleartext := decrypt(packet.enc-part) using negotiated key;
if (decryption_error()) then
error_out(KRB_AP_ERR_BAD_INTEGRITY);
endif
if ((packet.r-address is present or required) and
(packet.s-address != O/S_sender(packet)) then
/* O/S report of sender not who claims to have sent it */
error_out(KRB_AP_ERR_BADADDR);
endif
if ((packet.r-address is present) and
(packet.r-address != local_host_address)) then
/* was not sent to proper place */
error_out(KRB_AP_ERR_BADADDR);
endif
if (not in_clock_skew(packet.timestamp,packet.usec)) then
error_out(KRB_AP_ERR_SKEW);
endif
if (repeated(packet.timestamp,packet.usec,packet.s-address)) then
error_out(KRB_AP_ERR_REPEAT);
endif
if (packet.nonce is required or present) and
(packet.nonce != expected-nonce) then
error_out(KRB_AP_ERR_MODIFIED);
endif
for (ticket[n] in tickets that were forwarded) do
save_for_later(ticket[n],key[n],principal[n],
server[n],times[n],flags[n]);
return
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A.20. KRB_ERROR generation
/* assemble packet: */
packet.pvno := protocol version; /* 5 */
packet.msg-type := message type; /* KRB_ERROR */
get system_time;
packet.stime, packet.susec := system_time;
packet.realm, packet.sname := server name;
if (client time available) then
packet.ctime, packet.cusec := client_time;
endif
packet.error-code := error code;
if (client name available) then
packet.cname, packet.crealm := client name;
endif
if (error text available) then
packet.e-text := error text;
endif
if (error data available) then
packet.e-data := error data;
endif
B. Definition of common authorization data elements
This appendix contains the definitions of common authorization data
elements. These common authorization data elements are recursivly defined,
meaning the ad-data for these types will itself contain a sequence of
authorization data whose interpretation is affected by the encapsulating
element. Depending on the meaning of the encapsulating element, the
encapsulated elements may be ignored, might be interpreted as issued
directly by the KDC, or they might be stored in a separate plaintext part
of the ticket. The types of the encapsulating elements are specified as
part of the Kerberos specification because the behavior based on these
values should be understood across implementations whereas other elements
need only be understood by the applications which they affect.
In the definitions that follow, the value of the ad-type for the element
will be specified in the subsection number, and the value of the ad-data
will be as shown in the ASN.1 structure that follows the subsection
heading.
B.1. KDC Issued
AD-KDCIssued SEQUENCE {
ad-checksum[0] Checksum,
i-realm[1] Realm OPTIONAL,
i-sname[2] PrincipalName OPTIONAL,
elements[3] AuthorizationData.
}
ad-checksum
A checksum over the elements field using a cryptographic checksum
method that is identical to the checksum used to protect the ticket
itself (i.e. using the same hash function and the same encryption
algorithm used to encrypt the ticket) and using a key derived from the
same key used to protect the ticket.
i-realm, i-sname
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The name of the issuing principal if different from the KDC itself.
This field would be used when the KDC can verify the authenticity of
elements signed by the issuing principal and it allows this KDC to
notify the application server of the validity of those elements.
elements
A sequence of authorization data elements issued by the KDC.
The KDC-issued ad-data field is intended to provide a means for Kerberos
principal credentials to embed within themselves privilege attributes and
other mechanisms for positive authorization, amplifying the priveleges of
the principal beyond what can be done using a credentials without such an
a-data element.
This can not be provided without this element because the definition of the
authorization-data field allows elements to be added at will by the bearer
of a TGT at the time that they request service tickets and elements may
also be added to a delegated ticket by inclusion in the authenticator.
For KDC-issued elements this is prevented because the elements are signed
by the KDC by including a checksum encrypted using the server's key (the
same key used to encrypt the ticket - or a key derived from that key).
Elements encapsulated with in the KDC-issued element will be ignored by the
application server if this "signature" is not present. Further, elements
encapsulated within this element from a ticket granting ticket may be
interpreted by the KDC, and used as a basis according to policy for
including new signed elements within derivative tickets, but they will not
be copied to a derivative ticket directly. If they are copied directly to a
derivative ticket by a KDC that is not aware of this element, the signature
will not be correct for the application ticket elements, and the field will
be ignored by the application server.
This element and the elements it encapulates may be safely ignored by
applications, application servers, and KDCs that do not implement this
element.
B.2. Intended for server
AD-INTENDED-FOR-SERVER SEQUENCE {
intended-server[0] SEQUENCE OF PrincipalName
elements[1] AuthorizationData
}
AD elements encapsulated within the intended-for-server element may be
ignored if the application server is not in the list of principal names of
intended servers. Further, a KDC issuing a ticket for an application server
can remove this element if the application server is not in the list of
intended servers.
Application servers should check for their principal name in the
intended-server field of this element. If their principal name is not
found, this element should be ignored. If found, then the encapsulated
elements should be evaluated in the same manner as if they were present in
the top level authorization data field. Applications and application
servers that do not implement this element should reject tickets that
contain authorization data elements of this type.
B.3. Intended for application class
AD-INTENDED-FOR-APPLICATION-CLASS SEQUENCE { intended-application-class[0]
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SEQUENCE OF GeneralString elements[1] AuthorizationData } AD elements
encapsulated within the intended-for-application-class element may be
ignored if the application server is not in one of the named classes of
application servers. Examples of application server classes include
"FILESYSTEM", and other kinds of servers.
This element and the elements it encapulates may be safely ignored by
applications, application servers, and KDCs that do not implement this
element.
B.4. If relevant
AD-IF-RELEVANT AuthorizationData
AD elements encapsulated within the if-relevant element are intended for
interpretation only by application servers that understand the particular
ad-type of the embedded element. Application servers that do not understand
the type of an element embedded within the if-relevant element may ignore
the uninterpretable element. This element promotes interoperability across
implementations which may have local extensions for authorization.
B.5. And-Or
AD-AND-OR SEQUENCE {
condition-count[0] INTEGER,
elements[1] AuthorizationData
}
When restrictive AD elements encapsulated within the and-or element are
encountered, only the number specified in condition-count of the
encapsulated conditions must be met in order to satisfy this element. This
element may be used to implement an "or" operation by setting the
condition-count field to 1, and it may specify an "and" operation by
setting the condition count to the number of embedded elements. Application
servers that do not implement this element must reject tickets that contain
authorization data elements of this type.
B.6. Mandatory ticket extensions
AD-Mandatory-Ticket-Extensions Checksum
An authorization data element of type mandatory-ticket-extensions specifies
a collision-proof checksum using the same hash algorithm used to protect
the integrity of the ticket itself. This checksum will be calculated over
an individual extension field. If there are more than one extension,
multiple Mandatory-Ticket-Extensions authorization data elements may be
present, each with a checksum for a different extension field. This
restriction indicates that the ticket should not be accepted if a ticket
extension is not present in the ticket for which the checksum does not
match that checksum specified in the authorization data element.
Application servers that do not implement this element must reject tickets
that contain authorization data elements of this type.
B.7. Authorization Data in ticket extensions
AD-IN-Ticket-Extensions Checksum
An authorization data element of type in-ticket-extensions specifies a
collision-proof checksum using the same hash algorithm used to protect the
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integrity of the ticket itself. This checksum is calculated over a separate
external AuthorizationData field carried in the ticket extensions.
Application servers that do not implement this element must reject tickets
that contain authorization data elements of this type. Application servers
that do implement this element will search the ticket extensions for
authorization data fields, calculate the specified checksum over each
authorization data field and look for one matching the checksum in this
in-ticket-extensions element. If not found, then the ticket must be
rejected. If found, the corresponding authorization data elements will be
interpreted in the same manner as if they were contained in the top level
authorization data field.
Note that if multiple external authorization data fields are present in a
ticket, each will have a corresponding element of type in-ticket-extensions
in the top level authorization data field, and the external entries will be
linked to the corresponding element by their checksums.
C. Definition of common ticket extensions
This appendix contains the definitions of common ticket extensions. Support
for these extensions is optional. However, certain extensions have
associated authorization data elements that may require rejection of a
ticket containing an extension by application servers that do not implement
the particular extension. Other extensions have been defined beyond those
described in this specification. Such extensions are described elswhere and
for some of those extensions the reserved number may be found in the list
of constants.
It is known that older versions of Kerberos did not support this field, and
that some clients will strip this field from a ticket when they parse and
then reassemble a ticket as it is passed to the application servers. The
presence of the extension will not break such clients, but any functionaly
dependent on the extensions will not work when such tickets are handled by
old clients. In such situations, some implementation may use alternate
methods to transmit the information in the extensions field.
C.1. Null ticket extension
TE-NullExtension OctetString -- The empty Octet String
The te-data field in the null ticket extension is an octet string of lenght
zero. This extension may be included in a ticket granting ticket so that
the KDC can determine on presentation of the ticket granting ticket whether
the client software will strip the extensions field.
C.2. External Authorization Data
TE-ExternalAuthorizationData AuthorizationData
The te-data field in the external authorization data ticket extension is
field of type AuthorizationData containing one or more authorization data
elements. If present, a corresponding authorization data element will be
present in the primary authorization data for the ticket and that element
will contain a checksum of the external authorization data ticket
extension.
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[TM] Project Athena, Athena, and Kerberos are trademarks of the
Massachusetts Institute of Technology (MIT). No commercial use of these
trademarks may be made without prior written permission of MIT.
Neuman, Ts'o, Kohl Expires: 18 May 1999
INTERNET-DRAFT draft-ietf-cat-kerberos-r-03 November 18 1998
[1] Note, however, that many applications use Kerberos' functions only upon
the initiation of a stream-based network connection. Unless an application
subsequently provides integrity protection for the data stream, the
identity verification applies only to the initiation of the connection, and
does not guarantee that subsequent messages on the connection originate
from the same principal.
[2] Secret and private are often used interchangeably in the literature. In
our usage, it takes two (or more) to share a secret, thus a shared DES key
is a secret key. Something is only private when no one but its owner knows
it. Thus, in public key cryptosystems, one has a public and a private key.
[3] Of course, with appropriate permission the client could arrange
registration of a separately-named prin- cipal in a remote realm, and
engage in normal exchanges with that realm's services. However, for even
small numbers of clients this becomes cumbersome, and more automatic
methods as described here are necessary.
[4] Though it is permissible to request or issue tick- ets with no network
addresses specified.
[5] The password-changing request must not be honored unless the requester
can provide the old password (the user's current secret key). Otherwise, it
would be possible for someone to walk up to an unattended ses- sion and
change another user's password.
[6] To authenticate a user logging on to a local system, the credentials
obtained in the AS exchange may first be used in a TGS exchange to obtain
credentials for a local server. Those credentials must then be verified by
a local server through successful completion of the Client/Server exchange.
[7] "Random" means that, among other things, it should be impossible to
guess the next session key based on knowledge of past session keys. This
can only be achieved in a pseudo-random number generator if it is based on
cryptographic principles. It is more desirable to use a truly random number
generator, such as one based on measurements of random physical phenomena.
[8] Tickets contain both an encrypted and unencrypted portion, so cleartext
here refers to the entire unit, which can be copied from one message and
replayed in another without any cryptographic skill.
[9] Note that this can make applications based on unreliable transports
difficult to code correctly. If the transport might deliver duplicated
messages, either a new authenticator must be generated for each retry, or
the application server must match requests and replies and replay the first
reply in response to a detected duplicate.
[10] This is used for user-to-user authentication as described in [8].
[11] 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.
[12] In the Kerberos version 4 protocol, the timestamp in the reply was the
client's timestamp plus one. This is not necessary in version 5 because
version 5 messages are formatted in such a way that it is not possible to
Neuman, Ts'o, Kohl Expires: 18 May 1999
INTERNET-DRAFT draft-ietf-cat-kerberos-r-03 November 18 1998
create the reply by judicious message surgery (even in encrypted form)
without knowledge of the appropriate encryption keys.
[13] Note that for encrypting the KRB_AP_REP message, the sub-session key
is not used, even if present in the Authenticator.
[14] Implementations of the protocol may wish to provide routines to choose
subkeys based on session keys and random numbers and to generate a
negotiated key to be returned in the KRB_AP_REP message.
[15]This can be accomplished in several ways. It might be known beforehand
(since the realm is part of the principal identifier), it might be stored
in a nameserver, or it might be obtained from a configura- tion 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 authenti-
cated. This might result in the use of a realm which has been compromised,
and would result in an attacker's ability to compromise the authentication
of the application server to the client.
[16] If the client selects a sub-session key, care must be taken to ensure
the randomness of the selected sub- session key. One approach would be to
generate a random number and XOR it with the session key from the
ticket-granting ticket.
[17] This allows easy implementation of user-to-user authentication [8],
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.
[18] For the purpose of appending, the realm preceding the first listed
realm is considered to be the null realm ("").
[19] 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.
[20] This means that a client and server running on the same host and
communicating with one another using the KRB_SAFE messages should not share
a common replay cache to detect KRB_SAFE replays.
[21] 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.
[22] See the discussion of the padata field in section 5.4.2 for details on
why this can be useful.
[23] Warning for implementations that unpack and repack data structures
during the generation and verification of embedded checksums: Because any
checksums applied to data structures must be checked against the original
data the length of bit strings must be preserved within a data structure
between the time that a checksum is generated through transmission to the
time that the checksum is verified.
[24] 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.
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INTERNET-DRAFT draft-ietf-cat-kerberos-r-03 November 18 1998
[25] 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.
[27] An application code in the encrypted part of a message provides an
additional check that the message was decrypted properly.
[29] An application code in the encrypted part of a message provides an
additional check that the message was decrypted properly.
[31] An application code in the encrypted part of a message provides an
additional check that the message was decrypted properly.
[32] If supported by the encryption method in use, an initialization vector
may be passed to the encryption procedure, in order to achieve proper
cipher chaining. The initialization vector might come from the last block
of the ciphertext from the previous KRB_PRIV message, but it is the
application's choice whether or not to use such an initialization vector.
If left out, the default initialization vector for the encryption algorithm
will be used.
[33] This prevents an attacker who generates an incorrect AS request from
obtaining verifiable plaintext for use in an off-line password guessing
attack.
[35] In the above specification, UNTAGGED OCTET STRING(length) is the
notation for an octet string with its tag and length removed. It is not a
valid ASN.1 type. The tag bits and length must be removed from the
confounder since the purpose of the confounder is so that the message
starts with random data, but the tag and its length are fixed. For other
fields, the length and tag would be redundant if they were included because
they are specified by the encryption type. [36] The ordering of the fields
in the CipherText is important. Additionally, messages encoded in this
format must include a length as part of the msg-seq field. This allows the
recipient to verify that the message has not been truncated. Without a
length, an attacker could use a chosen plaintext attack to generate a
message which could be truncated, while leaving the checksum intact. Note
that if the msg-seq is an encoding of an ASN.1 SEQUENCE or OCTET STRING,
then the length is part of that encoding.
[37] In some cases, it may be necessary to use a different "mix-in" string
for compatibility reasons; see the discussion of padata in section 5.4.2.
[38] In some cases, it may be necessary to use a different "mix-in" string
for compatibility reasons; see the discussion of padata in section 5.4.2.
[39] A variant of the key is used to limit the use of a key to a particular
function, separating the functions of generating a checksum from other
encryption performed using the session key. The constant F0F0F0F0F0F0F0F0
was chosen because it maintains key parity. The properties of DES precluded
the use of the complement. The same constant is used for similar purpose in
the Message Integrity Check in the Privacy Enhanced Mail standard.
[40] This error carries additional information in the e- data field. The
contents of the e-data field for this message is described in section
5.9.1.
Neuman, Ts'o, Kohl Expires: 18 May 1999
Datatracker
draft-ietf-cat-kerberos-revisions-03
Internet-Draft
