must be loosely synchronized), the authenticator, and the ticket.
Several errors are possible. If an error occurs, the server is
expected to reply to the client with a KRB_ERROR message. This
message may be encapsulated in the application protocol if its "raw"
form is not acceptable to the protocol. The format of error messages
is described in section 5.9.1.
The algorithm for verifying authentication information is as follows.
If the message type is not KRB_AP_REQ, the server returns the
KRB_AP_ERR_MSG_TYPE error. If the key version indicated by the Ticket
in the KRB_AP_REQ is not one the server can use (e.g., it indicates
an old key, and the server no longer possesses a copy of the old
key), the KRB_AP_ERR_BADKEYVER error is returned. If the USE-
SESSION-KEY flag is set in the ap-options field, it indicates to the
server that the ticket is encrypted in the session key from the
server's ticket-granting ticket rather than its secret key (This is
used for user-to-user authentication as described in [6]). Since it
is possible for the server to be registered in multiple realms, with
different keys in each, the srealm field in the unencrypted portion
of the ticket in the KRB_AP_REQ is used to specify which secret key
the server should use to decrypt that ticket. The KRB_AP_ERR_NOKEY
error code is returned if the server doesn't have the proper key to
decipher the ticket.
The ticket is decrypted using the version of the server's key
specified by the ticket. If the decryption routines detect a
modification of the ticket (each encryption system must provide
safeguards to detect modified ciphertext; see section 6), the
KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that
different keys were used to encrypt and decrypt).
The authenticator is decrypted using the session key extracted from
the decrypted ticket. If decryption shows it to have been modified,
the KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm
of the client from the ticket are compared against the same fields in
the authenticator. If they don't match, the KRB_AP_ERR_BADMATCH
error is returned (they might not match, for example, if the wrong
session key was used to encrypt the authenticator). The addresses in
the ticket (if any) are then searched for an address matching the
operating-system reported address of the client. If no match is
found or the server insists on ticket addresses but none are present
in the ticket, the KRB_AP_ERR_BADADDR error is returned.
If the local (server) time and the client time in the authenticator
differ by more than the allowable clock skew (e.g., 5 minutes), the
KRB_AP_ERR_SKEW error is returned. If the server name, along with
the client name, time and microsecond fields from the Authenticator
match any recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
returned (Note that the rejection here is restricted to
authenticators from the same principal to the same server. Other
client principals communicating with the same server principal should
not be have their authenticators rejected if the time and microsecond
fields happen to match some other client's authenticator.). The
server must remember any authenticator presented within the allowable
clock skew, so that a replay attempt is guaranteed to fail. If a
server loses track of any authenticator presented within the
allowable clock skew, it must reject all requests until the clock
skew interval has passed. This assures that any lost or re-played
authenticators will fall outside the allowable clock skew and can no
longer be successfully replayed (If this is not done, an attacker
could conceivably record the ticket and authenticator sent over the
network to a server, then disable the client's host, pose as the
disabled host, and replay the ticket and authenticator to subvert the
authentication.). If a sequence number is provided in the
authenticator, the server saves it for later use in processing
KRB_SAFE and/or KRB_PRIV messages. If a subkey is present, the
server either saves it for later use or uses it to help generate its
own choice for a subkey to be returned in a KRB_AP_REP message.
The server computes the age of the ticket: local (server) time minus
the start time inside the Ticket. If the start time is later than
the current time by more than the allowable clock skew or if the
INVALID flag is set in the ticket, the KRB_AP_ERR_TKT_NYV error is
returned. Otherwise, if the current time is later than end time by
more than the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED error
is returned.
If all these checks succeed without an error, the server is assured
that the client possesses the credentials of the principal named in
the ticket and thus, the client has been authenticated to the server.
See section A.10 for pseudocode.
3.2.4. Generation of a KRB_AP_REP message
Typically, a client's request will include both the authentication
information and its initial request in the same message, and the
server need not explicitly reply to the KRB_AP_REQ. However, if
mutual authentication (not only authenticating the client to the
server, but also the server to the client) is being performed, the
KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options
field, and a KRB_AP_REP message is required in response. As with the
error message, this message may be encapsulated in the application
protocol if its "raw" form is not acceptable to the application's
protocol. The timestamp and microsecond field used in the reply must
be the client's timestamp and microsecond field (as provided in the
authenticator). [Note: In the Kerberos version 4 protocol, the
timestamp in the reply was the client's timestamp plus one. This is
not necessary in version 5 because version 5 messages are formatted
in such a way that it is not possible to create the reply by
judicious message surgery (even in encrypted form) without knowledge
of the appropriate encryption keys.] If a sequence number is to be
included, it should be randomly chosen as described above for the
authenticator. A subkey may be included if the server desires to
negotiate a different subkey. The KRB_AP_REP message is encrypted in
the session key extracted from the ticket. See section A.11 for
pseudocode.
3.2.5. Receipt of KRB_AP_REP message
If a KRB_AP_REP message is returned, the client uses the session key
from the credentials obtained for the server (Note that for
encrypting the KRB_AP_REP message, the sub-session key is not used,
even if present in the Authenticator.) to decrypt the message, and
verifies that the timestamp and microsecond fields match those in the
Authenticator it sent to the server. If they match, then the client
is assured that the server is genuine. The sequence number and subkey
(if present) are retained for later use. See section A.12 for
pseudocode.
3.2.6. Using the encryption key
After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and
server share an encryption key which can be used by the application.
The "true session key" to be used for KRB_PRIV, KRB_SAFE, or other
application-specific uses may be chosen by the application based on
the subkeys in the KRB_AP_REP message and the authenticator
(Implementations of the protocol may wish to provide routines to
choose subkeys based on session keys and random numbers and to
orchestrate a negotiated key to be returned in the KRB_AP_REP
message.). In some cases, the use of this session key will be
implicit in the protocol; in others the method of use must be chosen
from a several alternatives. We leave the protocol negotiations of
how to use the key (e.g., selecting an encryption or checksum type)
to the application programmer; the Kerberos protocol does not
constrain the implementation options.
With both the one-way and mutual authentication exchanges, the peers
should take care not to send sensitive information to each other
without proper assurances. In particular, applications that require
privacy or integrity should use the KRB_AP_REP or KRB_ERROR responses
from the server to client to assure both client and server of their
peer's identity. If an application protocol requires privacy of its
messages, it can use the KRB_PRIV message (section 3.5). The KRB_SAFE
message (section 3.4) can be used to assure integrity.
3.3. The Ticket-Granting Service (TGS) Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_TGS_REQ 5.4.1
2. Kerberos to client KRB_TGS_REP or 5.4.2
KRB_ERROR 5.9.1
The TGS exchange between a client and the Kerberos Ticket-Granting
Server is initiated by a client when it wishes to obtain
authentication credentials for a given server (which might be
registered in a remote realm), when it wishes to renew or validate an
existing ticket, or when it wishes to obtain a proxy ticket. In the
first case, the client must already have acquired a ticket for the
Ticket-Granting Service using the AS exchange (the ticket-granting
ticket is usually obtained when a client initially authenticates to
the system, such as when a user logs in). The message format for the
TGS exchange is almost identical to that for the AS exchange. The
primary difference is that encryption and decryption in the TGS
exchange does not take place under the client's key. Instead, the
session key from the ticket-granting ticket or renewable ticket, or
sub-session key from an Authenticator is used. As is the case for
all application servers, expired tickets are not accepted by the TGS,
so once a renewable or ticket-granting ticket expires, the client
must use a separate exchange to obtain valid tickets.
The TGS exchange consists of two messages: A request (KRB_TGS_REQ)
from the client to the Kerberos Ticket-Granting Server, and a reply
(KRB_TGS_REP or KRB_ERROR). The KRB_TGS_REQ message includes
information authenticating the client plus a request for credentials.
The authentication information consists of the authentication header
(KRB_AP_REQ) which includes the client's previously obtained ticket-
granting, renewable, or invalid ticket. In the ticket-granting
ticket and proxy cases, the request may include one or more of: a
list of network addresses, a collection of typed authorization data
to be sealed in the ticket for authorization use by the application
server, or additional tickets (the use of which are described later).
The TGS reply (KRB_TGS_REP) contains the requested credentials,
encrypted in the session key from the ticket-granting ticket or
renewable ticket, or if present, in the subsession key from the
Authenticator (part of the authentication header). The KRB_ERROR
message contains an error code and text explaining what went wrong.
The KRB_ERROR message is not encrypted. The KRB_TGS_REP message
contains information which can be used to detect replays, and to
associate it with the message to which it replies. The KRB_ERROR
message also contains information which can be used to associate it
with the message to which it replies, but the lack of encryption in
the KRB_ERROR message precludes the ability to detect replays or
fabrications of such messages.
3.3.1. Generation of KRB_TGS_REQ message
Before sending a request to the ticket-granting service, the client
must determine in which realm the application server is registered
[Note: This can be accomplished in several ways. It might be known
beforehand (since the realm is part of the principal identifier), or
it might be stored in a nameserver. Presently, however, this
information is obtained from a configuration file. If the realm to
be used is obtained from a nameserver, there is a danger of being
spoofed if the nameservice providing the realm name is not
authenticated. This might result in the use of a realm which has
been compromised, and would result in an attacker's ability to
compromise the authentication of the application server to the
client.]. If the client does not already possess a ticket-granting
ticket for the appropriate realm, then one must be obtained. This is
first attempted by requesting a ticket-granting ticket for the
destination realm from the local Kerberos server (using the
KRB_TGS_REQ message recursively). The Kerberos server may return a
TGT for the desired realm in which case one can proceed.
Alternatively, the Kerberos server may return a TGT for 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; as long as the secret keys exchanged by realms
are kept secret, only denial of service results from a false Kerberos
server.
As in the AS exchange, the client may specify a number 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 (If the client selects a sub-session key, care must be
taken to ensure the randomness of the selected subsession key. One
approach would be to generate a random number and XOR it with the
session key from the ticket-granting ticket.). If the sub-session key
is not specified, the session key from the ticket-granting ticket
will be used. If the enc-authorization-data is present, it must be
encrypted in the sub-session key, if present, from the authenticator
portion of the authentication header, or if not present in the
session key from the ticket-granting ticket.
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 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 (endtimestarttime) of the old
ticket.
If the FORWARDED option has been requested, then the resulting ticket
will contain the addresses specified by the client. This option will
only be honored if the FORWARDABLE flag is set in the TGT. The PROXY
option is similar; the resulting ticket will contain the addresses
specified by the client. It will be honored only if the PROXIABLE
flag in the TGT is set. The PROXY option will not be honored on
requests for additional ticket-granting tickets.
If the requested start time is absent or indicates a time in the
past, then the start time of the ticket is set to the authentication
server's current time. If it indicates a time in the future, but the
POSTDATED option has not been specified or the MAY-POSTDATE flag is
not set in the TGT, then the error KDC_ERR_CANNOT_POSTDATE is
returned. Otherwise, if the ticket-granting ticket has the
MAYPOSTDATE flag set, then the resulting ticket will be postdated and
the requested starttime is checked against the policy of the local
realm. If acceptable, the ticket's start time is set as requested,
and the INVALID flag is set. The postdated ticket must be validated
before use by presenting it to the KDC after the starttime has been
reached. However, in no case may the starttime, endtime, or renew-
till time of a newly-issued postdated ticket extend beyond the
renew-till time of the ticket-granting ticket.
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 (This allows easy implementation of user-to-
user authentication [6], which uses ticket-granting ticket session
keys in lieu of secret server keys in situations where such secret
keys could be easily compromised.).
If the name of the server in the ticket that is presented to the KDC
as part of the authentication header is not that of the ticket-
granting server itself, and the server is registered in the realm of
the KDC, If the RENEW option is requested, then the KDC will verify
that the RENEWABLE flag is set in the ticket and that the renew_till
time is still in the future. If the VALIDATE option is rqeuested,
the KDC will check that the starttime has passed and the INVALID flag
is set. If the PROXY option is requested, then the KDC will check
that the PROXIABLE flag is set in the ticket. If the tests succeed,
the KDC will issue the appropriate new ticket.
Whenever a request is made to the ticket-granting server, the
presented ticket(s) is(are) checked against a hot-list of tickets
which have been canceled. This hot-list might be implemented by
storing a range of issue dates for "suspect tickets"; if a presented
ticket had an authtime in that range, it would be rejected. In this
way, a stolen ticket-granting ticket or renewable ticket cannot be
used to gain additional tickets (renewals or otherwise) once the
theft has been reported. Any normal ticket obtained before it was
reported stolen will still be valid (because they require no
interaction with the KDC), but only until their normal expiration
time.
The ciphertext part of the response in the KRB_TGS_REP message is
encrypted in the sub-session key from the Authenticator, if present,
or the session key key from the ticket-granting ticket. It is not
encrypted using the client's secret key. Furthermore, the client's
key's expiration date and the key version number fields are left out
since these values are stored along with the client's database
record, and that record is not needed to satisfy a request based on a
ticket-granting ticket. See section A.6 for pseudocode.
3.3.3.1. 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 preceding them with a "\".
A realm name ending with a "." is interpreted as being prepended to
the previous realm. For example, we can encode traversal of EDU,
MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:
"EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".
Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were endpoints,
that they would not be included in this field, and we would have:
"EDU,MIT.,WASHINGTON.EDU"
A realm name beginning with a "/" is interpreted as being appended to
the previous realm (For the purpose of appending, the realm preceding
the first listed realm is considered to be the null realm ("")). If
it is to stand by itself, then it should be preceded by a space ("
"). For example, we can encode traversal of /COM/HP/APOLLO, /COM/HP,
/COM, and /COM/DEC as:
"/COM,/HP,/APOLLO, /COM/DEC".
Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints,
they they would not be included in this field, and we would have:
"/COM,/HP"
A null subfield preceding or following a "," indicates that all
realms between the previous realm and the next realm have been
traversed (For the purpose of interpreting null subfields, the
client's realm is considered to precede those in the transited field,
and the server's realm is considered to follow them.). Thus, ","
means that all realms along the path between the client and the
server have been traversed. ",EDU, /COM," means that that all realms
from the client's realm up to EDU (in a domain style hierarchy) have
been traversed, and that everything from /COM down to the server's
realm in an X.500 style has also been traversed. This could occur if
the EDU realm in one hierarchy shares an inter-realm key directly
with the /COM realm in another hierarchy.
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 collisionproof 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 some sort of
keyed one-way hash function (such as the RSA-MD5-DES checksum
algorithm specified in section 6.4.5, or the DES MAC), generated
using the sub-session key if present, or the session key. Different
algorithms may be selected by changing the checksum type in the
message. Unkeyed or non-collision-proof checksums are not suitable
for this use.
The control information for the KRB_SAFE message includes both a
timestamp and a sequence number. The designer of an application
using the KRB_SAFE message must choose at least one of the two
mechanisms. This choice should be based on the needs of the
application protocol.
Sequence numbers are useful when all messages sent will be received
by one's peer. Connection state is presently required to maintain
the session key, so maintaining the next sequence number should not
present an additional problem.
If the application protocol is expected to tolerate lost messages
without them being resent, the use of the timestamp is the
appropriate replay detection mechanism. Using timestamps is also the
appropriate mechanism for multi-cast protocols where all of one's
peers share a common sub-session key, but some messages will be sent
to a subset of one's peers.
After computing the checksum, the client then transmits the
information and checksum to the recipient in the message format
specified in section 5.6.1.
3.4.2. Receipt of KRB_SAFE message
When an application receives a KRB_SAFE message, it verifies it as
follows. If any error occurs, an error code is reported for use by
the application.
The message is first checked by verifying that the protocol version
and type fields match the current version and KRB_SAFE, respectively.
A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
error. The application verifies that the checksum used is a
collisionproof keyed checksum, and if it is not, a
KRB_AP_ERR_INAPP_CKSUM error is generated. The recipient verifies
that the operating system's report of the sender's address matches
the sender's address in the message, and (if a recipient address is
specified or the recipient requires an address) that one of the
recipient's addresses appears as the recipient's address in the
message. A failed match for either case generates a
KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the
sequence number fields are checked. If timestamp and usec are
expected and not present, or they are present but not current, the
KRB_AP_ERR_SKEW error is generated. If the server name, along with
the client name, time and microsecond fields from the Authenticator
match any recently-seen such tuples, the KRB_AP_ERR_REPEAT error is
generated. If an incorrect sequence number is included, or a
sequence number is expected but not present, the KRB_AP_ERR_BADORDER
error is generated. If neither a timestamp and usec or a sequence
number is present, a KRB_AP_ERR_MODIFIED error is generated.
Finally, the checksum is computed over the data and control
information, and if it doesn't match the received checksum, a
KRB_AP_ERR_MODIFIED error is generated.
If all the checks succeed, the application is assured that the
message was generated by its peer and was not modified in transit.
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.
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 timestamp 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 raddress fields, are placed in the encrypted part of the
KRB_CRED message which is then encrypted under an encryption key
previosuly exchanged in the KRB_AP exchange (usually the last key
negotiated via subkeys, or the session key if no negotiation has
occured).
3.6.2. Receipt of KRB_CRED message
When an application receives a KRB_CRED message, it verifies it. If
any error occurs, an error code is reported for use by the
application. The message is verified by checking that the protocol
version and type fields match the current version and KRB_CRED,
respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or
KRB_AP_ERR_MSG_TYPE error. The application then decrypts the
ciphertext and processes the resultant plaintext. If decryption shows
the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY error is
generated.
If present or required, the recipient 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 containing the
principal identifiers and secret keys of principals to be
authenticated (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.).
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.
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 [7]), or to identify the "string to key" conversion algorithm
used for a principal's key. (See the discussion of the padata field
in section 5.4.2 for details on why this can be useful.) Other bits
are used to indicate that certain ticket options should not be
allowed in tickets encrypted under a principal's key (one bit each):
Disallow issuing postdated tickets, disallow issuing forwardable
tickets, disallow issuing tickets based on TGT authentication,
disallow issuing renewable tickets, disallow issuing proxiable
tickets, and disallow issuing tickets for which the principal is the
server.
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).
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.
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 [8].
5.2. 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 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 General
String. 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 SEQUENCE {
addr-type[0] INTEGER,
address[1] OCTET STRING
}
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
}
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.
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)
}
KDCOptions ::= BIT STRING {
reserved(0),
forwardable(1),
forwarded(2),
proxiable(3),
proxy(4),
allow-postdate(5),
postdated(6),
unused7(7),
renewable(8),
unused9(9),
unused10(10),
unused11(11),
renewable-ok(27),
enc-tkt-in-skey(28),
renew(30),
validate(31)
}
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).
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
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.