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,
                 padata[2]                  SEQUENCE OF PA-DATA OPTIONAL,
                 crealm[3]                  Realm,
                 cname[4]                   PrincipalName,
                 ticket[5]                  Ticket,
                 enc-part[6]                EncryptedData
   }

   EncASRepPart ::=    [APPLICATION 25[25]] 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
   }

NOTE: In EncASRepPart, the application code in the encrypted part of a message provides an additional check that the message was decrypted properly.

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 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 expira tion 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.