3.1.3. Generation of KRB_AS_REP message

The authentication server looks up the client and server principals named in the KRB_AS_REQ in its database, extracting their respective keys. If required, the server pre-authenticates the request, and if the pre-authentication check fails, an error message with the code KDC_ERR_PREAUTH_FAILED is returned. If the server cannot accommodate the requested encryption type, an error message with code KDC_ERR_ETYPE_NOSUPP is returned. Otherwise it generates a "random" session key ("Random" means that, among other things, it should be impossible to guess the next session key based on knowledge of past session keys. This can only be achieved in a pseudo-random number generator if it is based on cryptographic principles. It would be more desirable to use a truly random number generator, such as one based on measurements of random physical phenomena.).

If the requested start time is absent or indicates a time in the past, then the start time of the ticket is set to the authentication server's current time. If it indicates a time in the future, but the POSTDATED option has not been specified, then the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise the requested start time is checked against the policy of the local realm (the administrator might decide to prohibit certain types or ranges of postdated tickets), and if acceptable, the ticket's start time is set as requested and the INVALID flag is set in the new ticket. The postdated ticket must be validated before use by presenting it to the KDC after the start time has been reached.

The expiration time of the ticket will be set to the minimum of the following:

• The expiration time (endtime) requested in the KRB_AS_REQ message.

• The ticket's start time plus the maximum allowable lifetime associated with the client principal (the authentication server's database includes a maximum ticket lifetime field in each principal's record; see section 4).

• The ticket's start time plus the maximum allowable lifetime associated with the server principal.

• The ticket's start time plus the maximum lifetime set by the policy of the local realm.

If the requested expiration time minus the start time (as determined above) is less than a site-determined minimum lifetime, an error message with code KDC_ERR_NEVER_VALID is returned. If the requested expiration time for the ticket exceeds what was determined as above, and if the "RENEWABLE-OK" option was requested, then the "RENEWABLE" flag is set in the new ticket, and the renew-till value is set as if the "RENEWABLE" option were requested (the field and option names are described fully in section 5.4.1). If the RENEWABLE option has been requested or if the RENEWABLE-OK option has been set and a renewable ticket is to be issued, then the renew-till field is set to the minimum of:

• Its requested value.

• The start time of the ticket plus the minimum of the two maximum renewable lifetimes associated with the principals' database entries.

• The start time of the ticket plus the maximum renewable lifetime set by the policy of the local realm.

The flags field of the new ticket will have the following options set if they have been requested and if the policy of the local realm allows: FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE. If the new ticket is postdated (the start time is in the future), its INVALID flag will also be set.

If all of the above succeed, the server formats a KRB_AS_REP message (see section 5.4.2), copying the addresses in the request into the caddr of the response, placing any required pre-authentication data into the padata of the response, and encrypts the ciphertext part in the client's key using the requested encryption method, and sends it to the client. See section A.2 for pseudocode.