1. Client sends a request to Application server KRB_AP_REQ
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
packet.ticket := ticket; /* ticket */
generate authenticator;
encode authenticator into OCTET STRING;
encrypt OCTET STRING into packet.authenticator
using session_key;
2. 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 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
/* caller must check decr_ticket.flags for any pertinent */
/* details */
return(OK, decr_ticket, packet.ap_options.MUTUAL-REQUIRED);
3. Application server send s a reply to client KRB_AP_REP or KRB_ERROR
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;
4. 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
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);
