Subject: TGS-- The Ticket-Granting Service Exchange
Author: Alex_Raj
In response to: AS -- The Authentication Service Exchange
Posted on: 11/08/2006 07:23:27 PM
1. Client sends a request to KDC: KRB_TGS_REQ
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));
send(packet,kerberos);
wait(for response);
if (timed_out) then
retry or use alternate server;
endif
2. KDC sends a reply to client: KRB_TGS_REP or KRB_ERROR
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(server)) then
server := best_intermediate_tgs(server);
else
/* no server in Database */
error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
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.POSTDATE is set) then
if (tgt.flags.POSTDATE is reset)
error_out(KDC_ERR_BADOPTION);
endif
set new_tkt.flags.POSTDATE;
endif
if (req.kdc-options.POSTDATED is set) then
if (tgt.flags.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,
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)
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);
3. KRB_TGS_REP verification
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);
>
> On 11/08/2006 07:10:51 PM
Alex_Raj wrote:
1. Client sends a request to KDC: KRB_AS_REQ
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
2. KDC sneds a reply to client: KRB_AS_REP or KRB_ERROR
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;
endif
if (req.kdc-options.ALLOW-POSTDATE is set) then
set new_tkt.flags.ALLOW-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.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,
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);
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
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);
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
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
References:
