What is the auth_tkt cookie format?

Question

The Apache module auth_tkt creates authentication cookies that can be verified cryptographically so a web server can generate REMOTE_USER without consulting a database. What is the specification for auth_tkt cookies?

Answer 1

From the README inside http://www.openfusion.com.au/labs/dist/mod_auth_tkt/mod_auth_tkt-2.1.0.tar.gz

Cookie Format

The TKTAuthCookieName cookie is constructed using following algorithm:`

('+' is concatenation operation)

cookie := digest + hextimestamp + user_id + '!' + user_data

or if using tokens:

cookie := digest + hextimestamp + user_id + '!' + token_list + '!' + user_data

digest := MD5(digest0 + key)

digest0 := MD5(iptstamp + key + user_id + '\0' + token_list + '\0' + user_data)

iptstamp is a 8 bytes long byte array, bytes 0-3 are filled with
client's IP address as a binary number in network byte order, bytes
4-7 are filled with timestamp as a binary number in network byte
order. 

hextimestamp is 8 character long hexadecimal number expressing
timestamp used in iptstamp. 

token_list is an optional comma-separated list of access tokens 
for this user. This list is checked if TKTAuthToken is set for a
particular area.

user_data is optional