The link below leads to a very secure login system, the way this system works is that it sets an encrypted cookie on your computer when you login. Since is it impossible to forge the encrypted cookie the admin status of the user is stored in the cookie. The system does however validate the username stored in the cookie.
You must forge a cookie that makes your user appear to be an admin.
Fortunately for you the system is still in debugging mode, so it spits out plenty of useful input. There's also a test-user available with both username and password set to 'test1'.
Just in case it would be needed you've also managed to sniff the cookie of an admin-user:
login_cookie=8bc190be479a1515a334bb5d04ded999c675891ac7d944eebd8b3425411ce09ac27c9c3e6a69be01c1f63d8550223a07884b0a6f4ef5553d8d7ea5ea7e13d3e5f0fd9817bec86b8c4d9add344451439216cae8fddb23873396c5f104caa63558fbd2cd2c15013475b73c78a1239aae2175c6a1ca0cf197f051ac1314c8e222346667ad84f80eae366450ee02a8d30f614b84117776941361d7f5d48992a9d4748927289b37ecd3626440437f7929c52f
The site is online at http://hack.tg13.gathering.org/content/mixed/webcrypt/index.php