Lecture 3.1: Handling Remote Access: RADIUS Motivation

poor PRNG implementations
Attack to Customer passwords /1
Passively monitor the network traffic allows to build a dictionary of Request Authenticators
and the associated (protected) User-Password attributes
Valid users NAS
Access-Request (Request authenticator)
Dictionary of ReqAuth/User-Password
Repeated Request Authenticator observed
XOR previous user-password with new user-password
From different users
Result: since ReqAuth is the same
(user-password #1) XOR (user-Password #2) =
= [pw_user1 XOR MD5(secret,ReqAuth)] XOR [ps_user2 XOR MD5(secret,ReqAuth)] =
= pw_user1 XOR pw_user2
BUT passwords from different users differ in length:
last characters of longer password are put in clear!!
Password sizes are known!!
Now intelligent dictionary attack (guided by improper habits to select
passwords with low entropy) may clear passwords
Giuseppe Bianchi
poor PRNG implementations
Attack to Customer passwords /2
ACTIVELY submit known passwords to add known passwords to the dictionary of Request
Authenticators and the associated (protected) User-Password attributes
Arbitrary pw NAS
Giuseppe Bianchi
Access-Request (Request authenticator)
Dictionary of ReqAuth/User-Password
If customer Access-Request uses a Request
Authenticator already in the dictionary linked to a
KNOWN password, customer password gets known!
Since ReqAuth is the same
(User-password from customer) XOR (user-Password from attacker) =
= [pw_user XOR MD5(secret,ReqAuth)] XOR
[known_pw XOR MD5(secret,ReqAuth)] =
= pw_user XOR known_pw pw_user in clear
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