Security tokens

07-09-2005, 10:52 PM

I wonder if you have different tokens with different keys, token A
and token B given the same challenge will create the cipher texts CA
and CB respectively. Even though the cipher texts differ they may still
give the same response if CA mod 10^8 = CB mod 10^8.The token is a 56
bit DES-key and the input is 8 digits sending from a webserver and the
output is 8 digits
If an attacker even with infinite computer power cannot decide the key
of the token given just one challenge-response pair. How is it possible
to count how many challenge response pairs are needed to be able with
great probability to decide the key of a token for an attacker with
infinite computer power?

Ok Before you have one challenge-response pair I suppouse that there
are 2 ^56 keys in the token and the when you have one challenge
-response pair you had used 10^8 keys = 2^27 and of the truncated
value you have used 2^8 and the if you want to count a new challenge
-response pair how should I continue

Please send me some information how to solve this

Your regards Tomas

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Security tokens