Given a closed public-key device such as a typical smart card with its limited set of operations (chiefly "sign"), is it possible to implement a challenge/response function such that
* Both the challenge and the response are short enough for an average user to be willing to type them when needed.
* The challenge can be generated, and the response verified using the cardholder's public key and a reasonable amount of computation.
My reasoning is that the full output of the signing function will almost always be as long as the key, if only response = f(signature) is given, with f having a range in some set of size ~ 2^32, verifying response must be nearly as hard as brute-force guessing.
Matt Crawford <[EMAIL PROTECTED]>
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