The 'fit' is determined by an algorithm 'known' to Bob and Alice. Alice must respond with the one string of characters which 'fits' the challenge Bob issued. Bob issues a challenge, perhaps '52w72y'.
Say, Bob is controlling access to some resource. For added security, each set of codes is only valid for a particular time period which is ordinarily 24 hours.Ī more interesting challenge–response technique works as follows. TRIAD includes a list of three-letter challenge codes, which the verifier is supposed to choose randomly from, and random three-letter responses to them. military uses the AKAC-1553 TRIAD numeral cipher to authenticate and encrypt some communications. Assuming that the passwords are chosen independently, an adversary who intercepts one challenge–response message pair has no clues to help with a different challenge at a different time.įor example, when other communications security methods are unavailable, the U.S. The verifier can ask for any of the passwords, and the prover must have that correct password for that identifier. One solution is to issue multiple passwords, each of them marked with an identifier. The simplest example of a challenge–response protocol is password authentication, where the challenge is asking for the password and the valid response is the correct password.Ĭlearly an adversary who can eavesdrop on a password authentication can then authenticate itself in the same way.
In computer security, challenge–response authentication is a family of protocols in which one party presents a question ('challenge') and another party must provide a valid answer ('response') to be authenticated.
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