phc-discussions - Suggestion: API should include a verifier function

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Date: Tue, 26 Mar 2013 19:57:18 -0700
From: Tony Arcieri <tony.arcieri@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Suggestion: API should include a verifier function

I think to be a "complete" solution, the API should include a verify
function which guarantees a constant time comparison, similar to what NaCl
provides for its authenticators:

http://nacl.cr.yp.to/auth.html

e.g.

    crypto_auth(a,m,mlen,k);
    crypto_auth_verify(a,m,mlen,k);

Without such an API, I am afraid that users of the API will screw up the
comparison and open themselves up to a timing attack.

The arguments of the verifier function are identical to the original
hashing function, except instead of calculating a password hash, you'd
provide a user-supplied one as input, and verify via a guaranteed constant
time comparison whether or not it's correct.

-- 
Tony Arcieri

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