phc-discussions - Halting Password Puzzle support

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Date: Thu, 6 Feb 2014 17:19:32 -0500
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Halting Password Puzzle support

I just added Halting Password Puzzle support to my code.  There are no
patent issues with it are there?  Catena and other entries supporting
garlic could use this as well.  I think it should be used in
TrueCrypt, since it is critical there to have deniability, so we can't
have any obvious parameters that look non-random.  The only two
parameters we can store that I can see are:

- salt
- expected hash

We just start with memory == small (1MiB), and iteratively increase
garlic like we do with client-independent update, doubling memory each
time.  When we get the expected hash, or reach a pre-set quitting
level, we're done.  This really is how TrueCrypt should work.  It's
password security for now is pretty weak against brute-force attacks.
The code looks like:

    resultHash = PasswordHash(password, salt)
    for(garlic = 0; garlic < quitGarlic; garlic++) {
        ApplyGarlic(resultHash, garlic)
        if(resultHash == expectedHash) {
            return true
        }
    }
    return false

Bill