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List:       otr-dev
Subject:    [OTR-dev] No hash truncation in DSA signatures
From:       Adam Langley <agl () imperialviolet ! org>
Date:       2011-11-29 21:37:41
Message-ID: CAMfhd9VZpavHY46rLk6dJHD_hkQn1MtGjOK4YCQvRSnZNFLKhw () mail ! gmail ! com
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In http://www.cypherpunks.ca/otr/Protocol-v2-3.1.0.html, it says:

"This is the signature, using the private part of the key pubB, of the
32-byte MB (which does not need to be hashed again to produce the
signature)."

In http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf, section 4.6:

"z = the leftmost min(N, outlen) bits of Hash(M)"

Where outlen is the output length of the hash function (256 here) and
N is the bit length of q (160 for OTR).

libgcrypt doesn't do this and, therefore, not does the OTR protocol. I
think it's worth making a note of that - it screwed me up for a while
:)


Cheers

AGL

-- 
Adam Langley agl@imperialviolet.org http://www.imperialviolet.org
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