[53658] in cryptography@c2.net mail archive
hashes on restricted domains: random functions or permutations?
daemon@ATHENA.MIT.EDU (Travis H.)
Tue Oct 17 23:18:30 2006
X-Original-To: cryptography@metzdowd.com
X-Original-To: cryptography@metzdowd.com
Date: Tue, 17 Oct 2006 19:13:11 -0500
From: "Travis H." <solinym@gmail.com>
To: Cryptography <cryptography@metzdowd.com>
So I was reading about the OTP system (based on S/Key) described in RFC 2289.
It basically hashes a secret several times (with salt to individualize
it) and stores
the value that the correct password will hash to.
Now my question is, if we restrict ourselves to, say, 160-bit inputs, is SHA-1
a permutation, or do collisions exist? If there are collisions, then iterating
the hash could lead to fewer possible values each time, potentially converging
on a set of inputs that form a permutation and are closed under composition.
Is that correct? What are the expected sizes of such sets?
Is it worth worrying about?
--
"The obvious mathematical breakthrough would be the development of an
easy way to factor large prime numbers.'' [sic] -- Bill Gates -><-
<URL:http://www.subspacefield.org/~travis/>
GPG fingerprint: 9D3F 395A DAC5 5CCC 9066 151D 0A6B 4098 0C55 1484
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