[53776] in cryptography@c2.net mail archive
Re: hashes on restricted domains: random functions or permutations?
daemon@ATHENA.MIT.EDU (James A. Donald)
Wed Oct 18 07:16:54 2006
X-Original-To: cryptography@metzdowd.com
X-Original-To: cryptography@metzdowd.com
Date: Wed, 18 Oct 2006 19:04:42 +1000
From: "James A. Donald" <jamesd@echeque.com>
To: "Travis H." <solinym@gmail.com>
Cc: Cryptography <cryptography@metzdowd.com>
In-Reply-To: <d4f1333a0610171713k414561fctdba16c14559e2356@mail.gmail.com>
Travis H. wrote:
> 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?
Yes.
> What are the expected sizes of such sets?
More relevant is how many iterations it takes to get to a significantly
smaller set.
> Is it worth worrying about?
No.
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