Re: Hashes from composite keys?
From: David BL <davidbl_at_iinet.net.au>
Date: Tue, 27 Jul 2010 06:50:30 -0700 (PDT)
Message-ID: <ad982404-ac5e-43b0-9aad-587dc06b05b8_at_w15g2000pro.googlegroups.com>
Date: Tue, 27 Jul 2010 06:50:30 -0700 (PDT)
Message-ID: <ad982404-ac5e-43b0-9aad-587dc06b05b8_at_w15g2000pro.googlegroups.com>
On Jul 26, 6:27 pm, Clifford Heath <n..._at_spam.please.net> wrote:
> You're missing the point here. You cannot "identify" your objects
> using any kind of general hash code. All general hash codes will
> (in fact, *must*) sometimes map two different key values into the
> same hash code.
Yes in theory but not always in practise. E.g. SHA-2. A "decent" hash function with n output values requires about sqrt(n) evaluations for a successful birthday attack. In certain application domains (e.g. content addressable file stores) the chance of a collision can be made small enough for a suitable hash to be regarded as unique for all practical purposes. Received on Tue Jul 27 2010 - 15:50:30 CEST