Date: Thu, 14 Aug 2003 22:36:00 +0100
From: Jamie Lokier <jamie@shareable.org>
To: David Wagner <daw@mozart.cs.berkeley.edu>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH] Make cryptoapi non-optional?
Message-ID: <20030814213600.GA12420@mail.jlokier.co.uk>
References: <20030809173329.GU31810@waste.org> <20030813032038.GA1244@think> <20030813040614.GP31810@waste.org> <20030814165320.GA2839@speare5-1-14> <bhgoj9$9ab$1@abraham.cs.berkeley.edu>
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David Wagner wrote:
> Val Henson  wrote:
> >Throwing away 80 bits of the 160 bit output is much better
> >than folding the two halves together.  In all the cases we've
> >discussed where folding might improve matters, throwing away half the
> >output would be even better.
> 
> I don't see where you are getting this from.  Define
>   F(x) = first80bits(SHA(x))
>   G(x) = first80bits(SHA(x)) xor last80bits(SHA(x)).
> What makes you think that F is a better (or worse) hash function than G?
> 
> I think there is little basis for discriminating between them.
> If SHA is cryptographically secure, both F and G are fine.
> If SHA is insecure, then all bets are off, and both F and G might be weak.

I still do not see why either F or G are any more secure than SHA.

F, G and SHA are all supposedly strong hash functions, and I don't see
why the postulated folks capable of getting useful information about
the inputs to SHA would have any more difficulty getting useful
information about the inputs to F or G.

Unless we're postulating that SHA is deliberately weak, so that the
designers have a back door, that is not present in F or G.

Could some explain, please?

-- Jamie
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