linux-kernel - Re: [PATCH 2/3] [CRYPTO] Add optimized SHA-1 implementation for i486+

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Message-ID: <466D9FDB.2010305@cs.cmu.edu>
Date:	Mon, 11 Jun 2007 15:17:47 -0400
From:	Benjamin Gilbert <bgilbert@...cmu.edu>
To:	linux@...izon.com
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] [CRYPTO] Add optimized SHA-1 implementation for i486+

linux@...izon.com wrote:
> /* Majority: (x^y)|(y&z)|(z&x) = (x & z) + ((x ^ z) & y)
> #define F3(x,y,z,dest)		\
> 	movl	z, TMP;		\
> 	andl	x, TMP;		\
> 	addl	TMP, dest;	\
> 	movl	z, TMP;		\
> 	xorl	x, TMP;		\
> 	andl	y, TMP;		\
> 	addl	TMP, dest
> 
> Since y is the most recently computed result (it's rotated in the
> previous round), I arranged the code to delay its use as late as
> possible.
> 
> 
> Now you have one more register to play with.

Okay, thanks.  It doesn't actually give one more register except in the 
F3 rounds (TMP2 is normally used to hold the magic constants) but it's a 
good cleanup.

> A faster way is to unroll 5 iterations and do:
> 	e += F(b, c, d) + K + rol32(a, 5) + W[i  ]; b = rol32(b, 30);
> 	d += F(a, b, c) + K + rol32(e, 5) + W[i+1]; a = rol32(a, 30);
> 	c += F(e, a, b) + K + rol32(d, 5) + W[i+2]; e = rol32(e, 30);
> 	b += F(d, e, a) + K + rol32(c, 5) + W[i+3]; d = rol32(d, 30);
> 	a += F(c, d, e) + K + rol32(b, 5) + W[i+4]; c = rol32(c, 30);
> then loop over that 4 times each.  This is somewhat larger, but
> still reasonably compact; only 20 of the 80 rounds are written out
> long-hand.

I got this code from Nettle, originally, and I never looked at the SHA-1 
round structure very closely.  I'll give that approach a try.

Thanks
--Benjamin Gilbert
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