Commit 750426d6 authored by Eric Biggers's avatar Eric Biggers Committed by Herbert Xu
Browse files

crypto: x86/ghash - add comment and fix broken link 



Add a comment that explains what ghash_setkey() is doing, as it's hard
to understand otherwise.  Also fix a broken hyperlink.

Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
parent f1740751

arch/x86/crypto/ghash-clmulni-intel_asm.S

+1 −1
Original line number Diff line number Diff line
@@ -4,7 +4,7 @@ 
 * instructions. This file contains accelerated part of ghash

 * implementation. More information about PCLMULQDQ can be found at:

 *

 * http://software.intel.com/en-us/articles/carry-less-multiplication-and-its-usage-for-computing-the-gcm-mode/

 * https://www.intel.com/content/dam/develop/external/us/en/documents/clmul-wp-rev-2-02-2014-04-20.pdf

 *

 * Copyright (c) 2009 Intel Corp.

 *   Author: Huang Ying <ying.huang@intel.com>

arch/x86/crypto/ghash-clmulni-intel_glue.c

+23 −4
Original line number Diff line number Diff line
@@ -60,16 +60,35 @@ static int ghash_setkey(struct crypto_shash *tfm, 
	if (keylen != GHASH_BLOCK_SIZE)

		return -EINVAL;



	/* perform multiplication by 'x' in GF(2^128) */

	/*

	 * GHASH maps bits to polynomial coefficients backwards, which makes it

	 * hard to implement.  But it can be shown that the GHASH multiplication

	 *

	 *	D * K (mod x^128 + x^7 + x^2 + x + 1)

	 *

	 * (where D is a data block and K is the key) is equivalent to:

	 *

	 *	bitreflect(D) * bitreflect(K) * x^(-127)

	 *		(mod x^128 + x^127 + x^126 + x^121 + 1)

	 *

	 * So, the code below precomputes:

	 *

	 *	bitreflect(K) * x^(-127) (mod x^128 + x^127 + x^126 + x^121 + 1)

	 *

	 * ... but in Montgomery form (so that Montgomery multiplication can be

	 * used), i.e. with an extra x^128 factor, which means actually:

	 *

	 *	bitreflect(K) * x (mod x^128 + x^127 + x^126 + x^121 + 1)

	 *

	 * The within-a-byte part of bitreflect() cancels out GHASH's built-in

	 * reflection, and thus bitreflect() is actually a byteswap.

	 */

	a = get_unaligned_be64(key);

	b = get_unaligned_be64(key + 8);



	ctx->shash.a = cpu_to_le64((a << 1) | (b >> 63));

	ctx->shash.b = cpu_to_le64((b << 1) | (a >> 63));



	if (a >> 63)

		ctx->shash.a ^= cpu_to_le64((u64)0xc2 << 56);



	return 0;

}