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

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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: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
This commit is contained in:
Eric Biggers 2022-12-19 21:40:42 -08:00 • committed by Herbert Xu
parent f1740751f7
commit 750426d633
2 changed files with 24 additions and 5 deletions
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2
arch/x86/crypto/ghash-clmulni-intel_asm.S
View file

@ -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>

27
arch/x86/crypto/ghash-clmulni-intel_glue.c
View file

@ -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;
}