From: Eric Biggers <ebiggers@xxxxxxxxxx> 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@xxxxxxxxxx> --- arch/x86/crypto/ghash-clmulni-intel_asm.S | 2 +- arch/x86/crypto/ghash-clmulni-intel_glue.c | 27 ++++++++++++++++++---- 2 files changed, 24 insertions(+), 5 deletions(-) diff --git a/arch/x86/crypto/ghash-clmulni-intel_asm.S b/arch/x86/crypto/ghash-clmulni-intel_asm.S index 9dfeb4d31b92..257ed9446f3e 100644 --- a/arch/x86/crypto/ghash-clmulni-intel_asm.S +++ b/arch/x86/crypto/ghash-clmulni-intel_asm.S @@ -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@xxxxxxxxx> diff --git a/arch/x86/crypto/ghash-clmulni-intel_glue.c b/arch/x86/crypto/ghash-clmulni-intel_glue.c index 9453b094bb3b..700ecaee9a08 100644 --- a/arch/x86/crypto/ghash-clmulni-intel_glue.c +++ b/arch/x86/crypto/ghash-clmulni-intel_glue.c @@ -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; } -- 2.39.0
