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cmac: factor multiply by u to its own function

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We're doing exactly the same operation for K1 and K2.
This commit is contained in:
Manuel Pégourié-Gonnard 2016-01-13 11:28:16 +00:00 committed by Simon Butcher
parent 3da5402a89
commit a610b4c04b
1 changed files with 17 additions and 24 deletions
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41
library/cmac.c
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@ -82,24 +82,28 @@ void mbedtls_cmac_init( mbedtls_cmac_context *ctx )
} }
/* /*
* Leftshift a 16-byte block by 1 bit * Multiply by u in GF(2^128)
* \note output can be same as input *
* As explained in the paper, this can be achieved as
* If MSB(p) = 0, then p = (p << 1)
* If MSB(p) = 1, then p = (p << 1) ^ Rb
* with Rb = 0x87
*
* Input and output MUST not point to the same buffer
*/ */
static void leftshift_onebit( unsigned char *input, unsigned char *output ) static void multiply_by_u( unsigned char *output, const unsigned char *input )
{ {
int i; static const unsigned char Rb[2] = { 0x00, 0x87 }; /* block size 16 only */
unsigned char temp;
unsigned char overflow = 0; unsigned char overflow = 0;
int i;
for( i = 15; i >= 0; i-- ) for( i = 15; i >= 0; i-- )
{ {
temp = input[i]; output[i] = input[i] << 1 | overflow;
output[i] = temp << 1; overflow = input[i] >> 7;
output[i] |= overflow;
overflow = temp >> 7;
} }
return; output[15] ^= Rb[input[0] >> 7]; /* "Constant-time" operation */
} }
/* /*
@ -107,7 +111,6 @@ static void leftshift_onebit( unsigned char *input, unsigned char *output )
*/ */
static int generate_subkeys( mbedtls_cmac_context *ctx ) static int generate_subkeys( mbedtls_cmac_context *ctx )
{ {
static const unsigned char Rb[2] = { 0x00, 0x87 }; /* block size 16 only */
int ret; int ret;
unsigned char L[16]; unsigned char L[16];
size_t olen; size_t olen;
@ -121,20 +124,10 @@ static int generate_subkeys( mbedtls_cmac_context *ctx )
} }
/* /*
* Generate K1 * Generate K1 and K2
* If MSB(L) = 0, then K1 = (L << 1)
* If MSB(L) = 1, then K1 = (L << 1) ^ Rb
*/ */
leftshift_onebit( L, ctx->K1 ); multiply_by_u( ctx->K1, L );
ctx->K1[15] ^= Rb[L[0] >> 7]; /* "Constant-time" operation */ multiply_by_u( ctx->K2, ctx->K1 );
/*
* Generate K2
* If MSB(K1) == 0, then K2 = (K1 << 1)
* If MSB(K1) == 1, then K2 = (K1 << 1) ^ Rb
*/
leftshift_onebit( ctx->K1, ctx->K2 );
ctx->K2[15] ^= Rb[ctx->K1[0] >> 7]; /* "Constant-time" operation */
return( 0 ); return( 0 );
} }