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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 a4dfa0550f
commit 756bcf7631
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
* \note output can be same as input
* Multiply by u in GF(2^128)
*
* 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;
unsigned char temp;
static const unsigned char Rb[2] = { 0x00, 0x87 }; /* block size 16 only */
unsigned char overflow = 0;
int i;
for( i = 15; i >= 0; i-- )
{
temp = input[i];
output[i] = temp << 1;
output[i] |= overflow;
overflow = temp >> 7;
output[i] = input[i] << 1 | overflow;
overflow = input[i] >> 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 const unsigned char Rb[2] = { 0x00, 0x87 }; /* block size 16 only */
int ret;
unsigned char L[16];
size_t olen;
@ -121,20 +124,10 @@ static int generate_subkeys( mbedtls_cmac_context *ctx )
}
/*
* Generate K1
* If MSB(L) = 0, then K1 = (L << 1)
* If MSB(L) = 1, then K1 = (L << 1) ^ Rb
* Generate K1 and K2
*/
leftshift_onebit( L, ctx->K1 );
ctx->K1[15] ^= Rb[L[0] >> 7]; /* "Constant-time" operation */
/*
* 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 */
multiply_by_u( ctx->K1, L );
multiply_by_u( ctx->K2, ctx->K1 );
return( 0 );
}