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Move window precomputation into an auxiliary function

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Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2022-11-16 20:12:49 +01:00 committed by Janos Follath
parent cf979b0fc1
commit 0de0a049f1
1 changed files with 32 additions and 17 deletions
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49
library/bignum_core.c
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@ -596,6 +596,34 @@ static size_t exp_mod_get_window_size( size_t Ebits )
return( wsize );
}
static void exp_mod_precompute_window( const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N,
size_t AN_limbs,
mbedtls_mpi_uint mm,
const mbedtls_mpi_uint *RR,
size_t welem,
mbedtls_mpi_uint *Wtable,
mbedtls_mpi_uint *temp )
{
/* pointers to table entries */
mbedtls_mpi_uint *Wcur, *Wlast, *W1;
/* W[0] = 1 (in Montgomery presentation) */
memset( Wtable, 0, AN_limbs * ciL );
Wtable[0] = 1;
mbedtls_mpi_core_montmul( Wtable, Wtable, RR, AN_limbs, N, AN_limbs, mm, temp );
Wcur = Wtable + AN_limbs;
/* W[1] = A * R^2 * R^-1 mod N = A * R mod N */
memcpy( Wcur, A, AN_limbs * ciL );
mbedtls_mpi_core_montmul( Wcur, Wcur, RR, AN_limbs, N, AN_limbs, mm, temp );
W1 = Wcur;
Wcur += AN_limbs;
/* W[i+1] = W[i] * W[1], i >= 2 */
Wlast = W1;
for( size_t i = 2; i < welem; i++, Wlast += AN_limbs, Wcur += AN_limbs )
mbedtls_mpi_core_montmul( Wcur, Wlast, W1, AN_limbs, N, AN_limbs, mm, temp );
}
int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
const mbedtls_mpi_uint *A,
const mbedtls_mpi_uint *N,
@ -635,23 +663,10 @@ int mbedtls_mpi_core_exp_mod( mbedtls_mpi_uint *X,
const mbedtls_mpi_uint mm = mbedtls_mpi_core_montmul_init( N );
/* pointers to table entries */
mbedtls_mpi_uint *Wcur, *Wlast, *W1;
/* W[0] = 1 (in Montgomery presentation) */
memset( Wtable, 0, AN_limbs * ciL );
Wtable[0] = 1;
mbedtls_mpi_core_montmul( Wtable, Wtable, RR, AN_limbs, N, AN_limbs, mm, temp );
Wcur = Wtable + AN_limbs;
/* W[1] = A * R^2 * R^-1 mod N = A * R mod N */
memcpy( Wcur, A, AN_limbs * ciL );
mbedtls_mpi_core_montmul( Wcur, Wcur, RR, AN_limbs, N, AN_limbs, mm, temp );
W1 = Wcur;
Wcur += AN_limbs;
/* W[i+1] = W[i] * W[1], i >= 2 */
Wlast = W1;
for( size_t i = 2; i < welem; i++, Wlast += AN_limbs, Wcur += AN_limbs )
mbedtls_mpi_core_montmul( Wcur, Wlast, W1, AN_limbs, N, AN_limbs, mm, temp );
/* Set Wtable[i] = A^(2^i) (in Montgomery representation) */
exp_mod_precompute_window( A, N, AN_limbs,
mm, RR,
welem, Wtable, temp );
/*
* Fixed window exponentiation