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mpi_exp_mod: move X next to the precomputed values

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With small exponents (for example, when doing RSA-1024 with CRT, each
prime is 512 bits and we'll use wsize = 5 which may be smaller that the
maximum - or even worse when doing public RSA operations which typically
have a 16-bit exponent so we'll use wsize = 1) the usage of W will have
pre-computed values, then empty space, then the accumulator at the very
end.

Move X next to the precomputed values to make accesses more efficient
and intuitive.

Signed-off-by: Janos Follath <janos.follath@arm.com>
This commit is contained in:
Janos Follath 2022-11-21 14:55:05 +00:00
parent 6632383993
commit aadbadbf42
1 changed files with 8 additions and 7 deletions
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15
library/bignum.c
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@ -2006,7 +2006,6 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
size_t bufsize, nbits;
mbedtls_mpi_uint ei, mm, state;
mbedtls_mpi RR, T, W[ ( 1 << MBEDTLS_MPI_WINDOW_SIZE ) + 1 ], WW, Apos;
const size_t w_table_size = sizeof( W ) / sizeof( W[0] );
int neg;
MPI_VALIDATE_RET( X != NULL );
@ -2037,6 +2036,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
window_bitsize = ( i > 671 ) ? 6 : ( i > 239 ) ? 5 :
( i > 79 ) ? 4 : ( i > 23 ) ? 3 : 1;
const size_t w_table_used_size = ( 1 << window_bitsize ) + 1;
#if( MBEDTLS_MPI_WINDOW_SIZE < 6 )
if( window_bitsize > MBEDTLS_MPI_WINDOW_SIZE )
@ -2055,7 +2055,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
* To achieve this, we make a copy of X and we use the table entry in each
* calculation from this point on.
*/
const size_t x_index = w_table_size - 1;
const size_t x_index = w_table_used_size - 1;
mbedtls_mpi_init( &W[x_index] );
mbedtls_mpi_copy( &W[x_index], X );
@ -2177,7 +2177,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
/*
* out of window, square W[x_index]
*/
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_size, x_index ) );
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_used_size, x_index ) );
mpi_montmul( &W[x_index], &WW, N, mm, &T );
continue;
}
@ -2197,14 +2197,15 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
*/
for( i = 0; i < window_bitsize; i++ )
{
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_size, x_index ) );
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_used_size,
x_index ) );
mpi_montmul( &W[x_index], &WW, N, mm, &T );
}
/*
* W[x_index] = W[x_index] * W[exponent_bits_in_window] R^-1 mod N
*/
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_size,
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_used_size,
exponent_bits_in_window ) );
mpi_montmul( &W[x_index], &WW, N, mm, &T );
@ -2219,14 +2220,14 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
*/
for( i = 0; i < nbits; i++ )
{
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_size, x_index ) );
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_used_size, x_index ) );
mpi_montmul( &W[x_index], &WW, N, mm, &T );
exponent_bits_in_window <<= 1;
if( ( exponent_bits_in_window & ( one << window_bitsize ) ) != 0 )
{
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_size, 1 ) );
MBEDTLS_MPI_CHK( mpi_select( &WW, W, w_table_used_size, 1 ) );
mpi_montmul( &W[x_index], &WW, N, mm, &T );
}
}