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Simplify x25519 reduction using internal bignum MLA helper

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Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker 2022-04-11 07:03:48 +01:00
parent aef9cc4f96
commit 25bb732ea7
1 changed files with 16 additions and 26 deletions
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42
library/ecp_curves.c
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@ -26,6 +26,7 @@
#include "mbedtls/error.h"
#include "bn_mul.h"
#include "bignum_internal.h"
#include "ecp_invasive.h"
#include <string.h>
@ -5213,40 +5214,29 @@ cleanup:
/*
* Fast quasi-reduction modulo p255 = 2^255 - 19
* Write N as A0 + 2^255 A1, return A0 + 19 * A1
* Write N as A0 + 2^256 A1, return A0 + 38 * A1
*/
static int ecp_mod_p255( mbedtls_mpi *N )
{
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
size_t i;
mbedtls_mpi M;
mbedtls_mpi_uint Mp[P255_WIDTH + 2];
mbedtls_mpi_uint Mp[P255_WIDTH];
if( N->n < P255_WIDTH )
/* Helper references for top part of N */
mbedtls_mpi_uint * const NT_p = N->p + P255_WIDTH;
unsigned const NT_n = N->n - P255_WIDTH;
if( NT_n > P255_WIDTH )
return( 0 );
/* M = A1 */
M.s = 1;
M.n = N->n - ( P255_WIDTH - 1 );
if( M.n > P255_WIDTH + 1 )
return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
M.p = Mp;
memset( Mp, 0, sizeof Mp );
memcpy( Mp, N->p + P255_WIDTH - 1, M.n * sizeof( mbedtls_mpi_uint ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &M, 255 % ( 8 * sizeof( mbedtls_mpi_uint ) ) ) );
M.n++; /* Make room for multiplication by 19 */
/* Split N as N + 2^256 M */
memset( Mp, 0, sizeof( Mp ) );
memcpy( Mp, NT_p, sizeof( mbedtls_mpi_uint ) * NT_n );
memset( NT_p, 0, sizeof( mbedtls_mpi_uint ) * NT_n );
/* N = A0 */
MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( N, 255, 0 ) );
for( i = P255_WIDTH; i < N->n; i++ )
N->p[i] = 0;
/* N = A0 + 38 * A1 */
mbedtls_mpi_core_mla( N->p, N->n,
Mp, P255_WIDTH,
38 );
/* N = A0 + 19 * A1 */
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_int( &M, &M, 19 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_add_abs( N, N, &M ) );
cleanup:
return( ret );
return( 0 );
}
#endif /* MBEDTLS_ECP_DP_CURVE25519_ENABLED */