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Use bit operations for mpi_safe_cond_swap()

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Unrelated to RSA (only used in ECP), but while improving one
mbedtls_safe_cond_xxx function, let's improve the other as well.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2021-06-03 10:54:01 +02:00
parent c3be399591
commit 464fe6a4d7
1 changed files with 20 additions and 4 deletions
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24
library/bignum.c
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@ -343,6 +343,8 @@ int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X, mbedtls_mpi *Y, unsigned char sw
{ {
int ret, s; int ret, s;
size_t i; size_t i;
unsigned int sign_mask;
mbedtls_mpi_uint limb_mask;
mbedtls_mpi_uint tmp; mbedtls_mpi_uint tmp;
MPI_VALIDATE_RET( X != NULL ); MPI_VALIDATE_RET( X != NULL );
MPI_VALIDATE_RET( Y != NULL ); MPI_VALIDATE_RET( Y != NULL );
@ -350,22 +352,36 @@ int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X, mbedtls_mpi *Y, unsigned char sw
if( X == Y ) if( X == Y )
return( 0 ); return( 0 );
/* MSVC has a warning about unary minus on unsigned integer types,
* but this is well-defined and precisely what we want to do here. */
#if defined(_MSC_VER)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
/* make sure swap is 0 or 1 in a time-constant manner */ /* make sure swap is 0 or 1 in a time-constant manner */
swap = (swap | (unsigned char)-swap) >> 7; swap = (swap | (unsigned char)-swap) >> 7;
/* all-bits 1 if swap is 1, all-bits 0 if swap is 0 */
sign_mask = -swap;
limb_mask = -swap;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( Y, X->n ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_grow( Y, X->n ) );
s = X->s; s = X->s;
X->s = X->s * ( 1 - swap ) + Y->s * swap; X->s = ( X->s & ~sign_mask ) | ( Y->s & sign_mask );
Y->s = Y->s * ( 1 - swap ) + s * swap; Y->s = ( Y->s & ~sign_mask ) | ( s & sign_mask );
for( i = 0; i < X->n; i++ ) for( i = 0; i < X->n; i++ )
{ {
tmp = X->p[i]; tmp = X->p[i];
X->p[i] = X->p[i] * ( 1 - swap ) + Y->p[i] * swap; X->p[i] = ( X->p[i] & ~limb_mask ) | ( Y->p[i] & limb_mask );
Y->p[i] = Y->p[i] * ( 1 - swap ) + tmp * swap; Y->p[i] = ( Y->p[i] & ~limb_mask ) | ( tmp & limb_mask );
} }
cleanup: cleanup: