diff --git a/library/bignum.c b/library/bignum.c index 9c6a91dc7..7cc42766e 100644 --- a/library/bignum.c +++ b/library/bignum.c @@ -343,6 +343,8 @@ int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X, mbedtls_mpi *Y, unsigned char sw { int ret, s; size_t i; + unsigned int sign_mask; + mbedtls_mpi_uint limb_mask; mbedtls_mpi_uint tmp; MPI_VALIDATE_RET( X != 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 ) 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 */ 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( Y, X->n ) ); s = X->s; - X->s = X->s * ( 1 - swap ) + Y->s * swap; - Y->s = Y->s * ( 1 - swap ) + s * swap; + X->s = ( X->s & ~sign_mask ) | ( Y->s & sign_mask ); + Y->s = ( Y->s & ~sign_mask ) | ( s & sign_mask ); for( i = 0; i < X->n; i++ ) { tmp = X->p[i]; - X->p[i] = X->p[i] * ( 1 - swap ) + Y->p[i] * swap; - Y->p[i] = Y->p[i] * ( 1 - swap ) + tmp * swap; + X->p[i] = ( X->p[i] & ~limb_mask ) | ( Y->p[i] & limb_mask ); + Y->p[i] = ( Y->p[i] & ~limb_mask ) | ( tmp & limb_mask ); } cleanup: