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Don't use multiplication by condition in even a semi-constant time function

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Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
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Tom Cosgrove
2022-08-30 17:41:23 +01:00
parent f0c8a8cf44
commit 9354990a54
1 changed files with 15 additions and 1 deletions
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16
library/bignum_core.c
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@ -300,9 +300,23 @@ mbedtls_mpi_uint mbedtls_mpi_core_add_if( mbedtls_mpi_uint *A,
{ {
mbedtls_mpi_uint c = 0; mbedtls_mpi_uint c = 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
/* all-bits 1 if cond is 1, all-bits 0 if cond is 0 */
const mbedtls_mpi_uint mask = -(mbedtls_mpi_uint)cond;
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
for( size_t i = 0; i < limbs; i++ ) for( size_t i = 0; i < limbs; i++ )
{ {
mbedtls_mpi_uint add = cond * B[i]; mbedtls_mpi_uint add = mask & B[i];
mbedtls_mpi_uint t = c + A[i]; mbedtls_mpi_uint t = c + A[i];
c = ( t < A[i] ); c = ( t < A[i] );
t += add; t += add;