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Remove multiplication from conditional assignments

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Multiplication is not constant flow on any CPU we are generally
targetting, so replace this with bit twiddling.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
This commit is contained in:
Paul Elliott 2021-03-03 15:31:17 +00:00
parent c48cb80b1f
commit 88f2eb664f
1 changed files with 29 additions and 7 deletions
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36
library/base64.c
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@ -71,22 +71,44 @@ static const unsigned char base64_dec_map[128] =
static void mbedtls_base64_cond_assign_uchar(unsigned char * dest, const unsigned char * const src, static void mbedtls_base64_cond_assign_uchar(unsigned char * dest, const unsigned char * const src,
unsigned char condition) unsigned char condition)
{ {
/* make sure assign is 0 or 1 in a time-constant manner */ /* MSVC has a warning about unary minus on unsigned integer types,
condition = (condition | (unsigned char)-condition) >> 7; * 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
*dest = ( *dest ) * ( 1 - condition ) + ( *src ) * condition; /* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
unsigned char mask = -( unsigned char )( ( condition | -condition ) >> 7 );
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
*dest = ( ( *src ) & mask ) | ( ( *dest ) & ~mask );
} }
/* /*
* Constant flow conditional assignment to uint_32 * Constant flow conditional assignment to uint_32
*/ */
static void mbedtls_base64_cond_assign_uint32(uint32_t * dest, const uint32_t src, static void mbedtls_base64_cond_assign_uint32(uint32_t * dest, const uint32_t src,
unsigned char condition) uint32_t condition)
{ {
/* make sure assign is 0 or 1 in a time-constant manner */ /* MSVC has a warning about unary minus on unsigned integer types,
condition = (condition | (unsigned char)-condition) >> 7; * 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
*dest = ( *dest ) * ( 1 - condition ) + ( src ) * condition; /* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
uint32_t mask = -( uint32_t )( ( condition | -condition ) >> 31 );
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
*dest = ( src & mask ) | ( ( *dest ) & ~mask );
} }