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Expose mbedtls_ct_size_gt and mbedtls_ct_mem_move_to_left in ct interface

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2023-05-09 11:09:52 +01:00
parent 2801f7fa8d
commit 0afe001871
2 changed files with 40 additions and 30 deletions
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32
library/constant_time.c
View File

@ -253,19 +253,7 @@ unsigned mbedtls_ct_size_bool_eq(size_t x,
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
/** Constant-flow "greater than" comparison:
* return x > y
*
* This is equivalent to \p x > \p y, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return 1 if \p x greater than \p y, otherwise 0.
*/
static unsigned mbedtls_ct_size_gt(size_t x,
size_t y)
unsigned mbedtls_ct_size_gt(size_t x, size_t y)
{
/* Return the sign bit (1 for negative) of (y - x). */
return (y - x) >> (sizeof(size_t) * 8 - 1);
@ -345,23 +333,7 @@ void mbedtls_ct_mpi_uint_cond_assign(size_t n,
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
/** Shift some data towards the left inside a buffer.
*
* `mbedtls_ct_mem_move_to_left(start, total, offset)` is functionally
* equivalent to
* ```
* memmove(start, start + offset, total - offset);
* memset(start + offset, 0, total - offset);
* ```
* but it strives to use a memory access pattern (and thus total timing)
* that does not depend on \p offset. This timing independence comes at
* the expense of performance.
*
* \param start Pointer to the start of the buffer.
* \param total Total size of the buffer.
* \param offset Offset from which to copy \p total - \p offset bytes.
*/
static void mbedtls_ct_mem_move_to_left(void *start,
void mbedtls_ct_mem_move_to_left(void *start,
size_t total,
size_t offset)
{

38
library/constant_time_internal.h
View File

@ -300,4 +300,42 @@ static inline unsigned char mbedtls_ct_uchar_in_range_if(unsigned char low,
#endif /* MBEDTLS_BASE64_C */
#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
/** Constant-flow "greater than" comparison:
* return x > y
*
* This is equivalent to \p x > \p y, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return 1 if \p x greater than \p y, otherwise 0.
*/
unsigned mbedtls_ct_size_gt(size_t x, size_t y);
/** Shift some data towards the left inside a buffer.
*
* `mbedtls_ct_mem_move_to_left(start, total, offset)` is functionally
* equivalent to
* ```
* memmove(start, start + offset, total - offset);
* memset(start + offset, 0, total - offset);
* ```
* but it strives to use a memory access pattern (and thus total timing)
* that does not depend on \p offset. This timing independence comes at
* the expense of performance.
*
* \param start Pointer to the start of the buffer.
* \param total Total size of the buffer.
* \param offset Offset from which to copy \p total - \p offset bytes.
*/
void mbedtls_ct_mem_move_to_left(void *start,
size_t total,
size_t offset);
#endif /* defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) */
#endif /* MBEDTLS_CONSTANT_TIME_INTERNAL_H */