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Make mbedtls_cf_size_mask_lt function static

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The mbedtls_cf_size_mask_lt is solely used as an auxiliary function
for mbedtls_cf_size_mask_ge.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
Gabor Mezei 2021-10-18 16:05:50 +02:00
parent 5b3a32d883
commit 1e64261da5
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GPG Key ID: 106F5A41ECC305BD
2 changed files with 14 additions and 20 deletions
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16
library/constant_time.c
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@ -103,8 +103,20 @@ mbedtls_mpi_uint mbedtls_cf_mpi_uint_mask( mbedtls_mpi_uint value )
#endif /* MBEDTLS_BIGNUM_C */
size_t mbedtls_cf_size_mask_lt( size_t x,
size_t y )
/** Constant-flow mask generation for "less than" comparison:
* - if \p x < \p y, return all-bits 1, that is (size_t) -1
* - otherwise, return all bits 0, that is 0
*
* This function can be used to write constant-time code by replacing branches
* with bit operations using masks.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return All-bits-one if \p x is less than \p y, otherwise zero.
*/
static size_t mbedtls_cf_size_mask_lt( size_t x,
size_t y )
{
/* This has the most significant bit set if and only if x < y */
const size_t sub = x - y;

18
library/constant_time.h
View File

@ -104,24 +104,6 @@ mbedtls_mpi_uint mbedtls_cf_mpi_uint_mask( mbedtls_mpi_uint value );
#endif /* MBEDTLS_BIGNUM_C */
/** Constant-flow mask generation for "less than" comparison:
* - if \p x < \p y, return all-bits 1, that is (size_t) -1
* - otherwise, return all bits 0, that is 0
*
* This function can be used to write constant-time code by replacing branches
* with bit operations using masks.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param x The first value to analyze.
* \param y The second value to analyze.
*
* \return All-bits-one if \p x is less than \p y, otherwise zero.
*/
size_t mbedtls_cf_size_mask_lt( size_t x,
size_t y );
/** Constant-flow mask generation for "greater or equal" comparison:
* - if \p x >= \p y, return all-bits 1, that is (size_t) -1
* - otherwise, return all bits 0, that is 0