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Move mbedtls_cf_size_bool_eq function to the constant-time module

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There were multiple functions called mbedtls_cf_size_bool_eq. They had exactly
the same behavior, so move the one in bignum.c and remove the other.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
gabor-mezei-arm
2021-09-27 12:15:19 +02:00
committed by Gabor Mezei
parent a2bcabceb2
commit 96584ddd4b
4 changed files with 39 additions and 71 deletions
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35
library/ssl_msg.c
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@ -1045,41 +1045,6 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
}
#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
/*
* Constant-flow boolean "equal" comparison:
* return x == y
*
* This function can be used to write constant-time code by replacing branches
* with bit operations - it can be used in conjunction with
* mbedtls_cf_size_mask().
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
static size_t mbedtls_cf_size_bool_eq( size_t x, size_t y )
{
/* diff = 0 if x == y, non-zero otherwise */
const size_t diff = x ^ y;
/* 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
/* diff_msb's most significant bit is equal to x != y */
const size_t diff_msb = ( diff | -diff );
#if defined(_MSC_VER)
#pragma warning( pop )
#endif
/* diff1 = (x != y) ? 1 : 0 */
const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
return( 1 ^ diff1 );
}
/*
* Constant-flow conditional memcpy: