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Move mbedtls_cf_memcpy_if_eq function to the constant-time module

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Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
gabor-mezei-arm 2021-09-27 13:34:25 +02:00 committed by Gabor Mezei
parent 7b23c0b46d
commit ee06febbb4
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GPG Key ID: 106F5A41ECC305BD
3 changed files with 28 additions and 23 deletions
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23
library/constant_time.c
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@ -395,3 +395,26 @@ void mbedtls_cf_mem_move_to_left( void *start,
buf[total-1] = mbedtls_cf_uint_if( no_op, buf[total-1], 0 );
}
}
/*
* Constant-flow conditional memcpy:
* - if c1 == c2, equivalent to memcpy(dst, src, len),
* - otherwise, a no-op,
* but with execution flow independent of the values of c1 and c2.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
void mbedtls_cf_memcpy_if_eq( unsigned char *dst,
const unsigned char *src,
size_t len,
size_t c1, size_t c2 )
{
/* mask = c1 == c2 ? 0xff : 0x00 */
const size_t equal = mbedtls_cf_size_bool_eq( c1, c2 );
const unsigned char mask = (unsigned char) mbedtls_cf_size_mask( equal );
/* dst[i] = c1 == c2 ? src[i] : dst[i] */
for( size_t i = 0; i < len; i++ )
dst[i] = ( src[i] & mask ) | ( dst[i] & ~mask );
}

5
library/constant_time.h
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@ -69,3 +69,8 @@ void mbedtls_cf_mpi_uint_cond_assign( size_t n,
void mbedtls_cf_mem_move_to_left( void *start,
size_t total,
size_t offset );
void mbedtls_cf_memcpy_if_eq( unsigned char *dst,
const unsigned char *src,
size_t len,
size_t c1, size_t c2 );

23
library/ssl_msg.c
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@ -1046,29 +1046,6 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
#if defined(MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC)
/*
* Constant-flow conditional memcpy:
* - if c1 == c2, equivalent to memcpy(dst, src, len),
* - otherwise, a no-op,
* but with execution flow independent of the values of c1 and c2.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
static void mbedtls_cf_memcpy_if_eq( unsigned char *dst,
const unsigned char *src,
size_t len,
size_t c1, size_t c2 )
{
/* mask = c1 == c2 ? 0xff : 0x00 */
const size_t equal = mbedtls_cf_size_bool_eq( c1, c2 );
const unsigned char mask = (unsigned char) mbedtls_cf_size_mask( equal );
/* dst[i] = c1 == c2 ? src[i] : dst[i] */
for( size_t i = 0; i < len; i++ )
dst[i] = ( src[i] & mask ) | ( dst[i] & ~mask );
}
/*
* Compute HMAC of variable-length data with constant flow.
*