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Make functions static

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These functions are only used as an auxiliary function for constant-time functions.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
Gabor Mezei 2021-10-18 16:35:23 +02:00
parent a316fc8eb0
commit a2d0f90c5a
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GPG Key ID: 106F5A41ECC305BD
2 changed files with 63 additions and 76 deletions
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74
library/constant_time.c
View File

@ -167,8 +167,19 @@ unsigned mbedtls_cf_size_bool_eq( size_t x,
return( 1 ^ diff1 );
}
unsigned mbedtls_cf_size_gt( size_t x,
size_t y )
/** 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_cf_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 ) );
@ -214,17 +225,42 @@ unsigned mbedtls_cf_uint_if( unsigned condition,
return( ( mask & if1 ) | (~mask & if0 ) );
}
size_t mbedtls_cf_size_if( unsigned condition,
size_t if1,
size_t if0 )
/** Choose between two integer values without branches.
*
* This is equivalent to `condition ? if1 : if0`, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* \param condition Condition to test.
* \param if1 Value to use if \p condition is nonzero.
* \param if0 Value to use if \p condition is zero.
*
* \return \c if1 if \p condition is nonzero, otherwise \c if0.
*/
static size_t mbedtls_cf_size_if( unsigned condition,
size_t if1,
size_t if0 )
{
size_t mask = mbedtls_cf_size_mask( condition );
return( ( mask & if1 ) | (~mask & if0 ) );
}
int mbedtls_cf_cond_select_sign( unsigned char condition,
int if1,
int if0 )
/** Select between two sign values witout branches.
*
* This is functionally equivalent to `condition ? if1 : if0` but uses only bit
* operations in order to avoid branches.
*
* \note if1 and if0 must be either 1 or -1, otherwise the result
* is undefined.
*
* \param condition Condition to test.
* \param if1 The first sign; must be either +1 or -1.
* \param if0 The second sign; must be either +1 or -1.
*
* \return \c if1 if \p condition is nonzero, otherwise \c if0.
* */
static int mbedtls_cf_cond_select_sign( unsigned char condition,
int if1,
int if0 )
{
/* In order to avoid questions about what we can reasonnably assume about
* the representations of signed integers, move everything to unsigned
@ -271,9 +307,25 @@ void mbedtls_cf_mpi_uint_cond_assign( size_t n,
#endif /* MBEDTLS_BIGNUM_C */
void mbedtls_cf_mem_move_to_left( void *start,
size_t total,
size_t offset )
/** Shift some data towards the left inside a buffer.
*
* `mbedtls_cf_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_cf_mem_move_to_left( void *start,
size_t total,
size_t offset )
{
volatile unsigned char *buf = start;
size_t i, n;

65
library/constant_time.h
View File

@ -122,19 +122,6 @@ size_t mbedtls_cf_size_mask_ge( size_t x,
unsigned mbedtls_cf_size_bool_eq( size_t x,
size_t y );
/** 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_cf_size_gt( size_t x,
size_t y );
#if defined(MBEDTLS_BIGNUM_C)
@ -168,38 +155,6 @@ unsigned mbedtls_cf_uint_if( unsigned condition,
unsigned if1,
unsigned if0 );
/** Choose between two integer values without branches.
*
* This is equivalent to `condition ? if1 : if0`, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* \param condition Condition to test.
* \param if1 Value to use if \p condition is nonzero.
* \param if0 Value to use if \p condition is zero.
*
* \return \c if1 if \p condition is nonzero, otherwise \c if0.
*/
size_t mbedtls_cf_size_if( unsigned condition,
size_t if1,
size_t if0 );
/** Select between two sign values witout branches.
*
* This is functionally equivalent to `condition ? if1 : if0` but uses only bit
* operations in order to avoid branches.
*
* \note if1 and if0 must be either 1 or -1, otherwise the result
* is undefined.
*
* \param condition Condition to test.
* \param if1 The first sign; must be either +1 or -1.
* \param if0 The second sign; must be either +1 or -1.
*
* \return \c if1 if \p condition is nonzero, otherwise \c if0. */
int mbedtls_cf_cond_select_sign( unsigned char condition,
int if1,
int if0 );
#if defined(MBEDTLS_BIGNUM_C)
/** Conditionally assign a value without branches.
@ -222,26 +177,6 @@ void mbedtls_cf_mpi_uint_cond_assign( size_t n,
#endif /* MBEDTLS_BIGNUM_C */
/** Shift some data towards the left inside a buffer.
*
* `mbedtls_cf_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_cf_mem_move_to_left( void *start,
size_t total,
size_t offset );
/** Conditional memcpy without branches.
*
* This is equivalent to `if ( c1 == c2 ) memcpy(dst, src, len)`, but is likely