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Move implementation specific comment

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This comment is about how the functions are implemented, not about their
public interface, so it doesn't belong in the header file.
It applies to everything in constant_time.c so moved there.

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
This commit is contained in:
Gabor Mezei 2021-10-18 16:09:41 +02:00
parent 1e64261da5
commit eab90bcc36
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GPG Key ID: 106F5A41ECC305BD
2 changed files with 5 additions and 38 deletions
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5
library/constant_time.c
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@ -17,6 +17,11 @@
* limitations under the License.
*/
/*
* The following functiona are implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*/
#include "common.h"
#include "constant_time.h"
#include "mbedtls/error.h"

38
library/constant_time.h
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@ -38,9 +38,6 @@
* 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 a Pointer to the first buffer.
* \param b Pointer to the second buffer.
* \param n The number of bytes to compare in the buffer.
@ -59,9 +56,6 @@ int mbedtls_cf_memcmp( const void *a,
* 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 value The value to analyze.
*
* \return Zero if \p value is zero, otherwise all-bits-one.
@ -75,9 +69,6 @@ unsigned mbedtls_cf_uint_mask( unsigned value );
* 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 value The value to analyze.
*
* \return Zero if \p value is zero, otherwise all-bits-one.
@ -93,9 +84,6 @@ size_t mbedtls_cf_size_mask( size_t value );
* 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 value The value to analyze.
*
* \return Zero if \p value is zero, otherwise all-bits-one.
@ -111,9 +99,6 @@ mbedtls_mpi_uint mbedtls_cf_mpi_uint_mask( mbedtls_mpi_uint value );
* 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.
*
@ -129,9 +114,6 @@ size_t mbedtls_cf_size_mask_ge( size_t x,
* This is equivalent to \p x == \p y, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* 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.
*
@ -146,9 +128,6 @@ unsigned mbedtls_cf_size_bool_eq( size_t x,
* This is equivalent to \p x > \p y, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* 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.
*
@ -164,9 +143,6 @@ unsigned mbedtls_cf_size_gt( size_t x,
* This is equivalent to \p x < \p y, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* 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.
*
@ -182,9 +158,6 @@ unsigned mbedtls_cf_mpi_uint_lt( const mbedtls_mpi_uint x,
* This is equivalent to `condition ? if1 : if0`, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \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.
@ -200,9 +173,6 @@ unsigned mbedtls_cf_uint_if( unsigned condition,
* This is equivalent to `condition ? if1 : if0`, but is likely to be compiled
* to code using bitwise operation rather than a branch.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \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.
@ -218,8 +188,6 @@ size_t mbedtls_cf_size_if( unsigned condition,
* This is functionally equivalent to `condition ? if1 : if0` but uses only bit
* operations in order to avoid branches.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param condition Condition to test.
* \param if1 The first sign; must be either +1 or -1.
@ -237,9 +205,6 @@ int mbedtls_cf_cond_select_sign( unsigned char condition,
* This is equivalent to `if ( condition ) dest = src`, but is likely
* to be compiled to code using bitwise operation rather than a branch.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param n \p dest and \p src must be arrays of limbs of size n.
* \param dest The MPI to conditionally assign to. This must point
* to an initialized MPI.
@ -280,9 +245,6 @@ void mbedtls_cf_mem_move_to_left( void *start,
* This is equivalent to `if ( c1 == c2 ) memcpy(dst, src, len)`, but is likely
* to be compiled to code using bitwise operation rather than a branch.
*
* This function is implemented without using comparison operators, as those
* might be translated to branches by some compilers on some platforms.
*
* \param dest The pointer to conditionally copy to.
* \param src The pointer to copy from.
* \param len The number of bytes to copy.