constant_time.c File Reference

#include <stdint.h>
#include <limits.h>
#include "common.h"
#include "constant_time_internal.h"
#include "mbedtls/constant_time.h"
#include "mbedtls/error.h"
#include "mbedtls/platform_util.h"
#include <string.h>

Include dependency graph for constant_time.c:

Functions
int	mbedtls_ct_memcmp (const void *a, const void *b, size_t n)
void	mbedtls_ct_memcpy_if (mbedtls_ct_condition_t condition, unsigned char *dest, const unsigned char *src1, const unsigned char *src2, size_t len)
void	mbedtls_ct_memcpy_offset (unsigned char *dest, const unsigned char *src, size_t offset, size_t offset_min, size_t offset_max, size_t len)

Variables
volatile mbedtls_ct_uint_t	mbedtls_ct_zero = 0

Function Documentation

mbedtls_ct_memcmp()

int mbedtls_ct_memcmp	(	const void *	a,
		const void *	b,
		size_t	n
	)

Constant-time functions Constant-time buffer comparison without branches.

This is equivalent to the standard memcmp function, but is likely to be compiled to code using bitwise operations rather than a branch, such that the time taken is constant w.r.t. the data pointed to by a and b, and w.r.t. whether a and b are equal or not. It is not constant-time w.r.t. n .

This function can be used to write constant-time code by replacing branches with bit operations using masks.

Parameters

a	Pointer to the first buffer, containing at least n bytes. May not be NULL.
b	Pointer to the second buffer, containing at least n bytes. May not be NULL.
n	The number of bytes to compare.

Returns

Zero if the contents of the two buffers are the same, otherwise non-zero.

mbedtls_ct_memcpy_if()

void mbedtls_ct_memcpy_if	(	mbedtls_ct_condition_t	condition,
		unsigned char *	dest,
		const unsigned char *	src1,
		const unsigned char *	src2,
		size_t	len
	)

Conditional memcpy.

Functionally equivalent to:

if (condition) { memcpy(dest, src1, len); } else { if (src2 != NULL) memcpy(dest, src2, len); }

It will always read len bytes from src1. If src2 != NULL, it will always read len bytes from src2. If src2 == NULL, it will instead read len bytes from dest (as if src2 == dest).

Parameters

condition	The condition
dest	Secret. Destination pointer.
src1	Secret. Pointer to copy from (if condition == MBEDTLS_CT_TRUE). This may be equal to dest, but may not overlap in other ways.
src2	Secret (contents only - may branch to determine if this parameter is NULL). Pointer to copy from (if condition == MBEDTLS_CT_FALSE and src2 is not NULL). May be NULL. This may be equal to dest, but may not overlap it in other ways. It may overlap with src1.
len	Number of bytes to copy.

mbedtls_ct_memcpy_offset()

void mbedtls_ct_memcpy_offset	(	unsigned char *	dest,
		const unsigned char *	src,
		size_t	offset,
		size_t	offset_min,
		size_t	offset_max,
		size_t	len
	)

Copy data from a secret position.

Functionally equivalent to:

memcpy(dst, src + offset, len)

This function copies len bytes from src + offset to dst, with a code flow and memory access pattern that does not depend on offset, but only on offset_min, offset_max and len.

Note

This function reads from dest, but the value that is read does not influence the result and this function's behavior is well-defined regardless of the contents of the buffers. This may result in false positives from static or dynamic analyzers, especially if dest is not initialized.

Parameters

dest	Secret. The destination buffer. This must point to a writable buffer of at least len bytes.
src	Secret. The base of the source buffer. This must point to a readable buffer of at least offset_max + len bytes. Shouldn't overlap with dest
offset	Secret. The offset in the source buffer from which to copy. This must be no less than offset_min and no greater than offset_max.
offset_min	The minimal value of offset.
offset_max	The maximal value of offset.
len	The number of bytes to copy.

Variable Documentation

mbedtls_ct_zero

volatile mbedtls_ct_uint_t mbedtls_ct_zero = 0

Constant-time functions

Copyright The Mbed TLS Contributors SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later