include/mbedtls/constant_time.h

0001 /**
0002  *  Constant-time functions
0003  */
0004 /*
0005  *  Copyright The Mbed TLS Contributors
0006  *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
0007  */
0008
0009 #ifndef MBEDTLS_CONSTANT_TIME_H
0010 #define MBEDTLS_CONSTANT_TIME_H
0011
0012 #include <stddef.h>
0013
0014 /** Constant-time buffer comparison without branches.
0015  *
0016  * This is equivalent to the standard memcmp function, but is likely to be
0017  * compiled to code using bitwise operations rather than a branch, such that
0018  * the time taken is constant w.r.t. the data pointed to by \p a and \p b,
0019  * and w.r.t. whether \p a and \p b are equal or not. It is not constant-time
0020  * w.r.t. \p n .
0021  *
0022  * This function can be used to write constant-time code by replacing branches
0023  * with bit operations using masks.
0024  *
0025  * \param a     Pointer to the first buffer, containing at least \p n bytes. May not be NULL.
0026  * \param b     Pointer to the second buffer, containing at least \p n bytes. May not be NULL.
0027  * \param n     The number of bytes to compare.
0028  *
0029  * \return      Zero if the contents of the two buffers are the same,
0030  *              otherwise non-zero.
0031  */
0032 int mbedtls_ct_memcmp(const void *a,
0033                       const void *b,
0034                       size_t n);
0035
0036 #endif /* MBEDTLS_CONSTANT_TIME_H */