verify.c

// SPDX-License-Identifier: MIT

#include <stddef.h>
#include <stdint.h>

#include "verify.h"

/*************************************************
 * Name:        verify
 *
 * Description: Compare two arrays for equality in constant time.
 *
 * Arguments:   - const uint8_t *a: pointer to first byte array
 *              - const uint8_t *b: pointer to second byte array
 *              - size_t len: length of the byte arrays
 *
 * Return: 0 if the byte arrays are equal, 1 otherwise
 **************************************************/
int verify(const uint8_t *a, const uint8_t *b, size_t len) {
    size_t i;
    uint8_t r = 0;

    for (i = 0; i < len; i++)
        r |= a[i] ^ b[i];

    return (-(uint64_t)r) >> 63;
}

/*************************************************
 * Name:        cmov
 *
 * Description: Copy len bytes from x to r if b is 1;
 *              don't modify x if b is 0. Requires b to be in {0,1};
 *              assumes two's complement representation of negative integers.
 *              Runs in constant time.
 *
 * Arguments:   - uint8_t *r: pointer to output byte array
 *              - const uint8_t *x: pointer to input byte array
 *              - size_t len: amount of bytes to be copied
 *              - uint8_t b: condition bit; has to be in {0,1}
 **************************************************/
void cmov(uint8_t *r, const uint8_t *x, size_t len, uint8_t b) {
    size_t i;

    b = -b;
    for (i = 0; i < len; i++)
        r[i] ^= b & (r[i] ^ x[i]);
}