pqcrypto/crypto_kem/kyber768/clean/verify.c

#include <stdint.h>
#include <string.h>

/*************************************************
 * Name:        verify
 *
 * Description: Compare two arrays for equality in constant time.
 *
 * Arguments:   const unsigned char *a: pointer to first byte array
 *              const unsigned char *b: pointer to second byte array
 *              size_t len:             length of the byte arrays
 *
 * Returns 0 if the byte arrays are equal, 1 otherwise
 **************************************************/
int PQCLEAN_KYBER768_verify(const unsigned char *a, const unsigned char *b,
                            size_t len) {
    uint64_t r;
    size_t i;
    r = 0;

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

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

/*************************************************
 * 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:   unsigned char *r:       pointer to output byte array
 *              const unsigned char *x: pointer to input byte array
 *              size_t len:             Amount of bytes to be copied
 *              unsigned char b:        Condition bit; has to be in {0,1}
 **************************************************/
void PQCLEAN_KYBER768_cmov(unsigned char *r, const unsigned char *x, size_t len,
                           unsigned char b) {
    size_t i;

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