verify.c

#include <string.h>
#include <stdint.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 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 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]);
}