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#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
*
* Returns 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]);
}
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