Constant-time byte compare

Challenge

Implement int ct_equals(const unsigned char *a, const unsigned char *b, size_t n) that compares n bytes in constant time — its execution time must not depend on where (or whether) a and b differ. Return 1 if equal, 0 otherwise.

Constant-time compares are how to defeat timing attacks on secret-equal-secret checks (HMAC, hash tags, etc.).

Starter code

#include <stddef.h>

int ct_equals(const unsigned char *a, const unsigned char *b, size_t n) {
    /* TODO */
    return 0;
}