libmincrypt/p256_ecdsa.c

/*
 * Copyright 2013 The Android Open Source Project
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 *
 * THIS SOFTWARE IS PROVIDED BY Google Inc. ``AS IS'' AND ANY EXPRESS OR
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 */

#include <string.h>

#include "mincrypt/p256_ecdsa.h"
#include "mincrypt/p256.h"

int p256_ecdsa_verify(const p256_int* key_x, const p256_int* key_y,
                      const p256_int* message,
                      const p256_int* r, const p256_int* s) {
  p256_int u, v;

  // Check public key.
  if (!p256_is_valid_point(key_x, key_y)) return 0;

  // Check r and s are != 0 % n.
  p256_mod(&SECP256r1_n, r, &u);
  p256_mod(&SECP256r1_n, s, &v);
  if (p256_is_zero(&u) || p256_is_zero(&v)) return 0;

  p256_modinv_vartime(&SECP256r1_n, s, &v);
  p256_modmul(&SECP256r1_n, message, 0, &v, &u);  // message / s % n
  p256_modmul(&SECP256r1_n, r, 0, &v, &v);  // r / s % n

  p256_points_mul_vartime(&u, &v,
                          key_x, key_y,
                          &u, &v);

  p256_mod(&SECP256r1_n, &u, &u);  // (x coord % p) % n
  return p256_cmp(r, &u) == 0;
}