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      1 /*
      2  * Copyright 2013 The Android Open Source Project
      3  *
      4  * Redistribution and use in source and binary forms, with or without
      5  * modification, are permitted provided that the following conditions are met:
      6  *     * Redistributions of source code must retain the above copyright
      7  *       notice, this list of conditions and the following disclaimer.
      8  *     * Redistributions in binary form must reproduce the above copyright
      9  *       notice, this list of conditions and the following disclaimer in the
     10  *       documentation and/or other materials provided with the distribution.
     11  *     * Neither the name of Google Inc. nor the names of its contributors may
     12  *       be used to endorse or promote products derived from this software
     13  *       without specific prior written permission.
     14  *
     15  * THIS SOFTWARE IS PROVIDED BY Google Inc. ``AS IS'' AND ANY EXPRESS OR
     16  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
     17  * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
     18  * EVENT SHALL Google Inc. BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
     19  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
     20  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
     21  * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
     22  * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
     23  * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
     24  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     25  */
     26 
     27 #include <string.h>
     28 
     29 #include "mincrypt/p256_ecdsa.h"
     30 #include "mincrypt/p256.h"
     31 
     32 int p256_ecdsa_verify(const p256_int* key_x, const p256_int* key_y,
     33                       const p256_int* message,
     34                       const p256_int* r, const p256_int* s) {
     35   p256_int u, v;
     36 
     37   // Check public key.
     38   if (!p256_is_valid_point(key_x, key_y)) return 0;
     39 
     40   // Check r and s are != 0 % n.
     41   p256_mod(&SECP256r1_n, r, &u);
     42   p256_mod(&SECP256r1_n, s, &v);
     43   if (p256_is_zero(&u) || p256_is_zero(&v)) return 0;
     44 
     45   p256_modinv_vartime(&SECP256r1_n, s, &v);
     46   p256_modmul(&SECP256r1_n, message, 0, &v, &u);  // message / s % n
     47   p256_modmul(&SECP256r1_n, r, 0, &v, &v);  // r / s % n
     48 
     49   p256_points_mul_vartime(&u, &v,
     50                           key_x, key_y,
     51                           &u, &v);
     52 
     53   p256_mod(&SECP256r1_n, &u, &u);  // (x coord % p) % n
     54   return p256_cmp(r, &u) == 0;
     55 }
     56 
     57