1 // SPDX-License-Identifier: BSD-2-Clause
2 /* LibTomCrypt, modular cryptographic library -- Tom St Denis
3 *
4 * LibTomCrypt is a library that provides various cryptographic
5 * algorithms in a highly modular and flexible manner.
6 *
7 * The library is free for all purposes without any express
8 * guarantee it works.
9 */
10
11 #include "tomcrypt_private.h"
12
13 #ifdef LTC_MECC
14
15 /** Returns whether [x,y] is a point on curve defined by dp
16 @param dp curve parameters
17 @param x x point coordinate
18 @param y y point coordinate
19 @return CRYPT_OK if valid
20 */
21
ltc_ecc_is_point(const ltc_ecc_dp * dp,void * x,void * y)22 int ltc_ecc_is_point(const ltc_ecc_dp *dp, void *x, void *y)
23 {
24 void *prime, *a, *b, *t1, *t2;
25 int err;
26
27 prime = dp->prime;
28 b = dp->B;
29 a = dp->A;
30
31 if ((err = mp_init_multi(&t1, &t2, NULL)) != CRYPT_OK) return err;
32
33 /* compute y^2 */
34 if ((err = mp_sqr(y, t1)) != CRYPT_OK) goto cleanup;
35
36 /* compute x^3 */
37 if ((err = mp_sqr(x, t2)) != CRYPT_OK) goto cleanup;
38 if ((err = mp_mod(t2, prime, t2)) != CRYPT_OK) goto cleanup;
39 if ((err = mp_mul(x, t2, t2)) != CRYPT_OK) goto cleanup;
40
41 /* compute y^2 - x^3 */
42 if ((err = mp_sub(t1, t2, t1)) != CRYPT_OK) goto cleanup;
43
44 /* compute y^2 - x^3 - a*x */
45 if ((err = mp_submod(prime, a, prime, t2)) != CRYPT_OK) goto cleanup;
46 if ((err = mp_mulmod(t2, x, prime, t2)) != CRYPT_OK) goto cleanup;
47 if ((err = mp_addmod(t1, t2, prime, t1)) != CRYPT_OK) goto cleanup;
48
49 /* adjust range (0, prime) */
50 while (mp_cmp_d(t1, 0) == LTC_MP_LT) {
51 if ((err = mp_add(t1, prime, t1)) != CRYPT_OK) goto cleanup;
52 }
53 while (mp_cmp(t1, prime) != LTC_MP_LT) {
54 if ((err = mp_sub(t1, prime, t1)) != CRYPT_OK) goto cleanup;
55 }
56
57 /* compare to b */
58 if (mp_cmp(t1, b) != LTC_MP_EQ) {
59 err = CRYPT_INVALID_PACKET;
60 } else {
61 err = CRYPT_OK;
62 }
63
64 cleanup:
65 mp_clear_multi(t1, t2, NULL);
66 return err;
67 }
68
69 #endif
70
71 /* ref: $Format:%D$ */
72 /* git commit: $Format:%H$ */
73 /* commit time: $Format:%ai$ */
74