1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis */
2 /* SPDX-License-Identifier: Unlicense */
3
4 #include "tomcrypt_private.h"
5
6 #ifdef LTC_MECC
7
8 /** Returns whether [x,y] is a point on curve defined by dp
9 @param dp curve parameters
10 @param x x point coordinate
11 @param y y point coordinate
12 @return CRYPT_OK if valid
13 */
14
ltc_ecc_is_point(const ltc_ecc_dp * dp,void * x,void * y)15 int ltc_ecc_is_point(const ltc_ecc_dp *dp, void *x, void *y)
16 {
17 void *prime, *a, *b, *t1, *t2;
18 int err;
19
20 prime = dp->prime;
21 b = dp->B;
22 a = dp->A;
23
24 if ((err = mp_init_multi(&t1, &t2, LTC_NULL)) != CRYPT_OK) return err;
25
26 /* compute y^2 */
27 if ((err = mp_sqr(y, t1)) != CRYPT_OK) goto cleanup;
28
29 /* compute x^3 */
30 if ((err = mp_sqr(x, t2)) != CRYPT_OK) goto cleanup;
31 if ((err = mp_mod(t2, prime, t2)) != CRYPT_OK) goto cleanup;
32 if ((err = mp_mul(x, t2, t2)) != CRYPT_OK) goto cleanup;
33
34 /* compute y^2 - x^3 */
35 if ((err = mp_sub(t1, t2, t1)) != CRYPT_OK) goto cleanup;
36
37 /* compute y^2 - x^3 - a*x */
38 if ((err = mp_submod(prime, a, prime, t2)) != CRYPT_OK) goto cleanup;
39 if ((err = mp_mulmod(t2, x, prime, t2)) != CRYPT_OK) goto cleanup;
40 if ((err = mp_addmod(t1, t2, prime, t1)) != CRYPT_OK) goto cleanup;
41
42 /* adjust range (0, prime) */
43 while (mp_cmp_d(t1, 0) == LTC_MP_LT) {
44 if ((err = mp_add(t1, prime, t1)) != CRYPT_OK) goto cleanup;
45 }
46 while (mp_cmp(t1, prime) != LTC_MP_LT) {
47 if ((err = mp_sub(t1, prime, t1)) != CRYPT_OK) goto cleanup;
48 }
49
50 /* compare to b */
51 if (mp_cmp(t1, b) != LTC_MP_EQ) {
52 err = CRYPT_INVALID_PACKET;
53 } else {
54 err = CRYPT_OK;
55 }
56
57 cleanup:
58 mp_clear_multi(t1, t2, LTC_NULL);
59 return err;
60 }
61
62 #endif
63