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 /**
14 @file ltc_ecc_map.c
15 ECC Crypto, Tom St Denis
16 */
17
18 #ifdef LTC_MECC
19
20 /**
21 Map a projective jacbobian point back to affine space
22 @param P [in/out] The point to map
23 @param modulus The modulus of the field the ECC curve is in
24 @param mp The "b" value from montgomery_setup()
25 @return CRYPT_OK on success
26 */
ltc_ecc_map(ecc_point * P,void * modulus,void * mp)27 int ltc_ecc_map(ecc_point *P, void *modulus, void *mp)
28 {
29 void *t1, *t2;
30 int err;
31
32 LTC_ARGCHK(P != NULL);
33 LTC_ARGCHK(modulus != NULL);
34 LTC_ARGCHK(mp != NULL);
35
36 if (mp_iszero(P->z)) {
37 return ltc_ecc_set_point_xyz(0, 0, 1, P);
38 }
39
40 if ((err = mp_init_multi(&t1, &t2, NULL)) != CRYPT_OK) {
41 return err;
42 }
43
44 /* first map z back to normal */
45 if ((err = mp_montgomery_reduce(P->z, modulus, mp)) != CRYPT_OK) { goto done; }
46
47 /* get 1/z */
48 if ((err = mp_invmod(P->z, modulus, t1)) != CRYPT_OK) { goto done; }
49
50 /* get 1/z^2 and 1/z^3 */
51 if ((err = mp_sqr(t1, t2)) != CRYPT_OK) { goto done; }
52 if ((err = mp_mod(t2, modulus, t2)) != CRYPT_OK) { goto done; }
53 if ((err = mp_mul(t1, t2, t1)) != CRYPT_OK) { goto done; }
54 if ((err = mp_mod(t1, modulus, t1)) != CRYPT_OK) { goto done; }
55
56 /* multiply against x/y */
57 if ((err = mp_mul(P->x, t2, P->x)) != CRYPT_OK) { goto done; }
58 if ((err = mp_montgomery_reduce(P->x, modulus, mp)) != CRYPT_OK) { goto done; }
59 if ((err = mp_mul(P->y, t1, P->y)) != CRYPT_OK) { goto done; }
60 if ((err = mp_montgomery_reduce(P->y, modulus, mp)) != CRYPT_OK) { goto done; }
61 if ((err = mp_set(P->z, 1)) != CRYPT_OK) { goto done; }
62
63 err = CRYPT_OK;
64 done:
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
75