1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis */
2 /* SPDX-License-Identifier: Unlicense */
3
4 #include "tomcrypt_private.h"
5
6 /**
7 @file ltc_ecc_map.c
8 ECC Crypto, Tom St Denis
9 */
10
11 #ifdef LTC_MECC
12
13 /**
14 Map a projective jacbobian point back to affine space
15 @param P [in/out] The point to map
16 @param modulus The modulus of the field the ECC curve is in
17 @param mp The "b" value from montgomery_setup()
18 @return CRYPT_OK on success
19 */
ltc_ecc_map(ecc_point * P,void * modulus,void * mp)20 int ltc_ecc_map(ecc_point *P, void *modulus, void *mp)
21 {
22 void *t1, *t2;
23 int err;
24
25 LTC_ARGCHK(P != NULL);
26 LTC_ARGCHK(modulus != NULL);
27 LTC_ARGCHK(mp != NULL);
28
29 if (mp_iszero(P->z)) {
30 return ltc_ecc_set_point_xyz(0, 0, 1, P);
31 }
32
33 if ((err = mp_init_multi(&t1, &t2, LTC_NULL)) != CRYPT_OK) {
34 return err;
35 }
36
37 /* first map z back to normal */
38 if ((err = mp_montgomery_reduce(P->z, modulus, mp)) != CRYPT_OK) { goto done; }
39
40 /* get 1/z */
41 if ((err = mp_invmod(P->z, modulus, t1)) != CRYPT_OK) { goto done; }
42
43 /* get 1/z^2 and 1/z^3 */
44 if ((err = mp_sqr(t1, t2)) != CRYPT_OK) { goto done; }
45 if ((err = mp_mod(t2, modulus, t2)) != CRYPT_OK) { goto done; }
46 if ((err = mp_mul(t1, t2, t1)) != CRYPT_OK) { goto done; }
47 if ((err = mp_mod(t1, modulus, t1)) != CRYPT_OK) { goto done; }
48
49 /* multiply against x/y */
50 if ((err = mp_mul(P->x, t2, P->x)) != CRYPT_OK) { goto done; }
51 if ((err = mp_montgomery_reduce(P->x, modulus, mp)) != CRYPT_OK) { goto done; }
52 if ((err = mp_mul(P->y, t1, P->y)) != CRYPT_OK) { goto done; }
53 if ((err = mp_montgomery_reduce(P->y, modulus, mp)) != CRYPT_OK) { goto done; }
54 if ((err = mp_set(P->z, 1)) != CRYPT_OK) { goto done; }
55
56 err = CRYPT_OK;
57 done:
58 mp_clear_multi(t1, t2, LTC_NULL);
59 return err;
60 }
61
62 #endif
63
64