1 /* LibTomCrypt, modular cryptographic library -- Tom St Denis */
2 /* SPDX-License-Identifier: Unlicense */
3
4 #include "tomcrypt_private.h"
5
6 #ifdef LTC_MDH
7
8 /**
9 Create a DH shared secret.
10 @param private_key The private DH key in the pair
11 @param public_key The public DH key in the pair
12 @param out [out] The destination of the shared data
13 @param outlen [in/out] The max size and resulting size of the shared data.
14 @return CRYPT_OK if successful
15 */
dh_shared_secret(const dh_key * private_key,const dh_key * public_key,unsigned char * out,unsigned long * outlen)16 int dh_shared_secret(const dh_key *private_key, const dh_key *public_key,
17 unsigned char *out, unsigned long *outlen)
18 {
19 void *tmp;
20 unsigned long x;
21 int err;
22
23 LTC_ARGCHK(private_key != NULL);
24 LTC_ARGCHK(public_key != NULL);
25 LTC_ARGCHK(out != NULL);
26 LTC_ARGCHK(outlen != NULL);
27
28 /* types valid? */
29 if (private_key->type != PK_PRIVATE) {
30 return CRYPT_PK_NOT_PRIVATE;
31 }
32
33 /* same DH group? */
34 if (mp_cmp(private_key->prime, public_key->prime) != LTC_MP_EQ) { return CRYPT_PK_TYPE_MISMATCH; }
35 if (mp_cmp(private_key->base, public_key->base) != LTC_MP_EQ) { return CRYPT_PK_TYPE_MISMATCH; }
36
37 /* init big numbers */
38 if ((err = mp_init(&tmp)) != CRYPT_OK) {
39 return err;
40 }
41
42 /* check public key */
43 if ((err = dh_check_pubkey(public_key)) != CRYPT_OK) {
44 goto error;
45 }
46
47 /* compute tmp = y^x mod p */
48 if ((err = mp_exptmod(public_key->y, private_key->x, private_key->prime, tmp)) != CRYPT_OK) {
49 goto error;
50 }
51
52 /* enough space for output? */
53 x = (unsigned long)mp_unsigned_bin_size(tmp);
54 if (*outlen < x) {
55 *outlen = x;
56 err = CRYPT_BUFFER_OVERFLOW;
57 goto error;
58 }
59 if ((err = mp_to_unsigned_bin(tmp, out)) != CRYPT_OK) {
60 goto error;
61 }
62 *outlen = x;
63 err = CRYPT_OK;
64
65 error:
66 mp_clear(tmp);
67 return err;
68 }
69
70 #endif /* LTC_MDH */
71