1 /**********************************************************************
2 * Copyright (c) 2015 Andrew Poelstra *
3 * Distributed under the MIT software license, see the accompanying *
4 * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
5 **********************************************************************/
6
7 #ifndef SECP256K1_MODULE_ECDH_MAIN_H
8 #define SECP256K1_MODULE_ECDH_MAIN_H
9
10 #include "include/secp256k1_ecdh.h"
11 #include "ecmult_const_impl.h"
12
ecdh_hash_function_sha256(unsigned char * output,const unsigned char * x32,const unsigned char * y32,void * data)13 static int ecdh_hash_function_sha256(unsigned char *output, const unsigned char *x32, const unsigned char *y32, void *data) {
14 unsigned char version = (y32[31] & 0x01) | 0x02;
15 secp256k1_sha256 sha;
16 (void)data;
17
18 secp256k1_sha256_initialize(&sha);
19 secp256k1_sha256_write(&sha, &version, 1);
20 secp256k1_sha256_write(&sha, x32, 32);
21 secp256k1_sha256_finalize(&sha, output);
22
23 return 1;
24 }
25
26 const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_sha256 = ecdh_hash_function_sha256;
27 const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_default = ecdh_hash_function_sha256;
28
secp256k1_ecdh(const secp256k1_context * ctx,unsigned char * output,const secp256k1_pubkey * point,const unsigned char * scalar,secp256k1_ecdh_hash_function hashfp,void * data)29 int secp256k1_ecdh(const secp256k1_context* ctx, unsigned char *output, const secp256k1_pubkey *point, const unsigned char *scalar, secp256k1_ecdh_hash_function hashfp, void *data) {
30 int ret = 0;
31 int overflow = 0;
32 secp256k1_gej res;
33 secp256k1_ge pt;
34 secp256k1_scalar s;
35 unsigned char x[32];
36 unsigned char y[32];
37
38 VERIFY_CHECK(ctx != NULL);
39 ARG_CHECK(output != NULL);
40 ARG_CHECK(point != NULL);
41 ARG_CHECK(scalar != NULL);
42
43 if (hashfp == NULL) {
44 hashfp = secp256k1_ecdh_hash_function_default;
45 }
46
47 secp256k1_pubkey_load(ctx, &pt, point);
48 secp256k1_scalar_set_b32(&s, scalar, &overflow);
49
50 overflow |= secp256k1_scalar_is_zero(&s);
51 secp256k1_scalar_cmov(&s, &secp256k1_scalar_one, overflow);
52
53 secp256k1_ecmult_const(&res, &pt, &s, 256);
54 secp256k1_ge_set_gej(&pt, &res);
55
56 /* Compute a hash of the point */
57 secp256k1_fe_normalize(&pt.x);
58 secp256k1_fe_normalize(&pt.y);
59 secp256k1_fe_get_b32(x, &pt.x);
60 secp256k1_fe_get_b32(y, &pt.y);
61
62 ret = hashfp(output, x, y, data);
63
64 memset(x, 0, 32);
65 memset(y, 0, 32);
66 secp256k1_scalar_clear(&s);
67
68 return !!ret & !overflow;
69 }
70
71 #endif /* SECP256K1_MODULE_ECDH_MAIN_H */
72