1 /*************************************************************************
2 * Written in 2024 by Sebastian Falbesoner *
3 * To the extent possible under law, the author(s) have dedicated all *
4 * copyright and related and neighboring rights to the software in this *
5 * file to the public domain worldwide. This software is distributed *
6 * without any warranty. For the CC0 Public Domain Dedication, see *
7 * EXAMPLES_COPYING or https://creativecommons.org/publicdomain/zero/1.0 *
8 *************************************************************************/
9 10 /** This file demonstrates how to use the ElligatorSwift module to perform
11 * a key exchange according to BIP 324. Additionally, see the documentation
12 * in include/secp256k1_ellswift.h and doc/ellswift.md.
13 */
14 15 #include <stdio.h>
16 #include <stdlib.h>
17 #include <assert.h>
18 #include <string.h>
19 20 #include <secp256k1.h>
21 #include <secp256k1_ellswift.h>
22 23 #include "examples_util.h"
24 25 int main(void) {
26 secp256k1_context* ctx;
27 unsigned char randomize[32];
28 unsigned char auxrand1[32];
29 unsigned char auxrand2[32];
30 unsigned char seckey1[32];
31 unsigned char seckey2[32];
32 unsigned char ellswift_pubkey1[64];
33 unsigned char ellswift_pubkey2[64];
34 unsigned char shared_secret1[32];
35 unsigned char shared_secret2[32];
36 int return_val;
37 38 /* Create a secp256k1 context */
39 ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
40 if (!fill_random(randomize, sizeof(randomize))) {
41 printf("Failed to generate randomness\n");
42 return EXIT_FAILURE;
43 }
44 /* Randomizing the context is recommended to protect against side-channel
45 * leakage. See `secp256k1_context_randomize` in secp256k1.h for more
46 * information about it. This should never fail. */
47 return_val = secp256k1_context_randomize(ctx, randomize);
48 assert(return_val);
49 50 /*** Generate secret keys ***/
51 if (!fill_random(seckey1, sizeof(seckey1)) || !fill_random(seckey2, sizeof(seckey2))) {
52 printf("Failed to generate randomness\n");
53 return EXIT_FAILURE;
54 }
55 /* If the secret key is zero or out of range (greater than secp256k1's
56 * order), we fail. Note that the probability of this occurring is negligible
57 * with a properly functioning random number generator. */
58 if (!secp256k1_ec_seckey_verify(ctx, seckey1) || !secp256k1_ec_seckey_verify(ctx, seckey2)) {
59 printf("Generated secret key is invalid. This indicates an issue with the random number generator.\n");
60 return EXIT_FAILURE;
61 }
62 63 /* Generate ElligatorSwift public keys. This should never fail with valid context and
64 verified secret keys. Note that providing additional randomness (fourth parameter) is
65 optional, but recommended. */
66 if (!fill_random(auxrand1, sizeof(auxrand1)) || !fill_random(auxrand2, sizeof(auxrand2))) {
67 printf("Failed to generate randomness\n");
68 return EXIT_FAILURE;
69 }
70 return_val = secp256k1_ellswift_create(ctx, ellswift_pubkey1, seckey1, auxrand1);
71 assert(return_val);
72 return_val = secp256k1_ellswift_create(ctx, ellswift_pubkey2, seckey2, auxrand2);
73 assert(return_val);
74 75 /*** Create the shared secret on each side ***/
76 77 /* Perform x-only ECDH with seckey1 and ellswift_pubkey2. Should never fail
78 * with a verified seckey and valid pubkey. Note that both parties pass both
79 * EllSwift pubkeys in the same order; the pubkey of the calling party is
80 * determined by the "party" boolean (sixth parameter). */
81 return_val = secp256k1_ellswift_xdh(ctx, shared_secret1, ellswift_pubkey1, ellswift_pubkey2,
82 seckey1, 0, secp256k1_ellswift_xdh_hash_function_bip324, NULL);
83 assert(return_val);
84 85 /* Perform x-only ECDH with seckey2 and ellswift_pubkey1. Should never fail
86 * with a verified seckey and valid pubkey. */
87 return_val = secp256k1_ellswift_xdh(ctx, shared_secret2, ellswift_pubkey1, ellswift_pubkey2,
88 seckey2, 1, secp256k1_ellswift_xdh_hash_function_bip324, NULL);
89 assert(return_val);
90 91 /* Both parties should end up with the same shared secret */
92 return_val = memcmp(shared_secret1, shared_secret2, sizeof(shared_secret1));
93 assert(return_val == 0);
94 95 printf( " Secret Key1: ");
96 print_hex(seckey1, sizeof(seckey1));
97 printf( "EllSwift Pubkey1: ");
98 print_hex(ellswift_pubkey1, sizeof(ellswift_pubkey1));
99 printf("\n Secret Key2: ");
100 print_hex(seckey2, sizeof(seckey2));
101 printf( "EllSwift Pubkey2: ");
102 print_hex(ellswift_pubkey2, sizeof(ellswift_pubkey2));
103 printf("\n Shared Secret: ");
104 print_hex(shared_secret1, sizeof(shared_secret1));
105 106 /* This will clear everything from the context and free the memory */
107 secp256k1_context_destroy(ctx);
108 109 /* It's best practice to try to clear secrets from memory after using them.
110 * This is done because some bugs can allow an attacker to leak memory, for
111 * example through "out of bounds" array access (see Heartbleed), or the OS
112 * swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
113 *
114 * Here we are preventing these writes from being optimized out, as any good compiler
115 * will remove any writes that aren't used. */
116 secure_erase(seckey1, sizeof(seckey1));
117 secure_erase(seckey2, sizeof(seckey2));
118 secure_erase(shared_secret1, sizeof(shared_secret1));
119 secure_erase(shared_secret2, sizeof(shared_secret2));
120 121 return EXIT_SUCCESS;
122 }
123