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| 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 <assert.h> |
| 17 | +#include <string.h> |
| 18 | + |
| 19 | +#include <secp256k1.h> |
| 20 | +#include <secp256k1_ellswift.h> |
| 21 | + |
| 22 | +#include "examples_util.h" |
| 23 | + |
| 24 | +int main(void) { |
| 25 | + secp256k1_context* ctx; |
| 26 | + unsigned char randomize[32]; |
| 27 | + unsigned char auxrand1[32]; |
| 28 | + unsigned char auxrand2[32]; |
| 29 | + unsigned char seckey1[32]; |
| 30 | + unsigned char seckey2[32]; |
| 31 | + unsigned char ellswift_pubkey1[64]; |
| 32 | + unsigned char ellswift_pubkey2[64]; |
| 33 | + unsigned char shared_secret1[32]; |
| 34 | + unsigned char shared_secret2[32]; |
| 35 | + int return_val; |
| 36 | + |
| 37 | + /* Create a secp256k1 context */ |
| 38 | + ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE); |
| 39 | + if (!fill_random(randomize, sizeof(randomize))) { |
| 40 | + printf("Failed to generate randomness\n"); |
| 41 | + return 1; |
| 42 | + } |
| 43 | + /* Randomizing the context is recommended to protect against side-channel |
| 44 | + * leakage. See `secp256k1_context_randomize` in secp256k1.h for more |
| 45 | + * information about it. This should never fail. */ |
| 46 | + return_val = secp256k1_context_randomize(ctx, randomize); |
| 47 | + assert(return_val); |
| 48 | + |
| 49 | + /*** Generate secret keys ***/ |
| 50 | + |
| 51 | + /* If the secret key is zero or out of range (bigger than secp256k1's |
| 52 | + * order), we try to sample a new key. Note that the probability of this |
| 53 | + * happening is negligible. */ |
| 54 | + while (1) { |
| 55 | + if (!fill_random(seckey1, sizeof(seckey1)) || !fill_random(seckey2, sizeof(seckey2))) { |
| 56 | + printf("Failed to generate randomness\n"); |
| 57 | + return 1; |
| 58 | + } |
| 59 | + if (secp256k1_ec_seckey_verify(ctx, seckey1) && secp256k1_ec_seckey_verify(ctx, seckey2)) { |
| 60 | + break; |
| 61 | + } |
| 62 | + } |
| 63 | + |
| 64 | + /* Generate ElligatorSwift public keys. This should never fail with valid context and |
| 65 | + verified secret keys. Note that providing additional randomness (fourth parameter) is |
| 66 | + optional, but recommended. */ |
| 67 | + if (!fill_random(auxrand1, sizeof(auxrand1)) || !fill_random(auxrand2, sizeof(auxrand2))) { |
| 68 | + printf("Failed to generate randomness\n"); |
| 69 | + return 1; |
| 70 | + } |
| 71 | + return_val = secp256k1_ellswift_create(ctx, ellswift_pubkey1, seckey1, auxrand1); |
| 72 | + assert(return_val); |
| 73 | + return_val = secp256k1_ellswift_create(ctx, ellswift_pubkey2, seckey2, auxrand2); |
| 74 | + assert(return_val); |
| 75 | + |
| 76 | + /*** Create the shared secret on each side ***/ |
| 77 | + |
| 78 | + /* Perform x-only ECDH with seckey1 and ellswift_pubkey2. Should never fail |
| 79 | + * with a verified seckey and valid pubkey. Note that both parties pass both |
| 80 | + * EllSwift pubkeys in the same order; the pubkey of the calling party is |
| 81 | + * determined by the "party" boolean (sixth parameter). */ |
| 82 | + return_val = secp256k1_ellswift_xdh(ctx, shared_secret1, ellswift_pubkey1, ellswift_pubkey2, |
| 83 | + seckey1, 0, secp256k1_ellswift_xdh_hash_function_bip324, NULL); |
| 84 | + assert(return_val); |
| 85 | + |
| 86 | + /* Perform x-only ECDH with seckey2 and ellswift_pubkey1. Should never fail |
| 87 | + * with a verified seckey and valid pubkey. */ |
| 88 | + return_val = secp256k1_ellswift_xdh(ctx, shared_secret2, ellswift_pubkey1, ellswift_pubkey2, |
| 89 | + seckey2, 1, secp256k1_ellswift_xdh_hash_function_bip324, NULL); |
| 90 | + assert(return_val); |
| 91 | + |
| 92 | + /* Both parties should end up with the same shared secret */ |
| 93 | + return_val = memcmp(shared_secret1, shared_secret2, sizeof(shared_secret1)); |
| 94 | + assert(return_val == 0); |
| 95 | + |
| 96 | + printf( " Secret Key1: "); |
| 97 | + print_hex(seckey1, sizeof(seckey1)); |
| 98 | + printf( "EllSwift Pubkey1: "); |
| 99 | + print_hex(ellswift_pubkey1, sizeof(ellswift_pubkey1)); |
| 100 | + printf("\n Secret Key2: "); |
| 101 | + print_hex(seckey2, sizeof(seckey2)); |
| 102 | + printf( "EllSwift Pubkey2: "); |
| 103 | + print_hex(ellswift_pubkey2, sizeof(ellswift_pubkey2)); |
| 104 | + printf("\n Shared Secret: "); |
| 105 | + print_hex(shared_secret1, sizeof(shared_secret1)); |
| 106 | + |
| 107 | + /* This will clear everything from the context and free the memory */ |
| 108 | + secp256k1_context_destroy(ctx); |
| 109 | + |
| 110 | + /* It's best practice to try to clear secrets from memory after using them. |
| 111 | + * This is done because some bugs can allow an attacker to leak memory, for |
| 112 | + * example through "out of bounds" array access (see Heartbleed), or the OS |
| 113 | + * swapping them to disk. Hence, we overwrite the secret key buffer with zeros. |
| 114 | + * |
| 115 | + * Here we are preventing these writes from being optimized out, as any good compiler |
| 116 | + * will remove any writes that aren't used. */ |
| 117 | + secure_erase(seckey1, sizeof(seckey1)); |
| 118 | + secure_erase(seckey2, sizeof(seckey2)); |
| 119 | + secure_erase(shared_secret1, sizeof(shared_secret1)); |
| 120 | + secure_erase(shared_secret2, sizeof(shared_secret2)); |
| 121 | + |
| 122 | + return 0; |
| 123 | +} |
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