31f84595c4
This should hopefully be useful as orientation for users implementing the key exchange part of BIP324. Conceptually the example is not very different to the ECDH one, so a lot of code/comments are just copied (e.g. context creation, secret key generation, shared secret comparison, console output, cleanup with secret key clearing).
124 lines
5.2 KiB
C
124 lines
5.2 KiB
C
/*************************************************************************
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* Written in 2024 by Sebastian Falbesoner *
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* To the extent possible under law, the author(s) have dedicated all *
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* copyright and related and neighboring rights to the software in this *
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* file to the public domain worldwide. This software is distributed *
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* without any warranty. For the CC0 Public Domain Dedication, see *
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* EXAMPLES_COPYING or https://creativecommons.org/publicdomain/zero/1.0 *
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*************************************************************************/
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/** This file demonstrates how to use the ElligatorSwift module to perform
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* a key exchange according to BIP 324. Additionally, see the documentation
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* in include/secp256k1_ellswift.h and doc/ellswift.md.
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*/
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#include <stdio.h>
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#include <assert.h>
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#include <string.h>
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#include <secp256k1.h>
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#include <secp256k1_ellswift.h>
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#include "examples_util.h"
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int main(void) {
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secp256k1_context* ctx;
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unsigned char randomize[32];
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unsigned char auxrand1[32];
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unsigned char auxrand2[32];
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unsigned char seckey1[32];
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unsigned char seckey2[32];
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unsigned char ellswift_pubkey1[64];
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unsigned char ellswift_pubkey2[64];
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unsigned char shared_secret1[32];
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unsigned char shared_secret2[32];
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int return_val;
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/* Create a secp256k1 context */
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ctx = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
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if (!fill_random(randomize, sizeof(randomize))) {
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printf("Failed to generate randomness\n");
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return 1;
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}
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/* Randomizing the context is recommended to protect against side-channel
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* leakage. See `secp256k1_context_randomize` in secp256k1.h for more
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* information about it. This should never fail. */
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return_val = secp256k1_context_randomize(ctx, randomize);
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assert(return_val);
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/*** Generate secret keys ***/
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/* If the secret key is zero or out of range (bigger than secp256k1's
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* order), we try to sample a new key. Note that the probability of this
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* happening is negligible. */
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while (1) {
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if (!fill_random(seckey1, sizeof(seckey1)) || !fill_random(seckey2, sizeof(seckey2))) {
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printf("Failed to generate randomness\n");
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return 1;
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}
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if (secp256k1_ec_seckey_verify(ctx, seckey1) && secp256k1_ec_seckey_verify(ctx, seckey2)) {
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break;
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}
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}
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/* Generate ElligatorSwift public keys. This should never fail with valid context and
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verified secret keys. Note that providing additional randomness (fourth parameter) is
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optional, but recommended. */
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if (!fill_random(auxrand1, sizeof(auxrand1)) || !fill_random(auxrand2, sizeof(auxrand2))) {
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printf("Failed to generate randomness\n");
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return 1;
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}
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return_val = secp256k1_ellswift_create(ctx, ellswift_pubkey1, seckey1, auxrand1);
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assert(return_val);
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return_val = secp256k1_ellswift_create(ctx, ellswift_pubkey2, seckey2, auxrand2);
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assert(return_val);
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/*** Create the shared secret on each side ***/
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/* Perform x-only ECDH with seckey1 and ellswift_pubkey2. Should never fail
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* with a verified seckey and valid pubkey. Note that both parties pass both
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* EllSwift pubkeys in the same order; the pubkey of the calling party is
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* determined by the "party" boolean (sixth parameter). */
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return_val = secp256k1_ellswift_xdh(ctx, shared_secret1, ellswift_pubkey1, ellswift_pubkey2,
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seckey1, 0, secp256k1_ellswift_xdh_hash_function_bip324, NULL);
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assert(return_val);
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/* Perform x-only ECDH with seckey2 and ellswift_pubkey1. Should never fail
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* with a verified seckey and valid pubkey. */
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return_val = secp256k1_ellswift_xdh(ctx, shared_secret2, ellswift_pubkey1, ellswift_pubkey2,
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seckey2, 1, secp256k1_ellswift_xdh_hash_function_bip324, NULL);
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assert(return_val);
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/* Both parties should end up with the same shared secret */
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return_val = memcmp(shared_secret1, shared_secret2, sizeof(shared_secret1));
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assert(return_val == 0);
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printf( " Secret Key1: ");
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print_hex(seckey1, sizeof(seckey1));
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printf( "EllSwift Pubkey1: ");
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print_hex(ellswift_pubkey1, sizeof(ellswift_pubkey1));
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printf("\n Secret Key2: ");
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print_hex(seckey2, sizeof(seckey2));
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printf( "EllSwift Pubkey2: ");
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print_hex(ellswift_pubkey2, sizeof(ellswift_pubkey2));
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printf("\n Shared Secret: ");
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print_hex(shared_secret1, sizeof(shared_secret1));
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/* This will clear everything from the context and free the memory */
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secp256k1_context_destroy(ctx);
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/* It's best practice to try to clear secrets from memory after using them.
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* This is done because some bugs can allow an attacker to leak memory, for
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* example through "out of bounds" array access (see Heartbleed), or the OS
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* swapping them to disk. Hence, we overwrite the secret key buffer with zeros.
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*
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* Here we are preventing these writes from being optimized out, as any good compiler
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* will remove any writes that aren't used. */
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secure_erase(seckey1, sizeof(seckey1));
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secure_erase(seckey2, sizeof(seckey2));
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secure_erase(shared_secret1, sizeof(shared_secret1));
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secure_erase(shared_secret2, sizeof(shared_secret2));
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return 0;
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}
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