Add a ecdh shared secret example

Co-authored-by: Jonas Nick <jonasd.nick@gmail.com>

Author: elichai

examples/ecdh.c

@@ -0,0 +1,113 @@
+/*************************************************************************
+ * Copyright (c) 2020-2021 Elichai Turkel                                *
+ * Distributed under the CC0 software license, see the accompanying file *
+ * EXAMPLES_COPYING or https://creativecommons.org/publicdomain/zero/1.0 *
+ *************************************************************************/
+
+#include <stdio.h>
+#include <assert.h>
+#include <string.h>
+
+#include "random.h"
+#include "secp256k1.h"
+#include "secp256k1_ecdh.h"
+
+
+int main(void) {
+    unsigned char seckey1[32];
+    unsigned char seckey2[32];
+    unsigned char compressed_pubkey1[33];
+    unsigned char compressed_pubkey2[33];
+    unsigned char shared_secret1[32];
+    unsigned char shared_secret2[32];
+    unsigned char randomize[32];
+    size_t len;
+    secp256k1_pubkey pubkey1;
+    secp256k1_pubkey pubkey2;
+
+    /* The docs in secp256k1.h above the `secp256k1_ec_pubkey_create` function
+     * say: "pointer to a context object, initialized for signing" which is why
+     * we create a context for signing with the SECP256K1_CONTEXT_SIGN flag.
+     * (The docs for `secp256k1_ecdh` don't require any special context, just
+     * some initialized context) */
+    secp256k1_context* ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN);
+    if (!fill_random(randomize, sizeof(randomize))) {
+        printf("Failed to generate randomness\n");
+        return 1;
+    }
+    /* Randomizing the context is recommended to protect against side-channel
+     * leakage See `secp256k1_context_randomize` in secp256k1.h for more
+     * information about it. This should never fail. */
+    assert(secp256k1_context_randomize(ctx, randomize));
+
+    /*** Key Generation ***/
+
+    /* If the secret key is zero or out of range (bigger than secp256k1's
+     * order), we try to sample a new key. Note that the probability of this
+     * happening is negligible. */
+    while (1) {
+        if (!fill_random(seckey1, sizeof(seckey1)) || !fill_random(seckey2, sizeof(seckey2))) {
+            printf("Failed to generate randomness\n");
+            return 1;
+        }
+        if (secp256k1_ec_seckey_verify(ctx, seckey1) && secp256k1_ec_seckey_verify(ctx, seckey2)) {
+            break;
+        }
+    }
+
+    /* Public key creation using a valid context with a verified secret key should never fail */
+    assert(secp256k1_ec_pubkey_create(ctx, &pubkey1, seckey1));
+    assert(secp256k1_ec_pubkey_create(ctx, &pubkey2, seckey2));
+
+    /* Serialize pubkey1 in a compressed form (33 bytes), should always return 1 */
+    len = sizeof(compressed_pubkey1);
+    assert(secp256k1_ec_pubkey_serialize(ctx, compressed_pubkey1, &len, &pubkey1, SECP256K1_EC_COMPRESSED));
+    /* Should be the same size as the size of the output, because we passed a 33 bytes array. */
+    assert(len == sizeof(compressed_pubkey1));
+
+    /* Serialize pubkey2 in a compressed form (33 bytes) */
+    len = sizeof(compressed_pubkey2);
+    secp256k1_ec_pubkey_serialize(ctx, compressed_pubkey2, &len, &pubkey2, SECP256K1_EC_COMPRESSED);
+    assert(len == sizeof(compressed_pubkey2));
+
+    /*** Creating the shared secret ***/
+
+    /* Perform ECDH with seckey1 and pubkey2. Should never fail with a verified
+     * seckey and valid pubkey */
+    assert(secp256k1_ecdh(ctx, shared_secret1, &pubkey2, seckey1, NULL, NULL));
+
+    /* Perform ECDH with seckey2 and pubkey1. Should never fail with a verified
+     * seckey and valid pubkey */
+    assert(secp256k1_ecdh(ctx, shared_secret2, &pubkey1, seckey2, NULL, NULL));
+
+    /* Both parties should end up with the same shared secret */
+    assert(memcmp(shared_secret1, shared_secret2, sizeof(shared_secret1)) == 0);
+
+    printf("Secret Key1: ");
+    print_hex(seckey1, sizeof(seckey1));
+    printf("Compressed Pubkey1: ");
+    print_hex(compressed_pubkey1, sizeof(compressed_pubkey1));
+    printf("\nSecret Key2: ");
+    print_hex(seckey2, sizeof(seckey2));
+    printf("Compressed Pubkey2: ");
+    print_hex(compressed_pubkey2, sizeof(compressed_pubkey2));
+    printf("\nShared Secret: ");
+    print_hex(shared_secret1, sizeof(shared_secret1));
+
+    /* This will clear everything from the context and free the memory */
+    secp256k1_context_destroy(ctx);
+
+    /* It's best practice to try to remove secrets from memory after using them.
+     * This is done because some bugs can allow an attacker leak memory, for
+     * example through out of bounds array access (see Heartbleed for example).
+     * Hence, we overwrite the secret key buffer with zeros.
+     *
+     * TODO: Prevent these writes from being optimized out, as any good compiler
+     * will remove any writes that aren't used. */
+    memset(seckey1, 0, sizeof(seckey1));
+    memset(seckey2, 0, sizeof(seckey2));
+    memset(shared_secret1, 0, sizeof(shared_secret1));
+    memset(shared_secret2, 0, sizeof(shared_secret2));
+
+    return 0;
+}