brute.c

#include <string.h>
#include <stdio.h>
#include <stdbool.h>
#include <openssl/sha.h>
#include <stdlib.h>

// compile with gcc -lcrypto -std=c11 brute.c -o brute
// run with ./brute <prevhash>

static char lastHash[2 * SHA_DIGEST_LENGTH + 1];
static const char *collide = "00";
typedef unsigned char byte;
#define MSGL 10

int hash(const char* msg) {
	SHA_CTX ctx;
	byte sha[SHA_DIGEST_LENGTH];
	SHA1_Init(&ctx);
	SHA1_Update(&ctx, lastHash, sizeof(lastHash));
	SHA1_Update(&ctx, msg, strlen(msg));
	SHA1_Final(sha, &ctx);
    
	char hex[2 * SHA_DIGEST_LENGTH + 1];
	for (size_t i = 0; i < SHA_DIGEST_LENGTH; ++i) {
		snprintf(&hex[2 * i], sizeof(hex) - 2 * i, "%02x", sha[i]);
	}
    // don't copy if no colliison
    if (strncmp(hex, collide, strlen(collide))) return false;
	strncpy(lastHash, hex, sizeof(lastHash) + 1);
    return true;
}

// write a certain number of random characters to the string
void write_rand_str(char* output, int length) {
    for (int i = 0; i < length; ++i) {
        // random char between 0 and 9
        char x = (rand() % 10) + '0';
        output[i] = x;
    }
}

int main(int argc, char** argv) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <hash>\n", argv[0]);
        return -1;
    }
    const char* prevHash = argv[1];

    char msg[MSGL+1] = {'\0'};
    strncpy(lastHash, prevHash, strlen(prevHash)+1);
    bool match = false;
    while (!match) {
        write_rand_str(msg, MSGL);
        match = hash(msg);
    }
    printf("%s\n", msg);
    
    return 0;
}