Generate git sha1 commit hash collision (used for Stripe CTF 3.0 level1 - gitcoin miner)

miner.c

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

static void compute_stream_hash(char *a, char *b, char *c)
{
    unsigned char sha1[20];
    unsigned char hash[(20 * 2) + 1];
    int i;
    SHA_CTX ctx;
    SHA1_Init(&ctx);
    SHA1_Update(&ctx, a, strlen(a) + 1);
    SHA1_Update(&ctx, b, strlen(b));
    SHA1_Update(&ctx, c, strlen(c));
    SHA1_Final(sha1, &ctx);
    for (i = 0; i < 20; i++)
      sprintf((char *)&(hash[i * 2]), "%02x", (unsigned int)sha1[i]);

    if (strcmp((char *)hash, "000001") < 0)
    {
      printf("%s %s", hash, c);  
      exit(EXIT_SUCCESS);
    }
}

static void do_setup(char *t, char *p, char *ti, char *m)
{
    char *tree_line = malloc(5 + strlen(t) + 1);
    char *parent_line = malloc(7 + strlen(p) + 1);
    char *author_line = malloc(100);
    char *committer_line = malloc(100);
    char *commit_msg = malloc(200);

    snprintf(tree_line, 10 + strlen(t), "tree %s\n", t);
    snprintf(parent_line, 10 + strlen(p), "parent %s\n", p);
    snprintf(author_line, 70 + strlen(ti), \
    "author CTF user <[email protected]> %s +0000\n", ti);
    snprintf(committer_line, 70 + strlen(ti), \
    "committer CTF user <[email protected]> %s +0000\n\n", ti);
    snprintf(commit_msg, 90, "Mined a Gitcoin\nnonce");

    int length = strlen(tree_line) + strlen(parent_line) + strlen(author_line) + \
    strlen(committer_line) + strlen(commit_msg);

    snprintf(m, length + 1, "%s%s%s%s%s", tree_line, parent_line,\
    author_line, committer_line, commit_msg);

}

int main(int argc, char *argv[])
{
    const char *hexchars = "0123456789abcdef";
    int j, index;
    char nonce[20];
    int len = strlen(hexchars);
    srand(time(NULL));
    char *message = malloc(1000);
    char *header = malloc(100);
    do_setup(argv[1], argv[2], argv[3], message);
    
    while (1)
    {
      int idx = 0;
      nonce[idx++] = ' ';
        
      for (j = 0; j < 8; j++)
      {
        index = rand() % len;
        nonce[idx++] = hexchars[index];
      }

      nonce[idx++] = '\n';
      nonce[idx] = '\0';  
      snprintf(header, 30, "commit %d", strlen(message) + strlen(nonce));
      compute_stream_hash(header, message, nonce);
      /*printf("%s", header);
      printf("%s", message);
      printf("%s", nonce);
      fgetc(stdin);*/
    }
    return 0;  
}