adler32 collider

adler32_col.c

/*
 * ----------------------------------------------------------------------------
 * "THE BEER-WARE LICENSE" (Revision 42):
 * @hops_ch wrote this file.  As long as you retain this notice you
 * can do whatever you want with this stuff. If we meet some day, and you think
 * this stuff is worth it, you can buy me a beer in return.    Michael Sprecher
 * ----------------------------------------------------------------------------
 */

// compile with: gcc -o adler32_col -lz -O3 adler32_col.c

#include <zlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define CHARS 0x7e - 0x2f
#define START 0x2f

int main (int argc, char* argv[])
{
  if (argc != 3) {
    fprintf (stderr, "usage: %s checksum length\n", argv[0]);
    return (-1);
  }

  uLong match = atol(argv[1]);
  int len = atoi(argv[2]);
  char buf[len + 1];

  memset(buf, 0, len + 1);

  while(1) {
    for (int i = 0; i < len; i++) {
      buf[i] = START + (random() % (CHARS));
    }

    uLong adler = adler32(0L, Z_NULL, 0);
    adler = adler32(adler, (Byte *)buf, len);

    if (adler == match) { 
      printf("found collision: %s\n", buf);
    }
  }
  return 0;
}