Question

I have the MD5 hash for a String, stored as a String. I’m writing a small program to find out the original string by brute force.

I’d like to loop through a subset of char.

The code below works for when String.length() == 0.

I can’t figure out how to edit this code to work with variable-length Strings. I feel like I’m on the right track with recursion, but can’t go any further.

I have the following code:

    public void attempt(String initial, String md5) {

    for (char c = ' '; c < '~'; ++c) {
        attempt = initial + Character.toString(c);
        generatedMd5 = generateMD5(attempt);
        System.out.println(attempt);
        if (hashesMatch(md5, generatedMd5)) {
            break;
        } else attempt(attempt, md5);
    }
}

_{Note: I should mention this is for an academic study on MD5.}

Answer 1

You are doing a “Depth first” search (and of unlimited depth!), this is almost guaranteed to fail ( exhausting your stack) if you do not add some depth check.

You should probably better might want to do a Breadth first search : your loop should first try all the combinations that results in adding a character, and only then, if no success, try to recursively call the method with each augmented string.

In any case, you should add some depth check, always.

Edited: thinking it twice, I’m not so sure you should not stick with depth first. Breadth first is only rasonable here for small depths and combinations (character ranges). A possible implementation

  // Breadth first returns null if not found
  public String bruteforce(List<String> prefixes, String md5,int availdepth) {
    if(availabledepth<0) return null;
    List<String> newprefixes = new ArrayList<String>();
    for(String prefix : prefixes) {
        for (char c = ' '; c < '~'; ++c) {
          String attempt = prefix + Character.toString(c);
          generatedMd5 = generateMD5(attempt);
          if (hashesMatch(md5, generatedMd5)) 
            return attempt;
          newprefixes.add(attempt);
       }
    }
    // no success in this level go for next
    return bruteforce(newprefixes,md5,availddepth-1);
  }


  // Depth first - returns null if not found
  public String bruteforce(String prefix, String md5,int availdepth) {
    if(availdepth <= 0) return null;
    for (char c = ' '; c < '~'; ++c) {
          String attempt = prefix + Character.toString(c);
          if (hashesMatch(md5, generateMD5(attempt))) 
            return attempt;
          String res = bruteforce(attempt, md5, availdepth-1);
          if(res != null) return res;
       }
    return null;
  }