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String Generation for Brute-force Attacks

A brute-force attack is one of the most trivial (and yet pretty useful) methods of cracking passwords and breaking access keys.

The idea is simply trying all possible sequences of input characters, until you guess the right combination. The thing is, that it might take some time. Actually, sometimes it might take literally ages, due to large number of possible outcomes. The faster our machines (and algorithms!) get, the lesser time it takes to break in using brute-force attack.

One of the key components in this technique is an algorithm that generates the input combinations. It’s run every time in the main loop. Well, the loop is pretty much generate a password, try it and try again. This article will present a couple of simple implementations of string sequence generators in various languages.

Python implementation

Here is code for a most basic example in Python. It’s a simple recursive function that is able to generate strings up to infinite length. I use a list instad of string in this example, because strings in python are immutable. You need to convert it to string ( "".join(list) ).

def next(string):
    if len(string)
        string.append(chr(0))
    else:
        string[0] = chr((ord(string[0]) + 1) % 256)
        if ord(string[0]) is 0:
            return list(string[0]) + next(string[1:])
    return string

Download the whole example from github.

This example is very simple. It tries every possible combination which takes it’s success rate up to 100 percent. You just can’t miss anything if you try them all, right? But a lot of the characters from ASCII table are non-printable, weird and people don’t use them for passwords. So you spend a great amount of time by trying out combinations that are extremely unlikely to ever occur. It would be pretty awesome if there was some way of saying what characters can be part of a password string and use only them. The number of possible outcomes lowers a lot by this optimization while the chance to miss is still almost zero. I made a some alternations to the next() function above:

import string

ALLOWED_CHARACTERS = string.printable
NUMBER_OF_CHARACTERS = len(ALLOWED_CHARACTERS)

def characterToIndex(char):
    return ALLOWED_CHARACTERS.index(char)

def indexToCharacter(index):
    if NUMBER_OF_CHARACTERS
        raise ValueError("Index out of range.")
    else:
        return ALLOWED_CHARACTERS[index]

def next(string):
    if len(string)
        string.append(indexToCharacter(0))
    else:
        string[0] = indexToCharacter((characterToIndex(string[0]) + 1) \
                                                   % NUMBER_OF_CHARACTERS)
        if characterToIndex(string[0]) is 0:
            return list(string[0]) + next(string[1:])
    return string

This snippet above works only with printable characters (as specified in python’s string module). You can also change the subset of characters it works with by changing the value of ALLOWED_CHARACTERS constant. The whole source is again available at github.

Next time I’ll look into a C implementation of the technique and a comparison of speed between the two languages.