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Brute force implementation / C#
class Program
#region Private variables
// the secret password which we will try to find via brute force
private static string password = "p123";
private static string result;
private static bool isMatched = false;
/* The length of the charactersToTest Array is stored in a
* additional variable to increase performance */
private static int charactersToTestLength = 0;
private static long computedKeys = 0;
/* An array containing the characters which will be used to create the brute force keys,
* if less characters are used (e.g. only lower case chars) the faster the password is matched */
private static char[] charactersToTest =
'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't',
'u', 'v', 'w', 'x', 'y', 'z','A','B','C','D','E',
static void Main(string[] args)
var timeStarted = DateTime.Now;
Console.WriteLine("Start BruteForce - {0}", timeStarted.ToString());
// The length of the array is stored permanently during runtime
charactersToTestLength = charactersToTest.Length;
// The length of the password is unknown, so we have to run trough the full search space
var estimatedPasswordLength = 0;
while (!isMatched)
/* The estimated length of the password will be increased and every possible key for this
* key length will be created and compared against the password */
Console.WriteLine("Password matched. - {0}", DateTime.Now.ToString());
Console.WriteLine("Time passed: {0}s", DateTime.Now.Subtract(timeStarted).TotalSeconds);
Console.WriteLine("Resolved password: {0}", result);
Console.WriteLine("Computed keys: {0}", computedKeys);
#region Private methods
/// <summary>
/// Starts the recursive method which will create the keys via brute force
/// </summary>
/// <param name="keyLength">The length of the key</param>
private static void startBruteForce(int keyLength)
var keyChars = createCharArray(keyLength, charactersToTest[0]);
// The index of the last character will be stored for slight perfomance improvement
var indexOfLastChar = keyLength - 1;
createNewKey(0, keyChars, keyLength, indexOfLastChar);
/// <summary>
/// Creates a new char array of a specific length filled with the defaultChar
/// </summary>
/// <param name="length">The length of the array</param>
/// <param name="defaultChar">The char with whom the array will be filled</param>
/// <returns></returns>
private static char[] createCharArray(int length, char defaultChar)
return (from c in new char[length] select defaultChar).ToArray();
/// <summary>
/// This is the main workhorse, it creates new keys and compares them to the password until the password
/// is matched or all keys of the current key length have been checked
/// </summary>
/// <param name="currentCharPosition">The position of the char which is replaced by new characters currently</param>
/// <param name="keyChars">The current key represented as char array</param>
/// <param name="keyLength">The length of the key</param>
/// <param name="indexOfLastChar">The index of the last character of the key</param>
private static void createNewKey(int currentCharPosition, char[] keyChars, int keyLength, int indexOfLastChar)
var nextCharPosition = currentCharPosition + 1;
// We are looping trough the full length of our charactersToTest array
for (int i = 0; i < charactersToTestLength; i++)
/* The character at the currentCharPosition will be replaced by a
* new character from the charactersToTest array => a new key combination will be created */
keyChars[currentCharPosition] = charactersToTest[i];
// The method calls itself recursively until all positions of the key char array have been replaced
if (currentCharPosition < indexOfLastChar)
createNewKey(nextCharPosition, keyChars, keyLength, indexOfLastChar);
// A new key has been created, remove this counter to improve performance
/* The char array will be converted to a string and compared to the password. If the password
* is matched the loop breaks and the password is stored as result. */
if ((new String(keyChars)) == password)
if (!isMatched)
isMatched = true;
result = new String(keyChars);
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mkaulfers commented Aug 29, 2019

Thanks for sharing this. I'm playing around with some of the logic behind this but I still can't wrap my head around it.

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chri-ops commented Aug 19, 2020

So elegant. I just wish I'd come up with it myself. Slowly unlocking my understanding of the logic. I just think it's brilliant. 👍

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chri-ops commented Aug 21, 2020

@jwoschitz I used this code and added a Write() that outputs the current key being tried,
and I found out that the search algoritm doesn't stop when it found a key; it rather stops when it went through all the
combinations (until zero '0') for the length of the correct key.

In other words: let's say your password is 'aaaa'.
The search algoritm actually finds aaaa (correctly)
but doesn't stop until it went all the way to '0000'.

I corrected this by copying the if-section at line 116: if (!isMatched).....
and inserting that control even at line 104, right after createNewKey() at line 103.
Now the search algoritm stops right when it found the correct key,
instead of searching to the end of all combinations for that length.

:) otherwise this solution is very elegant, that's why I wanted to use it.
Thanx and welcome.

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ALEEF02 commented Jan 7, 2021

Can you multithread this?

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