How can I "randomize" an array of Colors in VB.NET (or C#) with a password?

Question

I've got an array containing lots of "Color" and I want to change the order of this colors (random) but with a password: so another user can get the "original" (and correct) sequence of the array only with the right password. How can I do this in Visual Basic .NET or C# ? Do have I to use a particular encryption engine?

Answer 1

Here is a simple method to scramble an array of colors using a password.

The key is to transform your password from a string to a number. Then, you can use that number as a seed for a random number generator. After that, you can use that random number generator to get a permutation of the same length as the color array. You can use that permutation to change the order of the colors and scramble the array.

When decrypting, if you are given the same password, you can generate the same permutation and unscramble the array to its original form.

Here is an example of that principle, written in C#:

int[] GetPermutation(int size, int seed)
{
    Random random = new Random(seed);
    int[] array = new int[size];
    for (int i = 0; i < size; i++)
        array[i] = i;
    for (int i = array.Length; i > 1; i--)
    {
        int j = random.Next(i);
        int tmp = array[j];
        array[j] = array[i - 1];
        array[i - 1] = tmp;
    }
    return array;
}

Color[] Encrypt(Color[] input, string password)
{
    int seed = password.GetHashCode();
    int[] perm = GetPermutation(input.Length, seed);
    Color[] encrypted = new Color[input.Length];
    for (int i = 0; i < input.Length; i++)
    {
        encrypted[perm[i]] = input[i];
    }
    return encrypted;
}

Color[] Decrypt(Color[] input, string password)
{
    int seed = password.GetHashCode();
    int[] perm = GetPermutation(input.Length, seed);
    Color[] decrypted = new Color[input.Length];
    for (int i = 0; i < input.Length; i++)
    {
        decrypted[i] = input[perm[i]];
    }
    return decrypted;
}

The GetPermutation function generates a permutation based on a seed passed as a parameter. The Encrypt and Decrypt functions actually scramble and unscramble the arrays.

Here is a usage example:

Color[] array = new Color[5] { Color.Red, Color.Green, Color.Blue, Color.Yellow, Color.Black };
string password = "secret";
Color[] enc = Encrypt(array, password); // will always return Blue, Green, Black, Yellow, Red for the "secret" password
Color[] dec = Decrypt(enc, password); // will return the original array: Red, Green, Blue, Yellow, Black if given the "secret" password
Color[] dec2 = Decrypt(enc, "incorrectpwd"); // will return Green, Blue, Yellow, Black, Red, which is incorrect because the password was incorrect

Notes:

• I used the .GetHashCode() method for simplicity. This method might generate different numbers for the same string in future versions of the .NET framework

• If the length of the array is small, you can get the same encryption order with different passwords, and decryption will be successful even if the password is incorrect. For example:

  Color[] array = new Color[2] {Color.Red, Color.Green};
  Color[] enc = Encrypt(array, "one"); // will return Green, Red
  Color[] dec = Decrypt(enc, "one"); // will return Red, Green
  Color[] dec2 = Decrypt(enc, "two"); // will also return Red, Green, even though the password was incorrect
  

• If you want to use colors as an alternative to passwords, you must note that you lose some security because the space of all the passwords is not as high as a password made up of characters, numbers and signs. Brute-forcing such a password will be much easier.

Answer 2

I'm guessing this may work:

1. Have user enter password
2. Hash the password (Some sample hashes in C++ here) and store the number result
3. Use a shuffle algorithm with the hash as the seed. See here for example, just initialize the Random object with the hash as the seed.

Again, hashing is not always perfect (as it can collide), so you may need to find a hashing algorithm that is suitable for your problem.

EDIT:

Here's some code that may help you:

String input = "User inputted string here";
int hashSeed = input.GetHashCode();
Random rnd = new Random(hashSeed);
Color[] MyRandomColorArray = MyColorArray.OrderBy(x => rnd.Next()).ToArray();

Note: GetHashCode() returns differnet values whether it's on a 32-bit vs. 64-bit system.