Reversible shuffle algorithm using a key

Question

How would I code a reversible shuffle algorithm in C# which uses a key to shuffle and can be reversed to the original state?

For instance, I have a string: "Hello world", how can I shuffle it so that later I could be able to reverse the shuffled string back to "Hello world".

Answer 1

Here's a simple implementation of what you need (if I got it well):

public static class ShuffleExtensions
{
    public static int[] GetShuffleExchanges(int size, int key)
    {
        int[] exchanges = new int[size - 1];
        var rand = new Random(key);
        for (int i = size - 1; i > 0; i--)
        {
            int n = rand.Next(i + 1);
            exchanges[size - 1 - i] = n;
        }
        return exchanges;
    }

    public static string Shuffle(this string toShuffle, int key)
    {
        int size = toShuffle.Length;
        char[] chars = toShuffle.ToArray();
        var exchanges = GetShuffleExchanges(size, key);
        for (int i = size - 1; i > 0; i--)
        {
            int n = exchanges[size - 1 - i];
            char tmp = chars[i];
            chars[i] = chars[n];
            chars[n] = tmp;
        }
        return new string(chars);
    }

    public static string DeShuffle(this string shuffled, int key)
    {
        int size = shuffled.Length;
        char[] chars = shuffled.ToArray();
        var exchanges = GetShuffleExchanges(size, key);
        for (int i = 1; i < size; i++)
        {
            int n = exchanges[size - i - 1];
            char tmp = chars[i];
            chars[i] = chars[n];
            chars[n] = tmp;
        }
        return new string(chars);
    }
}

usage:

var originalString = "Hello world";
var shuffled = originalString.Shuffle(123);
var deShuffled = shuffled.DeShuffle(123);

// shuffled = "lelooH rwld";
// deShuffled = "Hello world";

The key must be an integer, if you need to use a string as password, just call GetHashCode() on it:

var shuffled = originalString.Shuffle(myStringKey.GetHashCode());

EDIT:

Now is exactly a Fisher–Yates shuffle algorithm implementation. Thanks to Jeff for the code