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3DES

The DES algorithm has been around for a long time, and the 56-bit version is now easily crackable (in less than a day on fairly modest equipment). An enhancement, and one which is still fairly compatible with DES, is the 3-DES algorithm. It has three phases, and splits the key into two. Overall the key size is typically 112 bits (with a combination of the three keys - of which two of the keys are the same). The algorithm is EncryptK3( DecryptK2( EncryptK1(message), where K1 and K3 are typically the same (to keep compatibility). 

In this case, if we try "test" as the key we get:

"Specified key is a known weak key for 'TripleDES' and cannot be used." and it will not encrypt.

As a test: "Hello how are you?" with a stronger key of "m65hhhgfd"  which should give:

B8841E83139EDF87AE0D033162CFE174A968ED820FA86C01

The code is:

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Collections;
using System.Security.Cryptography;
using System.IO;
using System.Text;

public partial class _Default5 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button3_Click(object sender, EventArgs e)
    {
        try
        {
            TripleDES threedes = new TripleDESCryptoServiceProvider();

            threedes.Key = StringToByte(this.tbKey.Text, 24); // convert to 24 characters - 192 bits
            threedes.IV = StringToByte("12345678");
            byte[] key = threedes.Key;
            byte[] IV = threedes.IV;

            ICryptoTransform encryptor = threedes.CreateEncryptor(key, IV);

            MemoryStream msEncrypt = new MemoryStream();
            CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write);

            // Write all data to the crypto stream and flush it.
            csEncrypt.Write(StringToByte(this.tbMessage.Text), 0, StringToByte(this.tbMessage.Text).Length);
            csEncrypt.FlushFinalBlock();

            // Get the encrypted array of bytes.
            byte[] encrypted = msEncrypt.ToArray();

            this.tbEncrypt.Text = ByteToString(encrypted);

            ICryptoTransform decryptor = threedes.CreateDecryptor(key, IV);

            // Now decrypt the previously encrypted message using the decryptor
            MemoryStream msDecrypt = new MemoryStream(encrypted);
            CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read);

            this.tbDecrypt.Text = ByteToString(csDecrypt);
        }
        catch (Exception ex)
        {
            this.tbEncrypt.Text = ex.Message.ToString();
        }
    }
    public static byte[] StringToByte(string StringToConvert)
    {

        char[] CharArray = StringToConvert.ToCharArray();
        byte[] ByteArray = new byte[CharArray.Length];
        for (int i = 0; i < CharArray.Length; i++)
        {
            ByteArray[i] = Convert.ToByte(CharArray[i]);
        }
        return ByteArray;
    }
    public static byte[] StringToByte(string StringToConvert, int length)
    {

        char[] CharArray = StringToConvert.ToCharArray();
        byte[] ByteArray = new byte[length];
        for (int i = 0; i < CharArray.Length; i++)
        {
            ByteArray[i] = Convert.ToByte(CharArray[i]);
        }
        return ByteArray;
    }
    public static string ByteToString(CryptoStream buff)
    {
        string sbinary = "";
        int b = 0;
        do
        {
            b = buff.ReadByte();
            if (b != -1) sbinary += ((char)b);

        } while (b != -1);
        return (sbinary);
    }
    public static string ByteToString(byte[] buff)
    {
        string sbinary = "";
        for (int i = 0; i < buff.Length; i++)
        {
            sbinary += buff[i].ToString("X2"); // hex format
        }
        return (sbinary);
    }
}