Here is a class that handles symmetric encryption and decryption using the .NET Rijndael provider. This works well when passing data that you need to keep encrypted over a URL. Remember to UrlEncode the resulting Base64 string with Server.UrlEncode() if you plan on passing your encrypted string as part of the Query String. I tried to boil it down to its simplest form.

You can download a sample solution:
http://download.binaryocean.com/SymmetricEncryptionSolution.zip

using System;

using System.IO;

using System.Security.Cryptography;

using System.Text;

/// <summary>

/// Simple symmetric encryption using the .NET Rijndael provider

/// </summary>

public class SymmetricEncryption

{

    public SymmetricEncryption()

    {

    }

    public SymmetricEncryption(string password)

    {

        GenerateKey(password);

    }

    public string Password

    {

        set { GenerateKey(value); }

    }

    private byte[] Key;

    private byte[] Vector;

    private void GenerateKey(string password)

    {

        SHA384Managed sha = new SHA384Managed();

        byte[] b = sha.ComputeHash(new ASCIIEncoding().GetBytes(password));

        Key = new byte[32];

        Vector = new byte[16];

        Array.Copy(b, 0, Key, 0, 32);

        Array.Copy(b, 32, Vector, 0, 16);

    }

    public string Encrypt(string plainText, string password)

    {

        GenerateKey(password);

        return Encrypt(plainText);

    }

    public string Encrypt(string plainText)

    {

        if (Key == null)

        {

            throw new InvalidOperationException("Password must be provided or set.");

        }

        byte[] data = new ASCIIEncoding().GetBytes(plainText);

        RijndaelManaged crypto = new RijndaelManaged();

        ICryptoTransform encryptor = crypto.CreateEncryptor(Key, Vector);

        MemoryStream memoryStream = new MemoryStream();

        CryptoStream crptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write);

        crptoStream.Write(data, 0, data.Length);

        crptoStream.FlushFinalBlock();

        crptoStream.Close();

        memoryStream.Close();

        return Convert.ToBase64String(memoryStream.ToArray());

    }

    public string Decrypt(string encryptedText, string password)

    {

        GenerateKey(password);

        return Decrypt(encryptedText);

    }

    public string Decrypt(string encryptedText)

    {

        if (Key == null)

        {

            throw new InvalidOperationException("Password must be provided or set.");

        }

        byte[] cipher = Convert.FromBase64String(encryptedText);

        RijndaelManaged crypto = new RijndaelManaged();

        ICryptoTransform encryptor = crypto.CreateDecryptor(Key, Vector);

        MemoryStream memoryStream = new MemoryStream(cipher);

        CryptoStream crptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Read);

        byte[] data = new byte[cipher.Length];

        int dataLength = crptoStream.Read(data, 0, data.Length);

        memoryStream.Close();

        crptoStream.Close();

        return (new ASCIIEncoding()).GetString(data, 0, dataLength);

    }

}