how to easy way of Encrypt and decrypt in asp.net?

using System.Text;
using System.Security.Cryptography;
using System.IO;

public static string EncryptQueryString(string str)
    {
        byte[] bytes = ASCIIEncoding.ASCII.GetBytes("proposal");
        DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
        MemoryStream memoryStream = new MemoryStream();
        CryptoStream cryptoStream = new CryptoStream(memoryStream, cryptoProvider.CreateEncryptor(bytes, bytes), CryptoStreamMode.Write);
        StreamWriter writer = new StreamWriter(cryptoStream);
        writer.Write(str);
        writer.Flush();
        cryptoStream.FlushFinalBlock();
        writer.Flush();
        return HttpUtility.UrlEncode(Convert.ToBase64String(memoryStream.GetBuffer(), 0, (int)memoryStream.Length));
    }


 public static string DecryptQueryString(string cryptedstr)
    {
        cryptedstr = cryptedstr.Replace(" ", "+");
        byte[] bytes = ASCIIEncoding.ASCII.GetBytes("proposal");
        if (String.IsNullOrEmpty(cryptedstr))
        {
            throw new ArgumentNullException("String can not be null.");
        }
        DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
        MemoryStream memoryStream = new MemoryStream(Convert.FromBase64String(cryptedstr));
        CryptoStream cryptoStream = new CryptoStream(memoryStream, cryptoProvider.CreateDecryptor(bytes, bytes), CryptoStreamMode.Read);
        StreamReader reader = new StreamReader(cryptoStream);
        return HttpUtility.UrlDecode(reader.ReadToEnd());
    }