Length of the data to decrypt is invalid

I am trying to simulate storing and retreiving encrypted data, I am able to encrypt data entered into a text box using Button1_click and output the encrypted string to another text both, but I have a problem when trying to decrypt the string from the text box and output the decrytped string to a further text box. The error I get is "Length of the data to decrypt is invalid"  on the line of code in Button2_Click

 "csDecrypt.Read(decryptedTextBytes, 0, bytestodecrypt.Length);"

 has anyone got any ideas?

using System;

using System.Data;

using System.Configuration;

using System.Web;

using System.Web.Sec ...