Encryption problem

I am running a visual basic.net code that encrypts and decrypts a value.It is a window's program.the form contains four controls viz two textboxes and two button controls.the textboxes are named txtvalue and txtresult.that when u enter a value in the txtvalue and click a button(that handles the click event for encrypt),it encrypts the value and displays the encrypted value in the txtresult textbox on the form.and when u also copy the encrypted value back to the txtvalue textbox and click on the second button (that handles the decrypt function) it displays the original value(decrypted text) in txtresult textbox.the encrypt function works well but the decrypt function throws an error when it tries to execute this statement
" cs.FlushFinalBlock()" (' that the char lenght is invalid)

the code i am running is attached.Pls run it in visual studio to help me fix the problem. thanks