encrypt string problem

[Techno]

Hi there

i seem to be having an "Invalid length of data" problem when i am trying to encrypt a string

I am using tripleDES

Public Function EncryptString(ByVal InputString As String) As String

        Dim data As Byte()
        Dim dest As Byte()
        Dim transform As ICryptoTransform

        data = Encoding.Default.GetBytes(InputString)

        transform = des.CreateDecryptor
        dest = transform.TransformFinalBlock(data, 0, data.Length) '<< HERE!!!!

        Return Convert.ToBase64String(dest, 0, dest.Length)    
End Function

any ideas?