When I Try To Decrypt The Url I Am Getting This Error "Invalid Length For A Base-64 Char Array " Where Do I Need Change The Method In Below Code

Public Function URLDecrypt(ByVal oKey As String, ByVal oValue As String) As String
       '***************************************
       '***Symmetric Encryption
       '***************************************
       Dim str As String
       If System.Configuration.ConfigurationSettings.AppSettings.Get("URLEncrypt.Mode").ToUpper = "ON" Then
           Dim objSym As New FE_SymmetricNamespace.FE_Symmetric
           'oValue = System.Web.HttpUtility.UrlDecode(oValue)
           str = objSym.DecryptData(oKey, System.Text.Encoding.ASCII.GetString(Convert.FromBase64String(Server.UrlDecode(oValue))))
           'str = objSym.DecryptData(oKey, HttpUtility.UrlDecode(oValue, System.Text.Encoding.Default))
       Else
           str = oValue
       End If

       Return str

   End Function

   Public Function URLEncrypt(ByVal oKey As String, ByVal oValue As String) As String
       '***Symmetric Encryption
       '***************************************
       Dim str As String
       If System.Configuration.ConfigurationSettings.AppSettings.Get("URLEncrypt.Mode").ToUpper = "ON" Then
           Dim objSym As New FE_SymmetricNamespace.FE_Symmetric
           'str = objSym.EncryptData(oKey, oValue)
           '''''''''''''''''''''''''''''''''''''''''''''''''''''
           ' convert to base64 before server.urlencode to remove special characters "+" and "=="
           '''''''''''''''''''''''''''''''''''''''''''''''''''''
           str = Server.UrlEncode(Convert.ToByte(System.Text.Encoding.ASCII.GetBytes(objSym.EncryptData(oKey, oValue))))
           'str = HttpUtility.UrlEncode(objSym.EncryptData(oKey, oValue).TrimEnd("=="), System.Text.Encoding.Default)
           'str = System.Web.HttpUtility.UrlEncode(str)
       Else
           str = oValue
       End If

       Return str

   End Function