encyption not defined

Type 'ExtractAndSerialize.Encryption64' is not defined.      

Public Function encryptQueryString(ByVal strQueryString As String) As String
            Dim oES As New ExtractAndSerialize.Encryption64()
            Return oES.Encrypt(strQueryString, "!#$a54?3")
        End Function

        Public Function decryptQueryString(ByVal strQueryString As String) As String
            Dim oES As New ExtractAndSerialize.Encryption64()
            Return oES.Decrypt(strQueryString, "&%#@?,:*")
        End Function