Category Archives: model

How to assure interoperability?

Motivation

Typically protocols are connecting two different systems. In an open system with several stakeholders interoperability between these systems is a fundamental requirement. To assure this interoperability there are various way. In this blog post I present you two popular approaches in cooperation with my colleague Dr. Guido Frank who works at the German Federal Office for Information Security (BSI).

Interoperability

Interoperability is a characteristic of a product or system, whose interfaces are completely understood, to work with other products or systems, present or future, in either implementation or access, without any restrictions (Source: Wikipedia).

puzzle - interoperability test

From a system perspective, this means that all implementations need to comply to the same technical specifications. Interoperability is essential because these systems are open systems with different stakeholders. It refers to the collaboration ability of cross-system protocols.

Crossover Testing vs. Conformity Testing

To ensure interoperability, implementations need to be tested. In general, there are different approaches to test systems or implementations.

Crossover Testing

The scope of a crossover test is to test every system component with all other system components. This procedure allows to detect incompatibilities between existing implementations with a fixed release status.

The efforts to perform this kind of test increases disproportionately with every additional instance of the system. Therefore these kinds of tests can practically be performed only with a small number of involved test partners. The following figure illustrates the interaction between different systems in a crossover test.

Crossover Testing

Crossover Testing

Another problem of crossover testing is maintenance: every new implementation or any new version of existing implementations must be tested with every corresponding system, which again increases the testing efforts significantly. A benefit of crossover testing can be found in the early phase of developing when crossover testing helps the developer to implement their own system and can be used as an indicator to use the right way. On the other side, this kind of testing only indicates a positive test case with two correct systems to test (“smoke test”). The behaviour of the systems in bad cases is not tested. Additionally, with two different systems to be tested in a crossover test it’s difficult to decide which system behaves correctly in case of a failure and which implementation has to be changed.

Conformity Testing

The purpose of conformity tests is to verify that a system implements the specifications correctly (i.e. it “conforms” to the specifications).  These specifications need to be defined by stakeholders and finally implemented e.g. by test labs to run their conformity test tools.

This way these test suites verify the implementation under test with protocol data units which mimic both “correct” and “incorrect” behaviour of the system. The figure below illustrates the interactions between the test suite implementation and the system in a conformity test. The test definitions have to be tested with regard to harmonised interpretation among test participants.

Conformity Testing

Conformity Testing

Conclusion

Both approaches of testing allow assuring interoperability to other systems components to a certain extent. But with increasing complexity of the systems to be tested and the increasing number of systems at all the crossover testing is getting more and more extensive. Only conformity testing scales well with the complexity and the number of systems in an adequate way. The following diagram illustrates the increasing efforts of crossover tests in relation to conformity test.

Compared efforts of crossover testing and conformity testing

Compared efforts of crossover testing and conformity testing

Direct tests between a subset of implementations are be useful during implementation or integration phase of a node.  Such tests could also be performed via bilateral appointment between different stakeholders, e.g. via pilots. Experience from such test could also be used as additional input for conformity tests. Crossover testing or a central coordination of such tests would not be necessary for this purpose. As soon as there are several system components to be tested, conformity testing should be chosen.

The benefits of such an approach of interoperability testing can also be seen in several so called ‘InterOp tests‘ that have been performed for more than a decade in context of eMRTD. Detailed failure analysis allows to improve the stability of the whole eMRTD system. Additionally, the results of ‘InterOp tests’ helps not only to improve the stability of ePassports and inspection systems but also to improve the quality of (test) specifications and test tools.

Another open system with several vendors is banking. All cash cards or credit cards must fulfill international test specifications. This way of interoperability testing allows customers to use their cards worldwide at cash machines of various banks.

To assure systematic interoperability, it is necessary to set up conformity test specifications that systematically test the requirements as defined in the technical specifications. The tests should not only define good cases but also bad cases that mirror the pitfalls typically occurring in a system. Only this way allows to assure real system-wide interoperability.

Setup of test specification

Important components of a conformance test specifications are:

  • Description of general test requirements
  • Test setup / Testing environment
  • Definition of suitable test profiles / implementation profiles
  • Implementation Conformance Statement (ICS)
  • Definition of testing or configuration data
  • Definition of test cases according to a unified data structure
  • Each test case should concentrate on a single feature to be tested

The following structure of a test specification has been established since the beginning of eMRTD testing in 2005. It is based on the ISO/OSI layer model where data is tested on layer 7 and protocols are tested on layer 6.

Typical structure of a test case in this context:

  • Test case ID: unique identifier for each test case
  • Purpose: objective of the test case
  • Version: current version of this test case independent from the test specification
  • Reference: where is this feature / behaviour specified
  • Preconditions: setup of test case
  • Test scenario: description of test case, step by step
  • Postconditions: setdown of test case

Test cases can be combined in test suites to cluster test cases of similar topics or objectives. As the test specifications need to be implemented in suitable testing tools, it is useful to define the test cases already in a way, that eases their implementation, e.g. via XML using a suitable XML scheme.

 

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Next generation of ePassport testing

Developing and implementing conformity tests is a time-consuming and fault-prone task. To reduce these efforts a new route must be tackled. The current way of specifying tests and implementing them includes too many manual steps. Based on the experience of testing electronic smart cards in ID documents like ePassports or ID cards, the author describes a new way of saving time to write new test specifications and to get test cases based on these specifications. It is possible, though, to improve the specification and implementation of tests significantly by using new technologies such as model based testing (MBT) and domain specific languages (DSL). I’m describing here my experience in defining a new language for testing smart cards based on DSL and models, and in using this language to generate both documents and test cases that can run in several test tools. The idea of using a DSL to define a test specification goes back to a tutorial of Markus Voelter and Peter Friese, hold during the conference Software Engineering 2010  in Paderborn.

With the introduction of smart cards in ID documents the verification of these electronic parts has become more and more important. The German Federal Office for Information Security (BSI) defines technical guidelines that specify several tests required to fulfill compliance. These guidelines include tests on the electrical and physical layer on the one hand, and tests on the application and data layer on the other hand. In this presentation the author focusses on the tests on the last two layers because these tests can be implemented completely in software.

In TR-03105 the BSI specifies several hundreds of test cases concerning the data format of smart cards and also the commands and protocols used to communicate with the chip.

In the past the typical approach was divided into several separate steps. At first the BSI specified a list of test cases and published them in a document that was written manually by an editor of the technical guideline. Then several test houses and vendors of test tools implemented all the test cases based on the specific guideline into their software solution. All these steps had to be done manually, which means: the software engineer of each institution read the guideline and implemented test case by test case in his special test environment. With every new version of the guideline this procedure had to be repeated again and again. At the beginning, the update cycle of these test specifications was very frequent because all the feedback collected in the field was included in the guideline and new versions were published in short intervals:

figure_1

This way of specifying test specifications is inefficient because of the large number of manual steps. Doing the transformation from the test specification to the implementation is not only inefficient but also fault-prone: every test case in the guideline must be formulated in “prose” by the editor; every engineer must implement the test case in the respective programming language. Also the consistency of the tests must be maintained by the editor manually.

Furthermore, the writing of test specifications is an extensive part of conformity testing. The editor of such a specification in general uses a word processing software that is useful for e.g. writing small letters. But this kind of software is not really convenient for writing technical specifications like TR-03105. A typical problem is versioning of different types. It would be most helpful for developers, if the editor used the track changes mode when he changes test cases. This way the developer can easily detect changes. But this advantage depends on the activated mode. As soon as the editor forgets to activate the track changes mode the implementation of these changes becomes more and more complicated.

Due to an increasing number of new requirements of the applications running on smart cards the complexity of these systems becomes higher and higher. In Walter Fumys “Handbook of eID Security” the history of eID documents from purely visible ones to future versions is illustrated. This complexity in these applications will result in so many test cases that the current approach of writing and implementing test specifications is a blind alley.

With recent results of Model Driven Software Development (MDSD) this blind alley can be avoided. New techniques and tools allow us now to switch from the manual parts to a more automated procedure. The goal is to write only one “text” that can be used as a source for all the test tools. The solution is a model that defines the test cases and a transformation of this model to other platforms or formats.

With this new approach, the process of specifying tests can be reduced to the interesting part where the editor can use his creativity to conceive new tests and not to use his office software to write tests.

Defining a language that describes the test case is the basis for this procedure. This grammar can be used to model test cases, and based on this model all the artifacts needed can be generated. The following figure visualizes this process: there is one Meta test specification that is used to generate not only the human-readable document but also the tool-specific test cases for every test environment.

figure_2

One solution to define a language is Xtext. With Xtext the user gets a complete environment based on Eclipse to develop his own domain specific language (DSL). One of the benefits of Xtext is the editor that is generated automatically by the tool itself. This editor includes code-completion, syntax coloring, code-folding and outline view. This editor is very helpful to write test cases. Every test case that is not compliant with the grammar is marked as faulty. So the editor of the specification can recognize this wrong test case directly like a software developer in Integrated Development Environments (IDE).

Additionally, the user can implement generators to generate code for the scope platform. These generators are called by the Modeling Workflow Engine (MWE). These generators are powerful and productive tools to provide test cases for different platforms.

In the public sector it is more and more important to write barrier-free documents. It takes a lot of time to write a barrier-free document based on a typical technical specification. With a generator that produces a human-readable document the author of the test specifications can use generic templates that produce barrier-free documents in an automatic way because the generator can use rules that fulfill even these standards.

Once the user has generated a new test specification or a new test case based on any test tool, he can modify this document by adding some special features, e.g. a special library to one test case. With a model based test specification it is possible to re-import this modified artifact into the model to assure persistency. The author presented and published his first experience at ICST 2011.

This approach helps to write test specifications in a technical way on a Meta level but it does not focus on the content of the test specification. Thus, the approach helps to write the document but it does not help to produce any content needed. Currently, the quality of a test specification is dependent from the background of the author. With his knowledge of protocols and corresponding pitfalls he can specify interesting test cases. But many test cases contain the same scenarios (wrong length, set a value to zero, use maximum or minimum value and so on). It would be more reasonable and economical if the author could focus on special test cases for the relevant protocols and their pitfalls and “standard” test cases would be generated automatically. On the other hand, test specifications written by humans always run the risk of being inconsistent, error-prone and imprecise. Additionally, it is always rather time-consuming to write test specifications manually.

To focus and solve problems as described above a consortium of BSI, HJP Consulting, s-lab Software Quality Lab (University of Paderborn) and TÜViT started a research project, namely MOTEMRI (Modellbasiertes Testen mit Referenzimplementierung). In MOTEMRI a model is developed that contains all relevant information of the popular protocol PACE. This model is specified in UML so everybody who is interested can read and modify the diagrams easily. In this way it is possible to adapt new protocols into the model like PACE or new versions developed in the future. Based on the model, algorithms generate test cases automatically. Thereby the knowledge of designing test cases is enacted into software, independent from the knowledge of the author of the test specification. By the way, this procedure also allows using various “wrong” values for negative test cases.  Negative test cases are generated automatically and access different “wrong” values. Using random values allows better testing and ensures better chip implementations.

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