Grammatical inference using the Z3 SMT solver

Z3GI is a Python tool and library that uses the Z3 SMT solver for learning minimal consistent state machine models from labeled strings or input/output taces. The ideas the tool is based on and the experiments conducted are described in the publication "Model Learning as a Satisfiability Modulo Theories Problem" due to appear at the LATA 2018 conference. A long version of this paper is included here, see extended.pdf.

Introduction

Grammatical inference is the research field that is concerned with learning the set of rules that describe the behavior of a system, such as a network protocol, a piece of software, or a (formal) language. Z3GI provides different ways of solving this (and similar) problem(s) using satisfiability modulo theories (SMT).

Installing with pip (recommended)

To be enabled.

Installing from sources

Alternatively, you can install Z3GI by cloning this repository, and installing using setuptools:

$ git clone https://gitlab.science.ru.nl/rick/z3gi.git
$ python z3gi/setup.py install

Getting started - learning from traces

Consider a deterministic finite automaton (DFA) corresponding to the regex (ab)*.

The training file at resources\traces\regex_example for this DFA reads:

1 a b
1 a b a b
0 a
0 b
0 a b a
0 a a
0 a b b
0 a b a a

Each line represents an observation comprising elements separated by spaces. For a DFA, the observation has the syntax:

label symbol1 symbol2 ... symboln

The symols form a string, the label indicates that the DFA accepts the string (if its value is 1) or that it rejects the string (if its value is 0). In the current version, we require that the first observation introduces all inputs in the alphabet.

We can use Z3GI to learn a model for the observations in regex_example as follows:

$ python z3gi -m traces -a DFA -f resources\traces\regex_example

This produces an output containing a textual description of the learned model:

Learned model:
 {'q0': False, 'q1': True, 'q2': True}
acc_seq(q0) =
acc_seq(q1) = q0 a -> q1
acc_seq(q2) = q0 a -> q1 , q1 b -> q2
q0
        a -> q1
        b -> q0
q1
        a -> q0
        b -> q2
q2
        a -> q1
        b -> q0

The model description is split into 3 (or 2) sections.

• the first section indicates acceptance/rejection of each state
• the second section indicates access sequences to each state, in later versions, this section may be ommitted
• the third section describes transitions of the learned model

Learning scalable systems

Z3GI implements several scalable systems such as Sets or Login systems. What these systems have in common is that they are configurable in size. A greater size results in a larger system. Z3GI can be used to learn these systems.

To give an example, suppose we want to learn a Login system with 2 users which is structured as a Mealy machine. To learn this system we run:

$ python z3gi -m scalable -a MealyMachine -sc Login -s 2

Each scalable system is implemented in many different formalisms. Say we want to learn a Register Automaton variant of the Login system with 1 user. Then we would have to run:

$ python z3gi -m scalable -a RegisterAutomaton -sc Login -s 1

Learning .dot models

We can apply Z3GI to learn simulations of .dot models. In particular, we can learn the Mealy machine models obtained by various case-studies which are included in the resources\models directory.

Say we want to learn the biometric passport. We then need to run:

$ python z3gi -m dot -a MealyMachine -f resources\models\biometric.dot

For bigger models which are more difficult to test, we may require an external test algorithm. We provide a Windows 64 bit binary for the Yannakakis test algorithm found in the resources\binaries. You can activate this algorithm using the -y path_bin option where path_bin is a path to the binary. By using -m dotnorst, Z3GI will attempt to learn without using resets.

Our implementation currently only supports learning .dot Mealy machine models, though functionality for other formalisms will be added in the future. Note there is currently no standard .dot format for Register Automata. Also note that the Yannakakis test algorithm only works on Mealy machines and requires resets.

Learning randomly generated Mealy machines without reset

Z3GI can be used to learn without reset randomly generated Mealy machines with the property that they are strongly connected (though not necessarily minimal).

Say we want to learn a randomly generated Mealy machine with 2 inputs, 2 outputs and 3 states. We then run:

$ python z3gi -m randnorst -a MealyMachine -ni 2 -no 2 -ns 3

Performing benchmarks

For the publication, we ran Z3GI through a series of benchmarks, which can be re-run by executing the .*benchmark.py python scripts in the z3gi directory. These scripts are well documented in the way relevant parameters (such as solver timeout) can be tweaked. Ensure these parameters are set to their desired values before running, and note that experiments may take minutes to a few hours to complete.