Verification of Neural Networks in Julia.

Installation

Prerequisites

NeuralVerifier uses Z3 as the backend SMT solver. To run NeuralVerifier, this module must be installed via python's pip interface:

pip install z3-solver

After the installation of Z3, you will be ready to install this library. Installation can either be made from the REPL or a script.

REPL

] add http://github.com/jaypmorgan/NeuralVerifier.jl.git

Script

using Pkg
Pkg.add("https://github.com/jaypmorgan/NeuralVerifier.jl.git")

Usage

Once installed, you can import the library:

using NeuralVerifier;

The normal process of using framework is as follows:

1. Create an SMT encoding of an existing network
2. Instantiate an SMT solver
3. Add your encoding and specifications to check to the solver
4. Check for satisfiability

Generating an Encoding

Create an SMT encoding of an existing Neural Network:

# build an network with FluxML
neural_network = Chain(
	Dense(30, 20, Flux.relu),
	Dense(20, 1));  # scalar output -- i.e. for regression

# train the network...

# generate the SMT encoding of the _trained_ network
encoding(x) = begin
	detach(x) = Tracker.data(x)
    y = dense(x,
              neural_network[1].W |> detach,
              neural_network[1].b |> detach) |> relu;
    y = dense(y,
              neural_network[2].W |> detach,
              neural_network[2].b |> detach);
end

In this example, we've created a function called encoding that takes a argument -- the input to the neural network. This function creates a very basic two-layer fully-connected network using the existing weights and biases from the real network. Moreover, we may apply the non-linearity functions to each layer (such as 'relu' or 'softmax') in the usual manner.

We have used a function to create the encoding to allow us to re-use the encoding under different constraints in the SMT solver, instead of having to re-write the encoding process multiple times.

However, for clarity, here is another way of accomplishing the same goal with a more function composition centric writing style:

W_1 = Tracker.data(neural_network[1].W)
b_1 = Tracker.data(neural_network[1].b)
W_2 = Tracker.data(neural_network[2].W)
b_2 = Tracker.data(neural_network[2].b)

y = relu(dense(x, W_1, b_1))
y = softmax(dense(y, W_2, b_2))

In these examples, we are using FluxML network (using Tracker.data to extract the matrices from the computational graph), it is feasible -- but untested -- to use other arbitrary matrices from other ML libraries.

Verify Against a Constraint

As per the installation requirements, this framework comes with Z3 and provides basic access to the python-api. This means that, using our encoding of the neural network, we can create and check for satisfiability of specifications directly in Julia.

To begin, we must first create our solver:

solver = Solver()

And add some variables that represent each dimension of the input

input = [Z3Float("x$i") for i = 1:INPUT_SIZE]

Notice how each input dimension was given a different name (i.e. x1, x2, and so on). If we didn't give the variable a unique name, it would have been overwritten in the Z3 solver.

We can then apply some additional constraint, such as the input must be within a particular range:

# all dimensions must be within the range (-10, 10)
for i = 1:INPUT_SIZE
    add!(solver, input[i] < 10 ∧ input[i] > -10)
    # or add!(solver, And(input[i] < 10, input[i] > -10)
end

And add out search condition (just another constraint).

# Is there some input within the range(-10, 10) that results in an output > 100
add!(solver, encoding(input) > 100)

Finally, we can start the verification process and check whether the constraints are satisfiable.

check(solver)  # probably will take a while for very large networks
print(m.is_sat)

In summary our example script is as follows:

# build an network with FluxML
INPUT_SIZE  = 30
HIDDEN_SIZE = 50

neural_network = Chain(
	Dense(INPUT_SIZE, HIDDEN_SIZE, Flux.relu),
	Dense(HIDDEN_SIZE, 1));  # scalar output -- i.e. for regression

# train the network...

# generate the SMT encoding of the _trained_ network
encoding(x) = begin
	detach(x) = Tracker.data(x)
    y = dense(x,
              neural_network[1].W |> detach,
              neural_network[1].b |> detach) |> relu;
    y = dense(y,
              neural_network[2].W |> detach,
              neural_network[2].b |> detach);
end

# instantiate our solver and add input variables
solver = Solver()
input  = [Z3Float("x$i") for i = 1:INPUT_SIZE]
for i = 1:n_dims
    add!(solver, input[i] < 10 ∧ input[i] > -10)
end

# add our final constraint to check for
add!(solver, encoding(input) > 100)

# check for satisfiability and print output
check(solver)
print(m.is_sat)

Functionality & Support

We have thus far included support for the following layers:

• Dense/Fully-connected (dense)
• 1D/2D convolutions (conv1D/conv2D respectively)
• Pooling operations (maxpool/avgpool)
• Matrix flattening (flatten)

And the following non-linearites:

• Sigmoid (sigmoid -- hidden non-linearities currently uses limited precision that is not yet production quality)
• ReLU (relu)
• Softmax (softmax)

While this covers a suprising number of basic configurations, contributions of more implementations are always welcome!