GitHub Project

## Introduction

In 2016, researchers from Google Brain published a paper showing how neural networks can learn symmetric encryption to protect information from AI attackers. In this article, we use Keras to implement the neural networks described in Learning to Protect Communications with Adversarial Neural Cryptography.

We also provide a second implementation, and some proof-of-concept code that can allow a user to use the encryption scheme learned by neural networks through adversarial training.

## Basic setup (symmetric)

Suppose Alice and Bob want to communicate securely over a public channel, and Eve is an attacker eavesdropping on the communication. For symmetric encryption, we may assume Alice and Bob both have a common secret key. Eve is able to capture all the encrypted messages Alice sends to Bob, but does not have access to the secret key. This scenario is depicted in Figure 1.

Figure 1: Symmetric encryption setup

## Implementation

To implement this scenario, we build 3 neural networks:

• Alice is the encryption algorithm.
• Bob is the decryption algorithm
• Eve is the attacker

We will need the following imports for our Keras implementation:

We also set up the model parameters. As per the research paper, we will use 16-bit messages and secret keys, and 16-dim ciphertexts.

### Alice/Bob net

The Alice network will need to have two input vectors: the message to be encrypted, and the secret key. These get concatenated along axis=1 and fed to a dense layer. The signal then passes through a sequence of four Conv1D layers to create the output.

Note that the output of the Dense layer will be 2-dimensional, with the mini-batch instances along the first axis. This needs to be reshaped to (batch_size, m_bits + k_bits, 1) before being fed to a Conv1D layer, introducing a third dimension which would normally correspond to the number of channels (in this case there is only 1).

The architecture for the Bob network is identical to Alice’s, except input0 now represents the ciphertext instead of the plaintext.

### Eve net

As per the specifications in the research paper, the Eve network will have TWO dense layers acting on the inputs. The idea is to give it a better chance at decrypting the message, since it doesn’t have access to the secret key - it only sees the ciphertext.

### Loss and optimizer

In order to train the networks, we first need link them according to Figure 1.

The loss for Eve is just the L1 distance between ainput0 and eoutput:

Instead of doing an average, sum is taken over all the bits in the message - this is the per-example loss talked about in the original paper, and its value represents the avg number of bits Eve guesses incorrectly. We then take the average across the entire mini-batch with K.mean(). The minimum value of the loss is 0 (Eve guesses all the bits correctly), while the maximum is 16 (Eve is wrong about all the bits, in which case we flip the prediction and recover the message).

The loss for Alice-Bob is slightly more complicated. First, we want Bob to successfully decrypt the ciphertext:

Would also like Alice to learn an encryption scheme which Eve can’t break. In an ideal situation, Eve should do no better than random guessing, in which case she would correctly guess half the bits, or m_bits/2 correctly (corresponding to a loss value of 8).

The square term ensures the error grows large when Eve starts to guess a few of the bits correctly. This forces Alice to adapt more rapidly to attackers, and change its encryption scheme. Overall the Alice-Bob loss will be:

As per the optimizer, we use RMSprop with a default learning rate of 0.001:

Finally, we create two training models with the custom loss functions defined earlier:

## Training

The research paper suggests to train the Alice-Bob stack first, followed by 2 cycles of training the Eve network. We build a custom training script to achieve this (note that the Jupyter notebook on GitHub contains more code for nice output during training).

## Results

During training, we kept track of not only the abeloss and eveloss values, but we also evaluated Bob’s ability to decrypt messages sent by Alice. Figure 2 shows the loss values as training progressed.

Figure 2: Loss values during training

Initially, all three loss values start at 8, which means neither Eve nor Bob do any better than random guessing. Eve’s loss decreases faster up to a point, after which Alice is able to learn an encryption scheme which Bob can understand, but which fools Eve. Notice that Eve’s loss doesn’t climb back up to 8, so Eve still does slightly better than random guessing.

We evaluated Bob and Eve on 10,000 random messages (encrypted with randomly selected keys), and found the following:

Bob is able to successfully decrypt a little under 90% of the messages he receives, while Eve cannot recover the full message in its current state. However, if we were to freeze the Alice-Bob model, and continue to train Eve, her performance would improve.

It’s important to realize that Eve still performs better than random guessing. For each bit, a purely random guess would have $$P(\text{1 bit correct}) = \frac{1}{2}$$

chance of guessing that bit correctly. Under the assumption of independent guesses for each ciphertext bit, the probability of guessing the entire message correctly would be $$P(\text{all bits correct}) = \pr{\frac{1}{2}}^{16} = 0.0015 \%$$ which is significantly worse than Eve’s actual performance.

It is interesting to analyze the performance of Eve when keeping Alice-Bob frozen. Figure 3 shows two such scenarios:

Figure 3: Eve’s performance for frozen A-B when starting untrained (left) or trained (right)

When starting with freshly initialized networks, and keeping Alice-Bob frozen, Eve is still able to gain a non-negligible advantage. She ends up with 3 incorrect bit guesses on average. When evaluated on a sample of 10,000 ciphertexts, we get

However, when keeping the trained version of Alice-Bob constant, we see that Eve’s advantage doesn’t begin to improve. The loss stays roughly constant, which suggests the encryption algorithm learned by Alice may not need to be continually improved.

## AI-based encryption (PoC)

We built and trained similar neural configuration on 8-bit messages/keys and 8-dimensional ciphertexts. Other than the bit size, we also added a dense layer after the sequence of 1D convolutions, and allowed the codings to be between $-1$ and $1$ by using the $tanh()$ activation function at output. Figure 4 shows the training loss of our setup.

Figure 4: Training progress for 2000 iterations (left) and overall (right)

The resulting performance of Bob and Eve when evaluated on a sample of 10,000 ciphertexts was:

The high level of accuracy by the decryption algorithm makes this a good model to build a toy-model cryptosystem that can operate on human-readable characters. To this end, one could use 8-bit ASCII character encodings, and convert them to binary arrays that can be fed into the Alice model.

Instead we limited ourselves to lowercase letters and a few punctuation marks by using a 5-bit binary encoding. We pad this with 3 random bits, resulting in an 8-bit binary encoding that can be fed to the Alice model (this also makes our encryption probabilistic).

The fact that our encodings are 32-bit floating point numbers means there’s no nice way to convert the ciphertext to something human-readable. To get around this, we convert each coordinate of the 8-dim encoding into a 32-bit binary string, and concatenate them together to get a binary encoding. This means that each human-readable character goes from an 8-bit padded binary representation to a $32 \cdot 8 = 256$-bit ciphertext representation. For example, the plaintext “adi” has (one of many) binary representation

but its ciphertext encoding is much, much worse. Mapped to human-readable output via the 5-bit map, it looks like

Bob is able to successfully decode the binary ciphertext, while Eve’s code-breaking attempt gave:

While somewhat wasteful memory-wise, this is just a PoC. There are various other options to create a robust cryptosystem using the trained Alice and Bob models. We leave it to the reader to use our code as a starting point, and modify it as they see fit to design their own AI-based encryption scheme.