Production deployment

This document explains the deployment workflow and the model serving pattern for deploying Fully Homomorphic Encryption machine learning models in a client/server setting using Concrete ML.

Deployment

The steps to prepare a model for encrypted inference in a client/server setting is illustrated as follows:

Model training and compilation

The training of the model and its compilation to FHE are performed on a development machine.

Three different files are created when saving the model:

• client.zip contains the following files:

  • client.specs.json lists the secure cryptographic parameters needed for the client to generate private and evaluation keys.

  • serialized_processing.json describes the pre-processing and post-processing required by the machine learning model, such as quantization parameters to quantize the input and de-quantize the output.

• server.zip contains the compiled model. This file is sufficient to run the model on a server. The compiled model is machine-architecture specific, for example, a model compiled on x86 cannot run on ARM.

Model deployment

The compiled model (server.zip) is deployed to a server. The cryptographic parameters (client.zip) are shared with the clients. In some settings, such as a phone application, the client.zip can be directly deployed on the client device and the server does not need to host it.

Important: In a client-server production using FHE, the server's output format depends on the model type:

• For regressors, the output matches the predict() method from scikit-learn, providing direct predictions.

• For classifiers, the output uses the predict_proba() method format, offering probability scores for each class, which allows clients to determine class membership by applying a threshold (commonly 0.5).

Using the API Classes

The FHEModelDev, FHEModelClient, and FHEModelServer classes in the concrete.ml.deployment module simplifies the deployment and interaction between the client and server:

• FHEModelDev:

  • This class handles the serialization of the underlying FHE circuit as well as the crypto-parameters used for generating the keys.

  • Use the save method of this class during the development phase to prepare and save the model artifacts (client.zip and server.zip). With save method, you can deploy a trained model or a training FHE program.
• FHEModelClient is used on the client side for the following actions:

  • Generate and serialize the cryptographic keys.

  • Encrypt the data before sending it to the server.

  • Decrypt the results received from the server.

  • Load quantization parameters and pre/post-processing from serialized_processing.json.

• FHEModelServer is used on the server side for the following actions:

  • Load the FHE circuit from server.zip .

  • Execute the model on encrypted data received from the client.

Example Usage

from concrete.ml.sklearn import DecisionTreeClassifier
from concrete.ml.deployment import FHEModelDev, FHEModelClient, FHEModelServer
import numpy as np

# Define the directory for FHE client/server files
fhe_directory = '/tmp/fhe_client_server_files/'

# Initialize the Decision Tree model
model = DecisionTreeClassifier(n_bits=8)

# Generate some random data for training
X = np.random.rand(100, 20)
y = np.random.randint(0, 2, size=100)

# Train and compile the model
model.fit(X, y)
model.compile(X)

# Setup the development environment
dev = FHEModelDev(path_dir=fhe_directory, model=model)
dev.save()

# Setup the client
client = FHEModelClient(path_dir=fhe_directory, key_dir="/tmp/keys_client")
serialized_evaluation_keys = client.get_serialized_evaluation_keys()

# Client pre-processes new data
X_new = np.random.rand(1, 20)
encrypted_data = client.quantize_encrypt_serialize(X_new)

# Setup the server
server = FHEModelServer(path_dir=fhe_directory)
server.load()

# Server processes the encrypted data
encrypted_result = server.run(encrypted_data, serialized_evaluation_keys)

# Client decrypts the result
result = client.deserialize_decrypt_dequantize(encrypted_result)

Data transfer overview:

• From Client to Server: serialized_evaluation_keys (once), encrypted_data.

• From Server to Client: encrypted_result.

These objects are serialized into bytes to streamline the data transfer between the client and server.

Ciphertext formats and keys

Two types of ciphertext formats are available in Concrete ML and both are available for deployment. To use the TFHE-rs radix format, pass the ciphertext_format option to the compilation call as follows:

from concrete.ml.common.utils import CiphertextFormat
model.compile(X, ciphertext_format=CiphertextFormat.TFHE_RS)

fhe_directory = '/tmp/fhe_client_server_files_tfhers/'

# Setup the development environment
dev = FHEModelDev(path_dir=fhe_directory, model=model)
dev.save()

# Setup the client
client = FHEModelClient(path_dir=fhe_directory, key_dir="/tmp/keys_client_tfhers")
serialized_evaluation_keys, tfhers_evaluation_keys = client.get_serialized_evaluation_keys(include_tfhers_key=True)

# Client pre-processes new data
X_new = np.random.rand(1, 20)
encrypted_data = client.quantize_encrypt_serialize(X_new)

# Setup the server
server = FHEModelServer(path_dir=fhe_directory)
server.load()

# Server processes the encrypted data
encrypted_result = server.run(encrypted_data, serialized_evaluation_keys)

# Client decrypts the result
result = client.deserialize_decrypt_dequantize(encrypted_result[0])

In the example above, a second evaluation key is obtained in the tfhers_evaluation_keys variable. This key can be loaded by TFHE-rs Rust programs to perform further computation on the model output ciphertexts.

Serving

The client-side deployment of a secured inference machine learning model is illustrated as follows:

The workflow contains the following steps:

1. Key generation: The client obtains the cryptographic parameters stored in client.zip and generates a private encryption/decryption key as well as a set of public evaluation keys.

2. Sending public keys: The public evaluation keys are sent to the server, while the secret key remains on the client.

3. Data encryption: The private data is encrypted by the client as described in the serialized_processing.json file in client.zip.

4. Data transmission: The encrypted data is sent to the server.

5. Encrypted inference: Server-side, the FHE model inference is run on encrypted inputs using the public evaluation keys.

6. Data transmission: The encrypted result is returned by the server to the client.

7. Data decryption: The client decrypts it using its private key.

8. Post-processing: The client performs any necessary post-processing of the decrypted result as specified in serialized_processing.json (part of client.zip).

The server-side implementation of a Concrete ML model is illustrated as follows:

The workflow contains the following steps:

1. Storing the public key: The public evaluation keys sent by clients are stored.

2. Model evaluation: The public evaluation keys are retrieved for the client that is querying the service and used to evaluate the machine learning model stored in server.zip.

3. Sending back the result: The server sends the encrypted result of the computation back to the client.

Example notebook

For a complete example, see the client-server notebook or the use-case examples.