Quick start

This document covers how to compute on encrypted data homomorphically using the Concrete framework. We will walk you through a complete example step-by-step.

The basic workflow of computation is as follows:

1. Define the function you want to compute

2. Compile the function into a Concrete Circuit

3. Use the Circuit to perform homomorphic evaluation

Here is the complete example, which we will explain step by step in the following paragraphs.

from concrete import fhe

def add(x, y):
    return x + y

compiler = fhe.Compiler(add, {"x": "encrypted", "y": "encrypted"})

inputset = [(2, 3), (0, 0), (1, 6), (7, 7), (7, 1), (3, 2), (6, 1), (1, 7), (4, 5), (5, 4)]

print(f"Compilation...")
circuit = compiler.compile(inputset)

print(f"Key generation...")
circuit.keygen()

print(f"Homomorphic evaluation...")
encrypted_x, encrypted_y = circuit.encrypt(2, 6)
encrypted_result = circuit.run(encrypted_x, encrypted_y)
result = circuit.decrypt(encrypted_result)

assert result == add(2, 6)

Decorator

Another simple way to compile a function is to use a decorator.

from concrete import fhe

@fhe.compiler({"x": "encrypted"})
def f(x):
    return x + 42

inputset = range(10)
circuit = f.compile(inputset)

assert circuit.encrypt_run_decrypt(10) == f(10)

This decorator is a way to add the compile method to the function object without changing its name elsewhere.

Importing the library

Import the fhe module, which includes everything you need to perform homomorphic evaluation:

from concrete import fhe

Defining the function to compile

Here we define a simple addition function:

def add(x, y):
    return x + y

Creating a compiler

To compile the function, you first need to create a Compiler by specifying the function to compile and the encryption status of its inputs:

compiler = fhe.Compiler(add, {"x": "encrypted", "y": "encrypted"})

For instance, to set the input y as clear:

compiler = fhe.Compiler(add, {"x": "encrypted", "y": "clear"})

Defining an inputset

An inputset is a collection representing the typical inputs of the function. It is used to determine the bit widths and shapes of the variables within the function.

The inputset should be an iterable that yields tuples of the same length as the number of arguments of the compiled function.

For example:

inputset = [(2, 3), (0, 0), (1, 6), (7, 7), (7, 1), (3, 2), (6, 1), (1, 7), (4, 5), (5, 4)]

Here, our inputset consists of 10 integer pairs, ranging from a minimum of (0, 0) to a maximum of (7, 7).

Choosing a representative inputset is critical to allow the compiler to find accurate bounds of all the intermediate values (see more details here). Evaluating the circuit with input values under or over the bounds may result in undefined behavior.

You can use the fhe.inputset(...) function to easily create random inputsets, see more details in this documentation.

Compiling the function

Use the compile method of the Compiler class with an inputset to perform the compilation and get the resulting circuit:

print(f"Compilation...")
circuit = compiler.compile(inputset)

Generating the keys

Use the keygen method of the Circuit class to generate the keys (public and private):

print(f"Key generation...")
circuit.keygen()

If you don't call the key generation explicitly, keys will be generated lazily when needed.

Performing homomorphic evaluation

Now you can easily perform the homomorphic evaluation using the encrypt, run and decrypt methods of the Circuit:

print(f"Homomorphic evaluation...")
encrypted_x, encrypted_y = circuit.encrypt(2, 6)
encrypted_result = circuit.run(encrypted_x, encrypted_y)
result = circuit.decrypt(encrypted_result)

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