Serialization/deserialization

This document explains the serialization and deserialization features that are useful to send data to a server to perform the computations.

Serialization/deserialization

TFHE-rs uses the Serde framework and implements Serde's Serialize and Deserialize traits.

To serialize the data, you need to choose a data format. In the following example, we use bincode for its binary format.

Here is a full example:

# Cargo.toml

[dependencies]
# ...
tfhe = { version = "0.7.5", features = ["integer","x86_64-unix"]}
bincode = "1.3.3"

// main.rs

use bincode;
use std::io::Cursor;
use tfhe::{ConfigBuilder, ServerKey, generate_keys, set_server_key, FheUint8};
use tfhe::prelude::*;

fn main() -> Result<(), Box<dyn std::error::Error>>{
    let config = ConfigBuilder::default().build();

    let ( client_key, server_key) = generate_keys(config);

    let msg1 = 1;
    let msg2 = 0;

    let value_1 = FheUint8::encrypt(msg1, &client_key);
    let value_2 = FheUint8::encrypt(msg2, &client_key);

    // Prepare to send data to the server
    // The ClientKey is _not_ sent
    let mut serialized_data = Vec::new();
    bincode::serialize_into(&mut serialized_data, &server_key)?;
    bincode::serialize_into(&mut serialized_data, &value_1)?;
    bincode::serialize_into(&mut serialized_data, &value_2)?;

    // Simulate sending serialized data to a server and getting
    // back the serialized result
    let serialized_result = server_function(&serialized_data)?;
    let result: FheUint8 = bincode::deserialize(&serialized_result)?;

    let output: u8 = result.decrypt(&client_key);
    assert_eq!(output, msg1 + msg2);
    Ok(())
}


fn server_function(serialized_data: &[u8]) -> Result<Vec<u8>, Box<dyn std::error::Error>> {
    let mut serialized_data = Cursor::new(serialized_data);
    let server_key: ServerKey = bincode::deserialize_from(&mut serialized_data)?;
    let ct_1: FheUint8 = bincode::deserialize_from(&mut serialized_data)?;
    let ct_2: FheUint8 = bincode::deserialize_from(&mut serialized_data)?;

    set_server_key(server_key);

    let result = ct_1 + ct_2;

    let serialized_result = bincode::serialize(&result)?;

    Ok(serialized_result)
}

Safe serialization/deserialization

When dealing with sensitive types, it's important to implement safe serialization and safe deserialization functions to prevent runtime errors and enhance security. The safe serialization and deserialization use bincode internally.

The safe deserialization must take the output of a safe-serialization as input. During the process, the following validation occurs:

• Type match: deserializing type A from a serialized type B raises an error indicating "On deserialization, expected type A, got type B".

• Version compatibility: deserializing type A of a newer version (for example, version 0.2) from a serialized type A of an older version (for example, version 0.1) raises an error indicating "On deserialization, expected serialization version 0.2, got version 0.1".

• Parameter compatibility: deserializing an object of type A with one set of crypto parameters from an object of type A with another set of crypto parameters raises an error indicating "Deserialized object of type A not conformant with given parameter set"

  • If both parameter sets have the same LWE dimension for ciphertexts, a ciphertext from param 1 may not fail this deserialization check with param 2.

  • This check can't distinguish ciphertexts/server keys from independent client keys with the same parameters.

  • This check is meant to prevent runtime errors in server homomorphic operations by checking that server keys and ciphertexts are compatible with the same parameter set.

  • You can use the standalone is_conformant method to check parameter compatibility. Besides, the safe_deserialize_conformant function includes the parameter compatibility check, and the safe_deserialize function does not include the compatibility check.

• Size limit: both serialization and deserialization processes expect a size limit (measured in bytes) for the serialized data:

  • On serialization, an error is raised if the serialized output exceeds the specific limit.

  • On deserialization, an error is raised if the serialized input exceeds the specific limit.

This feature aims to gracefully return an error in case of an attacker trying to cause an out-of-memory error on deserialization.

Here is an example:

// main.rs

use tfhe::conformance::ParameterSetConformant;
use tfhe::integer::parameters::RadixCiphertextConformanceParams;
use tfhe::prelude::*;
use tfhe::safe_deserialization::{safe_deserialize_conformant, safe_serialize};
use tfhe::shortint::parameters::{PARAM_MESSAGE_2_CARRY_2_KS_PBS, PARAM_MESSAGE_2_CARRY_2_PBS_KS};
use tfhe::conformance::ListSizeConstraint;
use tfhe::{
    generate_keys, FheUint8, CompactCiphertextList, FheUint8ConformanceParams,
    CompactPublicKey, ConfigBuilder, CompactCiphertextListConformanceParams
};

fn main() {
    let config = ConfigBuilder::default().build();

    let params_1 = PARAM_MESSAGE_2_CARRY_2_KS_PBS;
    let params_2 = PARAM_MESSAGE_2_CARRY_2_PBS_KS;
    
    let (client_key, server_key) = generate_keys(
        ConfigBuilder::with_custom_parameters(params_1, None).build()
    );
    
    let conformance_params_1 = FheUint8ConformanceParams::from(params_1);
    let conformance_params_2 = FheUint8ConformanceParams::from(params_2);
    
    let public_key = CompactPublicKey::new(&client_key);

    let msg = 27u8;

    let ct = FheUint8::try_encrypt(msg, &client_key).unwrap();
    
    assert!(ct.is_conformant(&conformance_params_1));
    assert!(!ct.is_conformant(&conformance_params_2));

    let mut buffer = vec![];

    safe_serialize(&ct, &mut buffer, 1 << 40).unwrap();
    
    assert!(safe_deserialize_conformant::<FheUint8>(
        buffer.as_slice(),
        1 << 20,
        &conformance_params_2
    ).is_err());

    let ct2 = safe_deserialize_conformant::<FheUint8>(
        buffer.as_slice(),
        1 << 20,
        &conformance_params_1
    ).unwrap();

    let dec: u8 = ct2.decrypt(&client_key);
    assert_eq!(msg, dec);
    
    
    // Example with a compact list:
    let msgs = [27, 188u8];
    let mut builder = CompactCiphertextList::builder(&public_key);
    builder.extend(msgs.iter().copied());
    let compact_list = builder.build();

    let mut buffer = vec![];
    safe_serialize(&compact_list, &mut buffer, 1 << 40).unwrap();
    
    let conformance_params = CompactCiphertextListConformanceParams {
        shortint_params: params_1.to_shortint_conformance_param(),
        num_elements_constraint: ListSizeConstraint::exact_size(2),
    };
    assert!(safe_deserialize_conformant::<CompactCiphertextList>(
        buffer.as_slice(),
        1 << 20,
        &conformance_params
    ).is_ok());
}

You can combine this serialization/deserialization feature with the data versioning feature by using the safe_serialize_versioned and safe_deserialize_conformant_versioned functions.