Use parallelized PBS

Parallelized Programmable Bootstrapping

The Programmable Bootstrapping(PBS) is a sequential operation by nature. However, some recent results showed that parallelism could be added at the cost of having larger keys. Overall, the performance of the PBS are improved. This new PBS is called a multi bit PBS. In TFHE-rs, since integer homomorphic operations are already parallelized, activating this feature may improve performance in the case of high core count CPUs if enough cores are available, or for small input message precision.

In what follows, an example on how to use the parallelized bootstrapping by choosing multi bit PBS parameters:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint32};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::default()
        .use_custom_parameters(
           tfhe::shortint::parameters::PARAM_MULTI_BIT_MESSAGE_2_CARRY_2_GROUP_3_KS_PBS,
           None,
        )
        .build();
        
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);
    
    let clear_a = 673u32;
    let clear_b = 6u32;
    let a = FheUint32::try_encrypt(clear_a, &keys)?;
    let b = FheUint32::try_encrypt(clear_b, &keys)?;

    let c = &a >> &b;
    let decrypted: u32 = c.decrypt(&keys);
    assert_eq!(decrypted, clear_a >> clear_b);

    Ok(())
}

Deterministic Parallelized Programmable Bootstrapping

By construction, the parallelized PBS might not be deterministic: the resulting ciphertext will always decrypt to the same plaintext, but the order of the operations could differ so the output ciphertext might differ. In order to activate the deterministic version, the suffix 'with_deterministic_execution()' should be added to the parameters, as shown in the following example:

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint32};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let config = ConfigBuilder::default()
        .use_custom_parameters(
           tfhe::shortint::parameters::PARAM_MULTI_BIT_MESSAGE_2_CARRY_2_GROUP_3_KS_PBS.with_deterministic_execution(),
           None,
        )
        .build();
        
    let (keys, server_keys) = generate_keys(config);
    set_server_key(server_keys);
    
    let clear_a = 673u32;
    let clear_b = 6u32;
    let a = FheUint32::try_encrypt(clear_a, &keys)?;
    let b = FheUint32::try_encrypt(clear_b, &keys)?;

    let c = &a >> &b;
    let decrypted: u32 = c.decrypt(&keys);
    assert_eq!(decrypted, clear_a >> clear_b);

    Ok(())
}