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//! Module with the definition of the ServerKey.
//!
//! This module implements the generation of the server public key, together with all the
//! available homomorphic integer operations.
pub mod comparator;
mod crt;
mod crt_parallel;
pub(crate) mod radix;
pub(crate) mod radix_parallel;
use crate::integer::client_key::ClientKey;
use crate::shortint::ciphertext::MaxDegree;
use serde::{Deserialize, Serialize};
/// Error returned when the carry buffer is full.
pub use crate::shortint::CheckError;
use crate::shortint::{CarryModulus, MessageModulus};
pub use radix::scalar_mul::ScalarMultiplier;
pub use radix::scalar_sub::TwosComplementNegation;
pub use radix_parallel::{MiniUnsignedInteger, Reciprocable};
/// A structure containing the server public key.
///
/// The server key is generated by the client and is meant to be published: the client
/// sends it to the server so it can compute homomorphic integer circuits.
#[derive(Serialize, Deserialize, Clone)]
pub struct ServerKey {
pub(crate) key: crate::shortint::ServerKey,
}
impl From<ServerKey> for crate::shortint::ServerKey {
fn from(key: ServerKey) -> Self {
key.key
}
}
impl MaxDegree {
/// Compute the [`MaxDegree`] for an integer server key (compressed or uncompressed).
/// To allow carry propagation between shortint blocks in a
/// [`RadixCiphertext`](`crate::integer::RadixCiphertext`) (which includes adding the extracted
/// carry from one shortint block to the next block), this formula provisions space to add a
/// carry.
pub(crate) fn integer_radix_server_key(
message_modulus: MessageModulus,
carry_modulus: CarryModulus,
) -> Self {
let full_max_degree = message_modulus.0 * carry_modulus.0 - 1;
let carry_max_degree = carry_modulus.0 - 1;
// We want to be have a margin to add a carry from another block
Self::new(full_max_degree - carry_max_degree)
}
}
impl MaxDegree {
/// Compute the [`MaxDegree`] for an integer server key (compressed or uncompressed).
/// This is tailored for [`CrtCiphertext`](`crate::integer::CrtCiphertext`) and not compatible
/// for use with [`RadixCiphertext`](`crate::integer::RadixCiphertext`).
fn integer_crt_server_key(
message_modulus: MessageModulus,
carry_modulus: CarryModulus,
) -> Self {
let full_max_degree = message_modulus.0 * carry_modulus.0 - 1;
Self::new(full_max_degree)
}
}
impl ServerKey {
/// Generates a server key.
///
/// # Example
///
/// ```rust
/// use tfhe::integer::{ClientKey, ServerKey};
/// use tfhe::shortint::parameters::PARAM_MESSAGE_2_CARRY_2_KS_PBS;
///
/// // Generate the client key:
/// let cks = ClientKey::new(PARAM_MESSAGE_2_CARRY_2_KS_PBS);
///
/// // Generate the server key:
/// let sks = ServerKey::new_radix_server_key(&cks);
/// ```
pub fn new_radix_server_key<C>(cks: C) -> Self
where
C: AsRef<ClientKey>,
{
// It should remain just enough space to add a carry
let client_key = cks.as_ref();
let max_degree = MaxDegree::integer_radix_server_key(
client_key.key.parameters.message_modulus(),
client_key.key.parameters.carry_modulus(),
);
let sks = crate::shortint::server_key::ServerKey::new_with_max_degree(
&client_key.key,
max_degree,
);
Self { key: sks }
}
pub fn new_crt_server_key<C>(cks: C) -> Self
where
C: AsRef<ClientKey>,
{
let client_key = cks.as_ref();
let max_degree = MaxDegree::integer_crt_server_key(
client_key.key.parameters.message_modulus(),
client_key.key.parameters.carry_modulus(),
);
let sks = crate::shortint::server_key::ServerKey::new_with_max_degree(
&client_key.key,
max_degree,
);
Self { key: sks }
}
/// Creates a ServerKey destined to be used with
/// [`RadixCiphertext`](`crate::integer::RadixCiphertext`) from an already generated
/// shortint::ServerKey.
///
/// # Example
///
/// ```rust
/// use tfhe::integer::{ClientKey, ServerKey};
/// use tfhe::shortint::parameters::PARAM_MESSAGE_2_CARRY_2_KS_PBS;
/// use tfhe::shortint::ServerKey as ShortintServerKey;
///
/// let size = 4;
///
/// // Generate the client key:
/// let cks = ClientKey::new(PARAM_MESSAGE_2_CARRY_2_KS_PBS);
///
/// // Generate the shortint server key:
/// let shortint_sks = ShortintServerKey::new(cks.as_ref());
///
/// // Generate the server key:
/// let sks = ServerKey::new_radix_server_key_from_shortint(shortint_sks);
/// ```
pub fn new_radix_server_key_from_shortint(
mut key: crate::shortint::server_key::ServerKey,
) -> Self {
// It should remain just enough space add a carry
let max_degree =
MaxDegree::integer_radix_server_key(key.message_modulus, key.carry_modulus);
key.max_degree = max_degree;
Self { key }
}
/// Creates a ServerKey destined to be used with
/// [`CrtCiphertext`](`crate::integer::CrtCiphertext`) from an already generated
/// shortint::ServerKey.
///
/// # Example
///
/// ```rust
/// use tfhe::integer::{ClientKey, ServerKey};
/// use tfhe::shortint::parameters::PARAM_MESSAGE_2_CARRY_2_KS_PBS;
/// use tfhe::shortint::ServerKey as ShortintServerKey;
///
/// let size = 4;
///
/// // Generate the client key:
/// let cks = ClientKey::new(PARAM_MESSAGE_2_CARRY_2_KS_PBS);
///
/// // Generate the shortint server key:
/// let shortint_sks = ShortintServerKey::new(cks.as_ref());
///
/// // Generate the server key:
/// let sks = ServerKey::new_crt_server_key_from_shortint(shortint_sks);
/// ```
pub fn new_crt_server_key_from_shortint(
mut key: crate::shortint::server_key::ServerKey,
) -> Self {
key.max_degree = MaxDegree::integer_crt_server_key(key.message_modulus, key.carry_modulus);
Self { key }
}
/// Deconstruct a [`ServerKey`] into its constituents.
pub fn into_raw_parts(self) -> crate::shortint::ServerKey {
self.key
}
/// Construct a [`ServerKey`] from its constituents.
pub fn from_raw_parts(key: crate::shortint::ServerKey) -> Self {
Self { key }
}
pub fn deterministic_pbs_execution(&self) -> bool {
self.key.deterministic_pbs_execution()
}
pub fn set_deterministic_pbs_execution(&mut self, new_deterministic_execution: bool) {
self.key
.set_deterministic_pbs_execution(new_deterministic_execution);
}
pub fn message_modulus(&self) -> MessageModulus {
self.key.message_modulus
}
pub fn carry_modulus(&self) -> CarryModulus {
self.key.carry_modulus
}
}
impl AsRef<crate::shortint::ServerKey> for ServerKey {
fn as_ref(&self) -> &crate::shortint::ServerKey {
&self.key
}
}
#[derive(Clone, Serialize, Deserialize)]
pub struct CompressedServerKey {
pub(crate) key: crate::shortint::CompressedServerKey,
}
impl CompressedServerKey {
pub fn new_radix_compressed_server_key(client_key: &ClientKey) -> Self {
let max_degree = MaxDegree::integer_radix_server_key(
client_key.key.parameters.message_modulus(),
client_key.key.parameters.carry_modulus(),
);
let key =
crate::shortint::CompressedServerKey::new_with_max_degree(&client_key.key, max_degree);
Self { key }
}
pub fn new_crt_compressed_server_key(client_key: &ClientKey) -> Self {
let key = crate::shortint::CompressedServerKey::new(&client_key.key);
Self { key }
}
/// Decompress a [`CompressedServerKey`] into a [`ServerKey`].
pub fn decompress(&self) -> ServerKey {
ServerKey {
key: self.key.decompress(),
}
}
/// Deconstruct a [`CompressedServerKey`] into its constituents.
pub fn into_raw_parts(self) -> crate::shortint::CompressedServerKey {
self.key
}
/// Construct a [`CompressedServerKey`] from its constituents.
pub fn from_raw_parts(key: crate::shortint::CompressedServerKey) -> Self {
Self { key }
}
}
#[cfg(test)]
mod test {
use super::*;
use crate::integer::RadixClientKey;
use crate::shortint::parameters::PARAM_MESSAGE_2_CARRY_2;
/// https://github.com/zama-ai/tfhe-rs/issues/460
/// Problem with CompressedServerKey degree being set to shortint MaxDegree not accounting for
/// the necessary carry bits for e.g. Radix carry propagation.
#[test]
fn test_compressed_server_key_max_degree() {
{
let cks = ClientKey::new(crate::shortint::parameters::PARAM_MESSAGE_2_CARRY_2_KS_PBS);
// msg_mod = 4, carry_mod = 4, (msg_mod * carry_mod - 1) - (carry_mod - 1) = 12
let expected_radix_max_degree = MaxDegree::new(12);
let sks = ServerKey::new_radix_server_key(&cks);
assert_eq!(sks.key.max_degree, expected_radix_max_degree);
let csks = CompressedServerKey::new_radix_compressed_server_key(&cks);
assert_eq!(csks.key.max_degree, expected_radix_max_degree);
let decompressed_sks: ServerKey = csks.decompress();
assert_eq!(decompressed_sks.key.max_degree, expected_radix_max_degree);
}
{
let cks = ClientKey::new(crate::shortint::parameters::PARAM_MESSAGE_2_CARRY_2_KS_PBS);
// msg_mod = 4, carry_mod = 4, msg_mod * carrymod - 1 = 15
let expected_crt_max_degree = MaxDegree::new(15);
let sks = ServerKey::new_crt_server_key(&cks);
assert_eq!(sks.key.max_degree, expected_crt_max_degree);
let csks = CompressedServerKey::new_crt_compressed_server_key(&cks);
assert_eq!(csks.key.max_degree, expected_crt_max_degree);
let decompressed_sks: ServerKey = csks.decompress();
assert_eq!(decompressed_sks.key.max_degree, expected_crt_max_degree);
}
// Repro case from the user
{
let client_key = RadixClientKey::new(PARAM_MESSAGE_2_CARRY_2, 14);
let compressed_eval_key =
CompressedServerKey::new_radix_compressed_server_key(client_key.as_ref());
let evaluation_key = compressed_eval_key.decompress();
let modulus = (client_key.parameters().message_modulus().0 as u128)
.pow(client_key.num_blocks() as u32);
let mut ct = client_key.encrypt(modulus - 1);
let mut res_ct = ct.clone();
for _ in 0..5 {
res_ct = evaluation_key.smart_add_parallelized(&mut res_ct, &mut ct);
}
let res: u128 = client_key.decrypt(&res_ct);
assert_eq!(modulus - 6, res);
}
}
}