tfhe 1.5.5

use crate::backward_compatibility::cpk_re_randomization::ReRandomizationMetadataVersions;
use crate::core_crypto::commons::math::random::XofSeed;
use crate::high_level_api::keys::CompactPublicKey;
use crate::high_level_api::tag::SmallVec;
use crate::integer::ciphertext::{ReRandomizationSeed, ReRandomizationSeedHasher};

use tfhe_versionable::Versionize;

/// Re-Randomization adds randomness to an existing ciphertext without changing the value it
/// encrypts.
///
/// It can be used to achieve sIND-CPAD security and needs to be called on every function inputs.
///
/// This works by encrypting zeros using a public key, then adding theses zeros to the ciphertext.
/// This process is seeded using the [`ReRandomizationContext`] and thus can be made deterministic.
///
/// The randomization seeds are built from the ciphertexts encrypted values, some metadata that tie
/// them to their origin (such as a zk-pok) and a sequence of bytes that uniquely describe the
/// function that will be applied to them.
///
/// More precisely, the hash function will be updated with, in order:
/// - the rerand seeder domain separator (e.g: TFHE_Rrd)
/// - the ciphertexts encrypted values
/// - the ciphertexts metadata
/// - the function description (e.g: "FheUint64+FheUint64" + a unique random nonce)
/// - a unique counter for each seed
///
/// For example, if we want to re-randomize the inputs of a function with two arguments ct1 and ct2,
/// respectively associated to metadata meta1 and meta2, what happens conceptually is:
///
/// ```text
/// seed1 = hash(rerand_domain_separator, ct1, ct2, meta1, meta2, fn_description, 0)
/// seed2 = hash(rerand_domain_separator, ct1, ct2, meta1, meta2, fn_description, 1)
/// ct1_rerand = ct1 + encrypt(0, pubkey, public_encryption_domain_separator, seed1)
/// ct2_rerand = ct2 + encrypt(0, pubkey, public_encryption_domain_separator, seed2)
/// function(ct1_rerand, ct2_rerand)
/// ```
///
/// # Example
///
/// ```rust
/// use tfhe::prelude::*;
/// use tfhe::shortint::parameters::*;
/// use tfhe::{
///     generate_keys, set_server_key, CompactPublicKey, ConfigBuilder, FheUint64,
///     ReRandomizationContext,
/// };
///
/// let params = PARAM_MESSAGE_2_CARRY_2_KS_PBS_TUNIFORM_2M128;
/// let cpk_params = (
///     PARAM_PKE_MESSAGE_2_CARRY_2_KS_PBS_TUNIFORM_2M128,
///     PARAM_KEYSWITCH_MESSAGE_2_CARRY_2_KS_PBS_TUNIFORM_2M128,
/// );
/// let re_rand_ks_params = PARAM_KEYSWITCH_TO_BIG_MESSAGE_2_CARRY_2_KS_PBS_TUNIFORM_2M128;
///
/// let config = ConfigBuilder::with_custom_parameters(params)
///     .use_dedicated_compact_public_key_parameters(cpk_params)
///     .enable_ciphertext_re_randomization(re_rand_ks_params)
///     .build();
///
/// let (cks, sks) = generate_keys(config);
/// let cpk = CompactPublicKey::new(&cks);
///
/// let compact_public_encryption_domain_separator = *b"TFHE_Enc";
/// let rerand_domain_separator = *b"TFHE_Rrd";
///
/// set_server_key(sks);
///
/// let clear_a = 12u64;
/// let clear_b = 37u64;
/// let mut a = FheUint64::encrypt(clear_a, &cks);
/// let mut b = FheUint64::encrypt(clear_b, &cks);
///
/// // Simulate a 256 bits hash added as metadata
/// let rand_a: [u8; 256 / 8] = core::array::from_fn(|_| rand::random());
/// let rand_b: [u8; 256 / 8] = core::array::from_fn(|_| rand::random());
/// a.re_randomization_metadata_mut().set_data(&rand_a);
/// b.re_randomization_metadata_mut().set_data(&rand_b);
///
/// // Simulate a 256 bits nonce
/// let nonce: [u8; 256 / 8] = core::array::from_fn(|_| rand::random());
///
/// let mut re_rand_context = ReRandomizationContext::new(
///     rerand_domain_separator,
///     // First is the function description, second is a nonce
///     [b"FheUint64+FheUint64".as_slice(), nonce.as_slice()],
///     compact_public_encryption_domain_separator,
/// );
///
/// // Add ciphertexts to the context
/// re_rand_context.add_ciphertext(&a);
/// re_rand_context.add_ciphertext(&b);
///
/// let mut seed_gen = re_rand_context.finalize();
///
/// a.re_randomize(&cpk, seed_gen.next_seed().unwrap()).unwrap();
/// b.re_randomize(&cpk, seed_gen.next_seed().unwrap()).unwrap();
///
/// let c = a + b;
/// let dec: u64 = c.decrypt(&cks);
///
/// assert_eq!(clear_a.wrapping_add(clear_b), dec);
/// ```
pub trait ReRandomize
where
    Self: Sized,
{
    fn add_to_re_randomization_context(&self, context: &mut ReRandomizationContext);

    /// Re-randomize the ciphertext using the provided public key and seed.
    ///
    /// The random elements of the ciphertexts will be changed but it will still encrypt the same
    /// value.
    fn re_randomize(
        &mut self,
        compact_public_key: &CompactPublicKey,
        seed: ReRandomizationSeed,
    ) -> crate::Result<()>;
}

/// The context in which the ciphertexts to re-randomized will be used.
///
/// It can be updated with user provided ciphertexts and will then be finalized into a
/// [`ReRandomizationSeedGen`].
pub struct ReRandomizationContext {
    pub(in crate::high_level_api) inner: crate::integer::ciphertext::ReRandomizationContext,
}

impl ReRandomizationContext {
    /// Create a new re-randomization context with the default seed hasher (blake3).
    ///
    /// `rerand_seeder_domain_separator` is the domain separator that will be fed into the
    /// seed generator.
    /// `public_encryption_domain_separator` is the domain separator that will be used along this
    /// seed to generate the encryptions of zero.
    /// `fn_description` is a unique sequence of bytes that represents the functions called on the
    /// re-randomized values.
    ///
    /// (See [`XofSeed`] for more information)
    ///
    /// # Example
    /// ```rust
    /// use tfhe::ReRandomizationContext;
    /// // Simulate a 256 bits nonce
    /// let nonce: [u8; 256 / 8] = core::array::from_fn(|_| rand::random());
    /// let _re_rand_context = ReRandomizationContext::new(
    ///     *b"TFHE_Rrd",
    ///     [b"FheUint64+FheUint64".as_slice(), &nonce],
    ///     *b"TFHE_Enc"
    ///  );
    pub fn new<'a>(
        rerand_seeder_domain_separator: [u8; XofSeed::DOMAIN_SEP_LEN],
        fn_description: impl IntoIterator<Item = &'a [u8]>,
        public_encryption_domain_separator: [u8; XofSeed::DOMAIN_SEP_LEN],
    ) -> Self {
        Self {
            inner: crate::integer::ciphertext::ReRandomizationContext::new(
                rerand_seeder_domain_separator,
                fn_description,
                public_encryption_domain_separator,
            ),
        }
    }

    /// Create a new re-randomization context with the provided seed hasher.
    pub fn new_with_hasher<'a>(
        fn_description: impl IntoIterator<Item = &'a [u8]>,
        public_encryption_domain_separator: [u8; XofSeed::DOMAIN_SEP_LEN],
        seed_hasher: ReRandomizationSeedHasher,
    ) -> Self {
        Self {
            inner: crate::integer::ciphertext::ReRandomizationContext::new_with_hasher(
                fn_description,
                public_encryption_domain_separator,
                seed_hasher,
            ),
        }
    }

    /// Adds a new ciphertext to the re-randomization context
    pub fn add_ciphertext<Data: ReRandomize>(&mut self, data: &Data) {
        data.add_to_re_randomization_context(self);
    }

    /// Consumes the context to create a seed generator
    pub fn finalize(self) -> ReRandomizationSeedGen {
        ReRandomizationSeedGen {
            inner: self.inner.finalize(),
        }
    }
}

/// A generator that can be used to obtain seeds needed to re-randomize individual ciphertexts.
///
/// It should only be used to create one seed per ciphertext that was added to the context
pub struct ReRandomizationSeedGen {
    inner: crate::integer::ciphertext::ReRandomizationSeedGen,
}

impl ReRandomizationSeedGen {
    /// Generate the next seed from the seeder.
    ///
    /// Returns an error if more seeds have been generated than the number of ciphertext added into
    /// the context.
    pub fn next_seed(&mut self) -> crate::Result<ReRandomizationSeed> {
        self.inner.next_seed()
    }
}

/// Metadata linked to a ciphertext that will be used when updating the [`ReRandomizationContext`]
#[derive(
    Default, Clone, Debug, serde::Serialize, serde::Deserialize, Versionize, PartialEq, Eq,
)]
#[versionize(ReRandomizationMetadataVersions)]
pub struct ReRandomizationMetadata {
    inner: SmallVec,
}

impl ReRandomizationMetadata {
    pub fn new(data: &[u8]) -> Self {
        let mut inner = SmallVec::default();
        inner.set_data(data);

        Self { inner }
    }

    pub fn data(&self) -> &[u8] {
        self.inner.data()
    }

    pub fn set_data(&mut self, data: &[u8]) {
        self.inner.set_data(data);
    }

    pub fn clear(&mut self) {
        self.inner.clear();
    }
}