Module transciphering 

Server-side homomorphic conversion of a symmetric-cipher ciphertext into an FHE ciphertext.

The client encrypts data with a lightweight symmetric stream cipher, using a key it generates locally, and ships an FHE encryption of that key to the server once. The server, holding only the encrypted key, applies a transcipher round that turns each symmetric ciphertext into an FHE ciphertext of the same plaintext.

End-to-end example using Kreyvium:

use rand::Rng;
use tfhe::shortint::prelude::*;
use tfhe::shortint::parameters::current_params::V1_7_PARAM_MESSAGE_2_CARRY_2_KS_PBS_TUNIFORM_2M128;
use tfhe::transciphering::{StreamCipher, Transcipherer};
use tfhe::transciphering::ciphers::kreyvium::{
    KreyviumFheState, KreyviumPlainKey, KreyviumPlainState,
};

let (client_key, server_key) =
    gen_keys(V1_7_PARAM_MESSAGE_2_CARRY_2_KS_PBS_TUNIFORM_2M128);

// Client: pick a symmetric key + IV and encrypt a u64 with plain Kreyvium.
let mut rng = rand::thread_rng();
let key_bits: [bool; 128] = std::array::from_fn(|_| rng.gen());
let iv_bits: [bool; 128] = std::array::from_fn(|_| rng.gen());
let input: u64 = 0xDEADBEEFCAFEBABE;
let input_bytes = input.to_le_bytes();

let mut sym = KreyviumPlainState::new(key_bits, iv_bits);
let sym_cipher = sym.encrypt(&input_bytes);

// Client → server: ship the FHE-encrypted Kreyvium key (one-time setup).
let enc_key = KreyviumPlainKey::from(key_bits).encrypt(&client_key);

// Server: warm up the FHE-side Kreyvium stream and transcipher.
let mut fhe_stream = KreyviumFheState::new(enc_key, iv_bits, &server_key);
let blocks = fhe_stream.transcipher(&server_key, &sym_cipher).unwrap();

// Client: decrypt to recover `input`.
let recovered: u64 = blocks
    .iter()
    .enumerate()
    .map(|(i, b)| client_key.decrypt(b) << (2 * i))
    .sum();
assert_eq!(recovered, input);

Modules

backward_compatibility

ciphers

Structs

FheKeyStream

An FHE encrypted keystream that can be xored with an input encrypted with a StreamCipher.

StreamCiphertext

Output of StreamCipher::encrypt / StreamCipher::encrypt_bits: a stream-cipher ciphertext that the client ships to the server for transciphering.

StreamCiphertextConformanceParams

Parameters used to check StreamCiphertext conformance: the expected cipher family and bit length.

Enums

StreamCipherKind

Identifier for a concrete stream-cipher family.

TranscipherError

Errors raised by StreamCipher / Transcipherer operations that consume a StreamCiphertext.

TranscipherSession

Owning, runtime-dispatched Transcipherer. Lets higher layers keep a single concrete type that can hold any in-tree cipher state (Self::Kreyvium, Self::Aes), plus arbitrary out-of-tree implementors via Self::Dynamic.

Traits

StreamCipher

Client-side: a stateful symmetric-cipher session, no FHE.

Transcipherer

Server-side: a stateful FHE-side session that mirrors a StreamCipher. Same shape as StreamCipher, FHE-evaluated.

Functions

apply_keystream

Xor an FHE keystream with a clear StreamCiphertext.