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Trivial ciphertexts

This document describes how to use trivial encryption in TFHE-rs to initialize server-side values.

Sometimes, the server side needs to initialize a value. For example, when computing the sum of a list of ciphertexts, you typically initialize the sum variable to 0.

Instead of asking the client to send an actual encrypted zero, the server can use a trivial encryption. A trivial encryption creates a ciphertext that contains the desired value but isn't securely encrypted - essentially anyone, any key can decrypt it.

Note that when you want to do an operation that involves a ciphertext and a clear value (often called scalar operation), you should only use trivial encryption of the clear value if the scalar operations that you want to run are not supported.

Trivial ciphertexts

This document describes how to use trivial encryption in TFHE-rs to initialize server-side values.

Sometimes, the server side needs to initialize a value. For example, when computing the sum of a list of ciphertexts, you typically initialize the sum variable to 0.

use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint8};

let config = ConfigBuilder::default().build();
let (client_key, sks) = generate_keys(config);

set_server_key(sks);

let a = FheUint8::try_encrypt_trivial(234u8).unwrap();

let clear: u8 = a.decrypt(&client_key);
assert_eq!(clear, 234);

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

let config = ConfigBuilder::default().build();
let (client_key, sks) = generate_keys(config);

set_server_key(sks);

// This is going to be faster
let a = FheUint32::try_encrypt(2097152u32, &client_key).unwrap();
let shift = 1u32;
let shifted = a << shift;
let clear: u32 = shifted.decrypt(&client_key);
assert_eq!(clear, 2097152 << 1);

// This is going to be slower
let a = FheUint32::try_encrypt(2097152u32, &client_key).unwrap();
let shift = FheUint32::try_encrypt_trivial(1u32).unwrap();
let shifted = a << shift;
let clear: u32 = shifted.decrypt(&client_key);
assert_eq!(clear, 2097152 << 1);