This contains the operations available in tfhe::boolean, along with code examples.

The NOT unary gate

Binary gates

The MUX ternary gate

Let ct_1, ct_2, ct_3 be three Boolean ciphertexts. Then, the MUX gate (abbreviation of MUltipleXer) is equivalent to the operation:

This example shows how to use the MUX ternary gate:

In tfhe::boolean, the available operations are mainly related to their equivalent Boolean gates (i.e., AND, OR... etc). What follows are examples of a unary gate (NOT) and a binary gate (XOR). The last one is about the ternary MUX gate, which allows homomorphic computation of conditional statements of the form If..Then..Else.

This library is meant to be used both on the server side and the client side. The typical use case should follow the subsequent steps:

1. On the client side, generate the client and server keys.

2. Send the server key to the server.

3. Then any number of times:

   • On the client side, encrypt the input data with the client key.

   • Transmit the encrypted input to the server.

   • On the server side, perform homomorphic computation with the server key.

   • Transmit the encrypted output to the client.

   • On the client side, decrypt the output data with the client key.

Setup

In the first step, the client creates two keys, the client key and the server key, with the tfhe::boolean::gen_keys function:

use tfhe::boolean::prelude::*;

fn main() {

// We generate the client key and the server key,
// using the default parameters:
    let (client_key, server_key): (ClientKey, ServerKey) = gen_keys();
}

• The client_key is of type ClientKey. It is secret and must never be transmitted. This key will only be used to encrypt and decrypt data.

• The server_key is of type ServerKey. It is a public key and can be shared with any party. This key has to be sent to the server because it is required for homomorphic computation.

Note that both the client_key and server_key implement the Serialize and Deserialize traits. This way you can use any compatible serializer to store/send the data. To store the server_key in a binary file, you can use the bincode library:

use std::fs::{File, create_dir_all};
use std::io::{Write, Read};
use tfhe::boolean::prelude::*;

fn main() {

//---------------------------- CLIENT SIDE ----------------------------

// We generate a client key and a server key, using the default parameters:
    let (client_key, server_key) = gen_keys();

// We serialize the server key to bytes, and store them in a file:
    let encoded: Vec<u8> = bincode::serialize(&server_key).unwrap();

// Create a tmp dir with the current user name to avoid cluttering the /tmp dir
    let user = std::env::var("USER").unwrap_or_else(|_| "unknown_user".to_string());
    let tmp_dir_for_user = &format!("/tmp/{user}");

    create_dir_all(tmp_dir_for_user).unwrap();

    let server_key_file = &format!("{tmp_dir_for_user}/tutorial_server_key.bin");

// We write the server key to a file:
    let mut file = File::create(server_key_file)
        .expect("failed to create server key file");
    file.write_all(encoded.as_slice()).expect("failed to write key to file");

// ...
// We send the key to server side
// ...


//---------------------------- SERVER SIDE ----------------------------

// We read the file:
    let mut file = File::open(server_key_file)
        .expect("failed to open server key file");
    let mut encoded: Vec<u8> = Vec::new();
    file.read_to_end(&mut encoded).expect("failed to read key");

// We deserialize the server key:
    let key: ServerKey = bincode::deserialize(&encoded[..])
        .expect("failed to deserialize");
}

Encrypting inputs

Once the server key is available on the server side, it is possible to perform some homomorphic computations. The client needs to encrypt some data and send it to the server. Again, the Ciphertext type implements the Serialize and the Deserialize traits, so that any serializer and communication tool suiting your use case can be employed:

use tfhe::boolean::prelude::*;

fn main() {
    // Don't consider the following line; you should follow the procedure above.
    let (client_key, _) = gen_keys();

//---------------------------- CLIENT SIDE

// We use the client key to encrypt the messages:
    let ct_1 = client_key.encrypt(true);
    let ct_2 = client_key.encrypt(false);

// We serialize the ciphertexts:
    let encoded_1: Vec<u8> = bincode::serialize(&ct_1).unwrap();
    let encoded_2: Vec<u8> = bincode::serialize(&ct_2).unwrap();

// ...
// And we send them to the server somehow
// ...
}

Encrypting inputs using a public key

Anyone (the server or a third party) with the public key can also encrypt some (or all) of the inputs. The public key can only be used to encrypt, not to decrypt.

use tfhe::boolean::prelude::*;

fn main() {
    // Don't consider the following line; you should follow the procedure above.
    let (client_key, _) = gen_keys();
    let public_key = PublicKey::new(&client_key);

//---------------------------- SERVER or THIRD_PARTY SIDE

// We use the public key to encrypt the messages:
    let ct_1 = public_key.encrypt(true);
    let ct_2 = public_key.encrypt(false);

// We serialize the ciphertexts (if not on the server already):
    let encoded_1: Vec<u8> = bincode::serialize(&ct_1).unwrap();
    let encoded_2: Vec<u8> = bincode::serialize(&ct_2).unwrap();

// ...
// And we send them to the server to be deserialized (if not on the server already)
// ...
}

Executing a Boolean circuit

Once the encrypted inputs are on the server side, the server_key can be used to homomorphically execute the desired Boolean circuit:

use tfhe::boolean::prelude::*;

fn main() {
    // Don't consider the following lines; you should follow the procedure above.
    let (client_key, server_key) = gen_keys();
    let ct_1 = client_key.encrypt(true);
    let ct_2 = client_key.encrypt(false);
    let encoded_1: Vec<u8> = bincode::serialize(&ct_1).unwrap();
    let encoded_2: Vec<u8> = bincode::serialize(&ct_2).unwrap();

    //---------------------------- ON SERVER SIDE ----------------------------

    // We deserialize the ciphertexts:
    let ct_1: Ciphertext = bincode::deserialize(&encoded_1[..])
        .expect("failed to deserialize");
    let ct_2: Ciphertext = bincode::deserialize(&encoded_2[..])
        .expect("failed to deserialize");

    // We use the server key to execute the boolean circuit:
    // if ((NOT ct_2) NAND (ct_1 AND ct_2)) then (NOT ct_2) else (ct_1 AND ct_2)
    let ct_3 = server_key.not(&ct_2);
    let ct_4 = server_key.and(&ct_1, &ct_2);
    let ct_5 = server_key.nand(&ct_3, &ct_4);
    let ct_6 = server_key.mux(&ct_5, &ct_3, &ct_4);

    // Then we serialize the output of the circuit:
    let encoded_output: Vec<u8> = bincode::serialize(&ct_6)
        .expect("failed to serialize output");

    // ...
    // And we send the output to the client
    // ...
}

Decrypting the output

Once the encrypted output is on the client side, the client_key can be used to decrypt it:

use tfhe::boolean::prelude::*;

fn main() {
    // Don't consider the following lines; you should follow the procedure above.
    let (client_key, server_key) = gen_keys();
    let ct_6 = client_key.encrypt(true);
    let encoded_output: Vec<u8> = bincode::serialize(&ct_6).unwrap();

//---------------------------- ON CLIENT SIDE

// We deserialize the output ciphertext:
    let output: Ciphertext = bincode::deserialize(&encoded_output[..])
        .expect("failed to deserialize");

// Finally, we decrypt the output:
    let output = client_key.decrypt(&output);

// And check that the result is the expected one:
    assert!(output);
}

Since the ServerKey and ClientKey types both implement the Serialize and Deserialize traits, you are free to use any serializer that suits you to save and load the keys to disk.

Here is an example using the bincode serialization library, which serializes to a binary format:

use std::fs::{File, create_dir_all};
use std::io::{Write, Read};
use tfhe::boolean::prelude::*;

fn main() {
// We generate a set of client/server keys, using the default parameters:
    let (client_key, server_key) = gen_keys();

// We serialize the keys to bytes:
    let encoded_server_key: Vec<u8> = bincode::serialize(&server_key).unwrap();
    let encoded_client_key: Vec<u8> = bincode::serialize(&client_key).unwrap();

// Create a tmp dir with the current user name to avoid cluttering the /tmp dir
    let user = std::env::var("USER").unwrap_or_else(|_| "unknown_user".to_string());
    let tmp_dir_for_user = &format!("/tmp/{user}");

    create_dir_all(tmp_dir_for_user).unwrap();

    let server_key_file = &format!("{tmp_dir_for_user}/ser_example_server_key.bin");
    let client_key_file = &format!("{tmp_dir_for_user}/ser_example_client_key.bin");

// We write the keys to files:
    let mut file = File::create(server_key_file)
        .expect("failed to create server key file");
    file.write_all(encoded_server_key.as_slice()).expect("failed to write key to file");
    let mut file = File::create(client_key_file)
        .expect("failed to create client key file");
    file.write_all(encoded_client_key.as_slice()).expect("failed to write key to file");

// We retrieve the keys:
    let mut file = File::open(server_key_file)
        .expect("failed to open server key file");
    let mut encoded_server_key: Vec<u8> = Vec::new();
    file.read_to_end(&mut encoded_server_key).expect("failed to read the key");

    let mut file = File::open(client_key_file)
        .expect("failed to open client key file");
    let mut encoded_client_key: Vec<u8> = Vec::new();
    file.read_to_end(&mut encoded_client_key).expect("failed to read the key");

// We deserialize the keys:
    let loaded_server_key: ServerKey = bincode::deserialize(&encoded_server_key[..])
        .expect("failed to deserialize");
    let loaded_client_key: ClientKey = bincode::deserialize(&encoded_client_key[..])
        .expect("failed to deserialize");


    let ct_1 = client_key.encrypt(false);

// We check for equality:
    assert!(!loaded_client_key.decrypt(&ct_1));
}

tfhe::shortint is dedicated to unsigned integers smaller than 8 bits. The steps to homomorphically evaluate a circuit are described below.

Key generation

tfhe::shortint provides 3 key types:

• ClientKey

• ServerKey

• PublicKey

The ClientKey is the key that encrypts and decrypts messages (integer values up to 8 bits here). It is meant to be kept private and should never be shared. This key is created from parameter values that will dictate both the security and efficiency of computations. The parameters also set the maximum number of bits of message encrypted in a ciphertext.

The ServerKey is the key that is used to evaluate the FHE computations. Most importantly, it contains a bootstrapping key and a keyswitching key. This key is created from a ClientKey that needs to be shared to the server (it is not meant to be kept private). A user with a ServerKey can compute on the encrypted data sent by the owner of the associated ClientKey.

Computation/operation methods are tied to the ServerKey type.

The PublicKey is the key used to encrypt messages. It can be publicly shared to allow users to encrypt data such that only the ClientKey holder will be able to decrypt. Encrypting with the PublicKey does not alter the homomorphic capabilities associated to the ServerKey.

use tfhe::shortint::prelude::*;

fn main()  {
    // We generate a set of client/server keys
    let (client_key, server_key) = gen_keys(PARAM_MESSAGE_2_CARRY_2_KS_PBS);
}

Encrypting values

Once the keys have been generated, the client key is used to encrypt data:

use tfhe::shortint::prelude::*;

fn main() {
    // We generate a set of client/server keys
   let (client_key, server_key) = gen_keys(PARAM_MESSAGE_2_CARRY_2_KS_PBS);

    let msg1 = 1;
    let msg2 = 0;

    // We use the client key to encrypt two messages:
    let ct_1 = client_key.encrypt(msg1);
    let ct_2 = client_key.encrypt(msg2);
}

Encrypting values using a public key

Once the keys have been generated, the client key is used to encrypt data:

use tfhe::shortint::prelude::*;

fn main() {
    // We generate a set of client/server keys
   let (client_key, _) = gen_keys(PARAM_MESSAGE_2_CARRY_2_KS_PBS);
   let public_key = PublicKey::new(&client_key);

    let msg1 = 1;
    let msg2 = 0;

    // We use the client key to encrypt two messages:
    let ct_1 = public_key.encrypt(msg1);
    let ct_2 = public_key.encrypt(msg2);
}

Computing and decrypting

Using the server_key, addition is possible over encrypted values. The resulting plaintext is recovered after the decryption via the secret client key.

use tfhe::shortint::prelude::*;

fn main() {
    // We generate a set of client/server keys
    let (client_key, server_key) = gen_keys(PARAM_MESSAGE_2_CARRY_2_KS_PBS);

    let msg1 = 1;
    let msg2 = 0;

    let modulus = client_key.parameters.message_modulus().0;

    // We use the client key to encrypt two messages:
    let ct_1 = client_key.encrypt(msg1);
    let ct_2 = client_key.encrypt(msg2);

    // We use the server public key to execute an integer circuit:
    let ct_3 = server_key.add(&ct_1, &ct_2);

    // We use the client key to decrypt the output of the circuit:
    let output = client_key.decrypt(&ct_3);
    assert_eq!(output, (msg1 + msg2) % modulus as u64);
}

This library makes it possible to execute homomorphic operations over encrypted data, where the data are either Booleans, short integers (named shortint in the rest of this documentation), or integers up to 256 bits. It allows you to execute a circuit on an untrusted server because both circuit inputs and outputs are kept private. Data are indeed encrypted on the client side, before being sent to the server. On the server side, every computation is performed on ciphertexts.

The server, however, has to know the circuit to be evaluated. At the end of the computation, the server returns the encryption of the result to the user. Then the user can decrypt it with the secret key.

General method to write an homomorphic circuit program

The overall process to write an homomorphic program is the same for all types. The basic steps for using the TFHE-rs library are the following:

1. Choose a data type (Boolean, shortint, integer)

2. Import the library

3. Create client and server keys

4. Encrypt data with the client key

5. Compute over encrypted data using the server key

6. Decrypt data with the client key

API levels.

This library has different modules, with different levels of abstraction.

There is the core_crypto module, which is the lowest level API with the primitive functions and types of the TFHE scheme.

Above the core_crypto module, there are the Boolean, shortint, and integer modules, which contain easy to use APIs enabling evaluation of Boolean, short integer, and integer circuits.

Finally, there is the high-level module built on top of the Boolean, shortint, integer modules. This module is meant to abstract cryptographic complexities: no cryptographical knowledge is required to start developing an FHE application. Another benefit of the high-level module is the drastically simplified development process compared to lower level modules.

high-level API

TFHE-rs exposes a high-level API by default that includes datatypes that try to match Rust's native types by having overloaded operators (+, -, ...).

Here is an example of how the high-level API is used:

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

fn main() {
    let config = ConfigBuilder::default()
        .build();

    let (client_key, server_key) = generate_keys(config);

    set_server_key(server_key);

    let clear_a = 27u8;
    let clear_b = 128u8;

    let a = FheUint8::encrypt(clear_a, &client_key);
    let b = FheUint8::encrypt(clear_b, &client_key);

    let result = a + b;

    let decrypted_result: u8 = result.decrypt(&client_key);

    let clear_result = clear_a + clear_b;

    assert_eq!(decrypted_result, clear_result);
}

Boolean example

Here is an example of how the library can be used to evaluate a Boolean circuit:

use tfhe::boolean::prelude::*;

fn main() {
    // We generate a set of client/server keys, using the default parameters:
    let (client_key, server_key) = gen_keys();

    // We use the client secret key to encrypt two messages:
    let ct_1 = client_key.encrypt(true);
    let ct_2 = client_key.encrypt(false);

    // We use the server public key to execute a boolean circuit:
    // if ((NOT ct_2) NAND (ct_1 AND ct_2)) then (NOT ct_2) else (ct_1 AND ct_2)
    let ct_3 = server_key.not(&ct_2);
    let ct_4 = server_key.and(&ct_1, &ct_2);
    let ct_5 = server_key.nand(&ct_3, &ct_4);
    let ct_6 = server_key.mux(&ct_5, &ct_3, &ct_4);

    // We use the client key to decrypt the output of the circuit:
    let output = client_key.decrypt(&ct_6);
    assert!(output);
}

shortint example

Here is a full example using shortint:

use tfhe::shortint::prelude::*;

fn main() {
    // We generate a set of client/server keys
    // using parameters with 2 bits of message and 2 bits of carry
    let (client_key, server_key) = gen_keys(PARAM_MESSAGE_2_CARRY_2);

    let msg1 = 1;
    let msg2 = 0;

    let modulus = client_key.parameters.message_modulus().0;

    // We use the client key to encrypt two messages:
    let ct_1 = client_key.encrypt(msg1);
    let ct_2 = client_key.encrypt(msg2);

    // We use the server public key to execute an integer circuit:
    let ct_3 = server_key.add(&ct_1, &ct_2);

    // We use the client key to decrypt the output of the circuit:
    let output = client_key.decrypt(&ct_3);
    assert_eq!(output, (msg1 + msg2) % modulus as u64);
}

integer example

use tfhe::integer::gen_keys_radix;
use tfhe::shortint::parameters::PARAM_MESSAGE_2_CARRY_2;

fn main() {
    // We generate keys to encrypt 16 bits radix-encoded integers
    // using 8 blocks of 2 bits
    let (cks, sks) = gen_keys_radix(PARAM_MESSAGE_2_CARRY_2, 8);

    let clear_a = 2382u16;
    let clear_b = 29374u16;

    let mut a = cks.encrypt(clear_a as u64);
    let mut b = cks.encrypt(clear_b as u64);

    let encrypted_max = sks.smart_max_parallelized(&mut a, &mut b);
    let decrypted_max: u64 = cks.decrypt(&encrypted_max);

    assert_eq!(decrypted_max as u16, clear_a.max(clear_b))
}

The library is simple to use and can evaluate homomorphic circuits of arbitrary length. The description of the algorithms can be found in the TFHE paper (also available as ePrint 2018/421).

tfhe::integer is dedicated to integers smaller than 256 bits. The steps to homomorphically evaluate an integer circuit are described here.

Key Types

integer provides 3 basic key types:

• ClientKey

• ServerKey

• PublicKey

The ClientKey is the key that encrypts and decrypts messages, thus this key is meant to be kept private and should never be shared. This key is created from parameter values that will dictate both the security and efficiency of computations. The parameters also set the maximum number of bits of message encrypted in a ciphertext.

The ServerKey is the key that is used to actually do the FHE computations. It contains a bootstrapping key and a keyswitching key. This key is created from a ClientKey that needs to be shared to the server, so it is not meant to be kept private. A user with a ServerKey can compute on the encrypted data sent by the owner of the associated ClientKey.

To reflect this, computation/operation methods are tied to the ServerKey type.

The PublicKey is a key used to encrypt messages. It can be publicly shared to allow users to encrypt data such that only the ClientKey holder will be able to decrypt. Encrypting with the PublicKey does not alter the homomorphic capabilities associated to the ServerKey.

1. Key Generation

To generate the keys, a user needs two parameters:

• A set of shortint cryptographic parameters.

• The number of ciphertexts used to encrypt an integer (we call them "shortint blocks").

We are now going to build a pair of keys that can encrypt 8-bit integers (signed or unsigned) by using 4 shortint blocks that store 2 bits of message each.

2. Encrypting values

Once we have our keys, we can encrypt values:

3. Encrypting values with the public key

Once the client key is generated, the public key can be derived and used to encrypt data.

4. Computing and decrypting

With our server_key, and encrypted values, we can now do an addition and then decrypt the result.

Default parameters

The TFHE cryptographic scheme relies on a variant of Regev cryptosystem and is based on a problem so difficult that it is even post-quantum resistant.

Some cryptographic parameters will require tuning to ensure both the correctness of the result and the security of the computation.

To make it simpler, we've provided two sets of parameters, which ensure correct computations for a certain probability with the standard security of 128 bits. There exists an error probability due to the probabilistic nature of the encryption, which requires adding randomness (noise) following a Gaussian distribution. If this noise is too large, the decryption will not give a correct result. There is a trade-off between efficiency and correctness: generally, using a less efficient parameter set (in terms of computation time) leads to a smaller risk of having an error during homomorphic evaluation.

In the two proposed sets of parameters, the only difference lies in this error probability. The default parameter set ensures an error probability of at most $2^{-40}$ when computing a programmable bootstrapping (i.e., any gates but the not). The other one is closer to the error probability claimed in the original TFHE paper, namely $2^{-165}$, but it is up-to-date regarding security requirements.

The following array summarizes this:

User-defined parameters

You can also create your own set of parameters. This is an unsafe operation as failing to properly fix the parameters will result in an incorrect and/or insecure computation:

use tfhe::boolean::prelude::*;

fn main() {
    // WARNING: might be insecure and/or incorrect
    // You can create your own set of parameters
    let parameters = BooleanParameters::new(
        LweDimension(586),
        GlweDimension(2),
        PolynomialSize(512),
        DynamicDistribution::new_gaussian_from_std_dev(
            StandardDev(0.00008976167396834998),
        ),
        DynamicDistribution::new_gaussian_from_std_dev(
            StandardDev(0.00000002989040792967434),
        ),
        DecompositionBaseLog(8),
        DecompositionLevelCount(2),
        DecompositionBaseLog(2),
        DecompositionLevelCount(5),
        EncryptionKeyChoice::Small,
    );
}

The structure and operations related to integers are described in this section.

How an integer is represented

In integer, the encrypted data is split amongst many ciphertexts encrypted with the shortint library. Below is a scheme representing an integer composed by k shortint ciphertexts.

This crate implements two ways to represent an integer:

• the Radix representation

• the CRT (Chinese Reminder Theorem) representation

Radix-based integers.

In this representation, the correctness of operations requires the carries to be propagated throughout the ciphertext. This operation is costly, since it relies on the computation of many programmable bootstrapping operations over shortints.

CRT-based integers.

This representation has many advantages: no carry propagation is required, cleaning the carry buffer of each ciphertext block is enough. This implies that operations can easily be parallelized. It also allows the efficient computation of PBS in the case where the function is CRT-compliant.

A variant of the CRT is proposed where each block might be associated to a different key couple. Here, a keychain to the computations is required, but this may result in a performance improvement.

List of available operations

The list of operations available in integer depends on the type of representation:

Types of operations

Much like shortint, the operations available via a ServerKey may come in different variants:

• operations that take their inputs as encrypted values.

• scalar operations take at least one non-encrypted value as input.

For example, the addition has both variants:

• ServerKey::unchecked_add, which takes two encrypted values and adds them.

• ServerKey::unchecked_scalar_add, which takes an encrypted value and a clear value (the so-called scalar) and adds them.

Each operation may come in different 'flavors':

• unchecked: always does the operation, without checking if the result may exceed the capacity of the plaintext space.

• checked: checks are done before computing the operation, returning an error if operation cannot be done safely.

• smart: always does the operation, if the operation cannot be computed safely, the smart operation will propagate the carry buffer to make the operation possible. Some of those will require a mutable reference as input: this is because the inputs' carry might be cleaned, but this will not change the underlying encrypted value.

• default: always compute the operation and always clear the carry. Could be slower than smart, but ensure that the timings are consistent from one call to another.

Not all operations have these 4 flavors, as some of them are implemented in a way that the operation is always possible without ever exceeding the plaintext space capacity.

How to use each operation type

Let's try to do a circuit evaluation using the different flavors of already introduced operations. For a very small circuit, the unchecked flavor may be enough to do the computation correctly. Otherwise, checked and smart are the best options.

As an example, let's do a scalar multiplication, a subtraction, and an addition.

During this computation the carry buffer has been overflowed, and the output may be incorrect as all the operations were unchecked.

If the same circuit is done but using the checked flavor, a panic will occur:

The checked flavor permits the manual management of the overflow of the carry buffer by raising an error if correctness is not guaranteed.

Using the smart flavor will output the correct result all the time. However, the computation may be slower as the carry buffer may be propagated during the computations.

The main advantage of the default flavor is to ensure predictable timings, as long as only this kind of operation is used. Only the parallelized version of the operations is provided.

The structure and operations related to short integers are described in this section.

How a shortint is represented

In shortint, the encrypted data is stored in an LWE ciphertext.

Conceptually, the message stored in an LWE ciphertext is divided into a carry buffer and a message buffer.

The message buffer is the space where the actual message is stored. This represents the modulus of the input messages (denoted by MessageModulus in the code). When doing computations on a ciphertext, the encrypted message can overflow the message modulus. The part of the message which exceeds the message modulus is stored in the carry buffer. The size of the carry buffer is defined by another modulus, called CarryModulus.

Together, the message modulus and the carry modulus form the plaintext space that is available in a ciphertext. This space cannot be overflowed, otherwise the computation may result in an incorrect output.

In order to ensure the correctness of the computation, we track the maximum value encrypted in a ciphertext via an associated attribute called the degree. When the degree reaches a defined threshold, the carry buffer may be emptied to safely resume the computations. In shortint the carry modulus is considered useful as a means to do more computations.

Types of operations

The operations available via a ServerKey may come in different variants:

• operations that take their inputs as encrypted values

• scalar operations that take at least one non-encrypted value as input

For example, the addition has two variants:

• ServerKey::unchecked_add, which takes two encrypted values and adds them.

• ServerKey::unchecked_scalar_add, which takes an encrypted value and a clear value (a so-called scalar) and adds them.

Each operation may come in different 'flavors':

• unchecked: always does the operation, without checking if the result may exceed the capacity of the plaintext space. Using this operation might have an impact on the correctness of the following operations;

• checked: checks are done before computing the operation, returning an error if operation cannot be done safely;

• smart: always does the operation. If the operation cannot be computed safely, the smart operation will clear the carry to make the operation possible. Some of those will require a mutable reference as input: this is to allow the modification of the carry, but this will not change the underlying encrypted value;

• default: always does the operation and always clears the carry. Could be slower than smart, but it ensures that the timings are consistent from one call to another.

Not all operations have these 4 flavors, as some of them are implemented in a way that the operation is always possible without ever exceeding the plaintext space capacity.

How to use operation types

Let's try to do a circuit evaluation using the different flavors of operations that we have already introduced. For a very small circuit, the unchecked flavour may be enough to do the computation correctly. Otherwise,checked and smart are the best options.

Let's do a scalar multiplication, a subtraction, and a multiplication.

During this computation, the carry buffer has been overflowed and, as all the operations were unchecked, the output may be incorrect.

If we redo this same circuit with the checked flavor, a panic will occur:

The checked flavor permits manual management of the overflow of the carry buffer by raising an error if correctness is not guaranteed.

Using the smart flavor will output the correct result all the time. However, the computation may be slower as the carry buffer may be cleaned during the computations.

The main advantage of the default flavor is to ensure predictable timings as long as this is the only kind of operation which is used.

#List of available operations

The list of implemented operations for shortint is:

• addition between two ciphertexts

• addition between a ciphertext and an unencrypted scalar

• comparisons <, <=, >, >=, ==, != between a ciphertext and an unencrypted scalar

• division of a ciphertext by an unencrypted scalar

• LSB multiplication between two ciphertexts returning the result truncated to fit in the message buffer

• multiplication of a ciphertext by an unencrypted scalar

• bitwise shift <<, >>

• subtraction of a ciphertext by another ciphertext

• subtraction of a ciphertext by an unencrypted scalar

• negation of a ciphertext

• bitwise and, or and xor (*)

• comparisons <, <=, >, >=, ==, != between two ciphertexts (*)

• division between two ciphertexts (*)

• MSB multiplication between two ciphertexts returning the part overflowing the message buffer (*)

Public key encryption.

TFHE-rs supports both private and public key encryption methods. The only difference between both lies in the encryption step: in this case, the encryption method is called using public_key instead of client_key.

Here is a small example on how to use public encryption:

Arithmetic operations.

Classical arithmetic operations are supported by shortint:

bitwise operations

Short homomorphic integer types support some bitwise operations.

A simple example on how to use these operations:

comparisons

Short homomorphic integer types support comparison operations.

A simple example on how to use these operations:

univariate function evaluations

A simple example on how to use this operation to homomorphically compute the hamming weight (i.e., the number of bits equal to one) of an encrypted number.

bi-variate function evaluations

Using the shortint types offers the possibility to evaluate bi-variate functions, or functions that take two ciphertexts as input. This requires choosing a parameter set such that the carry buffer size is at least as large as the message (i.e., PARAM_MESSAGE_X_CARRY_Y with X <= Y).

Here is a simple code example:

integer does not come with its own set of parameters. Instead, it relies on parameters from shortint. Currently, parameter sets having the same space dedicated to the message and the carry (i.e. PARAM_MESSAGE_{X}_CARRY_{X} with X in [1,4]) are recommended. See for more details about cryptographic parameters, and to see how to properly instantiate integers depending on the chosen representation.

All parameter sets provide at least 128-bits of security according to the , with an error probability equal to when using programmable bootstrapping. This error probability is due to the randomness added at each encryption (see for more details about the encryption process).

Parameters and message precision

shortint comes with sets of parameters that permit the use of the library functionalities securely and efficiently. Each parameter set is associated to the message and carry precisions. Therefore, each key pair is entangled to precision.

The user is allowed to choose which set of parameters to use when creating the pair of keys.

The difference between the parameter sets is the total amount of space dedicated to the plaintext, how it is split between the message buffer and the carry buffer, and the order in which the keyswitch (KS) and bootstrap (PBS) are computed. The syntax chosen for the name of a parameter is: PARAM_MESSAGE_{number of message bits}_CARRY_{number of carry bits}_{KS_PBS | PBS_KS}. For example, the set of parameters for a message buffer of 5 bits, a carry buffer of 2 bits and where the keyswitch is computed before the bootstrap is PARAM_MESSAGE_5_CARRY_2_KS_PBS.

Note that the KS_PBS order should have better performance at the expense of ciphertext size, PBS_KS is the opposite.

This example contains keys that are generated to have messages encoded over 2 bits (i.e., computations are done modulus ) with 2 bits of carry.

The PARAM_MESSAGE_2_CARRY_2_KS_PBS parameter set is the default shortint parameter set that you can also use through the tfhe::shortint::prelude::DEFAULT_PARAMETERS constant.

Impact of parameters on the operations

Generic bi-variate functions.

The computations of bi-variate functions is based on a trick: concatenating two ciphertexts into one. Where the carry buffer is not at least as large as the message buffer, this trick no longer works. In this case, many bi-variate operations, such as comparisons, cannot be correctly computed. The only exception concerns multiplication.

Multiplication.

User-defined parameter sets

It is possible to define new parameter sets. To do so, it is sufficient to use the function new() or to manually fill the ClassicPBSParameters structure fields.

For instance:

As explained in the introduction, some types (Serverkey, Ciphertext) are meant to be shared with the server that performs the computations.

The easiest way to send these data to a server is to use the serialization and deserialization features. tfhe::shortint uses the framework. Serde's Serialize and Deserialize are then implemented on the tfhe::shortint types.

To serialize the data, we need to pick a . For our use case, is a good choice, mainly because it is a binary format.