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Types & Operations
TFHE-rs includes two main types to represent encrypted data:FheUint: this is the homomorphic equivalent of Rust unsigned integersu8, u16, ...FheInt: this is the homomorphic equivalent of Rust (signed) integersi8, i16, ...
In the same manner as many programming languages, the number of bits used to represent the data must be chosen when declaring a variable. For instance:
// let clear_a: u64 = 7;
let mut a = FheUint64::try_encrypt(clear_a, &keys)?;
// let clear_b: i8 = 3;
let mut b = FheInt8::try_encrypt(clear_b, &keys)?;
// let clear_c: u128 = 2;
let mut c = FheUint128::try_encrypt(clear_c, &keys)?;
The table below contains an overview of the available operations in
TFHE-rs. The notation Enc (for Encypted) either refers to FheInt or FheUint, for any size between 1 and 256-bits.More details, and further examples, are given in the following sections.
name | symbol | Enc/Enc | Enc/ Int |
Neg | - | ✔ | ✔ |
Add | + | ✔ | ✔ |
Sub | - | ✔ | ✔ |
Mul | * | ✔ | ✔ |
Div | / | ✔ | ✔ |
Rem | % | ✔ | ✔ |
Not | ! | ✔ | ✔ |
BitAnd | & | ✔ | ✔ |
BitOr | | | ✔ | ✔ |
BitXor | ^ | ✔ | ✔ |
Shr | >> | ✔ | ✔ |
Shl | << | ✔ | ✔ |
Min | min | ✔ | ✔ |
Max | max | ✔ | ✔ |
Greater than | gt | ✔ | ✔ |
Greater or equal than | ge | ✔ | ✔ |
Lower than | lt | ✔ | ✔ |
Lower or equal than | le | ✔ | ✔ |
Equal | eq | ✔ | ✔ |
Cast (into dest type) | cast_into | ✔ | ✖ |
Cast (from src type) | cast_from | ✔ | ✖ |
Ternary operator | if_then_else | ✔ | ✖ |
In
TFHE-rs, integers are used to encrypt all messages which are larger than 4 bits. All supported operations are listed below.Homomorphic integer types support arithmetic operations.
The list of supported operations is:
For division by 0, the convention is to return
modulus - 1. For instance, for FheUint8, the modulus is , so a division by 0 will return an encryption of 255. For the remainder operator, the convention is to return the first input without any modification. For instance, if
ct1 = FheUint8(63) and ct2 = FheUint8(0) then ct1 % ct2 will return FheUint8(63).A simple example of how to use these operations:
use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheInt8, FheUint8};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let config = ConfigBuilder::all_disabled().enable_default_integers().build();
let (keys, server_keys) = generate_keys(config);
set_server_key(server_keys);
let clear_a = 15_u64;
let clear_b = 27_u64;
let clear_c = 43_u64;
let clear_d = -87_i64;
let mut a = FheUint8::try_encrypt(clear_a, &keys)?;
let mut b = FheUint8::try_encrypt(clear_b, &keys)?;
let mut c = FheUint8::try_encrypt(clear_c, &keys)?;
let mut d = FheInt8::try_encrypt(clear_d, &keys)?;
a = a * &b; // Clear equivalent computations: 15 * 27 mod 256 = 149
b = &b + &c; // Clear equivalent computations: 27 + 43 mod 256 = 70
b = b - 76u8; // Clear equivalent computations: 70 - 76 mod 256 = 250
d = d - 13i8; // Clear equivalent computations: -87 - 13 = 100 in [-128, 128[
let dec_a: u8 = a.decrypt(&keys);
let dec_b: u8 = b.decrypt(&keys);
let dec_d: i8 = d.decrypt(&keys);
assert_eq!(dec_a, ((clear_a * clear_b) % 256_u64) as u8);
assert_eq!(dec_b, (((clear_b + clear_c).wrapping_sub(76_u64)) % 256_u64) as u8);
assert_eq!(dec_d, (clear_d - 13) as i8);
Ok(())
}
Homomorphic integer types support some bitwise operations.
The list of supported operations is:
name | symbol | type |
|---|---|---|
! | Unary | |
& | Binary | |
| | Binary | |
^ | Binary | |
>> | Binary | |
<< | Binary | |
rotate_right | Binary | |
rotate_left | Binary |
A simple example of how to use these operations:
use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint8};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let config = ConfigBuilder::all_disabled().enable_default_integers().build();
let (keys, server_keys) = generate_keys(config);
set_server_key(server_keys);
let clear_a = 164;
let clear_b = 212;
let mut a = FheUint8::try_encrypt(clear_a, &keys)?;
let mut b = FheUint8::try_encrypt(clear_b, &keys)?;
a = a ^ &b;
b = b ^ &a;
a = a ^ &b;
let dec_a: u8 = a.decrypt(&keys);
let dec_b: u8 = b.decrypt(&keys);
// We homomorphically swapped values using bitwise operations
assert_eq!(dec_a, clear_b);
assert_eq!(dec_b, clear_a);
Ok(())
}
Homomorphic integers support comparison operations.
Due to some Rust limitations, it is not possible to overload the comparison symbols because of the inner definition of the operations. This is because Rust expects to have a Boolean as an output, whereas a ciphertext is returned when using homomorphic types.
You will need to use different methods instead of using symbols for the comparisons. These methods follow the same naming conventions as the two standard Rust traits:
The list of supported operations is:
name | symbol | type |
|---|---|---|
eq | Binary | |
ne | Binary | |
gt | Binary | |
ge | Binary | |
lt | Binary | |
le | Binary |
A simple example of how to use these operations:
use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheInt8};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let config = ConfigBuilder::all_disabled().enable_default_integers().build();
let (keys, server_keys) = generate_keys(config);
set_server_key(server_keys);
let clear_a: i8 = -121;
let clear_b: i8 = 87;
let mut a = FheInt8::try_encrypt(clear_a, &keys)?;
let mut b = FheInt8::try_encrypt(clear_b, &keys)?;
let greater = a.gt(&b);
let greater_or_equal = a.ge(&b);
let lower = a.lt(&b);
let lower_or_equal = a.le(&b);
let equal = a.eq(&b);
let dec_gt: i8 = greater.decrypt(&keys);
let dec_ge: i8 = greater_or_equal.decrypt(&keys);
let dec_lt: i8 = lower.decrypt(&keys);
let dec_le: i8 = lower_or_equal.decrypt(&keys);
let dec_eq: i8 = equal.decrypt(&keys);
// We homomorphically swapped values using bitwise operations
assert_eq!(dec_gt, (clear_a > clear_b ) as i8);
assert_eq!(dec_ge, (clear_a >= clear_b) as i8);
assert_eq!(dec_lt, (clear_a < clear_b ) as i8);
assert_eq!(dec_le, (clear_a <= clear_b) as i8);
assert_eq!(dec_eq, (clear_a == clear_b) as i8);
Ok(())
}
Homomorphic integers support the min/max operations.
name | symbol | type |
|---|---|---|
Min | min | Binary |
Max | max | Binary |
A simple example of how to use these operations:
use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint8};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let config = ConfigBuilder::all_disabled().enable_default_integers().build();
let (keys, server_keys) = generate_keys(config);
set_server_key(server_keys);
let clear_a:u8 = 164;
let clear_b:u8 = 212;
let mut a = FheUint8::try_encrypt(clear_a, &keys)?;
let mut b = FheUint8::try_encrypt(clear_b, &keys)?;
let min = a.min(&b);
let max = a.max(&b);
let dec_min : u8 = min.decrypt(&keys);
let dec_max : u8 = max.decrypt(&keys);
// We homomorphically swapped values using bitwise operations
assert_eq!(dec_min, u8::min(clear_a, clear_b));
assert_eq!(dec_max, u8::max(clear_a, clear_b));
Ok(())
}
The ternary conditional operator allows computing conditional instructions of the form
if cond { choice_if } else { choice_else }.name | symbol | type |
|---|---|---|
Ternary operator | if_then_else | Ternary |
The syntax is
encrypted_condition.if_then_else(encrypted_choice_if, encrypted_choice_else). The encrypted_condition should be an encryption of 0 or 1 in order to be valid.use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheInt32};
fn main() -> Result<(), Box<dyn std::error::Error>> {
// Basic configuration to use homomorphic integers
let config = ConfigBuilder::all_disabled()
.enable_default_integers()
.build();
// Key generation
let (client_key, server_keys) = generate_keys(config);
let clear_a = 32i32;
let clear_b = -45i32;
// Encrypting the input data using the (private) client_key
// FheInt32: Encrypted equivalent to i32
let encrypted_a = FheInt32::try_encrypt(clear_a, &client_key)?;
let encrypted_b = FheInt32::try_encrypt(clear_b, &client_key)?;
// On the server side:
set_server_key(server_keys);
// Clear equivalent computations: 32 > -45
let encrypted_comp = &encrypted_a.gt(&encrypted_b);
let clear_res: i32 = encrypted_comp.decrypt(&client_key);
assert_eq!(clear_res, (clear_a > clear_b) as i32);
// `encrypted_comp` contains the result of the comparison, i.e.,
// a boolean value. This acts as a condition on which the
// `if_then_else` function can be applied on.
// Clear equivalent computations:
// if 32 > -45 {result = 32} else {result = -45}
let encrypted_res = &encrypted_comp.if_then_else(&encrypted_a, &encrypted_b);
let clear_res: i32 = encrypted_res.decrypt(&client_key);
assert_eq!(clear_res, clear_a);
Ok(())
}
Casting between integer types is possible via the
cast_from associated function or the cast_into method.use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheInt16, FheUint8, FheUint32, FheUint16};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let config = ConfigBuilder::all_disabled()
.enable_default_integers()
.build();
let (client_key, server_key) = generate_keys(config);
// Casting requires server_key to set
// (encryptions/decryptions do not need server_key to be set)
set_server_key(server_key);
{
let clear = 12_837u16;
let a = FheUint16::encrypt(clear, &client_key);
// Downcasting
let a: FheUint8 = a.cast_into();
let da: u8 = a.decrypt(&client_key);
assert_eq!(da, clear as u8);
// Upcasting
let a: FheUint32 = a.cast_into();
let da: u32 = a.decrypt(&client_key);
assert_eq!(da, (clear as u8) as u32);
}
{
let clear = 12_837u16;
let a = FheUint16::encrypt(clear, &client_key);
// Upcasting
let a = FheUint32::cast_from(a);
let da: u32 = a.decrypt(&client_key);
assert_eq!(da, clear as u32);
// Downcasting
let a = FheUint8::cast_from(a);
let da: u8 = a.decrypt(&client_key);
assert_eq!(da, (clear as u32) as u8);
}
{
let clear = 12_837i16;
let a = FheInt16::encrypt(clear, &client_key);
// Casting from FheInt16 to FheUint16
let a = FheUint16::cast_from(a);
let da: u16 = a.decrypt(&client_key);
assert_eq!(da, clear as u16);
}
Ok(())
}
Native small homomorphic integer types (e.g., FheUint3 or FheUint4) can easily compute various operations. In general, computing over encrypted data in
TFHE-rs is as easy as computing over clear data, since the same operation symbol is used. The addition between two ciphertexts is done using the symbol + between two FheUint values. Many operations can be computed between a clear value (i.e. a scalar) and a ciphertext.In Rust, operations on native types are modular. For example, computations on
u8 are carried out modulo . A similar idea applies for FheUintX, where operations are done modulo
. For FheUint3, operations are done modulo
.
Small homomorphic integer types support all common arithmetic operations, meaning
+, -, x, /, mod.The division operation presents a subtlety: since data is encrypted, it is possible to compute a division by 0. In this case, the division is tweaked so that dividing by 0 returns the maximum possible value for the message.
The list of supported operations is:
A simple example of how to use these operations:
use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint3};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let config = ConfigBuilder::all_disabled().enable_default_uint3().build();
let (keys, server_keys) = generate_keys(config);
set_server_key(server_keys);
let clear_a = 7;
let clear_b = 3;
let clear_c = 2;
let mut a = FheUint3::try_encrypt(clear_a, &keys)?;
let mut b = FheUint3::try_encrypt(clear_b, &keys)?;
let mut c = FheUint3::try_encrypt(clear_c, &keys)?;
a = a * &b; // Clear equivalent computations: 7 * 3 mod 8 = 5
b = &b + &c; // Clear equivalent computations: 3 + 2 mod 8 = 5
b = b - 5; // Clear equivalent computations: 5 - 5 mod 8 = 0
let dec_a = a.decrypt(&keys);
let dec_b = b.decrypt(&keys);
// We homomorphically swapped values using bitwise operations
assert_eq!(dec_a, (clear_a * clear_b) % 8);
assert_eq!(dec_b, ((clear_b + clear_c) - 5) % 8);
Ok(())
}
Small homomorphic integer types support some bitwise operations.
The list of supported operations is:
name | symbol | type |
|---|---|---|
! | Unary | |
& | Binary | |
| | Binary | |
^ | Binary | |
>> | Binary | |
<< | Binary | |
rotate_right | Binary | |
rotate_left | Binary |
A simple example of how to use these operations:
use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint3};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let config = ConfigBuilder::all_disabled().enable_default_uint3().build();
let (keys, server_keys) = generate_keys(config);
set_server_key(server_keys);
let clear_a = 7;
let clear_b = 3;
let mut a = FheUint3::try_encrypt(clear_a, &keys)?;
let mut b = FheUint3::try_encrypt(clear_b, &keys)?;
a = a ^ &b;
b = b ^ &a;
a = a ^ &b;
let dec_a = a.decrypt(&keys);
let dec_b = b.decrypt(&keys);
// We homomorphically swapped values using bitwise operations
assert_eq!(dec_a, clear_b);
assert_eq!(dec_b, clear_a);
Ok(())
}
Small homomorphic integer types support comparison operations.
Due to some Rust limitations, it is not possible to overload the comparison symbols because of the inner definition of the operations. Rust expects to have a Boolean as an output, whereas a ciphertext is returned when using homomorphic types.
You will need to use different methods instead of using symbols for the comparisons. These methods follow the same naming conventions as the two standard Rust traits:
The list of supported operations is:
name | symbol | type |
|---|---|---|
eq | Binary | |
ne | Binary | |
gt | Binary | |
ge | Binary | |
lt | Binary | |
le | Binary |
A simple example of how to use these operations:
use tfhe::prelude::*;
use tfhe::{generate_keys, set_server_key, ConfigBuilder, FheUint3};
fn main() -> Result<(), Box<dyn std::error::Error>> {
let config = ConfigBuilder::all_disabled().enable_default_uint3().build();
let (keys, server_keys) = generate_keys(config);
set_server_key(server_keys);
let clear_a = 7;
let clear_b = 3;
let mut a = FheUint3::try_encrypt(clear_a, &