This crate provides a straightforward implementation of the ElGamal cryptosystem over the ristretto255 elliptic curve group using the curve25519-dalek crate. This crate is not intended for general developers, but rather for cryptographers implementing protocols that use ElGamal.

Should I use ElGamal?

The main reason to use the ElGamal cryptosystem is when one needs the homomorphic property: given messages m1 and m2, the sum of their encryptions is an encryption of m1 + m2. Here, addition is defined in the elliptic curve group; you may be more used to ElGamal over a finite field, in which the homomorphism is multiplicative.

In other cases, ElGamal is generally not suitable. This is because it is not secure against chosen-ciphertext attacks (a direct consequence of the homomorphic property).

Warning: while the author of this crate is educated in cryptography, they make no guarantees as to the security of the implementation. Use at your own risk.

Acknowledgements

The author thanks Henry de Valence for his feedback and suggestions.

Use

To import rust-elgamal, add the following dependency to your Cargo.toml file:

rust-elgamal = "0.2"

Because this crate is in a pre-release state (major version 0), minor versions may introduce breaking changes. Thus, you should not use rust_elgamal = "0".

Example

use rand::rngs::StdRng;
use rand::SeedableRng;
use rust_elgamal::{DecryptionKey, Scalar, GENERATOR_TABLE};

const N: usize = 100;

let mut rng = StdRng::from_entropy();
let dec_key = DecryptionKey::new(&mut rng);
let enc_key = dec_key.encryption_key();

let message = &Scalar::from(5u32) * &GENERATOR_TABLE;
let encrypted = enc_key.encrypt(message, &mut rng);
let decrypted = dec_key.decrypt(encrypted);
assert_eq!(message, decrypted);

Features

• std (enabled by default): include the Rust standard library. Disable this feature for embedded environments.
• enable-serde: Turn on serde support.
• simd_backend: This crate uses curve25519-dalek to implement elliptic curve group operations. With nightly Rust, curve25519-dalek supports SIMD (single-instruction, multiple-data) instructions for extra efficiency. Enable this feature with a nightly version of Rust to use SIMD operations (either avx2 or ifma). For example:

# Requires nightly, RUSTFLAGS="-C target_feature=+avx2" to use avx2
cargo build --features "simd_backend"
# Requires nightly, RUSTFLAGS="-C target_feature=+avx512ifma" to use ifma
cargo build --features "simd_backend"

• nightly: used for building documentation and for SIMD support.