Add ElGamal cipher

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Add `ElGamal` encryption module [#162]
+
+### Changed
+
+- Restructure `Encryption` module.
+
 ## [0.27.0] - 2024-04-24
 
 ### Added
@@ -284,6 +292,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Canonical implementation shielded by feature.
 
 <!-- ISSUES -->
+[#162]: https://github.com/dusk-network/phoenix-core/issues/162
 [#156]: https://github.com/dusk-network/phoenix-core/issues/156
 [#155]: https://github.com/dusk-network/phoenix-core/issues/155
 [#152]: https://github.com/dusk-network/phoenix-core/issues/152

Cargo.toml

@@ -11,6 +11,7 @@ exclude = [".github/workflows/dusk-ci.yml", ".gitignore"]
 [dependencies]
 rand_core = { version = "0.6", default-features = false }
 dusk-bytes = "0.1"
+dusk-plonk = { version = "0.19", default-features = false, optional = true }
 dusk-bls12_381 = { version = "0.13", default-features = false }
 bls12_381-bls = { version = "0.3", default-features = false }
 dusk-jubjub = { version = "0.14", default-features = false, features = ["zeroize"] }
@@ -40,3 +41,6 @@ rkyv-impl = [
     "rkyv",
     "bytecheck"
 ]
+zk = [
+    "dusk-plonk",
+]

src/encryption/elgamal.rs

@@ -0,0 +1,60 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+//
+// Copyright (c) DUSK NETWORK. All rights reserved.
+
+//! This module implements the ElGamal asymmetric cipher. It allows to
+//! encrypt, decrypt, and prove encryption in Zero-Knowledge of JubJub points.
+//!
+//! Reference: https://link.springer.com/chapter/10.1007/3-540-39568-7_2
+
+use dusk_jubjub::{JubJubExtended, JubJubScalar, GENERATOR};
+
+#[cfg(feature = "zk")]
+use dusk_plonk::prelude::*;
+
+/// Encrypts a JubJubExtended plaintext given a public key and a fresh random
+/// number 'r', returning a ciphertext (JubJubExtended, JubJubExtended)
+pub fn encrypt(
+    public_key: &JubJubExtended,
+    plaintext: &JubJubExtended,
+    r: &JubJubScalar,
+) -> (JubJubExtended, JubJubExtended) {
+    let ciphertext_1 = GENERATOR * r;
+    let ciphertext_2 = plaintext + public_key * r;
+
+    (ciphertext_1, ciphertext_2)
+}
+
+/// Decrypts a ciphertext given a secret key,
+/// returning a JubJubExtended plaintext
+pub fn decrypt(
+    secret_key: &JubJubScalar,
+    ciphertext_1: &JubJubExtended,
+    ciphertext_2: &JubJubExtended,
+) -> JubJubExtended {
+    ciphertext_2 - ciphertext_1 * secret_key
+}
+
+/// Encrypt in-circuit a plaintext, returning
+/// a ciphertext (WitnessPoint, WitnessPoint)
+#[cfg(feature = "zk")]
+pub fn zk_encrypt(
+    composer: &mut Composer,
+    public_key: &JubJubAffine,
+    plaintext: &JubJubAffine,
+    r: &JubJubScalar,
+) -> Result<(WitnessPoint, WitnessPoint), Error> {
+    // IMPORT INPUTS
+    let public_key = composer.append_point(*public_key);
+    let plaintext = composer.append_point(*plaintext);
+    let r = composer.append_witness(*r);
+
+    // ENCRYPT
+    let R = composer.component_mul_point(r, public_key);
+    let ciphertext_1 = composer.component_mul_generator(r, GENERATOR)?;
+    let ciphertext_2 = composer.component_add_point(plaintext, R);
+
+    Ok((ciphertext_1, ciphertext_2))
+}

src/encryption/mod.rs

@@ -0,0 +1,11 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+//
+// Copyright (c) DUSK NETWORK. All rights reserved.
+
+/// AES symmetric cipher
+pub mod aes;
+
+/// ElGamal asymmetric cipher
+pub mod elgamal;

src/lib.rs

@@ -27,11 +27,13 @@ pub mod note;
 /// Phoenix Core Keys & Addresses
 mod keys;
 
-/// Encryption and decryption methods
+/// Encryption algorithms
 mod encryption;
 
-/// Encryption and decryption methods
-pub use encryption::{decrypt, encrypt, ENCRYPTION_EXTRA_SIZE};
+/// AES symmetric cipher
+pub use encryption::aes;
+/// ElGamal asymmetric cipher
+pub use encryption::elgamal;
 /// Hash function
 pub use keys::hash;
 /// Public (Spend) Key

src/note.rs

@@ -14,7 +14,7 @@ use dusk_jubjub::{
     GENERATOR_NUMS_EXTENDED,
 };
 
-use crate::encryption::{decrypt, encrypt, ENCRYPTION_EXTRA_SIZE};
+use crate::aes;
 
 use dusk_poseidon::sponge::hash;
 use ff::Field;
@@ -31,7 +31,7 @@ pub(crate) const PLAINTEXT_SIZE: usize = 40;
 
 /// Size of the Phoenix notes encryption
 pub(crate) const ENCRYPTION_SIZE: usize =
-    PLAINTEXT_SIZE + ENCRYPTION_EXTRA_SIZE;
+    PLAINTEXT_SIZE + aes::ENCRYPTION_EXTRA_SIZE;
 
 /// The types of a Note
 #[derive(Debug, Clone, Copy, Eq, PartialEq)]
@@ -123,7 +123,7 @@ impl Note {
                 let mut plaintext = value.to_bytes().to_vec();
                 plaintext.append(&mut blinding_factor.to_bytes().to_vec());
 
-                encrypt(&shared_secret, &plaintext, rng)
+                aes::encrypt(&shared_secret, &plaintext, rng)
                     .expect("Encrypted correctly.")
             }
         };
@@ -200,7 +200,7 @@ impl Note {
         let shared_secret = dhke(vk.a(), R);
 
         let dec_plaintext: [u8; PLAINTEXT_SIZE] =
-            decrypt(&shared_secret, &self.encryption)?;
+            aes::decrypt(&shared_secret, &self.encryption)?;
 
         let value = u64::from_slice(&dec_plaintext[..u64::SIZE])?;
 

tests/encryption.rs

@@ -4,25 +4,127 @@
 //
 // Copyright (c) DUSK NETWORK. All rights reserved.
 
-use dusk_jubjub::{JubJubAffine, JubJubScalar, GENERATOR};
+use dusk_jubjub::{JubJubAffine, JubJubScalar, GENERATOR, GENERATOR_EXTENDED};
+use ff::Field;
 use rand_core::OsRng;
 
-use phoenix_core::{decrypt, encrypt, ENCRYPTION_EXTRA_SIZE};
+use phoenix_core::{aes, elgamal, PublicKey, SecretKey};
+
+#[cfg(feature = "zk")]
+use dusk_plonk::prelude::*;
+#[cfg(feature = "zk")]
+static LABEL: &[u8; 12] = b"dusk-network";
+#[cfg(feature = "zk")]
+const CAPACITY: usize = 12; // capacity required for the setup
+
+#[cfg(feature = "zk")]
+#[derive(Default, Debug)]
+pub struct ElGamalCircuit {
+    public_key: JubJubAffine,
+    plaintext: JubJubAffine,
+    r: JubJubScalar,
+    ciphertext_1: JubJubAffine,
+    ciphertext_2: JubJubAffine,
+}
+
+#[cfg(feature = "zk")]
+impl ElGamalCircuit {
+    pub fn new(
+        public_key: &JubJubExtended,
+        plaintext: &JubJubExtended,
+        r: &JubJubScalar,
+        ciphertext_1: &JubJubExtended,
+        ciphertext_2: &JubJubExtended,
+    ) -> Self {
+        Self {
+            public_key: JubJubAffine::from(public_key),
+            plaintext: JubJubAffine::from(plaintext),
+            r: *r,
+            ciphertext_1: JubJubAffine::from(ciphertext_1),
+            ciphertext_2: JubJubAffine::from(ciphertext_2),
+        }
+    }
+}
+
+#[cfg(feature = "zk")]
+impl Circuit for ElGamalCircuit {
+    fn circuit(&self, composer: &mut Composer) -> Result<(), Error> {
+        let (ciphertext_1, ciphertext_2) = elgamal::zk_encrypt(
+            composer,
+            &self.public_key,
+            &self.plaintext,
+            &self.r,
+        )?;
+
+        // ASSERT RESULT MAKING THE CIPHERTEXT PUBLIC
+        composer.assert_equal_public_point(ciphertext_1, self.ciphertext_1);
+        composer.assert_equal_public_point(ciphertext_2, self.ciphertext_2);
+
+        Ok(())
+    }
+}
 
 #[test]
-fn test_encrypt_and_decrypt() {
+fn test_aes_encrypt_and_decrypt() {
     const PLAINTEXT_SIZE: usize = 20;
-    const ENCRYPTION_SIZE: usize = PLAINTEXT_SIZE + ENCRYPTION_EXTRA_SIZE;
+    const ENCRYPTION_SIZE: usize = PLAINTEXT_SIZE + aes::ENCRYPTION_EXTRA_SIZE;
 
     let shared_secret_key =
         JubJubAffine::from(GENERATOR * JubJubScalar::from(1234u64));
 
     let plaintext = b"00112233445566778899";
     let encryption: [u8; ENCRYPTION_SIZE] =
-        encrypt(&shared_secret_key, plaintext, &mut OsRng)
+        aes::encrypt(&shared_secret_key, plaintext, &mut OsRng)
             .expect("Encrypted correctly.");
-    let dec_plaintext =
-        decrypt(&shared_secret_key, &encryption).expect("Decrypted correctly.");
+    let dec_plaintext = aes::decrypt(&shared_secret_key, &encryption)
+        .expect("Decrypted correctly.");
 
     assert_eq!(&dec_plaintext, plaintext);
 }
+
+#[test]
+fn test_elgamal_encrypt_and_decrypt() {
+    let sk = SecretKey::random(&mut OsRng);
+    let pk = PublicKey::from(&sk);
+
+    let message = GENERATOR_EXTENDED * JubJubScalar::from(1234u64);
+
+    // Encrypt using a fresh random value 'r'
+    let r = JubJubScalar::random(&mut OsRng);
+    let (c1, c2) = elgamal::encrypt(pk.A(), &message, &r);
+
+    // Assert decryption
+    let dec_message = elgamal::decrypt(sk.a(), &c1, &c2);
+    assert_eq!(message, dec_message);
+
+    // Assert decryption using an incorrect key
+    let dec_message_wrong = elgamal::decrypt(sk.b(), &c1, &c2);
+    assert_ne!(message, dec_message_wrong);
+}
+
+#[cfg(feature = "zk")]
+#[test]
+fn test_elgamal_zk_encrypt() {
+    let sk = SecretKey::random(&mut OsRng);
+    let pk = PublicKey::from(&sk);
+
+    let message = GENERATOR_EXTENDED * JubJubScalar::from(1234u64);
+    let r = JubJubScalar::random(&mut OsRng);
+    let (c1, c2) = elgamal::encrypt(pk.A(), &message, &r);
+
+    let pp = PublicParameters::setup(1 << CAPACITY, &mut OsRng).unwrap();
+
+    let (prover, verifier) = Compiler::compile::<ElGamalCircuit>(&pp, LABEL)
+        .expect("failed to compile circuit");
+
+    let (proof, public_inputs) = prover
+        .prove(
+            &mut OsRng,
+            &ElGamalCircuit::new(&pk.A(), &message, &r, &c1, &c2),
+        )
+        .expect("failed to prove");
+
+    verifier
+        .verify(&proof, &public_inputs)
+        .expect("failed to verify proof");
+}