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// Copyright 2024 The Matrix.org Foundation C.I.C.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//! Implementation of an integrated encryption scheme.
//!
//! This module implements
//! [ECIES](https://en.wikipedia.org/wiki/Integrated_Encryption_Scheme), the
//! elliptic curve variant of the Integrated Encryption Scheme. This is a hybrid
//! encryption scheme, using elliptic curve Diffie-Hellman for shared secret
//! establishment and a symmetric algorithm for encryption of individual
//! messages. It is instantiated with X25519 (Curve25519-based Diffie-Hellman),
//! HMAC-SHA256 as the KDF and ChaCha20-Poly1305 for symmetric encryption.
//!
//! ECIES allows a party (the initiator) to establish a communication channel
//! toward another party (the recipient) given knowledge of only its public key.
//! We assume that this key was obtained in a secure way. This implies that the
//! initiator side is able to tell for sure whether there is an active MITM
//! attack in progress once the channel is established.
//!
//! On the other hand, the initiator's key pair is ephemeral and generated anew
//! for each new channel. This implies the initiator must send their ephemeral
//! public key to the recipient *unauthenticated* so that the recipient can
//! complete the channel establishment on its end. From this it follows that the
//! recipient has no way of knowing who is contacting them, allowing for active
//! MITM attacks on the recipient side.
//!
//! In order to close this vector, an out-of-band confirmation is required to be
//! sent from the initiator device to the recipient device, after which the
//! channel is considered *secure*. The module provides the [`CheckCode`]
//! facility which can be used for this purpose.
//!
//! Throughout this document, we use a naming convention which designates the
//! device initiating an ECIES channel as device S, while the device on the
//! other side (towards which the channel is opened) is designated device G.
//!
//! # Examples
//!
//! ```
//! use vodozemac::ecies::{Ecies, InboundCreationResult, OutboundCreationResult};
//!
//! let plaintext = b"It's a secret to everybody";
//!
//! let alice = Ecies::new();
//! let bob = Ecies::new();
//!
//! let OutboundCreationResult { ecies: mut alice, message } = alice
//! .establish_outbound_channel(bob.public_key(), plaintext)?;
//!
//! let InboundCreationResult { mut ecies, message } = bob
//! .establish_inbound_channel(&message)
//! .expect("We should be able to create an inbound channel");
//!
//! assert_eq!(
//! message, plaintext,
//! "The decrypted plaintext should match our initial plaintext"
//! );
//!
//! // We now exchange the check code out-of-band and compare it.
//! if alice.check_code() != ecies.check_code() {
//! panic!("The check code must match; possible active MITM attack in progress");
//! }
//!
//! let message = ecies.encrypt(b"Another plaintext");
//! let decrypted = alice.decrypt(&message)?;
//!
//! assert_eq!(decrypted, b"Another plaintext");
//! # Ok::<(), anyhow::Error>(())
//! ```
use chacha20poly1305::{aead::Aead, ChaCha20Poly1305, Key as Chacha20Key, KeyInit, Nonce};
use hkdf::Hkdf;
use rand::thread_rng;
use sha2::Sha512;
use thiserror::Error;
use x25519_dalek::{EphemeralSecret, SharedSecret};
use zeroize::{Zeroize, ZeroizeOnDrop};
pub use self::messages::{InitialMessage, Message, MessageDecodeError};
use crate::Curve25519PublicKey;
mod messages;
const MATRIX_QR_LOGIN_INFO_PREFIX: &str = "MATRIX_QR_CODE_LOGIN";
/// The Error type for the ECIES submodule.
#[derive(Debug, Error)]
pub enum Error {
/// At least one of the keys did not have contributory behaviour and the
/// resulting shared secret would have been insecure.
#[error("At least one of the keys did not have contributory behaviour")]
NonContributoryKey,
/// Message decryption failed. Either the message was corrupted, the message
/// was replayed, or the wrong key is being used to decrypt the message.
#[error("Failed decrypting the message")]
Decryption,
}
/// A nonce that is used for the [`EstablishedEcies`] channel.
///
/// The nonce is internally represented as a [`u128`]. Each time a new value is
/// retrieved, the counter will get incremented.
struct EciesNonce {
inner: u128,
}
impl EciesNonce {
/// Create a new [`EciesNonce`], starting the count from 0.
const fn new() -> Self {
Self { inner: 0 }
}
/// Get the next nonce value.
///
/// This will increment the underlying counter and return a 12 byte
/// [`Nonce`] value.
fn get(&mut self) -> Nonce {
let current = self.inner;
let (new_nonce, _) = self.inner.overflowing_add(1);
self.inner = new_nonce;
let mut nonce = [0u8; 12];
nonce.copy_from_slice(¤t.to_le_bytes()[..12]);
Nonce::from_exact_iter(nonce)
.expect("We should be able to construct the correct nonce from a 12 byte slice")
}
}
/// A check code that can be used to confirm that two [`EstablishedEcies`]
/// objects share the same secret. This is supposed to be shared out-of-band to
/// protect against active MITM attacks.
///
/// Since the initiator device can always tell whether a MITM attack is in
/// progress after channel establishment, this code technically carries only a
/// single bit of information, representing whether the initiator has determined
/// that the channel is "secure" or "not secure".
///
/// However, given this will need to be interactively confirmed by the user,
/// there is risk that the user would confirm the dialogue without paying
/// attention to its content. By expanding this single bit into a deterministic
/// two-digit check code, the user is forced to pay more attention by having to
/// enter it instead of just clicking through a dialogue.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct CheckCode {
bytes: [u8; 2],
}
impl CheckCode {
/// Convert the check code to an array of two bytes.
///
/// The bytes can be converted to a more user-friendly representation. The
/// [`CheckCode::to_digit`] converts the bytes to a two-digit number.
pub const fn as_bytes(&self) -> &[u8; 2] {
&self.bytes
}
/// Convert the check code to two base-10 numbers.
///
/// The number should be displayed with a leading 0 in case the first digit
/// is a 0.
///
/// # Examples
///
/// ```no_run
/// # use vodozemac::ecies::CheckCode;
/// # let check_code: CheckCode = unimplemented!();
/// let check_code = check_code.to_digit();
///
/// println!("The check code of the IECS channel is: {check_code:02}");
/// ```
pub const fn to_digit(&self) -> u8 {
let first = (self.bytes[0] % 10) * 10;
let second = self.bytes[1] % 10;
first + second
}
}
/// The result of an inbound ECIES channel establishment.
#[derive(Debug)]
pub struct InboundCreationResult {
/// The established ECIES channel.
pub ecies: EstablishedEcies,
/// The plaintext of the initial message.
pub message: Vec<u8>,
}
/// The result of an outbound ECIES channel establishment.
#[derive(Debug)]
pub struct OutboundCreationResult {
/// The established ECIES channel.
pub ecies: EstablishedEcies,
/// The initial message.
pub message: InitialMessage,
}
/// An unestablished ECIES session.
pub struct Ecies {
secret_key: EphemeralSecret,
application_info_prefix: String,
}
/// The possible device roles for an ECIES channel, indicating whether the
/// device is initiating the channel or receiving/responding as the other side
/// of the initiation.
#[derive(Debug, Clone, Copy)]
enum Role {
Initiator,
Recipient,
}
impl Ecies {
/// Create a new, random, unestablished ECIES session.
///
/// This method will use the `MATRIX_QR_CODE_LOGIN` info. If you are using
/// this for a different purpose, consider using the [`Ecies::with_info()`]
/// method.
#[allow(clippy::new_without_default)]
pub fn new() -> Self {
Self::with_info(MATRIX_QR_LOGIN_INFO_PREFIX)
}
/// Create a new, random, unestablished ECIES session with the given
/// application info.
///
/// The application info will be used to derive the various secrets and
/// provide domain separation.
pub fn with_info(info: &str) -> Self {
let rng = thread_rng();
let secret_key = EphemeralSecret::random_from_rng(rng);
let application_info_prefix = info.to_owned();
Self { secret_key, application_info_prefix }
}
/// Create an [`EstablishedEcies`] session using the other side's Curve25519
/// public key and an initial plaintext.
///
/// After the channel has been established, we can encrypt messages to send
/// to the other side. The other side uses the initial message to
/// establishes the same channel on its side.
pub fn establish_outbound_channel(
self,
their_public_key: Curve25519PublicKey,
initial_plaintext: &[u8],
) -> Result<OutboundCreationResult, Error> {
let our_public_key = self.public_key();
let shared_secret = self.secret_key.diffie_hellman(&their_public_key.inner);
if shared_secret.was_contributory() {
let mut ecies = EstablishedEcies::new(
&shared_secret,
our_public_key,
their_public_key,
&self.application_info_prefix,
Role::Initiator,
);
let message = ecies.encrypt(initial_plaintext);
let message =
InitialMessage { public_key: our_public_key, ciphertext: message.ciphertext };
Ok(OutboundCreationResult { ecies, message })
} else {
Err(Error::NonContributoryKey)
}
}
/// Create a [`EstablishedEcies`] from an [`InitialMessage`] encrypted by
/// the other side.
pub fn establish_inbound_channel(
self,
message: &InitialMessage,
) -> Result<InboundCreationResult, Error> {
let our_public_key = self.public_key();
let shared_secret = self.secret_key.diffie_hellman(&message.public_key.inner);
if shared_secret.was_contributory() {
let mut ecies = EstablishedEcies::new(
&shared_secret,
our_public_key,
message.public_key,
&self.application_info_prefix,
Role::Recipient,
);
let nonce = ecies.decryption_nonce.get();
let message = ecies.decrypt_helper(&nonce, &message.ciphertext)?;
Ok(InboundCreationResult { ecies, message })
} else {
Err(Error::NonContributoryKey)
}
}
/// Get our [`Curve25519PublicKey`].
///
/// This public key needs to be sent to the other side to be able to
/// establish an ECIES channel.
pub fn public_key(&self) -> Curve25519PublicKey {
Curve25519PublicKey::from(&self.secret_key)
}
}
/// An established ECIES session.
///
/// This session can be used to encrypt and decrypt messages between the two
/// sides of the channel.
#[derive(Zeroize, ZeroizeOnDrop)]
pub struct EstablishedEcies {
/// Our own Curve25519 public key which was used to establish the ECIES
/// channel.
#[zeroize(skip)]
our_public_key: Curve25519PublicKey,
/// The other side's Curve25519 public key which was used to establish the
/// ECIES channel.
#[zeroize(skip)]
their_public_key: Curve25519PublicKey,
/// A counter which we'll use to create a [`Nonce`] every time we want to
/// encrypt a message.
#[zeroize(skip)]
encryption_nonce: EciesNonce,
/// A counter which we'll use to create a [`Nonce`] every time we want to
/// decrypt a message. The other side uses an analogous counter to encrypt
/// messages.
#[zeroize(skip)]
decryption_nonce: EciesNonce,
/// The key used to encrypt our messages.
encryption_key: Box<[u8; 32]>,
/// The key used by the other party to encrypt messages.
decryption_key: Box<[u8; 32]>,
/// The check code, generated on both devices and shared out-of-band, which
/// needs to match to ensure both sides are using the same secret.
#[zeroize(skip)]
check_code: CheckCode,
/// Our device's role in the ECIES channel, i.e. are we the initiator
/// (device S) or the recipient (device G)?
#[zeroize(skip)]
role: Role,
}
impl std::fmt::Debug for EstablishedEcies {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.debug_struct("EstablishedEcies")
.field("our_public_key", &self.our_public_key)
.field("their_public_key", &self.their_public_key)
.field("check_code", &self.check_code)
.field("role", &self.role)
.finish()
}
}
impl EstablishedEcies {
fn create_check_code(
shared_secret: &SharedSecret,
our_public_key: Curve25519PublicKey,
their_public_key: Curve25519PublicKey,
info: &str,
role: Role,
) -> CheckCode {
let mut bytes = [0u8; 2];
let kdf: Hkdf<Sha512> = Hkdf::new(None, shared_secret.as_bytes());
let info = Self::get_check_code_info(info, role, our_public_key, their_public_key);
kdf.expand(info.as_bytes(), bytes.as_mut_slice())
.expect("We should be able to expand the shared secret into a 32 byte key.");
CheckCode { bytes }
}
fn create_key(info: &str, shared_secret: &SharedSecret) -> Box<[u8; 32]> {
let mut key = Box::new([0u8; 32]);
let kdf: Hkdf<Sha512> = Hkdf::new(None, shared_secret.as_bytes());
kdf.expand(info.as_bytes(), key.as_mut_slice())
.expect("We should be able to expand the shared secret into a 32 byte key.");
key
}
/// Create the encryption key for messages we send into the channel.
fn create_encryption_key(
shared_secret: &SharedSecret,
our_public_key: Curve25519PublicKey,
their_public_key: Curve25519PublicKey,
app_info: &str,
role: Role,
) -> Box<[u8; 32]> {
let info = Self::get_encryption_key_info(app_info, role, our_public_key, their_public_key);
Self::create_key(&info, shared_secret)
}
/// Create the decryption key for messages received from the other side of
/// the channel.
///
/// The decryption key for G is the encryption key for S and vice versa.
fn create_decryption_key(
shared_secret: &SharedSecret,
our_public_key: Curve25519PublicKey,
their_public_key: Curve25519PublicKey,
app_info: &str,
role: Role,
) -> Box<[u8; 32]> {
let info = Self::get_decryption_key_info(app_info, role, our_public_key, their_public_key);
Self::create_key(&info, shared_secret)
}
fn new(
shared_secret: &SharedSecret,
our_public_key: Curve25519PublicKey,
their_public_key: Curve25519PublicKey,
app_info: &str,
role: Role,
) -> Self {
let (encryption_nonce, decryption_nonce) = (EciesNonce::new(), EciesNonce::new());
let encryption_key = Self::create_encryption_key(
shared_secret,
our_public_key,
their_public_key,
app_info,
role,
);
let decryption_key = Self::create_decryption_key(
shared_secret,
our_public_key,
their_public_key,
app_info,
role,
);
let check_code = Self::create_check_code(
shared_secret,
our_public_key,
their_public_key,
app_info,
role,
);
Self {
encryption_key,
decryption_key,
encryption_nonce,
decryption_nonce,
our_public_key,
their_public_key,
check_code,
role,
}
}
/// Get our [`Curve25519PublicKey`].
///
/// This public key needs to be sent to the other side so that it can
/// complete the ECIES channel establishment.
pub const fn public_key(&self) -> Curve25519PublicKey {
self.our_public_key
}
/// Get the [`CheckCode`] which uniquely identifies this
/// [`EstablishedEcies`] session.
///
/// This check code can be used to check that both sides of the session are
/// indeed using the same shared secret.
pub const fn check_code(&self) -> &CheckCode {
&self.check_code
}
fn encryption_key(&self) -> &Chacha20Key {
Chacha20Key::from_slice(self.encryption_key.as_slice())
}
fn decryption_key(&self) -> &Chacha20Key {
Chacha20Key::from_slice(self.decryption_key.as_slice())
}
/// Encrypt the given plaintext using this [`EstablishedEcies`] session.
pub fn encrypt(&mut self, plaintext: &[u8]) -> Message {
let nonce = self.encryption_nonce.get();
let cipher = ChaCha20Poly1305::new(self.encryption_key());
let ciphertext = cipher.encrypt(&nonce, plaintext).expect(
"We should always be able to encrypt a message since we provide the correct nonce",
);
Message { ciphertext }
}
/// Decrypt the given message using this [`EstablishedEcies`] session.
pub fn decrypt(&mut self, message: &Message) -> Result<Vec<u8>, Error> {
let nonce = self.decryption_nonce.get();
self.decrypt_helper(&nonce, &message.ciphertext)
}
fn decrypt_helper(&self, nonce: &Nonce, ciphertext: &[u8]) -> Result<Vec<u8>, Error> {
let cipher = ChaCha20Poly1305::new(self.decryption_key());
let plaintext = cipher.decrypt(nonce, ciphertext).map_err(|_| Error::Decryption)?;
Ok(plaintext)
}
fn get_check_code_info(
app_info: &str,
role: Role,
our_public_key: Curve25519PublicKey,
their_public_key: Curve25519PublicKey,
) -> String {
let partial_info = format!("{app_info}_CHECKCODE");
Self::construct_info_string(&partial_info, role, our_public_key, their_public_key)
}
fn get_encryption_key_info(
app_info: &str,
role: Role,
our_public_key: Curve25519PublicKey,
their_public_key: Curve25519PublicKey,
) -> String {
let partial_info = match role {
Role::Initiator => format!("{app_info}_ENCKEY_S"),
Role::Recipient => format!("{app_info}_ENCKEY_G"),
};
Self::construct_info_string(&partial_info, role, our_public_key, their_public_key)
}
fn get_decryption_key_info(
app_info: &str,
role: Role,
our_public_key: Curve25519PublicKey,
their_public_key: Curve25519PublicKey,
) -> String {
// The decryption key for G is the encryption key for S and vice versa.
let partial_info = match role {
Role::Initiator => format!("{app_info}_ENCKEY_G"),
Role::Recipient => format!("{app_info}_ENCKEY_S"),
};
Self::construct_info_string(&partial_info, role, our_public_key, their_public_key)
}
fn construct_info_string(
partial_info: &str,
role: Role,
our_public_key: Curve25519PublicKey,
their_public_key: Curve25519PublicKey,
) -> String {
match role {
Role::Recipient => {
// we are Device G. Gp = our_public_key, Sp = their_public_key
format!(
"{partial_info}|{}|{}",
our_public_key.to_base64(),
their_public_key.to_base64(),
)
}
Role::Initiator => {
// we are Device S. Gp = their_public_key, Sp = our_public_key
format!(
"{partial_info}|{}|{}",
their_public_key.to_base64(),
our_public_key.to_base64(),
)
}
}
}
}
#[cfg(test)]
mod test {
use proptest::prelude::*;
use super::*;
#[test]
fn channel_creation() {
let plaintext = b"It's a secret to everybody";
let alice = Ecies::new();
let bob = Ecies::new();
let OutboundCreationResult { ecies: mut alice, message } = alice
.establish_outbound_channel(bob.public_key(), plaintext)
.expect("We should be able to create an outbound channel");
let InboundCreationResult { ecies: mut bob, message } = bob
.establish_inbound_channel(&message)
.expect("We should be able to create an inbound channel");
assert_eq!(
message, plaintext,
"The decrypted plaintext should match our initial plaintext"
);
assert_eq!(alice.check_code(), bob.check_code());
assert_eq!(alice.check_code().to_digit(), bob.check_code().to_digit());
let message = bob.encrypt(b"Another plaintext");
let decrypted =
alice.decrypt(&message).expect("We should be able to decrypt the second message");
assert_eq!(decrypted, b"Another plaintext");
}
#[test]
fn invalid_check_code() {
let plaintext = b"It's a secret to everybody";
let alice = Ecies::new();
let bob = Ecies::new();
let malory = Ecies::new();
let OutboundCreationResult { mut message, .. } = alice
.establish_outbound_channel(bob.public_key(), plaintext)
.expect("We should be able to create an outbound channel");
message.public_key = malory.public_key();
bob.establish_inbound_channel(&message).expect_err(
"The decryption should fail since Malory inserted the \
wrong public key into the message",
);
}
#[test]
fn nonce() {
let mut nonce = EciesNonce::new();
assert_eq!(nonce.inner, 0, "The nonce should start the counter from 0");
let first = nonce.get();
assert_eq!(
nonce.inner, 1,
"After the first nonce is returned, the counter should have been incremented"
);
assert_eq!(first.as_slice(), [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
let second = nonce.get();
assert_eq!(
nonce.inner, 2,
"After the first nonce is returned, the counter should have been incremented"
);
assert_eq!(second.as_slice(), [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])
}
#[test]
fn check_code() {
let check_code = CheckCode { bytes: [0x0, 0x0] };
let digit = check_code.to_digit();
assert_eq!(digit, 0, "Two zero bytes should generate a 0 digit");
assert_eq!(
check_code.as_bytes(),
&[0x0, 0x0],
"CheckCode::as_bytes() should return the exact bytes we generated."
);
let check_code = CheckCode { bytes: [0x9, 0x9] };
let digit = check_code.to_digit();
assert_eq!(
check_code.as_bytes(),
&[0x9, 0x9],
"CheckCode::as_bytes() should return the exact bytes we generated."
);
assert_eq!(digit, 99);
let check_code = CheckCode { bytes: [0xff, 0xff] };
let digit = check_code.to_digit();
assert_eq!(
check_code.as_bytes(),
&[0xff, 0xff],
"CheckCode::as_bytes() should return the exact bytes we generated."
);
assert_eq!(digit, 55, "u8::MAX should generate 55");
}
#[test]
fn test_info_construction() {
use crate::types::Curve25519Keypair;
let app_info = "foobar";
let our_public_key = Curve25519Keypair::new().public_key;
let their_public_key = Curve25519Keypair::new().public_key;
let check_code_info1 = EstablishedEcies::get_check_code_info(
app_info,
Role::Initiator,
our_public_key,
their_public_key,
);
assert_eq!(
check_code_info1,
format!("foobar_CHECKCODE|{their_public_key}|{our_public_key}")
);
let check_code_info2 = EstablishedEcies::get_check_code_info(
app_info,
Role::Recipient,
our_public_key,
their_public_key,
);
assert_eq!(
check_code_info2,
format!("foobar_CHECKCODE|{our_public_key}|{their_public_key}")
);
}
proptest! {
#[test]
fn check_code_proptest(bytes in prop::array::uniform2(0u8..) ) {
let check_code = CheckCode {
bytes
};
let digit = check_code.to_digit();
prop_assert!(
(0..=99).contains(&digit),
"The digit should be in the 0-99 range"
);
}
}
}