vodozemac/megolm/group_session.rs
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// Copyright 2021 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.
use serde::{Deserialize, Serialize};
use super::{
default_config, message::MegolmMessage, ratchet::Ratchet, session_config::Version,
session_keys::SessionKey, SessionConfig,
};
use crate::{
cipher::Cipher,
types::Ed25519Keypair,
utilities::{pickle, unpickle},
PickleError,
};
/// A Megolm group session represents a single sending participant in an
/// encrypted group communication context containing multiple receiving parties.
///
/// A group session consists of a ratchet, used for encryption, and an Ed25519
/// signing key pair, used for authenticity.
///
/// A group session containing the signing key pair is also known as an
/// "outbound" group session. We differentiate this from an *inbound* group
/// session where this key pair has been removed and which can be used solely
/// for receipt and decryption of messages.
///
/// Such an inbound group session is typically sent by the outbound group
/// session owner to each of the receiving parties via a secure peer-to-peer
/// channel (e.g. an Olm channel).
pub struct GroupSession {
ratchet: Ratchet,
signing_key: Ed25519Keypair,
config: SessionConfig,
}
impl Default for GroupSession {
fn default() -> Self {
Self::new(Default::default())
}
}
impl GroupSession {
/// Construct a new group session, with a random ratchet state and signing
/// key pair.
pub fn new(config: SessionConfig) -> Self {
let signing_key = Ed25519Keypair::new();
Self { signing_key, ratchet: Ratchet::new(), config }
}
/// Returns the globally unique session ID, in base64-encoded form.
///
/// A session ID is the public part of the Ed25519 key pair associated with
/// the group session. Due to the construction, every session ID is
/// (probabilistically) globally unique.
pub fn session_id(&self) -> String {
self.signing_key.public_key().to_base64()
}
/// Return the current message index.
///
/// The message index is incremented each time a message is encrypted with
/// the group session.
pub const fn message_index(&self) -> u32 {
self.ratchet.index()
}
/// Get the [`SessionConfig`] that this [`GroupSession`] is configured
/// to use.
pub const fn session_config(&self) -> SessionConfig {
self.config
}
/// Encrypt the given `plaintext` with the group session.
///
/// The resulting ciphertext is MAC-ed, then signed with the group session's
/// Ed25519 key pair and finally base64-encoded.
pub fn encrypt(&mut self, plaintext: impl AsRef<[u8]>) -> MegolmMessage {
let cipher = Cipher::new_megolm(self.ratchet.as_bytes());
let message = match self.config.version {
Version::V1 => MegolmMessage::encrypt_truncated_mac(
self.message_index(),
&cipher,
&self.signing_key,
plaintext.as_ref(),
),
Version::V2 => MegolmMessage::encrypt_full_mac(
self.message_index(),
&cipher,
&self.signing_key,
plaintext.as_ref(),
),
};
self.ratchet.advance();
message
}
/// Export the group session into a session key.
///
/// The session key contains the key version constant, the current message
/// index, the ratchet state and the *public* part of the signing key pair.
/// It is signed by the signing key pair for authenticity.
///
/// The session key is in a portable format, suitable for sending over the
/// network. It is typically sent to other group participants so that they
/// can reconstruct an inbound group session in order to decrypt messages
/// sent by this group session.
pub fn session_key(&self) -> SessionKey {
let mut session_key = SessionKey::new(&self.ratchet, self.signing_key.public_key());
let signature = self.signing_key.sign(&session_key.to_signature_bytes());
session_key.signature = signature;
session_key
}
/// Convert the group session into a struct which implements
/// [`serde::Serialize`] and [`serde::Deserialize`].
pub fn pickle(&self) -> GroupSessionPickle {
GroupSessionPickle {
ratchet: self.ratchet.clone(),
signing_key: self.signing_key.clone(),
config: self.config,
}
}
/// Restore a [`GroupSession`] from a previously saved
/// [`GroupSessionPickle`].
pub fn from_pickle(pickle: GroupSessionPickle) -> Self {
pickle.into()
}
/// Creates a [`GroupSession`] object by unpickling a session in the legacy
/// libolm pickle format.
///
/// These pickles are encrypted and must be decrypted using the provided
/// `pickle_key`.
#[cfg(feature = "libolm-compat")]
pub fn from_libolm_pickle(
pickle: &str,
pickle_key: &[u8],
) -> Result<Self, crate::LibolmPickleError> {
use crate::{megolm::group_session::libolm_compat::Pickle, utilities::unpickle_libolm};
const PICKLE_VERSION: u32 = 1;
unpickle_libolm::<Pickle, _>(pickle, pickle_key, PICKLE_VERSION)
}
}
#[cfg(feature = "libolm-compat")]
mod libolm_compat {
use matrix_pickle::Decode;
use zeroize::{Zeroize, ZeroizeOnDrop};
use super::GroupSession;
use crate::{
megolm::{libolm::LibolmRatchetPickle, SessionConfig},
utilities::LibolmEd25519Keypair,
Ed25519Keypair,
};
#[derive(Zeroize, ZeroizeOnDrop, Decode)]
pub(super) struct Pickle {
version: u32,
ratchet: LibolmRatchetPickle,
ed25519_keypair: LibolmEd25519Keypair,
}
impl TryFrom<Pickle> for GroupSession {
type Error = crate::LibolmPickleError;
fn try_from(pickle: Pickle) -> Result<Self, Self::Error> {
// Removing the borrow doesn't work and clippy complains about
// this on nightly.
#[allow(clippy::needless_borrow)]
let ratchet = (&pickle.ratchet).into();
let signing_key =
Ed25519Keypair::from_expanded_key(&pickle.ed25519_keypair.private_key)?;
Ok(Self { ratchet, signing_key, config: SessionConfig::version_1() })
}
}
}
/// A format suitable for serialization which implements [`serde::Serialize`]
/// and [`serde::Deserialize`]. Obtainable by calling [`GroupSession::pickle`].
#[derive(Serialize, Deserialize)]
pub struct GroupSessionPickle {
ratchet: Ratchet,
signing_key: Ed25519Keypair,
#[serde(default = "default_config")]
config: SessionConfig,
}
impl GroupSessionPickle {
/// Serialize and encrypt the pickle using the given key.
///
/// This is the inverse of [`GroupSessionPickle::from_encrypted`].
pub fn encrypt(self, pickle_key: &[u8; 32]) -> String {
pickle(&self, pickle_key)
}
/// Obtain a pickle from a ciphertext by decrypting and deserializing using
/// the given key.
///
/// This is the inverse of [`GroupSessionPickle::encrypt`].
pub fn from_encrypted(ciphertext: &str, pickle_key: &[u8; 32]) -> Result<Self, PickleError> {
unpickle(ciphertext, pickle_key)
}
}
impl From<GroupSessionPickle> for GroupSession {
fn from(pickle: GroupSessionPickle) -> Self {
Self { ratchet: pickle.ratchet, signing_key: pickle.signing_key, config: pickle.config }
}
}
#[cfg(test)]
mod test {
use crate::megolm::{GroupSession, SessionConfig};
#[test]
fn create_with_session_config() {
assert_eq!(
GroupSession::new(SessionConfig::version_1()).session_config(),
SessionConfig::version_1()
);
assert_eq!(
GroupSession::new(SessionConfig::version_2()).session_config(),
SessionConfig::version_2()
);
}
}