Olm unwedging | Discrepency between what the spec says and rust-sdk implementation

[BillCarsonFr]

Reference: Olm unwedging MSC

When a device receives an olm-encrypted message that it cannot decrypt, it should assume that the olm session has become corrupted and create a new olm session to replace it. It should then send a dummy message, using that session, to the other party in order to inform them of the new session. To send a dummy message, clients may send an event with type m.dummy, and with empty contents.

As per spec:

If a client has multiple sessions established with another device, it should use the session from which it last received and successfully decrypted a message. For these purposes, a session that has not received any messages should use its creation time as the time that it last received a message.

However the rust sdk is not using storing any timestamp for "last received and successfully decrypted" message.

There is a last_used_time timestamp, that is updated after any encryption or decryption:

matrix-rust-sdk/crates/matrix-sdk-crypto/src/olm/session.rs

Lines 82 to 94 in dddc607

	pub async fn decrypt(&mut self, message: &OlmMessage) -> Result<String, DecryptionError> {
	let mut inner = self.inner.lock().await;
	Span::current().record("session_id", inner.session_id());
	let plaintext = inner.decrypt(message)?;
	debug!(session=?inner, "Decrypted an Olm message");
	let plaintext = String::from_utf8_lossy(&plaintext).to_string();
	self.last_use_time = SecondsSinceUnixEpoch::now();
	Ok(plaintext)
	}

matrix-rust-sdk/crates/matrix-sdk-crypto/src/olm/session.rs

Lines 126 to 132 in dddc607

	pub(crate) async fn encrypt_helper(&mut self, plaintext: &str) -> OlmMessage {
	let mut session = self.inner.lock().await;
	let message = session.encrypt(plaintext);
	self.last_use_time = SecondsSinceUnixEpoch::now();
	debug!(?session, "Successfully encrypted an event");
	message
	}

But this last_used_time doesn't appear to be used anywhere.

And to decide what session to use when encrypting, the rust-sdk relies on the session creation_time to pick up the best one:

matrix-rust-sdk/crates/matrix-sdk-crypto/src/identities/device.rs

Line 692 in dddc607

sessions.sort_by_key(|s| s.creation_time);

This issue is not to say that there could be an issue by using creation_time instead of last successful decryption time to pick up a session, but to note the difference with the spec.

Outcome could be to:

• Update the spec
• Make the rust-sdk uselast successful decryption time