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Ensure that we're not creating the same session twice

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Usually it's impossible to create a Olm session from a pre-key message
twice. The one-time key that should be used for the 3DH step will be
used up and we're going to throw a `MissingOneTimeKey` error.

This used to be true and unproblematic until we added fallback keys,
these keys will not get discarded immediately after they have been used
once.

This means that a pre-key message, for which we already have a Session,
but decryption for it fails, might create a new Session overwriting the
existing one which will essentially reset the ratchet.
This commit is contained in:
Damir Jelić
2023-06-16 15:27:38 +02:00
parent 817dcde4d8
commit a94e2f06ee
1 changed files with 18 additions and 0 deletions
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18
crates/matrix-sdk-crypto/src/olm/account.rs
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@@ -273,6 +273,24 @@ impl Account {
if let Ok(p) = session.decrypt(message).await {
decrypted = Some((session.clone(), p));
break;
} else if let OlmMessage::PreKey(message) = message {
if message.session_id() == session.session_id() {
// The message was intended for this session, but we weren't able to decrypt it.
//
// We're going to return early here since no other session will be able to
// decrypt this message, nor should we try to create a new one since we already
// created a `Session` with such a pre-key message.
//
// Creating a new session will likely fail since the one-time key that should be
// used to establish the session will be used up.
//
// The one exception where creating a new session won't fail is if a fallback
// key was used for this `Session`.
return Err(OlmError::SessionWedged(
session.user_id.to_owned(),
session.sender_key(),
));
}
} else {
// An error here is completely normal, after all we don't know
// which session was used to encrypt a message. We will log a