We change the protocol to accomodate publishing multiple bundles, in
order to propagate bundles for group chats and have a way to extend it
further.
This commit re-introduces backward compatibility for direct messages,
to be removed once that is not an issue anymore.
Change to support sending multiple bundles, as needed for group chats,
limit number of devices to 3 as already done in the UI and refresh
bundle daily.
- Skipped keys
The purpose of limiting the number of skipped keys generated is to avoid a dos
attack whereby an attacker would send a large N, forcing the device to
compute all the keys between currentN..N .
Previously the logic for handling skipped keys was:
- If in the current receiving chain there are more than maxSkip keys,
throw an error
This is problematic as in long-lived session dropped/unreceived messages starts
piling up, eventually reaching the threshold (1000 dropped/unreceived
messages).
This logic has been changed to be more inline with signals spec, and now
it is:
- If N is > currentN + maxSkip, throw an error
The purpose of limiting the number of skipped keys stored is to avoid a dos
attack whereby an attacker would force us to store a large number of
keys, filling up our storage.
Previously the logic for handling old keys was:
- Once you have maxKeep ratchet steps, delete any key from
currentRatchet - maxKeep.
This, in combination with the maxSkip implementation, capped the number of stored keys to
maxSkip * maxKeep.
The logic has been changed to:
- Keep a maximum of MaxMessageKeysPerSession
and additionally we delete any key that has a sequence number <
currentSeqNum - maxKeep
- Version
We check now the version of the bundle so that when we get a bundle from
the same installationID with a higher version, we mark the previous
bundle as expired and use the new bundle the next time a message is sent
Andrea Maria Piana
Ivan Daniluk