messages sent while offline cannot be decrypted

[huguesdk]

Steps to reproduce

1. being offline, send several messages to an existing contact (to which you didn’t send a message in a little while, in my case, if it matters)
2. go back online, and wait for the messages to be sent

Outcome

what did you expect?

that the recipient could decrypt the messages

what happened instead?

the message could not be decrypted by the recipient. sending extra messages while being online did not solve the problem. i eventually used /discardsession and messages sent after that were able to be decrypted, but not the previous ones.

note that this happened with existing devices (sessions) that sent many messages successfully to each other for a long time. no new sessions are involved in this.

it seems to me that element created a new cryptographic session, but because it was offline, it could not ask the server for the device (session) list of the other user. having something like matrix-org/matrix-doc#3354 would probably help with this. 😉

this happened between users on separate homeservers: synapse 1.44.0 sending to synapse 1.33.2.

Your phone model

No response

Operating system version

android 9

Application version and app store

element 1.3.12 from google play

Homeserver

No response

Will you send logs?

No

[ouchadam]

I've also encountered this and it's caused by the senders device being offline during a key rotation.

The key requests fail along with the sending of the original messages (due to being offline), the failed messages will retry when the device reconnects but the keys either aren't sent or are sent too late, which ends up with the end user receiving encrypted messages without the keys needed to decrypt them