60 lines
2.8 KiB
Markdown
60 lines
2.8 KiB
Markdown
# Olm unwedging
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Olm sessions sometimes get out of sync, resulting in undecryptable messages.
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This can happen for several reasons. For example, if a user restores their
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client state from a backup, the client will be using an old ratchet state
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([riot-web#3822](https://github.com/vector-im/riot-web/issues/3822)). Or a
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client might expire a one-time key that another client is trying to use
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([riot-web#3309](https://github.com/vector-im/riot-web/issues/3309)). This
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proposal documents a method for devices to create a new session to replace the
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broken session.
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## Proposal
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When a device receives an olm-encrypted message that it cannot decrypt, it
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should assume that the olm session has become corrupted and create a new olm
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session to replace it. It should then send a dummy message, using that
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session, to the other party in order to inform them of the new session. To
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send a dummy message, clients may send an event with type `m.dummy`, and with
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empty contents.
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In order to avoid creating too many extra sessions, a client should rate-limit
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the number of new sessions it creates per device that it receives a message
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from; the client should not create a new session with another device if it has
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already created one for that given device in the past 1 hour.
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Clients may wish to take steps to mitigate the loss of the undecryptable
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messages. For example, megolm sessions that were sent using the old session
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would have been lost, so the client can send
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[`m.room_key_request`](https://matrix.org/docs/spec/client_server/r0.6.1.html#m-room-key-request)
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messages to re-request any megolm sessions that it is unable to decrypt.
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The spec currently says, "If a client has multiple sessions established with
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another device, it should use the session from which it last received a
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message." (the last paragraph of the [`m.olm.v1.curve25519-aes-sha2`
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section](https://matrix.org/docs/spec/client_server/r0.4.0.html#m-olm-v1-curve25519-aes-sha2)).
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When comparing the time of the last received message for each session, the
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client should only consider messages that were successfully decrypted,
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and for sessions that have never received a message, it should use the creation
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time of the session. The spec will be changed to read:
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> If a client has multiple sessions established with another device, it should
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> use the session from which it last received and successfully decrypted a
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> message. For these purposes, a session that has not received any messages
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> should use its creation time as the time that it last received a message.
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## Tradeoffs
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## Potential issues
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## Security considerations
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An attacker could use this to create a new session on a device that they are
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able to read. However, this would require the attacker to have compromised the
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device's keys.
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## Conclusion
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This proposal outlines how wedged olm sessions can be replaced by a new
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session.
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