MLS support

[uhoreg]

• [x] initial plain MLS-over-Matrix test
• [x] initial Decentralized MLS test
• [x] backfilling MLS messages
• [x] comparison of MLS and Olm/Megolm
• [x] investigate suitability of OpenMLS for DMLS
  • write a wrapper library that implements DMLS
  • encrypt and decrypt messages with linear MLS group
  • merge multiple forks and decrypt
  • add/remove users with linear MLS group
  • add/remove users with forked MLS group
  • may need to (temporarily?) fork OpenMLS, or else write our own implementation
• [ ] trial implementation in a client (Hydrogen?/Element Web?)
  • possible synapse changes
• [ ] update proto-MSC and publish as an MSC
• [ ] add preliminary support to Rust matrix-sdk-crypto
• [ ] add support via matrix-sdk-crypto to clients
• [ ] support re-decryption of historical messages (MLS requires throwing away keys once they are used. But to re-decrypt messages, we need to keep them around. Since OpenMLS implements the MLS behaviour, this will require changes)
• [ ] determine necessary information for key forwarding/key backup
• [ ] key forwarding test
• [ ] update key backup endpoints as needed
• [ ] external commits (being able to send a message to the group before you've been added by a member)
• [ ] ...?
• [ ] profit!

[ara4n]

guess i should go register arewemlsyet.com? :P