Device dehydration v2

[BillCarsonFr]

User story

As a user, if I log out my last device, other users should still be able to send messages to me and I should be able to read them once I come back and log in with some device.

Concept

Initial device dehydration feature has several issues:

• Tricky to implement, as it requires that the device rehydration be done before any other API calls
• User loses the advantage of verifying their other devices via emoji or QR code (passphrase needed anyhow to rehydrate)

It's now replaced with MSC3814.

Tasks

Groundwork

• Ruma support for MSC3814 #1898
• Rust crypto SDK Support for MSC3814 - Dehydrated devices v2 matrix-org/matrix-rust-sdk#2349
• https://github.com/vector-im/crypto-internal/issues/126
• MSC updates/Polishing MSC3814: Document new dehydrated device format (probably just a concatenation of the various bits of information needed, similar to existing pickle format) #2602
• Synapse support part 2 Support MSC3814: Dehydrated Devices Part 2 matrix-org/synapse#16010
• Fix endpoint improperly declaring support for MSC3814 matrix-org/synapse#16068

Initial Dehydrated device support

• IOS SDK Device dehydration v2 matrix-org/matrix-ios-sdk#1807
• iOS App Device dehydration v2 element-ios#7630
• WebR full support for dehydrated device v2 #2278
• Implement new dehydration format for Dehydrated Devices #2601
• KeySharing, DevicebasedStrategy | Exclude insecure dehydrated devices when sending messages matrix-org/matrix-rust-sdk#4313
• Dehydrated devices: drop incoming to-device messages from dehydrated devices matrix-org/matrix-rust-sdk#4466
• Device Dehydration: Cache Dehydration Key #2603

Production ready on Web

• Dehydrated devices: drop .well-known check element-web#29387
• Dehydrated devices: gossip the dehydration key between devices when you verify #2703
• Dehydrated devices: gossip the dehydration key to verified devices when you create/change the dehydration key #2753
• Dehydrated devices: API change POST->GET #2704
• Dehydrated devices: API change to make paging consistent with other APIs #2799
• Dehydrated devices: when resetting SSSS, if some prior step fails, we won't reset device dehydration element-web#29084
• Dehydrated devices: Add some stuff to the crypto dev tools element-web#29081
• Device dehydration: upgrade existing user accounts to add a dehydrated device element-web#29080
• Device dehydration: users should be able to see their own dehydrated device(s) element-web#29264
• Dehydrated devices: does not detect if another client changed the dehydration key in 4S element-web#28760
• Device dehydration: detect and handle when user has a dehydrated device for which we don't have the key element-web#29579
• Device dehydration: state explicitly on UI that recovery enables/disables dehydration #2828

Future work

• Rust SDK: Support the resumption of the rehydration process #1978
• Deprecate and remove legacy device dehydration (MSC2697)
• EX: indicate when I have a dehydrated device #2635
• EX full support for dehydrated device v2
• https://github.com/element-hq/crypto-internal/issues/402

[pmaier1]

Design/UX inputs needed for settings and option to enable disable

As discussed today, we want to make this feature transparent to the user as far as possible. As it requires key backup being enabled, we'd bind the availability of dehydrated devices to the setting to enable/disable key backup.

[emmaburton1]

Work on this can be timesheeted to the project on https://element-io.atlassian.net/browse/PSB-87

[poljar]

So a status update here:
Ruma PR: ruma/ruma#1605
Rust SDK branch: https://github.com/matrix-org/matrix-rust-sdk/tree/poljar/device-dehydration
Synapse branch that can be used for testing: https://github.com/poljar/synapse/tree/poljar/msc3814-v2

@stefanceriu above are the main ingredients that you'll need to get started on this on the iOS side, the Rust SDK branch depends on the Ruma PR, so you only need the later two.

[poljar]

Work on this can be timesheeted to the project on https://element-io.atlassian.net/browse/PSB-87

Where exactly do I find this in the myhours thingy? I can't seem to find a PSB-87 in the task list.

[pmaier1]

Where exactly do I find this in the myhours thingy? I can't seem to find a PSB-87 under the BWI task list.

It should just be the title of this ticket.

[emmaburton1]

@poljar I've added the task to relevant customer retainer project.

[pmaier1]

Considering that device dehydration

• will first be a labs feature and we probably can't ask all the end users to turn it on over and over again (when logging out and logging in again)
• the server-side support will also be an experimental feature at first

can we automatically enable the feature on the client if the server-side setting is enabled? Probably doesn't make sense to have two switches to enable it and this way we could control availability centrally.

[t3chguy]

@pmaier1 we don't tend to have HS-driven labs features due to the potential for data loss or work preventing bugs whilst thinking you are running a stable release of Element

[pmaier1]

then we need a separate switch on the backend to force enabling the labs feature on the client. otherwise this will be unusable.

[t3chguy]

Element web/desktop already support Such via config.json but not via anything HS driven.