feat(sdk): Rotate session keys when a member leaves the room by kaylendog · Pull Request #6292 · matrix-org/matrix-rust-sdk

kaylendog · 2026-03-16T11:08:21Z

This PR adds an event handler on start that discards (and hence rotates) the current session whenever we see a leave membership event.

Part of element-hq/element-meta#3078

I've documented the public API Changes in the appropriate CHANGELOG.md files.
This PR was made with the help of AI.

Signed-off-by: Skye Elliot [email protected]

codspeed-hq · 2026-03-16T11:33:32Z

Merging this PR will not alter performance

✅ 50 untouched benchmarks

_{Comparing kaylendog/history-sharing/respect-visibility (9082cb9) with main (60ce062)¹}

No successful run was found on main (9082cb9) during the generation of this report, so 60ce062 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

kaylendog · 2026-03-16T11:48:06Z

crates/matrix-sdk/src/encryption/mod.rs

+            let Some(user_id) = client.user_id() else {
+                // We aren't logged in, so this shouldn't ever happen.
+                return;
+            };


I'm tempted to use unwrap here, but I am wary that if any of these assumptions are wrong, this will crash the client entirely.

codecov · 2026-03-16T12:40:39Z

Codecov Report

❌ Patch coverage is 65.21739% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.80%. Comparing base (60ce062) to head (9082cb9).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/encryption/mod.rs	63.63%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6292      +/-   ##
==========================================
- Coverage   89.81%   89.80%   -0.02%     
==========================================
  Files         376      376              
  Lines      102447   102469      +22     
  Branches   102447   102469      +22     
==========================================
+ Hits        92017    92022       +5     
- Misses       6850     6863      +13     
- Partials     3580     3584       +4

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

richvdh · 2026-03-16T13:13:09Z

@kaylendog in the description of the PR, could you link the issue that explains why we need to do this?

richvdh

mostly lgtm, a couple of small comments

crates/matrix-sdk/src/encryption/mod.rs

testing/matrix-sdk-integration-testing/src/tests/e2ee/shared_history.rs

crates/matrix-sdk/src/encryption/mod.rs

richvdh

LGTM otherwise.

I do think it's worth discussing the potential sliding sync a bit further

Not quite sure what you're referring to here.

richvdh · 2026-03-18T12:48:00Z

testing/matrix-sdk-integration-testing/src/tests/e2ee/shared_history.rs

+/// Test that when a user leaves a room that uses history sharing, the room key
+/// is rotated so they cannot decrypt future messages.


This is the same description as test_history_share_on_invite_room_key_rotation. Better add an explanation of the difference.

kaylendog · 2026-03-18T13:57:39Z

LGTM otherwise.

I do think it's worth discussing the potential sliding sync a bit further

Not quite sure what you're referring to here.

With EX in particular, sliding sync may mean we don't get all of the state updates that happened while we were offline, which might mean we miss a join & leave pair and skip rotating the room key?

Regardless of any issues, I don't think they would belong in this PR and can come later.

Signed-off-by: Skye Elliot <[email protected]>

This method is now only used by tests, so I opted to lock it behind the test configuration to appease CI. Signed-off-by: Skye Elliot <[email protected]>

Signed-off-by: Skye Elliot <[email protected]>

andybalaam · 2026-03-19T09:33:04Z

... sliding sync may mean we don't get all of the state updates that happened while we were offline, which might mean we miss a join & leave pair and skip rotating the room key?

If I understand right, in sliding sync, we should get at least one state event for each piece of state that changed inside required_state https://github.com/matrix-org/matrix-spec-proposals/blob/erikj/sss/proposals/4186-simplified-sliding-sync.md#currently-or-previously-joined-rooms , assuming the client asked for m.room.member events, which it should in this case.

kaylendog self-assigned this Mar 16, 2026

kaylendog force-pushed the kaylendog/history-sharing/respect-visibility branch 2 times, most recently from d15f1b2 to 11a27af Compare March 16, 2026 11:25

kaylendog force-pushed the kaylendog/history-sharing/respect-visibility branch from 11a27af to f0c2d33 Compare March 16, 2026 11:46

kaylendog commented Mar 16, 2026

View reviewed changes

kaylendog force-pushed the kaylendog/history-sharing/respect-visibility branch from f0c2d33 to f0d12dd Compare March 16, 2026 12:30

kaylendog marked this pull request as ready for review March 16, 2026 12:42

kaylendog requested a review from a team as a code owner March 16, 2026 12:42

kaylendog requested review from poljar and removed request for a team March 16, 2026 12:42

richvdh requested changes Mar 16, 2026

View reviewed changes

richvdh reviewed Mar 16, 2026

View reviewed changes

crates/matrix-sdk/src/encryption/mod.rs Outdated Show resolved Hide resolved

kaylendog requested a review from a team as a code owner March 17, 2026 14:44

kaylendog force-pushed the kaylendog/history-sharing/respect-visibility branch from 02f3839 to 177f38a Compare March 17, 2026 15:17

kaylendog requested a review from richvdh March 18, 2026 11:47

poljar removed their request for review March 18, 2026 12:43

richvdh approved these changes Mar 18, 2026

View reviewed changes

kaylendog added 5 commits March 18, 2026 14:25

feat(sdk): Rotate session key when member leaves the room

bfe24d9

Signed-off-by: Skye Elliot <[email protected]>

docs(sdk): Outline key rotation scenario on discard handler

ba9e876

Signed-off-by: Skye Elliot <[email protected]>

refactor(crypto): Lock GroupSessionCache::get to cfg(test)

635fdd3

This method is now only used by tests, so I opted to lock it behind the test configuration to appease CI. Signed-off-by: Skye Elliot <[email protected]>

tests: Assert room key is rotated when a member leaves the room

7a26db6

Signed-off-by: Skye Elliot <[email protected]>

tests: Assert room key rotated on leave event after being offline

fd806a9

Signed-off-by: Skye Elliot <[email protected]>

kaylendog force-pushed the kaylendog/history-sharing/respect-visibility branch from 0a9cc5d to fd806a9 Compare March 18, 2026 14:25

docs(sdk): Update CHANGELOG.md

9082cb9

kaylendog enabled auto-merge March 18, 2026 14:31

kaylendog merged commit 81bed18 into main Mar 18, 2026
52 checks passed

kaylendog deleted the kaylendog/history-sharing/respect-visibility branch March 18, 2026 14:44

kaylendog mentioned this pull request Mar 18, 2026

History sharing: sessions are not rotated after a shared-with user leaves element-hq/element-meta#3078

Open

2 tasks

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

feat(sdk): Rotate session keys when a member leaves the room#6292

feat(sdk): Rotate session keys when a member leaves the room#6292
kaylendog merged 6 commits intomainfrom
kaylendog/history-sharing/respect-visibility

kaylendog commented Mar 16, 2026 •

edited

Loading

Uh oh!

codspeed-hq bot commented Mar 16, 2026 •

edited

Loading

Uh oh!

kaylendog Mar 16, 2026

Uh oh!

codecov bot commented Mar 16, 2026 •

edited

Loading

Uh oh!

richvdh commented Mar 16, 2026

Uh oh!

richvdh left a comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

richvdh left a comment

Uh oh!

richvdh Mar 18, 2026

Uh oh!

kaylendog commented Mar 18, 2026 •

edited

Loading

Uh oh!

Uh oh!

andybalaam commented Mar 19, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

		/// Test that when a user leaves a room that uses history sharing, the room key
		/// is rotated so they cannot decrypt future messages.

Conversation

kaylendog commented Mar 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

codspeed-hq bot commented Mar 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Merging this PR will not alter performance

Footnotes

Uh oh!

kaylendog Mar 16, 2026

Choose a reason for hiding this comment

Uh oh!

codecov bot commented Mar 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codecov Report

Uh oh!

richvdh commented Mar 16, 2026

Uh oh!

richvdh left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

richvdh left a comment

Choose a reason for hiding this comment

Uh oh!

richvdh Mar 18, 2026

Choose a reason for hiding this comment

Uh oh!

kaylendog commented Mar 18, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Uh oh!

andybalaam commented Mar 19, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

kaylendog commented Mar 16, 2026 •

edited

Loading

codspeed-hq bot commented Mar 16, 2026 •

edited

Loading

codecov bot commented Mar 16, 2026 •

edited

Loading

kaylendog commented Mar 18, 2026 •

edited

Loading