Element can't decrypt some messages despite another active session having them and being connected to key backup

[davidmehren]

Steps to reproduce

1. I logged in to a new Element Web session
2. I verified that session using my existing Element Desktop session
3. I scrolled up in a room

Outcome

What did you expect?

All messages that the old session can decrypt, can be decrypted in the new session

What happened instead?

The new session can not decrypt many old messages

Operating system

Fedora 42

Browser information

Firefox 139.0b3

URL for webapp

element.kif.rocks

Application version

Element: 1.11.99 Krypto-Version: Rust SDK 0.10.0 (3cc301d), Vodozemac 0.9.0

Homeserver

kif.rocks

Will you send logs?

Yes

[davidmehren]

Added rageshakes from both Element sessions

[davidmehren]

Note: Fractal and Nheko can both decrypt the messages in question, but seem to be using gossiping.

[andybalaam]

I see the new client initialising backup here and it looks fine:

2025-05-04T11:19:40.789Z D Checking key backup status...
2025-05-04T11:19:40.789Z D FetchHttpApi: --> GET https://redacted/_matrix/client/v3/room_keys/version
2025-05-04T11:19:40.912Z D FetchHttpApi: <-- GET https://redacted/_matrix/client/v3/room_keys/version [123ms 200]
2025-05-04T11:19:40.919Z D Found usable key backup v3: enabling key backups
2025-05-04T11:19:40.919Z I INFO matrix_sdk_crypto::backups: Activated a backup
    backup_key=MegolmV1BackupKey { key: "redacted", version: Some("3") }
    at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/matrix-sdk-crypto-0.10.0/src/backups/mod.rs:390
2025-05-04T11:19:40.919Z D Backup: Starting keys upload loop for backup version:3.
2025-05-04T11:19:40.920Z D [PerSessionKeyBackupDownloader] Got current backup version from server: 3

[andybalaam]

In the new client, I see lots of This message was sent before this device logged in. Key backup is working, but we still do not (yet) have the key which is expected initially, and indicates that the key backup is (supposedly ) working.

[andybalaam]

Still on the new client, soon afterwards, I see this:

2025-05-04T11:19:42.033Z D [PerSessionKeyBackupDownloader] Got key from backup for sessionId:TacETC
2025-05-04T11:19:42.034Z I INFO matrix_sdk_crypto::store: Successfully imported room keys
    total_count=0 imported_count=0 room_keys={}
    at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/matrix-sdk-crypto-0.10.0/src/store/mod.rs:1867

which is interesting, because it says we imported a key, but the total_count=0 part looks suspicious.

[andybalaam]

In both the bad and the good client, recent attempts to download messages keys are reporting total_count=0.

[andybalaam]

In the new/bad client:

We had a backup at version 1:

2025-05-04T09:13:37.513Z D [PerSessionKeyBackupDownloader] Got current backup version from server: 1

Then someone else changed it (?) so we didn't trust it:

2025-05-04T09:24:05.603Z D [PerSessionKeyBackupDownloader] Got current backup version from server: 3
...
2025-05-04T09:24:05.604Z I [PerSessionKeyBackupDownloader] The current backup version on the server (3) is not trusted. Version we are currently backing up to: null

Then it starts claiming it works again, still version 3:

2025-05-04T09:24:05.604Z I [PerSessionKeyBackupDownloader] The current backup version on the server (3) is not trusted. Version we are currently backing up to: null
...
2025-05-04T09:33:49.115Z D [PerSessionKeyBackupDownloader] Got key from backup for sessionId:Tac6MQcyL8jhWHfvUq1LILXKIV/0jhaUTeN2fVXWsok

But actually it sees every backup as empty.

[andybalaam]

These total_count=0 lines come from us failing to deserialise keys we have downloaded from backup at https://github.com/matrix-org/matrix-rust-sdk-crypto-wasm/blob/main/src/machine.rs#L1086 . Unfortunately, we don't log what went wrong when this happens because we don't pass any progressListener in at https://github.com/matrix-org/matrix-js-sdk/blob/develop/src/rust-crypto/PerSessionKeyBackupDownloader.ts#L401

I am not sure how common/expected this is.

[andybalaam]

@davidmehren I don't suppose you are using FluffyChat in addition to Element Web & Desktop are you?

[davidmehren]

I'm not currently using it, but I sometimes try another client for a few days, so I surely have used Fluffychat with this account in the past.