What is being backed up to iCloud?

[aaronraimist]

@faenil:matrix.org noticed this a few days ago https://matrix.to/#/!pMBteVpcoJRdCJxDmn:matrix.org/$oJZGCUKoKELfzzljZ5Liq3PxQ8h9Qx-vybxdp6oLKwg?via=matrix.org&via=privacytools.io&via=tchncs.de

Steps to reproduce

1. Have iCloud Backup enabled
2. Login to Element iOS
3. Open Settings.app
4. Tap your name
5. Tap iCloud
6. Tap Manage Storage
7. Tap Backups
8. Tap on your device
9. Scroll down to Element

Outcome

What did you expect?

Element iOS shouldn't be backing up message contents, encryption keys, or media since to iCloud Backup since it is not end-to-end encrypted https://support.apple.com/en-us/HT202303

Alternatively if does backup things to iCloud, it should be encrypted by the app before it is backed up. I'm not sure what there would be to backup to iCloud though considering Matrix already has it's own key backup feature and the homeserver should be storing all of the other stuff.

What happened instead?

Element iOS has backed up a somewhat large amount of data. For me it was 58 MB.

I then sent a 6 MB image in an encrypted room and backed up to iCloud again. The backup increased to 88 MB.

I'm not sure what exactly is being backed up but I searched the repos for https://developer.apple.com/documentation/foundation/urlresourcekey/1408756-isexcludedfrombackupkey and didn't see anything marked as being excluded from backups. I'm guessing this means everything stored locally is being backed up.

For comparison my Signal iCloud Backup is only 300KB. It looks like they have excluded most stuff from being backed up signalapp/Signal-iOS@fb0281f.

Your phone model

No response

Operating system version

15.3

Application version

1.7.0

Homeserver

No response

Will you send logs?

No

[ara4n]

This looks like a thinko; we shouldn't be storing anything in iCloud backup.

[faenil]

@aaronraimist thanks for creating the report following my questions on the Matrix channel 🎩
I was planning to sync with the developers before raising the issue here, but thanks for taking the lead 😃 👍

First of all, I would like to thank the Element (and Matrix) teams for the awesome project they have created and all the effort they have put into it.
I have been visiting your booth at FOSDEM for a few years, I hope to see you there again next year, in person! 🤞

Back to the ticket:
I have been self-hosting my server for some time.
Some of the users on the server own iOS devices.

Given the core values of Matrix and Element, I fully trust the app not to share any data with iCloud (unless explicitly requested by the user).
However, I have recently borrowed an OS device for a few minutes to try and verify those assumptions myself.

Unfortunately I did not have the needed equip to do actual debugging, so I only did a few user-level tests.

I created a E2EE room with only myself and the (just created) iOS user and shared one high res video and about 10 pictures.

While running the experiment, I checked the iCloud storage overview in the Settings app on the iOS device and noticed that the iCloud data for Element kept increasing, as I sent more messages.

After sending the 10pics and the video, the Element iCloud backup was about 2MB (if I remember correctly).

Disclaimer: I am a developer/tinkerer, but no background on iOS :-)
I could not find any info on Element's website/docs about what data the iOS client is backing up to iCloud.
I went through Apple documentation for iCloud and CloudKit, but that did not lead to a clear answer.

I would like to propose the creation of a document which would provide info on:

• what data is backed up to iCloud and in what format (encrypted messages? thumbnails? metadata?)
• pointers to the component (snippets of source code, or at file/folder level) managing each data type
• options for admins to minimize the amount of data shared with iCloud (without trusting the user to disable iCloud backup for the app)

I will be happy to contribute to this effort as I can, toddler allowing :)
(no iOS dev experience though, so writing it from scratch won't be feasible, but I'll be happy to review, edit and/or provide feedback)

Thanks again for all your effort on this project!

[faenil]

Great! Thanks for working on this with high prio! :)