Support mtls on IOS

How to use GitHub

• Please use the 👍 reaction to show that you are interested into the same feature.
• Please don't comment if you have no relevant information to add. It's just extra noise for everyone subscribed to this issue.
• Subscribe to receive notifications on status change and new comments.

Feature request

Is your feature request related to a problem? Please describe.
The Nextcloud app on IOS cannot authenticate against a webserver/proxy using mtls. This is due to the restrictions of IOS. Installed certificates work only with system apps (e.g. safari). Nextcloud app has no access to the certificate storage of IOS and thus cannot authenticate. On Android this just works fine.

Describe the solution you'd like
Implement mtls authentication for Nextcloud on IOS. One should be able to import an mtls certificate chain (PKCS12) on a per account basis. Alternatively Nextcloud must be able to pick the right certificate for an account.

Describe alternatives you've considered
We use mtls like a vpn. Without a valid client certificate there is no way of communicating with the Nextcloud server. Other vpn solutions are no alternative as the vpn has to be always on (battery usage) and it introduces a whole lot of other problems.

Additional context
Funding the necessary development is possible.

No file or test is named in the issue, so first locate the iOS account authentication flow and certificate handling. Define the account-level certificate import or selection behavior, then verify that an account using a PKCS12 client certificate can authenticate through an mTLS-protected server.