Client Side Certificate Auth

Due to security restrictions in some companies, the Matrix server is only accessible behind a VPN. It requires VPN to be always enabled on the phone. However, sometimes, a client side certificate auth is also available. iOS supports natively client side certificate auth for Apple apps using the main Keychain. Unfortunately, other apps can't access to this Keychain. Instead, they must implement their own certificate "keychain". It would be great to see this in Riot iOS (and Android) app !

Some references:
https://stackoverflow.com/questions/3749429/how-to-use-client-certificate-authentication-in-ios-app
https://stackoverflow.com/questions/30174052/swift-ios-client-certificate-authentication

Thanks

The issue names no files or tests; begin by reviewing the iOS and Android authentication entry points and the two linked client-certificate references. Confirm the required certificate storage and connection behavior with maintainers, then define done as client-certificate authentication working for the stated Matrix server scenario on the supported mobile platforms.

Written by the indexing model from the issue text.