🔑 Use SSL Client Certificate to improve security

Expected behaviour

Option to configure a Nextcloud account to include an SSL User's Private Key and Certificate to connect to the server.

The use of an SSL Client certificate greatly improves the security. It protects the SSL connection against SSL decryptors deployed here and there and many other threats. It also improves the security in the mobile device by moving the private key to a memory space where nothing can touch it.

As a first step, it should be easy to add this as an extra option to account but still require the password or the access token. In a further release, it would be possible to use the certificate as the only authentication but that requires more effort and more config in the SSL engine facing the Nextcloud service as well as in the Nextcloud config itself to map certificates names to usernames.

Actual behaviour

To use such a client side certificate is not an option as of now

Steps to reproduce

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iOS version

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App version

Latest

Server configuration

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Operating system:
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Web server:
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Database:
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PHP version:
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Nextcloud version: (see Nextcloud admin page)
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