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External Storage authentication mechanisms
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Nextcloud storage backends accept one or more authentication schemes such as
passwords, OAuth, or token-based, to name a few examples. Each authentication
scheme may be implemented by combining multiple authentication mechanisms. Different
mechanisms require different configuration parameters, depending on their
behavior.


.. figure:: images/authentication-types.png
   :alt: Authentication types

Special mechanisms
------------------

The **None** authentication mechanism requires no configuration parameters, and
is used when a backend requires no authentication.

The **Built-in** authentication mechanism itself requires no configuration
parameters, but is used as a placeholder for legacy storages that have not been
migrated to the new system and do not take advantage of generic authentication
mechanisms. The authentication parameters are provided directly by the backend.

Password-based mechanisms
-------------------------

The **Username and password** mechanism requires a manually-defined username and
password. These get passed directly to the backend and are specified during the
setup of the mount point.

The **Log-in credentials, save in session** mechanism uses the Nextcloud login
credentials of the user to connect to the storage. These are not stored anywhere
on the server, but rather in the user session, giving increased security. The
drawbacks are that sharing is disabled when this mechanism is in use, as
Nextcloud has no access to the storage credentials, and background file scanning
does not work.

The **Log-in credentials, save in database** mechanism uses the Nextcloud login
credentials of the user to connect to the storage. These are stored in the
database encrypted with the shared secret. This allows to share files from
within this mount point.

The **User entered, store in database** mechanism work in the same way as the
"Username and password" mechanism but the credentials need to be specified by
each user individually. Before the first access to that mount point the user
will be prompted to enter the credentials.

The **Global credentials** mechanism uses the general input field for "Global
credentials" in the external storage settings section as source for the
credentials instead of individual credentials for a mount point.

Public-key mechanisms
---------------------

Currently only the RSA mechanism is implemented, where a public/private
keypair is generated by Nextcloud and the public half shown in the GUI. The keys
are generated in the SSH format, and are currently 1024 bits in length. Keys
can be regenerated with a button in the GUI.

.. figure:: images/auth_rsa.png
   :alt: Form on admin page for generating RSA keys.

OAuth
-----

OAuth 1.0 and OAuth 2.0 are both implemented, but currently limited to the
Dropbox and Google Drive backends respectively. These mechanisms require
additional configuration at the service provider, where an app ID and app
secret are provided and then entered into Nextcloud. Then Nextcloud can
perform an authentication request, establishing the storage connection.

.. figure:: images/dropbox-oc.png
   :alt: Dropbox storage mount configuration.
