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External Storage Authentication mechanisms
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ownCloud 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 multiple authentication mechanisms. Different 
mechanisms require different configuration parameters, depending on their 
behaviour.

Special Mechanisms
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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.

The **Log-in credentials, save in session** mechanism uses the ownCloud 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 
ownCloud has no access to the storage credentials, and background file scanning 
does not work.

Public-key Mechanisms
---------------------

Currently only the RSA mechanism is implemented, where a public/private
keypair is generated by ownCloud 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 ownCloud. Then ownCloud can
perform an authentication request, establishing the storage connection.

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