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-Data channel cipher negotiation
-===============================
-
-OpenVPN 2.4 and higher have the capability to negotiate the data cipher that
-is used to encrypt data packets. This section describes the mechanism in more detail and the
-different backwards compatibility mechanism with older server and clients.
-
-OpenVPN 2.5 and higher behaviour
---------------------------------
-When both client and server are at least running OpenVPN 2.5, that the order of
-the ciphers of the server's ``--data-ciphers`` is used to pick the the data cipher.
-That means that the first cipher in that list that is also in the client's
-``--data-ciphers`` list is chosen. If no common cipher is found the client is rejected
-with a AUTH_FAILED message (as seen in client log):
-
-    AUTH: Received control message: AUTH_FAILED,Data channel cipher negotiation failed (no shared cipher)
-
-OpenVPN 2.5 will only allow the ciphers specified in ``--data-ciphers``. To ensure
-backwards compatibility also if a cipher is specified using the ``--cipher`` option
-it is automatically added to this list. If both options are unset the default is
-:code:`AES-256-GCM:AES-128-GCM`.
-
-OpenVPN 2.4 clients
--------------------
-The negotiation support in OpenVPN 2.4 was the first iteration of the implementation
-and still had some quirks. Its main goal was "upgrade to AES-256-GCM when possible".
-An OpenVPN 2.4 client that is built against a crypto library that supports AES in GCM
-mode and does not have ``--ncp-disable`` will always announce support for
-`AES-256-GCM` and `AES-128-GCM` to a server by sending :code:`IV_NCP=2`.
-
-This only causes a problem if ``--ncp-ciphers`` option has been changed from the
-default of :code:`AES-256-GCM:AES-128-GCM` to a value that does not include
-these two ciphers. When a OpenVPN servers try to use `AES-256-GCM` or
-`AES-128-GCM` the connection will then fail. It is therefore recommended to
-always have the `AES-256-GCM` and `AES-128-GCM` ciphers to the ``--ncp-ciphers``
-options to avoid this behaviour.
-
-OpenVPN 3 clients
------------------
-Clients based on the OpenVPN 3.x library (https://github.com/openvpn/openvpn3/)
-do not have a configurable ``--ncp-ciphers`` or ``--data-cipher`` option. Instead
-these clients will announce support for all their supported AEAD ciphers
-(`AES-256-GCM`, `AES-128-GCM` and in newer versions also `Chacha20-Poly1305`).
-
-To support OpenVPN 3.x based clients at least one of these ciphers needs to be
-included in the server's ``--data-ciphers`` option.
-
-
-OpenVPN 2.3 and older clients (and clients with ``--ncp-disable``)
-------------------------------------------------------------------
-When a client without cipher negotiation support connects to a server the
-cipher specified with the ``--cipher`` option in the client configuration
-must be included in the ``--data-ciphers`` option of the server to allow
-the client to connect. Otherwise the client will be sent the ``AUTH_FAILED``
-message that indicates no shared cipher.
-
-If the client is 2.3 or older and has been configured with the
-``--enable-small``  :code:`./configure` argument, using
-``data-ciphers-fallback cipher`` in the server config file with the explicit
-cipher used by the client is necessary.
-
-OpenVPN 2.4 server
-------------------
-When a client indicates support for `AES-128-GCM` and `AES-256-GCM`
-(with ``IV_NCP=2``) an OpenVPN 2.4 server will send the first
-cipher of the ``--ncp-ciphers`` to the OpenVPN client regardless of what
-the cipher is. To emulate the behaviour of an OpenVPN 2.4 client as close
-as possible and have compatibility to a setup that depends on this quirk,
-adding  `AES-128-GCM` and `AES-256-GCM` to the client's ``--data-ciphers``
-option is required. OpenVPN 2.5+ will only announce the ``IV_NCP=2`` flag if
-those ciphers are present.
-
-OpenVPN 2.3 and older servers (and servers with ``--ncp-disable``)
-------------------------------------------------------------------
-The cipher used by the server must be included in ``--data-ciphers`` to
-allow the client connecting to a server without cipher negotiation
-support.
-(For compatibility OpenVPN 2.5 will also accept the cipher set with
-``--cipher``)
-
-If the server is 2.3 or older and  has been configured with the
-``--enable-small`` :code:`./configure` argument, adding
-``data-ciphers-fallback cipher`` to the client config with the explicit
-cipher used by the server is necessary.
-
-Blowfish in CBC mode (BF-CBC) deprecation
-------------------------------------------
-The ``--cipher`` option defaulted to ``BF-CBC`` in OpenVPN 2.4 and older
-version. The default was never changed to ensure backwards compatibility.
-In OpenVPN 2.5 this behaviour has now been changed so that if the ``--cipher``
-is not explicitly set it does not allow the weak ``BF-CBC`` cipher any more
-and needs to explicitly added as ``--cipher BFC-CBC`` or added to
-``-data-ciphers``.
-
-We strongly recommend to switching away from BF-CBC to a
-more secure cipher as soon as possible instead.