Deprecate non TLS mode in OpenVPN

Deprecate non TLS mode in OpenVPN

The non-TLS mode is a relict from OpenVPN 1.x or 2.0. When TLS mode was
introduced the advantages of TLS over non-TLS were small but TLS mode
evolved to include a lot more features (NCP, multipeer, AEAD ciphers to
name a few).

Today VPN setups that use --secret are mainly used because this mode is
easier to setup and does not require setting up a PKI.  This shortcoming
of TLS mode should be addressed now with the peer-fingerprint option.

The primary reason to deprecate --secret is that it is not secure enough
anymore for modern environments.  This mode uses a fixed pre-shared key and
no session keys.  Thus, no forward secrecy is possible, which means that
any captured VPN traffic can be decrypted later should the --secret key
get into the wrong hands.  The cryptography overall used here was okay
when --secret was introduced but is not acceptable by today's standard
anymore.

Finally, modern hardware-accelerated crypto modes like AES-GCM can only
be used in TLS mode (due to IV requirements).

Patch V2: Improve commit message

Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210328090530.10653-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21868.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>