Add support for CHACHA20-POLY1305 in the data channel

Add support for CHACHA20-POLY1305 in the data channel

We explicitly only supported GCM as a valid AEAD mode, change that to also
allow ChaCha20-Poly1305 as an AEAD cipher.  That works nicely with our new
(GCM) data channel format, because is has the same 96-bit IV.

Note that we need some tricks to not treat the cipher as insecure, because
we used to only look at the block size of a cipher to determine if find a
cipher insecure.  But ChaCha20-Poly1305 is a stream cipher, which
essentially
has a 'block size' of 1 byte and is reported as such.  So, special-case
this
cipher to be in the list of secure ciphers.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20181007223035.21179-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17629.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>