man: add security considerations to --compress section

As Ahamed Nafeez reported to the OpenVPN security team, we did not
sufficiently inform our users about the risks of combining encryption
and compression.  This patch adds a "Security Considerations" paragraph
to the --compress section of the manpage to point the risks out to our
users.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1528020718-12721-1-git-send-email-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16919.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

doc/openvpn.8

@@ -2516,6 +2516,16 @@ If the
 parameter is empty, compression will be turned off, but the packet
 framing for compression will still be enabled, allowing a different
 setting to be pushed later.
+
+.B Security Considerations
+
+Compression and encryption is a tricky combination.  If an attacker knows or is
+able to control (parts of) the plaintext of packets that contain secrets, the
+attacker might be able to extract the secret if compression is enabled.  See
+e.g. the CRIME and BREACH attacks on TLS which also leverage compression to
+break encryption.  If you are not entirely sure that the above does not apply
+to your traffic, you are advised to *not* enable compression.
+
 .\"*********************************************************
 .TP
 .B \-\-comp\-lzo [mode]