Deprecate --keysize

The --keysize option can only be used with already deprecated ciphers,
such as CAST5, RC2 or BF.  Deviating from the default keysize is
generally not a good idea (see man page text), and otherwise only
complicates our code.

Since we will also remove the support for weak ciphers (ciphers with
cipher block length less than 128 bits) in OpenVPN 2.6 as well, we
start the deprecation of this option instantly.

[DS: Slightly amended the patch, referencing OpenVPN 2.6 and added
     a few more details to Changes.rst and the commit message]

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20170701112951.19119-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15004.html
Signed-off-by: David Sommerseth <davids@openvpn.net>

Changes.rst

@@ -178,6 +178,9 @@ Deprecated features
 
 - ``--no-iv`` is deprecated in 2.4 and will be removed in 2.5.
 
+- ``--keysize`` is deprecated and will be removed in v2.6 together
+  with the support of ciphers with cipher block size less than 128 bits.
+
 
 User-visible Changes
 --------------------

doc/openvpn.8

@@ -4217,6 +4217,9 @@ negotiation.
 .\"*********************************************************
 .TP
 .B \-\-keysize n
+.B DEPRECATED
+This option will be removed in OpenVPN 2.6.
+
 Size of cipher key in bits (optional).
 If unspecified, defaults to cipher-specific default.  The
 .B \-\-show\-ciphers

src/openvpn/options.c

@@ -2484,6 +2484,11 @@ options_postprocess_verify_ce(const struct options *options, const struct connec
         msg(M_USAGE, "NCP cipher list contains unsupported ciphers.");
     }
 
+    if (options->keysize)
+    {
+        msg(M_WARN, "WARNING: --keysize is DEPRECATED and will be removed in OpenVPN 2.6");
+    }
+
     /*
      * Check consistency of replay options
      */