Respect algorithm support announced by management client

Respect algorithm support announced by management client

Support for padding algorithms in management-client is indicated
in the optional argument to --management-external-key as "pkcs1",
"pss" etc. We currently use it only for an early exit based on heuristics
that a required algorithm may not be handled by the client. When
signature is requested we do not check whether the padding is indeed
supported by the client. This leads to situations like the client
announcing "nopadding" support but we request "pss" signature.

Here we add a check while requesting signature as well. If the padding
treat it as an error instead of submitting the request to the
management-interface regardless.

This change is made only when xkey provider is in use, though such a check
would be appropriate always.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211214165928.30676-11-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23441.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>