Use XOR instead of concatenation for calculation of IV from implicit IV

Use XOR instead of concatenation for calculation of IV from implicit IV

This change prepares the extended packet id data where also the packet id
part of the IV will be derived using xor.  Using xor also in the AEAD
case where this degenerates to a concatenation allows using the same
IV generation code later.

Change-Id: I74216d776d3e0a8dc987ec7b1671c8e8dcccdbd6
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: MaxF <max@max-fillinger.net>
Acked-by: Antonio Quartulli <antonio@mandelbit.com>
Acked-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20241212143845.4090-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg30097.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>