of the encrypted data, use the keyed checksum length if the enctype
supports a keyed checksum. This only matter for aes, for all other
enctypes the key and unkeyed checksum have the same length.
git-svn-id: svn://svn.h5l.se/heimdal/trunk/heimdal@12450 ec53bebd-3082-4978-b11e-865c3cabbd6b