mc-gnus-snarf-keys switches back to the *Original Article*
before looking for keys. If the message in the original
article was encrypted, mc-snarf-keys can't find any keys
there. The keys can, of course, be found in the decrypted
article, one just has to invoke mc-snarf-keys directly,
or switch to some other mode before running mc-snarf.
I'm sure there must be some valid reasons why the search
is done on the original article, but in case of an
message this seems to be the wrong thing to do.