PGP/MIME ([RFC 3156](https://datatracker.ietf.org/doc/html/rfc3156#section-5)) specifies a `micalg=` parameter that's present for the sake of one-pass processing of the message. With the salt added to v6 signatures, `micalg=` alone is insufficient to permit one-pass processing of a PGP/MIME `multipart/signed` message. Maybe we want to have this document update RFC 3156 by adding a `pgpsalt=` parameter?