Jim,
I must admit to being surprized by the "requirement" to not
disclose public keys in message headers, irrespective of the debate
over whether the public key is the "right" choice for a key selector.
Public key technology was based, from the beginning, on the notion
that the public keys would be publically available. Yes, I am willing
to believe that keeping public keys secret does, in principle, add to
the work involved in attacking the system, but in practice it seems
unlikely to be a critical factor. Moreover, one of the major
advantages of public key technology is the (relative) ease of
distributing keys, and the need for an integrity secure, but not
confidential, channel for such distribution. Considering the possible
implications of this constraint on the design, it would really be
nice to have a strong justification cited, for the record.
Steve