Summary: This message tosses out an idea for conveying within an { email | usenet } message the datum "the author of this message prefers to receive PGP encrypted communications" in a standard machine-readable form. Motivation: In the most recent version of my <a href="http://www.openmarket.com/personal/tjic/emacs.html"> mail-secure.el package </a> I included a feature that allows users to maintain a list of correspondants who prefer to receive encrypted mail. This list is then used by the package at email-send-time to see if a piece of email should be encrypted. I and others on this list include tags along the lines of "PGP encrypted mail preferred" in our .sigs. It occured to me that this could be automated: (1) a standard "I-like-encrypted-mail" tag could be defined (2) cryptography-capable mail tools could scan incoming messages for this tag and add the originators of any message containing the tag to a list (3) cryptography-capable mail tools could then use this list when sending (as mail-secure.el currently does). Possible Implementations: A couple of different methods occur to me to implement step #1: (a) a new header could be defined and added to messages (as per section 3 of <a href="http://www.cis.ohio-state.edu/htbin/rfc/rfc1505.html"> RFC 1505 </a> ), along the lines of X-Rcv-Security-Prefered: PGP-Encryption (b) a certain keyword or phrase could be defined that could be inserted in the .sig block of a message. Ex: -- foo@bar.com PGP-Encrypted-Mail-Preferred I am tempted to suggest something either compatible with or in the spirit of <a href="http://www.eit.com/creations/s-http/draft-ietf-wts-shttp-00.txt"> SHTTP </a> negotiation blocks, but restrain myself from doing so because this (a) seems needlessly baroque for the information being conveyed (b) falsely implies that a negotiation is taking place, when in fact a mere advisory is being issues. Security concerns: A temporary denial-of-service/annoyance attack could be mounted by an adversary who issues one forged message from individual A with the "PGP-encrypted-mail prefered tag" present, and then puts a fake key on a keyserver. Anyone who uses a tag-aware mail-reading package and sees this message would send future mail to individual A encrypted in a manner that didn't allow A to read it. Please mail or post any thoughts on this proposal. Thanks, TJIC