If we split our aims into layers then we can meet the aims of PGP,
Steve Crocker and Steve Kent within a unified design. Here is a
possible layering.
1. Continuity, integrity and confidentiality.
For this the mail message only needs the sender's public key to be
included in the message, and for the message to be signed by that
public key.
a. We can be sure that multiple messages in this form are from the
same sender (continuity of conversation), even if they are
actually posted from different e-mail addresses.
b. We know the message hasn't been modified since signed.
c. While we don't know for sure how to get messages back to the
person concerned, still we can save the embarrassment of sending a
reply to someone other than the sender by encrypting it so that no
one other than the sender can receive and understand it.
The fact that we don't have any idea who we are talking to is not
necessarily a problem. We do business with shop-keepers on that basis
every day. Why should we tell people we deal with exactly who we are
until/unless we need to?
2. E-mail forgery protection.
Internet e-mail is trivial to forge. Apart from all other aspects of
identifying the sender we would like to know that the e-mail address
on the From header actually sent the mail in question.
The natural way to do this is a certificate identifying the public key
with the e-mail address. The natural person to sign this is the owner
of the domain of the e-mail address (or a parent thereof).
The change from (1) is that we now know an e-mail address for a reply.
The address and/or the associated comment field might provide
additional information about who the person is, but that is not the
general intention of this certificate.
3. Sender Identification.
If the sender wants to let the recipient know who she really is then
she can include one or more certificates that link her public key
to external (non-email) information that will help the sender identify
her. Examples of useful certificates to include would be:
a. X.509 stating that the public key is associated wiht a particular
point in the X.500 Directory.
b. A certificate associating the public key with a picture of the
sender.
c. A certificate associating the public key with some financial
information (like a bankcard number).
What sort of identification you send depends on the objective of the
message.
Bob Smart