Matthew wrote:
1) domain.com uses a self generated private key to sign each
message that originates at one of its MTA. The signature might
include include: the from address, the to address, the subject of the
message, a hash of the message body (and more?). The signature would
be added to the message as a header.
This is pretty much what GPG and S/MIME already do, if you consider the
protected "from" address to be the email address on the public key, not
the From address on the message (which the signature obsoletes anyway.)
2) domain.com uses DNS TXT records to publish the URL from
which its public key(s) can be downloaded.
Such a system, which operates only at the endpoints, allows
the receiving MTA to verify that signed messages are valid, no matter
how many times they have been forwarded. This prevents forged frome
headers and allows whitelist and blacklisting at the domain level.
Yes, you can certainly publish the keys or even set up your own private,
alternative PKI hierarchy, but this is not strictly necessary. Many
email users have a relatively static white list, so simply including the
keys in the signatures and caching them at the receiver the first time
they're seen (as Mozilla Mail does with S/MIME) would permit them to
automatically verify all future mail from those senders. No elaborate
certificate hierarchy or key publication scheme is needed.
This was the brilliant insight behind SSH -- that all (or nearly all) of
the benefits of public key cryptography can be had for many important
applications without a formal PKI. And it's the formal PKI that
ordinarily makes public key cryptography so complicated and creates
choke-points and opportunities for monopolistic abuse by companies like
Verisign.
Phil
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Description: S/MIME Cryptographic Signature