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From bear at sonic.net Fri Jan 30 16:47:23 2009
From: bear at sonic.net (Ray Dillinger)
Date: Fri, 30 Jan 2009 13:47:23 -0800
Subject: UCE - a simpler approach using just digital signing?
In-Reply-To: <1b587cab0901291832kd132e29uc7a58c4abbc5e7d@mail.gmail.com>
References: <CHILKAT-MID-fe8337f6-2f9d-265e-4a2c-9535ff0028cb@server123>
<200901252240.n0PMejwU002817@new.toad.com>
<1b587cab0901291832kd132e29uc7a58c4abbc5e7d@mail.gmail.com>
Message-ID: <1233352043.28107.143.camel@localhost>

I have a disgustingly simple proposal. It seems to me that one
of the primary reasons why UCE-limiting systems fail is the
astonishing complexity of having a trust infrastructure
maintained by trusted third parties or shared by more than
one user. Indeed, "trusted third party" and "trust shared by
multiple users" may be oxymorons here. Trust, by nature, is
not really knowable by any third party, not the same for any
set of more than one user, and in fact the people most willing
to pay for it at least where UCE is concerned, experience shows
to be usually the _least_ trustworthy parties.

So why hasn't anybody tried direct implementation of user-
managed digital signatures yet?

A key list maintained by individual recipients for themselves
alone could be astonishingly simpler in practice, probably
to the point of actually being practical.

In fact, it is _necessary_ to eliminate third parties and
shared infrastructure almost entirely in order to allow mail
recipients to have the kind of fine-grained control that
they actually need to address the problem by creating
social and business feedback loops that promote good security.

As matters stand today, there is no protection from UCE.
If I know there is a user account named 'fred' on the host
'example.com', then I have an email address and I can send
all the UCE I want. And poor fred has the same email address
he gives everybody, so he gets spam from people who've gotten
his address and he has no idea where they got it. All his
legitimate correspondents are using the same email address,
so he can't abandon it without abandoning *all* of them,
and he doesn't know which of them gave his address to the
spammers. What if email accounts weren't that simple?

Consider the implications of a third field, or "trust token,"
which works like a "password" to fred's mail box. Your
mailer's copy of fred's email address would look like
"fred#token at example.com" where "token" was a field that
was your own personal password to fred's mailbox. Your
system would still send mail to "fred at example.com" but
it would include a "Trust:" header based on the token.

The simplest solution I can think of would be a direct
application of digital signatures; the trust token would
be (used as) a cryptographic key, and the headers of any
message would have to include a "Trust" field containing a
digital signature (a keyed cryptographic hash of the message,
generated by that key). Messages to multiple recipients
would need to contain one "Trust" field per recipient.

Its use would follow simple rules:

Each time Fred gives out his email address to a new sender,
he creates a trust token for that sender. They must use it
when they send him mail. So fred gives his bank a key when
he gives them his email address. If fred were willing to
recieve mail from strangers, he could publish a trust token
on his webpage or on usenet or whatever - it would be painless
to revoke it later, so why not? If fred trusted someone to
give out his email address, he could give that person multiple
trust tokens to pass along to others. Again, an error in
judgement would be painless to revoke later.