From: "gtoal@gtoal.com" <gtoal@pizzabox.demon.co.uk>
Date: Mon, 11 Apr 94 21:59:24 PDT
To: cypherpunks@toad.com
Subject: Re: New anon mailer idea?
Message-ID: <9404120451.AA29748@pizzabox.demon.co.uk>
MIME-Version: 1.0
Content-Type: text/plain


Following up my own post, because I'm rather pleased with it and have had
some more ideas... :-)

: I want to mail fred@somesite anonymously.  I know fred@somesite's
: public key.  I encrypt my message for fred, then send it to a
: remailer address with instructions to pass it on to fred.  For a little
: eavesdropping security, I include an anonymous pgp key of mine
: in the mail to fred so that he can reply to me without the remailer
: operators reading his mail.  You can choose your favourite syntax
: for how I ask the remailer to send this mail to fred - I don't care
: what it is.

: The remailer then encrypts *my reply mail address* with the remailers own
: key, and inserts this as a header in the mail which only it can read.
: It attaches a little message to this header saying 'when you reply to
: this message, be sure to include this opaque header I'm giving
: you here...'

: The recipient gets the mail, decodes it, reads it, and replies.
: (Maybe encrypted with an anonymous public key I included in the
: mail, maybe in cleartext - doesn't matter for the scheme) When
: he replies, he included the small encrypted block that the remailer
: gave him at the top of his message, as he was asked to do by the
: remailer.

: The reply goes to the anonymous remailer.  The anonymous remailer
: decrypts the header block that it searches the mail for, and
: extracts my email address from it again.  The remailer then passes
: the mail back to me - this time including an encrypted block with
: the fred@somesite's address in it.  (Or some other address if
: fred replied from another account; or perhaps I mailed a mail
: to news gateway - well, my encrypted address will still work
: even if a dozen people reply to the news article by mailing
: via the remailer, and now I *don't* know who the encrypted
: sender is)

: In this way, once a conversation has been established, replies
: can keep going backwards and forwards without much fancy protocol
: at all - all you ever do is remember not to delete the encrypted
: block that the remailer keeps inserting at the top of your mail.


I've thought of another thing that cypherpunks like that this scheme can do:
return postage.  Where the remailer encrypts the reply address and puts
it in the body of the destination mail, so that the adressee can reply,
I'd been assuming a public key system like pgp, just because that's what
we're used to.  But in fact the encryption is entirely private to the
remailer and might as well be secret key like DES.

So instead of having a single master key, let's use a key that's generated
for each message.  So, the remailer encrypts the return block with a secret
DES key, and includes something like the MD5 hash of the DES key in
cleartext so that it can find the correct DES key when the reply comes
back.  (Hmmm.  doesn't have to be the md5 hash - could even be the filename
that stores the key - yeah, that works better...)

Anyway, we now have a way of doing postage.  Let's say that the outgoing
message was to a mail2news gateway, and the sender asked for a limit of
10 replies.  Then as each reply comes in, the reply count stored in the
file that holds the DES key is decremented to 0, and when it hits 0 the
file is deleted.  Unless the remailer operator made illicit backups, that
return address can *never* be used again - it may be out there on the
net in an encrypted version, but *no-one* can now decrypt it.

More likely you'd use this feature to guarantee single-shot replies.

Similarly, the same thing can be applied to timed-expiry keys - the file
storing the DES key can be removed after a certain time has elapsed.  If
you post a message on some timely event, you arrange that only answers
posted in the next 48 hours will be delivered - anything that arrives
too late is bounced by the remailer. I can see *lots* of uses for this
feature :-)  One nice use of this is to foil traffic analysis - if you
send off something to someone with a reply address and force an
immediate reply, they don't have time to get stuff in place to watch
all the feed sites and follow a message through the net if they later
want to trace who you are by sending you lots of mail (a technique I
realised would work against Julf's mailer) - as soon as one of the
remailer-chain's keys has expired (and been deleted), that part of
the routing is permanently broken.

Couple this implementation of postage-limitation with the Magic-money stuff
for payment and I think we have quite a nice extension of the cypherpunk
mailing ethos.  I wonder if it could be hacked in to the existing
remailers?  Anyone interested?

G