From: tcmay@netcom.com (Timothy C. May)
Date: Sat, 1 Oct 94 11:44:32 PDT
To: hfinney@shell.portal.com (Hal)
Subject: Re: Technical Remailer Analysis.
In-Reply-To: <199410011756.KAA17377@jobe.shell.portal.com>
Message-ID: <199410011844.LAA16070@netcom15.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


Hal wrote:

> Good point.  There is a related attack which Chaum pointed out in his
> 1981 CACM paper: the attacker intercepts and keeps a copy of an incoming
> message, then later re-sends it.  This one will go to the same place and
> by repeating this multiple times we can figure out where the original
> message went.

Thanks to Louis Cypher and Hal Finney for discussing this. Our
remailers are, as several of us have discussed, at a primitive level
of security...in fact, most of the security is illusory and would
collapse under serious scrutiny.

Here are some fixes to consider, as I see them. Chaum, in his Feb.
1981 paper on Untraceable E-Mail (Comm. of the ACM) remains the key
paper, though some of the later DC-Net papers also deal with such
attacks (under the rubric of "collusion" and "flooding" types of
attacks).

Sorry for the format, as I'm using the tools I use for the FAQ.

157.3.  Some possible fixes:

	157.3.1.  remailers can recognize duplicates and agree not to
remail them, or to remail them off in different directions (adding their own 
hop-wrappers)

	157.3.2.  digital postage helps a bit, as the attacker at
least has to spend money

	157.3.3.  (If the inner layers of a message each have some
digital money, or a "one-use" coupon, then an attacker who copies and resends
the whole message is effectively double-spending and this should be detected.
Most simply, the "use once" coupon will only allow one passage through the
remailer.)

--Tim May


-- 
..........................................................................
Timothy C. May         | Crypto Anarchy: encryption, digital money,  
tcmay@netcom.com       | anonymous networks, digital pseudonyms, zero
408-688-5409           | knowledge, reputations, information markets, 
W.A.S.T.E.: Aptos, CA  | black markets, collapse of governments.
Higher Power: 2^859433 | Public Key: PGP and MailSafe available.
Cypherpunks list: majordomo@toad.com with body message of only: 
subscribe cypherpunks. FAQ available at ftp.netcom.com in pub/tcmay