Re: Technical Remailer Analysis.

[tcmay@netcom.com (Timothy C. May)]

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


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