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