[Cryptography] prism proof email, namespaces, and anonymity

John Kelsey crypto.jmk at gmail.com
Fri Sep 13 16:55:05 EDT 2013


The more I think about it, the more important it seems that any anonymous email like communications system *not* include people who don't want to be part of it, and have lots of defenses to prevent its anonymous communications from becoming a nightmare for its participants.  If the goal is to make PRISM stop working and make the email part of the internet go dark for spies (which definitely includes a lot more than just US spies!), then this system has to be something that lots of people will want to use.  

There should be multiple defenses against spam and phishing and other nasty things being sent in this system, with enough designed-in flexibility to deal with changes in attacker behavior over tome.  If someone can send participants in the system endless spam or credible death threats, then few people are going to want to participate, and that diminishes the privacy of everyone remaining in the system, along with just making the system a blight in general.  If nonparticipants start getting spam from the system, it will either be shunned or shut down, and at any rate won't have the kind of reputation that will move a lot of people onto the system.  An ironclad anonymous email system with 10,000 users is a whole lot less privacy-preserving than one with 10,000,000 users.  As revelations of more and more eavesdropping come out, we might actually see millions of users want to have something really secure and anonymous, but not if it's widely seen as a firehose o' spam.  

A lot of the tools we use on the net everyday suffer from having been designed without thinking very far ahead into how they might be exploited or misused--hence spam, malware in PDF files, browser hijacking sorts of attacks, etc.  My thought is that we should be thinking of multiple independent defenses against spamming and malware and all the rest, because parasites adapt to their environment.  We can't count on "and then you go to jail" as a final step in any protocol, and we can't count on having some friendly utility read millions of peoples' mail to filter the spam if we want this to be secure.  So what can we count on to stop spam and malware and other nastiness?  

Some thoughts off the top of my head.  Note that while I think all these can be done with crypto somehow, I am not thinking of how to do them yet, except in very general terms.  

a.  You can't freely send messages to me unless you're on my whitelist.  

b.  This means an additional step of sending me a request to be added to your whitelist.  This needs to be costly in something the sender cares about--money, processing power, reputation, solving a captcha, rate-limits to these requests, whatever.  (What if the system somehow limited you to only, say, five outstanding requests at a time?). 

c.  Make account creation costly somehow (processing, money, solving a captcha, whatever).  Or maybe make creating a receive-only account cheap but make it costly to have an account that can request to communicate with strangers.  

d.  Make sending a message in general cost something.  Let receiver addresses indicate what proof of payment of the desired cost they require to accept emails.  

e.  Enable some kind of reputation tracking for senders?  I'm not sure if this would work or be a good idea, but it's worth thinking about.  

f.  All this needs to be made flexible, so that as attackers evolve, so can defenses.  Ideally, my ppe (prism proof email) address would carry an indication of what proofs your request to communicate needed to carry in order for me to consider it.  

g.  The format of messages needs to be restricted to block malware, both the kind that wants to take over your machine and the kind that wants to help the attacker track you down.  Plain text email only?  Some richer format to allow foreign language support?  

h.  Attachments should become links to files in an anonymizing cloud storage system.  Among other things, this will make it easier to limit the size of the emails in the system, which is important for ensuring anonymity without breaking stuff.  

What else?  I see this as the defining thing that can kill an anonymous encrypted communications system--it can become a swamp of spam and malware and nutcases stalking people, and then nobody sensible will want to come within a hundred meters of it.  Alternatively, if users are *more* in control of who contacts them in the prism-proof scheme than with the current kind of email, we can get a lot more people joining.  



More information about the cryptography mailing list