I want opportunistic, identity-less encryption on the Internet

November 13, 2014

The tech news has recently been full of reports that some ISPs are modifying SMTP conversations passing through them in order to remove encryption, or more specifically to remove a note that the server is offering it (this goes by the name of 'STARTTLS'). Some people are up in arms over it; others are saying that this is no big deal because it's not as if TLS on SMTP conversations means much in the first place. This latter viewpoint may raise some eyebrows. The reason why TLS SMTP means less than you think is that essentially no clients require signed and verified TLS certificates from the SMTP server; instead they'll almost always accept any random TLS certificate. In one set of jargon, this is called 'opportunistic' encryption; you could also call it 'unauthenticated' encryption. Its drawback is that this lack of authentication of the server means that the server could be anyone, including an attacker playing man in the middle to read your 'TLS encrypted' email.

This general issue is a long-standing dispute in Internet cryptography. On one side are people who say that opportunistic encryption is better than nothing; on the other side are people who say that it's effectively nothing because of its vulnerability to MITM attacks (and that it has other bad side effects, like causing people to think that they're more secure than they are). Once upon a time I might have been reasonably sympathetic to the second view, but these days I have come completely around to the first view.

As far as the second view goes: yes, there are many places that are perfectly happy to perform MITM attacks on you at the drop of a hat. Most such places generally don't call them MITM attacks, of course; instead they're 'captive portals' and so on. Those ISPs that are stripping STARTTLS markers are effectively doing a MITM attack. But in practice, in the real world, this is not equivalent to having no encryption at all. The big difference between opportunistic encryption and no encryption is that opportunistic encryption completely defeats passive monitoring. And in the real world, passive monitoring is now pervasive.

That is why I want as much opportunistic encryption as possible; I want that pervasive passive monitoring to get as little as possible. Sure, in theory the opportunistic encryption could be MITMd. In practice no significant amount of it is likely to be, because the resources required to do so go up much faster than with passive monitoring (and in some situations so do the chances of getting noticed and caught). Opportunistic encryption is not theoretically clean but it is practically useful, and it is much, much easier to design in and set up than authenticated encryption is.

(And this is why ISPs stripping STARTTLS matters quite a bit.)

Written on 13 November 2014.
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Last modified: Thu Nov 13 00:57:03 2014
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