Securing the future net

Today I had the fortune to attend a group discussion ambitiously entitled Future of Internet Security at Mozilla. What this was mostly about was, given that a recent incident has severely shaken everyone’s confidence in the PKIX (PDF, say sorry) mechanism that everyone currently uses to decide that a secure website is who it says it is, what can we do about it? I’m not going to attempt to summarize; instead I’m going to point at the Etherpad log and [2016: the Etherpad log is no longer available either from Mozilla or the Internet Archive] video record of the discussion, then plow boldly forward with my own (incontrovertibly correct, of course) opinion on the way forward, on the assumption that everyone who reads this will already be familiar enough with the context to know what I’m talking about.

I will quote in full the principles with which the discussion was kicked off, though (really they’re more like constraints on solutions acceptable to all parties).

  • Performance - large sites will not adopt solutions which bulk up the amount of data required to be exchanged to establish an secure connection.
  • Independence/Availability - large sites will not accept tying the uptime of their site to the uptime of infrastructure over which they have no control (e.g. an OCSP responder)
  • Accessibility/Usability - solutions should not put the cost of security, either in terms of single sites or large deployments, out of the reach of ordinary people
  • Simplicity - solutions should be simple to deploy, or capable of being made simple.
  • Privacy - ideally, web users should not have to reveal their browsing habits to a third party.
  • Fail-closed - new mechanisms should allow us to treat mechanism and policy failures as hard failures (not doing so is why revocation is ineffective) (however this is trading off security for availability, which has historically proven almost impossible).
  • Disclosure - the structure of the system should be knowable by all parties, and users must know the identities of who they are trusting

I should probably emphasize that this is a walk, do not run, to the exits situation. The status quo is dire, but we can afford to take the time to come up with a solution that solves the problem thoroughly; we do not need an emergency stopgap. Despite that, I think the short-term solution will be different from the long-term solution.

In the short term, the solution with the most traction, and IMO the best chance of actually helping, is DANE, an IETF draft standard for putting TLS server keys in the DNS. This can (at least on paper) completely replace the common DV certificates issued by traditional certificate authorities. However, to offer real security improvements relative to the status quo, I assert that the final version of the spec needs to:

  • Require clients to fail closed on any sort of validation failure. The current text of the spec does say this, but not clearly and not with enough RFC2119 MUSTs.
  • Provide exclusion (trust no server keys but these, possibly also trust no CA but these) rather than inclusion (you should trust this server key). The current text of the spec can be read either way. A vocal minority of the DANE working group wants inclusion. It is my considered opinion that inclusion is completely useless—all it does is add the DNS root signing key to the existing pool of trusted CAs, which doesn’t solve the untrustworthy CA problem.
  • Require the use of DNSSEC. It has recently been suggested that a signed DNS zone is not necessary for exclusion, but then a DNS-tampering attacker can deny service by injecting a bogus DANE record, which will deter deployment. (It doesn’t matter that a DNS-tampering attacker can also deny service by messing up the A records; this is a new risk, which scares people more than an existing risk.)
  • Clearly indicate that it does not provide EV-level validation, leaving a business model for traditional CAs to retreat to.

In the longer term, I think we’re going to want to move to some sort of content-based addressing. DANE gets rid of the CA mess, but it substitutes the DNS as a single point of failure. Here’s a half-baked scheme that we could start rolling out real soon for URIs that don’t need to be user comprehensible:

<!-- jQuery 1.5.2 -->
<script src="h:sha1,b8dcaa1c866905c0bdb0b70c8e564ff1c3fe27ad"></script>

The browser somehow knows how to expand h: URIs to something it can go ask a server on the net for. What the server produces MUST be discarded if it does not have the specified hash (and the browser can go try some other server). We don’t need to worry about where the browser got the content or whether it was transferred under encryption—if it’s not what was wanted, it’ll fail the hash check. Still to be worked out: how to do the expansion without reintroducing that single point of failure; how to disseminate these URIs; how to fit dynamic content into the scheme; under what circumstances h: URIs should, or should not, be considered same-origin with the requesting page.