Today at W2SP I presented a new paper making the case for distributing security policy in hyperlinks. The basic idea is old, but I think the time is right to re-examine it. After the DigiNotar debacle, the community is getting serious about fixing PKI on the web. It was hot topic at this week’s IEEE Security & Privacy (Oakland), highlighted by Jeremy Clark and Paul van Oorschot’s excellent survey paper. There are a slew of protocols under development like key pinning (HPKP), Certificate Transparency, TACK, and others. To these I add s-links, a complementary mechanism to declare support for new proposals in HTML links.
Though it’s unclear which proposals will take hold, deployment will probably be fragmented: some servers will require HTTPS (using HSTS), some may pin keys or use another new protocol, and many will continue to not support HTTPS at all. Clients must know what the server supports prior to initially connecting, or else a middleperson attacker can simulate a server which only supports insecure HTTP (often called a stripping attack). Thus hardening HTTPS includes an enormous policy distribution problem.
The consensus is that querying a new out-of-band trusted server to learn security policy is a non-starter. OCSP, a protocol to check if certificates are revoked, provides a painful example. It was never reliable enough for browsers to fail closed if OCSP servers couldn’t be reached, so it provided negligible security and Chrome eventually disabled it. This leaves very few channels to distribute security policy prior to initial connections. Browser preloads are great, but can’t scale indefinitely. DNSSEC (via extensions like DANE) is a promising approach, but many deployment issues remain.
S-links aren’t a panacea: they can’t protect users who manually type a new URL. Still, compared to the alternatives s-links are an efficient and easy-to-deploy channel for security policy. An important lesson from past PKI failures is to build for robustness: multiple protocols will have to be supported and we should build multiple ways of advertising security upgrades. S-links is still a very early-stage project with important details to get right about the user experience and some subtle interactions with the browser’s same-origin policy. I would greatly appreciate feedback.