Mozilla Debates Whether to Trust Chinese CA

By Ed Felten 

Sometimes geeky technical details matter only to engineers. But sometimes a seemingly arcane technical decision exposes deep social or political divisions. A classic example is being debated within the Mozilla project now, as designers decide whether the Mozilla Firefox browser should trust a Chinese certification authority by default.

Here's the technical background: When you browse to a secure website (typically at a URL starting with "https:"), your browser takes two special security precautions: it sets up a private, encrypted "channel" to the server, and it authenticates the server's identity. The second step, authentication, is necessary because a secure channel is useless if you don't know who is on the other end. Without authentication, you might be talking to an impostor.

Suppose you're connecting to https://mail.google.com, to pick up your Gmail. To authenticate itself to you, the server will (1) do some fancy math to prove to you that it knows a certain encryption key, and (2) present you with a digital certificate (or "cert") attesting that only Google knows that encryption key. The cert is created by a Certification Authority ("CA"), which asserts that it has done the necessary due diligence to establish that the designated encryption key is known only to Google Inc.

If the CA is competent and honest, then you can rely on the cert, and your connection will be secure. But a dishonest CA can trick you into talking to an impostor site, so you need to be cautious about which CAs you trust. Your browser comes preinstalled with a list of CAs whom it will trust. In principle you can change this list, but almost nobody does. So browser vendors effectively decide which CAs their users will trust.

With this background in mind, let's unpack the Mozilla debate. What set off the debate was the addition of the China Internet Network Information Center (CNNIC) as a trusted CA in Firefox. CNNIC is not part of the Chinese government but many people assert that it would be willing to act in concert with the Chinese government.

To see why this is worrisome, let's suppose, just for the sake of argument, that CNNIC were a puppet of the Chinese government. Then CNNIC's status as a trusted CA would give it the technical power to let the Chinese government spy on its citizens' "secure" web connections. If a Chinese citizen tried to make a secure connection to Gmail, their connection could be directed to an impostor Gmail site run by the Chinese government, and CNNIC could give the impostor a cert saying that the government impostor was the real Gmail site. The Chinese citizen would be fooled by the fake Gmail site (having no reason to suspect anything was wrong) and would happily enter his Gmail password into the impostor site, giving the Chinese government free run of the citizen's email archive.

CNNIC's defenders respond that any CA could do such a thing. If the problem is that CNNIC is too close to a government, what about the CAs already on the Firefox CA list that are governments? Isn't CNNIC being singled out because it is Chinese? Doesn't the country with the largest Internet population deserve at least one slot among the dozens of already trusted CAs? These are all good questions, even if they're not the whole story.

Mozilla's decision touches deep questions of fairness, trust, and institutional integrity that I won't even pretend to address in this post. No single answer will be right for all users.

Part of the problem is that the underlying technical design is fragile. Any CA can certify to any user that any server owns any name, so the consequences of a misplaced trust decision are about as bad as they can be. It's tempting to write this off as bonehead design, but in truth the available design options are all unattractive.