Last month, Wired published a long article about Ray Ozzie and his supposed new scheme for adding a backdoor in encrypted devices. It's a weird article. It paints Ozzie's proposal as something that "attains the impossible" and "satisfies both law enforcement and privacy purists," when (1) it's barely a proposal, and (2) it's essentially the same key escrow scheme we've been hearing about for decades.
Basically, each device has a unique public/private key pair and a secure processor. The public key goes into the processor and the device, and is used to encrypt whatever user key encrypts the data. The private key is stored in a secure database, available to law enforcement on demand. The only other trick is that for law enforcement to use that key, they have to put the device in some sort of irreversible recovery mode, which means it can never be used again. That's basically it.
I have no idea why anyone is talking as if this were anything new. Several cryptographers have already explained why this key escrow scheme is no better than any other key escrow scheme. The short answer is (1) we won't be able to secure that database of backdoor keys, (2) we don't know how to build the secure coprocessor the scheme requires, and (3) it solves none of the policy problems around the whole system. This is the typical mistake non-cryptographers make when they approach this problem: they think that the hard part is the cryptography to create the backdoor. That's actually the easy part. The hard part is ensuring that it's only used by the good guys, and there's nothing in Ozzie's proposal that addresses any of that.
I worry that this kind of thing is damaging in the long run. There should be some rule that any backdoor or key escrow proposal be a fully specified proposal, not just some cryptography and hand-waving notions about how it will be used in practice. And before it is analyzed and debated, it should have to satisfy some sort of basic security analysis. Otherwise, we'll be swatting pseudo-proposals like this one, while those on the other side of this debate become increasingly convinced that it's possible to design one of these things securely.
Already people are using the National Academies report on backdoors for law enforcement as evidence that engineers are developing workable and secure backdoors. Writing in Lawfare, Alan Z. Rozenshtein claims that the report -- and a related New York Times story -- "undermine the argument that secure third-party access systems are so implausible that it's not even worth trying to develop them." Susan Landau effectively corrects this misconception, but the damage is done.
Here's the thing: it's not hard to design and build a backdoor. What's hard is building the systems -- both technical and procedural -- around them. Here's Rob Graham:
He's only solving the part we already know how to solve. He's deliberately ignoring the stuff we don't know how to solve. We know how to make backdoors, we just don't know how to secure them.
A bunch of us cryptographers have already explained why we don't think this sort of thing will work in the foreseeable future. We write:
Exceptional access would force Internet system developers to reverse "forward secrecy" design practices that seek to minimize the impact on user privacy when systems are breached. The complexity of today's Internet environment, with millions of apps and globally connected services, means that new law enforcement requirements are likely to introduce unanticipated, hard to detect security flaws. Beyond these and other technical vulnerabilities, the prospect of globally deployed exceptional access systems raises difficult problems about how such an environment would be governed and how to ensure that such systems would respect human rights and the rule of law.
Finally, Matthew Green:
The reason so few of us are willing to bet on massive-scale key escrow systems is that we've thought about it and we don't think it will work. We've looked at the threat model, the usage model, and the quality of hardware and software that exists today. Our informed opinion is that there's no detection system for key theft, there's no renewability system, HSMs are terrifically vulnerable (and the companies largely staffed with ex-intelligence employees), and insiders can be suborned. We're not going to put the data of a few billion people on the line an environment where we believe with high probability that the system will fail.