In a blog post, cryptographer Matthew Green summarized the technical problems with this GCHQ proposal. Basically, making this backdoor work requires not only changing the cloud computers that oversee communications, but it also means changing the client program on everyone’s phone and computer. And that change makes all of those systems less secure. Levy and Robinson make a big deal of the fact that their backdoor would only be targeted against specific individuals and their communications, but it’s still a general backdoor that could be used against anybody.
The basic problem is that a backdoor is a technical capability — a vulnerability — that is available to anyone who knows about it and has access to it. Surrounding that vulnerability is a procedural system that tries to limit access to that capability. Computers, especially internet-connected computers, are inherently hackable, limiting the effectiveness of any procedures. The best defense is to not have the vulnerability at all.
Lest we ever forget why security backdoors, however
weasely well-worded, are a terrible idea, we’ve got Schneier calling them out. Spooks in democratic nations the world over keep coming up with “innovative” suggestions like this one from GCHQ but they keep solving the same problem, the technical problem of key distribution or key weakening or whatever it is that they want to achieve this week, without solving the actual underlying problem which is that any weakness introduced to a secure system, even a weakness that was created outwardly for the benefit of the “good guys”, can and eventually will be used by the “bad guys” too.
Furthermore: any known weakness introduced into a system for the purpose of helping the “good guys” will result in the distrust of that system by the people they’re trying to catch. It’s pretty trivial for criminals, foreign agents and terrorists to switch from networks that their enemies have rooted to networks that they (presumably) haven’t, which tends to mean a drift towards open-source security systems. Ultimately, any backdoor that gets used in a country with transparent judicial processes becomes effectively public knowledge, and ceases to be useful for the “good guys” any more. Only the non-criminals suffer, in the long run.