How do we build encryption backdoors?

End-to-end encryption 101

Modern encryption schemes break down into several categories. For the purposes of this discussion we'll consider two: those systems for which the provider holds the key, and the set of systems where the provider doesn't.

We're not terribly interested in the first type of encryption, which includes protocols like SSL/TLS and Google Hangouts, since those only protect data at the the link layer, i.e., until it reaches your provider's servers. I think it's fairly well established that if Facebook, Apple, Google or Yahoo can access your data, then the government can access it as well -- simply by subpoenaing or compelling those companies. We've even seen how this can work.

The encryption systems we're interested all belong to the second class -- protocols where even the provider can't decrypt your information. This includes:

    Apple and Android device encryption (based on user passwords and/or a key that never leaves the device).

    End-to-end messaging applications such as WhatsApp, iMessage and Telegram*.

    Encrypted phone/videochat applications such as Facetime and Signal.

    Encrypted email systems like PGP, or Google/Yahoo's end-to-end.

This seems like a relatively short list, but in practice w're talking about an awful lot of data. The iMessage and WhatsApp systems alone process billions of instant messages every day, and Apple's device encryption is on by default for everyone with a recent(ly updated) iPhone.

How to defeat end-to-end encryption

If you've decided to go after end-to-end encryption through legal means, there are a relatively small number of ways to proceed.

By far the simplest is to simply ban end-to-end crypto altogether, or to mandate weak encryption. There's some precedent for this: throughout the 1990s, the NSA forced U.S. companies to ship 'export' grade encryption that was billed as being good enough for commercial use, but weak enough for governments to attack. The problem with this strategy is that attacks only get better -- but legacy crypto never dies.

Fortunately for this discussion, we have some parameters to work with. One of these is that Washington seems to genuinely want to avoid dictating technological designs to Silicon Valley. More importantly, President Obama himself has stated that "there’s no scenario in which we don’t want really strong encryption". Taking these statements at face value should mean that we can exclude outright crypto bans, mandated designs, and any modification has the effect of fundamentally weakening encryption against outside attackers.

If we mix this all together, we're left with only two real options:

    Attacks on key distribution. In systems that depend on centralized, provider-operated key servers, such as WhatsApp, Facetime, Signal and iMessage,** governments can force providers to distribute illegitimate public keys, or register shadow devices to a user's account. This is essentially a man-in-the-middle attack on encrypted communication systems.

    Key escrow. Just about any encryption scheme can be modified to encrypt a copy of a decryption (or session) key such that a 'master keyholder' (e.g., Apple, or the U.S. government) can still decrypt. A major advantage is that this works even for device encryption systems, which have no key servers to suborn.

Each approach requires some modifications to clients, servers or other components of the system.