Protecting E-Mail from Eavesdropping
In the wake of the Snowden NSA documents, reporters have been asking me whether encryption can solve the problem. Leaving aside the fact that much of what the NSA is collecting can’t be encrypted by the user—telephone metadata, e-mail headers, phone calling records, e-mail you’re reading from a phone or tablet or cloud provider, anything you post on Facebook—it’s hard to give good advice.
In theory, an e-mail program will protect you, but the reality is much more complicated.
- The program has to be vulnerability-free. If there is some back door in the program that bypasses, or weakens, the encryption, it’s not secure. It’s very difficult, almost impossible, to verify that a program is vulnerability-free.
- The user has to choose a secure password. Luckily, there’s advice on how to do this.
- The password has to be managed securely. The user can’t store it in a file somewhere. If he’s worried about security for after the FBI has arrested him and searched his house, he shouldn’t write it on a piece of paper, either.
- Actually, he should understand the threat model he’s operating under. Is it the NSA trying to eavesdrop on everything, or an FBI investigation that specifically targets him—or a targeted attack, like dropping a Trojan on his computer, that bypasses e-mail encryption entirely?
This is simply too much for the poor reporter, who wants an easy-to-transcribe answer.
We’ve known how to send cryptographically secure e-mail since the early 1990s. Twenty years later, we’re still working on the security engineering of e-mail programs. And if the NSA is eavesdropping on encrypted e-mail, and if the FBI is decrypting messages from suspects’ hard drives, they’re both breaking the engineering, not the underlying cryptographic algorithms.
On the other hand, the two adversaries can be very different. The NSA has to process a ginormous amount of traffic. It’s the “drinking from a fire hose” problem; they cannot afford to devote a lot of time to decrypting everything, because they simply don’t have the computing resources. There’s just too much data to collect. In these situations, even a modest level of encryption is enough—until you are specifically targeted. This is why the NSA saves all encrypted data it encounters; it might want to devote cryptanalysis resources to it at some later time.