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Last week, Edward Snowden spoke to a packed crowd at SXSW about the many problems (and limited solutions) facing those of us who want to keep our communications private. Snowden said a number of things — including a shout out to Moxie’s company Whisper Systems, who certainly deserve it. But instead of talking about that, I wanted to focus on (in my opinion) one of Snowden’s most important quotes:

We need all those brilliant Belgian cryptographers to go “alright we know that these encryption algorithms we are using today work, typically it is the random number generators that are attacked as opposed to the encryption algorithms themselves. How can we make them [secure], how can we test them?”

Now it’s possible I’m a little biased, but it seems to me this cuts to the core of our problems with building secure systems in an increasingly hostile world. Namely: most encryption relies on some source of “random” numbers, either to generate keys or (particularly in the case of public key encryption) to provide semantic security for our ciphertexts.

What this means is that an attacker who can predict the output of your RNG — perhaps by taking advantage of a bug, or even compromising it at a design level — can often completely decrypt your communications. The Debian project learned this firsthand, as have many others. This certainly hasn’t escaped NSA’s notice, if the allegations regarding its Dual EC random number generator are true.

All of this brings us back to Snowden’s quote above, and the question he throws open for us. How do you know that an RNG is working? What kind of tests can we run on our code to avoid flaws ranging from the idiotic to the highly malicious? Unfortunately this question does not have an easy answer. In the rest of this post I’m going to try to explain why.