I just got back from CCS (ACM’s conference on Computer and Communications Security) in Vienna, Austria. I really enjoyed the conference and the city.
At the conference we presented our Build it, Break it, Fix it paper. In the same session was the presentation on a really interesting paper called Practical Detection of Entropy Loss in Pseudo-Random Number Generators, by Felix Dörre and Vladimir Klebanov. 1 This paper presents a program analysis that aims to find entropy-compromising flaws in PRNG implementations; such flaws could make the PRNG’s outputs more predictable (“entropy” is basically a synonym for “uncertainty”). When a PRNG is used to generate secrets like encryption keys, less entropy means those secrets can be more easily guessed by an attacker.
A PRNG can be viewed as a deterministic function from an actually-random seed to a pseudo-random output. The key insight of the work is that this function should not “lose entropy” — i.e., its outputs should contain all of the randomness present in its seed. Another way of saying this is that the PRNG must be injective. The injectivity of a function can be checked by an automatic program analysis.
Using the tool they developed, the paper’s authors were able to find known bugs, and an important unknown bug in libgcrypt’s PRNG. Their approach was essentially a variation of prior work checking noninterference; I found the connection to this property surprising and insightful. As discussed in a prior post, security properties can often be challenging to state and verify; in this case the property is pleasingingly simple to state and practical to verify.
Both injectivity and noninterference are examples of K-safety properties, which involve relations between multiple (i.e., K) program executions. New technology to check such properties is being actively researched, e.g., in DARPA’s STAC program. This paper is another example of the utility of that general thrust of work, and of using programming languages techniques (automated program analysis) for proving security properties.