Hi folks, The Linux kernel RNG currently pretends to care about the "premature next" RNG threat model. I'm wondering whether this is sensible and corresponds to anything real. "Premature next" is the scenario in which: - Attacker compromises the current state of a fully initialized RNG with a wild 'n crazy kernel infoleak. - New bits of entropy are added directly to the key used to generate the /dev/urandom stream, without any buffering or pooling. - Attacker then, somehow having read access to /dev/urandom, samples RNG output and brute forces the individual new bits that were added. - Result: the RNG never "recovers" from the initial compromise, a so-called violation of what academics term "post-compromise security". (Note that this is a different scenario from "premature first", which relates to boot-time concerns; this email isn't about "premature first".) There are other varied scenarios to this, usually involving some combination of: a) Attacker has access to /dev/urandom output continuously or at some interesting interval. b) Attacker controls one or more entropy sources providing some subset of varying size of those new bits of entropy. The Linux kernel currently pretends to mitigate this for scenario (a) at least, using "entropy estimation". The idea is that it waits until 256 estimated "bits" of new entropy are pooled before mixing them into the key used to generate the /dev/urandom stream. Never mind the fact that entropy estimation is an impossible proposition and thus flawed, it certainly does nothing in the way of (b), since there's just one pool. The NT kernel is a bit more robust, by way of their Fortuna RNG, in which there are several pools, and entropy sources round-robin into those pools. When it's time to reseed, the first pool is used every time, the second pool is used every other time, the third pool is used every third time, the forth pool is used every forth time, and so on. In theory this should handle both (a) and (b) without needing entropy estimation, and this sort of scheduler prompts interesting questions for academics with regards to different types of scheduling (a random walk instead of round-robin? sounds like a paper topic.) and trying to characterize the rate of inputs (continuous? sporadic? modelable?). While the above "problem" maps pretty clearly to things academics are interested in -- post-compromise security for a system with a clear model and various creative solutions -- I'm wondering whether any of this matters in the real world. From conversations over the last several months with various security experts and cryptographers, including those who work on the "premature next" problem, the impression I get is that nobody actually thinks this matters back on planet Earth, even from people who write papers on it. So the purpose of this email is to solicit feedback on whether anybody can think of a plausible scenario in which it does matter. If it does matter, the next step will be to determine how much it matters exactly, in order for me to gauge the cost-benefit ratio of mitigating the issue more robustly in the kernel (e.g. Fortuna requires non-zero code complexity; does the benefit outweigh the cost of such complexity?). On the other hand, if nobody can think of any reason why this matters, then there are some nice improvements that I'm eager to make in a different direction. To review, this attack model concerns: - An attacker who compromises the RNG at one point in time via a kernel infoleak. - After that infoleak, the attacker somehow no longer has access to the system, but can prevent the RNG from recovering from the compromise by having pretty rapid access to /dev/urandom (and possibly also having compromised zero or more entropy sources). The questions are thus: 1) When does an attacker with a kernel infoleak exercise it just once, and then attempt to maintain the leak with some sort of network access to lots of /dev/urandom output (from, e.g., large nonces)? 2) Or, if it's a local user attacker, when does that attacker infoleak once, but rather than just running the exploit again, cats /dev/urandom continuously? 3) More broadly speaking, what kernel infoleak is actually acceptable to the degree that anybody would feel okay in the first place about the system continuing to run after it's been compromised? Looking forward to hearing opinions on this. There's certainly a lot to split hairs about above -- incomplete/inaccurate description of the "premature next" model, what Fortuna actually achieves, my entropy estimation remark, and so forth -- but hopefully this at least throws enough things at the board to begin the discussion. Is "premature next" just an academic exercise, rather than a real world RNG concern? Regards, Jason
