A riddle wrapped in a curve

If you’re looking for a nice dose of crypto conspiracy theorizing and want to read a paper by some very knowledgeable cryptographers, I have just the paper for you. Titled “A Riddle Wrapped in an Enigma” by Neal Koblitz and Alfred J. Menezes, it tackles one of the great mysteries of the year 2015. Namely: why did the NSA just freak out and throw its Suite B program down the toilet?

Koblitz and Menezes are noted cryptographers and mathematicians, together responsible for scores of papers, including a recent series of position papers such as “Another Look at Provable Security” (mentioned previously on this blog) and “The Brave New World of Bodacious Assumptions in Cryptography”. (Yes, that is a real paper title.)

Their latest paper is highly accessible and contains almost no mathematics, so it’s worth your time to go read it now. If you don’t, or can’t handle reading things typeset in LaTeX, I’m going to sum up the gist of their arguments as best I can, then talk about why this all matters.

Let’s start with some background.

What is Suite B, and what do you mean by NSA ‘freaking out’?

For most of the past century, the NSA (and its predecessors) have jealously guarded a monopoly on advanced cryptography. This approach worked reasonably well through the 1980s, then rapidly became unsustainable. There were a couple of reasons for this. First, the PC revolution made it impossible for the NSA to keep encryption out of the hands of ordinary users and industry — as much as the NSA tried to make this so. Second, Congress got fed up with paying for expensive bespoke computer systems, and decided the DoD should buy its computing equipment off the shelf like everyone else.

The result was the Cryptographic Modernization Program, and Suite B — the first public cryptography standard to include non-classified algorithms certified for encrypting Secret and Top Secret data. Suite B was standardized for industry in the mid/late 1990s, and was notable for its exclusive reliance on (then new) field of elliptic curve cryptography (ECC) for public key encryption and key agreement.

At the time of Suite B’s adoption, ECC was relatively new to the non-classified world. Many industry and academic cryptographers didn’t feel it had been reasonably studied (Koblitz and Menezes’ anecdotes on this are priceless). ECC was noteworthy for using dramatically shorter keys than alternative public-key algorithms such as RSA and “classical” Diffie-Hellman, largely because new sub-exponential attacks that worked in those settings did not seem to have any analogue in the ECC world.

The NSA pushed hard for adoption. Since they had the best mathematicians, and moreover, clearly had the most to lose if things went south, the standards bodies were persuaded to go along. The algorithms of Suite B were standardized and — aside from a few intellectual property concerns — have been gradually adopted even by the non-classified community.

Then, in August of this year, NSA freaked out.

It was a quiet freakout, as you would expect from the world’s largest spy agency. But it made big waves with cryptographers. Citing concerns about future developments in quantum computing, the Agency updated the Suite B website to announce a rapid transition away from Suite B, and to a new set of quantum-resistant algorithms in the coming years.

Why all the sudden and immediate concern about quantum computers? Well, the NSA obviously isn’t willing to talk about that. But as of 2013, leaked slides from the Snowden cache show no real evidence of any massive quantum breakthroughs on the horizon that would necessitate a rapid transition from ECC.

Which quantum-resistant algorithms are we supposed to move to? Good question. Cryptographers haven’t agreed on any yet. Which makes the timing of this announcement so particularly mysterious.

So let me spell this out: despite the fact that quantum computers seem to be a long ways off and reasonable quantum-resistant replacement algorithms are nowhere to be seen, NSA decided to make this announcement publicly and not quietly behind the scenes. Weirder still, if you haven’t yet upgraded to Suite B, you are now being urged not to. In practice, that means some firms will stay with algorithms like RSA rather than transitioning to ECC at all. And RSA is also vulnerable to quantum attacks.

When reporters finally reached the NSA for comment, the only sound on the line was breaking glass and the sound of digging. (No, I’m kidding about that — but NSA did update their site again just to make it clear they weren’t playing some kind of deeply misguided April Fool’s prank.)

So that’s the background of the Koblitz/Menezes paper. The riddle is: what does the NSA know that we don’t?

No quantum advance?

Koblitz and Menezes exhaustively detail the possible reasons for the NSA’s announcement, ranging from the straightforward — that NSA actually has made a huge advance in developing quantum computers — to the downright bizarre. They even moot the possibility that the NSA is so embarrassed about getting caught tampering with national standards that they’ve just decided to flip the table and act like crazy people.

I’m going to skip most of the details, not because it isn’t interesting, but rather because I’d like to focus on what I think is the most intriguing hypotheses in the paper. Namely, that the NSA isn’t worried about quantum computers at all, but rather, that they’ve made a major advance in classical cryptanalysis of the elliptic curve discrete logarithm problem — and panic is the result.

Let me lay the groundwork. The security of most EC cryptosystems rests on the presumed intractability of a few basic mathematical problems. The most important of these is called the Elliptic Curve Discrete Logarithm Problem (ECDLP). This problem must be supremely hard to solve, or virtually every cryptosystem built on ECC unravels very quickly.

The definition of “hard” is important here. What we mean in the context of ECC is that the best known algorithm for solving the ECDLP requires a number of operations that is fully exponential in the security parameter. In practice, this means we can achieve 128-bit equivalent security using an elliptic curve set in a 256 bit field — which implies similarly-small keys and ciphertexts. To achieve equivalent security with RSA, where sub-exponential algorithms apply, your keys need to be at least 3072 bits (!) long.

But while the ability to use (relatively) tiny elliptic curve points is wonderful for implementers, it leaves no room for error. If NSA’s mathematicians began to make even modest, but sustained advances in the state of the art for solving the ECDLP, it would put the entire field at risk. Beginning with the smallest of the standard curves, P-256, which would now provide less than the required 128-bit security.

Did I mention that as part of the recent announcement, NSA also deprecated P-256?

Of course, there’s no reason to believe that classical cryptanalysis is at fault here. It’s possible that NSA really has reason to believe that quantum computing is moving faster than it was in 2013. It’s also possible that they’re just sloppy, and didn’t realize that having a very public freakout about cryptography you just promoted for years might not be such a good idea. But that seems doubtful.

You can read the Koblitz and Menezes paper for their take on all of the alternative hypotheses. I want to take a last few paragraphs to cover a slightly tangential issue, one that’s been the subject of a great deal of panic on the part of our community.

Did the NSA really backdoor the NIST elliptic curves????1??1

One of the nicer points of the Koblitz and Menezes paper is that it tackles the argument that the NSA deliberately backdoored the NIST elliptic curves.

This argument can be found in various places around the Internet. It originates with accomplished cryptographers who are quite rightly applying skepticism to an oddball and ad hoc process. It then heads off into crazytown. Whether people admit it or not, this concern has been hugely influential in the recent adoption of non-NIST alternative curves by the CFRG and IETF.

The accusation being leveled at NSA can be described simply. It’s known that the NIST pseudorandom curves were generated by the NSA hashing “seeds” through SHA1 to generate the curve parameters. This process was supposed to reassure the public, by eliminating the NSA’s ability to tamper with the parameters at will. However, nobody thought to restrict the NSA’s choice of input seeds — leading to a potential attack where they tried many different random seeds until they found a curve that contained a weakness the NSA could later exploit.

Koblitz and Menezes tackle this argument on grounds of basic common sense, history, and mathematics. The common sense argument, of course, is that you don’t sh*t where you eat. In other words, the NSA wouldn’t deliberately choose weak elliptic curves given that they planned to use them for encrypting Secret and Top Secret data for the next 20 years.

But common sense convinces nobody. The mathematical argument is a bit more subtle, and may work where self interest didn’t. Roughly speaking, Koblitz and Menezes point out that the NSA, despite their vast computing power, can’t try every possible seed. This is especially true given that they were generating these curves using 1990s equipment, and the process of evaluating (counting points on) an elliptic curve is computationally intensive. Thus, they assume that the NSA could try only test 2^{48} or so different curves for their hypothetical weakness.

By calculating the number of possible curve families, Koblitz and Menezes show that a vast proportion of curves (for P-256, around 2^{209} out of 2^{257}) would have to be weak in order for the NSA to succeed in this attack. The implications of such a large class of vulnerable curves is very bad for the field of ECC. It dwarfs every previous known weak curve class and would call into question the decision to use ECC at all.

In other words, Koblitz and Menezes are saying that if you accept the weak curve hypothesis into your heart, the solution is not to replace the NIST elliptic curves with anything at all, but rather, to leave the building as rapidly as possible and perhaps not shut the door on the way out. No joke.

On the gripping hand, this sounds very much like the plan NSA is currently implementing. Perhaps we should be worried.