Saturday, May 16, 2009

AES-256 and Reputational Risk

Reputational risk is something that everyone understands, particularly businesses who regard their brand as one of their most critical assets. As there is considerable trust in the security of AES-256 both in the public and commercial sectors, reputational risk to AES-256 has a very high impact, and we therefore hope, a very low likelihood of occurrence.

But I will argue in this post that the likelihood of reputational damage to AES-256 is far from low, and perhaps even quite high. AES-256 keys are so large that it is next to impossible to argue that a cryptanalyst will not be able to find some shortcut to recovering keys that saves time over exhaustive search – simply because the cryptanalyst has so much time to play with.

For example, an attack that requires 2^{200} operations is a “huge” saving over the 2^{255} operations for a full exhaustive search of a 256-bit key space. But is a cryptanalyst with a computational arsenal of 2^{200} operations any more of a credible threat than one armed with 2^{255} operations? I think not. But the 2^{200} attack will seem quite menacing since it “saved” a massive 2^{55} operations, and there will be reputational damage to AES-256.

In short 256-bit keys are simply not defensible within the framework of traditional notions of security. I have been intending to post on this topic for sometime, and thanks to Eric Rescorla’s post for prompting me to action.

Let's start by talking about absolute and relative breaks on ciphers. Absolute breaks (like the name suggests) signal the end-of-life for a cipher, while relative breaks signal improvements over exhaustive search. Reputational risk for a cipher starts gaining momentum in the latter case.

Absolute Breaks

By an absolute break we mean, for a family of ciphers or for a cipher at a given key length, a known method to efficiently recover keys. We can measure efficiency in several ways. RSA, like other public key systems, represents a family of ciphers since key generation can be scaled to an arbitrary size. RSA keys can have lengths of 512, 1024 or 2048 bits, and even larger lengths in principle. In practice computational cost is the limiting factor that prevents public keys from becoming arbitrarily large. So to break a public key cipher in some absolute sense we require an attack that works efficiently for all potential key sizes. Or put anther way, an absolute break prevents the cipher designer from simply escaping the attack by increasing the key length.

Many of the early public key systems based on the knapsack problem (including the famous Merkle-Hellman system) were broken efficiently for all key sizes by applying the LLL basis reduction algorithm (you can read a detailed history here). Knapsack ciphers have essentially disappeared from the cryptographic landscape, as their reputation has been damaged beyond repair. Another example of an absolute break is Shor's algorithm, which assuming a large scale quantum computer, can factor an RSA N-bit modulus in time proportional to N^3 operations. Since doing an RSA decryption today requires N^3 bit operations we can say that Shor's algorithm breaks RSA in time proportional to what it costs to setup up an SSL session today. Now we may be very far away from having quantum computers, but even the remote threat of such a device has caused enduring reputational damage for popular public key ciphers in use today.

Absolute Breaks for Symmetric Ciphers

The other way to demonstrate an absolute break of a cipher is to show that a fixed key of a given length can be recovered in a short amount of elapsed time. So the 56-bit key length of DES was conclusively shown to be too short in 1998 by the EFF DES Cracker device which demonstrated key recovery in 56 hours (a bit an hour!). When we have a symmetric key cipher with a fixed key length, then this is what an absolute attack usually means – key recovery in a few weeks or months (or even days) when the expected time was thousands of years. When such an attack is successful the cipher is deemed broken or we must move to a variant, or an entirely new cipher with a longer key length.

Relative Breaks

Well-designed ciphers will typically not succumb to absolute breaks (barring a breakthrough attack), but even the best ciphers have some blemishes which reduce the effort of brute forcing key recovery. By a relative break we mean an attack which is faster than exhaustive search of the key space (the set of all possible keys), but not does break the cipher in any absolute sense. Relative breaks show that the implied margin of security for the key length is not as large as we thought. But what is the implied margin of security?

When a symmetric key cipher has a key length of n bits this is interpreted to mean that, in the absence of any weaknesses, the security of the cipher is proportional to the cost of a brute force attack on a key space of size 2^{n}. That is, a needle-in-a-haystack search problem where there are of 2^{n} pieces of hay. In the worst case an attacker will have to try all possible keys to find the right one (he guesses the right key last) but on average he will need only examine half the possible keys since


So we lose one bit of security from the key length in the average case of exhaustive search. Many attacks on symmetric ciphers are of the relative type, identifying a weakness which can be translated into some computational saving over exhaustive search. Let's translate the saving into R bits over the average case of exhaustive search

The question here is how large should R be before we think a relative attack is worth worrying about?

In the case of DES the complementary property gave us R = 1 for free. In the 70's, when DES was essentially the only commercial cipher in town, shaving off just a few bits was significant. This was true because many people thought that a 56-bit key was already teetering on the brink of feasible recovery, and saving a few more bits would definitely tip the scale towards an absolute break. But while saving say 5 bits for DES was significant, the same 5 bits of cryptanalytic advantage means less for a 128-bit cipher and practically nothing for AES-256. Nothing more than a mosquito hitting a bullet train.

Relative breaks can point to small cracks in the design of a cipher which can potentially be developed into absolute breaks. This is what we are now seeing with SHA-1 where the idea behind an initial saving in the cost of finding collisions of just a few bits has been developed into serious attacks. The relative is on the verge of being converted to the absolute.

This is the period of reputational damage for a cipher, since for a given relative attack we cannot be certain if it will be converted into an absolute break over time. In February last year a new attack on the 64-bit key space of the A5/1 cipher used in GSM devices was announced, with some fanfare, at a Black Hat conference. The attack claimed to provide a relative break by pre-computing a special “rainbow table” of size 2^{58}, but the promised details have yet to materialize.

Relative breaks for AES-256

Let us now turn specifically to AES-256. AES-256 should be secure against an attacker who has materially less than 2^{255} = 10^{76} resources at their disposal. Since 10^{76} is to within a few powers of 10 of the estimated number of atoms in the known universe, then it is essentially impossible for AES-256 to succumb to pure computational attacks on its key space.

So ruling out absolute breaks (in the absence of discovering a major weakness in AES), this only leaves relative breaks for consideration. Again, how big should the relative computational saving R be before we have a serious or material relative break?

Well let’s be generous and assume that when R = 55 we have a material relative break. This means that the cryptanalyst has saved a computational effort on the order of recovering a DES key. With this saving the cryptanalyst still has 2^{255-55} = 2^{200} resources at their disposal to break an AES-256 key.

You could read the headlines now “AES-256 Broken - researcher knocks off 17 zeroes from exhaustive search”, where 2^{55} is approximately 10^{17}. But would this result really imply a meaningful threat? All that has been done is change an absolutely absurd large computational task to a slightly less absurdly large computational task.

We are ceding too much to the cryptanalyst here by a giving them a massive 2^{200} sandbox for toying around with how to break AES-256. Every scrap of current computing power in the world running for a year would amount to less than 2^{100} operations and we are postulating that our cryptanalyst has access to 2^{100} times that power, or 10^{29} centuries of continuous computation at current rates.

Something is wrong here in the analysis – no one will have 2^200 resources up their sleeve. The problem with a 256-bit key is that it can shed 55 bits and still lose no real effective security. So the reputational risk is that someone emerges from this sandbox with a 2^{200}-idea that has credence.

Managing the reputation of AES-256

Google Trends is a service which estimates search and news activity for a given keyword. For “AES 256” the graph below was produced


It is evident that there has been a considerable increase in search and news activity since 2007. The letters in the upper graph are links to news stories, which are mainly product announcements for AES-256 support. Vendors are certainly adopting AES-256, and in a previous post I wrote that

… AES-256 is being widely deployed since it conveniently lies at the intersection of good marketing and pragmatic security. In upgrading from AES-128 to AES-256 vendors can legitimately claim that their products use maximum strength cryptography, and key lengths can be doubled (thus squaring the effort for brute force attacks) for a modest 40% performance hit.

However the only possible security threat which would warrant the deployment of 256-bits keys is the existence of large scale quantum computing devices. If this were the case then there is a quantum algorithm that effectively reduces the key space to 128-bits. But if this is the case, then we will be living in a very different world from the one we inhabit now.

Imagine you posed the following question to a group of top physicists. You asked them to present you with ideas for new research projects where they could assume that the budget included all the money that we have, all the money that has ever been, and the total financial assets of the world for the next 10 million years. Would the resulting proposals be credible?

AES-256 puts cryptanalysts on the same research agenda.

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