Entries Tagged "SHA-3"

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Skein News

Skein is one of the 14 SHA-3 candidates chosen by NIST to advance to the second round. As part of the process, NIST allowed the algorithm designers to implement small “tweaks” to their algorithms. We’ve tweaked the rotation constants of Skein. This change does not affect Skein’s performance in any way.

The revised Skein paper contains the new rotation constants, as well as information about how we chose them and why we changed them, the results of some new cryptanalysis, plus new IVs and test vectors. Revised source code is here.

The latest information on Skein is always here.

Tweaks were due today, September 15. Now the SHA-3 process moves into the second round. According to NIST’s timeline, they’ll choose a set of final round candidate algorithms in 2010, and then a single hash algorithm in 2012. Between now and then, it’s up to all of us to evaluate the algorithms and let NIST know what we want. Cryptanalysis is important, of course, but so is performance.

Here’s my 2008 essay on SHA-3. The second-round algorithms are: BLAKE, Blue Midnight Wish, CubeHash, ECHO, Fugue, Grøstl, Hamsi, JH, Keccak, Luffa, Shabal, SHAvite-3, SIMD, and Skein. You can find details on all of them, as well as the current state of their cryptanalysis, here.

In other news, we’re making Skein shirts available to the public. Those of you who attended the First Hash Function Candidate Conference in Leuven, Belgium, earlier this year might have noticed the stylish black Skein polo shirts worn by the Skein team. Anyone who wants one is welcome to buy it, at cost. Details (with photos) are here. All orders must be received before 1 October, and then we’ll have all the shirts made in one batch.

Posted on September 15, 2009 at 6:10 AM • View Comments

SHA-3 Second Round Candidates Announced

NIST has announced the 14 SHA-3 candidates that have advanced to the second round: BLAKE, Blue Midnight Wish, CubeHash, ECHO, Fugue, Grøstl, Hamsi, JH, Keccak, Luffa, Shabal, SHAvite-3, SIMD, and Skein.

In February, I chose my favorites: Arirang, BLAKE, Blue Midnight Wish, ECHO, Grøstl, Keccak, LANE, Shabal, and Skein. Of the ones NIST eventually chose, I am most surprised to see CubeHash and most surprised not to see LANE.

Here’s my 2008 essay on SHA-3. Here’s NIST’s SHA-3 page. And here’s the page on my own submission, Skein.

Posted on July 24, 2009 at 12:15 PM • View Comments

MD6 Withdrawn from SHA-3 Competition

In other SHA-3 news, Ron Rivest seems to have withdrawn MD6 from the SHA-3 competition. From an e-mail to a NIST mailing list:

We suggest that MD6 is not yet ready for the next SHA-3 round, and we also provide some suggestions for NIST as the contest moves forward.

Basically, the issue is that in order for MD6 to be fast enough to be competitive, the designers have to reduce the number of rounds down to 30-40, and at those rounds, the algorithm loses its proofs of resistance to differential attacks.

Thus, while MD6 appears to be a robust and secure cryptographic hash algorithm, and has much merit for multi-core processors, our inability to provide a proof of security for a reduced-round (and possibly tweaked) version of MD6 against differential attacks suggests that MD6 is not ready for consideration for the next SHA-3 round.

EDITED TO ADD (7/1): This is a very classy withdrawal, as we expect from Ron Rivest—especially given the fact that there are no attacks on it, while other algorithms have been seriously broken and their submitters keep trying to pretend that no one has noticed.

EDITED TO ADD (7/6): From the MD6 website:

We are not withdrawing our submission; NIST is free to select MD6 for further consideration in the next round if it wishes. But at this point MD6 doesn’t meet our own standards for what we believe should be required of a SHA-3 candidate, and we suggest that NIST might do better looking elsewhere. In particular, we feel that a minimum “ticket of admission” for SHA-3 consideration should be a proof of resistance to basic differential attacks, and we don’t know how to make such a proof for a reduced-round MD6.

Posted on July 1, 2009 at 2:27 PM • View Comments

New Attack on AES

There’s a new cryptanalytic attack on AES that is better than brute force:

Abstract. In this paper we present two related-key attacks on the full AES. For AES-256 we show the first key recovery attack that works for all the keys and has complexity 2119, while the recent attack by Biryukov-Khovratovich-Nikolic works for a weak key class and has higher complexity. The second attack is the first cryptanalysis of the full AES-192. Both our attacks are boomerang attacks, which are based on the recent idea of finding local collisions in block ciphers and enhanced with the boomerang switching techniques to gain free rounds in the middle.

In an e-mail, the authors wrote:

We also expect that a careful analysis may reduce the complexities. As a preliminary result, we think that the complexity of the attack on AES-256 can be lowered from 2119 to about 2110.5 data and time.

We believe that these results may shed a new light on the design of the key-schedules of block ciphers, but they pose no immediate threat for the real world applications that use AES.

Agreed. While this attack is better than brute force—and some cryptographers will describe the algorithm as “broken” because of it—it is still far, far beyond our capabilities of computation. The attack is, and probably forever will be, theoretical. But remember: attacks always get better, they never get worse. Others will continue to improve on these numbers. While there’s no reason to panic, no reason to stop using AES, no reason to insist that NIST choose another encryption standard, this will certainly be a problem for some of the AES-based SHA-3 candidate hash functions.

EDITED TO ADD (7/14): An FAQ.

Posted on July 1, 2009 at 11:49 AM • View Comments

More SHA-3 News

NIST has published all 51 first-round candidates in its hash algorithm competition. (Presumably the other submissions—we heard they received 64—were rejected because they weren’t complete.) You can download the submission package from the NIST page. The SHA-3 Zoo is still the best source for up-to-date cryptanalysis information.

Various people have been trying to benchmark the performance of the candidates, but—of course—results depend on what metrics you choose.

And there’s news about Skein’s performance. And two Java implementations. (Does anyone want to do an implementation of Threefish?) In general, the Skein website is the place to go for up-to-date Skein information.

Posted on December 11, 2008 at 1:16 PM • View Comments

Skein and SHA-3 News

There are two bugs in the Skein code. They are subtle and esoteric, but they’re there. We have revised both the reference and optimized code—and provided new test vectors—on the Skein website. A revision of the paper—Version 1.1—has new IVs, new test vectors, and also fixes a few typos in the paper.

Errata: Version 1.1 of the paper, reference, and optimized code corrects an error in which the length of the configuration string was passed in as the size of the internal block (256 bits for Skein-256, 512 for Skein-512, and 1024 for Skein-1024), instead of a constant 256 bits for all three sizes. This error has no cryptographic significance, but affected the test vectors and the initialization values. The revised code also fixes a bug in the MAC mode key processing. This bug does not affect the NIST submission in any way.

NIST has received 64 submissions. (This article interviews one of the submitters, who is fifteen.) Of those, 28 are public and six have been broken. NIST is going through the submissions right now, making sure they are complete and proper. Their goal is to publish the accepted submissions by the end of the month, in advance of the Third Cryptographic Hash Workshop to be held in Belgium right after FSE in February. They expect to quickly make a first cut of algorithms—hopefully to about a dozen—and then give the community about a year of cryptanalysis before making a second cut in 2010.

Lastly, this is a really nice article on Skein.

These submissions make some accommodation to the Core 2 processor. They operate in “little-endian” mode (a quirk of the Intel-like processors that reads some bytes in reverse order). They also allow a large file to be broken into chunks to split the work across multiple processors.

However, virtually all of the contest submissions share the performance problem mentioned above. The logic they use won’t optimally fit within the constraints of a Intel Core 2 processor. Most will perform as bad or worse than the existing SHA-1 algorithm.

One exception to this is Skein, created by several well-known cryptographers and noted pundit Bruce Schneier. It was designed specifically to exploit all three of the Core 2 execution units and to run at a full 64-bits. This gives it roughly four to 10 times the logic density of competing submissions.

This is what I meant by the Matrix quote above. They didn’t bend the spoon; they bent the crypto algorithm. They moved the logic operations around in a way that wouldn’t weaken the crypto, but would strengthen its speed on the Intel Core 2.

In their paper (PDF), the authors of Skein express surprise that a custom silicon ASIC implementation is not any faster than the software implementation. They shouldn’t be surprised. Every time you can redefine a problem to run optimally in software, you will reach the same speeds you get with optimized ASIC hardware. The reason software has a reputation of being slow is because people don’t redefine the original problem.

That’s exactly what we were trying to do.

EDITED TO ADD (11/20): I wrote an essay for Wired.com on the process.

Posted on November 19, 2008 at 6:14 AM • View Comments

The Skein Hash Function

NIST is holding a competition to replace the SHA family of hash functions, which have been increasingly under attack. (I wrote about an early NIST hash workshop here.)

Skein is our submission (myself and seven others: Niels Ferguson, Stefan Lucks, Doug Whiting, Mihir Bellare, Tadayoshi Kohno, Jon Callas, and Jesse Walker). Here’s the paper:

Executive Summary

Skein is a new family of cryptographic hash functions. Its design combines speed, security, simplicity, and a great deal of flexibility in a modular package that is easy to analyze.

Skein is fast. Skein-512—our primary proposal—hashes data at 6.1 clock cycles per byte on a 64-bit CPU. This means that on a 3.1 GHz x64 Core 2 Duo CPU, Skein hashes data at 500 MBytes/second per core—almost twice as fast as SHA-512 and three times faster than SHA-256. An optional hash-tree mode speeds up parallelizable implementations even more. Skein is fast for short messages, too; Skein-512 hashes short messages in about 1000 clock cycles.

Skein is secure. Its conservative design is based on the Threefish block cipher. Our current best attack on Threefish-512 is on 25 of 72 rounds, for a safety factor of 2.9. For comparison, at a similar stage in the standardization process, the AES encryption algorithm had an attack on 6 of 10 rounds, for a safety factor of only 1.7. Additionally, Skein has a number of provably secure properties, greatly increasing confidence in the algorithm.

Skein is simple. Using only three primitive operations, the Skein compression function can be easily understood and remembered. The rest of the algorithm is a straightforward iteration of this function.

Skein is flexible. Skein is defined for three different internal state sizes—256 bits, 512 bits, and 1024 bits—and any output size. This allows Skein to be a drop-in replacement for the entire SHA family of hash functions. A completely optional and extendable argument system makes Skein an efficient tool to use for a very large number of functions: a PRNG, a stream cipher, a key derivation function, authentication without the overhead of HMAC, and a personalization capability. All these features can be implemented with very low overhead. Together with the Threefish large-block cipher at Skein core, this design provides a full set of symmetric cryptographic primitives suitable for most modern applications.

Skein is efficient on a variety of platforms, both hardware and software. Skein-512 can be implemented in about 200 bytes of state. Small devices, such as 8-bit smart cards, can implement Skein-256 using about 100 bytes of memory. Larger devices can implement the larger versions of Skein to achieve faster speeds.

Skein was designed by a team of highly experienced cryptographic experts from academia and industry, with expertise in cryptography, security analysis, software, chip design, and implementation of real-world cryptographic systems. This breadth of knowledge allowed them to create a balanced design that works well in all environments.

Here’s source code, text vectors, and the like for Skein. Watch the Skein website for any updates—new code, new results, new implementations, the proofs.

NIST’s deadline is Friday. It seems as if everyone—including many amateurs—is working on a hash function, and I predict that NIST will receive at least 80 submissions. (Compare this to the sixteen NIST submissions received for the AES competition in 1998.) I expect people to start posting their submissions over the weekend. (Ron Rivest already presented MD6 at Crypto in August.) Probably the best place to watch for new hash functions is here; I’ll try to keep a listing of the submissions myself.

The selection process will take around four years. I’ve previously called this sort of thing a cryptographic demolition derby—last one left standing wins—but that’s only half true. Certainly all the groups will spend the next couple of years trying to cryptanalyze each other, but in the end there will be a bunch of unbroken algorithms; NIST will select one based on performance and features.

NIST has stated that the goal of this process is not to choose the best standard but to choose a good standard. I think that’s smart of them; in this process, “best” is the enemy of “good.” My advice is this: immediately sort them based on performance and features. Ask the cryptographic community to focus its attention on the top dozen, rather than spread its attention across all 80—although I also expect that most of the amateur submissions will be rejected by NIST for not being “complete and proper.” Otherwise, people will break the easy ones and the better ones will go unanalyzed.

EDITED TO ADD (10/30): Here is a single website for all information, including cryptanalysis, of all the SHA-3 submissions. A spoke to a reporter who told me that, as of yesterday, NIST had received 30 submissions. And three news articles about Skein.

Posted on October 29, 2008 at 6:35 AM • View Comments

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Sidebar photo of Bruce Schneier by Joe MacInnis.