the skein hash function

Eugen Leitl eugen at
Wed Oct 29 13:24:13 EDT 2008

October 29, 2008

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

    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

    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

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 21 submissions NIST received --
five were rejected as not being complete -- 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.

Posted on October 29, 2008 at 4:35 AM ? 22 Comments ? View Blog Reactions

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