Skein (hash function)

Source: Wikipedia, the free encyclopedia.
For other uses, see Skein.
Skein
Core 2.[1]
Threefish Mix Function

Skein is a

Keccak.[2]

The name Skein refers to how the Skein function intertwines the input, similar to a skein of yarn.[1]

History

Skein was created by Bruce Schneier, Niels Ferguson, Stefan Lucks, Doug Whiting, Mihir Bellare, Tadayoshi Kohno, Jon Callas and Jesse Walker.

Skein is based on the

Matyas–Meyer–Oseas hash mode,[3]
while leveraging an optional low-overhead argument-system for flexibility.

Skein's algorithm and a reference implementation was given to public domain.[4]

Functionality

Skein supports internal state sizes of 256, 512 and 1024 bits, and arbitrary output sizes.[5]

The authors claim 6.1

cycles per byte for any output size on an Intel Core 2 Duo in 64-bit mode.[6]

The core of Threefish is based on a MIX function that transforms 2 64-bit words using a single addition, rotation by a constant and XOR. The UBI chaining mode combines an input chaining value with an arbitrary length input string and produces a fixed size output.

Threefish's

parallelizable tree hashing, a stream cipher, personalization, and a key derivation function
.

Cryptanalysis

In October 2010, an attack that combines rotational cryptanalysis with the rebound attack was published. The attack finds rotational collisions for 53 of 72 rounds in Threefish-256, and 57 of 72 rounds in Threefish-512. It also affects the Skein hash function.[7] This is a follow-up to the earlier attack published in February, which breaks 39 and 42 rounds respectively.[8]

The Skein team tweaked the key schedule constant for round 3 of the NIST hash function competition, to make this attack less effective, even though they believe the hash would still be secure without these tweaks.[1]

Examples of Skein hashes

Hash values of empty string. 

Skein-256-256("")
c8877087da56e072870daa843f176e9453115929094c3a40c463a196c29bf7ba
Skein-512-256("")
39ccc4554a8b31853b9de7a1fe638a24cce6b35a55f2431009e18780335d2621
Skein-512-512("")
bc5b4c50925519c290cc634277ae3d6257212395cba733bbad37a4af0fa06af41fca7903d06564fea7a2d3730dbdb80c1f85562dfcc070334ea4d1d9e72cba7a

Even a small change in the message will (with overwhelming probability) result in a mostly different hash, due to the avalanche effect. For example, adding a period to the end of the sentence: 

Skein-512-256("The quick brown fox jumps over the lazy dog")
b3250457e05d3060b1a4bbc1428bc75a3f525ca389aeab96cfa34638d96e492a
Skein-512-256("The quick brown fox jumps over the lazy dog.")
41e829d7fca71c7d7154ed8fc8a069f274dd664ae0ed29d365d919f4e575eebb
Skein-512-512("The quick brown fox jumps over the lazy dog")
94c2ae036dba8783d0b3f7d6cc111ff810702f5c77707999be7e1c9486ff238a7044de734293147359b4ac7e1d09cd247c351d69826b78dcddd951f0ef912713
Skein-512-512("The quick brown fox jumps over the lazy dog.")
658223cb3d69b5e76e3588ca63feffba0dc2ead38a95d0650564f2a39da8e83fbb42c9d6ad9e03fbfde8a25a880357d457dbd6f74cbcb5e728979577dbce5436

References

  1. ^ a b c Ferguson; et al. (2010-10-01). "The Skein Hash Function Family" (PDF). Archived from the original (PDF) on 2014-08-24. Retrieved 2010-12-15. {{cite journal}}: Cite journal requires |journal= (help)
  2. NIST
    . 2012-10-02. Retrieved 2012-10-02.
  3. ^ http://www.skein-hash.info/sites/default/files/skein1.3.pdf Archived 2014-08-24 at the Wayback Machine p. 6
  4. ^ skein_NIST_CD_121508.zip Archived 2016-06-10 at the Wayback Machine on skein-hash.info, skein.c "Implementation of the Skein hash function. Source code author: Doug Whiting, 2008. This algorithm and source code is released to the public domain."
  5. ^ "Now From Bruce Schneier, the Skein Hash Function". Slashdot. 2008-10-31. Retrieved 2008-10-31.
  6. ^ "Paper describing the hash function, Version 1.3 (2010-10-01)" (PDF). Archived from the original (PDF) on 2014-08-24. Retrieved 2010-12-15.
  7. ^ Dmitry Khovratovich; Ivica Nikolic; Christian Rechberger (2010-10-20). "Rotational Rebound Attacks on Reduced Skein". Cryptology ePrint Archive.
  8. ^ Dmitry Khovratovich & Ivica Nikolić (2010). "Rotational Cryptanalysis of ARX" (PDF). University of Luxembourg. Archived from the original (PDF) on 2013-01-26. Retrieved 2010-10-25. {{cite journal}}: Cite journal requires |journal= (help)

External links

Implementations

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