Has threefish successfully been attacked (practically or theoretically)? Reading Schneier's "The Doghouse: Crypteto" dated September 30, 2009, I noticed Bruce Schneier stating:


  Threefish, the block cipher inside Skein, encrypts data at 7.6 clock cycles/byte with a 256-bit key, 6.1 clock cycles/byte with a 512-bit key, and 6.5 clock cycles/byte with a 1024-bit key. 


and


  I'm not claiming that Threefish is secure and ready for commercial use -- at any keylength -- but there simply isn't a chance that encryption speed will drop by half for every key bit added.


If I understand things correctly, "Skein" (with Threefish at it's core) was an SHA3 candidate. So I expect that Threefish has seen some cryptanalysis since 2009. 

Can you tell me if Threefish has successfully been attacked yet (either practically or theoretically) and point me to the related paper(s) so I can learn about those attacks and related security implications?
Full disclosure — I'm a Skein/Threefish co-author. Also, when I mention Skein/Threefish without any other qualification, I mean Skein/Threefish-512.

The security proofs we did for Skein prove that if there's a weakness in Skein, it implies an underlying weakness in its components (Threefish or UBI). As Dmitry says above, Threefish is very strong, and there are very few attacks on it. I don't know of one that's better than the 35-round attack we mention in V1.3 of the paper.

Our goal in Skein was that we know a lot more about designing block ciphers than hash functions so we'd build a strong hash function by starting with a strong cipher. Threefish builds on Twofish, (P)Helix, and other things. The philosophy was lots of fast rounds, and that has held up.

Enough time has passed that I'm starting to use Threefish in some of my own work. I think it's a very good way to go because tweakability is a powerful feature. If you are interested, also look at McOE mode which can turn a cipher into an authenticated function, and is designed to be resistant to misuse. It is built with AES and Threefish as examples, and I think Threefish in McOE mode is powerful.