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.
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?