TWOFISH

Twofish
The Twofish algorithm
General
Designer(s)	Bruce Schneier
First published	1993
Derived from	Blowfish, SAFER
Cipher(s) based on this design	none
Algorithm detail
Block size(s)	128 bits
Key size(s)	128, 192 or 256 bits
Structure	Feistel network
Number of rounds	16
Best cryptanalysis
Full 16-round Twofish is susceptible to a truncated differential cryptanalysis, requiring roughly 251 chosen plaintexts.[1]

In cryptography, Twofish is a symmetric key block cipher with a block size of 128 bits and key sizes up to 256 bits. It was one of the five finalists of the Advanced Encryption Standard contest, but was not selected for standardisation. Twofish is related to the earlier block cipher Blowfish.

Twofish's distinctive features are the use of pre-computed key-dependent S-boxes, and a relatively complex key schedule. Twofish borrows some elements from other designs; for example, the pseudo-Hadamard transform (PHT) from the SAFER family of ciphers. Twofish uses the same Feistel structure as DES.

On most software platforms Twofish is slightly slower than Rijndael (the chosen algorithm for Advanced Encryption Standard) for 128-bit keys, but somewhat faster for 256-bit keys.^[2]

Twofish was designed by Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Chris Hall, and Niels Ferguson; the "extended Twofish team" who met to perform further cryptanalysis of Twofish and other AES contest entrants included Stefan Lucks, Tadayoshi Kohno, and Mike Stay.

Cryptanalysis

As of 2006, the best cryptanalysis on the Twofish block cipher finds the full 16-round version to be susceptible to a truncated differential cryptanalysis attack. The paper finds that the probability of truncated differentials is 2^-57.3 per block and that it will take roughly 2⁵¹ chosen plaintexts (32 PiB worth of data) to find a good pair of truncated differentials.^[1]

References

1. ^ ^a b Shiho Moriai, Yiqun Lisa Yin (2000). "Cryptanalysis of Twofish (II)" (PDF). Retrieved on 2006-08-13.
2. ^ Bruce Schneier, Doug Whiting (2000-04-07). "A Performance Comparison of the Five AES Finalists" (PDF/PostScript). Retrieved on 2006-08-13.

1. Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Chris Hall, Niels Ferguson (1998-06-15). "The Twofish Encryption Algorithm" (PDF/PostScript). Wiley ISBN 0-471-35381-7. Retrieved on 2006-08-13.

External links

• Twofish web page with full specifications, free source code, and other Twofish resources.
• David Wagner's sci.crypt post recommending AES over Twofish — Wagner was one of the designers of Twofish.
• SCAN's entry for Twofish