Twofish is a symmetric-key block cipher with a block size of 128 bits and a key size of up to 256 bits. This algorithm was one of the five finalists of the AES public contest that was announced by National Institute of Standards and Technology (NIST) in 1997. The cipher is the further development of the well-known and popular Blowfish encryption algorithm developed in 1993 by a group of specialists headed by Bruce Schneier, the author of the bestseller “Applied Cryptography”. No successful cryptanalysis of this algorithm has been publicly published since the development of the Blowfish algorithm. Almost the entire company “Counterpane Systems” headed by Bruce Schneier participated in the development of the Twofish algorithm. To underline the cryptographic strength of the Twofish algorithm, its developers promised a $10,000 prize for the best cryptanalytic attack against it.

Twofish is perfectly implemented on 8, 32 and 64-bit architectures, which is its main advantage together with an amazing cryptographic strength. The cryptanalysis of Twofish showed that the algorithm has a much larger strength margin as compared to the rest of finalists in the AES contest. However, the complicated structure of the algorithm and its complicated analysis for weak keys and hidden relations together with being slower on most platforms than Rijndael did not let it win. As of 2008, the best cryptanalysis of Twofish is the one published by Shiho Moriai and Yiqun Lisa Yin in Japan in 2000. However, it was only theoretical research and no actual attack took place. The author of Twofish, Bruce Schneier, claims that it would be impossible to carry out such an attack in real life.

Complete Twofish-related information is available on the Twofish home page http://www.schneier.com/twofish.html.