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In cryptography, Blowfish is a keyed, symmetric block cipher, designed in 1993 by Bruce Schneier and included in a large number of cipher suites and encryption products. While no effective cryptanalysis of Blowfish has been found to date, more attention is now given to block ciphers with a larger block size, such as AES or Twofish. Schneier designed Blowfish as a general-purpose algorithm, intended as a replacement for the aging DES and free of the problems associated with other algorithms. At the time, many other designs were proprietary, encumbered by patents or kept as government secrets. Schneier has stated that, "Blowfish is unpatented, and will remain so in all countries. The algorithm is hereby placed in the public domain, and can be freely used by anyone." Notable features of the design include key-dependent S-boxes and a highly complex key schedule.
The algorithm
Cryptanalysis of Blowfish There is no effective cryptanalysis on the full-round version of Blowfish known publicly as of 2006, although the 64-bit block size is now considered too short, because encrypting more than 232 data blocks can begin to leak information about the plaintext in most modes of operation due to the birthday attack. Despite this, Blowfish seems thus far to be secure. While the short block size does not pose any serious concerns for routine consumer applications like e-mail, Blowfish may not be suitable in situations where large plaintexts must be encrypted, as in data archival. In 1996, Serge Vaudenay found a known-plaintext attack requiring 28r + 1 known plaintexts to break, where r is the number of rounds. Moreover, he also found a class of weak keys that can be detected and broken by the same attack with only 24r + 1 known plaintexts. This attack cannot be used against the full 16-round Blowfish; Vaudenay used a reduced-round variant of Blowfish. Vincent Rijmen, in his Ph.D. thesis, introduced a second-order differential attack that can break four rounds and no more. There remains no known way to break the full 16 rounds, apart from a brute-force search. • Blowfish in practice Blowfish is one of the fastest block ciphers in widespread use, except when changing keys. Each new key requires pre-processing equivalent to encrypting about 4 kilobytes of text, which is very slow compared to other block ciphers. This prevents its use in certain applications, but is not a problem in others. In one application, it is actually a benefit: the password-hashing method used in OpenBSD uses an algorithm derived from Blowfish that makes use of the slow key schedule; the idea is that the extra computational effort required gives protection against dictionary attacks. In some implementations, Blowfish has a relatively large memory footprint of just over 4 kilobytes of RAM. This is not a problem even for older smaller desktop and laptop computers, but it does prevent use in the smallest embedded systems such as early smartcards. Blowfish is not subject to any patents and is therefore freely available for anyone to use. This has contributed to its popularity in cryptographic software. See also | |||||||||||||||||||||||||||||||
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