- Blowfish (cipher)
Infobox block cipher
name = Blowfish
caption = The round function (Feistel function) of Blowfish
designers =Bruce Schneier
publish date = 1993
derived from =
derived to =Twofish
key size = 32-448 bits in steps of 8 bits; default 128 bits
block size = 64 bits
structure =Feistel network
rounds = 16
cryptanalysis = Four rounds of Blowfish are susceptible to a second-orderdifferential attack (Rijmen, 1997); for a class ofweak key s, 14 rounds of Blowfish can be distinguished from apseudorandom permutation (Vaudenay, 1996).Incryptography , Blowfish is a keyed, symmetricblock cipher , designed in 1993 byBruce Schneier and included in a large number of cipher suites and encryption products. Blowfish provides a good encryption rate in software and no effectivecryptanalysis of it has been found to date. However, theAdvanced Encryption Standard now receives more attention.Schneier designed Blowfish as a general-purpose algorithm, intended as a replacement for the aging DES and free of the problems and constraints associated with other algorithms. At the time Blowfish was released, many other designs were proprietary, encumbered by
patent s or were commercial/government secrets. Schneier has stated that, "Blowfish is unpatented, and will remain so in all countries. The algorithm is hereby placed in thepublic domain , and can be freely used by anyone."Notable features of the design include key-dependent
S-box es and a highly complexkey schedule .The algorithm
Blowfish has a 64-bit block size and a variable
key length from 0 up to 448 bits [http://www.schneier.com/paper-blowfish-fse.html] . It is a 16-roundFeistel cipher and uses large key-dependent S-boxes. It is similar in structure toCAST-128 , which uses fixed S-boxes.The diagram to the left shows the action of Blowfish. Each line represents 32 bits. The algorithm keeps two subkey arrays: the 18-entry P-array and four 256-entry S-boxes. The S-boxes accept 8-bit input and produce 32-bit output. One entry of the P-array is used every round, and after the final round, each half of the data block is XORed with one of the two remaining unused P-entries.
The diagram to the right shows Blowfish's F-function. The function splits the 32-bit input into four eight-bit quarters, and uses the quarters as input to the S-boxes. The outputs are added modulo 232 and XORed to produce the final 32-bit output.
Since Blowfish is a Feistel network, it can be inverted simply by XORing P17 and P18 to the ciphertext block, then using the P-entries in reverse order.
Blowfish's
key schedule starts by initializing the P-array and S-boxes with values derived from thehexadecimal digits ofpi , which contain no obvious pattern (seenothing up my sleeve number ). The secret key is then XORed with the P-entries in order (cycling the key if necessary). A 64-bit all-zero block is then encrypted with the algorithm as it stands. The resultant ciphertext replaces P1 and P2. The ciphertext is then encrypted again with the new subkeys, and P3 and P4 are replaced by the new ciphertext. This continues, replacing the entire P-array and all the S-box entries. In all, the Blowfish encryption algorithm will run 521 times to generate all the subkeys - about 4KB of data is processed.Cryptanalysis of Blowfish
There is no effective
cryptanalysis on the full-round version of Blowfish known publicly as of 2008update after|2009|1|1. Asign extension bug in one publication of C code has been identified. [http://www.schneier.com/blowfish-bug.txt]In 1996,
Serge Vaudenay found a known-plaintext attack requiring 28"r" + 1 known plaintexts to break, where "r" is the number of rounds. Moreover, he also found a class ofweak key s that can be detected and broken by the same attack with only 24"r" + 1 known plaintexts. This attack cannot be used against the regular Blowfish; it assumes knowledge of the key-dependent S-boxes.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 abrute-force search . cite paper
author = Serge Vaudenay
title = On the Weak Keys of Blowfish
date = 1996
url = http://lasecwww.epfl.ch/php_code/publications/search.php?ref=Vau96a
format =PostScript
accessdate = 2006-08-23]Bruce Schneier notes that while Blowfish is still in use, he recommends using the more recent
Twofish algorithm instead cite web
url= http://www.computerworld.com.au/index.php/id;1891124482;pp;1;fp;4194304;fpid;1
title= Bruce Almighty: Schneier preaches security to Linux faithful
accessdate= 2007-12-31
author= Dahna McConnachie
last= Dahna
first= McConnachie
date= 2007-12-27
work= Computerworld
pages= 3
doi=
archiveurl=
archivedate=
quote= At this point, though, I'm amazed it's still being used. If people ask, I recommend Twofish instead.]Blowfish in practice
Blowfish is one of the fastest
block cipher s 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: thepassword -hashing method used inOpenBSD 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 againstdictionary attack s. "See"key strengthening .In some implementations, Blowfish has a memory footprint of just over 4 kilobytes of RAM. This constraint is not a problem even for older desktop and laptop computers, though it does prevent use in the smallest
embedded systems such as earlysmartcard s.Blowfish is not subject to any patents and is therefore freely available for anyone to use. This benefit has contributed to its popularity in cryptographic software.
ee also
*
Twofish
* MacGuffin
*Advanced Encryption Standard Notes and references
* Vincent Rijmen, "Cryptanalysis and design of iterated block ciphers", doctoral dissertation, October 1997.
* Bruce Schneier, Description of a New Variable-Length Key, 64-bit Block Cipher (Blowfish). Fast Software Encryption 1993: 191-204 [http://www.schneier.com/paper-blowfish-fse.html] .
* Bruce Schneier, The Blowfish Encryption Algorithm -- One Year Later, "Dr. Dobb's Journal ", 20(9), p. 137, September 1995 [http://www.schneier.com/paper-blowfish-oneyear.html] .
* Serge Vaudenay, "On the weak keys of Blowfish," Fast Software Encryption (FSE'96), LNCS 1039, D. Gollmann, Ed., Springer-Verlag, 1996, pp. 27--32.External links
* [http://www.schneier.com/blowfish.html Official Blowfish website]
* [http://www.schneier.com/blowfish-products.html List of products using Blowfish]
* [http://www.users.zetnet.co.uk/hopwood/crypto/scan/cs.html#Blowfish SCAN's entry for Blowfish]
* [http://www.php-einfach.de/blowfish_en.php Blowfish PHP and JavaScript implementation with online demo]
* [http://aam.ugpl.de/?q=node/1060 Blowfish JavaScript implementation and Page Encryption]
* [http://pear.php.net/package/Crypt_Blowfish/ Blowfish PHP implementation]
* [http://www.aurigalogic.com/auriga/avs/home/downloads/crypto.html Blowfish Java implementation (LGPL)]
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