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Blowfish (cipher)
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Blowfish (cipher)

Blowfish

The round function (Feistel function) of Blowfish
General
Designer(s) Bruce Schneier
First published 1993
Derived from -
Cipher(s) based on this design -
Algorithm detail
Block size(s) 64 bits
Key size(s) 32-448 bits in steps of 8 bits; default 128 bits
Structure Feistel network
Number of rounds 16
Best cryptanalysis
Four rounds can be broken using a second-order differential attack (Rijmen, 1997); for a class of weak keys, 14 rounds of Blowfish can be distinguished from a random permutation (Vaudenay, 1996).
In cryptography, Blowfish is a symmetric key, secret key, block cipher designed in 1993 by Bruce Schneier and is included in a large number of cipher-suites and encryption products.

Schneier intended it as a general-purpose algorithm to replace the aging DES that had none of the problems associated with other algorithms available at the time. Blowfish is unencumbered by patents (as opposed to Khufu, REDOC II and IDEA), non-proprietary (as opposed to RC2 and RC4), and open to the public (unlike the then-secret Skipjack cipher).

Table of contents
1 The algorithm
2 Cryptanalysis of Blowfish
3 Blowfish in practice
4 See also
5 References
6 External links

The algorithm

Blowfish has a 64-bit block size and a key length of anywhere from 32 bits to 448 bits. It is a 16-round Feistel cipher and uses large key-dependent S-boxes. It is similar in structure to CAST-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 the hexadecimal digits of pi, which seem to be random. 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 of Blowfish known publicly as of 2004, 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. Despite this, Blowfish seems thus far to be secure, although specific implementations may not be.

Serge Vaudenay, in 1996, found a known-plaintext attack that requires 28r + 1 known plaintexts to break, where r is the number of rounds. However, 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 is still no known way to break the full 16 rounds other than brute force.

Blowfish in practice

It is one of the faster 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 a 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

References

External links


Block ciphers
Algorithms: 3-Way | AES | Blowfish | Camellia | CAST-128 | CAST-256 | CMEA | DEAL | DES | DES-X | FEAL | G-DES | GOST | IDEA | Iraqi | KASUMI | KHAZAD | Khufu and Khafre; | LOKI89/91 | LOKI97 | Lucifer | MacGuffin | Madryga | MAGENTA | MARS | MISTY1 | MMB | NewDES | RC2 | RC5 | RC6 | Red Pike; | S-1 | SAFER | Serpent | SHARK | Skipjack | Square | TEA | Triple DES; | Twofish | XTEA
Design: Feistel network; | Key schedule; | Product cipher; | S-box | SPN   Attacks: Brute force; | Linear / Differential cryptanalysis | Mod n; | XSL   Standardisation: AES process; | CRYPTREC | NESSIE   Misc: Avalanche effect | Block size; | IV | Key size; | Modes of operation; | Piling-up lemma; | Weak key;