| Feistel Network |
Shopping Cipher |
Information AboutFeistel Network |
| CATEGORIES ABOUT FEISTEL CIPHER | |
| block ciphers | |
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Feistel networks and similar constructions are Product Cipher s, and so combine multiple rounds of repeated operations, such as:
to produce a function with large amounts of what Claude Shannon described as " Confusion And Diffusion ". Bit shuffling creates the diffusion effect, while substitution is used for confusion. HISTORY Feistel networks were first seen commercially in IBM's Lucifer cipher, designed by Feistel and Don Coppersmith . Feistel networks gained respectability when the US Federal Government adopted the DES (a cipher based on Lucifer, with changes made by the NSA ). Like other components of the DES, the iterative nature of the Feistel construction makes implementing the cryptosystem in hardware easier (particularly on the hardware available at the time of DES' design). Things have changed in the decades since as hardware has become more capable. THEORETICAL WORK Many modern symmetric block ciphers are based on Feistel networks, and the structure and properties of Feistel ciphers have been extensively explored by Cryptographer s. Specifically, Michael Luby and Charles Rackoff analyzed the Feistel block cipher construction and proved that if the round function is a Cryptographically Secure Pseudorandom Number Generator , with Ki used as the seed, then 3 rounds is sufficient to make the block cipher secure, while 4 rounds is sufficient to make the block cipher "strongly" secure, meaning that it is secure against Chosen-ciphertext Attack s. Because of this very important result of Luby and Rackoff, Feistel ciphers are sometimes inaccurately called Luby-Rackoff block ciphers. Further theoretical work has generalized the construction somewhat, and given more precise bounds for security [3 . CONSTRUCTION DETAILS The basic operation is as follows: Split the plaintext block into two equal pieces, (, ) For each round , compute : : where is the round function and is the sub-key. Then the ciphertext is (, ). Regardless of the function , decryption is accomplished via : : One advantage of this model is that the function used does not have to be invertible, and can be very complex. This diagram illustrates both encryption and decryption. Note the reversal of the subkey order for decryption; this is the only difference between encryption and decryption: Unbalanced Feistel Cipher s use a modified structure where L0 and R0 are not of equal lengths. The Skipjack Encryption Algorithm is an example of such a cipher. The Texas Instruments Digital Signature Transponder {Link without Title} uses a proprietary unbalanced Feistel cipher to perform Challenge-response Authentication . The Feistel construction is also used in cryptographic algorithms other than block ciphers. For example, the Optimal Asymmetric Encryption Padding (OAEP) scheme uses a simple Feistel network to randomize ciphertexts in certain Asymmetric Key Encryption schemes. LIST OF FEISTEL CIPHERS Feistel or modified Feistel: Blowfish , Camellia , CAST-128 , DES , FEAL , KASUMI , LOKI97 , Lucifer , MAGENTA , MISTY1 , RC5 , TEA , Triple DES , Twofish , XTEA Generalised Feistel: CAST-256 , MacGuffin , RC2 , RC6 , Skipjack =References= {Link without Title} M. Luby and C. Rackoff. "How to Construct Pseudorandom Permutations and Pseudorandom Functions." In ''SIAM J. Comput.," vol. 17, 1988, pp. 373-386. S. Bono, M. Green, A. Stubblefield, A. Rubin, A. Juels, M. Szydlo. "Security Analysis of a Cryptographically-Enabled RFID Device". In ''Proceedings of the USENIX Security Symposium'', August 2005. [http://www.usenix.org/events/sec05/tech/bono/bono.pdf (pdf) {Link without Title} Jacques Patarin, Luby-Rackoff: 7 Rounds Are Enough for Security, Lecture Notes in Computer Science, Volume 2729, Oct 2003, Pages 513 - 529 SEE ALSO |
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