In cryptography, DES-X (or DESX) is a variant on the DES (Data Encryption Standard) block cipher intended to increase the complexity of a brute force attack using a technique called key whitening. Cryptography (or cryptology; from Greek grc κρυπτός kryptos, "hidden secret" and grc γράφω gráphō, "I write" The Data Encryption Standard ( DES) is a Cipher (a method for Encrypting information selected by NBS as an official Federal Information In Cryptography, a block cipher is a symmetric key Cipher which operates on fixed-length groups of Bits termed blocks, with an In Cryptanalysis, a brute force attack is a method of defeating a Cryptographic scheme by trying a large number of possibilities for example possible keys In Cryptography, key whitening is a technique intended to increase the security of an iterated block cipher.

The original DES algorithm was specified in 1976 with a 56-bit key size: 2⁵⁶ possibilities for the key. In Cryptography, key size or key length is the size (usually measured in bits or bytes of the key used in a cryptographic algorithm (such as a Cipher In Cryptography, a key is a piece of information (a Parameter) that determines the functional output of a cryptographic algorithm There was criticism that an exhaustive search might be within the capabilities of large governments, particularly the United States' National Security Agency (NSA). The National Security Agency/ Central Security Service ( NSA/CSS) is a cryptologic intelligence agency of the United States government One scheme to increase the key size of DES without substantially altering the algorithm was DES-X, proposed by Ron Rivest in May 1984. Ronald Linn Rivest (born 1947, Schenectady, New York) is a cryptographer.

The algorithm was included in RSA Security's BSAFE cryptographic library since the late 1980s. RSA The Security Division of EMC Corporation, is headquartered in Bedford Massachusetts, United States and maintains offices in Ireland, Israel

DES-X augments DES by XORing an extra 64 bits of key (K₁) to the plaintext before applying DES, and then XORing another 64 bits of key (K₂) after the encryption:

The key size is thereby increased to 56 + 2 × 64 = 184 bits. In Cryptography, plaintext is the information which the sender wishes to transmit to the receiver(s

However, the effective key size (security) is only increased to 56+64-1-lg(M) =119 - lg(M) = ~119 bits, where M is the number of known plaintext/ciphertext pairs the adversary can obtain,and lg() denotes the binary logarithm. The known-plaintext attack (KPA is an Attack model for Cryptanalysis where the attacker has samples of both the Plaintext and its encrypted In Mathematics, the binary logarithm (log2 n) is the Logarithm for Base 2 (Because of this, some implementations actually make K₂ a strong one way function of K₁ and K. )

DES-X also increases the strength of DES against differential cryptanalysis and linear cryptanalysis, although the improvement is much smaller than in the case of brute force attacks. Differential cryptanalysis is a general form of Cryptanalysis applicable primarily to Block ciphers but also to Stream ciphers and Cryptographic hash In Cryptography, linear cryptanalysis is a general form of Cryptanalysis based on finding affine approximations to the action of a Cipher. It is estimated that differential cryptanalysis would require 2⁶¹ chosen plaintexts (vs. 2⁴⁷ for DES), while linear cryptanalysis would require 2⁶⁰ known plaintexts (vs. 2⁴³ for DES. ) Note that with 2⁶⁴ plaintexts (known or chosen being the same in this case), DES (or indeed any other block cipher with a 64 bit block size) is totally broken via the elementary codebook attack. In Cryptography, a block cipher is a symmetric key Cipher which operates on fixed-length groups of Bits termed blocks, with an In modern Cryptography, symmetric key Ciphers are generally divided into Stream ciphers and Block ciphers Block ciphers operate on a

See also

• G-DES
• Triple DES
• Meet-in-the-middle attack

References

• Joe Kilian and Phillip Rogaway, How to protect DES against exhaustive key search (PostScript), Advances in Cryptology - Crypto '96, Springer-Verlag (1996), pp. In Cryptography, the Generalized DES Scheme ( GDES or G-DES) is a variant of the DES Block cipher designed to speed-up the Encryption In Cryptography, Triple DES is a Block cipher formed from the Data Encryption Standard (DES Cipher by using it three times The Meet-in-the-middle attack is a cryptographic attack which like the Birthday attack, makes use of a Space-time tradeoff. 252–267.
• P. Rogaway, The security of DESX (PostScript), CryptoBytes 2(2) (Summer 1996).

External links

• RSA FAQ Entry
• Discussion of DES-X and source code samples (Dead link: May 2007)

---

© 2009 citizendia.org; parts available under the terms of GNU Free Documentation License, from http://en.wikipedia.org
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic