NewDES

From Academic Kids

In cryptography, NewDES is a symmetric key block cipher. It was created in 19841985 by Robert Scott as a potential DES replacement. Despite its name, it is not derived from DES and has a quite different structure. Its intended niche as a DES replacement has now mostly been filled by AES. The algorithm was revised with a modified key schedule in 1996 to counter a related-key vulnerability; this version is sometimes referred to as NewDES-96.

In 2004, Scott posted some comments on sci.crypt reflecting on the motivation behind NewDES's design and what he might have done differently to make the cipher more secure [1] (http://groups.google.com/groups?selm=418062d6.30341101%40news.provide.net).

Contents

The algorithm

NewDES, unlike DES, has no bit-level permutations, making it easy to implement in software. All operations are performed on whole bytes. It is a product cipher, consisting of 17 rounds performed on a 64-bit data block and makes use of a 120-bit key. In each round, subkey material is XORed with the 1-byte sub-blocks of data, then fed through a round function, the result of which is then XORed with another sub-block of data. In total, 8 XORs are performed in each round. The round function is derived from the United States Declaration of Independence.

Each set of two rounds uses seven 1-byte subkeys, which are derived by splitting 56 bits of the key into bytes. The key is then rotated 56 bits for use in the next two rounds.

Cryptanalysis of NewDES

Only a small amount of cryptanalysis has been published on NewDES. Scott, the designer of NewDES, showed that NewDES exhibits the full avalanche effect after seven rounds: every ciphertext bit depends on every plaintext bit and key bit.

NewDES has the same complementation property that DES has: namely, that if

<math>E_K(P)=C,<math>

then

<math>E_\overline{K}(\overline{P})=\overline{C},<math>

where

<math>\overline{x}<math>

is the bitwise complement of x. This means that the work factor for a brute force attack is reduced by a factor of 2.

Eli Biham's related-key attack can break NewDES with 233 chosen-key chosen plaintexts, meaning that NewDES is not as secure as DES.

John Kelsey, Bruce Schneier and David Wagner used related-key cryptanalysis to develop another attack on NewDES: it requires 232 known plaintexts and one related key. NewDES therefore does not seem very secure: using Triple DES or AES instead would be more prudent.

External links

References

  • R. Scott, "Wide Open Encryption Design Offers Flexible Implementations," Cryptologia, v. 9, n. 1, Jan 1985, pp. 75-90.
  • John Kelsey, Bruce Schneier, and David Wagner. Related-key cryptanalysis of 3-WAY, Biham-DES, CAST, DES-X, NewDES, RC2, and TEA. Lecture Notes in Computer Science 1334, pp233246, 1997 (PS or PDF) (http://www.schneier.com/paper-relatedkey.html).


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