# NewDES

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

[itex]E_K(P)=C,[itex]

then

[itex]E_\overline{K}(\overline{P})=\overline{C},[itex]

where

[itex]\overline{x}[itex]

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.

## 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, pp233–246, 1997 (PS or PDF) (http://www.schneier.com/paper-relatedkey.html).

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