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General | |
---|---|

Designers | Robert Scott |

First published | 1985 |

Cipher detail | |

Key sizes | 120 bits |

Block sizes | 64 bits |

Rounds | 17 |

Best public cryptanalysis | |

A related-key attack succeeds with 2^{32} known plaintexts |

In cryptography, **NewDES** is a symmetric key block cipher. It was created in 1984–1985 by Robert Scott as a potential DES replacement.

Despite its name, it is not derived from DES and has quite a 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 attack; 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 so as to make the cipher more secure.^{[1]}

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 an S-box, the output of which is then XORed with another sub-block of data. In total, 8 XORs are performed in each round. The S-box is derived from the United States Declaration of Independence (to show that Scott had nothing up his sleeve).

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.

Only a small amount of cryptanalysis has been published on NewDES. The designer 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

then

where

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 also noticed that changing a full byte in all the key and data bytes leads to another complementation property. This reduces the work factor by 2^{8}.

Biham's related-key attack can break NewDES with 2^{33} 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 2^{32} known plaintexts and one related key.^{[2]}

**^**[1]**^**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).

- R. Scott, "Wide Open Encryption Design Offers Flexible Implementations," Cryptologia, v. 9, n. 1, Jan 1985, pp. 75–90.
- Schneier, Bruce (1996).
*Applied Cryptography, Second Edition*. John Wiley & Sons. pp. 306–308. ISBN 0-471-11709-9.