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NewDES
From Wikipedia, the free encyclopedia
| Designer(s): | Robert Scott |
|---|---|
| First published: | 1985 |
| Key size(s): | 120 bits |
| Block size(s): | 64 bits |
| Rounds: | 17 |
| Best public cryptanalysis: | |
| A related-key attack succeeds with 232 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 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 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 to make the cipher more secure [1].
Contents |
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
- EK(P) = C,
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 28.
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.
- 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).
- Schneier, Bruce (1996). Applied Cryptography, Second Edition. John Wiley & Sons, 306-308. ISBN 0-471-11709-9.