G-DES

GDES or G-DES (for Generalized OF the Design ) is a algorithm of Chiffrement per block derived from Data Encryption Standard and designed by Ingrid Schaumuller-Bichl in 1981. Intended to accelerate coding and to offer more safety, G-DES proved to be vulnerable to the Cryptanalyse differential. This attack will be presented by Eli Biham and Adi Shamir within their famous paper on the differential attacks (conference CRYPTO in 1990).

This publication proved that G-DES was an alternative much less robust than and than increased speed in term of coding was done with the detriment of safety.

Optimization

To accelerate coding, Schamuller-Bichl proposed to work with blocks while limiting the number of expensive operations. The principal block is subdivided in words of 32 bits. In OF traditional, the function known as F is destined for each turn on a block of 64 bits.

The diagram of G-DES consists in employing F only for the first under-block and to chain XOR with the remainder of the under-blocks of 32 bits. It follows a rotation towards the line of these blocks for the following turn. Thus G-DES manages to treat a greater number of bits but with the same number of calls to F as OF the original.

Biham and Shamir will show that a G-DES with 8 under-blocks of 32 bits (either 256 bits on the whole) and 16 turns could be broken with only 6 quantified texts (in a few seconds on a PC of the time). An attack on a more complex version with 8 under-blocks and 31 turns required 2⁵² quantified texts.

References

Eli Biham, Adi Shamir: Differential Cryptanalysis off Of-like Cryptosystems. CRYPTO 1990:2 - 21
Ingrid Schaumuller-Bichl, Zur Analyzes Data Encryption Standard und Synthese Verwandter Chiffriersysteme, Ph.D. Thesis, Linz university, May 1981. (In German).
I. Schaumuller-Bichl, " One the Design and Analysis off New Cipher Systems Related to OF, " Technical Carryforward, Linz University, 1983.

External bonds

Differential Cryptanalysis off Of-like Cryptosystems (chapter 9)

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