Introduction

This project presents an implementation of a cryptanalytic attack against 6-round DES. An attack on 6-round DES is a probabilistic attack. By testing significantly many plaintext/ciphertext pairs, many of the key bits become statistically apparent. This attack centers on the use of characteristics. Please see [1] for a thorough discussion of characteristics as they apply to differential cryptanalysis. Using the properties of the characteristics and the known output from the last DES encryption round for each pair of inputs, it is possible to work backwards to determine the key bits that were XORed with the right-hand side of the output of the previous round to form the SBox input. In this case, I use two characteristics that allow the determination of 30 of the 56 key bits each (excluding parity bits). This corresponds to the input to 5 different SBoxes. Unfortunately 3 of the SBoxes (18 bits) for each characteristic overlap which leaves 42 bits that are known and 14 bits yet to determine. From this point, however, it is very simple to exhaustively search the remaining 2^14 possibilities and determine the correct key.

In researching this project, most of the references contained a high level description of the cryptanalysis on reduced round DES, but there were enough details missing to make it unclear as to how the attack could actually be implemented. This project is an attempt to show an implementation of the attack that fills in some of the gaps left by other sources for anyone who is new to this subject area.

To implement this attack, I first implemented the DES algorithm as described in [1]. The implementation of the cryptanalysis followed after much studying of [2] and [3].

Building

The project can be built with ant using the provided build.xml file.

ant

Cleaning

ant clean

Try it out

Thanks to the magic of github pages you can try it out here.

References

• [1] Eli Biham, Adi Shamir: Differential Cryptanalysis of DES-like Cryptosystems. CRYPTO 1990: 2-21
• [2] Eli Biham: Tutorial on Differenetial Cryptanalysis, 2005
• [3] Menezes, A. J., van Oorschot, P. C., and Vanstone, S. A. Handbook of Applied Cryptography. CRC Press, 1997