The purpose of this project is to demonstrate the necessity of each step in a Feistel network by performing practical attacks on a cipher with those steps removed.

In a cipher based on a Feistel network, there are two stages that each defeat particular classes of attacks:

• The S-boxes provide non-linearity in the ciphertext with respect to the plaintext and key, ensuring that linear relationships do not exist between the ciphertext and the inputs. A cipher without non-linearity is vulnerable to linear cryptanalytic attacks.
• The bit shuffle and plaintext expansion functions provide diffusion, ensuring that small changes in input lead to unpredictable changes in output. A cipher without diffusion is vulnerable to differential cryptanalytic attacks.

We will use DES as our cipher, since it is well-known, well-studied, and easy to implement. We will perform a practical attack resulting in either key or plaintext retrieval on two modified versions of DES; each version will have one of the above steps omitted to demonstrate the kind of attacks each property protects against.

#####Copyright (C) 2015 Zachary Price

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