Cryptology II.1 Composition of Ciphers

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Contents

1. Multiple ciphers and group structures [PDF].
2. Examples of multiple ciphers [PDF].
3. Cryptanalysis of double ciphers [PDF].
4. Triple ciphers [PDF].
5. Cascades of different ciphers [PDF].

The complete Section II.1 as an PDF file.

Introduction

A first approach to constructing strong ciphers is the composition of several simple transformation steps. We saw some examples of this approach already in classical cryptography, and we saw how this often resulted in much stronger ciphers. But this effect is not guaranteed, and does not crop up in all cases. In this section we consider some basic aspects of this approach that is an essential ingredient of the construction of strong bitblock ciphers.

Terminologically we distinguish between

• Multiple ciphers: combinations of instances of the same encryption function but with different keys.
• Cascades: combinations of different encryption functions (also called product ciphers).