Introduction to Cryptography with Coding Theory, 3rd Edition

7.2 A Simplified DES-Type Algorithm

The DES algorithm is rather unwieldy to use for examples, so in the present section we present an algorithm that has many of the same features, but is much smaller. Like DES, the present algorithm is a block cipher. Since the blocks are encrypted separately, we assume throughout the present discussion that the full message consists of only one block.

The message has 12 bits and is written in the form $L_0{R}_0$, where $L_0$ consists of the first six bits and $R_0$ consists of the last six bits. The key $K$ has nine bits. The $i$th round of the algorithm transforms an input $L_{i-1}{R}_{i-1}$ to the output $L_i{R}_i$ using an eight-bit key $K_i$ derived from $K$.

The main part of the encryption process is a function $f(R_{i-1}\text{,}{K}_i)$ that takes a six-bit ...