Block Ciphers - Modern Cryptanalysis: Techniques for Advanced Code Breaking - page 80

Figure 4-5 EASY1 SPN cipher for a single round.
As Reference [4] points out, SPN ciphers are ideal for demonstrating several cryptanalytic techniques.
4.4.1 EASY1 Cipher
Thereisaneedforacipherthatiseasyenoughforyoutoeasilyimplementandseeresultsofvarioustechniques,
while still being complicated enough to be useful. For that, we shall create a cipher call EASY1 in this section.
We'll use it in later chapters to demonstrate some of the methods of cryptanalysis.
EASY1 is a 36-bit block cipher, with an 18-bit key. EASY1 works by splitting its input into 6-bit segments,
running them through S-boxes, concatenating the results back together, permuting them, and then XORing the
results with the key. The key is just copied side-by-side to become a 36-bit key (both the “left half” and “right
half” of the cipher bits are XORed with the same key).
We will use various numbers of rounds at different times, usually low numbers.
The 6-bit S-boxes in EASY1 are all the same:
[16, 42, 28, 3, 26, 0, 31, 46, 27, 14, 49, 62, 37, 56, 23, 6, 40, 48,
53, 8,
20, 25, 33, 1, 2, 63, 15, 34, 55, 21, 39, 57, 54, 45, 47, 13, 7, 44,
61, 9,
60, 32, 22, 29, 52, 19, 12, 50, 5, 51, 11, 18, 59, 41, 36, 30, 17, 38,
10, 4,
58, 43, 35, 24]
This is represented in the more compact, array form: The first element is the substitution for 0, the second
for 1, and so forth. Hence, an input of 0 is replaced by 16, 1 by 42, and so on. The P-box is a large, 36-bit P-box,
represented by