Prehistory of Cryptography - A Classical Introduction to Cryptography: Applications for Communications Security - page 9

Figure 1.4. Visual cryptography example.

26 3 ,welet For a plaintext x = x 1 ,..., x m with m <

y i = σ − 1 ◦ α − 1

i 1

◦ β − 1
i 2

◦ γ − 1
i 3

◦ π ◦ γ i 3 ◦ β i 2 ◦ α i 1 ◦ σ ( x i )

where i 3 i 2 i 1 are the last three digits of the basis 26 numeration of i + a . Finally, we let

y = y 1 ,..., y m be the ciphertext.

This is a mathematical definition of an electromechanical device. In practice, α ,

β , and γ are rotors that permute 26 electrical signals, α i simply consists of α turned

by i positions, S is a box of five rotors, π is a cabled permutation which sends back

electrical signals, σ is a configurable wire connection, and a is the initial position of

the rotors.

Here the secret consists of

the specification of manufactured permutations like

π and the five permutations

of S ,
the choice of the initial offset a ,
the choice of

σ .

Once again, we notice that the Enigma cipher consists of a keyed substitution. Here,
the key is rather short.