Why is it not practical to use an arbitrary reversible

Authors:

William Stallings

Chapter:

Block Ciphers And The Data Encryption Standard

Exercise:

Review Questions

Question:3 | ISBN:9781292158587 | Edition: 7

Why is it not practical to use an arbitrary reversible substitution cipher of the kind shown in Table 4.1?

Table 4.1:Encryption and Decryption Tables for Substitution Cipher of Figure 4.2 in text book.

Plaintext	Ciphertext
0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111	1110 0100 1101 0001 0010 1111 1011 1000 0011 1010 0110 1100 0101 1001 0000 0111

Ciphertext	Plaintext
0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111	1110 0100 1101 0001 0010 1111 1011 1000 0011 1010 0110 1100 0101 1001 0000 0111