In this section, we'll look at a number of cryptographical techniques that have been used in historical ciphers. These ad hoc ciphers are not secure enough to be used in modern applications, but because of their simplicity, they can encourage us to learn more about cryptography. Exploring the weaknesses of classical cryptography also helps us to learn more about some of the principles of cryptography. Take a look at the following diagram:
Figure 2.1 shows the conventional encryption model used to encrypt a plaintext using a secret key that is shared with the other user over a secure channel. The user who wants to read the text will decrypt the ciphertext using the secret key, which will return the original plaintext. The key is private, and the encryption and decryption algorithms are made public because...
