Classical encryption algorithm

There are two main types of classical cryptographic algorithms, substitution and permutation. Substitution ciphers can be further categorized into single-table substitution ciphers and multi-table substitution ciphers. In addition, there are also product ciphers that combine the methods of substitution and substitution.

Single-meter substitution codes

Substitution cipher is the replacement of characters in the plaintext with other characters. Single-table substitution cipher, as the name suggests, this encryption algorithm has only one deterministic mapping from the plaintext alphabet to the ciphertext alphabet during the encryption process. The main single-table substitution ciphers are additive ciphers, multiplicative ciphers and affine ciphers.

Multi-meter substitution algorithm

Multi-table substitution ciphers have several mappings from the plaintext alphabet to the ciphertext alphabet during the encryption process. The key determines the mapping relationship between each letter in the plaintext and each letter in the ciphertext, so the same letter appearing in different positions in the plaintext can be replaced by different letters in the ciphertext.

The Virginia cipher is one of the most representative multi-table substitution algorithms.

Permutation cipher

A substitution cipher is a cipher that does not change the letters in the plaintext, but merely rearranges them according to some rules.

Fence cipher is a typical substitution cipher, which means that the plaintext is divided into n groups, and then the first letter of each group is connected, and then the second letter of each group is connected, and so on and so forth, and finally connected to the whole ciphertext.

Product code

A simple substitution or replacement cipher is not secure, so you can consider using a number of such ciphers in succession to make them difficult to crack, such as a substitution followed by a replacement, which can generate a new and more difficult to crack the cipher, which is the product cipher.

This is the product cipher. The product cipher is a bridge from classical ciphers to modern ciphers.

Your task

Can you give examples of applications of classical encryption algorithms in your life?

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