Permutation cipher is also called transposition cipher. This kind of cipher changes the relative position of the elements of the plaintext message, but the value or content form of the plaintext message element itself remains unchanged; In the previous alternative cipher, it can be considered that the order of plaintext symbols is maintained, but they are replaced by other symbols.

This kind of cipher is a kind of cipher system which rearranges the position order of characters in plaintext to get ciphertext. There are many ways to realize it. It is the simplest permutation cipher to directly reverse the order of plaintext and then arrange it into a set of fixed length letters as ciphertext. For example, the plaintext is this cryptosystem is not secure. The ciphertext is eruc, ESTO, nsim, Etsy, sotp, yrcs, IHT.

Typical permutation cipher algorithms: periodic permutation cipher, column permutation cipher.

### Periodic permutation cipher

Periodic permutation cipher is a ciphertext encryption method, which divides plaintext characters into groups according to a certain length N and rearranges the position order of characters in each group according to a permutation of 1, 2,…, n. The key is substitution, which contains the information of group length. When decrypting, the ciphertext is divided into groups of length N, and each group of characters is rearranged according to the replacement of pi to get the plaintext.

### Column replacement password

In short, the encryption method of this password is to fill the plaintext into a table or matrix with fixed column width (set as m) by row, then exchange the position order of columns by a permutation of (1, 2,…, m), and then read the ciphertext by column. When decrypting, the ciphertext is filled in a table or matrix with a fixed number of rows (also m) by columns, and the position order of columns is exchanged according to the inverse permutation of permutation PI, and then the plaintext is read out by rows. Permutation PI can be regarded as the key of the algorithm.

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