Classical encryption algorithms are divided into substitution algorithm and permutation shift method.

1. substitution algorithm

substitution algorithm uses plaintext letters to be replaced by other letters or numbers or symbols. The most famous alternative algorithm is Caesar cipher. The principle of Caesar's password is very simple, in fact, it is a single letter replacement.

example:

plaintext: abcdefghijklmnopq

ciphertext: defghijklmnopqrst

2. permutation and shift method

The most famous password using permutation and shift method is called Virginia password. It periodically replaces passwords based on permutation shift.

in the Virginia password, the encryption key is a string that can be arbitrarily specified. Encryption key characters act on plaintext information characters one by one in turn. The length of plaintext information is often longer than the length of key string, and each character of plaintext needs a corresponding key character, so the key needs to be continuously circulated until each character of plaintext corresponds to a key character.

Other common encryption algorithms

1. DES algorithm is a symmetric cryptosystem, which changes a 64-bit plaintext input block into a 64-bit ciphertext output block, and its key is also 64 bits.

2 and 3DES are symmetric algorithms based on DES, and a piece of data is encrypted three times with three different keys, with higher strength.

3, RC2 and RC4 are symmetric algorithms. It is faster than DES to encrypt a large amount of data with variable-length keys.

4. The IDEA algorithm is developed on the basis of DES algorithm, and is implemented as an iterative block cipher, using a 128-bit key and 8 cycles.

5. RSA was invented by RSA Company. It is a public key algorithm that supports variable-length keys, and the length of the file block to be encrypted is also variable, which is an asymmetric algorithm.

6. DSA (digital signature algorithm) is a standard DSS (digital signature standard), which is not strictly an encryption algorithm.

7. AES is an advanced encryption standard symmetric algorithm, which is the next generation encryption algorithm standard with high speed and high security level. One realization of AES standard in the 21st century is Rijndael algorithm.