This topic examines the basic knowledge of key algorithms. According to whether the encryption key and decryption key are the same, the existing encryption systems can be divided into two types: single-key encryption system and double-key encryption system. The encryption technology using single-key encryption system is called symmetric key encryption, and the encryption technology using double-key encryption system is called asymmetric key encryption, also known as public key encryption. Common symmetric key encryption algorithms mainly include DES algorithm, IDEA algorithm, AES algorithm, RC4 algorithm and so on. Common asymmetric key encryption algorithms mainly include RSA algorithm, E 1 Gamal algorithm, Rabin algorithm and so on. The message digest algorithm uses one-way hash algorithm to digest the plaintext to be encrypted and generate a fixed-length one-way hash value. Among them, Hash Functions is a formula to convert messages with different lengths into a digital string (message digest), which does not need a key, and the formula determines the length of the message digest. Together with asymmetric key encryption, message digest algorithm can provide digital signature. Message digest algorithms mainly include secure hash standard and MD series standard. The hash value generated by the secure hash algorithm (SHA for short) is 160 bits. SHA has been approved as a standard by the US government. MD5 encoding generates 128-bit ciphertext. In the SET electronic payment model, RSA public key encryption is adopted, and a 160-bit digital digest is generated by secure hash algorithm SHA.