Symmetric cryptosystem

Symmetric key cryptosystem, also called key cryptosystem, means that the sender of a message and the receiver of the message must use the same key, which must be kept secret. The sender encrypts the message to be sent with this key and then transmits the message to the receiver, who then decrypts the received message with the same key. This process can be expressed in mathematical form. The encryption function encrypt used by the message sender has two parameters: the key K and the message to be encrypted M, encrypted message for E, E can be expressed as E = encrypt (K, M) message receiver to use the decryption function decrypt to reverse this process, it produced the original message:

M = decrypt (K, E) = decrypt(K, encrypt(K, M))

2. Asymmetric-key cryptosystem

Asymmetric-key cryptosystem, also known as public-key cryptosystem, uses two keys: a public **** key PK and a private key SK. These two keys are mathematically related, and the corresponding private key cannot be computed from the public key, and likewise cannot be computed from the private key. private key to compute the corresponding public key. This method of encryption and decryption with two keys is expressed in the following mathematical form. Suppose M denotes a message, pub-a denotes user a's public *** key, and prv-a denotes user a's private key, then:

M=decrypt(pub-a, encrypt(prv- a, M))