This topic examines the encryption and decryption knowledge points in security. When sending a message, the sender uses a hash function to generate a message digest from the message text, and then encrypts the digest with its own private key. The encrypted digest will be sent to the receiver as a message digital signature. The receiver first uses the same hash function as the sender to calculate the message digest from the received original message, and then decrypts the message with the sender's public key. If the two digests are the same, the receiver can confirm that the digital signature belongs to the sender.