Answer: (1) Use the hash algorithm to calculate the original message and get a fixed-length number string called MessageDigest. Different messages get different message digests, but for the same message, the message digest is unique. Mathematically, it is guaranteed that as long as any bit in the message changes, the recalculated message digest value will not match the original value, thus ensuring the invariance of the message.

(2) The sender encrypts the abstract with his own private key to form a digital signature.

(3) The digital signature will be sent to the receiver together with the message as an attachment of the message.

(4) The receiver first calculates a new message digest for the received original message with the same algorithm, then decrypts the digital signature of the message attachment with the sender's public key, and compares the two message digests. If the values are the same, the receiver can confirm that the digital signature belongs to the sender, otherwise, the received message is considered to be forged or tampered with.