(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.