The file sent by (1) is encrypted with SHA code to generate a digital summary of 128 bits (see the previous section).

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

(3) Send the original text and the encrypted abstract to the other party at the same time.

(4) The other party decrypts the digest with the sender's public key and encrypts the received file with SHA code to generate another digest.

(5) Comparing the decrypted digest with the digest generated by re-encrypting the received file at the receiver. If the two are consistent, it means that the information has not been destroyed or tampered with during transmission. Otherwise, otherwise.

The principle of digital digest: the sender encrypts the original information into a digest with a hash function, and then sends the digital digest and the original information to the receiver. The receiver also uses a hash function to encrypt the original message into a digest to see if the two digests are the same. If they are the same, the information is complete. Otherwise, it is incomplete.

It is used to ensure the integrity of information.