Answer: D

If the message may be very large, such as a 10M message, if the message is encrypted, the decryption will be very slow, and the message digest is often of fixed length and cannot Reversely push out the message. Different messages have different message digests. Encrypting the message digest can play a signature role and improve the efficiency of encryption and decryption. The application process of digital signature is as follows: 1. The information sender uses a single hash function to generate a message digest; 2. The message sender uses his own private key to sign the message digest; 3. The message sender combines the message itself with the signed The digest information is sent together; 4. The information recipient generates a new information digest for the received information itself by using the same one-way hash function as the information sender, and then uses the information sender's public key to verify the information sent by the sender. Digest to confirm whether the identity of the sender of the message has been modified.