Algorithm: Convert data of any length into a fixed-length data string (usually expressed by hexadecimal 16 string) through a function.
Common summarization algorithms are MD5, SHA 1, SHA256 and SHA5 12.
Function: detect whether the content has been modified.
There is a problem: in the process of sending a message, the content of the message can be modified by the middleman and a new digest can be generated, and the message receiver cannot determine whether the message has been tampered with.
Digital signature is the content generated by encrypting the digest with the private key of the message sender. The content can only be decrypted by the sender's public key, so it can prevent others from modifying it.
The purpose of digital signature is to ensure that the message is sent by the sender and has not been modified or replaced in the middle.
Problem: Digital signature can solve the problem that the message is modified by the middleman, but only if the sender's public key obtained by the message receiver is true. If the obtained public key is not the sender's, but the middleman's, then the message can still be tampered with.
Digital certificates are issued by a third party (generally called CA), and are generally generated by encrypting the public key and related information of the message sender with the private key of the third party. In this way, the message receiver decrypts the certificate with the public key of the third party and obtains the public key of the message sender.
The problem of digital certificate is to prevent the public key of the message sender from being replaced in the middle, thus solving the problems of simple digest and digital signature.