CA Certification Center is the authority responsible for issuing and managing digital certificates. Certification centers usually adopt a multi-level hierarchical structure. The superior certification center is responsible for issuing and managing the certificates of the subordinate certification centers, and the lowest certification center directly faces the end users. The main functions of China Biometric Portal Authentication Center are: issuing, updating, inquiring, invalidation and filing of certificates. Digital signature is an alphanumeric string obtained by processing the information to be transmitted through a one-way function, which is used to authenticate the source of the information and verify whether the information has changed during transmission. Digital signature provides the determination of the source of information and can detect whether the information has been tampered with. Digital certificate is a series of data that identifies the identity information of all parties in network communication, and its function is similar to that of ID card in real life. It is published by an authoritative organization, and people can use it to identify each other in communication. How to achieve the purpose of signing an electronic document? We can use digital signatures. RSA public key system can realize digital signature of digital information. The method is as follows: the sender of information uses his own private key to perform RSA algorithm operation on the characteristic data (or digital fingerprint) extracted from the transmitted message, so as to ensure that the sender can not deny the sending of the message (i.e. non-repudiation) and ensure that the information message has not been tampered with (i.e. integrity) during transmission. When the information receiver receives the message, he can verify the digital signature with the sender's public key.