Answer: X.509

The format of digital certificates generally adheres to the X.509 international standard. X.509 is one of the most widely used certificate formats today. The X.509 user public key certificate is created by a trustworthy certificate authority (Certificate Authority, CA), who or the user stores the certificate in an X.500 format directory. In the X.509 format, digital certificates usually include version number, serial number (the serial number of each certificate issued by the CA is unique), signature algorithm identifier, issuer name, validity, subject name, subject Public key information, issuer unique identifier, subject unique identifier, extended fields, and signature (the result of the CA digitally signing the above fields with its own private key can also be understood as the CA's signature on the user certificate).