Digital certificates are encrypted and decrypted by a public key system. Each user has a private key for (); At the same time, each user also has a public key of (). X.509 standard stipulates that digital certificates are issued by () and put into public directories for users to access. The signature field in the X.509 digital certificate refers to (please leave it blank). If the user UA obtains the certificate from the authority in place A and the user UB obtains the certificate from the authority in place B, then ().