What is the standard used by most digital certificates?

The correct choice is X.509, which is the standard that is widely used for digital certificates. This standard defines the format for public key certificates and specifies how public keys are organized and managed. X.509 certificates are an essential part of the Public Key Infrastructure (PKI) framework, which is utilized to secure communications and verify identities across networks such as the internet.

X.509 certificates outline critical information, such as the subject's public key, the identity of the entity the certificate is issued to, the authority that issued the certificate, and the certificate's expiration date. These attributes ensure that a secure and trusted authentication process can take place, safeguarding data transmission and validation of identities.

The other options refer to different concepts within digital security. RFC 2298 is a standard concerning Internet message format for the delivery of digital certificates but is not specifically focused on the certificates themselves. OCSP (Online Certificate Status Protocol) provides real-time certificate status checking, and while it is essential for determining if a certificate is still valid, it does not describe the certificate's format. CRL (Certificate Revocation List) is a list of certificates that have been revoked by the issuing authority before their expiration date, which also does not pertain directly to the standard format