Which standard is commonly used for digital certificates?

Digital certificates are a crucial component of secure online communication, and the X.509 standard is widely recognized for defining the format of public key certificates. These certificates associate a public key with the identity of an entity (such as a person, organization, or device), enabling secure communications over networks like the internet.

X.509 specifies both the structure of the certificate itself and the hierarchy of trust through Certificate Authorities (CAs). It includes information such as the issuer, the subject (the entity to which the certificate has been issued), the public key, and a digital signature from the CA. This standard plays a vital role in enabling SSL/TLS protocols, which secure web traffic, by ensuring that users can verify the authenticity of the entities they are communicating with.

In contrast, the other options relate to different areas of cryptography or encryption but do not pertain specifically to digital certificates. DES and RC5 are encryption algorithms, primarily used for securing data rather than establishing identities through certificates. Diffie-Hellman is a key exchange protocol used to securely share cryptographic keys over a public channel, not a standard for digital certificates. Thus, X.509 stands out as the foundational standard for digital certificates, making it the correct answer.