Which format is most popular for public key certificates?

The most popular format for public key certificates is x.509 v3. This format is widely used for defining the structure and content of a public key certificate, which is essential for establishing secure communications in various network protocols, including SSL/TLS.

x.509 v3 certificates include critical information such as the public key of the certificate holder, the identity of the entity that issued the certificate (the Certificate Authority), the validity period, and various extensions that can provide additional information about the certificate's use and validation methods. The versatility and comprehensive structure of x.509 v3 make it the standard in both internet and enterprise security contexts.

PKI v.1 and PKI v.2 refer to different iterations of Public Key Infrastructure concepts and do not represent specific certificate formats used in practice, which is why they are less relevant in this context. SSL v.3, while related to secure communications, pertains to a protocol rather than a specific format for public key certificates. Thus, x.509 v3 remains the definitive answer as it encapsulates all the necessary attributes and conventions used for public key certificates in modern cybersecurity practices.