X.509 The most widely used standard for defining digital certificates. X.509 is actually an ITU Recommendation, which means that it has not yet been officially defined or approved. As a result, companies have implemented the standard in different ways. For example, both Netscape and Microsoft use X.509 certificates to implement SSL in their Web servers and browsers. However, an X.509 certificate generated by Netscape may not be readable by Microsoft products, and vice versa.