X.509 is a standard defining the format of public-key certificates. X.509 certificates are used in many Internet protocols, including TLS/SSL, which is the basis for HTTPS, the secure protocol for web browsing and transactions. An X.509 certificate, also known as a digital certificate, contains a public key and an identity (a hostname, an organization, or an individual), and is either signed by a certificate authority (CA) or self-signed.