X509 SSL certificate

 x509 specifies the format of SSL server certificates :

No matter its intended application(s), each X.509 certificate includes a public key, digital signature, and information about both the identity associated with the certificate and its issuing certificate authority (CA):

• The public key is part of a key pair that also includes a private key. The private key is kept secure, and the public key is included in the certificate. This public/private key pair:
  • Allows the owner of the private key to digitally sign documents; these signatures can be verified by anyone with the corresponding public key.
  • Allows third parties to send messages encrypted with the public key that only the owner of the private key can decrypt.
• A digital signature is an encoded hash (fixed-length digest) of a document that has been encrypted with a private key. When an X.509 certificate is signed by a publicly trusted CA, such as SSL.com, the certificate can be used by a third party to verify the identity of the entity presenting it.

https://www.ssl.com/faqs/what-is-an-x-509-certificate/