Ainfrastructure is a framework that consists of programs, procedures, and security policies and employs public key cryptography and the x.509 standard (digital certificates) for secure communications.

The answer is "Public Key".

Explanation:

PKI stands for public key infrastructure. It is a collection of functions, protocols, equipment, code, and procedures that require to create, maintain, transmit, store and cancel encrypted certs and handle the authentication of a public key.