An authentication protocol is a predefined set of rules and procedures that govern the verification of an entity's identity—whether a user, device, or system—when requesting access to digital resources. These protocols dictate the specific methodologies, data formats, and cryptographic exchanges required to confirm legitimacy, typically involving the secure transmission and validation of credentials such as passwords, digital certificates, or multi-factor authentication tokens.

Within cybersecurity, authentication protocols serve as foundational security controls and the front line of defense for protecting servers, applications, databases, and network infrastructure. By ensuring only authenticated entities can establish connections or elevate privileges, they play a critical role in preventing unauthorized access, identity theft, and data breaches. Common examples include Kerberos, LDAP, OAuth, and RADIUS, each designed for specific use cases ranging from enterprise network authentication to web application security.