Authentication is the process of determining whether someone or something is, in fact, who or what it declares itself to be.
A backdoor is a means to access a computer system or encrypted data that bypasses the system's customary security mechanisms.
Remote Authentication Dial-In User Service (RADIUS) is a client/server protocol and software that enables remote access servers to communicate with a central server to authenticate dial-in users and authorize their access to the requested system or service.
Secure Shell (SSH) is a network protocol that secures data communications between computers on an insecure network using strong authentication and encryption. SSH also refers to the utility suite that implements the protocol, and which can be used to enable terminal sessions, file transfers and remote command execution.
Two-factor authentication (2FA), sometimes referred to as two-step verification or dual factor authentication, is a security process in which the user provides two different authentication factors to verify themselves to better protect both the user's credentials and the resources the user can access.