Which authentication mechanism uses public key authentication by specifying a remote user's public key in a locally stored list of authorized keys on the server?

The concept described in the question pertains to how certain authentication mechanisms leverage public key cryptography to validate a user's identity. In this case, the correct answer pertains to SSH key authentication.

SSH, or Secure Shell, is widely used for secure remote access to servers. The SSH key authentication mechanism involves generating a pair of cryptographic keys: a public key and a private key. The public key is added to the authorized_keys file on the server, while the private key is maintained securely by the user. When a user attempts to connect to the server, the server uses the public key to verify that the user possesses the corresponding private key without actually transmitting the private key over the network. This mechanism provides a robust method of authentication because it relies on cryptographic principles, making it extremely difficult for unauthorized users to gain access.

In contrast, digital signatures and code signing use public key cryptography but are primarily aimed at ensuring the integrity and authenticity of data rather than user authentication. Network Access Control (NAC) authentication involves more comprehensive policies and rules for device access rather than solely using public keys for user authentication. Thus, SSH key authentication is unique in its specific functionality and purpose related to establishing a secure connection between a user and a remote server.