Device authentication is implemented at the application layer of a computer communication model to add a factor to user authentication without requiring any action by the user. User space applications, such as web browsers, e-mail readers, and such, can remain completely unaffected. Instead, the additional authentication factor is provided at the application layer, typically in an operating system, where protocols such as HTTP(s), FTP(s), POP, SMTP, SNMP and DNS are implemented. Authentication is performed by a challenge/response transaction and the client device's digital fingerprint is compared to a whitelist of digital fingerprints of authorized client devices.