A system and method are provided for using a mobile device to authenticate access to a private network. The mobile device may operate to receive a challenge from an authentication server, the challenge having being generated according to a request to access a private network; obtain a private value; use the private value, the challenge, and a private key to generate a response to the challenge; and send the response to the authentication server. An authentication server may operate to generate a challenge; send the challenge to a mobile device; receive a response from the mobile device, the response having been generated by the mobile device using a private value, the challenge, and a private key; verify the response; and confirm verification of the response with a VPN gateway to permit a computing device to access a private network.