Transparent two-factor authentication via mobile communication device

Two-factor authentication can be provided transparently to a user by virtue of proof information available at a mobile communication device. For example, after an access request for a service is sent, an authentication code can be intercepted from a responsive incoming message. The technologies can incorporate a cost proof as part of a cost optimization. Other features such as obfuscation and separate channels can be incorporated into the technologies to provide a superior user experience while implementing superior security.