Two-factor authentication protects online accounts even if passwords are leaked. Most users, however, still prefer password-only authentication. One of the reasons behind two-factor authentication being unpopular is the extra steps that the user must complete in order to log in. Current two-factor authentication mechanisms require the user to interact with his phone, and e.g., copy a verification code to the browser. In this paper we propose Sound-Proof, a two-factor authentication mechanism that does not require interaction between the user and his phone. In Sound-Proof the second authentication factor is the proximity of the user's phone to the device being used to log in. The proximity of the two devices is verified by comparing the ambient noise recorded by their microphones. Audio recording and comparison are transparent to the user. Sound-Proof can be easily deployed as it works with major browsers without plugins. We build a prototype for both Android and iOS. We provide empirical evidence that ambient noise is a robust discriminant to determine the proximity of two devices both indoors and outdoors, and even if the phone is in a pocket or purse. We further conduct a user study designed to compare the perceived usability of Sound-Proof with Google 2-Step Verification. Participants ranked Sound-Proof as more usable and the majority would be willing to use Sound-Proof even for scenarios in which two-factor authentication is optional.
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