PassChords

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Abstract
Blind mobile device users face security risks such as inaccessible authentication methods, and aural and visual eavesdropping. We interviewed 13 blind smartphone users and found that most participants were unaware of or not concerned about potential security threats. Not a single participant used optional authentication methods such as a password-protected screen lock. We addressed the high risk of unauthorized user access by developing PassChords, a non-visual authentication method for touch surfaces that is robust to aural and visual eavesdropping. A user enters a PassChord by tapping several times on a touch surface with one or more fingers. The set of fingers used in each tap defines the password. We give preliminary evidence that a four-tap PassChord has about the same entropy, a measure of password strength, as a four-digit personal identification number (PIN) used in the iPhone's Passcode Lock. We conducted a study with 16 blind participants that showed that PassChords were nearly three times as fast as iPhone's Passcode Lock with VoiceOver, suggesting that PassChords are a viable accessible authentication method for touch screens.