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