A user inputs a pattern consisting of a plurality of lines. The lines are classified by relative length, overall direction and degree of curvature. Where a line is started from a new position, the direction from the previous starting point is taken into account. The series of lines is then serialized into a key value, which may then be used to decrypt data stored on a device. This enables data to be securely stored since the key is supplied by the user at runtime and is not itself stored on the device.