"For example, a thief sitting on a bus may notice the four characters that an unsuspecting person entered on the person's Smartphone. As another example, a thief may pick up a tablet computer in a public place and discover, based on finger prints on the display of the tablet computer, which characters were recently selected by the owner of the tablet computer," Apple added.

The new system proposed would not necessarily be based on identifying picture of people or places, for instance, a user can name picture with digits he could remember easily, and those digits must be entered whenever that image is displayed as part of the authentication process, Apple explained.

The firm is believed to be exploring other ways it could replace the iOS passcode system. In 2011 Apple also acquired Authentec, a biometrics company, hinting at the possibility of fingerprint recognition. It has also filed several parents related to facial recognition.

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