Passwords and PINs are still the most deployed authentication mechanisms and their protection is a classical branch of research in computer security. Several password schemes, as well as more sophisticated tokens, algorithms, and protocols, have been proposed during the last years. Some proposals require dedicated devices, such as biometric sensors, whereas, others of them have high computational requirements. Graphical passwords are a promising research branch, but implementation of many proposed schemes often requires considerable resources (e.g., data storage, high quality displays) making difficult their usage on small devices, like old fashioned ATM terminals, smart cards and many low-price cellular phones. In this paper we present a graphical mechanism that handles authentication by means of a numerical PIN, that users have to type on the basis of a secret sequence of objects and a graphical challenge. The proposed scheme can be instantiated in a way to require low computation capabilities, making it also suitable for small devices with limited resources. We prove that our scheme is effective against "shoulder surfing" attacks.
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