This paper from Mike Bond and Piotr Zielinski of Cambridge University presents an attack on hardware security modules used by retail banks for the secure storage and verification of customer PINs in cash machine infrastructures.
By using adaptive decimalisation tables and guesses, the maximum amount of information is learnt about the true PIN upon each guess. It takes an average of 15 guesses to determine a four digit PIN using this technique, instead of the 5000 guesses intended, claim the researchers
In a single 30 minute lunch-break, a corrupt bank employee can thus discover approximately 7000 PINs rather than 24 with the brute force method. With a $300 withdrawal limit per card, the potential bounty is raised from $7200 to $2.1 million and a single motivated attacker could withdraw up to $50,000 each day.
The paper explains the methodology of the attack which is described as a serious threat to bank security.
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