Blunting Differential Attacks on PIN Processing APIs

We propose a countermeasure for a class of known attacks on the PIN processing API used in the ATM (cash machine) network. This API controls access to the tamper-resistant Hardware Security Modules where PIN encryption, decryption and verification takes place. The attacks are differential attacks, whereby an attacker gains information about the plaintext values of encrypted customer PINs by making changes to the non-confidential inputs to a command. Our proposed fix adds an integrity check to the parameters passed to the command. It is novel in that it involves very little change to the existing ATM network infrastructure