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An EMV smartcard contains a (typically 16-bit) transaction counter that is incremented with each payment or CAP transaction. The response displayed by a CAP reader essentially consists of the various parts of the card's response (Application Transaction Counter, MAC, etc.) which is then reduced to specific bits as determined by the Issuer Authentication Indicator (IAI) record stored in the card (this is set on a per-issuer basis, although should an issuer desire, it could be set randomly for each card providing a database of each card's IAI is kept), finally, after unwanted bits are discarded (essentially the absolute position of bits is irrelevant, a bit in the IAI that is 0 means the corresponding bit in the card response will be dropped rather than merely being set to 0). Finally the value is converted from binary into a decimal number and displayed to the user. A truncated example is provided below:
# CAP device selects EMV application, reads IAI info from card and the user selects an action to perform (in this example, IAI will be 111011011000<sub>[[binary
# After successful PIN entry, CAP device sends challenge of 011100111010<sub>2</sub> as an Authorization Request Cryptogram (ARQC) transaction.
# Smartcard gives a response of 110101110110<sub>2</sub> and CAP device cancels the fake transaction.
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