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[[Image:Nordea e-kod.jpg|right|thumb|A Nordea E-code reader]]
In all three modes, the CAP reader asks the EMV card to output a data packet that confirms the cancellation of a fictitious EMV payment transaction, which involves the details entered by the user. This confirmation message contains a [[message authentication code]] (typically [[CBC-MAC]]/[[Triple DES]]) that is generated with the help of a card-specific secret key stored securely in the smartcard. Such cancellation messages pose no security risk to the regular EMV payment application, but can be cryptographically verified and are generated by an EMV card only after the correct PIN has been entered. It provided the CAP designers a way to create strong cryptographic evidence that a PIN-activated EMV card is present and has seen some given input data, without having to add any new software functions to
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:
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