The present invention is directed toward using patterns in APDU to perform identification data substitution. According to one or more embodiments of the present invention, a user inserts a smart card into a card reader connected to a client computing device. Then, the user enters a PIN. The PIN is embedded into an APDU which is sent to the card reader and is presented to the smart card. The APDU contains special patterns that specify to the card reader where and in what format the PIN should be embedded into a prototype APDU that is constructed in the card reader and presented to the card for verification.