A user accesses a merchant system website via a user computing device, selects items for purchase, and selects an option to checkout using a digital wallet account. The user selects payment information associated with a payment card device for use in an online transaction. The merchant system transmits an unpredictable number to the user computing device. The user taps the payment card device to the user computing device to establish a wireless communication channel over which the payment card device receives the unpredictable number. The payment card device transmits payment card information and a check sum calculated from the unpredictable number and by the payment card device to the merchant system via the user computing device. The merchant system transmits the check sum and payment card information in a transaction authorization request to the issuer system, which verifies the check sum using the shared secret and the unpredictable number.