A method of reconstructing a packet that includes financial card information from a mobile device uses a card reader. The card reader has a read head, a slot for swiping a magnetic stripe of a card and device electronics that includes a microcontroller with logic resources with code. The card reader has a unique identifier and associated cryptographic key. The same code is used to produce an encrypted and signed signal. The encrypted and signed signal, that includes the card reader identifier, is sent to the mobile device. The mobile device sends an encrypted and signed packet to a back end of a payment system. A cryptographic key is used to decrypt and verify the contents of the packet at the back end. Non-encrypted financial card information is sent to a payment gateway.