Tamper-proofing and secure identity validation techniques in a transaction processing system and secure electronic payment techniques are disclosed. A tamper-proof transaction processing device is provided and comprises at least two different strength adhesives to secure parts of the device together and a housing comprising at least a first and second protective layer. An electronic component comprising a secure element chip storing unique information relating to the chip is located between the first and second protective layer in the housing. In another aspect, a transaction processing system includes a payment instrument that is configured to approve only negative value and/or zero value transaction requests. Another aspect provides an identity card checking system and method where the identity card is brought into proximity of a data processing device and identity information is displayed on the screen of the data processing device for the period of time while the card is in proximity.