A trusted input channel, such as a numeric keypad, allows the smart card to present trusted output over an untrusted communications channel. The customer feeds the output from the untrusted path back to the smart card via the trusted input channel. If the smart card receives an unexpected value from the customer, it uses a single bit of trusted output, such as an integral LED, to signal the problem to the customer. (This method does not address the customer authentication problem for stolen smart cards.)