1. A secure data processing method that is executed by a microcircuit card and a host external to the microcircuit card, comprising: in the microcircuit card, filling an area of a volatile memory with a filling value; in the microcircuit card, reading a first data item from a non-volatile memory; in the microcircuit card, transmitting the first data item to a host device; in the microcircuit card, writing the first data item into said area of the volatile memory to replace said filling value; in the microcircuit card, reading from said area of the volatile memory a second data item that repeats at least a portion of the first data item as written in said area; in the microcircuit card, executing an operation using the second data item to produce a second data item; in the host, executing an operation using the first data item received from the microcircuit card to produce a third data item; in the microcircuit card, receiving the third data item from the host device; and in the microcircuit card, comparing the second data item to the third data item; and in the microcircuit card, executing a protected operation if and only if the comparing step determines a positive match between the second data item and the third data item.