Patent ID: 8653938

Claim:
A method of protection of communication between a contactless card reader and a card, the method comprising: exchange between the reader and the card of information on a chosen value of duration T, measured with respect to a starting instant t 0 seen from the reader side; the decoding of this information of a value of duration T by the card; sending by the card of a return signal, comprising a pseudo-random sequence known to the reader, temporally set with respect to an instant t′ 0 +T, where t′ 0 is a starting instant as seen by the card taking into account delays in propagation or processing of signals received from the reader; detecting the return signal by the reader; determining the return signal with respect to the starting instant t 0 , including a search for maximum temporal correlation between the return signal and a pseudo-random sequence temporally offset with respect to the instant t 0 , the temporal setting of the return signal being the temporal offsetting of the sequence which provides a maximum of correlation; and interrupting communication by the reader if the temporal setting of the return signal detected is not equal to a theoretical instant of arrival of the return signal with a predetermined margin of error.