A method for protecting a key used, by an electronic circuit, in a symmetrical algorithm for ciphering or deciphering a message, including the steps of complementing to one the key and the message; executing the algorithm twice, respectively with the key and the message and with the key and the message complemented to one, the selection between that of the executions which processes the key and the message and that which processes the key and the message complemented to one being random; and checking the consistency between the two executions.