A method for protecting a key implemented, by an electronic circuit, in a symmetrical algorithm for ciphering or deciphering a message, including the steps of: selecting in non-deterministic fashion a pair of different masks from a set of at least four different masks, the masks having the property of representing different bit combinations, at least by pairs of bits; executing the algorithm twice by applying, to the key or to the message, one of the masks of the selected pair at each execution; checking the consistency between the two executions.