1. Apparatus for comparing pairs of binary words (u, u′), the apparatus comprising: an intermediate value determiner configured to determine an intermediate binary word (z), so that the intermediate binary word (z) is equal to a reference binary word for a first pair of equal or inverted binary words, so that the intermediate binary word (z) is equal to an inversion of the reference binary word for a second pair of equal or inverted binary words and so that the intermediate binary word (z) is unequal to the reference binary word and the inverted reference binary word for a pair of unequal and uninverted binary words, if the intermediate value determiner works faultlessly, wherein the first pair of equal or inverted binary words is different from the second pair of equal or inverted binary words; and an error detector configured to provide an error signal (y) based on the intermediate binary word(z) so that the error signal (y) indicates whether or not the binary words (u, u′) of a pair of binary words are equal or inverted, if the intermediate value determiner works faultlessly, and indicates whether or not the intermediate value determiner works faultlessly, if the binary words (u, u′) of a pair of binary words are equal or inverted.