A first partner connected to a channel collects samples of a physical variable on the basis of a time-variable property of the channel; stores a first array of at least bivalent elements; stores a second array of at least bivalent elements, each element in the second array corresponding to a remaining element in the first array and representing a first state if the sample, to which the remaining element in the first array corresponds, is outside a limit range and representing a second state if the sample is within the limit range; receives a parity check bit from the second partner; subjects elements in the first array to a parity check using the parity check bit; and, if the parity check fails, determines a checked element in the first array whose corresponding element in the second array represents the second state, and inverts the determined element in the first array.