A method for authenticating at least one subscriber during a data interchange between at least two subscribers is described. In the method, a first subscriber transmits a first data item (a challenge signal) to a second subscriber, the second subscriber processes the first data item using an algorithm to give a second data item and transmits it to the first subscriber, which checks it for its correctness. At the same time as the first data item is being processed using the algorithm, at least one other processing operation on the first data item is carried out.