The present invention relates to arrangements and methods for generating keys for cryptographic processing of communication between a first communication unit (200) and a second communication unit (300). The first communication unit (200) and second communication unit (300) are adapted to obtain knowledge about a secret function, wherein the first communication unit comprises: -means for selecting a value z (210), means for calculating the secret function as a function of the selected value z (220) means for processing data with the calculated secret function (230), and means for transmitting the processed data in association with the selected z to the second communication unit (240), wherein the secret function is selected from a set of functions that are almost k-wise independent.