The present invention includes a method and device for calculating an encryption key, K_c, for use in the initialization of an encryption algorithm. The present invention discloses a method for calculating the encryption key, K_c(x), by obtaining a ciphering key, K_c(x), determining a maximum ciphering key length, L, based on the requirements of the two devices wishing to communicate, and determining a maximum usable encryption key length, L_max. The ciphering key, K_c(x), is then manipulated through an addition function or an exclusively-or function with a polynomial g₁(x). The result is multiplied with a polynomial g₂(x), which spreads the starting points of said encryption key, K_c(x). The encryption key is thus determined according to K_c(x)=g₂^(L)(x)[K_c(x) mod g₁^(L)(x)]. The method of calculating the encryption key, K_c, ensures that a maximum ciphering key length is not exceeded. In addition, the method provides new possible starting points each time a new encryption key, K_c, is generated.