A NEW VERSION OF FEAL, STRONGER AGAINST DIFFERENTIAL CRYPTANALYSIS

We create a new version of the FEAL-N(X) cryptographic function, calle d FEAL-N(X)S, by introducing a dynamic swapping function. FEAL-N(X)S i s stronger against Differential Cryptanalysis in the sense that any ch aracteristic for FEAL-N(X) is less effective when applied to FEAL-N(X) S. Furthermore, the only iterative characteristics that may attack the same number of rounds for the two versions are the symmetric ones, wh ich have an average probability bounded above by 2(-4) per round, i.e. , the FEAL-N(X)S is at least as strong as DES with respect to this typ e of characteristic. We also show that in general the probability of a n iterative characteristic for the FEAL-N(X) that is still valid for F EAL-N(X)S is decreased by 1/2 per round. Some of the best characterist ics are shown. Experimental results show that the running time require d by FEAL-N(X)S is around 10% greater compared to FEAL-N(X), in softwa re; but this price is small compared to the gained strength against Di fferential Cryptanalysis.