We create a new version of the FEAL-N(X) cryptographic function, called FEAL-N(X)S, by introducing a dynamic swapping function. FEAL-N(X)S is stronger against Differential Cryptanalysis in the sense that any characteristic 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, which 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 type of characteristic. We also show that in general the probability of an iterative characteristic for the FEAL-N(X) that is still valid for FEAL-N(X)S is decreased by 1/2 per round. Some of the best characteristics are shown. Experimental results show that the running time required by FEAL-N(X)S is around 10% greater compared to FEAL-N(X), in software; but this price is small compared to the gained strength against Differential Cryptanalysis.