FeistelFunctions.java

import java.util.*;

public class FeistelFunctions {

    public static String xorIt(String bin1, String bin2) {
        if (bin1.length() != bin2.length()) {
            throw new IllegalArgumentException("xorIt requires same-length binary strings: " + bin1.length() + " vs " + bin2.length());
        }
        StringBuilder result = new StringBuilder();
        for (int i = 0; i < bin1.length(); i++) {
            result.append(bin1.charAt(i) == bin2.charAt(i) ? '0' : '1');
        }
        return result.toString();
    }


    public static String functionF(String rightHalf, String subkey) {
        String xorResult = xorIt(rightHalf, subkey);

        // apply SBox substitution
        StringBuilder sBoxResult = new StringBuilder();
        for (int i = 0; i < 32; i += 8) {
            sBoxResult.append(SBox.substitution(xorResult.substring(i, i + 8)));
        }

        return Permutation.permute(sBoxResult.toString());
    }

}