start with the K function

src/test/java/ch/fhnw/kry/SPNTest.java

@@ -0,0 +1,18 @@
+package ch.fhnw.kry;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SPNTest {
+
+    @Test
+    void k() {
+        var spn = new SPN();
+
+        final int key = 0xFFFFFFFF;
+        assertEquals(0xFFFF,  spn.k(key, 0));
+        assertEquals(0xFFFF,  spn.k(key, 1));
+        assertEquals(0xFFFF,  spn.k(key, 2));
+    }
+}