start with the K function

.idea/modules/kry-spn.test.iml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module version="4">
+  <component name="CheckStyle-IDEA-Module">
+    <option name="configuration">
+      <map />
+    </option>
+  </component>
+</module>

.idea/vcs.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>

src/main/java/ch/fhnw/kry/Main.java

@@ -5,6 +5,9 @@ package ch.fhnw.kry;
  */
 public class Main {
 
+    private static final String KEY = "00111010100101001101011000111111";
+    private static final String CHIFFRE = "00000100110100100000101110111000000000101000111110001110011111110110000001010001010000111010000000010011011001110010101110110000";
+
     public static void main(String[] args)  {
 
     }

src/main/java/ch/fhnw/kry/SPN.java

@@ -0,0 +1,10 @@
+package ch.fhnw.kry;
+
+public class SPN {
+    public int k(int key, int i) {
+        i *= 4;
+        int mask = 0xFFFF >>> i;
+
+        return key & mask;
+    }
+}

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));
+    }
+}