fix k function

.idea/gradle.xml

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
+  <component name="GradleMigrationSettings" migrationVersion="1" />
   <component name="GradleSettings">
     <option name="linkedExternalProjectsSettings">
       <GradleProjectSettings>

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

@@ -1,10 +1,12 @@
 package ch.fhnw.kry;
 
 public class SPN {
+    private final static int ROUND_KEY_LENGTH = 16;
+
     public int k(int key, int i) {
         i *= 4;
-        int mask = 0xFFFF >>> i;
+        int mask = 0xFFFF_0000 >>> i;
 
-        return key & mask;
+        return key & mask >>> ROUND_KEY_LENGTH - i;
     }
 }