Make helper class final.

Author: dkocher

hub/src/main/java/ch/iterate/hub/crypto/KeyHelper.java

@@ -25,7 +25,7 @@
 import ch.iterate.hub.crypto.exceptions.NotECKeyException;
 import com.google.common.io.BaseEncoding;
 
-public class KeyHelper {
+public final class KeyHelper {
 
     // adapted from org.cryptomator.ui.keyloading.hub.HubKeyLoadingModule
     public static String getDeviceIdFromDeviceKeyPair(final ECKeyPair deviceKeyPair) {