diff --git a/src/main/java/com/pusher/client/crypto/nacl/SecretBoxOpener.java b/src/main/java/com/pusher/client/crypto/nacl/SecretBoxOpener.java
index fb2ecb04..dd5f8a8b 100644
--- a/src/main/java/com/pusher/client/crypto/nacl/SecretBoxOpener.java
+++ b/src/main/java/com/pusher/client/crypto/nacl/SecretBoxOpener.java
@@ -24,6 +24,9 @@ a copy of this software and associated documentation files (the "Software"),
 
 package com.pusher.client.crypto.nacl;
 
+import static com.pusher.client.util.internal.Preconditions.checkArgument;
+import static com.pusher.client.util.internal.Preconditions.checkNotNull;
+
 import java.util.Arrays;
 
 public class SecretBoxOpener {
@@ -33,19 +36,15 @@ public class SecretBoxOpener {
     private byte[] key;
 
     public SecretBoxOpener(byte[] key) {
-        // TODO: add a a little preconditions lib/class for that
-        if (key == null) throw new IllegalArgumentException("null key passed");
-        if (key.length != 32) {
-            throw new IllegalArgumentException("key should be 32B, is: " + key.length + "B");
-        }
+        checkNotNull(key, "null key passed");
+        checkArgument(key.length == 32, "key length must be 32 bytes, but is " +
+                key.length + " bytes");
 
         this.key = key;
     }
 
     public byte[] open(byte box[], byte nonce[]) throws AuthenticityException {
-        if (key == null) {
-            throw new IllegalStateException("key has been cleared, create new instance");
-        }
+        checkNotNull(key, "key has been cleared, create new instance");
 
         byte subKey[] = new byte[32];
         byte counter[] = new byte[16];
diff --git a/src/main/java/com/pusher/client/util/internal/Preconditions.java b/src/main/java/com/pusher/client/util/internal/Preconditions.java
new file mode 100644
index 00000000..9841efe4
--- /dev/null
+++ b/src/main/java/com/pusher/client/util/internal/Preconditions.java
@@ -0,0 +1,23 @@
+package com.pusher.client.util.internal;
+
+public class Preconditions {
+
+    public static void checkArgument(boolean expression, Object errorMessage) {
+        if (!expression) {
+            throw new IllegalArgumentException(String.valueOf(errorMessage));
+        }
+    }
+
+    public static void checkState(boolean expression, Object errorMessage) {
+        if (!expression) {
+            throw new IllegalStateException(String.valueOf(errorMessage));
+        }
+    }
+
+    public static <T> T checkNotNull(T reference, Object errorMessage) {
+        if (reference == null) {
+            throw new NullPointerException(String.valueOf(errorMessage));
+        }
+        return reference;
+    }
+}