Add a test for crypto authenticity failure

src/test/java/com/pusher/client/crypto/nacl/SecretBoxOpenerTest.java

@@ -1,6 +1,7 @@
 package com.pusher.client.crypto.nacl;
 
 import static com.google.common.truth.Truth.assertThat;
+import static java.util.Arrays.copyOf;
 
 import com.pusher.client.util.internal.Base64;
 import org.junit.Before;
@@ -9,6 +10,10 @@
 public class SecretBoxOpenerTest {
 
     byte[] key = Base64.decode("6071zp2l/GPnDPDXNWTJDHyIZ8pZMvQrYsa4xuTKK2c=");
+
+    byte[] nonce = Base64.decode("xsbOS0KylAV2ziTDHrP/7rSFqpCOah3p");
+    byte[] cipher = Base64.decode("tvttPE2PRQp0bWDmaPyiEU8YJGztmTvTN77OoPwftTNTdDgJXwxHQPE=");
+
     SecretBoxOpener subject;
 
     @Before
@@ -18,11 +23,16 @@ public void setUp() {
 
     @Test
     public void open() {
-        byte[] cipher = Base64.decode("tvttPE2PRQp0bWDmaPyiEU8YJGztmTvTN77OoPwftTNTdDgJXwxHQPE=");
-        byte[] nonce = Base64.decode("xsbOS0KylAV2ziTDHrP/7rSFqpCOah3p");
-
         byte[] clearText = subject.open(cipher, nonce);
 
         assertThat(new String(clearText)).isEqualTo("{\"message\":\"hello world\"}");
     }
+
+    @Test(expected = AuthenticityException.class)
+    public void openFailsForTamperedCipher() {
+        byte[] tamperedCipher = copyOf(cipher, cipher.length);
+        tamperedCipher[0] ^= tamperedCipher[0];
+
+        subject.open(tamperedCipher, nonce);
+    }
 }