Add AutoCloseable to MutableBytes interface

Author: patrickfav

CHANGELOG

@@ -7,6 +7,7 @@
 * add `encodeBase64()` supporting padding-less encoding
 * add `toIntArray()` converter #28
 * add `toLongArray()` converter #29
+* add `AutoCloseable` to MutableBytes interface #31
 
 ### Breaking
 

README.md

@@ -580,6 +580,18 @@ Bytes b = Bytes.from(array).mutable().copy();
 assertTrue(b.isMutable());
 ```
 
+##### AutoClosable for try-with-resources
+
+In security-relevant environments it is best practice to wipe the memory of secret data, such as
+secret keys. This can be used with Java 7 feature try-with-resource like this:
+
+```java
+try (MutableBytes b = Bytes.wrap(aesBytes).mutable()) {
+    SecretKey s = new SecretKeySpec(b.array(), "AES");
+    ...
+}
+```
+
 #### Readonly Bytes
 
 On the other hand, if you want a export a instance with limited access,

src/main/java/at/favre/lib/bytes/MutableBytes.java

@@ -32,7 +32,7 @@
  * Adds additional mutator, which may change the internal array in-place, like {@link #wipe()}
  */
 @SuppressWarnings("WeakerAccess")
-public final class MutableBytes extends Bytes {
+public final class MutableBytes extends Bytes implements AutoCloseable {
 
     MutableBytes(byte[] byteArray, ByteOrder byteOrder) {
         super(byteArray, byteOrder, new Factory());
@@ -147,6 +147,11 @@ public boolean equals(Object o) {
         return super.equals(o);
     }
 
+    @Override
+    public void close() {
+        secureWipe();
+    }
+
     /**
      * Factory creating mutable byte types
      */

src/test/java/at/favre/lib/bytes/MutableBytesTest.java

@@ -23,6 +23,8 @@
 
 import org.junit.Test;
 
+import javax.crypto.SecretKey;
+import javax.crypto.spec.SecretKeySpec;
 import java.security.SecureRandom;
 import java.util.Arrays;
 
@@ -167,4 +169,18 @@ public void testTransformerShouldBeMutable() {
         assertTrue(b.append(3).isMutable());
         assertTrue(b.hashSha256().isMutable());
     }
+
+    @Test
+    public void testAutoCloseable() {
+        MutableBytes leak;
+
+        try (MutableBytes b = Bytes.wrap(new byte[16]).mutable()) {
+            assertArrayEquals(new byte[16], b.array());
+            SecretKey s = new SecretKeySpec(b.array(), "AES");
+            leak = b;
+        }
+
+        assertArrayNotEquals(new byte[16], leak.array());
+
+    }
 }