*Issue #, if available:* #108

*Description of changes:*

Add a basic example for encrypting and decrypting with a KMS CMK.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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- [ ] Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.

src/examples/java/com/amazonaws/crypto/examples/BasicEncryptionExample.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"). You may not use this file except
+ * in compliance with the License. A copy of the License is located at
+ *
+ * http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+
+package com.amazonaws.crypto.examples;
+
+import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.Map;
+
+import com.amazonaws.encryptionsdk.AwsCrypto;
+import com.amazonaws.encryptionsdk.CryptoResult;
+import com.amazonaws.encryptionsdk.kms.KmsMasterKey;
+import com.amazonaws.encryptionsdk.kms.KmsMasterKeyProvider;
+
+/**
+ * <p>
+ * Encrypts and then decrypts data using an AWS KMS customer master key.
+ *
+ * <p>
+ * Arguments:
+ * <ol>
+ * <li>Key ARN: For help finding the Amazon Resource Name (ARN) of your KMS customer master
+ *    key (CMK), see 'Viewing Keys' at http://docs.aws.amazon.com/kms/latest/developerguide/viewing-keys.html
+ * </ol>
+ */
+public class BasicEncryptionExample {
+
+    private static final byte[] EXAMPLE_DATA = "Hello World".getBytes(StandardCharsets.UTF_8);
+
+    public static void main(final String[] args) {
+        final String keyArn = args[0];
+
+        encryptAndDecrypt(keyArn);
+    }
+
+    static void encryptAndDecrypt(final String keyArn) {
+        // 1. Instantiate the SDK
+        final AwsCrypto crypto = new AwsCrypto();
+
+        // 2. Instantiate a KMS master key provider
+        final KmsMasterKeyProvider prov = KmsMasterKeyProvider.builder().withKeysForEncryption(keyArn).build();
+
+        // 3. Create an encryption context
+        //
+        // Most encrypted data should have an associated encryption context
+        // to protect integrity. This sample uses placeholder values.
+        //
+        // For more information see:
+        // blogs.aws.amazon.com/security/post/Tx2LZ6WBJJANTNW/How-to-Protect-the-Integrity-of-Your-Encrypted-Data-by-Using-AWS-Key-Management
+        final Map<String, String> context = Collections.singletonMap("ExampleContextKey", "ExampleContextValue");
+
+        // 4. Encrypt the data
+        final CryptoResult<byte[], KmsMasterKey> encryptResult = crypto.encryptData(prov, EXAMPLE_DATA, context);
+        final byte[] ciphertext = encryptResult.getResult();
+
+        // 5. Decrypt the data
+        final CryptoResult<byte[], KmsMasterKey> decryptResult = crypto.decryptData(prov, ciphertext);
+
+        // 6. Before verifying the plaintext, verify that the customer master key that
+        // was used in the encryption operation was the one supplied to the master key provider.
+        if (!decryptResult.getMasterKeyIds().get(0).equals(keyArn)) {
+            throw new IllegalStateException("Wrong key ID!");
+        }
+
+        // 7. Also, verify that the encryption context in the result contains the
+        // encryption context supplied to the encryptData method. Because the
+        // SDK can add values to the encryption context, don't require that
+        // the entire context matches.
+        if (!context.entrySet().stream()
+                .allMatch(e -> e.getValue().equals(decryptResult.getEncryptionContext().get(e.getKey())))) {
+            throw new IllegalStateException("Wrong Encryption Context!");
+        }
+
+        // 8. Verify that the decrypted plaintext matches the original plaintext
+        assert Arrays.equals(decryptResult.getResult(), EXAMPLE_DATA);
+    }
+}

src/test/java/com/amazonaws/crypto/examples/BasicEncryptionExampleTest.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"). You may not use this file except
+ * in compliance with the License. A copy of the License is located at
+ *
+ * http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+
+package com.amazonaws.crypto.examples;
+
+import com.amazonaws.encryptionsdk.kms.KMSTestFixtures;
+import org.junit.Test;
+
+public class BasicEncryptionExampleTest {
+
+    @Test
+    public void testEncryptAndDecrypt() {
+        BasicEncryptionExample.encryptAndDecrypt(KMSTestFixtures.TEST_KEY_IDS[0]);
+    }
+}

src/test/java/com/amazonaws/encryptionsdk/kms/KMSTestFixtures.java

@@ -1,6 +1,19 @@
+/*
+ * Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"). You may not use this file except
+ * in compliance with the License. A copy of the License is located at
+ *
+ * http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+
 package com.amazonaws.encryptionsdk.kms;
 
-final class KMSTestFixtures {
+final public class KMSTestFixtures {
     private KMSTestFixtures() {
         throw new UnsupportedOperationException(
                 "This class exists to hold static constants and cannot be instantiated."
@@ -14,7 +27,7 @@ private KMSTestFixtures() {
      * This should go without saying, but never use these keys for production purposes (as anyone in the world can
      * decrypt data encrypted using them).
      */
-    static final String[] TEST_KEY_IDS = new String[] {
+    public static final String[] TEST_KEY_IDS = new String[] {
             "arn:aws:kms:us-west-2:658956600833:key/b3537ef1-d8dc-4780-9f5a-55776cbb2f7f",
             "arn:aws:kms:eu-central-1:658956600833:key/75414c93-5285-4b57-99c9-30c1cf0a22c2"
     };