Encryption support for backup and restore

Also moved to the _ prefix in backup.js for all private methods exported
for testing.

test/crypto_test.js

@@ -0,0 +1,83 @@
+'use strict';
+
+describe('Crypto', function() {
+  it('roundtrip symmetric encryption succeeds', async function() {
+    var message = 'this is my message';
+    var plaintext = new dcodeIO.ByteBuffer.wrap(message, 'binary').toArrayBuffer();
+    var key = textsecure.crypto.getRandomBytes(32);
+
+    var encrypted = await Signal.Crypto.encryptSymmetric(key, plaintext);
+    var decrypted = await Signal.Crypto.decryptSymmetric(key, encrypted);
+
+    var equal = Signal.Crypto.constantTimeEqual(plaintext, decrypted);
+    if (!equal) {
+      throw new Error('The output and input did not match!');
+    }
+  });
+
+  it('roundtrip fails if nonce is modified', async function() {
+    var message = 'this is my message';
+    var plaintext = new dcodeIO.ByteBuffer.wrap(message, 'binary').toArrayBuffer();
+    var key = textsecure.crypto.getRandomBytes(32);
+
+    var encrypted = await Signal.Crypto.encryptSymmetric(key, plaintext);
+    var uintArray = new Uint8Array(encrypted);
+    uintArray[2] = 9;
+
+    try {
+      var decrypted = await Signal.Crypto.decryptSymmetric(key, uintArray.buffer);
+    } catch (error) {
+      assert.strictEqual(
+        error.message,
+        'decryptSymmetric: Failed to decrypt; MAC verification failed'
+      );
+      return;
+    }
+
+    throw new Error('Expected error to be thrown');
+  });
+
+  it('fails if mac is modified', async function() {
+    var message = 'this is my message';
+    var plaintext = new dcodeIO.ByteBuffer.wrap(message, 'binary').toArrayBuffer();
+    var key = textsecure.crypto.getRandomBytes(32);
+
+    var encrypted = await Signal.Crypto.encryptSymmetric(key, plaintext);
+    var uintArray = new Uint8Array(encrypted);
+    uintArray[uintArray.length - 3] = 9;
+
+    try {
+      var decrypted = await Signal.Crypto.decryptSymmetric(key, uintArray.buffer);
+    } catch (error) {
+      assert.strictEqual(
+        error.message,
+        'decryptSymmetric: Failed to decrypt; MAC verification failed'
+      );
+      return;
+    }
+
+    throw new Error('Expected error to be thrown');
+  });
+
+  it('fails if encrypted contents are modified', async function() {
+    var message = 'this is my message';
+    var plaintext = new dcodeIO.ByteBuffer.wrap(message, 'binary').toArrayBuffer();
+    var key = textsecure.crypto.getRandomBytes(32);
+
+    var encrypted = await Signal.Crypto.encryptSymmetric(key, plaintext);
+    var uintArray = new Uint8Array(encrypted);
+    uintArray[35] = 9;
+
+    try {
+      var decrypted = await Signal.Crypto.decryptSymmetric(key, uintArray.buffer);
+    } catch (error) {
+      assert.strictEqual(
+        error.message,
+        'decryptSymmetric: Failed to decrypt; MAC verification failed'
+      );
+      return;
+    }
+
+    throw new Error('Expected error to be thrown');
+  });
+});