Add CryptoUtils class.

lib/crypto/crypto_utils.dart

Index: lib/crypto/crypto_utils.dart
diff --git a/lib/crypto/crypto_utils.dart b/lib/crypto/crypto_utils.dart
new file mode 100644
index 0000000000000000000000000000000000000000..dbd0e5ea4d9d3881f2af7eef7ad997b1af94b1fe
--- /dev/null
+++ b/lib/crypto/crypto_utils.dart
@@ -0,0 +1,13 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class _CryptoUtils {
+  static String digestToHex(List<int> digest) {
+    var result = new StringBuffer();
+    for (var part in digest) {
+      result.add('${part < 16 ? '0' : ''}${part.toRadixString(16)}');
+    }
+    return result.toString();
+  }
+}