diff --git a/examples/fitness/sky.yaml b/examples/fitness/sky.yaml
index ebb7dc228008a..3eb1406efe186 100644
--- a/examples/fitness/sky.yaml
+++ b/examples/fitness/sky.yaml
@@ -1,6 +1,6 @@
 name: fitness
 version: 0.0.1
-update_url: http://localhost:9888/examples/fitness/
+update-url: http://localhost:9888/examples/fitness/
 material-design-icons:
   - name: action/assessment
   - name: action/help
diff --git a/examples/stocks/sky.yaml b/examples/stocks/sky.yaml
index 7e3bf1d81cc10..e71bce607a8d4 100644
--- a/examples/stocks/sky.yaml
+++ b/examples/stocks/sky.yaml
@@ -1,3 +1,6 @@
+name: stocks
+version: 0.0.2
+update-url: http://localhost:9888/examples/stocks/
 material-design-icons:
   - name: action/account_balance
   - name: action/assessment
diff --git a/sky/packages/sky/pubspec.yaml b/sky/packages/sky/pubspec.yaml
index 638ce8b56de66..5851cef4099a6 100644
--- a/sky/packages/sky/pubspec.yaml
+++ b/sky/packages/sky/pubspec.yaml
@@ -11,7 +11,7 @@ dependencies:
   newton: '>=0.1.4 <0.2.0'
   sky_engine: 0.0.38
   sky_services: 0.0.38
-  sky_tools: '>=0.0.20 <0.1.0'
+  sky_tools: '>=0.0.25 <0.1.0'
   vector_math: '>=1.4.3 <2.0.0'
   intl: '>=0.12.4+2 <0.13.0'
 environment:
diff --git a/sky/packages/updater/lib/bundle.dart b/sky/packages/updater/lib/bundle.dart
new file mode 100644
index 0000000000000..9e72a259215d9
--- /dev/null
+++ b/sky/packages/updater/lib/bundle.dart
@@ -0,0 +1,75 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+import 'dart:typed_data';
+
+const String kBundleMagic = '#!mojo ';
+
+Future<List<int>> _readBytesWithLength(RandomAccessFile file) async {
+  ByteData buffer = new ByteData(4);
+  await file.readInto(buffer.buffer.asUint8List());
+  int length = buffer.getUint32(0, Endianness.LITTLE_ENDIAN);
+  return await file.read(length);
+}
+
+const int kMaxLineLen = 10*1024;
+const int kNewline = 0x0A;
+Future<String> _readLine(RandomAccessFile file) async {
+  String line = '';
+  while (line.length < kMaxLineLen) {
+    int byte = await file.readByte();
+    if (byte == -1 || byte == kNewline)
+      break;
+    line += new String.fromCharCode(byte);
+  }
+  return line;
+}
+
+// Represents a parsed .flx Bundle. Contains information from the bundle's
+// header, as well as an open File handle positioned where the zip content
+// begins.
+// The bundle format is:
+// #!mojo <any string>\n
+// <32-bit length><signature of the manifest data>
+// <32-bit length><manifest data>
+// <zip content>
+//
+// The manifest is a JSON string containing the following keys:
+// (optional) name: the name of the package.
+// version: the package version.
+// update-url: the base URL to download a new manifest and bundle.
+// key: a BASE-64 encoded DER-encoded ASN.1 representation of the Q point of the
+//   ECDSA public key that was used to sign this manifest.
+// content-hash: an integer SHA-256 hash value of the <zip content>.
+class Bundle {
+  Bundle(this.path);
+
+  final String path;
+  List<int> signatureBytes;
+  List<int> manifestBytes;
+  Map<String, dynamic> manifest;
+  RandomAccessFile content;
+
+  Future<bool> _readHeader() async {
+    content = await new File(path).open();
+    String magic = await _readLine(content);
+    if (!magic.startsWith(kBundleMagic))
+      return false;
+    signatureBytes = await _readBytesWithLength(content);
+    manifestBytes = await _readBytesWithLength(content);
+    String manifestString = UTF8.decode(manifestBytes);
+    manifest = JSON.decode(manifestString);
+    return true;
+  }
+
+  static Future<Bundle> readHeader(String path) async {
+    Bundle bundle = new Bundle(path);
+    if (!await bundle._readHeader())
+      return null;
+    return bundle;
+  }
+}
diff --git a/sky/packages/updater/lib/main.dart b/sky/packages/updater/lib/main.dart
index 9a748eb6e1a6e..307984c0d04a6 100644
--- a/sky/packages/updater/lib/main.dart
+++ b/sky/packages/updater/lib/main.dart
@@ -3,19 +3,38 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
+import 'dart:math';
 import 'dart:io';
+import 'dart:typed_data';
 
 import 'package:mojo/core.dart';
-import 'package:flutter/services.dart';
+// TODO(mpcomplete): Remove this 'hide' when we remove the conflicting
+// UpdateService from activity.mojom.
+import 'package:flutter/services.dart' hide UpdateServiceProxy;
 import 'package:sky_services/updater/update_service.mojom.dart';
 import 'package:path/path.dart' as path;
 import 'package:yaml/yaml.dart' as yaml;
+import 'package:asn1lib/asn1lib.dart';
+import 'package:bignum/bignum.dart';
+import 'package:cipher/cipher.dart';
+import 'package:cipher/impl/client.dart';
 
-import 'version.dart';
+import 'bundle.dart';
 import 'pipe_to_file.dart';
+import 'version.dart';
+
+const String kManifestFile = 'sky.yaml';
+const String kBundleFile = 'app.flx';
 
-const String kManifestFile = 'ui.yaml';
-const String kBundleFile = 'app.skyx';
+// Number of bytes to read at a time from a file.
+const int kReadBlockSize = 32*1024;
+
+// The ECDSA algorithm parameters we're using. These match the parameters used
+// by the signing tool in flutter_tools.
+final ECDomainParameters _ecDomain = new ECDomainParameters('prime256v1');
+final String kSignerAlgorithm = 'SHA-256/ECDSA';
+final String kHashAlgorithm = 'SHA-256';
 
 UpdateServiceProxy _initUpdateService() {
   UpdateServiceProxy updateService = new UpdateServiceProxy.unbound();
@@ -32,20 +51,45 @@ Future<String> getDataDir() async {
   return cachedDataDir;
 }
 
+// Parses a DER-encoded ASN.1 ECDSA signature block.
+ECSignature _asn1ParseSignature(Uint8List signature) {
+  ASN1Parser parser = new ASN1Parser(signature);
+  ASN1Object object = parser.nextObject();
+  if (object is! ASN1Sequence)
+    return null;
+  ASN1Sequence sequence = object;
+  if (!(sequence.elements.length == 2 &&
+        sequence.elements[0] is ASN1Integer &&
+        sequence.elements[1] is ASN1Integer))
+    return null;
+  ASN1Integer r = sequence.elements[0];
+  ASN1Integer s = sequence.elements[1];
+  return new ECSignature(r.valueAsPositiveBigInteger, s.valueAsPositiveBigInteger);
+}
+
+class UpdateFailure extends Error {
+  UpdateFailure(this._message);
+  String _message;
+  String toString() => _message;
+}
+
 class UpdateTask {
-  UpdateTask() {}
+  UpdateTask();
 
-  run() async {
+  Future run() async {
     try {
       await _runImpl();
-    } catch(e) {
+    } on UpdateFailure catch (e) {
+      print('Update failed: $e');
+    } catch (e, stackTrace) {
       print('Update failed: $e');
+      print('Stack: $stackTrace');
     } finally {
       _updateService.ptr.notifyUpdateCheckComplete();
     }
   }
 
-  _runImpl() async {
+  Future _runImpl() async {
     _dataDir = await getDataDir();
 
     await _readLocalManifest();
@@ -54,27 +98,25 @@ class UpdateTask {
       print('Update skipped. No new version.');
       return;
     }
-    MojoResult result = await _fetchBundle();
-    if (!result.isOk) {
-      print('Update failed while fetching new skyx bundle.');
-      return;
-    }
+    await _fetchBundle();
+    await _validateBundle();
     await _replaceBundle();
     print('Update success.');
   }
 
-  yaml.YamlMap _currentManifest;
+  Map _currentManifest;
   String _dataDir;
   String _tempPath;
 
-  _readLocalManifest() async {
-    String manifestPath = path.join(_dataDir, kManifestFile);
-    String manifestData = await new File(manifestPath).readAsString();
-    _currentManifest = yaml.loadYaml(manifestData, sourceUrl: manifestPath);
+  Future _readLocalManifest() async {
+    String bundlePath = path.join(_dataDir, kBundleFile);
+    Bundle bundle = await Bundle.readHeader(bundlePath);
+    _currentManifest = bundle.manifest;
+    bundle.content.close();
   }
 
   Future<yaml.YamlMap> _fetchManifest() async {
-    String manifestUrl = _currentManifest['update_url'] + '/' + kManifestFile;
+    String manifestUrl = _currentManifest['update-url'] + '/' + kManifestFile;
     String manifestData = await fetchString(manifestUrl);
     return yaml.loadYaml(manifestData, sourceUrl: manifestUrl);
   }
@@ -85,21 +127,74 @@ class UpdateTask {
     return (currentVersion < remoteVersion);
   }
 
-  Future<MojoResult> _fetchBundle() async {
+  Future _fetchBundle() async {
     // TODO(mpcomplete): Use the cache dir. We need an equivalent of mkstemp().
     _tempPath = path.join(_dataDir, 'tmp.skyx');
-    String bundleUrl = _currentManifest['update_url'] + '/' + kBundleFile;
+    String bundleUrl = _currentManifest['update-url'] + '/' + kBundleFile;
     UrlResponse response = await fetchUrl(bundleUrl);
-    return PipeToFile.copyToFile(response.body, _tempPath);
+    MojoResult result = await PipeToFile.copyToFile(response.body, _tempPath);
+    if (!result.isOk)
+      throw new UpdateFailure('Failure fetching new package: ${response.statusLine}');
+  }
+
+  Future _validateBundle() async {
+    Bundle bundle = await Bundle.readHeader(_tempPath);
+
+    if (bundle == null)
+      throw new UpdateFailure('Remote package not a valid FLX file.');
+    if (bundle.manifest['key'] != _currentManifest['key'])
+      throw new UpdateFailure('Remote package key does not match.');
+
+    await _verifyManifestSignature(bundle);
+    await _verifyContentHash(bundle);
+
+    bundle.content.close();
+  }
+
+  Future _verifyManifestSignature(Bundle bundle) async {
+    ECSignature ecSignature = _asn1ParseSignature(bundle.signatureBytes);
+    if (ecSignature == null)
+      throw new UpdateFailure('Corrupt package signature.');
+
+    List keyBytes = BASE64.decode(_currentManifest['key']);
+    ECPoint q = _ecDomain.curve.decodePoint(keyBytes);
+    ECPublicKey ecPublicKey = new ECPublicKey(q, _ecDomain);
+
+    Signer signer = new Signer(kSignerAlgorithm);
+    signer.init(false, new PublicKeyParameter(ecPublicKey));
+    if (!signer.verifySignature(bundle.manifestBytes, ecSignature))
+      throw new UpdateFailure('Invalid package signature. This package has been tampered with.');
+  }
+
+  Future _verifyContentHash(Bundle bundle) async {
+    // Hash the bundle contents.
+    Digest hasher = new Digest(kHashAlgorithm);
+    RandomAccessFile content = bundle.content;
+    int remainingLen = await content.length() - await content.position();
+    while (remainingLen > 0) {
+      List<int> chunk = await content.read(min(remainingLen, kReadBlockSize));
+      hasher.update(chunk, 0, chunk.length);
+      remainingLen -= chunk.length;
+    }
+    Uint8List hashBytes = new Uint8List(hasher.digestSize);
+    int len = hasher.doFinal(hashBytes, 0);
+    hashBytes = hashBytes.sublist(0, len);
+    BigInteger actualHash = new BigInteger.fromBytes(1, hashBytes);
+
+    // Compare to our expected hash from the manifest.
+    BigInteger expectedHash = new BigInteger(bundle.manifest['content-hash'], 10);
+    if (expectedHash != actualHash)
+      throw new UpdateFailure('Invalid package content hash. This package has been tampered with.');
   }
 
-  _replaceBundle() async {
+  Future _replaceBundle() async {
     String bundlePath = path.join(_dataDir, kBundleFile);
     await new File(_tempPath).rename(bundlePath);
   }
 }
 
 void main() {
-  var task = new UpdateTask();
+  initCipher();
+  UpdateTask task = new UpdateTask();
   task.run();
 }
diff --git a/sky/packages/updater/lib/pipe_to_file.dart b/sky/packages/updater/lib/pipe_to_file.dart
index f6a62d0a0b77e..511a7a369e153 100644
--- a/sky/packages/updater/lib/pipe_to_file.dart
+++ b/sky/packages/updater/lib/pipe_to_file.dart
@@ -30,13 +30,13 @@ class PipeToFile {
     return _consumer.endRead(thisRead.lengthInBytes);
   }
 
-  Future<MojoResult> drain() async {
-    var completer = new Completer();
+  Future drain() async {
+    Completer completer = new Completer();
     // TODO(mpcomplete): Is it legit to pass an async callback to listen?
     _eventStream.listen((List<int> event) async {
-      var mojoSignals = new MojoHandleSignals(event[1]);
+      MojoHandleSignals mojoSignals = new MojoHandleSignals(event[1]);
       if (mojoSignals.isReadable) {
-        var result = await _doRead();
+        MojoResult result = await _doRead();
         if (!result.isOk) {
           _eventStream.close();
           _eventStream = null;
@@ -58,7 +58,7 @@ class PipeToFile {
   }
 
   static Future<MojoResult> copyToFile(MojoDataPipeConsumer consumer, String outputPath) {
-    var drainer = new PipeToFile(consumer, outputPath);
+    PipeToFile drainer = new PipeToFile(consumer, outputPath);
     return drainer.drain();
   }
 }
diff --git a/sky/packages/updater/lib/version.dart b/sky/packages/updater/lib/version.dart
index 975927736cb5d..15c1ab7d5c23f 100644
--- a/sky/packages/updater/lib/version.dart
+++ b/sky/packages/updater/lib/version.dart
@@ -9,7 +9,7 @@ import 'dart:math';
 // Usage: assert(new Version('1.1.0') < new Version('1.2.1'));
 class Version {
   Version(String versionStr) :
-    _parts = versionStr.split('.').map((val) => int.parse(val)).toList();
+    _parts = versionStr.split('.').map((String val) => int.parse(val)).toList();
 
   List<int> _parts;
 
@@ -28,5 +28,5 @@ class Version {
     return _parts.length - other._parts.length;  // results in 1.0 < 1.0.0
   }
 
-  int get hashCode => _parts.fold(373, (acc, part) => 37*acc + part);
+  int get hashCode => _parts.fold(373, (int acc, int part) => 37*acc + part);
 }
diff --git a/sky/packages/updater/pubspec.yaml b/sky/packages/updater/pubspec.yaml
index 198081cc74bd6..a1eb2a6f57ebc 100644
--- a/sky/packages/updater/pubspec.yaml
+++ b/sky/packages/updater/pubspec.yaml
@@ -9,6 +9,8 @@ dependencies:
   sky_services: any
   path: any
   yaml: any
+  cipher: any
+  asn1lib: any
 dependency_overrides:
   flutter:
     path: ../sky
diff --git a/sky/packages/workbench/pubspec.yaml b/sky/packages/workbench/pubspec.yaml
index 5d7ec55f90576..bd035f07db39f 100644
--- a/sky/packages/workbench/pubspec.yaml
+++ b/sky/packages/workbench/pubspec.yaml
@@ -6,6 +6,8 @@ homepage: https://github.com/flutter/engine/tree/master/sky/packages/workbench
 dependencies:
   flutter: ">=0.0.3 <0.1.0"
   sky_tools: any
+  cipher: any
+  asn1lib: any
 dependency_overrides:
   material_design_icons:
     path: ../material_design_icons
diff --git a/sky/tools/sky_build.py b/sky/tools/sky_build.py
old mode 100644
new mode 100755
index 49152d1ebd335..21eb820ebe187
--- a/sky/tools/sky_build.py
+++ b/sky/tools/sky_build.py
@@ -33,6 +33,7 @@ def main():
         '--output-file', os.path.abspath(args.output_file),
         '--package-root', os.path.abspath(args.package_root),
         '--snapshot', os.path.abspath(args.snapshot),
+        '--private-key', os.path.abspath(os.path.join(args.package_root, '..', 'privatekey.der')),
     ]
 
     if args.manifest: