Allow MessageEncryptor to take advantage of authenticated encryption modes

AEAD modes like `aes-256-gcm` provide both confidentiality and data authenticity, eliminating the need to use MessageVerifier to check if the encrypted data has been tampered with.

Signed-off-by: Jeremy Daer <jeremydaer@gmail.com>

activesupport/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Allow MessageEncryptor to take advantage of authenticated encryption modes.
+
+    AEAD modes like `aes-256-gcm` provide both confidentiality and data
+    authenticity, eliminating the need to use MessageVerifier to check if the
+    encrypted data has been tampered with. This speeds up encryption/decryption
+    and results in shorter cipher text.
+
+    *Bart de Water*
+
 *   Introduce `assert_changes` and `assert_no_changes`.
 
     `assert_changes` is a more general `assert_difference` that works with any

activesupport/lib/active_support/message_encryptor.rb

@@ -1,6 +1,7 @@
 require 'openssl'
 require 'base64'
 require 'active_support/core_ext/array/extract_options'
+require 'active_support/message_verifier'
 
 module ActiveSupport
   # MessageEncryptor is a simple way to encrypt values which get stored
@@ -28,6 +29,16 @@ module ActiveSupport
       end
     end
 
+    module NullVerifier #:nodoc:
+      def self.verify(value)
+        value
+      end
+
+      def self.generate(value)
+        value
+      end
+    end
+
     class InvalidMessage < StandardError; end
     OpenSSLCipherError = OpenSSL::Cipher::CipherError
 
@@ -40,7 +51,8 @@ module ActiveSupport
     # Options:
     # * <tt>:cipher</tt>     - Cipher to use. Can be any cipher returned by
     #   <tt>OpenSSL::Cipher.ciphers</tt>. Default is 'aes-256-cbc'.
-    # * <tt>:digest</tt> - String of digest to use for signing. Default is +SHA1+.
+    # * <tt>:digest</tt> - String of digest to use for signing. Default is
+    #   +SHA1+. Ignored when using an AEAD cipher like 'aes-256-gcm'.
     # * <tt>:serializer</tt> - Object serializer to use. Default is +Marshal+.
     def initialize(secret, *signature_key_or_options)
       options = signature_key_or_options.extract_options!
@@ -48,7 +60,8 @@ module ActiveSupport
       @secret = secret
       @sign_secret = sign_secret
       @cipher = options[:cipher] || 'aes-256-cbc'
-      @verifier = MessageVerifier.new(@sign_secret || @secret, digest: options[:digest] || 'SHA1', serializer: NullSerializer)
+      @digest = options[:digest] || 'SHA1' unless aead_mode?
+      @verifier = resolve_verifier
       @serializer = options[:serializer] || Marshal
     end
 
@@ -73,20 +86,28 @@ module ActiveSupport
 
       # Rely on OpenSSL for the initialization vector
       iv = cipher.random_iv
+      cipher.auth_data = "" if aead_mode?
 
       encrypted_data = cipher.update(@serializer.dump(value))
       encrypted_data << cipher.final
 
-      "#{::Base64.strict_encode64 encrypted_data}--#{::Base64.strict_encode64 iv}"
+      blob = "#{::Base64.strict_encode64 encrypted_data}--#{::Base64.strict_encode64 iv}"
+      blob << "--#{::Base64.strict_encode64 cipher.auth_tag}" if aead_mode?
+      blob
     end
 
     def _decrypt(encrypted_message)
       cipher = new_cipher
-      encrypted_data, iv = encrypted_message.split("--".freeze).map {|v| ::Base64.strict_decode64(v)}
+      encrypted_data, iv, auth_tag = encrypted_message.split("--".freeze).map {|v| ::Base64.strict_decode64(v)}
+      raise InvalidMessage if aead_mode? && auth_tag.bytes.length != 16
 
       cipher.decrypt
       cipher.key = @secret
       cipher.iv  = iv
+      if aead_mode?
+        cipher.auth_tag = auth_tag
+        cipher.auth_data = ""
+      end
 
       decrypted_data = cipher.update(encrypted_data)
       decrypted_data << cipher.final
@@ -103,5 +124,17 @@ module ActiveSupport
     def verifier
       @verifier
     end
+
+    def aead_mode?
+      @aead_mode ||= new_cipher.authenticated?
+    end
+
+    def resolve_verifier
+      if aead_mode?
+        NullVerifier
+      else
+        MessageVerifier.new(@sign_secret || @secret, digest: @digest, serializer: NullSerializer)
+      end
+    end
   end
 end

activesupport/test/message_encryptor_test.rb

@@ -70,6 +70,24 @@ class MessageEncryptorTest < ActiveSupport::TestCase
     assert_not_verified([iv,  message] * bad_encoding_characters)
   end
 
+  def test_aead_mode_encryption
+    encryptor = ActiveSupport::MessageEncryptor.new(@secret, cipher: 'aes-256-gcm')
+    message = encryptor.encrypt_and_sign(@data)
+    assert_equal @data, encryptor.decrypt_and_verify(message)
+  end
+
+  def test_messing_with_aead_values_causes_failures
+    encryptor = ActiveSupport::MessageEncryptor.new(@secret, cipher: 'aes-256-gcm')
+    text, iv, auth_tag = encryptor.encrypt_and_sign(@data).split("--")
+    assert_not_decrypted([iv, text, auth_tag] * "--")
+    assert_not_decrypted([munge(text), iv, auth_tag] * "--")
+    assert_not_decrypted([text, munge(iv), auth_tag] * "--")
+    assert_not_decrypted([text, iv, munge(auth_tag)] * "--")
+    assert_not_decrypted([munge(text), munge(iv), munge(auth_tag)] * "--")
+    assert_not_decrypted([text, iv] * "--")
+    assert_not_decrypted([text, iv, auth_tag[0..-2]] * "--")
+  end
+
   private
 
   def assert_not_decrypted(value)