Add a MessageEncryptor, just like MessageVerifier but using symmetric key encryption.

The use of encryption prevents people from seeing any potentially secret values you've used.  It also supports and encrypt_and_sign model to prevent people from tampering with the bits and creating random junk that gets fed to

A motivated coder could use this to add an :encrypt=>true option to the cookie store.

activesupport/CHANGELOG

@@ -1,6 +1,6 @@
 *2.3.0 [Edge]*
 
-* Added ActiveSupport::MessageVerifier to aid users who need to store signed messages. [Koz]
+* Added ActiveSupport::MessageVerifier and MessageEncryptor to aid users who need to store signed and/or encrypted messages. [Koz]
 
 * Added ActiveSupport::BacktraceCleaner to cut down on backtrace noise according to filters and silencers [DHH]
 

activesupport/lib/active_support.rb

@@ -38,6 +38,7 @@ module ActiveSupport
   autoload :Gzip, 'active_support/gzip'
   autoload :Inflector, 'active_support/inflector'
   autoload :Memoizable, 'active_support/memoizable'
+  autoload :MessageEncryptor, 'active_support/message_encryptor'
   autoload :MessageVerifier, 'active_support/message_verifier'
   autoload :Multibyte, 'active_support/multibyte'
   autoload :OptionMerger, 'active_support/option_merger'

activesupport/lib/active_support/message_encryptor.rb

@@ -0,0 +1,69 @@
+require 'openssl'
+
+module ActiveSupport
+  # MessageEncryptor is a simple way to encrypt values which get stored somewhere 
+  # you don't trust.
+  # 
+  # The cipher text and initialization vector are base64 encoded and returned to you.
+  #
+  # This can be used in situations similar to the MessageVerifier, but where you don't
+  # want users to be able to determine the value of the payload.
+  class MessageEncryptor
+    class InvalidMessage < StandardError; end
+    
+    def initialize(secret, cipher = 'aes-256-cbc')
+      @secret = secret
+      @cipher = cipher
+    end
+    
+    def encrypt(value)
+      cipher = new_cipher
+      # Rely on OpenSSL for the initialization vector
+      iv = cipher.random_iv
+      
+      cipher.encrypt 
+      cipher.key = @secret
+      cipher.iv  = iv
+      
+      encrypted_data = cipher.update(Marshal.dump(value)) 
+      encrypted_data << cipher.final
+
+      [encrypted_data, iv].map {|v| ActiveSupport::Base64.encode64s(v)}.join("--")
+    end
+    
+    def decrypt(encrypted_message)
+      cipher = new_cipher
+      encrypted_data, iv = encrypted_message.split("--").map {|v| ActiveSupport::Base64.decode64(v)}
+      
+      cipher.decrypt
+      cipher.key = @secret
+      cipher.iv  = iv
+
+      decrypted_data = cipher.update(encrypted_data)
+      decrypted_data << cipher.final
+      
+      Marshal.load(decrypted_data)
+    rescue OpenSSL::CipherError, TypeError
+      raise InvalidMessage
+    end
+    
+    def encrypt_and_sign(value)
+      verifier.generate(encrypt(value))
+    end
+    
+    def decrypt_and_verify(value)
+      decrypt(verifier.verify(value))
+    end
+    
+    
+    
+    private 
+      def new_cipher
+        OpenSSL::Cipher::Cipher.new(@cipher)
+      end
+      
+      def verifier
+        MessageVerifier.new(@secret)
+      end
+  end
+end

activesupport/test/message_encryptor_test.rb

@@ -0,0 +1,46 @@
+require 'abstract_unit'
+
+class MessageEncryptorTest < Test::Unit::TestCase
+  def setup
+    @encryptor = ActiveSupport::MessageEncryptor.new(ActiveSupport::SecureRandom.hex(64))
+    @data = {:some=>"data", :now=>Time.now}
+  end
+  
+  def test_simple_round_tripping
+    message = @encryptor.encrypt(@data)
+    assert_equal @data, @encryptor.decrypt(message)
+  end
+  
+  def test_encrypting_twice_yields_differing_cipher_text
+    first_messqage = @encryptor.encrypt(@data)
+    second_message = @encryptor.encrypt(@data)
+    assert_not_equal first_messqage, second_message
+  end
+  
+  def test_messing_with_either_value_causes_failure
+    text, iv = @encryptor.encrypt(@data).split("--")
+    assert_not_decrypted([iv, text] * "--")
+    assert_not_decrypted([text, munge(iv)] * "--")
+    assert_not_decrypted([munge(text), iv] * "--")
+    assert_not_decrypted([munge(text), munge(iv)] * "--")
+  end
+  
+  def test_signed_round_tripping
+    message = @encryptor.encrypt_and_sign(@data)
+    assert_equal @data, @encryptor.decrypt_and_verify(message)
+  end
+  
+  
+  private
+    def assert_not_decrypted(value)
+      assert_raises(ActiveSupport::MessageEncryptor::InvalidMessage) do
+        @encryptor.decrypt(value)
+      end
+    end
+    
+    def munge(base64_string)
+      bits = ActiveSupport::Base64.decode64(base64_string)
+      bits.reverse!
+      ActiveSupport::Base64.encode64s(bits)
+    end
+end