Add ActiveSupport::MessageVerifier to aid users who need to store tamper-proof messages in cookies etc.

This is particularly useful for things like remember-me tokens in web applications and auto-unsubscribe links in emails.

4 files changed, 73 insertions, 0 deletions

diff --git a/activesupport/CHANGELOG b/activesupport/CHANGELOG
index 5e1b1cb4b0..755ec246ab 100644
--- a/activesupport/CHANGELOG
+++ b/activesupport/CHANGELOG
@@ -1,5 +1,7 @@
 *2.3.0 [Edge]*
 
+* Added ActiveSupport::MessageVerifier to aid users who need to store signed messages. [Koz]
+
 * Added ActiveSupport::BacktraceCleaner to cut down on backtrace noise according to filters and silencers [DHH]
 
 * Added Object#try. ( Taken from http://ozmm.org/posts/try.html ) [Chris Wanstrath]
diff --git a/activesupport/lib/active_support.rb b/activesupport/lib/active_support.rb
index cbfd95f092..7337b55a21 100644
--- a/activesupport/lib/active_support.rb
+++ b/activesupport/lib/active_support.rb
@@ -56,6 +56,7 @@ require 'active_support/base64'
 require 'active_support/time_with_zone'
 
 require 'active_support/secure_random'
+require 'active_support/message_verifier'
 
 require 'active_support/rescuable'
 
diff --git a/activesupport/lib/active_support/message_verifier.rb b/activesupport/lib/active_support/message_verifier.rb
new file mode 100644
index 0000000000..f3f905c15e
--- /dev/null
+++ b/activesupport/lib/active_support/message_verifier.rb
@@ -0,0 +1,45 @@
+module ActiveSupport
+  # MessageVerifier makes it easy to generate and verify messages which are signed
+  # to prevent tampering.
+  # 
+  # This is useful for cases like remember-me tokens and auto-unsubscribe links where the
+  # session store isn't suitable or available.
+  #
+  # Remember Me:
+  #   cookies[:remember_me] = @verifier.generate_message([@user.id, 2.weeks.from_now])
+  # 
+  # In the authentication filter:
+  #
+  #   id, time = @verifier.verify_message(cookies[:remember_me])
+  #   if time < Time.now
+  #     self.current_user = User.find(id)
+  #   end
+  # 
+  class MessageVerifier
+    class InvalidSignature < StandardError; end
+    
+    def initialize(secret, digest = 'SHA1')
+      @secret = secret
+      @digest = digest
+    end
+    
+    def verify_message(signed_message)
+      data, digest = signed_message.split("--")
+      if digest != generate_digest(data)
+        raise InvalidSignature
+      else
+        Marshal.load(ActiveSupport::Base64.decode64(data))
+      end
+    end
+    
+    def generate_message(value)
+      data = ActiveSupport::Base64.encode64s(Marshal.dump(value))
+      "#{data}--#{generate_digest(data)}"
+    end
+    
+    private
+      def generate_digest(data)
+        OpenSSL::HMAC.hexdigest(OpenSSL::Digest::Digest.new(@digest), @secret, data)
+      end
+  end
+end
\ No newline at end of file
diff --git a/activesupport/test/message_verifier_test.rb b/activesupport/test/message_verifier_test.rb
new file mode 100644
index 0000000000..c7ea4cb962
--- /dev/null
+++ b/activesupport/test/message_verifier_test.rb
@@ -0,0 +1,25 @@
+require 'abstract_unit'
+
+class MessageVerifierTest < Test::Unit::TestCase
+  def setup
+    @verifier = ActiveSupport::MessageVerifier.new("Hey, I'm a secret!")
+    @data = {:some=>"data", :now=>Time.now}
+  end
+  
+  def test_simple_round_tripping
+    message = @verifier.generate_message(@data)
+    assert_equal @data, @verifier.verify_message(message)
+  end
+  
+  def test_tampered_data_raises
+    data, hash = @verifier.generate_message(@data).split("--")
+    assert_not_verified("#{data.reverse}--#{hash}")
+    assert_not_verified("#{data}--#{hash.reverse}")
+  end
+  
+  def assert_not_verified(message)
+    assert_raises(ActiveSupport::MessageVerifier::InvalidSignature) do
+      @verifier.verify_message(message)
+    end
+  end
+end