2 files changed, 17 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/message_encryptor.rb b/activesupport/lib/active_support/message_encryptor.rb
index ab8b2f8ded..7b33dc3481 100644
--- a/activesupport/lib/active_support/message_encryptor.rb
+++ b/activesupport/lib/active_support/message_encryptor.rb
@@ -19,6 +19,8 @@ module ActiveSupport
   #   encrypted_data = crypt.encrypt_and_sign('my secret data')              # => "NlFBTTMwOUV5UlA1QlNEN2xkY2d6eThYWWh..."
   #   crypt.decrypt_and_verify(encrypted_data)                               # => "my secret data"
   class MessageEncryptor
+    DEFAULT_CIPHER = "aes-256-cbc"
+
     module NullSerializer #:nodoc:
       def self.load(value)
         value
@@ -77,6 +79,11 @@ module ActiveSupport
       _decrypt(verifier.verify(value))
     end
 
+    # Given a cipher, returns the key length of the cipher to help generate the key of desired size
+    def self.key_len(cipher = DEFAULT_CIPHER)
+      OpenSSL::Cipher.new(cipher).key_len
+    end
+
     private
 
       def _encrypt(value)
diff --git a/activesupport/test/message_encryptor_test.rb b/activesupport/test/message_encryptor_test.rb
index fb0b1e377d..f8282c89ca 100644
--- a/activesupport/test/message_encryptor_test.rb
+++ b/activesupport/test/message_encryptor_test.rb
@@ -48,6 +48,16 @@ class MessageEncryptorTest < ActiveSupport::TestCase
     assert_equal @data, @encryptor.decrypt_and_verify(message)
   end
 
+  def test_backwards_compat_for_64_bytes_key
+    # 64 bit key
+    secret = ["3942b1bf81e622559ed509e3ff274a780784fe9e75b065866bd270438c74da822219de3156473cc27df1fd590e4baf68c95eeb537b6e4d4c5a10f41635b5597e"].pack("H*")
+    # Encryptor with 32 bit key, 64 bit secret for verifier
+    encryptor = ActiveSupport::MessageEncryptor.new(secret[0..31], secret)
+    # Message generated with 64 bit key
+    message = "eHdGeExnZEwvMSt3U3dKaFl1WFo0TjVvYzA0eGpjbm5WSkt5MXlsNzhpZ0ZnbWhBWFlQZTRwaXE1bVJCS2oxMDZhYVp2dVN3V0lNZUlWQ3c2eVhQbnhnVjFmeVVubmhRKzF3WnZyWHVNMDg9LS1HSisyakJVSFlPb05ISzRMaXRzcFdBPT0=--831a1d54a3cda8a0658dc668a03dedcbce13b5ca"
+    assert_equal "data", encryptor.decrypt_and_verify(message)[:some]
+  end
+
   def test_alternative_serialization_method
     prev = ActiveSupport.use_standard_json_time_format
     ActiveSupport.use_standard_json_time_format = true