Fix for AEAD auth_tag check in MessageEncryptor

When MessageEncryptor tries to +decrypt_and_verify+ ciphertexts
generated in a different mode (such CBC-HMAC), the +auth_tag+ may be
+nil+ and must explicitly check for it.

See the discussion here:
https://github.com/rails/rails/pull/28132#discussion_r116388462

1 files changed, 8 insertions, 0 deletions

diff --git a/activesupport/test/message_encryptor_test.rb b/activesupport/test/message_encryptor_test.rb
index 56a436f751..c67ada5f20 100644
--- a/activesupport/test/message_encryptor_test.rb
+++ b/activesupport/test/message_encryptor_test.rb
@@ -86,6 +86,14 @@ class MessageEncryptorTest < ActiveSupport::TestCase
     assert_equal @data, encryptor.decrypt_and_verify(message)
   end
 
+  def test_aead_mode_with_hmac_cbc_cipher_text
+    encryptor = ActiveSupport::MessageEncryptor.new(@secret, cipher: "aes-256-gcm")
+
+    assert_raise ActiveSupport::MessageEncryptor::InvalidMessage do
+      encryptor.decrypt_and_verify "eHdGeExnZEwvMSt3U3dKaFl1WFo0TjVvYzA0eGpjbm5WSkt5MXlsNzhpZ0ZnbWhBWFlQZTRwaXE1bVJCS2oxMDZhYVp2dVN3V0lNZUlWQ3c2eVhQbnhnVjFmeVVubmhRKzF3WnZyWHVNMDg9LS1HSisyakJVSFlPb05ISzRMaXRzcFdBPT0=--831a1d54a3cda8a0658dc668a03dedcbce13b5ca"
+    end
+  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("--")