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

2 files changed, 9 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/message_encryptor.rb b/activesupport/lib/active_support/message_encryptor.rb
index 0671469788..9589dba3ca 100644
--- a/activesupport/lib/active_support/message_encryptor.rb
+++ b/activesupport/lib/active_support/message_encryptor.rb
@@ -110,7 +110,7 @@ module ActiveSupport
         # Currently the OpenSSL bindings do not raise an error if auth_tag is
         # truncated, which would allow an attacker to easily forge it. See
         # https://github.com/ruby/openssl/issues/63
-        raise InvalidMessage if aead_mode? && auth_tag.bytes.length != 16
+        raise InvalidMessage if aead_mode? && (auth_tag.nil? || auth_tag.bytes.length != 16)
 
         cipher.decrypt
         cipher.key = @secret
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("--")