Bug #8439: test_aes_gcm_wrong_tag(OpenSSL::TestCipher) fails randomly

• Status: Closed
• Assignee: MartinBosslet (Martin Bosslet)
• Target version: 2.1.0
• ruby -v: ruby -v: ruby 2.0.0p195 (2013-05-14 revision 40734) [x86_64-linux]
• Backport: 1.9.3: DONTNEED, 2.0.0: DONE

[vo.x (Vit Ondruch)]

=begin
7) Error:
test_aes_gcm_wrong_tag(OpenSSL::TestCipher):
OpenSSL::Cipher::CipherError: unable to set GCM tag
/builddir/build/BUILD/ruby-2.0.0-p195/test/openssl/test_cipher.rb:190:in auth_tag=' /builddir/build/BUILD/ruby-2.0.0-p195/test/openssl/test_cipher.rb:190:in test_aes_gcm_wrong_tag'

I am building ruby against openssl-1.0.1e-8.el7.x86_64
=end

[zzak (zzak _)]

• Category set to ext
• Status changed from Open to Assigned
• Assignee set to MartinBosslet (Martin Bosslet)
• Target version set to 2.1.0

Can you reproduce this with trunk?

[vo.x (Vit Ondruch)]

Might be related/duplicate to #8221

[MartinBosslet (Martin Bosslet)]

vo.x (Vit Ondruch) wrote:

Might be related/duplicate to #8221

Yes, very much sounds like it. I need to take a deeper look at what happens in the OpenSSL implementation that might explain the random failures.

[akr (Akira Tanaka)]

I tracked down the random failure.

It seems that the failure occur when tag[-1].succ is 2byte.
I.e. it fails when tag[-1] is "9", "Z", "z" or "\xFF".

I modified test_aes_gcm_wrong_tag as follows:

Index: test_cipher.rb

--- test_cipher.rb (revision 43555)
+++ test_cipher.rb (working copy)
@@ -187,6 +187,7 @@ class OpenSSL::TestCipher < Test::Unit::
tag = cipher.auth_tag

   decipher = new_decryptor('aes-128-gcm', key, iv)

•  p [tag[-1], tag[-1].succ]
   decipher.auth_tag = tag[0..-2] << tag[-1].succ
   decipher.auth_data = "aad"
  

When the test fails, ["9", "10"], ["Z", "AA"], ["z", "aa"] or ["\xFF", "\x01\x00"] are shown.
Assuming tag[-1] is a random byte, I guess the test fails once per 64 times on average because 4/256=1/64.

I'm not sure the intent of tag[-1].succ, though.

[akr (Akira Tanaka)]

• Status changed from Assigned to Closed
• % Done changed from 0 to 100

This issue was solved with changeset r43676.
Vit, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.

---

• test/openssl/test_cipher.rb (test_aes_gcm_wrong_tag): Don't use
  String#succ because it can make modified (wrong) auth_tag longer
  than 16 bytes. The longer auth_tag makes that
  EVP_CIPHER_CTX_ctrl (and internally aes_gcm_ctrl) fail.
  [ruby-core:55143] [Bug #8439] reported by Vit Ondruch.

[vo.x (Vit Ondruch)]

• Backport changed from 1.9.3: UNKNOWN, 2.0.0: UNKNOWN to 1.9.3: UNKNOWN, 2.0.0: REQUIRED

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 1.9.3: UNKNOWN, 2.0.0: REQUIRED to 1.9.3: UNKNOWN, 2.0.0: DONE

r43676 was backported to ruby_2_0_0 at r44566.

[usa (Usaku NAKAMURA)]

• Backport changed from 1.9.3: UNKNOWN, 2.0.0: DONE to 1.9.3: DONTNEED, 2.0.0: DONE