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

• Status: Closed
• Priority: Normal
• 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:intest_aes_gcm_wrong_tag'

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

[zzak (Zachary Scott)]

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

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

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. [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