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Date: Mon, 19 Sep 2016 15:03:23 -0500
From: Brandon Perry <bperry.volatile@...il.com>
To: oss-security@...ts.openwall.com
Cc: Mike Santillana <michael.santillana@...ork.com>,
 'Apple' via <infosec@...ork.com>
Subject: Re: CVE Request - Ruby OpenSSL Library - IV Reuse in GCM Mode


> On Sep 19, 2016, at 2:53 PM, Seth Arnold <seth.arnold@...onical.com> wrote:
> 
> On Mon, Sep 19, 2016 at 03:20:02PM -0400, Mike Santillana wrote:
>> An IV reuse bug was discovered in Ruby's OpenSSL library when using
>> aes-gcm. When encrypting data with aes-*-gcm, if the IV is set before
>> setting the key, the cipher will default to using a static IV. This creates
>> a static nonce and since aes-gcm is a stream cipher, this can lead to known
>> cryptographic issues.
>> 
>> The documentation does not appear to specify the order of operations when
>> setting the key and IV [1]. As an example, see the following insecure code
>> snippet below:
>> 
>> Vulnerable Code:
>> 
>> def encrypt(plaintext)
>>    cipher = OpenSSL::Cipher.new('aes-256-gcm')
>>    iv = cipher.random_iv # Notice here the IV is set before the key
>>    cipher.key = '11111111111111111111111111111111'
>>    cipher.auth_data = ""
>>    ciphertext = cipher.update(plaintext) + cipher.final
>>    tag = cipher.auth_tag
>> 
>>    puts "[+] Encrypting: #{plaintext}"
>>    puts "[+] CipherMessage (IV | Tag | Ciphertext): #{bin2hex(iv)} |
>> #{bin2hex(tag)} | #{bin2hex(ciphertext)}"
>> end
> 
> Hello,
> 
> I think you have a mistake in this sample code, 'iv' is assigned but never
> used (aside from being printed).

Ruby really likes side-effects. Calling #random_iv generates and sets a new IV on the cipher, then returns it to the caller. Very magical.

https://ruby-doc.org/stdlib-2.0.0/libdoc/openssl/rdoc/OpenSSL/Cipher.html#method-i-random_iv <https://ruby-doc.org/stdlib-2.0.0/libdoc/openssl/rdoc/OpenSSL/Cipher.html#method-i-random_iv>
> 
> Your github code is far more complicated but looks like it is doing the
> right thing.
> 
> Thanks


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