Re: [TLS] Rizzo claims implementation attach, should be interesting

"Yngve N. Pettersen" <yngve@opera.com> Thu, 29 September 2011 13:53 UTC

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To: "Marsh Ray" <marsh@extendedsubset.com>, "Martin Rex" <mrex@sap.com>
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Date: Thu, 29 Sep 2011 15:55:49 +0200
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From: "Yngve N. Pettersen" <yngve@opera.com>
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Subject: Re: [TLS] Rizzo claims implementation attach, should be interesting
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On Thu, 29 Sep 2011 15:44:31 +0200, Martin Rex <mrex@sap.com> wrote:

> Marsh Ray wrote:
>>
>> >    - avoid CBC-based cipher suites (at least when SSLv3 or TLSv1.0
>> >      is negotiated), and use RC4-128 instead.
>>
>> Does TLS's use of RC4 still take the first bytes after initialization
>> with the key? Or does it discard the proper amount?
>>
>> I believe there was a mitigation put in place by OpenSSL: sending an
>> empty (just padding) message before each app data message. The document
>> at eprint.iacr.org/2006/136 suggests that this could be a server-side
>> only change. I don't see how that would work since the session cookie
>> recovery attack is clearly happening on the client->server channel.
>>
>> I read somewhere that this mitigation was off by default in OpenSSL
>> because it some software (an old MSIE IIRC).
>
> Why using a EMPTY (just padding) SSL record?  That looks like an
> obvious untested border case.
>
> How about using an initial SSL record with one byte of real data
> for SSLv3 and TLSv1.0 SSL with CBC cipher suites?

According to my testing, this breaks about as many websites,  
unfortunately, approximately 0.15% of tested websites.

There is some uncertainty in the numbers, as I have observed sites that  
has been reported as breaking still work with the patched versions, whil  
others doesn't work. At present I do not why this is happening.

-- 
Sincerely,
Yngve N. Pettersen

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