Re: [TLS] Question about TLS_RSA_WITH_3DES_EDE_CBC_SHA

Martin Rex <mrex@sap.com> Mon, 04 July 2011 13:12 UTC

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From: Martin Rex <mrex@sap.com>
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To: stefan.winter@restena.lu (Stefan Winter)
Date: Mon, 4 Jul 2011 15:12:38 +0200 (MEST)
In-Reply-To: <4E11B217.4060504@restena.lu> from "Stefan Winter" at Jul 4, 11 02:29:11 pm
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Subject: Re: [TLS] Question about TLS_RSA_WITH_3DES_EDE_CBC_SHA
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Stefan Winter wrote:
> 
> there I was, naively hoping for a simple "Yes or No" answer :-)
> 
> My hopefully acceptable takeaway of this thread is that
> TLS_RSA_WITH_3DES_EDE_CBC_SHA *supports* three-key operation, which
> would be acceptable as per NIST.
> 
> I understand there is also a *risk* that if the dice fall very badly,
> the actual encryption strength in that specific TLS session may exhibit
> weaker cryptographic strength.
> 
> Thanks to all who responded!

I'm sorry for causing the confusion.

In _my_ interpretation of the NIST guidance, TLS_RSA_WITH_3DES_EDE_CBC_SHA
is a 3-key triple des.  While they recommend weak key avoidance for
DES, they do not require it.  the sub-key collisions for 3-DES could
be viewed as additional weak keys for 3DES compared to single-DES.

Dealing with weak keys and non-dense key spaces is common for
traditional key generation (assymetric and symmetric DES), but seems
fairly uncommon for algorithmic key derivation of symmetric and MAC
traffic keys.

-Martin