Re: [TLS] New drafts: adding input to the TLS master secret

Martin Rex <mrex@sap.com> Wed, 03 February 2010 02:17 UTC

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From: Martin Rex <mrex@sap.com>
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To: marsh@extendedsubset.com (Marsh Ray)
Date: Wed, 3 Feb 2010 03:18:06 +0100 (MET)
In-Reply-To: <4B68D672.2060609@extendedsubset.com> from "Marsh Ray" at Feb 2, 10 07:50:42 pm
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Cc: paul.hoffman@vpnc.org, tls@ietf.org
Subject: Re: [TLS] New drafts: adding input to the TLS master secret
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Marsh Ray wrote:
> 
> Paul Hoffman wrote:
> > 
> > This proposed protocol change is only relevant to scenarios where
> > there is a cryptographic reason to mix inherently non-sensitive data
> > passed before the change_cipher_spec message into the master secret.
> 
> You said the data is "not sensitive" and it is sent in plain text. So an
> attacker has it as soon as the endpoints do.
> 
> What does this accomplish over just hashing your extra data into the 28
> byte random field? (which would not require a new protocol structure)

I had the same thought.  Have the endpoints hash it into the
random fields that are sent in the ClientHellos.

Adding extra data onto every TLS handshake that will _only_ be
used during full handshake and completely useless for all session
resumptions seems like a waste.  (For large&busy servers, a
session resumption rate of >95% is desired).


> 
> 224 bits of from each endpoint ... is 448 bits really not enough entropy
> somehow? 448 bits ought to be enough for anybody :-)
> 
> It looks to me (but IANAC) like the PRF maxes out at 672 bits of entropy
> doing key block expansion (master_secret[48]*8 + 128md5 + 160sha), so
> you can only effectively add 224 bits (only 50% more) from any source.

The PRF will have to do some kind of compression of all the entropy
from the inputs.


> 
> How big were you planning to make those symmetric keys anyway?

I would prefer to _not_ call anything "key" that is going to
travel in the clear.  Personally, I always think of "keys" being
secret or even private information.   I would prefer the term
"random" or "entropy".


-Martin