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

Marsh Ray <marsh@extendedsubset.com> Wed, 03 February 2010 16:22 UTC

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Date: Wed, 03 Feb 2010 11:23:17 -0500
From: Marsh Ray <marsh@extendedsubset.com>
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To: Paul Hoffman <paul.hoffman@vpnc.org>
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Subject: Re: [TLS] New drafts: adding input to the TLS master secret
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Paul Hoffman wrote:
> At 10:19 AM -0500 2/3/10, Marsh Ray wrote:
>> I was thinking of TLS 1.0, since that's what seems to be mostly
>> used.
> 
> draft-hoffman-tls-master-secret-input explicitly says it is modifies
> TLS 1.2.

Oh, sorry I didn't see that wording.
I guess it's the "Updates: 4347, 5246" part.

> Please say whether or not you think that there is a
> cryptographic flaw in draft-hoffman-tls-master-secret-input that
> applies to TLS 1.2. If there is, then there is probably an underlying
> flaw in TLS 1.2 by itself, and that should be discussed seriously. I
> suspect that there is not.

I know of no flaw in TLS 1.2.

>> Well, you could check for duplicated extension IDs with a hash
>> table data structure that is O(1) in space and time.
> 
> The proposal in draft-hoffman-tls-master-secret-input does not change
> this; the one-use-of-each-extension rule comes from the TLS 1.2 spec,
> not from the draft.

Hmm, I wonder how many implementations actually do it that way?

> If there are thousands of extensions that do master secret input, and
> you have an implementation that is able to do all those thousands,
> then I would agree with you. However, I believe that the complexity
> of adding the code for those thousands of extensions is probably
> higher than the complexity of your algorithm that sorts two-byte
> numbers

Good point, we only need to sort the extensions that we successfully
negotiate and that modify PRF.

> It is justified because the two sides need to know what order to add
> in the values to the master secret PRF calculation. If the spec had
> said "in the order that the extensions came in", then you would be
> complaining that you need to add a new data structure to remember the
> order that they came in. Sorting on a value that is already in the
> extension data seemed like a better choice, yes?

Oh yes, I see the need for the proposal to specify the order.

I'm just not sure I see how the proposal helps all that much if the real
bottleneck on entropy lies deeper in the protocol.

- Marsh