Re: [TLS] The MCSV hack is a compliment to SSL/TLS designers

Marsh Ray <marsh@extendedsubset.com> Thu, 10 December 2009 16:54 UTC

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Date: Thu, 10 Dec 2009 10:54:26 -0600
From: Marsh Ray <marsh@extendedsubset.com>
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To: James Manger <James@Manger.com.au>
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Subject: Re: [TLS] The MCSV hack is a compliment to SSL/TLS designers
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James Manger wrote:
> The Magic Cipher Spec Value (MCSV) is a hack.

Yes.

> An empty RI extension is also a hack.

No, it's a well-defined and appropriate way of communicating an
important bit of information over the protocol, namely: "I support the
renegotiation fix and my belief is that we have had no previous
connection state on this connection".

> The only non-hack solution is a new version number, but that is
> impractical.

Which is why most well-designed nontrivial protocols have an extension
mechanism.

> The designers of SSL/TLS should take the fact that a hack is needed
> to fix this bug as a compliment.

I wouldn't.

> It indicates that SSL/TLS has been so successful, is used by so many
> application protocols, on so many devices, in so many situations that
> minimising disruption to this unknowable collection of systems trumps
> all other criteria for choosing a fix (other than actually fixing the
> bug, of course).

It has been very successful yes, and that is at least in part because
the designers did many things right.

> It trumps architectural purity; it trumps saving a few bytes; it
> trumps a few weeks heads start in developing code.

That's not a good thing.

> I recommend the approach in draft-mrex-tls-secure-renegotiation-03
> (MCSV in every ClientHello).

It also requires the use of an extension.

> This approach simply takes out of the equation all the uncertainties
> about the prevalence, behaviour, and importance or otherwise of old
> versions or limited implementations.

No it doesn't.

- Marsh