Re: [TLS] Wrapping up cached info

Ben Laurie <benl@google.com> Mon, 17 May 2010 17:19 UTC

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Date: Mon, 17 May 2010 18:17:37 +0100
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From: Ben Laurie <benl@google.com>
To: Nicolas Williams <Nicolas.Williams@oracle.com>
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Subject: Re: [TLS] Wrapping up cached info
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On 17 May 2010 18:08, Nicolas Williams <Nicolas.Williams@oracle.com> wrote:

> On Mon, May 17, 2010 at 12:02:55PM -0500, Marsh Ray wrote:
> > On 5/17/2010 11:29 AM, Joseph Salowey (jsalowey) wrote:
> > > I agree with Uri, that if you determine you need SHA-256 then you
> should
> > > plan for hash agility.  TLS 1.2 plans for hash agility.
> > >
> > > What about Nico's proposal where a checksum is used to identify the
> > > cached data and the actual handshake contains the actual data hashed
> > > with the algorithm used in the PRF negotiated with the cipher suite?
> > >
> > > This way we don't have to introduce hash agility into the extension,
> but
> > > we have cryptographic hash agility where it matters in the Finished
> > > computation.  Does it solve the problem?
> >
> > It could perhaps be made to solve the problem, but IMHO it would require
> > so much additional complexity (aka attack surface) that it wouldn't be
> > worth it.
>
> Really?  It'd be a single extra message consisting of the hash of all
> the objects' data, concatenated in some canonical order (or maybe the
> hash of the XOR of the object data hashes, to avoid having to define a
> canonical order), that the first node to sent a Finished message would
> have to send, and which the other node would have to check.  Where's all
> that additional complexity, and how does it compare with the complexity
> of the protocol's security analysis as it stands now?
>

This is, of course, equivalent to using the hashes as identifiers in the
first place :-)

Why is agility a problem, though - could we not say that any hash that is
currently required as a signature hash can be used, and identify the hash in
the extension message?


>
> > While I don't like to reject anything out-of-hand, injecting data into
>
> But you just did.
>
> > the calculation of the Finished messages raises a lot of concerns. The
>
> You mis-read.  Re-read.
>
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