Re: [TLS] draft-ietf-tls-cached-info-02 / New "Fast-Track" draft

Martin Rex <mrex@sap.com> Fri, 19 February 2010 14:02 UTC

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From: Martin Rex <mrex@sap.com>
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To: brian@briansmith.org (Brian Smith)
Date: Fri, 19 Feb 2010 15:03:52 +0100 (MET)
In-Reply-To: <4B7D811D.50909@briansmith.org> from "Brian Smith" at Feb 18, 10 12:04:13 pm
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Cc: tls@ietf.org
Subject: Re: [TLS] draft-ietf-tls-cached-info-02 / New "Fast-Track" draft
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Brian Smith wrote:
> 
> Stefan Santesson wrote:
> > Well,
> >
> > I went through my change list and have now updated and reposted the draft as
> > draft-03.
> >
> > Changes can be viewed from:
> > http://tools.ietf.org/rfcdiff?url2=draft-ietf-tls-cached-info-03.txt
> >    
> I think it is especially important to have the SHA-1 requirement 
> changed. It is a big hassle to require SHA-1 for compliance when now 
> every use of SHA-1 has to be reviewed. Also, mandating SHA-1 would be in 
> conflict with other requirements--especially requirements to follow NIST 
> and NSA recommendations regarding algorithms. At a minimum, make SHA-1 
> support mandatory only if/when the connection's negotiated version is 
> less than TLS 1.2.


I assumed that NIST deprecated the use of SHA-1 only for digital signatures.
Implementations of TLS v1.0 and v1.1 may not support anything besides
SHA-1 within TLS yet.

The use of SHA-1 is not as security critical as you might think it is.
The _real_ data, identified by the hash, must still be processed, but
it is taken from the clients cache instead of being transferred
over the wire.


> 
> The server already told the client what hash algorithms it supports in 
> CertificateRequest message from which the client extracted the cached 
> certificate_authorities.

That is one of the non-obvious changes in TLS v1.2.
Prior versions (TLSv1.1,TLSv1.0,SSLv3) do not have a signature
algorithms element in the CertificateRequest message.

One IMHO of the biggest problems in the revised TLS specifications
is that it doesn't clearly document changes from prior protocol
versions.  rfc-5246 7.4.4. CertificateRequest ought to really
have mentioned in the description of "supported_signature_algorithms"
that this element was newly inserted into the CertificateRequest
message with TLSv1.2 and does not exist in prior versions of TLS/SSLv3.



> ... the server could indicate which items it supports by including 
> empty extension(s) with the corresponding type(s), which is what most 
> other extensions do.

I also think that reporting back in ServerHello for which elements
it supports caching would be a good idea.


-Martin