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

Stefan Santesson <stefan@aaa-sec.com> Fri, 19 February 2010 18:26 UTC

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Date: Fri, 19 Feb 2010 19:28:13 +0100
From: Stefan Santesson <stefan@aaa-sec.com>
To: <mrex@sap.com>, Brian Smith <brian@briansmith.org>, Tim Polk <wpolk@nist.gov>
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Thread-Topic: [TLS] draft-ietf-tls-cached-info-02 / New "Fast-Track" draft
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Cc: DPKemp@missi.ncsc.mil, tls@ietf.org
Subject: Re: [TLS] draft-ietf-tls-cached-info-02 / New "Fast-Track" draft
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Added Tim Polk,

As we are so fortunate that we have a security AD working for NIST maybe Tim
could help us clarify NIST intention.

/Stefan


On 10-02-19 7:21 PM, "Martin Rex" <mrex@sap.com>; wrote:

> Brian Smith wrote:
>> 
>> Stefan Santesson wrote:
>>> Brian,
>>> 
>>> The use of hash functions in this draft does not require collision
>>> resistance.
>>> This is actually quite easy to prove.
>> 
>> Nobody is questioning that at all.
>> 
>> My points are:
>> 
>> 1. As long as this web page on the NIST website says, basically, "don't
>> use SHA-1 for anything," people will want to disable SHA-1 whenever they
>> can. It doesn't matter that this page on the website isn't the official
>> NIST recommendation on the matter. It doesn't matter that NIST
>> recommendations are only binding for federal government work. It doesn't
>> matter that the official NIST recommendation (SP 800-107) is more
>> limited in its recommendations against SHA-1. People will still want to
>> disable it. Like I said many messages before, even if it isn't a
>> technical problem, it is a social problem. And, that problem can be
>> easily avoided.
> 
> 
> There is some confusion about what problems there are with SHA-1.
> 
> But doing as you suggest, you are just adding massively to that
> confusion, depicting SHA-1 as being universally bad.
> 
> 
> My memory was, that NIST's recommendation was to no longer
> issue certs with sha-1 as a signature algorithm after 2010.
> 
> 
> I don't know what you mean by "disabling SHA-1", but most software
> is likely to not have such a configuration settings -- and those
> software that does is likely to no longer interoperate securely
> with others if SHA-1 would be universally disabled.
> 
> What you are asking for is a policy like this:
> 
> "We can no longer let you use HTTPS because it makes use of
>  TLSv1.0 with contains the universally insecure SHA-1 algorithm.
>  Please use HTTP URLs instead."
> 
> 
>> 
>> 2. It doesn't make sense to require SHA-1 support to be added to
>> implementations that don't need SHA-1 for any other purpose. TLS 1.2
>> doesn't require SHA-1 for anything, so you can build a compliant and
>> useful implementation without SHA-1 support. There are technical
>> advantages to doing so. Obviously, this isn't practical for
>> general-purpose web browsers, but it is practical for many other uses.
> 
> You are mistaken.  SHA-1 is a MUST implement for TLSv1.2.
> 
> TLS uses a SHA-256 only PRF and provides cipher suites with SHA-256,
> but limiting an implmentation to that subset of TLSv1.2 is only
> an option for the consumer of the technology, not an option
> for the implementor.
> 
> Limiting your application to allow only protocol version TLSv1.2
> and only ciphersuites with SHA-256 hash and support of Certs
> with signature algorithms using sha-256 is going to make your
> application non-interoperable with 99,8% of the installed base
> of TLS-enables apps.  And that situation is going to change
> only at a fairly slow pace in the future.
> 
> So it could be that the particular isolated environment that
> you are looking at might be happy with such a configuration,
> but for the average consumer of this technology it just would not
> make any sense. 
> 
> And mind you, for your isolated environment, you can certainly
> implement and use the caching extension with SHA-256 only,
> the proposal has the necessary protocol options.
> 
> 
> -Martin