Re: [TLS] WGLC for draft-ietf-tls-md5-sha1-deprecate

David Benjamin <davidben@chromium.org> Sun, 24 November 2019 16:27 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Sun, 24 Nov 2019 11:27:26 -0500
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
Cc: "<tls@ietf.org>" <tls@ietf.org>
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Subject: Re: [TLS] WGLC for draft-ietf-tls-md5-sha1-deprecate
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On Sat, Nov 23, 2019 at 8:40 AM Ilari Liusvaara <ilariliusvaara@welho.com>;
wrote:

> On Fri, Nov 22, 2019 at 08:18:47PM +0100, Hubert Kario wrote:
> > On Friday, 22 November 2019 03:25:24 CET, David Benjamin wrote:
> > > On Fri, Nov 22, 2019 at 8:35 AM Salz, Rich <rsalz@akamai.com>; wrote:
> > >
> > > > > ...
> > > > SHA-1 signature hashes in TLS 1.2" draft available
> > > > https://datatracker.ietf.org/doc/draft-ietf-tls-md5-sha1-deprecate/.
> > > > Please review the document and send your comments to the list by
> 2359 UTC
> > > > on 13 December 2019.
> > > >
> > > > I just re-read this.  Looks good. Perhaps a sentence of rationale in
> ...
> > >
> > > To that end, the combination of client advice in sections 2 and 4 is a
> bit
> > > odd. Section 2 uses SHOULD NOT include MD5 and SHA-1, but section 4
> says
> > > the client MUST NOT accept the MD5 SHA-1, even if it included it. Why
> would
> > > the client include it in that case? It seems the two should either
> both be
> > > MUST NOT or both be SHOULD NOT.
> >
> > because it also influences certificate selection, and getting a
> certificate
> > signed with SHA-1 isn't an automatically disqualifying property?
> > (it may be an intermediate CA that's not used, it may be an explicitly
> > trusted
> > certificate, etc.)
>
> If you don't want SHA-1 exchange signatures, you darn sure do not want
> actual SHA-1 certificates that are not trust anchors anyway. And because
> TLS 1.2 does not have separate lists for exchange signatures and
> certificate signatures, the client needs to withdraw advertisment for
> both in order to not send a misleading offer.
>

Right, I had a longer discussion of the certificate-but-not-TLS case but
omitted it. :-) Basically what Ilari said. In particular, I believe older
versions of Schannel will, despite being able to sign SHA-256,
preferentially sign SHA-1 if the client offers it. This is inconvenient
when it comes to predicting breakage but is perfectly consistent with the
client's offer. When I last looked at this a few years ago, this accounted
for a nontrivial portion of SHA-1-negotiating servers on the web, so
rejecting SHA-1 while still advertising it is probably not the best
strategy.

Fortunately, we've already distrusted SHA-1 X.509 signatures on the web, so
hopefully that will simplify things. There is a risk that some servers'
trust anchors' (otherwise irrelevant) signatures are SHA-1 and they are
trying to match it against the signature algorithms list, but I expect the
SHA-1-preferring servers to be the deciding concern. Issues with
trust-anchor-checking servers can likely be worked around by configuring
the server to not send the trust anchor, which is desirable anyway.

(All of this may not apply to non-web deployments, of course.)


> And I expect that in practice, not sending SHA-1 in
> signature_algorithms would cause very little breakage on top of what
> is already broken due to using SHA-1 exchange signatures.



So I think both should be MUST NOT.
>
>
>
> -Ilari
>
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