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

[Christopher Wood <[e-mail]>]

FYI: This PR has been merged.

Best,
Chris, on behalf of the chairs

On Mon, May 4, 2020, at 10:08 AM, Christopher Wood wrote:
> Thanks, Alessandro! We'll aim to merge this PR on Friday. We ask that 
> folks review it before then. 
> 
>    https://github.com/tlswg/draft-ietf-tls-md5-sha1-deprecate/pull/5
> 
> Thanks,
> Chris, on behalf of the chairs
> 
> On Thu, Apr 23, 2020, at 10:45 AM, Alessandro Ghedini wrote:
> > On Sun, Nov 24, 2019 at 11:27:26AM -0500, David Benjamin wrote:
> > > On Sat, Nov 23, 2019 at 8:40 AM Ilari Liusvaara <[e-mail]>
> > > 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 <[e-mail]> 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.
> > 
> > So, based on this discussion, I made the following PR to change this to MUST NOT
> > https://github.com/tlswg/draft-ietf-tls-md5-sha1-deprecate/pull/5
> > 
> > Thanks for the review!
> > 
> > Cheers
> > 
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> >
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