[TLS] Obsolete SCSV!? (was Re: AD review of draft-ietf-tls-oldversions-deprecate-06)

Sean Turner <sean@sn3rd.com> Wed, 23 September 2020 12:43 UTC

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From: Sean Turner <sean@sn3rd.com>
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Date: Wed, 23 Sep 2020 08:43:03 -0400
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Subject: [TLS] Obsolete SCSV!? (was Re: AD review of draft-ietf-tls-oldversions-deprecate-06)
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Hi! this issue was buried in the Ben’s review, but I think it is worth getting some attention on.

> On Aug 13, 2020, at 13:54, Benjamin Kaduk <kaduk@mit.edu> wrote:
> On Wed, Aug 12, 2020 at 04:29:56PM -0400, Kathleen Moriarty wrote:
>> On Sun, Jul 26, 2020 at 5:22 PM Benjamin Kaduk <kaduk@mit.edu> wrote:
>>> - Similarly, the downgrade protection provided by the SCSV of RFC 7507
>>>  seems to be entirely obsolete.  Any implementation that is compliant
>>>  with this document will support only 1.2 or higher.  If it only
>>>  supports one version, no downgrade is possible; if it also supports
>>>  1.3 or newer, the new downgrade-detection mechanism defined by TLS 1.3
>>>  applies, so the SCSV mechanism is entirely redundant (i.e., obsolete).
>>>  We could effectuate that status change in this document as well.
>> Has this been addressed in RFC8446?  If not, the specific downgrade
>> examples are just listed as examples.  If a gap is left, then the full
>> document should not be deprecated and made obsolete.  The text infers other
>> versions in my read.  I have not looked to see if this was addressed in
>> RFC8446 yet though.
> I'd really like to get a few more people to weigh in on this one -- IIRC
> David Benjamin and Martin Thomson had mentioned some thoughts in the chat
> during the session at 108, and Ekr as author of 8446 would be expected to
> have a good sense of what it does.  
> The specific RFC 8446 mechanism here is described at
> https://tools.ietf.org/html/rfc8446#section-4.1.3 : "TLS 1.3 has a
> downgrade protection mechanism embedded in the server's random value.
> [...]"
> While the RFC 8446 mechanism has the client do the actual detection of
> downgrade, there's a MUST-level requirement on clients to make the check,
> so from a specification point of view the check can be treated as reliable.
> The RFC 7507 mechanism has the server do the detection, but I think the end
> result is still the same: in an "downgraded" exchange between two honest
> participants, the handshake fails and the downgrade is detected.
> Since the functionality is still useful, just superseded, this one seems
> like a better fit for "obsoletes" (vs. "historic).

Right now, we have a list of RFCs draft-ietf-tls-oldversions-deprecate will update. RFC 7507 "TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks" (https://datatracker.ietf.org/doc/rfc7507/) is in this list. If you agree with Ben’s logic then we would be move 7507 out of the list of “updates” and adding an obsoletes header, i.e., “Obsoletes: 7507 (if approved)”, and moving 7507 down in s1.1 to the obsoletes paragraph. While this might seem like a minor point, this is the kind of the IESG loves to sink its teeth into so have a WG opinion on this matter can make overcoming later hurdles easier for the AD and doc shepherd.

Thanks for the your time,

spt (doc shepherd)