Re: [TLS] Spec issue with RFC 7627 (EMS) and resumption

[David Benjamin <[e-mail]>]

It depends on what the server is trying to do. If the server is trying to
mandate EMS, aborting the connection is correct. E.g. the full handshake
section then says:

   If the server receives a ClientHello without the extension, it SHOULD
   abort the handshake if it does not wish to interoperate with legacy
   clients.

Though I suppose the abbreviated handshake version is missing a "if it does
not wish to interoperate with legacy clients". Yeah, okay, this probably
also needs an erratum to add the right qualifier. Gosh this document is
confusingly organized.

On Wed, Oct 27, 2021 at 4:55 AM Achim Kraus <[e-mail]> wrote:

> Hi David,
>
> my question considers 2. ,
> if one peer uses RFC 7627 and the other not (legacy).
>
> Case 1:
> - client using RFC 7627, server not (legacy)
> -- client fallsback to full-handshakes
>
> Case 2:
> - server using RFC 7627, client not (legacy)
> -- if the client tries a resumption, the current definition is, that the
> server SHOULD abort.
>
> In my opinion in case 2, the server SHOULD also just fallback to a
> full-handshake. That's done by sending a ServerHello with a new Session
> ID and the other handshake messages of a full handshake.
> In the end, that prevents, that a client to have to execute a second,
> then full handshake, as fallback.
>
> best regards
> Achim
>
>
> Am 26.10.21 um 17:50 schrieb David Benjamin:
> > At least for an erratum, I don't think it makes sense to change that as
> > part of this.
> >
> > I think your question is conflating a few things. Let me try to untangle
> > this, as this document is little confusing. It seems to be describing,
> > via SHOULDs and MUSTs, three different implementation profiles
> concurrently:
> > 1. A client or server that negotiates EMS when available, but continues
> > to be compatible with legacy peers, resumption and all.
> > 2. A client or server that negotiates EMS when available, continues to
> > be compatible with legacy peers, but only allows full handshakes with
> > them. I forget how 3SHAKE worked, but I vaguely recall that cutting out
> > legacy resumptions avoided a source of issues
> > 3. A client or server that requires EMS on all connections, and is not
> > compatible with legacy peers at all. This way you can rely on the EMS
> > bugfix being applied.
> >
> > Because these are all interleaved, it's a very hard to tell what bits
> > correspond to this profile and what bits correspond to EMS itself. I
> > think this was overambitious. Anything but (1) was implausible for most
> > deployments in 2016, because EMS only just started existing. I expect it
> > is still implausible for most, though I don't have metrics for the web.
> > Hence, if we were completely redoing this document (probably not worth
> > it?), I think it should be reorganized. :-)
> >
> > Now, as to your question, I don't believe this is right:
> >
> >  > If the client follows this guide, it falls-back to use a full
> handshake.
> >  > If the client doesn't follow this (maybe, the client is not aware of
> RFC
> >  > 7627), the server SHOULD aborts.
> >
> > The client SHOULD you are referring to is for a client that implements
> > (2). If a client does not follow this, it implements (1). The server
> > SHOULD, however, says "If neither the original session /nor the new
> > ClientHello/ uses the extension, the server SHOULD abort the handshake."
> > That clause doesn't apply to the client SHOULD in the first place
> > because, whether the client is (1) or (2), the ClientHello will signal
> EMS.
> >
> > It does apply to legacy clients that do not implement EMS at all.
> > Whether the server continues, does a full handshake, or aborts depends
> > on whether the server wants to do (1), (2), or (3). Yes, if it picks (3)
> > and aborts, the server will not interoperate with a legacy client. That
> > is exactly what those servers would want. It is odd that the text only
> > describes (1) and (3) for the server in this case, not (2). But adding
> > (2) in without a rework will only make this complicated situation worse,
> > so I do not propose to address it as part of this fix.
> >
> > On Tue, Oct 26, 2021 at 1:05 AM Achim Kraus <[e-mail]
> > <mailto:[e-mail]>> wrote:
> >
> >     Hi David,
> >
> >     if you're on it, maybe it's worth to consider my question from
> January
> >     2021 as well.
> >
> >       > If the client follows this guide, it falls-back to use a full
> >     handshake.
> >       > If the client doesn't follow this (maybe, the client is not
> >     aware of RFC
> >     7627), the server SHOULD aborts.
> >
> >       > Why SHOULD the server not (also) just fall-back to use a full
> >     handshake?
> >
> >     For more details see:
> >
> >
> https://mailarchive.ietf.org/arch/msg/tls/gjBFHWwp1k-w1KdBkotp496zaf8/
> >     <
> https://mailarchive.ietf.org/arch/msg/tls/gjBFHWwp1k-w1KdBkotp496zaf8/>
> >
> >     best regards
> >     Achim Kraus
> >
> >     Am 26.10.21 um 00:51 schrieb David Benjamin:
> >      > Here's some possible replacement text for that paragraph:
> >      >
> >      > """
> >      > In some deployments, a legacy client or server may be exposed to a
> >      > session using extended master secret. For example, a group of
> servers
> >      > sharing a ticket encryption key may be in the process of enabling
> >     this
> >      > extension. If such a session is used in an abbreviated handshake
> >     without
> >      > the extension, a newer peer will fail the connection, as
> described in
> >      > Section 5.3. To avoid this, legacy servers SHOULD ignore such
> >     sessions
> >      > and continue with a full handshake, and legacy clients SHOULD NOT
> >     offer
> >      > such sessions in the ClientHello.
> >      >
> >      > This can be implemented by ensuring legacy implementations do not
> >      > recognize sessions using extended master secret. For example, the
> >      > sessions may have a higher internal version number or, if the
> older
> >      > implementation rejects unrecognized fields, include a new field.
> >     If this
> >      > is not possible, deployments may deploy a new session cache or
> ticket
> >      > encryption key alongside the new version.
> >      > """
> >      >
> >      > Unfortunately, "session using extended master secret" is a bit of
> a
> >      > mouthful, but section 5.4 didn't define a more concise term.
> >      >
> >      > On Mon, Oct 25, 2021 at 4:01 PM David Benjamin
> >     <[e-mail] <mailto:[e-mail]>
> >      > <mailto:[e-mail] <mailto:[e-mail]>>>
> wrote:
> >      >
> >      >     Hi all,
> >      >
> >      >     In diagnosing an interop issue, I noticed RFC 7627 did not
> >     describe
> >      >     the correct server behavior for EMS very well. Seemingly as a
> >      >     result, some server implementation has gotten this wrong. I'd
> >     like
> >      >     to fix this in the spec so this doesn't happen again. I
> think, at
> >      >     minimum, we need to replace the last paragraph of section 5.4.
> >      >
> >      >     The issue is a server that /doesn't/ implement EMS, when
> >     presented a
> >      >     ClientHello containing a ticket or session ID by a server that
> >      >     /did/ implement EMS, must ignore the session and continue
> with a
> >      >     full handshake. Failing to do so will trip the client check in
> >      >     Section 5.3, "If a client receives a ServerHello that accepts
> an
> >      >     abbreviated handshake, [...]". This is important to meet
> >     these three
> >      >     properties:
> >      >
> >      >     - If the client and server both support EMS, the connection
> must
> >      >     negotiate it.
> >      >     - On resumption, the EMS status of the connection must match
> >     the EMS
> >      >     status of the session
> >      >     - In order for EMS to be safely deployable, it must be
> >     possible to
> >      >     roll EMS out gradually, or roll it back, without breaking
> >      >     connections. This means a mixed pre-EMS and post-EMS server
> >      >     deployment must work.
> >      >
> >      >     Note that, although this behavior is only visible at the
> pre-EMS
> >      >     server (not directly in scope for this document), it is
> >     actually a
> >      >     requirement on the post-EMS server. When the post-EMS server
> >     issues
> >      >     a session, it must arrange for the pre-EMS server to ignore
> >     it. For
> >      >     example, if the pre-EMS server rejects sessions with
> unparsable
> >      >     fields (the safest option), the post-EMS server can add a new
> >     field
> >      >     to the session state serialization. Failing that, it can bump
> >     some
> >      >     internal version number. Another strategy is to rotate session
> >      >     ticket keys alongside the version, but this can be tricky the
> way
> >      >     deployments and software updates are often split.
> >      >
> >      >     There's an analogous, though less likely, client scenario
> that a
> >      >     pre-EMS client must not offer a post-EMS session. Otherwise
> >     it will
> >      >     run afoul of a server requirement. This can be relevant for
> >     clients
> >      >     that serialize their session cache.
> >      >
> >      >     As far as I can tell, RFC 7627 does not specify any of this.
> The
> >      >     first paragraph of section 5.4 talks about adding a flag, but
> >      >     doesn't talk about how pre-EMS servers interact with that
> >     flag. The
> >      >     last paragraph discusses this scenario, but says something
> very
> >      >     strange, if not plain wrong:
> >      >
> >      >         If the original session uses an extended master secret
> >     but the
> >      >         ClientHello or ServerHello in the abbreviated handshake
> >     does not
> >      >         include the extension, it MAY be safe to continue the
> >     abbreviated
> >      >         handshake since it is protected by the extended master
> >     secret of the
> >      >         original session.  This scenario may occur, for example,
> >     when a
> >      >         server that implements this extension establishes a
> >     session but the
> >      >         session is subsequently resumed at a different server
> >     that does not
> >      >         support the extension.  Since such situations are unusual
> >     and likely
> >      >         to be the result of transient or inadvertent
> >     misconfigurations, this
> >      >         document recommends that the client and server MUST abort
> >     such
> >      >         handshakes.
> >      >
> >      > https://datatracker.ietf.org/doc/html/rfc7627#section-5.4
> >     <https://datatracker.ietf.org/doc/html/rfc7627#section-5.4>
> >      >     <https://datatracker.ietf.org/doc/html/rfc7627#section-5.4
> >     <https://datatracker.ietf.org/doc/html/rfc7627#section-5.4>>
> >      >
> >      >     First, the "MAY" is immediately contradicted by the following
> >      >     "MUST", and by section 5.3. It seems it should have been an
> >     English
> >      >     lowercase "may", not a normative RFC 2119 "MAY". It is also
> >     wrong in
> >      >     calling this situation "unusual and likely to be the result of
> >      >     transient or inadvertent misconfigurations". Rather, it is the
> >      >     natural transition state of any large server rollout. I think
> we
> >      >     need to delete that entire paragraph and replace it with text
> >     that
> >      >     describes the rules above. If we were doing a whole new
> >     version of
> >      >     the document, I think the text could do with reorganization.
> But
> >      >     that may not be worth doing, given folks should be using TLS
> >     1.3 now.
> >      >
> >      >     Thoughts? I can put together some replacement text if folks
> >     agree.
> >      >     What would be the best way to do this? Just an erratum?
> >      >
> >      >     David
> >      >
> >      >
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