Re: [TLS] Updating for non-X.509 certificate types

Eric Rescorla <ekr@rtfm.com> Fri, 10 March 2017 15:03 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 10 Mar 2017 07:02:22 -0800
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Subject: Re: [TLS] Updating for non-X.509 certificate types
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On Fri, Mar 10, 2017 at 4:40 AM, Ilari Liusvaara <ilariliusvaara@welho.com>
wrote:

> On Thu, Mar 09, 2017 at 04:43:19PM -0800, Eric Rescorla wrote:
> > As noted in https://github.com/tlswg/tls13-spec/issues/722, the new
> fancy
> > TLS 1.3 Certificate structure doesn't map well to the various non-X.509
> > cert structures we have defined, specifically:
> >
> > - Raw Public Keys
> > - Cached Info
> > - OpenPGP
> >
> > Probably mapping each of these to 1.3 is relatively straightforward
> > (Raw public keys == a list with one key, Cached info == the hash of
> > each cert + its extensions, and so on), but I tend to think that given
> the
> > modest/specialized deployment of these extensions, it's better to do a
> > set of small bis RFCs to define each of these, rather than add a bunch
> > of clutter to TLS 1.3 proper.
> >
> > Does anyone object to this? Volunteers.
>
> Ugh, the situation is way worse than what I thought.
>

That makes it an opportunity!


Basically, all three assume they have full control of certificate
> message, worst of all being OpenPGP, which modifies it in more
> complex ways (it isn't a pure element or list anymore).
>
> And even with RPK, which appiles least severe modifications,
> still modifies the structure in ways that are not obvious in
> implications w.r.t. TLS 1.3.
>
> Oh, and turns out I had implemented RPK in TLS 1.2 wrong by assuming
> that it doesn't actually modify the certificate message format.
> Which turned out to be wrong assumption (I fixed this after
> discovering the format changes).
>
>
> And then certificate types don't currently work sanely for client
> certs, even if you knew how those map to Certificate message.
>
>
> Client_certificate_type and server_certificate type aren't the
> only problematic extensions w.r.t. TLS 1.3. The table of
> extensions has the following too (all marked as allowed, I
> added short reason I think those are problematic):
>
> - client_certificate_url: Replaces certificate message. Hardcodes
>   SHA-1 (which is now provably broken).
>

So that's an easy fix :)



> - user_mapping: Has extra handshake message.
> - cert_type: All the problems of CCertT and SCertT, combined with
>   fixing both to be the same.
>

Does anyone use this?


>
> With user_mapping, applying similar trick as in status_request is
> not completely trivial because extensions that are answered in client
> Certificate are offered in CertificateRequest. Okay, except that
> extension is not an answer to ClientHello extensions, and the
> extension assumes offer-answer relationship between client and server
> extensions. Might need some special-casing.
>

Yeah, I think we should probably just consider banning user_mapping,
at least until someone comes up with a way to use it here.


Could be useful to have explicit list of extensions (no registry, since
> this list can be never updated) that lists extensions that are
> deprecated in TLS 1.3.
>

Currently this is by exclusion. I.e., these aren't listed as usable with
1.3. It does seem to me that we shouldn't ban cached_info and
the cert type ones, because if/when they become usable with 1.3
then they should be permissible. So I think I would rather say
"don't advertise these with 1.3 unless you're willing to do them
with 1.3"

-Ekr




That is, only allowed in ClientHello and only if also advertising
> prior versions.
>
> AFAICT, the list would be:
>
> - client_certificate_url?
> - trusted_ca_keys
> - truncated_hmac
> - client_authz
> - server_authz
> - cert_type?
> - srp
> - status_request_v2
> - client_certificate_type?
> - server_certificate_type?
> - encrypt_then_mac
> - extended_master_secret
> - cached_info?
> - SessionTicket TLS (don't ask why it is written like this)
> - renegotiation_info
>
>
>
> -Ilari
>