Re: [OAUTH-WG] [UNVERIFIED SENDER] Re: MTLS token endoint & discovery

[Dominick Baier <[e-mail]>]

Sorry - this was not meant to be snide at all.

It was honest feedback that you also need to keep software complexity in
mind when creating a spec. Every MAY or OPTIONAL, or do it like this OR
that, or send values in arbitrary order adds to complexity. Complexity is
the natural enemy of security.

Cheers
———
Dominick

On 13. February 2019 at 20:39:16, Richard Backman, Annabelle (
[e-mail]) wrote:

The issue is that the proposal violates the standard by changing the
semantics of a parameter it defines. We should be very, very, VERY careful
about telling implementers to violate an existing standard. This change
might prove incompatible with existing or future implementations of 8414,
it might not, but by violating the standard the proposal creates an
opportunity for incompatibility that didn’t exist before.



As far as simplicity is concerned, I find a solution that reuses the
existing data model and naturally supports existing and future extensions
to be far simpler than one that introduces ambiguous semantics to existing
parameters. Generally speaking, data models with properties that mean
different things in different contexts tend to be fragile and require more
complex code to work with. But that’s just my experience as, you know, an
actual developer.



Let’s keep the assumptions and snide remarks about others’ backgrounds off
the list, please.



-- 

Annabelle Richard Backman

AWS Identity





*From: *Dominick Baier <[e-mail]>
*Date: *Wednesday, February 13, 2019 at 4:18 AM
*To: *"Richard Backman, Annabelle" <[e-mail]>, Filip Skokan <
[e-mail]>
*Cc: *Brian Campbell <[e-mail]>, "Richard Backman,
Annabelle" <[e-mail]>, oauth <[e-mail]>
*Subject: *[UNVERIFIED SENDER] Re: [OAUTH-WG] MTLS token endoint & discovery



I am for keeping it simple and not introducing another link to follow.



I sometimes wonder how many of the spec authors are actually developers -
these suggestions make software unnecessary complex ;)



———

Dominick



On 13. February 2019 at 12:25:13, Filip Skokan ([e-mail]) wrote:

Hello,



Additionally, a metadata document that omits token_endpoint in favor of
mtls_endpoints since only mTLS is supported would violate 8414, as 8414
says token_endpoint “is REQUIRED unless only the implicit grant type is
supported.”


mtls only AS doesn't get anything out of using mtls_endpoints, its use
should not be recommended for such AS and regular endpoints will be used,
this satisfies the requirement

Here is one example, based on my understanding of the “straw man” format
presented in this thread: RFC8414 defines the value of
token_endpoint_auth_methods_supported as a “JSON array containing a list of
client authentication methods supported by this token endpoint.” If that
array contains “tls_client_auth” and the endpoint specified in
token_endpoint does not support mTLS, then that metadata violates 8414. You
could address this by adding a token_endpoint_auth_methods_supported
parameter to the mtls_endpoints object, and likewise for the other
endpoints and auth methods. If you take that to its logical conclusion, you
end up with a complete metadata document hanging off of mtls_endpoints.
It’s that line of thought that led me to suggest just pointing to an
alternate document.


Assuming we go with using the same root auth methods property, what's the
issue? Clients not having mtls capabilities won't care about the additional
method members being present. Clients that do implement mtls by the
specification will know to look for mtls_endpoints and fall back to regular
ones if the specific endpoint or mtls_endpoints root property is not
present.



I gave `mtls_alternate_metadata` route a thought and don't see how it
helps, given the two above are non-issues (my $.02) another discovery
document only brings more of them since every property can now be
different, not just the endpoints.


S pozdravem,
*Filip Skokan*





On Wed, 13 Feb 2019 at 00:00, Richard Backman, Annabelle <
[e-mail]> wrote:

Here is one example, based on my understanding of the “straw man” format
presented in this thread: RFC8414 defines the value of
token_endpoint_auth_methods_supported as a “JSON array containing a list of
client authentication methods supported by this token endpoint.” If that
array contains “tls_client_auth” and the endpoint specified in
token_endpoint does not support mTLS, then that metadata violates 8414. You
could address this by adding a token_endpoint_auth_methods_supported
parameter to the mtls_endpoints object, and likewise for the other
endpoints and auth methods. If you take that to its logical conclusion, you
end up with a complete metadata document hanging off of mtls_endpoints.
It’s that line of thought that led me to suggest just pointing to an
alternate document.



Additionally, a metadata document that omits token_endpoint in favor of
mtls_endpoints since only mTLS is supported would violate 8414, as 8414
says token_endpoint “is REQUIRED unless only the implicit grant type is
supported.”



It is possible to define the mtls_endpoints parameter such that the above
use cases are invalid, but that does make the document more complicated. If
we go the “mtls_alternate_metadata” route, we can skip past all of these
issues, because it brings us back to just parsing the same metadata that we
do today.



-- 

Annabelle Richard Backman

AWS Identity





*From:* OAuth <[e-mail]> on behalf of Filip Skokan <
[e-mail]>
*Date:* Tuesday, February 12, 2019 at 1:13 PM
*To:* "Richard Backman, Annabelle" <richanna=[e-mail]>
*Cc:* Brian Campbell <bcampbell=[e-mail]>, oauth <
[e-mail]>
*Subject:* Re: [OAUTH-WG] MTLS token endoint & discovery



Hi Annabelle,



can you elaborate what would be the violation / negative impact of usage of
RFC8414?



As it already makes use of `signed_metadata` and this language is present
...



> Consumers of the metadata MAY ignore the signed metadata if they do not
support this feature.  If the consumer of the metadata supports signed
metadata, metadata values conveyed in the signed metadata MUST take
precedence over the corresponding values conveyed using plain JSON elements.



... would mtls_endpoints be any different? Clients are free to ignore this
if they don't support/use mtls, and if they do those values must take
precedence over the root ones. the use of mtls_endpoints would not be
recommended for mtls-only AS but recommended for one with both mtls/regular
authentication methods. token_endpoint remains required for all intents and
purposes. And as for the usable client authentication methods - they should
all be listed, or do you think we should separate the ones for each
hostname/port location? To what end? Is there a risk having the mtls
hostname/port accepting regular requests? Other then secret/key _jwt auth
methods assertion aud claim the endpoint location doesn't play a role in
the authentication process.


Best,
*Filip*





On Tue, 12 Feb 2019 at 20:47, Richard Backman, Annabelle <richanna=
[e-mail]> wrote:

I’m not opposed to introducing a metadata change, if the scenario is
relevant and other approaches can’t adequately address it – and I’ll
readily grant that we probably don’t want to depend on consistency of
browser behavior at the intersection of client certificates and
Access-Control-Allow-Credentials. But care needs to be taken in designing
that metadata to avoid violating or otherwise negatively impacting usage of
RFC8414.



Along those lines, instead of adding an “mtls_endpoints” metadata
parameter, we could add an “mtls_alternate_metadata” parameter whose value
is the URL of an alternate metadata document intended for clients using
mTLS. An mTLS-optional AS could have two different metadata documents for
mTLS and non-mTLS clients, reducing the mTLS-optional scenario to the
mTLS-only scenario. This sidesteps the challenges of aligning the
“either/or” semantics of mTLS-optional with some of the rigid parameter
definitions in RFC8414 (see: token_endpoint,
token_endpoint_auth_methods_supported).



-- 

Annabelle Richard Backman

AWS Identity





*From:* OAuth <[e-mail]> on behalf of Brian Campbell
<bcampbell=[e-mail]>
*Date:* Tuesday, February 12, 2019 at 10:58 AM
*To:* Dominick Baier <[e-mail]>
*Cc:* oauth <[e-mail]>
*Subject:* Re: [OAUTH-WG] MTLS token endoint & discovery



Thanks for the input, Dominick.



I'd said previously that I was having trouble adequately gauging whether or
not there's sufficient consensus to go ahead with the update. I was even
thinking of asking the chairs about a consensus call during the office
hours meeting yesterday. But it got canceled. And looking again back over
the discussion, I don't believe a consensus call is necessary. There's been
a lot of general discussion over the last several weeks during which
several folks have stated support for the proposal while there's been only
one voice of direct dissent. That seems like rough enough consensus and, as
such, I plan to make the change in the next revision of the document (which
should be done soon). Chairs, please let me know, if you believe the
situation warrants a different course of action.







On Mon, Feb 11, 2019 at 11:01 PM Dominick Baier <[e-mail]>
wrote:

IMHO that’s a reasonable and pragmatic option.



Thanks

———

Dominick



On 11. February 2019 at 13:26:37, Brian Campbell ([e-mail])
wrote:

It's been pointed out that the potential issue is not isolated to the just
token endpoint but that revocation, introspection, etc. could be impacted
as well. So,  at this point, the proposal on the table is to add a new
optional AS metadata parameter named 'mtls_endpoints' that's value we be a
JSON object which itself contains endpoints that, when present, a client
doing MTLS would use rather than the regular endpoints.  A straw-man
example might look like this

{
  "issuer":"https://server.example.com",
  "authorization_endpoint":"https://server.example.com/authz",
  "token_endpoint":"https://server.example.com/token",
  "token_endpoint_auth_methods_supported":[
 "client_secret_basic","tls_client_auth", "none"],
  "userinfo_endpoint":"https://server.example.com/userinfo",
  "revocation_endpoint":"https://server.example.com/revo",
  "jwks_uri":"https://server.example.com/jwks.json",




*"mtls_endpoints":{     "token_endpoint":"https://mtls.example.com/token
<https://mtls.example.com/token>",
"userinfo_endpoint":"https://mtls.example.com/userinfo
<https://mtls.example.com/userinfo>",
"revocation_endpoint":"https://mtls.example.com/revo
<https://mtls.....example.com/revo>"   }*
}



A client doing MTLS would look for and give precedence to an endpoint under
"mtls_endpoints" while falling back to use the normal endpoint from the top
level of metadata when/if its not in "mtls_endpoints".


The idea being that "regular" clients (those not doing MTLS) will use the
regular endpoints. And only the host/port of the endpoints listed in
mtls_endpoints will be set up to request TLS client certificates during
handshake. Thus any potential impact of the CertificateRequest message
being sent in the TLS handshake can be avoided for all the other regular
clients that are not going to do MTLS - including and most importantly
in-browser javascript clients where there can be less than desirable UI
presented to the end-user.



I'm struggling, however, to adequately gauge whether or not there's
sufficient consensus to go ahead with the update. There's been some support
for it voiced. As well as talk of other approaches that could be
alternatives or additional measures. And also some vocal opposition to it.





On Sat, Feb 9, 2019 at 3:09 AM Dominick Baier <[e-mail]>
wrote:

We are currently implementing MTLS in IdentityServer.



Our approach will be that we’ll offer a separate token endpoint that
supports client certs. Are you planning on adding an official endpoint name
for discovery? Right now we are using “mtls_token_endpoint”.



Thanks

———

Dominick



On 7. February 2019 at 22:36:55, Brian Campbell (
bcampbell=[e-mail]) wrote:

Ah yes, that makes sense. Thanks for clarifying. And it is indeed a good
reminder that even a seemingly innocuous change that should be backwards
compatible can break things unexpectedly..











On Thu, Feb 7, 2019 at 8:57 AM Richard Backman, Annabelle <
[e-mail]> wrote:

The case I’m referencing didn’t specifically involve AS metadata. We had
clients in the wild that assumed that all the properties in the JSON object
returned from our userinfo endpoint were scalar strings. This broke when we
introduced a new property whose value was a JSON object. It was a good
reminder that even a seemingly innocuous change that should be backwards
compatible can force more work on clients than we expect.



-- 

Annabelle Richard Backman

AWS Identity





*From:* Brian Campbell <[e-mail]>
*Date:* Wednesday, February 6, 2019 at 11:30 AM
*To:* "Richard Backman, Annabelle" <richanna=[e-mail]>
*Cc:* "Richard Backman, Annabelle" <[e-mail]>, oauth <
[e-mail]>
*Subject:* Re: [OAUTH-WG] [UNVERIFIED SENDER] Re: MTLS and in-browser
clients using the token endpoint



And I'm honestly really struggling to see what could have gone wrong with
or how token_endpoint_auth_methods broke something with the user info
endpoint. If you have the time and/or it'd be informative to this here
little discussion, please explain that one a bit more.



On Wed, Feb 6, 2019 at 12:15 PM Brian Campbell <[e-mail]>
wrote:

"far" should have said "fair" in the previous message











On Tue, Feb 5, 2019 at 4:35 PM Brian Campbell <[e-mail]>
wrote:

It may well be due to my own intellectual shortcomings but these
issues/questions/confusion-points are not resonating for me as being
problematic.



The more general stance of "this isn't needed or worth it in this document"
(I think that's far?) is being heard though.







On Tue, Feb 5, 2019 at 1:42 PM Richard Backman, Annabelle <richanna=
[e-mail] <[e-mail]....org>> wrote:

(TL;DR: punt AS metadata to a separate draft)

AS points #1-3 are all questions I would have as an implementer:

   1. Section 2 of RFC8414 <https://tools.ietf.org/html/rfc8414#section-2>
   says token_endpoint “is REQUIRED unless only the implicit grant type is
   supported.” So what does the mTLS-only AS put here?
   2. The claims for these other endpoints are OPTIONAL, potentially
   leading to inconsistency depending on how #1 gets decided.
   3. The example usage of the token_endpoint_auth_methods property given
   earlier is incompatible with RFC8414, since some of its contents are only
   valid for the non-mTLS endpoints, and others are only valid for the mTLS
   endpoints. Hence this question.
   4. This introduces a new metadata property that could impact how other
   specs should extend AS metadata. That needs to be addressed.



I could go on for client points but you already get the point. Though I’ll
share that #3 is real and once forced me to roll back an update to the
Login with Amazon userinfo endpoint…good times. 😃



I don’t necessarily think an AS metadata property is wrong per se, but
understand that you’re bolting a layer of flexibility onto a standard that
wasn’t designed for that, and I don’t think the metadata proposal as it’s
been discussed here sufficiently deals with the fallout from that. In my
view this is a complex enough issue and it’s for a nuanced enough use case
(as far as I can tell from discussion? Please correct me if I’m wrong) that
we should punt it to a separate draft (e.g., “Authorization Server Metadata
Extensions for mTLS Hybrids”) and get mTLS out the door.



-- 

Annabelle Richard Backman

AWS Identity





*From:* OAuth <[e-mail]> on behalf of Brian Campbell
<bcampbell=[e-mail]>
*Date:* Monday, February 4, 2019 at 5:28 AM
*To:* "Richard Backman, Annabelle" <richanna=[e-mail]>
*Cc:* oauth <[e-mail]>
*Subject:* Re: [OAUTH-WG] [UNVERIFIED SENDER] Re: MTLS and in-browser
clients using the token endpoint



Those points of confusion strike me as somewhat hypothetical or hyperbolic.
But your general point is taken and your position of being anti additional
metadata on this issue is noted.



All of which leaves me a bit uncertain about how to proceed. There seem to
be a range of opinions on this point and gauging consensus is proving
elusive for me. That's confounded by my own opinion on it being somewhat
fluid.



And I'd really like to post an update to this draft about a month or two
ago.













On Fri, Feb 1, 2019 at 5:03 PM Richard Backman, Annabelle <richanna=
[e-mail] <[e-mail]....org>> wrote:

Confusion from the AS’s perspective:

   1. If I only support mTLS, do I need to include both token_endpoint_uri
   and mtls_endpoints? Should I omit token_endpoint_uri? Or set it to the
   empty string?
   2. What if I only support mTLS for the token endpoint, but not
   revocation or user info?
   3. How do I specify authentication methods for the mTLS token endpoint?
   Does token_endpoint_auth_methods apply to both the mTLS and non-mTLS
   endpoints?
   4. I’m using the OAuth 2.0 Device Flow. Do I include a mTLS-enabled
   device_authorization_endpoint under mtls_endpoints?



Confusion from the client’s perspective:

   1. As far as I know, I’m a public client, and don’t know anything about
   mTLS, but the IT admins installed client certs in their users’ browsers and
   the AS expects to use that to authenticate me.
   2. My AS metadata parser crashed because the mTLS-only AS omitted
   token_endpoint_uri.
   3. My AS metadata parser crashed because it didn’t expect to encounter a
   JSON object as a parameter value.
   4. The mTLS-only AS didn’t provide a value for mtls_endpoints, what do I
   do?
   5. I don’t know what that “m” means, but they told me to use HTTPS, so I
   should use the one with “tls” in its name, right?



Yes, you can write normative text that answers most of these. But you’ll
have to clearly cover a lot of similar-but-slightly-different scenarios and
be very explicit. And implementers will still get it wrong. The metadata
change introduces opportunities for confusion and failure that do not exist
now, and forces them on everyone who supports mTLS. In contrast, the 307
redirect is only required when an AS wants to support both, and is
unambiguous in its behavior and meaning.



-- 

Annabelle Richard Backman

AWS Identity





*From:* Brian Campbell <bcampbell=[e-mail]>
*Date:* Friday, February 1, 2019 at 3:17 PM
*To:* "Richard Backman, Annabelle" <[e-mail]>
*Cc:* George Fletcher <[e-mail]>, oauth <[e-mail]>
*Subject:* [UNVERIFIED SENDER] Re: [OAUTH-WG] MTLS and in-browser clients
using the token endpoint



It doesn't seem like that confusing or large of a change to me - if the
client is doing MTLS and the given endpoint is present in `mtls_endpoints`,
then it uses that one.  Otherwise it uses the regular endpoint. It gives an
AS a lot of flexibility in deployment options. I personally think getting a
307 approach deployed and working would be more complicated and error
prone.



It is a minority use case at the moment but there are forces in play, like
the push for increased security in general and to have javascript clients
use the code flow, that suggest it won't be terribly unusual to see an AS
that wants to support MTLS clients and javascript/spa clients at the same
time.



I've personally wavered back and forth in this thread on whether or not to
add the new metadata (or something like it). With my reasoning each way
kinda explained somewhere back in the 40 or so messages that make up this
thread.  But it seems like the rough consensus of the group here is in
favor of it.









On Fri, Feb 1, 2019 at 3:18 PM Richard Backman, Annabelle <richanna=
[e-mail] <[e-mail]....org>> wrote:

This strikes me as a very prominent and confusing change to support what
seems to be a minority use case. I’m getting a headache just thinking about
the text needed to clarify when the AS should provide `mtls_endpoints` and
when the client should use that versus using `token_endpoint.` Why is the
307 status code insufficient to cover the case where a single AS supports
both mTLS and non-mTLS?



-- 

Annabelle Richard Backman

AWS Identity





*From:* OAuth <[e-mail]> on behalf of Brian Campbell
<bcampbell=[e-mail]>
*Date:* Friday, February 1, 2019 at 1:31 PM
*To:* George Fletcher <gffletch=[e-mail]
<40aol.com@dmarc.........ietf.org>>
*Cc:* oauth <[e-mail]>
*Subject:* Re: [OAUTH-WG] MTLS and in-browser clients using the token
endpoint



Yes, that would work.



On Fri, Feb 1, 2019 at 2:28 PM George Fletcher <gffletch=
[e-mail]> wrote:

What if the AS wants to ONLY support MTLS connections. Does it not specify
the optional "mtls_endpoints" and just use the normal metadata values?

On 1/15/19 8:48 AM, Brian Campbell wrote:

It would definitely be optional, apologies if that wasn't made clear. It'd
be something to the effect of optional for the AS to include and clients
doing MTLS would use it when present in AS metadata.



On Tue, Jan 15, 2019 at 2:04 AM Dave Tonge <[e-mail]>
wrote:

I'm in favour of the `mtls_endpoints` metadata parameter - although it
should be optional.


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