Minutes IETF100: acme
Automated Certificate Management Environment
||Minutes IETF100: acme
ACME Working Group meeting at IETF 100
Thursday, November 16 at 15:50 local time (7:50 UTC)
- Published acme-08;
- OPS and Gen-ART reviews;
- New versions for all new drafts.
TLS certificates for email services.
See slides (#2) for a summary of the changes since Prague.
Open issues: - Include port and service (e.g., smtp, imaps) in challange
MT: Port under the hash: OK. Are you suggesting or even requiring that
the ACME server insists on a particular port number for these protocols?
AM: in theory you could put any of these on a non standard port. I
should probably say: CA may enforce that port number correlates to the
MT: Ability to run TCP server on my machine on port 5000 is not a good
indication that person controls the machine. Definitely want to have
some text around this and maybe strongly recommend there be some policy
at the CA that says: if the port number is different from what I say
here then not a good idea to accept this. We did the same thing for
RB: The analogy MT was pointing at is the HTTP challenge: the CA will
only connect to the validation server on port 80, which is supposedly
privileged. Use the standard port for the services we are talking about.
- Cover both TLS and non TLS ports (start plain, try STARTTLS and
upgrade to TLS if available, e.g.: IMAP)? In theory same certificate
could be used for both ports, in practice services and ports are
different. Therefore difficult to get a certificate that covers both
in one go.
RB: Does not seem like an ACME issue: if certificate you are issuing is
agnostic as to port number it should be fine. Only if using an SRV name
or specifying the port number in the certificate then you need some
AM: Yes, I was thinking whether it is possible to get SAN and SRV name
in the same certificate in one go. The answer is probably no. It is an
optimisation, I don't think it matters in practice.
JL: IMAP server I am using I don't think it has provision for doing
separate certficates for the two services, so combining is important.
JL: Unrelated question: any reason why POP is missing?
AM: No particular reason.
JL: Tiny data point: I use it. Anything that applies to IMAP equally
applies for POP.
AM: It'd need new capability challenge. Only DNS challenge would work
out of the box.
JL: Adding cap challenge should be same effort as adding it to IMAP.
AM: OK will do.
- Support LMTP?
AM: A variant of SMTP for final delivery, some of these services are
public facing. Trouble is LMTP doesn't have registered port number, and
neither is the lmtp name registered. Could do registration in a separate
document, no problem.
Next steps: - More reviews (Richard should review); - Call for
implementers, could be something in London on top of letsencrypt for
Certificates for S/MIME users.
AM: Still rather raw. Done some changes but really need to flesh out how
the messages are going to look like.
See slides (#5) for a summary of the changes since Prague.
RB: one thing not clear from the document is whether you are trying to
verify someone is able to receive mail at the claimed address, or send
email from that address, or both?
AM: open question, but I suspect the answer is both (and sending is more
powerful assertion than receiving.)
JL: just being able to receive is not sufficient: I could probably get
an ACME certificate for every IETF mailing list... sending is
problematic as well. Probably need text in security considerations
saying that a "certain amount" of spoofing of identities is unavoidable.
HN: Same point as JL. For receiving OK. For sending we need to make sure
an email validation system like DMARC is mentioned in the document.
AM: OK, sounds good.
RB: I should observe that verification of receipt is used in typical web
based workflows where email is sent to an address with a web link to
click on for verification. I don't know if it's evidence of sufficiency
but it's evidence that this works in some other cases.
JL: (not intelligible)
RB: Last observation: looking at JP here as there seems to be some
overlap between this and the empirical telephone number verification
JP: Return routability tests of various kinds have been used for some
time - stolen from practices used for email. If the endpoint is smart
enough you could add more on top to strengthen the verification (e.g.,
crypto in the SIM card), or identifiers (IMEI, IMSI) that are maintained
by the network therefore the network can be queried. If we want to make
this more generic by defining a profile that encompasses both return
routability tests, I'd be willing to work with you (Alexey) on that, if
you are interested.
See slides (#6) for open issues.
Main document wrap-up
On proactive issuance, i.e., when to send the CSR? (See slides for
details about requirements and alternative flows.)
RB: One approach is to always send the CSR twice (PR#342). Simple logic
(no branching), but unnecessary caching on client side if client always
talk to a CA that caches the CSR + retransmit cost.
(?): How do you know the two CSRs are the same?
RB: This issue always exists when you send it twice
EKR: Maybe... (long argument which I could not get.)
RB: The other approach is CA signals client whether to do the second
transmission. Another way you can view this dicothomy is whether the
first and second requests are part of the same flow, or decoupled
things. The first request is more or less a stateful thing that just
gets advice and the second one (when you need a second one) as the
actual issuance transaction. The more decoupled case that EKR was
mentioning just now has the benefit that you only spend as much caching
and bandwidth as you need to for a given CA and transaction - you don't
cache the CSR if you don't need to. As EKR was mentioning you get this
decoupling where there doesn't need to be any coupling between the first
and the second requests and so you can have a slightly simpler logic on
the server by just considering each request on its own without having to
bind sequence of requests together. In this case the logic on the client
is a little more complicated because it has to switch between the two
JL: why do you need the first CSR?
RB: one of the use cases for getting the CSR upfront is that of legacy
APIs that you might have behind an ACME interface that require the CSR
at the beginning of the process. In this case first and second CSR are
required to be the same because the backend has done everything based on
the first CSR.
RB: The question I have for the working group is which of these two
approaches do we prefer? Had some discussion already on the list on
PR#342. Personally I prefer PR#350 because it keeps things a little
more decoupled and avoids unnecessary retransmits, but not a strong
JHA: strongly prefer the former (PR#342) or, should I say, strongly
disprefer the latter (PR#350). Main reason: I want to avoid that "this
CA does something this way and that CA does something that way"; the
risk is proliferation of clients that only implement one behaviour and
not the other and then we don't have an interoperable ecosystem. Prefer
simple and straightforward solution.
EKR: not a crazy concern, but this is what interop testing is for, and
actually having sensible protocol hygiene is more important. And what
the security requirements are in term of what the CA needs to check
between the two CSRs?
JHA: CA can hash the two and make sure the hash is the same.
EKR: That means you think they have to be the same, why do they have to
be the same?
JHA: I don't think this is necessarily true.
EKR: This is concerning.
JHA: In our setup and policy it's OK if they are different. I proposed
hash equality because I thought you wanted them to be identical, but if
you don't think so, I don't either.
RB: There are cases (legacy API) where the CA requires them to be
RB: If you do the checks before issuance on the second one then it
doesn't matter what the first one was...
EKR: yes, but only as long as the protocol says you have to issue off
the second one.
RB: yeah. you say you do all your pre-issuance checks on the 2nd one.
EKR: but again, protocol has to say if they are the same or not. I see
great opportunities for screwing things up in a bunch of different ways.
JHA: I agree. Would you be happy with PR#342 if we said the in the 1st
CSR the public key and extensions are ignored and the only that matter
is the 2nd one?
EKR: I'd be happy-er
RB: we could say: whatever checks the CA does on an issuance request
have to be done with regard to the CSR that is sent in the 2nd request?
MT: what aspects of the CSR are needed in order to tell the client what
to do? It's only the identifiers, right?
RB: issue with legacy APIs that need a real CSR.
EKR: why unify the two flows? Can't this be separated in two different
flows with the server advertising which one to follow?
JHA: I think only one of the flows would get implemented. There are
50-100 ACME clients which are essentially letsencrypt clients... how to
build the protocol so that clients have an incentive to work with
EKR: (summary) this seems the wrong way to go
JHA: ideally, first round send just the identifiers, second round send
the full CSR. we'd need to hear from CAs that have an API that needs CSR
first and would have trouble adopting ACME without this flow.
EKR: it seems you and I prefer the same thing (PR#342). I still need to
be convinced we need "CSR first" flow.
RS (no hat): could we make the error code generic so whenever you get
this error code, resubmit with the CSR and if the CSR is resent it MUST
be the same one?
JHA: could work
RB: need to see the PR to understand exactly how it'd work
RB: surprised that PR#342 works with LE workflows with wildcards?
JHA: the identifier notion introduced in PR#342 is a little different
from the identifiers in the authorisation objects.
RB: so basically in PR#342 you'd take the collection of DNS name SANs,
translate them to identifier objects and send them in? JHA: yes
MT: I like PR#342 as well, an editorial note: be more precise the
definition of identifiers (e.g., define what it means to have a wildcard
ID and the format of it)
RB: there seems to be a bit of mass behind PR#342. Having a signal in
the directory object to take care of legacy whereas the the core ACME is
focused on going forward use cases. Then clients can implement the
legacy affordance or not.
YN: let's take a hum. Proposed way forward is accept PR#342. Hum if you
YN: Hum if you don't
YN: hum if you agree this is the right way forward
YN: hum if you don't agree this is the right way forward
RB: I'll work with Daniel to get PR#342 landed and talk with folks who
have legacy APIs and see if we need to add some affordance for backwards
RB: Have this done and review comments incorporated by early December.
RS: Kathleen, assuming this is the only substantive technical issue, do
you want us to do another WGLC? Or just move it forward after like a
KM (AD): as in TLS, do a narrowly focused WGLC on only areas of
question, not the whole thing. Would be nice to have this done soon so
that it doesn't need another AD review. The sooner the better.
RB: Hope to get this done well before March.
JP: both I and Mary are presenting basically the same material and
trying to pose the same questions. Trying to cover both as much as I
JP: in STIR we have defined certificate profiles and a certain number of
certificate extension. Two branches: TN (telephone numbers, maybe
associated with an end-user) and SPC (service provider codes). It'd be
very difficult, maybe impossible to administer those without something
See slides (#3-7) for a description of how STIR works, and how STIR
plans to make use of ACME, and what the alternatives to prove identities
JP: Generalising off the telephone use cases: an ACME server has a
preexisting relationship with an identity authority of some kind; when a
user claims a certain identity, the ACME server can challenge the user
asking it to go to the authority that controls the relevant name space
and get a token; if the authority thinks user is legit, they will give
back something cryptographically signed that the user can present to the
ACME server; if the ACME server verifies that "something" it issues the
user a new certificate.
JP: Other possible use case: E-ZPass / FasTrack with license plate
JP: Therefore define a generic token, with an explicit type governed by
some registry and some more things in it (like a hint).
JP: Mary's approach is slightly different, we want to check with the
ACME WG whether this is a good idea, and which way is preferable.
(?): clarifying question: you sort of imply ACME service reaching out to
the authority to get the token?
See slides (#2-?) for an overview.
MB: (audio breaks lots)
JP: I'm arguing for a model with sub-typing and an IANA registry that
specifies how these tokens work in particular. The approach in Mary's
draft is more focused on identifying that entity designation than on
something that could potentially use different forms of tokens.
CW: Jon covered it pretty well. Service provider corresponds to the
SHAKEN stuff that we've done in the STIR domain in terms of the service
provider code. If I understand Jon's proposal, the difference is that
instead of having a predefined entity code, we have a type that is
defined in a profile document, or something like that, and that
characterises all the claims and other things. My personal opinion is
I'm fine with it. Wish Mary was here to give her position as well.
MB: I have two drafts, one generic and another which provides an example
of using the former. And in this model you woudn't need the registration
and is therefore lighter weight in my opinion. It doesn't put as much
knowledge of how we're doing this in the ACME server. So, it's totally
up to the authority and its relationship with the end-entity to figure
out the value of the token and the unique identifier.
JP: yes, I guess it's lighter weight but since we have to assume a
pre-existing association between the authority and the CA anyway, being
lighter weight limits the kind of things you can end up talking about.
MB: I see your point. I was doing it in the original service provider
code document, where the perspective was that the PA and CA already have
that trust relationship and in the SHAKEN model we absolutely have that.
We have a more restricted model. I can see your point, OK.
JP: I think this can be useful in lots of cases in ACME, but this is a
question for the room, that's what I'm interested to hear about.
MB: it doesn't cost anything to not be generic. My concern is if we are
too generic we are going to get use cases that we need to evaluate and
make sure it works for those situations. People have already implemented
the SKAKEN stuff and it works...
RB: I think this is worth doing generically. I've had a similar use case
in mind for DNS names for a while. Wouldn't it be nice even for the DV
case if the CA could get information directly from a registry as to who
holds which domain name? It's worth at least including that case. The
thing we need to struggle with here is what the right axes for
generalisation are. The only draft I read is Jon's and it looks roughly
correct to me in terms of branching on token type. I'm broadly
supportive of this approach.
CW: I agree it'd be good to have a generic approach. One thing we could
do is having sort of a default mode where there is e.g, an UUID. I can
see a scenario where 80-90% of the folks using this may have pretty much
the same mechanism: you don't have to necessarily define every single
use case - may be something to consider.
RS: we already adopted both. If they are converging, authors should get
together and merge.
JP: if we merge Mary's and mine proposal, both the service provider
draft and the TN document would then rely on the generic one. And I'm
happy to put in Chris's UUID case as I think it makes a lot of sense. I
love the DV case. I think it's a win-win.
RS: Mary opinions?
MB: Jon, Chris and I will work together.
TH: I generally like the mechanism you guys are converging on. A slight
thing on the amount of domain knowledge that you need and I think that
you still require some amount of it in the CA to know something about
(let's call it) the number space that you are eventually going to issue
the certificate for, because I don't think you can get away from the
fact that in STIR a wildcard means something and in regular domain name
FQDNs it means another thing. Let's just try and not be so generic that
we expect every CA to be able to have the knowledge to issue certificate
of fundamentally different types.
RB: just to clarify the context: the context is within an authorisation
transaction at which point you have already specified this is the
specific identifier you are validating. So, I don't think we run into
the questions of wildcards, etc.
TH: I agree, I'm just trying to say that the amount of state can only
get so low, because you are going to be able to match it to what kind of
identifier is going to be issued. There are some of these identifiers
which are functionally issued with only one reference authority and the
amount of state you are willing to take is whatever the reference
CW: you also have to assume that the ACME implementation needs to
support STIR certificates as well so there are a couple of levels of
qualification that have to happen before you can actually give a
YN (chair): guidance is talk together and come back with a single
submission and then we can call for adoption, hopefully before London.
RS (chair): do we need to hum on that? Is everyone fine? If the charter
has to be adjusted we'll deal with that. Idea is to have this as a WG
doc before London.
RB: there shouldn't be any need for charter adjustments.
RS: It depends on how generic you guys make it.
See slides (#3-8) for a summary of the changes since Prague.
JP: with my use case which is not domain names. I do appreciate the
de-lurkage that you did thus far. The term DNO, and the notion that
there is a domain name owner that cancels an automatic renewal still
doesn't fit the language of my use case, but I'd love doing exactly what
you are doing. So, just a little bit less specificy of the domain name
part of this and STIR would totally use this. Happy to help.
TF: OK, thanks.
TF: interaction with certificate transparency is TBD. Diego went to the
TRANS WG session to present the problem.
DL: No conclusive answer whether this would pose a problem to CT or not.
What I've heard from Melinda was they are OK for documenting the problem
and solution space. What is not completely clear is where this work
would be done since TRANS may be closing down in the next few months.
DL: If others that were there (EKR?) could share their impression as
EKR: AD hat off. Given clock skews forces short-term to be not below
7-days-ish, the increase in ingestion rate should be fine. So, no actual
problem here, you should just go forward.
RB: CT shall arrive at some solution for short-lived certificates
anyway, regardless of this document, because there are multiple
different fronts from which this is coming. So it seems it's their
problem, not a problem for this document to solve.
RB: One quick question out of curiosity: any CAs interested in deploying
TF: Yes, Digicert contacted us; they said they have "short" as short as