Re: [Acme] ACME breaking change: Most GETs become POSTs

[Richard Barnes <[e-mail]>]

Hey all,

This thread forked into a couple of different issues, so I wanted to post a
little end-of-day summary of the issues and where we stand.  I've updated
the PR [1] to reflect most of today's discussion.

===

ISSUE 1. Should we do POST-as-GET at all, vs. keeping GET and doing the
privacy analysis?

It seems like there's pretty strong agreement that we should get rid of
GET, as the architecturally cleanest option.

===

ISSUE 2: How should we signal that POST-as-GET request is different from
other POST requests?

The current PR signals this by sending a JWS with an empty (zero-octet)
payload, instead of a JSON object.  Jacob and Daniel suggested that we
should instead use the payload being an empty JSON object as the signal.
An earlier draft PR used a field in the protected header.

===

ISSUE 3: Should servers be required to allow GET requests for certificate
URLs?

I had proposed this earlier today; Jacob and Daniel pushed back.  I have
implemented a compromise in the latest PR, where servers MAY accept GET
requests.

===

ISSUE 4: How should we address the risk that an attacker can discover URLs
by probing for Unauthorized vs. Not Found?

There seemed to be agreement on the list that this should be addressed with
some guidance to servers on how to assign URLs.  I have just added some
text to the PR for this.

===

It seems to me we're pretty much closed on the first issue, and the other
three are still open.  Please send comments, so we can resolve this issue
and get the document back in motion!

Thanks,
--Richard

[1] https://github.com/ietf-wg-acme/acme/pull/445

On Thu, Aug 30, 2018 at 7:20 PM Jacob Hoffman-Andrews <[e-mail]> wrote:

> ACME currently has unauthenticated GETs for some resources. This was
> originally discussed in January 2015[1]. We decided to put all sensitive
> data in the account resource and consider all GET resources public, with
> a slant towards transparency.
>
> Adam Roach recently pointed out in his Area Director review that even
> when the contents of GET URLs aren’t sensitive, their correlation may
> be. For instance, some CAs might consider the grouping of certificates
> by account to be sensitive.
>
> Richard Barnes proposes[2] to change all GETs to POSTs (except directory
> and new-nonce). This will be a breaking change. Clients that were
> compatible with previous drafts, informally called ACMEv1 and ACMEv2,
> will not be compatible with a draft that mandates POSTs everywhere. It
> will be a painful change, since the ecosystem just started switching to
> ACMEv2, which looked to be near-final.
>
> I think this is the right path forwards. ACME will be a simpler, better
> protocol long-term if all requests are authenticated. However, if we’re
> taking this path we should aim to come to consensus and land the final
> spec quickly to reduce uncertainty for ACME client implementers.
>
> [1] https://github.com/letsencrypt/acme-spec/pull/48#issuecomment-70169712
> [2] https://github.com/ietf-wg-acme/acme/pull/445/files
>
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