ACME for Subdomains
draft-friel-acme-subdomains-00

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Network Working Group                                           O. Friel
Internet-Draft                                                 R. Barnes
Intended status: Standards Track                                   Cisco
Expires: April 25, 2020                                     T. Hollebeek
                                                                DigiCert
                                                        October 23, 2019

                          ACME for Subdomains
                     draft-friel-acme-subdomains-00

Abstract

   This document outlines how ACME can be used by a client to obtain a
   certificate for a subdomain identifier from a certificate authority.
   The client has fulfilled a challenge against a parent domain but does
   not need to fulfil a challenge against the explicit subdomain as
   certificate authority policy allows issuance of the subdomain
   certificate without explicit subdomain ownership proof.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on April 25, 2020.

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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  ACME Workflow and Identifier Requirements . . . . . . . . . .   3
   4.  ACME Issuance of Subdomain Certificates . . . . . . . . . . .   4
     4.1.  newOrder and newAuthz Handling  . . . . . . . . . . . . .   5
     4.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Directory Object Metadata Fields Registry . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  CA Browser Forum Baseline Requirements . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   ACME [RFC8555] defines a protocol that a certificate authority (CA)
   and an applicant can use to automate the process of domain name
   ownership validation and X.509 (PKIX) certificate issuance.  The
   protocol is rich and flexible and enables multiple use cases that are
   not immediately obvious from reading the specification.

   This document explicitly outlines how ACME can be used to issue
   subdomain certificates, without requiring the ACME client to
   explicitly fulfil an ownership challenge against the subdomain
   identifiers - the ACME client need only fulfil an ownership challenge
   against a parent domain identifier.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are used in this document:

   o  CA: Certificate Authority

   o  CSR: Certificate Signing Request

   o  FQDN: Fully Qualified Domain Name

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3.  ACME Workflow and Identifier Requirements

   A typical ACME workflow for issuance of certificates is as follows:

   1.  client POSTs a newOrder request that contains a set of
       "identifiers"

   2.  server replies with a set of "authorizations" and a "finalize"
       URI

   3.  client sends POST-as-GET requests to retrieve the
       "authorizations", with the downloaded "authorization" object(s)
       containing the "identifier" that the client must prove control of

   4.  client proves control over the "identifier" in the
       "authorization" object by completing the specified challenge, for
       example, by publishing a DNS TXT record

   5.  client POSTs a CSR to the "finalize" API

   6.  server replies with an updated order object that includes a
       "certificate" URI

   7.  client sends POST-as-GET request to the "certificate" URI to
       download the certificate

   ACME places the following restrictions on "identifiers":

   o  section 7.1.4: the only type of "identifier" defined by the ACME
      specification is a fully qualified domain name: "The only type of
      identifier defined by this specification is a fully qualified
      domain name (type: "dns").  The domain name MUST be encoded in the
      form in which it would appear in a certificate."

   o  Section 7.4: the "identifier" in the CSR request must match the
      "identifier" in the newOrder request: "The CSR MUST indicate the
      exact same set of requested identifiers as the initial newOrder
      request."

   o  Sections 8.3: the "identifier", or FQDN, in the "authorization"
      object must be used when fulfilling challenges via HTTP:
      "Construct a URL by populating the URL template ... where the
      domain field is set to the domain name being verified"

   o  Section 8.4: the "identifier", or FQDN, in the "authorization"
      object must be used when fulfilling challenges via DNS: "The
      client constructs the validation domain name by prepending the
      label "_acme-challenge" to the domain name being validated."

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   ACME does not mandate that the "identifier" in a newOrder request
   matches the "identifier" in "authorization" objects.

4.  ACME Issuance of Subdomain Certificates

   As noted in the previous section, ACME does not mandate that the
   "identifier" in a newOrder request matches the "identifier" in
   "authorization" objects.  This means that the ACME specification does
   not preclude an ACME server processing newOrder requests and issuing
   certificates for a subdomain without requiring a challenge to be
   fulfilled against that explicit subdomain.  ACME server policy could
   allow issuance of certificates for a subdomain to a client where the
   client only has to fulfil an authorization challenge for the parent
   domain.  The relevant sections from current CA/Browser baseline
   requirements are given in section Appendix A.

   This allows a flow where a client proves ownership of, for example,
   "example.com" and then successfully obtains a certificate for
   "sub.example.com".  The ACME pre-authorization flow makes most sense
   for this use case, and that is what is illustrated in the following
   call flow.

   The client could pre-authorize for the parent domain once, and then
   issue multiple newOrder requests for certificates for multiple
   subdomains.  This call flow illustrates the client only placing one
   newOrder request.

   The call flow illustrates the DNS-based proof of ownership mechanism,
   but the subdomain workflow is equally valid for HTTP based proof of
   ownership.

   +--------+             +------+     +-----+
   | Client |             | ACME |     | DNS |
   +--------+             +------+     +-----+
       |                      |           |
    STEP 1: Pre-Authorization of parent domain
       |                      |           |
       | POST /newAuthz       |           |
       | "example.com"        |           |
       |--------------------->|           |
       |                      |           |
       | 201 authorizations   |           |
       |<---------------------|           |
       |                      |           |
       | Publish DNS TXT      |           |
       | "example.com"        |           |
       |--------------------------------->|
       |                      |           |

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       | POST /challenge      |           |
       |--------------------->|           |
       |                      | Verify    |
       |                      |---------->|
       | 200 status=valid     |           |
       |<---------------------|           |
       |                      |           |
       | Delete DNS TXT       |           |
       | "example.com"        |           |
       |--------------------------------->|
       |                      |           |
    STEP 2: Place order for subdomain
       |                      |           |
       | POST /newOrder       |           |
       | "sub.example.com"    |           |
       |--------------------->|           |
       |                      |           |
       | 201 status=ready     |           |
       |<---------------------|           |
       |                      |           |
       | POST /finalize       |           |
       | CSR "sub.example.com"|           |
       |--------------------->|           |
       |                      |           |
       | 200 OK status=valid  |           |
       |<---------------------|           |
       |                      |           |
       | POST /certificate    |           |
       |--------------------->|           |
       |                      |           |
       | 200 OK               |           |
       | PKI "sub.example.com"|           |
       |<---------------------|           |

4.1.  newOrder and newAuthz Handling

   Servers may consider validation of a parent domain sufficient
   authorization for a subdomain.  If a server has such a policy and a
   client is already authorized for the parent domain then:

   o  If the client submits a newAuthz request for a subdomain: The
      server MUST return status 200 (OK) response.  The response body is
      the existing authorization object for the parent domain with
      status set to "valid".

   o  If the client submits a newOrder request for a subdomain: The
      server MUST return a 201 (Created) response.  The response body is

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      an order object with status set to "ready" and links to the
      unexpired authorizations against the parent domain.

   If a server has such a policy and a client is not authorized for the
   parent domain then:

   o  If the client submits a newAuthz request for a subdomain: The
      server MUST return a status 201 (Created) response.  The response
      body is a newly created authorization object for the parent domain
      with status set to "pending".

   o  If the client submits a newOrder request for a subdomain: The
      server MUST return a status 201 (Created) response.  The response
      body is an order object with status set to "pending" and links to
      newly created authorizations objects against the parent domain.

   [[ TODO: This section documents a change from RFC8555, which states
   that the identifier in the newAuthz request MUST match that in the
   authorization object.

   Additionally, 200 response code is used here in one scenario instead
   of a 201 response.  However, this is arguably an under-specification
   in RFC8555, and has been reported in https://www.rfc-
   editor.org/errata/eid5861.

   These two items need a review. ]]

4.2.  Examples

   In order to illustrate subdomain behaviour, let us assume that a
   client wishes to get certificates for subdomain identifiers
   "sub0.example.com", "sub1.example.com" and "sub2.example.com" under
   parent domain "example.com", and CA policy allows certificate
   issuance of these subdomain identifiers while only requiring the
   client to fulfil an ownership challenge for parent domain
   "example.com".  Let us also assume that the client has not yet proven
   ownership of parent domain "example.com".

   1.  The client POSTs a newOrder request for identifier
       "sub0.example.com"

   The server creates an authorization object for identifier
   "example.com".  The server replies with a 201 (Created) response.
   The response body is an order object with status set to "pending" and
   a link to newly created authorization object against the parent
   domain "example.com".  Therefore, the server is instructing the
   client to fulfil a challenge against domain identifier "example.com"

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   in order to obtain a certificate including identifier
   "sub0.example.com".

   The client completes the challenge for "example.com", POSTs a CSR to
   the order finalize URI, and downloads the certificate.

   1.  The client POSTs a newOrder request for identifier
       "sub1.example.com"

   The server replies with a 201 (Created) response.  The response body
   is an order object with status set to "ready" and a link to the
   unexpired authorization against the parent domain "example.com".

   The client POSTs a CSR to the order finalize URI, and downloads the
   certificate.

   1.  The client POSTs a newAuthz request for identifier
       "sub2.example.com"

   The server replies with a 200 (OK) response.  The response body is
   the previously created authorization object for "example.com" with
   status set to "valid".

5.  Directory Object Metadata Fields Registry

   [[ TODO: is this required? ]]

   An ACME server can advertise support of issuance of subdomain
   certificates by including the boolean field
   "implicitSubdomainAuthorization" in its "ACME Directory Metadata
   Fields" registry.  If not specified, then no default value is
   assumed.  If an ACME server supports issuance of subdomain
   certificates, it can indicate this by including this field with a
   value of "true".

        +--------------------------------+------------+-----------+
        | Field Name                     | Field Type | Reference |
        +--------------------------------+------------+-----------+
        | implicitSubdomainAuthorization | boolean    | RFC XXXX  |
        +--------------------------------+------------+-----------+

6.  IANA Considerations

   [[TODO: register implicitSubdomainAuthorization? ]]

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7.  Security Considerations

   [[TODO]]

8.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

Appendix A.  CA Browser Forum Baseline Requirements

   The CA/Browser Forum Baseline Requirements version 1.6.5 states:

   o  Section: "1.6.1 Definitions": Authorization Domain Name: The
      Domain Name used to obtain authorization for certificate issuance
      for a given FQDN.  The CA may use the FQDN returned from a DNS
      CNAME lookup as the FQDN for the purposes of domain validation.
      If the FQDN contains a wildcard character, then the CA MUST remove
      all wildcard labels from the left most portion of requested FQDN.
      The CA may prune zero or more labels from left to right until
      encountering a Base Domain Name and may use any one of the
      intermediate values for the purpose of domain validation.

   o  Section: "3.2.2.4.7 DNS Change": Once the FQDN has been validated
      using this method, the CA MAY also issue Certificates for other
      FQDNs that end with all the labels of the validated FQDN.  This
      method is suitable for validating Wildcard Domain Names.

Authors' Addresses

   Owen Friel
   Cisco

   Email: ofriel@cisco.com

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   Richard Barnes
   Cisco

   Email: rlb@ipv.sx

   Tim Hollebeek
   DigiCert

   Email: tim.hollebeek@digicert.com

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