Network Working Group O. Friel
Internet-Draft R. Barnes
Intended status: Standards Track Cisco
Expires: September 7, 2020 T. Hollebeek
DigiCert
M. Richardson
Sandelman Software Works
March 06, 2020
ACME for Subdomains
draft-friel-acme-subdomains-02
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
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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. Pre-Authorization . . . . . . . . . . . . . . . . . . . . 4
4.2. Illustrative Call Flow . . . . . . . . . . . . . . . . . 5
4.3. newOrder and newAuthz Handling . . . . . . . . . . . . . 6
4.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Resource Enhancements . . . . . . . . . . . . . . . . . . . . 8
5.1. Authorization Object . . . . . . . . . . . . . . . . . . 8
5.2. Directory Object Metadata . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. Authorization Object Fields Registry . . . . . . . . . . 8
6.2. Directory Object Metadata Fields Registry . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Informative References . . . . . . . . . . . . . . . . . . . 9
Appendix A. ACME Server Policy Considerations . . . . . . . . . 9
A.1. CA Browser Forum Baseline Requirements Extracts . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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. This
document 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:
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o CA: Certificate Authority
o CSR: Certificate Signing Request
o FQDN: Fully Qualified Domain Name
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:
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"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."
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 a parent domain of that subdomain. This
allows a flow where a client proves ownership of, for example,
"example.com" and then successfully obtains a certificate for
"sub.example.com".
ACME server policy is out of scope of this document, however some
commentary is provided in Appendix A.
4.1. Pre-Authorization
The standard ACME workflow has authorization objects created
reactively in response to a certificate order. ACME also allows for
pre-authorization, where clients obtain authorization for an
identifier proactively, outside of the context of a specific
issuance. This document allows for both workflows, and Section 4.3
outlines how the ACME server handles newOrder and newAuthz requests
for both workflows.
It may make sense to use the ACME pre-authorization flow for the
subdomain use case, however, that is an operator implementation and
deployment decision. With the ACME pre-authorization flow, the
client could pre-authorize for the parent domain once, and then issue
multiple newOrder requests for certificates for multiple subdomains.
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4.2. Illustrative Call Flow
The call flow illustrated here uses the ACME pre-authorization flow.
The call flow also 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" | |
|--------------------------------->|
| | |
| 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"| |
|--------------------->| |
| | |
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| 200 OK status=valid | |
|<---------------------| |
| | |
| POST /certificate | |
|--------------------->| |
| | |
| 200 OK | |
| PKI "sub.example.com"| |
|<---------------------| |
4.3. 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
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 section 7.4.1
which states "Note that because the identifier in a pre-authorization
request is the exact identifier to be included in the authorization
object, pre-authorization cannot be used to authorize issuance of
certificates containing wildcard domain names."
Additionally, 200 response code is used here in one scenario instead
of a 201 response. However, this is arguably an under-specification
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in RFC8555, and has been reported in https://www.rfc-
editor.org/errata/eid5861.
These two items need a review. ]]
4.4. 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" 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.
2. 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.
3. 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".
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5. Resource Enhancements
This document defines enhancements to the authorization and directory
objects.
5.1. Authorization Object
If an ACME server allows issuance of certificates for subdomains of a
parent domain, then the authorization object for the parent domain
MUST include the optional "basedomain" field, with a value of true.
The structure of an ACME authorization resource is enhanced to
include the following optional field:
basedomain (optional, boolean): This field MUST be present and true
for authorizations where ACME server policy allows certificates to to
be issued for subdomains of the identifier in the authorization
object without explicit authorization of the subdomain
5.2. Directory Object Metadata
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".
6. IANA Considerations
6.1. Authorization Object Fields Registry
The following field is added to the "ACME Authorization Object
Fields" registry defined in ACME [RFC8555].
+------------+-----------------+--------------+-----------+
| Field Name | Field Type | Configurable | Reference |
+------------+-----------------+--------------+-----------+
| basedomain | boolean | false | RFC XXXX |
+------------+-----------------+--------------+-----------+
6.2. Directory Object Metadata Fields Registry
The following field is added to the "ACME Directory Metadata Fields"
registry defined in ACME [RFC8555].
+---------------------+--------+-------+ | Field Name | Field Type |
Reference | +---------------------+--------+-------+ |
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implicitSubdomainAuthorization | boolean | RFC XXXX |
+---------------------+--------+-------+
7. Security Considerations
[[TODO]]
8. Informative References
[CAB] CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates", n.d.,
<https://cabforum.org/baseline-requirements-documents/>.
[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. ACME Server Policy Considerations
The ACME specification does not mandate any specific ACME server or
CA policies, or any specific use cases for issuance of certificates.
For example, an ACME server could be used:
o to issue Web PKI certificates where the ACME server must comply
with CA/Browser [CAB] Baseline Requirements.
o as a Private CA for issuance of certificates within an
organisation. The organisation could enforce whatever policies
they desire on the ACME server.
o for issuance of IoT device certificates. There are currently no
IoT device certificate policies that are generally enforced across
the industry. Organsations issuing IoT device certificates can
enforce whatever policies they desire on the ACME server.
ACME server policy could specify whether:
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o issuance of subdomain certificates is allowed based on proof of
ownership of a parent domain
o whether DNS based proof of ownership, or HTTP based proof of
ownership, or both, are allowed
ACME server policy specification is exlpicitly out of scope of this
document.
A.1. CA Browser Forum Baseline Requirements Extracts
The CA/Browser Forum Baseline Requirements [CAB] allow issuance of
subdomain certificates where authorization is only required for a
parent domain. 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
Richard Barnes
Cisco
Email: rlb@ipv.sx
Tim Hollebeek
DigiCert
Email: tim.hollebeek@digicert.com
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Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
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