An ACME Profile for Generating Delegated Certificates
draft-ietf-acme-star-delegation-07
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9115.
|
|
|---|---|---|---|
| Authors | Yaron Sheffer , Diego Lopez , Antonio Pastor , Thomas Fossati | ||
| Last updated | 2021-04-08 (Latest revision 2021-03-26) | ||
| Replaces | draft-sheffer-acme-star-delegation | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-06)
by Suresh Krishnan
Ready w/nits
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Rich Salz | ||
| Shepherd write-up | Show Last changed 2021-01-19 | ||
| IESG | IESG state | Became RFC 9115 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs 4 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Roman Danyliw | ||
| Send notices to | ynir.ietf@gmail.com, rsalz@akamai.com | ||
| IANA | IANA review state | IANA - Not OK | |
| IANA expert review state | Issues identified | ||
| IANA expert review comments | Authors: A couple of points for you: 1. The "delegation" field is currently attached to the "identifier" object, which is a bad semantic fit in a few ways. ACME orders can have multiple identifiers, and delegations can describe multiple SAN values, yet this design assumes singularity on both sides. This field should be moved to the order object; in fact, if you wanted to be more radical, you could even use it to replace the "identifiers" field in the newOrder request. 2. The "allow-certificate-get" field is listed as configurable. It seems like this is a matter of CA policy, so it should either be non-configurable, or if you allow the client to request a value for it, there should be a clear specification that the server is allowed to ignore the client's preference. |
draft-ietf-acme-star-delegation-07
ACME Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track D. López
Expires: 27 September 2021 A. Pastor Perales
Telefonica I+D
T. Fossati
ARM
26 March 2021
An ACME Profile for Generating Delegated Certificates
draft-ietf-acme-star-delegation-07
Abstract
This memo defines a profile of the Automatic Certificate Management
Environment (ACME) protocol by which the owner of an identifier
(e.g., a domain name) can allow a third party to obtain an X.509
certificate such that the certificate subject is the delegated
identifier while the certified public key corresponds to a private
key controlled by the third party. A primary use case is that of a
Content Delivery Network (CDN, the third party) terminating TLS
sessions on behalf of a content provider (the owner of a domain
name). The presented mechanism allows the owner of the identifier to
retain control over the delegation and revoke it at any time. A key
property of this mechanism is it does not require any modification to
the deployed TLS ecosystem.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 27 September 2021.
Sheffer, et al. Expires 27 September 2021 [Page 1]
Internet-Draft ACME Delegation March 2021
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Conventions used in this document . . . . . . . . . . . . 5
2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Preconditions . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Delegated Identity Profile . . . . . . . . . . . . . . . 8
2.3.1. Delegation Configuration . . . . . . . . . . . . . . 8
2.3.2. Order Object Transmitted from NDC to IdO and to ACME
Server (STAR) . . . . . . . . . . . . . . . . . . . . 10
2.3.3. Order Object Transmitted from NDC to IdO and to ACME
Server (non-STAR) . . . . . . . . . . . . . . . . . . 13
2.3.4. Capability Discovery . . . . . . . . . . . . . . . . 16
2.3.5. Terminating the Delegation . . . . . . . . . . . . . 16
2.4. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . 17
3. CSR Template . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1. Template Syntax . . . . . . . . . . . . . . . . . . . . . 19
3.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 20
4. Further Use Cases . . . . . . . . . . . . . . . . . . . . . . 21
4.1. CDN Interconnection (CDNI) . . . . . . . . . . . . . . . 21
4.1.1. Multiple Parallel Delegates . . . . . . . . . . . . . 22
4.1.2. Chained Delegation . . . . . . . . . . . . . . . . . 22
4.2. Secure Telephone Identity Revisited (STIR) . . . . . . . 25
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
5.1. New ACME Identifier Object Fields . . . . . . . . . . . . 26
5.2. New Fields in the "meta" Object within a Directory
Object . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.3. New Fields in the Order Object . . . . . . . . . . . . . 27
5.4. New Fields in the Account Object . . . . . . . . . . . . 28
5.5. New Error Types . . . . . . . . . . . . . . . . . . . . . 28
5.6. CSR Template Extensions . . . . . . . . . . . . . . . . . 28
6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
Sheffer, et al. Expires 27 September 2021 [Page 2]
Internet-Draft ACME Delegation March 2021
6.1. Trust Model . . . . . . . . . . . . . . . . . . . . . . . 29
6.2. Delegation Security Goal . . . . . . . . . . . . . . . . 29
6.3. New ACME Channels . . . . . . . . . . . . . . . . . . . . 30
6.4. Restricting CDNs to the Delegation Mechanism . . . . . . 31
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
8.1. Normative References . . . . . . . . . . . . . . . . . . 32
8.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Document History . . . . . . . . . . . . . . . . . . 35
A.1. draft-ietf-acme-star-delegation-07 . . . . . . . . . . . 35
A.2. draft-ietf-acme-star-delegation-06 . . . . . . . . . . . 35
A.3. draft-ietf-acme-star-delegation-05 . . . . . . . . . . . 35
A.4. draft-ietf-acme-star-delegation-04 . . . . . . . . . . . 35
A.5. draft-ietf-acme-star-delegation-03 . . . . . . . . . . . 36
A.6. draft-ietf-acme-star-delegation-02 . . . . . . . . . . . 36
A.7. draft-ietf-acme-star-delegation-01 . . . . . . . . . . . 36
A.8. draft-ietf-acme-star-delegation-00 . . . . . . . . . . . 36
A.9. draft-sheffer-acme-star-delegation-01 . . . . . . . . . . 36
A.10. draft-sheffer-acme-star-delegation-00 . . . . . . . . . . 36
Appendix B. CSR Template: CDDL . . . . . . . . . . . . . . . . . 36
Appendix C. CSR Template: JSON Schema . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
1. Introduction
This document is a companion document to [RFC8739]. To avoid
duplication, we give here a bare-bones description of the motivation
for this solution. For more details and further use cases, please
refer to the introductory sections of [RFC8739].
An Identifier Owner (IdO) has agreements in place with one or more
NDC (Name Delegation Consumer) to use and attest its identity.
In the primary use case the IdO is a content provider, and we
consider a Content Delivery Network (CDN) provider contracted to
serve the content over HTTPS. The CDN terminates the HTTPS
connection at one of its edge cache servers and needs to present its
clients (browsers, mobile apps, set-top-boxes) a certificate whose
name matches the domain name of the URL that is requested, i.e., that
of the IdO. Understandably, some IdOs may balk at sharing their
long-term private keys with another organization and, equally,
delegates would rather not have to handle other parties' long-term
secrets. Other relevant use cases are discussed in Section 4.
This document describes a profile of the ACME protocol [RFC8555] that
allows the NDC to request from the IdO, acting as a profiled ACME
server, a certificate for a delegated identity - i.e., one belonging
to the IdO. The IdO then uses the ACME protocol (with the extensions
Sheffer, et al. Expires 27 September 2021 [Page 3]
Internet-Draft ACME Delegation March 2021
described in [RFC8739]) to request issuance of a Short-Term,
Automatically Renewed (STAR) certificate for the same delegated
identity. The generated short-term certificate is automatically
renewed by the ACME Certification Authority (CA), periodically
fetched by the NDC and used to terminate HTTPS connections in lieu of
the IdO. The IdO can end the delegation at any time by simply
instructing the CA to stop the automatic renewal and letting the
certificate expire shortly thereafter.
While the primary use case we address is delegation of STAR
certificates, the mechanism proposed here accommodates also long-
lived certificates managed with the ACME protocol. The most
noticeable difference between long-lived and STAR certificates is the
way the termination of the delegation is managed. In the case of
long-lived certificates, the IdO uses the revokeCert URL exposed by
the ACME CA and waits for the explicit revocation based on CRL and
OCSP to propagate to the relying parties.
In case the delegated identity is a domain name, this document also
provides a way for the NDC to inform the IdO about the CNAME mappings
that need to be installed in the IdO's DNS zone to enable the
aliasing of the delegated name, thus allowing the complete name
delegation workflow to be handled using a single interface.
We note that other ongoing efforts address the problem of certificate
delegation for TLS connections, specifically [I-D.ietf-tls-subcerts]
and [I-D.mglt-lurk-tls13]. Compared to these other solutions, the
current document does not introduce additional latency to the TLS
connection, nor does it require changes to the TLS network stack of
either the client or the server.
1.1. Terminology
IdO Identifier Owner, the owner of an identifier (e.g., a domain
name) that needs to be delegated.
NDC Name Delegation Consumer, the entity to which the domain name is
delegated for a limited time. This is a CDN in the primary use
case (in fact, readers may note the symmetry of the two acronyms).
CDN Content Delivery Network, a widely distributed network that
serves the domain's web content to a wide audience at high
performance.
STAR Short-Term, Automatically Renewed X.509 certificates.
ACME Automated Certificate Management Environment, a certificate
management protocol [RFC8555].
Sheffer, et al. Expires 27 September 2021 [Page 4]
Internet-Draft ACME Delegation March 2021
CA A Certification Authority that implements the ACME protocol. In
this document, the term is synonymous with "ACME server".
CSR A PKCS#10 [RFC2986] Certificate Signing Request, as supported by
ACME.
FQDN Fully Qualified Domain Name.
1.2. Conventions used in this document
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.
2. Protocol Flow
This section presents the protocol flow. For completeness, we
include the ACME profile proposed in this document as well as the
extended ACME protocol described in [RFC8739].
2.1. Preconditions
The protocol assumes the following preconditions are met:
* The IdO exposes an ACME server interface to the NDC(s) comprising
the account management interface;
* The NDC has registered an ACME account with the IdO;
* NDC and IdO have agreed on a "CSR template" to use, including at a
minimum: subject name (e.g., "somesite.example.com"), requested
algorithms and key length, key usage, extensions (e.g.,
TNAuthList). The NDC is required to use this template for every
CSR created under the same delegation;
* IdO has registered an ACME account with the Certification
Authority (CA)
Note that even if the IdO implements the ACME server role, it is not
acting as a CA: in fact, from the point of view of the certificate
issuance process, the IdO only works as a "policing" forwarder of the
NDC's key-pair and is responsible for completing the identity
verification process towards the ACME server.
Sheffer, et al. Expires 27 September 2021 [Page 5]
Internet-Draft ACME Delegation March 2021
2.2. Overview
For clarity, the protocol overview presented here covers the main use
case of this protocol, namely delegation of STAR certificates.
Protocol behavior for non-STAR certificates is similar, and the
detailed differences are listed in the following sections.
The interaction between the NDC and the IdO is governed by the
profiled ACME workflow detailed in Section 2.3. The interaction
between the IdO and the CA is ruled by ACME [RFC8555], ACME STAR
[RFC8739] as well as any other ACME extension that applies (e.g.,
[I-D.ietf-acme-authority-token-tnauthlist] for STIR).
The outline of the combined protocol for STAR certificates is as
follow (Figure 1):
* NDC sends an order Order1 for the delegated identifier to IdO;
* IdO creates an Order1 resource in state "ready" with a "finalize"
URL;
* NDC immediately sends a finalize request (which includes the CSR)
to the IdO;
* IdO verifies the CSR according to the agreed upon CSR template;
* If the CSR verification fails, Order1 is moved to an "invalid"
state and everything stops;
* If the CSR verification is successful, IdO moves Order1 to state
"processing", and sends a new Order2 (using its own account) for
the delegated identifier to the CA;
* If the ACME STAR protocol fails, Order2 moves to "invalid" and the
same state is reflected in Order1 (i.e., the NDC Order);
* If the ACME STAR run is successful (i.e., Order2 is "valid"), IdO
copies the "star-certificate" URL from Order2 to Order1 and
updates the Order1 state to "valid".
The NDC can now download, install and use the short-term certificate
bearing the name delegated by the IdO. This can continue until the
STAR certificate expires or the IdO decides to cancel the automatic
renewal process with the CA.
Note that the interactive identifier authorization phase described in
Section 7.5 of [RFC8555] is suppressed on the NDC-IdO side because
the delegated identity contained in the CSR presented to the IdO is
validated against the configured CSR template (Section 2.3.1).
Therefore, the NDC sends the finalize request, including the CSR, to
the IdO immediately after Order1 has been acknowledged. The IdO
SHALL buffer a (valid) CSR until the Validation phase completes
successfully.
Sheffer, et al. Expires 27 September 2021 [Page 6]
Internet-Draft ACME Delegation March 2021
.------. .---------------. .------.
| NDC | | IdO | | ACME |
+--------+ +--------+--------+ +--------+
| Client | | Server | Client | | Server |
'---+----' '----+---+---+----' '----+---'
| | | |
| Order1 | | |
| Signature | | |
o------------------->| | |
| | | |
| [ No identity ] | | |
| [ validation via ] | | |
| [ authorizations ] | | |
| | | |
| CSR | | |
| Signature | | |
o------------------->| | |
| Acknowledgement | | Order2 |
|<-------------------o | Signature |
| | o------------------->|
| | | Required |
| | | Authorizations |
| | |<-------------------o
| | | Responses |
| | | Signature |
| | o------------------->|
| | | |
| | |<~~~~Validation~~~~>|
| | | |
| | | CSR |
| | | Signature |
| | o------------------->|
| | | Acknowledgement |
| | |<-------------------o
| | | |
|<~~Await issuance~->| |<~~Await issuance~~>|
| |
| (unauthenticated) GET STAR certificate |
o------------------------------------------------>|
| Certificate #1 |
|<------------------------------------------------o
| (unauthenticated) GET STAR certificate |
o------------------------------------------------>|
| Certificate #2 |
|<------------------------------------------------o
| [...] |
| (unauthenticated) GET STAR certificate |
o------------------------------------------------>|
Sheffer, et al. Expires 27 September 2021 [Page 7]
Internet-Draft ACME Delegation March 2021
| Certificate #n |
|<------------------------------------------------o
Figure 1: End to end STAR delegation flow
2.3. Delegated Identity Profile
This section defines a profile of the ACME protocol, to be used
between the NDC and IdO.
2.3.1. Delegation Configuration
The IdO must be preconfigured to recognize one or more NDCs, and
present them with details about certificate delegations that apply to
each one.
2.3.1.1. Account Object Extensions
An NDC identifies itself to the IdO as an ACME account. The IdO can
delegate multiple names to a NDC, and these configurations are
described through "delegation" objects associated with the NDC's
Account object on the IdO.
As shown in Figure 2, the ACME account resource on the IdO is
extended with a new "delegations" attribute:
* delegations (required, string): A URL from which a list of
delegations configured for this account can be fetched via a POST-
as-GET request.
{
"status": "valid",
"contact": [
"mailto:delegation-admin@ido.example"
],
"termsOfServiceAgreed": true,
"orders": "https://example.com/acme/orders/rzGoeA",
"delegations": "https://acme.ido.example/acme/delegations/adFqoz"
}
Figure 2: Example Account object with delegations
2.3.1.2. Delegation Objects
This profile extends the ACME resource model with a new read-only
delegation object that represents a delegation configuration that
applies to a given NDC.
Sheffer, et al. Expires 27 September 2021 [Page 8]
Internet-Draft ACME Delegation March 2021
A delegation object contains the CSR template (see Section 3) that
applies to that delegation, and optionally any related CNAME mapping
for the delegated identifiers. Its structure is as follows:
* csr-template (required, object): CSR template as defined in
Section 3.
* cname-map (optional, object): a map of FQDN pairs. In each pair,
the name is the delegated identifier, the value is the
corresponding IdO name that is aliased in the IdO's zone file to
redirect the resolvers to the delegated entity. Both names and
values MUST be FQDNs with a terminating '.'. This field is only
meaningful for identifiers of type "dns".
An example delegation object is shown in Figure 3.
{
"csr-template": {
"keyTypes": [
{
"PublicKeyType": "id-ecPublicKey",
"namedCurve": "secp256r1",
"SignatureType": "ecdsa-with-SHA256"
}
],
"subject": {
"country": "CA",
"stateOrProvince": "**",
"locality": "**",
"commonName": "**"
},
"extensions": {
"subjectAltName": {
"DNS": [
"abc.ndc.ido.example"
]
},
"keyUsage": [
"digitalSignature"
],
"extendedKeyUsage": [
"serverAuth"
]
}
},
"cname-map": {
"abc.ndc.ido.example.": "abc.ndc.example."
}
}
Sheffer, et al. Expires 27 September 2021 [Page 9]
Internet-Draft ACME Delegation March 2021
Figure 3: Example Delegation Configuration object
In order to indicate which specific delegation applies to the
requested certificate a new "delegation" attribute is added to the
identifier in the Order object on the NDC-IdO side (see Figure 4).
The value of this attribute is the URL pointing to the delegation
configuration object that is to be used for this certificate request.
If the "delegation" attribute in the Order object contains a URL that
does not correspond to a configuration available to the requesting
NDC, the IdO MUST return an error response with status code 403
(Forbidden) and type "urn:ietf:params:acme:error:unknownDelegation".
2.3.2. Order Object Transmitted from NDC to IdO and to ACME Server
(STAR)
If the delegation is for a STAR certificate, the Order object created
by the NDC:
* MUST have the delegated name as the identifier value with a
"delegation" attribute indicating the configuration used for the
identifier.
* MUST NOT contain the "notBefore" and "notAfter" fields;
* MUST contain an "auto-renewal" object and inside it, the fields
listed in Section 3.1.1 of [RFC8739].
Sheffer, et al. Expires 27 September 2021 [Page 10]
Internet-Draft ACME Delegation March 2021
POST /acme/new-order HTTP/1.1
Host: acme.ido.example
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://acme.ido.example/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://acme.ido.example/acme/new-order"
}),
"payload": base64url({
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.ido.example.",
"delegation":
"https://acme.ido.example/acme/delegations/adFqoz/2"
}
],
"auto-renewal": {
"end-date": "2020-04-20T00:00:00Z",
"lifetime": 345600, // 4 days
"allow-certificate-get": true
}
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
Figure 4: New STAR Order from NDC
The Order object that is created on the IdO:
* MUST start in the "ready" state;
* MUST contain an "authorizations" array with zero elements;
* MUST contain the indicated "delegation" configurations.
* MUST NOT contain the "notBefore" and "notAfter" fields.
Sheffer, et al. Expires 27 September 2021 [Page 11]
Internet-Draft ACME Delegation March 2021
{
"status": "ready",
"expires": "2019-05-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.ido.example.",
"delegation":
"https://acme.ido.example/acme/delegations/adFqoz/2"
}
],
"auto-renewal": {
"end-date": "2020-04-20T00:00:00Z",
"lifetime": 345600,
"allow-certificate-get": true
},
"authorizations": [],
"finalize": "https://acme.ido.example/acme/order/TO8rfgo/finalize"
}
Figure 5: STAR Order Resource Created on IdO
The Order is then finalized by the NDC supplying the CSR containing
the delegated identifiers. The IdO checks the provided CSR against
the template that applies to each delegated identifier, as described
in Section 3.1. If the CSR fails validation for any of the
identifiers, the IdO MUST return an error response with status code
403 (Forbidden) and an appropriate type, e.g., "rejectedIdentifier"
or "badCSR". The error response SHOULD contain subproblems
(Section 6.7.1 of [RFC8555]) for each failed identifier. If the CSR
is successfully validated, the Order object status moves to
"processing" and the twin ACME protocol instance is initiated on the
IdO-CA side.
The Order object created by the IdO:
* MUST copy the identifiers sent by the NDC and strip the
"delegation" attribute;
* MUST carry a copy of the "auto-renewal" object sent by the NDC and
augment it with an "allow-certificate-get" attribute set to true.
When the validation of the identifiers has been successfully
completed and the certificate has been issued by the CA, the IdO:
Sheffer, et al. Expires 27 September 2021 [Page 12]
Internet-Draft ACME Delegation March 2021
* MUST move its Order resource status to "valid".
* MUST copy the "star-certificate" field from the STAR Order. The
latter indirectly includes (via the NotBefore and NotAfter HTTP
headers) the renewal timers needed by the NDC to inform its
certificate reload logic.
{
"status": "valid",
"expires": "2019-05-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.ido.example.",
"delegation":
"https://acme.ido.example/acme/delegations/adFqoz/2"
}
],
"auto-renewal": {
"end-date": "2020-04-20T00:00:00Z",
"lifetime": 345600,
"allow-certificate-get": true
},
"authorizations": [],
"finalize": "https://acme.ido.example/acme/order/TO8rfgo/finalize",
"star-certificate": "https://acme.ca.example/acme/order/yTr23sSDg9"
}
Figure 6: STAR Order Resource Updated on IdO
2.3.2.1. CNAME Installation
If an identifier object of type "dns" was included, the IdO can add
the corresponding CNAME records to its zone, e.g.:
abc.ndc.ido.example. CNAME abc.ndc.example.
2.3.3. Order Object Transmitted from NDC to IdO and to ACME Server
(non-STAR)
If the delegation is for a non-STAR certificate, the Order object
created by the NDC:
Sheffer, et al. Expires 27 September 2021 [Page 13]
Internet-Draft ACME Delegation March 2021
* MUST have the delegated name as the identifier value with a
"delegation" attribute indicating the configuration used for the
identifier.
POST /acme/new-order HTTP/1.1
Host: acme.ido.example
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://acme.ido.example/acme/acct/evOfKhNU60wg",
"nonce": "IYBkoQfaCS80UcCn9qH8Gt",
"url": "https://acme.ido.example/acme/new-order"
}),
"payload": base64url({
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.ido.example.",
"delegation":
"https://acme.ido.example/acme/delegations/adFqoz/2"
}
]
}),
"signature": "H6ZyWqg8aaKEkYca...dudoz4igiMvUBJ9j"
}
Figure 7: New Non-STAR Order from NDC
The Order object that is created on the IdO:
* MUST start in the "ready" state;
* MUST contain an "authorizations" array with zero elements;
* MUST contain the indicated "delegation" configurations.
Sheffer, et al. Expires 27 September 2021 [Page 14]
Internet-Draft ACME Delegation March 2021
{
"status": "ready",
"expires": "2019-05-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.ido.example.",
"delegation":
"https://acme.ido.example/acme/delegations/adFqoz/2"
}
],
"authorizations": [],
"finalize": "https://acme.ido.example/acme/order/to8RFGO/finalize"
}
Figure 8: Non-STAR Order Resource Created on IdO
The Order finalization by the NDC and the subsequent validation of
the CSR by the IdO proceed in the same way as for the STAR case. If
the CSR is successfully validated, the Order object status moves to
"processing" and the twin ACME protocol instance is initiated on the
IdO-CA side.
The Order object created by the IdO:
* MUST copy the identifiers sent by the NDC and strip the
"delegation" attribute;
* MUST include the "allow-certificate-get" attribute set to true.
When the validation of the identifiers has been successfully
completed and the certificate has been issued by the CA, the IdO:
* MUST move its Order resource status to "valid".
* MUST copy the "certificate" field from the Order, as well as
"notBefore" and "notAfter" if these fields exist.
Sheffer, et al. Expires 27 September 2021 [Page 15]
Internet-Draft ACME Delegation March 2021
{
"status": "valid",
"expires": "2019-05-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.ido.example.",
"delegation":
"https://acme.ido.example/acme/delegations/adFqoz/2"
}
],
"allow-certificate-get": true,
"authorizations": [],
"finalize": "https://acme.ido.example/acme/order/to8RFGO/finalize",
"certificate": "https://acme.ca.example/acme/order/YtR23SsdG9"
}
Figure 9: Non-STAR Order Resource Updated on IdO
At this point of the protocol flow, the same considerations as in
Section 2.3.2.1 apply.
2.3.4. Capability Discovery
In order to help a client to discover support for this profile, the
directory object of an ACME server MUST contain the following
attribute in the "meta" field:
* delegation-enabled: boolean flag indicating support for the
profile specified in this memo. An ACME server that supports this
delegation profile MUST include this key, and MUST set it to true.
The "delegation-enabled" flag may be specified regardless of the
existence or setting of the "auto-renewal" flag.
2.3.5. Terminating the Delegation
Identity delegation is terminated differently, depending on whether
this is a STAR certificate or not.
Sheffer, et al. Expires 27 September 2021 [Page 16]
Internet-Draft ACME Delegation March 2021
2.3.5.1. By Cancellation (STAR)
The IdO can terminate the delegation of a STAR certificate by
requesting its cancellation (see Section 3.1.2 of [RFC8739]).
Cancellation of the ACME STAR certificate is a prerogative of the
IdO. The NDC does not own the relevant account key on the ACME
server, therefore it can't issue a cancellation request for the STAR
certificate. Potentially, since it holds the STAR certificate's
private key, it could request the revocation of a single STAR
certificate. However, STAR explicitly disables the revokeCert
interface.
Shortly after the automatic renewal process is stopped by the IdO,
the last issued STAR certificate expires and the delegation
terminates.
2.3.5.2. By Revocation (non-STAR)
The IdO can terminate the delegation of a non-STAR certificate by
requesting it to be revoked using the revokeCert URL exposed by the
ACME server.
According to Section 7.6 of [RFC8555], the revocation endpoint can be
used with either the account keypair, or the certificate keypair. In
other words, an NDC that learns the revokeCert URL of the CA (which
is publicly available via the CA's Directory object) would be able to
revoke the certificate using the associated private key. However,
given the trust relationship between NDC and IdO expected by the
delegation trust model (Section 6.1), as well as the lack of
incentives for the NDC to prematurely terminate the delegation, this
does not represent a security risk.
2.4. Proxy Behavior
There are cases where the ACME Delegation flow should be proxied,
such as the use case described in Section 4.1.2. This section
describes the behavior of such proxies.
An entity implementing the IdO server role - an "ACME Delegation
server" - can decide, on a per-identity case, whether to act as a
proxy into another ACME Delegation server, or to behave as an IdO and
obtain a certificate directly. The determining factor is whether it
can successfully be authorized by the ACME server for the identity
associated with the certificate request.
The identities supported by each server and the disposition for each
of them are preconfigured.
Sheffer, et al. Expires 27 September 2021 [Page 17]
Internet-Draft ACME Delegation March 2021
Following is the proxy's behavior for each of the messages exchanged
in the ACME Delegation process:
* New-order request:
- The complete "identifiers" object MUST be copied as-is.
- Similarly, the "auto-renewal" object MUST be copied as-is.
* New-order response:
- The "status", "expires", "authorizations", "identifiers" and
"auto-renewal" attributes/objects MUST be copied as-is.
- The "finalize" URL is rewritten, so that the "finalize" request
will be made to the proxy.
- Similarly, the "Location" header MUST be rewritten to point to
an Order object on the proxy.
- And similarly, any "Link" relations.
* Get Order response:
- The "status", "expires", "authorizations", "identifiers" and
"auto-renewal" attributes/objects MUST be copied as-is.
- Similarly, the "star-certificate" URL (or the "certificate" URL
in case of non-STAR requests) MUST be copied as-is.
- The "finalize" URL is rewritten, so that the "finalize" request
will be made to the proxy.
- The "Location" header MUST be rewritten to point to an Order
object on the proxy.
- Any "Link" relations MUST be rewritten to point to the proxy.
* Finalize request:
- The CSR MUST be copied as-is.
* Finalize response:
- The "Location" header, "Link" relations and the "finalize" URLs
are rewritten as for Get Order.
We note that all the above messages are authenticated, and therefore
each proxy must be able to authenticate any subordinate server.
3. CSR Template
The CSR template is used to express and constrain the shape of the
CSR that the NDC uses to request the certificate. The CSR is used
for every certificate created under the same delegation. Its
validation by the IdO is a critical element in the security of the
whole delegation mechanism.
Instead of defining every possible CSR attribute, this document takes
a minimalist approach by declaring only the minimum attribute set and
deferring the registration of further, more specific, attributes to
future documents.
Sheffer, et al. Expires 27 September 2021 [Page 18]
Internet-Draft ACME Delegation March 2021
3.1. Template Syntax
The template is a JSON document. Each field (with the exception of
"keyTypes", see below) denotes one of:
* A mandatory field, where the template specifies the literal value
of that field. This is denoted by a literal string, such as
"client1.ndc.ido.example.com".
* A mandatory field, where the content of the field is defined by
the client. This is denoted by "**".
* An optional field, where the client decides whether the field is
included in the CSR and if so, what its value is. This is denoted
by "*".
The NDC MUST NOT include in the CSR any fields, including any
extensions, unless they are specified in the template.
The structure of the template object is defined by the CDDL [RFC8610]
document in Appendix B.
An alternative, non-normative JSON Schema syntax is given in
Appendix C.
The "subject" field and its subfields are mapped into the "subject"
field of the CSR, as per [RFC5280], Section 4.1.2.6. Other extension
fields of the CSR template are mapped into the CSR according to the
table in Section 5.6.
The "subjectAltName" field is currently defined for the following
identifiers: DNS names, email addresses, and URIs. New identifier
types may be added in the future by documents that extend this
specification. Each new identifier type SHALL have an associated
identifier validation challenge that the ACME CA can use to obtain
proof of the requester's control over it.
The "keyTypes" property is not copied into the CSR. Instead, this
property constrains the "SubjectPublicKeyInfo" field of the CSR,
which MUST have the type/size defined by one of the array members of
the "keyTypes" property.
Sheffer, et al. Expires 27 September 2021 [Page 19]
Internet-Draft ACME Delegation March 2021
When the IdO receives the CSR, it MUST verify that the CSR is
consistent with the template contained in the "delegation" object
referenced in the Order. The IdO MAY enforce additional constraints,
e.g. by restricting field lengths. In this regard, note that a
"subjectAltName" of type "DNS" can be specified using the wildcard
notation, meaning that the NDC can be required ("**") or offered the
possibility ("*") to define the delegated domain name by itself. If
this is the case, the IdO needs to have a further layer of checks on
top of the control rules already provided by the CSR template to
fully validate the CSR input.
3.2. Example
The CSR template in Figure 10 represents one possible CSR template
governing the delegation exchanges provided in the rest of this
document.
Sheffer, et al. Expires 27 September 2021 [Page 20]
Internet-Draft ACME Delegation March 2021
{
"keyTypes": [
{
"PublicKeyType": "rsaEncryption",
"PublicKeyLength": 2048,
"SignatureType": "sha256WithRSAEncryption"
},
{
"PublicKeyType": "id-ecPublicKey",
"namedCurve": "secp256r1",
"SignatureType": "ecdsa-with-SHA256"
}
],
"subject": {
"country": "CA",
"stateOrProvince": "**",
"locality": "**",
"commonName": "**"
},
"extensions": {
"subjectAltName": {
"DNS": [
"client1.ndc.ido.example"
]
},
"keyUsage": [
"digitalSignature"
],
"extendedKeyUsage": [
"serverAuth",
"clientAuth"
]
}
}
Figure 10: Example CSR template
4. Further Use Cases
This non-normative section describes additional use cases that use
STAR certificate delegation in non-trivial ways.
4.1. CDN Interconnection (CDNI)
[I-D.ietf-cdni-interfaces-https-delegation] discusses several
solutions addressing different delegation requirements for the CDNI
(CDN Interconnection) environment. This section discusses two of the
stated requirements in the context of the STAR delegation workflow.
Sheffer, et al. Expires 27 September 2021 [Page 21]
Internet-Draft ACME Delegation March 2021
This section uses specifically CDNI terminology, e.g. "uCDN" and
"dCDN", as defined in [RFC7336].
4.1.1. Multiple Parallel Delegates
In some cases the content owner (IdO) would like to delegate
authority over a web site to multiple NDCs (CDNs). This could happen
if the IdO has agreements in place with different regional CDNs for
different geographical regions, or if a "backup" CDN is used to
handle overflow traffic by temporarily altering some of the CNAME
mappings in place. The STAR delegation flow enables this use case
naturally, since each CDN can authenticate separately to the IdO (via
its own separate account) specifying its CSR, and the IdO is free to
allow or deny each certificate request according to its own policy.
4.1.2. Chained Delegation
In other cases, a content owner (IdO) delegates some domains to a
large CDN (uCDN), which in turn delegates to a smaller regional CDN,
dCDN. The IdO has a contractual relationship with uCDN, and uCDN has
a similar relationship with dCDN. However IdO may not even know
about dCDN.
If needed, the STAR protocol can be chained to support this use case:
uCDN could forward requests from dCDN to IdO, and forward responses
back to dCDN. Whether such proxying is allowed is governed by policy
and contracts between the parties.
A mechanism is necessary at the interface between uCDN and dCDN by
which the uCDN can advertise:
* The namespace that is made available to the dCDN to mint its
delegated names;
* The policy for creating the key material (allowed algorithms,
minimum key lengths, key usage, etc.) that the dCDN needs to
satisfy.
Note that such mechanism is provided by the CSR template.
4.1.2.1. Two-Level Delegation in CDNI
A User Agent (UA), browser or set-top-box, wants to fetch the video
resource at the following URI: "https://video.cp.example/movie".
Redirection between Content Provider (CP), upstream, and downstream
CDNs is arranged as a CNAME-based aliasing chain as illustrated in
Figure 11.
Sheffer, et al. Expires 27 September 2021 [Page 22]
Internet-Draft ACME Delegation March 2021
.------------.
video.cp.example ? | .-----. |
.---------------------------------->| | |
| (a) | | DNS | CP |
| .-------------------------------+ | |
| | CNAME video.ucdn.example | '-----' |
| | '------------'
| |
| |
.-----------|---v--. .------------.
| .-----.-+-----. | video.ucdn.example ? | .-----. |
| | | +----------------------------->| | |
| UA | TLS | DNS | | (b) | | DNS | uCDN |
| | | |<-----------------------------+ | |
| '--+--'-----+-' | CNAME video.dcdn.example | '-----' |
'------|----^---|--' '------------'
| | |
| | |
| | | .------------.
| | | video.dcdn.example ? | .-----. |
| | '------------------------------>| | |
| | (c) | | DNS | |
| '-----------------------------------+ | |
| A 192.0.2.1 | +-----+ dCDN |
| | | | |
'--------------------------------------->| TLS | |
SNI: video.cp.example | | | |
| '-----' |
'------------'
Figure 11: DNS Redirection
Unlike HTTP based redirection, where the original URL is supplanted
by the one found in the Location header of the 302 response, DNS
redirection is completely transparent to the User Agent. As a
result, the TLS connection to the dCDN edge is done with an SNI equal
to the "host" in the original URI - in the example,
"video.cp.example". So, in order to successfully complete the
handshake, the landing dCDN node has to be configured with a
certificate whose subjectAltName matches "video.cp.example", i.e., a
Content Provider's name.
Figure 12 illustrates the cascaded delegation flow that allows dCDN
to obtain a STAR certificate that bears a name belonging to the
Content Provider with a private key that is only known to the dCDN.
Sheffer, et al. Expires 27 September 2021 [Page 23]
Internet-Draft ACME Delegation March 2021
.--------------------.
| .------.------. |
| | STAR | ACME |<-------------.
.------->| CP | dele | STAR | | |
| | | srv | cli +-----. |
| | '---+--'------' | | 6
| '---------|------^---' 5 |
| | | | .--|-------.
| | | | | .-+----. |
| 7 | '---->| ACME | |
| | | | | STAR | C |
0 | 4 | +------| A |
| | | | | HTTP | |
| | | | '----+-' |
| | .-' '--^--|----'
| .--------------v--|--. | |
| | .------.----+-. | | 10
| | | | STAR | | | |
'-->| uCDN | CDNI | dele | | | |
| | | fwd | | | |
| '----+-'-+----' | | |
'-------^--|---|--^--' | |
| | | | | |
| 2 8 | | |
1 | | 3 | |
| | | | 9 |
.-------|--v---v--|---------. | |
| .-+----.----+-.------. | | |
| | | STAR | +------------' |
| dCDN | CDNI | dele | HTTP | | |
| | | cli | |<--------------'
| '------'------'------' |
'---------------------------'
Figure 12: Two levels delegation in CDNI
uCDN is configured to delegate to dCDN, and CP is configured to
delegate to uCDN, both as defined in Section 2.3.1.
1. dCDN requests CDNI path metadata to uCDN;
2. uCDN replies with, among other CDNI metadata, the STAR
delegation configuration, which includes the delegated Content
Provider's name;
3. dCDN creates a key-pair and the CSR with the delegated name. It
then places an order for the delegated name to uCDN;
4. uCDN forwards the received order to the Content Provider (CP);
Sheffer, et al. Expires 27 September 2021 [Page 24]
Internet-Draft ACME Delegation March 2021
5. CP creates an order for a STAR certificate and sends it to the
CA. The order also requests unauthenticated access to the
certificate resource;
6. After all authorizations complete successfully, the STAR
certificate is issued;
7. CP notifies uCDN that the STAR certificate is available at the
order's star-certificate URL;
8. uCDN forwards the information to dCDN. At this point the ACME
signalling is complete;
9. dCDN requests the STAR certificate using unauthenticated GET
from the ACME server;
10. the CA returns the certificate. Now dCDN is fully configured to
handle HTTPS traffic in-lieu of the Content Provider.
Note that 9. and 10. repeat until the delegation expires or is
terminated.
4.2. Secure Telephone Identity Revisited (STIR)
As a second use case, we consider the delegation of credentials in
the STIR ecosystem [I-D.ietf-stir-cert-delegation].
This section uses STIR terminology. The term PASSPorT is defined in
[RFC8225], and "TNAuthList" in [RFC8226].
In the STIR "delegated" mode, a service provider SP2 - the NDC -
needs to sign PASSPorT's [RFC8225] for telephone numbers (e.g.,
TN=+123) belonging to another service provider, SP1 - the IdO. In
order to do that, SP2 needs a STIR certificate, and private key, that
includes TN=+123 in the TNAuthList [RFC8226] certificate extension.
In details (Figure 13):
1. SP1 and SP2 agree on the configuration of the delegation - in
particular, the CSR template that applies;
2. SP2 generates a private/public key-pair and sends a CSR to SP1
requesting creation of a certificate with: SP1 name, SP2 public
key, and a TNAuthList extension with the list of TNs that SP1
delegates to SP2. (Note that the CSR sent by SP2 to SP1 needs to
be validated against the CSR template agreed upon in step 1.);
3. SP1 sends an Order for the CSR to the CA;
4. Subsequently, after the required TNAuthList authorizations are
successfully completed, the CA moves the Order to a "valid"
state; at the same time the star-certificate endpoint is
populated.
5. The Order contents are forwarded from SP1 to SP2 by means of the
paired "delegation" Order.
Sheffer, et al. Expires 27 September 2021 [Page 25]
Internet-Draft ACME Delegation March 2021
6. SP2 dereferences the star-certificate URL in the Order to fetch
the rolling STAR certificate bearing the delegated identifiers.
.-------------------.
| .------.------. |
| | STAR | STAR |<--------------.
.-->| SP1 | dele | dele | | |
| | | srv | cli +-----. |
| | '----+-'------' | | 4
| '------^--|---------' 3 |
| | | | .----|-----.
| | 5 | | .---+--. |
| | | '--->| ACME | |
| | | | | STAR | C |
1 | | | +------| A |
| | | .--->| HTTP | |
| 2 | | | '---+--' |
| | | | '----|-----'
| .------|--v---------. 6 |
| | .-+----.------. | | 7
| | | STAR | +-----' |
'-->| SP2 | dele | HTTP | | |
| | cli | |<--------------'
| '----+-'-+----' |
'-------------------'
Figure 13: Delegation in STIR
As shown, the STAR delegation profile described in this document
applies straightforwardly, the only extra requirement being the
ability to instruct the NDC about the allowed TNAuthList values.
This can be achieved by a simple extension to the CSR template.
5. IANA Considerations
[[RFC Editor: please replace XXXX below by the RFC number.]]
5.1. New ACME Identifier Object Fields
This document requests that IANA create the following new registry
under the Automated Certificate Management Environment (ACME)
Protocol:
* ACME Identifier Object Fields
This registry is administered under a Specification Required policy
[RFC8126].
Sheffer, et al. Expires 27 September 2021 [Page 26]
Internet-Draft ACME Delegation March 2021
The "ACME Identifier Object Fields" registry lists field names that
are defined for use in the ACME identifier object.
Template:
* Field name: The string to be used as a field name in the JSON
object
* Field type: The type of value to be provided, e.g., string,
boolean, array of string
* Reference: Where this field is defined
+============+============+===========================+
| Field Name | Field Type | Reference |
+============+============+===========================+
| type | string | Section 7.1.3 of RFC 8555 |
+------------+------------+---------------------------+
| value | string | Section 7.1.3 of RFC 8555 |
+------------+------------+---------------------------+
| delegation | string | RFC XXXX |
+------------+------------+---------------------------+
Table 1
Note: this registry was not created at the time [RFC8555] was
standardized likely because it was not anticipated that the
identifier object would be extended. It is retrospectively
introduced to record the status quo and allow controlled
extensibility of the identifier object.
5.2. New Fields in the "meta" Object within a Directory Object
This document adds the following entries to the ACME Directory
Metadata Fields registry:
+====================+============+===========+
| Field Name | Field Type | Reference |
+====================+============+===========+
| delegation-enabled | boolean | RFC XXXX |
+--------------------+------------+-----------+
Table 2
5.3. New Fields in the Order Object
This document adds the following entries to the ACME Order Object
Fields registry:
Sheffer, et al. Expires 27 September 2021 [Page 27]
Internet-Draft ACME Delegation March 2021
+=======================+============+==============+===========+
| Field Name | Field Type | Configurable | Reference |
+=======================+============+==============+===========+
| allow-certificate-get | boolean | true | RFC XXXX |
+-----------------------+------------+--------------+-----------+
Table 3
5.4. New Fields in the Account Object
This document adds the following entries to the ACME Account Object
Fields registry:
+=============+============+==========+===========+
| Field Name | Field Type | Requests | Reference |
+=============+============+==========+===========+
| delegations | string | none | RFC XXXX |
+-------------+------------+----------+-----------+
Table 4
Note that the "delegations" field is only reported by ACME servers
that have "delegation-enabled" set to true in their meta Object.
5.5. New Error Types
This document adds the following entries to the ACME Error Type
registry:
+===================+================================+===========+
| Type | Description | Reference |
+===================+================================+===========+
| unknownDelegation | An unknown configuration is | RFC XXXX |
| | listed in the "delegations" | |
| | attribute of the request Order | |
+-------------------+--------------------------------+-----------+
Table 5
5.6. CSR Template Extensions
IANA is requested to establish a registry "STAR Delegation CSR
Template Extensions", with "Expert Review" as its registration
procedure.
Each extension registered must specify:
* An extension name.
Sheffer, et al. Expires 27 September 2021 [Page 28]
Internet-Draft ACME Delegation March 2021
* An extension syntax, as a reference to a JSON Schema document that
defines this extension.
* The extension's mapping into an X.509 certificate extension.
The initial contents of this registry are the extensions defined by
the CDDL in Appendix B.
+==================+===========+===============================+
| Extension Name | Extension | Mapping to X.509 Certificate |
| | Syntax | Extension |
+==================+===========+===============================+
| keyUsage | See | [RFC5280], Section 4.2.1.3 |
| | Appendix | |
| | B | |
+------------------+-----------+-------------------------------+
| extendedKeyUsage | See | [RFC5280], Section 4.2.1.12 |
| | Appendix | |
| | B | |
+------------------+-----------+-------------------------------+
| subjectAltName | See | [RFC5280], Section 4.2.1.6 |
| | Appendix | (note that only specific name |
| | B | formats are allowed: URI, DNS |
| | | name, email address) |
+------------------+-----------+-------------------------------+
Table 6
6. Security Considerations
6.1. Trust Model
The ACME trust model needs to be extended to include the trust
relationship between NDC and IdO. Note that once this trust link is
established, it potentially becomes recursive. Therefore, there has
to be a trust relationship between each of the nodes in the
delegation chain; for example, in case of cascading CDNs this is
contractually defined. Note that using standard [RFC6125] identity
verification there are no mechanisms available to the IdO to restrict
the use of the delegated name once the name has been handed over to
the first NDC.
6.2. Delegation Security Goal
Delegation introduces a new security goal: only an NDC that has been
authorised by the IdO, either directly or transitively, can obtain a
certificate with an IdO identity.
Sheffer, et al. Expires 27 September 2021 [Page 29]
Internet-Draft ACME Delegation March 2021
From a security point of view, the delegation process has five
separate parts:
1. Enabling a specific third party (the intended NDC) to submit
requests for delegated certificates;
2. Making sure that any request for a delegated certificate matches
the intended "shape" in terms of delegated identities as well as
any other certificate metadata, e.g., key length, x.509
extensions, etc.;
3. Serving the certificate back to the NDC;
4. A process for handling revocation of the delegation;
5. A process for handling revocation of the certificate itself.
The first part is covered by the NDC's ACME account that is
administered by the IdO, whose security relies on the correct
handling of the associated key pair. When a compromise of the
private key is detected, the delegate MUST use the account
deactivation procedures defined in Section 7.3.6 of [RFC8555].
The second part is covered by the act of checking an NDC's
certificate request against the intended CSR template. The steps of
shaping the CSR template correctly, selecting the right CSR template
to check against the presented CSR, and making sure that the
presented CSR matches the selected CSR template are all security
relevant.
The third part builds on the trust relationship between NDC and IdO
that is responsible for correctly forwarding the certificate URL from
the Order returned by the ACME server.
The fourth part is associated with the ability of the IdO to
unilaterally remove the delegation object associated with the revoked
identity, therefore disabling any further NDC requests for such
identity. Note that, in more extreme circumstances, the IdO might
decide to disable the NDC account thus entirely blocking any further
interaction.
The fifth is covered by two different mechanisms, depending on the
nature of the certificate. For STAR, the IdO shall use the
cancellation interface defined in Section 2.3 of [RFC8739]. For non-
STAR, the certificate revocation interface defined in Section 7.6 of
[RFC8555]).
6.3. New ACME Channels
Using the model established in Section 10.1 of [RFC8555], we can
decompose the interactions of the basic delegation workflow as shown
in Figure 14.
Sheffer, et al. Expires 27 September 2021 [Page 30]
Internet-Draft ACME Delegation March 2021
ACME Channel
.------------>------------.
.-----. ACME Channel .--+--. .--+----------.
| NDC +------------->| IdO | | ACME server |
'--+--' '--+--' '--+-+--------'
| '-----------<-------------' |
| Validation Channel |
'-------------------->---------------------------'
(subset of) ACME Channel [1]
[1] Unauthenticated certificate fetch and non-STAR certificate
revocation.
Figure 14: Delegation Channels Topology
The considerations regarding the security of the ACME Channel and
Validation Channel discussed in [RFC8555] apply verbatim to the IdO/
ACME server leg. The same can be said for the ACME channel on the
NDC/IdO leg. A slightly different set of considerations apply to the
ACME Channel between NDC and ACME server, which consists of a subset
of the ACME interface comprising two API endpoints: the
unauthenticated certificate retrieval and, potentially, non-STAR
revocation via certificate private key. No specific security
considerations apply to the former, but the privacy considerations in
Section 6.3 of [RFC8739] do. With regards to the latter, it should
be noted that there is currently no means for an IdO to disable
authorising revocation based on certificate private keys. So, in
theory, an NDC could use the revocation API directly with the ACME
server, therefore bypassing the IdO. The NDC SHOULD NOT directly use
the revocation interface exposed by the ACME server unless failing to
do so would compromise the overall security, for example if the
certificate private key is compromised and the IdO is not currently
reachable.
All other security considerations from [RFC8555] and [RFC8739] apply
as-is to the delegation topology.
6.4. Restricting CDNs to the Delegation Mechanism
When a web site is delegated to a CDN, the CDN can in principle
modify the web site at will, create and remove pages. This means
that a malicious or breached CDN can pass the ACME (as well as common
non-ACME) HTTPS-based validation challenges and generate a
certificate for the site. This is true regardless of whether the
CNAME mechanisms defined in the current document is used or not.
Sheffer, et al. Expires 27 September 2021 [Page 31]
Internet-Draft ACME Delegation March 2021
In some cases, this is the desired behavior: the domain owner trusts
the CDN to have full control of the cryptographic credentials for the
site. The current document however assumes that the domain owner
only wants to delegate restricted control, and wishes to retain the
capability to cancel the CDN's credentials at a short notice.
The following is a possible mitigation when the IdO wishes to ensure
that a rogue CDN cannot issue unauthorized certificates:
* The domain owner makes sure that the CDN cannot modify the DNS
records for the domain. The domain owner should ensure it is the
only entity authorized to modify the DNS zone. Typically, it
establishes a CNAME resource record from a subdomain into a CDN-
managed domain.
* The domain owner uses a CAA record [RFC8659] to restrict
certificate issuance for the domain to specific CAs that comply
with ACME and are known to implement [RFC8657].
* The domain owner uses the ACME-specific CAA mechanism [RFC8657] to
restrict issuance to a specific account key which is controlled by
it, and MUST require "dns-01" as the sole validation method.
We note that the above solution may need to be tweaked depending on
the exact capabilities and authorisation flows supported by the
selected CAs.
7. Acknowledgments
We would like to thank the following people who contributed
significantly to this document with their review comments and design
proposals: Roman Danyliw, Frédéric Fieau, Russ Housley, Sanjay
Mishra, Jon Peterson, Ryan Sleevi, Emile Stephan.
This work is partially supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI). This support does not imply
endorsement.
8. References
8.1. Normative 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>.
Sheffer, et al. Expires 27 September 2021 [Page 32]
Internet-Draft ACME Delegation March 2021
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
DOI 10.17487/RFC2986, November 2000,
<https://www.rfc-editor.org/info/rfc2986>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8657] Landau, H., "Certification Authority Authorization (CAA)
Record Extensions for Account URI and Automatic
Certificate Management Environment (ACME) Method Binding",
RFC 8657, DOI 10.17487/RFC8657, November 2019,
<https://www.rfc-editor.org/info/rfc8657>.
[RFC8659] Hallam-Baker, P., Stradling, R., and J. Hoffman-Andrews,
"DNS Certification Authority Authorization (CAA) Resource
Record", RFC 8659, DOI 10.17487/RFC8659, November 2019,
<https://www.rfc-editor.org/info/rfc8659>.
[RFC8739] Sheffer, Y., Lopez, D., Gonzalez de Dios, O., Pastor
Perales, A., and T. Fossati, "Support for Short-Term,
Automatically Renewed (STAR) Certificates in the Automated
Certificate Management Environment (ACME)", RFC 8739,
DOI 10.17487/RFC8739, March 2020,
<https://www.rfc-editor.org/info/rfc8739>.
Sheffer, et al. Expires 27 September 2021 [Page 33]
Internet-Draft ACME Delegation March 2021
8.2. Informative References
[I-D.ietf-acme-authority-token-tnauthlist]
Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
"TNAuthList profile of ACME Authority Token", Work in
Progress, Internet-Draft, draft-ietf-acme-authority-token-
tnauthlist-07, 22 February 2021,
<https://www.ietf.org/archive/id/draft-ietf-acme-
authority-token-tnauthlist-07.txt>.
[I-D.ietf-cdni-interfaces-https-delegation]
Fieau, F., Stephan, E., and S. Mishra, "CDNI extensions
for HTTPS delegation", Work in Progress, Internet-Draft,
draft-ietf-cdni-interfaces-https-delegation-05, 12 March
2021, <https://www.ietf.org/archive/id/draft-ietf-cdni-
interfaces-https-delegation-05.txt>.
[I-D.ietf-stir-cert-delegation]
Peterson, J., "STIR Certificate Delegation", Work in
Progress, Internet-Draft, draft-ietf-stir-cert-delegation-
04, 22 February 2021, <https://www.ietf.org/archive/id/
draft-ietf-stir-cert-delegation-04.txt>.
[I-D.ietf-tls-subcerts]
Barnes, R., Iyengar, S., Sullivan, N., and E. Rescorla,
"Delegated Credentials for TLS", Work in Progress,
Internet-Draft, draft-ietf-tls-subcerts-10, 24 January
2021, <https://www.ietf.org/archive/id/draft-ietf-tls-
subcerts-10.txt>.
[I-D.mglt-lurk-tls13]
Migault, D., "LURK Extension version 1 for (D)TLS 1.3
Authentication", Work in Progress, Internet-Draft, draft-
mglt-lurk-tls13-04, 25 January 2021,
<https://www.ietf.org/archive/id/draft-mglt-lurk-
tls13-04.txt>.
[json-schema-07]
Wright, A., Andrews, H., and G. Luff, "JSON Schema
Validation: A Vocabulary for Structural Validation of
JSON", 2018, <https://datatracker.ietf.org/doc/html/draft-
handrews-json-schema-validation-01>.
Sheffer, et al. Expires 27 September 2021 [Page 34]
Internet-Draft ACME Delegation March 2021
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC7336] Peterson, L., Davie, B., and R. van Brandenburg, Ed.,
"Framework for Content Distribution Network
Interconnection (CDNI)", RFC 7336, DOI 10.17487/RFC7336,
August 2014, <https://www.rfc-editor.org/info/rfc7336>.
[RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.
[RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", RFC 8226,
DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/info/rfc8226>.
Appendix A. Document History
[[Note to RFC Editor: please remove before publication.]]
A.1. draft-ietf-acme-star-delegation-07
* SecDir comments by Russ Housley.
* In particular, reorganized some parts of the document to clarify
handling of non-STAR certificates.
* And changed the document's title accordingly.
A.2. draft-ietf-acme-star-delegation-06
* CDDL schema to address Roman's remaining comments.
A.3. draft-ietf-acme-star-delegation-05
* Detailed AD review by Roman Danyliw.
* Some comments that were left unaddressed in Ryan Sleevi's review.
* Numerous other edits for clarity and consistency.
A.4. draft-ietf-acme-star-delegation-04
* Delegation of non-STAR certificates.
* More IANA clarity, specifically on certificate extensions.
* Add delegation configuration object and extend account and order
objects accordingly.
Sheffer, et al. Expires 27 September 2021 [Page 35]
Internet-Draft ACME Delegation March 2021
* A lot more depth on Security Considerations.
A.5. draft-ietf-acme-star-delegation-03
* Consistency with the latest changes in the base ACME STAR
document, e.g. star-delegation-enabled capability renamed and
moved.
* Proxy use cases (recursive delegation) and the definition of proxy
behavior.
* More detailed analysis of the CDNI and STIR use cases, including
sequence diagrams.
A.6. draft-ietf-acme-star-delegation-02
* Security considerations: review by Ryan Sleevi.
* CSR template simplified: instead of being a JSON Schema document
itself, it is now a simple JSON document which validates to a JSON
Schema.
A.7. draft-ietf-acme-star-delegation-01
* Refinement of the CDNI use case.
* Addition of the CSR template (partial, more work required).
* Further security considerations (work in progress).
A.8. draft-ietf-acme-star-delegation-00
* Republished as a working group draft.
A.9. draft-sheffer-acme-star-delegation-01
* Added security considerations about disallowing CDNs from issuing
certificates for a delegated domain.
A.10. draft-sheffer-acme-star-delegation-00
* Initial version, some text extracted from draft-sheffer-acme-star-
requests-02
Appendix B. CSR Template: CDDL
Following is the normative definition of the CSR template, using CDDL
[RFC8610]. The CSR template MUST be a valid JSON document, compliant
with the syntax defined here.
Sheffer, et al. Expires 27 September 2021 [Page 36]
Internet-Draft ACME Delegation March 2021
An additional constraint that is not expressed in CDDL but MUST be
validated by the recipient is that all objects (e.g.
"distinguishedName") MUST NOT be empty when they are included, even
when each separate property is optional.
csr-template-schema = {
keyTypes: [ 1* $keyType ]
? subject: distinguishedName
extensions: extensions
}
mandatory-wildcard = "**"
optional-wildcard = "*"
wildcard = mandatory-wildcard / optional-wildcard
; regtext matches all text strings but "*" and "**"
regtext = text .regexp "([^\*].*)|([\*][^\*].*)|([\*][\*].+)"
regtext-or-wildcard = regtext / wildcard
distinguishedName = {
? country: regtext-or-wildcard
? stateOrProvince: regtext-or-wildcard
? locality: regtext-or-wildcard
? organization: regtext-or-wildcard
? organizationalUnit: regtext-or-wildcard
? emailAddress: regtext-or-wildcard
? commonName: regtext-or-wildcard
}
$keyType /= rsaKeyType
$keyType /= ecdsaKeyType
rsaKeyType = {
PublicKeyType: "rsaEncryption" ; OID: 1.2.840.113549.1.1.1
PublicKeyLength: rsaKeySize
SignatureType: $rsaSignatureType
}
rsaKeySize = int .ge 2048
; RSASSA-PKCS1-v1_5 with SHA-256
$rsaSignatureType /= "sha256WithRSAEncryption"
; RSASSA-PCKS1-v1_5 with SHA-384
$rsaSignatureType /= "sha384WithRSAEncryption"
; RSASSA-PCKS1-v1_5 with SHA-512
$rsaSignatureType /= "sha512WithRSAEncryption"
; RSASSA-PSS with SHA-256, MGF-1 with SHA-256, and a 32 byte salt
Sheffer, et al. Expires 27 September 2021 [Page 37]
Internet-Draft ACME Delegation March 2021
$rsaSignatureType /= "sha256WithRSAandMGF1"
; RSASSA-PSS with SHA-384, MGF-1 with SHA-384, and a 48 byte salt
$rsaSignatureType /= "sha384WithRSAandMGF1"
; RSASSA-PSS with SHA-512, MGF-1 with SHA-512, and a 64 byte salt
$rsaSignatureType /= "sha512WithRSAandMGF1"
ecdsaKeyType = {
PublicKeyType: "id-ecPublicKey" ; OID: 1.2.840.10045.2.1
namedCurve: $ecdsaCurve
SignatureType: $ecdsaSignatureType
}
$ecdsaCurve /= "secp256r1" ; OID: 1.2.840.10045.3.1.7
$ecdsaCurve /= "secp384r1" ; OID: 1.3.132.0.34
$ecdsaCurve /= "secp521r1" ; OID: 1.3.132.0.3
$ecdsaSignatureType /= "ecdsa-with-SHA256" ; paired with secp256r1
$ecdsaSignatureType /= "ecdsa-with-SHA384" ; paired with secp384r1
$ecdsaSignatureType /= "ecdsa-with-SHA512" ; paired with secp521r1
subjectaltname = {
? DNS: [ 1* regtext-or-wildcard ]
? Email: [ 1* regtext ]
? URI: [ 1* regtext ]
* $$subjectaltname-extension
}
extensions = {
? keyUsage: [ 1* keyUsageType ]
? extendedKeyUsage: [ 1* extendedKeyUsageType ]
subjectAltName: subjectaltname
}
keyUsageType /= "digitalSignature"
keyUsageType /= "nonRepudiation"
keyUsageType /= "keyEncipherment"
keyUsageType /= "dataEncipherment"
keyUsageType /= "keyAgreement"
keyUsageType /= "keyCertSign"
keyUsageType /= "cRLSign"
keyUsageType /= "encipherOnly"
keyUsageType /= "decipherOnly"
extendedKeyUsageType /= "serverAuth"
extendedKeyUsageType /= "clientAuth"
extendedKeyUsageType /= "codeSigning"
extendedKeyUsageType /= "emailProtection"
extendedKeyUsageType /= "timeStamping"
Sheffer, et al. Expires 27 September 2021 [Page 38]
Internet-Draft ACME Delegation March 2021
extendedKeyUsageType /= "OCSPSigning"
extendedKeyUsageType /= oid
oid = text .regexp "[0-9]+(\\.[0-9]+)*"
Appendix C. CSR Template: JSON Schema
This appendix includes an alternative, non-normative, JSON Schema
definition of the CSR template. The syntax used is that of draft 7
of JSON Schema, which is documented in [json-schema-07]. Note that
later versions of this (now expired) draft describe later versions of
the JSON Schema syntax. At the time of writing, a stable reference
for this syntax is not yet available, and we have chosen to use the
draft version which is currently best supported by tool
implementations.
While the CSR template must follow the syntax defined here, neither
the IdO nor the NDC are expected to validate it at run-time.
{
"title": "JSON Schema for the STAR Delegation CSR template",
"$schema": "http://json-schema.org/draft-07/schema#",
"$id": "http://ietf.org/acme/drafts/star-delegation/csr-template",
"$defs": {
"distinguished-name": {
"$id": "#distinguished-name",
"type": "object",
"minProperties": 1,
"properties": {
"country": {
"type": "string"
},
"stateOrProvince": {
"type": "string"
},
"locality": {
"type": "string"
},
"organization": {
"type": "string"
},
"organizationalUnit": {
"type": "string"
},
"emailAddress": {
"type": "string"
},
"commonName": {
Sheffer, et al. Expires 27 September 2021 [Page 39]
Internet-Draft ACME Delegation March 2021
"type": "string"
}
},
"additionalProperties": false
},
"rsaKeyType": {
"$id": "#rsaKeyType",
"type": "object",
"properties": {
"PublicKeyType": {
"type": "string",
"const": "rsaEncryption"
},
"PublicKeyLength": {
"type": "integer",
"minimum": 2048,
"multipleOf": 8
},
"SignatureType": {
"type": "string",
"enum": [
"sha256WithRSAEncryption",
"sha384WithRSAEncryption",
"sha512WithRSAEncryption",
"sha256WithRSAandMGF1",
"sha384WithRSAandMGF1",
"sha512WithRSAandMGF1"
]
}
},
"required": [
"PublicKeyType",
"PublicKeyLength",
"SignatureType"
],
"additionalProperties": false
},
"ecdsaKeyType": {
"$id": "#ecdsaKeyType",
"type": "object",
"properties": {
"PublicKeyType": {
"type": "string",
"const": "id-ecPublicKey"
},
"namedCurve": {
"type": "string",
"enum": [
Sheffer, et al. Expires 27 September 2021 [Page 40]
Internet-Draft ACME Delegation March 2021
"secp256r1",
"secp384r1",
"secp521r1"
]
},
"SignatureType": {
"type": "string",
"enum": [
"ecdsa-with-SHA256",
"ecdsa-with-SHA384",
"ecdsa-with-SHA512"
]
}
},
"required": [
"PublicKeyType",
"namedCurve",
"SignatureType"
],
"additionalProperties": false
}
},
"type": "object",
"properties": {
"keyTypes": {
"type": "array",
"items": {
"oneOf": [
{
"$ref": "#rsaKeyType"
},
{
"$ref": "#ecdsaKeyType"
}
]
}
},
"subject": {
"$ref": "#distinguished-name"
},
"extensions": {
"type": "object",
"properties": {
"keyUsage": {
"type": "array",
"minItems": 1,
"items": {
"type": "string",
Sheffer, et al. Expires 27 September 2021 [Page 41]
Internet-Draft ACME Delegation March 2021
"enum": [
"digitalSignature",
"nonRepudiation",
"keyEncipherment",
"dataEncipherment",
"keyAgreement",
"keyCertSign",
"cRLSign",
"encipherOnly",
"decipherOnly"
]
}
},
"extendedKeyUsage": {
"type": "array",
"minItems": 1,
"items": {
"oneOf": [
{
"type": "string",
"enum": [
"serverAuth",
"clientAuth",
"codeSigning",
"emailProtection",
"timeStamping",
"OCSPSigning"
]
},
{
"type": "string",
"pattern": "^[0-9]+(\\.[0-9]+)*$",
"description": "Used for OID values"
}
]
}
},
"subjectAltName": {
"type": "object",
"minProperties": 1,
"properties": {
"DNS": {
"type": "array",
"minItems": 1,
"items": {
"type": "string",
"format": "hostname"
}
Sheffer, et al. Expires 27 September 2021 [Page 42]
Internet-Draft ACME Delegation March 2021
},
"Email": {
"type": "array",
"minItems": 1,
"items": {
"type": "string",
"format": "email"
}
},
"URI": {
"type": "array",
"minItems": 1,
"items": {
"type": "string",
"format": "uri"
}
}
},
"additionalProperties": false
}
},
"required": [
"subjectAltName"
],
"additionalProperties": false
}
},
"required": [
"extensions",
"keyTypes"
],
"additionalProperties": false
}
Authors' Addresses
Yaron Sheffer
Intuit
Email: yaronf.ietf@gmail.com
Diego López
Telefonica I+D
Email: diego.r.lopez@telefonica.com
Sheffer, et al. Expires 27 September 2021 [Page 43]
Internet-Draft ACME Delegation March 2021
Antonio Agustín Pastor Perales
Telefonica I+D
Email: antonio.pastorperales@telefonica.com
Thomas Fossati
ARM
Email: thomas.fossati@arm.com
Sheffer, et al. Expires 27 September 2021 [Page 44]