An ACME Profile for Generating Delegated STAR Certificates

1.1. Terminology

IdO

Identifier Owner, the owner of an identifier (e.g., a domain name) that needs to be delegated.¶

DNO

Domain Name Owner, a specific kind of IdO whose identifier is a domain name¶

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

The IETF Automated Certificate Management Environment, a certificate management protocol.¶

CA

A Certificate Authority that implements the ACME protocol.¶

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.¶

2.1. 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 Certificate 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 CA.¶

2.2. Overview

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 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 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 ACME STAR CA;¶
• If the ACME STAR protocol fails, Order2 moves to invalid and the same state is reflected in 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 moves its 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 ACME STAR CA.¶

Note that, because the identity validation is suppressed, the NDC sends the finalize request, including the CSR, to the IdO immediately after Order1 has been acknowledged. The IdO must buffer a (valid) CSR until the Validation phase completes successfully.¶

2.3. Delegated Identity Profile

This section defines a profile of the ACME protocol, to be used between the NDC and IdO.¶

{
  "csr-template": {
    "keyTypes": [
      {
        "PublicKeyType": "ecPublicKey",
        "Curve": "secp521r1",
        "SignatureType": "ecdsa-with-SHA256"
      }
    ],
    "subject": {
      "country": "CA",
      "stateOrProvince": "**",
      "locality": "**",
      "commonName": "**"
    },
    "extensions": {
      "subjectAltName": {
        "DNS": [
          "abc.ndc.dno.example"
        ]
      },
      "keyUsage": [
        "digitalSignature"
      ],
      "extendedKeyUsage": [
        "serverAuth"
      ]
    }
  }
}

{
  "status": "valid",
  "contact": [
    "mailto:delegation-admin@ido.example"
  ],
  "termsOfServiceAgreed": true,
  "orders": "https://example.com/acme/orders/rzGoeA",
  "delegations": [
    "https://acme.dno.example/acme/acct/ndc/delegations/1",
    "https://acme.dno.example/acme/acct/ndc/delegations/2"
  ]
}

POST /acme/new-order HTTP/1.1
Host: acme.dno.example
Content-Type: application/jose+json

{
  "protected": base64url({
    "alg": "ES256",
    "kid": "https://acme.dno.example/acme/acct/evOfKhNU60wg",
    "nonce": "5XJ1L3lEkMG7tR6pA00clA",
    "url": "https://acme.dno.example/acme/new-order"
  }),
  "payload": base64url({
    "identifiers": [
      {
        "type": "dns",
        "value": "abc.ndc.dno.example.",
        "delegated": true,
        "cname": "abc.ndc.example."
      }
    ],
    "auto-renewal": {
      "end-date": "2020-04-20T00:00:00Z",
      "lifetime": 345600,          // 4 days
      "allow-certificate-get": true
    },
    "delegation":
      "https://acme.dno.example/acme/acct/ndc/delegations/2"
  }),
  "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}

{
  "status": "ready",
  "expires": "2019-05-01T00:00:00Z",

  "identifiers": [
   {
     "type": "dns",
     "value": "abc.ndc.dno.example.",
     "delegated": true,
     "cname": "abc.ndc.example."
   }
  ],

  "auto-renewal": {
    "end-date": "2020-04-20T00:00:00Z",
    "lifetime": 345600,
    "allow-certificate-get": true
  },

  "delegation":
    "https://acme.dno.example/acme/acct/ndc/delegations/2",

  "authorizations": [],

  "finalize": "https://acme.dno.example/acme/order/TO8rfgo/finalize"
}

{
  "status": "valid",
  "expires": "2019-05-01T00:00:00Z",

  "identifiers": [
   {
     "type": "dns",
     "value": "abc.ndc.dno.example.",
     "delegated": true,
     "cname": "abc.ndc.example."
   }
  ],

  "auto-renewal": {
    "end-date": "2020-04-20T00:00:00Z",
    "lifetime": 345600,
    "allow-certificate-get": true
  },

  "delegation":
    "https://acme.dno.example/acme/acct/ndc/delegations/2",

  "authorizations": [],

  "finalize": "https://acme.dno.example/acme/order/TO8rfgo/finalize",

  "star-certificate": "https://acme.ca.example/acme/order/yTr23sSDg9"
}

   abc.ndc.dno.example. CNAME abc.ndc.example.

2.4. Delegation of Non-STAR Certificates

The mechanism defined here can be used to delegate regular ACME certificates whose expiry is not "short term".¶

To allow delegation of non-STAR certificates, this document allows use of allow-certificate-get directly in the Order object and independently of the auto-renewal object, so that the NDC can fetch the certificate without having to authenticate into the ACME server.¶

The following differences exist between STAR and non-STAR certificate delegation:¶

• With STAR certificates, the star-certificate field is copied by the IdO; with non-STAR certificates, the certificate field is copied.¶
• The auto-renewal object is not used (either in the request or response) for non-STAR certificates. The field allow-certificate-get MUST be included in the order object, and its value MUST be true.¶
• The notBefore and notAfter order fields are omitted only in STAR certificates.¶

When delegating a non-STAR certificate, standard certificate revocation still applies. The ACME certificate revocation endpoint is explicitly unavailable for STAR certificates but it is available for all other certificates. We note that according to Sec. 7.6 of [RFC8555], the revocation endpoint can be used with either the account keypair, or the certificate keypair. In other words, the NDC would be able to revoke the certificate. The authors believe that this is a very minor security risk.¶

2.5. 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 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 the server 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.¶

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 is rewritten.¶

• Get Order response:¶

  • The status, expires, authorizations, identifiers and auto-renewal attributes/objects MUST be copied as-is.¶
  • Similarly, the star-certificate URL 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.¶

• Finalize request:¶

  • The CSR MUST be copied as-is.¶

• Finalize response:¶

  • Both the Location header and the finalize URLs are rewritten.¶

We note that all the above messages are authenticated, and therefore each proxy must be able to authenticate any subordinate server.¶

3.1. Template Syntax

The template is a JSON document. Each field 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.dno.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 what its value is. This is denoted by \*.¶

The NDC MUST NOT include in the CSR any fields that are not specified in the template, and in particular MUST NOT add any extensions unless those were previously negotiated out of band with the IdO.¶

The mapping between X.509 CSR fields and the template will be defined in a future revision of this document.¶

When the CSR is received by the IdO, it MUST verify that the CSR is consistent with the template that the IdO sent earlier. The IdO MAY enforce additional constraints, e.g. by restricting field lengths.¶

3.2. Example

The CSR template in Figure 4 represents one possible CSR template governing the delegation exchanges provided in the rest of this document.¶

{
  "keyTypes": [
    {
      "PublicKeyType": "RSA",
      "PublicKeyLength": 4096,
      "SignatureType": "sha256WithRSAEncryption"
    }
  ],
  "subject": {
    "country": "CA",
    "stateOrProvince": "**",
    "locality": "**",
    "commonName": "**"
  },
  "extensions": {
    "subjectAltName": {
      "DNS": [
        "client1.ndc.dno.example"
      ],
      "IP": [
        "1.2.3.4",
        "13::17"
      ]
    },
    "keyUsage": [
      "digitalSignature"
    ],
    "extendedKeyUsage": [
      "serverAuth",
      "timeStamping"
    ]
  }
}

The template syntax is defined in Appendix B.¶

4.1. 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.¶

4.2. STIR

As a second use case, we consider the delegation of credentials in the STIR ecosystem [I-D.ietf-stir-cert-delegation].¶

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] cert extension.¶

In details (Figure 7):¶

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 ACME STAR CA;¶
4. Subsequently, after the required TNAuthList authorizations are successfully completed, the ACME STAR 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.¶
6. SP2 dereferences the star-certificate URL in the Order to fetch the rolling STAR certificate bearing the delegated identifiers.¶

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.¶

5.1. New Fields in the "meta" Object within a Directory Object

This document adds the following entries to the ACME Directory Metadata Fields:¶

5.2. New Fields in the Order Object

This document adds the following entries to the ACME Order Object Fields:¶

Note that the delegation field is only meaningful in interactions with ACME servers that have delegation-enabled set to true in their meta Object.¶

5.3. New Fields in the Account Object

This document adds the following entries to the ACME Account Object Fields:¶

Note that the delegations field is only reported by ACME servers that have delegation-enabled set to true in their meta Object.¶

5.4. New Fields for Identifiers

This document adds the following entries to each element of the ACME identifiers array of objects:¶

We note that [RFC8555] does not define a registry for these objects.¶

5.5. 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¶
• An extension syntax, as a JSON Schema snippet that defines a type¶
• Mapping into an X.509 certificate extension.¶

The initial contents of this registry are the extensions defined by the JSON Schema document in Appendix B.¶

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 cert with an IdO identity.¶

From a security point of view, the delegation process has two 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.¶

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.¶

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 8.¶

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.¶

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.¶

Following is the proposed solution where 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 [RFC6844] 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.¶

A.1. 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.¶
• A lot more depth on Security Considerations.¶

A.2. 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.3. 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.4. 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.5. draft-ietf-acme-star-delegation-00

• Republished as a working group draft.¶

A.6. draft-sheffer-acme-star-delegation-01

• Added security considerations about disallowing CDNs from issuing certificates for a delegated domain.¶

A.7. draft-sheffer-acme-star-delegation-00

• Initial version, some text extracted from draft-sheffer-acme-star-requests-02¶