An ACME Profile for Generating Delegated STAR Certificates
draft-sheffer-acme-star-delegation-00

Abstract

This memo proposes a profile of the ACME protocol that allows the owner of an identifier (e.g., a domain name) to delegate to a third party access to a certificate associated with said identifier. A primary use case is that of a 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 by cancelling the associated STAR certificate renewal with the ACME CA. Another 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.

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This Internet-Draft will expire on April 22, 2019.

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Table of Contents

• 1. Introduction
• 1.1. Terminology
• 1.2. Conventions used in this document
• 2. Protocol Flow
• 2.1. Preconditions
• 2.2. Overview
• 2.3. Delegated Identity Profile
• 2.3.1. Order Object on the NDC-IdO side
• 2.3.2. Order Object on the IdO-CA side
• 2.3.3. Capability Discovery
• 2.3.4. On Cancelation
• 3. CDNI Use Cases
• 3.1. Multiple Parallel Delegates
• 3.2. Chained Delegation
• 4. IANA Considerations
• 4.1. New fields in the “meta” Object within a Directory Object
• 5. Security Considerations
• 6. Acknowledgments
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Document History
• A.1. draft-sheffer-acme-star-delegation-00
• Authors' Addresses

1. Introduction

This document is a companion document to [I-D.ietf-acme-star]. 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 [I-D.ietf-acme-star].

An Identifier Owner (IdO), that we can associate in the primary use case to a content provider (also referred to as Domain Name Owner, DNO), has agreements in place with one or more NDC (Name Delegation Consumer) to use and attest its identity. In the primary use case, we consider a CDN provider contracted to serve the IdO 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 authority of the URL that is requested, i.e., that of the IdO. Understandably, most IdOs 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.

This document describes a profile of the ACME protocol [I-D.ietf-acme-acme] that allows the NDC to request 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 described in [I-D.ietf-acme-star]) to request issuance of a 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.

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.

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. Protocol Flow

This section presents the protocol flow. For completeness, we include the ACME profile proposed in this draft as well as the extended ACME protocol described in [I-D.ietf-acme-star].

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, key length, key usage. 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 [I-D.ietf-acme-star].

The outline of the combined protocol is as follow (Figure 1):

• NDC sends an Order for the delegated identifier to IdO;
• IdO creates an Order 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 CSR template;
• If the CSR verification fails, the Order is moved to an “invalid” state and everything stops;
• If the CSR verification is successful, IdO moves the Order to state “processing”, and sends an Order’ (using its own account) for the delegated identifier to the ACME STAR CA;
• If the ACME STAR protocol fails, Order’ moves to “invalid” and the same state is reflected in the NDC Order;
• If the ACME STAR run is successful (i.e., Order’ is “valid”), IdO copies the “star-certificate” URL from Order’ to Order and moves its state “valid”.

The NDC can now download, install and use the certificate bearing the name delegated by the IdO.

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

     NDC                      IdO                   CA
     Client              Server  Client             Server

     Order
     Signature ------->

     [ No identity validation ]

     CSR
     Signature ------->

                                 Order'
                                 Signature ------->
                                           <------- Required
                                                    Authorizations

                                 Responses
                                 Signature ------->

                                 <~~~~~~~~Validation~~~~~~~~>


                                 CSR
                                 Signature ------->

    <~~~~~~Await issuance~~~~~~> <~~~~~~Await issuance~~~~~~>

              <------------------------------------ Certificate

Figure 1: End to end flow

2.3. Delegated Identity Profile

2.3.1. Order Object on the NDC-IdO side

The Order object created by the NDC:

• MUST contain identifiers with the new “delegated” field set to true;
• MUST NOT contain the notBefore and notAfter fields;
• MAY contain any of the “recurrent-*” fields listed in Section 3.1.1 of [I-D.ietf-acme-star];
• In case the identifier type is “dns”, it MAY contain a “cname” field with the alias of the identifier in the NDC domain. This field is used by the IdO to create the DNS aliasing needed to redirect the resolvers to the delegated entity.

   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."
         }
       ],
     }),
     "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
   }

The Order object that is created on the IdO:

• MUST start in the “ready” state;
• MUST contain an “authorizations” array with zero elements;
• MUST NOT contain the “notBefore” and “notAfter” fields.

   {
     "status": "ready",
     "expires": "2016-01-01T00:00:00Z",

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

     "authorizations": [],

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

The IdO SHOULD copy any “recurrent-*” field from the NDC request into the related STAR request to the ACME CA.

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 “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": "2016-01-01T00:00:00Z",

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

     "authorizations": [],

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

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

If an “identifier” object of type “dns” was included, the IdO MUST validate the specified CNAME at this point in the flow. The NDC and IdO may have a pre-established list of valid CNAME values. At the minimum, the IdO MUST verify that both DNS names are syntactically valid.

Following this validation, the IdO can add the CNAME records to its zone:

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

2.3.2. Order Object on the IdO-CA side

When sending the Order to the ACME CA, the IdO SHOULD strip the “delegated” and “cname” attributes sent by the NDC (Section 2.3.1). The IdO MUST add the necessary STAR extensions to the Order. In addition, to allow the NDC to download the certificate using unauthenticated GET, the IdO MUST add the recurrent-certificate-get attribute and set it to true.

2.3.3. 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 inside the “meta” field:

• star-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.

2.3.4. On Cancelation

It is worth noting that cancelation of the ACME STAR certificate is a prerogative of the IdO. The NDC does not own the relevant account key on the ACME CA, therefore it can’t issue a cancelation request for the STAR cert. Potentially, since it holds the STAR cert private key, it could request the revocation of a single STAR certificate. However, STAR explicitly disables the revokeCert interface.

3. CDNI Use Cases

Members of the IETF CDNI (Content Delivery Network Interconnection) working group are interested in delegating authority over web content to CDNs. Their requirements are described in a draft [I-D.fieau-cdni-https-delegation] that considers several solutions addressing different delegation requirements. This section discusses two of these particular requirements in the context of the STAR delegation workflow.

3.1. 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.

3.2. 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 DNO has a contractual relationship with uCDN, and uCDN has a similar relationship with dCDN. However IdO may not even know about dCDN.

The STAR protocol does not prevent this use case, although there is no special support for it: uCDN could forward requests from dCDN to DNO, and forward responses back to dCDN. Whether such proxying is allowed is governed by policy and contracts between the parties.

One thing that might be necessary at the interface between uCDN and dCDN is a mechanism 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.

4. IANA Considerations

[[RFC Editor: please replace XXXX below by the RFC number.]]

4.1. New fields in the “meta” Object within a Directory Object

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

5. Security Considerations

TBD

6. Acknowledgments

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.

7. References

7.1. Normative References

7.2. Informative References

Appendix A. Document History

[[Note to RFC Editor: please remove before publication.]]

A.1. draft-sheffer-acme-star-delegation-00

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

Authors' Addresses