STAR                    STAR                   ACME/STAR
   Client                  Proxy                  Server
     |                       |                       |
     |                       |  Terminate STAR Id    |
     |                       +---------------------->|
     |                       |                       +-------.
     |                       |                       |       |
     |                       |                       |  End auto renewal  
     |                       |                       |  Remove cert link
     |                       |                       |  etc.
     |                       |                       |       |
     |                       |         Done          |<------'
     |                       |<----------------------+
     |                       |                       |
     |                                               |
     |                 Retrieve cert                 |
     +---------------------------------------------->|
     |                 Error: terminated             |
     |<----------------------------------------------+
     |                                               |

Figure 3: Termination

4. Protocol Details

This section describes the protocol’s details. We start with the STAR API between the STAR Client and the STAR Proxy. Then we describe a few extensions to the ACME protocol running between the STAR Proxy and the ACME Server.

4.1. STAR API

This API allows the STAR Client to request a STAR certificate via the STAR Proxy, using a previously agreed-upon CSR template.

The API consists of a single resource, “registration”. A new Registration is created with a POST and then the Registration instance is polled to obtain its details.

4.1.1. Creating a Registration

To create a registration, use:

POST /star/registration
Host: star-proxy.example.net
Content-Type: application/json

{
    "csr": "...", // CSR in PEM format
    "lifetime": 365 // requested registration lifetime in days,
                    // between 1 and 1095
}

Upon success, the call returns the new Registration resource.

HTTP/1.1 201 Created
Location: https://star-proxy.example.net/star/registration/567

4.1.2. Polling the Registration

The returned Registration can be polled until the information is available from the ACME server.

GET /star/registration/567
Host: star-proxy.example.net

In responding to poll requests while the validation is still in progress, the server MUST return a 200 (OK) response and MAY include a Retry-After header field to suggest a polling interval to the client. The Retry-After value MUST be expressed in seconds. If the Retry-After header is present, in order to avoid surprising interactions with heuristic expiration times, a max-age Cache-Control SHOULD also be present and set to a value slightly smaller than the Retry-After value.

HTTP/1.1 200 OK
Retry-After: 10
Cache-Control: max-age=9

{
    "status": "pending"
}

When the operation is successfully completed, the ACME Proxy returns:

HTTP/1.1 200 OK
Expires: Sun, 09 Sep 2018 14:09:00 GMT

{
    "status": "valid", // or "failed"
    "lifetime": 365, // lifetime of the registration in days,
                     //  possibly less than requested
    "certificates": "https://acme-server.example.org/certificates/A51A3"
}

The Expires header applies to the registration resource itself, and may be as small as a few minutes. It is unrelated to the order’s lifetime which is measured in days or longer. The “certificates” attribute contains a URL of the certificate pull endpoint, see Section 4.4.

If the registration fails for any reason, the server returns a “200 OK” response, with the status as “failed” and a “reason” attribute containing a human readable error message.

4.2. Transport Security for the STAR Protocol Leg

Traffic between the STAR Client and the STAR Proxy MUST be protected with HTTPS. For interoperability, all implementations MUST support HTTP Basic Authentication [RFC7617]. However some deployments MAY prefer mutually- authenticated HTTPS or two-legged OAUTH.

4.3. ACME Extensions between Proxy and Server

We propose to extend the ACME protocol slightly, by allowing recurrent orders.

4.3.1. Extending the Order Resource

The Order resource is extended with the following attributes:

{
    "recurrent": true,
    "recurrent-total-lifetime": 365, // requested lifetime of the
                                     // recurrent registration, in days
    "recurrent-certificate-validity": 7
       // requested validity of each certificate, in days
}

These attributes are included in a POST message when creating the order, as part of the “payload” encoded object. They are returned when the order has been created, possibly with adjusted values.

4.3.2. Canceling a Recurrent Order

An important property of the recurrent order is that it can be cancelled by the domain name owner, with no need for certificate revocation. We use the DELETE message for that:

DELETE /acme/order/1 HTTP/1.1
Host: acme-server.example.org

Which returns:

HTTP/1.1 202 Deleted

The server MUST NOT issue any additional certificates for this Order, beyond the certificate that is available for collection at the time of deletion.

4.3.3. Indicating Support of Recurrent Orders

ACME supports sending arbitrary extensions when creating an Order, and as a result, there is no need to explicitly indicate support of this extension. The Proxy MUST verify that the “recurrent” attribute was understood, as indicated by the “recurrent” attribute included in the created Order. Since the standard ACME protocol does not allow to explicitly cancel a pending Order (the DELETE operation above is an extension), an unhappy Proxy will probably let the Order expire instead of following through with the authorization process.

4.4. Fetching the Certificates

The certificate is fetched from the certificate endpoint, as per [I-D.ietf-acme-acme], Sec. 7.4.2 “Downloading the Certificate”. The server MUST include an Expires header that indicates expiry of the specific certificate. When the certificate expires, the client MAY assume that a newer certificate is already in place.

A certificate MUST be replaced by its successor at the latest 24 hours before its “Not After” time.

5. 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 compares several solutions. This section discusses two particular requirements in the context of the STAR protocol.

5.1. Multiple Parallel Delegates

In some cases the DNO would like to delegate authority over a web site to multiple CDNs. This could happen if the DNO has agreements in place with different regional CDNs for different geographical regions. STAR enables this use case naturally, since each CDN can authenticate separately to the DNO specifying its CSR, and the DNO is free to allow or deny each certificate request according to its own policy.

5.2. Chained Delegation

In other cases, a content owner (DNO) delegates some domains to a large CDN (CDN1), which in turn delegates to a smaller regional CDN, CDN2. The DNO has a contractual relationship with CDN1, and CDN1 has a similar relationship with CDN2. However DNO may not even know about CDN2.

The STAR protocol does not prevent this use case, although there is no special support for it. CDN1 can forward requests from CDN2 to DNO, and forward responses back to CDN2. Whether such proxying is allowed is governed by policy and contracts between the parties.

6. Security Considerations

• CDN’s client certificate key is first order security asset and MUST be protected. Absent 2FA/MFA, an attacker that can compromise the key might be able to obtain certificates bearing DNO’s identity.
• Consider collusion of two or more CDNs with contracts with the same DNO (?)

6.1. Restricting CDNs to the Delegation Mechanism

Currently there are no standard methods for the DNO to ensure that the CDN cannot issue a certificate through mechanisms other than the one described here, for the URLs under the CDN’s control. For example, regardless of the STAR solution, a rogue CDN employee can use the ACME protocol (or proprietary mechanisms used by various CAs) to create a fake certificate for the DNO’s content.

The best solution currently being worked on would consist of several related configuration steps:

• Make sure that the CDN cannot modify the DNS records for the domain. Typically this would mean that the content owner establishes a CNAME resource record from a subdomain into a CDN-managed domain.
• Restrict certificate issuance for the domain to specific CAs that comply with ACME. This assumes universal deployment of CAA [RFC6844] by CAs, which is not the case yet.
• Deploy ACME-specific methods to restrict issuance to a specific authorization key which is controlled by the content owner [I-D.landau-acme-caa], and/or to specific ACME authorization methods.

This solution is recommended in general, even if an alternative to the mechanism described here is used.

7. 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.

8. References

8.1. Normative References

[I-D.ietf-acme-acme]	Barnes, R., Hoffman-Andrews, J. and J. Kasten, "Automatic Certificate Management Environment (ACME)", Internet-Draft draft-ietf-acme-acme-06, March 2017.
[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC7617]	Reschke, J., "The 'Basic' HTTP Authentication Scheme", RFC 7617, DOI 10.17487/RFC7617, September 2015.

8.2. Informative References

[I-D.cairns-tls-session-key-interface]	Cairns, K., Mattsson, J., Skog, R. and D. Migault, "Session Key Interface (SKI) for TLS and DTLS", Internet-Draft draft-cairns-tls-session-key-interface-01, October 2015.
[I-D.erb-lurk-rsalg]	Erb, S. and R. Salz, "A PFS-preserving protocol for LURK", Internet-Draft draft-erb-lurk-rsalg-01, May 2016.
[I-D.fieau-cdni-https-delegation]	Fieau, F., Emile, S. and S. Mishra, "HTTPS delegation in CDNI", Internet-Draft draft-fieau-cdni-https-delegation-01, March 2017.
[I-D.iab-web-pki-problems]	Housley, R. and K. O'Donoghue, "Improving the Public Key Infrastructure (PKI) for the World Wide Web", Internet-Draft draft-iab-web-pki-problems-05, October 2016.
[I-D.landau-acme-caa]	Landau, H., "CA Account URI Binding for CAA Records", Internet-Draft draft-landau-acme-caa-01, October 2016.
[RFC6844]	Hallam-Baker, P. and R. Stradling, "DNS Certification Authority Authorization (CAA) Resource Record", RFC 6844, DOI 10.17487/RFC6844, January 2013.
[Topalovic]	Topalovic, E., Saeta, B., Huang, L., Jackson, C. and D. Boneh, "Towards Short-Lived Certificates", 2012.

Appendix A. Document History

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

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

• Renamed draft to prevent confusion with other work in this space.
• Added an initial STAR protocol: a REST API.
• Discussion of CDNI use cases.

A.2. draft-sheffer-acme-star-lurk-00

• Initial version.

Authors' Addresses