Support for Short-Term, Automatically-Renewed (STAR) Certificates in Automated Certificate Management Environment (ACME)
draft-ietf-acme-star-01

ACME Working Group                                            Y. Sheffer
Internet-Draft                                                    Intuit
Intended status: Standards Track                                D. Lopez
Expires: May 16, 2018                                O. Gonzalez de Dios
                                                       A. Pastor Perales
                                                          Telefonica I+D
                                                              T. Fossati
                                                                   Nokia
                                                       November 12, 2017

  Support for Short-Term, Automatically-Renewed (STAR) Certificates in
          Automated Certificate Management Environment (ACME)
                        draft-ietf-acme-star-01

Abstract

   This memo proposes an ACME extension to enable the issuance of short-
   term and automatically renewed certificates.

   [RFC Editor: please remove before publication]

   While the draft is being developed, the editor's version can be found
   at https://github.com/yaronf/I-D/tree/master/STAR.

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 http://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 May 16, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Sheffer, et al.           Expires May 16, 2018                  [Page 1]
Internet-Draft                  ACME STAR                  November 2017

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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.  Name Delegation Use Case  . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Conventions used in this document . . . . . . . . . . . .   4
   2.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Bootstrap . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Refresh . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Termination . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  ACME Extensions . . . . . . . . . . . . . . . . . . . . .   7
       3.1.1.  Extending the Order Resource  . . . . . . . . . . . .   7
       3.1.2.  Canceling a Recurrent Order . . . . . . . . . . . . .   8
     3.2.  Indicating Support of Recurrent Orders  . . . . . . . . .   9
     3.3.  Fetching the Certificates . . . . . . . . . . . . . . . .   9
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .  10
     4.1.  Certificate Transparency (CT) Logs  . . . . . . . . . . .  10
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  11
       5.1.1.  ACME Server with STAR extension . . . . . . . . . . .  11
       5.1.2.  Proxy STAR  . . . . . . . . . . . . . . . . . . . . .  11
     5.2.  Level of Maturity . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Coverage  . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.4.  Version Compatibility . . . . . . . . . . . . . . . . . .  12
     5.5.  Licensing . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.6.  Implementation experience . . . . . . . . . . . . . . . .  12
     5.7.  Contact Information . . . . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  New ACME Error Types  . . . . . . . . . . . . . . . . . .  13
     6.2.  New ACME Order Object Fields  . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  16
     A.1.  draft-ietf-acme-star-01 . . . . . . . . . . . . . . . . .  16

Sheffer, et al.           Expires May 16, 2018                  [Page 2]
Internet-Draft                  ACME STAR                  November 2017

     A.2.  draft-ietf-acme-star-00 . . . . . . . . . . . . . . . . .  16
     A.3.  draft-sheffer-acme-star-02  . . . . . . . . . . . . . . .  16
     A.4.  draft-sheffer-acme-star-01  . . . . . . . . . . . . . . .  16
     A.5.  draft-sheffer-acme-star-00  . . . . . . . . . . . . . . .  16
     A.6.  draft-sheffer-acme-star-lurk-00 . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   The ACME protocol [I-D.ietf-acme-acme] automates the process of
   issuing a certificate to a Domain Name Owner (DNO).  However, if the
   DNO wishes to obtain a string of short-term certificates originating
   from the same private key (see [Topalovic] for the rationale), she
   must go through the whole ACME protocol each time a new short-term
   certificate is needed - e.g., every 2-3 days.  If done this way, the
   process would involve frequent interactions between the registration
   function of the ACME Certification Authority (CA) and the user's
   backing infrastructure (e.g.: DNS, web servers), therefore making the
   issuance of short-term certificates exceedingly dependent on the
   reliability of both.

   This document presents an extension of the ACME protocol that
   optimizes this process by making short-term certificates first class
   objects in the ACME ecosystem.  Once the order for a string of short-
   term certificates is accepted, the CA is responsible for publishing
   the next certificate at an agreed upon URL before the previous one
   expires.  The DNO can terminate the automatic renewal before the
   natural deadline, if needed - e.g., on key compromise.

   For a more generic treatment of STAR certificates, readers are
   referred to [I-D.nir-saag-star].

1.1.  Name Delegation Use Case

   The proposed mechanism can be used as a building block of an
   efficient name-delegation protocol, for example one that exists
   between a CDN or a cloud provider and its users
   [I-D.sheffer-acme-star-request], in a way that makes the delegator
   (i.e., the DNO) in full control of the delegation by simply
   instructing the CA to stop the automatic renewal and letting the
   currently active certificate expire shortly thereafter.

1.2.  Terminology

   DNO  Domain Name Owner, the owner of a domain.
   STAR  Short-Term, Automatically Renewed X.509 certificates.
   NDC  Name Delegation Client, an entity to which the domain name owned
      by the DNO is delegated for a limited time.  This might be a CDN

Sheffer, et al.           Expires May 16, 2018                  [Page 3]
Internet-Draft                  ACME STAR                  November 2017

      edge cache, a cloud provider's load balancer or Web Application
      Firewall (WAF).

1.3.  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
   [RFC2119].

2.  Protocol Flow

   The following subsections describe the three main phases of the
   protocol:

   o  Bootstrap: the DNO asks an ACME CA to create a short-term and
      automatically-renewed (STAR) certificate (Section 2.1);
   o  Auto-renewal: the ACME CA periodically re-issues the short-term
      certificate and posts it to a public URL (Section 2.2);
   o  Termination: the DNO requests the ACME CA to discontinue the
      automatic renewal of the certificate (Section 2.3).

   This diagram presents the entities that are (or may be) involved in
   the protocol and their interactions during the different phases.

                            Refresh
             . . . . . . . . . . . . . . . . . . . .
         . '                                         ` v
      .-----.        Bootstrap / Terminate         .---------.
      | DNO |------------------------------------->| ACME CA |
      `-----'                                      `---------'
         ^                  .- - -.                    ^
          ` . . . . . . . . : NDC : . . . . . . . . . '
               Request      `- - -'    Refresh
               Delegation

   Note that there might be a distinct NDC entity (e.g., a CDN edge
   cache) that uses a separate channel to request the DNO to set up a
   name delegation.  The protocol described in
   [I-D.sheffer-acme-star-request] might be used for this purpose.

2.1.  Bootstrap

   The DNO, in its role as an ACME client, requests the CA to issue a
   STAR certificate, i.e., one that:

   o  Has a short validity (e.g., 24 to 72 hours);
   o  Is automatically renewed by the CA for a certain period of time;

Sheffer, et al.           Expires May 16, 2018                  [Page 4]
Internet-Draft                  ACME STAR                  November 2017

   o  Is downloadable from a (highly available) public link without
      requiring any special authorization.

   Other than that, the ACME protocol flows as normal between DNO and
   CA.  In particular, DNO is responsible for satisfying the requested
   ACME challenges until the CA is willing to issue the requested
   certificate.  Per normal ACME processing, the DNO is given back an
   Order ID for the issued STAR certificate to be used in subsequent
   interaction with the CA (e.g., if the certificate needs to be
   terminated.)

   The bootstrap phase ends when the DNO obtains the OK from the ACME
   CA.

2.2.  Refresh

   The CA automatically re-issues the certificate using the same CSR
   (and therefore the same name and public key) before it expires and
   publishes it to the URL that was returned to the DNO at the end of
   the bootstrap phase.  The certificate user, which could be either the
   DNO itself or a delegated third party, as described in
   [I-D.sheffer-acme-star-request], obtains the certificate and uses it.

   The refresh process (Figure 1) goes on until either:

   o  DNO terminates the delegation, or
   o  Automatic renewal expires.

Sheffer, et al.           Expires May 16, 2018                  [Page 5]
Internet-Draft                  ACME STAR                  November 2017

        Certificate              ACME/STAR
           User                    Server
             |     Retrieve cert     |                     [...]
             |---------------------->|                      |
             |                       +------.              /
             |                       |      |             /
             |                       | Automatic renewal :
             |                       |      |             \
             |                       |<-----'              \
             |     Retrieve cert     |                      |
             |---------------------->|                   72 hours
             |                       |                      |
             |                       +------.              /
             |                       |      |             /
             |                       | Automatic renewal :
             |                       |      |             \
             |                       |<-----'              \
             |     Retrieve cert     |                      |
             |---------------------->|                   72 hours
             |                       |                      |
             |                       +------.              /
             |                       |      |             /
             |                       | Automatic renewal :
             |                       |      |             \
             |                       |<-----'              \
             |                       |                      |
             |         [...]         |                    [...]

                          Figure 1: Auto renewal

2.3.  Termination

   The DNO may request early termination of the STAR certificate by
   including the Order ID in a certificate termination request to the
   ACME interface, defined below.  After the CA receives and verifies
   the request, it shall:

   o  Cancel the automatic renewal process for the STAR certificate;
   o  Change the certificate publication resource to return an error
      indicating the termination of the delegation to any external
      client.

   Note that it is not necessary to explicitly revoke the short-term
   certificate.

Sheffer, et al.           Expires May 16, 2018                  [Page 6]
Internet-Draft                  ACME STAR                  November 2017

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

                           Figure 2: Termination

3.  Protocol Details

   This section describes the protocol details, namely the extensions to
   the ACME protocol required to issue STAR certificates.

3.1.  ACME Extensions

   This protocol extends the ACME protocol, to allow for recurrent
   orders.

3.1.1.  Extending the Order Resource

   The Order resource is extended with the following attributes:

   {
       "recurrent": true,
       "recurrent-start-date": "2016-01-01T00:00:00Z",
       "recurrent-end-date": "2017-01-01T00:00:00Z",
       "recurrent-certificate-validity": 604800
   }

   o  recurrent: MUST be "true" for STAR certificates.
   o  recurrent-start-date: the earliest date of validity of the first
      certificate issued, in [RFC3339] format.  This attribute is

Sheffer, et al.           Expires May 16, 2018                  [Page 7]
Internet-Draft                  ACME STAR                  November 2017

      optional.  When omitted, the start date is as soon as
      authorization is complete.
   o  recurrent-end-date: the latest date of validity of the last
      certificate issued, in [RFC3339] format.
   o  recurrent-certificate-validity: the maximum validity period of
      each STAR certificate, an integer that denotes a number of
      seconds.

   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, and the ACME server MAY adjust them
   at will, according to its local policy.

   ACME defines the following values for the Order resource's status:
   "invalid", "pending", "processing", "valid".  In the case of
   recurrent orders, the status MUST be "valid" as long as STAR
   certificates are being issued.  We add a new status value,
   "canceled", see below.

3.1.2.  Canceling a Recurrent Order

   An important property of the recurrent Order is that it can be
   canceled by the DNO, with no need for certificate revocation.  To
   cancel the Order, the ACME client sends a POST:

   POST /acme/order/1 HTTP/1.1
   Host: acme-server.example.org
   Content-Type: application/jose+json

   {
    "protected": base64url({
      "alg": "ES256",
      "kid": "https://example.com/acme/acct/1",
      "nonce": "5XJ1L3lEkMG7tR6pA00clA",
      "url": "https://example.com/acme/order/1"
    }),
    "payload": base64url({
      "status": "canceled"
    }),
    "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
   }

   The server MUST NOT issue any additional certificates for this Order,
   beyond the certificate that is available for collection at the time
   of deletion.  Immediately after the Order is canceled, the server
   SHOULD respond with 403 (Forbidden) to any requests to the
   certificate endpoint.  The response SHOULD provide additional

Sheffer, et al.           Expires May 16, 2018                  [Page 8]
Internet-Draft                  ACME STAR                  November 2017

   information using a problem document [RFC7807] with type
   "urn:ietf:params:acme:error:recurrentOrderCanceled".

3.2.  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 DNO MUST verify that the "recurrent" attribute
   was understood, as indicated by the "recurrent" attribute included by
   the CA in the created Order.  Since the standard ACME protocol does
   not allow to explicitly cancel a pending Order (the POST operation in
   Section 3.1.2 is an extension), a DNO that encounters an non-
   supporting server will probably let the Order expire instead of
   following through with the authorization process.

3.3.  Fetching the Certificates

   The certificate is fetched from the certificate endpoint, as per
   [I-D.ietf-acme-acme], Section 7.4.2.

      GET /acme/cert/asdf HTTP/1.1
      Host: acme-server.example.org
      Accept: application/pkix-cert

      HTTP/1.1 200 OK
      Content-Type: application/pem-certificate-chain
      Link: <https://example.com/acme/some-directory>;rel="index"
      Not-Before: Mon, 1 Feb 2016 00:00:00 GMT
      Not-After: Mon, 8 Feb 2016 00:00:00 GMT

      -----BEGIN CERTIFICATE-----
      [End-entity certificate contents]
      -----END CERTIFICATE-----
      -----BEGIN CERTIFICATE-----
      [Issuer certificate contents]
      -----END CERTIFICATE-----
      -----BEGIN CERTIFICATE-----
      [Other certificate contents]
      -----END CERTIFICATE-----

   The Server SHOULD include the "Not-Before" and "Not-After" headers.
   When they exist, they MUST be equal to the respective fields inside
   the certificate.  Their format is "HTTP-date" as defined in
   Section 7.1.1.2 of [RFC7231].  Their purpose is to enable client
   implementations that do not parse the certificate.

   To improve robustness, the next certificate MUST be made available by
   the ACME CA at the latest halfway through the lifetime of the

Sheffer, et al.           Expires May 16, 2018                  [Page 9]
Internet-Draft                  ACME STAR                  November 2017

   currently active certificate.  It is worth noting that this has an
   implication in case of cancellation: in fact, from the time the next
   certificate is made available, the cancellation is not completely
   effective until the latter also expires.

   The server MUST NOT issue any additional certificates for this Order
   beyond its recurrent-end-date.

   Immediately after the Order expires, the server SHOULD respond with
   403 (Forbidden) to any requests to the certificate endpoint.  The
   response SHOULD provide additional information using a problem
   document [RFC7807] with type
   "urn:ietf:params:acme:error:recurrentOrderExpired".

4.  Operational Considerations

4.1.  Certificate Transparency (CT) Logs

   TBD: larger logs and how to deal with them.

5.  Implementation Status

   Note to RFC Editor: please remove this section before publication,
   including the reference to [RFC7942].

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

Sheffer, et al.           Expires May 16, 2018                 [Page 10]
Internet-Draft                  ACME STAR                  November 2017

5.1.  Overview

   The implementation is constructed around 3 elements: Client STAR for
   NDC, Proxy STAR for DNO and Server ACME for CA.  The communication
   between them is over an IP network and the HTTPS protocol.

   The software of the implementation is available at:
   https://github.com/mami-project/lurk

   The following subsections offer a basic description, detailed
   information is available in https://github.com/mami-
   project/lurk/blob/master/proxySTAR_v1/README.md

5.1.1.  ACME Server with STAR extension

   This is a fork of the Let's Encrypt Boulder project that implements
   an ACME compliant CA.  It includes modifications to extend the ACME
   protocol as it is specified in this draft, to support recurrent
   orders and cancelling orders.

   The implementation understands the new "recurrent" attributes as part
   of the Certificate issuance in the POST request for a new resource.
   An additional process "renewalManager.go" has been included in
   parallel that reads the details of each recurrent request,
   automatically produces a "cron" Linux based task that issues the
   recurrent certificates, until the lifetime ends or the order is
   canceled.  This process is also in charge of maintaining a fixed URI
   to enable the NDC to download certificates, unlike Boulder's regular
   process of producing a unique URI per certificate.

5.1.2.  Proxy STAR

   The Proxy STAR, has a double role as ACME client and STAR Server.
   The former is a fork of the EFF Certbot project that implements an
   ACME compliant client with the STAR extension.  The latter is a basic
   HTTP REST API server.

   The proxy STAR understands the basic API request with a server.  The
   current implementation of the API is defined in draft-sheffer-acme-
   star-request-00.  Registration or order cancellation triggers the
   modified Certbot client that requests, or cancels, the recurrent
   generation of certificates using the STAR extension over ACME
   protocol.  The URI with the location of the recurrent certificate is
   delivered to the STAR client as a response.

Sheffer, et al.           Expires May 16, 2018                 [Page 11]
Internet-Draft                  ACME STAR                  November 2017

5.2.  Level of Maturity

   This is a prototype.

5.3.  Coverage

   Client STAR is not included in this implementation, but done by
   direct HTTP request with any open HTTP REST API tool.  This is
   expected to be covered as part of [I-D.sheffer-acme-star-request]
   implementation.

   This implementation completely covers Proxy STAR and Server ACME with
   STAR extension

5.4.  Version Compatibility

   The implementation is compatible with version draft-ietf-acme-star-
   00.  The implementation is based on the Boulder and Certbot code
   release from 7-Aug-2017.

5.5.  Licensing

   This implementation inherits the Boulder license (Mozilla Public
   License 2.0) and Certbot license (Apache License Version 2.0 ).

5.6.  Implementation experience

   To prove the concept all the implementation has been done with a
   self-signed CA, to avoid impact on real domains.  To be able to do it
   we use the FAKE_DNS property of Boulder and static /etc/hosts entries
   with domains names.  Nonetheless this implementation should run with
   real domains.

   Most of the implementation has been made to avoid deep changes inside
   of Boulder or Certbot, for example, the recurrent certificates
   issuance by the CA is based on an external process that auto-
   configures the standard Linux "cron" daemon in the ACME CA server.

   The reference setup recommended is one physical host with 3 virtual
   machines, one for each of the 3 components (client, proxy and server)
   and the connectivity based on host bridge.

   No security is enabled (iptables default policies are "accept" and
   all rules removed) in this implementation to simplify and test the
   protocol.

Sheffer, et al.           Expires May 16, 2018                 [Page 12]
Internet-Draft                  ACME STAR                  November 2017

5.7.  Contact Information

   See author details below.

6.  IANA Considerations

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

6.1.  New ACME Error Types

   This document adds the following entry to the ACME Error Type
   registry:

   +------------------------+------------------------------+-----------+
   | Type                   | Description                  | Reference |
   +------------------------+------------------------------+-----------+
   | recurrentOrderCanceled | The short-term certificate   | RFC XXXX  |
   |                        | is no longer available       |           |
   |                        | because the recurrent order  |           |
   |                        | has been explicitly canceled |           |
   |                        | by the DNO                   |           |
   | recurrentOrderExpired  | The short-term certificate   | RFC XXXX  |
   |                        | is no longer available       |           |
   |                        | because the recurrent order  |           |
   |                        | has expired                  |           |
   +------------------------+------------------------------+-----------+

6.2.  New ACME Order Object Fields

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

   +-------------------------------+--------+--------------+-----------+
   | Field Name                    | Field  | Configurable | Reference |
   |                               | Type   |              |           |
   +-------------------------------+--------+--------------+-----------+
   | recurrent                     | string | true         | RFC XXXX  |
   | recurrent-start-date          | string | true         | RFC XXXX  |
   | recurrent-end-date            | string | true         | RFC XXXX  |
   | recurrent-certificate-        | string | true         | RFC XXXX  |
   | validity                      |        |              |           |
   +-------------------------------+--------+--------------+-----------+

7.  Security Considerations

   TBD

Sheffer, et al.           Expires May 16, 2018                 [Page 13]
Internet-Draft                  ACME STAR                  November 2017

8.  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.

9.  References

9.1.  Normative References

   [I-D.ietf-acme-acme]
              Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
              Certificate Management Environment (ACME)", draft-ietf-
              acme-acme-08 (work in progress), October 2017.

   [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>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014, <https://www.rfc-
              editor.org/info/rfc7231>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <https://www.rfc-editor.org/info/rfc7807>.

9.2.  Informative References

   [I-D.nir-saag-star]
              Nir, Y., Fossati, T., and Y. Sheffer, "Considerations For
              Using Short Term Certificates", draft-nir-saag-star-00
              (work in progress), October 2017.

   [I-D.sheffer-acme-star-request]
              Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T.
              Fossati, "Generating Certificate Requests for Short-Term,
              Automatically-Renewed (STAR) Certificates", draft-sheffer-
              acme-star-request-01 (work in progress), June 2017.

Sheffer, et al.           Expires May 16, 2018                 [Page 14]
Internet-Draft                  ACME STAR                  November 2017

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [Topalovic]
              Topalovic, E., Saeta, B., Huang, L., Jackson, C., and D.
              Boneh, "Towards Short-Lived Certificates", 2012,
              <http://www.w2spconf.com/2012/papers/w2sp12-final9.pdf>.

Sheffer, et al.           Expires May 16, 2018                 [Page 15]
Internet-Draft                  ACME STAR                  November 2017

Appendix A.  Document History

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

A.1.  draft-ietf-acme-star-01

   o  Generalized the introduction, separating out the specifics of
      CDNs.
   o  Clean out LURK-specific text.
   o  Using a POST to ensure cancellation is authenticated.
   o  First and last date of recurrent cert, as absolute dates.
      Validity of certs in seconds.
   o  Use RFC7807 "Problem Details" in error responses.
   o  Add IANA considerations.
   o  Changed the document's title.

A.2.  draft-ietf-acme-star-00

   o  Initial working group version.
   o  Removed the STAR interface, the protocol between NDC and DNO.
      What remains is only the extended ACME protocol.

A.3.  draft-sheffer-acme-star-02

   o  Using a more generic term for the delegation client, NDC.
   o  Added an additional use case: public cloud services.
   o  More detail on ACME authorization.

A.4.  draft-sheffer-acme-star-01

   o  A terminology section.
   o  Some cleanup.

A.5.  draft-sheffer-acme-star-00

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

A.6.  draft-sheffer-acme-star-lurk-00

   o  Initial version.

Authors' Addresses

Sheffer, et al.           Expires May 16, 2018                 [Page 16]
Internet-Draft                  ACME STAR                  November 2017

   Yaron Sheffer
   Intuit

   EMail: yaronf.ietf@gmail.com

   Diego Lopez
   Telefonica I+D

   EMail: diego.r.lopez@telefonica.com

   Oscar Gonzalez de Dios
   Telefonica I+D

   EMail: oscar.gonzalezdedios@telefonica.com

   Antonio Agustin Pastor Perales
   Telefonica I+D

   EMail: antonio.pastorperales@telefonica.com

   Thomas Fossati
   Nokia

   EMail: thomas.fossati@nokia.com

Sheffer, et al.           Expires May 16, 2018                 [Page 17]