Internet DRAFT - draft-ietf-acme-star

draft-ietf-acme-star







ACME Working Group                                            Y. Sheffer
Internet-Draft                                                    Intuit
Intended status: Standards Track                                D. Lopez
Expires: April 15, 2020                              O. Gonzalez de Dios
                                                       A. Pastor Perales
                                                          Telefonica I+D
                                                              T. Fossati
                                                                     ARM
                                                        October 13, 2019


  Support for Short-Term, Automatically-Renewed (STAR) Certificates in
          Automated Certificate Management Environment (ACME)
                        draft-ietf-acme-star-10

Abstract

   Public-key certificates need to be revoked when they are compromised,
   that is, when the associated private key is exposed to an
   unauthorized entity.  However the revocation process is often
   unreliable.  An alternative to revocation is issuing a sequence of
   certificates, each with a short validity period, and terminating this
   sequence upon compromise.  This memo proposes an ACME extension to
   enable the issuance of short-term and automatically renewed (STAR)
   X.509 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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 April 15, 2020.




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Copyright Notice

   Copyright (c) 2019 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.  Name Delegation Use Case  . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Conventions used in this document . . . . . . . . . . . .   4
   2.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Bootstrap . . . . . . . . . . . . . . . . . . . . . . . .   5
     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 an Auto-renewal Order . . . . . . . . . . .   8
     3.2.  Capability Discovery  . . . . . . . . . . . . . . . . . .  10
     3.3.  Fetching the Certificates . . . . . . . . . . . . . . . .  11
     3.4.  Negotiating an unauthenticated GET  . . . . . . . . . . .  13
     3.5.  Computing notBefore and notAfter of STAR Certificates . .  14
       3.5.1.  Example . . . . . . . . . . . . . . . . . . . . . . .  15
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .  15
     4.1.  The Meaning of "Short Term" and the Impact of Skewed
           Clocks  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.2.  Impact on Certificate Transparency (CT) Logs  . . . . . .  16
     4.3.  HTTP Caching and Dependability  . . . . . . . . . . . . .  16
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .  17
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  17
       5.1.1.  ACME Server with STAR extension . . . . . . . . . . .  18
       5.1.2.  STAR Proxy  . . . . . . . . . . . . . . . . . . . . .  18
     5.2.  Level of Maturity . . . . . . . . . . . . . . . . . . . .  18
     5.3.  Coverage  . . . . . . . . . . . . . . . . . . . . . . . .  18
     5.4.  Version Compatibility . . . . . . . . . . . . . . . . . .  19
     5.5.  Licensing . . . . . . . . . . . . . . . . . . . . . . . .  19
     5.6.  Implementation experience . . . . . . . . . . . . . . . .  19



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     5.7.  Contact Information . . . . . . . . . . . . . . . . . . .  19
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  New Registries  . . . . . . . . . . . . . . . . . . . . .  19
     6.2.  New Error Types . . . . . . . . . . . . . . . . . . . . .  20
     6.3.  New fields in Order Objects . . . . . . . . . . . . . . .  20
     6.4.  Fields in the "auto-renewal" Object within an Order
           Object  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     6.5.  New fields in the "meta" Object within a Directory Object  21
     6.6.  Fields in the "auto-renewal" Object within a Directory
           Metadata Object . . . . . . . . . . . . . . . . . . . . .  22
     6.7.  Cert-Not-Before and Cert-Not-After HTTP Headers . . . . .  22
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
     7.1.  No revocation . . . . . . . . . . . . . . . . . . . . . .  22
     7.2.  Denial of Service Considerations  . . . . . . . . . . . .  23
     7.3.  Privacy Considerations  . . . . . . . . . . . . . . . . .  24
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  24
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  24
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  25
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  27
     A.1.  draft-ietf-acme-star-10 . . . . . . . . . . . . . . . . .  27
     A.2.  draft-ietf-acme-star-09 . . . . . . . . . . . . . . . . .  27
     A.3.  draft-ietf-acme-star-08 . . . . . . . . . . . . . . . . .  27
     A.4.  draft-ietf-acme-star-07 . . . . . . . . . . . . . . . . .  27
     A.5.  draft-ietf-acme-star-06 . . . . . . . . . . . . . . . . .  27
     A.6.  draft-ietf-acme-star-05 . . . . . . . . . . . . . . . . .  27
     A.7.  draft-ietf-acme-star-04 . . . . . . . . . . . . . . . . .  28
     A.8.  draft-ietf-acme-star-03 . . . . . . . . . . . . . . . . .  28
     A.9.  draft-ietf-acme-star-02 . . . . . . . . . . . . . . . . .  28
     A.10. draft-ietf-acme-star-01 . . . . . . . . . . . . . . . . .  28
     A.11. draft-ietf-acme-star-00 . . . . . . . . . . . . . . . . .  28
     A.12. draft-sheffer-acme-star-02  . . . . . . . . . . . . . . .  28
     A.13. draft-sheffer-acme-star-01  . . . . . . . . . . . . . . .  29
     A.14. draft-sheffer-acme-star-00  . . . . . . . . . . . . . . .  29
     A.15. draft-sheffer-acme-star-lurk-00 . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   The ACME protocol [RFC8555] automates the process of issuing a
   certificate to a named entity (an Identifier Owner or IdO).
   Typically, but not always, the identifier is a domain name.

   If the IdO wishes to obtain a string of short-term certificates
   originating from the same private key (see [Topalovic] about why
   using short-lived certificates might be preferable to explicit
   revocation), 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



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   this way, the process would involve frequent interactions between the
   registration function of the ACME Certification Authority (CA) and
   the identity provider 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 IdO can terminate the automatic renewal before the
   negotiated 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 customers
   [I-D.ietf-acme-star-delegation].  At any time, the service customer
   (i.e., the IdO) can terminate the delegation by simply instructing
   the CA to stop the automatic renewal and letting the currently active
   certificate expire shortly thereafter.

   Note that in the name delegation use case the delegated entity needs
   to access the auto-renewed certificate without being in possession of
   the ACME account key that was used for initiating the STAR issuance.
   This leads to the optional use of unauthenticated GET in this
   protocol (Section 3.4).

1.2.  Terminology

   IdO  Identifier Owner, the owner of an identifier, e.g.: a domain
      name, a telephone number.
   STAR  Short-Term and Automatically Renewed X.509 certificates.

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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.





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2.  Protocol Flow

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

   o  Bootstrap: the IdO 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 the star-certificate URL
      (Section 2.2);
   o  Termination: the IdO requests the ACME CA to discontinue the
      automatic renewal of the certificate (Section 2.3).

2.1.  Bootstrap

   The IdO, 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.  Note that the exact
      definition of "short" depends on the use case;
   o  Is automatically renewed by the CA for a certain period of time;
   o  Is downloadable from a (highly available) location.

   Other than that, the ACME protocol flows as usual between IdO and CA.
   In particular, IdO is responsible for satisfying the requested ACME
   challenges until the CA is willing to issue the requested
   certificate.  Per normal ACME processing, the IdO is given back an
   Order resource associated with the 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 ACME CA updates the Order resource
   to include the URL for the issued STAR certificate.

2.2.  Refresh

   The CA issues the initial certificate after the authorization
   completes successfully.  It then automatically re-issues the
   certificate using the same CSR (and therefore the same identifier and
   public key) before the previous one expires, and publishes it to the
   URL that was returned to the IdO at the end of the bootstrap phase.
   The certificate user, which could be either the IdO itself or a
   delegated third party, as described in
   [I-D.ietf-acme-star-delegation], obtains the certificate
   (Section 3.3) and uses it.

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




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   o  IdO explicitly terminates the automatic renewal (Section 2.3); or
   o  Automatic renewal expires.

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

                          Figure 1: Auto renewal

2.3.  Termination

   The IdO may request early termination of the STAR certificate by
   sending a cancellation request to the Order resource, as described in
   Section 3.1.2.  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 issuance;
   o  Change the status of the Order to "canceled".

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




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     Certificate                                     ACME/STAR
     User                    IdO                     Server
     |                       |                       |
     |                       |      Cancel Order     |
     |                       +---------------------->|
     |                       |                       +-------.
     |                       |                       |       |
     |                       |                       |  End auto renewal
     |                       |                       |  Remove cert link
     |                       |                       |  etc.
     |                       |                       |       |
     |                       |         Done          |<------'
     |                       |<----------------------+
     |                       |                       |
     |                                               |
     |              Retrieve cert                    |
     +---------------------------------------------->|
     |              Error: autoRenewalCanceled       |
     |<----------------------------------------------+
     |                                               |

                           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 automatically
   renewed Orders.

3.1.1.  Extending the Order Resource

   The Order resource is extended with a new "auto-renewal" object that
   MUST be present for STAR certificates.  The "auto-renewal" object has
   the following structure:

   o  start-date (optional, string): the earliest date of validity of
      the first certificate issued, in [RFC3339] format.  When omitted,
      the start date is as soon as authorization is complete.
   o  end-date (required, string): the latest date of validity of the
      last certificate issued, in [RFC3339] format.
   o  lifetime (required, integer): the maximum validity period of each
      STAR certificate, an integer that denotes a number of seconds.
      This is a nominal value which does not include any extra validity
      time due to server or client adjustment (see below).



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   o  lifetime-adjust (optional, integer): amount of "left pad" added to
      each STAR certificate, an integer that denotes a number of
      seconds.  The default is 0.  If present, the value of the
      notBefore field that would otherwise appear in the STAR
      certificates is pre-dated by the specified number of seconds.  See
      also Section 4.1 for why a client might want to use this control
      and Section 3.5 for how the effective certificate lifetime is
      computed.  The value reflected by the server, together with the
      value of the lifetime attribute, can be used by the client as a
      hint to configure its polling timer.
   o  allow-certificate-get (optional, boolean): see Section 3.4.

   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 (see also Section 3.2).

   The optional notBefore and notAfter fields defined in Section 7.1.3
   of [RFC8555] MUST NOT be present in a STAR Order.  If they are
   included, the server MUST return an error with status code 400 "Bad
   Request" and type "malformedRequest".

   Section 7.1.6 of [RFC8555] defines the following values for the Order
   resource's status: "pending", "ready", "processing", "valid", and
   "invalid".  In the case of auto-renewal Orders, the status MUST be
   "valid" as long as STAR certificates are being issued.  We add a new
   status value: "canceled", see Section 3.1.2.

   A STAR certificate is by definition a dynamic resource, i.e., it
   refers to an entity that varies over time.  Instead of overloading
   the semantics of the "certificate" attribute, this document defines a
   new attribute "star-certificate" to be used instead of "certificate".

   o  star-certificate (optional, string): A URL for the (rolling) STAR
      certificate that has been issued in response to this Order.

3.1.2.  Canceling an Auto-renewal Order

   An important property of the auto-renewal Order is that it can be
   canceled by the IdO, with no need for certificate revocation.  To
   cancel the Order, the ACME client sends a POST to the Order URL as
   shown in Figure 3.









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     POST /acme/order/ogfr8EcolOT HTTP/1.1
     Host: example.org
     Content-Type: application/jose+json

     {
       "protected": base64url({
         "alg": "ES256",
         "kid": "https://example.com/acme/acct/gw06UNhKfOve",
         "nonce": "Alc00Ap6Rt7GMkEl3L1JX5",
         "url": "https://example.com/acme/order/ogfr8EcolOT"
       }),
       "payload": base64url({
         "status": "canceled"
       }),
       "signature": "g454e3hdBlkT4AEw...nKePnUyZTjGtXZ6H"
     }

                 Figure 3: Canceling an Auto-renewal Order

   After a successful cancellation, the server MUST NOT issue any
   additional certificates for this Order.

   When the Order is canceled, the server:

   o  MUST update the status of the Order resource to "canceled" and
      MUST set an appropriate "expires" date;
   o  MUST respond with 403 (Forbidden) to any requests to the star-
      certificate endpoint.  The response SHOULD provide additional
      information using a problem document [RFC7807] with type
      "urn:ietf:params:acme:error:autoRenewalCanceled".

   Issuing a cancellation for an Order that is not in "valid" state is
   not allowed.  A client MUST NOT send such a request, and a server
   MUST return an error response with status code 400 (Bad Request) and
   type "urn:ietf:params:acme:error:autoRenewalCancellationInvalid".

   The state machine described in Section 7.1.6 of [RFC8555] is extended
   as illustrated in Figure 4 (State Transitions for Order Objects).













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       pending --------------+
          |                  |
          | All authz        |
          | "valid"          |
          V                  |
        ready ---------------+
          |                  |
          | Receive          |
          | finalize         |
          | request          |
          V                  |
      processing ------------+
          |                  |
          | First            |
          | certificate      | Error or
          | issued           | Authorization failure
          V                  V
        valid             invalid
          |
          | STAR
          | Certificate
          | canceled
          V
       canceled


                                 Figure 4

   Explicit certificate revocation using the revokeCert interface
   (Section 7.6 of [RFC8555]) is not supported for STAR certificates.  A
   server receiving a revocation request for a STAR certificate MUST
   return an error response with status code 403 (Forbidden) and type
   "urn:ietf:params:acme:error:autoRenewalRevocationNotSupported".

3.2.  Capability Discovery

   In order to support the discovery of STAR capabilities, the "meta"
   field inside the directory object defined in Section 9.7.6 of
   [RFC8555] is extended with a new "auto-renewal" object.  The "auto-
   renewal" object MUST be present if the server supports STAR.  Its
   structure is as follows:

   o  min-lifetime (required, integer): minimum acceptable value for
      auto-renewal lifetime, in seconds.
   o  max-duration (required, integer): maximum delta between the auto-
      renewal end-date and start-date, in seconds.
   o  allow-certificate-get (optional, boolean): see Section 3.4.




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   An example directory object advertising STAR support with one day
   min-lifetime and one year max-duration, and supporting certificate
   fetching with an HTTP GET is shown in Figure 5.

    {
       "new-nonce": "https://example.com/acme/new-nonce",
       "new-account": "https://example.com/acme/new-account",
       "new-order": "https://example.com/acme/new-order",
       "new-authz": "https://example.com/acme/new-authz",
       "revoke-cert": "https://example.com/acme/revoke-cert",
       "key-change": "https://example.com/acme/key-change",
       "meta": {
         "terms-of-service": "https://example.com/acme/terms/2017-5-30",
         "website": "https://www.example.com/",
         "caa-identities": ["example.com"],
         "auto-renewal": {
           "min-lifetime": 86400,
           "max-duration":  31536000,
           "allow-certificate-get": true
         }
       }
    }

               Figure 5: Directory object with STAR support

3.3.  Fetching the Certificates

   The certificate is fetched from the star-certificate endpoint with
   POST-as-GET as per [RFC8555] Section 7.4.2, unless client and server
   have successfully negotiated the "unauthenticated GET" option
   described in Section 3.4.  In such case, the client can simply issue
   a GET to the star-certificate resource without authenticating itself
   to the server as illustrated in Figure 6.


















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     GET /acme/cert/mAt3xBGaobw HTTP/1.1
     Host: example.org
     Accept: application/pem-certificate-chain

     HTTP/1.1 200 OK
     Content-Type: application/pem-certificate-chain
     Link: <https://example.com/acme/some-directory>;rel="index"
     Cert-Not-Before: Thu, 3 Oct 2019 00:00:00 GMT
     Cert-Not-After: Thu, 10 Oct 2019 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-----

      Figure 6: Fetching a STAR certificate with unauthenticated GET

   The Server SHOULD include the "Cert-Not-Before" and "Cert-Not-After"
   HTTP header fields in the response.  When they exist, they MUST be
   equal to the respective fields inside the end-entity 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.

   Following are further clarifications regarding usage of these header
   fields, as per [RFC7231] Sec. 8.3.1.  All apply to both headers.

   o  This header field is a single value, not a list.
   o  The header field is used only in responses to GET, HEAD and POST-
      as-GET requests, and only for MIME types that denote public key
      certificates.
   o  Header field semantics are independent of context.
   o  The header field is not hop-by-hop.
   o  Intermediaries MAY insert or delete the value;
   o  If an intermediary inserts the value, it MUST ensure that the
      newly added value matches the corresponding value in the
      certificate.
   o  The header field is not appropriate for a Vary field.
   o  The header field is allowed within message trailers.
   o  The header field is not appropriate within redirects.
   o  The header field does not introduce additional security
      considerations.  It discloses in a simpler form information that
      is already available inside the certificate.



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   To improve robustness, the next certificate MUST be made available by
   the ACME CA at the URL pointed by "star-certificate" at the latest
   halfway through the lifetime of the 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 "next" certificate
   also expires.  To avoid the client accidentally entering a broken
   state, the notBefore of the "next" certificate MUST be set so that
   the certificate is already valid when it is published at the "star-
   certificate" URL.  Note that the server might need to increase the
   auto-renewal lifetime-adjust value to satisfy the latter requirement.
   For a detailed description of the renewal scheduling logic, see
   Section 3.5.  For further rationale on the need for adjusting the
   certificate validity, see Section 4.1.

   The server MUST NOT issue any certificates for this Order with
   notAfter after the auto-renewal end-date.

   For expired Orders, the server MUST respond with 403 (Forbidden) to
   any requests to the star-certificate endpoint.  The response SHOULD
   provide additional information using a problem document [RFC7807]
   with type "urn:ietf:params:acme:error:autoRenewalExpired".  Note that
   the Order resource's state remains "valid", as per the base protocol.

3.4.  Negotiating an unauthenticated GET

   In order to enable the name delegation workflow defined in
   [I-D.ietf-acme-star-delegation] as well as to increase the
   reliability of the STAR ecosystem (see Section 4.3 for details), this
   document defines a mechanism that allows a server to advertise
   support for accessing star-certificate resources via unauthenticated
   GET (in addition to POST-as-GET), and a client to enable this service
   with per-Order granularity.

   Specifically, a server states its availability to grant
   unauthenticated access to a client's Order star-certificate by
   setting the allow-certificate-get attribute to true in the auto-
   renewal object of the meta field inside the Directory object:

   o  allow-certificate-get (optional, boolean): If this field is
      present and set to true, the server allows GET (and HEAD) requests
      to star-certificate URLs.

   A client states its desire to access the issued star-certificate via
   unauthenticated GET by adding an allow-certificate-get attribute to
   the auto-renewal object of the payload of its newOrder request and
   setting it to true.




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   o  allow-certificate-get (optional, boolean): If this field is
      present and set to true, the client requests the server to allow
      unauthenticated GET (and HEAD) to the star-certificate associated
      with this Order.

   If the server accepts the request, it MUST reflect the attribute
   setting in the resulting Order object.

   Note that even when the use of unauthenticated GET has been agreed,
   the server MUST also allow POST-as-GET requests to the star-
   certificate resource.

3.5.  Computing notBefore and notAfter of STAR Certificates

   We define "nominal renewal date" as the point in time when a new
   short-term certificate for a given STAR Order is due.  Its cadence is
   a multiple of the Order's auto-renewal lifetime that starts with the
   issuance of the first short-term certificate and is upper-bounded by
   the Order's auto-renewal end-date (Figure 7).

       T      - STAR Order's auto-renewal lifetime
       end    - STAR Order's auto-renewal end-date
       nrd[i] - nominal renewal date of the i-th STAR certificate


                    .- T -.   .- T -.   .- T -.   .__.
                   /       \ /       \ /       \ /  end
       -----------o---------o---------o---------o----X-------> t
                 nrd[0]    nrd[1]    nrd[2]    nrd[3]

                      Figure 7: Nominal Renewal Date

   The rules to determine the notBefore and notAfter values of the i-th
   STAR certificate are as follows:

       notAfter  = min(nrd[i] + T, end)
       notBefore = nrd[i] - max(adjust_client, adjust_server)

   Where "adjust_client" is the min between the auto-renewal lifetime-
   adjust value ("la"), optionally supplied by the client, and the auto-
   renewal lifetime of each short-term certificate ("T");
   "adjust_server" is the amount of padding added by the ACME server to
   make sure that all certificates being published are valid at the time
   of publication.  The server padding is a fraction f of T (i.e., f * T
   with .5 <= f < 1, see Section 3.3):

       adjust_client = min(T, la)
       adjust_server = f * T



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   Note that the ACME server MUST NOT set the notBefore of the first
   STAR certificate to a date prior to the auto-renewal start-date.

3.5.1.  Example

   Given a server that intends to publish the next STAR certificate
   halfway through the lifetime of the previous one, and a STAR Order
   with the following attributes:

        "auto-renewal": {
          "start-date": "2019-01-10T00:00:00Z",
          "end-date": "2019-01-20T00:00:00Z",
          "lifetime": 345600,          // 4 days
          "lifetime-adjust": 259200    // 3 days
        }

   The amount of time that needs to be subtracted from each nominal
   renewal date is 3 days - i.e., max(min(345600, 259200), 345600 * .5).

   The notBefore and notAfter of each short-term certificate are:

              +----------------------+----------------------+
              | notBefore            | notAfter             |
              +----------------------+----------------------+
              | 2019-01-10T00:00:00Z | 2019-01-14T00:00:00Z |
              | 2019-01-11T00:00:00Z | 2019-01-18T00:00:00Z |
              | 2019-01-15T00:00:00Z | 2019-01-20T00:00:00Z |
              +----------------------+----------------------+

   The value of the notBefore is also the time at which the client
   should expect the new certificate to be available from the star-
   certificate endpoint.

4.  Operational Considerations

4.1.  The Meaning of "Short Term" and the Impact of Skewed Clocks

   "Short Term" is a relative concept, therefore trying to define a cut-
   off point that works in all cases would be a useless exercise.  In
   practice, the expected lifetime of a STAR certificate will be counted
   in minutes, hours or days, depending on different factors: the
   underlying requirements for revocation, how much clock
   synchronization is expected among relying parties and the issuing CA,
   etc.

   Nevertheless, this section attempts to provide reasonable suggestions
   for the Web use case, informed by current operational and research
   experience.



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   Acer et al.  [Acer] find that one of the main causes of "HTTPS error"
   warnings in browsers is misconfigured client clocks.  In particular,
   they observe that roughly 95% of the "severe" clock skews - the 6.7%
   of clock-related breakage reports which account for clients that are
   more than 24 hours behind - happen to be within 6-7 days.

   In order to avoid these spurious warnings about a not (yet) valid
   server certificate, site owners could use the auto-renewal lifetime-
   adjust attribute to control the effective lifetime of their Web
   facing certificates.  The exact number depends on the percentage of
   the "clock-skewed" population that the site owner expects to protect
   - 5 days cover 97.3%, 7 days cover 99.6% - as well as the nominal
   auto-renewal lifetime of the STAR Order.  Note that exact choice is
   also likely to depend on the kinds of client that is prevalent for a
   given site or app - for example, Android and Mac OS clients are known
   to behave better than Windows clients.  These considerations are
   clearly out of scope of the present document.

   In terms of security, STAR certificates and certificates with OCSP
   must-staple [RFC7633] can be considered roughly equivalent if the
   STAR certificate's and the OCSP response's lifetimes are the same.
   Given OCSP responses can be cached on average for 4 days [Stark], it
   is RECOMMENDED that a STAR certificate that is used on the Web has an
   "effective" lifetime (excluding any adjustment to account for clock
   skews) no longer than 4 days.

4.2.  Impact on Certificate Transparency (CT) Logs

   Even in the highly unlikely case STAR becomes the only certificate
   issuance model, discussion with the IETF TRANS Working Group and
   Certificate Transparency (CT) logs implementers suggests that
   existing CT Log Server implementations are capable of sustaining the
   resulting 100-fold increase in ingestion rate.  Additionally, such a
   future, higher load could be managed with a variety of techniques
   (e.g., sharding by modulo of certificate hash, using "smart" load-
   balancing CT proxies, etc.).  With regards to the increase in the log
   size, current CT log growth is already being managed with schemes
   like Chrome's Log Policy [OBrien] which allow Operators to define
   their log life-cycle; and allowing the CAs, User Agents, Monitors,
   and any other interested entities to build-in support for that life-
   cycle ahead of time.

4.3.  HTTP Caching and Dependability

   When using authenticated POST-as-GET, the HTTPS endpoint from where
   the STAR certificate is fetched can't be easily replicated by an on-
   path HTTP cache.  Reducing the caching properties of the protocol
   makes STAR clients increasingly dependent on the ACME server



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   availability.  This might be problematic given the relatively high
   rate of client-server interactions in a STAR ecosystem and especially
   when multiple endpoints (e.g., a high number of CDN edge nodes) end
   up requesting the same certificate.  Clients and servers should
   consider using the mechanism described in Section 3.4 to mitigate the
   risk.

   When using unauthenticated GET to fetch the STAR certificate, the
   server SHALL use the appropriate cache directives to set the
   freshness lifetime of the response (Section 5.2 of [RFC7234]) such
   that on-path caches will consider it stale before or at the time its
   effective lifetime is due to expire.

5.  Implementation Status

   Note to RFC Editor: please remove this section before publication,
   including the reference to [RFC7942] and
   [I-D.sheffer-acme-star-request].

   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".

5.1.  Overview

   The implementation is constructed around 3 elements: STAR Client for
   the Name Delegation Client (NDC), STAR Proxy for IdO and ACME Server
   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



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   The following subsections offer a basic description, detailed
   information is available in https://github.com/mami-
   project/lurk/blob/master/proxySTAR_v2/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.  STAR Proxy

   The STAR Proxy 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 STAR Proxy understands the basic API request with a server.  The
   current implementation of the API is defined in draft-ietf-acme-star-
   01.  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.

5.2.  Level of Maturity

   This is a prototype.

5.3.  Coverage

   A STAR Client 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 the [I-D.sheffer-acme-star-request]
   implementation.





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

5.4.  Version Compatibility

   The implementation is compatible with version draft-ietf-acme-star-
   01.  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.

   Network security is not enabled (iptables default policies are
   "accept" and all rules removed) in this implementation to simplify
   and test the protocol.

5.7.  Contact Information

   See author details below.

6.  IANA Considerations

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

6.1.  New Registries

   This document requests that IANA create the following new registries:

   o  ACME Order Auto Renewal Fields (Section 6.4)



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   o  ACME Directory Metadata Auto Renewal Fields (Section 6.6)

   All of these registries are administered under a Specification
   Required policy [RFC8126].

6.2.  New Error Types

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

   +-----------------------------------+-------------------+-----------+
   | Type                              | Description       | Reference |
   +-----------------------------------+-------------------+-----------+
   | autoRenewalCanceled               | The short-term    | RFC XXXX  |
   |                                   | certificate is no |           |
   |                                   | longer available  |           |
   |                                   | because the auto- |           |
   |                                   | renewal Order has |           |
   |                                   | been explicitly   |           |
   |                                   | canceled by the   |           |
   |                                   | IdO               |           |
   | autoRenewalExpired                | The short-term    | RFC XXXX  |
   |                                   | certificate is no |           |
   |                                   | longer available  |           |
   |                                   | because the auto- |           |
   |                                   | renewal Order has |           |
   |                                   | expired           |           |
   | autoRenewalCancellationInvalid    | A request to      | RFC XXXX  |
   |                                   | cancel a auto-    |           |
   |                                   | renewal Order     |           |
   |                                   | that is not in    |           |
   |                                   | state "valid" has |           |
   |                                   | been received     |           |
   | autoRenewalRevocationNotSupported | A request to      | RFC XXXX  |
   |                                   | revoke a auto-    |           |
   |                                   | renewal Order has |           |
   |                                   | been received     |           |
   +-----------------------------------+-------------------+-----------+

6.3.  New fields in Order Objects

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








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       +------------------+------------+--------------+-----------+
       | Field Name       | Field Type | Configurable | Reference |
       +------------------+------------+--------------+-----------+
       | auto-renewal     | object     | true         | RFC XXXX  |
       | star-certificate | string     | false        | RFC XXXX  |
       +------------------+------------+--------------+-----------+

6.4.  Fields in the "auto-renewal" Object within an Order Object

   The "ACME Order Auto Renewal Fields" registry lists field names that
   are defined for use in the JSON object included in the "auto-renewal"
   field of an ACME order object.

   Template:

   o  Field name: The string to be used as a field name in the JSON
      object
   o  Field type: The type of value to be provided, e.g., string,
      boolean, array of string
   o  Configurable: Boolean indicating whether the server should accept
      values provided by the client
   o  Reference: Where this field is defined

   Initial contents: The fields and descriptions defined in
   Section 3.1.1.

     +-----------------------+------------+--------------+-----------+
     | Field Name            | Field Type | Configurable | Reference |
     +-----------------------+------------+--------------+-----------+
     | start-date            | string     | true         | RFC XXXX  |
     | end-date              | string     | true         | RFC XXXX  |
     | lifetime              | integer    | true         | RFC XXXX  |
     | lifetime-adjust       | integer    | true         | RFC XXXX  |
     | allow-certificate-get | boolean    | true         | RFC XXXX  |
     +-----------------------+------------+--------------+-----------+

6.5.  New fields in the "meta" Object within a Directory Object

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

                 +--------------+------------+-----------+
                 | Field Name   | Field Type | Reference |
                 +--------------+------------+-----------+
                 | auto-renewal | object     | RFC XXXX  |
                 +--------------+------------+-----------+





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6.6.  Fields in the "auto-renewal" Object within a Directory Metadata
      Object

   The "ACME Directory Metadata Auto Renewal Fields" registry lists
   field names that are defined for use in the JSON object included in
   the "auto-renewal" field of an ACME directory "meta" object.

   Template:

   o  Field name: The string to be used as a field name in the JSON
      object
   o  Field type: The type of value to be provided, e.g., string,
      boolean, array of string
   o  Reference: Where this field is defined

   Initial contents: The fields and descriptions defined in Section 3.2.

            +-----------------------+------------+-----------+
            | Field Name            | Field Type | Reference |
            +-----------------------+------------+-----------+
            | min-lifetime          | integer    | RFC XXXX  |
            | max-duration          | integer    | RFC XXXX  |
            | allow-certificate-get | boolean    | RFC XXXX  |
            +-----------------------+------------+-----------+

6.7.  Cert-Not-Before and Cert-Not-After HTTP Headers

   The "Message Headers" registry should be updated with the following
   additional values:

    +-------------------+----------+----------+-----------------------+
    | Header Field Name | Protocol | Status   | Reference             |
    +-------------------+----------+----------+-----------------------+
    | Cert-Not-Before   | http     | standard | RFC XXXX, Section 3.3 |
    | Cert-Not-After    | http     | standard | RFC XXXX, Section 3.3 |
    +-------------------+----------+----------+-----------------------+

7.  Security Considerations

7.1.  No revocation

   STAR certificates eliminate an important security feature of PKI
   which is the ability to revoke certificates.  Revocation allows the
   administrator to limit the damage done by a rogue node or an
   adversary who has control of the private key.  With STAR
   certificates, expiration replaces revocation so there is potential
   for lack of timeliness in the revocation taking effect.  To that end,
   see also the discussion on clock skew in Section 4.1.



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   It should be noted that revocation also has timeliness issues,
   because both CRLs and OCSP responses have nextUpdate fields that tell
   relying parties (RPs) how long they should trust this revocation
   data.  These fields are typically set to hours, days, or even weeks
   in the future.  Any revocation that happens before the time in
   nextUpdate goes unnoticed by the RP.

   One situation where the lack of explicit revocation could create a
   security risk to the IdO is when the Order is created with start-date
   some appreciable amount of time in the future.  Recall that when
   authorizations have been fulfilled, the Order moves to the "valid"
   state and the star-certificate endpoint is populated with the first
   cert (Figure 4).  So, if an attacker manages to get hold of the
   private key as well as of the first (post-dated) certificate, there
   is a time window in the future when they will be able to successfully
   impersonate the IdO.  Note that cancellation is pointless in this
   case.  In order to mitigate the described threat, it is RECOMMENDED
   that IdO place their Orders at a time that is close to the Order's
   start-date.

   More discussion of the security of STAR certificates is available in
   [Topalovic].

7.2.  Denial of Service Considerations

   STAR adds a new attack vector that increases the threat of denial of
   service attacks, caused by the change to the CA's behavior.  Each
   STAR request amplifies the resource demands upon the CA, where one
   Order produces not one, but potentially dozens or hundreds of
   certificates, depending on the auto-renewal "lifetime" parameter.  An
   attacker can use this property to aggressively reduce the auto-
   renewal "lifetime" (e.g. 1 sec.) jointly with other ACME attack
   vectors identified in Sec. 10 of [RFC8555].  Other collateral impact
   is related to the certificate endpoint resource where the client can
   retrieve the certificates periodically.  If this resource is external
   to the CA (e.g. a hosted web server), the previous attack will be
   reflected to that resource.

   Mitigation recommendations from ACME still apply, but some of them
   need to be adjusted.  For example, applying rate limiting to the
   initial request, by the nature of the auto-renewal behavior cannot
   solve the above problem.  The CA server needs complementary
   mitigation and specifically, it SHOULD enforce a minimum value on
   auto-renewal "lifetime".  Alternatively, the CA can set an internal
   certificate generation processes rate limit.  Note that this limit
   has to take account of already-scheduled renewal issuances as well as
   new incoming requests.




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7.3.  Privacy Considerations

   In order to avoid correlation of certificates by account, if
   unauthenticated GET is negotiated (Section 3.4) the recommendation in
   Section 10.5 of [RFC8555] regarding the choice of URL structure
   applies, i.e. servers SHOULD choose URLs of certificate resources in
   a non-guessable way, for example using capability URLs
   [W3C.WD-capability-urls-20140218].

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.

   Thanks to Ben Kaduk, Richard Barnes, Roman Danyliw, Jon Peterson,
   Eric Rescorla, Ryan Sleevi, Sean Turner, Alexey Melnikov, Adam Roach,
   Martin Thomson and Mehmet Ersue for helpful comments and discussions
   that have shaped this document.

9.  References

9.1.  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>.

   [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>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [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>.




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   [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>.

9.2.  Informative References

   [Acer]     Acer, M., Stark, E., Felt, A., Fahl, S., Bhargava, R.,
              Dev, B., Braithwaite, M., Sleevi, R., and P. Tabriz,
              "Where the Wild Warnings Are: Root Causes of Chrome HTTPS
              Certificate Errors", DOI 10.1145/3133956.3134007, 2017,
              <https://acmccs.github.io/papers/p1407-acerA.pdf>.

   [I-D.ietf-acme-star-delegation]
              Sheffer, Y., Lopez, D., Pastor, A., and T. Fossati, "An
              ACME Profile for Generating Delegated STAR Certificates",
              draft-ietf-acme-star-delegation-01 (work in progress),
              August 2019.

   [I-D.nir-saag-star]
              Nir, Y., Fossati, T., Sheffer, Y., and T. Eckert,
              "Considerations For Using Short Term Certificates", draft-
              nir-saag-star-01 (work in progress), March 2018.

   [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-02 (work in progress), June 2018.

   [OBrien]   O'Brien, D. and R. Sleevi, "Chromium Certificate
              Transparency Log Policy", 2017,
              <https://github.com/chromium/ct-policy>.

   [RFC7633]  Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS)
              Feature Extension", RFC 7633, DOI 10.17487/RFC7633,
              October 2015, <https://www.rfc-editor.org/info/rfc7633>.





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   [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>.

   [Stark]    Stark, E., Huang, L., Israni, D., Jackson, C., and D.
              Boneh, "The case for prefetching and prevalidating TLS
              server certificates", 2012,
              <http://crypto.stanford.edu/~dabo/pubs/abstracts/ssl-
              prefetch.html>.

   [Topalovic]
              Topalovic, E., Saeta, B., Huang, L., Jackson, C., and D.
              Boneh, "Towards Short-Lived Certificates", 2012,
              <http://www.ieee-security.org/TC/W2SP/2012/papers/
              w2sp12-final9.pdf>.

   [W3C.WD-capability-urls-20140218]
              Tennison, J., "Good Practices for Capability URLs", World
              Wide Web Consortium WD WD-capability-urls-20140218,
              February 2014,
              <http://www.w3.org/TR/2014/WD-capability-urls-20140218>.





























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Appendix A.  Document History

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

A.1.  draft-ietf-acme-star-10

   IESG processing:

   o  More clarity on IANA registration (Alexey);
   o  HTTP header requirements adjustments (Adam);
   o  Misc editorial (Ben)

A.2.  draft-ietf-acme-star-09

   Richard and Ryan's review resulted in the following updates:

   o  STAR Order and Directory Meta attributes renamed slightly and
      grouped under two brand new "auto-renewal" objects;
   o  IANA registration updated accordingly (note that two new
      registries have been added as a consequence);
   o  Unbounded pre-dating of certificates removed so that STAR certs
      are never issued with their notBefore in the past;
   o  Changed "recurrent" to "autoRenewal" in error codes;
   o  Changed "recurrent" to "auto-renewal" in reference to Orders;
   o  Added operational considerations for HTTP caches.

A.3.  draft-ietf-acme-star-08

   o  Improved text on interaction with CT Logs, responding to Mehmet
      Ersue's review.

A.4.  draft-ietf-acme-star-07

   o  Changed the HTTP headers names and clarified the IANA
      registration, following feedback from the IANA expert reviewer

A.5.  draft-ietf-acme-star-06

   o  Roman's AD review

A.6.  draft-ietf-acme-star-05

   o  EKR's AD review
   o  A detailed example of the timing of certificate issuance and
      predating
   o  Added an explicit client-side parameter for predating
   o  Security considerations around unauthenticated GET




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A.7.  draft-ietf-acme-star-04

   o  WG last call comments by Sean Turner
   o  revokeCert interface handling
   o  Allow negotiating plain-GET for certs
   o  In STAR Orders, use star-certificate instead of certificate

A.8.  draft-ietf-acme-star-03

   o  Clock skew considerations
   o  Recommendations for "short" in the Web use case
   o  CT log considerations

A.9.  draft-ietf-acme-star-02

   o  Discovery of STAR capabilities via the directory object
   o  Use the more generic term Identifier Owner (IdO) instead of Domain
      Name Owner (DNO)
   o  More precision about what goes in the order
   o  Detail server side behavior on cancellation

A.10.  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.11.  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.12.  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.







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A.13.  draft-sheffer-acme-star-01

   o  A terminology section.
   o  Some cleanup.

A.14.  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.15.  draft-sheffer-acme-star-lurk-00

   o  Initial version.

Authors' Addresses

   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
   ARM

   EMail: thomas.fossati@arm.com






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