Support for Short-Term, Automatically Renewed (STAR) Certificates in the Automated Certificate Management Environment (ACME)
RFC 8739

Document Type RFC - Proposed Standard (March 2020; No errata)
Last updated 2020-03-11
Replaces draft-sheffer-acme-star
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Internet Engineering Task Force (IETF)                        Y. Sheffer
Request for Comments: 8739                                        Intuit
Category: Standards Track                                       D. Lopez
ISSN: 2070-1721                                      O. Gonzalez de Dios
                                                       A. Pastor Perales
                                                          Telefonica I+D
                                                              T. Fossati
                                                                     ARM
                                                              March 2020

Support for Short-Term, Automatically Renewed (STAR) Certificates in the
          Automated Certificate Management Environment (ACME)

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 the
   sequence upon compromise.  This memo proposes an Automated
   Certificate Management Environment (ACME) extension to enable the
   issuance of Short-Term, Automatically Renewed (STAR) X.509
   certificates.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8739.

Copyright Notice

   Copyright (c) 2020 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
     1.1.  Name Delegation Use Case
     1.2.  Terminology
     1.3.  Conventions Used in This Document
   2.  Protocol Flow
     2.1.  Bootstrap
     2.2.  Auto Renewal
     2.3.  Termination
   3.  Protocol Details
     3.1.  ACME Extensions
       3.1.1.  Extending the Order Resource
       3.1.2.  Canceling an Auto-renewal Order
     3.2.  Capability Discovery
     3.3.  Fetching the Certificates
     3.4.  Negotiating an Unauthenticated GET
     3.5.  Computing notBefore and notAfter of STAR Certificates
       3.5.1.  Example
   4.  Operational Considerations
     4.1.  The Meaning of "Short Term" and the Impact of Skewed Clocks
     4.2.  Impact on Certificate Transparency (CT) Logs
     4.3.  HTTP Caching and Dependability
   5.  IANA Considerations
     5.1.  New Registries
     5.2.  New Error Types
     5.3.  New Fields in Order Objects
     5.4.  Fields in the "auto-renewal" Object within an Order Object
     5.5.  New Fields in the "meta" Object within a Directory Object
     5.6.  Fields in the "auto-renewal" Object within a Directory
           Metadata Object
     5.7.  Cert-Not-Before and Cert-Not-After HTTP Headers
   6.  Security Considerations
     6.1.  No Revocation
     6.2.  Denial-of-Service Considerations
     6.3.  Privacy Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgments
   Authors' Addresses

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
   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 [SHORT-TERM-CERTS].

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 Content Distribution Network (CDN) or a cloud provider and
   its customers [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, etc.
   STAR    Short-Term, 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.

2.  Protocol Flow

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

   *  Bootstrap: the IdO asks an ACME CA to create a short-term,
      automatically renewed (STAR) certificate (Section 2.1);
   *  Auto-renewal: the ACME CA periodically reissues the short-term
      certificate and posts it to the star-certificate URL
      (Section 2.2);
   *  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:

   *  Has a short validity, e.g., 24 to 72 hours.  Note that the exact
      definition of "short" depends on the use case;
   *  Is automatically renewed by the CA for a certain period of time;
   *  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.  Auto Renewal

   The CA issues the initial certificate after the authorization
   completes successfully.  It then automatically reissues the
   certificate using the same Certificate Signing Request (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
   [STAR-DELEGATION], obtains the certificate (Section 3.3) and uses it.

   The auto-renewal process (Figure 1) goes on until either:

   *  IdO explicitly terminates the automatic renewal (Section 2.3); or
   *  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:

   *  Cancel the automatic renewal process for the STAR certificate;
   *  Change the certificate publication resource to return an error
      indicating the termination of the issuance;
   *  Change the status of the Order to "canceled".

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

      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:

   *  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.
   *  end-date (required, string): The latest date of validity of the
      last certificate issued, in [RFC3339] format.
   *  lifetime (required, integer): The maximum validity period of each
      STAR certificate, an integer that denotes a number of seconds.
      This is a nominal value that does not include any extra validity
      time due to server or client adjustment (see below).
   *  lifetime-adjust (optional, integer): The 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
      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.
   *  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 object encoded as "payload".  They are returned
   when the Order has been created.  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.  This document
   adds 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".

   *  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.

     POST /acme/order/ogfr8EcolOT HTTP/1.1
     Host: example.com
     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:

   *  MUST update the status of the Order resource to "canceled" and
      MUST set an appropriate "expires" date;
   *  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.

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

             Figure 4: State Transitions for STAR Order Objects

   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:

   *  min-lifetime (required, integer): Minimum acceptable value for
      auto-renewal lifetime, in seconds.
   *  max-duration (required, integer): Maximum allowed delta between
      the end-date and start-date attributes of the Order's auto-renewal
      object.
   *  allow-certificate-get (optional, boolean): See Section 3.4.

   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 Section 7.4.2 of [RFC8555] unless the 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.

     GET /acme/cert/g7m3ZQeTEqa HTTP/1.1
     Host: example.com
     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.

   The following are further clarifications regarding usage of these
   header fields as per Section 8.3.1 of [RFC7231].  All apply to both
   headers.

   *  This header field is a single value, not a list.
   *  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.
   *  Header field semantics are independent of context.
   *  The header field is not hop-by-hop.
   *  Intermediaries MAY insert or delete the value;
   *  If an intermediary inserts the value, it MUST ensure that the
      newly added value matches the corresponding value in the
      certificate.
   *  The header field is not appropriate for a Vary field.
   *  The header field is allowed within message trailers.
   *  The header field is not appropriate within redirects.
   *  The header field does not introduce additional security
      considerations.  It discloses in a simpler form information that
      is already available inside the certificate.

   To improve robustness, the next certificate MUST be made available by
   the ACME CA at the URL indicated by "star-certificate" halfway
   through the lifetime of the currently active certificate at the
   latest.  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
   [STAR-DELEGATION] and 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:

   *  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".

   *  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
   upon, 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 minimum value 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

   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 |
              +----------------------+----------------------+

                                  Table 1

   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
   cutoff 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.

   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 that 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 are 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 this document.

   In terms of security, STAR certificates and certificates with the
   Online Certificate Status Protocol (OCSP) "must-staple" flag asserted
   [RFC7633] can be considered roughly equivalent if the STAR
   certificate's and the OCSP response's lifetimes are the same.  (Here,
   "must-staple" refers to a certificate carrying a TLS feature
   extension with the "status_request" extension identifier [RFC6066].)
   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
   implementers of Certificate Transparency (CT) logs 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, as well as 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
   availability.  This might be problematic given the relatively high
   rate of client-server interactions in a STAR ecosystem, 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.  IANA Considerations

5.1.  New Registries

   Per this document, IANA has created the following new registries:

   *  ACME Order Auto-Renewal Fields (Section 5.4)
   *  ACME Directory Metadata Auto-Renewal Fields (Section 5.6)

   These registries are administered under a Specification Required
   policy [RFC8126].

5.2.  New Error Types

   Per this document, IANA has added the following entries to the "ACME
   Error Types" registry:

   +-----------------------------------+-------------------+-----------+
   | Type                              | Description       | Reference |
   +===================================+===================+===========+
   | autoRenewalCanceled               | The short-term    | RFC 8739  |
   |                                   | certificate is    |           |
   |                                   | no longer         |           |
   |                                   | available         |           |
   |                                   | because the       |           |
   |                                   | auto-renewal      |           |
   |                                   | Order has been    |           |
   |                                   | explicitly        |           |
   |                                   | canceled by       |           |
   |                                   | the IdO           |           |
   +-----------------------------------+-------------------+-----------+
   | autoRenewalExpired                | The short-term    | RFC 8739  |
   |                                   | certificate is    |           |
   |                                   | no longer         |           |
   |                                   | available         |           |
   |                                   | because the       |           |
   |                                   | auto-renewal      |           |
   |                                   | Order has         |           |
   |                                   | expired           |           |
   +-----------------------------------+-------------------+-----------+
   | autoRenewalCancellationInvalid    | A request to      | RFC 8739  |
   |                                   | cancel an         |           |
   |                                   | auto-renewal      |           |
   |                                   | Order that is     |           |
   |                                   | not in state      |           |
   |                                   | "valid" has       |           |
   |                                   | been received     |           |
   +-----------------------------------+-------------------+-----------+
   | autoRenewalRevocationNotSupported | A request to      | RFC 8739  |
   |                                   | revoke an         |           |
   |                                   | auto-renewal      |           |
   |                                   | Order has been    |           |
   |                                   | received          |           |
   +-----------------------------------+-------------------+-----------+

                                  Table 2

5.3.  New Fields in Order Objects

   Per this document, IANA has added the following entries to the "ACME
   Order Object Fields" registry:

       +------------------+------------+--------------+-----------+
       | Field Name       | Field Type | Configurable | Reference |
       +==================+============+==============+===========+
       | auto-renewal     | object     | true         | RFC 8739  |
       +------------------+------------+--------------+-----------+
       | star-certificate | string     | false        | RFC 8739  |
       +------------------+------------+--------------+-----------+

                                 Table 3

5.4.  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:

   *  Field name: The string to be used as a field name in the JSON
      object
   *  Field type: The type of value to be provided, e.g., string,
      boolean, array of string
   *  Configurable: Boolean indicating whether the server should accept
      values provided by the client
   *  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 8739  |
     +-----------------------+------------+--------------+-----------+
     | end-date              | string     | true         | RFC 8739  |
     +-----------------------+------------+--------------+-----------+
     | lifetime              | integer    | true         | RFC 8739  |
     +-----------------------+------------+--------------+-----------+
     | lifetime-adjust       | integer    | true         | RFC 8739  |
     +-----------------------+------------+--------------+-----------+
     | allow-certificate-get | boolean    | true         | RFC 8739  |
     +-----------------------+------------+--------------+-----------+

                                  Table 4

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

   Per this document, IANA has added the following entry to the "ACME
   Directory Metadata Fields":

                 +--------------+------------+-----------+
                 | Field Name   | Field Type | Reference |
                 +==============+============+===========+
                 | auto-renewal | object     | RFC 8739  |
                 +--------------+------------+-----------+

                                  Table 5

5.6.  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:

   *  Field name: The string to be used as a field name in the JSON
      object
   *  Field type: The type of value to be provided, e.g., string,
      boolean, array of string
   *  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 8739  |
            +-----------------------+------------+-----------+
            | max-duration          | integer    | RFC 8739  |
            +-----------------------+------------+-----------+
            | allow-certificate-get | boolean    | RFC 8739  |
            +-----------------------+------------+-----------+

                                 Table 6

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

   The "Message Headers" registry has been updated with the following
   additional values:

    +-------------------+----------+----------+-----------------------+
    | Header Field Name | Protocol | Status   | Reference             |
    +===================+==========+==========+=======================+
    | Cert-Not-Before   | http     | standard | RFC 8739, Section 3.3 |
    +-------------------+----------+----------+-----------------------+
    | Cert-Not-After    | http     | standard | RFC 8739, Section 3.3 |
    +-------------------+----------+----------+-----------------------+

                                  Table 7

6.  Security Considerations

6.1.  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.

   It should be noted that revocation also has timeliness issues because
   both Certificate Revocation Lists (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 a start-
   date of 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 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].

6.2.  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 second) jointly with other ACME attack
   vectors identified in Section 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, due to 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 a rate limit
   for internal certificate generation processes.  Note that this limit
   has to take account of already scheduled renewal issuances as well as
   new incoming requests.

6.3.  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.CAPABILITY-URLS].

7.  References

7.1.  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>.

   [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>.

7.2.  Informative References

   [ACER]     Acer, M.E., Stark, E., Felt, A.P., 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, October
              2017, <https://acmccs.github.io/papers/p1407-acerA.pdf>.

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

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [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>.

   [SHORT-TERM-CERTS]
              Nir, Y., Fossati, T., Sheffer, Y., and T. Eckert,
              "Considerations For Using Short Term Certificates", Work
              in Progress, Internet-Draft, draft-nir-saag-star-01, 5
              March 2018,
              <https://tools.ietf.org/html/draft-nir-saag-star-01>.

   [STAR-DELEGATION]
              Sheffer, Y., Lopez, D., Pastor, A., and T. Fossati, "An
              ACME Profile for Generating Delegated STAR Certificates",
              Work in Progress, Internet-Draft, draft-ietf-acme-star-
              delegation-03, 8 March 2020, <https://tools.ietf.org/html/
              draft-ietf-acme-star-delegation-03>.

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

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

   [W3C.CAPABILITY-URLS]
              Tennison, J., "Good Practices for Capability URLs", W3C
              First Public Working Draft, Latest version available at
              <https://www.w3.org/TR/capability-urls/>, February 2014,
              <https://www.w3.org/TR/2014/WD-capability-urls-20140218>.

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.

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