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Versions: (draft-sheffer-acme-star) 00 01 02 03 04

ACME Working Group                                            Y. Sheffer
Internet-Draft                                                    Intuit
Intended status: Standards Track                                D. Lopez
Expires: April 22, 2019                              O. Gonzalez de Dios
                                                       A. Pastor Perales
                                                          Telefonica I+D
                                                              T. Fossati
                                                                   Nokia
                                                        October 19, 2018


  Support for Short-Term, Automatically-Renewed (STAR) Certificates in
          Automated Certificate Management Environment (ACME)
                        draft-ietf-acme-star-04

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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




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Copyright Notice

   Copyright (c) 2018 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 . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Bootstrap . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Refresh . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Termination . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  ACME Extensions . . . . . . . . . . . . . . . . . . . . .   7
       3.1.1.  Extending the Order Resource  . . . . . . . . . . . .   7
       3.1.2.  Canceling a Recurrent Order . . . . . . . . . . . . .   8
     3.2.  Capability Discovery  . . . . . . . . . . . . . . . . . .   9
     3.3.  Fetching the Certificates . . . . . . . . . . . . . . . .  10
     3.4.  Negotiate unauthenticated GET . . . . . . . . . . . . . .  12
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .  12
     4.1.  The Meaning of "Short Term" and the Impact of Skewed
           Clocks  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Impact on Certificate Transparency (CT) Logs  . . . . . .  13
     4.3.  Dependability . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  14
       5.1.1.  ACME Server with STAR extension . . . . . . . . . . .  14
       5.1.2.  STAR Proxy  . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  Level of Maturity . . . . . . . . . . . . . . . . . . . .  15
     5.3.  Coverage  . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.4.  Version Compatibility . . . . . . . . . . . . . . . . . .  15
     5.5.  Licensing . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.6.  Implementation experience . . . . . . . . . . . . . . . .  16
     5.7.  Contact Information . . . . . . . . . . . . . . . . . . .  16
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16



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     6.1.  New Error Types . . . . . . . . . . . . . . . . . . . . .  16
     6.2.  New fields in Order Objects . . . . . . . . . . . . . . .  17
     6.3.  New fields in the "meta" Object within a Directory Object  18
     6.4.  Not-Before and Not-After HTTP Headers . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
     7.1.  Denial of Service Considerations  . . . . . . . . . . . .  18
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  21
     A.1.  draft-ietf-acme-star-04 . . . . . . . . . . . . . . . . .  21
     A.2.  draft-ietf-acme-star-03 . . . . . . . . . . . . . . . . .  21
     A.3.  draft-ietf-acme-star-02 . . . . . . . . . . . . . . . . .  21
     A.4.  draft-ietf-acme-star-01 . . . . . . . . . . . . . . . . .  21
     A.5.  draft-ietf-acme-star-00 . . . . . . . . . . . . . . . . .  21
     A.6.  draft-sheffer-acme-star-02  . . . . . . . . . . . . . . .  21
     A.7.  draft-sheffer-acme-star-01  . . . . . . . . . . . . . . .  22
     A.8.  draft-sheffer-acme-star-00  . . . . . . . . . . . . . . .  22
     A.9.  draft-sheffer-acme-star-lurk-00 . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   The ACME protocol [I-D.ietf-acme-acme] 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
   natural deadline, if needed - e.g., on key compromise.





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   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.sheffer-acme-star-request].  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 this 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.

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.

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:




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   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 URL for the issued STAR certificate to be used in subsequent
   interaction with the CA (e.g., if the certificate needs to be
   terminated.)

   The bootstrap phase ends when the IdO obtains a confirmation from the
   ACME CA that includes a star-certificate endpoint.

2.2.  Refresh

   The CA issues the initial certificate, typically when the
   authorization is done.  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.sheffer-acme-star-request], obtains the certificate
   (Section 3.3) and uses it.

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

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



















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

                          Figure 1: Auto renewal

2.3.  Termination

   The 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: recurrentOrderCanceled    |
     |<----------------------------------------------+
     |                                               |

                           Figure 2: Termination

3.  Protocol Details

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

3.1.  ACME Extensions

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

3.1.1.  Extending the Order Resource

   The Order resource is extended with the following attributes:

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

   o  recurrent (required, boolean): MUST be true for STAR certificates.
   o  recurrent-start-date (optional, string): the earliest date of
      validity of the first certificate issued, in [RFC3339] format.




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      When omitted, the start date is as soon as authorization is
      complete.
   o  recurrent-end-date (required, string): the latest date of validity
      of the last certificate issued, in [RFC3339] format.
   o  recurrent-certificate-validity (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 which is due to pre-dating.  The
      client can use this value as a hint to configure its polling
      timer.
   o  recurrent-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 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".

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

   A STAR certificate is by definition a mutable resource.  Instead of
   overloading the semantics of the "certificate" key, this document
   defines a new key "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 a Recurrent Order

   An important property of the recurrent 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:











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     POST /acme/order/TOlocE8rfgo HTTP/1.1
     Host: example.org
     Content-Type: application/jose+json

     {
       "protected": base64url({
         "alg": "ES256",
         "kid": "https://example.com/acme/acct/evOfKhNU60wg",
         "nonce": "5XJ1L3lEkMG7tR6pA00clA",
         "url": "https://example.com/acme/order/TOlocE8rfgo"
       }),
       "payload": base64url({
         "status": "canceled"
       }),
       "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
     }

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

   Immediately after the order is canceled, the server:

   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:recurrentOrderCanceled".

   Issuing a cancellation for an order that is not in "valid" state has
   undefined semantics.  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:recurrentCancellationInvalid".

   Explicit certificate revocation using the revokeCert interface
   (Section 7.6 of [I-D.ietf-acme-acme]) 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:recurrentRevocationNotSupported".

3.2.  Capability Discovery

   In order to support the discovery of STAR capabilities, The directory
   object of an ACME STAR server is extended with the following
   attributes inside the "meta" field:



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   o  star-enabled (required, boolean): indicates STAR support.  An ACME
      STAR server MUST include this key, and MUST set it to true if the
      feature is enabled.
   o  star-min-cert-validity (required, integer): minimum acceptable
      value for recurrent-certificate-validity, in seconds.
   o  star-max-renewal (required, integer): maximum delta between
      recurrent-end-date and recurrent-start-date, in seconds.
   o  star-allow-certificate-get (optional, boolean): see Section 3.4.

   An example directory object advertising STAR support with one day
   star-min-cert-validity and one year star-max-renewal, and supporting
   certificate fetching with an HTTP GET:

    {
       "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"],
         "star-enabled": true,
         "star-min-cert-validity": 86400,
         "star-max-renewal":  31536000,
             "star-allow-certificate-get": true
       }
    }

3.3.  Fetching the Certificates

   The certificate is fetched from the star-certificate endpoint with
   POST-as-GET as per [I-D.ietf-acme-acme] 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 the following example:












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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"
     Not-Before: Mon, 1 Feb 2016 00:00:00 GMT
     Not-After: Mon, 8 Feb 2016 00:00:00 GMT

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

   The Server SHOULD include the "Not-Before" and "Not-After" HTTP
   headers 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.

   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 latter also
   expires.  To avoid the client accidentally entering a broken state,
   the "next" certificate MUST be pre-dated so that it is already valid
   when it is published at the "star-certificate" URL.  For further
   discussion on pre-dating, see Section 4.1.

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

   Immediately after the order expires, 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:recurrentOrderExpired".  Note that the
   Order resource's state remains "valid", as per the base protocol.




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3.4.  Negotiate unauthenticated GET

   In order to enable the name delegation workflow defined in
   [I-D.sheffer-acme-star-request] 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 (instead of, or 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 star-allow-certificate-get key to true in the meta field
   of the Directory object:

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

   A client states its will to access the issued star-certificate via
   unauthenticated GET by adding a recurrent-certificate-get key to its
   Order and setting it to true.

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

   If the server accepts the request, it MUST reflect the key in the
   Order.

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, it is RECOMMENDED that site owners pre-date their
   Web facing certificates by 5 to 7 days.  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%.  Note
   that exact choice is also likely to depend on the kind of clients
   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 pre-dating to account for clock
   skews) no longer than 4 days.

4.2.  Impact on Certificate Transparency (CT) Logs

   Provided that the recommendations in Section 4.1 are followed, the
   increase in Certificate Transparency (CT) [RFC6962] log ingestion
   should be one order of magnitude in the worst case compared to the
   current state.

   The input received from most members of the CT community when the
   issue was raised was that this should not represent a problem for the
   CT architecture.

4.3.  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.  Clients and
   servers should consider using the mechanism described in Section 3.4
   to mitigate the risk.





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5.  Implementation Status

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

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

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

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

   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.



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   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.

   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 ).




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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 Error Types

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


















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   +---------------------------------+---------------------+-----------+
   | Type                            | Description         | Reference |
   +---------------------------------+---------------------+-----------+
   | recurrentOrderCanceled          | The short-term      | RFC XXXX  |
   |                                 | certificate is no   |           |
   |                                 | longer available    |           |
   |                                 | because the         |           |
   |                                 | recurrent order has |           |
   |                                 | been explicitly     |           |
   |                                 | canceled by the IdO |           |
   | recurrentOrderExpired           | The short-term      | RFC XXXX  |
   |                                 | certificate is no   |           |
   |                                 | longer available    |           |
   |                                 | because the         |           |
   |                                 | recurrent order has |           |
   |                                 | expired             |           |
   | recurrentCancellationInvalid    | A request to cancel | RFC XXXX  |
   |                                 | a recurrent order   |           |
   |                                 | that is not in      |           |
   |                                 | state "valid" has   |           |
   |                                 | been received       |           |
   | recurrentRevocationNotSupported | A request to revoke | RFC XXXX  |
   |                                 | a recurrent order   |           |
   |                                 | has been received   |           |
   +---------------------------------+---------------------+-----------+

6.2.  New fields in Order Objects

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

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








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

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

          +----------------------------+------------+-----------+
          | Field Name                 | Field Type | Reference |
          +----------------------------+------------+-----------+
          | star-enabled               | boolean    | RCF XXXX  |
          | star-min-cert-validity     | integer    | RCF XXXX  |
          | star-max-renewal           | integer    | RCF XXXX  |
          | star-allow-certificate-get | boolean    | RFC XXXX  |
          +----------------------------+------------+-----------+

6.4.  Not-Before and Not-After HTTP Headers

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

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

7.  Security Considerations

7.1.  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 "recurrent-certificate-validity"
   parameter.  An attacker can use this property to aggressively reduce
   the "recurrent-certificate-validity" (e.g. 1 sec.) jointly with other
   ACME attack vectors identified in Sec. 10 of [I-D.ietf-acme-acme].
   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 recurrent behavior cannot solve
   the above problem.  The CA server needs complementary mitigation and
   specifically, it SHOULD enforce a minimum value on "recurrent-



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   certificate-validity".  Alternatively, the CA can set an internal
   certificate generation processes rate limit.

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 Jon Peterson, Sean Turner and Martin Thomson for helpful
   comments and discussions that have shaped this document.

9.  References

9.1.  Normative References

   [I-D.ietf-acme-acme]
              Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", draft-ietf-acme-acme-16 (work in progress),
              October 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

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

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

   [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>.







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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.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.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <https://www.rfc-editor.org/info/rfc6962>.

   [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>.

   [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.w2spconf.com/2012/papers/w2sp12-final9.pdf>.









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Appendix A.  Document History

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

A.1.  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.2.  draft-ietf-acme-star-03

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

A.3.  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.4.  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.5.  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.6.  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.7.  draft-sheffer-acme-star-01

   o  A terminology section.
   o  Some cleanup.

A.8.  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.9.  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
   Nokia

   EMail: thomas.fossati@nokia.com






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