ACME Challenges Using an Authority Token
draft-ietf-acme-authority-token-07

Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                               M. Barnes
Expires: April 28, 2022                                      Independent
                                                              D. Hancock
                                                                C. Wendt
                                                                 Comcast
                                                        October 25, 2021

                ACME Challenges Using an Authority Token
                   draft-ietf-acme-authority-token-07

Abstract

   Some proposed extensions to the Automated Certificate Management
   Environment (ACME) rely on proving eligibility for certificates
   through consulting an external authority that issues a token
   according to a particular policy.  This document specifies a generic
   Authority Token challenge for ACME which supports subtype claims for
   different identifiers or namespaces that can be defined separately
   for specific applications.

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
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 28, 2022.

Copyright Notice

   Copyright (c) 2021 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

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  ACME Authority Token Challenge  . . . . . . . . . . . . . . .   3
     3.1.  Token Type Requirements . . . . . . . . . . . . . . . . .   4
     3.2.  Authority Token Scope . . . . . . . . . . . . . . . . . .   5
     3.3.  Binding Challenges  . . . . . . . . . . . . . . . . . . .   5
   4.  Authority Token Challenge tkauth-type Registration  . . . . .   6
   5.  Acquiring a Token . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Basic REST Interface  . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  ACME Validation Method Registration . . . . . . . . . . .   9
     7.2.  JSON Web Token Claim Registration . . . . . . . . . . . .   9
     7.3.  Creation of ACME Authority Token Challenge Type Registry    9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   ACME [RFC8555] is a mechanism for automating certificate management
   on the Internet.  It enables administrative entities to prove
   effective control over resources like domain names, and automates the
   process of generating and issuing certificates that attest control or
   ownership of those resources.

   In some cases, proving effective control over an identifier requires
   an attestation from a third party who has authority over the
   resource, for example, an external policy administrator for a
   namespace other than the DNS application ACME was originally designed
   to support.  In order to automate the process of issuing certificates
   for those resources, this specification defines a generic Authority
   Token challenge that ACME servers can issue in order to require
   clients to return such a token.  The challenge contains a type
   indication that tells the client what sort of token it needs to
   acquire.  It is expected that the Authority Token challenge will be
   usable for a variety of identifier types.  In particular, this

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   document describes an architecture for Authority Tokens, defines a
   the Authority Token format along with a protocol for token
   acquisition, and shows how to integrate these tokens into an ACME
   challenge.

   For example, the system of [I-D.ietf-acme-authority-token-tnauthlist]
   provides a mechanism that allows service providers to acquire
   certificates corresponding to a Service Provider Code (SPC) as
   defined in [RFC8226] by consulting an external authority responsible
   for those codes.  Furthermore, Communications Service Providers
   (CSPs) can delegate authority over numbers to their customers, and
   those CSPs who support ACME can then help customers to acquire
   certificates for those numbering resources with ACME.  This can
   permit number acquisition flows compatible with those shown in
   [RFC8396].

2.  Terminology

   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.

3.  ACME Authority Token Challenge

   Proving that a device on the Internet has effective control over a
   non-Internet resource is not as straightforward as proving control
   over an Internet resources like a DNS zone or a web page.  Provided
   that the issuer of identifiers in a namespace, or someone acting on
   the issuer's behalf, can implement a service that grants Authority
   Tokens to the people to whom it has issued identifiers, a generic
   token could be used as a response to an ACME challenge.  This
   specification, therefore, defines an Authority Token issued by an
   authority over a namespace to an ACME client for delivery to a CA in
   response to a challenge.  Authority over a hierarchical namespace can
   also be delegated, so that delegates of a root authority can
   themselves act as Token Authorities for certain types of names.

   This architecture assumes a trust relationship between CAs and Token
   Authorities: that CAs are willing to accept the attestation of Token
   Authorities for particular types of identifiers as sufficient proof
   to issue a credential.  It furthermore assumes that ACME clients have
   a relationship with Token Authorities which permits them to
   authenticate and authorize the issuance of Authority Tokens to the
   proper entities.  This ACME challenge has no applicability to
   identifiers or authorities where those pre-associations cannot be
   assumed.

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   The ACME Authority Token Challenge type, "tkauth-01", is here
   specified for use with the "TNAuthList" ACME Identifier Type
   described in [I-D.ietf-acme-authority-token-tnauthlist]; in order to
   use "tkauth-01" Validation Method with an ACME Identifier type other
   than "TNAuthList," that identifier type would need to be registered
   separately with IANA. "tkauth-01" furthermore supports different
   token subtypes.  The token subtype is determined by a new ACME
   challenge field, tkauth-type.  An IANA registry is used to manage the
   values of tkauth-type, see Section 7.3.  Additionally, this challenge
   type also has a new "token-authority" field to designate a location
   where a token can be acquired.

3.1.  Token Type Requirements

   The IANA will maintain a registry of tkauth-types under a policy of
   Specification Required.  In order to register a new tkauth-type,
   specifications must address the following requirements; in cases
   where a tkauth-type admits of its own subtypes, subtypes inherit
   these requirements.

   While Authority Token types do not need to be specific to a
   namespace, every token must carry enough information for a CA to
   determine the name that it will issue a certificate for.  Some types
   of Authority Token types might be reusable for a number of different
   namespaces; other might be specific to a particular type of name.
   Therefore, in defining tkauth-types, future specifications must
   indicate how a token conveys to the CA the name(s) that the Token
   Authority is attesting that the ACME client controls.

   While nothing precludes use cases where an ACME client is itself a
   Token Authority, an ACME client will typically need a protocol to
   request and retrieve an Authority Token.  The Token Authority will
   require certain information from an ACME client in order to ascertain
   that it is the right entity to request a certificate for a particular
   name.  The protocols used to request an Authority Token MUST convey
   to the Token Authority the identifier type and value from or what
   will be used in the ACME challenge, as well as the binding (see
   Section 3.3), and those MUST be reflected in the Authority Token.  A
   baseline mechanism for how the Token Authority authenticates and
   authorizes ACME clients to receive Authority Tokens is given in
   Section 5.

   Because the assignment of resources can change over time,
   demonstrations of authority must be regularly refreshed.  Definitions
   of a tkauth-type MUST specify how they manage the freshness of
   authority assignments.  Typically, a CA will expect a regular
   refreshing of the token.

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3.2.  Authority Token Scope

   An Authority Token is used to answer a challenge from an ACME server,
   upon a request for the issuance of a certificate.  It could be that
   the Authority Token is requested from the Token Authority after a
   challenge has been received, or it could be that the Authority Token
   was acquired prior to the initial ACME client request.  A Token
   Authority could grant to a client an Authority Token that has the
   exact same scope as the requested certificate; alternatively, an
   Authority Token could attest to all of the resources that the client
   is eligible to receive certificates for, which could be a superset of
   the scope of the requested certificate.

   For example, imagine a case where an Authority for DNS names knows
   that a client is eligible to receive certificates for "example.com"
   and "example.net".  The client asks an ACME server for a certificate
   for "example.com", the server directs the client to acquire an
   Authority Token from the Token Authority.  When the client sends an
   acquisition request (see Section 5) to the Token Authority, the Token
   Authority could issue a token scoped just to "example.com", or a
   token that attests the client is eligible to receive certificates for
   both "example.com" or "example.net".  The advantage of the latter is
   that if, at a later time (but one within the expiry of the JWT), the
   client wanted to acquire a certificate for "example.net", it would
   not have to return to the Token Authority, as the Token effectively
   pre-authorized the issuance of that certificate.

   Applications of the Authority Token to different identifier types
   might require different scopes, so registrations of tkauth-types
   should be clear if and how a scope greater than that of the requested
   certificate would be conveyed in a token.

3.3.  Binding Challenges

   Applications that use the Authority Token need a way to correlate
   tokens issued by a Token Authority with the proper ACME client, to
   prevent replay or cut-and-paste attacks using a token issued for a
   different purpose.  To mitigate this, Authority Tokens contain a
   binding signed by a Token Authority; an ACME server can use the
   binding to determine that a Token presented by a client was in fact
   granted by the Token Authority based on a request from the client,
   and not from some other entity.

   Creating a binding from an Authority Token to a particular ACME
   account entails that the Token could be reused up until its expiry
   for multiple challenges issued by an ACME server.  This might be a
   desirable property when using short-lived certificates, for example,
   or in any cases where the ACME server issues challenges more

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   frequently that an Authority Token can or should issue tokens, or in
   cases where the Authority Token scope (see Section 3.2) is broad, so
   certificates with a more narrow scope may periodically be issued.

   For some identifier types, it may be more appropriate to bind the
   Authority Token to a nonce specific to the challenge rather than to
   an ACME account fingerprint.  Any specification of the use of the
   nonce for this purpose is left to the identifier type profile for the
   Authority Token.

4.  Authority Token Challenge tkauth-type Registration

   This draft specifies a tkauth-type of "atc" which contains a standard
   JWT token [RFC7519] using a JWS-defined signature string [RFC7515].
   The "atc" tkauth-type MAY be used for any number of different ACME
   identifier types in the ACME challenge.

   A new JWT claim, "atc", is defined below and lists the identifier
   type used in this Authority Token.  The "atc" tkauth-type is
   restricted to the JWTs; if a non-JWT token format is desired for the
   ACME Authority Token Challenge, a different tkauth-type should be
   specified and registered in the "ACME Authority Token Challenge
   Types" registry defined in Section 8.

   For this ACME Authority Token usage of JWT, the payload of the JWT
   OPTIONALLY contain an "iss" indicating the Token Authority that
   generated the token, if the "x5u" element in the header does not
   already convey that information; typically, this will be the same
   location that appeared in the "token-authority" field of the ACME
   challenge.  In order to satisfy the requirement for replay prevention
   the JWT MUST contain a "jti" element, and an "exp" claim; the "exp"
   claim manages token freshness.  In addition to helping to manage
   replay, the "jti" provides a convenient way to reliably track with
   the same "atc" Authority Token is being used for multiple challenges
   over time within its set expiry.

   The JWT payload MUST also contain a new JWT claim, "atc", for
   Authority Token Challenge, which contains three mandatory elements in
   an array: the ATC identifier type ("tktype"), the identifier value
   ("tkvalue"), and the binding ("fingerprint").  The use of "tktype" is
   restricted to the values in the "ACME Identifier Types" registry as
   defined by [RFC8555].  The "tkvalue" indicates the scope of the
   authority that the token, and its semantics are outside the scope of
   this document.  The identifier type and value are those given in the
   ACME challenge and conveyed to the Token Authority by the ACME
   client.

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   Following the example of [I-D.ietf-acme-authority-token-tnauthlist],
   the "tkvalue" identifier type could be the TNAuthList, with a
   "tkvalue" as defined in [RFC8226] that the Token Authority is
   attesting.  Practically speaking, that scope may comprise a list of
   Service Provider Code elements, telephone number range elements, and/
   or individual telephone numbers.  For the purposes of the "atc"
   tkauth-type, the binding "fingerprint" is assumed to be a fingerprint
   of the ACME credential for the account used to request the
   certificate, but the specification of how the binding is generated is
   left to the identifier type profile for the Authority Token.  So for
   example:

        {
    "protected": base64url({
         "typ":"JWT",
     "alg":"ES256",
     "x5u":"https://authority.example.org/cert"
        }),
    "payload": base64url({
         "iss":"https://authority.example.org/authz",
     "exp":1300819380,
     "jti":"id6098364921",
     "atc":{"tktype":"TnAuthList","tkvalue":"F83n2a...avn27DN3==","fingerprint":
     "SHA256 56:3E:CF:AE:83:CA:4D:15:B0:29:FF:1B:71:D3:BA:B9:19:81:F8:50:
     9B:DF:4A:D4:39:72:E2:B1:F0:B9:38:E3"}
        }),
     "signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
        }

   Optionally, the "atc" claim may contain a fourth element, "ca".  If
   set to "true", the "ca" element indicates that the Token Authority is
   granting permission to issue a certification authority certificate
   rather than an end-entity certificate for the names in question.
   This permits subordinate delegations from the issued certificate.  If
   the "ca" element is absent, the Token Authority is explicitly
   withholding permission.  The "atc" object in the example above would
   then look like:

   "atc":{"tktype":"TnAuthList","tkvalue":"F83n2a...avn27DN3==","ca":true,
   "fingerprint":"SHA256 56:3E:CF:AE:83:CA:4D:15:B0:29:FF:1B:71:D3:BA:B9:19:81:F8:50:
   9B:DF:4A:D4:39:72:E2:B1:F0:B9:38:E3"} }

   Specifications of "tktype" identifier types may define additional
   optional "atc" elements.

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5.  Acquiring a Token

   The acquisition of an Authority Token requires a network interface,
   apart from potential use cases where the entity that acts as an ACME
   client itself also acts as a Token Authority trusted by the ACME
   server.  Implementations compliant with this specification MUST
   support an HTTPS REST interface for Authority Token acquisition as
   described below, though other interfaces MAY be supported as well.

5.1.  Basic REST Interface

   In order to request an Authority Token from a Token Authority, a
   client sends an HTTPS POST request.  This specification assumes that
   Token Authority URIs are known to clients through preexisting
   business relationships, and that credentials and related
   authentication and authorization for Authority Token acquisition
   encompassed in that relationship.  Different services may organize
   their web resources in domain-specific ways, but the resource locator
   should specify the account of the client, an identifier for the
   service provider, and finally a locator for the token.

      POST /at/account/:id/token HTTP/1.1
      Host: authority.example.com
      Content-Type: application/json

   The body of the POST request MUST contain the Authority Token
   Challenge element that the client is requesting the Token Authority
   generate.  In the way, the client proposes the scope of the Authority
   Token it would like to receive from the Token Authority.

   In common use cases, the "tkvalue" in this request is asking that the
   Token Authority issue a token that attests the entire scope of
   authority to which the client is entitled.  The client may also
   request an Authority Token with some subset of its own authority via
   the "tkvalue" element in the Authority Token Challenge object.  The
   way that "tkvalue" is defined will necessarily be specific to the
   identifier type.  For the TNAuthlist identifier type, for example, an
   object requesting an Authority Token could request authority for only
   a single telephone number in a way that is defined in the TNAuthList
   specification.

   Finally, the JSON object MAY also contain an optional boolean element
   "ca" which signifies that the client is requesting that the Token
   Authority issue an Authority Token with the "ca" flag set, as
   described in Section 4.

   After an HTTPS-level challenge (e.g. a 401 HTTP response code) to
   verify the identity of the client and subsequently making an

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   authorization decision about whether the client should receive an
   Authority Token with the requested scope, then in the success case,
   the Token Authority MUST return a 200 OK with a body of type
   "application/json" containing the Authority Token.

   A full example of "atc" token acquisition using the HTTP interface,
   with the "tktype" of "TNAuthList", is given in
   [I-D.ietf-acme-authority-token-tnauthlist] Section 5.5.

6.  Acknowledgements

   We would like to Roman Danyliw for contributions to this problem
   statement and framework.

7.  IANA Considerations

7.1.  ACME Validation Method Registration

   This document requests that IANA populate a new ACME Validation
   Method (again per [RFC8555]) for the label "tkauth-01", identifier
   type "TNAuthList", an ACME value of "Y", and a reference pointing to
   [RFCThis].

7.2.  JSON Web Token Claim Registration

   This document asks IANA to populate a new claim in the "JSON Web
   Token Claims" registry as defined in [RFC7519] as follows:

      Claim name: atc

      Claim Description: Authority Token Challenge

      Change Controller: IESG

      Specification document(s): [RFCThis]

7.3.  Creation of ACME Authority Token Challenge Type Registry

   This document requests that the IANA create a new registry for "ACME
   Authority Token Challenge Types" as used in these challenges, under a
   policy of Specification Required and following the requirements in
   Section 3.1, with two columns, Label and Reference.  The registry
   should be pre-populated with a Label of "atc" per Section 4 with a
   Reference value of [RFCThis].

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8.  Security Considerations

   Per the guidance in [RFC8555], ACME transactions MUST use TLS, and
   similarly the HTTPS REST transactions used to request and acquire
   Authority Tokens MUST use TLS.  These measures are intended to
   prevent the capture of Authority Tokens by eavesdroppers.  The
   security considerations of [RFC8555] apply to the use of the
   mechanism in this specification.

   As described in Section 3.2, an Authority Token can either have a
   scope that attests all of the resources which a client is eligible to
   receive certificates for, or potentially a more limited scope that is
   intended to capture only those resources for which a client will
   receive a certificate from a particular certification authority.  Any
   certification authority that sees an Authority Token can learn
   information about the resources a client can claim.  In cases where
   this incurs a privacy risk, Authority Token scopes should be limited
   to only the resources that will be attested by the requested ACME
   certificate.

   In cases where a tkauth-type as defined in Section 4 admits of its
   own subtypes, the security of features like binding challenges (see
   Section 3.3) will depend on the subtype specification.

   The capture of Authority Tokens by an adversary could enable an
   attacker to acquire a certificate from a CA.  Therefore, all
   Authority Tokens MUST contain a field that identifies to the CA which
   ACME client requested the token from the Token Authority; here that
   is the fingerprint specified in Section 4).  All Authority Tokens
   must specify an expiry (of the token itself as proof for a CA, as
   opposed to the expiry of the name), and for some application, it may
   make sense of that expiry to be quite short.  Any protocol used to
   retrieve Authority Tokens from a Token Authority MUST use
   confidentiality to prevent eavesdroppers from acquiring an Authority
   Token.

9.  References

9.1.  Normative References

   [I-D.ietf-acme-authority-token-tnauthlist]
              Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
              "TNAuthList profile of ACME Authority Token", draft-ietf-
              acme-authority-token-tnauthlist-08 (work in progress),
              March 2021.

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   [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>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

9.2.  Informative References

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/info/rfc8226>.

   [RFC8396]  Peterson, J. and T. McGarry, "Managing, Ordering,
              Distributing, Exposing, and Registering Telephone Numbers
              (MODERN): Problem Statement, Use Cases, and Framework",
              RFC 8396, DOI 10.17487/RFC8396, July 2018,
              <https://www.rfc-editor.org/info/rfc8396>.

Authors' Addresses

   Jon Peterson
   Neustar, Inc.
   1800 Sutter St Suite 570
   Concord, CA  94520
   US

   Email: jon.peterson@team.neustar

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   Mary Barnes
   Independent

   Email: mary.ietf.barnes@gmail.com

   David Hancock
   Comcast

   Email: davidhancock.ietf@gmail.com

   Chris Wendt
   Comcast
   One Comcast Center
   Philadelphia, PA  19103
   USA

   Email: chris-ietf@chriswendt.net

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