Network Working Group J. Peterson Internet-Draft Neustar Intended status: Informational M. Barnes Expires: April 25, 2019 iconectiv D. Hancock C. Wendt Comcast October 22, 2018 ACME Challenges Using an Authority Token draft-ietf-acme-authority-token-01.txt Abstract A number of proposed challenges for the Automated Certificate Management Environment (ACME) effectively rely on an external authority issuing 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 of this Authority Token challenge. 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 25, 2019. 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 Peterson, et al. Expires April 25, 2019 [Page 1] Internet-Draft ACME Authority Token October 2018 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Challenges for an Authority Token . . . . . . . . . . . . . . 3 3.1. Token Type Requirements . . . . . . . . . . . . . . . . . 4 3.2. Authority Token Scope . . . . . . . . . . . . . . . . . . 4 3.3. Binding Challenges . . . . . . . . . . . . . . . . . . . 5 4. Registration . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. 'ATC' Token Type . . . . . . . . . . . . . . . . . . . . 7 5. Acquiring a Token . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 9. Informative References . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction ACME [I-D.ietf-acme-acme] 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. 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. 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 [I-D.ietf-stir-certificates] by consulting an external Peterson, et al. Expires April 25, 2019 [Page 2] Internet-Draft ACME Authority Token October 2018 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 [I-D.ietf-modern-problem-framework]. Another, similar example would a mechanism that permits CSPs to delegate authority for particular telephone numbers to customers, as described in [I-D.ietf-acme-telephone]. 2. Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119]. 3. Challenges for an Authority Token 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. There has been considerable interest in using ACME to issue certificates associated with telephone numbers and service provider identifiers used in the telephone network, for example. 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 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. ACME challenges that support Authority Tokens therefore need to specify the type of token they require; CAs can even provide a hint in their challenges to ACME clients that tells them how to find a Peterson, et al. Expires April 25, 2019 [Page 3] Internet-Draft ACME Authority Token October 2018 Token Authority who can issue tokens for a given namespace. This challenge type thus requires a new "tkauth-type" element, and may optionally supply a "token-authority" designating a location where tokens can be acquired. The set of "tkauth-type" values and the semantic requirements for those tokens are tracked by an IANA registry. 3.1. Token Type Requirements The IANA will control 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. 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 that the Token Authority is attesting that the ACME client controls. In most cases, an ACME client will 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 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. 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. A Token Authority could grant to a client a 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. Peterson, et al. Expires April 25, 2019 [Page 4] Internet-Draft ACME Authority Token October 2018 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 Authority. When the client sends an acquisition request (see Section 5) to the Authority, the 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 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 an 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 an Authority; an ACME server can use the binding to determine that a Token presented by a client was in fact granted by the Authority based on a request from the client, and not from some other entity. Binding 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 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. Peterson, et al. Expires April 25, 2019 [Page 5] Internet-Draft ACME Authority Token October 2018 4. Registration This draft registers a tkauth-type of "ATC", for the Authority Token Challenge, a JWT usage which is further documented below. Taking the identifier example of TNAuthList from [I-D.ietf-acme-authority-token-tnauthlist], an ACME for this tkauth- type challenge might for example look as follows: HTTP/1.1 200 OK Content-Type: application/json Link: ;rel="directory" { "status": "pending", "identifier": { "type": "TNAuthList", "value": "F83n2a...avn27DN3==" }, "challenges": [ { "type": "tkauth-01", "tkauth-type": "ATC", "token-authority": "https://authority.example.org/authz", "url": "https://boulder.example.com/authz/asdf/0" "token": "IlirfxKKXAsHtmzK29Pj8A" } ], } Entities receiving this challenge know that they can, as a proof, acquire an ATC token from the designated Token Authority (specified in the "token-authority" field), and that this authority can provide tokens corresponding to the identifier type of "TNAuthList". Once the ATC has been acquired by the ACME Client, it can be posted back to the URL given by the ACME challenge. Peterson, et al. Expires April 25, 2019 [Page 6] Internet-Draft ACME Authority Token October 2018 POST /acme/authz/asdf/0 HTTP/1.1 Host: boulder.example.com Content-Type: application/jose+json { "protected": base64url({ "alg": "ES256", "kid": "https://boulder.example.com/acme/reg/asdf", "nonce": "Q_s3MWoqT05TrdkM2MTDcw", "url": "https://boulder.example.com/acme/authz/asdf/0" }), "payload": base64url({ "ATC": "evaGxfADs...62jcerQ" }), "signature": "5wUrDI3eAaV4wl2Rfj3aC0Pp--XB3t4YYuNgacv_D3U" } The "ATC" field in this response contains the Authority Token. 4.1. 'ATC' Token Type This specification pre-populates the tkauth-type registry with a type for "ATC". Here the "ATC" tkauth-type signifies a standard JWT token [RFC7519] using a JWS-defined signature string [RFC7515]. This may be used for any number of different identifier types given in ACME challenges. The "atc" element (defined below) lists the identifier type used by tokens based on ATC. The use of "ATC" is restricted to JWTs, if non- JWT tokens were desired for ACME challenges, a different tkauth-type should be defined for them. 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. 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 elements in an array: the identifier type, the identifier value, and the binding. The identifier type and value are those given in the ACME challenge and Peterson, et al. Expires April 25, 2019 [Page 7] Internet-Draft ACME Authority Token October 2018 conveyed to the Token Authority by the ACME client. Again, following the example of [I-D.ietf-acme-authority-token-tnauthlist], this could be the TNAuthList, as defined in [RFC8226], that the Token Authority is attesting. Practically speaking, that may contain 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 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: { "typ":"JWT", "alg":"ES256", "x5u":"https://authority.example.org/cert"} { "iss":"https://authority.example.org/authz", "exp":1300819380, "jti":"id6098364921", "atc":{"TnAuthList","F83n2a...avn27DN3==", "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"} } 5. Acquiring a Token [TBD. Show protocol flow for token acquisition.] 5.1. Example [TBD.] 6. Acknowledgements We would like to thank you for your contributions to this problem statement and framework. 7. IANA Considerations Future versions of this specification will include registrations for the ACME Challenge type registries here. It will also create a registry for "token types" as used in these challenges, following the requirements in Section 3.1, pre-populated with the value for "ATC" per Section 4.1. Peterson, et al. Expires April 25, 2019 [Page 8] Internet-Draft ACME Authority Token October 2018 8. Security Considerations 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 authority; here that is the fingerprint specified in Section 4.1). 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 an authority MUST use confidentiality to prevent eavesdroppers from acquiring an Authority Token. More TBD. 9. Informative 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. [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-00 (work in progress), July 2018. [I-D.ietf-acme-service-provider] Barnes, M. and C. Wendt, "ACME Identifiers and Challenges for VoIP Service Providers", draft-ietf-acme-service- provider-02 (work in progress), October 2017. [I-D.ietf-acme-star] Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T. Fossati, "Support for Short-Term, Automatically-Renewed (STAR) Certificates in Automated Certificate Management Environment (ACME)", draft-ietf-acme-star-04 (work in progress), October 2018. [I-D.ietf-acme-telephone] Peterson, J. and R. Barnes, "ACME Identifiers and Challenges for Telephone Numbers", draft-ietf-acme- telephone-01 (work in progress), October 2017. Peterson, et al. Expires April 25, 2019 [Page 9] Internet-Draft ACME Authority Token October 2018 [I-D.ietf-modern-problem-framework] Peterson, J. and T. McGarry, "Modern Problem Statement, Use Cases, and Framework", draft-ietf-modern-problem- framework-04 (work in progress), March 2018. [I-D.ietf-stir-certificates] Peterson, J. and S. Turner, "Secure Telephone Identity Credentials: Certificates", draft-ietf-stir- certificates-18 (work in progress), December 2017. [I-D.ietf-stir-passport] Wendt, C. and J. Peterson, "Personal Assertion Token (PASSporT)", draft-ietf-stir-passport-11 (work in progress), February 2017. [I-D.ietf-stir-rfc4474bis] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt, "Authenticated Identity Management in the Session Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-16 (work in progress), February 2017. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure Telephone Identity Problem Statement and Requirements", RFC 7340, DOI 10.17487/RFC7340, September 2014, . [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015, . [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, . [RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity Credentials: Certificates", RFC 8226, DOI 10.17487/RFC8226, February 2018, . Peterson, et al. Expires April 25, 2019 [Page 10] Internet-Draft ACME Authority Token October 2018 Authors' Addresses Jon Peterson Neustar, Inc. 1800 Sutter St Suite 570 Concord, CA 94520 US Email: jon.peterson@team.neustar Mary Barnes iconectiv 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 Peterson, et al. Expires April 25, 2019 [Page 11]