Internet Engineering Task Force (IETF) J. Peterson
Request for Comments: 9447 M. Barnes
Category: Standards Track Neustar
ISSN: 2070-1721 D. Hancock
C. Wendt
Somos
September 2023
Automated Certificate Management Environment (ACME) Challenges Using an
Authority Token
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 that supports subtype claims for
different identifiers or namespaces that can be defined separately
for specific applications.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9447.
Copyright Notice
Copyright (c) 2023 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
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Table of Contents
1. Introduction
2. Requirements Language
3. ACME Authority Token Challenge
3.1. Token Type Requirements
3.2. Authority Token Scope
3.3. Binding Challenges
4. Authority Token Challenge tkauth-type Registration
5. Acquiring a Token
5.1. Basic REST Interface
6. IANA Considerations
6.1. ACME Validation Method Registration
6.2. JSON Web Token Claim Registration
6.3. Creation of ACME Authority Token Challenge Types Registry
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
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 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 an Authority Token that authorizes a given
issuance. 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 document describes an
architecture for Authority Tokens, defines a JSON Web Token (JWT)
[RFC7519] Authority Token format along with a protocol for token
acquisition, and shows how to integrate these tokens into an ACME
challenge.
As a concrete example, [RFC9448] 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. Requirements Language
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 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 an ACME
server 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 certification
authorities (CAs) and Token Authorities, i.e., 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.
The ACME Authority Token Challenge type, "tkauth-01", is here
specified for use with the "TNAuthList" (Telephone Number
Authentication List) ACME Identifier Type described in [RFC9448]; in
order to use the "tkauth-01" Validation Method with an ACME
Identifier Type other than "TNAuthList", that identifier type would
need to be listed in a new registration in the ACME Validation
Methods registry maintained by 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 6.3). 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
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 for which certificate issuance is authorized.
Some types of Authority Token types might be reusable for a number of
different namespaces; others 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 an authorized 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 that
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.
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 an Authority Token that has the exact same
scope as the requested certificate to a client; 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 a Token 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", and 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 token), 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 about 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. It is RECOMMENDED that the ACME account
fingerprint be used for this purpose.
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,
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 or other factors for this purpose is left to the identifier
type profile for the Authority Token.
Note that the fingerprint value in the client's JWT is reflected in
the Authority Token returned by the Token Authority; the Token
Authority has no requirement to validate that fingerprint. Were a
fingerprint to be captured by an attacker that had its own account
with the Token Authority, it could replay that fingerprint in its own
JWT in order to receive an Authority Token with that fingerprint.
However, were the attacker to present that Authority Token to an ACME
service, the service would see the fingerprint does not match the
attacker's ACME account fingerprint. So unless an attacker can
compromise a target ACME account or gain similar privileges, the
binding would be secure.
4. Authority Token Challenge tkauth-type Registration
This document specifies a tkauth-type of "atc", which contains a
standard JWT [RFC7519] using a signature string defined by a JSON Web
Signature (JWS) [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 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 6.3.
For this ACME Authority Token usage of a JWT, it is OPTIONAL for the
payload of the JWT to contain an "iss", indicating the Token
Authority that generated the token if the "x5u" or "x5c" 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, when present. 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 when the same "atc" Authority Token
is being used for multiple challenges over time within its set
lifetime.
The JWT payload MUST also contain a new JWT claim, "atc", for
Authority Token Challenge, which contains three mandatory elements in
a JSON map: 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 identifier type and value are those given
in the ACME challenge and conveyed to the Token Authority by the ACME
client. 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 (see Section 3.3).
The "tkvalue" indicates the scope of the authority that the token and
its semantics are outside the scope of this document, as they will be
specified by the "tkvalue" identifier in a separate specification.
Following the example of [RFC9448], the "tktype" identifier type
could be the TNAuthList (as defined in [RFC8226]), which would be the
value for the "tkvalue" element 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. 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 boolean 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 (using [RFC9115] or similar mechanisms). 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.
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 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 a HTTPS POST request [RFC9110]. This specification
assumes that Token Authority URIs are known to clients through
preexisting business relationships and that the credentials and
related authentication and authorization for Authority Token
acquisition are 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
Note that ":id" here is a placeholder for an actual account
identifier. The body of the POST request MUST contain the Authority
Token Challenge element (the key "atc", colon, and its value) that
the client is requesting the Token Authority generate. In this 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
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 Section 5.5 of
[RFC9448].
6. IANA Considerations
6.1. ACME Validation Method Registration
IANA has added a new ACME Validation Method (per [RFC8555]) in the
"ACME Validation Methods" subregistry of the "Automated Certificate
Management Environment (ACME) Protocol" registry group as follows:
Label: tkauth-01
Identifier Type: TNAuthList
ACME: Y
Reference: RFC 9447
6.2. JSON Web Token Claim Registration
IANA has added 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: IETF
Specification document(s): RFC 9447
6.3. Creation of ACME Authority Token Challenge Types Registry
IANA has created 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 three
columns: Label, Description, and Reference. The initial content of
the registry is as follows:
Label: atc (as defined in Section 4)
Description: JSON Web Token (JWT) challenge type
Reference: RFC 9447
7. 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. A
preexisting trust relationship between the HTTPS REST client and the
Token Authority must also exist in order for the parties to
meaningfully authenticate one another. The security considerations
of [RFC8555] apply to the use of the mechanism in this specification.
Implementations should follow the best practices identified in
[RFC8725].
As described in Section 3.2, an Authority Token can either have a
scope that attests all of the resources that 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 applications, it may make
sense for that expiry to be quite short. ACME services relying on
Authority Tokens SHOULD NOT issue certificates with a longer expiry
than the expiry of the Authority Token. Any protocol used to
retrieve Authority Tokens from a Token Authority MUST use
confidentiality to prevent eavesdroppers from acquiring an Authority
Token. The details of this protocol are out of the scope of this
specification.
This document only specifies SHA256 for the fingerprint hash.
However, the syntax of the fingerprint object would permit other keys
if, due to concerns about algorithmic agility, a more robust
algorithm were required at a future time. Future specifications can
define new keys for the fingerprint object as needed.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC8725] Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
Current Practices", BCP 225, RFC 8725,
DOI 10.17487/RFC8725, February 2020,
<https://www.rfc-editor.org/info/rfc8725>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC9448] Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
"TNAuthList Profile of Automated Certificate Management
Environment (ACME) Authority Token", RFC 9448,
DOI 10.17487/RFC9448, September 2023,
<https://www.rfc-editor.org/info/rfc9448>.
8.2. 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>.
[RFC9115] Sheffer, Y., López, D., Pastor Perales, A., and T.
Fossati, "An Automatic Certificate Management Environment
(ACME) Profile for Generating Delegated Certificates",
RFC 9115, DOI 10.17487/RFC9115, September 2021,
<https://www.rfc-editor.org/info/rfc9115>.
Acknowledgements
We would like to Roman Danyliw and Ben Kaduk for contributions to
this problem statement and framework.
Authors' Addresses
Jon Peterson
Neustar, Inc.
Email: jon.peterson@team.neustar
Mary Barnes
Neustar, Inc.
Email: mary.ietf.barnes@gmail.com
David Hancock
Somos
Email: davidhancock.ietf@gmail.com