Network Working Group J. Peterson
Internet-Draft Neustar
Intended status: Standards Track M. Barnes
Expires: January 13, 2022 Independent
D. Hancock
C. Wendt
Comcast
July 12, 2021
ACME Challenges Using an Authority Token
draft-ietf-acme-authority-token-06
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.
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This Internet-Draft will expire on January 13, 2022.
Copyright Notice
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document authors. All rights reserved.
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Provisions Relating to IETF Documents
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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 . . . . . . . . . . . . . . . . . . 4
3.3. Binding Challenges . . . . . . . . . . . . . . . . . . . 5
4. Authority Token Challenge tkauth-type Registration . . . . . 6
5. Acquiring a Token . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Basic REST Interface . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
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
document describes an architecture for Authority Tokens, defines a
the Authority Token format along with a protocol for token
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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", 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. Additionally, this
challenge 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.
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
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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
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.
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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 "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.
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
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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 type 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 REST interface for Authority Token acquisition as
described below, though other interfaces MAY be supported as well.
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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
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.
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6. Acknowledgements
We would like to Roman Danyliw for contributions to this problem
statement and framework.
7. IANA Considerations
This document requests that IANA populate a new ACME Validation
Method (again per [RFC8555]) for the label "tkauth-01", identifier
type "atc", an ACME value of "Y", and a reference pointing to
[RFCThis].
Furthermore, 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]
This document further 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].
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
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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.
[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>.
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[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>.
[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>.
Authors' Addresses
Jon Peterson
Neustar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
US
Email: jon.peterson@team.neustar
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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