Internet DRAFT - draft-sheffer-acme-star-delegation
draft-sheffer-acme-star-delegation
ACME Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track D. Lopez
Expires: May 17, 2019 A. Pastor Perales
Telefonica I+D
T. Fossati
Nokia
November 13, 2018
An ACME Profile for Generating Delegated STAR Certificates
draft-sheffer-acme-star-delegation-01
Abstract
This memo proposes a profile of the ACME protocol that allows the
owner of an identifier (e.g., a domain name) to delegate to a third
party access to a certificate associated with said identifier. A
primary use case is that of a CDN (the third party) terminating TLS
sessions on behalf of a content provider (the owner of a domain
name). The presented mechanism allows the owner of the identifier to
retain control over the delegation and revoke it at any time by
cancelling the associated STAR certificate renewal with the ACME CA.
Another key property of this mechanism is it does not require any
modification to the deployed TLS ecosystem.
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 May 17, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Conventions used in this document . . . . . . . . . . . . 4
2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Preconditions . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Delegated Identity Profile . . . . . . . . . . . . . . . 6
2.3.1. Order Object on the NDC-IdO side . . . . . . . . . . 6
2.3.2. Order Object on the IdO-CA side . . . . . . . . . . . 9
2.3.3. Capability Discovery . . . . . . . . . . . . . . . . 9
2.3.4. On Cancelation . . . . . . . . . . . . . . . . . . . 9
3. CDNI Use Cases . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Multiple Parallel Delegates . . . . . . . . . . . . . . . 9
3.2. Chained Delegation . . . . . . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
4.1. New fields in the "meta" Object within a Directory Object 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5.1. Restricting CDNs to the Delegation Mechanism . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Document History . . . . . . . . . . . . . . . . . . 13
A.1. draft-sheffer-acme-star-delegation-01 . . . . . . . . . . 13
A.2. draft-sheffer-acme-star-delegation-00 . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This document is a companion document to [I-D.ietf-acme-star]. To
avoid duplication, we give here a bare-bones description of the
motivation for this solution. For more details and further use
cases, please refer to the introductory sections of
[I-D.ietf-acme-star].
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An Identifier Owner (IdO), that we can associate in the primary use
case to a content provider (also referred to as Domain Name Owner,
DNO), has agreements in place with one or more NDC (Name Delegation
Consumer) to use and attest its identity. In the primary use case,
we consider a CDN provider contracted to serve the IdO content over
HTTPS. The CDN terminates the HTTPS connection at one of its edge
cache servers and needs to present its clients (browsers, mobile
apps, set-top-boxes) a certificate whose name matches the authority
of the URL that is requested, i.e., that of the IdO. Understandably,
most IdOs balk at sharing their long-term private keys with another
organization and, equally, delegates would rather not have to handle
other parties' long-term secrets.
This document describes a profile of the ACME protocol
[I-D.ietf-acme-acme] that allows the NDC to request the IdO, acting
as a profiled ACME server, a certificate for a delegated identity -
i.e., one belonging to the IdO. The IdO then uses the ACME protocol
(with the extensions described in [I-D.ietf-acme-star]) to request
issuance of a STAR certificate for the same delegated identity. The
generated short-term certificate is automatically renewed by the ACME
Certification Authority (CA), periodically fetched by the NDC and
used to terminate HTTPS connections in lieu of the IdO. The IdO can
end the delegation at any time by simply instructing the CA to stop
the automatic renewal and letting the certificate expire shortly
thereafter.
In case the delegated identity is a domain name, this document also
provides a way for the NDC to inform the IdO about the CNAME mappings
that need to be installed in the IdO's DNS zone to enable the
aliasing of the delegated name, thus allowing the complete name
delegation workflow to be handled using a single interface.
1.1. Terminology
IdO Identifier Owner, the owner of an identifier (e.g., a domain
name) that needs to be delegated.
DNO Domain Name Owner, a specific kind of IdO whose identifier is a
domain name
NDC Name Delegation Consumer, the entity to which the domain name is
delegated for a limited time. This is a CDN in the primary use
case (in fact, readers may note the symmetry of the two acronyms).
CDN Content Delivery Network, a widely distributed network that
serves the domain's web content to a wide audience at high
performance.
STAR Short-Term, Automatically Renewed X.509 certificates.
ACME The IETF Automated Certificate Management Environment, a
certificate management protocol.
CA A Certificate Authority that implements the ACME protocol.
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1.2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Protocol Flow
This section presents the protocol flow. For completeness, we
include the ACME profile proposed in this draft as well as the
extended ACME protocol described in [I-D.ietf-acme-star].
2.1. Preconditions
The protocol assumes the following preconditions are met:
o The IdO exposes an ACME server interface to the NDC(s) comprising
the account management interface;
o The NDC has registered an ACME account with the IdO;
o NDC and IdO have agreed on a "CSR template" to use, including at a
minimum: subject name (e.g., "somesite.example.com"), requested
algorithms, key length, key usage. The NDC is required to use
this template for every CSR created under the same delegation;
o IdO has registered an ACME account with the Certificate Authority
(CA)
Note that even if the IdO implements the ACME server role, it is not
acting as a CA: in fact, from the point of view of the certificate
issuance process, the IdO only works as a "policing" forwarder of the
NDC's key-pair and is responsible for completing the identity
verification process towards the ACME CA.
2.2. Overview
The interaction between the NDC and the IdO is governed by the
profiled ACME workflow detailed in Section 2.3. The interaction
between the IdO and the CA is ruled by ACME STAR
[I-D.ietf-acme-star].
The outline of the combined protocol is as follow (Figure 1):
o NDC sends an Order for the delegated identifier to IdO;
o IdO creates an Order resource in state "ready" with a "finalize"
URL;
o NDC immediately sends a finalize request (which includes the CSR)
to the IdO;
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o IdO verifies the CSR according to the agreed CSR template;
o If the CSR verification fails, the Order is moved to an "invalid"
state and everything stops;
o If the CSR verification is successful, IdO moves the Order to
state "processing", and sends an Order' (using its own account)
for the delegated identifier to the ACME STAR CA;
o If the ACME STAR protocol fails, Order' moves to "invalid" and the
same state is reflected in the NDC Order;
o If the ACME STAR run is successful (i.e., Order' is "valid"), IdO
copies the "star-certificate" URL from Order' to Order and moves
its state "valid".
The NDC can now download, install and use the certificate bearing the
name delegated by the IdO.
Note that, because the identity validation is suppressed, the NDC
sends the finalize request, including the CSR, to the IdO immediately
after the Order has been acknowledged. The IdO must buffer a (valid)
CSR until the Validation phase completes successfully.
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NDC IdO CA
Client Server Client Server
Order
Signature ------->
[ No identity validation ]
CSR
Signature ------->
Order'
Signature ------->
<------- Required
Authorizations
Responses
Signature ------->
<~~~~~~~~Validation~~~~~~~~>
CSR
Signature ------->
<~~~~~~Await issuance~~~~~~> <~~~~~~Await issuance~~~~~~>
<------------------------------------ Certificate
Figure 1: End to end flow
2.3. Delegated Identity Profile
2.3.1. Order Object on the NDC-IdO side
The Order object created by the NDC:
o MUST contain identifiers with the new "delegated" field set to
true;
o MUST NOT contain the notBefore and notAfter fields;
o MAY contain any of the "recurrent-*" fields listed in
Section 3.1.1 of [I-D.ietf-acme-star];
o In case the identifier type is "dns", it MAY contain a "cname"
field with the alias of the identifier in the NDC domain. This
field is used by the IdO to create the DNS aliasing needed to
redirect the resolvers to the delegated entity.
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POST /acme/new-order HTTP/1.1
Host: acme.dno.example
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://acme.dno.example/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://acme.dno.example/acme/new-order"
}),
"payload": base64url({
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.dno.example.",
"delegated": true,
"cname": "abc.ndc.example."
}
],
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
The Order object that is created on the IdO:
o MUST start in the "ready" state;
o MUST contain an "authorizations" array with zero elements;
o MUST NOT contain the "notBefore" and "notAfter" fields.
{
"status": "ready",
"expires": "2016-01-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.dno.example.",
"delegated": true,
"cname": "abc.ndc.example."
}
],
"authorizations": [],
"finalize": "https://acme.dno.example/acme/order/TO8rfgo/finalize"
}
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The IdO SHOULD copy any "recurrent-*" field from the NDC request into
the related STAR request to the ACME CA.
When the validation of the identifiers has been successfully
completed and the certificate has been issued by the CA, the IdO:
o MUST move its Order resource status to "valid";
o MUST copy the "star-certificate" field from the STAR Order;
The latter indirectly includes (via the NotBefore and NotAfter HTTP
headers) the renewal timers needed by the NDC to inform its
certificate reload logic.
{
"status": "valid",
"expires": "2016-01-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.dno.example.",
"delegated": true,
"cname": "abc.ndc.example."
}
],
"authorizations": [],
"finalize": "https://acme.dno.example/acme/order/TO8rfgo/finalize",
"star-certificate": "https://acme.ca.example/acme/order/yTr23sSDg9"
}
If an "identifier" object of type "dns" was included, the IdO MUST
validate the specified CNAME at this point in the flow. The NDC and
IdO may have a pre-established list of valid CNAME values. At the
minimum, the IdO MUST verify that both DNS names are syntactically
valid.
Following this validation, the IdO can add the CNAME records to its
zone:
abc.ndc.dno.example. CNAME abc.ndc.example.
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2.3.2. Order Object on the IdO-CA side
When sending the Order to the ACME CA, the IdO SHOULD strip the
"delegated" and "cname" attributes sent by the NDC (Section 2.3.1).
The IdO MUST add the necessary STAR extensions to the Order. In
addition, to allow the NDC to download the certificate using
unauthenticated GET, the IdO MUST add the recurrent-certificate-get
attribute and set it to true.
2.3.3. Capability Discovery
In order to help a client to discover support for this profile, the
directory object of an ACME server MUST contain the following
attribute inside the "meta" field:
o star-delegation-enabled: boolean flag indicating support for the
profile specified in this memo. An ACME server that supports this
delegation profile MUST include this key, and MUST set it to true.
2.3.4. On Cancelation
It is worth noting that cancelation of the ACME STAR certificate is a
prerogative of the IdO. The NDC does not own the relevant account
key on the ACME CA, therefore it can't issue a cancelation request
for the STAR cert. Potentially, since it holds the STAR cert private
key, it could request the revocation of a single STAR certificate.
However, STAR explicitly disables the revokeCert interface.
3. CDNI Use Cases
Members of the IETF CDNI (Content Delivery Network Interconnection)
working group are interested in delegating authority over web content
to CDNs. Their requirements are described in a draft
[I-D.fieau-cdni-https-delegation] that considers several solutions
addressing different delegation requirements. This section discusses
two of these particular requirements in the context of the STAR
delegation workflow.
3.1. Multiple Parallel Delegates
In some cases the content owner (IdO) would like to delegate
authority over a web site to multiple NDCs (CDNs). This could happen
if the IdO has agreements in place with different regional CDNs for
different geographical regions, or if a "backup" CDN is used to
handle overflow traffic by temporarily altering some of the CNAME
mappings in place. The STAR delegation flow enables this use case
naturally, since each CDN can authenticate separately to the IdO (via
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its own separate account) specifying its CSR, and the IdO is free to
allow or deny each certificate request according to its own policy.
3.2. Chained Delegation
In other cases, a content owner (IdO) delegates some domains to a
large CDN (uCDN), which in turn delegates to a smaller regional CDN,
dCDN. The DNO has a contractual relationship with uCDN, and uCDN has
a similar relationship with dCDN. However IdO may not even know
about dCDN.
The STAR protocol does not prevent this use case, although there is
no special support for it: uCDN could forward requests from dCDN to
DNO, and forward responses back to dCDN. Whether such proxying is
allowed is governed by policy and contracts between the parties.
One thing that might be necessary at the interface between uCDN and
dCDN is a mechanism by which the uCDN can advertise:
o The namespace that is made available to the dCDN to mint its
delegated names;
o The policy for creating the key material (allowed algorithms,
minimum key lengths, key usage, etc.) that the dCDN needs to
satisfy.
4. IANA Considerations
[[RFC Editor: please replace XXXX below by the RFC number.]]
4.1. New fields in the "meta" Object within a Directory Object
This document adds the following entries to the ACME Directory
Metadata Fields:
+-------------------------+------------+-----------+
| Field Name | Field Type | Reference |
+-------------------------+------------+-----------+
| star-delegation-enabled | boolean | RFC XXXX |
+-------------------------+------------+-----------+
5. Security Considerations
5.1. Restricting CDNs to the Delegation Mechanism
When a web site is delegated to a CDN, the CDN can in principle
modify the web site at will, create and remove pages. This means
that a malicious or breached CDN can pass the ACME (as well as common
non-ACME) HTTPS-based validation challenges and generate a
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certificate for the site. This is true regardless of whether the
CNAME mechanisms defined in the current document is used or not.
In some cases, this is the desired behavior: the domain owner trusts
the CDN to have full control of the cryptographic credentials for the
site. The current document however assumes that the domain owner
only wants to delegate restricted control, and wishes to retain the
capability to cancel the CDN's credentials at a short notice.
To restrict certificate delegation only to the protocol defined here:
o The domain owner MUST make sure that the CDN cannot modify the DNS
records for the domain. The domain owner should ensure it is the
only entity authorized to modify the DNS zone. Typically, it
establishes a CNAME resource record from a subdomain into a CDN-
managed domain.
o The domain owner MUST use a CAA record [RFC6844] to restrict
certificate issuance for the domain to specific CAs that comply
with ACME.
o The domain owner MUST use the ACME-specific CAA mechanism
[I-D.ietf-acme-caa] to restrict issuance to a specific account key
which is controlled by it, and MUST require "dns-01" as the sole
validation method.
6. Acknowledgments
This work is partially supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI). This support does not imply
endorsement.
7. References
7.1. Normative References
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", draft-ietf-acme-acme-16 (work in progress),
October 2018.
[I-D.ietf-acme-caa]
Landau, H., "CAA Record Extensions for Account URI and
ACME Method Binding", draft-ietf-acme-caa-05 (work in
progress), June 2018.
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[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.
[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>.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
<https://www.rfc-editor.org/info/rfc6844>.
[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>.
7.2. Informative References
[I-D.fieau-cdni-https-delegation]
Fieau, F., Emile, S., and S. Mishra, "HTTPS delegation in
CDNI", draft-fieau-cdni-https-delegation-02 (work in
progress), July 2017.
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Appendix A. Document History
[[Note to RFC Editor: please remove before publication.]]
A.1. draft-sheffer-acme-star-delegation-01
o Added security considerations about disallowing CDNs from issuing
certificates for a delegated domain.
A.2. draft-sheffer-acme-star-delegation-00
o Initial version, some text extracted from draft-sheffer-acme-star-
requests-02
Authors' Addresses
Yaron Sheffer
Intuit
EMail: yaronf.ietf@gmail.com
Diego Lopez
Telefonica I+D
EMail: diego.r.lopez@telefonica.com
Antonio Agustin Pastor Perales
Telefonica I+D
EMail: antonio.pastorperales@telefonica.com
Thomas Fossati
Nokia
EMail: thomas.fossati@nokia.com
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