ACME Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track D. Lopez
Expires: December 18, 2017 O. Gonzalez de Dios
A. Pastor Perales
Telefonica I+D
T. Fossati
Nokia
June 16, 2017
Generating Certificate Requests for Short-Term, Automatically-Renewed
(STAR) Certificates
draft-sheffer-acme-star-request-01
Abstract
This memo proposes a protocol that allows a domain name owner to
delegate to a third party (such as a CDN) control over a certificate
that bears one or more names in that domain. Specifically the third
party creates a Certificate Signing Request for the domain, which can
then be used by the domain owner to request a short term and
automatically renewed (STAR) certificate.
This is a component in a solution where a third-party such as a CDN
can terminate TLS sessions on behalf of a domain name owner (e.g., a
content provider), and the domain owner can cancel this delegation at
any time without having to rely on certificate revocation mechanisms.
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 http://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 December 18, 2017.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Conventions used in this document . . . . . . . . . . . . 4
2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Preconditions . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Bootstrap . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Refresh . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Termination . . . . . . . . . . . . . . . . . . . . . . . 7
3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 8
3.1. STAR API . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.1. Creating a Registration . . . . . . . . . . . . . . . 8
3.1.2. Polling the Registration . . . . . . . . . . . . . . 9
3.2. Transport Security for the STAR Protocol . . . . . . . . 10
4. CDNI Use Cases . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Multiple Parallel Delegates . . . . . . . . . . . . . . . 10
4.2. Chained Delegation . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5.1. STAR Protocol Authentication . . . . . . . . . . . . . . 11
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-request-01 . . . . . . . . . . . 13
A.2. draft-sheffer-acme-star-request-00 . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
This document is a companion document to [I-D.ietf-acme-star]. To
avoid duplication, we give here a barebones 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].
A content provider (referred to in this document as Domain Name
Owner, DNO) has agreements in place with one or more Content Delivery
Networks (CDNs) that are contracted to serve its content over HTTPS.
The CDN terminates the HTTPS connection at one of its edge cache
servers and needs to present its clients (browsers, set-top-boxes) a
certificate whose name matches the authority of the URL that is
requested, i.e. that of the DNO. However, many DNOs balk at sharing
their long-term private keys with another organization and, equally,
delegates (henceforth referred to as NDC, Name Delegation Consumer)
would rather not have to handle other parties' long-term secrets.
This document describes a protocol where the DNO and the NDC agree on
a CSR template and the NDC generates a CSR for a private key that it
holds. The DNO then uses the ACME protocol (as extended in
[I-D.ietf-acme-star] to issue the STAR certificate.
The generated short-term certificate is automatically renewed by an
ACME Certification Authority (CA) [I-D.ietf-acme-acme] and routinely
fetched into the NDC and used for HTTPS connections. The DNO can end
the delegation at any time by simply instructing the CA to stop the
automatic renewal and letting the certificate expire shortly
thereafter.
1.1. Terminology
DNO Domain Name Owner, the owner of a domain that needs to be
delegated.
NDC Name Delegation Consumer, the entity to which the domain name is
delegated for a limited time. This is often a CDN (in fact,
readers may note the similarity 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
[RFC2119].
2. Protocol Flow
This section presents the protocol flow. For completeness, we
include the STAR Interface proposed in this draft, as well as the
extended ACME protocol as described in [I-D.ietf-acme-star].
2.1. Preconditions
The protocol assumes the following preconditions are met:
o A mutually authenticated channel between NDC and DNO pre-exists.
This is called "STAR channel" and all STAR protocol exchanges
between NDC and DNO are run over it. It provides the guarantee
that requests and responses are authentic.
o NDC and DNO 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 DNO has registered through the ACME interface exposed by the
Certificate Authority (CA) using the usual ACME registration
procedure. In ACME terms, the DNO has an Account on the server
and is ready to issue Orders.
2.2. Bootstrap
The NDC (STAR Client) generates a key-pair, wraps it into a
Certificate Signing Request (CSR) according to the agreed upon CSR
template, and sends it to the DNO (STAR Proxy) over the pre-
established STAR channel. The DNO uses the NDC identity provided on
the STAR channel to look up the CSR template that applies to the
requesting NDC and decides whether or not to accept the request.
Assuming everything is in order, it then "forwards" the NDC request
to the ACME CA by means of the usual ACME application procedure.
Specifically, the DNO, in its role as an ACME client, requests the CA
to issue a STAR certificate, i.e., one that:
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o Has a short validity (e.g., 24 to 72 hours);
o Is automatically renewed by the CA for a certain period of time;
o Is downloadable from a (highly available) public link without
requiring any special authorization.
Other than that, the ACME protocol flows as normal between DNO and
CA, in particular DNO is responsible for satisfying the requested
ACME challenges until the CA is willing to issue the requested
certificate. Per normal ACME processing, the DNO is given back an
Order ID for the issued STAR certificate to be used in subsequent
interaction with the CA (e.g., if the certificate needs to be
terminated.)
Concurrently, a response is sent back to the NDC with an endpoint to
poll for completion of the certificate generation process.
The bootstrap phase ends when the DNO obtains the OK from the ACME CA
and posts the certificate's URL to the "completion endpoint" where
the NDC can retrieve it.
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...........................
STAR : STAR Proxy / : ACME/STAR
Client : ACME Client : Server
| : | | : |
| : | | ACME registration |
+-------. : | |<--------------------->|
| | : | | STAR capabilities |
| generate CSR : | | : |
| | : | | : |
|<------' : | | : |
| : | | : |
| Request new : | | : |
+---------------------->| | : |
| cert for CSR : | | : |
| : +-------. | : |
| : | | | : |
| : | Verify CSR | : |
| : | | | : |
| : +<------' | : |
| Accepted, poll at | | : |
|<----------------------+ | : |
| "completion URL" |- - - - - - - >| Application for |
| : | +---------------------->|
| : | | STAR certificate |
| : | | : |
| GET "completion URL" | | : Challenge |
|<--------------------->| |<--------------------->|
| in progress : | | : Response |
| : | | : |
| : | | Finalize/Certificate |
| : | |<----------------------+
| GET "completion URL" |< - - - - - - -| : + Order Id |
+---------------------->| | : |
| : | | : |
| 200, certificate URL | | : |
|<----------------------+ | : |
| and other metadata | | : |
| : | | : |
`.........................'
Figure 1: Bootstrap
2.3. Refresh
The CA automatically re-issues the certificate (using the same CSR)
before it expires and publishes it to the URL that the NDC has come
to know at the end of the bootstrap phase. The NDC downloads and
installs it. This process goes on until either:
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o DNO terminates the delegation, or
o Automatic renewal expires.
STAR ACME/STAR
Client Server
| Retrieve cert | [...]
|<--------------------->| |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|<--------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|<--------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| | |
| [...] | [...]
Figure 2: Auto renewal
2.4. Termination
The DNO may request early termination of the STAR certificate by
including the Order ID in a certificate termination request to the
ACME interface, defined below. After the CA receives and verifies
the request, it shall:
o Cancel the automatic renewal process for the STAR certificate;
o Change the certificate publication resource to return an error
indicating the termination of the delegation to external clients,
including the NDC.
Note that it is not necessary to explicitly revoke the short-term
certificate.
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STAR STAR ACME/STAR
Client Proxy Server
| | |
| | Terminate Order ID |
| +---------------------->|
| | +-------.
| | | |
| | | End auto renewal
| | | Remove cert link
| | | etc.
| | | |
| | Done |<------'
| |<----------------------+
| | |
| |
| Retrieve cert |
+---------------------------------------------->|
| Error: terminated |
|<----------------------------------------------+
| |
Figure 3: Termination
3. Protocol Details
This section describes the STAR API between the STAR Client and the
STAR Proxy.
3.1. STAR API
This API allows the STAR Client to request a STAR certificate via the
STAR Proxy, using a previously agreed-upon CSR template.
The API consists of a single resource, "registration". A new
Registration is created with a POST request, and the Registration
instance is polled to obtain its details.
3.1.1. Creating a Registration
To create a registration, use:
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POST /star/registration
Host: star-proxy.example.net
Content-Type: application/json
{
"csr": "...", // CSR in PEM format
"lifetime": 365, // requested registration lifetime in days,
// between 1 and 1095
"validity": 7 // requested certificate validity in days
}
The STAR Proxy MAY treat both "lifetime" and "validity" periods as
hints. Upon success, the call returns the new Registration resource.
HTTP/1.1 201 Created
Location: https://star-proxy.example.net/star/registration/567
3.1.2. Polling the Registration
The returned Registration can be polled until the information is
available from the ACME server.
GET /star/registration/567
Host: star-proxy.example.net
In responding to poll requests while the validation is still in
progress, the server MUST return a 200 (OK) response and MAY include
a Retry-After header field to suggest a polling interval to the
client. The Retry-After value MUST be expressed in seconds. If the
Retry-After header is present, in order to avoid surprising
interactions with heuristic expiration times, a max-age Cache-Control
SHOULD also be present and set to a value slightly smaller than the
Retry-After value.
HTTP/1.1 200 OK
Retry-After: 10
Cache-Control: max-age=9
{
"status": "pending"
}
When the operation is successfully completed, the ACME Proxy returns:
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HTTP/1.1 200 OK
Expires: Sun, 09 Sep 2018 14:09:00 GMT
{
"status": "valid", // or "failed"
"lifetime": 365, // lifetime of the registration in days,
// possibly less than requested
"certificates": "https://acme-server.example.org/certificates/A51A3"
}
The Expires header applies to the Registration resource itself, and
may be as small as a few minutes. It is unrelated to the Order's
lifetime which is measured in days or longer. The "certificates"
attribute contains a URL of the certificate pull endpoint, received
from the ACME Server.
If the registration fails for any reason, the server returns a "200
OK" response, with the status as "failed" and a "reason" attribute
containing a human readable error message.
3.2. Transport Security for the STAR Protocol
Traffic between the STAR Client and the STAR Proxy MUST be protected
with HTTPS. For interoperability, all implementations MUST support
HTTP Basic Authentication [RFC7617]. However some deployments MAY
prefer mutually- authenticated HTTPS or two-legged OAUTH.
4. 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 compares several solutions.
This section discusses two particular requirements in the context of
the STAR protocol.
4.1. Multiple Parallel Delegates
In some cases the DNO would like to delegate authority over a web
site to multiple CDNs. This could happen if the DNO has agreements
in place with different regional CDNs for different geographical
regions. STAR enables this use case naturally, since each CDN can
authenticate separately to the DNO specifying its CSR, and the DNO is
free to allow or deny each certificate request according to its own
policy.
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4.2. Chained Delegation
In other cases, a content owner (DNO) delegates some domains to a
large CDN (CDN1), which in turn delegates to a smaller regional CDN,
CDN2. The DNO has a contractual relationship with CDN1, and CDN1 has
a similar relationship with CDN2. However DNO may not even know
about CDN2.
The STAR protocol does not prevent this use case, although there is
no special support for it. CDN1 can forward requests from CDN2 to
DNO, and forward responses back to CDN2. Whether such proxying is
allowed is governed by policy and contracts between the parties.
5. Security Considerations
5.1. STAR Protocol Authentication
The STAR protocol allows its client to obtain certificates bearing
the DNO's identity. Therefore strong client authentication is
mandatory.
When multiple NDCs may connect to the same DNO, the STAR protocol's
authentication MUST allow the DNO to distinguish between different
NDCs, and the DNO MUST associate different Registration objects to
different clients. Among other benefits, this allows the DNO to
cancel a STAR registration for one of its clients instead of all of
them.
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., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf-
acme-acme-06 (work in progress), March 2017.
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[I-D.ietf-acme-star]
Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T.
Fossati, "Use of Short-Term, Automatically-Renewed (STAR)
Certificates to Delegate Authority over Web Sites", draft-
ietf-acme-star-00 (work in progress), June 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme",
RFC 7617, DOI 10.17487/RFC7617, September 2015,
<http://www.rfc-editor.org/info/rfc7617>.
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-01 (work in
progress), March 2017.
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Appendix A. Document History
[[Note to RFC Editor: please remove before publication.]]
A.1. draft-sheffer-acme-star-request-01
o Correct reference to WG draft.
A.2. draft-sheffer-acme-star-request-00
o Initial version, the STAR API extracted from draft-sheffer-acme-
star-02.
Authors' Addresses
Yaron Sheffer
Intuit
EMail: yaronf.ietf@gmail.com
Diego Lopez
Telefonica I+D
EMail: diego.r.lopez@telefonica.com
Oscar Gonzalez de Dios
Telefonica I+D
EMail: oscar.gonzalezdedios@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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