| ACME Working Group | 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 |
Use of Short-Term, Automatically-Renewed (STAR) Certificates to Delegate Authority over Web Sites
draft-ietf-acme-star-00
This memo proposes an ACME extension to enable the issuance of short-term and automatically renewed certificates. This allows a domain name owner to delegate the use of certificates to another party, while retaining the capability to cancel this delegation at any time with no need to rely on certificate revocation mechanisms.
[RFC Editor: please remove before publication]
While the draft is being developed, the editor’s version can be found at https://github.com/yaronf/I-D/tree/master/STAR.
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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, CDN providers would rather not have to handle other parties’ long-term secrets. This problem has been discussed at the IETF under the LURK (limited use of remote keys) title.
This document proposes a solution to the above problem that involves the use of short-term certificates with a DNO’s name on them, and a scheme for handling the naming delegation from the DNO to the CDN. The generated short-term credentials are automatically renewed by an ACME Certification Authority (CA) [I-D.ietf-acme-acme] and routinely rotated by the CDN on its edge cache servers. The DNO can end the delegation at any time by simply instructing the CA to stop the automatic renewal and let the certificate expire shortly thereafter.
Using short-term certificates makes revocation cheap and effective [Topalovic] [I-D.iab-web-pki-problems] in case of key compromise or of termination of the delegation; seamless certificate issuance and renewal enable the level of workflow automation that is expected in today’s cloud environments. Also, compared to other keyless-TLS solutions [I-D.cairns-tls-session-key-interface] [I-D.erb-lurk-rsalg], the proposed approach doesn’t suffer from scalability issues or increase in connection setup latency, while requiring virtually no changes to existing COTS caching software used by the CDN.
This document describes the ACME extension. A companion document [I-D.sheffer-acme-star-request] describes how the CDN can request the DNO to initiate the protocol with the ACME server.
A similar use case is that of cloud infrastructure components, such as load balancers and Web Application Firewalls (WAF). These components are typically provisioned with the DNO’s certificate, and similarly to the CDN use case, many organizations would prefer to manage the private key only on their own cloud-based or on-premise hosts, often on Hardware Security Modules (HSMs).
Here again, the STAR solution allows the DNO to delegate authority over the domain to the cloud provider, with the ability to revoke this authority at any time.
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].
For clarity, we describe how the proposed ACME extension can be used in a system that consists of an NDC, an ACME Client (the DNO) and an ACME Server. Only the latter part (ACME Client to ACME Server) is in scope of this document.
The protocol flow can be split into two: a STAR interface, used by NDC and DNO to agree on the name delegation, and the extended ACME interface, used by DNO to obtain the short-term and automatically renewed certificate from the CA, which is eventually consumed by the NDC. The latter is also used to terminate the delegation, if so needed.
Communication between the NDC and the DNO (the STAR interface) is out of scope of this document. It may take the form described in [I-D.sheffer-acme-star-request], some other online protocol, or may even be through manual generation of the CSR.
The following subsections describe the three main phases of the protocol:
This diagram presents the entities involved in the protocol and their interactions during the different phases.
+-----------------+
| STAR Proxy |
| (DNO) |
Bootstrap +-----------------+ Bootstrap
+---------->+ STAR | ACME +-----------+
| | Server | Client | Terminate |
| +--------+--------+ |
| v
+--------+ +--------+
| STAR | Refresh | ACME |
| Client +------------------------------->| Server |
| (NDC) | | (CA) |
+--------+ +--------+
The DNO, in its role as an ACME client, requests the CA to issue a STAR certificate, i.e., one that:
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.)
The bootstrap phase ends when the DNO obtains the OK from the ACME CA.
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:
STAR ACME/STAR
Client Server
| Retrieve cert | [...]
|<--------------------->| |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|<--------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|<--------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| | |
| [...] | [...]
Figure 1: Auto renewal
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:
Note that it is not necessary to explicitly revoke the short-term certificate.
STAR STAR ACME/STAR
Client Proxy Server
| | |
| | Terminate Order ID |
| +---------------------->|
| | +-------.
| | | |
| | | End auto renewal
| | | Remove cert link
| | | etc.
| | | |
| | Done |<------'
| |<----------------------+
| | |
| |
| Retrieve cert |
+---------------------------------------------->|
| Error: terminated |
|<----------------------------------------------+
| |
Figure 2: Termination
This section describes the protocol’s details, namely the extensions to the ACME protocol required to issue STAR certificates.
This protocol extends the ACME protocol, to allow for recurrent orders.
The Order resource is extended with the following attributes:
{
"recurrent": true,
"recurrent-total-lifetime": 365, // requested lifetime of the
// recurrent registration, in days
"recurrent-certificate-validity": 7
// requested validity of each certificate, in days
}
These attributes are included in a POST message when creating the order, as part of the “payload” encoded object. They are returned when the order has been created, and the ACME server MAY adjust them at will, according to its local policy.
An important property of the recurrent Order is that it can be cancelled by the domain name owner, with no need for certificate revocation. We use the DELETE message to cancel the Order:
DELETE /acme/order/1 HTTP/1.1 Host: acme-server.example.org
Which returns:
HTTP/1.1 202 Deleted
The server MUST NOT issue any additional certificates for this Order, beyond the certificate that is available for collection at the time of deletion.
ACME supports sending arbitrary extensions when creating an Order, and as a result, there is no need to explicitly indicate support of this extension. The Proxy MUST verify that the “recurrent” attribute was understood, as indicated by the “recurrent” attribute included in the created Order. Since the standard ACME protocol does not allow to explicitly cancel a pending Order (the DELETE operation above is an extension), a Proxy that encounters an non-supporting server will probably let the Order expire instead of following through with the authorization process.
The DNO MUST restrict the authorizations it requests from the ACME server to only those that cannot be spoofed by a malicious NDC. In most cases the NDC will have strong control of HTTP content under the delegated domain, and therefore HTTPS-based authorization MUST NOT be used. See also Section 5.1.
The certificate is fetched from the certificate endpoint, as per [I-D.ietf-acme-acme], Sec. 7.4.2 “Downloading the Certificate”. The server MUST include an Expires header that indicates expiry of the specific certificate. When the certificate expires, the client MAY assume that a newer certificate is already in place.
A certificate MUST be replaced by its successor at the latest halfway through its lifetime (the period between its notBefore and notAfter times).
TBD: larger logs and how to deal with them.
Currently there are no standard methods for the DNO to ensure that the CDN cannot issue a certificate through mechanisms other than the one described here, for the URLs under the CDN’s control. For example, regardless of the STAR solution, a rogue CDN employee can use the ACME protocol (or proprietary mechanisms used by various CAs) to create a fake certificate for the DNO’s content because ACME authorizes its requests using information that may be under the adversary’s control.
The best solution currently being worked on would consist of several related configuration steps:
This solution is recommended in general, even if an alternative to the mechanism described here is used.
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.
| [I-D.ietf-acme-acme] | Barnes, R., Hoffman-Andrews, J. and J. Kasten, "Automatic Certificate Management Environment (ACME)", Internet-Draft draft-ietf-acme-acme-06, March 2017. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
[[Note to RFC Editor: please remove before publication.]]