Internet DRAFT - draft-sheffer-lurk-cert-delegation
draft-sheffer-lurk-cert-delegation
Network Working Group Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track May 12, 2016
Expires: November 13, 2016
Delegating TLS Certificates to a CDN
draft-sheffer-lurk-cert-delegation-00
Abstract
An organization that owns web content often prefers to delegate
hosting of this content to a Content Delivery Network (CDN). To
serve HTTP content securely, it needs to be protected with TLS. This
document proposes a way for the CDN to request constrained
certificates so that it can serve web content on behalf of the
content owner, without having the owner's long term certificate.
Status of This Memo
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This Internet-Draft will expire on November 13, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Advantages . . . . . . . . . . . . . . . . . . . . . . . 3
3. LURK Operations . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Request a Certificate . . . . . . . . . . . . . . . . . . 4
3.2. Poll for a Certificate . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4.1. Certificate Details . . . . . . . . . . . . . . . . . . . 5
4.2. Revocation . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Restricting CDNs to the Delegation Mechanism . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Normative References . . . . . . . . . . . . . . . . . . 6
5.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Document History . . . . . . . . . . . . . . . . . . 8
A.1. draft-sheffer-lurk-cert-delegation-00 . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Content owners frequently prefer a Content Delivery Network (CDN) to
host their content. CDNs typically have very large networks, and are
designed to serve content to a global audience with a high aggregate
bandwidth.
To protect this traffic, the CDN uses HTTPS and presents a
certificate that usually bears the content owner's name. However,
many content owners balk at sharing their long-term private keys with
another organization.
This document proposes a way for the CDN to obtain short-term
credentials (an end-entity certificate along with the associated
private key), allowing the content owner to revoke this authority at
short notice.
We note that there are other solutions to this problem:
- The CDN could contact the content owner on each TLS handshake and
have the content owner take part in completing the TLS handshake.
Such a solution is described in e.g.
[I-D.cairns-tls-session-key-interface].
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- We could extend ACME [I-D.ietf-acme-acme] by allowing the content
owner to share an authorization "ticket" with the CDN, the CDN
then using it to obtain short-term certificates directly from the
ACME server. This alternative is possibly easier to deploy than
the one described in this document, but it would require a non-
trivial change to the ACME protocol.
- The current proposal has the content owner generate the
certificate's private key, although the best practice would have
the CDN generate it and create a Certificate Signing Request
(CSR). Note however that it would be difficult for the content
owner to validate the correctness of a CSR, potentially allowing a
malicious CDN to obtain fraudulent certificates.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Overview
We define the interaction between the CDN and the content owner,
where the CDN requests a short-term certificate periodically, and the
content owner obtains it on the CDN's behalf and returns it to the
CDN.
We expect the content owner to use the ACME protocol to obtain a
short-term certificate, but this is not strictly required by the
protocol.
2.1. Advantages
- Compared with solutions that require the CDN to have the content
owner sign each handshake, this solution does not require the
content owner to set up its own scalable infrastructure.
- Moreover, the need to scale the content owner's web service could
result in the content owner ending up by sharing the private keys
with the CDN and abdicating its responsibility for its own
security.
3. LURK Operations
This section lists the REST APIs that the content owner needs to
provide to the CDN.
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3.1. Request a Certificate
POST /.well-known/lurk/certificate/1234 HTTP/1.1
Content-Type: application/json
{
"password":"fb2831d6607124286a7b439f2f09793a"
}
There is no negotiation of key type (RSA or ECDSA), key length or
validity dates, and the client and server must coordinate these
details in advance. Similarly, the server MUST be able to determine
the FQDN to be included in the certificate based on the authenticated
client's identity.
The URI contains a request ID, which MAY be sequential or generated
randomly by the client.
The given password MUST be randomly generated and SHOULD have at
least 128-bits of entropy.
The server responds with one of:
- A "200 OK" status code, and response body containing a PKCS #12
[RFC7292] structure (private key and certificate), with the
content type: "application/x-pkcs12". The structure is protected
by the given password.
- A "201 Accepted" status code if the certificate is not yet ready.
The CDN should poll the content owner periodically (see below),
but not more often than once every 5 seconds.
- Other responses if the request is not acceptable or not allowed.
3.2. Poll for a Certificate
GET /.well-known/lurk/certificate/1234 HTTP/1.1
The server responds with one of:
- A "200 OK" status code, and response body containing the PKCS #12
response, with the content type: "application/x-pkcs12".
- A "204 No Content" status code if the certificate is not yet
ready.
- Other responses if the request is not acceptable or not allowed.
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Access to these resources MUST be protected by TLS.
Both requests MUST be authenticated, using one of the following
methods:
- Mutual TLS authentication with a client certificate. This is the
RECOMMENDED option.
- TLS with preshared secret authentication or TLS-SRP.
- TLS with HTTP-Basic or Digest authentication.
The client cannot assume that the sever will cache the certificate
beyond a few seconds after it is first fetched.
4. Security Considerations
This section presents additional considerations beyond those strictly
required by the protocol.
4.1. Certificate Details
- It is RECOMMENDED to restrict the certificate's scope as much as
possible. Specifically, the certificate request SHOULD specify
restrictive Key Usage.
- The certificate SHOULD NOT be for a wildcard DN.
- The RECOMMENDED validity period for certificates provisioned using
this mechanism is 3 days, and the certificate SHOULD be valid
immediately when it is fetched.
4.2. Revocation
When the content owner decides it no longer trusts the CDN, the
content owner MUST:
- Revoke any extant short-term certificates already handed to the
CDN. This implies that all such certificates MUST be logged.
- Immediately block the certificate issuance operations described
above.
4.3. Restricting CDNs to the Delegation Mechanism
Currently there are no standard methods for the content owner to
ensure that the CDN cannot issue a certificate through mechanisms
other than the one described here, for the URLs under the CDN's
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control. The best solution currently being worked on would consist
of several related configuration steps:
- Make sure that the CDN cannot modify the DNS records for the
domain. Typically this would mean that the content owner
establishes a CNAME resource record from a subdomain into a CDN-
managed domain.
- Restrict certificate issuance for the domain to specific CAs that
comply with ACME. This assumes universal deployment of CAA
[RFC6844] by CAs, which is not the case yet.
- Deploy ACME-specific methods to restrict issuance to a specific
authorization key which is controlled by the content owner
[I-D.landau-acme-caa].
This solution is recommended in general, even if an alternative to
the mechanism described here (e.g.
[I-D.cairns-tls-session-key-interface]) is used.
5. References
5.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7292] Moriarty, K., Ed., Nystrom, M., Parkinson, S., Rusch, A.,
and M. Scott, "PKCS #12: Personal Information Exchange
Syntax v1.1", RFC 7292, DOI 10.17487/RFC7292, July 2014,
<http://www.rfc-editor.org/info/rfc7292>.
5.2. Informative References
[I-D.cairns-tls-session-key-interface]
Cairns, K., Mattsson, J., Skog, R., and D. Migault,
"Session Key Interface (SKI) for TLS and DTLS", draft-
cairns-tls-session-key-interface-01 (work in progress),
October 2015.
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf-
acme-acme-02 (work in progress), March 2016.
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[I-D.landau-acme-caa]
Landau, H., "ACME Account Key Binding via CAA Records",
draft-landau-acme-caa-00 (work in progress), April 2016.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
<http://www.rfc-editor.org/info/rfc6844>.
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Appendix A. Document History
A.1. draft-sheffer-lurk-cert-delegation-00
Initial version.
Author's Address
Yaron Sheffer
Intuit
EMail: yaronf.ietf@gmail.com
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