Internet DRAFT - draft-sheffer-acme-star-lurk
draft-sheffer-acme-star-lurk
ACME Working Group Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track D. Lopez
Expires: April 28, 2017 O. Gonzalez de Dios
Telefonica I+D
T. Fossati
Nokia
October 25, 2016
Use of Short-Term, Automatically-Renewed (STAR) Certificates to address
the LURK problem
draft-sheffer-acme-star-lurk-00
Abstract
This memo proposes two mechanisms that work in concert to address the
LURK problem statement, allowing a third party (e.g., a content
delivery network) to terminate TLS sessions on behalf of a domain
name owner (e.g., a content provider).
The proposed mechanisms are:
1. An extension to the ACME protocol to enable the issuance of
short-term and automatically renewed certificates, and
2. A protocol that allows a domain name owner to delegate to a third
party control over a certificate that bears its own name.
It should be noted that these are in fact independent building blocks
that could be used separately to solve completely different problems.
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
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This Internet-Draft will expire on April 28, 2017.
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Copyright Notice
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Table of Contents
1. A Solution for the HTTPS CDN Use Case . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Preconditions . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Bootstrap . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Refresh . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Termination . . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4.1. Restricting CDNs to the Delegation Mechanism . . . . . . 8
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Normative References . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. A Solution for the HTTPS CDN Use Case
A content provider, and Domain Name Owner (DNO), has agreements in
place with one or more Content Delivery Networks (CDN) 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
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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 after.
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.
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].
TODO: glossary.
3. Protocol Flow
The protocol flow can be split into two: a LURK interface, used by
CDN and DNO to agree on the name delegation, and the ACME STAR
interface, used by DNO to obtain the short-term and automatically
renewed certificate from the CA, which is eventually consumed by the
CDN. The latter is also used to terminate the delegation, if so
needed.
The following subsections describe the preconditions (Section 3.1),
and the three main phases of the protocol:
o Bootstrap: the CDN requests from the DNO the delegation of a
specific name and in turn DNO asks an ACME CA to create the
corresponding short-term and auto-renewed (STAR) certificate
(Section 3.2);
o Auto-renewal: the ACME CA periodically re-issues the short-term
certificate and posts it to a public URL (Section 3.3);
o Termination: the DNO (indirectly) stops name delegation by
explicitly requesting the ACME CA to discontinue the automatic
renewal of the certificate (Section 3.4).
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3.1. Preconditions
The protocol assumes the following preconditions are met:
o A mutually authenticated channel between CDN and DNO pre-exists.
This is called "LURK channel" and all LURK protocol exchanges
between CDN and DNO are run over it. It provides the guarantee
that the LURK requests and responses are authentic [[_1: Note
that, under this assumption, the key used to authenticate the CDN
to the DNO becomes a critical asset for the security of the
proposed protocol, and that certain interactions (e.g., CSR
submission) might require a stronger authentication mechanism.
For example, stacking a further authentication factor on top of
CDN's LURK key would allow to distinguish an attacker that has
only managed to successfully attack the CDN's LURK key from the
legitimate CDN. --tf]].
o CDN and DNO have agreed on a "CSR template" to use, including at a
minimum:
- Subject name (e.g., "somesite.DNO.com"),
- Validity (e.g., 24 to 72 hours),
- Requested algorithms,
- Key length,
- Key usage.
The CDN 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. The DNO shall, at the registration stage, query the
ACME server for the supported STAR capabilities - for example: the
minimum validity period of the issued certificate, the maximum
duration of the automatic renewal process (either as a maximum
number of renewal events, or as its maximum absolute life-span).
3.2. Bootstrap
CDN (LURK client) generates a key-pair, wraps it into a Certificate
Signing Request (CSR) according to the agreed CSR template, and sends
it to the DNO (LURK server) over the pre-established LURK channel.
The DNO uses the CDN identity provided on the LURK channel to look up
the CSR template that applies to the requesting CDN and decides
whether or not to accept the request. (TBD: This is probably a case
that would require a further authentication stage over the one
provided by the mutual-authenticated LURK channel?) Assuming
everything is in order, it then "forwards" the CDN request to the
ACME CA by means of the usual ACME application procedure.
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Specifically, DNO, in its role as a ACME/STAR client, requests the CA
a STAR certificate, i.e., one that:
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 authorisation.
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. The DNO is given back a unique identifier 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 202 response has been sent back to the CDN 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 CDN can retrieve it. The information that is passed on to the
CDN at this stage also includes details about how much time before
the certificate expires can the CDN expect the replacement to be
ready.
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...........................
LURK : LURK ACME/STAR: ACME/STAR
Client : Server 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 |
|<--------------------->| |<--------------------->|
| 202, in progress | | : Response |
| : | | : |
| : | | Finalize/Certificate |
| : | |<----------------------+
| GET "completion URL" |< - - - - - - -| : + STAR Id |
+---------------------->| | : |
| : | | : |
| 200, certificate URL | | : |
|<----------------------+ | : |
| and other metadata | | : |
| : | | : |
`.........................'
Figure 1: Bootstrap
3.3. Refresh
The CA automatically re-issues the certificate (using the same CSR)
before it expires and publishes it to the URL that the CDN has come
to know at the end of the bootstrap phase. The CDN downloads and
installs it. This process goes on until either:
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o DNO terminates the delegation, or
o Automatic renewal expires.
LURK ACME/STAR
Client Server
| Retrieve cert | [...]
|<--------------------->| |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|<--------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|<--------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| | |
| [...] | [...]
Figure 2: Auto renewal
3.4. Termination
DNO requests termination of the STAR certificate by including the
previously obtained identifier in a STAR certificate termination
request to the ACME interface. After CA receives and verifies the
request, it shall:
o Cancel the automatic renewal process for the LURK certificate;
o Change the certificate publication resource to return an error
indicating the termination of the delegation to external clients,
including the CDN;
Note that it is not necessary to explicitly revoke the short-term
certificate.
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LURK ACME/STAR ACME/STAR
Client Client Server
| | |
| | Terminate STAR Id |
| +---------------------->|
| | +-------.
| | | |
| | | End auto renewal
| | | Remove cert link
| | | etc.
| | | |
| | Done |<------'
| |<----------------------+
| | |
| |
| Retrieve cert |
+---------------------------------------------->|
| Error: terminated |
|<----------------------------------------------+
| |
Figure 3: Termination
4. Security Considerations
o CDN's client certificate key is first order security asset and
MUST be protected. Absent 2FA/MFA, an attacker that can
compromise the key might be able to obtain certificates bearing
DNO's identity.
o Consider collusion of two or more CDNs with contracts with the
same DNO (?)
4.1. Restricting CDNs to the Delegation Mechanism
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.
The best solution currently being worked on would consist of several
related configuration steps:
o 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.
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o 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.
o Deploy ACME-specific methods to restrict issuance to a specific
authorization key which is controlled by the content owner
[I-D.landau-acme-caa], and/or to specific ACME authorization
methods.
This solution is recommended in general, even if an alternative to
the mechanism described here is used.
5. 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.
6. References
6.1. Normative References
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf-
acme-acme-03 (work in progress), July 2016.
[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>.
6.2. 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.erb-lurk-rsalg]
Erb, S. and R. Salz, "A PFS-preserving protocol for LURK",
draft-erb-lurk-rsalg-01 (work in progress), May 2016.
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[I-D.iab-web-pki-problems]
Housley, R. and K. O'Donoghue, "Problems with the Public
Key Infrastructure (PKI) for the World Wide Web", draft-
iab-web-pki-problems-03 (work in progress), September
2016.
[I-D.landau-acme-caa]
Landau, H., "CA Account URI Binding for CAA Records",
draft-landau-acme-caa-01 (work in progress), October 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>.
[Topalovic]
Topalovic, E., Saeta, B., Huang, L., Jackson, C., and D.
Boneh, "Towards Short-Lived Certificates", 2012,
<http://www.w2spconf.com/2012/papers/w2sp12-final9.pdf>.
Authors' Addresses
Yaron Sheffer
Intuit
EMail: yaronf.ietf@gmail.com
Diego Lopez
Telefonica I+D
EMail: diego@telefonica.es
Oscar Gonzalez de Dios
Telefonica I+D
EMail: oscar.gonzalezdedios@telefonica.com
Thomas Fossati
Nokia
EMail: thomas.fossati@nokia.com
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