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Versions: (draft-sheffer-acme-star) 00 01 02
03 04 05
ACME Working Group Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track D. Lopez
Expires: April 22, 2019 O. Gonzalez de Dios
A. Pastor Perales
Telefonica I+D
T. Fossati
Nokia
October 19, 2018
Support for Short-Term, Automatically-Renewed (STAR) Certificates in
Automated Certificate Management Environment (ACME)
draft-ietf-acme-star-04
Abstract
Public-key certificates need to be revoked when they are compromised,
that is, when the associated private key is exposed to an
unauthorized entity. However the revocation process is often
unreliable. An alternative to revocation is issuing a sequence of
certificates, each with a short validity period, and terminating this
sequence upon compromise. This memo proposes an ACME extension to
enable the issuance of short-term and automatically renewed (STAR)
X.509 certificates.
[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.
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 https://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 April 22, 2019.
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Name Delegation Use Case . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Conventions used in this document . . . . . . . . . . . . 4
2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Bootstrap . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Refresh . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Termination . . . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 7
3.1. ACME Extensions . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Extending the Order Resource . . . . . . . . . . . . 7
3.1.2. Canceling a Recurrent Order . . . . . . . . . . . . . 8
3.2. Capability Discovery . . . . . . . . . . . . . . . . . . 9
3.3. Fetching the Certificates . . . . . . . . . . . . . . . . 10
3.4. Negotiate unauthenticated GET . . . . . . . . . . . . . . 12
4. Operational Considerations . . . . . . . . . . . . . . . . . 12
4.1. The Meaning of "Short Term" and the Impact of Skewed
Clocks . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2. Impact on Certificate Transparency (CT) Logs . . . . . . 13
4.3. Dependability . . . . . . . . . . . . . . . . . . . . . . 13
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 14
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1.1. ACME Server with STAR extension . . . . . . . . . . . 14
5.1.2. STAR Proxy . . . . . . . . . . . . . . . . . . . . . 15
5.2. Level of Maturity . . . . . . . . . . . . . . . . . . . . 15
5.3. Coverage . . . . . . . . . . . . . . . . . . . . . . . . 15
5.4. Version Compatibility . . . . . . . . . . . . . . . . . . 15
5.5. Licensing . . . . . . . . . . . . . . . . . . . . . . . . 15
5.6. Implementation experience . . . . . . . . . . . . . . . . 16
5.7. Contact Information . . . . . . . . . . . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
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6.1. New Error Types . . . . . . . . . . . . . . . . . . . . . 16
6.2. New fields in Order Objects . . . . . . . . . . . . . . . 17
6.3. New fields in the "meta" Object within a Directory Object 18
6.4. Not-Before and Not-After HTTP Headers . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7.1. Denial of Service Considerations . . . . . . . . . . . . 18
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Document History . . . . . . . . . . . . . . . . . . 21
A.1. draft-ietf-acme-star-04 . . . . . . . . . . . . . . . . . 21
A.2. draft-ietf-acme-star-03 . . . . . . . . . . . . . . . . . 21
A.3. draft-ietf-acme-star-02 . . . . . . . . . . . . . . . . . 21
A.4. draft-ietf-acme-star-01 . . . . . . . . . . . . . . . . . 21
A.5. draft-ietf-acme-star-00 . . . . . . . . . . . . . . . . . 21
A.6. draft-sheffer-acme-star-02 . . . . . . . . . . . . . . . 21
A.7. draft-sheffer-acme-star-01 . . . . . . . . . . . . . . . 22
A.8. draft-sheffer-acme-star-00 . . . . . . . . . . . . . . . 22
A.9. draft-sheffer-acme-star-lurk-00 . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
The ACME protocol [I-D.ietf-acme-acme] automates the process of
issuing a certificate to a named entity (an Identifier Owner or IdO).
Typically, but not always, the identifier is a domain name.
If the IdO wishes to obtain a string of short-term certificates
originating from the same private key (see [Topalovic] about why
using short-lived certificates might be preferable to explicit
revocation), she must go through the whole ACME protocol each time a
new short-term certificate is needed - e.g., every 2-3 days. If done
this way, the process would involve frequent interactions between the
registration function of the ACME Certification Authority (CA) and
the identity provider infrastructure (e.g.: DNS, web servers),
therefore making the issuance of short-term certificates exceedingly
dependent on the reliability of both.
This document presents an extension of the ACME protocol that
optimizes this process by making short-term certificates first class
objects in the ACME ecosystem. Once the order for a string of short-
term certificates is accepted, the CA is responsible for publishing
the next certificate at an agreed upon URL before the previous one
expires. The IdO can terminate the automatic renewal before the
natural deadline, if needed - e.g., on key compromise.
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For a more generic treatment of STAR certificates, readers are
referred to [I-D.nir-saag-star].
1.1. Name Delegation Use Case
The proposed mechanism can be used as a building block of an
efficient name-delegation protocol, for example one that exists
between a CDN or a cloud provider and its customers
[I-D.sheffer-acme-star-request]. At any time, the service customer
(i.e., the IdO) can terminate the delegation by simply instructing
the CA to stop the automatic renewal and letting the currently active
certificate expire shortly thereafter. Note that in this case the
delegated entity needs to access the auto-renewed certificate without
being in possession of the ACME account key that was used for
initiating the STAR issuance.
1.2. Terminology
IdO Identifier Owner, the owner of an identifier, e.g.: a domain
name, a telephone number.
STAR Short-Term and Automatically Renewed X.509 certificates.
1.3. 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
The following subsections describe the three main phases of the
protocol:
o Bootstrap: the IdO asks an ACME CA to create a short-term and
automatically-renewed (STAR) certificate (Section 2.1);
o Auto-renewal: the ACME CA periodically re-issues the short-term
certificate and posts it to the star-certificate URL
(Section 2.2);
o Termination: the IdO requests the ACME CA to discontinue the
automatic renewal of the certificate (Section 2.3).
2.1. Bootstrap
The IdO, 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. Note that the exact
definition of "short" depends on the use case;
o Is automatically renewed by the CA for a certain period of time;
o Is downloadable from a (highly available) location.
Other than that, the ACME protocol flows as usual between IdO and CA.
In particular, IdO is responsible for satisfying the requested ACME
challenges until the CA is willing to issue the requested
certificate. Per normal ACME processing, the IdO is given back an
order URL 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 IdO obtains a confirmation from the
ACME CA that includes a star-certificate endpoint.
2.2. Refresh
The CA issues the initial certificate, typically when the
authorization is done. It then automatically re-issues the
certificate using the same CSR (and therefore the same identifier and
public key) before the previous one expires, and publishes it to the
URL that was returned to the IdO at the end of the bootstrap phase.
The certificate user, which could be either the IdO itself or a
delegated third party, as described in
[I-D.sheffer-acme-star-request], obtains the certificate
(Section 3.3) and uses it.
The refresh process (Figure 1) goes on until either:
o IdO explicitly terminates the automatic renewal (Section 2.3); or
o Automatic renewal expires.
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Certificate ACME/STAR
User Server
| Retrieve cert | [...]
|---------------------->| |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|---------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|---------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| | |
| [...] | [...]
Figure 1: Auto renewal
2.3. Termination
The IdO may request early termination of the STAR certificate by
sending a cancellation request to the Order resource, as described in
Section 3.1.2. 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 issuance;
o Change the status of the Order to "canceled".
Note that it is not necessary to explicitly revoke the short-term
certificate.
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Certificate ACME/STAR
User IdO Server
| | |
| | Cancel Order |
| +---------------------->|
| | +-------.
| | | |
| | | End auto renewal
| | | Remove cert link
| | | etc.
| | | |
| | Done |<------'
| |<----------------------+
| | |
| |
| Retrieve cert |
+---------------------------------------------->|
| Error: recurrentOrderCanceled |
|<----------------------------------------------+
| |
Figure 2: Termination
3. Protocol Details
This section describes the protocol details, namely the extensions to
the ACME protocol required to issue STAR certificates.
3.1. ACME Extensions
This protocol extends the ACME protocol, to allow for recurrent
Orders.
3.1.1. Extending the Order Resource
The Order resource is extended with the following attributes:
{
"recurrent": true,
"recurrent-start-date": "2016-01-01T00:00:00Z",
"recurrent-end-date": "2017-01-01T00:00:00Z",
"recurrent-certificate-validity": 604800
}
o recurrent (required, boolean): MUST be true for STAR certificates.
o recurrent-start-date (optional, string): the earliest date of
validity of the first certificate issued, in [RFC3339] format.
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When omitted, the start date is as soon as authorization is
complete.
o recurrent-end-date (required, string): the latest date of validity
of the last certificate issued, in [RFC3339] format.
o recurrent-certificate-validity (required, integer): the maximum
validity period of each STAR certificate, an integer that denotes
a number of seconds. This is a nominal value which does not
include any extra validity time which is due to pre-dating. The
client can use this value as a hint to configure its polling
timer.
o recurrent-certificate-get (optional, boolean): see Section 3.4.
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 (see also Section 3.2).
The optional notBefore and notAfter fields MUST NOT be present in a
STAR Order. If they are included, the server MUST return an error
with status code 400 "Bad Request" and type "malformedRequest".
ACME defines the following values for the Order resource's status:
"pending", "ready", "processing", "valid", and "invalid". In the
case of recurrent Orders, the status MUST be "valid" as long as STAR
certificates are being issued. We add a new status value:
"canceled", see Section 3.1.2.
A STAR certificate is by definition a mutable resource. Instead of
overloading the semantics of the "certificate" key, this document
defines a new key "star-certificate" to be used instead of
"certificate".
o star-certificate (optional, string): A URL for the (rolling) STAR
certificate that has been issued in response to this Order.
3.1.2. Canceling a Recurrent Order
An important property of the recurrent Order is that it can be
canceled by the IdO, with no need for certificate revocation. To
cancel the Order, the ACME client sends a POST to the Order URL:
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POST /acme/order/TOlocE8rfgo HTTP/1.1
Host: example.org
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://example.com/acme/order/TOlocE8rfgo"
}),
"payload": base64url({
"status": "canceled"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
The server MUST NOT issue any additional certificates for this order,
beyond the certificate that is available for collection at the time
of deletion.
Immediately after the order is canceled, the server:
o MUST update the status of the order resource to "canceled" and
MUST set an appropriate "expires" date;
o MUST respond with 403 (Forbidden) to any requests to the star-
certificate endpoint. The response SHOULD provide additional
information using a problem document [RFC7807] with type
"urn:ietf:params:acme:error:recurrentOrderCanceled".
Issuing a cancellation for an order that is not in "valid" state has
undefined semantics. A client MUST NOT send such a request, and a
server MUST return an error response with status code 400 (Bad
Request) and type
"urn:ietf:params:acme:error:recurrentCancellationInvalid".
Explicit certificate revocation using the revokeCert interface
(Section 7.6 of [I-D.ietf-acme-acme]) is not supported for STAR
certificates. A server receiving a revocation request for a STAR
certificate MUST return an error response with status code 403
(Forbidden) and type
"urn:ietf:params:acme:error:recurrentRevocationNotSupported".
3.2. Capability Discovery
In order to support the discovery of STAR capabilities, The directory
object of an ACME STAR server is extended with the following
attributes inside the "meta" field:
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o star-enabled (required, boolean): indicates STAR support. An ACME
STAR server MUST include this key, and MUST set it to true if the
feature is enabled.
o star-min-cert-validity (required, integer): minimum acceptable
value for recurrent-certificate-validity, in seconds.
o star-max-renewal (required, integer): maximum delta between
recurrent-end-date and recurrent-start-date, in seconds.
o star-allow-certificate-get (optional, boolean): see Section 3.4.
An example directory object advertising STAR support with one day
star-min-cert-validity and one year star-max-renewal, and supporting
certificate fetching with an HTTP GET:
{
"new-nonce": "https://example.com/acme/new-nonce",
"new-account": "https://example.com/acme/new-account",
"new-order": "https://example.com/acme/new-order",
"new-authz": "https://example.com/acme/new-authz",
"revoke-cert": "https://example.com/acme/revoke-cert",
"key-change": "https://example.com/acme/key-change",
"meta": {
"terms-of-service": "https://example.com/acme/terms/2017-5-30",
"website": "https://www.example.com/",
"caa-identities": ["example.com"],
"star-enabled": true,
"star-min-cert-validity": 86400,
"star-max-renewal": 31536000,
"star-allow-certificate-get": true
}
}
3.3. Fetching the Certificates
The certificate is fetched from the star-certificate endpoint with
POST-as-GET as per [I-D.ietf-acme-acme] Section 7.4.2, unless client
and server have successfully negotiated the "unauthenticated GET"
option described in Section 3.4. In such case, the client can simply
issue a GET to the star-certificate resource without authenticating
itself to the server as illustrated in the following example:
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GET /acme/cert/mAt3xBGaobw HTTP/1.1
Host: example.org
Accept: application/pem-certificate-chain
HTTP/1.1 200 OK
Content-Type: application/pem-certificate-chain
Link: <https://example.com/acme/some-directory>;rel="index"
Not-Before: Mon, 1 Feb 2016 00:00:00 GMT
Not-After: Mon, 8 Feb 2016 00:00:00 GMT
-----BEGIN CERTIFICATE-----
[End-entity certificate contents]
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
[Issuer certificate contents]
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
[Other certificate contents]
-----END CERTIFICATE-----
The Server SHOULD include the "Not-Before" and "Not-After" HTTP
headers in the response. When they exist, they MUST be equal to the
respective fields inside the end-entity certificate. Their format is
"HTTP-date" as defined in Section 7.1.1.2 of [RFC7231]. Their
purpose is to enable client implementations that do not parse the
certificate.
To improve robustness, the next certificate MUST be made available by
the ACME CA at the URL pointed by "star-certificate" at the latest
halfway through the lifetime of the currently active certificate. It
is worth noting that this has an implication in case of cancellation:
in fact, from the time the next certificate is made available, the
cancellation is not completely effective until the latter also
expires. To avoid the client accidentally entering a broken state,
the "next" certificate MUST be pre-dated so that it is already valid
when it is published at the "star-certificate" URL. For further
discussion on pre-dating, see Section 4.1.
The server MUST NOT issue any additional certificates for this order
beyond its recurrent-end-date.
Immediately after the order expires, the server MUST respond with 403
(Forbidden) to any requests to the star-certificate endpoint. The
response SHOULD provide additional information using a problem
document [RFC7807] with type
"urn:ietf:params:acme:error:recurrentOrderExpired". Note that the
Order resource's state remains "valid", as per the base protocol.
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3.4. Negotiate unauthenticated GET
In order to enable the name delegation workflow defined in
[I-D.sheffer-acme-star-request] as well as to increase the
reliability of the STAR ecosystem (see Section 4.3 for details), this
document defines a mechanism that allows a server to advertise
support for accessing star-certificate resources via unauthenticated
GET (instead of, or in addition to, POST-as-GET), and a client to
enable this service with per-Order granularity.
Specifically, a server states its availability to grant
unauthenticated access to a client's Order star-certificate by
setting the star-allow-certificate-get key to true in the meta field
of the Directory object:
o star-allow-certificate-get (optional, boolean): If this field is
present and set to true, the server allows GET requests to star-
certificate URLs.
A client states its will to access the issued star-certificate via
unauthenticated GET by adding a recurrent-certificate-get key to its
Order and setting it to true.
o recurrent-certificate-get (optional, boolean): If this field is
present and set to true, the client requests the server to allow
unauthenticated GET to the star-certificate associated with this
Order.
If the server accepts the request, it MUST reflect the key in the
Order.
4. Operational Considerations
4.1. The Meaning of "Short Term" and the Impact of Skewed Clocks
"Short Term" is a relative concept, therefore trying to define a cut-
off point that works in all cases would be a useless exercise. In
practice, the expected lifetime of a STAR certificate will be counted
in minutes, hours or days, depending on different factors: the
underlying requirements for revocation, how much clock
synchronization is expected among relying parties and the issuing CA,
etc.
Nevertheless, this section attempts to provide reasonable suggestions
for the Web use case, informed by current operational and research
experience.
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Acer et al. [Acer] find that one of the main causes of "HTTPS error"
warnings in browsers is misconfigured client clocks. In particular,
they observe that roughly 95% of the "severe" clock skews - the 6.7%
of clock-related breakage reports which account for clients that are
more than 24 hours behind - happen to be within 6-7 days.
In order to avoid these spurious warnings about a not (yet) valid
server certificate, it is RECOMMENDED that site owners pre-date their
Web facing certificates by 5 to 7 days. The exact number depends on
the percentage of the "clock-skewed" population that the site owner
expects to protect - 5 days cover 97.3%, 7 days cover 99.6%. Note
that exact choice is also likely to depend on the kind of clients
that is prevalent for a given site or app - for example, Android and
Mac OS clients are known to behave better than Windows clients.
These considerations are clearly out of scope of the present
document.
In terms of security, STAR certificates and certificates with OCSP
must-staple [RFC7633] can be considered roughly equivalent if the
STAR certificate's and the OCSP response's lifetimes are the same.
Given OCSP responses can be cached on average for 4 days [Stark], it
is RECOMMENDED that a STAR certificate that is used on the Web has an
"effective" lifetime (excluding any pre-dating to account for clock
skews) no longer than 4 days.
4.2. Impact on Certificate Transparency (CT) Logs
Provided that the recommendations in Section 4.1 are followed, the
increase in Certificate Transparency (CT) [RFC6962] log ingestion
should be one order of magnitude in the worst case compared to the
current state.
The input received from most members of the CT community when the
issue was raised was that this should not represent a problem for the
CT architecture.
4.3. Dependability
When using authenticated POST-as-GET, the HTTPS endpoint from where
the STAR certificate is fetched can't be easily replicated by an on-
path HTTP cache. Reducing the caching properties of the protocol
makes STAR clients increasingly dependent on the ACME server
availability. This might be problematic given the relatively high
rate of client-server interactions in a STAR ecosystem. Clients and
servers should consider using the mechanism described in Section 3.4
to mitigate the risk.
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5. Implementation Status
Note to RFC Editor: please remove this section before publication,
including the reference to [RFC7942].
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC7942], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
5.1. Overview
The implementation is constructed around 3 elements: STAR Client for
the Name Delegation Client (NDC), STAR Proxy for IdO and ACME Server
for CA. The communication between them is over an IP network and the
HTTPS protocol.
The software of the implementation is available at:
https://github.com/mami-project/lurk
The following subsections offer a basic description, detailed
information is available in https://github.com/mami-
project/lurk/blob/master/proxySTAR_v2/README.md
5.1.1. ACME Server with STAR extension
This is a fork of the Let's Encrypt Boulder project that implements
an ACME compliant CA. It includes modifications to extend the ACME
protocol as it is specified in this draft, to support recurrent
orders and cancelling orders.
The implementation understands the new "recurrent" attributes as part
of the Certificate issuance in the POST request for a new resource.
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An additional process "renewalManager.go" has been included in
parallel that reads the details of each recurrent request,
automatically produces a "cron" Linux based task that issues the
recurrent certificates, until the lifetime ends or the order is
canceled. This process is also in charge of maintaining a fixed URI
to enable the NDC to download certificates, unlike Boulder's regular
process of producing a unique URI per certificate.
5.1.2. STAR Proxy
The STAR Proxy has a double role as ACME client and STAR Server. The
former is a fork of the EFF Certbot project that implements an ACME
compliant client with the STAR extension. The latter is a basic HTTP
REST API server.
The STAR Proxy understands the basic API request with a server. The
current implementation of the API is defined in draft-ietf-acme-star-
01. Registration or order cancellation triggers the modified Certbot
client that requests, or cancels, the recurrent generation of
certificates using the STAR extension over ACME protocol. The URI
with the location of the recurrent certificate is delivered to the
STAR client as a response.
5.2. Level of Maturity
This is a prototype.
5.3. Coverage
A STAR Client is not included in this implementation, but done by
direct HTTP request with any open HTTP REST API tool. This is
expected to be covered as part of the [I-D.sheffer-acme-star-request]
implementation.
This implementation completely covers STAR Proxy and ACME Server with
STAR extension.
5.4. Version Compatibility
The implementation is compatible with version draft-ietf-acme-star-
01. The implementation is based on the Boulder and Certbot code
release from 7-Aug-2017.
5.5. Licensing
This implementation inherits the Boulder license (Mozilla Public
License 2.0) and Certbot license (Apache License Version 2.0 ).
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5.6. Implementation experience
To prove the concept all the implementation has been done with a
self-signed CA, to avoid impact on real domains. To be able to do it
we use the FAKE_DNS property of Boulder and static /etc/hosts entries
with domains names. Nonetheless this implementation should run with
real domains.
Most of the implementation has been made to avoid deep changes inside
of Boulder or Certbot, for example, the recurrent certificates
issuance by the CA is based on an external process that auto-
configures the standard Linux "cron" daemon in the ACME CA server.
The reference setup recommended is one physical host with 3 virtual
machines, one for each of the 3 components (client, proxy and server)
and the connectivity based on host bridge.
Network security is not enabled (iptables default policies are
"accept" and all rules removed) in this implementation to simplify
and test the protocol.
5.7. Contact Information
See author details below.
6. IANA Considerations
[[RFC Editor: please replace XXXX below by the RFC number.]]
6.1. New Error Types
This document adds the following entries to the ACME Error Type
registry:
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+---------------------------------+---------------------+-----------+
| Type | Description | Reference |
+---------------------------------+---------------------+-----------+
| recurrentOrderCanceled | The short-term | RFC XXXX |
| | certificate is no | |
| | longer available | |
| | because the | |
| | recurrent order has | |
| | been explicitly | |
| | canceled by the IdO | |
| recurrentOrderExpired | The short-term | RFC XXXX |
| | certificate is no | |
| | longer available | |
| | because the | |
| | recurrent order has | |
| | expired | |
| recurrentCancellationInvalid | A request to cancel | RFC XXXX |
| | a recurrent order | |
| | that is not in | |
| | state "valid" has | |
| | been received | |
| recurrentRevocationNotSupported | A request to revoke | RFC XXXX |
| | a recurrent order | |
| | has been received | |
+---------------------------------+---------------------+-----------+
6.2. New fields in Order Objects
This document adds the following entries to the ACME Order Object
Fields registry:
+------------------------------+---------+--------------+-----------+
| Field Name | Field | Configurable | Reference |
| | Type | | |
+------------------------------+---------+--------------+-----------+
| recurrent | string | true | RFC XXXX |
| recurrent-start-date | string | true | RFC XXXX |
| recurrent-end-date | string | true | RFC XXXX |
| recurrent-certificate- | string | true | RFC XXXX |
| validity | | | |
| recurrent-certificate-get | boolean | true | RFC XXXX |
| star-certificate | string | false | RFC XXXX |
+------------------------------+---------+--------------+-----------+
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6.3. 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-enabled | boolean | RCF XXXX |
| star-min-cert-validity | integer | RCF XXXX |
| star-max-renewal | integer | RCF XXXX |
| star-allow-certificate-get | boolean | RFC XXXX |
+----------------------------+------------+-----------+
6.4. Not-Before and Not-After HTTP Headers
The "Message Headers" registry should be updated with the following
additional values:
+-------------------+----------+----------+-----------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-----------+
| Not-Before | http | standard | RFC XXXX |
| Not-After | http | standard | RFC XXXX |
+-------------------+----------+----------+-----------+
7. Security Considerations
7.1. Denial of Service Considerations
STAR adds a new attack vector that increases the threat of denial of
service attacks, caused by the change to the CA's behavior. Each
STAR request amplifies the resource demands upon the CA, where one
order produces not one, but potentially dozens or hundreds of
certificates, depending on the "recurrent-certificate-validity"
parameter. An attacker can use this property to aggressively reduce
the "recurrent-certificate-validity" (e.g. 1 sec.) jointly with other
ACME attack vectors identified in Sec. 10 of [I-D.ietf-acme-acme].
Other collateral impact is related to the certificate endpoint
resource where the client can retrieve the certificates periodically.
If this resource is external to the CA (e.g. a hosted web server),
the previous attack will be reflected to that resource.
Mitigation recommendations from ACME still apply, but some of them
need to be adjusted. For example, applying rate limiting to the
initial request, by the nature of the recurrent behavior cannot solve
the above problem. The CA server needs complementary mitigation and
specifically, it SHOULD enforce a minimum value on "recurrent-
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certificate-validity". Alternatively, the CA can set an internal
certificate generation processes rate limit.
8. 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.
Thanks to Jon Peterson, Sean Turner and Martin Thomson for helpful
comments and discussions that have shaped this document.
9. References
9.1. 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.
[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>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7807] Nottingham, M. and E. Wilde, "Problem Details for HTTP
APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
<https://www.rfc-editor.org/info/rfc7807>.
[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>.
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9.2. Informative References
[Acer] Acer, M., Stark, E., Felt, A., Fahl, S., Bhargava, R.,
Dev, B., Braithwaite, M., Sleevi, R., and P. Tabriz,
"Where the Wild Warnings Are: Root Causes of Chrome HTTPS
Certificate Errors", DOI 10.1145/3133956.3134007, 2017,
<https://acmccs.github.io/papers/p1407-acerA.pdf>.
[I-D.nir-saag-star]
Nir, Y., Fossati, T., Sheffer, Y., and T. Eckert,
"Considerations For Using Short Term Certificates", draft-
nir-saag-star-01 (work in progress), March 2018.
[I-D.sheffer-acme-star-request]
Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T.
Fossati, "Generating Certificate Requests for Short-Term,
Automatically-Renewed (STAR) Certificates", draft-sheffer-
acme-star-request-02 (work in progress), June 2018.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
[RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS)
Feature Extension", RFC 7633, DOI 10.17487/RFC7633,
October 2015, <https://www.rfc-editor.org/info/rfc7633>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[Stark] Stark, E., Huang, L., Israni, D., Jackson, C., and D.
Boneh, "The case for prefetching and prevalidating TLS
server certificates", 2012,
<http://crypto.stanford.edu/~dabo/pubs/abstracts/
ssl-prefetch.html>.
[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>.
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Appendix A. Document History
[[Note to RFC Editor: please remove before publication.]]
A.1. draft-ietf-acme-star-04
o WG last call comments by Sean Turner
o revokeCert interface handling
o Allow negotiating plain-GET for certs
o In STAR Orders, use star-certificate instead of certificate
A.2. draft-ietf-acme-star-03
o Clock skew considerations
o Recommendations for "short" in the Web use case
o CT log considerations
A.3. draft-ietf-acme-star-02
o Discovery of STAR capabilities via the directory object
o Use the more generic term Identifier Owner (IdO) instead of Domain
Name Owner (DNO)
o More precision about what goes in the order
o Detail server side behavior on cancellation
A.4. draft-ietf-acme-star-01
o Generalized the introduction, separating out the specifics of
CDNs.
o Clean out LURK-specific text.
o Using a POST to ensure cancellation is authenticated.
o First and last date of recurrent cert, as absolute dates.
Validity of certs in seconds.
o Use RFC7807 "Problem Details" in error responses.
o Add IANA considerations.
o Changed the document's title.
A.5. draft-ietf-acme-star-00
o Initial working group version.
o Removed the STAR interface, the protocol between NDC and DNO.
What remains is only the extended ACME protocol.
A.6. draft-sheffer-acme-star-02
o Using a more generic term for the delegation client, NDC.
o Added an additional use case: public cloud services.
o More detail on ACME authorization.
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A.7. draft-sheffer-acme-star-01
o A terminology section.
o Some cleanup.
A.8. draft-sheffer-acme-star-00
o Renamed draft to prevent confusion with other work in this space.
o Added an initial STAR protocol: a REST API.
o Discussion of CDNI use cases.
A.9. draft-sheffer-acme-star-lurk-00
o Initial version.
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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