Internet DRAFT - draft-ietf-acme-star
draft-ietf-acme-star
ACME Working Group Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track D. Lopez
Expires: April 26, 2020 O. Gonzalez de Dios
A. Pastor Perales
Telefonica I+D
T. Fossati
ARM
October 24, 2019
Support for Short-Term, Automatically-Renewed (STAR) Certificates in
Automated Certificate Management Environment (ACME)
draft-ietf-acme-star-11
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 26, 2020.
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Copyright Notice
Copyright (c) 2019 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
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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 . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Bootstrap . . . . . . . . . . . . . . . . . . . . . . . . 5
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 an Auto-renewal Order . . . . . . . . . . . 8
3.2. Capability Discovery . . . . . . . . . . . . . . . . . . 10
3.3. Fetching the Certificates . . . . . . . . . . . . . . . . 11
3.4. Negotiating an unauthenticated GET . . . . . . . . . . . 13
3.5. Computing notBefore and notAfter of STAR Certificates . . 14
3.5.1. Example . . . . . . . . . . . . . . . . . . . . . . . 15
4. Operational Considerations . . . . . . . . . . . . . . . . . 15
4.1. The Meaning of "Short Term" and the Impact of Skewed
Clocks . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Impact on Certificate Transparency (CT) Logs . . . . . . 16
4.3. HTTP Caching and Dependability . . . . . . . . . . . . . 16
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 17
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1.1. ACME Server with STAR extension . . . . . . . . . . . 18
5.1.2. STAR Proxy . . . . . . . . . . . . . . . . . . . . . 18
5.2. Level of Maturity . . . . . . . . . . . . . . . . . . . . 18
5.3. Coverage . . . . . . . . . . . . . . . . . . . . . . . . 18
5.4. Version Compatibility . . . . . . . . . . . . . . . . . . 19
5.5. Licensing . . . . . . . . . . . . . . . . . . . . . . . . 19
5.6. Implementation experience . . . . . . . . . . . . . . . . 19
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5.7. Contact Information . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
6.1. New Registries . . . . . . . . . . . . . . . . . . . . . 19
6.2. New Error Types . . . . . . . . . . . . . . . . . . . . . 20
6.3. New fields in Order Objects . . . . . . . . . . . . . . . 20
6.4. Fields in the "auto-renewal" Object within an Order
Object . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.5. New fields in the "meta" Object within a Directory Object 21
6.6. Fields in the "auto-renewal" Object within a Directory
Metadata Object . . . . . . . . . . . . . . . . . . . . . 22
6.7. Cert-Not-Before and Cert-Not-After HTTP Headers . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
7.1. No revocation . . . . . . . . . . . . . . . . . . . . . . 22
7.2. Denial of Service Considerations . . . . . . . . . . . . 23
7.3. Privacy Considerations . . . . . . . . . . . . . . . . . 24
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Document History . . . . . . . . . . . . . . . . . . 27
A.1. draft-ietf-acme-star-11 . . . . . . . . . . . . . . . . . 27
A.2. draft-ietf-acme-star-10 . . . . . . . . . . . . . . . . . 27
A.3. draft-ietf-acme-star-09 . . . . . . . . . . . . . . . . . 27
A.4. draft-ietf-acme-star-08 . . . . . . . . . . . . . . . . . 27
A.5. draft-ietf-acme-star-07 . . . . . . . . . . . . . . . . . 27
A.6. draft-ietf-acme-star-06 . . . . . . . . . . . . . . . . . 27
A.7. draft-ietf-acme-star-05 . . . . . . . . . . . . . . . . . 28
A.8. draft-ietf-acme-star-04 . . . . . . . . . . . . . . . . . 28
A.9. draft-ietf-acme-star-03 . . . . . . . . . . . . . . . . . 28
A.10. draft-ietf-acme-star-02 . . . . . . . . . . . . . . . . . 28
A.11. draft-ietf-acme-star-01 . . . . . . . . . . . . . . . . . 28
A.12. draft-ietf-acme-star-00 . . . . . . . . . . . . . . . . . 28
A.13. draft-sheffer-acme-star-02 . . . . . . . . . . . . . . . 29
A.14. draft-sheffer-acme-star-01 . . . . . . . . . . . . . . . 29
A.15. draft-sheffer-acme-star-00 . . . . . . . . . . . . . . . 29
A.16. draft-sheffer-acme-star-lurk-00 . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
The ACME protocol [RFC8555] 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
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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
negotiated deadline, if needed - e.g., on key compromise.
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.ietf-acme-star-delegation]. 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 the name delegation use 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.
This leads to the optional use of unauthenticated GET in this
protocol (Section 3.4).
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.
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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:
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 resource associated with the 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 ACME CA updates the Order resource
to include the URL for the issued STAR certificate.
2.2. Refresh
The CA issues the initial certificate after the authorization
completes successfully. 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.ietf-acme-star-delegation], obtains the certificate
(Section 3.3) and uses it.
The refresh process (Figure 1) goes on until either:
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o IdO explicitly terminates the automatic renewal (Section 2.3); or
o Automatic renewal expires.
Certificate ACME/STAR
User Server
| Retrieve cert | [...]
|---------------------->| |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|---------------------->| short validity period
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|---------------------->| short validity period
| | |
| +------. /
| | | /
| | 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: autoRenewalCanceled |
|<----------------------------------------------+
| |
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 automatically
renewed Orders.
3.1.1. Extending the Order Resource
The Order resource is extended with a new "auto-renewal" object that
MUST be present for STAR certificates. The "auto-renewal" object has
the following structure:
o start-date (optional, string): the earliest date of validity of
the first certificate issued, in [RFC3339] format. When omitted,
the start date is as soon as authorization is complete.
o end-date (required, string): the latest date of validity of the
last certificate issued, in [RFC3339] format.
o lifetime (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 due to server or client adjustment (see below).
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o lifetime-adjust (optional, integer): amount of "left pad" added to
each STAR certificate, an integer that denotes a number of
seconds. The default is 0. If present, the value of the
notBefore field that would otherwise appear in the STAR
certificates is pre-dated by the specified number of seconds. See
also Section 4.1 for why a client might want to use this control
and Section 3.5 for how the effective certificate lifetime is
computed. The value reflected by the server, together with the
value of the lifetime attribute, can be used by the client as a
hint to configure its polling timer.
o allow-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 defined in Section 7.1.3
of [RFC8555] 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".
Section 7.1.6 of [RFC8555] defines the following values for the Order
resource's status: "pending", "ready", "processing", "valid", and
"invalid". In the case of auto-renewal 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 dynamic resource, i.e., it
refers to an entity that varies over time. Instead of overloading
the semantics of the "certificate" attribute, this document defines a
new attribute "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 an Auto-renewal Order
An important property of the auto-renewal 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 as
shown in Figure 3.
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POST /acme/order/ogfr8EcolOT HTTP/1.1
Host: example.org
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/gw06UNhKfOve",
"nonce": "Alc00Ap6Rt7GMkEl3L1JX5",
"url": "https://example.com/acme/order/ogfr8EcolOT"
}),
"payload": base64url({
"status": "canceled"
}),
"signature": "g454e3hdBlkT4AEw...nKePnUyZTjGtXZ6H"
}
Figure 3: Canceling an Auto-renewal Order
After a successful cancellation, the server MUST NOT issue any
additional certificates for this Order.
When 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:autoRenewalCanceled".
Issuing a cancellation for an Order that is not in "valid" state is
not allowed. 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:autoRenewalCancellationInvalid".
The state machine described in Section 7.1.6 of [RFC8555] is extended
as illustrated in Figure 4 (State Transitions for Order Objects).
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pending --------------+
| |
| All authz |
| "valid" |
V |
ready ---------------+
| |
| Receive |
| finalize |
| request |
V |
processing ------------+
| |
| First |
| certificate | Error or
| issued | Authorization failure
V V
valid invalid
|
| STAR
| Certificate
| canceled
V
canceled
Figure 4
Explicit certificate revocation using the revokeCert interface
(Section 7.6 of [RFC8555]) 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:autoRenewalRevocationNotSupported".
3.2. Capability Discovery
In order to support the discovery of STAR capabilities, the "meta"
field inside the directory object defined in Section 9.7.6 of
[RFC8555] is extended with a new "auto-renewal" object. The "auto-
renewal" object MUST be present if the server supports STAR. Its
structure is as follows:
o min-lifetime (required, integer): minimum acceptable value for
auto-renewal lifetime, in seconds.
o max-duration (required, integer): maximum delta between the auto-
renewal end-date and start-date, in seconds.
o allow-certificate-get (optional, boolean): see Section 3.4.
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An example directory object advertising STAR support with one day
min-lifetime and one year max-duration, and supporting certificate
fetching with an HTTP GET is shown in Figure 5.
{
"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"],
"auto-renewal": {
"min-lifetime": 86400,
"max-duration": 31536000,
"allow-certificate-get": true
}
}
}
Figure 5: Directory object with STAR support
3.3. Fetching the Certificates
The certificate is fetched from the star-certificate endpoint with
POST-as-GET as per [RFC8555] 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 Figure 6.
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GET /acme/cert/g7m3ZQeTEqa 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"
Cert-Not-Before: Thu, 3 Oct 2019 00:00:00 GMT
Cert-Not-After: Thu, 10 Oct 2019 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-----
Figure 6: Fetching a STAR certificate with unauthenticated GET
The Server SHOULD include the "Cert-Not-Before" and "Cert-Not-After"
HTTP header fields 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.
Following are further clarifications regarding usage of these header
fields, as per [RFC7231] Sec. 8.3.1. All apply to both headers.
o This header field is a single value, not a list.
o The header field is used only in responses to GET, HEAD and POST-
as-GET requests, and only for MIME types that denote public key
certificates.
o Header field semantics are independent of context.
o The header field is not hop-by-hop.
o Intermediaries MAY insert or delete the value;
o If an intermediary inserts the value, it MUST ensure that the
newly added value matches the corresponding value in the
certificate.
o The header field is not appropriate for a Vary field.
o The header field is allowed within message trailers.
o The header field is not appropriate within redirects.
o The header field does not introduce additional security
considerations. It discloses in a simpler form information that
is already available inside the certificate.
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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 "next" certificate
also expires. To avoid the client accidentally entering a broken
state, the notBefore of the "next" certificate MUST be set so that
the certificate is already valid when it is published at the "star-
certificate" URL. Note that the server might need to increase the
auto-renewal lifetime-adjust value to satisfy the latter requirement.
For a detailed description of the renewal scheduling logic, see
Section 3.5. For further rationale on the need for adjusting the
certificate validity, see Section 4.1.
The server MUST NOT issue any certificates for this Order with
notAfter after the auto-renewal end-date.
For expired Orders, 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:autoRenewalExpired". Note that
the Order resource's state remains "valid", as per the base protocol.
3.4. Negotiating an unauthenticated GET
In order to enable the name delegation workflow defined in
[I-D.ietf-acme-star-delegation] 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 (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 allow-certificate-get attribute to true in the auto-
renewal object of the meta field inside the Directory object:
o allow-certificate-get (optional, boolean): If this field is
present and set to true, the server allows GET (and HEAD) requests
to star-certificate URLs.
A client states its desire to access the issued star-certificate via
unauthenticated GET by adding an allow-certificate-get attribute to
the auto-renewal object of the payload of its newOrder request and
setting it to true.
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o allow-certificate-get (optional, boolean): If this field is
present and set to true, the client requests the server to allow
unauthenticated GET (and HEAD) to the star-certificate associated
with this Order.
If the server accepts the request, it MUST reflect the attribute
setting in the resulting Order object.
Note that even when the use of unauthenticated GET has been agreed,
the server MUST also allow POST-as-GET requests to the star-
certificate resource.
3.5. Computing notBefore and notAfter of STAR Certificates
We define "nominal renewal date" as the point in time when a new
short-term certificate for a given STAR Order is due. Its cadence is
a multiple of the Order's auto-renewal lifetime that starts with the
issuance of the first short-term certificate and is upper-bounded by
the Order's auto-renewal end-date (Figure 7).
T - STAR Order's auto-renewal lifetime
end - STAR Order's auto-renewal end-date
nrd[i] - nominal renewal date of the i-th STAR certificate
.- T -. .- T -. .- T -. .__.
/ \ / \ / \ / end
-----------o---------o---------o---------o----X-------> t
nrd[0] nrd[1] nrd[2] nrd[3]
Figure 7: Nominal Renewal Date
The rules to determine the notBefore and notAfter values of the i-th
STAR certificate are as follows:
notAfter = min(nrd[i] + T, end)
notBefore = nrd[i] - max(adjust_client, adjust_server)
Where "adjust_client" is the min between the auto-renewal lifetime-
adjust value ("la"), optionally supplied by the client, and the auto-
renewal lifetime of each short-term certificate ("T");
"adjust_server" is the amount of padding added by the ACME server to
make sure that all certificates being published are valid at the time
of publication. The server padding is a fraction f of T (i.e., f * T
with .5 <= f < 1, see Section 3.3):
adjust_client = min(T, la)
adjust_server = f * T
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Note that the ACME server MUST NOT set the notBefore of the first
STAR certificate to a date prior to the auto-renewal start-date.
3.5.1. Example
Given a server that intends to publish the next STAR certificate
halfway through the lifetime of the previous one, and a STAR Order
with the following attributes:
"auto-renewal": {
"start-date": "2019-01-10T00:00:00Z",
"end-date": "2019-01-20T00:00:00Z",
"lifetime": 345600, // 4 days
"lifetime-adjust": 259200 // 3 days
}
The amount of time that needs to be subtracted from each nominal
renewal date is 3 days - i.e., max(min(345600, 259200), 345600 * .5).
The notBefore and notAfter of each short-term certificate are:
+----------------------+----------------------+
| notBefore | notAfter |
+----------------------+----------------------+
| 2019-01-10T00:00:00Z | 2019-01-14T00:00:00Z |
| 2019-01-11T00:00:00Z | 2019-01-18T00:00:00Z |
| 2019-01-15T00:00:00Z | 2019-01-20T00:00:00Z |
+----------------------+----------------------+
The value of the notBefore is also the time at which the client
should expect the new certificate to be available from the star-
certificate endpoint.
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, site owners could use the auto-renewal lifetime-
adjust attribute to control the effective lifetime of their Web
facing certificates. 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% - as well as the nominal
auto-renewal lifetime of the STAR Order. Note that exact choice is
also likely to depend on the kinds of client 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 adjustment to account for clock
skews) no longer than 4 days.
4.2. Impact on Certificate Transparency (CT) Logs
Even in the highly unlikely case STAR becomes the only certificate
issuance model, discussion with the IETF TRANS Working Group and
Certificate Transparency (CT) logs implementers suggests that
existing CT Log Server implementations are capable of sustaining the
resulting 100-fold increase in ingestion rate. Additionally, such a
future, higher load could be managed with a variety of techniques
(e.g., sharding by modulo of certificate hash, using "smart" load-
balancing CT proxies, etc.). With regards to the increase in the log
size, current CT log growth is already being managed with schemes
like Chrome's Log Policy [OBrien] which allow Operators to define
their log life-cycle; and allowing the CAs, User Agents, Monitors,
and any other interested entities to build-in support for that life-
cycle ahead of time.
4.3. HTTP Caching and 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
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availability. This might be problematic given the relatively high
rate of client-server interactions in a STAR ecosystem and especially
when multiple endpoints (e.g., a high number of CDN edge nodes) end
up requesting the same certificate. Clients and servers should
consider using the mechanism described in Section 3.4 to mitigate the
risk.
When using unauthenticated GET to fetch the STAR certificate, the
server SHALL use the appropriate cache directives to set the
freshness lifetime of the response (Section 5.2 of [RFC7234]) such
that on-path caches will consider it stale before or at the time its
effective lifetime is due to expire.
5. Implementation Status
Note to RFC Editor: please remove this section before publication,
including the reference to [RFC7942] and
[I-D.sheffer-acme-star-request].
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
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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.
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.
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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 ).
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 Registries
This document requests that IANA create the following new registries:
o ACME Order Auto Renewal Fields (Section 6.4)
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o ACME Directory Metadata Auto Renewal Fields (Section 6.6)
All of these registries are administered under a Specification
Required policy [RFC8126].
6.2. New Error Types
This document adds the following entries to the ACME Error Type
registry:
+-----------------------------------+-------------------+-----------+
| Type | Description | Reference |
+-----------------------------------+-------------------+-----------+
| autoRenewalCanceled | The short-term | RFC XXXX |
| | certificate is no | |
| | longer available | |
| | because the auto- | |
| | renewal Order has | |
| | been explicitly | |
| | canceled by the | |
| | IdO | |
| autoRenewalExpired | The short-term | RFC XXXX |
| | certificate is no | |
| | longer available | |
| | because the auto- | |
| | renewal Order has | |
| | expired | |
| autoRenewalCancellationInvalid | A request to | RFC XXXX |
| | cancel a auto- | |
| | renewal Order | |
| | that is not in | |
| | state "valid" has | |
| | been received | |
| autoRenewalRevocationNotSupported | A request to | RFC XXXX |
| | revoke a auto- | |
| | renewal Order has | |
| | been received | |
+-----------------------------------+-------------------+-----------+
6.3. New fields in Order Objects
This document adds the following entries to the ACME Order Object
Fields registry:
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+------------------+------------+--------------+-----------+
| Field Name | Field Type | Configurable | Reference |
+------------------+------------+--------------+-----------+
| auto-renewal | object | true | RFC XXXX |
| star-certificate | string | false | RFC XXXX |
+------------------+------------+--------------+-----------+
6.4. Fields in the "auto-renewal" Object within an Order Object
The "ACME Order Auto Renewal Fields" registry lists field names that
are defined for use in the JSON object included in the "auto-renewal"
field of an ACME order object.
Template:
o Field name: The string to be used as a field name in the JSON
object
o Field type: The type of value to be provided, e.g., string,
boolean, array of string
o Configurable: Boolean indicating whether the server should accept
values provided by the client
o Reference: Where this field is defined
Initial contents: The fields and descriptions defined in
Section 3.1.1.
+-----------------------+------------+--------------+-----------+
| Field Name | Field Type | Configurable | Reference |
+-----------------------+------------+--------------+-----------+
| start-date | string | true | RFC XXXX |
| end-date | string | true | RFC XXXX |
| lifetime | integer | true | RFC XXXX |
| lifetime-adjust | integer | true | RFC XXXX |
| allow-certificate-get | boolean | true | RFC XXXX |
+-----------------------+------------+--------------+-----------+
6.5. New fields in the "meta" Object within a Directory Object
This document adds the following entry to the ACME Directory Metadata
Fields:
+--------------+------------+-----------+
| Field Name | Field Type | Reference |
+--------------+------------+-----------+
| auto-renewal | object | RFC XXXX |
+--------------+------------+-----------+
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6.6. Fields in the "auto-renewal" Object within a Directory Metadata
Object
The "ACME Directory Metadata Auto Renewal Fields" registry lists
field names that are defined for use in the JSON object included in
the "auto-renewal" field of an ACME directory "meta" object.
Template:
o Field name: The string to be used as a field name in the JSON
object
o Field type: The type of value to be provided, e.g., string,
boolean, array of string
o Reference: Where this field is defined
Initial contents: The fields and descriptions defined in Section 3.2.
+-----------------------+------------+-----------+
| Field Name | Field Type | Reference |
+-----------------------+------------+-----------+
| min-lifetime | integer | RFC XXXX |
| max-duration | integer | RFC XXXX |
| allow-certificate-get | boolean | RFC XXXX |
+-----------------------+------------+-----------+
6.7. Cert-Not-Before and Cert-Not-After HTTP Headers
The "Message Headers" registry should be updated with the following
additional values:
+-------------------+----------+----------+-----------------------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-----------------------+
| Cert-Not-Before | http | standard | RFC XXXX, Section 3.3 |
| Cert-Not-After | http | standard | RFC XXXX, Section 3.3 |
+-------------------+----------+----------+-----------------------+
7. Security Considerations
7.1. No revocation
STAR certificates eliminate an important security feature of PKI
which is the ability to revoke certificates. Revocation allows the
administrator to limit the damage done by a rogue node or an
adversary who has control of the private key. With STAR
certificates, expiration replaces revocation so there is potential
for lack of timeliness in the revocation taking effect. To that end,
see also the discussion on clock skew in Section 4.1.
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It should be noted that revocation also has timeliness issues,
because both CRLs and OCSP responses have nextUpdate fields that tell
relying parties (RPs) how long they should trust this revocation
data. These fields are typically set to hours, days, or even weeks
in the future. Any revocation that happens before the time in
nextUpdate goes unnoticed by the RP.
One situation where the lack of explicit revocation could create a
security risk to the IdO is when the Order is created with start-date
some appreciable amount of time in the future. Recall that when
authorizations have been fulfilled, the Order moves to the "valid"
state and the star-certificate endpoint is populated with the first
cert (Figure 4). So, if an attacker manages to get hold of the
private key as well as of the first (post-dated) certificate, there
is a time window in the future when they will be able to successfully
impersonate the IdO. Note that cancellation is pointless in this
case. In order to mitigate the described threat, it is RECOMMENDED
that IdO place their Orders at a time that is close to the Order's
start-date.
More discussion of the security of STAR certificates is available in
[Topalovic].
7.2. 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 auto-renewal "lifetime" parameter. An
attacker can use this property to aggressively reduce the auto-
renewal "lifetime" (e.g. 1 sec.) jointly with other ACME attack
vectors identified in Sec. 10 of [RFC8555]. 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 auto-renewal behavior cannot
solve the above problem. The CA server needs complementary
mitigation and specifically, it SHOULD enforce a minimum value on
auto-renewal "lifetime". Alternatively, the CA can set an internal
certificate generation processes rate limit. Note that this limit
has to take account of already-scheduled renewal issuances as well as
new incoming requests.
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7.3. Privacy Considerations
In order to avoid correlation of certificates by account, if
unauthenticated GET is negotiated (Section 3.4) the recommendation in
Section 10.5 of [RFC8555] regarding the choice of URL structure
applies, i.e. servers SHOULD choose URLs of certificate resources in
a non-guessable way, for example using capability URLs
[W3C.WD-capability-urls-20140218].
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 Ben Kaduk, Richard Barnes, Roman Danyliw, Jon Peterson,
Eric Rescorla, Ryan Sleevi, Sean Turner, Alexey Melnikov, Adam Roach,
Martin Thomson and Mehmet Ersue for helpful comments and discussions
that have shaped this document.
9. References
9.1. 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,
<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>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[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>.
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[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.
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.ietf-acme-star-delegation]
Sheffer, Y., Lopez, D., Pastor, A., and T. Fossati, "An
ACME Profile for Generating Delegated STAR Certificates",
draft-ietf-acme-star-delegation-01 (work in progress),
August 2019.
[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.
[OBrien] O'Brien, D. and R. Sleevi, "Chromium Certificate
Transparency Log Policy", 2017,
<https://github.com/chromium/ct-policy>.
[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>.
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[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.ieee-security.org/TC/W2SP/2012/papers/
w2sp12-final9.pdf>.
[W3C.WD-capability-urls-20140218]
Tennison, J., "Good Practices for Capability URLs", World
Wide Web Consortium WD WD-capability-urls-20140218,
February 2014,
<http://www.w3.org/TR/2014/WD-capability-urls-20140218>.
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Appendix A. Document History
[[Note to RFC Editor: please remove before publication.]]
A.1. draft-ietf-acme-star-11
o One more nit re: random URL
A.2. draft-ietf-acme-star-10
IESG processing:
o More clarity on IANA registration (Alexey);
o HTTP header requirements adjustments (Adam);
o Misc editorial (Ben)
A.3. draft-ietf-acme-star-09
Richard and Ryan's review resulted in the following updates:
o STAR Order and Directory Meta attributes renamed slightly and
grouped under two brand new "auto-renewal" objects;
o IANA registration updated accordingly (note that two new
registries have been added as a consequence);
o Unbounded pre-dating of certificates removed so that STAR certs
are never issued with their notBefore in the past;
o Changed "recurrent" to "autoRenewal" in error codes;
o Changed "recurrent" to "auto-renewal" in reference to Orders;
o Added operational considerations for HTTP caches.
A.4. draft-ietf-acme-star-08
o Improved text on interaction with CT Logs, responding to Mehmet
Ersue's review.
A.5. draft-ietf-acme-star-07
o Changed the HTTP headers names and clarified the IANA
registration, following feedback from the IANA expert reviewer
A.6. draft-ietf-acme-star-06
o Roman's AD review
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A.7. draft-ietf-acme-star-05
o EKR's AD review
o A detailed example of the timing of certificate issuance and
predating
o Added an explicit client-side parameter for predating
o Security considerations around unauthenticated GET
A.8. 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.9. draft-ietf-acme-star-03
o Clock skew considerations
o Recommendations for "short" in the Web use case
o CT log considerations
A.10. 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.11. 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.12. 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.
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A.13. 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.
A.14. draft-sheffer-acme-star-01
o A terminology section.
o Some cleanup.
A.15. 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.16. 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
Sheffer, et al. Expires April 26, 2020 [Page 29]
Internet-Draft ACME STAR October 2019
Thomas Fossati
ARM
EMail: thomas.fossati@arm.com
Sheffer, et al. Expires April 26, 2020 [Page 30]
