An ACME Profile for Generating Delegated STAR Certificates
draft-ietf-acme-star-delegation-04
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9115.
|
|
|---|---|---|---|
| Authors | Yaron Sheffer , Diego Lopez , Antonio Pastor , Thomas Fossati | ||
| Last updated | 2021-02-04 (Latest revision 2020-08-25) | ||
| Replaces | draft-sheffer-acme-star-delegation | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-06)
by Suresh Krishnan
Ready w/nits
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Rich Salz | ||
| Shepherd write-up | Show Last changed 2021-01-19 | ||
| IESG | IESG state | Became RFC 9115 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Roman Danyliw | ||
| Send notices to | ynir.ietf@gmail.com, rsalz@akamai.com |
draft-ietf-acme-star-delegation-04
ACME Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track D. Lopez
Expires: 26 February 2021 A. Pastor Perales
Telefonica I+D
T. Fossati
ARM
25 August 2020
An ACME Profile for Generating Delegated STAR Certificates
draft-ietf-acme-star-delegation-04
Abstract
This memo proposes a profile of the ACME protocol that allows the
owner of an identifier (e.g., a domain name) to delegate to a third
party access to a certificate associated with said identifier. A
primary use case is that of a CDN (the third party) terminating TLS
sessions on behalf of a content provider (the owner of a domain
name). The presented mechanism allows the owner of the identifier to
retain control over the delegation and revoke it at any time by
cancelling the associated STAR certificate renewal with the ACME CA.
Another key property of this mechanism is it does not require any
modification to the deployed TLS ecosystem.
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 26 February 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Conventions used in this document . . . . . . . . . . . . 4
2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Preconditions . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Delegated Identity Profile . . . . . . . . . . . . . . . 7
2.3.1. Delegation Configuration . . . . . . . . . . . . . . 7
2.3.2. Order Object on the NDC-IdO side . . . . . . . . . . 9
2.3.3. Order Object on the IdO-CA side . . . . . . . . . . . 13
2.3.4. Capability Discovery . . . . . . . . . . . . . . . . 13
2.3.5. On Cancellation . . . . . . . . . . . . . . . . . . . 13
2.4. Delegation of Non-STAR Certificates . . . . . . . . . . . 13
2.5. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . 14
3. CSR Template . . . . . . . . . . . . . . . . . . . . . . . . 15
3.1. Template Syntax . . . . . . . . . . . . . . . . . . . . . 15
3.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 16
4. Further Use Cases . . . . . . . . . . . . . . . . . . . . . . 17
4.1. CDNI . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1.1. Multiple Parallel Delegates . . . . . . . . . . . . . 17
4.1.2. Chained Delegation . . . . . . . . . . . . . . . . . 17
4.2. STIR . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
5.1. New Fields in the "meta" Object within a Directory
Object . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2. New Fields in the Order Object . . . . . . . . . . . . . 22
5.3. New Fields in the Account Object . . . . . . . . . . . . 22
5.4. New Fields for Identifiers . . . . . . . . . . . . . . . 22
5.5. CSR Template Extensions . . . . . . . . . . . . . . . . . 23
6. Security Considerations . . . . . . . . . . . . . . . . . . . 23
6.1. Trust Model . . . . . . . . . . . . . . . . . . . . . . . 24
6.2. Delegation Security Goal . . . . . . . . . . . . . . . . 24
6.3. New ACME Channels . . . . . . . . . . . . . . . . . . . . 24
6.4. Restricting CDNs to the Delegation Mechanism . . . . . . 25
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.1. Normative References . . . . . . . . . . . . . . . . . . 26
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8.2. Informative References . . . . . . . . . . . . . . . . . 27
Appendix A. Document History . . . . . . . . . . . . . . . . . . 28
A.1. draft-ietf-acme-star-delegation-04 . . . . . . . . . . . 28
A.2. draft-ietf-acme-star-delegation-03 . . . . . . . . . . . 28
A.3. draft-ietf-acme-star-delegation-02 . . . . . . . . . . . 29
A.4. draft-ietf-acme-star-delegation-01 . . . . . . . . . . . 29
A.5. draft-ietf-acme-star-delegation-00 . . . . . . . . . . . 29
A.6. draft-sheffer-acme-star-delegation-01 . . . . . . . . . . 29
A.7. draft-sheffer-acme-star-delegation-00 . . . . . . . . . . 29
Appendix B. CSR Template Schema . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
This document is a companion document to [RFC8739]. To avoid
duplication, we give here a bare-bones description of the motivation
for this solution. For more details and further use cases, please
refer to the introductory sections of [RFC8739].
An Identifier Owner (IdO), that we can associate in the primary use
case to a content provider (also referred to as Domain Name Owner,
DNO), has agreements in place with one or more NDC (Name Delegation
Consumer) to use and attest its identity. In the primary use case,
we consider a CDN provider contracted to serve the IdO content over
HTTPS. The CDN terminates the HTTPS connection at one of its edge
cache servers and needs to present its clients (browsers, mobile
apps, set-top-boxes) a certificate whose name matches the authority
of the URL that is requested, i.e., that of the IdO. Understandably,
most IdOs balk at sharing their long-term private keys with another
organization and, equally, delegates would rather not have to handle
other parties' long-term secrets.
Other relevant use cases are discussed in Section 4.
This document describes a profile of the ACME protocol [RFC8555] that
allows the NDC to request the IdO, acting as a profiled ACME server,
a certificate for a delegated identity - i.e., one belonging to the
IdO. The IdO then uses the ACME protocol (with the extensions
described in [RFC8739]) to request issuance of a STAR certificate for
the same delegated identity. The generated short-term certificate is
automatically renewed by the ACME Certification Authority (CA),
periodically fetched by the NDC and used to terminate HTTPS
connections in lieu of the IdO. The IdO can end the delegation at
any time by simply instructing the CA to stop the automatic renewal
and letting the certificate expire shortly thereafter.
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In case the delegated identity is a domain name, this document also
provides a way for the NDC to inform the IdO about the CNAME mappings
that need to be installed in the IdO's DNS zone to enable the
aliasing of the delegated name, thus allowing the complete name
delegation workflow to be handled using a single interface.
While the primary use case we address is delegation of STAR
certificates, the mechanism proposed here accommodates any
certificate managed with the ACME protocol. See Section 2.4 for
details.
1.1. Terminology
IdO Identifier Owner, the owner of an identifier (e.g., a domain
name) that needs to be delegated.
DNO Domain Name Owner, a specific kind of IdO whose identifier is a
domain name
NDC Name Delegation Consumer, the entity to which the domain name is
delegated for a limited time. This is a CDN in the primary use
case (in fact, readers may note the symmetry of the two acronyms).
CDN Content Delivery Network, a widely distributed network that
serves the domain's web content to a wide audience at high
performance.
STAR Short-Term, Automatically Renewed X.509 certificates.
ACME The IETF Automated Certificate Management Environment, a
certificate management protocol.
CA A Certificate Authority that implements the ACME protocol.
1.2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Protocol Flow
This section presents the protocol flow. For completeness, we
include the ACME profile proposed in this draft as well as the
extended ACME protocol described in [RFC8739].
2.1. Preconditions
The protocol assumes the following preconditions are met:
* The IdO exposes an ACME server interface to the NDC(s) comprising
the account management interface;
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* The NDC has registered an ACME account with the IdO;
* NDC and IdO have agreed on a "CSR template" to use, including at a
minimum: subject name (e.g., "somesite.example.com"), requested
algorithms and key length, key usage, extensions (e.g.,
TNAuthList). The NDC is required to use this template for every
CSR created under the same delegation;
* IdO has registered an ACME account with the Certificate Authority
(CA)
Note that even if the IdO implements the ACME server role, it is not
acting as a CA: in fact, from the point of view of the certificate
issuance process, the IdO only works as a "policing" forwarder of the
NDC's key-pair and is responsible for completing the identity
verification process towards the ACME CA.
2.2. Overview
The interaction between the NDC and the IdO is governed by the
profiled ACME workflow detailed in Section 2.3. The interaction
between the IdO and the CA is ruled by ACME STAR [RFC8739] as well as
any other ACME extension that applies (e.g.,
[I-D.ietf-acme-authority-token-tnauthlist] for STIR).
The outline of the combined protocol is as follow (Figure 1):
* NDC sends an order Order1 for the delegated identifier to IdO;
* IdO creates an Order1 resource in state "ready" with a "finalize"
URL;
* NDC immediately sends a finalize request (which includes the CSR)
to the IdO;
* IdO verifies the CSR according to the agreed upon CSR template;
* If the CSR verification fails, Order1 is moved to an "invalid"
state and everything stops;
* If the CSR verification is successful, IdO moves Order1 to state
"processing", and sends a new Order2 (using its own account) for
the delegated identifier to the ACME STAR CA;
* If the ACME STAR protocol fails, Order2 moves to "invalid" and the
same state is reflected in the NDC Order;
* If the ACME STAR run is successful (i.e., Order2 is "valid"), IdO
copies the "star-certificate" URL from Order2 to Order1 and moves
its state to "valid".
The NDC can now download, install and use the short-term certificate
bearing the name delegated by the IdO. This can continue until the
STAR certificate expires or the IdO decides to cancel the automatic
renewal process with the ACME STAR CA.
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Note that, because the identity validation is suppressed, the NDC
sends the finalize request, including the CSR, to the IdO immediately
after Order1 has been acknowledged. The IdO must buffer a (valid)
CSR until the Validation phase completes successfully.
.------. .---------------. .------.
| NDC | | IdO | | CA |
+--------+ +--------+--------+ +--------+
| Client | | Server | Client | | Server |
'---+----' '----+---+---+----' '----+---'
| | | |
| Order1 | | |
| Signature | | |
o------------------->| | |
| | | |
| [ No identity ] | | |
| [ validation ] | | |
| | | |
| CSR | | |
| Signature | | |
o------------------->| | |
| Acknowledgement | | Order2 |
|<-------------------o | Signature |
| | o------------------->|
| | | Required |
| | | Authorizations |
| | |<-------------------o
| | | Responses |
| | | Signature |
| | o------------------->|
| | | |
| | |<~~~~Validation~~~~>|
| | | |
| | | CSR |
| | | Signature |
| | o------------------->|
| | | Acknowledgement |
| | |<-------------------o
| | | |
|<~~Await issuance~->| |<~~Await issuance~~>|
| |
| (unauthenticated) GET STAR certificate |
o------------------------------------------------>|
| Certificate #1 |
|<------------------------------------------------o
| (unauthenticated) GET STAR certificate |
o------------------------------------------------>|
| Certificate #2 |
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|<------------------------------------------------o
| [...] |
| (unauthenticated) GET STAR certificate |
o------------------------------------------------>|
| Certificate #n |
|<------------------------------------------------o
Figure 1: End to end STAR delegation flow
2.3. Delegated Identity Profile
This section defines a profile of the ACME protocol, to be used
between the NDC and IdO.
2.3.1. Delegation Configuration
An NDC identifies itself to the IdO as an ACME account. The IdO can
delegate multiple names through each NDC, and these configurations
are described through "delegation" objects associated with the NDC's
Account object on the IdO. A delegation configuration object
contains the CSR template (see Section 3) that applies to that
delegation. Its structure is as follows:
* csr-template (required, object): CSR template as defined in
Section 3.
An example delegation object is shown in Figure 2.
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{
"csr-template": {
"keyTypes": [
{
"PublicKeyType": "ecPublicKey",
"Curve": "secp521r1",
"SignatureType": "ecdsa-with-SHA256"
}
],
"subject": {
"country": "CA",
"stateOrProvince": "**",
"locality": "**",
"commonName": "**"
},
"extensions": {
"subjectAltName": {
"DNS": [
"abc.ndc.dno.example"
]
},
"keyUsage": [
"digitalSignature"
],
"extendedKeyUsage": [
"serverAuth"
]
}
}
}
Figure 2: Example Delegation Configuration object
In order to list all the delegation configuration objects that are
associated with the NDC account, a new (read-only) "delegations"
attribute is added to the Account object. The value of this
attribute is an array of URLs each pointing to a delegation
configuration object as shown in Figure 3.
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{
"status": "valid",
"contact": [
"mailto:delegation-admin@ido.example"
],
"termsOfServiceAgreed": true,
"orders": "https://example.com/acme/orders/rzGoeA",
"delegations": [
"https://acme.dno.example/acme/acct/ndc/delegations/1",
"https://acme.dno.example/acme/acct/ndc/delegations/2"
]
}
Figure 3: Example Account object with delegations
In order to indicate which specific delegation applies to the
requested certificate a new "delegation" attribute is added to the
Order object on the NDC-IdO side (see Section 2.3.2). The value of
this attribute is the URL pointing to the delegation configuration
object that is to be used for this certificate request.
2.3.2. Order Object on the NDC-IdO side
The Order object created by the NDC:
* MUST contain a "delegation" attribute indicating the configuration
used for this request;
* MUST contain identifiers with the new "delegated" field set to
true;
* MUST NOT contain the "notBefore" and "notAfter" fields;
* MUST contain an "auto-renewal" object and inside it, the fields
listed in Section 3.1.1 of [RFC8739];
* In case the identifier type is "dns", it MAY contain a "cname"
field with the alias of the identifier in the NDC domain. This
field is used by the IdO to create the DNS aliasing needed to
redirect the resolvers to the delegated entity.
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POST /acme/new-order HTTP/1.1
Host: acme.dno.example
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://acme.dno.example/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://acme.dno.example/acme/new-order"
}),
"payload": base64url({
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.dno.example.",
"delegated": true,
"cname": "abc.ndc.example."
}
],
"auto-renewal": {
"end-date": "2020-04-20T00:00:00Z",
"lifetime": 345600, // 4 days
"allow-certificate-get": true
},
"delegation":
"https://acme.dno.example/acme/acct/ndc/delegations/2"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
The Order object that is created on the IdO:
* MUST start in the "ready" state;
* MUST contain an "authorizations" array with zero elements;
* MUST NOT contain the "notBefore" and "notAfter" fields;
* MUST contain the indicated "delegation" configuration.
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{
"status": "ready",
"expires": "2019-05-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.dno.example.",
"delegated": true,
"cname": "abc.ndc.example."
}
],
"auto-renewal": {
"end-date": "2020-04-20T00:00:00Z",
"lifetime": 345600,
"allow-certificate-get": true
},
"delegation":
"https://acme.dno.example/acme/acct/ndc/delegations/2",
"authorizations": [],
"finalize": "https://acme.dno.example/acme/order/TO8rfgo/finalize"
}
The IdO MUST copy the "auto-renewal" object from the NDC request into
the related STAR request to the ACME CA.
When the validation of the identifiers has been successfully
completed and the certificate has been issued by the CA, the IdO:
* MUST move its Order resource status to "valid";
* MUST copy the "star-certificate" field from the STAR Order;
The latter indirectly includes (via the NotBefore and NotAfter HTTP
headers) the renewal timers needed by the NDC to inform its
certificate reload logic.
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{
"status": "valid",
"expires": "2019-05-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.dno.example.",
"delegated": true,
"cname": "abc.ndc.example."
}
],
"auto-renewal": {
"end-date": "2020-04-20T00:00:00Z",
"lifetime": 345600,
"allow-certificate-get": true
},
"delegation":
"https://acme.dno.example/acme/acct/ndc/delegations/2",
"authorizations": [],
"finalize": "https://acme.dno.example/acme/order/TO8rfgo/finalize",
"star-certificate": "https://acme.ca.example/acme/order/yTr23sSDg9"
}
If an "identifier" attribute of type "dns" was included, the IdO MUST
validate the specified CNAME at this point in the flow. At the
minimum, the IdO MUST verify that both DNS names are syntactically
valid, to prevent a malicious NDC from injecting arbitrary data into
a DNS zone file.
Following this validation, the IdO can add the CNAME records to its
zone:
abc.ndc.dno.example. CNAME abc.ndc.example.
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2.3.3. Order Object on the IdO-CA side
When sending the Order to the ACME CA, the IdO SHOULD strip the
"delegated" and "cname" attributes sent by the NDC (Section 2.3.2).
The IdO MUST add the necessary STAR extensions to the Order. In
addition, to allow the NDC to download the certificate using
unauthenticated GET, the IdO MUST add the "auto-renewal" object and
inside it, include the "allow-certificate-get" attribute and set it
to true.
2.3.4. Capability Discovery
In order to help a client to discover support for this profile, the
directory object of an ACME server MUST contain the following
attribute in the "meta" field:
* delegation-enabled: boolean flag indicating support for the
profile specified in this memo. An ACME server that supports this
delegation profile MUST include this key, and MUST set it to true.
The "delegation-enabled" flag may be specified regardless of the
existence or setting of the "auto-renewal" flag.
2.3.5. On Cancellation
It is worth noting that cancellation of the ACME STAR certificate is
a prerogative of the IdO. The NDC does not own the relevant account
key on the ACME CA, therefore it can't issue a cancellation request
for the STAR cert. Potentially, since it holds the STAR
certificate's private key, it could request the revocation of a
single STAR certificate. However, STAR explicitly disables the
revokeCert interface.
2.4. Delegation of Non-STAR Certificates
The mechanism defined here can be used to delegate regular ACME
certificates whose expiry is not "short term".
To allow delegation of non-STAR certificates, this document allows
use of "allow-certificate-get" directly in the Order object and
independently of the "auto-renewal" object, so that the NDC can fetch
the certificate without having to authenticate into the ACME server.
The following differences exist between STAR and non-STAR certificate
delegation:
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* With STAR certificates, the "star-certificate" field is copied by
the IdO; with non-STAR certificates, the "certificate" field is
copied.
* The "auto-renewal" object is not used (either in the request or
response) for non-STAR certificates. The field "allow-
certificate-get" MUST be included in the order object, and its
value MUST be "true".
* The "notBefore" and "notAfter" order fields are omitted only in
STAR certificates.
When delegating a non-STAR certificate, standard certificate
revocation still applies. The ACME certificate revocation endpoint
is explicitly unavailable for STAR certificates but it is available
for all other certificates. We note that according to Sec. 7.6 of
[RFC8555], the revocation endpoint can be used with either the
account keypair, or the certificate keypair. In other words, the NDC
would be able to revoke the certificate. The authors believe that
this is a very minor security risk.
2.5. Proxy Behavior
There are cases where the ACME Delegation flow should be proxied,
such as the use case described in Section 4.1.2. This section
describes the behavior of such proxies.
An ACME Delegation server can decide, on a per-identity case, whether
to act as a proxy into another ACME Delegation server, or to behave
as an IdO and obtain a certificate directly. The determining factor
is whether the server can successfully be authorized by the ACME
Server for the identity associated with the certificate request.
The identities supported by each server and the disposition for each
of them are preconfigured.
Following is the proxy's behavior for each of the messages exchanged
in the ACME Delegation process:
* New-order request:
- The complete "identifiers" object MUST be copied as-is.
- Similarly, the "auto-renewal" object MUST be copied as-is.
* New-order response:
- The "status", "expires", "authorizations", "identifiers" and
"auto-renewal" attributes/objects MUST be copied as-is.
- The "finalize" URL is rewritten, so that the "finalize" request
will be made to the proxy.
- Similarly, the Location header is rewritten.
* Get Order response:
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- The "status", "expires", "authorizations", "identifiers" and
"auto-renewal" attributes/objects MUST be copied as-is.
- Similarly, the "star-certificate" URL MUST be copied as-is.
- The "finalize" URL is rewritten, so that the "finalize" request
will be made to the proxy.
- The "Location" header must be rewritten.
* Finalize request:
- The CSR MUST be copied as-is.
* Finalize response:
- Both the "Location" header and the "finalize" URLs are
rewritten.
We note that all the above messages are authenticated, and therefore
each proxy must be able to authenticate any subordinate server.
3. CSR Template
The CSR template is used to express and constrain the shape of the
CSR that the NDC uses to request the certificate. The CSR is used
for every certificate created under the same delegation. Its
validation by the IdO is a critical element in the security of the
whole delegation mechanism.
Instead of defining every possible CSR attribute, this document takes
a minimalist approach by declaring only the minimum attribute set and
deferring the registration of further, more specific, attributes to
future documents.
3.1. Template Syntax
The template is a JSON document. Each field denotes one of:
* A mandatory field, where the template specifies the literal value
of that field. This is denoted by a literal string, such as
"client1.ndc.dno.example.com".
* A mandatory field, where the content of the field is defined by
the client. This is denoted by "\*\*".
* An optional field, where the client decides whether the field is
included in the CSR and what its value is. This is denoted by
"\*".
The NDC MUST NOT include in the CSR any fields that are not specified
in the template, and in particular MUST NOT add any extensions unless
those were previously negotiated out of band with the IdO.
The mapping between X.509 CSR fields and the template will be defined
in a future revision of this document.
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When the CSR is received by the IdO, it MUST verify that the CSR is
consistent with the template that the IdO sent earlier. The IdO MAY
enforce additional constraints, e.g. by restricting field lengths.
3.2. Example
The CSR template in Figure 4 represents one possible CSR template
governing the delegation exchanges provided in the rest of this
document.
{
"keyTypes": [
{
"PublicKeyType": "RSA",
"PublicKeyLength": 4096,
"SignatureType": "sha256WithRSAEncryption"
}
],
"subject": {
"country": "CA",
"stateOrProvince": "**",
"locality": "**",
"commonName": "**"
},
"extensions": {
"subjectAltName": {
"DNS": [
"client1.ndc.dno.example"
],
"IP": [
"1.2.3.4",
"13::17"
]
},
"keyUsage": [
"digitalSignature"
],
"extendedKeyUsage": [
"serverAuth",
"timeStamping"
]
}
}
Figure 4: Example CSR template
The template syntax is defined in Appendix B.
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4. Further Use Cases
4.1. CDNI
[I-D.ietf-cdni-interfaces-https-delegation] discusses several
solutions addressing different delegation requirements for the CDNI
(CDN Interconnection) environment. This section discusses two of the
stated requirements in the context of the STAR delegation workflow.
4.1.1. Multiple Parallel Delegates
In some cases the content owner (IdO) would like to delegate
authority over a web site to multiple NDCs (CDNs). This could happen
if the IdO has agreements in place with different regional CDNs for
different geographical regions, or if a "backup" CDN is used to
handle overflow traffic by temporarily altering some of the CNAME
mappings in place. The STAR delegation flow enables this use case
naturally, since each CDN can authenticate separately to the IdO (via
its own separate account) specifying its CSR, and the IdO is free to
allow or deny each certificate request according to its own policy.
4.1.2. Chained Delegation
In other cases, a content owner (IdO) delegates some domains to a
large CDN (uCDN), which in turn delegates to a smaller regional CDN,
dCDN. The DNO has a contractual relationship with uCDN, and uCDN has
a similar relationship with dCDN. However IdO may not even know
about dCDN.
If needed, the STAR protocol can be chained to support this use case:
uCDN could forward requests from dCDN to DNO, and forward responses
back to dCDN. Whether such proxying is allowed is governed by policy
and contracts between the parties.
A mechanism is necessary at the interface between uCDN and dCDN by
which the uCDN can advertise:
* The namespace that is made available to the dCDN to mint its
delegated names;
* The policy for creating the key material (allowed algorithms,
minimum key lengths, key usage, etc.) that the dCDN needs to
satisfy.
Note that such mechanism is provided by the CSR template.
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4.1.2.1. Two-Level Delegation in CDNI
A User Agent (browser or set-top-box) wants to fetch the video
resource at the following URI: "https://video.cp.example/movie".
Redirection between Content Provider, upstream, and downstream CDNs
is arranged as a CNAME-based aliasing chain as illustrated in
Figure 5.
.------------.
video.cp.example ? | .-----. |
.---------------------------------->| | |
| (a) | | DNS | CP |
| .-------------------------------+ | |
| | CNAME video.ucdn.example | '-----' |
| | '------------'
| |
| |
.-----------|---v--. .------------.
| .-----.-+-----. | video.ucdn.example ? | .-----. |
| | | +----------------------------->| | |
| UA | TLS | DNS | | (b) | | DNS | uCDN |
| | | |<-----------------------------+ | |
| '--+--'-----+-' | CNAME video.dcdn.example | '-----' |
'------|----^---|--' '------------'
| | |
| | |
| | | .------------.
| | | video.dcdn.example ? | .-----. |
| | '------------------------------>| | |
| | (c) | | DNS | |
| '-----------------------------------+ | |
| A 192.0.2.1 | +-----+ dCDN |
| | | | |
'--------------------------------------->| TLS | |
SNI: video.cp.example | | | |
| '-----' |
'------------'
Figure 5: DNS Redirection
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Unlike HTTP based redirection, where the original URL is supplanted
by the one found in the Location header of the 302 response, DNS
redirection is completely transparent to the User Agent. As a
result, the TLS connection to the dCDN edge is done with an SNI equal
to the "host" in the original URI - in the example,
"video.cp.example". So, in order to successfully complete the
handshake, the landing dCDN node has to be configured with a
certificate whose SAN matches "video.cp.example", i.e., a Content
Provider's name.
Figure 6 illustrates the cascaded delegation flow that allows dCDN to
obtain a STAR certificate that bears a name belonging to the Content
Provider with a private key that is only known to the dCDN.
.--------------------.
| .------.------. |
| | STAR | ACME |<-------------.
.------->| CP | dele | STAR | | |
| | | srv | cli +-----. |
| | '---+--'------' | | 6
| '---------|------^---' 5 |
| | | | .--|-------.
| | | | | .-+----. |
| 7 | '---->| ACME | |
| | | | | STAR | C |
0 | 4 | +------| A |
| | | | | HTTP | |
| | | | '----+-' |
| | .-' '--^--|----'
| .--------------v--|--. | |
| | .------.----+-. | | 10
| | | | STAR | | | |
'-->| uCDN | CDNI | dele | | | |
| | | fwd | | | |
| '----+-'-+----' | | |
'-------^--|---|--^--' | |
| | | | | |
| 2 8 | | |
1 | | 3 | |
| | | | 9 |
.-------|--v---v--|---------. | |
| .-+----.----+-.------. | | |
| | | STAR | +------------' |
| dCDN | CDNI | dele | HTTP | | |
| | | cli | |<--------------'
| '------'------'------' |
'---------------------------'
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Figure 6: Two levels delegation in CDNI
TBD bootstrap, see https://github.com/yaronf/I-D/issues/47
1. dCDN requests CDNI path metadata to uCDN;
2. uCDN replies with, among other CDNI things, the STAR delegation
configuration, which includes the delegated Content Provider's
name;
3. dCDN creates a key-pair and the CSR with the delegated name. It
then places an order for the delegated name to uCDN;
4. uCDN forwards the received order to the Content Provider (CP);
5. CP creates an order for a STAR certificate and sends it to the
ACME CA. The order also requests unauthenticated access to the
certificate resource;
6. After all authorizations complete successfully, the STAR
certificate is issued;
7. CP notifies uCDN that the STAR cert is available at the order's
star-certificate URL;
8. uCDN forwards the information to dCDN. At this point the ACME
signalling is complete;
9. dCDN requests the STAR cert using unauthenticated GET from the
ACME CA;
10. the CA returns the certificate. Now dCDN is fully configured to
handle HTTPS traffic in-lieu of the Content Provider.
Note that 9. and 10. repeat until the delegation expires or is
terminated.
4.2. STIR
As a second use case, we consider the delegation of credentials in
the STIR ecosystem [I-D.ietf-stir-cert-delegation].
In the STIR "delegated" mode, a service provider SP2 - the NDC -
needs to sign PASSPorT's [RFC8225] for telephone numbers (e.g.,
TN=+123) belonging to another service provider, SP1 - the IdO. In
order to do that, SP2 needs a STIR certificate, and private key, that
includes TN=+123 in the TNAuthList [RFC8226] cert extension.
In details (Figure 7):
1. SP1 and SP2 agree on the configuration of the delegation - in
particular, the CSR template that applies;
2. SP2 generates a private/public key-pair and sends a CSR to SP1
requesting creation of a certificate with: SP1 name, SP2 public
key, and a TNAuthList extension with the list of TNs that SP1
delegates to SP2. (Note that the CSR sent by SP2 to SP1 needs to
be validated against the CSR template agreed upon in step 1.);
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3. SP1 sends an Order for the CSR to the ACME STAR CA;
4. Subsequently, after the required TNAuthList authorizations are
successfully completed, the ACME STAR CA moves the Order to a
"valid" state; at the same time the star-certificate endpoint is
populated.
5. The Order contents are forwarded from SP1 to SP2 by means of the
paired "delegation" Order.
6. SP2 dereferences the star-certificate URL in the Order to fetch
the rolling STAR certificate bearing the delegated identifiers.
.-------------------.
| .------.------. |
| | STAR | STAR |<--------------.
.-->| SP1 | dele | dele | | |
| | | srv | cli +-----. |
| | '----+-'------' | | 4
| '------^--|---------' 3 |
| | | | .----|-----.
| | 5 | | .---+--. |
| | | '--->| ACME | |
| | | | | STAR | C |
1 | | | +------| A |
| | | .--->| HTTP | |
| 2 | | | '---+--' |
| | | | '----|-----'
| .------|--v---------. 6 |
| | .-+----.------. | | 7
| | | STAR | +-----' |
'-->| SP2 | dele | HTTP | | |
| | cli | |<--------------'
| '----+-'-+----' |
'-------------------'
Figure 7: Delegation in STIR
As shown, the STAR delegation profile described in this document
applies straightforwardly, the only extra requirement being the
ability to instruct the NDC about the allowed TNAuthList values.
This can be achieved by a simple extension to the CSR template.
5. IANA Considerations
[[RFC Editor: please replace XXXX below by the RFC number.]]
5.1. New Fields in the "meta" Object within a Directory Object
This document adds the following entries to the ACME Directory
Metadata Fields:
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+====================+============+===========+
| Field Name | Field Type | Reference |
+====================+============+===========+
| delegation-enabled | boolean | RFC XXXX |
+--------------------+------------+-----------+
Table 1
5.2. New Fields in the Order Object
This document adds the following entries to the ACME Order Object
Fields:
+=======================+============+==============+===========+
| Field Name | Field Type | Configurable | Reference |
+=======================+============+==============+===========+
| allow-certificate-get | boolean | true | RFC XXXX |
+-----------------------+------------+--------------+-----------+
| delegation | string | true | RFC XXXX |
+-----------------------+------------+--------------+-----------+
Table 2
Note that the delegation field is only meaningful in interactions
with ACME servers that have "delegation-enabled" set to true in their
meta Object.
5.3. New Fields in the Account Object
This document adds the following entries to the ACME Account Object
Fields:
+=============+==================+==========+===========+
| Field Name | Field Type | Requests | Reference |
+=============+==================+==========+===========+
| delegations | array of strings | none | RFC XXXX |
+-------------+------------------+----------+-----------+
Table 3
Note that the delegations field is only reported by ACME servers that
have "delegation-enabled" set to true in their meta Object.
5.4. New Fields for Identifiers
This document adds the following entries to each element of the ACME
"identifiers" array of objects:
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+============+============+
| Field Name | Field Type |
+============+============+
| delegated | boolean |
+------------+------------+
| cname | string |
+------------+------------+
Table 4
We note that [RFC8555] does not define a registry for these objects.
5.5. CSR Template Extensions
IANA is requested to establish a registry "STAR Delegation CSR
Template Extensions", with "Expert Review" as its registration
procedure.
Each extension registered must specify:
* An extension name
* An extension syntax, as a JSON Schema snippet that defines a type
* Mapping into an X.509 certificate extension.
The initial contents of this registry are the extensions defined by
the JSON Schema document in Appendix B.
+==================+============+=================================+
| Extension Name | Type | Mapping to X.509 |
+==================+============+=================================+
| keyUsage | See | [RFC5280], Sec. 4.2.1.3 |
| | Appendix B | |
+------------------+------------+---------------------------------+
| extendedKeyUsage | See | [RFC5280], Sec. 4.2.1.12 |
| | Appendix B | |
+------------------+------------+---------------------------------+
| subjectAltName | See | [RFC5280], Sec. 4.2.1.6 (only |
| | Appendix B | for the supported name formats) |
+------------------+------------+---------------------------------+
Table 5
6. Security Considerations
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6.1. Trust Model
The ACME trust model needs to be extended to include the trust
relationship between NDC and IdO. Note that once this trust link is
established, it potentially becomes recursive. Therefore, there has
to be a trust relationship between each of the nodes in the
delegation chain; for example, in case of cascading CDNs this is
contractually defined. Note that using standard [RFC6125] identity
verification there are no mechanisms available to the IdO to restrict
the use of the delegated name once the name has been handed over to
the first NDC.
6.2. Delegation Security Goal
Delegation introduces a new security goal: only an NDC that has been
authorised by the IdO, either directly or transitively, can obtain a
cert with an IdO identity.
From a security point of view, the delegation process has two
separate parts:
1. Enabling a specific third party (the intended NDC) to submit
requests for delegated certificates;
2. Making sure that any request for a delegated certificate matches
the intended "shape" in terms of delegated identities as well as
any other certificate metadata, e.g., key length, x.509
extensions, etc.
The first part is covered by the NDC's ACME account that is
administered by the IdO, whose security relies on the correct
handling of the associated key pair. When a compromise of the
private key is detected, the delegate MUST use the account
deactivation procedures defined in Section 7.3.6 of [RFC8555].
The second part is covered by the act of checking an NDC's
certificate request against the intended CSR template. The steps of
shaping the CSR template correctly, selecting the right CSR template
to check against the presented CSR, and making sure that the
presented CSR matches the selected CSR template are all security
relevant.
6.3. New ACME Channels
Using the model established in Section 10.1 of [RFC8555], we can
decompose the interactions of the basic delegation workflow as shown
in Figure 8.
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ACME Channel
.------------>------------.
.-----. ACME Channel .--+--. .--+----------.
| NDC +------------->| IdO | | ACME server |
'--+--' '--+--' '--+-+--------'
| '-----------<-------------' |
| Validation Channel |
'-------------------->---------------------------'
(subset of) ACME Channel [1]
[1] Unauthenticated certificate fetch and non-STAR certificate
revocation.
Figure 8: Delegation Channels Topology
The considerations regarding the security of the ACME Channel and
Validation Channel discussed in [RFC8555] apply verbatim to the IdO/
ACME server leg. The same can be said for the ACME channel on the
NDC/IdO leg. A slightly different set of considerations apply to the
ACME Channel between NDC and ACME server, which consists of a subset
of the ACME interface comprising two API endpoints: the
unauthenticated certificate retrieval and, potentially, non-STAR
revocation via certificate private key. No specific security
considerations apply to the former, but the privacy considerations in
Section 6.3 of [RFC8739] do. With regards to the latter, it should
be noted that there is currently no means for an IdO to disable
authorising revocation based on certificate private keys. So, in
theory, an NDC could use the revocation API directly with the ACME
server, therefore bypassing the IdO. The NDC SHOULD NOT directly use
the revocation interface exposed by the ACME server unless failing to
do so would compromise the overall security, for example if the
certificate private key is compromised and the IdO is not currently
reachable.
All other security considerations from [RFC8555] and [RFC8739] apply
as-is to the delegation topology.
6.4. Restricting CDNs to the Delegation Mechanism
When a web site is delegated to a CDN, the CDN can in principle
modify the web site at will, create and remove pages. This means
that a malicious or breached CDN can pass the ACME (as well as common
non-ACME) HTTPS-based validation challenges and generate a
certificate for the site. This is true regardless of whether the
CNAME mechanisms defined in the current document is used or not.
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In some cases, this is the desired behavior: the domain owner trusts
the CDN to have full control of the cryptographic credentials for the
site. The current document however assumes that the domain owner
only wants to delegate restricted control, and wishes to retain the
capability to cancel the CDN's credentials at a short notice.
Following is the proposed solution where the IdO wishes to ensure
that a rogue CDN cannot issue unauthorized certificates:
* The domain owner makes sure that the CDN cannot modify the DNS
records for the domain. The domain owner should ensure it is the
only entity authorized to modify the DNS zone. Typically, it
establishes a CNAME resource record from a subdomain into a CDN-
managed domain.
* The domain owner uses a CAA record [RFC6844] to restrict
certificate issuance for the domain to specific CAs that comply
with ACME and are known to implement [RFC8657].
* The domain owner uses the ACME-specific CAA mechanism [RFC8657] to
restrict issuance to a specific account key which is controlled by
it, and MUST require "dns-01" as the sole validation method.
We note that the above solution may need to be tweaked depending on
the exact capabilities and authorisation flows supported by the
selected CAs.
7. 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.
8. References
8.1. Normative References
[I-D.handrews-json-schema]
Wright, A., Andrews, H., Hutton, B., and G. Dennis, "JSON
Schema: A Media Type for Describing JSON Documents", Work
in Progress, Internet-Draft, draft-handrews-json-schema-
02, 17 September 2019, <http://www.ietf.org/internet-
drafts/draft-handrews-json-schema-02.txt>.
[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>.
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[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
<https://www.rfc-editor.org/info/rfc6844>.
[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>.
[RFC8657] Landau, H., "Certification Authority Authorization (CAA)
Record Extensions for Account URI and Automatic
Certificate Management Environment (ACME) Method Binding",
RFC 8657, DOI 10.17487/RFC8657, November 2019,
<https://www.rfc-editor.org/info/rfc8657>.
[RFC8739] Sheffer, Y., Lopez, D., Gonzalez de Dios, O., Pastor
Perales, A., and T. Fossati, "Support for Short-Term,
Automatically Renewed (STAR) Certificates in the Automated
Certificate Management Environment (ACME)", RFC 8739,
DOI 10.17487/RFC8739, March 2020,
<https://www.rfc-editor.org/info/rfc8739>.
8.2. Informative References
[I-D.ietf-acme-authority-token-tnauthlist]
Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
"TNAuthList profile of ACME Authority Token", Work in
Progress, Internet-Draft, draft-ietf-acme-authority-token-
tnauthlist-06, 9 March 2020, <http://www.ietf.org/
internet-drafts/draft-ietf-acme-authority-token-
tnauthlist-06.txt>.
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[I-D.ietf-cdni-interfaces-https-delegation]
Fieau, F., Emile, S., and S. Mishra, "CDNI extensions for
HTTPS delegation", Work in Progress, Internet-Draft,
draft-ietf-cdni-interfaces-https-delegation-03, 9 March
2020, <http://www.ietf.org/internet-drafts/draft-ietf-
cdni-interfaces-https-delegation-03.txt>.
[I-D.ietf-stir-cert-delegation]
Peterson, J., "STIR Certificate Delegation", Work in
Progress, Internet-Draft, draft-ietf-stir-cert-delegation-
03, 13 July 2020, <http://www.ietf.org/internet-drafts/
draft-ietf-stir-cert-delegation-03.txt>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.
[RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", RFC 8226,
DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/info/rfc8226>.
Appendix A. Document History
[[Note to RFC Editor: please remove before publication.]]
A.1. draft-ietf-acme-star-delegation-04
* Delegation of non-STAR certificates.
* More IANA clarity, specifically on certificate extensions.
* Add delegation configuration object and extend account and order
objects accordingly.
* A lot more depth on Security Considerations.
A.2. draft-ietf-acme-star-delegation-03
* Consistency with the latest changes in the base ACME STAR
document, e.g. star-delegation-enabled capability renamed and
moved.
* Proxy use cases (recursive delegation) and the definition of proxy
behavior.
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* More detailed analysis of the CDNI and STIR use cases, including
sequence diagrams.
A.3. draft-ietf-acme-star-delegation-02
* Security considerations: review by Ryan Sleevi.
* CSR template simplified: instead of being a JSON Schema document
itself, it is now a simple JSON document which validates to a JSON
Schema.
A.4. draft-ietf-acme-star-delegation-01
* Refinement of the CDNI use case.
* Addition of the CSR template (partial, more work required).
* Further security considerations (work in progress).
A.5. draft-ietf-acme-star-delegation-00
* Republished as a working group draft.
A.6. draft-sheffer-acme-star-delegation-01
* Added security considerations about disallowing CDNs from issuing
certificates for a delegated domain.
A.7. draft-sheffer-acme-star-delegation-00
* Initial version, some text extracted from draft-sheffer-acme-star-
requests-02
Appendix B. CSR Template Schema
Following is a JSON Schema definition of the CSR template. The
syntax used is that of draft 7 of JSON Schema, which may not be the
latest version of the corresponding Internet Draft
[I-D.handrews-json-schema] at the time of publication.
While the CSR template must follow the syntax defined here, neither
the IdO nor the NDC are expected to validate it at run-time.
{
"title": "JSON Schema for the STAR Delegation CSR template",
"$schema": "http://json-schema.org/draft-07/schema#",
"$id": "http://ietf.org/acme/drafts/star-delegation/csr-template",
"$def": {
"distinguished-name": {
"$id": "#distinguished-name",
"type": "object",
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"properties": {
"country": {
"type": "string"
},
"stateOrProvince": {
"type": "string"
},
"locality": {
"type": "string"
},
"organization": {
"type": "string"
},
"organizationalUnit": {
"type": "string"
},
"emailAddress": {
"type": "string"
},
"commonName": {
"type": "string"
}
},
"additionalProperties": false
},
"rsaKeyType": {
"$id": "#rsaKeyType",
"type": "object",
"properties": {
"PublicKeyType": {
"type": "string",
"const": "RSA"
},
"PublicKeyLength": {
"type": "integer"
},
"SignatureType": {
"type": "string",
"enum": [
"sha256WithRSAEncryption"
]
}
},
"additionalProperties": false
},
"ecKeyTYpe": {
"$id": "#ecKeyType",
"type": "object",
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"properties": {
"PublicKeyType": {
"type": "string",
"const": "ecPublicKey"
},
"Curve": {
"type": "string",
"enum": [
"secp521r1"
]
},
"SignatureType": {
"type": "string",
"enum": [
"ecdsa-with-SHA256"
]
}
},
"additionalProperties": false
}
},
"type": "object",
"properties": {
"keyTypes": {
"type": "array",
"items": {
"oneOf": [
{
"$ref": "#rsaKeyType"
},
{
"$ref": "#ecKeyType"
}
]
}
},
"subject": {
"$ref": "#distinguished-name"
},
"extensions": {
"type": "object",
"properties": {
"keyUsage": {
"type": "array",
"items": {
"type": "string",
"enum": [
"digitalSignature",
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"nonRepudiation",
"keyEncipherment",
"dataEncipherment",
"keyAgreement",
"keyCertSign",
"cRLSign",
"encipherOnly",
"decipherOnly"
]
}
},
"extendedKeyUsage": {
"type": "array",
"items": {
"type": "string",
"enum": [
"serverAuth",
"clientAuth",
"codeSigning",
"emailProtection",
"timeStamping",
"OCSPSigning"
]
}
},
"subjectAltName": {
"type": "object",
"properties": {
"DNS": {
"type": "array",
"items": {
"type": "string",
"format": "hostname"
}
},
"IP": {
"type": "array",
"items": {
"oneOf": [
{
"type": "string",
"format": "ipv4"
},
{
"type": "string",
"format": "ipv6"
}
]
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}
},
"Email": {
"type": "array",
"items": {
"type": "string",
"format": "email"
}
}
},
"additionalProperties": false
}
},
"additionalProperties": false
}
},
"additionalProperties": false
}
Authors' Addresses
Yaron Sheffer
Intuit
Email: yaronf.ietf@gmail.com
Diego Lopez
Telefonica I+D
Email: diego.r.lopez@telefonica.com
Antonio Agustin Pastor Perales
Telefonica I+D
Email: antonio.pastorperales@telefonica.com
Thomas Fossati
ARM
Email: thomas.fossati@arm.com
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