ACME Integrations
draft-ietf-acme-integrations-04
Network Working Group O. Friel
Internet-Draft R. Barnes
Intended status: Informational Cisco
Expires: December 25, 2021 R. Shekh-Yusef
Auth0
M. Richardson
Sandelman Software Works
June 23, 2021
ACME Integrations
draft-ietf-acme-integrations-04
Abstract
This document outlines multiple advanced use cases and integrations
that ACME facilitates without any modifications or enhancements
required to the base ACME specification. The use cases include ACME
integration with EST, BRSKI and TEAP.
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 December 25, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. ACME Integration with EST . . . . . . . . . . . . . . . . . . 4
4. ACME Integration with BRSKI . . . . . . . . . . . . . . . . . 7
5. ACME Integration with BRSKI Default Cloud Registrar . . . . . 9
6. ACME Integration with TEAP . . . . . . . . . . . . . . . . . 11
7. ACME Integration Considerations . . . . . . . . . . . . . . . 14
7.1. Service Operators . . . . . . . . . . . . . . . . . . . . 14
7.2. CSR Attributes . . . . . . . . . . . . . . . . . . . . . 15
7.3. Certificate Chains and Trust Anchors . . . . . . . . . . 15
7.3.1. EST /cacerts . . . . . . . . . . . . . . . . . . . . 15
7.3.2. TEAP PKCS#7 TLV . . . . . . . . . . . . . . . . . . . 16
7.4. id-kp-cmcRA . . . . . . . . . . . . . . . . . . . . . . . 16
7.5. Error Handling . . . . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9.1. Denial of Service against ACME infrastructure . . . . . . 18
10. Informative References . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
ACME [RFC8555] defines a protocol that a certificate authority (CA)
and an applicant can use to automate the process of domain name
ownership validation and X.509 (PKIX) certificate issuance. The
protocol is rich and flexible and enables multiple use cases that are
not immediately obvious from reading the specification. This
document explicitly outlines multiple advanced ACME use cases
including:
o ACME integration with EST [RFC7030]
o ACME integration with BRSKI [RFC8995]
o ACME integration with BRSKI Default Cloud Registrar
[I-D.ietf-anima-brski-cloud]
o ACME integration with TEAP [RFC7170] and TEAP Update and
Extensions for Bootstrapping [I-D.lear-eap-teap-brski]
The integrations with EST, BRSKI (which is based upon EST), and TEAP
enable automated certificate enrollment for devices.
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ACME for subdomains [I-D.friel-acme-subdomains] outlines how ACME can
be used by a client to obtain a certificate for a subdomain
identifier from a certificate authority where the client has
fulfilled a challenge against a parent domain, but does not need to
fulfil a challenge against the explicit subdomain. This is a useful
optimization when ACME is used to issue certificates for large
numbers of devices as it reduces the domain ownership proof traffic
(DNS or HTTP) and ACME traffic overhead, but is not a necessary
requirement.
2. Terminology
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.
The following terms are defined in the CA/Browser Forum Baseline
Requirements [CAB] and are reproduced here:
o Authorization Domain Name (ADN): The Domain Name used to obtain
authorization for certificate issuance for a given FQDN. The CA
may use the FQDN returned from a DNS CNAME lookup as the FQDN for
the purposes of domain validation. If the FQDN contains a
wildcard character, then the CA MUST remove all wildcard labels
from the left most portion of requested FQDN. The CA may prune
zero or more labels from left to right until encountering a Base
Domain Name and may use any one of the intermediate values for the
purpose of domain validation
o Base Domain Name: The portion of an applied-for FQDN that is the
first domain name node left of a registry-controlled or public
suffix plus the registry-controlled or public suffix (e.g.
"example.co.uk" or "example.com"). For FQDNs where the right-most
domain name node is a gTLD having ICANN Specification 13 in its
registry agreement, the gTLD itself may be used as the Base Domain
Name.
o Certification Authority (CA): An organization that is responsible
for the creation, issuance, revocation, and management of
Certificates. The term applies equally to both Roots CAs and
Subordinate CAs
o Domain Name: The label assigned to a node in the Domain Name
System
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o Domain Namespace: The set of all possible Domain Names that are
subordinate to a single node in the Domain Name System
o Fully-Qualified Domain Name (FQDN): A Domain Name that includes
the labels of all superior nodes in the Internet Domain Name
System.
The following terms are used in this document:
o BRSKI: Bootstrapping Remote Secure Key Infrastructures [RFC8995]
o CMC: Certificate Management over CMS
o CSR: Certificate Signing Request
o EST: Enrollment over Secure Transport [RFC7030]
o RA: PKI Registration Authority
o TEAP: Tunneled Extensible Authentication Protocol [RFC7170]
3. ACME Integration with EST
EST [RFC7030] defines a mechanism for clients to enroll with a PKI
Registration Authority by sending CMC messages over HTTP. EST
section 1 states:
"Architecturally, the EST service is located between a Certification
Authority (CA) and a client. It performs several functions
traditionally allocated to the Registration Authority (RA) role in a
PKI."
EST section 1.1 states that:
"For certificate issuing services, the EST CA is reached through the
EST server; the CA could be logically "behind" the EST server or
embedded within it."
When the CA is logically "behind" the EST RA, EST does not specify
how the RA communicates with the CA. EST section 1 states:
"The nature of communication between an EST server and a CA is not
described in this document."
This section outlines how ACME could be used for communication
between the EST RA and the CA. The example call flow leverages
[I-D.friel-acme-subdomains] and shows the RA proving ownership of a
parent domain, with individual client certificates being subdomains
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under that parent domain. This is an optimization that reduces DNS
and ACME traffic overhead. The RA could of course prove ownership of
every explicit client certificate identifier.
The call flow illustrates the client calling the EST /csrattrs API
before calling the EST /simpleenroll API.
The call flow illustrates the EST RA returning a 202 Retry-After
response to the client's simpleenroll request. This is an optional
step and may be necessary if the interactions between the RA and the
ACME server take some time to complete. The exact details of when
the RA returns a 202 Retry-After are implementation specific.
+--------+ +--------+ +------+ +-----+
| Pledge | | EST RA | | ACME | | DNS |
+--------+ +--------+ +------+ +-----+
| | | |
STEP 1: Pre-Authorization of parent domain
| | | |
| | POST /newAuthz | |
| | "example.com" | |
| |--------------------->| |
| | | |
| | 201 authorizations | |
| |<---------------------| |
| | | |
| | Publish DNS TXT | |
| | "example.com" | |
| |--------------------------------->|
| | | |
| | POST /challenge | |
| |--------------------->| |
| | | Verify |
| | |---------->|
| | 200 status=valid | |
| |<---------------------| |
| | | |
| | Delete DNS TXT | |
| | "example.com" | |
| |--------------------------------->|
| | | |
STEP 2: Pledge enrolls against RA
| | | |
| GET /csrattrs | | |
|--------------------->| | |
| | | |
| 200 OK | | |
| SEQUENCE {AttrOrOID} | | |
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| SAN OID: | | |
| "pledge.example.com" | | |
|<---------------------| | |
| | | |
| POST /simpleenroll | | |
| PCSK#10 CSR | | |
| "pledge.example.com" | | |
|--------------------->| | |
| | | |
| 202 Retry-After | | |
|<---------------------| | |
| | | |
STEP 3: RA places ACME order
| | | |
| | POST /newOrder | |
| | "pledge.example.com" | |
| |--------------------->| |
| | | |
| | 201 status=ready | |
| |<---------------------| |
| | | |
| | POST /finalize | |
| | PKCS#10 CSR | |
| | "pledge.example.com" | |
| |--------------------->| |
| | | |
| | 200 OK status=valid | |
| |<---------------------| |
| | | |
| | POST /certificate | |
| |--------------------->| |
| | | |
| | 200 OK | |
| | PKCS#7 | |
| | "pledge.example.com" | |
| |<---------------------| |
| | | |
STEP 4: Pledge retries enroll
| | | |
| POST /simpleenroll | | |
| PCSK#10 CSR | | |
| "pledge.example.com" | | |
|--------------------->| | |
| | | |
| 200 OK | | |
| PKCS#7 | | |
| "pledge.example.com" | | |
|<---------------------| | |
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4. ACME Integration with BRSKI
BRSKI [RFC8995] is based upon EST [RFC7030] and defines how to
autonomically bootstrap PKI trust anchors into devices via means of
signed vouchers. EST certificate enrollment may then optionally take
place after trust has been established. BRKSI voucher exchange and
trust establishment are based on EST extensions and the certificate
enrollment part of BRSKI is fully based on EST. Similar to EST,
BRSKI does not define how the EST RA communicates with the CA.
Therefore, the mechanisms outlined in the previous section for using
ACME as the communications protocol between the EST RA and the CA are
equally applicable to BRSKI.
The following call flow shows how ACME may be integrated into a full
BRSKI voucher plus EST enrollment workflow. For brevity, it assumes
that the EST RA has previously proven ownership of a parent domain
and that pledge certificate identifiers are a subdomain of that
parent domain. The domain ownership exchanges between the RA, ACME
and DNS are not shown. Similarly, not all BRSKI interactions are
shown and only the key protocol flows involving voucher exchange and
EST enrollment are shown.
Similar to the EST section above, the client calls EST /csrattrs API
before calling the EST /simpleenroll API. This enables the server to
indicate what fields the pledge should include in the CSR that the
client sends in the /simpleenroll API.
The call flow illustrates the RA returning a 202 Retry-After response
to the initial EST /simpleenroll API. This may be appropriate if
processing of the /simpleenroll request and ACME interactions takes
some timme to complete.
+--------+ +--------+ +------+ +------+
| Pledge | | EST RA | | ACME | | MASA |
+--------+ +--------+ +------+ +------+
| | | |
NOTE: Pre-Authorization of "example.com" is complete
| | | |
STEP 1: Pledge requests Voucher
| | | |
| POST /requestvoucher | | |
|--------------------->| | |
| | POST /requestvoucher | |
| |--------------------------------->|
| | | |
| | 200 OK Voucher | |
| |<---------------------------------|
| 200 OK Voucher | | |
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|<---------------------| | |
| | | |
STEP 2: Pledge enrolls against RA
| | | |
| GET /csrattrs | | |
|--------------------->| | |
| | | |
| 200 OK | | |
| SEQUENCE {AttrOrOID} | | |
| SAN OID: | | |
| "pledge.example.com" | | |
|<---------------------| | |
| | | |
| POST /simpleenroll | | |
| PCSK#10 CSR | | |
| "pledge.example.com" | | |
|--------------------->| | |
| | | |
| 202 Retry-After | | |
|<---------------------| | |
| | | |
STEP 3: RA places ACME order
| | | |
| | POST /newOrder | |
| | "pledge.example.com" | |
| |--------------------->| |
| | | |
| | 201 status=ready | |
| |<---------------------| |
| | | |
| | POST /finalize | |
| | PKCS#10 CSR | |
| | "pledge.example.com" | |
| |--------------------->| |
| | | |
| | 200 OK status=valid | |
| |<---------------------| |
| | | |
| | POST /certificate | |
| |--------------------->| |
| | | |
| | 200 OK | |
| | PKCS#7 | |
| | "pledge.example.com" | |
| |<---------------------| |
| | | |
STEP 4: Pledge retries enroll
| | | |
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| POST /simpleenroll | | |
| PCSK#10 CSR | | |
| "pledge.example.com" | | |
|--------------------->| | |
| | | |
| 200 OK | | |
| PKCS#7 | | |
| "pledge.example.com" | | |
|<---------------------| | |
5. ACME Integration with BRSKI Default Cloud Registrar
BRSKI Cloud Registrar [I-D.ietf-anima-brski-cloud] specifies the
behaviour of a BRSKI Cloud Registrar, and how a pledge can interact
with a BRSKI Cloud Registrar when bootstrapping. Similar to the
local domain registrar BRSKI flow, ACME can be easily integrated with
a cloud registrar bootstrap flow.
BRSKI cloud registrar is flexible and allows for multiple different
local domain discovery and redirect scenarios. In the example
illustrated here, the extension to [RFC8366] Vouchers which is
defined in [I-D.ietf-anima-brski-cloud], and allows the specification
of a bootstrap EST domain, is leveraged. This extension allows the
cloud registrar to specify the local domain RA that the pledge should
connect to for the purposes of EST enrollment.
Similar to the sectioms above, the client calls EST /csrattrs API
before calling the EST /simpleenroll API.
+--------+ +--------+ +------+ +----------+
| Pledge | | EST RA | | ACME | | Cloud RA |
+--------+ +--------+ +------+ | / MASA |
| +----------+
| |
NOTE: Pre-Authorization of "example.com" is complete
| |
STEP 1: Pledge requests Voucher from Cloud Registrar
| |
| POST /requestvoucher |
|-------------------------------------------------------->|
| |
| 200 OK Voucher (includes 'est-domain') |
|<--------------------------------------------------------|
| | | |
STEP 2: Pledge enrolls against local domain RA
| | | |
| GET /csrattrs | | |
|--------------------->| | |
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| | | |
| 200 OK | | |
| SEQUENCE {AttrOrOID} | | |
| SAN OID: | | |
| "pledge.example.com" | | |
|<---------------------| | |
| | | |
| POST /simpleenroll | | |
| PCSK#10 CSR | | |
| "pledge.example.com" | | |
|--------------------->| | |
| | | |
| 202 Retry-After | | |
|<---------------------| | |
| | | |
STEP 3: RA places ACME order
| | | |
| | POST /newOrder | |
| | "pledge.example.com" | |
| |--------------------->| |
| | | |
| | 201 status=ready | |
| |<---------------------| |
| | | |
| | POST /finalize | |
| | PKCS#10 CSR | |
| | "pledge.example.com" | |
| |--------------------->| |
| | | |
| | 200 OK status=valid | |
| |<---------------------| |
| | | |
| | POST /certificate | |
| |--------------------->| |
| | | |
| | 200 OK | |
| | PKCS#7 | |
| | "pledge.example.com" | |
| |<---------------------| |
| | | |
STEP 4: Pledge retries enroll
| | | |
| POST /simpleenroll | | |
| PCSK#10 CSR | | |
| "pledge.example.com" | | |
|--------------------->| | |
| | | |
| 200 OK | | |
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| PKCS#7 | | |
| "pledge.example.com" | | |
|<---------------------| | |
6. ACME Integration with TEAP
TEAP [RFC7170] defines a tunnel-based EAP method that enables secure
communication between a peer and a server by using TLS to establish a
mutually authenticated tunnel. TEAP enables certificate provisioning
within the tunnel. TEAP Update and Extensions for Bootstrapping
[I-D.lear-eap-teap-brski] defines extensions to TEAP that includes
additional TLVs for certificate enrollment and BRSKI handling inside
the TEAP tunnel. Neither TEAP [RFC7170] or TEAP Update and
Extensions for Bootstrapping [I-D.lear-eap-teap-brski] define how the
TEAP server communicates with the CA.
This section outlines how ACME could be used for communication
between the TEAP server and the CA. The example call flow leverages
[I-D.friel-acme-subdomains] and shows the TEAP server proving
ownership of a parent domain, with individual client certificates
being subdomains under that parent domain.
The example illustrates the TEAP server sending a Request-Action TLV
including a CSR-Attributes TLV instructing the peer to send a CSR-
Attributes TLV to the server. This enables the server to indicate
what fields the peer should include in the CSR that the peer sends in
the PKCS#10 TLV. For example, the TEAP server could instruct the
peer what Subject or SAN entries to include in its CSR.
Althought not explicitly illustrated in this call flow, the Peer and
TEAP Server could exchange BRSKI TLVs, and a BRSKI integration and
voucher exchange with a MASA server could take place over TEAP.
Whether a BRSKI TLV exchange takes place or not does not impact the
ACME specific message exchanges.
+------+ +-------------+ +------+ +-----+
| Peer | | TEAP-Server | | ACME | | DNS |
+------+ +-------------+ +------+ +-----+
| | | |
STEP 1: Pre-Authorization of parent domain
| | | |
| | POST /newAuthz | |
| | "example.com" | |
| |--------------------->| |
| | | |
| | 201 authorizations | |
| |<---------------------| |
| | | |
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| | Publish DNS TXT | |
| | "example.com" | |
| |--------------------------------->|
| | | |
| | POST /challenge | |
| |--------------------->| |
| | | Verify |
| | |---------->|
| | 200 status=valid | |
| |<---------------------| |
| | | |
| | Delete DNS TXT | |
| | "example.com" | |
| |--------------------------------->|
| | | |
| | | |
STEP 2: Establsh EAP Outer Tunnel
| | | |
| EAP-Request/ | | |
| Type=Identity | | |
|<------------------------| | |
| | | |
| EAP-Response/ | | |
| Type=Identity | | |
|------------------------>| | |
| | | |
| EAP-Request/ | | |
| Type=TEAP, | | |
| TEAP Start, | | |
| Authority-ID TLV | | |
|<------------------------| | |
| | | |
| EAP-Response/ | | |
| Type=TEAP, | | |
| TLS(ClientHello) | | |
|------------------------>| | |
| | | |
| EAP-Request/ | | |
| Type=TEAP, | | |
| TLS(ServerHello, | | |
| Certificate, | | |
| ServerKeyExchange, | | |
| CertificateRequest, | | |
| ServerHelloDone) | | |
|<------------------------| | |
| | | |
| EAP-Response/ | | |
| Type=TEAP, | | |
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| TLS(Certificate, | | |
| ClientKeyExchange, | | |
| CertificateVerify, | | |
| ChangeCipherSpec, | | |
| Finished) | | |
|------------------------>| | |
| | | |
| EAP-Request/ | | |
| Type=TEAP, | | |
| TLS(ChangeCipherSpec, | | |
| Finished), | | |
| {Crypto-Binding TLV, | | |
| Result TLV=Success} | | |
|<------------------------| | |
| | | |
| EAP-Response/ | | |
| Type=TEAP, | | |
| {Crypto-Binding TLV, | | |
| Result TLV=Success} | | |
|------------------------>| | |
| | | |
| EAP-Request/ | | |
| Type=TEAP, | | |
| {Request-Action TLV: | | |
| Status=Failure, | | |
| Action=Process-TLV, | | |
| TLV=CSR-Attributes, | | |
| TLV=PKCS#10} | | |
|<------------------------| | |
| | | |
STEP 3: Enroll for certificate
| | | |
| EAP-Response/ | | |
| Type=TEAP, | | |
| {CSR-Attributes TLV} | | |
|------------------------>| | |
| | | |
| EAP-Request/ | | |
| Type=TEAP, | | |
| {CSR-Attributes TLV} | | |
|<------------------------| | |
| | | |
| EAP-Response/ | | |
| Type=TEAP, | | |
| {PKCS#10 TLV: | | |
| "pledge.example.com"} | | |
|------------------------>| | |
| | POST /newOrder | |
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| | "pledge.example.com" | |
| |--------------------->| |
| | | |
| | 201 status=ready | |
| |<---------------------| |
| | | |
| | POST /finalize | |
| | PKCS#10 CSR | |
| | "pledge.example.com" | |
| |--------------------->| |
| | | |
| | 200 OK status=valid | |
| |<---------------------| |
| | | |
| | POST /certificate | |
| |--------------------->| |
| | | |
| | 200 OK | |
| | PKCS#7 | |
| | "pledge.example.com" | |
| |<---------------------| |
| | | |
| EAP-Request/ | | |
| Type=TEAP, | | |
| {PKCS#7 TLV, | | |
| Result TLV=Success} | | |
|<------------------------| | |
| | | |
| EAP-Response/ | | |
| Type=TEAP, | | |
| {Result TLV=Success} | | |
|------------------------>| | |
| | | |
| EAP-Success | | |
|<------------------------| | |
7. ACME Integration Considerations
7.1. Service Operators
The goal of these integrations is enabling issuance of certificates
with identitiers in a given domain by an ACME server to a client. It
is expected that the EST RA or TEAP servers that the client sends
certificate enrollment requests to are operated by the organization
that controls the domains. The ACME server is not necessarily
operated by the organization that controls the domain.
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7.2. CSR Attributes
In all integrations, the client MUST send a CSR Attributes request to
the EST or TEAP server prior to sending a certificate enrollment
request. This enables the server to indicate to the client what
attributes it expects the client to include in the subsequent CSR
request.
Servers MUST use this mechanism to tell the client what identifiers
to include in CSR request. ACME [RFC8555] allows the identifier to
be included in either CSR Subject or Subject Alternative Name fields,
however [I-D.ietf-uta-use-san] states that Subject Alternative Name
field MUST be used. This document aligns with [I-D.ietf-uta-use-san]
and Subject Alternate Name field MUST be used. The identifier must
be a Domain Name in a Domain Namespace that the server has control
over and can fulfill ACME challenges against. The leftmost part of
the identifier MAY be a field that the client presented to the server
in an IEEE 802.1AR [IDevID].
Servers MAY use this field to instruct the client to include other
attributes such as specific policy OIDs. Refer to EST [RFC7030]
section 2.6 for further details.
7.3. Certificate Chains and Trust Anchors
ACME [RFC8555] section 9.1 states that ACME servers may return a
certificate chain to an ACME client where an end entity certificate
is followed by certificates that certify it. The trust anchor
certificate MAY be ommitted from the chain as it is assumed that the
trust anchor is already known by the ACME client i.e. the EST or TEAP
server.
7.3.1. EST /cacerts
EST [RFC7030] section 4.2.3 states that the /simpleenroll response
contains "only the certificate that was issued". EST [RFC7030]
section 4.1.3 states that the /cacerts response "MUST include any
additional certificates the client would need to build a chain from
an EST CA-issued certificate to the current EST CA TA".
Therefore, the EST server MUST return only the ACME end entity
certificate in the /simpleenroll response. The EST server MUST
return the remainder of the chain returned by the ACME server to the
EST server in the /cacerts response to the client, appending the
trust anchor root CA if necessary.
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7.3.2. TEAP PKCS#7 TLV
TEAP [RFC7170] section 4.2.16 allows for download of a PKCS#7
certificate chain in response to a TEAP PKCS#10 TLV request. TEAP
also allows for download of multiple PKCS#7 certificates in response
to a TEAP Trusted-Server-Root TLV request.
The TEAP server MUST return the full ACME client certificate chain in
the PKCS#7 response to the PKCS#10 TLV request. The TEAP server MUST
return the ACME server trust anchor in a PKCS#7 response to a
Trusted-Server-Root TLV request. As outlined in Section 7.4, the
TEAP server SHOULD also return the trust anchor that was used for
issuing its own identity certificate, if different from the ACME
server trust anchor.
7.4. id-kp-cmcRA
BRSKI [RFC8995] mandates that the id-kp-cmcRA extended key usage bit
is set in the Registrar (or EST RA) end entity certificate that the
Registrar uses when signing voucher request messages sent to the
MASA. Public ACME servers may not be willing to issue end entity
certificates that have the id-kp-cmcRA extended key usage bit set.
In these scenarios, the EST RA may be used by the pledge to get
issued certificates by a public ACME server, but the EST RA itself
will need an end entity certificate that has been issued by a
different CA (e.g. an operator deployed private CA) and that has the
id-kp-cmcRA bit set.
7.5. Error Handling
ACME [RFC8555] section 6.7 defines multiple errors that may be
returned by an ACME server to an ACME client. TEAP [RFC7170] section
4.2.6 defines multiple errors that may be returned by a TEAP server
to a client in an Error TLV. EST [RFC7030] section 4.2.3 defines how
an EST server may return an error encoded in a CMC response, or may
return a human readable error in the response body.
The following mapping from ACME errors to CMC [RFC5272] section 6.1.4
CMCFailInfo and TEAP [RFC7170] section 4.2.6 error codes is
RECOMMENDED.
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+--------------------+-----------------+--------------------------+
| ACME | CMCFailInfo | TEAP Error Code |
+--------------------+-----------------+--------------------------+
| badCSR | badRequest | 1025 Bad CSR |
| caa | badRequest | 1025 Bad CSR |
| rejectedIdentifier | badIdentity | 1024 Bad Identity In CSR |
| all other errors | internalCAError | 1026 Internal CA Error |
+--------------------+-----------------+--------------------------+
8. IANA Considerations
This document does not make any requests to IANA.
9. Security Considerations
This draft is informational and makes no changes to the referenced
specifications. All security considerations from these referenced
documents are applicable here:
o EST [RFC7030]
o BRSKI [RFC8995]
o BRSKI Default Cloud Registrar [I-D.ietf-anima-brski-cloud]
o TEAP [RFC7170] and TEAP Update and Extensions for Bootstrapping
[I-D.lear-eap-teap-brski]
Additionally, all Security Considerations in ACME in the following
areas are equally applicable to ACME Integrations.
The integration mechanisms proposed here will primarily use the
DNS-01 challenge documented in [RFC8555] section 8.4. The security
considerations in RFC8555 says:
The DNS is a common point of vulnerability for all of these
challenges. An entity that can provision false DNS records for a
domain can attack the DNS challenge directly and can provision false
A/AAAA records to direct the ACME server to send its HTTP validation
query to a remote server of the attacker's choosing.
It is expected that the TEAP-EAP server/EST Registrar will perform
DNS dynamic updates to a DNS primary server using [RFC3007] Dynamic
updates, secured with with either SIG(0), or TSIG keys.
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A major source of vulnerability is the disclosure of these DNS key
records. An attacker that has access to them, can provision their
own certificates into the the name space of the entity.
For many uses, this may allow the attacker to get access to some
enterprise resource. When used to provision, for instance, a (SIP)
phone system this would permit an attacker to impersonate a
legitimate phone. Not only does this allow for redirection of phone
calls, but possibly also toll fraud.
Operators should consider restricting the integration server such
that it can only update the DNS records for a specific zone or zones
where ACME is required for client certificate enrollment automation.
For example, if all IoT devices in an organisation enroll using EST
against an EST RA, and all IoT devices will be issued certificates in
a subdomain under iot.example.com, then the integration server could
be issued a credential that only allows updating of DNS records in a
zone that includes domains in the iot.example.com namespace, but does
not allow updating of DNS records under any other example.com DNS
namespace.
When performing challenge fulfilment via writing files to HTTP
webservers, write access should only be granted to a specific set of
servers, and only to a specific set of directories for storage of
challenge files.
9.1. Denial of Service against ACME infrastructure
The intermdiate node (the TEAP-EAP server, or the EST Registrar)
should cache the resulting certificates such that if the
communication with the pledge is lost, subsequent attempts to enroll
will result in the cache certificate being returned.
As many ACME servers have per-day, per-IP and per-subjectAltName
limits, it is prudent not to request identical certificates too
often. This could be due to operator or installer error, with
multiple configuration resets occuring within a short period of time.
The cache should be indexed by the complete contents of the
Certificate Signing Request, and should not persist beyond the
notAfter date in the certificate.
This means that if the private/public keypair changes on the pledge,
then a new certificate will be issued. If the the requested
SubjectAltName changes, then a new certificate will be requested.
In a case where a device is simply factory reset, and enrolls again,
then the same certificate can be returned.
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10. Informative References
[CAB] CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates", n.d.,
<https://cabforum.org/wp-content/uploads/CA-Browser-Forum-
BR-1.7.1.pdf>.
[I-D.friel-acme-subdomains]
Friel, O., Barnes, R., Hollebeek, T., and M. Richardson,
"ACME for Subdomains", draft-friel-acme-subdomains-04
(work in progress), March 2021.
[I-D.ietf-anima-brski-cloud]
Friel, O., Shekh-Yusef, R., and M. Richardson, "BRSKI
Cloud Registrar", draft-ietf-anima-brski-cloud-00 (work in
progress), April 2021.
[I-D.ietf-uta-use-san]
Salz, R., "Update to Verifying TLS Server Identities with
X.509 Certificates", draft-ietf-uta-use-san-00 (work in
progress), April 2021.
[I-D.lear-eap-teap-brski]
Lear, E., Friel, O., Cam-Winget, N., and D. Harkins, "TEAP
Update and Extensions for Bootstrapping", draft-lear-eap-
teap-brski-05 (work in progress), November 2019.
[IDevID] IEEE, "IEEE Standard for Local and metropolitan area
networks - Secure Device Identity", n.d.,
<https://1.ieee802.org/security/802-1ar>.
[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>.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
<https://www.rfc-editor.org/info/rfc3007>.
[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
<https://www.rfc-editor.org/info/rfc5272>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<https://www.rfc-editor.org/info/rfc7030>.
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[RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
"Tunnel Extensible Authentication Protocol (TEAP) Version
1", RFC 7170, DOI 10.17487/RFC7170, May 2014,
<https://www.rfc-editor.org/info/rfc7170>.
[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>.
[RFC8366] Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
"A Voucher Artifact for Bootstrapping Protocols",
RFC 8366, DOI 10.17487/RFC8366, May 2018,
<https://www.rfc-editor.org/info/rfc8366>.
[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>.
[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/info/rfc8995>.
Authors' Addresses
Owen Friel
Cisco
Email: ofriel@cisco.com
Richard Barnes
Cisco
Email: rlb@ipv.sx
Rifaat Shekh-Yusef
Auth0
Email: rifaat.s.ietf@gmail.com
Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
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