ACME-Based Provisioning of IoT Devices
draft-sweet-iot-acme-05
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Michael Sweet | ||
| Last updated | 2024-01-30 | ||
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draft-sweet-iot-acme-05
IOT Operations M. Sweet, Ed.
Internet-Draft Lakeside Robotics Corporation
Intended status: Experimental 30 January 2024
Expires: 2 August 2024
ACME-Based Provisioning of IoT Devices
draft-sweet-iot-acme-05
Abstract
This document extends the Automatic Certificate Management
Environment (ACME) [RFC8555] to provision X.509 certificates for
local Internet of Things (IoT) devices that are accepted by existing
web browsers and other software running on End User client devices.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 August 2024.
Copyright Notice
Copyright (c) 2024 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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Connecting to the Network . . . . . . . . . . . . . . . . 3
1.2. Trusting IoT Devices on the Network . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. ACME Server Discovery . . . . . . . . . . . . . . . . . . 4
3.2. ACME Server Extensions . . . . . . . . . . . . . . . . . 5
3.2.1. Root (CA) Certificate . . . . . . . . . . . . . . . . 5
3.2.2. Accounts . . . . . . . . . . . . . . . . . . . . . . 6
3.2.3. IoT Device Certificate Signing Requests . . . . . . . 6
3.2.4. IoT Device Certificates . . . . . . . . . . . . . . . 6
3.3. Client Device Configuration . . . . . . . . . . . . . . . 6
3.4. IoT Device Configuration . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4.1. Trusting Local DHCP and DNS Infrastructure . . . . . . . 7
4.2. Certificate Signing Request Validation . . . . . . . . . 7
4.3. Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 8
4.4. Storage of Key Material . . . . . . . . . . . . . . . . . 8
4.5. Revocation and Reissuance/Regeneration . . . . . . . . . 8
4.6. Use of mDNS . . . . . . . . . . . . . . . . . . . . . . . 8
4.7. mDNS Domain Name Collisions . . . . . . . . . . . . . . . 8
4.8. Network Identification and Validation . . . . . . . . . . 9
4.9. Multiple Network Support . . . . . . . . . . . . . . . . 9
4.10. Protection of Certificates and Key Material . . . . . . . 9
4.11. Reuse of Key Material . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5.1. DHCP Option . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Service Name . . . . . . . . . . . . . . . . . . . . . . 10
6. Normative References . . . . . . . . . . . . . . . . . . . . 10
7. Informative References . . . . . . . . . . . . . . . . . . . 12
Appendix A. Change History . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
IoT Devices are common on local networks and often utilize TLS
[RFC8446] with self-signed X.509 certificates [RFC5280] to provide
HTTPS [RFC2818] based web pages and services. Unfortunately, web
browsers typically do not trust such certificates and show error
messages intended to deter usage. Some IoT Devices also have
manufacturer-supplied X.509 certificates, however due to the service
life of such devices, the need for crypto-agility, and well-known
challenges of secure key management, those certificates are better
suited to attestation and secure network connection than direct use
with TLS.
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The goal of this document is to provide a method for providing
trusted X.509 certificates for use with TLS that does not depend on
an Internet connection or a service provided by the original device
manufacturer, does not depend on credentials or key material provided
with the device from the factory, allows a device to be moved to
different networks or domains without interaction with the device
manufacturer, and supports the needs of both simple home networks and
complex enterprise networks.
1.1. Connecting to the Network
This document is not concerned with the method of connecting Client
or IoT Devices to the network. That said, the level of security and/
or trust of the network and the proposed solution necessarily depends
on the access controls, confidentiality, and validation provided by
the network, as well as the quality of any identification and/or key
material supplied by the manufacturer.
1.2. Trusting IoT Devices on the Network
This document uses existing infrastructure, namely the network's DHCP
[RFC2131] and DNS [RFC1034] services, to discover the local Automatic
Certificate Management Environment (ACME) [RFC8555] Server to use for
that network. Local ACME Servers are discovered using either a DHCP
option or a DNS-SD [RFC6763] service record from the network's DNS
service, and the local ACME Server's X.509 certificate provides a
usable and verifiable network identifier as well as the trust anchor
for issued IoT Device X.509 certificates.
ACME defines a protocol for network services to obtain trusted X.509
credentials for use with TLS [RFC8446]. However, since existing ACME
Servers depend on public Internet connectivity to the ACME Client for
validation, and since those same servers cannot issue X.509
certificates for the ".local" domain, some changes are needed to
support a local ACME Server. X.509 certificates issued by the local
ACME server are only valid when accessing the IoT Device for the
local DNS domain, the mDNS (".local") domain, or any link-local or
private IP addresses. Local ACME Servers can be standalone servers
(common in enterprise networks) or software that runs on a consumer
Internet router/modem.
Because devices often connect to multiple, unconnected networks,
trust and usage of X.509 certificates provided by a local ACME server
is limited to that network, essentially creating an intermediate
trust level below global Certificate Authorities (CAs).
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in "Key words for use in
RFCs to Indicate Requirement Levels" [RFC2119].
ACME Client: A device that uses the ACME protocol to request
certificate management actions, such as issuance or revocation.
ACME Server: A device that implements the ACME protocol to respond
to ACME Client requests, performing the requested actions if the
client is authorized.
Certificate Authority (CA): A trusted source for X.509 certificates
used during negotiation of a TLS session. (TODO: Update from
current TLS/X.509 specifications)
Client Device: A computer, tablet, phone, or other End User device
that accesses an IoT Device.
End User: A person or software process that is authorized to use
Client Devices and, through the Client Device, access and use IoT
Devices.
IoT Device: A camera, printer, switch, or other local device that
provides services or functions to a Client Device.
Media Access Control (MAC) Address: A unique identifier assigned to
a network interface controller for use as a network address in
communications within a network segment.
Service Set Identifier (SSID): The name associated with a wireless
network.
Trust On First Use (TOFU): An unauthenticated public key obtained on
first contact (and retained for future use) will be good enough to
secure future communication [RFC7435].
Uniform Resource Identifier (URI): A compact sequence of characters
that identifies an abstract or physical resource [RFC3986].
3. Specification
3.1. ACME Server Discovery
Client and IoT devices discover the local ACME Server using one of
two methods (in order of precedence):
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1. Via DHCP Option NNN (ACME Server) when obtaining IPv4/IPv6
addresses. _Note:DHCP Option 60 (Vender Class Identifier
[RFC3925]) with enterprise number 55357 (Lakeside Robotics
Corporation) shall be used for purposes of prototyping this
document._
2. Via a subsequent DNS-SD query sent to the configured DNS server
for the "_acme-server._tcp.domain" SRV record.
Most home networks will use the DHCP Option, while larger
(enterprise) networks providing a dedicated DNS domain will use the
DNS-SD query.
Note: DNS-SD queries MUST NOT be performed using Multicast DNS (mDNS)
[RFC6762] for security reasons.
3.2. ACME Server Extensions
ACME [RFC8555] defines a protocol for managing trusted X.509
certificates. Organizations such as "Let's Encrypt" provide publicly
available ACME servers, and such servers have led to the ubiquitous
usage of TLS for internet web and email servers. However, public
ACME servers typically cannot access local (private) devices and will
not issue certificates for the mDNS ".local" domain. A local ACME
server can both access local devices and issue certificates for local
domains.
3.2.1. Root (CA) Certificate
A local ACME server will typically generate a self-signed X.509
certificate as its root (CA) certificate and the local network's
trust anchor. The certificate MUST use a SHA2 hash of at least 256
bits and MUST use either RSA encryption with a key length of at least
3072 bits or ECDSA encryption with the secp384r1 (P-384) or secp521r1
(P-521) curves. The expiration of the self-signed certificate MUST
be between 1 and 10 years, inclusive. The certificate MUST contain
subjectAltName extensions for the mDNS (".local") and local domain
name(s), and MAY contain subjectAltName extensions for the current IP
address(es) of the server. For example, if the local ACME server
name is "router-fdb531" and the local domain is "example.com", the
certificate will at least contain two subjectAltName extensions with
values "DNS:router-fdb531.example.com" and "DNS:router-fdb531.local".
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3.2.2. Accounts
ACME account objects contain an array of contact strings. Normally
this array consists of "mailto:" URIs, however for local IoT devices
an array of "https:" URIs should be used instead, one for each fully-
qualified domain name used by the device.
3.2.3. IoT Device Certificate Signing Requests
The certificate signing request supplied by the IoT Device MUST use a
SHA2 hash of at least 256 bits and MUST use either RSA encryption
with a key length of at least 3072 bits or ECDSA encryption with the
secp384r1 (P-384) or secp521r1 (P-521) curves. The request MUST also
contain subjectAltName extensions for the mDNS (".local") and any
local domain name(s), MAY contain subjectAltName extensions for the
current IP address(es) of the device, and MUST NOT contain
subjectAltName extensions for "localhost". For example, if the
device name is "device-12cd56" and the local domain is "example.com",
the signing request will at least contain two subjectAltName
extensions with values "DNS:device-12cd56.example.com" and
"DNS:device-12cd56.local".
3.2.4. IoT Device Certificates
Certificates generated by the local ACME server MUST have an
expiration of three months or less.
3.3. Client Device Configuration
Client Devices, upon connecting to a network, MUST use ACME Server
Discovery to determine whether the local network has an ACME Server.
If it does, the Client Device connects to the server using HTTPS and
copies the X.509 certificates for use in validating future
connections to IoT Devices. The Client Device SHOULD utilize a TOFU
validation policy for self-signed X.509 certificates unless otherwise
configured, for example in a managed enterprise network environment.
Client Devices can present UI informing and/or obtaining consent from
the user to use or trust the root certificate, however such UI is
beyond the scope of this document.
The Client Device MUST NOT use the supplied X.509 certificate when
validating certificates on other networks. The certificate is
typically associated with the network interface name, network SSID,
and/or MAC address of the default router and MAY be associated with
the local domain name. Client Devices MUST validate the host name(s)
and/or IP address(es) to validate the CA certificate against the
name(s) or IP address(es) supplied by the DHCP or DNS server during
discovery. Since a certificate MAY be used for multiple networks,
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for example with a wireless cable modem that provides both Wi-Fi and
Ethernet connectivity, such validation MUST allow for the presence of
subjectAltName extensions containing values other than those provided
by the DHCP or DNS server the Client uses.
3.4. IoT Device Configuration
IoT Devices, upon connecting to a network, MUST use ACME Server
Discovery to determine whether the local network has an ACME Server.
If it does, the IoT Device connects to the server using HTTPS and
uses the ACME protocol to obtain, renew, or verify an X.509
certificate for each network the device is connected to. The IoT
Device SHOULD utilize a TOFU validation policy for self-signed X.509
certificates unless otherwise configured, for example in a managed
enterprise network environment.
The IoT Device MAY share/reuse certificates between networks when
those networks utilize the same ACME server and X.509 certificate.
4. Security Considerations
The security considerations of IoT provisioning are similar to those
described in [RFC1034], [RFC2131], [RFC6763], [RFC8446], and
[RFC8555]. The following subsections describe additional security
considerations.
4.1. Trusting Local DHCP and DNS Infrastructure
This specification necessarily depends on all devices trusting the
underlying network infrastructure, specifically the local DHCP and
DNS servers. Sites can utilize Network Endpoint Assessment [RFC5209]
and Trusted Network Connect (TNC) [RFC5792][RFC5793] to provide
enhanced security and trust in the local network.
4.2. Certificate Signing Request Validation
The local ACME Server MUST validate the subjectAltName values in
certificate signing requests from IoT Devices. DNS name suffixes
MUST be restricted to ".local" and the configured local domain
name(s), and the leftmost label MUST NOT be the name of the local
ACME Server or "localhost". IP addresses MUST be limited to link-
local, loopback, and private use addresses.
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4.3. Man-in-the-Middle Attacks
Because the local ACME Server will often rely on a self-signed
certificate and TOFU validation policy, a man-in-the-middle attack is
possible with successful DHCP, DNS, and/or mDNS request interception
and/or redirection. Such attacks can be detected using network
monitoring tools, and the use of a long-lived root certificate helps
to mitigate the possibility that compromised network connections or
infrastructure will go undetected by the Client Device.
4.4. Storage of Key Material
It is important for all devices to protect stored encryption keys
from disclosure. Disclosure of the local ACME Server's private key
will compromise all encrypted traffic on the local network.
Disclosure of an IoT Device's private key will only affect that
device's traffic.
4.5. Revocation and Reissuance/Regeneration
All devices MUST provide a way for an End User to re-issue X.509
certificates and regenerate a new private/public key pair for
certificates and certificate requests. The most common way is
through a so-called "factory reset" process that restores a device to
its original, factory configuration/state.
All devices SHOULD provide a way for an End User to revoke X.509
certificates.
4.6. Use of mDNS
Multicast DNS (mDNS) [RFC6762] has a number of known security
limitations. DHCP Option NNN provides the local ACME Server's fully-
qualified domain name which can be resolved using mDNS, providing a
small window for a man-in-the-middle attack during initial device
connection. Such attacks can be detected using network monitoring
tools and/or through the use of a root X.509 certificate from a
trusted, public CA on the local ACME Server.
4.7. mDNS Domain Name Collisions
Multicast DNS (mDNS) domain names ("example.local.") can collide with
other network devices. While mDNS does define an algorithm to
resolve name collisions, IoT Devices SHOULD use a default name with a
unique identifier, e.g., "device-12cd56.local.", so that name changes
are less likely. When an IoT Device's mDNS changes, it MUST revoke
all certificates for the old name with the (current) local ACME
Server and request new certificate(s) for the new name. Portable IoT
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Devices that connect to many different networks MUST track their mDNS
hostname separately for each network and only revoke certificates for
the currently connected network(s).
4.8. Network Identification and Validation
Client and IoT Devices SHOULD identify networks using the local
network interface name, MAC address of the default router, and/or the
Wi-Fi SSID and validate the local ACME Server's root certificate when
connecting. Wi-Fi validation is necessarily limited since Wi-Fi
SSIDs are not unique. Client Devices MUST and IoT Devices SHOULD
notify the End User when the root certificate changes for a network.
4.9. Multiple Network Support
Multiple network configurations pose an interesting implementation
challenge. The most typical multiple-network configurations are Wi-
Fi + cellular and Wi-Fi + Ethernet, where cellular networks are
usually public-facing with no mDNS while Ethernet networks are
usually private with mDNS support.
Client Devices MUST separately track and validate the root X.509
certificate for each local ACME Server. Similarly, IoT Devices MUST
separately track, store, and use X.509 certificates for each local
ACME Server. Client and IoT Devices MAY purge "old" network
information if sufficient storage space is not available.
4.10. Protection of Certificates and Key Material
IoT Devices and local ACME Servers MUST protect access to certificate
and key material, allowing access only to approved software running
on the device or server. The private key for an X.509 certificate
MUST NOT be accessible outside of the corresponding device or server.
4.11. Reuse of Key Material
IoT Devices MUST NOT reuse key material when generating an X.509
certificate signing request. Local ACME Servers MUST NOT reuse key
material when generating the root X.509 certificate.
5. IANA Considerations
5.1. DHCP Option
In accordance with [RFC2132], IANA has added the following new DHCP
option to the BOOTP Vendor Extensions and DHCP Options
[DHCP-REGISTRY] registry:
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Tag: NNN
Name: ACME Server
Data Length: N (variable length)
Meaning: Fully-qualified domain name of the local ACME server
Reference: This document
5.2. Service Name
In accordance with [RFC6335], IANA has added the following new
service name to the Service Name and Transport Protocol Port Number
Registry [SERVICE-REGISTRY]:
Service Name: acme-server
Port Number: None
Transport Protocol: tcp
Description: Automatic Certificate Management Environment (ACME)
server
Assignee: Michael Sweet
Contact: Michael Sweet
Reference: This document
Assignment Notes: Defined TXT keys: None
6. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[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>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
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[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/info/rfc2132>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5209] Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J.
Tardo, "Network Endpoint Assessment (NEA): Overview and
Requirements", RFC 5209, DOI 10.17487/RFC5209, June 2008,
<https://www.rfc-editor.org/info/rfc5209>.
[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>.
[RFC5792] Sangster, P. and K. Narayan, "PA-TNC: A Posture Attribute
(PA) Protocol Compatible with Trusted Network Connect
(TNC)", RFC 5792, DOI 10.17487/RFC5792, March 2010,
<https://www.rfc-editor.org/info/rfc5792>.
[RFC5793] Sahita, R., Hanna, S., Hurst, R., and K. Narayan, "PB-TNC:
A Posture Broker (PB) Protocol Compatible with Trusted
Network Connect (TNC)", RFC 5793, DOI 10.17487/RFC5793,
March 2010, <https://www.rfc-editor.org/info/rfc5793>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
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[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://www.rfc-editor.org/info/rfc7435>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[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>.
7. Informative References
[DHCP-REGISTRY]
IANA, "BOOTP Vendor Extensions and DHCP Options",
<https://www.iana.org/assignments/bootp-dhcp-parameters/
bootp-dhcp-parameters.xhtml#options>.
[RFC3925] Littlefield, J., "Vendor-Identifying Vendor Options for
Dynamic Host Configuration Protocol version 4 (DHCPv4)",
RFC 3925, DOI 10.17487/RFC3925, October 2004,
<https://www.rfc-editor.org/info/rfc3925>.
[SERVICE-REGISTRY]
IANA, "Service Name and Transport Protocol Port Number
Registry", <https://www.iana.org/assignments/service-
names-port-numbers/service-names-port-numbers.xhtml>.
Appendix A. Change History
[ RFC Editor: This section to be deleted before RFC publication ]
January 30, 2024 - draft-sweet-acme-iot-05
* Clarified the contents and validation of X.509 root (CA)
certificates.
* Added note that a Client Device can display UI when connecting to
a network with an ACME server.
* Clarified key material security considerations.
* Clarified that devices may purge old network information.
* Updated X.509 certificate revocation requirement to SHOULD.
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* Clarified that X.509 certificates are only revokes on the current
network and the mDNS hostname is tracked separately per network.
August 2, 2023 - draft-sweet-acme-iot-04
* Updated introduction to provide a clearer explanation of the scope
and purpose of the document.
* Added security considerations for protecting the certificate and
key materials to address concerns of malicious software running on
the device.
* Added security considerations for reusing key materials to address
concerns that manufacturers might hardcode keys.
February 6, 2023 - draft-sweet-acme-iot-03
* Added security considerations for trusting the local network
infrastructure with references to NEA and TNC.
July 14, 2022 - draft-sweet-acme-iot-02
* Added clarifications and more detail per Printer Working Group
review at May 2022 face-to-face meeting, specifically more detail
in the introduction and security considerations for mDNS Domain
Name Collisions, Network Identification and Validation, and
Multiple Network Support.
April 14, 2022 - draft-sweet-acme-iot-01
* Added temporary use of DHCP vendor class option (60), per guidance
from DHCP WG chair
April 6, 2022 - draft-sweet-acme-iot-00
* Initial revision.
Author's Address
Michael Sweet (editor)
Lakeside Robotics Corporation
1094 Valecrest St
Blezard Valley Ontario P0M 1E0
Canada
Email: msweet@msweet.org
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