| RFC draft-sweet-iot-acme-00 | ACME IoT Provisioning | April 2022 |
| Sweet | Expires 8 October 2022 | [Page] |
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.¶
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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.¶
This document defines a extensions to the Automatic Certificate Management Environment (ACME) [RFC8555], a DHCP [RFC2131] option that provides the host name of the local ACME service, a DNS-SD [RFC6763] service for the local ACME service, and implementation guidance for client devices and software.¶
Client Devices access the local ACME Server to obtain the local site's signing certificate, which is then used as a local trust anchor to validate IoT Device X.509 certificates. IoT Devices access the local ACME Server to obtain X.509 certificates for use on local network(s). 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.¶
Because devices often connect to multiple, unconnected networks, trust and usage of X.509 certificates provided by a local ACME server is limited to the currently connected network, essentially creating an intermediate trust level below global Certificate Authorities (CAs).¶
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].¶
Client and IoT devices discover the local ACME Server using one of two methods (in order of precedence):¶
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.¶
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.¶
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 ".local" and the local domain name(s), and MAY contain subjectAltName extensions for the current IP address(es) of the server.¶
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.¶
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 ".local" and the 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".¶
Certificates generated by the local ACME server MUST have an expiration of three months or less.¶
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.¶
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 SSID and/or MAC address of the default router and MAY be associated with the local domain name. Client Devices MUST NOT use ".local" host names or IP addresses to validate the CA certificate since those values are not unique. A certificate MAY be used for multiple networks, for example with a wireless cable modem that provides both Wi-Fi and Ethernet connectivity.¶
IoT Devices, upon connecting to a network, MUST use ACME Server Discovery to determine whether the local network as 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 certificates between networks when those networks utilize the same ACME server and X.509 certificate.¶
The security considerations of IoT provisioning are similar to those described in [RFC8555]. The following subsections describe additional security considerations.¶
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.¶
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 a compromised network connection will go undetected by the Client Device.¶
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.¶
All devices MUST provide a way for an End User to revoke or 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.¶
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.¶
In accordance with [RFC2132], IANA has added the following new DHCP option to the BOOTP Vendor Extensions and DHCP Options [DHCP-REGISTRY] registry:¶
Tag: NNN¶
Name: ACME Server¶
Data Length: N (variable length)¶
Meaning: Fully-qualified domain name of the local ACME server¶
Reference: This document¶
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¶
[ RFC Editor: This section to be deleted before RFC publication ]¶
April 6, 2022 - draft-sweet-acme-iot-00¶