ACME-Based Provisioning of IoT Devices

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).¶

3.1. ACME Server Discovery

Client and IoT devices discover the local ACME Server using one of two methods (in order of precedence):¶

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.¶

3.3. 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, 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.¶

4.1. 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.¶

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 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.¶

5.1. 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:¶

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¶