| Internet-Draft | SWORN: Secure Wake on Radio Nudging | October 2021 |
| Bormann & Li | Expires 25 April 2022 | [Page] |
Normally off devices (RFC7228) would need to expend considerable energy resources to be reachable at all times. Instead, MAC layer mechanisms are often employed that allow the last hop router of the device to "wake" the device via radio when needed. Activating these devices even for a short time still does expend energy and thus should be available to authorized correspondents only. Traditionally, this has been achieved by heavy firewalling, allowing only authorized hosts to reach the device at all. This may be too inflexible for an Internet of Things.¶
The present report describes how to use a combination of currently standardized technologies to securely effect this authorization.¶
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/.¶
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This Internet-Draft will expire on 25 April 2022.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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The term "byte" is used in its now customary sense as a synonym for "octet".¶
Messages defined in this document employ CBOR [RFC8949] and are described in CDDL [RFC8610].¶
Terms used in this draft:¶
Client, or Correspondent host. The node that wants to effect "Wake on Radio" on D by sending a message to D.¶
Device. This is typically battery operated and "Normally off" [RFC7228].¶
Router. The router that is adjacent to D, sharing an energy-saving link with D, and serving as a ("parent") router to D.¶
D is a normally off [RFC7228] device, waking up very briefly to communicate with its first hop router R. R and D share a MAC layer that allows R to keep D in extended wake periods.¶
R and D have a security association. (This may have been created in network onboarding, or be setup dynamically from the device-to-network security association when D chose R as a parent router.)¶
D wants to authorize a client (or correspondent host) C to ask R to initiate wake periods in D.¶
Because of changes in the radio environment, D needs to be able to change its parent router from R1 to R2 occasionally. This should not cause a need to notify all its clients; which parent router is used by D is therefore opaque to its clients as usual in IP.¶
Since packets with wake tokens are kept in R for extended periods, the limited size buffer provided in R for this is a resource that needs to be protected to protect availability.¶
D uses up battery for a wake period, which would make it susceptible to battery depletion attacks. To protect availability, D should only undergo wake periods that R has commanded based on previous authorization by D.¶
There may be confidentiality requirements (e.g., for privacy); this is not addressed in the present version of this report.¶
A wake grant is a CWE [RFC8152], packaging a grant key, provided from D
or D's authorization manager to C.
(Possibly the grant key can be conveyed within a larger confidentiality
protected data structure or channel, such as a CWT [RFC8392] employing
a cnf claim for the key [RFC8747].)¶
A wake grant may then be used by C for initiating (a possibly limited number or total duration of) wake periods, employing Wake-Tokens.¶
Information about the wake grant is also made available to R, so it knows the grant key and the parameters of the wake grant. (Upon a change of parent router, D will need to make that information available to its new parent router as well.)¶
A wake token is a CWS, MACed with the Wake-Grant's key, containing a CBOR data item of the form:¶
[serial: uint, wake-period: duration]¶
The CWS is additionally marked by tagging it with a CBOR tag 1398230866 (a value that becomes visible in a packet dump as ASCII "SWOR").¶
(Discussion: Should this be a CWE for confidentiality?)¶
The serial is used for replay detection, based on the usual window mechanism. Wake-Tokens for a fresh wake grant start out with serial numbers at zero.¶
A Wake-Token instructs R to use MAC mechanisms to provide an extended wake period to D the next time it wakes up.¶
The wake token is sent from C to D; R finds it be examining packets that it would need to forward to D.¶
As C is addressing D with the wake token, R needs to find it in traffic purportedly for D.¶
As described in [I-D.bormann-intarea-alfi], this cannot be reasonably done with IP options (which originally would have carried this kind of information in the IP architecture).¶
Instead, R finds the wake token by deep packet inspection. The wake token is found by a heuristic that may have false positives; this is not a problem as the wake token is then verified by its MAC.¶
SWORN requests are carried in UDP packets that also may have a payload function. To this end, they are conveyed as CoAP messages [RFC7252]. The wake token is carried in a CoAP option, Wake-Token. R can find the option by decoding the CoAP packet in the UDP payload or simply by scanning for the 5-byte signature 0xda53574f52 created by the CBOR wake token tag. Any potential wake token so found is then validated as a CWS.¶
This works well with [RFC8613] as the CoAP security mechanism for any payload function that this packet may have. To be able to use DTLS as well, we define a media type "application/dtls-payload" that can be used in a CoAP POST request to send a DTLS payload as payload of a CoAP message (in other words, the CoAP POST request carries a Wake-Token and a Content-Format option). (Any return packet can be similarly sent back in the POST response.) (TODO: This media type has to define the port number juggling needed.)¶
Define CBOR Wake-Token Tag 1398230866 in [IANA.cbor-tags].¶
Define CoAP option Wake-Token in the CoAP Option Numbers Registry of [IANA.core-parameters] (Section 12.2 of [RFC7252]. (The option is safe, no-cache-key, elective, repeatable, of type opaque 0-255 bytes.)¶
Define media-type "application/dtls-payload", with an associated CoAP Content-Format in the CoAP Content-Formats Registry of [IANA.core-parameters] (Section 12.3 of [RFC7252].¶
The purpose of the security mechanisms described is primarily to protect availability (obviously, any symmetric keys employed also need to be confidentiality protected for the sake of the integrity of the mechanism). For the purposes of this kind of availability protection, occasional false positives of the per-packet authorization mechanisms may be acceptable, as long as they don't reach a probability of success that is application dependent (say, success in one out of a million of brute force attempts, equivalent to 20-bit security). This may offer optimization opportunities that need further study.¶
TBD¶