| Internet-Draft | EDHOC + OSCORE | November 2020 |
| Palombini, et al. | Expires 6 May 2021 | [Page] |
This document defines possible optimization approaches for combining the lightweight authenticated key exchange protocol EDHOC run over CoAP with the first subsequent OSCORE transaction. This combination reduces the number of round trips required to set up an OSCORE Security Context and complete an OSCORE transaction using that context.¶
This note is to be removed before publishing as an RFC.¶
Source for this draft and an issue tracker can be found at https://github.com/EricssonResearch/oscore-edhoc.¶
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This document presents possible optimization approaches to combine the lightweight authenticated key exchange protocol EDHOC [I-D.ietf-lake-edhoc], when running over CoAP [RFC7252], with the first subsequent OSCORE [RFC8613] transaction.¶
This allows for a minimum number of round trips necessary to setup the OSCORE Security Context and complete an OSCORE transaction, for example when an IoT device gets configured in a network for the first time.¶
The number of protocol round trips impacts the minimum number of flights, which can have a substantial impact on performance with certain radio technologies.¶
Without this optimization, it is not possible, not even in theory, to achieve the minimum number of flights. This optimization makes it possible also in practice, since the last message of the EDHOC protocol can be made relatively small (see Section 1 of [I-D.ietf-lake-edhoc]), thus allowing additional OSCORE protected CoAP data within target MTU sizes.¶
The goal of this document is to provide details on different alternatives for transporting and processing the necessary data, gather opinions on the different approaches, and select only one of those.¶
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 reader is expected to be familiar with terms and concepts defined in CoAP [RFC7252], CBOR [I-D.ietf-cbor-7049bis], OSCORE [RFC8613] and EDHOC [I-D.ietf-lake-edhoc].¶
EDHOC is a 3-message key exchange protocol. Section 7.1 of [I-D.ietf-lake-edhoc] specifies how to transport EDHOC over CoAP: the EDHOC data (referred to as "EDHOC messages") are transported in the payload of CoAP requests and responses.¶
This draft deals with the case of the Initiator acting as CoAP Client and the Responder acting as CoAP Server. (The case of the Initiator acting as CoAP server cannot be optimized in this way.) That is, the CoAP Client sends a POST request containing the EDHOC message 1 to a reserved resource at the CoAP Server. This triggers the EDHOC exchange on the CoAP Server, which replies with a 2.04 (Changed) Response containing the EDHOC message 2. Finally, the EDHOC message 3 is sent by the CoAP Client in a CoAP POST request to the same resource used for the EDHOC message 1. The Content-Format of these CoAP messages is set to "application/edhoc".¶
After this exchange takes place, and after successful verifications specified in the EDHOC protocol, the Client and Server derive the OSCORE Security Context, as specified in Section 7.1.1 of [I-D.ietf-lake-edhoc]. Then, they are ready to use OSCORE.¶
This sequential way of running EDHOC and then OSCORE is specified in Figure 1. As shown in the figure, this mechanism is executed in 3 round trips.¶
CoAP Client CoAP Server
| ------------- EDHOC message_1 ------------> |
| |
| <------------ EDHOC message_2 ------------- |
| |
EDHOC verification |
| |
| ------------- EDHOC message_3 ------------> |
| |
| EDHOC verification
| |
OSCORE Sec Ctx OSCORE Sec Ctx
Derivation Derivation
| |
| -------------- OSCORE Request ------------> |
| |
| <------------ OSCORE Response ------------- |
| |
The number of roundtrips can be minimized: after receiving the EDHOC message 2, the CoAP Client has all the information needed to derive the OSCORE Security Context before sending the EDHOC message 3.¶
This means that the Client can potentially send at the same time both the EDHOC message 3 and the subsequent OSCORE Request. On a semantic level, this approach practically requires to send two separate REST requests at the same time.¶
The high level message flow of running EDHOC and OSCORE combined is shown in Figure 2.¶
Defining the specific details of how to transport the data and of their processing order is the goal of this specification.¶
CoAP Client CoAP Server
| ------------- EDHOC message_1 ------------> |
| |
| <------------ EDHOC message_2 ------------- |
| |
EDHOC verification + |
OSCORE Sec Ctx |
Derivation |
| |
| ------------- EDHOC message_3 ------------> |
| + OSCORE Request |
| |
| EDHOC verification +
| OSCORE Sec Ctx
| Derivation
| |
| <------------ OSCORE Response ------------- |
| |
This approach consists in sending the EDHOC message 3 inside an OSCORE message (i.e., an OSCORE protected CoAP message).¶
The resulting OSCORE + EDHOC request is in practice the OSCORE Request from Figure 1, sent to a protected resource and with the correct CoAP method and options, with the addition that it also transports the EDHOC message 3.¶
As the EDHOC message 3 may be too large to be included in a CoAP Option, e.g. if containing a large public key certificate chain, it would have to be transported in the CoAP payload.¶
In particular, the payload of the OSCORE + EDHOC request is formatted as a CBOR sequence of two CBOR byte strings: the EDHOC message 3 and the OSCORE ciphertext of the original OSCORE Request, in this order, both encoded as CBOR byte strings.¶
Note that the OSCORE ciphertext is not computed over the EDHOC message 3, which is not protected by OSCORE. That is, the client first prepares the OSCORE Request as in Figure 1. Then, it reformats the payload to include also the EDHOC message 3, as defined above. The result is the OSCORE + EDHOC request to send.¶
The usage of this approach is indicated by a signalling information in the OSCORE + EDHOC request, which can be either a new EDHOC Option (see Section 3.1) or the OSCORE Option with a particular Flag Bit set (see Section 3.2).¶
When receiving such a request, the Server needs to perform the following processing, in addition to the EDHOC, OSCORE and CoAP processing:¶
The following sections expand on the two ways of signalling that the EDHOC message is transported in the OSCORE message.¶
One way to signal that the Server has to extract and process the EDHOC message 3 before processing the OSCORE protected CoAP request is to define a new CoAP Option, called the EDHOC Option.¶
The presence of this option means that the message contains EDHOC data in the payload, that must be extracted and processed before the rest of the message can be processed.¶
In particular, the EDHOC message 3 has to be extracted from the CoAP payload, as the first element of a CBOR sequence wrapped in a CBOR byte string.¶
The Option is critical, Safe-to-Forward, and part of the Cache-Key.¶
The Option value is always empty. If any value is sent, the value is simply ignored.¶
The Option MUST occur at most once.¶
The Option is of Class U for OSCORE.¶
Figure 3 shows the format for a CoAP message containing both the OSCORE ciphertext and EDHOC message 3, using the newly defined EDHOC option for signaling.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Ver| T | TKL | Code | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Token (if any, TKL bytes) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OSCORE option | EDHOC option | other options (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 1 1 1 1 1| Payload +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An example based on the OSCORE test vector from Appendix C.4 of [RFC8613] and the EDHOC test vector from Appendix B.2 of [I-D.ietf-lake-edhoc] is given in Figure 4. The example assumes that the EDHOC option is registered with CoAP option number 13.¶
o OSCORE option value: 0x0914 (2 bytes)
o ciphertext: 0x612f1092f1776f1c1668b3825e (13 bytes)
o EDHOC option value: - (0 bytes)
o EDHOC message 3: 085253c3991999a5ffb86921e99b607c067770e0
(20 bytes)
From there:
o Protected CoAP request (OSCORE message): 0x44025d1f0000397439
6c6f63616c686f737462 0914 04 ff 54085253C3991999A5FFB86921E99
B607C067770E0 4d612f1092f1776f1c1668b3825e (58 bytes)
Another way to signal that the EDHOC message 3 is to be extracted from the CoAP payload as the first element of a CBOR sequence wrapped in a CBOR byte string, and that the processing defined in Section 3 is to be executed, is to use one of the OSCORE Flag Bits of the OSCORE Option.¶
Bit Position: 1¶
Name: EDHOC¶
Description: Set to 1 if the payload is a sequence of EDHOC message 3 and OSCORE ciphertext.¶
Reference: this document¶
The OSCORE Option value with the EDHOC bit set is given in Figure 5.¶
0 1 2 3 4 5 6 7 <------------- n bytes --------------> +-+-+-+-+-+-+-+-+-------------------------------------- |0|1|0|h|k| n | Partial IV (if any) ... +-+-+-+-+-+-+-+-+-------------------------------------- <- 1 byte -> <----- s bytes ------> +------------+----------------------+------------------+ | s (if any) | kid context (if any) | kid (if any) ... | +------------+----------------------+------------------+
Figure 6 shows the format for a CoAP message containing both the OSCORE ciphertext and EDHOC message 3, using the Flag Bit 1 in the OSCORE Option for signaling.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Ver| T | TKL | Code | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Token (if any, TKL bytes) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OSCORE opt (with EDHOC bit set) | other options (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 1 1 1 1 1| Payload +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An example based on the OSCORE test vector from Appendix C.4 of [RFC8613] and the EDHOC test vector from Appendix B.2 of [I-D.ietf-lake-edhoc] is given in Figure 7.¶
o OSCORE option value without EDHOC bit set: 0x0914 (2 bytes)
o OSCORE option value with EDHOC bit set: 0x4914 (2 bytes)
o ciphertext: 0x612f1092f1776f1c1668b3825e (13 bytes)
o EDHOC message 3: 085253c3991999a5ffb86921e99b607c067770e0
(20 bytes)
From there:
o Protected CoAP request (OSCORE message): 0x44025d1f000039743
96c6f63616c686f737462 4914 ff 54085253C3991999A5FFB86921E99B
607C067770E0 4d612f1092f1776f1c1668b3825e (58 bytes)
The same security considerations from OSCORE [RFC8613] and EDHOC [I-D.ietf-lake-edhoc] hold for this document.¶
TODO (more considerations)¶
Depending on the option chosen, this document will either register a new CoAP Option number to the CoAP Option Number registry, or a new bit to the OSCORE Flag Bits registry.¶
The authors sincerely thank Christian Amsuess, Klaus Hartke, Jim Schaad and Malisa Vucinic for their feedback and comments in the discussion leading up to this draft.¶
The work on this document has been partly supported by VINNOVA and the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home (Grant agreement 952652).¶