Ericsson

francesca.palombini@ericsson.com

RISE AB

Isafjordsgatan 22 Kista SE-16440 Stockholm Sweden marco.tiloca@ri.se

RISE AB

Isafjordsgatan 22 Kista SE-16440 Stockholm Sweden rikard.hoglund@ri.se

Fraunhofer AISEC

stefan.hristozov@aisec.fraunhofer.de

Ericsson

goran.selander@ericsson.com

Internet CoRE Working Group Internet-Draft This document defines an optimization approach 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 to complete an OSCORE transaction using that Security Context.

Ephemeral Diffie-Hellman Over COSE (EDHOC) is a lightweight authenticated key exchange protocol, especially intended for use in constrained scenarios. In particular, EDHOC messages can be transported over the Constrained Application Protocol (CoAP) and used for establishing a Security Context for Object Security for Constrained RESTful Environments (OSCORE) . This document defines an optimization approach that combines EDHOC run over CoAP with the first subsequent OSCORE 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. This optimization is desirable, since the number of protocol round trips impacts on the minimum number of flights, which in turn can have a substantial impact on the latency of conveying the first OSCORE request, when using 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 ), thus allowing additional OSCORE protected CoAP data within target MTU sizes.

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 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 , CBOR , CBOR sequences , OSCORE and EDHOC .

The EDHOC protocol allows two peers to agree on a cryptographic secret, in a mutually-authenticated way and by using Diffie-Hellman ephemeral keys to achieve perfect forward secrecy. The two peers are denoted as Initiator and Responder, as the one sending or receiving the initial EDHOC message_1, respectively. After successful processing of EDHOC message_3, both peers agree on a cryptographic secret that can be used to derive further security material, and especially to establish an OSCORE Security Context . The Responder can also send an optional EDHOC message_4 to achieve key confirmation, e.g., in deployments where no protected application message is sent from the Responder to the Initiator. Appendix A.3 of specifies how to transport EDHOC over CoAP. That is, the EDHOC data (referred to as “EDHOC messages”) are transported in the payload of CoAP requests and responses. The default message flow consists in the CoAP Client acting as Initiator and the CoAP Server acting as Responder. Alternatively, the two roles can be reversed. In the rest of this document, EDHOC messages are considered to be transported over CoAP. shows a Client and Server running EDHOC as Initiator and Responder, respectively. That is, the Client sends a POST request with payload EDHOC message_1 to a reserved resource at the CoAP Server, by default at Uri-Path “/.well-known/edhoc”. This triggers the EDHOC exchange at the Server, which replies with a 2.04 (Changed) Response with payload EDHOC message_2. Finally, the Client sends a CoAP POST request to the same resource used for EDHOC message_1, with payload EDHOC message_3. The Content-Format of these CoAP messages may be set to “application/edhoc”. After this exchange takes place, and after successful verifications as specified in the EDHOC protocol, the Client and Server can derive an OSCORE Security Context, as defined in Appendix A.2 of . After that, they can use OSCORE to protect their communications.

| | Header: POST (Code=0.02) | | Uri-Path: "/.well-known/edhoc" | | Content-Format: application/edhoc | | | | <------------ EDHOC message_2 ------------- | | Header: 2.04 Changed | | Content-Format: application/edhoc | | | EDHOC verification | | | | ------------- EDHOC message_3 ------------> | | Header: POST (Code=0.02) | | Uri-Path: "/.well-known/edhoc" | | Content-Format: application/edhoc | | | | EDHOC verification | + OSCORE Sec Ctx OSCORE Sec Ctx Derivation Derivation | | | ------------- OSCORE Request -------------> | | Header: POST (Code=0.02) | | | | <------------ OSCORE Response ------------- | | Header: 2.04 Changed | | | ]]>

As shown in , this purely-sequential way of first running EDHOC and then using OSCORE takes three round trips to complete. defines an optimization for combining EDHOC with the first subsequent OSCORE transaction. This reduces the number of round trips required to set up an OSCORE Security Context and to complete an OSCORE transaction using that Security Context.

This section defines an optimization for combining the EDHOC exchange with the first subsequent OSCORE transaction, thus minimizing the number of round trips between the two peers. This approach can be used only if the default EDHOC message flow is used, i.e., when the Client acts as Initiator and the Server acts as Responder, while it cannot be used in the case with reversed roles. When running the purely-sequential flow of , the Client has all the information to derive the OSCORE Security Context already after receiving EDHOC message_2 and before sending EDHOC message_3. Hence, the Client can potentially send both EDHOC message_3 and the subsequent OSCORE Request at the same time. On a semantic level, this requires sending two REST requests at once, as in .

| | Header: POST (Code=0.02) | | Uri-Path: "/.well-known/edhoc" | | Content-Format: application/edhoc | | | | <------------ EDHOC message_2 ------------- | | Header: 2.04 Changed | | Content-Format: application/edhoc | | | EDHOC verification | + | OSCORE Sec Ctx | Derivation | | | | ---- EDHOC message_3 + OSCORE Request ----> | | Header: POST (Code=0.02) | | | | EDHOC verification | + | OSCORE Sec Ctx | Derivation | | | <------------ OSCORE Response ------------- | | Header: 2.04 Changed | | | ]]>

To this end, the specific approach defined in this section consists of sending EDHOC message_3 inside an OSCORE protected CoAP message. The resulting EDHOC + OSCORE request is in practice the OSCORE Request from , as still sent to a protected resource and with the correct CoAP method and options, but with the addition that it also transports EDHOC message_3. As 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 has to be transported in the CoAP payload of the EDHOC + OSCORE request. The rest of this section specifies how to transport the data in the EDHOC + OSCORE request and their processing order. In particular, the use of this approach is explicitly signalled by including an EDHOC Option (see ) in the EDHOC + OSCORE request. The processing of the EDHOC + OSCORE request is specified in for the Client side and in for the Server side.

This section defines the EDHOC Option. The option is used in a CoAP request, to signal that the request payload conveys both an EDHOC message_3 and OSCORE protected data, combined together. The EDHOC Option has the properties summarized in , which extends Table 4 of . The option is Critical, Safe-to-Forward, and part of the Cache-Key. The option MUST occur at most once and is always empty. If any value is sent, the value is simply ignored. The option is intended only for CoAP requests and is of Class U for OSCORE .

The presence of this option means that the message payload contains also EDHOC data, that must be extracted and processed as defined in , before the rest of the message can be processed. shows the format of a CoAP message containing both the EDHOC data and the OSCORE ciphertext, using the newly defined EDHOC option for signalling.

The Client prepares an EDHOC + OSCORE request as follows. Compose EDHOC message_3 as per Section 5.4.2 of . Since the Client is the EDHOC Initiator, the EDHOC message_3 always includes the connection identifier C_R and CIPHERTEXT_3. Note that C_R is the OSCORE Sender ID of the Client, encoded as per . Encrypt the original CoAP request as per Section 8.1 of , using the new OSCORE Security Context established after receiving EDHOC message_2. Note that the OSCORE ciphertext is not computed over EDHOC message_3, which is not protected by OSCORE. That is, the result of this step is the OSCORE Request as in . Build a CBOR sequence composed of two CBOR byte strings in the following order. The first CBOR byte string is the CIPHERTEXT_3 of the EDHOC message_3 resulting from step 3. The second CBOR byte string has as value the OSCORE ciphertext of the OSCORE protected CoAP request resulting from step 2. Compose the EDHOC + OSCORE request, as the OSCORE protected CoAP request resulting from step 2, where the payload is replaced with the CBOR sequence built at step 3. Signal the usage of this approach within the EDHOC + OSCORE request, by including the new EDHOC Option defined in .

When receiving a request containing the EDHOC option, i.e., an EDHOC + OSCORE request, the Server MUST perform the following steps. Check that the payload of the EDHOC + OSCORE request is a CBOR sequence composed of two CBOR byte strings. If this is not the case, the Server MUST stop processing the request and MUST respond with a 4.00 (Bad Request) error message. Extract CIPHERTEXT_3 from the payload of the EDHOC + OSCORE request, as the first CBOR byte string in the CBOR sequence. Rebuild EDHOC message_3, as a CBOR sequence composed of two CBOR byte strings in the following order. The first CBOR byte string is the ‘kid’ of the Client indicated in the OSCORE option of the EDHOC + OSCORE request (i.e., the OSCORE Sender ID of the Client), encoded as per . The second CBOR byte string is the CIPHERTEXT_3 retrieved at step 2. Perform the EDHOC processing on the EDHOC message_3 rebuilt at step 3, including verifications as per Section 5.4.3 of and the OSCORE Security Context derivation as per Appendix A.2 of . If the applicability statement used in the EDHOC session specifies that EDHOC message_4 shall be sent, the Server MUST stop the EDHOC processing and consider it failed, as due to a client error. Extract the OSCORE ciphertext from the payload of the EDHOC + OSCORE request, as the value of the second CBOR byte string in the CBOR sequence. Rebuild the OSCORE protected CoAP request as the EDHOC + OSCORE request, where the payload is replaced with the OSCORE ciphertext resulting from step 5. Decrypt and verify the OSCORE protected CoAP request resulting from step 6, as per Section 8.2 of , by using the new OSCORE Security Context established at step 4. Process the CoAP request resulting from step 7. If steps 4 (EDHOC processing) and 7 (OSCORE processing) are both successfully completed, the Server MUST reply with an OSCORE protected response, in order for the Client to achieve key confirmation (see Section 5.4.2 of ). The usage of EDHOC message_4 as defined in Section 5.5 of is not applicable to the approach defined in this document. If step 4 (EDHOC processing) fails, the server discontinues the protocol as per Section 5.4.3 of and responds with an EDHOC error message, formatted as defined in Section 6.2 of . In particular, the CoAP response conveying the EDHOC error message MUST have Content-Format set to application/edhoc defined in Section 8.9 of . If step 4 (EDHOC processing) is successfully completed but step 7 (OSCORE processing) fails, the same OSCORE error handling applies as defined in Section 8.2 of .

shows an example of EDHOC + OSCORE Request, based on the OSCORE test vector from Appendix C.4 of and the EDHOC test vector from Appendix D.2 of . In particular, the example assumes that: The used OSCORE Partial IV is 0, consistently with the first request protected with the new OSCORE Security Context. The OSCORE Sender ID of the Client is 0x00. This corresponds to the numeric EDHOC connection identifier C_R with value 0, which in EDHOC message_3 is encoded as the CBOR integer 0, hence as 0x00. The EDHOC option is registered with CoAP option number 21.

The same security considerations from OSCORE and EDHOC hold for this document. TODO (more considerations)

RFC Editor: Please replace “[[this document]]” with the RFC number of this document and delete this paragraph. This document has the following actions for IANA.

IANA is asked to enter the following option numbers to the “CoAP Option Numbers” registry defined in within the “CoRE Parameters” registry. [ The CoAP option numbers 13 and 21 are both consistent with the properties of the EDHOC Option defined in , and they both allow the EDHOC Option to always result in an overall size of 1 byte. This is because: The EDHOC option is always empty, i.e., with zero-length value; and Since the OSCORE option with option number 9 is always present in the CoAP request, the EDHOC option would be encoded with a maximum delta of 4 or 12, depending on its option number being 13 or 21. At the time of writing, the CoAP option numbers 13 and 21 are both unassigned in the “CoAP Option Numbers” registry, as first available and consistent option numbers for the EDHOC option. This document suggests 21 (TBD21) as option number to be assigned to the new EDHOC option, since both 13 and 21 are consistent for the use case in question, but different use cases or protocols may make better use of the option number 13. ]

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation.CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols.Although an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates RFC 7252. This document describes the Concise Binary Object Representation (CBOR) Sequence format and associated media type "application/cbor-seq". A CBOR Sequence consists of any number of encoded CBOR data items, simply concatenated in sequence.Structured syntax suffixes for media types allow other media types to build on them and make it explicit that they are built on an existing media type as their foundation. This specification defines and registers "+cbor-seq" as a structured syntax suffix for CBOR Sequences. The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack.This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. Ericsson AB Ericsson AB Ericsson AB This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, perfect forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios and a main use case is to establish an OSCORE security context. By reusing COSE for cryptography, CBOR for encoding, and CoAP for transport, the additional code size can be kept very low.

Appendix A.1 in defines a rule for converting from EDHOC connection identifier to OSCORE Sender/Recipient ID. This appendix defines the rule for converting from OSCORE Sender/Recipient ID to EDHOC connection identifier, and related processing.

The process defined in this section ensures that every OSCORE Sender/Recipient ID is converted to only one of the two corresponding, equivalent EDHOC connection identifiers, see Appendix A.1 in . An OSCORE Sender/Recipient ID, OSCORE_ID, is converted to an EDHOC connection identifier, EDHOC_ID, as follows. If OSCORE_ID is 0 bytes in size, it is converted to the empty byte string EDHOC_ID (0x40 in CBOR encoding). If OSCORE_ID is longer than 5 bytes in size, it is converted to a byte-valued EDHOC_ID, i.e., a CBOR byte string with value OSCORE_ID. For example, the OSCORE_ID 0x001122334455 is converted to the byte-valued EDHOC_ID 0x001122334455 (0x46001122334455 in CBOR encoding). If OSCORE_ID is 1-5 bytes in size, the following applies. If OSCORE_ID is a valid CBOR encoding for an integer value (i.e., it can be correctly decoded as a CBOR integer), then it is converted to a numeric EDHOC_ID having OSCORE_ID as its CBOR encoded form. For example, the OSCORE_ID 0x01 is converted to the numeric EDHOC_ID 1 (0x01 in CBOR encoding), while the OSCORE_ID 0x2B is converted to the numeric EDHOC_ID -12 (0x2B in CBOR encoding). If OSCORE_ID is not a valid CBOR encoding for an integer value (i.e., it cannot be correctly decoded as a CBOR integer), then it is converted to a byte-valued EDHOC_ID having OSCORE_ID as its value. For example, the OSCORE_ID 0xFF is converted to the byte-valued EDHOC_ID 0xFF (0x41FF in CBOR encoding). Implementations can easily determine which case holds for a given OSCORE_ID with no need to try to actually CBOR-decode it, e.g., by using the approach in .

This section specifies additional EDHOC message processing in addition to what is specified in Section 5 of .

The Initiator selects C_I as follows. The Initiator initializes a set ID_SET as the empty set. The Initiator selects an available OSCORE Recipient ID, ID*, which is not included in ID_SET. The Initiator converts ID* to the EDHOC connection identifier C_I, as per . If the resulting C_I is already used, the Initiator adds ID* to ID_SET and moves back to step 2. Otherwise, it uses C_I as its EDHOC connection identifier.

The Responder MUST discontinue the protocol and reply with an EDHOC error message, if C_I is a CBOR byte string and it has as value a valid CBOR encoding of an integer value (e.g., C_I is CBOR encoded as 0x4100). In fact, this would mean that the Initiator has not followed the conversion rule in when converting its (to be) OSCORE Recipient ID to C_I.

The Responder selects C_R as follows. The Responder initializes a set ID_SET as the empty set. The Responder selects an available OSCORE Recipient ID, ID*, which is not included in ID_SET. The Responder converts ID* to the EDHOC connection identifier C_R, as per . If the resulting C_R is already used or it is equal byte-by-byte to the C_I specified in EDHOC message_1, the Initiator adds ID* to ID_SET and moves back to step 2. Otherwise, it uses C_R as its EDHOC connection identifier.

The Initiator MUST discontinue the protocol and reply with an EDHOC error message, if any of the following conditions holds. C_R is equal byte-by-byte to the C_I that was specified in EDHOC message_1. C_R is a CBOR byte string and it has as value a valid CBOR encoding of an integer value (e.g., C_R is CBOR encoded as 0x4100). In fact, this would mean that the Responder has not followed the conversion rule in when converting its (to be) OSCORE Recipient ID to C_R.

Given a binary string of N bytes in size, it is a valid CBOR encoding of an integer value if and only if, for that size N, its first byte is equal to one of the byte values specified in the “First byte” column of the table below.

RFC Editor: Please remove this section.

Improved background overview of EDHOC. Added explicit rules for converting OSCORE Sender/Recipient IDs to EDHOC connection identifiers following the removal of bstr_identifier from EDHOC. Revised section organization. Recommended number for EDHOC option changed to 21. Editorial improvements.

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).