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Security LAKE Working Group Internet-Draft This document provides considerations for guiding the implementation of the authenticated key exchange protocol Ephemeral Diffie-Hellman Over COSE (EDHOC). Discussion Venues Discussion of this document takes place on the Lightweight Authenticated Key Exchange Working Group mailing list (lake@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction Ephemeral Diffie-Hellman Over COSE (EDHOC) is a lightweight authenticated key exchange protocol, especially intended for use in constrained scenarios. During the development of EDHOC, a number of side topics were raised and discussed, as emerging from reviews of the protocol latest design and from implementation activities. These topics were identified as strongly pertaining to the implementation of EDHOC rather than to the protocol in itself. Hence, they are not discussed in , which rightly focuses on specifying the actual protocol. At the same time, implementors of an application using the EDHOC protocol or of an "EDHOC library" enabling its use cannot simply ignore such topics, and will have to take them into account throughout their implementation work. In order to prevent multiple, independent re-discoveries and assessments of those topics, as well as to facilitate and guide implementation activities, this document collects such topics and discusses them through considerations about the implementation of EDHOC. At a high-level, the topics in question are summarized below.

• Handling of completed EDHOC sessions when they become invalid, and of application keys derived from an EDHOC session when those become invalid. This topic is discussed in .
• Enforcing of different trust models, with respect to learning new authentication credentials during an execution of EDHOC. This topic is discussed in .
• Branched-off, side processing of incoming EDHOC messages, with particular reference to: i) fetching and validation of authentication credentials; and ii) processing of External Authorization Data (EAD) items, which in turn might play a role in the fetching and validation of authentication credentials. This topic is discussed in .
• Effectively using EDHOC over the Constrained Application Protocol (CoAP) in combination with Block-Wise Transfers for CoAP , possibly together with the optimized EDHOC execution workflow defined in . This topic is discussed in .

Terminology 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 related to the EDHOC protocol , the Constrained Application Protocol (CoAP) , and Block-Wise Transfers for CoAP .

Handling of Invalid EDHOC Sessions and Application Keys This section considers the most common situation where, given a certain peer, only the application at that peer has visibility and control of both:

• The EDHOC sessions at that peer; and
• The application keys for that application at that peer, including the knowledge of whether they have been derived from an EDHOC session, i.e., by means of the EDHOC_Exporter interface after the successful completion of an execution of EDHOC (see ).

Building on the above, the following expands on three relevant cases concerning the handling of EDHOC sessions and application keys, in the event that any of those becomes invalid. As a case in point to provide more concrete guidance, the following also considers the specific case where "applications keys" stands for the keying material and parameters that compose an OSCORE Security Context and that are derived from an EDHOC session (see ). Nevertheless, the same considerations are applicable in case EDHOC is used to derive other application keys, e.g., to key different security protocols than OSCORE or to provide the application with secure values bound to an EDHOC session.

EDHOC Sessions Become Invalid The application at a peer P may have learned that a completed EDHOC session S has to be invalidated. When S is marked as invalid, the application at P purges S and deletes each set of application keys (e.g., the OSCORE Security Context) that was generated from S. Then, the applications runs a new execution of the EDHOC protocol with the other peer. Upon successfully completing the EDHOC execution, the two peers derive and install a new set of application keys from this latest EDHOC session. The flowchart in shows the handling of an EDHOC session that has become invalid.

Handling of an EDHOC Session that has Become Invalid and the application keys --> EDHOC --> install new session derived from it application keys ]]>

An EDHOC session may have become invalid, for example, because an authentication credential CRED_X may have expired, or because P may have learned from a trusted source that CRED_X has been revoked. This effectively invalidates CRED_X, and therefore also invalidates any EDHOC session where CRED_X was used as authentication credential of either peer in the session (i.e., P itself or the other peer). In such a case, the application at P has to additionally delete CRED_X and any stored, corresponding credential identifier.

Application Keys Become Invalid The application at a peer P may have learned that a set of application keys is not safe to use anymore. When such a set is specifically an OSCORE Security Context, the application may have learned that from the used OSCORE library or from an OSCORE layer that takes part to the communication stack. A current set SET of application keys shared with another peer can become unsafe to use, for example, due to the following reasons.

• SET has reached its pre-determined expiration time; or
• SET has been established for a pre-defined, now elapsed amount of time, according to enforced application policies; or
• Some elements of SET have been used enough times to approach cryptographic limits that should not be passed, e.g., according to the properties of the specifically used security algorithms. With particular reference to an OSCORE Security Context, such limits are discussed in .

When this happens, the application at the peer P proceeds as follows.

1. If the following conditions both hold, then the application moves to step 2. Otherwise, it moves to step 3.
   • Let us define S as the EDHOC session from which the peer P has derived SET or the eldest SET's ancestor set of application keys. Then, since the completion of S with the other peer, the application at P has received from the other peer at least one message protected with any set of application keys derived from S. That is, P has persisted S (see ).
   • The peer P supports a key update protocol, as an alternative to performing a new execution of EDHOC with the other peer. When SET is specifically an OSCORE Security Context, this means that the peer P supports the key update protocol KUDOS defined in .
2. The application at P runs the key update protocol mentioned at step 1 with the other peer, in order to update SET. When SET is specifically an OSCORE Security Context, this means that the application at P runs KUDOS with the other peer. If the key update protocol terminates successfully, the updated application keys are installed and no further actions are taken. Otherwise, the application at P moves to step 3.
3. The application at the peer P performs the following actions.
   • It deletes SET.
   • It deletes the EDHOC session from which SET was generated, or from which the eldest SET's ancestor set of application keys was generated before any key update occurred (e.g., by means of the EDHOC_KeyUpdate interface defined in or other key update methods).
   • It runs a new execution of the EDHOC protocol with the other peer. Upon successfully completing the EDHOC execution, the two peers derive and install a new set of application keys from this latest EDHOC session.

The flowchart in shows the handling of a set of application keys that has become invalid.

Handling of a set of Application Keys that has Become Invalid Delete the application ----> Rerun application keys keys and the EDHOC session EDHOC persisted? ^ ^ | | | | | | YES | | v v | | | | Derive and install Is KUDOS NO | | new application keys supported? ------------------+ | | | | | YES | v | | Run KUDOS | | | | | | v | | Has KUDOS NO | succeeded? ---------------------------+ | | YES v Install the updated application keys ]]>

Application Keys or Bound Access Rights Become Invalid The following considers two peers that use the ACE framework for authentication and authorization in constrained environments , and specifically the EDHOC and OSCORE profile of ACE defined in . When doing so, one of the two peers acts as ACE Resource Server (RS) hosting protected resources. The other peer acts as ACE Client, requests from an ACE Authorization Server (AS) an Access Token that specifies access rights for accessing protected resources at the RS, and uploads the Access Token to the RS as part of the ACE workflow. Consistent with the used EDHOC and OSCORE profile of ACE, the two peers run EDHOC in order to specifically derive an OSCORE Security Context as their shared set of application keys (see ). The successfully completed EDHOC session is bound to the Access Token. After that, the peer acting as ACE Client can access the protected resources hosted at the other peer, according to the access rights specified in the Access Token. The communications between the two peers are protected by means of the established OSCORE Security Context, which is also bound to the used Access Token. Later on, the application at one of the two peers P may have learned that the established OSCORE Security Context CTX is not safe to use anymore, e.g., from the used OSCORE library or from an OSCORE layer that takes part to the communication stack. The reasons that make CTX not safe to use anymore are the same ones discussed in when considering a set of application keys in general, plus the event where the Access Token bound to CTX becomes invalid (e.g., it has expired or it has been revoked). When this happens, the application at the peer P proceeds as follows.

1. If the following conditions both hold, then the application moves to step 2. Otherwise, it moves to step 3.
   • The Access Token is still valid. That is, it has not expired yet and the peer P is not aware that it has been revoked.
   • Let us define S as the EDHOC session from which the peer P has derived CTX or the eldest CTX's ancestor OSCORE Security Context. Then, since the completion of S with the other peer, the application at P has received from the other peer at least one message protected with any set of application keys derived from S. That is, P has persisted S (see ).
2. If the peer P supports the key update protocol KUDOS , then P runs KUDOS with the other peer, in order to update CTX. If the execution of KUDOS terminates successfully, the updated OSCORE Security Context is installed and no further actions are taken. If the execution of KUDOS does not terminate successfully or if the peer P does not support KUDOS altogether, then the application at P moves to step 3.
3. The application at the peer P performs the following actions.
   • If the Access Token is not valid anymore, the peer P deletes all the EDHOC sessions associated with the Access Token, as well as the OSCORE Security Context derived from each of those sessions. If the peer P acted as ACE Client, then P obtains a new Access Token from the ACE AS, and uploads it to the other peer acting as ACE RS. Finally, the application at P moves to step 4.
   • If the Access Token is valid while the OSCORE Security Context CTX is not, then the peer P deletes CTX. After that, the peer P deletes the EDHOC session from which CTX was generated, or from which the eldest CTX's ancestor OSCORE Security Context was generated before any key update occurred (e.g., by means of KUDOS or other key update methods). Finally, the application at P moves to step 4.
4. The peer P runs a new execution of the EDHOC protocol with the other peer. Upon successfully completing the EDHOC execution, the two peers derive and install a new OSCORE Security Context from this latest EDHOC session.

The flowchart in shows the handling of an Access Token or of a set of application keys that have become invalid.

Handling of an Access Token or of a set of Application Keys that have Become Invalid Delete the associated --> Obtain and --> Rerun ---+ still valid? EDHOC sessions and upload a EDHOC | the application keys new token | | derived from those ^ | | | | | YES | | v | | | | The application keys | | are not valid anymore | | | | | | | | | | v | | | | Are the NO | | application keys -----> Delete the application keys and ------+ | persisted? the associated EDHOC session | | | ^ ^ | | YES | | | v | | | | | | Is KUDOS NO | | | supported? ---------------------+ | v | | | Derive and install | YES | new application keys v | | Run KUDOS | | | | | | v | | Has KUDOS NO | succeeded? ------------------------------+ | | YES v Install the updated application keys ]]>

Trust Models for Learning New Authentication Credentials A peer P relies on authentication credentials of other peers, in order to authenticate those peers when running EDHOC with them. There are different ways for P to acquire an authentication credential CRED of another peer. For example, CRED can be supplied to P out-of-band by a trusted provider. Alternatively, CRED can be specified by the other peer during the EDHOC execution with P. This relies on EDHOC message_2 or message_3, whose respective ID_CRED_R and ID_CRED_I can specify CRED by value, or instead a URI or other external reference where CRED can be retrieved from (see ). Also during the EDHOC execution, an External Authorization Data (EAD) field might include an EAD item that specifies CRED by value or reference. This is the case, e.g., for the EAD item defined in , which is used in the EAD_3 field of EDHOC message_3 and transports (a reference to) an Access Token that in turn specifies CRED_I by value or by reference. When obtaining a new credential CRED, the peer P has to validate it before storing it. The validation steps to perform depend on the specific type of CRED (e.g., a public key certificate ), and can rely on (the authentication credential of) a trusted third party acting as a trust anchor. Upon retrieving a new CRED through the processing of a received EDHOC message and following the successful validation of CRED, the peer P stores CRED only if it assesses CRED to be also trusted, and must not store CRED otherwise. An exception applies for the two unauthenticated operations described in , where a trust relationship with an unknown or not-yet-trusted endpoint is established later. That is, CRED is verified out-of-band at a later stage, or an EDHOC session key is bound to an identity out-of-band at a later stage. If P stores CRED, then P will consider CRED as valid and trusted until it possibly becomes invalid, e.g., because it expires or because P gains knowledge that it has been revoked. When storing CRED, the peer P should generate the authentication credential identifier(s) corresponding to CRED and store them as associated with CRED. For example, if CRED is a public key certificate, an identifier of CRED can be the hash of the certificate. In general, P should generate and associate with CRED one corresponding identifier for each type of authentication credential identifier that P supports and that is compatible with CRED. In future executions of EDHOC with the other peer associated with CRED, this allows such other peer to specify CRED by reference, e.g., by indicating its credential identifier as ID_CRED_R/ID_CRED_I in the EDHOC message_2 or message_3 addressed to the peer P. In turn, this allows P to retrieve CRED from its local storage. When processing a received EDHOC message M that specifies an authentication credential CRED, the peer P can enforce one of the following trust policies in order to determine whether to trust CRED.

• NO-LEARNING: according to this policy, the peer P trusts CRED if and only if P is already storing CRED at message reception time. That is, upon receiving M, the peer P can continue the execution of EDHOC only if both the following conditions hold.
  • P currently stores CRED, as specified by reference or by value in ID_CRED_I/ID_CRED_R or in the value of an EAD item; and
  • CRED is still valid, i.e., P believes CRED to not be expired or revoked.
• LEARNING: according to this policy, the peer P trusts CRED even if P is not already storing CRED at message reception time. That is, upon receiving M, the peer P performs the following steps.
  1. P retrieves CRED, as specified by reference or by value in ID_CRED_I/ID_CRED_R or in the value of an EAD item.
  2. P checks whether CRED is already being stored and if it is still valid. In such a case, P trusts CRED and can continue the EDHOC execution. Otherwise, P moves to step 3.
  3. P attempts to validate CRED. If the validation process is not successful, P aborts the EDHOC session with the other peer. Otherwise, P trusts and stores CRED, and can continue the EDHOC execution.

Irrespective of the adopted trust policy, P actually uses CRED only if it is determined to be fine to use in the context of the ongoing EDHOC session, also depending on the specific identity of the other peer (see Sections and of ). If this is not the case, P aborts the EDHOC session with the other peer.

Enforcement in the EDHOC and OSCORE Profile of ACE As discussed in , two EDHOC peers can be using the ACE framework and specifically the EDHOC and OSCORE profile of ACE defined in . In particular:

• One of the two EDHOC peers, namely PEER_RS, acts as ACE Resource Server (RS).
• The other EDHOC peer, namely PEER_C, acts as ACE Client, and obtains from the ACE Authorization Server (AS) an Access Token for accessing protected resources at PEER_RS. Together with other information, the Access Token specifies (by value or by reference) the public authentication credential AUTH_CRED_C that PEER_C is going to use when running EDHOC with PEER_RS. Note that AUTH_CRED_C will be used as either CRED_I or CRED_R, depending on whether the two peers use the EDHOC forward message flow (i.e., PEER_C is the EDHOC Initiator) or the EDHOC reverse message flow (i.e., PEER_C is the EDHOC Responder), respectively (see ).

When using the EDHOC and OSCORE profile of ACE, it is also possible for PEER_C to upload the Access Token at PEER_RS by means of a dedicated EDHOC EAD item, while running EDHOC with PEER_RS. In order to ensure that this effectively works, PEER_RS has to support the trust policy "LEARNING", at least for its EDHOC resource used for exchanging the EDHOC messages of the EDHOC session in question. That is, PEER_RS is expected to not store AUTH_CRED_C in advance, and instead to gain knowledge of it as specified in the Access Token issued by the AS and uploaded by PEER_C. If PEER_RS has to learn AUTH_CRED_C as a new public authentication credential during an EDHOC session, indeed this requires PEER_RS to enforce the trust policy "LEARNING". In fact, when the AS issues the first Access Token ever such that it specifies AUTH_CRED_C and is intended to be uploaded to PEER_RS, it is expected that the Access Token specifies AUTH_CRED_C by value and that PEER_RS is not currently storing AUTH_CRED_C.

Side Processing of Incoming EDHOC Messages This section describes an approach that EDHOC peers can use upon receiving EDHOC messages, in order to fetch/validate authentication credentials and to process External Authorization Data (EAD) items. As per , the EDHOC protocol provides a transport mechanism for conveying EAD items, but specifications defining those items have to set the ground for "agreeing on the surrounding context and the meaning of the information passed to and from the application". The approach described in this section aims to help implementors navigate the surrounding context mentioned above, irrespective of the specific EAD items conveyed in the EDHOC messages. In particular, the described approach takes into account the following points.

• The fetching and validation of the other peer's authentication credential relies on ID_CRED_I in EDHOC message_2, or on ID_CRED_R in EDHOC message_3, or on the value of an EAD item. When this occurs upon receiving EDHOC message_2 or message_3, the decryption of the EDHOC message has to be completed first. The validation of the other peer's authentication credential might depend on using the value of an EAD item, which in turn has to be validated first. For instance, an EAD item within the EAD_2 field may contain a voucher to be used for validating the other peer's authentication credential (see ).
• Some EAD items may be processed only after having successfully verified the EDHOC message, i.e., after a successful verification of the Signature_or_MAC field. For instance, an EAD item within the EAD_3 field may contain a Certificate Signing Request (CSR) . Hence, such an EAD item can be processed only once the recipient peer has attained proof of the other peer possessing its private key.

In order to conveniently handle such processing, the application can prepare in advance one "side-processor object" (SPO), which takes care of the operations above during the EDHOC execution. In particular, the application provides EDHOC with the SPO before starting an EDHOC execution, during which EDHOC will temporarily transfer control to the SPO at the right point in time, in order to perform the required side-processing of an incoming EDHOC message. Furthermore, the application has to instruct the SPO about how to prepare any EAD item such that: it has to be included in an outgoing EDHOC message; and it is independent of the processing of other EAD items included in incoming EDHOC messages. This includes, for instance, the preparation of padding EAD items. At the right point in time during the processing of an incoming EDHOC message M at the peer P, EDHOC invokes the SPO and provides it with the following input:

• When M is EDHOC message_2 or message_3, an indication of whether this invocation is happening before or after the message verification (i.e., before or after having verified the Signature_or_MAC field).
• The full set of information related to the current EDHOC session. This especially includes the selected cipher suite and the ephemeral Diffie-Hellman public keys G_X and G_Y that the two peers have exchanged in the EDHOC session.
• The other peers' authentication credentials that the peer P stores.
• All the decrypted information elements retrieved from M.
• The EAD items included in M.
  • Note that EDHOC might do some preliminary work on M before invoking the SPO, in order to provide the SPO only with actually relevant EAD items. This requires the application to additionally provide EDHOC with the ead_labels of the EAD items that the peer P recognizes (see ). With such information available, EDHOC can early abort the current session in case M includes any EAD item which is both critical and not recognized by the peer P. If no such EAD items are found, EDHOC can remove any padding EAD item (see ), as well as any EAD item which is neither critical nor recognized (since the SPO is going to ignore it anyway). As a result, EDHOC is able to provide the SPO only with EAD items that will be recognized and that require actual processing.
  • Note that, after having processed the EAD items, the SPO might actually need to store them throughout the whole EDHOC execution, e.g., in order to refer to them also when processing later EDHOC messages in the current EDHOC session.

The SPO performs the following tasks on an incoming EDHOC message M.

• The SPO fetches and/or validates the other peer's authentication credential CRED, based on a dedicated EAD item of M or on the ID_CRED field of M (for EDHOC message_2 or message_3).
• The SPO processes the EAD items conveyed in the EAD field of M.
• The SPO stores the results of the performed operations, and makes such results available to the application.
• When the SPO has completed its side processing and transfers control back to EDHOC, the SPO provides EDHOC with the produced EAD items to include in the EAD field of the next outgoing EDHOC message. The production of such EAD items can be triggered, e.g., by:
  • The consumption of EAD items included in M; and
  • The execution of instructions that the SPO has received from the application, concerning EAD items to produce irrespective of other EAD items included in M.

In the following, to describe more in detail the actions performed by the SPO on the different, incoming EDHOC messages. Then, describes further, special handling of incoming EDHOC messages in particular situations.

EDHOC message_1 During the processing of an incoming EDHOC message_1, EDHOC invokes the SPO only once, after the Responder peer has successfully decoded the message and accepted the selected cipher suite. If the EAD_1 field is present, the SPO processes the EAD items included therein. Once all such EAD items have been processed the SPO transfers control back to EDHOC. When doing so, the SPO also provides EDHOC with any produced EAD items to include in the EAD field of the next outgoing EDHOC message. Then, EDHOC resumes its execution and advances its protocol state.

EDHOC message_4 During the processing of an incoming EDHOC message_4, EDHOC invokes the SPO only once, after the Initiator peer has successfully decrypted the message. If the EAD_4 field is present, the SPO processes the EAD items included therein. Once all such EAD items have been processed, the SPO transfers control back to EDHOC, which resumes its execution and advances its protocol state.

EDHOC message_2 and message_3 The following refers to "message_X" as an incoming EDHOC message_2 or message_3, and to "message verification" as the verification of Signature_or_MAC_X in message_X. During the processing of a message_X, EDHOC invokes the SPO two times:

• Right after message_X has been decrypted and before its verification starts. Following this invocation, the SPO performs the actions described in .
• Right after message_X has been successfully verified. Following this invocation, the SPO performs the actions described in .

The flowcharts in show the high-level interaction between the core EDHOC processing and the SPO, as well as the different steps taken for processing an incoming message_X.

Pre-Verification Side Processing The pre-verification side processing occurs in two sequential phases, namely PHASE_1 and PHASE_2. PHASE_1 - During PHASE_1, the SPO at the recipient peer P determines CRED, i.e., the other peer's authentication credential to use in the ongoing EDHOC session. In particular, the SPO performs the following steps.

1. The SPO determines CRED based on ID_CRED_X or on an EAD item in message_X. Those may specify CRED by value or by reference, including a URI or other external reference where CRED can be retrieved from. If CRED is already installed, the SPO moves to step 2. Otherwise, the SPO moves to step 3.
2. The SPO determines if the stored CRED is currently valid, e.g., by asserting that CRED has not expired and has not been revoked. Performing such a validation may require the SPO to first process an EAD item included in message_X. For example, it can be an EAD item in EDHOC message_2, which confirms or revokes the validity of CRED_R specified by ID_CRED_R, as the result of an OCSP process . In case CRED is determined to be valid, the SPO moves to step 9. Otherwise, the SPO moves to step 11.
3. The SPO attempts to retrieve CRED, and then moves to step 4.
4. If the retrieval of CRED has succeeded, the SPO moves to step 5. Otherwise, the SPO moves to step 11.
5. If the enforced trust policy for new authentication credentials is "NO-LEARNING" (see ), the SPO moves to step 11. Otherwise, the SPO moves to step 6.
6. If this step has been reached, the peer P enforces the trust policy "LEARNING" (see ) and it is not already storing the retrieved CRED. Consistently, the SPO determines if CRED is currently valid, e.g., by asserting that CRED has not expired and has not been revoked. Then, the SPO moves to step 7. Validating CRED may require the SPO to first process an EAD item included in message_X. For example, it can be an EAD item in EDHOC message_2 that: i) specifies a voucher for validating CRED_R as a CWT Claims Set (CCS) transported by value in ID_CRED_R (see ); or instead ii) an OCSP response for validating CRED_R as a certificate transported by value or reference in ID_CRED_R.
7. If CRED has been determined valid, the SPO moves to step 8. Otherwise, the SPO moves to step 11.
8. The SPO stores CRED as a valid and trusted authentication credential associated with the other peer, together with corresponding authentication credential identifiers (see ). Then, the SPO moves to step 9.
9. The SPO checks if CRED is fine to use in the context of the ongoing EDHOC session, also depending on the specific identity of the other peer (see Sections and of ). If this is the case, the SPO moves to step 10. Otherwise, the SPO moves to step 11.
10. P uses CRED as authentication credential of the other peer in the ongoing EDHOC session. Then, PHASE_1 ends, and the pre-verification side processing moves to the next PHASE_2 (see below).
11. The SPO has not found a valid authentication credential associated with the other peer that can be used in the ongoing EDHOC session. Therefore, the EDHOC session with the other peer is aborted.

PHASE_2 - During PHASE_2, the SPO processes any EAD item included in message_X such that both the following conditions hold.

• The EAD item has not been already processed during PHASE_1.
• The EAD item can be processed before performing the verification of message_X.

Once all such EAD items have been processed, the SPO transfers control back to EDHOC, which either aborts the ongoing EDHOC session or continues the processing of message_X with its corresponding message verification.

Post-Verification Side Processing During the post-verification side processing, the SPO processes any EAD item included in message_X such that the processing of that EAD item had to wait for completing the successful message verification. The late processing of such EAD items is typically due to the fact that a pre-requirement has to be fulfilled first. For example, the recipient peer P has to have first asserted that the other peer does possess the private key corresponding to the public key of the other peer's authentication credential CRED determined during the pre-verification side processing (see ). This requirement is fulfilled after a successful message verification of message_X. Once all such EAD items have been processed, the SPO transfers control back to EDHOC. When doing so, the SPO also provides EDHOC with any produced EAD items to include in the EAD field of the next outgoing EDHOC message. Then, EDHOC resumes its execution and advances its protocol state.

Flowcharts The flowchart in shows the high-level interaction between the core EDHOC processing and the SPO, with particular reference to an incoming EDHOC message_2 or message_3.

High-Level Interaction Between the Core EDHOC Processing and the Side-Processor Object (SPO) | Retrieve the | | Advance the | | | | message_X | | protocol state | | protocol state | | | +-----------+ +----------------+ +----------------+ | | | ^ | | | | | | v | | | +--------------+ +--------------------+ | | | | Decrypt | | Verify | | | | | CYPHERTEXT_X | | Signature_or_MAC_X | | | | +--------------+ +--------------------+ | | | | ^ | | | | | | | | | +----------------|-----------|-----------|---------|------------------+ | | | | | | | | ................. Divert | Get | Divert | Get | : EAD items : | back | | back | : for the next : | | | | : EDHOC message : | | | | :...............: | | | | +----------------|-----------|-----------|---------|------------------+ | | | | | | | v | v | | | +---------------------------+ +-----------------------------+ | | | a) Retrieval and | | Processing of | | | | validation of CRED_X; | | post-verification EAD items | | | | b) Trust assessment | +-----------------------o-----+ | | | of CRED_X; | | | | | c) Processing of o-------- Shared state -------o | | | pre-verification | | | | EAD items | ...................... | | | | : Instructions about : | | | - (a) and (c) might have | : EAD items to : | | | to occur in parallel | : unconditionally : | | | - (b) depends on the | : produce for the : | | | used trust model | : next EDHOC message : | | +---------------------------+ :....................: | | | | Side-Processor Object | +---------------------------------------------------------------------+ ]]>

The flowchart in shows the different steps taken for processing an incoming EDHOC message_2 and message_3.

Processing steps for EDHOC message_2 and message_3 | CRED via |--->| retrieval | | | | point to an already | | ID_CRED_X or | | of CRED | | | | stored CRED? | | an EAD item | | successful? | | | +---------------------+ +--------------+ +-------------+ | | | | | | | | | NO | YES | | | +--------------+ | | | | YES | | | | v v v | | +-----------------+ NO +-----------+ YES +----------------+ | | | 2. Is this CRED |-------->| 11. Abort |<-----| 5. Is the | | | | still valid? | | the EDHOC | | used policy | | | +-----------------+ | session | | "NO-LEARNING"? | | | | | | +----------------+ | | | YES | | | | | v | | The used policy | NO | | +--------------------+ NO | | is "LEARNING" | | | | 9. Is this CRED |----->| | v | | | good to use in the | +-----------+ +-------------+ | | | context of this | ^ | 6. Validate | | | | EDHOC session? |<--+ | | CRED | | | +--------------------+ | | +-------------+ | | | | | | | | | YES | | NO | | | | | | v | | | | +-----------------------------+ | | | | | 7. Is CRED valid? | | | | | +-----------------------------+ | | | | | | | | | | YES | | | | v | | v | +-----------------------------+ | | +------------------+ | | 8. Store CRED as valid and | | | | 10. Continue by | +--------| trusted. | | | | considering this | | | | | | CRED as the | | Pair CRED with consistent | | | | authentication | | credential identifiers, for | | | | credential of | | each supported type of | | | | the other peer | | credential identifier. | | | +------------------+ +-----------------------------+ | | | | +---------|---------------------------------------------------------+ | | +---------|---------------------------------------------------------+ | | Pre-verification side processing (PHASE_2) | | v | | +---------------------------------------------------------+ | | | Process the EAD items that have not been processed yet, | | | | and that can be processed before message verification | | | +---------------------------------------------------------+ | | | | +---------|---------------------------------------------------------+ | | v Control transferred back to the core EDHOC processing | | v +------------------+ | Verify message_X | (core EDHOC processing) +------------------+ | | v Control transferred to the side-processor object | +---------|----------------------------------------+ | | Post-verification processing | | v | | +---------------------------------------------+ | | | Process the EAD items that have to be | | | | processed (also) after message verification | | | +---------------------------------------------+ | | | | | | | | v | | +--------------------------------------------+ | | | Make all the results of the EAD processing | | | | available to build the next EDHOC message | | | +--------------------------------------------+ | | | | +---------|----------------------------------------+ | | v Control transferred back to the core EDHOC processing | | v +----------------+ | Advance the | (core EDHOC processing) | protocol state | +----------------+ ]]>

Side Processing in Particular Situations This section describes methods to perform special handling of incoming EDHOC messages in particular situations.

EDHOC and OSCORE Profile of ACE discusses the case where two EDHOC peer use the ACE framework and specifically the EDHOC and OSCORE profile of ACE defined in . In particular, considers a PEER_C that, when running EDHOC with PEER_RS, uses a dedicated EDHOC EAD item for uploading at PEER_RS an ACE Access Token, which in turn includes its own public authentication credential AUTH_CRED_C. Also as noted in , this practically builds on PEER_RS supporting the trust policy "LEARNING" (see ), at least for its EDHOC resource used for exchanging the EDHOC messages of the EDHOC session in question.

• Editor's note: TODO PEER_RS might have reasons to enforce the trust policy "NO_LEARNING" anyway. In that case, unless PEER_RS already stores AUTH_CRED_C, the upload of the Access Token by means of the EAD item has to fail, and consequently the EDHOC session is aborted. Explain how the SPO can practically ensure that to be the outcome.
• Editor's note: TODO AUTH_CRED_C is always specified (by value or by reference) in ID_CRED_R (ID_CRED_I) of EDHOC message_2 (EDHOC message_3). AUTH_CRED_C can also be specified (by value or by reference) within an Access Token, which is conveyed by an EDHOC EAD item in an EDHOC message that PEER_C sends to PEER_RS. The details also depend on the two EDHOC peers using either the EDHOC forward message flow or the EDHOC reverse message flow (see ). When such an Access Token is uploaded by means of an EDHOC EAD item, PEER_RS has to perform consistency checks between the AUTH_CRED_C specified in ID_CRED_R or ID_CRED_I, and the AUTH_CRED_C specified within the Access Token. Explain when PEER_RS becomes able to perform the consistency check, for the different cases, depending on using the EDHOC forward message flow or the EDHOC reverse message flow, and on the specific EDHOC message including the EAD item that conveys the Access Token including AUTH_CRED_C. In the end, some of this content might better fit in , while this document can keep only implementation-specific details.

Using EDHOC over CoAP with Block-Wise specifies how to transfer EDHOC over CoAP . In such a case, the EDHOC messages (possibly prepended by an EDHOC connection identifier) are transported in the payload of CoAP requests and responses, according to the EDHOC forward message flow or the EDHOC reverse message flow. Furthermore, specifies how to derive an OSCORE Security Context from an EDHOC session. Building on that, further details the use of EDHOC with CoAP and OSCORE, and specifies an optimization approach for the EDHOC forward message flow that combines the EDHOC execution with the first subsequent OSCORE transaction. This is achieved by means of an "EDHOC + OSCORE request", i.e., a single CoAP request message that conveys both EDHOC message_3 of the ongoing EDHOC session and an OSCORE-protected application request, where the latter is protected with the OSCORE Security Context derived from that EDHOC session. This section provides guidelines and recommendations for CoAP clients supporting Block-wise transfers for CoAP and the EDHOC + OSCORE request defined in . The following especially considers a CoAP client that may perform only "inner" Block-wise, but not "outer" Block-wise operations (see ). That is, the considered CoAP client does not (further) split an OSCORE-protected request like an intermediary (e.g., a proxy) might do. This is the typical case for OSCORE endpoints (see ).

Notation The rest of this section refers to the following notation.

• SIZE_APP: the size in bytes of the application data to be included in a CoAP request. When Block-wise is used, this is referred to as the "body" to be fragmented into blocks.
• SIZE_EDHOC: the size in bytes of EDHOC message_3, if this is sent as part of the EDHOC + OSCORE request. Otherwise, the size of EDHOC message_3 plus the size in bytes of the EDHOC Connection Identifier C_R, encoded as per .
• SIZE_MTU: the maximum amount of transmittable bytes before having to use Block-wise. This is, for example, 64 KiB as maximum datagram size when using UDP, or 1280 bytes as the maximum size for an IPv6 MTU.
• SIZE_OH: the size in bytes of the overall overhead due to all the communication layers underlying the application. This takes into account also the overhead introduced by the OSCORE processing.
• LIMIT = (SIZE_MTU - SIZE_OH): the practical maximum size in bytes to be considered by the application before using Block-wise.
• SIZE_BLOCK: the size in bytes of inner blocks.
• ceil(): the ceiling function.

Pre-requirements Before sending an EDHOC + OSCORE request, the CoAP client has to perform the following checks. Note that, while the CoAP client is able to fragment the application data, it cannot fragment the EDHOC + OSCORE request or the EDHOC message_3 added therein.

• If inner Block-wise is not used, hence SIZE_APP <= LIMIT, the CoAP client must verify whether all the following conditions hold:
  • COND1: SIZE_EDHOC <= LIMIT
  • COND2: (SIZE_APP + SIZE_EDHOC) <= LIMIT
• If inner Block-wise is used, the CoAP client must verify whether all the following conditions hold:
  • COND3: SIZE_EDHOC <= LIMIT
  • COND4: (SIZE_BLOCK + SIZE_EDHOC) <= LIMIT

In either case, if not all the corresponding conditions hold, the CoAP client should not send the EDHOC + OSCORE request. Instead, the CoAP client can continue by switching to the purely sequential, original EDHOC workflow (see ). That is, the CoAP client first sends EDHOC message_3 prepended by the EDHOC Connection Identifier C_R encoded as per , and then sends the OSCORE-protected CoAP request once the EDHOC execution is completed.

Effectively Using Block-Wise In order to avoid further fragmentation at lower layers when sending an EDHOC + OSCORE request, the CoAP client has to use inner Block-wise if any of the following conditions holds:

• COND5: SIZE_APP > LIMIT
• COND6: (SIZE_APP + SIZE_EDHOC) > LIMIT

In particular, consistently with , the used SIZE_BLOCK has to be such that the following condition also holds:

• COND7: (SIZE_BLOCK + SIZE_EDHOC) <= LIMIT

Note that the CoAP client might still use Block-wise due to reasons different from exceeding the size indicated by LIMIT. The following shows the number of round trips for completing both the EDHOC execution and the first OSCORE-protected exchange, under the assumption that the exchange of EDHOC message_1 and EDHOC message_2 do not result in using Block-wise. If both the conditions COND5 and COND6 hold, the use of Block-wise results in the following number of round trips experienced by the CoAP client.

• If the original EDHOC execution workflow is used (see ), the number of round trips RT_ORIG is equal to 1 + ceil(SIZE_EDHOC / SIZE_BLOCK) + ceil(SIZE_APP / SIZE_BLOCK).
• If the optimized EDHOC execution workflow is used (see ), the number of round trips RT_COMB is equal to 1 + ceil(SIZE_APP / SIZE_BLOCK).

It follows that RT_COMB < RT_ORIG, i.e., the optimized EDHOC execution workflow always yields a lower number of round trips. Instead, the convenience of using the optimized EDHOC execution workflow becomes questionable if both the following conditions hold:

• COND8: SIZE_APP <= LIMIT
• COND9: (SIZE_APP + SIZE_EDHOC) > LIMIT

That is, since SIZE_APP <= LIMIT, using Block-wise would not be required when using the original EDHOC execution workflow, provided that SIZE_EDHOC <= LIMIT still holds. At the same time, using the combined workflow is in itself what actually triggers the use of Block-wise, since (SIZE_APP + SIZE_EDHOC) > LIMIT. Therefore, the following round trips are experienced by the CoAP client.

• The original EDHOC execution workflow run without using Block-wise results in a number of round trips RT_ORIG equal to 3.
• The optimized EDHOC execution workflow run using Block-wise results in a number of round trips RT_COMB equal to 1 + ceil(SIZE_APP / SIZE_BLOCK).

It follows that RT_COMB >= RT_ORIG, i.e., the optimized EDHOC execution workflow might still be not worse than the original EDHOC execution workflow in terms of round trips. This is the case only if the used SIZE_BLOCK is such that ceil(SIZE_APP / SIZE_BLOCK) is equal to 2, i.e., the EDHOC + OSCORE request is fragmented into only 2 inner blocks. However, even in such a case, there would be no advantage in terms or round trips compared to the original workflow, while still requiring the CoAP client and the CoAP server to perform the processing due to using the EDHOC + OSCORE request and Block-wise transferring. Therefore, if both the conditions COND8 and COND9 hold, the CoAP client should not send the EDHOC + OSCORE request. Instead, the CoAP client should continue by switching to the original EDHOC execution workflow. That is, the CoAP client first sends EDHOC message_3 prepended by the EDHOC Connection Identifier C_R encoded as per , and then sends the OSCORE-protected CoAP request once the EDHOC execution is completed.

Security Considerations TBD

IANA Considerations This document has no actions for IANA.

References Normative References 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. The Constrained Application Protocol (CoAP) is a RESTful transfer protocol for constrained nodes and networks. Basic CoAP messages work well for small payloads from sensors and actuators; however, applications will need to transfer larger payloads occasionally -- for instance, for firmware updates. In contrast to HTTP, where TCP does the grunt work of segmenting and resequencing, CoAP is based on datagram transports such as UDP or Datagram Transport Layer Security (DTLS). These transports only offer fragmentation, which is even more problematic in constrained nodes and networks, limiting the maximum size of resource representations that can practically be transferred. Instead of relying on IP fragmentation, this specification extends basic CoAP with a pair of "Block" options for transferring multiple blocks of information from a resource representation in multiple request-response pairs. In many important cases, the Block options enable a server to be truly stateless: the server can handle each block transfer separately, with no need for a connection setup or other server-side memory of previous block transfers. Essentially, the Block options provide a minimal way to transfer larger representations in a block-wise fashion. A CoAP implementation that does not support these options generally is limited in the size of the representations that can be exchanged, so there is an expectation that the Block options will be widely used in CoAP implementations. Therefore, this specification updates RFC 7252. 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 specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios, and a main use case is to establish an Object Security for Constrained RESTful Environments (OSCORE) security context. By reusing CBOR Object Signing and Encryption (COSE) for cryptography, Concise Binary Object Representation (CBOR) for encoding, and Constrained Application Protocol (CoAP) for transport, the additional code size can be kept very low. Ericsson RISE AB RISE AB Fraunhofer AISEC Ericsson The lightweight authenticated key exchange protocol Ephemeral Diffie- Hellman Over COSE (EDHOC) can be run over the Constrained Application Protocol (CoAP) and used by two peers to establish a Security Context for the security protocol Object Security for Constrained RESTful Environments (OSCORE). This document details this use of the EDHOC protocol, by specifying a number of additional and optional mechanisms. These especially include an optimization approach for combining the execution of EDHOC with the first 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. 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. 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. Informative References This memo represents a republication of PKCS #10 v1.7 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from the PKCS #9 v2.0 or the PKCS #10 v1.7 document. This memo provides information for the Internet community. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document specifies a protocol useful in determining the current status of a digital certificate without requiring Certificate Revocation Lists (CRLs). Additional mechanisms addressing PKIX operational requirements are specified in separate documents. This document obsoletes RFCs 2560 and 6277. It also updates RFC 5912. CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. This specification defines a framework for authentication and authorization in Internet of Things (IoT) environments called ACE-OAuth. The framework is based on a set of building blocks including OAuth 2.0 and the Constrained Application Protocol (CoAP), thus transforming a well-known and widely used authorization solution into a form suitable for IoT devices. Existing specifications are used where possible, but extensions are added and profiles are defined to better serve the IoT use cases. Ericsson Ericsson RISE RISE This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Ephemeral Diffie-Hellman Over COSE (EDHOC) for achieving mutual authentication between an ACE-OAuth Client and Resource Server, and it binds an authentication credential of the Client to an ACE-OAuth access token. EDHOC also establishes an Object Security for Constrained RESTful Environments (OSCORE) Security Context, which is used to secure communications when accessing protected resources according to the authorization information indicated in the access token. This profile can be used to delegate management of authorization information from a resource-constrained server to a trusted host with less severe limitations regarding processing power and memory. RISE AB RISE AB This document defines Key Update for OSCORE (KUDOS), a lightweight procedure that two CoAP endpoints can use to update their keying material by establishing a new OSCORE Security Context. Accordingly, it updates the use of the OSCORE flag bits in the CoAP OSCORE Option as well as the protection of CoAP response messages with OSCORE, and it deprecates the key update procedure specified in Appendix B.2 of RFC 8613. Thus, this document updates RFC 8613. Also, this document defines a procedure that two endpoints can use to update their OSCORE identifiers, run either stand-alone or during a KUDOS execution. RISE AB RISE AB Object Security for Constrained RESTful Environments (OSCORE) uses AEAD algorithms to ensure confidentiality and integrity of exchanged messages. Due to known issues allowing forgery attacks against AEAD algorithms, limits should be followed on the number of times a specific key is used for encryption or decryption. Among other reasons, approaching key usage limits requires updating the OSCORE keying material before communications can securely continue. This document defines how two OSCORE peers can follow these key usage limits and what steps they should take to preserve the security of their communications. Ericsson AB Ericsson AB RISE AB RISE AB Nexus Group This document specifies a CBOR encoding of X.509 certificates. The resulting certificates are called C509 Certificates. The CBOR encoding supports a large subset of RFC 5280 and all certificates compatible with the RFC 7925, IEEE 802.1AR (DevID), CNSA, RPKI, GSMA eUICC, and CA/Browser Forum Baseline Requirements profiles. When used to re-encode DER encoded X.509 certificates, the CBOR encoding can in many cases reduce the size of RFC 7925 profiled certificates with over 50% while also significantly reducing memory and code size compared to ASN.1. The CBOR encoded structure can alternatively be signed directly ("natively signed"), which does not require re- encoding for the signature to be verified. The document also specifies C509 Certificate Signing Requests, C509 COSE headers, a C509 TLS certificate type, and a C509 file format. Ericsson AB Ericsson AB INRIA INRIA Sandelman Software Works This document describes a procedure for authorizing enrollment of new devices using the lightweight authenticated key exchange protocol Ephemeral Diffie-Hellman Over COSE (EDHOC). The procedure is applicable to zero-touch onboarding of new devices to a constrained network leveraging trust anchors installed at manufacture time.

Document Updates

Version -00 to -01

• Added considerations on trust policies when using the EDHOC and OSCORE profile of the ACE framework.
• Placeholder section on special processing when using the EDHOC and OSCORE profile of the ACE framework.
• Added considerations on using EDHOC with CoAP and Block-wise.
• Editorial improvements.

Acknowledgments The author sincerely thanks , , , , , and for their comments and feedback. The work on this document has been partly supported by the Sweden's Innovation Agency VINNOVA and the Celtic-Next project CYPRESS.