]> Inria

elsa.lopez-perez@inria.fr

Ericsson

goran.selander@ericsson.com

Ericsson

john.mattsson@ericsson.com

University of Murcia

rafa@um.es

Security LAKE Working Group This document specifies a Pre-Shared Key (PSK) authentication method for the Ephemeral Diffie-Hellman Over COSE (EDHOC) key exchange protocol. The PSK method enhances computational efficiency while providing mutual authentication, ephemeral key exchange, identity protection, and quantum resistance. It is particularly suited for systems where nodes share a PSK provided out-of-band (external PSK) and enables efficient session resumption with less computational overhead when the PSK is provided from a previous EDHOC session (resumption PSK). This document details the PSK method flow, key derivation changes, message formatting, processing, and security considerations. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the LAKE Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction This document defines a Pre-Shared Key (PSK) authentication method for the Ephemeral Diffie-Hellman Over COSE (EDHOC) key exchange protocol . The PSK method balances the complexity of credential distribution with computational efficiency. While symmetrical key distribution is more complex than asymmetrical approaches, PSK authentication offers greater computational efficiency compared to the methods outlined in . The PSK method retains mutual authentication, asymmetric ephemeral key exchange, and identity protection established by . EDHOC with PSK authentication benefits use cases where two nodes share a Pre-Shared Key (PSK) provided out-of-band (external PSK). This applies to scenarios like the Authenticated Key Management Architecture (AKMA) in mobile systems or the Peer and Authenticator in Extensible Authentication Protocol (EAP) systems. The PSK method enables the nodes to perform ephemeral key exchange, achieving Perfect Forward Secrecy (PFS). This ensures that even if the PSK is compromised, past communications remain secure against active attackers, while future communications are protected from passive attackers. Additionally, by leveraging the PSK for both authentication and key derivation, the method offers quantum resistance key exchange and authentication even when used with ECDHE. Another key use case of PSK authentication in the EDHOC protocol is session resumption. This enables previously connected parties to quickly reestablish secure communication using pre-shared keys from a prior session, reducing the overhead associated with key exchange and asymmetric authentication. By using PSK authentication, EDHOC allows session keys to be refreshed with significantly lower computational overhead compared to public-key authentication. In this case, the PSK (resumption PSK) is provisioned after the establishment of a previous EDHOC session by using EDHOC_Exporter (resumption PSK). Therefore, the external PSK is supposed to be a long-term credential while the resumption PSK is a session key. Section 3 provides an overview of the PSK method flow and credentials. Section 4 outlines the changes to key derivation compared to . Section 5 details message formatting and processing, and Section 6 describes the usage of PSK for resumption. Section 7 defines the use of EDHOC-PSK with OSCORE. Security considerations are described in Section 8, and Section 9 outlines the IANA considerations.

Conventions and Definitions 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. Readers are expected to be familiar with the terms and concepts described in EDHOC , CBOR , CBOR Sequences , COSE Structures and Processing , COSE Algorithms , CWT and CCS , and the Concise Data Definition Language (CDDL) , which is used to express CBOR data structures.

Protocol In this method, the Pre-Shared Key identifier (ID_CRED_PSK), which allows retrieval of the CRED_PSK, is sent in message_3. CRED_PSK is an authentication credential associated with the PSK. Although it may use a COSE_Key representation for compatibility, the format is not restricted to COSE_Key and can vary depending on the implementation. Through this document we will refer to the Pre-Shared Key authentication method as EDHOC-PSK.

Credentials The Initiator and Responder are assumed to share a PSK (external PSK or resumption PSK) with good amount of entropy and the following requirements:

• Only the Initiator and the Responder have access to the PSK.
• The Responder is able to retrieve CRED_PSK and the PSK, using ID_CRED_PSK.

ID_CRED_PSK ID_CRED_PSK is a COSE header map containing header parameters that can identify a pre-shared key. For example: The purpose of ID_CRED_PSK is to facilitate retrieval of the correct PSK. It is RECOMMENDED that ID_CRED_PSK uniquely identifies the corresponding CRED_PSK, since ambiguity may require the recipient to try multiple keys.

CRED_PSK CRED_PSK is an authentication credential that identifies and encapsulates the PSK. It substitutes the public-key based credentials CRED_I and CRED_R defined in . While CRED_PSK may adopt encoding and representation patterns from Section and Section of , it differs fundamentally in that:

• CRED_PSK contains or identifies a symmetric key, not a public authentication key.
• Authentication is achieved implicitly via the successful use of the PSK to derive keying material and encrypt and posteriorly decrypt protected messages.

A common representation of CRED_PSK is a CBOR Web Token (CWT) or CWT Claims Set (CCS) whose 'cnf' claim uses the confirmation method 'COSE_Key' to carry the PSK. An example of CRED_PSK would be: Alternative formats for CRED_PSK MAY be used, as long as they allow the recipient to identify and retrieve the correct PSK. When the authentication credential includes only a COSE_Key (e.g., a raw key reference), it SHOULD be wrapped as a CCS by prefixing it with a 'cnf' claim. In this case, the resulting CRED_PSK contains no identity beyond the key itself. Implementations MUST ensure that CRED_PSK values used by the Initiator and the Responder carry distinct identities in their sub claims (e.g., "42-50-31-FF-EF-37-32-39" and "23-11-58-AA-B3-7F-10"). This enables correct identification of parties and prevents misbinding attacks, as per . Note that sub is used solely for identity binding and MUST NOT be included in the key derivation function, such as EDHOC_Extract(), where only the cryptographic key material (e.g., cnf) is used.

Encoding and processing guidelines The following guidelines apply to the encoding and handling of CRED_PSK and ID_CRED_PSK.

• If CRED_PSK is CBOR-encoded, it SHOULD use deterministic encoding as specified in Sections and of . This ensures consistent identification and avoids interoperability issues due to non-deterministic CBOR variants.
• If CRED_PSK is provisioned out-of-band and transported by value, it SHOULD be used as-is without re-encoding. Re-encoding might cause mismatches when comparing identifiers such as hash values or 'kid' references.
• ID_CRED_PSK SHOULD uniquely identify the correspoding CRED_PSK to avoid ambiguity. In cases where ID_CRED_PSK is a reference to a key identifier, care must be taken to ensure that 'kid' is globally unique for the PSK.
• When ID_CRED_PSK consists solely of a 'kid' parameter (i.e., { 4 : kid }), the compact encoding optimization defined in MUST be applied in plaintext fields (such as PLAINTEXT_3A). For example:
  • { 4 : h'0f' } encoded as h'0f' (CBOR byte string)
  • { 4 : 21 } encoded as 0x15 (CBOR integer)

These optimizations MUST NOT be applied in COSE header parameters or other contexts where full map structure is required.

• To mitigate misbinding attacks, identity information such as a 'sub' (subject) claim MUST be included in CRED_PSK. If no such identity is present, the authentication credential binds only to the key.
• Guidelines in related to certificate chains, X.509 DER encoding, or public key validation do not apply to CRED_PSK, since it encapsulates a symmetric key and is not used for explicit signature or certificate validation.

Message Flow of EDHOC-PSK The ID_CRED_PSK is sent in message_3, encrypted using a key derived from the ephemeral shared secret, G_XY, calculated from the Initiator's ephemeral public key G_X or from the encapsulation G_Y and the private key corresponding to G_X. When ECDHE is used, the encapsulation G_Y is the Responder's ephemeral public key. The Responder authenticates the Initiator first. shows the message flow of the EDHOC-PSK authentication method.

Overview of Message Flow of EDHOC-PSK. | | message_1 | | | | G_Y, Enc( C_R, EAD_2 ) | |<------------------------------------------------------------------+ | message_2 | | | | Enc( ID_CRED_PSK, AEAD( EAD_3 ) ) | +------------------------------------------------------------------>| | message_3 | | | | AEAD( EAD_4 ) | |<------------------------------------------------------------------+ | message_4 | ]]>

This approach provides identity protection against passive attackers for both Initiator and Responder. message_4 remains optional, but is needed to authenticate the Responder and achieve mutual authentication in EDHOC if not relying on external applications, such as OSCORE. With this fourth message, the protocol achieves both explicit key confirmation and mutual authentication.

Key Derivation The pseudorandom keys (PRKs) used for the PSK authentication method in EDHOC are derived using EDHOC_Extract, as done in . EDHOC_Extract allows to derive fixed-length uniformly pseudorandom keys (PRKs) from ECDH shared secrets. where salt and input keying material (IKM) are defined for each key. The definition of EDHOC_Extract depends on the EDHOC hash algorithm selected in the cipher suite, see . To maintain a uniform key schedule across the different EDHOC authentication methods, the same pseudorandom key notation (PRK_2e, PRK_3e2m, and PRK_4e3m) is retained. In the case of EDHOC with PSK-based authentication, no MACs are used. Consequently, while the naming convention suggests involvement in both encryption and MAC operations, only the parts related to encryption (2e, 3e, and 4e) are relevant. The index notation does not fully reflect the functional role of the keys in this method but is preserved for consistency with other EDHOC authentication variants The transcript hash TH_2 = H( G_Y, H(message_1) ) is defined as in , the others are modified as specified below. lists the key derivations that differ from those specified in . The following data is used as in :

• PRK_2e is extracted with
  • salt = TH_2, and
  • IKM = G_XY.
• SALT_4e3m is derived from PRK_3e2m and TH_3, see Figure 6 of .

Key Derivation of EDHOC-PSK.

where:

• KEYSTREAM_3A is used to encrypt the PLAINTEXT_3A, a concatenation of ID_CRED_PSK and CIPHERTEXT_3B, in message_3.
• KEYSTREAM_2A is used to encrypt PLAINTEXT_2A in message_2.
• TH_3 = H( TH_2, PLAINTEXT_2A ).
• plaintext_length_2a is the length of PLAINTEXT_2A in message_2.
• plaintext_length_3a is the length of PLAINTEXT_3A in message_3.

Additionally, the definition of the transcript hash TH_4 is modified as:

• TH_4 = H( TH_3, ID_CRED_PSK, PLAINTEXT_3B, CRED_PSK )

Message Formatting and Processing This section specifies the differences in message formatting and processing compared to .

Message 1 Message 1 is formatted and processed as specified in .

Message 2

Formatting of Message 2 Message 2 is formatted as specified in .

Responder Composition of Message 2 CIPHERTEXT_2A is calculated with a binary additive stream cipher, using a keystream generated with EDHOC_Expand, and the following plaintext:

• PLAINTEXT_2A = ( C_R, ? EAD_2 )
• CIPHERTEXT_2A = PLAINTEXT_2A XOR KEYSTREAM_2A

Contrary to , ID_CRED_R, MAC_2, and Signature_or_MAC_2 are not used. C_R, EAD_2 are defined in . KESYTREAM_2A is defined in .

Initiator Processing of Message 2 Upon receiving message_2, the Initiator processes it as follows:

• It computes KEYSTREAM_2A, following , where plaintext_length_2a is the length of PLAINTEXT_2A.
• It decrypts CIPHERTEXT_2A using binary XOR, i.e., PLAINTEXT_2A = CIPHERTEXT_2A XOR KEYSTREAM_2A

Compared to , ID_CRED_R is not made available to the application in step 4, and steps 5 and 6 are skipped

Message 3

Formatting of Message 3 Message 3 is formatted as specified in .

Initiator Composition of Message 3

• CIPHERTEXT_3A is calculated with a binary additive stream cipher, using a keystream generated with EDHOC_Expand, and the following plaintext:
  • PLAINTEXT_3A = ( ID_CRED_PSK / bstr / -24..23, CIPHERTEXT_3B )
    • If ID_CRED_PSK contains a single 'kid' parameter, i.e., ID_CRED_PSK = { 4 : kid_PSK }, then the compact encoding is applied, see .
    • If the length of PLAINTEXT_3A exceeds the output of EDHOC_KDF, then applies.
  • Compute KEYSTREAM_3A as in , where plaintext_length_3a is the length of PLAINTEXT_3A.
  • CIPHERTEXT_3A = PLAINTEXT_3A XOR KEYSTREAM_3A
• CIPHERTEXT_3B is the 'ciphertext' of COSE_Encrypt0 object as defined in Section and Section of , with the EDHOC AEAD algorithm of the selected cipher suite, using the encryption key K_3, the initialization vector IV_3 (if used by the AEAD algorithm), the parameters described in , plaintext PLAINTEXT_3B and the following parameters as input:
  • protected = h''
  • external_aad = << ID_CRED_PSK, TH_3, CRED_PSK >>
  • K_3 and IV_3 as defined in
  • PLAINTEXT_3B = ( ? EAD_3 )

The Initiator computes TH_4 = H( TH_3, ID_CRED_PSK, PLAINTEXT_3B, CRED_PSK ), defined in . There is no need for MAC_3 or signature, since AEAD's built-in integrity and the use of PSK-based key derivation provides implicit authentication of the Initiator.

Responder Processing of Message 3 Upon receiving message_3, the Responder performs the following steps:

• Derive the decryption key K_3 and IV_3 as defined in .
• Parse the structure of message_3, which consists of a stream-cipher encrypted structure, CIPHERTEXT_3A = PLAINTEXT_3A XOR KEYSTREAM_3A, where PLAINTEXT_3A = ( ID_CRED_PSK, CIPHERTEXT_3B ) and CIPHERTEXT_3B is the inner AEAD-encrypted object.
• Generate KEYSTREAM_3A with the same method the initiator used.
• Decrypt CIPHERTEXT_3A using binary XOR with KEYSTREAM_3A, resulting in PLAINTEXT_3A = ( ID_CRED_PSK, CIPHERTEXT_3B ).
• Validate or match ID_CRED_PSK to identify which PSK to use. If the ID is unrecognized, the Responder aborts.
• AEAD-decrypt CIPHERTEXT_3B using:
  • K_3, IV_3
  • external_aad = << ID_CRED_PSK, TH_3, CRED_PSK >>
  • protected = h''
  • AEAD algorithm from cipher suite

If verification fails, this indicates either the Initiator does not know the correct PSK, or the message was tampered with. Contrary, if verification succeeds, the Responder conlcudes that the Initiator knows the correct PSK, has correctly derived the transcript hash TH_3 and is actively participating in the protocol. Lastly, the Responder computes TH_4 as defined in No MAC_3 or signature is needed, as the AEAD tag guarantees both integrity and authenticity in this symmetric setting.

Message 4 Message 4 is formatted and processed as specified in . Compared to , a fourth message does not only provide key confirmation but also Responder authentication. To authenticate the Responder and achieve mutual authentication, a fourth message is mandatory. After verifying message_4, the Initiator is assured that the Responder has calculated the key PRK_out (key confirmation) and that no other party can derive the key. The Initiator MUST NOT persistently store PRK_out or application keys until the Initiator has verified message_4 or a message protected with a derived application key, such as an OSCORE message, from the Responder and the application has authenticated the Responder.

PSK usage for Session Resumption This section defines how PSKs are used for session resumption in EDHOC. Following , EDHOC_Exporter can be used to derive both rPSK and rID_CRED_PSK: where:

• resumption_psk_length is by default the key_length (length of the encryption key of the EDHOC AEAD algorithm of the selected cipher suite) of the session in which the EDHOC_Exporter is called.
• id_cred_psk_length is by default 2.

A peer that has successfully completed an EDHOC session, regardless of the used authentication method, MUST generate a resumption key to use for the next resumption in the present "session series", as long as it supports PSK resumption. To guarantee that both peers share the same resumption key, when a session is run using rPSK_i as a resumption key:

• The Initiator can delete rPSK_i after having successfully verified EDHOC message_4. In this case, the Responder will have derived the next rPSK_(i+1), which the Initiator can know for sure, upon receiving EDHOC message_4.
• The Responder can delete rPSK_(i-1), if any, after having successfully sent EDHOC message_4. That is, upon receiving EDHOC message_3, the Responder knows for sure that the other peer did derive rPSK_i at the end of the previous session in the "session series", thus making it safe to delete the previous resumption key rPSK_(i-1).

Cipher Suite Requirements for Resumption When using a resumption PSK derived from a previous EDHOC exchange:

1. The resumption PSK MUST only be used with the same cipher suite that was used in the original EDHOC exchange, or with a cipher suite that provides equal or higher security guarantees.
2. Implementations SHOULD maintain a mapping between the resumption PSK and its originating cipher suite to enforce this requirement.
3. If a resumption PSK is offered with a cipher suite different from the one used in the original EDHOC session, the recipient can fail the present EDHOC session according to application-specific policies.

Privacy Considerations for Resumption When using resumption PSKs:

• The same ID_CRED_PSK is reused each time EDHOC is executed with a specific resumption PSK.
• To prevent long-term tracking, implementations SHOULD periodically initiate a full EDHOC exchange to generate a new resumption PSK and corresponding ID_CRED_PSK. Alternatively, as stated in , EDHOC_KeyUpdate can be used to derive a new PRK_out, and consequently a new CRED_PSK and ID_CRED_PSK for session resumption.

Security Considerations for Resumption

• Resumption PSKs MUST NOT be used for purposes other than EDHOC session resumption.
• Resumption PSKs MUST be securely stored with the same level of protection as the original session keys.
• Parties SHOULD implement mechanisms to detect and prevent excessive reuse of the same resumption PSK.

EDHOC PSK and OSCORE When PSK authenitcation is used and the Initiator is able to derive PRK_out before sending message_3, then the optimization described in applies. In this scenario, the Initiator MAY concatenate EDHOC message_3 and the first OSCORE request in a single CoAP message.

Security Considerations The EDHOC-PSK authentication method introduces changes with respect to the current specification of EDHOC . This section analyzes the security implications of these changes.

Identity protection EDHOC-PSK encrypts ID_CRED_PSK in message 3 with a keystream derived from the ephemeral shared secret G_XY. As a consequence, contrary to the current EDHOC methods that protect the Initiator’s identity against active attackers and the Responder’s identity against passive attackers (See ), EDHOC-PSK provides identity protection for both the Initiator and the Responder against passive attackers. In symmetric key setups, using the same CRED_PSK for both parties or omitting role identity (e.g., sub) makes the protocol vulnerable to reflection or Selfie attacks. Separate identities in sub serve as non-cryptographic role binders and MUST be distinct.

Mutual Authentication EDHOC-PSK provides mutual authentication, assuming the PSK remains secret. However, if the optional fourth message (message_4) is omitted, mutual authentication is not guaranteed—unless the Responder is later authenticated through an OSCORE message or other application data demonstrating possession of the PSK. When message_4 is included, the protocol ensures both mutual authentication and explicit key confirmation.

External Authorization Data Protection Similarly to , EDHOC-PSK provides external authorization data protection. The integrity and confidentiality of EAD fields follow the same security guarantees as in the original EDHOC specification.

Post Quantum Considerations Recent advancements in quantum computing suggest that the development of a Cryptographically Relevant Quantum Computer (CRQC) is likely feasible long-term. If realized, such a machine would render many currently deployed asymmetric cryptographic algorithms—such as Elliptic Curve Diffie-Hellman (ECDH)—insecure. By leveraging a symmetric PSK for both authentication and key derivation, EDHOC-PSK provides quantum-resistant key exchange and authentication, even when used with ECDHE. However, if a cryptographically relevant quantum computer (CRQC) is realized, the ECDHE component would be broken and contribute only randomness. Consequently, EDHOC-PSK with ECDHE does not offer identity protection or Perfect Forward Secrecy (PFS) against quantum-capable adversaries. If the PSK is compromised, a passive quantum attacker could decrypt both past and future sessions. In contrast, EDHOC-PSK combined with a quantum-resistant Key Encapsulation Mechanism (KEM), such as ML-KEM, provides identity protection and PFS even in the presence of a quantum attacker.

Independence of Session Keys NIST mandates that an ephemeral private key shall be used in exactly one key-establishment transaction (see Section 5.6.3.3 of ). This requirement is essential for preserving session key independence and ensuring forward secrecy. The EDHOC-PSK protocol complies with this NIST requirement. In other protocols, the reuse of ephemeral keys, particularly when combined with implementation flaws such as the absence of public key validation, has resulted in critical security vulnerabilities. Such weaknesses have allowed attackers to recover the so called “ephemeral” private key from a compromised session, thereby enabling them to compromise the security of both past and future sessions between legitimate parties. Assuming breach and minimizing the impact of compromise are fundamental zero-trust principles.

Unified Approach and Recommendations For use cases involving the transmission of application data, application data can be sent concurrently with message_3, maintaining the protocol's efficiency. In applications such as EAP-EDHOC, where application data is not sent, message_4 is mandatory. Thus, the EDHOC-PSK authentication method does not include any extra messages. Other implementations may continue using OSCORE in place of EDHOC message_4, with a required change in the protocol's language to: The Initiator SHALL NOT persistently store PRK_out or application keys until the Initiator has verified message_4 or a message protected with a derived application key, such as an OSCORE message. This change ensures that key materials are only stored once their integrity and authenticity are confirmed, thereby enhancing privacy by preventing early storage of potentially compromised keys. Lastly, whether the Initiator or Responder authenticates first is not relevant when using symmetric keys. This consideration was important for the privacy properties when using asymmetric authentication but is not significant in the context of symmetric key usage.

IANA Considerations This document has IANA actions.

EDHOC Method Type Registry IANA is requested to register the following entry in the "EDHOC Method Type" registry under the group name "Ephemeral Diffie-Hellman Over OCSE (EDHOC)". Addition to the EDHOC Method Type Registry.

Value	Initiator Authentication Key	Responder Authentication Key
4	PSK	PSK

EDHOC Exporter Label Registry IANA is requested to register the following entry in the "EDHOC Exporter Label" registry under the group name "Ephemeral Diffie-Hellman Over OCSE (EDHOC)". Additions to the EDHOC Exporter Label Registry.

Label	Description	Change Controller	Reference
2	Resumption CRED_PSK	IETF	Section 7
3	Resumption ID_CRED_PSK	IETF	Section 7

Normative References 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. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. 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. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. 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, including 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. 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. 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.

CDDL Definitions This section compiles the CDDL definitions for easy reference, incorporating errata filed against .

Test Vectors

message_1 Both endpoints are authenticated with Pre-Shared Keys (METHOD = 4) The initiator selects cipher suite 0. A single cipher suite is encoded as an int: The Initiator creates an ephemeral key pair for use with the EDHOC key exchange algorithm: The Initiator selects its connection identifier C_I to be the byte string 0xA, which is encoded as 0xA since it is represented by the 1-byte CBOR int 10: No external authorization data The Initiator constructs message_1:

message_2 The Responder supports the most preferred and selected cipher suite 0, so SUITES_I is acceptable. The Responder creates an ephemeral key pair for use with the EDHOC key exchange algorithm: The Responder selects its connection identifier C_R to be the byte string 0x5, which is encoded as 0x5 since it is represented by the 1-byte CBOR int -10: The transcript hash TH_2 is calculated using the EDHOC hash algorithm: TH_2 = H( G_Y, H(message_1) ), where H(message_1) is: PRK_2e is specified in . To compute it, the Elliptic Curve Diffie-Hellman (ECDH) shared secret G_XY is needed. It is computed from G_X and Y or G_Y and X: Then, PRK_2e is calculated as defined in Since the Responder authenticates using PSK, PRK_3e2m = PRK_2e. No external authorization data: The Responder constructs PLAINTEXT_2A: The Responder computes KEYSTREAM_2 as defined in The Responder calculates CIPHERTEXT_2B as XOR between PLAINTEXT_2A and KEYSTREAM_2: The Responder constructs message_2 as defined in :

message_3 The Initiator computes PRK_4e3m, as described in Section 4, using SALT_4e3m and CRED_PSK: The transcript hash TH_3 is calculated using the EDHOC hash algorithm: TH_3 = H( TH_2, PLAINTEXT_2A ) No external authorization data: The Initiator constructs firstly PLAINTEXT_3B as defined in Section 5.3.1.: It then computes CIPHERTEXT_3B as defined in Section 5.3.2. It uses ID_CRED_PSK, CRED_PSK and TH_3 as external_aad: The initiator computes K_3 and IV_3 It then computes CIPHERTEXT_3B: The Initiator computes KEYSTREAM_3 as defined in Section 4: It then calculates PLAINTEXT_3A as stated in Section 5.3.2.: It then uses KEYSTREAM_3 to derive CIPHERTEXT_3A: The Initiator computes message_3 as defined in Section 5.3.2.: The transcript hash TH_4 is calculated using the EDHOC hash algorithm: TH_4 = H( TH_3, ID_CRED_PSK, ? EAD_3, CRED_PSK )

message_4 No external authorization data: The Responder constructs PLAINTEXT_4: The Responder computes K_4 and IV_4 The Responder computes message_4:

Change Log RFC Editor: Please remove this appendix.

• From -03 to -04
  • Test Vectors
  • Editorial changes
• From -02 to -03
  • Updated abstract and Introduction
  • Changed message_3 to hide the identity length from passive attackers
  • CDDL Definitions
  • Security considerations of independence of Session Keys
  • Editorial changes
• From -01 to -02
  • Changes to message_3 formatting and processing
• From -00 to -01
  • Editorial changes and corrections

Acknowledgments The authors want to thank , , , , and for reviewing and commenting on intermediate versions of the draft. This work has been partly funded by PID2023-148104OB-C43 funded by MICIU/AEI/10.13039/501100011033 (ONOFRE4), FEDER/UE EU HE CASTOR under Grant Agreement No 101167904 and EU CERTIFY under Grant Agreement No 101069471. This work was supported partially by Vinnova - the Swedish Agency for Innovation Systems - through the EUREKA CELTIC-NEXT project CYPRESS.