Ephemeral Diffie-Hellman Over COSE (EDHOC)

1.1. Motivation

Many Internet of Things (IoT) deployments require technologies which are highly performant in constrained environments [RFC7228]. IoT devices may be constrained in various ways, including memory, storage, processing capacity, and power. The connectivity for these settings may also exhibit constraints such as unreliable and lossy channels, highly restricted bandwidth, and dynamic topology. The IETF has acknowledged this problem by standardizing a range of lightweight protocols and enablers designed for the IoT, including the Constrained Application Protocol (CoAP, [RFC7252]), Concise Binary Object Representation (CBOR, [RFC8949]), and Static Context Header Compression (SCHC, [RFC8724]).¶

The need for special protocols targeting constrained IoT deployments extends also to the security domain [I-D.ietf-lake-reqs]. Important characteristics in constrained environments are the number of round trips and protocol message sizes, which if kept low can contribute to good performance by enabling transport over a small number of radio frames, reducing latency due to fragmentation or duty cycles, etc. Another important criteria is code size, which may be prohibitive for certain deployments due to device capabilities or network load during firmware update. Some IoT deployments also need to support a variety of underlying transport technologies, potentially even with a single connection.¶

Some security solutions for such settings exist already. CBOR Object Signing and Encryption (COSE, [I-D.ietf-cose-rfc8152bis-struct]) specifies basic application-layer security services efficiently encoded in CBOR. Another example is Object Security for Constrained RESTful Environments (OSCORE, [RFC8613]) which is a lightweight communication security extension to CoAP using CBOR and COSE. In order to establish good quality cryptographic keys for security protocols such as COSE and OSCORE, the two endpoints may run an authenticated Diffie-Hellman key exchange protocol, from which shared secret key material can be derived. Such a key exchange protocol should also be lightweight; to prevent bad performance in case of repeated use, e.g., due to device rebooting or frequent rekeying for security reasons; or to avoid latencies in a network formation setting with many devices authenticating at the same time.¶

This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a lightweight authenticated key exchange protocol providing good security properties including forward secrecy, identity protection, and cipher suite negotiation. Authentication can be based on raw public keys (RPK) or public key certificates and requires the application to provide input on how to verify that endpoints are trusted. This specification focuses on referencing instead of transporting credentials to reduce message overhead. EDHOC does currently not support pre-shared key (PSK) authentication as authentication with static Diffie-Hellman public keys by reference produces equally small message sizes but with much simpler key distribution and identity protection.¶

EDHOC makes use of known protocol constructions, such as SIGMA [SIGMA] and Extract-and-Expand [RFC5869]. EDHOC uses COSE for cryptography and identification of credentials (including COSE_Key, CWT, CCS, X.509, C509, see Section 3.5.3). COSE provides crypto agility and enables the use of future algorithms and credentials targeting IoT.¶

1.2. Use of EDHOC

EDHOC is designed for highly constrained settings making it especially suitable for low-power wide area networks [RFC8376] such as Cellular IoT, 6TiSCH, and LoRaWAN. A main objective for EDHOC is to be a lightweight authenticated key exchange for OSCORE, i.e., to provide authentication and session key establishment for IoT use cases such as those built on CoAP [RFC7252]. CoAP is a specialized web transfer protocol for use with constrained nodes and networks, providing a request/response interaction model between application endpoints. As such, EDHOC is targeting a large variety of use cases involving 'things' with embedded microcontrollers, sensors, and actuators.¶

A typical setting is when one of the endpoints is constrained or in a constrained network, and the other endpoint is a node on the Internet (such as a mobile phone) or at the edge of the constrained network (such as a gateway). Thing-to-thing interactions over constrained networks are also relevant since both endpoints would then benefit from the lightweight properties of the protocol. EDHOC could e.g., be run when a device connects for the first time, or to establish fresh keys which are not revealed by a later compromise of the long-term keys. Further security properties are described in Section 8.1.¶

EDHOC enables the reuse of the same lightweight primitives as OSCORE: CBOR for encoding, COSE for cryptography, and CoAP for transport. By reusing existing libraries, the additional code size can be kept very low. Note that, while CBOR and COSE primitives are built into the protocol messages, EDHOC is not bound to a particular transport. Transfer of EDHOC messages in CoAP payloads is detailed in Appendix A.3.¶

1.3. Message Size Examples

Compared to the DTLS 1.3 handshake [I-D.ietf-tls-dtls13] with ECDHE and connection ID, the number of bytes in EDHOC + CoAP can be less than 1/6 when RPK authentication is used, see [I-D.ietf-lwig-security-protocol-comparison]. Figure 1 shows examples of message sizes for EDHOC with different kinds of authentication keys and different COSE header parameters for identification: static Diffie-Hellman keys or signature keys, either in CBOR Web Token (CWT) / CWT Claims Set (CCS) [RFC8392] identified by a key identifier using 'kid' [I-D.ietf-cose-rfc8152bis-struct], or in X.509 certificates identified by a hash value using 'x5t' [I-D.ietf-cose-x509].¶

========================================================
                    Static DH Keys        Signature Keys
                    --------------        --------------
                    kid        x5t        kid        x5t
--------------------------------------------------------
message_1            37         37         37         37
message_2            45         58        102        115
message_3            19         33         77         90
--------------------------------------------------------
Total               101        128        216        242
========================================================

1.4. Document Structure

The remainder of the document is organized as follows: Section 2 outlines EDHOC authenticated with digital signatures, Section 3 describes the protocol elements of EDHOC, including formatting of the ephemeral public keys, Section 4 specifies the key derivation, Section 5 specifies message processing for EDHOC authenticated with signature keys or static Diffie-Hellman keys, Section 6 describes the error messages, and Appendix A shows how to transfer EDHOC with CoAP and establish an OSCORE security context.¶

1.5. Terminology and Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

Readers are expected to be familiar with the terms and concepts described in CBOR [RFC8949], CBOR Sequences [RFC8742], COSE structures and processing [I-D.ietf-cose-rfc8152bis-struct], COSE algorithms [I-D.ietf-cose-rfc8152bis-algs], CWT and CWT Claims Set [RFC8392], and CDDL [RFC8610]. The Concise Data Definition Language (CDDL) is used to express CBOR data structures [RFC8949]. Examples of CBOR and CDDL are provided in Appendix C.1. When referring to CBOR, this specification always refers to Deterministically Encoded CBOR as specified in Sections 4.2.1 and 4.2.2 of [RFC8949]. The single output from authenticated encryption (including the authentication tag) is called "ciphertext", following [RFC5116].¶

3.1. General

The EDHOC protocol consists of three mandatory messages (message_1, message_2, message_3) between Initiator and Responder, an optional fourth message (message_4), and an error message. All EDHOC messages are CBOR Sequences [RFC8742]. Figure 3 illustrates an EDHOC message flow with the optional fourth message as well as the content of each message. The protocol elements in the figure are introduced in Section 3 and Section 5. Message formatting and processing is specified in Section 5 and Section 6.¶

Application data may be protected using the agreed application algorithms (AEAD, hash) in the selected cipher suite (see Section 3.6) and the application can make use of the established connection identifiers C_I and C_R (see Section 3.3). EDHOC may be used with the media type application/edhoc defined in Section 9.¶

The Initiator can derive symmetric application keys after creating EDHOC message_3, see Section 4.3. Protected application data can therefore be sent in parallel or together with EDHOC message_3. EDHOC message_4 is typically not sent.¶

Initiator                                                   Responder
|                 METHOD, SUITES_I, G_X, C_I, EAD_1                 |
+------------------------------------------------------------------>|
|                             message_1                             |
|                                                                   |
|       G_Y, Enc( ID_CRED_R, Signature_or_MAC_2, EAD_2 ), C_R       |
|<------------------------------------------------------------------+
|                             message_2                             |
|                                                                   |
|            AEAD( ID_CRED_I, Signature_or_MAC_3, EAD_3 )           |
+------------------------------------------------------------------>|
|                             message_3                             |
|                                                                   |
|                           AEAD( EAD_4 )                           |
|<- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
|                             message_4                             |

3.2. Method

The data item METHOD in message_1 (see Section 5.2.1), is an integer specifying the authentication method. EDHOC supports authentication with signature or static Diffie-Hellman keys, as defined in the four authentication methods: 0, 1, 2, and 3, see Figure 4. When using a static Diffie-Hellman key the authentication is provided by a Message Authentication Code (MAC) computed from an ephemeral-static ECDH shared secret which enables significant reductions in message sizes.¶

The Initiator and the Responder need to have agreed on a single method to be used for EDHOC, see Section 3.9.¶

+-------+-------------------+-------------------+-------------------+
| Value | Initiator         | Responder         | Reference         |
+-------+-------------------+-------------------+-------------------+
|     0 | Signature Key     | Signature Key     | [[this document]] |
|     1 | Signature Key     | Static DH Key     | [[this document]] |
|     2 | Static DH Key     | Signature Key     | [[this document]] |
|     3 | Static DH Key     | Static DH Key     | [[this document]] |
+-------+-------------------+-------------------+-------------------+

3.3. Connection Identifiers

EDHOC includes the selection of connection identifiers (C_I, C_R) identifying a connection for which keys are agreed.¶

Connection identifiers may be used to correlate EDHOC messages and facilitate the retrieval of protocol state during EDHOC protocol execution (see Section 3.4) or in a subsequent application protocol, e.g., OSCORE (see Section 3.3.2). The connection identifiers do not have any cryptographic purpose in EDHOC.¶

Connection identifiers in EDHOC are byte strings or integers, encoded in CBOR. One byte connection identifiers (the integers -24 to 23 and the empty CBOR byte string h'') are realistic in many scenarios as most constrained devices only have a few connections.¶

3.4. Transport

Cryptographically, EDHOC does not put requirements on the lower layers. EDHOC is not bound to a particular transport layer and can even be used in environments without IP. The transport is responsible, where necessary, to handle:¶

• message loss,¶
• message reordering,¶
• message duplication,¶
• fragmentation,¶
• demultiplex EDHOC messages from other types of messages,¶
• denial-of-service protection,¶
• message correlation.¶

The Initiator and the Responder need to have agreed on a transport to be used for EDHOC, see Section 3.9.¶

3.5. Authentication Parameters

EDHOC supports various settings for how the other endpoint's authentication (public) key is transported, identified, and trusted as described in this section.¶

The authentication key (see Section 3.5.2) is used in several parts of EDHOC:¶

1. as part of the authentication credential included in the integrity calculation¶
2. for verification of the Signature_or_MAC field in message_2 and message_3 (see Section 5.3.2 and Section 5.4.2)¶
3. in the key derivation (in case of a static Diffie-Hellman key, see Section 4).¶

The authentication credential (CRED_x) contains, in addition to the authentication key, also the authentication key algorithm and optionally other parameters such as identity, key usage, expiry, issuer, etc. (see Section 3.5.3). Identical authentication credentials need to be established in both endpoints to be able to verify integrity. For many settings it is not necessary to transport the authentication credential within EDHOC over constrained links, for example, it may be pre-provisioned or acquired out-of-band over less constrained links.¶

EDHOC relies on COSE for identification of authentication credentials (using ID_CRED_x, see Section 3.5.4) and supports all credential types for which COSE header parameters are defined (see Section 3.5.3).¶

The choice of authentication credential depends also on the trust model (see Section 3.5.1). For example, a certificate or CWT may rely on a trusted third party, whereas a CCS or a self-signed certificate/CWT may be used when trust in the public key can be achieved by other means, or in the case of trust-on-first-use.¶

The type of authentication key, authentication credential, and the way to identify the credential have a large impact on the message size. For example, the signature_or_MAC field is much smaller with a static DH key than with a signature key. A CCS is much smaller than a self-signed certificate/CWT, but if it is possible to reference the credential with a COSE header like 'kid', then that is typically much smaller than to transport a CCS.¶

{                                              /CCS/
  2 : "42-50-31-FF-EF-37-32-39",               /sub/
  8 : {                                        /cnf/
    1 : {                                      /COSE_Key/
      1 : 1,                                   /kty/
      2 : 0,                                   /kid/
     -1 : 4,                                   /crv/
     -2 : h'b1a3e89460e88d3a8d54211dc95f0b90   /x/
            3ff205eb71912d6db8f4af980d2db83a'
    }
  }
}

3.6. Cipher Suites

An EDHOC cipher suite consists of an ordered set of algorithms from the "COSE Algorithms" and "COSE Elliptic Curves" registries as well as the EDHOC MAC length. Algorithms need to be specified with enough parameters to make them completely determined. EDHOC is only specified for use with key exchange algorithms of type ECDH curves. Use with other types of key exchange algorithms would likely require a specification updating EDHOC. Note that for most signature algorithms, the signature is determined by the signature algorithm and the authentication key algorithm together, see Section 3.5.2. The authentication key algorithm needs to be compatible with the EDHOC key exchange algorithm when static Diffie-Hellman authentication is used and compatible with the EDHOC signature algorithm when signature authentication is used.¶

• EDHOC AEAD algorithm¶
• EDHOC hash algorithm¶
• EDHOC MAC length in bytes (Static DH)¶
• EDHOC key exchange algorithm (ECDH curve)¶
• EDHOC signature algorithm¶
• Application AEAD algorithm¶
• Application hash algorithm¶

Each cipher suite is identified with a pre-defined int label.¶

EDHOC can be used with all algorithms and curves defined for COSE. Implementation can either use one of the pre-defined cipher suites (Section 9.2) or use any combination of COSE algorithms and parameters to define their own private cipher suite. Private cipher suites can be identified with any of the four values -24, -23, -22, -21.¶

The following CCM cipher suites are for constrained IoT where message overhead is a very important factor. Cipher suites 1 and 3 use a larger tag length (128-bit) in EDHOC than in the Application AEAD algorithm (64-bit):¶

   0. ( 10, -16, 8, 4, -8, 10, -16 )
      (AES-CCM-16-64-128, SHA-256, 8, X25519, EdDSA,
       AES-CCM-16-64-128, SHA-256)

   1. ( 30, -16, 16, 4, -8, 10, -16 )
      (AES-CCM-16-128-128, SHA-256, 16, X25519, EdDSA,
       AES-CCM-16-64-128, SHA-256)

   2. ( 10, -16, 8, 1, -7, 10, -16 )
      (AES-CCM-16-64-128, SHA-256, 8, P-256, ES256,
       AES-CCM-16-64-128, SHA-256)

   3. ( 30, -16, 16, 1, -7, 10, -16 )
      (AES-CCM-16-128-128, SHA-256, 16, P-256, ES256,
       AES-CCM-16-64-128, SHA-256)

The following ChaCha20 cipher suites are for less constrained applications and only use 128-bit tag lengths.¶

   4. ( 24, -16, 16, 4, -8, 24, -16 )
      (ChaCha20/Poly1305, SHA-256, 16, X25519, EdDSA,
       ChaCha20/Poly1305, SHA-256)

   5. ( 24, -16, 16, 1, -7, 24, -16 )
      (ChaCha20/Poly1305, SHA-256, 16, P-256, ES256,
       ChaCha20/Poly1305, SHA-256)

The following GCM cipher suite is for general non-constrained applications. It uses high performance algorithms that are widely supported:¶

   6. ( 1, -16, 16, 4, -7, 1, -16 )
      (A128GCM, SHA-256, 16, X25519, ES256,
       A128GCM, SHA-256)

The following two cipher suites are for high security application such as government use and financial applications. The two cipher suites do not share any algorithms. The first of the two cipher suites is compatible with the CNSA suite [CNSA].¶

  24. ( 3, -43, 16, 2, -35, 3, -43 )
      (A256GCM, SHA-384, 16, P-384, ES384,
       A256GCM, SHA-384)

  25. ( 24, -45, 16, 5, -8, 24, -45 )
      (ChaCha20/Poly1305, SHAKE256, 16, X448, EdDSA,
       ChaCha20/Poly1305, SHAKE256)

The different methods use the same cipher suites, but some algorithms are not used in some methods. The EDHOC signature algorithm is not used in methods without signature authentication.¶

The Initiator needs to have a list of cipher suites it supports in order of preference. The Responder needs to have a list of cipher suites it supports. SUITES_I contains cipher suites supported by the Initiator, formatted and processed as detailed in Section 5.2.1 to secure the cipher suite negotiation. Examples of cipher suite negotiation are given in Section 6.3.2.¶

3.7. Ephemeral Public Keys

EDHOC always uses compact representation of elliptic curve points, see Appendix B. In COSE compact representation is achieved by formatting the ECDH ephemeral public keys as COSE_Keys of type EC2 or OKP according to Sections 7.1 and 7.2 of [I-D.ietf-cose-rfc8152bis-algs], but only including the 'x' parameter in G_X and G_Y. For Elliptic Curve Keys of type EC2, compact representation MAY be used also in the COSE_Key. If the COSE implementation requires an 'y' parameter, the value y = false SHALL be used. COSE always use compact output for Elliptic Curve Keys of type EC2.¶

3.8. External Authorization Data (EAD)

In order to reduce round trips and number of messages or to simplify processing, external security applications may be integrated into EDHOC by transporting authorization related data in the messages. One example is third-party identity and authorization information protected out of scope of EDHOC [I-D.selander-ace-ake-authz]. Another example is a certificate enrolment request or the resulting issued certificate.¶

EDHOC allows opaque external authorization data (EAD) to be sent in the EDHOC messages. External authorization data sent in message_1 (EAD_1) or message_2 (EAD_2) should be considered unprotected by EDHOC, see Section 8.4. External authorization data sent in message_3 (EAD_3) or message_4 (EAD_4) is protected between Initiator and Responder.¶

External authorization data is a CBOR sequence (see Appendix C.1) consisting of one or more (ead_label, ead_value) pairs as defined below:¶

ead = 1* (
  ead_label : int,
  ead_value : any,
)

Applications using external authorization data need to specify format, processing, and security considerations and register the (ead_label, ead_value) pair, see Section 9.5. The CDDL type of ead_value is determined by the int ead_label and MUST be specified.¶

The EAD fields of EDHOC are not intended for generic application data. Since data carried in EAD_1 and EAD_2 fields may not be protected, special considerations need to be made such that it does not violate security and privacy requirements of the service which uses this data. Moreover, the content in an EAD field may impact the security properties provided by EDHOC. Security applications making use of the EAD fields must perform the necessary security analysis.¶

3.9. Applicability Statement

EDHOC requires certain parameters to be agreed upon between Initiator and Responder. Some parameters can be agreed through the protocol execution (specifically cipher suite negotiation, see Section 3.6) but other parameters may need to be known out-of-band (e.g., which authentication method is used, see Section 3.2).¶

The purpose of the applicability statement is to describe the intended use of EDHOC to allow for the relevant processing and verifications to be made, including things like:¶

1. How the endpoint detects that an EDHOC message is received. This includes how EDHOC messages are transported, for example in the payload of a CoAP message with a certain Uri-Path or Content-Format; see Appendix A.3.¶

   • The method of transporting EDHOC messages may also describe data carried along with the messages that are needed for the transport to satisfy the requirements of Section 3.4, e.g., connection identifiers used with certain messages, see Appendix A.3.¶
2. Authentication method (METHOD; see Section 3.2).¶
3. Profile for authentication credentials (CRED_I, CRED_R; see Section 3.5.3), e.g., profile for certificate or CCS, including supported authentication key algorithms (subject public key algorithm in X.509 or C509 certificate).¶
4. Type used to identify authentication credentials (ID_CRED_I, ID_CRED_R; see Section 3.5.4).¶
5. Use and type of external authorization data (EAD_1, EAD_2, EAD_3, EAD_4; see Section 3.8).¶
6. Identifier used as identity of endpoint; see Section 3.5.1.¶
7. If message_4 shall be sent/expected, and if not, how to ensure a protected application message is sent from the Responder to the Initiator; see Section 5.5.¶

The applicability statement may also contain information about supported cipher suites. The procedure for selecting and verifying cipher suite is still performed as described in Section 5.2.1 and Section 6.3, but it may become simplified by this knowledge.¶

An example of an applicability statement is shown in Appendix D.¶

For some parameters, like METHOD, ID_CRED_x, type of EAD, the receiver is able to verify compliance with applicability statement, and if it needs to fail because of incompliance, to infer the reason why the protocol failed.¶

For other parameters, like CRED_x in the case that it is not transported, it may not be possible to verify that incompliance with applicability statement was the reason for failure: Integrity verification in message_2 or message_3 may fail not only because of wrong authentication credential. For example, in case the Initiator uses public key certificate by reference (i.e., not transported within the protocol) then both endpoints need to use an identical data structure as CRED_I or else the integrity verification will fail.¶

Note that it is not necessary for the endpoints to specify a single transport for the EDHOC messages. For example, a mix of CoAP and HTTP may be used along the path, and this may still allow correlation between messages.¶

The applicability statement may be dependent on the identity of the other endpoint, or other information carried in an EDHOC message, but it then applies only to the later phases of the protocol when such information is known. (The Initiator does not know identity of Responder before having verified message_2, and the Responder does not know identity of the Initiator before having verified message_3.)¶

Other conditions may be part of the applicability statement, such as target application or use (if there is more than one application/use) to the extent that EDHOC can distinguish between them. In case multiple applicability statements are used, the receiver needs to be able to determine which is applicable for a given session, for example based on URI or external authorization data type.¶

4.1. Extract

The pseudorandom keys (PRKs) are derived using Extract.¶

   PRK = Extract( salt, IKM )

where the input keying material (IKM) and salt are defined for each PRK below.¶

The definition of Extract depends on the EDHOC hash algorithm of the selected cipher suite:¶

• if the EDHOC hash algorithm is SHA-2, then Extract( salt, IKM ) = HKDF-Extract( salt, IKM ) [RFC5869]¶
• if the EDHOC hash algorithm is SHAKE128, then Extract( salt, IKM ) = KMAC128( salt, IKM, 256, "" )¶
• if the EDHOC hash algorithm is SHAKE256, then Extract( salt, IKM ) = KMAC256( salt, IKM, 512, "" )¶

   G_XY = X25519( Y, G_X ) = X25519( X, G_Y )

   PRK_2e = HMAC-SHA-256( salt, G_XY )

4.2. Expand

The keys, IVs and MACs used in EDHOC are derived from the PRKs using Expand, and instantiated with the EDHOC AEAD algorithm in the selected cipher suite.¶

   OKM = EDHOC-KDF( PRK, transcript_hash, label, context, length )
       = Expand( PRK, info, length )

where info is encoded as the CBOR sequence¶

info = (
  transcript_hash : bstr,
  label : tstr,
  context : bstr,
  length : uint,
)

where¶

• transcript_hash is a bstr set to one of the transcript hashes TH_2, TH_3, or TH_4 as defined in Sections 5.3.1, 5.4.1, and 4.3.¶
• label is a tstr set to the name of the derived key, IV or MAC; i.e., "KEYSTREAM_2", "MAC_2", "K_3", "IV_3", or "MAC_3".¶
• context is a bstr¶
• length is the length of output keying material (OKM) in bytes¶

The definition of Expand depends on the EDHOC hash algorithm of the selected cipher suite:¶

• if the EDHOC hash algorithm is SHA-2, then Expand( PRK, info, length ) = HKDF-Expand( PRK, info, length ) [RFC5869]¶
• if the EDHOC hash algorithm is SHAKE128, then Expand( PRK, info, length ) = KMAC128( PRK, info, L, "" )¶
• if the EDHOC hash algorithm is SHAKE256, then Expand( PRK, info, length ) = KMAC256( PRK, info, L, "" )¶

where L = 8*length, the output length in bits.¶

The keys, IVs and MACs are derived as follows:¶

• KEYSTREAM_2 is derived using the transcript hash TH_2 and the pseudorandom key PRK_2e.¶
• MAC_2 is derived using the transcript hash TH_2 and the pseudorandom key PRK_3e2m.¶
• K_3 and IV_3 are derived using the transcript hash TH_3 and the pseudorandom key PRK_3e2m. IVs are only used if the EDHOC AEAD algorithm uses IVs.¶
• MAC_3 is derived using the transcript hash TH_3 and the pseudorandom key PRK_4x3m.¶

KEYSTREAM_2, K_3, and IV_3 use an empty CBOR byte string h'' as context. MAC_2 and MAC_3 use context as defined in Section 5.3.2 and Section 5.4.2, respectively.¶

4.3. EDHOC-Exporter

Application keys and other application specific data can be derived using the EDHOC-Exporter interface defined as:¶

   EDHOC-Exporter(label, context, length)
     = EDHOC-KDF(PRK_4x3m, TH_4, label, context, length)

where label is a registered tstr from the EDHOC Exporter Label registry (Section 9.1), context is a bstr defined by the application, and length is a uint defined by the application. The (label, context) pair must be unique, i.e., a (label, context) MUST NOT be used for two different purposes. However an application can re-derive the same key several times as long as it is done in a secure way. For example, in most encryption algorithms the same key kan be reused with different nonces. The context can for example be the empty (zero-length) sequence or a single CBOR byte string.¶

The transcript hash TH_4 is a CBOR encoded bstr and the input to the hash function is a CBOR Sequence.¶

   TH_4 = H( TH_3, CIPHERTEXT_3 )

where H() is the hash function in the selected cipher suite. Examples of use of the EDHOC-Exporter are given in Section 5.5.2 and Appendix A.¶

• K_4 and IV_4 are derived with the EDHOC-Exporter using the empty CBOR byte string h'' as context, and labels "EDHOC_K_4" and "EDHOC_IV_4", respectively. IVs are only used if the EDHOC AEAD algorithm uses IVs.¶

4.4. EDHOC-KeyUpdate

To provide forward secrecy in an even more efficient way than re-running EDHOC, EDHOC provides the function EDHOC-KeyUpdate. When EDHOC-KeyUpdate is called the old PRK_4x3m is deleted and the new PRK_4x3m is calculated as a "hash" of the old key using the Extract function as illustrated by the following pseudocode:¶

   EDHOC-KeyUpdate( nonce ):
      PRK_4x3m = Extract( nonce, PRK_4x3m )

The EDHOC-KeyUpdate takes a nonce as input to guarantee that there are no short cycles. The Initiator and the Responder need to agree on the nonce, which can e.g., be a counter or a random number. While the KeyUpdate method provides forward secrecy it does not give as strong security properties as re-running EDHOC, see Section 8.¶

5.1. Message Processing Outline

This section outlines the message processing of EDHOC.¶

For each session, the endpoints are assumed to keep an associated protocol state containing identifiers, keys, etc. used for subsequent processing of protocol related data. The protocol state is assumed to be associated to an applicability statement (Section 3.9) which provides the context for how messages are transported, identified, and processed.¶

EDHOC messages SHALL be processed according to the current protocol state. The following steps are expected to be performed at reception of an EDHOC message:¶

1. Detect that an EDHOC message has been received, for example by means of port number, URI, or media type (Section 3.9).¶
2. Retrieve the protocol state according to the message correlation provided by the transport, see Section 3.4. If there is no protocol state, in the case of message_1, a new protocol state is created. The Responder endpoint needs to make use of available Denial-of-Service mitigation (Section 8.5).¶
3. If the message received is an error message, then process according to Section 6, else process as the expected next message according to the protocol state.¶

If the processing fails for some reason then, typically, an error message is sent, the protocol is discontinued, and the protocol state erased. Further details are provided in the following subsections and in Section 6.¶

Different instances of the same message MUST NOT be processed in one session. Note that processing will fail if the same message appears a second time for EDHOC processing because the state of the protocol has moved on and now expects something else. This assumes that message duplication due to re-transmissions is handled by the transport protocol, see Section 3.4. The case when the transport does not support message deduplication is addressed in Appendix E.¶

5.2. EDHOC Message 1

message_1 = (
  METHOD : int,
  SUITES_I : suites,
  G_X : bstr,
  C_I : bstr / int,
  ? EAD_1 : ead,
)

suites = [ 2* int ] / int

5.3. EDHOC Message 2

message_2 = (
  G_Y_CIPHERTEXT_2 : bstr,
  C_R : bstr / int,
)

5.4. EDHOC Message 3

message_3 = (
  CIPHERTEXT_3 : bstr,
)

5.5. EDHOC Message 4

This section specifies message_4 which is OPTIONAL to support. Key confirmation is normally provided by sending an application message from the Responder to the Initiator protected with a key derived with the EDHOC-Exporter, e.g., using OSCORE (see Appendix A). In deployments where no protected application message is sent from the Responder to the Initiator, the Responder MUST send message_4. Two examples of such deployments:¶

1. When EDHOC is only used for authentication and no application data is sent.¶
2. When application data is only sent from the Initiator to the Responder.¶

Further considerations are provided in Section 3.9.¶

message_4 = (
  CIPHERTEXT_4 : bstr,
)

6.1. Success

Error code 0 MAY be used internally in an application to indicate success, e.g., in log files. ERR_INFO can contain any type of CBOR item. Error code 0 MUST NOT be used as part of the EDHOC message exchange flow.¶

6.2. Unspecified

Error code 1 is used for errors that do not have a specific error code defined. ERR_INFO MUST be a text string containing a human-readable diagnostic message written in English. The diagnostic text message is mainly intended for software engineers that during debugging need to interpret it in the context of the EDHOC specification. The diagnostic message SHOULD be provided to the calling application where it SHOULD be logged.¶

6.3. Wrong Selected Cipher Suite

Error code 2 MUST only be used in a response to message_1 in case the cipher suite selected by the Initiator is not supported by the Responder, or if the Responder supports a cipher suite more preferred by the Initiator than the selected cipher suite, see Section 5.2.3. ERR_INFO is in this case denoted SUITES_R and is of type suites, see Section 5.2.1. If the Responder does not support the selected cipher suite, then SUITES_R MUST include one or more supported cipher suites. If the Responder supports a cipher suite in SUITES_I other than the selected cipher suite (independently of if the selected cipher suite is supported or not) then SUITES_R MUST include the supported cipher suite in SUITES_I which is most preferred by the Initiator. SUITES_R MAY include a single cipher suite, i.e., be encoded as an int. If the Responder does not support any cipher suite in SUITES_I, then it SHOULD include all its supported cipher suites in SUITES_R in any order.¶

Initiator                                                   Responder
|              METHOD, SUITES_I = 5, G_X, C_I, EAD_1                |
+------------------------------------------------------------------>|
|                             message_1                             |
|                                                                   |
|                   ERR_CODE = 2, SUITES_R = 6                      |
|<------------------------------------------------------------------+
|                               error                               |
|                                                                   |
|             METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1            |
+------------------------------------------------------------------>|
|                             message_1                             |

Initiator                                                   Responder
|            METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1             |
+------------------------------------------------------------------>|
|                             message_1                             |
|                                                                   |
|                  ERR_CODE = 2, SUITES_R = [9, 8]                  |
|<------------------------------------------------------------------+
|                               error                               |
|                                                                   |
|           METHOD, SUITES_I = [5, 6, 7, 8], G_X, C_I, EAD_1        |
+------------------------------------------------------------------>|
|                             message_1                             |

8.1. Security Properties

EDHOC inherits its security properties from the theoretical SIGMA-I protocol [SIGMA]. Using the terminology from [SIGMA], EDHOC provides forward secrecy, mutual authentication with aliveness, consistency, and peer awareness. As described in [SIGMA], peer awareness is provided to the Responder, but not to the Initiator.¶

EDHOC protects the credential identifier of the Initiator against active attacks and the credential identifier of the Responder against passive attacks. The roles should be assigned to protect the most sensitive identity/identifier, typically that which is not possible to infer from routing information in the lower layers.¶

Compared to [SIGMA], EDHOC adds an explicit method type and expands the message authentication coverage to additional elements such as algorithms, external authorization data, and previous messages. This protects against an attacker replaying messages or injecting messages from another session.¶

EDHOC also adds selection of connection identifiers and downgrade protected negotiation of cryptographic parameters, i.e., an attacker cannot affect the negotiated parameters. A single session of EDHOC does not include negotiation of cipher suites, but it enables the Responder to verify that the selected cipher suite is the most preferred cipher suite by the Initiator which is supported by both the Initiator and the Responder.¶

As required by [RFC7258], IETF protocols need to mitigate pervasive monitoring when possible. EDHOC therefore only supports methods with ephemeral Diffie-Hellman and provides a KeyUpdate function for lightweight application protocol rekeying with forward secrecy, in the sense that compromise of the private authentication keys does not compromise past session keys, and compromise of a session key does not compromise past session keys.¶

While the KeyUpdate method can be used to meet cryptographic limits and provide partial protection against key leakage, it provides significantly weaker security properties than re-running EDHOC with ephemeral Diffie-Hellman. Even with frequent use of KeyUpdate, compromise of one session key compromises all future session keys, and an attacker therefore only needs to perform static key exfiltration [RFC7624]. Frequently re-running EDHOC with ephemeral Diffie-Hellman forces attackers to perform dynamic key exfiltration instead of static key exfiltration [RFC7624]. In the dynamic case, the attacker must have continuous interactions with the collaborator, which is more complicated and has a higher risk profile than the static case.¶

To limit the effect of breaches, it is important to limit the use of symmetrical group keys for bootstrapping. EDHOC therefore strives to make the additional cost of using raw public keys and self-signed certificates as small as possible. Raw public keys and self-signed certificates are not a replacement for a public key infrastructure but SHOULD be used instead of symmetrical group keys for bootstrapping.¶

Compromise of the long-term keys (private signature or static DH keys) does not compromise the security of completed EDHOC exchanges. Compromising the private authentication keys of one party lets an active attacker impersonate that compromised party in EDHOC exchanges with other parties but does not let the attacker impersonate other parties in EDHOC exchanges with the compromised party. Compromise of the long-term keys does not enable a passive attacker to compromise future session keys. Compromise of the HDKF input parameters (ECDH shared secret) leads to compromise of all session keys derived from that compromised shared secret. Compromise of one session key does not compromise other session keys. Compromise of PRK_4x3m leads to compromise of all exported keying material derived after the last invocation of the EDHOC-KeyUpdate function.¶

EDHOC provides a minimum of 64-bit security against online brute force attacks and a minimum of 128-bit security against offline brute force attacks. This is in line with IPsec, TLS, and COSE. To break 64-bit security against online brute force an attacker would on average have to send 4.3 billion messages per second for 68 years, which is infeasible in constrained IoT radio technologies.¶

After sending message_3, the Initiator is assured that no other party than the Responder can compute the key PRK_4x3m (implicit key authentication). The Initiator does however not know that the Responder has actually computed the key PRK_4x3m. While the Initiator can securely send protected application data, the Initiator SHOULD NOT permanently store the keying material PRK_4x3m and TH_4 until the Initiator is assured that the Responder has actually computed the key PRK_4x3m (explicit key confirmation). Explicit key confirmation is e.g., assured when the Initiator has verified an OSCORE message or message_4 from the Responder. After verifying message_3, the Responder is assured that the Initiator has calculated the key PRK_4x3m (explicit key confirmation) and that no other party than the Responder can compute the key. The Responder can securely send protected application data and store the keying material PRK_4x3m and TH_4.¶

Key compromise impersonation (KCI): In EDHOC authenticated with signature keys, EDHOC provides KCI protection against an attacker having access to the long-term key or the ephemeral secret key. With static Diffie-Hellman key authentication, KCI protection would be provided against an attacker having access to the long-term Diffie-Hellman key, but not to an attacker having access to the ephemeral secret key. Note that the term KCI has typically been used for compromise of long-term keys, and that an attacker with access to the ephemeral secret key can only attack that specific session.¶

Repudiation: In EDHOC authenticated with signature keys, the Initiator could theoretically prove that the Responder performed a run of the protocol by presenting the private ephemeral key, and vice versa. Note that storing the private ephemeral keys violates the protocol requirements. With static Diffie-Hellman key authentication, both parties can always deny having participated in the protocol.¶

Two earlier versions of EDHOC have been formally analyzed [Norrman20] [Bruni18] and the specification has been updated based on the analysis.¶

8.2. Cryptographic Considerations

The SIGMA protocol requires that the encryption of message_3 provides confidentiality against active attackers and EDHOC message_4 relies on the use of authenticated encryption. Hence the message authenticating functionality of the authenticated encryption in EDHOC is critical: authenticated encryption MUST NOT be replaced by plain encryption only, even if authentication is provided at another level or through a different mechanism.¶

To reduce message overhead EDHOC does not use explicit nonces and instead rely on the ephemeral public keys to provide randomness to each session. A good amount of randomness is important for the key generation, to provide liveness, and to protect against interleaving attacks. For this reason, the ephemeral keys MUST NOT be used in more than one EDHOC message, and both parties SHALL generate fresh random ephemeral key pairs. Note that an ephemeral key may be used to calculate several ECDH shared secrets. When static Diffie-Hellman authentication is used the same ephemeral key is used in both ephemeral-ephemeral and ephemeral-static ECDH.¶

As discussed in [SIGMA], the encryption of message_2 does only need to protect against passive attacker as active attackers can always get the Responders identity by sending their own message_1. EDHOC uses the Expand function (typically HKDF-Expand) as a binary additive stream cipher. HKDF-Expand provides better confidentiality than AES-CTR but is not often used as it is slow on long messages, and most applications require both IND-CCA confidentiality as well as integrity protection. For the encryption of message_2, any speed difference is negligible, IND-CCA does not increase security, and integrity is provided by the inner MAC (and signature depending on method).¶

Requirement for how to securely generate, validate, and process the ephemeral public keys depend on the elliptic curve. For X25519 and X448, the requirements are defined in [RFC7748]. For secp256r1, secp384r1, and secp521r1, the requirements are defined in Section 5 of [SP-800-56A]. For secp256r1, secp384r1, and secp521r1, at least partial public-key validation MUST be done.¶

8.3. Cipher Suites and Cryptographic Algorithms

When using private cipher suite or registering new cipher suites, the choice of key length used in the different algorithms needs to be harmonized, so that a sufficient security level is maintained for certificates, EDHOC, and the protection of application data. The Initiator and the Responder should enforce a minimum security level.¶

The hash algorithms SHA-1 and SHA-256/64 (SHA-256 truncated to 64-bits) SHALL NOT be supported for use in EDHOC except for certificate identification with x5t and c5t. Note that secp256k1 is only defined for use with ECDSA and not for ECDH. Note that some COSE algorithms are marked as not recommended in the COSE IANA registry.¶

8.4. Unprotected Data

The Initiator and the Responder must make sure that unprotected data and metadata do not reveal any sensitive information. This also applies for encrypted data sent to an unauthenticated party. In particular, it applies to EAD_1, ID_CRED_R, EAD_2, and error messages. Using the same EAD_1 in several EDHOC sessions allows passive eavesdroppers to correlate the different sessions. Another consideration is that the list of supported cipher suites may potentially be used to identify the application.¶

The Initiator and the Responder must also make sure that unauthenticated data does not trigger any harmful actions. In particular, this applies to EAD_1 and error messages.¶

8.5. Denial-of-Service

EDHOC itself does not provide countermeasures against Denial-of-Service attacks. By sending a number of new or replayed message_1 an attacker may cause the Responder to allocate state, perform cryptographic operations, and amplify messages. To mitigate such attacks, an implementation SHOULD rely on lower layer mechanisms such as the Echo option in CoAP [I-D.ietf-core-echo-request-tag] that forces the initiator to demonstrate reachability at its apparent network address.¶

An attacker can also send faked message_2, message_3, message_4, or error in an attempt to trick the receiving party to send an error message and discontinue the session. EDHOC implementations MAY evaluate if a received message is likely to have been forged by an attacker and ignore it without sending an error message or discontinuing the session.¶

8.6. Implementation Considerations

The availability of a secure random number generator is essential for the security of EDHOC. If no true random number generator is available, a truly random seed MUST be provided from an external source and used with a cryptographically secure pseudorandom number generator. As each pseudorandom number must only be used once, an implementation needs to get a new truly random seed after reboot, or continuously store state in nonvolatile memory, see ([RFC8613], Appendix B.1.1) for issues and solution approaches for writing to nonvolatile memory. Intentionally or unintentionally weak or predictable pseudorandom number generators can be abused or exploited for malicious purposes. [RFC8937] describes a way for security protocol implementations to augment their (pseudo)random number generators using a long-term private key and a deterministic signature function. This improves randomness from broken or otherwise subverted random number generators. The same idea can be used with other secrets and functions such as a Diffie-Hellman function or a symmetric secret and a PRF like HMAC or KMAC. It is RECOMMENDED to not trust a single source of randomness and to not put unaugmented random numbers on the wire.¶

If ECDSA is supported, "deterministic ECDSA" as specified in [RFC6979] MAY be used. Pure deterministic elliptic-curve signatures such as deterministic ECDSA and EdDSA have gained popularity over randomized ECDSA as their security do not depend on a source of high-quality randomness. Recent research has however found that implementations of these signature algorithms may be vulnerable to certain side-channel and fault injection attacks due to their determinism. See e.g., Section 1 of [I-D.mattsson-cfrg-det-sigs-with-noise] for a list of attack papers. As suggested in Section 6.1.2 of [I-D.ietf-cose-rfc8152bis-algs] this can be addressed by combining randomness and determinism.¶

All private keys, symmetric keys, and IVs MUST be secret. Implementations should provide countermeasures to side-channel attacks such as timing attacks. Intermediate computed values such as ephemeral ECDH keys and ECDH shared secrets MUST be deleted after key derivation is completed.¶

The Initiator and the Responder are responsible for verifying the integrity of certificates. The selection of trusted CAs should be done very carefully and certificate revocation should be supported. The private authentication keys MUST be kept secret, only the Responder SHALL have access to the Responder's private authentication key and only the Initiator SHALL have access to the Initiator's private authentication key.¶

The Initiator and the Responder are allowed to select the connection identifiers C_I and C_R, respectively, for the other party to use in the ongoing EDHOC protocol as well as in a subsequent application protocol (e.g., OSCORE [RFC8613]). The choice of connection identifier is not security critical in EDHOC but intended to simplify the retrieval of the right security context in combination with using short identifiers. If the wrong connection identifier of the other party is used in a protocol message it will result in the receiving party not being able to retrieve a security context (which will terminate the protocol) or retrieve the wrong security context (which also terminates the protocol as the message cannot be verified).¶

If two nodes unintentionally initiate two simultaneous EDHOC message exchanges with each other even if they only want to complete a single EDHOC message exchange, they MAY terminate the exchange with the lexicographically smallest G_X. If the two G_X values are equal, the received message_1 MUST be discarded to mitigate reflection attacks. Note that in the case of two simultaneous EDHOC exchanges where the nodes only complete one and where the nodes have different preferred cipher suites, an attacker can affect which of the two nodes' preferred cipher suites will be used by blocking the other exchange.¶

If supported by the device, it is RECOMMENDED that at least the long-term private keys are stored in a Trusted Execution Environment (TEE) and that sensitive operations using these keys are performed inside the TEE. To achieve even higher security it is RECOMMENDED that additional operations such as ephemeral key generation, all computations of shared secrets, and storage of the PRK keys can be done inside the TEE. The use of a TEE enforces that code within that environment cannot be tampered with, and that any data used by such code cannot be read or tampered with by code outside that environment.¶

9.1. EDHOC Exporter Label Registry

IANA has created a new registry titled "EDHOC Exporter Label" under the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The registration procedure is "Expert Review". The columns of the registry are Label, Description, and Reference. All columns are text strings where Label consists only of the printable ASCII characters 0x21 - 0x7e. Labels beginning with "PRIVATE" MAY be used for private use without registration. All other label values MUST be registered. The initial contents of the registry are:¶

Label: EDHOC_K_4
Description: Key used to protect EDHOC message_4
Reference: [[this document]]

Label: EDHOC_IV_4
Description: IV used to protect EDHOC message_4
Reference: [[this document]]

Label: OSCORE_Master_Secret
Description: Derived OSCORE Master Secret
Reference: [[this document]]

Label: OSCORE_Master_Salt
Description: Derived OSCORE Master Salt
Reference: [[this document]]

9.2. EDHOC Cipher Suites Registry

IANA has created a new registry titled "EDHOC Cipher Suites" under the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The registration procedure is "Expert Review". The columns of the registry are Value, Array, Description, and Reference, where Value is an integer and the other columns are text strings. The initial contents of the registry are:¶

Value: -24
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]

Value: -23
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]

Value: -22
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]

Value: -21
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]

Value: 0
Array: 10, -16, 8, 4, -8, 10, -16
Desc: AES-CCM-16-64-128, SHA-256, 8, X25519, EdDSA,
      AES-CCM-16-64-128, SHA-256
Reference: [[this document]]

Value: 1
Array: 30, -16, 16, 4, -8, 10, -16
Desc: AES-CCM-16-128-128, SHA-256, 16, X25519, EdDSA,
      AES-CCM-16-64-128, SHA-256
Reference: [[this document]]

Value: 2
Array: 10, -16, 8, 1, -7, 10, -16
Desc: AES-CCM-16-64-128, SHA-256, 8, P-256, ES256,
      AES-CCM-16-64-128, SHA-256
Reference: [[this document]]

Value: 3
Array: 30, -16, 16, 1, -7, 10, -16
Desc: AES-CCM-16-128-128, SHA-256, 16, P-256, ES256,
      AES-CCM-16-64-128, SHA-256
Reference: [[this document]]

Value: 4
Array: 24, -16, 16, 4, -8, 24, -16
Desc: ChaCha20/Poly1305, SHA-256, 16, X25519, EdDSA,
      ChaCha20/Poly1305, SHA-256
Reference: [[this document]]

Value: 5
Array: 24, -16, 16, 1, -7, 24, -16
Desc: ChaCha20/Poly1305, SHA-256, 16, P-256, ES256,
      ChaCha20/Poly1305, SHA-256
Reference: [[this document]]

Value: 6
Array: 1, -16, 16, 4, -7, 1, -16
Desc: A128GCM, SHA-256, 16, X25519, ES256,
      A128GCM, SHA-256
Reference: [[this document]]

Value: 24
Array: 3, -43, 16, 2, -35, 3, -43
Desc: A256GCM, SHA-384, 16, P-384, ES384,
      A256GCM, SHA-384
Reference: [[this document]]

Value: 25
Array: 24, -45, 16, 5, -8, 24, -45
Desc: ChaCha20/Poly1305, SHAKE256, 16, X448, EdDSA,
      ChaCha20/Poly1305, SHAKE256
Reference: [[this document]]

9.3. EDHOC Method Type Registry

IANA has created a new registry entitled "EDHOC Method Type" under the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The registration procedure is "Expert Review". The columns of the registry are Value, Description, and Reference, where Value is an integer and the other columns are text strings. The initial contents of the registry are shown in Figure 4.¶

9.4. EDHOC Error Codes Registry

IANA has created a new registry entitled "EDHOC Error Codes" under the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The registration procedure is "Expert Review". The columns of the registry are ERR_CODE, ERR_INFO Type and Description, where ERR_CODE is an integer, ERR_INFO is a CDDL defined type, and Description is a text string. The initial contents of the registry are shown in Figure 7.¶

9.5. EDHOC External Authorization Data Registry

IANA has created a new registry entitled "EDHOC External Authorization Data" under the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The registration procedure is "Expert Review". The columns of the registry are Label, Description, Value Type, and Reference, where Label is an integer and the other columns are text strings.¶

9.6. COSE Header Parameters Registry

IANA has registered the following entries in the "COSE Header Parameters" registry under the group name "CBOR Object Signing and Encryption (COSE)". The value of the 'kcwt' header parameter is a COSE Web Token (CWT) [RFC8392], and the value of the 'kccs' header parameter is an CWT Claims Set (CCS), see Section 1.5. The CWT/CCS must contain a COSE_Key in a 'cnf' claim [RFC8747]. The Value Registry for this item is empty and omitted from the table below.¶

+-----------+-------+----------------+---------------------------+
| Name      | Label | Value Type     | Description               |
+===========+=======+================+===========================+
| kcwt      | TBD1  | COSE_Messages  | A CBOR Web Token (CWT)    |
|           |       |                | containing a COSE_Key in  |
|           |       |                | a 'cnf' claim             |
+-----------+-------+----------------+---------------------------+
| kccs      | TBD2  | map / #6(map)  | A CWT Claims Set (CCS)    |
|           |       |                | containing a COSE_Key in  |
|           |       |                | a 'cnf' claim             |
+-----------+-------+----------------+---------------------------+

9.7. COSE Header Parameters Registry

IANA has extended the Value Type of 'kid' in the "COSE Header Parameters" registry under the group name "CBOR Object Signing and Encryption (COSE)" to also allow the Value Type int. The resulting Value Type is bstr / int. The Value Registry for this item is empty and omitted from the table below.¶

+------+-------+------------+----------------+-------------------+
| Name | Label | Value Type | Description    | Reference         |
+------+-------+------------+----------------+-------------------+
| kid  |   4   | bstr / int | Key identifier | [RFC9052]         |
|      |       |            |                | [[This document]] |
+------+-------+------------+----------------+-------------------+

9.8. COSE Key Common Parameters Registry

IANA has extended the Value Type of 'kid' in the "COSE Key Common Parameters" registry under the group name "CBOR Object Signing and Encryption (COSE)" to also allow the Value Type int. The resulting Value Type is bstr / int. The Value Registry for this item is empty and omitted from the table below.¶

+------+-------+------------+----------------+-------------------+
| Name | Label | Value Type | Description    | Reference         |
+------+-------+------------+----------------+-------------------+
| kid  |   2   | bstr / int | Key identifi-  | [RFC9052]         |
|      |       |            | cation value - | [[This document]] |
|      |       |            | match to kid   |                   |
|      |       |            | in message     |                   |
+------+-------+------------+----------------+-------------------+

9.9. CWT Confirmation Methods Registry

IANA has extended the Value Type of 'kid' in the "CWT Confirmation Methods" registry under the group name "CBOR Web Token (CWT) Claims" to also allow the Value Type int. The incorrect term binary string has been corrected to bstr. The resulting Value Type is bstr / int. The new updated content for the 'kid' method is shown in the list below.¶

• Confirmation Method Name: kid¶
• Confirmation Method Description: Key Identifier¶
• JWT Confirmation Method Name: kid¶
• Confirmation Key: 3¶
• Confirmation Value Type(s): bstr / int¶
• Change Controller: IESG¶
• Specification Document(s): Section 3.4 of RFC 8747 [[This document]]¶

9.10. The Well-Known URI Registry

IANA has added the well-known URI "edhoc" to the "Well-Known URIs" registry under the group name "Well-Known URIs".¶

• URI suffix: edhoc¶
• Change controller: IETF¶
• Specification document(s): [[this document]]¶
• Related information: None¶

9.11. Media Types Registry

IANA has added the media type "application/edhoc" to the "Media Types" registry.¶

• Type name: application¶
• Subtype name: edhoc¶
• Required parameters: N/A¶
• Optional parameters: N/A¶
• Encoding considerations: binary¶
• Security considerations: See Section 7 of this document.¶
• Interoperability considerations: N/A¶
• Published specification: [[this document]] (this document)¶
• Applications that use this media type: To be identified¶
• Fragment identifier considerations: N/A¶

• Additional information:¶

  • Magic number(s): N/A¶
  • File extension(s): N/A¶
  • Macintosh file type code(s): N/A¶
• Person & email address to contact for further information: See "Authors' Addresses" section.¶
• Intended usage: COMMON¶
• Restrictions on usage: N/A¶
• Author: See "Authors' Addresses" section.¶
• Change Controller: IESG¶

9.12. CoAP Content-Formats Registry

IANA has added the media type "application/edhoc" to the "CoAP Content-Formats" registry under the group name "Constrained RESTful Environments (CoRE) Parameters".¶

• Media Type: application/edhoc¶
• Encoding:¶
• ID: TBD42¶
• Reference: [[this document]]¶

9.13. Resource Type (rt=) Link Target Attribute Values Registry

IANA has added the resource type "core.edhoc" to the "Resource Type (rt=) Link Target Attribute Values" registry under the group name "Constrained RESTful Environments (CoRE) Parameters".¶

• Value: "core.edhoc"¶
• Description: EDHOC resource.¶
• Reference: [[this document]]¶

Client applications can use this resource type to discover a server's resource for EDHOC, where to send a request for executing the EDHOC protocol.¶

9.14. Expert Review Instructions

The IANA Registries established in this document is defined as "Expert Review". This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason so they should be given substantial latitude.¶

Expert reviewers should take into consideration the following points:¶

• Clarity and correctness of registrations. Experts are expected to check the clarity of purpose and use of the requested entries. Expert needs to make sure the values of algorithms are taken from the right registry, when that is required. Expert should consider requesting an opinion on the correctness of registered parameters from relevant IETF working groups. Encodings that do not meet these objective of clarity and completeness should not be registered.¶
• Experts should take into account the expected usage of fields when approving point assignment. The length of the encoded value should be weighed against how many code points of that length are left, the size of device it will be used on, and the number of code points left that encode to that size.¶
• Specifications are recommended. When specifications are not provided, the description provided needs to have sufficient information to verify the points above.¶

A.1. Selecting EDHOC Connection Identifier

This section specifies a rule for converting from EDHOC connection identifier to OSCORE Sender/Recipient ID. (An identifier is Sender ID or Recipient ID depending on from which endpoint is the point of view, see Section 3.1 of [RFC8613].)¶

• If the EDHOC connection identifier is numeric, i.e., encoded as a CBOR integer on the wire, it is converted to a (naturally byte-string shaped) OSCORE Sender/Recipient ID equal to its CBOR encoded form.¶

For example, a numeric C_R equal to 10 (0x0A in CBOR encoding) is converted to a (typically client) Sender ID equal to 0x0A, while a numeric C_I equal to -12 (0x2B in CBOR encoding) is converted to a (typically client) Sender ID equal to 0x2B.¶

• If the EDHOC connection identifier is byte-valued, hence encoded as a CBOR byte string on the wire, it is converted to an OSCORE Sender/Recipient ID equal to the byte string.¶

For example, a byte-string valued C_R equal to 0xFF (0x41FF in CBOR encoding) is converted to a (typically client) Sender ID equal to 0xFF.¶

Two EDHOC connection identifiers are called "equivalent" if and only if, by applying the conversion above, they both result in the same OSCORE Sender/Recipient ID. For example, the two EDHOC connection identifiers with CBOR encoding 0x0A (numeric) and 0x410A (byte-valued) are equivalent since they both result in the same OSCORE Sender/Recipient ID 0x0A.¶

When EDHOC is used to establish an OSCORE security context, the connection identifiers C_I and C_R MUST NOT be equivalent. Furthermore, in case of multiple OSCORE security contexts with potentially different endpoints, to facilitate retrieval of the correct OSCORE security context, an endpoint SHOULD select an EDHOC connection identifier that when converted to OSCORE Recipient ID does not coincide with its other Recipient IDs.¶

A.2. Deriving the OSCORE Security Context

This section specifies how to use EDHOC output to derive the OSCORE security context.¶

After successful processing of EDHOC message_3, Client and Server derive Security Context parameters for OSCORE as follows (see Section 3.2 of [RFC8613]):¶

• The Master Secret and Master Salt are derived by using the EDHOC-Exporter interface, see Section 4.3.¶

The EDHOC Exporter Labels for deriving the OSCORE Master Secret and the OSCORE Master Salt, are "OSCORE_Master_Secret" and "OSCORE_Master_Salt", respectively.¶

The context parameter is h'' (0x40), the empty CBOR byte string.¶

By default, key_length is the key length (in bytes) of the application AEAD Algorithm of the selected cipher suite for the EDHOC session. Also by default, salt_length has value 8. The Initiator and Responder MAY agree out-of-band on a longer key_length than the default and on a different salt_length.¶

Master Secret = EDHOC-Exporter("OSCORE_Master_Secret", h'', key_length)
Master Salt   = EDHOC-Exporter("OSCORE_Master_Salt", h'', salt_length)

• The AEAD Algorithm is the application AEAD algorithm of the selected cipher suite for the EDHOC session.¶
• The HKDF Algorithm is the one based on the application hash algorithm of the selected cipher suite for the EDHOC session. For example, if SHA-256 is the application hash algorithm of the selected cipher suite, HKDF SHA-256 is used as HKDF Algorithm in the OSCORE Security Context.¶
• In case the Client is Initiator and the Server is Responder, the Client's OSCORE Sender ID and the Server's OSCORE Sender ID are determined from the EDHOC connection identifiers C_R and C_I for the EDHOC session, respectively, by applying the conversion in Appendix A.1. The reverse applies in case the Client is the Responder and the Server is the Initiator.¶

Client and Server use the parameters above to establish an OSCORE Security Context, as per Section 3.2.1 of [RFC8613].¶

From then on, Client and Server retrieve the OSCORE protocol state using the Recipient ID, and optionally other transport information such as the 5-tuple.¶

A.3. Transferring EDHOC over CoAP

This section specifies one instance for how EDHOC can be transferred as an exchange of CoAP [RFC7252] messages. CoAP provides a reliable transport that can preserve packet ordering and handle message duplication. CoAP can also perform fragmentation and protect against denial-of-service attacks. The underlying CoAP transport should be used in reliable mode, in particular when fragmentation is used, to avoid, e.g., situations with hanging endpoints waiting for each other.¶

By default, the CoAP client is the Initiator and the CoAP server is the Responder, but the roles SHOULD be chosen to protect the most sensitive identity, see Section 8. According to this specification, EDHOC is transferred in POST requests and 2.04 (Changed) responses to the Uri-Path: "/.well-known/edhoc". An application may define its own path that can be discovered, e.g., using resource directory [I-D.ietf-core-resource-directory].¶

By default, the message flow is as follows: EDHOC message_1 is sent in the payload of a POST request from the client to the server's resource for EDHOC. EDHOC message_2 or the EDHOC error message is sent from the server to the client in the payload of a 2.04 (Changed) response. EDHOC message_3 or the EDHOC error message is sent from the client to the server's resource in the payload of a POST request. If needed, an EDHOC error message is sent from the server to the client in the payload of a 2.04 (Changed) response. Alternatively, if EDHOC message_4 is used, it is sent from the server to the client in the payload of a 2.04 (Changed) response analogously to message_2.¶

In order to correlate a message received from a client to a message previously sent by the server, messages sent by the client are prepended with the CBOR serialization of the connection identifier which the server has chosen. This applies independently of if the CoAP server is Responder or Initiator. For the default case when the server is Responder, the prepended connection identifier is C_R, and C_I if the server is Initiator. If message_1 is sent to the server, the CBOR simple value "true" (0xf5) is sent in its place (given that the server has not selected C_R yet).¶

These identifiers are encoded in CBOR and thus self-delimiting. They are sent in front of the actual EDHOC message, and only the part of the body following the identifier is used for EDHOC processing.¶

Consequently, the application/edhoc media type does not apply to these messages; their media type is unnamed.¶

An example of a successful EDHOC exchange using CoAP is shown in Figure 10. In this case the CoAP Token enables correlation on the Initiator side, and the prepended C_R enables correlation on the Responder (server) side.¶

Client    Server
  |          |
  +--------->| Header: POST (Code=0.02)
  |   POST   | Uri-Path: "/.well-known/edhoc"
  |          | Payload: true, EDHOC message_1
  |          |
  |<---------+ Header: 2.04 Changed
  |   2.04   | Content-Format: application/edhoc
  |          | Payload: EDHOC message_2
  |          |
  +--------->| Header: POST (Code=0.02)
  |   POST   | Uri-Path: "/.well-known/edhoc"
  |          | Payload: C_R, EDHOC message_3
  |          |
  |<---------+ Header: 2.04 Changed
  |   2.04   |
  |          |

The exchange in Figure 10 protects the client identity against active attackers and the server identity against passive attackers.¶

An alternative exchange that protects the server identity against active attackers and the client identity against passive attackers is shown in Figure 11. In this case the CoAP Token enables the Responder to correlate message_2 and message_3, and the prepended C_I enables correlation on the Initiator (server) side. If EDHOC message_4 is used, C_I is prepended, and it is transported with CoAP in the payload of a POST request with a 2.04 (Changed) response.¶

Client    Server
  |          |
  +--------->| Header: POST (Code=0.02)
  |   POST   | Uri-Path: "/.well-known/edhoc"
  |          |
  |<---------+ Header: 2.04 Changed
  |   2.04   | Content-Format: application/edhoc
  |          | Payload: EDHOC message_1
  |          |
  +--------->| Header: POST (Code=0.02)
  |   POST   | Uri-Path: "/.well-known/edhoc"
  |          | Payload: C_I, EDHOC message_2
  |          |
  |<---------+ Header: 2.04 Changed
  |   2.04   | Content-Format: application/edhoc
  |          | Payload: EDHOC message_3
  |          |

To protect against denial-of-service attacks, the CoAP server MAY respond to the first POST request with a 4.01 (Unauthorized) containing an Echo option [I-D.ietf-core-echo-request-tag]. This forces the initiator to demonstrate its reachability at its apparent network address. If message fragmentation is needed, the EDHOC messages may be fragmented using the CoAP Block-Wise Transfer mechanism [RFC7959].¶

EDHOC does not restrict how error messages are transported with CoAP, as long as the appropriate error message can to be transported in response to a message that failed (see Section 6). EDHOC error messages transported with CoAP are carried in the payload.¶

C.1. CBOR and CDDL

The Concise Binary Object Representation (CBOR) [RFC8949] is a data format designed for small code size and small message size. CBOR builds on the JSON data model but extends it by e.g., encoding binary data directly without base64 conversion. In addition to the binary CBOR encoding, CBOR also has a diagnostic notation that is readable and editable by humans. The Concise Data Definition Language (CDDL) [RFC8610] provides a way to express structures for protocol messages and APIs that use CBOR. [RFC8610] also extends the diagnostic notation.¶

CBOR data items are encoded to or decoded from byte strings using a type-length-value encoding scheme, where the three highest order bits of the initial byte contain information about the major type. CBOR supports several different types of data items, in addition to integers (int, uint), simple values, byte strings (bstr), and text strings (tstr), CBOR also supports arrays [] of data items, maps {} of pairs of data items, and sequences [RFC8742] of data items. Some examples are given below.¶

The EDHOC specification sometimes use CDDL names in CBOR dignostic notation as in e.g., << ID_CRED_R, ? EAD_2 >>. This means that EAD_2 is optional and that ID_CRED_R and EAD_2 should be substituted with their values before evaluation. I.e., if ID_CRED_R = { 4 : h'' } and EAD_2 is omitted then << ID_CRED_R, ? EAD_2 >> = << { 4 : h'' } >>, which encodes to 0x43a10440.¶

For a complete specification and more examples, see [RFC8949] and [RFC8610]. We recommend implementors to get used to CBOR by using the CBOR playground [CborMe].¶

Diagnostic          Encoded              Type
------------------------------------------------------------------
1                   0x01                 unsigned integer
24                  0x1818               unsigned integer
-24                 0x37                 negative integer
-25                 0x3818               negative integer
true                0xf5                 simple value
h''                 0x40                 byte string
h'12cd'             0x4212cd             byte string
'12cd'              0x4431326364         byte string
"12cd"              0x6431326364         text string
{ 4 : h'cd' }       0xa10441cd           map
<< 1, 2, true >>    0x430102f5           byte string
[ 1, 2, true ]      0x830102f5           array
( 1, 2, true )      0x0102f5             sequence
1, 2, true          0x0102f5             sequence
------------------------------------------------------------------

C.2. CDDL Definitions

This sections compiles the CDDL definitions for ease of reference.¶

suites = [ 2* int ] / int

ead = 1* (
  ead_label : int,
  ead_value : any,
)

message_1 = (
  METHOD : int,
  SUITES_I : suites,
  G_X : bstr,
  C_I : bstr / int,
  ? EAD_1 : ead,
)

message_2 = (
  G_Y_CIPHERTEXT_2 : bstr,
  C_R : bstr / int,
)

message_3 = (
  CIPHERTEXT_3 : bstr,
)

message_4 = (
  CIPHERTEXT_4 : bstr,
)

error = (
  ERR_CODE : int,
  ERR_INFO : any,
)

info = (
  transcript_hash : bstr,
  label : tstr,
  context : bstr,
  length : uint,
)

C.3. COSE

CBOR Object Signing and Encryption (COSE) [I-D.ietf-cose-rfc8152bis-struct] describes how to create and process signatures, message authentication codes, and encryption using CBOR. COSE builds on JOSE, but is adapted to allow more efficient processing in constrained devices. EDHOC makes use of COSE_Key, COSE_Encrypt0, and COSE_Sign1 objects in the message processing:¶

• ECDH ephemeral public keys of type EC2 or OKP in message_1 and message_2 consist of the COSE_Key parameter named 'x', see Section 7.1 and 7.2 of [I-D.ietf-cose-rfc8152bis-algs]¶
• Certain ciphertexts in message_2 and message_3 consist of a subset of the single recipient encrypted data object COSE_Encrypt0, which is described in Sections 5.2-5.3 of [I-D.ietf-cose-rfc8152bis-struct]. The ciphertext is computed over the plaintext and associated data, using an encryption key and a nonce. The associated data is an Enc_structure consisting of protected headers and externally supplied data (external_aad).¶
• Signatures in message_2 of method 0 and 2, and in message_3 of method 0 and 1, consist of a subset of the single signer data object COSE_Sign1, which is described in Sections 4.2-4.4 of [I-D.ietf-cose-rfc8152bis-struct]. The signature is computed over a Sig_structure containing payload, protected headers and externally supplied data (external_aad) using a private signature key and verified using the corresponding public signature key.¶

Different header parameters to identify X.509 or C509 certificates by reference are defined in [I-D.ietf-cose-x509] and [I-D.ietf-cose-cbor-encoded-cert]:¶

• by a hash value with the 'x5t' or 'c5t' parameters, respectively:¶

  • ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R,¶
  • ID_CRED_x = { TBD3 : COSE_CertHash }, for x = I or R;¶

• or by a URI with the 'x5u' or 'c5u' parameters, respectively:¶

  • ID_CRED_x = { 35 : uri }, for x = I or R,¶
  • ID_CRED_x = { TBD4 : uri }, for x = I or R.¶

When ID_CRED_x does not contain the actual credential, it may be very short, e.g., if the endpoints have agreed to use a key identifier parameter 'kid':¶

• ID_CRED_x = { 4 : key_id_x }, where key_id_x : kid, for x = I or R.¶

Note that a COSE header map can contain several header parameters, for example { x5u, x5t } or { kid, kid_context }.¶

ID_CRED_x MAY also identify the authentication credential by value. For example, a certificate chain can be transported in ID_CRED_x with COSE header parameter c5c or x5chain, defined in [I-D.ietf-cose-cbor-encoded-cert] and [I-D.ietf-cose-x509] and credentials of type CWT and CCS can be transported with the COSE header parameters registered in Section 9.6.¶