ACE Working Group G. Selander Internet-Draft J. Mattsson Intended status: Standards Track F. Palombini Expires: August 29, 2019 Ericsson AB February 25, 2019 Ephemeral Diffie-Hellman Over COSE (EDHOC) draft-selander-ace-cose-ecdhe-12 Abstract This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact, and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, perfect forward secrecy, and identity protection. A main use case for EDHOC is to establish an OSCORE security context. EDHOC uses COSE for cryptography, CBOR for encoding, and CoAP for transport. By reusing existing libraries, the additional code footprint can be kept very low. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on August 29, 2019. Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Selander, et al. Expires August 29, 2019 [Page 1] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Rationale for EDHOC . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology and Requirements Language . . . . . . . . . . 5 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. EDHOC Overview . . . . . . . . . . . . . . . . . . . . . . . 7 3.1. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Ephemeral Public Keys . . . . . . . . . . . . . . . . . . 9 3.3. Key Derivation . . . . . . . . . . . . . . . . . . . . . 9 4. EDHOC Authenticated with Asymmetric Keys . . . . . . . . . . 11 4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 11 4.2. EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . . 13 4.3. EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . . 14 4.4. EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . . 17 5. EDHOC Authenticated with Symmetric Keys . . . . . . . . . . . 19 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 19 5.2. EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . . 20 5.3. EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . . 21 5.4. EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . . 21 6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 21 6.1. EDHOC Error Message . . . . . . . . . . . . . . . . . . . 21 7. Transferring EDHOC and Deriving Application Keys . . . . . . 23 7.1. Transferring EDHOC in CoAP . . . . . . . . . . . . . . . 23 7.2. Transferring EDHOC over Other Protocols . . . . . . . . . 26 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 8.1. EDHOC Cipher Suites Registry . . . . . . . . . . . . . . 26 8.2. EDHOC Method Type Registry . . . . . . . . . . . . . . . 26 8.3. The Well-Known URI Registry . . . . . . . . . . . . . . . 26 8.4. Media Types Registry . . . . . . . . . . . . . . . . . . 26 8.5. CoAP Content-Formats Registry . . . . . . . . . . . . . . 27 9. Security Considerations . . . . . . . . . . . . . . . . . . . 28 9.1. Security Properties . . . . . . . . . . . . . . . . . . . 28 9.2. Cryptographic Considerations . . . . . . . . . . . . . . 28 9.3. Mandatory to Implement Cipher Suite . . . . . . . . . . . 29 9.4. Unprotected Data . . . . . . . . . . . . . . . . . . . . 29 9.5. Denial-of-Service . . . . . . . . . . . . . . . . . . . . 30 9.6. Implementation Considerations . . . . . . . . . . . . . . 30 9.7. Other Documents Referencing EDHOC . . . . . . . . . . . . 31 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 10.1. Normative References . . . . . . . . . . . . . . . . . . 31 10.2. Informative References . . . . . . . . . . . . . . . . . 33 Appendix A. Use of CBOR, CDDL and COSE in EDHOC . . . . . . . . 34 Selander, et al. Expires August 29, 2019 [Page 2] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 A.1. CBOR and CDDL . . . . . . . . . . . . . . . . . . . . . . 35 A.2. COSE . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Appendix B. Example Messages and Sizes . . . . . . . . . . . . . 39 B.1. Message Sizes RPK . . . . . . . . . . . . . . . . . . . . 39 B.2. Message Sizes Certificates . . . . . . . . . . . . . . . 40 B.3. Message Sizes PSK . . . . . . . . . . . . . . . . . . . . 40 B.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 42 Appendix C. Test Vectors . . . . . . . . . . . . . . . . . . . . 43 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 43 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43 1. Introduction Security at the application layer provides an attractive option for protecting Internet of Things (IoT) deployments, for example where transport layer security is not sufficient [I-D.hartke-core-e2e-security-reqs] or where the protocol needs to work on a variety of underlying protocols. IoT devices may be constrained in various ways, including memory, storage, processing capacity, and energy [RFC7228]. A method for protecting individual messages at the application layer suitable for constrained devices, is provided by CBOR Object Signing and Encryption (COSE) [RFC8152]), which builds on the Concise Binary Object Representation (CBOR) [I-D.ietf-cbor-7049bis]. Object Security for Constrained RESTful Environments (OSCORE) [I-D.ietf-core-object-security] is a method for application-layer protection of the Constrained Application Protocol (CoAP), using COSE. In order for a communication session to provide forward secrecy, the communicating parties can run an Elliptic Curve Diffie-Hellman (ECDH) key exchange protocol with ephemeral keys, from which shared key material can be derived. This document specifies Ephemeral Diffie- Hellman Over COSE (EDHOC), a lightweight key exchange protocol providing perfect forward secrecy and identity protection. Authentication is based on credentials established out of band, e.g. from a trusted third party, such as an Authorization Server as specified by [I-D.ietf-ace-oauth-authz]. EDHOC supports authentication using pre-shared keys (PSK), raw public keys (RPK), and public key certificates. After successful completion of the EDHOC protocol, application keys and other application specific data can be derived using the EDHOC-Exporter interface. A main use case for EDHOC is to establish an OSCORE security context. EDHOC uses COSE for cryptography, CBOR for encoding, and CoAP for transport. By reusing existing libraries, the additional code footprint can be kept very low. Note that this document focuses on authentication and key establishment: for integration with authorization of resource access, refer to [I-D.ietf-ace-oscore-profile]. Selander, et al. Expires August 29, 2019 [Page 3] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 EDHOC is designed to work in highly constrained scenarios making it especially suitable for network technologies such as Cellular IoT, 6TiSCH [I-D.ietf-6tisch-dtsecurity-zerotouch-join], LoRaWAN [LoRa1][LoRa2]. These network technologies are characterized by their low throughput, low power consumption, and small frame sizes. Compared to the DTLS 1.3 handshake [I-D.ietf-tls-dtls13] with ECDH and connection ID, the number of bytes in EDHOC is less than 1/4 when PSK authentication is used and less than 1/3 when RPK authentication is used, see Appendix B. The ECDH exchange and the key derivation follow [SIGMA], NIST SP- 800-56A [SP-800-56A], and HKDF [RFC5869]. CBOR [I-D.ietf-cbor-7049bis] and COSE [RFC8152] are used to implement these standards. The use of COSE enables use of future COSE algorithms and headers designed for constrained IoT. This document is organized as follows: Section 2 describes how EDHOC builds on SIGMA-I, Section 3 specifies general properties of EDHOC, including message flow, formatting of the ephemeral public keys, and key derivation, Section 4 specifies EDHOC with asymmetric key authentication, Section 5 specifies EDHOC with symmetric key authentication, Section 6 specifies the EDHOC error message, and Section 7 describes how EDHOC can be transferred in CoAP and used to establish an OSCORE security context. 1.1. Rationale for EDHOC Many constrained IoT systems today do not use any security at all, and when they do, they often do not follow best practices. One reason is that many current security protocols are not designed with constrained IoT in mind. Constrained IoT systems often deals with personal information, valuable business data, and actuators interacting with the physical world. Not only do such systems need security and privacy, they often need end-to-end protection with source authentication and perfect-forward secrecy. EDHOC and OSCORE [I-D.ietf-core-object-security] enables security following current best practices to devices and systems where current security protocols are impractical. EDHOC is optimized for small message sizes and can therefore be sent over a small number of radio frames. The message size of a key exchange protocol may have a large impact on the performance of an IoT deployment, especially in noisy environments. For example, in a network bootstrapping setting a large number of devices turned on in a short period of time may result in large latencies caused by parallel key exchanges. Requirements on network formation time can in constrained environments be translated into key exchange overhead. Selander, et al. Expires August 29, 2019 [Page 4] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 Power consumption for wireless devices is highly dependent on message transmission, listening, and reception. For devices that only send a few bytes occasionally, the battery lifetime may be significantly reduced by a heavy key exchange protocol. Moreover, a key exchange may need to be executed more than once, e.g. due to a device losing power or rebooting for other reasons. EDHOC is adapted to primitives and protocols designed for the Internet of Things: EDHOC is built on CBOR and COSE which enables small message overhead and efficient parsing in constrained devices. EDHOC is not bound to a particular transport layer, but it is recommended to transport the EDHOC message in CoAP payloads. By reusing already existing IoT primitives in the device (CBOR, CoAP, and COSE encryption and signature formats) the additional code footprint can be kept very low. EDHOC is not bound to a particular communication security protocol but works off-the-shelf with OSCORE [I-D.ietf-core-object-security] providing the necessary input parameters with required properties. Since EDHOC builds on the same IoT primitives and protocols as OSCORE (CoAP, CBOR, COSE encryption and signature formats) the device footprint for EDHOC + OSCORE can be kept very low. The use of compact native encoding formats reduces the need for a general- purpose compression algorithm with associated footprint. 1.2. 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. The word "encryption" without qualification always refers to authenticated encryption, in practice implemented with an Authenticated Encryption with Additional Data (AEAD) algorithm, see [RFC5116]. Readers are expected to be familiar with the terms and concepts described in CBOR [I-D.ietf-cbor-7049bis], COSE [RFC8152], and CDDL [I-D.ietf-cbor-cddl]. The Concise Data Definition Language (CDDL) to express CBOR data structures [I-D.ietf-cbor-7049bis]. The use of the CDDL unwrap operator "~" is extended to unwrapping of byte strings. It is the inverse of "bstr .cbor" that wraps a data item in a bstr, i.e. ~ bstr .cbor T = T. Examples of CBOR and CDDL are provided in Appendix A.1. Selander, et al. Expires August 29, 2019 [Page 5] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 2. Background SIGMA (SIGn-and-MAc) is a family of theoretical protocols with a large number of variants [SIGMA]. Like IKEv2 and (D)TLS 1.3, EDHOC is built on a variant of the SIGMA protocol which provide identity protection of the initiator (SIGMA-I), and like (D)TLS 1.3, EDHOC implements the SIGMA-I variant as Sign-then-MAC. The SIGMA-I protocol using an authenticated encryption algorithm is shown in Figure 1. Party U Party V | X_U | +-------------------------------------------------------->| | | | X_V, AEAD( K_2; ID_CRED_V, Sig(V; CRED_V, X_U, X_V) ) | |<--------------------------------------------------------+ | | | AEAD( K_3; ID_CRED_U, Sig(U; CRED_U, X_V, X_U) ) | +-------------------------------------------------------->| | | Figure 1: Authenticated encryption variant of the SIGMA-I protocol. The parties exchanging messages are called "U" and "V". They exchange identities and ephemeral public keys, compute the shared secret, and derive symmetric application keys. o X_U and X_V are the ECDH ephemeral public keys of U and V, respectively. o CRED_U and CRED_V are the credentials containing the public authentication keys of U and V, respectively. o ID_CRED_U and ID_CRED_V are data enabling the recipient party to retrieve the credential of U and V, respectively o Sig(U; . ) and S(V; . ) denote signatures made with the private authentication key of U and V, respectively. o AEAD(K; . ) denotes authenticated encryption with additional data using the key K derived from the shared secret. The authenticated encryption MUST NOT be replaced by plain encryption, see Section 9. In order to create a "full-fledged" protocol some additional protocol elements are needed. EDHOC adds: Selander, et al. Expires August 29, 2019 [Page 6] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 o Explicit connection identifiers C_U, C_V chosen by U and V, respectively, enabling the recipient to find the protocol state. o An Authenticated Encryption with Additional Data (AEAD) algorithm is used. o Computationally independent keys derived from the ECDH shared secret and used for encryption of different messages. o Verification of a common preferred cipher suite (AEAD algorithm, ECDH algorithm, ECDH curve, signature algorithm): * U lists supported cipher suites in order of preference * V verifies that the selected cipher suite is the first supported cipher suite o Method types and error handling. o Transport of opaque application defined data. EDHOC is designed to encrypt and integrity protect as much information as possible, and all symmetric keys are derived using as much previous information as possible. EDHOC is furthermore designed to be as compact and lightweight as possible, in terms of message sizes, processing, and the ability to reuse already existing CBOR, COSE, and CoAP libraries. To simplify for implementors, the use of CBOR and COSE in EDHOC is summarized in Appendix A and example messages in CBOR diagnostic notation are given in Appendix B. 3. EDHOC Overview EDHOC consists of three flights (message_1, message_2, message_3) that maps directly to the three messages in SIGMA-I, plus an EDHOC error message. All EDHOC messages consists of a sequence of CBOR encoded data items, where the first data item of message_1 is an int specifying the method type (asymmetric, symmetric, error). The messages may be viewed as a CBOR encoding of an indefinite-length array without the first and last byte, see Appendix A.1. While EDHOC uses the COSE_Key, COSE_Sign1, and COSE_Encrypt0 structures, only a subset of the parameters is included in the EDHOC messages. After creating EDHOC message_3, Party U can derive symmetric application keys, and application protected data can therefore be sent in parallel with EDHOC message_3. The application may protect data using the algorithms (AEAD, HKDF, etc.) in the Selander, et al. Expires August 29, 2019 [Page 7] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 selected cipher suite and the connection identifiers (C_U, C_V). EDHOC may be used with the media type application/edhoc defined in Section 8. Party U Party V | | | ------------------ EDHOC message_1 -----------------> | | | | <----------------- EDHOC message_2 ------------------ | | | | ------------------ EDHOC message_3 -----------------> | | | | <----------- Application Protected Data ------------> | | | Figure 2: EDHOC message flow The EDHOC message exchange may be authenticated using pre-shared keys (PSK), raw public keys (RPK), or public key certificates. EDHOC assumes the existence of mechanisms (certification authority, manual distribution, etc.) for binding identities with authentication keys (public or pre-shared). When a public key infrastructure is used, the identity is included in the certificate and bound to the authentication key by trust in the certification authority. When the credential is manually distributed (PSK, RPK, self-signed certificate), the identity and authentication key is distributed out- of-band and bound together by trust in the distribution method. EDHOC with symmetric key authentication is very similar to EDHOC with asymmetric key authentication, the difference being that information is only MACed, not signed. EDHOC allows opaque application data (UAD and PAD) to be sent in the EDHOC messages. Unprotected Application Data (UAD_1, UAD_2) may be sent in message_1 and message_2 and can be e.g. be used to transfer access tokens that are protected outside of EDHOC. Protected application data (PAD_3) may be used to transfer any application data in message_3. Cryptographically, EDHOC does not put requirement on the lower layers. EDHOC is not bound to a particular transport layer, and can be used in environments without IP. It is recommended is to transport the EDHOC message in CoAP payloads, see Section 7. An implementation may support only Party U or only Party V. Selander, et al. Expires August 29, 2019 [Page 8] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 3.1. Cipher Suites EDHOC cipher suites consists of a set of COSE algorithms: an AEAD algorithm, an ECDH algorithm (including HKDF algorithm), an ECDH curve, and a signature algorithm. The signature algorithm is not used when EDHOC is authenticated with symmetric keys. Each cipher suite is associated with an integer value. Currently two cipher suites are defined. 0. AES-CCM-64-64-128, ECDH-SS + HKDF-256, X25519, and Ed25519 1. AES-CCM-64-64-128, ECDH-SS + HKDF-256, P-256, and ES256 Two additional numbers are registered for application defined cipher suites. Application defined cipher suites MUST only use algorithms specified for COSE, are not interoperable with other deployments and can therefore only be used in local networks. -24. First application defined cipher suite. -23. Second application defined cipher suite. 3.2. Ephemeral Public Keys The ECDH ephemeral public keys are formatted as a COSE_Key of type EC2 or OKP according to Sections 13.1 and 13.2 of [RFC8152], but only a subset of the parameters is included in the EDHOC messages. For Elliptic Curve Keys of type EC2, compact representation as per [RFC6090] MAY be used also in the COSE_Key. If the COSE implementation requires an y-coordinate, any of the possible values of the y-coordinate can be used, see Appendix C of [RFC6090]. COSE [RFC8152] always use compact output for Elliptic Curve Keys of type EC2. 3.3. Key Derivation Key and IV derivation SHALL be performed as specified in Section 11 of [RFC8152] with the following input: o The KDF SHALL be the HKDF [RFC5869] in the in the selected cipher suite (SUITE). o The secret (Section 11.1 of [RFC8152]) SHALL be the ECDH shared secret as defined in Section 12.4.1 of [RFC8152]. o The salt (Section 11.1 of [RFC8152]) SHALL be the PSK when EDHOC is authenticated with symmetric keys, and the empty byte string when EDHOC is authenticated with asymmetric keys. Note that [RFC5869] specifies that if the salt is not provided, it is set to a string of zeros (see Section 2.2 of [RFC5869]). For Selander, et al. Expires August 29, 2019 [Page 9] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 implementation purposes, not providing the salt is the same as setting the salt to the empty byte string. o The fields in the context information COSE_KDF_Context (Section 11.2 of [RFC8152]) SHALL have the following values: * AlgorithmID is an int or tstr, see below * PartyUInfo = PartyVInfo = ( null, null, null ) * keyDataLength is a uint, see below * protected SHALL be a zero length bstr * other is a bstr and SHALL be aad_2, aad_3, or exchange_hash; see below * SuppPrivInfo is omitted where exchange_hash, in non-CDDL notation, is: exchange_hash = H( bstr .cborseq [ aad_3, CIPHERTEXT_3 ] ) and where aad_2 and aad_3 are hashes of previous messages and data, defined in Sections 4.3.1 and 4.4.1. H() is the hash function in the HKDF, which takes a CBOR byte string (bstr) as input and produces a CBOR byte string as output. The use of '.cborseq' is exemplified in Appendix A.1. We define EDHOC-Key-Derivation to be the function which produces the output as described in [RFC5869] and [RFC8152] depending on the variable input AlgorithmID, keyDataLength, and other: output = EDHOC-Key-Derivation(AlgorithmID, keyDataLength, other) For message_i the key, called K_i, SHALL be derived using other = aad_i, where i = 2 or 3. The key SHALL be derived using AlgorithmID set to the integer value of the AEAD in the selected cipher suite (SUITE), and keyDataLength equal to the key length of the AEAD. If the AEAD algorithm uses an IV, then IV_i for message_i SHALL be derived using other = aad_i, where i = 2 or 3. The IV SHALL be derived using AlgorithmID = "IV-GENERATION" as specified in Section 12.1.2. of [RFC8152], and keyDataLength equal to the IV length of the AEAD. Selander, et al. Expires August 29, 2019 [Page 10] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 3.3.1. EDHOC-Exporter Interface Application keys and other application specific data can be derived using the EDHOC-Exporter interface defined as: EDHOC-Exporter(label, length) = EDHOC-Key-Derivation(label, 8 * length, exchange_hash) The output of the EDHOC-Exporter function SHALL be derived using other = exchange_hash, AlgorithmID = label, and keyDataLength = 8 * length, where label is a tstr defined by the application and length is a uint defined by the application. The label SHALL be different for each different exporter value. An example use of the EDHOC- Exporter is given in Section 7.1.1). 3.3.2. EDHOC PSK Chaining An application using EDHOC may want to derive new PSKs to use for authentication in future EDHOC exchanges. In this case, the new PSK and KID SHOULD be derived as follows where length is the key length (in bytes) of the AEAD Algorithm. PSK = EDHOC-Exporter("EDHOC Chaining PSK", length) KID = EDHOC-Exporter("EDHOC Chaining KID", 4) 4. EDHOC Authenticated with Asymmetric Keys 4.1. Overview EDHOC supports authentication with raw public keys (RPK) and public key certificates with the requirements that: o Party U SHALL be able to retrieve Party V's public authentication key using ID_CRED_V, o Party V SHALL be able to retrieve Party U's public authentication key using ID_CRED_U, where ID_CRED_x, for x = U or V, is encoded in a COSE map, see Appendix A.2. In the following we give some examples of possible COSE map labels. Raw public keys are most optimally stored as COSE_Key objects and identified with a 'kid' value (see [RFC8152]): o kid : ID_CRED_x, for x = U or V. Selander, et al. Expires August 29, 2019 [Page 11] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 Public key certificates can be identified in different ways, for example (see [I-D.schaad-cose-x509]): o by a hash value; * x5t : ID_CRED_x, for x = U or V, o by a URL; * x5u : ID_CRED_x, for x = U or V, o by a certificate chain; * x5chain : ID_CRED_x, for x = U or V, o or by a bag of certificates. * x5bag : ID_CRED_x, for x = U or V. In the latter two examples, ID_CRED_U and ID_CRED_V contains the actual credential used for authentication. ID_CRED_U and ID_CRED_V do not need to uniquely identify the public authentication key, but doing so is recommended as the recipient may otherwise have to try several public keys. ID_CRED_U and ID_CRED_V are transported in the ciphertext, see Section 4.3.2 and Section 4.4.2. The actual credentials CRED_U and CRED_V (e.g. a COSE_Key or a single X.509 certificate) are signed by party U and V, respectively, see Section 4.4.1 and Section 4.3.1. Party U and Party V MAY use different type of credentials, e.g. one uses RPK and the other uses certificate. EDHOC with asymmetric key authentication is illustrated in Figure 3. Party U Party V | TYPE, C_U, SUITES_U, SUITE, X_U, UAD_1 | +------------------------------------------------------------------>| | message_1 | | | | C_U, C_V, X_V, AEAD(K_2; ID_CRED_V, Sig(V; CRED_V, aad_2), UAD_2) | |<------------------------------------------------------------------+ | message_2 | | | | C_V, AEAD(K_3; ID_CRED_U, Sig(U; CRED_U, aad_3), PAD_3) | +------------------------------------------------------------------>| | message_3 | Figure 3: Overview of EDHOC with asymmetric key authentication. Selander, et al. Expires August 29, 2019 [Page 12] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 4.2. EDHOC Message 1 4.2.1. Formatting of Message 1 message_1 SHALL be a sequence of CBOR data items (see Appendix A.1) as defined below message_1 = ( TYPE : int, C_U : bstr, SUITES_U : suites, SUITE : uint, X_U : bstr, ? UAD_1 : bstr, ) suites : int / [ 2* int ] where: o TYPE = 1 o C_U - variable length connection identifier o SUITES_U - cipher suites which Party U supports, in order of decreasing preference. If a single cipher suite is conveyed, an int is used, if multiple cipher suites are conveyed, an array of ints is used. o SUITE - a single chosen cipher suite from SUITES_U (zero-based index, i.e. 0 for the first or only, 1 for the second, etc.) o X_U - the x-coordinate of the ephemeral public key of Party U o UAD_1 - bstr containing unprotected opaque application data 4.2.2. Party U Processing of Message 1 Party U SHALL compose message_1 as follows: o The supported cipher suites and the order of preference MUST NOT be changed based on previous error messages. However, the list SUITES_U sent to Party V MAY be truncated such that cipher suites which are the least preferred are omitted. The amount of truncation MAY be changed between sessions, e.g. based on previous error messages (see next bullet), but all cipher suites which are more preferred than the least preferred cipher suite in the list MUST be included in the list. Selander, et al. Expires August 29, 2019 [Page 13] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 o Determine the cipher suite SUITE to use with Party V in message_1. If Party U previously received from Party V an error message to message_1 with diagnostic payload identifying a cipher suite that U supports, then U SHALL use that cipher suite. Otherwise the first cipher suite in SUITES_U MUST be used. o Generate an ephemeral ECDH key pair as specified in Section 5 of [SP-800-56A] using the curve in the cipher suite SUITE. Let X_U be the x-coordinate of the ephemeral public key. o Choose a connection identifier C_U and store it for the length of the protocol. Party U MUST be able to retrieve the protocol state using the connection identifier C_U and optionally other information such as the 5-tuple. The connection identifier MAY be used with a protocol for which EDHOC establishes application keys, in which case C_U SHALL adhere to the requirements for that protocol. o Format message_1 as the sequence of CBOR data items specified in Section 4.2.1 and encode it to a byte string (see Appendix A.1). 4.2.3. Party V Processing of Message 1 Party V SHALL process message_1 as follows: o Decode message_1 (see Appendix A.1). o Verify that the cipher suite SUITE is supported and that no prior cipher suites in SUITES_U are supported. o Validate that there is a solution to the curve definition for the given x-coordinate X_U. o Pass UAD_1 to the application. If any verification step fails, Party V MUST send an EDHOC error message back, formatted as defined in Section 6, and the protocol MUST be discontinued. If V does not support the cipher suite SUITE, then SUITES_V MUST include one or more supported cipher suites. If V does not support the cipher suite SUITE, but supports another cipher suite in SUITES_U, then SUITES_V MUST include the first supported cipher suite in SUITES_U. 4.3. EDHOC Message 2 Selander, et al. Expires August 29, 2019 [Page 14] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 4.3.1. Formatting of Message 2 message_2 SHALL be a sequence of CBOR data items (see Appendix A.1) as defined below message_2 = ( data_2, CIPHERTEXT_2 : bstr, ) data_2 = ( C_U : bstr / nil, C_V : bstr, X_V : bstr, ) aad_2 : bstr where aad_2, in non-CDDL notation, is: aad_2 = H( bstr .cborseq [ message_1, data_2 ] ) where: o C_V - variable length connection identifier o X_V - the x-coordinate of the ephemeral public key of Party V o H() - the hash function in the HKDF, which takes a CBOR byte string (bstr) as input and produces a CBOR byte string as output. The use of '.cborseq' is exemplified in Appendix A.1. 4.3.2. Party V Processing of Message 2 Party V SHALL compose message_2 as follows: o Generate an ephemeral ECDH key pair as specified in Section 5 of [SP-800-56A] using the curve in the cipher suite SUITE. Let X_V be the x-coordinate of the ephemeral public key. o Choose a connection identifier C_V and store it for the length of the protocol. Party V MUST be able to retrieve the protocol state using the connection identifier C_V and optionally other information such as the 5-tuple. The connection identifier MAY be used with a protocol for which EDHOC establishes application keys, in which case C_V SHALL adhere to the requirements for that protocol. To reduce message overhead, party V can set the message field C_U in message_2 to null (still storing the actual value of Selander, et al. Expires August 29, 2019 [Page 15] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 C_U) if there is an external correlation mechanism (e.g. the Token in CoAP) that enables Party U to correlate message_1 and message_2. o Compute COSE_Sign1 as defined in Section 4.4 of [RFC8152], using the signature algorithm in the cipher suite SUITE, the private authentication key of Party V, and the following parameters (further clarifications in Appendix A.2.2). The unprotected header MAY contain parameters (e.g. 'alg'). * protected = bstr .cbor { abc : ID_CRED_V } * payload = CRED_V * external_aad = aad_2 * abc - any COSE map label that can identify a public authentication key, see Section 4.1 * ID_CRED_V - a CBOR type that can be used with the COSE map label. Enables the retrieval of the public authentication key of Party V, see Section 4.1 * CRED_V - bstr credential containing the public authentication key of Party V, see Section 4.1 Note that only 'protected' and 'signature' of the COSE_Sign1 object are used in message_2, see next bullet. o Compute COSE_Encrypt0 as defined in Section 5.3 of [RFC8152], with the AEAD algorithm in the cipher suite SUITE, K_2, IV_2, and the following parameters (further clarifications in Appendix A.2.2). The protected header SHALL be empty. The unprotected header MAY contain parameters (e.g. 'alg'). * plaintext = bstr .cborseq [ ~protected, signature, ? UAD_2 ] * external_aad = aad_2 * UAD_2 = bstr containing opaque unprotected application data Note that protected and signature in the plaintext are taken from the COSE_Sign1 object, and that that only 'ciphertext' of the COSE_Encrypt0 object are used in message_2, see next bullet. o Format message_2 as the sequence of CBOR data items specified in Section 4.3.1 and encode it to a byte string (see Appendix A.1). CIPHERTEXT_2 is the COSE_Encrypt0 ciphertext. Selander, et al. Expires August 29, 2019 [Page 16] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 4.3.3. Party U Processing of Message 2 Party U SHALL process message_2 as follows: o Decode message_2 (see Appendix A.1). o Retrieve the protocol state using the connection identifier C_U and optionally other information such as the 5-tuple. o Validate that there is a solution to the curve definition for the given x-coordinate X_V. o Decrypt and verify COSE_Encrypt0 as defined in Section 5.3 of [RFC8152], with the AEAD algorithm in the cipher suite SUITE, K_2, and IV_2. o Verify COSE_Sign1 as defined in Section 4.4 of [RFC8152], using the signature algorithm in the cipher suite SUITE and the public authentication key of Party V. If any verification step fails, Party U MUST send an EDHOC error message back, formatted as defined in Section 6, and the protocol MUST be discontinued. 4.4. EDHOC Message 3 4.4.1. Formatting of Message 3 message_3 SHALL be a sequence of CBOR data items (see Appendix A.1) as defined below message_3 = ( data_3, CIPHERTEXT_3 : bstr, ) data_3 = ( C_V : bstr / nil, ) aad_3 : bstr where aad_3, in non-CDDL notation, is: aad_3 = H( bstr .cborseq [ aad_2, CIPHERTEXT_2, data_3 ] ) Selander, et al. Expires August 29, 2019 [Page 17] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 4.4.2. Party U Processing of Message 3 Party U SHALL compose message_3 as follows: o To reduce message overhead, party U can set the message field C_V in message_3 to null (still storing the actual value of C_V) if there is an external correlation mechanism (e.g. the Token in CoAP) that enables Party V to correlate message_2 and message_3. o Compute COSE_Sign1 as defined in Section 4.4 of [RFC8152], using the signature algorithm in the cipher suite SUITE, the private authentication key of Party U, and the following parameters. The unprotected header MAY contain parameters (e.g. 'alg'). * protected = bstr .cbor { abc : ID_CRED_U } * payload = CRED_U * external_aad = aad_3 * abc - any COSE map label that can identify a public authentication key, see Section 4.1 * ID_CRED_U - a CBOR type that can be used with the COSE map label. Enables the retrieval of the public authentication key of Party U, see Section 4.1 * CRED_U - bstr credential containing the public authentication key of Party U, see Section 4.1 Note that only 'protected' and 'signature' of the COSE_Sign1 object are used in message_3, see next bullet. o Compute COSE_Encrypt0 as defined in Section 5.3 of [RFC8152], with the AEAD algorithm in the cipher suite SUITE, K_3, and IV_3 and the following parameters. The protected header SHALL be empty. The unprotected header MAY contain parameters (e.g. 'alg'). * plaintext = bstr .cborseq [ ~protected, signature, ? PAD_3 ] * external_aad = aad_3 * PAD_3 = bstr containing opaque protected application data Note that protected and signature in the plaintext are taken from the COSE_Sign1 object, and that only 'ciphertext' of the COSE_Encrypt0 object are used in message_3, see next bullet. Selander, et al. Expires August 29, 2019 [Page 18] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 o Format message_3 as the sequence of CBOR data items specified in Section 4.4.1 and encode it to a byte string (see Appendix A.1). CIPHERTEXT_3 is the COSE_Encrypt0 ciphertext. o Pass the connection identifiers (C_U, C_V) and the negotiated cipher suite SUITE to the application. The application can now derive application keys using the EDHOC-Exporter interface. 4.4.3. Party V Processing of Message 3 Party V SHALL process message_3 as follows: o Decode message_3 (see Appendix A.1). o Retrieve the protocol state using the connection identifier C_V and optionally other information such as the 5-tuple. o Decrypt and verify COSE_Encrypt0 as defined in Section 5.3 of [RFC8152], with the AEAD algorithm in the cipher suite SUITE, K_3, and IV_3. o Verify COSE_Sign1 as defined in Section 4.4 of [RFC8152], using the signature algorithm in the cipher suite SUITE and the public authentication key of Party U. If any verification step fails, Party V MUST send an EDHOC error message back, formatted as defined in Section 6, and the protocol MUST be discontinued. o Pass PAD_3, the connection identifiers (C_U, C_V), and the negotiated cipher suite SUITE to the application. The application can now derive application keys using the EDHOC-Exporter interface. 5. EDHOC Authenticated with Symmetric Keys 5.1. Overview EDHOC supports authentication with pre-shared keys. Party U and V are assumed to have a pre-shared key (PSK) with a good amount of randomness and the requirement that: o Party V SHALL be able to retrieve the PSK using KID. KID may optionally contain information about how to retrieve the PSK. KID does not need to uniquely identify the PSK, but doing so is recommended as the recipient may otherwise have to try several PSKs. Selander, et al. Expires August 29, 2019 [Page 19] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 EDHOC with symmetric key authentication is illustrated in Figure 4. Party U Party V | TYPE, C_U, SUITES_U, SUITE, X_U, KID, UAD_1 | +------------------------------------------------------------------>| | message_1 | | | | C_U, C_V, X_V, AEAD(K_2; aad_2, UAD_2) | |<------------------------------------------------------------------+ | message_2 | | | | C_V, AEAD(K_3; aad_3, PAD_3) | +------------------------------------------------------------------>| | message_3 | Figure 4: Overview of EDHOC with symmetric key authentication. EDHOC with symmetric key authentication is very similar to EDHOC with asymmetric key authentication. In the following subsections the differences compared to EDHOC with asymmetric key authentication are described. 5.2. EDHOC Message 1 5.2.1. Formatting of Message 1 message_1 SHALL be a sequence of CBOR data items (see Appendix A.1) as defined below message_1 = ( TYPE : int, C_U : bstr, SUITES_U : suites, SUITE : uint, X_U : bstr, KID : bstr, ? UAD_1 : bstr, ) where: o TYPE = 2 o KID - bstr enabling the retrieval of the pre-shared key Selander, et al. Expires August 29, 2019 [Page 20] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 5.3. EDHOC Message 2 5.3.1. Processing of Message 2 o COSE_Sign1 is not used. o COSE_Encrypt0 is computed as defined in Section 5.3 of [RFC8152], with the AEAD algorithm in the cipher suite SUITE, K_2, IV_2, and the following parameters. The protected header SHALL be empty. The unprotected header MAY contain parameters (e.g. 'alg'). * external_aad = aad_2 * plaintext = h'' / UAD_2 * UAD_2 = bstr containing opaque unprotected application data 5.4. EDHOC Message 3 5.4.1. Processing of Message 3 o COSE_Sign1 is not used. o COSE_Encrypt0 is computed as defined in Section 5.3 of [RFC8152], with the AEAD algorithm in the cipher suite SUITE, K_3, IV_3, and the following parameters. The protected header SHALL be empty. The unprotected header MAY contain parameters (e.g. 'alg'). * external_aad = aad_3 * plaintext = h'' / PAD_3 * PAD_3 = bstr containing opaque protected application data 6. Error Handling 6.1. EDHOC Error Message This section defines a message format for the EDHOC error message, used during the protocol. An EDHOC error message can be send by both parties as a response to any non-error EDHOC message. After sending an error message, the protocol MUST be discontinued. Errors at the EDHOC layer are sent as normal successful messages in the lower layers (e.g. CoAP POST and 2.04 Changed). An advantage of using such a construction is to avoid issues created by usage of cross protocol proxies (e.g. UDP to TCP). Selander, et al. Expires August 29, 2019 [Page 21] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 error SHALL be a sequence of CBOR data items (see Appendix A.1) as defined below error = ( TYPE : int, ERR_MSG : tstr, ? SUITES_V : suites, ) suites : int / [ 2* int ] where: o TYPE = 0 o ERR_MSG - text string containing the diagnostic payload, defined in the same way as in Section 5.5.2 of [RFC7252] o SUITES_V - cipher suites from SUITES_U or the EDHOC cipher suites registry that V supports. Note that SUITEs_V contains the values from the EDHOC cipher suites registry and not indexes. 6.1.1. Example Use of EDHOC Error Message with SUITES_V Assuming that Party U supports the five cipher suites {0, 1, 2, 3, 4} in decreasing order of preference, Figures 5 and 6 show examples of how Party U can truncate SUITES_U and how SUITES_V is used by Party V to give Party U information about the cipher suites that Party V supports. In Figure 5, Party V supports cipher suite 1 but not cipher suite 0. Party U Party V | TYPE, C_U, SUITES_U {0, 1, 2}, SUITE {0}, X_U, UAD_1 | +------------------------------------------------------------------>| | message_1 | | | | TYPE, ERR_MSG, SUITES_V {1} | |<------------------------------------------------------------------+ | error | | | | TYPE, C_U, SUITES_U {0, 1}, SUITE {1}, X_U, UAD_1 | +------------------------------------------------------------------>| | message_1 | Figure 5: Example use of error message with SUITES_V. In Figure 6, Party V supports cipher suite 2 but not cipher suites 0 and 1. Selander, et al. Expires August 29, 2019 [Page 22] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 Party U Party V | TYPE, C_U, SUITES_U {0, 1}, SUITE {0}, X_U, UAD_1 | +------------------------------------------------------------------>| | message_1 | | | | TYPE, ERR_MSG, SUITES_V {2, 4} | |<------------------------------------------------------------------+ | error | | | | TYPE, C_U, SUITES_U {0, 1, 2}, SUITE {2}, X_U, UAD_1 | +------------------------------------------------------------------>| | message_1 | Figure 6: Example use of error message with SUITES_V. As Party U's list of supported cipher suites and order of preference is fixed, and Party V only accepts message_1 if the selected cipher suite SUITE is the first cipher suite in SUITES_U that Party V supports, the parties can verify that the selected cipher suite SUITE is the most preferred (by Party U) cipher suite supported by both parties. If SUITE is not the first cipher suite in SUITES_U that Party V supports, Party V will discontinue the protocol. 7. Transferring EDHOC and Deriving Application Keys 7.1. Transferring EDHOC in CoAP It is recommended is to transport EDHOC as an exchange of CoAP [RFC7252] messages. CoAP is a reliable transport that can preserve packet ordering and handle message duplication. CoAP can also perform fragmentation and protect against denial of service attacks. It is recommended to carry the EDHOC flights in Confirmable messages, especially if fragmentation is used. By default, the CoAP client is Party U and the CoAP server is Party V, but the roles SHOULD be chosen to protect the most sensitive identity, see Section 9. By default, EDHOC is transferred in POST requests and 2.04 (Changed) responses to the Uri-Path: "/.well-known/ edhoc", but 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. Selander, et al. Expires August 29, 2019 [Page 23] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 If needed, an EDHOC error message is sent from the server to the client in the payload of a 2.04 (Changed) response. An example of a successful EDHOC exchange using CoAP is shown in Figure 7. Client Server | | +--------->| Header: POST (Code=0.02) | POST | Uri-Path: "/.well-known/edhoc" | | Content-Format: application/edhoc | | Payload: 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" | | Content-Format: application/edhoc | | Payload: EDHOC message_3 | | |<---------+ Header: 2.04 Changed | 2.04 | | | Figure 7: Transferring EDHOC in CoAP The exchange in Figure 7 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 8. Selander, et al. Expires August 29, 2019 [Page 24] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 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" | | Content-Format: application/edhoc | | Payload: EDHOC message_2 | | |<---------+ Header: 2.04 Changed | 2.04 | Content-Format: application/edhoc | | Payload: EDHOC message_3 | | Figure 8: Transferring EDHOC in CoAP 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]. 7.1.1. Deriving an OSCORE Context from EDHOC When EDHOC is used to derive parameters for OSCORE [I-D.ietf-core-object-security], the parties must make sure that the EDHOC connection identifiers are unique, i.e. C_V MUST NOT be equal to C_U. The CoAP client and server MUST be able to retrieve the OCORE protocol state using its chosen connection identifier and optionally other information such as the 5-tuple. In case that the CoAP client is party U and the CoAP server is party V: o The client's OSCORE Sender ID is C_V and the server's OSCORE Sender ID is C_U, as defined in this document o The AEAD Algorithm and the HMAC-based Key Derivation Function (HKDF) are the AEAD and HKDF algorithms in the cipher suite SUITE. o The Master Secret and Master Salt are derived as follows where length is the key length (in bytes) of the AEAD Algorithm. Selander, et al. Expires August 29, 2019 [Page 25] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 Master Secret = EDHOC-Exporter("OSCORE Master Secret", length) Master Salt = EDHOC-Exporter("OSCORE Master Salt", 8) 7.2. Transferring EDHOC over Other Protocols EDHOC may be transported over a different transport than CoAP. In this case the lower layers need to handle message loss, reordering, message duplication, fragmentation, and denial of service protection. 8. IANA Considerations 8.1. EDHOC Cipher Suites Registry IANA has created a new registry titled "EDHOC Cipher Suites". TODO 8.2. EDHOC Method Type Registry IANA has created a new registry titled "EDHOC Method Type". TODO 8.3. The Well-Known URI Registry IANA has added the well-known URI 'edhoc' to the Well-Known URIs registry. o URI suffix: edhoc o Change controller: IETF o Specification document(s): [[this document]] o Related information: None 8.4. Media Types Registry IANA has added the media type 'application/edhoc' to the Media Types registry. o Type name: application o Subtype name: edhoc o Required parameters: N/A o Optional parameters: N/A Selander, et al. Expires August 29, 2019 [Page 26] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 o Encoding considerations: binary o Security considerations: See Section 7 of this document. o Interoperability considerations: N/A o Published specification: [[this document]] (this document) o Applications that use this media type: To be identified o Fragment identifier considerations: N/A o Additional information: * Magic number(s): N/A * File extension(s): N/A * Macintosh file type code(s): N/A o Person & email address to contact for further information: See "Authors' Addresses" section. o Intended usage: COMMON o Restrictions on usage: N/A o Author: See "Authors' Addresses" section. o Change Controller: IESG 8.5. CoAP Content-Formats Registry IANA has added the media type 'application/edhoc' to the CoAP Content-Formats registry. o Media Type: application/edhoc o Encoding: o ID: TBD42 o Reference: [[this document]] Selander, et al. Expires August 29, 2019 [Page 27] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 9. Security Considerations 9.1. Security Properties EDHOC inherits its security properties from the theoretical SIGMA-I protocol [SIGMA]. Using the terminology from [SIGMA], EDHOC provides perfect forward secrecy, mutual authentication with aliveness, consistency, peer awareness, and identity protection. As described in [SIGMA], peer awareness is provided to Party V, but not to Party U. EDHOC with asymmetric authentication offers identity protection of Party U against active attacks and identity protection of Party V against passive attacks. The roles should be assigned to protect the most sensitive identity, 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, application data, and previous messages. This protects against an attacker replaying messages or injecting messages from another session. EDHOC also adds negotiation 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 Party V to verify that the selected cipher suite is the most preferred cipher suite by U which is supported by both U and V. As required by [RFC7258], IETF protocols need to mitigate pervasive monitoring when possible. One way to mitigate pervasive monitoring is to use a key exchange that provides perfect forward secrecy. EDHOC therefore only supports methods with perfect forward secrecy. 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. 9.2. Cryptographic Considerations The security of the SIGMA protocol requires the MAC to be bound to the identity of the signer. Hence the message authenticating functionality of the authenticated encryption in EDHOC is critical: authenticated encryption MUST NOT be replaced by plain encryption Selander, et al. Expires August 29, 2019 [Page 28] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 only, even if authentication is provided at another level or through a different mechanism. EDHOC implements SIGMA-I using the same Sign- then-MAC approach as TLS 1.3. 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 aliveness, and to protect against interleaving attacks. For this reason, the ephemeral keys MUST NOT be reused, and both parties SHALL generate fresh random ephemeral key pairs. 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. Party U and V should enforce a minimum security level. The data rates in many IoT deployments are very limited. Given that the application keys are protected as well as the long-term authentication keys they can often be used for years or even decades before the cryptographic limits are reached. If the application keys established through EDHOC need to be renewed, the communicating parties can derive application keys with other labels or run EDHOC again. 9.3. Mandatory to Implement Cipher Suite Cipher suite number 1 (AES-CCM-64-64-128, ECDH-SS + HKDF-256, X25519, Ed25519) is mandatory to implement. For many constrained IoT devices it is problematic to support more than one cipher suites, so some deployments with P-256 may not support the mandatory cipher suite. This is not a problem for local deployments. 9.4. Unprotected Data Party U and V 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 UAD_1, ID_CRED_V, UAD_2, and ERR_MSG in the asymmetric case, and KID, UAD_1, and ERR_MSG in the symmetric case. Using the same KID or UAD_1 in several EDHOC sessions allows passive eavesdroppers to correlate the different sessions. The communicating parties may therefore anonymize KID. Another consideration is that the list of supported cipher suites may be used to identify the application. Party U and V must also make sure that unauthenticated data does not trigger any harmful actions. In particular, this applies to UAD_1 Selander, et al. Expires August 29, 2019 [Page 29] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 and ERR_MSG in the asymmetric case, and KID, UAD_1, and ERR_MSG in the symmetric case. 9.5. 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 Party V 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. 9.6. Implementation Considerations The availability of a secure pseudorandom number generator and truly random seeds are 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. If ECDSA is supported, "deterministic ECDSA" as specified in [RFC6979] is RECOMMENDED. The referenced processing instructions in [SP-800-56A] must be complied with, including deleting the intermediate computed values along with any ephemeral ECDH secrets after the key derivation is completed. The ECDH shared secret, keys (K_2, K_3), and IVs (IV_2, IV_3) MUST be secret. Implementations should provide countermeasures to side-channel attacks such as timing attacks. Party U and V 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. Party U and V are allowed to select the connection identifiers C_U and C_V, respectively, for the other party to use in the ongoing EDHOC protocol as well as in a subsequent application protocol (e.g. OSCORE [I-D.ietf-core-object-security]). 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). Selander, et al. Expires August 29, 2019 [Page 30] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 9.7. Other Documents Referencing EDHOC EDHOC has been analyzed in several other documents. A formal verification of EDHOC was done in [SSR18], an analysis of EDHOC for certificate enrollment was done in [Kron18], the use of EDHOC in LoRaWAN is analyzed in [LoRa1] and [LoRa2], the use of EDHOC in IoT bootstrapping is analyzed in [Perez18], and the use of EDHOC in 6TiSCH is described in [I-D.ietf-6tisch-dtsecurity-zerotouch-join]. 10. References 10.1. Normative References [I-D.ietf-cbor-7049bis] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", draft-ietf-cbor-7049bis-05 (work in progress), January 2019. [I-D.ietf-cbor-cddl] Birkholz, H., Vigano, C., and C. Bormann, "Concise data definition language (CDDL): a notational convention to express CBOR and JSON data structures", draft-ietf-cbor- cddl-07 (work in progress), February 2019. [I-D.ietf-core-echo-request-tag] Amsuess, C., Mattsson, J., and G. Selander, "Echo and Request-Tag", draft-ietf-core-echo-request-tag-03 (work in progress), October 2018. [I-D.ietf-core-object-security] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", draft-ietf-core-object-security-15 (work in progress), August 2018. [I-D.schaad-cose-x509] Schaad, J., "CBOR Object Signing and Encryption (COSE): Headers for carrying and referencing X.509 certificates", draft-schaad-cose-x509-03 (work in progress), December 2018. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Selander, et al. Expires August 29, 2019 [Page 31] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, . [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010, . [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic Curve Cryptography Algorithms", RFC 6090, DOI 10.17487/RFC6090, February 2011, . [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2013, . [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, . [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, August 2016, . [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, July 2017, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [SIGMA] Krawczyk, H., "SIGMA - The 'SIGn-and-MAc' Approach to Authenticated Diffie-Hellman and Its Use in the IKE- Protocols (Long version)", June 2003, . [SP-800-56A] Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R. Davis, "Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography", NIST Special Publication 800-56A Revision 3, April 2018, . Selander, et al. Expires August 29, 2019 [Page 32] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 10.2. Informative References [CborMe] Bormann, C., "CBOR Playground", May 2018, . [I-D.hartke-core-e2e-security-reqs] Selander, G., Palombini, F., and K. Hartke, "Requirements for CoAP End-To-End Security", draft-hartke-core-e2e- security-reqs-03 (work in progress), July 2017. [I-D.ietf-6tisch-dtsecurity-zerotouch-join] Richardson, M., "6tisch Zero-Touch Secure Join protocol", draft-ietf-6tisch-dtsecurity-zerotouch-join-03 (work in progress), October 2018. [I-D.ietf-ace-oauth-authz] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "Authentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-21 (work in progress), February 2019. [I-D.ietf-ace-oscore-profile] Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson, "OSCORE profile of the Authentication and Authorization for Constrained Environments Framework", draft-ietf-ace- oscore-profile-07 (work in progress), February 2019. [I-D.ietf-core-resource-directory] Shelby, Z., Koster, M., Bormann, C., Stok, P., and C. Amsuess, "CoRE Resource Directory", draft-ietf-core- resource-directory-19 (work in progress), January 2019. [I-D.ietf-tls-dtls13] Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", draft-ietf-tls-dtls13-30 (work in progress), November 2018. [Kron18] Krontiris, A., "Evaluation of Certificate Enrollment over Application Layer Security", May 2018, . Selander, et al. Expires August 29, 2019 [Page 33] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 [LoRa1] Sanchez-Iborra, R., Sanchez-Gomez, J., Perez, S., Fernandez, P., Santa, J., Hernandez-Ramos, J., and A. Skarmeta, "Enhancing LoRaWAN Security through a Lightweight and Authenticated Key Management Approach", June 2018, . [LoRa2] Sanchez-Iborra, R., Sanchez-Gomez, J., Perez, S., Fernandez, P., Santa, J., Hernandez-Ramos, J., and A. Skarmeta, "Internet Access for LoRaWAN Devices Considering Security Issues", June 2018, . [Perez18] Perez, S., Garcia-Carrillo, D., Marin-Lopez, R., Hernandez-Ramos, J., Marin-Perez, R., and A. Skarmeta, "Architecture of security association establishment based on bootstrapping technologies for enabling critical IoT infrastructures", October 2018, . [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014, . [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [SSR18] Bruni, A., Sahl Joergensen, T., Groenbech Petersen, T., and C. Schuermann, "Formal Verification of Ephemeral Diffie-Hellman Over COSE (EDHOC)", November 2018, . Appendix A. Use of CBOR, CDDL and COSE in EDHOC This Appendix is intended to simplify for implementors not familiar with CBOR [I-D.ietf-cbor-7049bis], CDDL [I-D.ietf-cbor-cddl], COSE [RFC8152], and HKDF [RFC5869]. Selander, et al. Expires August 29, 2019 [Page 34] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 A.1. CBOR and CDDL The Concise Binary Object Representation (CBOR) [I-D.ietf-cbor-7049bis] 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) [I-D.ietf-cbor-cddl] provides a way to express structures for protocol messages and APIs that use CBOR. [I-D.ietf-cbor-cddl] 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 (e.g. null), byte strings (bstr), and text strings (tstr), CBOR also supports arrays [] of data items and maps {} of pairs of data items. Some examples are given below. For a complete specification and more examples, see [I-D.ietf-cbor-7049bis] and [I-D.ietf-cbor-cddl]. 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 null 0xf6 simple value h'12cd' 0x4212cd byte string '12cd' 0x4431326364 byte string "12cd" 0x6431326364 text string << 1, 2, null >> 0x430102f6 byte string [ 1, 2, null ] 0x830102f6 array [_ 1, 2, null ] 0x9f0102f6ff array (indefinite-length) ( 1, 2, null ) 0x0102f6 group { 4: h'cd' } 0xa10441cd map ------------------------------------------------------------------ All EDHOC messages consist of a sequence of CBOR encoded data items. While an EDHOC message in itself is not a CBOR data item, it may be viewed as the CBOR encoding of an indefinite-length array [_ message_i ] without the first byte (0x9f) and the last byte (0xff), for i = 1, 2 and 3. The same applies to the EDHOC error message. Selander, et al. Expires August 29, 2019 [Page 35] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 The message format specification uses the constructs '.cbor', '.cborseq' and '~' enabling conversion between different CDDL types matching different CBOR items with different encodings. Some examples are given below. A type (e.g. an uint) may be wrapped in a byte string (bstr), and back again: CDDL Type Diagnostic Encoded ------------------------------------------------------------------ uint 24 0x1818 bstr .cbor uint << 24 >> 0x421818 ~ bstr .cbor uint 24 0x1818 ------------------------------------------------------------------ An array, say of an uint and a byte string, may be converted into a byte string (bstr): CDDL Type Diagnostic Encoded -------------------------------------------------------------------- bstr h'cd' 0x41cd [ uint, bstr ] [ 24, h'cd' ] 0x82181841cd bstr .cborseq [ uint, bstr ] << 24, h'cd' >> 0x44181841cd -------------------------------------------------------------------- A.2. COSE CBOR Object Signing and Encryption (COSE) [RFC8152] 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, COSE_Sign1, and COSE_KDF_Context objects. A.2.1. Encryption and Decryption The COSE parameters used in COSE_Encrypt0 (see Section 5.2 of [RFC8152]) are constructed as described below. Note that "i" in "K_i", "IV_i" and "aad_i" is a variable with value i = 2 or 3, depending on whether the calculation is made over message_2 or message_3. o The secret key K_i is a CBOR bstr, generated with the EDHOC-Key- Derivation function as defined in Section 3.3. o The initialization vector IV_i is a CBOR bstr, also generated with the EDHOC-Key-Derivation function as defined in Section 3.3. Selander, et al. Expires August 29, 2019 [Page 36] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 o The plaintext is a CBOR bstr. If the application data (UAD and PAD) is omitted, then plaintext = h'' in the symmetric case, and plaintext = << ~protected, signature >> in the asymmetric case. For instance, if protected = h'a10140' and signature = h'050607' (CBOR encoding 0x43050607), then plaintext = h'a1014043050607'. o The external_aad is a CBOR bstr. It is always set to aad_i. COSE constructs the input to the AEAD [RFC5116] as follows: o The key K is the value of the key K_i. o The nonce N is the value of the initialization vector IV_i. o The plaintext P is the value of the COSE plaintext. E.g. if the COSE plaintext = h'010203', then P = 0x010203. o The associated data A is the CBOR encoding of: [ "Encrypt0", h'', aad_i ] This equals the concatenation of 0x8368456e63727970743040 and the CBOR encoding of aad_i. For instance if aad_2 = h'010203' (CBOR encoding 0x43010203), then A = 0x8368456e6372797074304043010203. A.2.2. Signing and Verification The COSE parameters used in COSE_Sign1 (see Section 4.2 of [RFC8152]) are constructed as described below. Note that "i" in "aad_i" is a variable with values i = 2 or 3, depending on whether the calculation is made over message_2 or message_3. Note also that "x" in "ID_CRED_x" and "CRED_x" is a variable with values x = U or V, depending on whether it is the credential of U or of V that is used in the relevant protocol message. o The key is the private authentication key of U or V. This may be stored as a COSE_KEY object or as a certificate. o The protected parameter is a map { abc : ID_CRED_x } wrapped in a byte string. o The payload is a bstr containing the CBOR encoding of a COSE_KEY or a single certificate. o external_aad = aad_i. COSE constructs the input to the Signature Algorithm as follows: Selander, et al. Expires August 29, 2019 [Page 37] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 o The key is the private authentication key of U or V. o The message to be signed M is the CBOR encoding of: [ "Signature1", << { abc : ID_CRED_x } >>, aad_i, CRED_x ] For instance, if abc = 4 (CBOR encoding 0x04), ID_CRED_U = h'1111' (CBOR encoding 0x421111), aad_3 = h'222222' (CBOR encoding 0x43222222), and CRED_U = h'55555555' (CBOR encoding 0x4455555555), then M = 0x846a5369676e61747572653145A104421111432222224455555555. A.2.3. Key Derivation Assuming use of the mandatory-to-implement algorithms HKDF SHA-256 and AES-CCM-16-64-128, the extract phase of HKDF produces a pseudorandom key (PRK) as follows: PRK = HMAC-SHA-256( salt, ECDH shared secret ) where salt = 0x in the asymmetric case and salt = PSK in the symmetric case. As the output length L is smaller than the hash function output size, the expand phase of HKDF consists of a single HMAC invocation, and K_i and IV_i are therefore the first 16 and 13 bytes, respectively, of output parameter = HMAC-SHA-256( PRK, info || 0x01 ) where || means byte string concatenation, and info is the CBOR encoding of COSE_KDF_Context = [ AlgorithmID, [ null, null, null ], [ null, null, null ], [ keyDataLength, h'', aad_i ] ] If AES-CCM-16-64-128 then AlgorithmID = 10 and keyDataLength = 128 for K_i, and AlgorithmID = "IV-GENERATION" (CBOR encoding 0x6d49562d47454e45524154494f4e) and keyDataLength = 104 for IV_i. Hence, if aad_2 = h'aaaa' then K_2 = HMAC-SHA-256( PRK, 0x840a83f6f6f683f6f6f68318804042aaaa01 ) IV_2 = HMAC-SHA-256( PRK, 0x846d49562d47454e45524154494f4e 83f6f6f683f6f6f68318804042aaaa01 ) Selander, et al. Expires August 29, 2019 [Page 38] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 Appendix B. Example Messages and Sizes This appendix gives an estimate of the message sizes of EDHOC with different authentication methods. It also gives examples of messages and plaintexts in CBOR diagnostic notation and hexadecimal to help implementors. Note that the examples in this appendix are not test vectors, the cryptographic parts are just replaced with byte strings of the same length. B.1. Message Sizes RPK B.1.1. message_1 message_1 = ( 1, h'c3', 0, 0, h'000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d 1e1f' ) message_1 (39 bytes): 01 41 C3 00 00 58 20 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F B.1.2. message_2 plaintext = << { 4 : 'acdc' }, h'000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d 1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b 3c3d3e3f' >> The protected header map is 7 bytes. The length of plaintext is 73 bytes so assuming a 64-bit MAC value the length of ciphertext is 81 bytes. message_2 = ( null, h'c4', h'000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d 1e1f', h'000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d 1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b 3c3d3e3f404142434445464748494a4b4c4d4e4f50' ) Selander, et al. Expires August 29, 2019 [Page 39] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 message_2 (120 bytes): F6 41 C4 58 20 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 58 51 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F 30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F 40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F 50 B.1.3. message_3 The plaintext and ciphertext in message_3 are assumed to be of equal sizes as in message_2. message_3 = ( h'c4', h'000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d 1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b 3c3d3e3f404142434445464748494a4b4c4d4e4f50' ) message_3 (85 bytes): 41 C4 58 51 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F 30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F 40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F 50 B.2. Message Sizes Certificates When the certificates are distributed out-of-band and identified with the x5t header and a SHA256/64 hash value, the protected header map will be 13 bytes instead of 7 bytes (assuming labels in the range -24...23). protected = << { TDB1 : [ TDB6, h'0001020304050607' ] } >> When the certificates are identified with the x5chain header, the message sizes depends on the size of the (truncated) certificate chains. The protected header map will be 3 bytes + the size of the certificate chain (assuming a label in the range -24...23). protected = << { TDB3 : h'0001020304050607...' } >> B.3. Message Sizes PSK Selander, et al. Expires August 29, 2019 [Page 40] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 B.3.1. message_1 message_1 = ( 2, h'c3', 0, 0, h'000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d 1e1f', 'abba' ) message_1 (44 bytes): 02 41 C3 00 00 58 20 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 44 61 63 64 63 B.3.2. message_2 Assuming a 0 byte plaintext and a 64-bit MAC value the ciphertext is 8 bytes message_2 = ( null, h'c4', h'000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d 1e1f', h'0001020304050607' ) message_2 (46 bytes): F6 41 C4 58 20 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 48 61 62 63 64 65 66 67 68 B.3.3. message_3 The plaintext and ciphertext in message_3 are assumed to be of equal sizes as in message_2. message_3 = ( h'c4', h'0001020304050607' ) message_3 (11 bytes): 41 C4 48 00 01 02 03 04 05 06 07 Selander, et al. Expires August 29, 2019 [Page 41] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 B.4. Summary The previous estimates of typical message sizes are summarized in Figure 9. ===================================================================== PSK RPK x5t x5chain --------------------------------------------------------------------- message_1 44 39 39 39 message_2 46 120 126 116 + Certificate chain message_3 11 85 91 81 + Certificate chain --------------------------------------------------------------------- Total 101 244 256 236 + Certificate chains ===================================================================== Figure 9: Typical message sizes in bytes In practice, most devices only have a few keys, so in deployments where assignment of key identifiers (KID, ID_CRED_V, ID_CRED_U) can be coordinated, the key identifiers can typically be much smaller (e.g. 1 byte). Figure 10 compares the message sizes of EDHOC with the DTLS 1.3 handshake [I-D.ietf-tls-dtls13] with connection ID. The comparison uses a minimum number of extensions and offered algorithms/cipher suites, 4 bytes key identifiers, 1 byte connection IDs, no DTLS message fragmentation, and DTLS RPK SubjectPublicKeyInfo with point compression. ===================================================================== Flight #1 #2 #3 Total --------------------------------------------------------------------- DTLS 1.3 RPK + ECDHE 150 373 213 736 DTLS 1.3 PSK + ECDHE 187 190 57 434 DTLS 1.3 PSK 137 150 57 344 --------------------------------------------------------------------- EDHOC RPK + ECDHE 39 120 85 244 EDHOC PSK + ECDHE 44 46 11 101 ===================================================================== Figure 10: Comparison of message sizes in bytes with Connection ID In reality the total overhead will be larger due to mechanisms for fragmentation, retransmission, and packet ordering. The overhead of fragmentation is roughly proportional to the number of fragments, while the expected overhead due to retransmission in noisy environments is a superlinear function of the flight sizes. Selander, et al. Expires August 29, 2019 [Page 42] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 Connection ID is not supported with TLS 1.3. Figure 11 compares the message sizes of EDHOC with the DTLS 1.3 [I-D.ietf-tls-dtls13] and TLS 1.3 [RFC8446] handshakes without connection ID. ===================================================================== Flight #1 #2 #3 Total --------------------------------------------------------------------- DTLS 1.3 RPK + ECDHE 144 364 212 722 DTLS 1.3 PSK + ECDHE 181 183 56 420 DTLS 1.3 PSK 131 143 56 330 --------------------------------------------------------------------- TLS 1.3 RPK + ECDHE 129 322 194 645 TLS 1.3 PSK + ECDHE 166 157 50 373 TLS 1.3 PSK 116 117 50 283 --------------------------------------------------------------------- EDHOC RPK + ECDHE 38 119 84 241 EDHOC PSK + ECDHE 44 45 10 98 ===================================================================== Figure 11: Comparison of message sizes in bytes without Connection ID Appendix C. Test Vectors This appendix provides a wealth of test vectors to ease implementation and ensure interoperability. TODO: This section needs to be updated. Acknowledgments The authors want to thank Alessandro Bruni, Theis Groenbech Petersen, Dan Harkins, Klaus Hartke, Alexandros Krontiris, Ilari Liusvaara, Karl Norrman, Salvador Perez, Michael Richardson, Thorvald Sahl Joergensen, Jim Schaad, Carsten Schuermann, Ludwig Seitz, Valery Smyslov, and Rene Struik for reviewing and commenting on intermediate versions of the draft. We are especially indebted to Jim Schaad for his continuous reviewing and implementation of different versions of the draft. Authors' Addresses Goeran Selander Ericsson AB Email: goran.selander@ericsson.com Selander, et al. Expires August 29, 2019 [Page 43] Internet-Draft Ephemeral Diffie-Hellman Over COSE (EDHOC) February 2019 John Mattsson Ericsson AB Email: john.mattsson@ericsson.com Francesca Palombini Ericsson AB Email: francesca.palombini@ericsson.com Selander, et al. Expires August 29, 2019 [Page 44]