| Internet-Draft | Ephemeral Diffie-Hellman Over COSE (EDHO | December 2020 |
| Selander, et al. | Expires 21 June 2021 | [Page] |
This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact, and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, perfect forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios and a main use case is to establish an OSCORE security context. By reusing COSE for cryptography, CBOR for encoding, and CoAP for transport, the additional code size can be kept very low.¶
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This Internet-Draft will expire on 21 June 2021.¶
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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Security at the application layer provides an attractive option for protecting Internet of Things (IoT) deployments, for example where protection needs to work over 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) [RFC8949]. Object Security for Constrained RESTful Environments (OSCORE) [RFC8613] 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]. The construction provided by EDHOC can be applied to authenticate raw public keys (RPK) and public key certificates. This version of the protocol is focusing on RPK and certificates by reference which is the initial focus for the LAKE WG (see Section 2.2 of [I-D.ietf-lake-reqs]).¶
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.¶
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. Compared to the DTLS 1.3 handshake [I-D.ietf-tls-dtls13] with ECDH 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 two examples of message sizes for EDHOC with different kinds of authentication keys and different COSE header parameters for identification: static Diffie-Hellman keys identified by 'kid' [RFC8152], and X.509 signature certificates identified by a hash value using 'x5t' [I-D.ietf-cose-x509]. Further reductions of message sizes are possible, for example by eliding redundant length indications.¶
=================================
kid x5t
---------------------------------
message_1 37 37
message_2 46 117
message_3 20 91
----------------------------------
Total 103 245
=================================
The ECDH exchange and the key derivation follow known protocol constructions such as [SIGMA], NIST SP-800-56A [SP-800-56A], and Extract-and-Expand [RFC5869]. CBOR [RFC8949] and COSE [RFC8152] are used to implement these standards. The use of COSE provides crypto agility and enables use of future algorithms and headers designed for constrained IoT.¶
This document is organized as follows: Section 2 describes how EDHOC authenticated with digital signatures builds on SIGMA-I, Section 3 specifies general properties of EDHOC, including message flow and formatting of the ephemeral public keys, Section 4 specifies the key derivation, Section 5 specifies EDHOC with signature key and static Diffie-Hellman 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.¶
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 deal 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 [RFC8613] 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 constrained 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 in constrained environments can be translated into key exchange overhead. In network technologies with duty cycle, each additional frame significantly increases the latency even if no other devices are transmitting.¶
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 impacted by a heavy key exchange protocol. A key exchange may need to be executed more than once, e.g. due to a device rebooting or for security reasons such as perfect forward secrecy.¶
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. EDHOC is not bound to a particular communication security protocol but works off-the-shelf with OSCORE [RFC8613] providing the necessary input parameters with required properties. Maximum code complexity (ROM/Flash) is often a constraint in many devices and by reusing already existing libraries, the additional code footprint for EDHOC + OSCORE can be kept very low.¶
EDHOC is designed as a lightweight AKE 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/device(s) connect(s) for the first time, or to establish fresh keys which are not compromised by a later compromise of the long-term keys. (Further security properties are described in Section 8.1.)¶
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 [RFC8152], 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 A.1.¶
EDHOC specifies different authentication methods of the Diffie-Hellman key exchange: digital signatures and static Diffie-Hellman keys. This section outlines the digital signature based method. Further details of protocol elements and other authentication methods are provided in the remainder of this document.¶
SIGMA (SIGn-and-MAc) is a family of theoretical protocols with a large number of variants [SIGMA]. Like IKEv2 [RFC7296] and (D)TLS 1.3 [RFC8446], EDHOC authenticated with digital signatures is built on a variant of the SIGMA protocol which provide identity protection of the initiator (SIGMA-I), and like IKEv2 [RFC7296], EDHOC implements the SIGMA-I variant as MAC-then-Sign. The SIGMA-I protocol using an authenticated encryption algorithm is shown in Figure 2.¶
Initiator Responder | G_X | +-------------------------------------------------------->| | | | G_Y, AEAD( K_2; ID_CRED_R, Sig(R; CRED_R, G_X, G_Y) ) | |<--------------------------------------------------------+ | | | AEAD( K_3; ID_CRED_I, Sig(I; CRED_I, G_Y, G_X) ) | +-------------------------------------------------------->| | |
The parties exchanging messages are called Initiator (I) and Responder (R). They exchange ephemeral public keys, compute the shared secret, and derive symmetric application keys.¶
In order to create a "full-fledged" protocol some additional protocol elements are needed. EDHOC adds:¶
Verification of a common preferred cipher suite:¶
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 test vectors including CBOR diagnostic notation are given in Appendix B.¶
EDHOC consists of three messages (message_1, message_2, message_3) between Initiator and Responder, plus an EDHOC error message. EDHOC messages are CBOR Sequences [RFC8742], see Figure 3. The protocol elements in the figure are introduced in the following sections. Message formatting and processing is specified in Section 5 and Section 6. An implementation may support only Initiator or only Responder.¶
Application data is protected using the agreed application algorithms (AEAD, hash) in the selected cipher suite (see Section 3.4) and the application can make use of the established connection identifiers C_I and C_R (see Section 3.2.4). 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 4.1. Application protected data can therefore be sent in parallel with EDHOC message_3, optionally in the same CoAP message [I-D.palombini-core-oscore-edhoc].¶
Initiator Responder | METHOD_CORR, SUITES_I, G_X, C_I, AD_1 | +------------------------------------------------------------------>| | message_1 | | | | C_I, G_Y, C_R, Enc(K_2e; ID_CRED_R, Signature_or_MAC_2, AD_2) | |<------------------------------------------------------------------+ | message_2 | | | | C_R, AEAD(K_3ae; ID_CRED_I, Signature_or_MAC_3, AD_3) | +------------------------------------------------------------------>| | message_3 |
The first data item of message_1, METHOD_CORR (see Section 5.2.1), is an integer specifying the method and the correlation properties of the transport, which are described in this section.¶
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. (Method 0 corresponds to the case outlined in Section 2 where both Initiator and Responder authenticate with signature keys.)¶
An implementation may support only a single method. The Initiator and the Responder need to have agreed on a single method to be used for EDHOC, see Appendix C.¶
+-------+-------------------+-------------------+-------------------+ | 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]] | +-------+-------------------+-------------------+-------------------+
EDHOC includes connection identifiers (C_I, C_R) to correlate messages. The connection identifiers C_I and C_R do not have any cryptographic purpose in EDHOC. They contain information facilitating retrieval of the protocol state and may therefore be very short. One byte connection identifiers are realistic in many scenarios as most constrained devices only have a few connections. In cases where a node only has one connection, the identifiers may even be the empty byte string.¶
The connection identifier MAY be used with an application protocol (e.g. OSCORE) for which EDHOC establishes keys, in which case the connection identifiers SHALL adhere to the requirements for that protocol. Each party choses a connection identifier it desires the other party to use in outgoing messages. (For OSCORE this results in the endpoint selecting its Recipient ID, see Section 3.1 of [RFC8613]).¶
Cryptographically, EDHOC does not put requirements on the lower layers. EDHOC is not bound to a particular transport layer, and can be used in environments without IP. The transport is responsible to handle message loss, reordering, message duplication, fragmentation, and denial of service protection, where necessary.¶
The Initiator and the Responder need to have agreed on a transport to be used for EDHOC, see Appendix C. It is recommended to transport EDHOC in CoAP payloads, see Section 7.¶
If the transport provides a mechanism for correlating messages, some of the connection identifiers may be omitted. There are four cases:¶
For example, if the key exchange is transported over CoAP, the CoAP Token can be used to correlate messages, see Section 7.1.¶
The authentication key MUST be a signature key or static Diffie-Hellman key. The Initiator and the Responder MAY use different types of authentication keys, e.g. one uses a signature key and the other uses a static Diffie-Hellman key. When using a signature key, the authentication is provided by a signature. 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 MAC is implemented with an AEAD algorithm. When using a static Diffie-Hellman keys the Initiator's and Responder's private authentication keys are called I and R, respectively, and the public authentication keys are called G_I and G_R, respectively.¶
EDHOC assumes the existence of mechanisms (certification authority, trusted third party, manual distribution, etc.) for specifying and distributing authentication keys and identities. Policies are set based on the identity of the other party, and parties typically only allow connections from a specific identity or a small restricted set of identities. For example, in the case of a device connecting to a network, the network may only allow connections from devices which authenticate with certificates having a particular range of serial numbers in the subject field and signed by a particular CA. On the other side, the device may only be allowed to connect to a network which authenticate with a particular public key (information of which may be provisioned, e.g., out of band or in the Auxiliary Data, see Section 3.6).¶
The EDHOC implementation must be able to receive and enforce information from the application about what is the intended peer endpoint, and in particular whether it is a specific identity or a set of identities.¶
The authentication credentials, CRED_I and CRED_R, contain the public authentication key of the Initiator and the Responder, respectively. The Initiator and the Responder MAY use different types of credentials, e.g. one uses an RPK and the other uses a public key certificate.¶
The credentials CRED_I and CRED_R are signed or MAC:ed (depending on method) by the Initiator and the Responder, respectively, see Section 5.4 and Section 5.3.¶
When the credential is a certificate, CRED_x is an end-entity certificate (i.e. not the certificate chain) encoded as a CBOR bstr. In X.509 certificates, signature keys typically have key usage "digitalSignature" and Diffie-Hellman keys typically have key usage "keyAgreement"¶
When the credential is a COSE_Key, CRED_x is a CBOR map only containing specific fields from the COSE_Key:¶
To prevent misbinding attacks in systems where an attacker can register public keys without proving knowledge of the private key, SIGMA [SIGMA] enforces a MAC to be calculated over the "Identity", which in case of a X.509 certificate would be the 'subject' and 'subjectAltName' fields. EDHOC follows SIGMA by calculating a MAC over the whole certificate. While SIGMA paper only focuses on the identity, the same principle is true for any information such as policies connected to the public key.¶
If the parties have agreed on an identity besides the public key, the identity is included in the CBOR map with the label "subject name", otherwise the subject name is the empty text string. The parameters SHALL be encoded in decreasing order with int labels first and text string labels last. An example of CRED_x when the RPK contains an X25519 static Diffie-Hellman key and the parties have agreed on an EUI-64 identity is shown below:¶
CRED_x = {
1: 1,
-1: 4,
-2: h'b1a3e89460e88d3a8d54211dc95f0b90
3ff205eb71912d6db8f4af980d2db83a',
"subject name" : "42-50-31-FF-EF-37-32-39"
}
¶
ID_CRED_I and ID_CRED_R are identifiers of the public authentication keys of the Initiator and the Responder, respectively. ID_CRED_I and ID_CRED_R do not have any cryptographic purpose in EDHOC.¶
The identifiers ID_CRED_I and ID_CRED_R are COSE header_maps, i.e. CBOR maps containing COSE Common Header Parameters, see Section 3.1 of [RFC8152]). In the following we give some examples of COSE header_maps.¶
Raw public keys are most optimally stored as COSE_Key objects and identified with a 'kid' parameter:¶
Public key certificates can be identified in different ways. Header parameters for identifying X.509 certificates are defined in [I-D.ietf-cose-x509], for example:¶
by a hash value with the 'x5t' parameter;¶
by a URL with the 'x5u' parameter;¶
ID_CRED_x MAY contain the actual credential used for authentication, CRED_x. It is RECOMMENDED that they uniquely identify the public authentication key as the recipient may otherwise have to try several keys. ID_CRED_I and ID_CRED_R are transported in the ciphertext, see Section 5.4 and Section 5.3.¶
When ID_CRED_x does not contain the actual credential it may be very short. One byte credential identifiers are realistic in many scenarios as most constrained devices only have a few keys. In cases where a node only has one key, the identifier may even be the empty byte string.¶
An EDHOC cipher suite consists of an ordered set of COSE code points from the "COSE Algorithms" and "COSE Elliptic Curves" registries:¶
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 9.1) or use any combination of COSE algorithms 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 cipher suites are for constrained IoT where message overhead is a very important factor:¶
0. ( 10, -16, 4, -8, 6, 10, -16 )
(AES-CCM-16-64-128, SHA-256, X25519, EdDSA, Ed25519,
AES-CCM-16-64-128, SHA-256)
1. ( 30, -16, 4, -8, 6, 10, -16 )
(AES-CCM-16-128-128, SHA-256, X25519, EdDSA, Ed25519,
AES-CCM-16-64-128, SHA-256)
2. ( 10, -16, 1, -7, 1, 10, -16 )
(AES-CCM-16-64-128, SHA-256, P-256, ES256, P-256,
AES-CCM-16-64-128, SHA-256)
3. ( 30, -16, 1, -7, 1, 10, -16 )
(AES-CCM-16-128-128, SHA-256, P-256, ES256, P-256,
AES-CCM-16-64-128, SHA-256)
¶
The following cipher suite is for general non-constrained applications. It uses very high performance algorithms that also are widely supported:¶
4. ( 1, -16, 4, -7, 1, 1, -16 )
(A128GCM, SHA-256, X25519, ES256, P-256,
A128GCM, SHA-256)
¶
The following cipher suite is for high security application such as government use and financial applications. It is compatible with the CNSA suite [CNSA].¶
5. ( 3, -43, 2, -35, 2, 3, -43 )
(A256GCM, SHA-384, P-384, ES384, P-384,
A256GCM, SHA-384)
¶
The different methods use the same cipher suites, but some algorithms are not used in some methods. The EDHOC signature algorithm and the EDHOC signature algorithm curve are 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 is a CBOR array containing cipher suites that the Initiator supports. SUITES_I is formatted and processed as detailed in Section 5.2.1 to secure the cipher suite negotation.¶
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 the 'x' parameter is included G_X and G_Y. 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' parameter, 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.¶
In order to reduce round trips and number of messages, and in some cases also streamline processing, certain security applications may be integrated into EDHOC by transporting auxiliary data together with the messages. One example is the transport of third-party authorization information protected outside of EDHOC [I-D.selander-ace-ake-authz]. Another example is the embedding of a certificate enrolment request or a newly issued certificate.¶
EDHOC allows opaque auxiliary data (AD) to be sent in the EDHOC messages. Unprotected Auxiliary Data (AD_1, AD_2) may be sent in message_1 and message_2, respectively. Protected Auxiliary Data (AD_3) may be sent in message_3.¶
Since data carried in AD_1 and AD_2 may not be protected, and the content of AD_3 is available to both the Initiator and the Responder, special considerations need to be made such that the availability of the data a) does not violate security and privacy requirements of the service which uses this data, and b) does not violate the security properties of EDHOC.¶
EDHOC allows the communication or negotiation of various protocol features during the execution of the protocol.¶
Editor's note: This section needs to be aligned with Appendix C.¶
EDHOC uses Extract-and-Expand [RFC5869] with the EDHOC hash algorithm in the selected cipher suite to derive keys. Extract is used to derive fixed-length uniformly pseudorandom keys (PRK) from ECDH shared secrets. Expand is used to derive additional output keying material (OKM) from the PRKs. The PRKs are derived using Extract.¶
PRK = Extract( salt, IKM )¶
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, "" ).¶
PRK_2e is used to derive key and IV to encrypt message_2. PRK_3e2m is used to derive keys and IVs produce a MAC in message_2 and to encrypt message_3. PRK_4x3m is used to derive keys and IVs produce a MAC in message_3 and to derive application specific data.¶
PRK_2e is derived with the following input:¶
Example: Assuming the use of SHA-256 the extract phase of HKDF produces PRK_2e as follows:¶
PRK_2e = HMAC-SHA-256( salt, G_XY )¶
where salt = 0x (the empty byte string).¶
The pseudorandom keys PRK_3e2m and PRK_4x3m are defined as follow:¶
Example: Assuming the use of curve25519, the ECDH shared secrets G_XY, G_RX, and G_IY are the outputs of the X25519 function [RFC7748]:¶
G_XY = X25519( Y, G_X ) = X25519( X, G_Y )¶
The keys and IVs used in EDHOC are derived from PRK using Expand [RFC5869] where the EDHOC-KDF is instantiated with the EDHOC AEAD algorithm in the selected cipher suite.¶
OKM = EDHOC-KDF( PRK, transcript_hash, label, length )
= Expand( PRK, info, length )
¶
where info is the CBOR encoding of¶
info = [ edhoc_aead_id : int / tstr, transcript_hash : bstr, label : tstr, length : uint ]¶
where¶
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, "" ).¶
K_2e and IV_2e are derived using the transcript hash TH_2 and the pseudorandom key PRK_2e. K_2m and IV_2m are derived using the transcript hash TH_2 and the pseudorandom key PRK_3e2m. K_3ae and IV_3ae are derived using the transcript hash TH_3 and the pseudorandom key PRK_3e2m. K_3m and IV_3m are derived using the transcript hash TH_3 and the pseudorandom key PRK_4x3m. IVs are only used if the EDHOC AEAD algorithm uses IVs.¶
Application keys and other application specific data can be derived using the EDHOC-Exporter interface defined as:¶
EDHOC-Exporter(label, length)
= EDHOC-KDF(PRK_4x3m, TH_4, label, 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. 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. Example use of the EDHOC-Exporter is given in Sections 7.1.1.¶
To provide forward secrecy in an even more efficient way than re-running EDHOC, EDHOC provides the function EDHOC-Exporter-FS. When EHDOC-Exporter-FS 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:¶
EHDOC-Exporter-FS( nonce ):
PRK_4x3m = Extract( [ "TH_4", nonce ], PRK_4x3m )
¶
This section specifies formatting of the messages and processing steps. Error messages are specified in Section 6.¶
An EDHOC message is encoded as a sequence of CBOR data (CBOR Sequence, [RFC8742]). Additional optimizations are made to reduce message overhead.¶
While EDHOC uses the COSE_Key, COSE_Sign1, and COSE_Encrypt0 structures, only a subset of the parameters is included in the EDHOC messages. The unprotected COSE header in COSE_Sign1, and COSE_Encrypt0 (not included in the EDHOC message) MAY contain parameters (e.g. 'alg').¶
Byte strings are encoded in CBOR as two or more bytes, whereas integers in the interval -24 to 23 are encoded in CBOR as one byte.¶
bstr_identifier is a special encoding of byte strings, used throughout the protocol to enable the encoding of the shortest byte strings as integers that only require one byte of CBOR encoding.¶
The bstr_identifier encoding is defined as follows: Byte strings in the interval h'00' to h'2f' are encoded as the corresponding integer minus 24, which are all represented by one byte CBOR ints. Other byte strings are encoded as CBOR byte strings.¶
For example, the byte string h'59e9' encoded as a bstr_identifier is equal to h'59e9', while the byte string h'2a' is encoded as the integer 18.¶
The CDDL definition of the bstr_identifier is given below:¶
bstr_identifier = bstr / int¶
Note that, despite what could be interpreted by the CDDL definition only, bstr_identifier once decoded are always byte strings.¶
message_1 SHALL be a CBOR Sequence (see Appendix A.1) as defined below¶
message_1 = ( METHOD_CORR : int, SUITES_I : [ selected : suite, supported : 2* suite ] / suite, G_X : bstr, C_I : bstr_identifier, ? AD_1 : bstr, ) suite = int¶
where:¶
The Initiator SHALL compose message_1 as follows:¶
The Responder SHALL process message_1 as follows:¶
If any verification step fails, the Responder MUST send an EDHOC error message back, formatted as defined in Section 6, and the protocol MUST be discontinued. If the Responder does not support the selected cipher suite, then SUITES_R MUST include one or more supported cipher suites. If the Responder does not support the selected cipher suite, but supports another cipher suite in SUITES_I, then SUITES_R MUST include the first supported cipher suite in SUITES_I.¶
message_2 and data_2 SHALL be CBOR Sequences (see Appendix A.1) as defined below¶
message_2 = ( data_2, CIPHERTEXT_2 : bstr, )¶
data_2 = ( ? C_I : bstr_identifier, G_Y : bstr, C_R : bstr_identifier, )¶
where:¶
The Responder SHALL compose message_2 as follows:¶
Compute an inner COSE_Encrypt0 as defined in Section 5.3 of [RFC8152], with the EDHOC AEAD algorithm in the selected cipher suite, K_2m, IV_2m, and the following parameters:¶
protected = << ID_CRED_R >>¶
external_aad = << TH_2, CRED_R, ? AD_2 >>¶
COSE constructs the input to the AEAD [RFC5116] as follows:¶
Associated data A =¶
[ "Encrypt0", << ID_CRED_R >>, << TH_2, CRED_R, ? AD_2 >> ]¶
MAC_2 is the 'ciphertext' of the inner COSE_Encrypt0.¶
If the Responder authenticates with a static Diffie-Hellman key (method equals 1 or 3), then Signature_or_MAC_2 is MAC_2. If the Responder authenticates with a signature key (method equals 0 or 2), then Signature_or_MAC_2 is the 'signature' of a COSE_Sign1 object as defined in Section 4.4 of [RFC8152] using the signature algorithm in the selected cipher suite, the private authentication key of the Responder, and the following parameters:¶
COSE constructs the input to the Signature Algorithm as:¶
Compute an outer COSE_Encrypt0 as defined in Section 5.3 of [RFC8152], with the EDHOC AEAD algorithm in the selected cipher suite, K_2e, IV_2e, and the following parameters. The protected header SHALL be empty.¶
plaintext = ( ID_CRED_R / bstr_identifier, Signature_or_MAC_2, ? AD_2 )¶
COSE constructs the input to the AEAD [RFC5116] as follows:¶
CIPHERTEXT_2 is the 'ciphertext' of the outer COSE_Encrypt0 with the tag removed.¶
The Initiator SHALL process message_2 as follows:¶
If any verification step fails, the Responder MUST send an EDHOC error message back, formatted as defined in Section 6, and the protocol MUST be discontinued.¶
message_3 and data_3 SHALL be CBOR Sequences (see Appendix A.1) as defined below¶
message_3 = ( data_3, CIPHERTEXT_3 : bstr, )¶
data_3 = ( ? C_R : bstr_identifier, )¶
The Initiator SHALL compose message_3 as follows:¶
Compute an inner COSE_Encrypt0 as defined in Section 5.3 of [RFC8152], with the EDHOC AEAD algorithm in the selected cipher suite, K_3m, IV_3m, and the following parameters:¶
protected = << ID_CRED_I >>¶
external_aad = << TH_3, CRED_I, ? AD_3 >>¶
COSE constructs the input to the AEAD [RFC5116] as follows:¶
Associated data A =¶
[ "Encrypt0", << ID_CRED_I >>, << TH_3, CRED_I, ? AD_3 >> ]¶
MAC_3 is the 'ciphertext' of the inner COSE_Encrypt0.¶
If the Initiator authenticates with a static Diffie-Hellman key (method equals 2 or 3), then Signature_or_MAC_3 is MAC_3. If the Initiator authenticates with a signature key (method equals 0 or 1), then Signature_or_MAC_3 is the 'signature' of a COSE_Sign1 object as defined in Section 4.4 of [RFC8152] using the signature algorithm in the selected cipher suite, the private authentication key of the Initiator, and the following parameters:¶
COSE constructs the input to the Signature Algorithm as:¶
Compute an outer COSE_Encrypt0 as defined in Section 5.3 of [RFC8152], with the EDHOC AEAD algorithm in the selected cipher suite, K_3ae, IV_3ae, and the following parameters. The protected header SHALL be empty.¶
plaintext = ( ID_CRED_I / bstr_identifier, Signature_or_MAC_3, ? AD_3 )¶
COSE constructs the input to the AEAD [RFC5116] as follows:¶
CIPHERTEXT_3 is the 'ciphertext' of the outer COSE_Encrypt0.¶
Pass the connection identifiers (C_I, C_R) and the application algorithms in the selected cipher suite to the application. The application can now derive application keys using the EDHOC-Exporter interface.¶
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 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 from the Responder.¶
The Responder SHALL process message_3 as follows:¶
If any verification step fails, the Responder MUST send an EDHOC error message back, formatted as defined in Section 6, and the protocol MUST be discontinued.¶
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.¶
This section defines a message format for the EDHOC error message, used during the protocol. An EDHOC error message can be sent by both parties as a reply 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).¶
error SHALL be a CBOR Sequence (see Appendix A.1) as defined below¶
error = ( ? C_x : bstr_identifier, ERR_MSG : tstr, ? SUITES_R : [ supported : 2* suite ] / suite, )¶
where:¶
After receiving SUITES_R, the Initiator can determine which selected cipher suite to use for the next EDHOC run with the Responder. If the Initiator intends to contact the Responder in the future, the Initiator SHOULD remember which selected cipher suite to use until the next message_1 has been sent, otherwise the Initiator and Responder will likely run into an infinite loop. After a successful run of EDHOC, the Initiator MAY remember the selected cipher suite to use in future EDHOC runs. Note that if the Initiator or Responder is updated with new cipher suite policies, any cached information may be outdated.¶
Assuming that the Initiator supports the five cipher suites 5, 6, 7, 8, and 9 in decreasing order of preference, Figures 5 and 6 show examples of how the Initiator can truncate SUITES_I and how SUITES_R is used by the Responder to give the Initiator information about the cipher suites that the Responder supports.¶
In Figure 5, the Responder supports cipher suite 6 but not the initially selected cipher suite 5.¶
Initiator Responder | METHOD_CORR, SUITES_I = 5, G_X, C_I, AD_1 | +------------------------------------------------------------------>| | message_1 | | | | C_I, ERR_MSG, SUITES_R = 6 | |<------------------------------------------------------------------+ | error | | | | METHOD_CORR, SUITES_I = [6, 5, 6], G_X, C_I, AD_1 | +------------------------------------------------------------------>| | message_1 |
In Figure 6, the Responder supports cipher suite 7 and 9 but not the more preferred (by the Initiator) cipher suites 5 and 6. The order of cipher suites in SUITES_R does not matter.¶
Initiator Responder | METHOD_CORR, SUITES_I = 5, G_X, C_I, AD_1 | +------------------------------------------------------------------>| | message_1 | | | | C_I, ERR_MSG, SUITES_R = [9, 7] | |<------------------------------------------------------------------+ | error | | | | METHOD_CORR, SUITES_I = [7, 5, 6, 7], G_X, C_I, AD_1 | +------------------------------------------------------------------>| | message_1 |
Note that the Initiator's list of supported cipher suites and order of preference is fixed (see Section 5.2.1 and Section 5.2.2). Furthermore, the Responder shall only accept message_1 if the selected cipher suite is the first cipher suite in SUITES_I that the Responder supports (see Section 5.2.3). Following this procedure ensures that the selected cipher suite is the most preferred (by the Initiator) cipher suite supported by both parties.¶
If the selected cipher suite is not the first cipher suite which the Responder supports in SUITES_I received in message_1, then Responder MUST discontinue the protocol, see Section 5.2.3. If SUITES_I in message_1 is manipulated then the integrity verification of message_2 containing the transcript hash TH_2 = H( message_1, data_2 ) will fail and the Initiator will discontinue the protocol.¶
It is recommended 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 messages in Confirmable messages, especially if fragmentation is used.¶
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. 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. 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. In this case the CoAP Token enables the Initiator to correlate message_1 and message_2 so the correlation parameter corr = 1.¶
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 | | |
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. In this case the CoAP Token enables the Responder to correlate message_2 and message_3 so the correlation parameter corr = 2.¶
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 | |
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].¶
When EDHOC is used to derive parameters for OSCORE [RFC8613], the parties make sure that the EDHOC connection identifiers are unique, i.e. C_R MUST NOT be equal to C_I. The CoAP client and server MUST be able to retrieve the OSCORE protocol state using its chosen connection identifier and optionally other information such as the 5-tuple. In case that the CoAP client is the Initiator and the CoAP server is the Responder:¶
Master Secret = EDHOC-Exporter( "OSCORE Master Secret", length ) Master Salt = EDHOC-Exporter( "OSCORE Master Salt", 8 )¶
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. 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, auxiliary 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 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. 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.¶
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.¶
If supported by the device, it is RECOMMENDED that at least the long-term private keys is stored in a Trusted Execution Environment (TEE) and that sensitive operations using these keys are performed inside the TEE. To achieve even higher security additional operation such as ephemeral key generation, all computations of shared secrets, and storage of the PRK keys can be done inside the TEE. Optimally, the whole EDHOC protocol can be implemented inside the TEE. Typically an adversary with physical access to a device can be assumed to gain access to all information outside of the TEE, but none of the information inside the TEE.¶
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 protocol run.¶
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.¶
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 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 liveness, 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. The Initiator and the Responder 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.¶
Cipher suite number 0 (AES-CCM-16-64-128, SHA-256, X25519, EdDSA, Ed25519, AES-CCM-16-64-128, SHA-256) is mandatory to implement. Implementations only need to implement the algorithms needed for their supported methods. 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.¶
The HMAC algorithm HMAC 256/64 (HMAC w/ SHA-256 truncated to 64 bits) SHALL NOT be supported for use in EDHOC.¶
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 AD_1, ID_CRED_R, AD_2, and ERR_MSG. Using the same AD_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 AD_1 and ERR_MSG.¶
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.¶
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. As each pseudorandom number must only be used once, an implementation need 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. 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, and IVs MUST be secret. Implementations should provide countermeasures to side-channel attacks such as timing attacks. Depending on the selected curve, the parties should perform various validations of each other's public keys, see e.g. Section 5 of [SP-800-56A].¶
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.¶
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).¶
The Responder MUST finish the verification step of message_3 before passing AD_3 to the application.¶
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.¶
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].¶
IANA has created a new registry titled "EDHOC Cipher Suites" under the new heading "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, 5, 4, -8, 6, 10, 5
Desc: AES-CCM-16-64-128, SHA-256, X25519, EdDSA, Ed25519,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
¶
Value: 1
Array: 30, 5, 4, -8, 6, 10, 5
Desc: AES-CCM-16-128-128, SHA-256, X25519, EdDSA, Ed25519,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
¶
Value: 2
Array: 10, 5, 1, -7, 1, 10, 5
Desc: AES-CCM-16-64-128, SHA-256, P-256, ES256, P-256,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
¶
Value: 3
Array: 30, 5, 1, -7, 1, 10, 5
Desc: AES-CCM-16-128-128, SHA-256, P-256, ES256, P-256,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
¶
Value: 4
Array: 1, -16, 4, -7, 1, 1, -16
Desc: A128GCM, SHA-256, X25519, ES256, P-256,
A128GCM, SHA-256
Reference: [[this document]]
¶
Value: 5
Array: 3, -43, 2, -35, 2, 3, -43
Desc: A256GCM, SHA-384, P-384, ES384, P-384,
A256GCM, SHA-384
Reference: [[this document]]
¶
IANA has created a new registry titled "EDHOC Method Type" under the new heading "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 is shown in Figure 4.¶
IANA has added the well-known URI 'edhoc' to the Well-Known URIs registry.¶
IANA has added the media type 'application/edhoc' to the Media Types registry.¶
Additional information:¶
IANA has added the media type 'application/edhoc' to the CoAP Content-Formats registry.¶
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:¶
This Appendix is intended to simplify for implementors not familiar with CBOR [RFC8949], CDDL [RFC8610], COSE [RFC8152], and HKDF [RFC5869].¶
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 (e.g. null), 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. 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
null 0xf6 simple value
h'12cd' 0x4212cd byte string
'12cd' 0x4431326364 byte string
"12cd" 0x6431326364 text string
{ 4 : h'cd' } 0xa10441cd map
<< 1, 2, null >> 0x430102f6 byte string
[ 1, 2, null ] 0x830102f6 array
( 1, 2, null ) 0x0102f6 sequence
1, 2, null 0x0102f6 sequence
------------------------------------------------------------------
¶
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, and COSE_Sign1 objects.¶
This appendix provides detailed test vectors to ease implementation and ensure interoperability. In addition to hexadecimal, all CBOR data items and sequences are given in CBOR diagnostic notation. The test vectors use the default mapping to CoAP where the Initiator acts as CoAP client (this means that corr = 1).¶
A more extensive test vector suite covering more combinations of authentication method used between Initiator and Responder and related code to generate them can be found at https://github.com/lake-wg/edhoc/tree/master/test-vectors .¶
EDHOC with signature authentication and X.509 certificates is used. In this test vector, the hash value 'x5t' is used to identify the certificate.¶
method (Signature Authentication) 0¶
CoAP is used as transport and the Initiator acts as CoAP client:¶
corr (the Initiator can correlate message_1 and message_2) 1¶
From there, METHOD_CORR has the following value:¶
METHOD_CORR (4 * method + corr) (int) 1¶
No unprotected opaque auxiliary data is sent in the message exchanges.¶
The list of supported cipher suites of the Initiator in order of preference is the following:¶
Supported Cipher Suites (4 bytes) 00 01 02 03¶
The cipher suite selected by the Initiator is the most preferred:¶
Selected Cipher Suite (int) 0¶
The mandatory-to-implement cipher suite 0 is supported by both the Initiator and the Responder, see Section 8.3.¶
X (Initiator's ephemeral private key) (32 bytes) 8f 78 1a 09 53 72 f8 5b 6d 9f 61 09 ae 42 26 11 73 4d 7d bf a0 06 9a 2d f2 93 5b b2 e0 53 bf 35¶
G_X (Initiator's ephemeral public key) (32 bytes) 89 8f f7 9a 02 06 7a 16 ea 1e cc b9 0f a5 22 46 f5 aa 4d d6 ec 07 6b ba 02 59 d9 04 b7 ec 8b 0c¶
The Initiator chooses a connection identifier C_I:¶
Connection identifier chosen by Initiator (0 bytes)¶
Since no unprotected opaque auxiliary data is sent in the message exchanges:¶
AD_1 (0 bytes)¶
Since the list of supported cipher suites needs to contain the selected cipher suite, the initiator truncates the list of supported cipher suites to one cipher suite only, 00.¶
Because one single selected cipher suite is conveyed, it is encoded as an int instead of an array:¶
SUITES_I (int) 0¶
With SUITES_I = 0, message_1 is constructed, as the CBOR Sequence of the CBOR data items above.¶
message_1 = ( 1, 0, h'898ff79a02067a16ea1eccb90fa52246f5aa4dd6ec076bba0259d904b7ec8b0c', h'' )¶
message_1 (CBOR Sequence) (37 bytes) 01 00 58 20 89 8f f7 9a 02 06 7a 16 ea 1e cc b9 0f a5 22 46 f5 aa 4d d6 ec 07 6b ba 02 59 d9 04 b7 ec 8b 0c 40¶
Since METHOD_CORR mod 4 equals 1, C_I is omitted from data_2.¶
Y (Responder's ephemeral private key) (32 bytes) fd 8c d8 77 c9 ea 38 6e 6a f3 4f f7 e6 06 c4 b6 4c a8 31 c8 ba 33 13 4f d4 cd 71 67 ca ba ec da¶
G_Y (Responder's ephemeral public key) (32 bytes) 71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0 19 52 81 75 4c 5e bc af 30 1e¶
From G_X and Y or from G_Y and X the ECDH shared secret is computed:¶
G_XY (ECDH shared secret) (32 bytes) 2b b7 fa 6e 13 5b c3 35 d0 22 d6 34 cb fb 14 b3 f5 82 f3 e2 e3 af b2 b3 15 04 91 49 5c 61 78 2b¶
The key and nonce for calculating the ciphertext are calculated as follows, as specified in Section 4.¶
HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).¶
PRK_2e = HMAC-SHA-256(salt, G_XY)¶
Salt is the empty byte string.¶
salt (0 bytes)¶
From there, PRK_2e is computed:¶
PRK_2e (32 bytes) ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f d8 2f be b7 99 71 39 4a¶
SK_R (Responders's private authentication key) (32 bytes) df 69 27 4d 71 32 96 e2 46 30 63 65 37 2b 46 83 ce d5 38 1b fc ad cd 44 0a 24 c3 91 d2 fe db 94¶
Since neither the Initiator nor the Responder authenticates with a static Diffie-Hellman key, PRK_3e2m = PRK_2e¶
PRK_3e2m (32 bytes) ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f d8 2f be b7 99 71 39 4a¶
The Responder chooses a connection identifier C_R.¶
Connection identifier chosen by Responder (1 byte) 2b¶
Note that since C_R is a byte string of length one, it is encoded as the corresponding integer subtracted by 24 (see bstr_identifier in Section 5.1). Thus 0x2b = 43, 43 - 24 = 19, and 19 in CBOR encoding is equal to 0x13.¶
C_R (1 byte) 13¶
Data_2 is constructed, as the CBOR Sequence of G_Y and C_R.¶
data_2 = ( h'71a3d599c21da18902a1aea810b2b6382ccd8d5f9bf0195281754c5ebcaf301e', h'13' )¶
data_2 (CBOR Sequence) (35 bytes) 58 20 71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0 19 52 81 75 4c 5e bc af 30 1e 13¶
From data_2 and message_1, compute the input to the transcript hash TH_2 = H( message_1, data_2 ), as a CBOR Sequence of these 2 data items.¶
Input to calculate TH_2 (CBOR Sequence) (72 bytes) 01 00 58 20 89 8f f7 9a 02 06 7a 16 ea 1e cc b9 0f a5 22 46 f5 aa 4d d6 ec 07 6b ba 02 59 d9 04 b7 ec 8b 0c 40 58 20 71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0 19 52 81 75 4c 5e bc af 30 1e 13¶
And from there, compute the transcript hash TH_2 = SHA-256( message_1, data_2 )¶
TH_2 (32 bytes) b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7¶
The Responder's subject name is the empty string:¶
Responders's subject name (text string) ""¶
CRED_R is the certificate (X509_R) encoded as a CBOR byte string:¶
X509_R (110 bytes) 47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e 4b f9 03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e 5c 50 db 78 97 4c 27 15 79 b0 16 33 a3 ef 62 71 be 5c 22 5e b2 8f 9c f6 18 0b 5a 6a f3 1e 80 20 9a 08 5c fb f9 5f 3f dc f9 b1 8b 69 3d 6c 0e 0d 0f fb 8e 3f 9a 32 a5 08 59 ec d0 bf cf f2 c2 18¶
CRED_R (112 bytes) 58 6e 47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e 4b f9 03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e 5c 50 db 78 97 4c 27 15 79 b0 16 33 a3 ef 62 71 be 5c 22 5e b2 8f 9c f6 18 0b 5a 6a f3 1e 80 20 9a 08 5c fb f9 5f 3f dc f9 b1 8b 69 3d 6c 0e 0d 0f fb 8e 3f 9a 32 a5 08 59 ec d0 bf cf f2 c2 18¶
And because certificates are identified by a hash value with the 'x5t' parameter, ID_CRED_R is the following:¶
ID_CRED_R = { 34 : COSE_CertHash }. In this example, the hash algorithm used is SHA-2 256-bit with hash truncated to 64-bits (value -15). The hash value is calculated over the certificate X509_R.¶
ID_CRED_R =
{
34: [-15, h'FC79990F2431A3F5']
}
¶
ID_CRED_R (14 bytes) a1 18 22 82 2e 48 fc 79 99 0f 24 31 a3 f5¶
Since no unprotected opaque auxiliary data is sent in the message exchanges:¶
AD_2 (0 bytes)¶
The Plaintext is defined as the empty string:¶
P_2m (0 bytes)¶
The Enc_structure is defined as follows: [ "Encrypt0", << ID_CRED_R >>, << TH_2, CRED_R >> ]¶
A_2m = [ "Encrypt0", h'A11822822E48FC79990F2431A3F5', h'5820B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF 7586E47624DC9CDC6824B2A4C52E95EC9D6B0534B71C2B49E4BF9031500CEE6869979 C297BB5A8B381E98DB714108415E5C50DB78974C271579B01633A3EF6271BE5C225EB 28F9CF6180B5A6AF31E80209A085CFBF95F3FDCF9B18B693D6C0E0D0FFB8E3F9A32A5 0859ECD0BFCFF2C218' ]¶
Which encodes to the following byte string to be used as Additional Authenticated Data:¶
A_2m (CBOR-encoded) (173 bytes) 83 68 45 6e 63 72 79 70 74 30 4e a1 18 22 82 2e 48 fc 79 99 0f 24 31 a3 f5 58 92 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 58 6e 47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e 4b f9 03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e 5c 50 db 78 97 4c 27 15 79 b0 16 33 a3 ef 62 71 be 5c 22 5e b2 8f 9c f6 18 0b 5a 6a f3 1e 80 20 9a 08 5c fb f9 5f 3f dc f9 b1 8b 69 3d 6c 0e 0d 0f fb 8e 3f 9a 32 a5 08 59 ec d0 bf cf f2 c2 18¶
info for K_2m is defined as follows:¶
info for K_2m = [ 10, h'B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF7', "K_2m", 16 ]¶
Which as a CBOR encoded data item is:¶
info for K_2m (CBOR-encoded) (42 bytes) 84 0a 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 64 4b 5f 32 6d 10¶
From these parameters, K_2m is computed. Key K_2m is the output of HKDF-Expand(PRK_3e2m, info, L), where L is the length of K_2m, so 16 bytes.¶
K_2m (16 bytes) b7 48 6a 94 a3 6c f6 9e 67 3f c4 57 55 ee 6b 95¶
info for IV_2m is defined as follows:¶
info for IV_2m = [ 10, h'B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF7', "IV_2m", 13 ]¶
Which as a CBOR encoded data item is:¶
info for IV_2m (CBOR-encoded) (43 bytes) 84 0a 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 65 49 56 5f 32 6d 0d¶
From these parameters, IV_2m is computed. IV_2m is the output of HKDF-Expand(PRK_3e2m, info, L), where L is the length of IV_2m, so 13 bytes.¶
IV_2m (13 bytes) c5 b7 17 0e 65 d5 4f 1a e0 5d 10 af 56¶
Finally, COSE_Encrypt0 is computed from the parameters above.¶
MAC_2 (8 bytes) cf 99 99 ae 75 9e c0 d8¶
To compute the Signature_or_MAC_2, the key is the private authentication key of the Responder and the message M_2 to be signed = [ "Signature1", << ID_CRED_R >>, << TH_2, CRED_R, ? AD_2 >>, MAC_2 ]¶
M_2 = [ "Signature1", h'A11822822E48FC79990F2431A3F5', h'5820B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF 7586E47624DC9CDC6824B2A4C52E95EC9D6B0534B71C2B49E4BF9031500CEE6869979 C297BB5A8B381E98DB714108415E5C50DB78974C271579B01633A3EF6271BE5C225EB 28F9CF6180B5A6AF31E80209A085CFBF95F3FDCF9B18B693D6C0E0D0FFB8E3F9A32A5 0859ECD0BFCFF2C218', h'CF9999AE759EC0D8' ]¶
Which as a CBOR encoded data item is:¶
M_2 (184 bytes) 84 6a 53 69 67 6e 61 74 75 72 65 31 4e a1 18 22 82 2e 48 fc 79 99 0f 24 31 a3 f5 58 92 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 58 6e 47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e 4b f9 03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e 5c 50 db 78 97 4c 27 15 79 b0 16 33 a3 ef 62 71 be 5c 22 5e b2 8f 9c f6 18 0b 5a 6a f3 1e 80 20 9a 08 5c fb f9 5f 3f dc f9 b1 8b 69 3d 6c 0e 0d 0f fb 8e 3f 9a 32 a5 08 59 ec d0 bf cf f2 c2 18 48 cf 99 99 ae 75 9e c0 d8¶
From there Signature_or_MAC_2 is a signature (since method = 0):¶
Signature_or_MAC_2 (64 bytes) 45 47 81 ec ef eb b4 83 e6 90 83 9d 57 83 8d fe 24 a8 cf 3f 66 42 8a a0 16 20 4a 22 61 84 4a f8 4f 98 b8 c6 83 4f 38 7f dd 60 6a 29 41 3a dd e3 a2 07 74 02 13 74 01 19 6f 6a 50 24 06 6f ac 0e¶
CIPHERTEXT_2 is the ciphertext resulting from XOR encrypting a plaintext constructed from the following parameters and the key K_2e.¶
The plaintext is the following:¶
P_2e (CBOR Sequence) (80 bytes) a1 18 22 82 2e 48 fc 79 99 0f 24 31 a3 f5 58 40 45 47 81 ec ef eb b4 83 e6 90 83 9d 57 83 8d fe 24 a8 cf 3f 66 42 8a a0 16 20 4a 22 61 84 4a f8 4f 98 b8 c6 83 4f 38 7f dd 60 6a 29 41 3a dd e3 a2 07 74 02 13 74 01 19 6f 6a 50 24 06 6f ac 0e¶
K_2e = HKDF-Expand( PRK, info, length ), where length is the length of the plaintext, so 80.¶
info for K_2e = [ 10, h'B0DC6C1BA0BAE6E2888610FA0B27BFC52E311A47B9CAFB609DE4F6A1760D6CF7', "K_2e", 80 ]¶
Which as a CBOR encoded data item is:¶
info for K_2e (CBOR-encoded) (43 bytes) 84 0a 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 64 4b 5f 32 65 18 50¶
From there, K_2e is computed:¶
K_2e (80 bytes) 38 cd 1a 83 89 6d 43 af 3d e8 39 35 27 42 0d ac 7d 7a 76 96 7e 85 74 58 26 bb 39 e1 76 21 8d 7e 5f e7 97 60 14 c9 ed ba c0 58 ee 18 cd 57 71 80 a4 4d de 0b 83 00 fe 8e 09 66 9a 34 d6 3e 3a e6 10 12 26 ab f8 5c eb 28 05 dc 00 13 d1 78 2a 20¶
Using the parameters above, the ciphertext CIPHERTEXT_2 can be computed:¶
CIPHERTEXT_2 (80 bytes) 99 d5 38 01 a7 25 bf d6 a4 e7 1d 04 84 b7 55 ec 38 3d f7 7a 91 6e c0 db c0 2b ba 7c 21 a2 00 80 7b 4f 58 5f 72 8b 67 1a d6 78 a4 3a ac d3 3b 78 eb d5 66 cd 00 4f c6 f1 d4 06 f0 1d 97 04 e7 05 b2 15 52 a9 eb 28 ea 31 6a b6 50 37 d7 17 86 2e¶
message_2 is the CBOR Sequence of data_2 and CIPHERTEXT_2, in this order:¶
message_2 = ( data_2, h'99d53801a725bfd6a4e71d0484b755ec383df77a916ec0dbc02bba7c21a200807b4f 585f728b671ad678a43aacd33b78ebd566cd004fc6f1d406f01d9704e705b21552a9eb 28ea316ab65037d717862e' )¶
Which as a CBOR encoded data item is:¶
message_2 (CBOR Sequence) (117 bytes) 58 20 71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0 19 52 81 75 4c 5e bc af 30 1e 13 58 50 99 d5 38 01 a7 25 bf d6 a4 e7 1d 04 84 b7 55 ec 38 3d f7 7a 91 6e c0 db c0 2b ba 7c 21 a2 00 80 7b 4f 58 5f 72 8b 67 1a d6 78 a4 3a ac d3 3b 78 eb d5 66 cd 00 4f c6 f1 d4 06 f0 1d 97 04 e7 05 b2 15 52 a9 eb 28 ea 31 6a b6 50 37 d7 17 86 2e¶
Since corr equals 1, C_R is not omitted from data_3.¶
SK_I (Initiator's private authentication key) (32 bytes) 2f fc e7 a0 b2 b8 25 d3 97 d0 cb 54 f7 46 e3 da 3f 27 59 6e e0 6b 53 71 48 1d c0 e0 12 bc 34 d7¶
HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).¶
PRK_4x3m = HMAC-SHA-256 (PRK_3e2m, G_IY)¶
PRK_4x3m (32 bytes) ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f d8 2f be b7 99 71 39 4a¶
data 3 is equal to C_R.¶
data_3 (CBOR Sequence) (1 bytes) 13¶
From data_3, CIPHERTEXT_2, and TH_2, compute the input to the transcript hash TH_3 = H(TH_2 , CIPHERTEXT_2, data_3), as a CBOR Sequence of these 3 data items.¶
Input to calculate TH_3 (CBOR Sequence) (117 bytes) 58 20 b0 dc 6c 1b a0 ba e6 e2 88 86 10 fa 0b 27 bf c5 2e 31 1a 47 b9 ca fb 60 9d e4 f6 a1 76 0d 6c f7 58 50 99 d5 38 01 a7 25 bf d6 a4 e7 1d 04 84 b7 55 ec 38 3d f7 7a 91 6e c0 db c0 2b ba 7c 21 a2 00 80 7b 4f 58 5f 72 8b 67 1a d6 78 a4 3a ac d3 3b 78 eb d5 66 cd 00 4f c6 f1 d4 06 f0 1d 97 04 e7 05 b2 15 52 a9 eb 28 ea 31 6a b6 50 37 d7 17 86 2e 13¶
And from there, compute the transcript hash TH_3 = SHA-256(TH_2 , CIPHERTEXT_2, data_3)¶
TH_3 (32 bytes) a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da¶
The initiator's subject name is the empty string:¶
Initiator's subject name (text string) ""¶
CRED_I is the certificate (X509_I) encoded as a CBOR byte string:¶
X509_I (101 bytes) fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f fc 79 5f 88 af c4 9c be 8a fd d1 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01 95 60 1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44 48 65 02 ff 7b dd a6 32 c7 88 37 00 16 b8 96 5b db 20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44 2b 87 ec 3f f2 45 b7¶
CRED_I (103 bytes) 58 65 fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f fc 79 5f 88 af c4 9c be 8a fd d1 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01 95 60 1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44 48 65 02 ff 7b dd a6 32 c7 88 37 00 16 b8 96 5b db 20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44 2b 87 ec 3f f2 45 b7¶
And because certificates are identified by a hash value with the 'x5t' parameter, ID_CRED_I is the following:¶
ID_CRED_I = { 34 : COSE_CertHash }. In this example, the hash algorithm used is SHA-2 256-bit with hash truncated to 64-bits (value -15). The hash value is calculated over the certificate X509_I.¶
ID_CRED_I =
{
34: [-15, h'FC79990F2431A3F5']
}
¶
ID_CRED_I (14 bytes) a1 18 22 82 2e 48 5b 78 69 88 43 9e bc f2¶
Since no opaque auxiliary data is exchanged:¶
AD_3 (0 bytes)¶
The Plaintext of the COSE_Encrypt is the empty string:¶
P_3m (0 bytes)¶
The external_aad is the CBOR Sequence od CRED_I and TH_3, in this order:¶
A_3m (CBOR-encoded) (164 bytes) 83 68 45 6e 63 72 79 70 74 30 4e a1 18 22 82 2e 48 5b 78 69 88 43 9e bc f2 58 89 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 58 65 fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f fc 79 5f 88 af c4 9c be 8a fd d1 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01 95 60 1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44 48 65 02 ff 7b dd a6 32 c7 88 37 00 16 b8 96 5b db 20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44 2b 87 ec 3f f2 45 b7¶
Info for K_3m is computed as follows:¶
info for K_3m = [ 10, h'A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358DA', "K_3m", 16 ]¶
Which as a CBOR encoded data item is:¶
info for K_3m (CBOR-encoded) (42 bytes) 84 0a 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 64 4b 5f 33 6d 10¶
From these parameters, K_3m is computed. Key K_3m is the output of HKDF-Expand(PRK_4x3m, info, L), where L is the length of K_2m, so 16 bytes.¶
K_3m (16 bytes) 3d bb f0 d6 01 03 26 e8 27 3f c6 c6 c3 b0 de cd¶
Nonce IV_3m is the output of HKDF-Expand(PRK_4x3m, info, L), where L = 13 bytes.¶
Info for IV_3m is defined as follows:¶
info for IV_3m = [ 10, h'A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358DA', "IV_3m", 13 ]¶
Which as a CBOR encoded data item is:¶
info for IV_3m (CBOR-encoded) (43 bytes) 84 0a 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 65 49 56 5f 33 6d 0d¶
From these parameters, IV_3m is computed:¶
IV_3m (13 bytes) 10 b6 f4 41 4a 2c 91 3c cd a1 96 42 e3¶
MAC_3 is the ciphertext of the COSE_Encrypt0:¶
MAC_3 (8 bytes) 5e ef b8 85 98 3c 22 d9¶
Since the method = 0, Signature_or_Mac_3 is a signature:¶
M_3 = [ "Signature1", h'A11822822E485B786988439EBCF2', h'5820A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358D A5865FA34B22A9CA4A1E12924EAE1D1766088098449CB848FFC795F88AFC49CBE8AFD D1BA009F21675E8F6C77A4A2C30195601F6F0A0852978BD43D28207D44486502FF7BD DA632C788370016B8965BDB2074BFF82E5A20E09BEC21F8406E86442B87EC3FF245 B7', h'5EEFB885983C22D9']¶
Which as a CBOR encoded data item is:¶
M_3 (175 bytes) 84 6a 53 69 67 6e 61 74 75 72 65 31 4e a1 18 22 82 2e 48 5b 78 69 88 43 9e bc f2 58 89 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 58 65 fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f fc 79 5f 88 af c4 9c be 8a fd d1 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01 95 60 1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44 48 65 02 ff 7b dd a6 32 c7 88 37 00 16 b8 96 5b db 20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44 2b 87 ec 3f f2 45 b7 48 5e ef b8 85 98 3c 22 d9¶
From there, the signature can be computed:¶
Signature_or_MAC_3 (64 bytes) b3 31 76 33 fa eb c7 f4 24 9c f3 ab 95 96 fd ae 2b eb c8 e7 27 5d 39 9f 42 00 04 f3 76 7b 88 d6 0f fe 37 dc f3 90 a0 00 d8 5a b0 ad b0 d7 24 e3 a5 7c 4d fe 24 14 a4 1e 79 78 91 b9 55 35 89 06¶
Finally, the outer COSE_Encrypt0 is computed.¶
The Plaintext is the following CBOR Sequence: plaintext = ( ID_CRED_I , Signature_or_MAC_3 )¶
P_3ae (CBOR Sequence) (80 bytes) a1 18 22 82 2e 48 5b 78 69 88 43 9e bc f2 58 40 b3 31 76 33 fa eb c7 f4 24 9c f3 ab 95 96 fd ae 2b eb c8 e7 27 5d 39 9f 42 00 04 f3 76 7b 88 d6 0f fe 37 dc f3 90 a0 00 d8 5a b0 ad b0 d7 24 e3 a5 7c 4d fe 24 14 a4 1e 79 78 91 b9 55 35 89 06¶
The Associated data A is the following: Associated data A = [ "Encrypt0", h'', TH_3 ]¶
A_3ae (CBOR-encoded) (45 bytes) 83 68 45 6e 63 72 79 70 74 30 40 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da¶
Key K_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).¶
info is defined as follows:¶
info for K_3ae = [ 10, h'A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358DA', "K_3ae", 16 ]¶
Which as a CBOR encoded data item is:¶
info for K_3ae (CBOR-encoded) (43 bytes) 84 0a 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 65 4b 5f 33 61 65 10¶
L is the length of K_3ae, so 16 bytes.¶
From these parameters, K_3ae is computed:¶
K_3ae (16 bytes) 58 b5 2f 94 5b 30 9d 85 4c a7 36 cd 06 a9 62 95¶
Nonce IV_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).¶
info is defined as follows:¶
info for IV_3ae = [ 10, h'A239A627ADA3802DB8DAE51EC392BFEB926D393EF6EEE4DDB32E4A27CE9358DA', "IV_3ae", 13 ]¶
Which as a CBOR encoded data item is:¶
info for IV_3ae (CBOR-encoded) (44 bytes) 84 0a 58 20 a2 39 a6 27 ad a3 80 2d b8 da e5 1e c3 92 bf eb 92 6d 39 3e f6 ee e4 dd b3 2e 4a 27 ce 93 58 da 66 49 56 5f 33 61 65 0d¶
L is the length of IV_3ae, so 13 bytes.¶
From these parameters, IV_3ae is computed:¶
IV_3ae (13 bytes) cf a9 a5 85 58 10 d6 dc e9 74 3c 3b c3¶
Using the parameters above, the ciphertext CIPHERTEXT_3 can be computed:¶
CIPHERTEXT_3 (88 bytes) 2d 88 ff 86 da 47 48 2c 0d fa 55 9a c8 24 a4 a7 83 d8 70 c9 db a4 78 05 e8 aa fb ad 69 74 c4 96 46 58 65 03 fa 9b bf 3e 00 01 2c 03 7e af 56 e4 5e 30 19 20 83 9b 81 3a 53 f6 d4 c5 57 48 0f 6c 79 7d 5b 76 f0 e4 62 f5 f5 7a 3d b6 d2 b5 0c 32 31 9f 34 0f 4a c5 af 9a¶
From the parameter above, message_3 is computed, as the CBOR Sequence of the following items: (C_R, CIPHERTEXT_3).¶
message_3 = ( h'13', h'' )¶
Which encodes to the following byte string:¶
message_3 (CBOR Sequence) (91 bytes) 13 58 58 2d 88 ff 86 da 47 48 2c 0d fa 55 9a c8 24 a4 a7 83 d8 70 c9 db a4 78 05 e8 aa fb ad 69 74 c4 96 46 58 65 03 fa 9b bf 3e 00 01 2c 03 7e af 56 e4 5e 30 19 20 83 9b 81 3a 53 f6 d4 c5 57 48 0f 6c 79 7d 5b 76 f0 e4 62 f5 f5 7a 3d b6 d2 b5 0c 32 31 9f 34 0f 4a c5 af 9a¶
EDHOC with static Diffie-Hellman keys is used.¶
method (Static DH Based Authentication) 3¶
CoAP is used as transport and the Initiator acts as CoAP client:¶
corr (the Initiator can correlate message_1 and message_2) 1¶
From there, METHOD_CORR has the following value:¶
METHOD_CORR (4 * method + corr) (int) 13¶
No unprotected opaque auxiliary data is sent in the message exchanges.¶
The list of supported cipher suites of the Initiator in order of preference is the following:¶
Supported Cipher Suites (4 bytes) 00 01 02 03¶
The cipher suite selected by the Initiator is the most preferred:¶
Selected Cipher Suite (int) 0¶
The mandatory-to-implement cipher suite 0 is supported by both the Initiator and the Responder, see Section 8.3.¶
X (Initiator's ephemeral private key) (32 bytes) ae 11 a0 db 86 3c 02 27 e5 39 92 fe b8 f5 92 4c 50 d0 a7 ba 6e ea b4 ad 1f f2 45 72 f4 f5 7c fa¶
G_X (Initiator's ephemeral public key) (32 bytes) 8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80 a5 38 a4 44 ee 9e 2b 57 e2 44 1a 7c¶
The Initiator chooses a connection identifier C_I:¶
Connection identifier chosen by Initiator (1 bytes) 16¶
Note that since C_I is a byte strings of length one, it is encoded as the corresponding integer - 24 (see bstr_identifier in Section 5.1), i.e. 0x16 = 22, 22 - 24 = -2, and -2 in CBOR encoding is equal to 0x21.¶
C_I (1 byte) 21¶
Since no unprotected opaque auxiliary data is sent in the message exchanges:¶
AD_1 (0 bytes)¶
Since the list of supported cipher suites needs to contain the selected cipher suite, the initiator truncates the list of supported cipher suites to one cipher suite only, 00.¶
Because one single selected cipher suite is conveyed, it is encoded as an int instead of an array:¶
SUITES_I (int) 0¶
With SUITES_I = 0, message_1 is constructed, as the CBOR Sequence of the CBOR data items above.¶
message_1 = ( 13, 0, h'8D3EF56D1B750A4351D68AC250A0E883790EFC80A538A444EE9E2B57E2441A7C', -2 )¶
message_1 (CBOR Sequence) (37 bytes) 0d 00 58 20 8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80 a5 38 a4 44 ee 9e 2b 57 e2 44 1a 7c 21¶
Since METHOD_CORR mod 4 equals 1, C_I is omitted from data_2.¶
Y (Responder's ephemeral private key) (32 bytes) c6 46 cd dc 58 12 6e 18 10 5f 01 ce 35 05 6e 5e bc 35 f4 d4 cc 51 07 49 a3 a5 e0 69 c1 16 16 9a¶
G_Y (Responder's ephemeral public key) (32 bytes) 52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db fc 33 01 04 70 69 45 1b af 35¶
From G_X and Y or from G_Y and X the ECDH shared secret is computed:¶
G_XY (ECDH shared secret) (32 bytes) de fc 2f 35 69 10 9b 3d 1f a4 a7 3d c5 e2 fe b9 e1 15 0d 90 c2 5e e2 f0 66 c2 d8 85 f4 f8 ac 4e¶
The key and nonce for calculating the ciphertext are calculated as follows, as specified in Section 4.¶
HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).¶
PRK_2e = HMAC-SHA-256(salt, G_XY)¶
Salt is the empty byte string.¶
salt (0 bytes)¶
From there, PRK_2e is computed:¶
PRK_2e (32 bytes) 93 9f cb 05 6d 2e 41 4f 1b ec 61 04 61 99 c2 c7 63 d2 7f 0c 3d 15 fa 16 71 fa 13 4e 0d c5 a0 4d¶
Since the Responder authenticates with a static Diffie-Hellman key, PRK_3e2m = HKDF-Extract( PRK_2e, G_RX ), where G_RX is the ECDH shared secret calculated from G_R and X, or G_X and R.¶
R (Responder's private authentication key) (32 bytes) bb 50 1a ac 67 b9 a9 5f 97 e0 ed ed 6b 82 a6 62 93 4f bb fc 7a d1 b7 4c 1f ca d6 6a 07 94 22 d0¶
G_R (Responder's public authentication key) (32 bytes) a3 ff 26 35 95 be b3 77 d1 a0 ce 1d 04 da d2 d4 09 66 ac 6b cb 62 20 51 b8 46 59 18 4d 5d 9a 32¶
From the Responder's authentication key and the Initiator's ephemeral key (see Appendix B.2.1), the ECDH shared secret G_RX is calculated.¶
G_RX (ECDH shared secret) (32 bytes) 21 c7 ef f4 fb 69 fa 4b 67 97 d0 58 84 31 5d 84 11 a3 fd a5 4f 6d ad a6 1d 4f cd 85 e7 90 66 68¶
PRK_3e2m (32 bytes) 75 07 7c 69 1e 35 01 2d 48 bc 24 c8 4f 2b ab 89 f5 2f ac 03 fe dd 81 3e 43 8c 93 b1 0b 39 93 07¶
The Responder chooses a connection identifier C_R.¶
Connection identifier chosen by Responder (1 byte) 20¶
Note that since C_R is a byte strings of length one, it is encoded as the corresponding integer - 24 (see bstr_identifier in Section 5.1), i.e. 0x20 = 32, 32 - 24 = 8, and 8 in CBOR encoding is equal to 0x08.¶
C_R (1 byte) 08¶
Data_2 is constructed, as the CBOR Sequence of G_Y and C_R.¶
data_2 = ( h'52FBA0BDC8D953DD86CE1AB2FD7C05A4658C7C30AFDBFC3301047069451BAF35', 08 )¶
data_2 (CBOR Sequence) (35 bytes) 58 20 52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db fc 33 01 04 70 69 45 1b af 35 08¶
From data_2 and message_1, compute the input to the transcript hash TH_2 = H( message_1, data_2 ), as a CBOR Sequence of these 2 data items.¶
Input to calculate TH_2 (CBOR Sequence) (72 bytes) 0d 00 58 20 8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80 a5 38 a4 44 ee 9e 2b 57 e2 44 1a 7c 21 58 20 52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db fc 33 01 04 70 69 45 1b af 35 08¶
And from there, compute the transcript hash TH_2 = SHA-256( message_1, data_2 )¶
TH_2 (32 bytes) 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30 0c af 9c 49 1a 52 e6 83 6a 25 64 ff¶
The Responder's subject name is the empty string:¶
Responders's subject name (text string) ""¶
ID_CRED_R is the following:¶
ID_CRED_R =
{
4: h'07'
}
¶
ID_CRED_R (4 bytes) a1 04 41 07¶
CRED_R is the following COSE_Key:¶
{
1: 1,
-1: 4,
-2: h'A3FF263595BEB377D1A0CE1D04DAD2D40966AC6BCB622051B84659184D5D9A32',
"subject name": ""
}
¶
Which encodes to the following byte string:¶
CRED_R (54 bytes) a4 01 01 20 04 21 58 20 a3 ff 26 35 95 be b3 77 d1 a0 ce 1d 04 da d2 d4 09 66 ac 6b cb 62 20 51 b8 46 59 18 4d 5d 9a 32 6c 73 75 62 6a 65 63 74 20 6e 61 6d 65 60¶
Since no unprotected opaque auxiliary data is sent in the message exchanges:¶
AD_2 (0 bytes)¶
The Plaintext is defined as the empty string:¶
P_2m (0 bytes)¶
The Enc_structure is defined as follows: [ "Encrypt0", << ID_CRED_R >>, << TH_2, CRED_R >> ]¶
A_2m = [ "Encrypt0", h'A1044107', h'58206A2878E84B2CC021CC1AEBA2965253EF42F7FA300CAF9C491A52E6836A2564FFA401012004215820A3FF263595BEB377D1A0CE1D04DAD2D40966AC6BCB622051B84659184D5D9A326C7375626A656374206E616D6560' ]¶
Which encodes to the following byte string to be used as Additional Authenticated Data:¶
A_2m (CBOR-encoded) (105 bytes) 83 68 45 6e 63 72 79 70 74 30 44 a1 04 41 07 58 58 58 20 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30 0c af 9c 49 1a 52 e6 83 6a 25 64 ff a4 01 01 20 04 21 58 20 a3 ff 26 35 95 be b3 77 d1 a0 ce 1d 04 da d2 d4 09 66 ac 6b cb 62 20 51 b8 46 59 18 4d 5d 9a 32 6c 73 75 62 6a 65 63 74 20 6e 61 6d 65 60¶
info for K_2m is defined as follows:¶
info for K_2m = [ 10, h'6A2878E84B2CC021CC1AEBA2965253EF42F7FA300CAF9C491A52E6836A2564FF', "K_2m", 16 ]¶
Which as a CBOR encoded data item is:¶
info for K_2m (CBOR-encoded) (42 bytes) 84 0a 58 20 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30 0c af 9c 49 1a 52 e6 83 6a 25 64 ff 64 4b 5f 32 6d 10¶
From these parameters, K_2m is computed. Key K_2m is the output of HKDF-Expand(PRK_3e2m, info, L), where L is the length of K_2m, so 16 bytes.¶
K_2m (16 bytes) 81 2a 48 87 d1 90 ff ed 2b 10 0b a7 a5 c2 5e 67¶
info for IV_2m is defined as follows:¶
info for IV_2m = [ 10, h'6A2878E84B2CC021CC1AEBA2965253EF42F7FA300CAF9C491A52E6836A2564FF', "IV_2m", 13 ]¶
Which as a CBOR encoded data item is:¶
info for IV_2m (CBOR-encoded) (43 bytes) 84 0a 58 20 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30 0c af 9c 49 1a 52 e6 83 6a 25 64 ff 65 49 56 5f 32 6d 0d¶
From these parameters, IV_2m is computed. IV_2m is the output of HKDF-Expand(PRK_3e2m, info, L), where L is the length of IV_2m, so 13 bytes.¶
IV_2m (13 bytes) 92 3c 0f 94 31 51 5b 69 21 30 49 2b 7f¶
Finally, COSE_Encrypt0 is computed from the parameters above.¶
MAC_2 (8 bytes) 64 21 0d 2e 18 b9 28 cd¶
From there Signature_or_MAC_2 is the MAC (since method = 3):¶
Signature_or_MAC_2 (8 bytes) 64 21 0d 2e 18 b9 28 cd¶
CIPHERTEXT_2 is the ciphertext resulting from XOR encrypting a plaintext constructed from the following parameters and the key K_2e.¶
The plaintext is the following:¶
P_2e (CBOR Sequence) (10 bytes) 30 48 64 21 0d 2e 18 b9 28 cd¶
K_2e = HKDF-Expand( PRK, info, length ), where length is the length of the plaintext, so 10.¶
info for K_2e = [ 10, h'6A2878E84B2CC021CC1AEBA2965253EF42F7FA300CAF9C491A52E6836A2564FF', "K_2e", 10 ]¶
Which as a CBOR encoded data item is:¶
info for K_2e (CBOR-encoded) (42 bytes) 84 0a 58 20 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30 0c af 9c 49 1a 52 e6 83 6a 25 64 ff 64 4b 5f 32 65 0a¶
From there, K_2e is computed:¶
K_2e (10 bytes) ec be 9a bd 5f 62 3a fc 65 26¶
Using the parameters above, the ciphertext CIPHERTEXT_2 can be computed:¶
CIPHERTEXT_2 (10 bytes) dc f6 fe 9c 52 4c 22 45 4d eb¶
message_2 is the CBOR Sequence of data_2 and CIPHERTEXT_2, in this order:¶
message_2 = ( data_2, h'DCF6FE9C524C22454DEB' )¶
Which as a CBOR encoded data item is:¶
message_2 (CBOR Sequence) (46 bytes) 58 20 52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db fc 33 01 04 70 69 45 1b af 35 08 4a dc f6 fe 9c 52 4c 22 45 4d eb¶
Since corr equals 1, C_R is not omitted from data_3.¶
SK_I (Initiator's private authentication key) (32 bytes) 2b be a6 55 c2 33 71 c3 29 cf bd 3b 1f 02 c6 c0 62 03 38 37 b8 b5 90 99 a4 43 6f 66 60 81 b0 8e¶
G_I (Initiator's public authentication key) (32 bytes) 2c 44 0c c1 21 f8 d7 f2 4c 3b 0e 41 ae da fe 9c aa 4f 4e 7a bb 83 5e c3 0f 1d e8 8a db 96 ff 71¶
HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).¶
From the Initiator's authentication key and the Responder's ephemeral key (see Appendix B.2.2), the ECDH shared secret G_IY is calculated.¶
G_IY (ECDH shared secret) (32 bytes) cb ff 8c d3 4a 81 df ec 4c b6 5d 9a 57 2e bd 09 64 45 0c 78 56 3d a4 98 1d 80 d3 6c 8b 1a 75 2a¶
PRK_4x3m = HMAC-SHA-256 (PRK_3e2m, G_IY).¶
PRK_4x3m (32 bytes) ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f d8 2f be b7 99 71 39 4a¶
data 3 is equal to C_R.¶
data_3 (CBOR Sequence) (1 bytes) 08¶
From data_3, CIPHERTEXT_2, and TH_2, compute the input to the transcript hash TH_3 = H(TH_2 , CIPHERTEXT_2, data_3), as a CBOR Sequence of these 3 data items.¶
Input to calculate TH_3 (CBOR Sequence) (46 bytes) 58 20 6a 28 78 e8 4b 2c c0 21 cc 1a eb a2 96 52 53 ef 42 f7 fa 30 0c af 9c 49 1a 52 e6 83 6a 25 64 ff 4a dc f6 fe 9c 52 4c 22 45 4d eb 08¶
And from there, compute the transcript hash TH_3 = SHA-256(TH_2 , CIPHERTEXT_2, data_3)¶
TH_3 (32 bytes) 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07 03 ee d9 c4 a1 bc b6 11¶
The initiator's subject name is the empty string:¶
Initiator's subject name (text string) ""¶
And its credential is:¶
ID_CRED_I =
{
4: h'24'
}
¶
ID_CRED_I (4 bytes) a1 04 41 24¶
CRED_I is the following COSE_Key:¶
{
1: 1,
-1: 4,
-2: h'2C440CC121F8D7F24C3B0E41AEDAFE9CAA4F4E7ABB835EC30F1DE88ADB96FF71',
"subject name": ""
}
¶
Which encodes to the following byte string:¶
CRED_I (54 bytes) a4 01 01 20 04 21 58 20 2c 44 0c c1 21 f8 d7 f2 4c 3b 0e 41 ae da fe 9c aa 4f 4e 7a bb 83 5e c3 0f 1d e8 8a db 96 ff 71 6c 73 75 62 6a 65 63 74 20 6e 61 6d 65 60¶
Since no opaque auxiliary data is exchanged:¶
AD_3 (0 bytes)¶
The Plaintext of the COSE_Encrypt is the empty string:¶
P_3m (0 bytes)¶
The external_aad is the CBOR Sequence of CRED_I and TH_3, in this order:¶
A_3m (CBOR-encoded) (105 bytes) 83 68 45 6e 63 72 79 70 74 30 44 a1 04 41 24 58 58 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07 03 ee d9 c4 a1 bc b6 11 a4 01 01 20 04 21 58 20 2c 44 0c c1 21 f8 d7 f2 4c 3b 0e 41 ae da fe 9c aa 4f 4e 7a bb 83 5e c3 0f 1d e8 8a db 96 ff 71 6c 73 75 62 6a 65 63 74 20 6e 61 6d 65 60¶
Info for K_3m is computed as follows:¶
info for K_3m = [ 10, h'51DD2243A6B83F1316DC53291AE191CD93B444CCE480160703EED9C4A1BCB611', "K_3m", 16 ]¶
Which as a CBOR encoded data item is:¶
info for K_3m (CBOR-encoded) (42 bytes) 84 0a 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07 03 ee d9 c4 a1 bc b6 11 64 4b 5f 33 6d 10¶
From these parameters, K_3m is computed. Key K_3m is the output of HKDF-Expand(PRK_4x3m, info, L), where L is the length of K_2m, so 16 bytes.¶
K_3m (16 bytes) 84 85 31 8a a3 08 6f d5 86 7a 02 8e 99 e2 40 30¶
Nonce IV_3m is the output of HKDF-Expand(PRK_4x3m, info, L), where L = 13 bytes.¶
Info for IV_3m is defined as follows:¶
info for IV_3m = [ 10, h'51DD2243A6B83F1316DC53291AE191CD93B444CCE480160703EED9C4A1BCB611', "IV_3m", 13 ]¶
Which as a CBOR encoded data item is:¶
info for IV_3m (CBOR-encoded) (43 bytes) 84 0a 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07 03 ee d9 c4 a1 bc b6 11 65 49 56 5f 33 6d 0d¶
From these parameters, IV_3m is computed:¶
IV_3m (13 bytes) 1e 10 5b 88 50 0e d5 ae b0 5d 00 6b ea¶
MAC_3 is the ciphertext of the COSE_Encrypt0:¶
MAC_3 (8 bytes) 1f b7 5a c1 aa d2 34 25¶
Since the method = 3, Signature_or_Mac_3 is the MAC_3:¶
Signature_or_MAC_3 (8 bytes) 1f b7 5a c1 aa d2 34 25¶
Finally, the outer COSE_Encrypt0 is computed.¶
The Plaintext is the following CBOR Sequence: plaintext = ( ID_CRED_I , Signature_or_MAC_3 ). Note that since ID_CRED_I contains a single 'kid' parameter, i.e., ID_CRED_I = { 4 : kid_I }, only the byte string kid_I is conveyed in the plaintext encoded as a bstr_identifier. kid_I is encoded as the corresponding integer - 24 (see bstr_identifier in Section 5.1), i.e. 0x24 = 36, 36 - 24 = 12, and 12 in CBOR encoding is equal to 0x0c.¶
P_3ae (CBOR Sequence) (10 bytes) 0c 48 1f b7 5a c1 aa d2 34 25¶
The Associated data A is the following: Associated data A = [ "Encrypt0", h'', TH_3 ]¶
A_3ae (CBOR-encoded) (45 bytes) 83 68 45 6e 63 72 79 70 74 30 40 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07 03 ee d9 c4 a1 bc b6 11¶
Key K_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).¶
info is defined as follows:¶
info for K_3ae = [ 10, h'51DD2243A6B83F1316DC53291AE191CD93B444CCE480160703EED9C4A1BCB611', "K_3ae", 16 ]¶
Which as a CBOR encoded data item is:¶
info for K_3ae (CBOR-encoded) (43 bytes) 84 0a 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07 03 ee d9 c4 a1 bc b6 11 65 4b 5f 33 61 65 10¶
L is the length of K_3ae, so 16 bytes.¶
From these parameters, K_3ae is computed:¶
K_3ae (16 bytes) bf 29 0b 7e e0 4b 86 5d e1 01 0a 81 1b 36 00 64¶
Nonce IV_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).¶
info is defined as follows:¶
info for IV_3ae = [ 10, h'51DD2243A6B83F1316DC53291AE191CD93B444CCE480160703EED9C4A1BCB611', "IV_3ae", 13 ]¶
Which as a CBOR encoded data item is:¶
info for IV_3ae (CBOR-encoded) (44 bytes) 84 0a 58 20 51 dd 22 43 a6 b8 3f 13 16 dc 53 29 1a e1 91 cd 93 b4 44 cc e4 80 16 07 03 ee d9 c4 a1 bc b6 11 66 49 56 5f 33 61 65 0d¶
L is the length of IV_3ae, so 13 bytes.¶
From these parameters, IV_3ae is computed:¶
IV_3ae (13 bytes) 0e 74 45 0a fc ec e9 73 af 64 e9 4d 46¶
Using the parameters above, the ciphertext CIPHERTEXT_3 can be computed:¶
CIPHERTEXT_3 (18 bytes) 53 c3 99 19 99 a5 ff b8 69 21 e9 9b 60 7c 06 77 70 e0¶
From the parameter above, message_3 is computed, as the CBOR Sequence of the following items: (C_R, CIPHERTEXT_3).¶
message_3 = ( h'08', h'53C3991999A5FFB86921E99B607C067770E0' )¶
Which encodes to the following byte string:¶
message_3 (CBOR Sequence) (20 bytes) 08 52 53 c3 99 19 99 a5 ff b8 69 21 e9 9b 60 7c 06 77 70 e0¶
EDHOC requires certain parameters to be agreed upon between Initiator and Responder. A cipher suite is negotiated with the protocol, but certain other parameters need to be agreed beforehand:¶
An example of an applicability statement is shown in the next section.¶
Note that for some of the parameters, like METHOD_CORR, ID_CRED_x, type of AD_x, the receiver is able to assert whether it supports the parameter or not and thus, if it fails, to infer why.¶
For other parameters, like type of authentication credential, it may be more difficult to detect if the receiver got the wrong type since the credential is not necessarily transported, and a failed integrity of the received message may be caused by other circumstances. For example in the case of public key certificates there is a large variety of profiles and alternative encodings, which the applicability statement needs to nail down.¶
Note also that it is not always necessary for the endpoints to agree on the transport for the EDHOC messages. For example, a mix of CoAP and HTTP may be used along the path and still allow correlation between message_1 and message_2.¶
For use of EDHOC in the XX protocol, the following assumptions are made on the parameters.¶
METHOD_CORR = 5¶
CRED_I is an 802.1AR IDevID encoded as a CBOR Certificate of type 0¶
CRED_R is a COSE_Key of type OKP as specified in Section 3.3.4.¶
Auxiliary Data is processed as specified in [I-D.ietf-ace-oauth-authz].¶
The authors want to thank Alessandro Bruni, Karthikeyan Bhargavan, Martin Disch, Theis Groenbech Petersen, Dan Harkins, Klaus Hartke, Russ Housley, Alexandros Krontiris, Ilari Liusvaara, Karl Norrman, Salvador Perez, Eric Rescorla, Michael Richardson, Thorvald Sahl Joergensen, Jim Schaad, Carsten Schuermann, Ludwig Seitz, Stanislav Smyshlyaev, Valery Smyslov, Rene Struik, Vaishnavi Sundararajan, Erik Thormarker, and Michel Veillette 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.¶