Ephemeral Diffie-Hellman Over COSE (EDHOC)
draft-ietf-lake-edhoc-13
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| Authors | Göran Selander , John Preuß Mattsson , Francesca Palombini | ||
| Last updated | 2022-04-18 (Latest revision 2021-10-20) | ||
| Replaces | draft-selander-lake-edhoc | ||
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draft-ietf-lake-edhoc-13
Network Working Group G. Selander
Internet-Draft J. Preuß Mattsson
Intended status: Standards Track F. Palombini
Expires: 20 October 2022 Ericsson
18 April 2022
Ephemeral Diffie-Hellman Over COSE (EDHOC)
draft-ietf-lake-edhoc-13
Abstract
This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
very compact and lightweight authenticated Diffie-Hellman key
exchange with ephemeral keys. EDHOC provides mutual authentication,
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 20 October 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Use of EDHOC . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Message Size Examples . . . . . . . . . . . . . . . . . . 5
1.4. Document Structure . . . . . . . . . . . . . . . . . . . 6
1.5. Terminology and Requirements Language . . . . . . . . . . 6
2. EDHOC Outline . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8
3.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Method . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Connection Identifiers . . . . . . . . . . . . . . . . . 10
3.4. Transport . . . . . . . . . . . . . . . . . . . . . . . . 11
3.5. Authentication Parameters . . . . . . . . . . . . . . . . 12
3.6. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 18
3.7. Ephemeral Public Keys . . . . . . . . . . . . . . . . . . 19
3.8. External Authorization Data (EAD) . . . . . . . . . . . . 20
3.9. Applicability Statement . . . . . . . . . . . . . . . . . 21
4. Key Derivation . . . . . . . . . . . . . . . . . . . . . . . 22
4.1. Extract . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.2. Expand . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3. EDHOC-Exporter . . . . . . . . . . . . . . . . . . . . . 25
4.4. EDHOC-KeyUpdate . . . . . . . . . . . . . . . . . . . . . 26
5. Message Formatting and Processing . . . . . . . . . . . . . . 26
5.1. Message Processing Outline . . . . . . . . . . . . . . . 27
5.2. EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . . 28
5.3. EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . . 29
5.4. EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . . 32
5.5. EDHOC Message 4 . . . . . . . . . . . . . . . . . . . . . 35
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 37
6.1. Success . . . . . . . . . . . . . . . . . . . . . . . . . 38
6.2. Unspecified . . . . . . . . . . . . . . . . . . . . . . . 38
6.3. Wrong Selected Cipher Suite . . . . . . . . . . . . . . . 38
7. Mandatory-to-Implement Compliance Requirements . . . . . . . 41
8. Security Considerations . . . . . . . . . . . . . . . . . . . 42
8.1. Security Properties . . . . . . . . . . . . . . . . . . . 42
8.2. Cryptographic Considerations . . . . . . . . . . . . . . 44
8.3. Cipher Suites and Cryptographic Algorithms . . . . . . . 45
8.4. Post-Quantum Considerations . . . . . . . . . . . . . . . 46
8.5. Unprotected Data . . . . . . . . . . . . . . . . . . . . 46
8.6. Denial-of-Service . . . . . . . . . . . . . . . . . . . . 47
8.7. Implementation Considerations . . . . . . . . . . . . . . 47
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
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9.1. EDHOC Exporter Label Registry . . . . . . . . . . . . . . 49
9.2. EDHOC Cipher Suites Registry . . . . . . . . . . . . . . 49
9.3. EDHOC Method Type Registry . . . . . . . . . . . . . . . 51
9.4. EDHOC Error Codes Registry . . . . . . . . . . . . . . . 51
9.5. EDHOC External Authorization Data Registry . . . . . . . 52
9.6. COSE Header Parameters Registry . . . . . . . . . . . . . 52
9.7. COSE Header Parameters Registry . . . . . . . . . . . . . 52
9.8. COSE Key Common Parameters Registry . . . . . . . . . . . 53
9.9. CWT Confirmation Methods Registry . . . . . . . . . . . . 53
9.10. The Well-Known URI Registry . . . . . . . . . . . . . . . 53
9.11. Media Types Registry . . . . . . . . . . . . . . . . . . 54
9.12. CoAP Content-Formats Registry . . . . . . . . . . . . . . 55
9.13. Resource Type (rt=) Link Target Attribute Values
Registry . . . . . . . . . . . . . . . . . . . . . . . . 55
9.14. Expert Review Instructions . . . . . . . . . . . . . . . 55
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 56
10.1. Normative References . . . . . . . . . . . . . . . . . . 56
10.2. Informative References . . . . . . . . . . . . . . . . . 59
Appendix A. Use with OSCORE and Transfer over CoAP . . . . . . . 61
A.1. Selecting EDHOC Connection Identifier . . . . . . . . . . 62
A.2. Deriving the OSCORE Security Context . . . . . . . . . . 62
A.3. Transferring EDHOC over CoAP . . . . . . . . . . . . . . 64
Appendix B. Compact Representation . . . . . . . . . . . . . . . 67
Appendix C. Use of CBOR, CDDL and COSE in EDHOC . . . . . . . . 67
C.1. CBOR and CDDL . . . . . . . . . . . . . . . . . . . . . . 68
C.2. CDDL Definitions . . . . . . . . . . . . . . . . . . . . 69
C.3. COSE . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Appendix D. Applicability Template . . . . . . . . . . . . . . . 72
Appendix E. EDHOC Message Deduplication . . . . . . . . . . . . 73
Appendix F. Transports Not Natively Providing Correlation . . . 74
Appendix G. Change Log . . . . . . . . . . . . . . . . . . . . . 74
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 79
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 80
1. Introduction
1.1. Motivation
Many Internet of Things (IoT) deployments require technologies which
are highly performant in constrained environments [RFC7228]. IoT
devices may be constrained in various ways, including memory,
storage, processing capacity, and power. The connectivity for these
settings may also exhibit constraints such as unreliable and lossy
channels, highly restricted bandwidth, and dynamic topology. The
IETF has acknowledged this problem by standardizing a range of
lightweight protocols and enablers designed for the IoT, including
the Constrained Application Protocol (CoAP, [RFC7252]), Concise
Binary Object Representation (CBOR, [RFC8949]), and Static Context
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Header Compression (SCHC, [RFC8724]).
The need for special protocols targeting constrained IoT deployments
extends also to the security domain [I-D.ietf-lake-reqs]. Important
characteristics in constrained environments are the number of round
trips and protocol message sizes, which if kept low can contribute to
good performance by enabling transport over a small number of radio
frames, reducing latency due to fragmentation or duty cycles, etc.
Another important criteria is code size, which may be prohibitive for
certain deployments due to device capabilities or network load during
firmware update. Some IoT deployments also need to support a variety
of underlying transport technologies, potentially even with a single
connection.
Some security solutions for such settings exist already. CBOR Object
Signing and Encryption (COSE, [I-D.ietf-cose-rfc8152bis-struct])
specifies basic application-layer security services efficiently
encoded in CBOR. Another example is Object Security for Constrained
RESTful Environments (OSCORE, [RFC8613]) which is a lightweight
communication security extension to CoAP using CBOR and COSE. In
order to establish good quality cryptographic keys for security
protocols such as COSE and OSCORE, the two endpoints may run an
authenticated Diffie-Hellman key exchange protocol, from which shared
secret key material can be derived. Such a key exchange protocol
should also be lightweight; to prevent bad performance in case of
repeated use, e.g., due to device rebooting or frequent rekeying for
security reasons; or to avoid latencies in a network formation
setting with many devices authenticating at the same time.
This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
lightweight authenticated key exchange protocol providing good
security properties including forward secrecy, identity protection,
and cipher suite negotiation. Authentication can be based on raw
public keys (RPK) or public key certificates and requires the
application to provide input on how to verify that endpoints are
trusted. This specification focuses on referencing instead of
transporting credentials to reduce message overhead. EDHOC does
currently not support pre-shared key (PSK) authentication as
authentication with static Diffie-Hellman public keys by reference
produces equally small message sizes but with much simpler key
distribution and identity protection.
EDHOC makes use of known protocol constructions, such as SIGMA
[SIGMA] and Extract-and-Expand [RFC5869]. EDHOC uses COSE for
cryptography and identification of credentials (including COSE_Key,
CWT, CCS, X.509, C509, see Section 3.5.3). COSE provides crypto
agility and enables the use of future algorithms and credentials
targeting IoT.
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1.2. Use of EDHOC
EDHOC is designed for highly constrained settings making it
especially suitable for low-power wide area networks [RFC8376] such
as Cellular IoT, 6TiSCH, and LoRaWAN. A main objective for EDHOC is
to be a lightweight authenticated key exchange for OSCORE, i.e., to
provide authentication and session key establishment for IoT use
cases such as those built on CoAP [RFC7252]. CoAP is a specialized
web transfer protocol for use with constrained nodes and networks,
providing a request/response interaction model between application
endpoints. As such, EDHOC is targeting a large variety of use cases
involving 'things' with embedded microcontrollers, sensors, and
actuators.
A typical setting is when one of the endpoints is constrained or in a
constrained network, and the other endpoint is a node on the Internet
(such as a mobile phone) or at the edge of the constrained network
(such as a gateway). Thing-to-thing interactions over constrained
networks are also relevant since both endpoints would then benefit
from the lightweight properties of the protocol. EDHOC could e.g.,
be run when a device connects for the first time, or to establish
fresh keys which are not revealed by a later compromise of the long-
term keys. Further security properties are described in Section 8.1.
EDHOC enables the reuse of the same lightweight primitives as OSCORE:
CBOR for encoding, COSE for cryptography, and CoAP for transport. By
reusing existing libraries, the additional code size can be kept very
low. Note that, while CBOR and COSE primitives are built into the
protocol messages, EDHOC is not bound to a particular transport.
Transfer of EDHOC messages in CoAP payloads is detailed in
Appendix A.3.
1.3. Message Size Examples
Compared to the DTLS 1.3 handshake [I-D.ietf-tls-dtls13] with ECDHE
and connection ID, the number of bytes in EDHOC + CoAP can be less
than 1/6 when RPK authentication is used, see
[I-D.ietf-lwig-security-protocol-comparison]. Figure 1 shows
examples of message sizes for EDHOC with different kinds of
authentication keys and different COSE header parameters for
identification: static Diffie-Hellman keys or signature keys, either
in CBOR Web Token (CWT) / CWT Claims Set (CCS) [RFC8392] identified
by a key identifier using 'kid' [I-D.ietf-cose-rfc8152bis-struct], or
in X.509 certificates identified by a hash value using 'x5t'
[I-D.ietf-cose-x509].
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========================================================
Static DH Keys Signature Keys
-------------- --------------
kid x5t kid x5t
--------------------------------------------------------
message_1 37 37 37 37
message_2 45 58 102 115
message_3 19 33 77 90
--------------------------------------------------------
Total 101 128 216 242
========================================================
Figure 1: Example of message sizes in bytes.
1.4. Document Structure
The remainder of the document is organized as follows: Section 2
outlines EDHOC authenticated with digital signatures, Section 3
describes the protocol elements of EDHOC, including formatting of the
ephemeral public keys, Section 4 specifies the key derivation,
Section 5 specifies message processing for EDHOC authenticated with
signature keys or static Diffie-Hellman keys, Section 6 describes the
error messages, and Appendix A shows how to transfer EDHOC with CoAP
and establish an OSCORE security context.
1.5. Terminology and Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are expected to be familiar with the terms and concepts
described in CBOR [RFC8949], CBOR Sequences [RFC8742], COSE
structures and processing [I-D.ietf-cose-rfc8152bis-struct], COSE
algorithms [I-D.ietf-cose-rfc8152bis-algs], CWT and CWT Claims Set
[RFC8392], and CDDL [RFC8610]. The Concise Data Definition Language
(CDDL) is used to express CBOR data structures [RFC8949]. Examples
of CBOR and CDDL are provided in Appendix C.1. When referring to
CBOR, this specification always refers to Deterministically Encoded
CBOR as specified in Sections 4.2.1 and 4.2.2 of [RFC8949]. The
single output from authenticated encryption (including the
authentication tag) is called "ciphertext", following [RFC5116].
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2. EDHOC Outline
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 provides identity protection
of the initiator (SIGMA-I) against active attackers, and like IKEv2
[RFC7296], EDHOC implements the MAC-then-Sign variant of the SIGMA-I
protocol shown in Figure 2.
Initiator Responder
| G_X |
+------------------------------------------------------------------>|
| |
| G_Y, Enc( ID_CRED_R, Sig( R; MAC( CRED_R, G_X, G_Y ) ) ) |
|<------------------------------------------------------------------+
| |
| AEAD( ID_CRED_I, Sig( I; MAC( CRED_I, G_Y, G_X ) ) ) |
+------------------------------------------------------------------>|
| |
Figure 2: MAC-then-Sign variant of the SIGMA-I protocol.
The parties exchanging messages are called Initiator (I) and
Responder (R). They exchange ephemeral public keys, compute a shared
secret, and derive symmetric application keys used to protect
application data.
* G_X and G_Y are the ECDH ephemeral public keys of I and R,
respectively.
* CRED_I and CRED_R are the credentials containing the public
authentication keys of I and R, respectively.
* ID_CRED_I and ID_CRED_R are credential identifiers enabling the
recipient party to retrieve the credential of I and R,
respectively.
* Sig(I; . ) and Sig(R; . ) denote signatures made with the private
authentication key of I and R, respectively.
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* Enc(), AEAD(), and MAC() denotes encryption, authenticated
encryption with additional data, and message authentication code
using keys derived from the shared secret.
In order to create a "full-fledged" protocol some additional protocol
elements are needed. EDHOC adds:
* Transcript hashes (hashes of message data) TH_2, TH_3, TH_4 used
for key derivation and as additional authenticated data.
* Computationally independent keys derived from the ECDH shared
secret and used for authenticated encryption of different
messages.
* An optional fourth message giving explicit key confirmation to I
in deployments where no protected application data is sent from R
to I.
* A key material exporter and a key update function with forward
secrecy.
* Verification of a common preferred cipher suite.
* Method types and error handling.
* Selection of connection identifiers C_I and C_R which may be used
to identify established keys or protocol state.
* Transport of external authorization data.
EDHOC is designed to encrypt and integrity protect as much
information as possible, and all symmetric keys are derived using as
much previous information as possible. EDHOC is furthermore designed
to be as compact and lightweight as possible, in terms of message
sizes, processing, and the ability to reuse already existing CBOR,
COSE, and CoAP libraries.
To simplify for implementors, the use of CBOR and COSE in EDHOC is
summarized in Appendix C. Test vectors including CBOR diagnostic
notation are provided in [I-D.selander-lake-traces].
3. Protocol Elements
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3.1. General
The EDHOC protocol consists of three mandatory messages (message_1,
message_2, message_3) between Initiator and Responder, an optional
fourth message (message_4), and an error message. All EDHOC messages
are CBOR Sequences [RFC8742]. Figure 3 illustrates an EDHOC message
flow with the optional fourth message as well as the content of each
message. The protocol elements in the figure are introduced in
Section 3 and Section 5. Message formatting and processing is
specified in Section 5 and Section 6.
Application data may be protected using the agreed application
algorithms (AEAD, hash) in the selected cipher suite (see
Section 3.6) and the application can make use of the established
connection identifiers C_I and C_R (see Section 3.3). EDHOC may be
used with the media type application/edhoc defined in Section 9.
The Initiator can derive symmetric application keys after creating
EDHOC message_3, see Section 4.3. Protected application data can
therefore be sent in parallel or together with EDHOC message_3.
EDHOC message_4 is typically not sent.
Initiator Responder
| METHOD, SUITES_I, G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| G_Y, Enc( ID_CRED_R, Signature_or_MAC_2, EAD_2 ), C_R |
|<------------------------------------------------------------------+
| message_2 |
| |
| AEAD( ID_CRED_I, Signature_or_MAC_3, EAD_3 ) |
+------------------------------------------------------------------>|
| message_3 |
| |
| AEAD( EAD_4 ) |
|<- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
| message_4 |
Figure 3: EDHOC Message Flow with the Optional Fourth Message
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3.2. Method
The data item METHOD in message_1 (see Section 5.2.1), is an integer
specifying the authentication method. EDHOC supports authentication
with signature or static Diffie-Hellman keys, as defined in the four
authentication methods: 0, 1, 2, and 3, see Figure 4. When using a
static Diffie-Hellman key the authentication is provided by a Message
Authentication Code (MAC) computed from an ephemeral-static ECDH
shared secret which enables significant reductions in message sizes.
The Initiator and the Responder need to have agreed on a single
method to be used for EDHOC, see Section 3.9.
+-------+-------------------+-------------------+-------------------+
| Value | Initiator | Responder | Reference |
+-------+-------------------+-------------------+-------------------+
| 0 | Signature Key | Signature Key | [[this document]] |
| 1 | Signature Key | Static DH Key | [[this document]] |
| 2 | Static DH Key | Signature Key | [[this document]] |
| 3 | Static DH Key | Static DH Key | [[this document]] |
+-------+-------------------+-------------------+-------------------+
Figure 4: Method Types
3.3. Connection Identifiers
EDHOC includes the selection of connection identifiers (C_I, C_R)
identifying a connection for which keys are agreed.
Connection identifiers may be used to correlate EDHOC messages and
facilitate the retrieval of protocol state during EDHOC protocol
execution (see Section 3.4) or in a subsequent application protocol,
e.g., OSCORE (see Section 3.3.2). The connection identifiers do not
have any cryptographic purpose in EDHOC.
Connection identifiers in EDHOC are byte strings or integers, encoded
in CBOR. One byte connection identifiers (the integers -24 to 23 and
the empty CBOR byte string h'') are realistic in many scenarios as
most constrained devices only have a few connections.
3.3.1. Selection of Connection Identifiers
C_I and C_R are chosen by I and R, respectively. The Initiator
selects C_I and sends it in message_1 for the Responder to use as a
reference to the connection in communications with the Initiator.
The Responder selects C_R and sends in message_2 for the Initiator to
use as a reference to the connection in communications with the
Responder.
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If connection identifiers are used by an application protocol for
which EDHOC establishes keys then the selected connection identifiers
SHALL adhere to the requirements for that protocol, see Section 3.3.2
for an example.
3.3.2. Use of Connection Identifiers with OSCORE
For OSCORE, the choice of a connection identifier results in the
endpoint selecting its Recipient ID, see Section 3.1 of [RFC8613],
for which certain uniqueness requirements apply, see Section 3.3 of
[RFC8613]. Therefore, the Initiator and the Responder MUST NOT
select connection identifiers such that it results in same OSCORE
Recipient ID. Since the Recipient ID is a byte string and a EDHOC
connection identifier is either a CBOR byte string or a CBOR integer,
care must be taken when selecting the connection identifiers and
converting them to Recipient IDs. A mapping from EDHOC connection
identifier to OSCORE Recipient ID is specified in Appendix A.1.
3.4. Transport
Cryptographically, EDHOC does not put requirements on the lower
layers. EDHOC is not bound to a particular transport layer and can
even be used in environments without IP. The transport is
responsible, where necessary, to handle:
* message loss,
* message reordering,
* message duplication,
* fragmentation,
* demultiplex EDHOC messages from other types of messages,
* denial-of-service protection,
* message correlation.
The Initiator and the Responder need to have agreed on a transport to
be used for EDHOC, see Section 3.9.
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3.4.1. Use of Connection Identifiers for EDHOC Message Correlation
The transport needs to support the correlation between EDHOC messages
and facilitate the retrieval of protocol state during EDHOC protocol
execution, including an indication of a message being message_1. The
correlation may reuse existing mechanisms in the transport protocol.
For example, the CoAP Token may be used to correlate EDHOC messages
in a CoAP response and an associated CoAP request.
Connection identifiers may be used to correlate EDHOC messages and
facilitate the retrieval of protocol state during EDHOC protocol
execution. EDHOC transports that do not inherently provide
correlation across all messages of an exchange can send connection
identifiers along with EDHOC messages to gain that required
capability, e.g., by prepending the appropriate connection identifier
(when available from the EDHOC protocol) to the EDHOC message.
Transport of EDHOC in CoAP payloads is described in Appendix A.3,
which also shows how to use connection identifiers and message_1
indication with CoAP.
3.5. Authentication Parameters
EDHOC supports various settings for how the other endpoint's
authentication (public) key is transported, identified, and trusted
as described in this section.
The authentication key (see Section 3.5.2) is used in several parts
of EDHOC:
1. as part of the authentication credential included in the
integrity calculation
2. for verification of the Signature_or_MAC field in message_2 and
message_3 (see Section 5.3.2 and Section 5.4.2)
3. in the key derivation (in case of a static Diffie-Hellman key,
see Section 4).
The authentication credential (CRED_x) contains, in addition to the
authentication key, also the authentication key algorithm and
optionally other parameters such as identity, key usage, expiry,
issuer, etc. (see Section 3.5.3). Identical authentication
credentials need to be established in both endpoints to be able to
verify integrity. For many settings it is not necessary to transport
the authentication credential within EDHOC over constrained links,
for example, it may be pre-provisioned or acquired out-of-band over
less constrained links.
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EDHOC relies on COSE for identification of authentication credentials
(using ID_CRED_x, see Section 3.5.4) and supports all credential
types for which COSE header parameters are defined (see
Section 3.5.3).
The choice of authentication credential depends also on the trust
model (see Section 3.5.1). For example, a certificate or CWT may
rely on a trusted third party, whereas a CCS or a self-signed
certificate/CWT may be used when trust in the public key can be
achieved by other means, or in the case of trust-on-first-use.
The type of authentication key, authentication credential, and the
way to identify the credential have a large impact on the message
size. For example, the signature_or_MAC field is much smaller with a
static DH key than with a signature key. A CCS is much smaller than
a self-signed certificate/CWT, but if it is possible to reference the
credential with a COSE header like 'kid', then that is typically much
smaller than to transport a CCS.
3.5.1. Identities and trust anchors
Policies for what connections to allow are typically 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 and signed by a particular CA. On the other hand, the device
may only be allowed to connect to a network which authenticates with
a particular public key (information of which may be provisioned,
e.g., out of band or in the external authorization data, see
Section 3.8). The EDHOC implementation or the application must
enforce information about the intended endpoint, and in particular
whether it is a specific identity or a set of identities. Either
EDHOC passes information about identity to the application for a
decision, or EDHOC needs to have access to relevant information and
makes the decision on its own.
EDHOC assumes the existence of mechanisms (certification authority,
trusted third party, pre-provisioning, etc.) for specifying and
distributing authentication credentials.
* When a Public Key Infrastructure (PKI) is used with certificates,
the trust anchor is a Certification Authority (CA) certificate,
and the identity is the subject whose unique name (e.g., a domain
name, NAI, or EUI) is included in the endpoint's certificate. In
order to run EDHOC each party needs at least one CA public key
certificate, or just the public key, and a specific identity or
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set of identities it is allowed to communicate with. Only
validated public-key certificates with an allowed subject name, as
specified by the application, are to be accepted. EDHOC provides
proof that the other party possesses the private authentication
key corresponding to the public authentication key in its
certificate. The certification path provides proof that the
subject of the certificate owns the public key in the certificate.
* Similarly, when a PKI is used with CWTs, each party needs to have
a trusted third party public key as trust anchor to verify the
end-entity CWTs, and a specific identity or set of identities in
the 'sub' (subject) claim of the CWT to determine if it is allowed
to communicate with. The trusted third party public key can,
e.g., be stored in a self-signed CWT or in a CCS.
* When PKI is not used (CCS, self-signed certificate/CWT), the trust
anchor is the authentication key of the other party. In this
case, the identity is typically directly associated to the
authentication key of the other party. For example, the name of
the subject may be a canonical representation of the public key.
Alternatively, if identities can be expressed in the form of
unique subject names assigned to public keys, then a binding to
identity can be achieved by including both public key and
associated subject name in the protocol message computation:
CRED_I or CRED_R may be a self-signed certificate/CWT or CCS
containing the authentication key and the subject name, see
Section 3.5.3. In order to run EDHOC, each endpoint needs a
specific authentication key/unique associated subject name, or a
set of public authentication keys/unique associated subject names,
which it is allowed to communicate with. EDHOC provides the proof
that the other party possesses the private authentication key
corresponding to the public authentication 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".
EDHOC follows SIGMA by calculating a MAC over the whole credential,
which in case of an X.509 or C509 certificate includes the "subject"
and "subjectAltName" fields, and in the case of CWT or CCS includes
the "sub" claim. While the SIGMA paper only focuses on the identity,
the same principle is true for other information such as policies
associated to the public key.
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3.5.2. Authentication Keys
The authentication key (i.e. the public key used for authentication)
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. The authentication key algorithm needs to be compatible
with the method and the cipher suite. The authentication key
algorithm needs to be compatible with the EDHOC key exchange
algorithm when static Diffie-Hellman authentication is used, and
compatible with the EDHOC signature algorithm when signature
authentication is used.
Note that for most signature algorithms, the signature is determined
by the signature algorithm and the authentication key algorithm
together. When using 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.
For X.509 the authentication key is represented with a
SubjectPublicKeyInfo field. For CWT and CCS, the authentication key
is represented with a 'cnf' claim [RFC8747] containing a COSE_Key
[I-D.ietf-cose-rfc8152bis-struct].
3.5.3. Authentication Credentials
The authentication credentials, CRED_I and CRED_R, contain the public
authentication key of the Initiator and the Responder, respectively.
EDHOC relies on COSE for identification of authentication credentials
(see Section 3.5.4) and supports all credential types for which COSE
header parameters are defined including X.509 [RFC5280], C509
[I-D.ietf-cose-cbor-encoded-cert], CWT [RFC8392] and CWT Claims Set
(CCS) [RFC8392]. When the identified credential is a chain or bag,
CRED_x is just the end-entity X.509 or C509 certificate / CWT. In
X.509 and C509 certificates, signature keys typically have key usage
"digitalSignature" and Diffie-Hellman public keys typically have key
usage "keyAgreement".
CRED_x needs to be defined such that it is identical when used by
Initiator or Responder. The Initiator and Responder are expected to
agree on a specific encoding of the credential, see Section 3.9. It
is RECOMMENDED that the COSE 'kid' parameter, when used, refers to a
specific encoding. The Initiator and Responder SHOULD use an
available authentication credential (transported in EDHOC or
otherwise provisioned) without re-encoding. If for some reason re-
encoding of the authentication credential may occur, then a potential
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common encoding for CBOR based credentials is bytewise lexicographic
order of their deterministic encodings as specified in Section 4.2.1
of [RFC8949].
* When the authentication credential is an X.509 certificate, CRED_x
SHALL be the end-entity DER encoded certificate, encoded as a bstr
[I-D.ietf-cose-x509].
* When the authentication credential is a C509 certificate, CRED_x
SHALL be the end-entity C509Certificate
[I-D.ietf-cose-cbor-encoded-cert]
* When the authentication credential is a COSE_Key in a CWT, CRED_x
SHALL be the untagged CWT.
* When the authentication credential is a COSE_Key but not in a CWT,
CRED_x SHALL be an untagged CCS.
- Naked COSE_Keys are thus dressed as CCS when used in EDHOC,
which is done by prefixing the COSE_Key with 0xA108A101.
An example of a CRED_x is shown below:
{ /CCS/
2 : "42-50-31-FF-EF-37-32-39", /sub/
8 : { /cnf/
1 : { /COSE_Key/
1 : 1, /kty/
2 : 0, /kid/
-1 : 4, /crv/
-2 : h'b1a3e89460e88d3a8d54211dc95f0b90 /x/
3ff205eb71912d6db8f4af980d2db83a'
}
}
}
Figure 5: A CCS Containing an X25519 Static Diffie-Hellman Key
and an EUI-64 Identity.
3.5.4. Identification of Credentials
ID_CRED_R and ID_CRED_I are transported in message_2 and message_3,
respectively (see Section 5.3.2 and Section 5.4.2). They are used to
identify and optionally transport the authentication keys of the
Initiator and the Responder, respectively. ID_CRED_I and ID_CRED_R
do not have any cryptographic purpose in EDHOC since EDHOC integrity
protects the authentication credential. EDHOC relies on COSE for
identification of authentication credentials and supports all types
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of COSE header parameters used to identify authentication credentials
including X.509, C509, CWT and CCS.
* ID_CRED_R is intended to facilitate for the Initiator to retrieve
the Responder's authentication key.
* ID_CRED_I is intended to facilitate for the Responder to retrieve
the Initiator's authentication key.
ID_CRED_I and ID_CRED_R are COSE header maps and contains one or more
COSE header parameter. ID_CRED_I and ID_CRED_R MAY contain different
header parameters. The header parameters typically provide some
information about the format of authentication credential.
Note that COSE header parameters in ID_CRED_x are used to identify
the sender's authentication credential. There is therefore no reason
to use the "-sender" header parameters, such as x5t-sender, defined
in Section 3 of [I-D.ietf-cose-x509]. Instead, the corresponding
parameter without "-sender", such as x5t, SHOULD be used.
Example: X.509 certificates can be identified by a hash value using
the 'x5t' parameter:
* ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R,
Example: CWT or CCS can be identified by a key identifier using the
'kid' parameter:
* ID_CRED_x = { 4 : key_id_x }, where key_id_x : kid, for x = I or
R.
Note that 'kid' is extended to support int values to allow more one-
byte identifiers (see Section 9.7 and Section 9.8) which may be
useful in many scenarios since constrained devices only have a few
keys. As stated in Section 3.1 of [I-D.ietf-cose-rfc8152bis-struct],
applications MUST NOT assume that 'kid' values are unique and several
keys associated with a 'kid' may need to be checked before the
correct one is found. Applications might use additional information
such as 'kid context' or lower layers to determine which key to try
first. Applications should strive to make ID_CRED_x as unique as
possible, since the recipient may otherwise have to try several keys.
See Appendix C.3 for more examples.
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3.6. Cipher Suites
An EDHOC cipher suite consists of an ordered set of algorithms from
the "COSE Algorithms" and "COSE Elliptic Curves" registries as well
as the EDHOC MAC length. Algorithms need to be specified with enough
parameters to make them completely determined. EDHOC is currently
only specified for use with key exchange algorithms of type ECDH
curves, but any Key Encapsulation Method (KEM), including Post-
Quantum Cryptography (PQC) KEMs, can be used in method 0, see
Section 8.4. Use of other types of key exchange algorithms to
replace static DH authentication (method 1,2,3) would likely require
a specification updating EDHOC with new methods.
EDHOC supports all signature algorithms defined by COSE, including
PQC signature algorithms such as HSS-LMS. Just like in TLS 1.3
[RFC8446] and IKEv2 [RFC7296], a signature in COSE is determined by
the signature algorithm and the authentication key algorithm
together, see Section 3.5.2. The exact details of the authentication
key algorithm depend on the type of authentication credential. COSE
supports different formats for storing the public authentication keys
including COSE_Key and X.509, which have different names and ways to
represent the authentication key and the authentication key
algorithm.
An EDHOC cipher suite consists of the following parameters:
* EDHOC AEAD algorithm
* EDHOC hash algorithm
* EDHOC MAC length in bytes (Static DH)
* EDHOC key exchange algorithm (ECDH curve)
* EDHOC signature algorithm
* Application AEAD algorithm
* Application hash algorithm
Each cipher suite is identified with a pre-defined int label.
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EDHOC can be used with all algorithms and curves defined for COSE.
Implementation can either use any combination of COSE algorithms and
parameters to define their own private cipher suite, or use one of
the pre-defined cipher suites. Private cipher suites can be
identified with any of the four values -24, -23, -22, -21. The pre-
defined cipher suites are listed in the IANA registry (Section 9.2)
with initial content outlined here:
* Cipher suites 0-3, based on AES-CCM, are intended for constrained
IoT where message overhead is a very important factor. Note that
AES-CCM-16-64-128 and AES-CCM-16-64-128 are compatible with the
IEEE CCM* mode.
- Cipher suites 1 and 3 use a larger tag length (128-bit) in
EDHOC than in the Application AEAD algorithm (64-bit).
* Cipher suites 4 and 5, based on ChaCha20, are intended for less
constrained applications and only use 128-bit tag lengths.
* Cipher suite 6, based on AES-GCM, is for general non-constrained
applications. It uses high performance algorithms that are widely
supported.
* Cipher suites 24 and 25 are intended for high security
applications such as government use and financial applications.
These cipher suites do not share any algorithms. Cipher suite 24
is compatible with the CNSA suite [CNSA].
The different methods (Section 3.2) use the same cipher suites, but
some algorithms are not used in some methods. The EDHOC signature
algorithm is not used in methods without signature authentication.
The Initiator needs to have a list of cipher suites it supports in
order of preference. The Responder needs to have a list of cipher
suites it supports. SUITES_I contains cipher suites supported by the
Initiator, formatted and processed as detailed in Section 5.2.1 to
secure the cipher suite negotiation. Examples of cipher suite
negotiation are given in Section 6.3.2.
3.7. Ephemeral Public Keys
EDHOC always uses compact representation of elliptic curve points,
see Appendix B. In COSE compact representation is achieved by
formatting the ECDH ephemeral public keys as COSE_Keys of type EC2 or
OKP according to Sections 7.1 and 7.2 of
[I-D.ietf-cose-rfc8152bis-algs], but only including the 'x' parameter
in G_X and G_Y. For Elliptic Curve Keys of type EC2, compact
representation MAY be used also in the COSE_Key. If the COSE
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implementation requires an 'y' parameter, the value y = false SHALL
be used. COSE always use compact output for Elliptic Curve Keys of
type EC2.
3.8. External Authorization Data (EAD)
In order to reduce round trips and number of messages or to simplify
processing, external security applications may be integrated into
EDHOC by transporting authorization related data in the messages.
One example is third-party identity and authorization information
protected out of scope of EDHOC [I-D.selander-ace-ake-authz].
Another example is a certificate enrolment request or the resulting
issued certificate.
EDHOC allows opaque external authorization data (EAD) to be sent in
the EDHOC messages. External authorization data sent in message_1
(EAD_1) or message_2 (EAD_2) should be considered unprotected by
EDHOC, see Section 8.5. External authorization data sent in
message_3 (EAD_3) or message_4 (EAD_4) is protected between Initiator
and Responder.
External authorization data is a CBOR sequence (see Appendix C.1)
consisting of one or more (ead_label, ead_value) pairs as defined
below:
ead = 1* (
ead_label : int,
ead_value : any,
)
Applications using external authorization data need to specify
format, processing, and security considerations and register the
(ead_label, ead_value) pair, see Section 9.5. The CDDL type of
ead_value is determined by the int ead_label and MUST be specified.
The EAD fields of EDHOC are not intended for generic application
data. Since data carried in EAD_1 and EAD_2 fields may not be
protected, special considerations need to be made such that it does
not violate security and privacy requirements of the service which
uses this data. Moreover, the content in an EAD field may impact the
security properties provided by EDHOC. Security applications making
use of the EAD fields must perform the necessary security analysis.
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3.9. Applicability Statement
EDHOC requires certain parameters to be agreed upon between Initiator
and Responder. Some parameters can be agreed through the protocol
execution (specifically cipher suite negotiation, see Section 3.6)
but other parameters may need to be known out-of-band (e.g., which
authentication method is used, see Section 3.2).
The purpose of the applicability statement is to describe the
intended use of EDHOC to allow for the relevant processing and
verifications to be made, including things like:
1. How the endpoint detects that an EDHOC message is received. This
includes how EDHOC messages are transported, for example in the
payload of a CoAP message with a certain Uri-Path or Content-
Format; see Appendix A.3.
* The method of transporting EDHOC messages may also describe
data carried along with the messages that are needed for the
transport to satisfy the requirements of Section 3.4, e.g.,
connection identifiers used with certain messages, see
Appendix A.3.
2. Authentication method (METHOD; see Section 3.2).
3. Profile for authentication credentials (CRED_I, CRED_R; see
Section 3.5.3), e.g., profile for certificate or CCS, including
supported authentication key algorithms (subject public key
algorithm in X.509 or C509 certificate).
4. Type used to identify authentication credentials (ID_CRED_I,
ID_CRED_R; see Section 3.5.4).
5. Use and type of external authorization data (EAD_1, EAD_2, EAD_3,
EAD_4; see Section 3.8).
6. Identifier used as identity of endpoint; see Section 3.5.1.
7. If message_4 shall be sent/expected, and if not, how to ensure a
protected application message is sent from the Responder to the
Initiator; see Section 5.5.
The applicability statement may also contain information about
supported cipher suites. The procedure for selecting and verifying
cipher suite is still performed as described in Section 5.2.1 and
Section 6.3, but it may become simplified by this knowledge.
An example of an applicability statement is shown in Appendix D.
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For some parameters, like METHOD, ID_CRED_x, type of EAD, the
receiver is able to verify compliance with applicability statement,
and if it needs to fail because of incompliance, to infer the reason
why the protocol failed.
For other parameters, like CRED_x in the case that it is not
transported, it may not be possible to verify that incompliance with
applicability statement was the reason for failure: Integrity
verification in message_2 or message_3 may fail not only because of
wrong authentication credential. For example, in case the Initiator
uses public key certificate by reference (i.e., not transported
within the protocol) then both endpoints need to use an identical
data structure as CRED_I or else the integrity verification will
fail.
Note that it is not necessary for the endpoints to specify a single
transport for the EDHOC messages. For example, a mix of CoAP and
HTTP may be used along the path, and this may still allow correlation
between messages.
The applicability statement may be dependent on the identity of the
other endpoint, or other information carried in an EDHOC message, but
it then applies only to the later phases of the protocol when such
information is known. (The Initiator does not know identity of
Responder before having verified message_2, and the Responder does
not know identity of the Initiator before having verified message_3.)
Other conditions may be part of the applicability statement, such as
target application or use (if there is more than one application/use)
to the extent that EDHOC can distinguish between them. In case
multiple applicability statements are used, the receiver needs to be
able to determine which is applicable for a given session, for
example based on URI or external authorization data type.
4. Key Derivation
EDHOC uses Extract-and-Expand [RFC5869] with the EDHOC hash algorithm
in the selected cipher suite to derive keys used in EDHOC and in the
application. 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.
This section defines Extract, Expand and other key derivation
functions based on these: Expand is used to define EDHOC-KDF and in
turn EDHOC-Exporter, whereas Extract is used to define EDHOC-
KeyUpdate.
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4.1. Extract
The pseudorandom keys (PRKs) are derived using Extract.
PRK = Extract( salt, IKM )
where the input keying material (IKM) and salt are defined for each
PRK below.
The definition of Extract depends on the EDHOC hash algorithm of the
selected cipher suite:
* if the EDHOC hash algorithm is SHA-2, then Extract( salt, IKM ) =
HKDF-Extract( salt, IKM ) [RFC5869]
* if the EDHOC hash algorithm is SHAKE128, then Extract( salt, IKM )
= KMAC128( salt, IKM, 256, "" )
* if the EDHOC hash algorithm is SHAKE256, then Extract( salt, IKM )
= KMAC256( salt, IKM, 512, "" )
4.1.1. PRK_2e
PRK_2e is used to derive a keystream to encrypt message_2. PRK_2e is
derived with the following input:
* The salt SHALL be a zero-length byte string. Note that [RFC5869]
specifies that if the salt is not provided, it is set to a string
of zeros (see Section 2.2 of [RFC5869]). For implementation
purposes, not providing the salt is the same as setting the salt
to the zero-length byte string (0x).
* The IKM SHALL be the ephemeral-ephemeral ECDH shared secret G_XY
(calculated from G_X and Y or G_Y and X) as defined in
Section 6.3.1 of [I-D.ietf-cose-rfc8152bis-algs]. The use of G_XY
gives forward secrecy, in the sense that compromise of the private
authentication keys does not compromise past session keys.
Example: Assuming the use of curve25519, the ECDH shared secret G_XY
is the output of the X25519 function [RFC7748]:
G_XY = X25519( Y, G_X ) = X25519( X, G_Y )
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 )
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where salt = 0x (zero-length byte string).
4.1.2. PRK_3e2m
PRK_3e2m is used to produce a MAC in message_2 and to encrypt
message_3. PRK_3e2m is derived as follows:
If the Responder authenticates with a static Diffie-Hellman key, then
PRK_3e2m = Extract( PRK_2e, G_RX ), where G_RX is the ECDH shared
secret calculated from G_R and X, or G_X and R, else PRK_3e2m =
PRK_2e.
4.1.3. PRK_4x3m
PRK_4x3m is used to produce a MAC in message_3, to encrypt message_4,
and to derive application specific data. PRK_4x3m is derived as
follows:
If the Initiator authenticates with a static Diffie-Hellman key, then
PRK_4x3m = Extract( PRK_3e2m, G_IY ), where G_IY is the ECDH shared
secret calculated from G_I and Y, or G_Y and I, else PRK_4x3m =
PRK_3e2m.
4.2. Expand
The keys, IVs and MACs used in EDHOC are derived from the PRKs using
Expand, and instantiated with the EDHOC AEAD algorithm in the
selected cipher suite.
OKM = EDHOC-KDF( PRK, transcript_hash, label, context, length )
= Expand( PRK, info, length )
where info is encoded as the CBOR sequence
info = (
transcript_hash : bstr,
label : tstr,
context : bstr,
length : uint,
)
where
* transcript_hash is a bstr set to one of the transcript hashes
TH_2, TH_3, or TH_4 as defined in Sections 5.3.1, 5.4.1, and 4.3.
* label is a tstr set to the name of the derived key, IV or MAC;
i.e., "KEYSTREAM_2", "MAC_2", "K_3", "IV_3", or "MAC_3".
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* context is a bstr
* length is the length of output keying material (OKM) in bytes
The definition of Expand depends on the EDHOC hash algorithm of the
selected cipher suite:
* if the EDHOC hash algorithm is SHA-2, then Expand( PRK, info,
length ) = HKDF-Expand( PRK, info, length ) [RFC5869]
* if the EDHOC hash algorithm is SHAKE128, then Expand( PRK, info,
length ) = KMAC128( PRK, info, L, "" )
* if the EDHOC hash algorithm is SHAKE256, then Expand( PRK, info,
length ) = KMAC256( PRK, info, L, "" )
where L = 8*length, the output length in bits.
The keys, IVs and MACs are derived as follows:
* KEYSTREAM_2 is derived using the transcript hash TH_2 and the
pseudorandom key PRK_2e.
* MAC_2 is derived using the transcript hash TH_2 and the
pseudorandom key PRK_3e2m.
* K_3 and IV_3 are derived using the transcript hash TH_3 and the
pseudorandom key PRK_3e2m. IVs are only used if the EDHOC AEAD
algorithm uses IVs.
* MAC_3 is derived using the transcript hash TH_3 and the
pseudorandom key PRK_4x3m.
KEYSTREAM_2, K_3, and IV_3 use an empty CBOR byte string h'' as
context. MAC_2 and MAC_3 use context as defined in Section 5.3.2 and
Section 5.4.2, respectively.
4.3. EDHOC-Exporter
Application keys and other application specific data can be derived
using the EDHOC-Exporter interface defined as:
EDHOC-Exporter(label, context, length)
= EDHOC-KDF(PRK_4x3m, TH_4, label, context, length)
where label is a registered tstr from the EDHOC Exporter Label
registry (Section 9.1), context is a bstr defined by the application,
and length is a uint defined by the application. The (label,
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context) pair must be unique, i.e., a (label, context) MUST NOT be
used for two different purposes. However an application can re-
derive the same key several times as long as it is done in a secure
way. For example, in most encryption algorithms the same key kan be
reused with different nonces. The context can for example be the
empty (zero-length) sequence or a single CBOR byte string.
The transcript hash TH_4 is a CBOR encoded bstr and the input to the
hash function is a CBOR Sequence.
TH_4 = H( TH_3, CIPHERTEXT_3 )
where H() is the hash function in the selected cipher suite.
Examples of use of the EDHOC-Exporter are given in Section 5.5.2 and
Appendix A.
* K_4 and IV_4 are derived with the EDHOC-Exporter using the empty
CBOR byte string h'' as context, and labels "EDHOC_K_4" and
"EDHOC_IV_4", respectively. IVs are only used if the EDHOC AEAD
algorithm uses IVs.
4.4. EDHOC-KeyUpdate
To provide forward secrecy in an even more efficient way than re-
running EDHOC, EDHOC provides the function EDHOC-KeyUpdate. When
EDHOC-KeyUpdate is called the old PRK_4x3m is deleted and the new
PRK_4x3m is calculated as a "hash" of the old key using the Extract
function as illustrated by the following pseudocode:
EDHOC-KeyUpdate( nonce ):
PRK_4x3m = Extract( nonce, PRK_4x3m )
The EDHOC-KeyUpdate takes a nonce as input to guarantee that there
are no short cycles. The Initiator and the Responder need to agree
on the nonce, which can e.g., be a counter or a random number. While
the KeyUpdate method provides forward secrecy it does not give as
strong security properties as re-running EDHOC, see Section 8.
5. Message Formatting and Processing
This section specifies formatting of the messages and processing
steps. Error messages are specified in Section 6. Annotated traces
of EDHOC protocol runs are provided in [I-D.selander-lake-traces].
An EDHOC message is encoded as a sequence of CBOR data items (CBOR
Sequence, [RFC8742]). Additional optimizations are made to reduce
message overhead.
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While EDHOC uses the COSE_Key, COSE_Sign1, and COSE_Encrypt0
structures, only a subset of the parameters is included in the EDHOC
messages, see Appendix C.3. The unprotected COSE header in
COSE_Sign1, and COSE_Encrypt0 (not included in the EDHOC message) MAY
contain parameters (e.g., 'alg').
5.1. Message Processing Outline
This section outlines the message processing of EDHOC.
For each new/ongoing session, the endpoints are assumed to keep an
associated protocol state containing identifiers, keying material,
etc. used for subsequent processing of protocol related data. The
protocol state is assumed to be associated to an applicability
statement (Section 3.9) which provides the context for how messages
are transported, identified, and processed.
EDHOC messages SHALL be processed according to the current protocol
state. The following steps are expected to be performed at reception
of an EDHOC message:
1. Detect that an EDHOC message has been received, for example by
means of port number, URI, or media type (Section 3.9).
2. Retrieve the protocol state according to the message correlation
provided by the transport, see Section 3.4. If there is no
protocol state, in the case of message_1, a new protocol state is
created. The Responder endpoint needs to make use of available
Denial-of-Service mitigation (Section 8.6).
3. If the message received is an error message, then process
according to Section 6, else process as the expected next message
according to the protocol state.
If the processing fails for some reason then, typically, an error
message is sent, the protocol is discontinued, and the protocol state
erased. Further details are provided in the following subsections
and in Section 6.
Different instances of the same message MUST NOT be processed in one
session. Note that processing will fail if the same message appears
a second time for EDHOC processing because the state of the protocol
has moved on and now expects something else. This assumes that
message duplication due to re-transmissions is handled by the
transport protocol, see Section 3.4. The case when the transport
does not support message deduplication is addressed in Appendix E.
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5.2. EDHOC Message 1
5.2.1. Formatting of Message 1
message_1 SHALL be a CBOR Sequence (see Appendix C.1) as defined
below
message_1 = (
METHOD : int,
SUITES_I : suites,
G_X : bstr,
C_I : bstr / int,
? EAD_1 : ead,
)
suites = [ 2* int ] / int
where:
* METHOD - authentication method, see Section 3.2.
* SUITES_I - array of cipher suites which the Initiator supports in
order of preference, starting with the most preferred and ending
with the cipher suite selected for this session. If the most
preferred cipher suite is selected then SUITES_I is encoded as
that cipher suite, i.e., as an int. The processing steps are
detailed below and in Section 6.3.
* G_X - the ephemeral public key of the Initiator
* C_I - variable length connection identifier
* EAD_1 - unprotected external authorization data, see Section 3.8.
5.2.2. Initiator Processing of Message 1
The Initiator SHALL compose message_1 as follows:
* SUITES_I contains a list of supported cipher suites, in order of
preference, truncated after the cipher suite selected for this
session.
- The Initiator MUST select its most preferred cipher suite,
conditioned on what it can assume to be supported by the
Responder.
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- The selected cipher suite MAY be changed between sessions,
e.g., based on previous error messages (see next bullet), but
all cipher suites which are more preferred than the selected
cipher suite in the list MUST be included in SUITES_I.
- If the Initiator previously received from the Responder an
error message with error code 2 (see Section 6.3) indicating
cipher suites supported by the Responder, then the Initiator
SHOULD select the most preferred supported cipher suite among
those (note that error messages are not authenticated and may
be forged).
- The supported cipher suites and the order of preference MUST
NOT be changed based on previous error messages.
* Generate an ephemeral ECDH key pair using the curve in the
selected cipher suite and format it as a COSE_Key. Let G_X be the
'x' parameter of the COSE_Key.
* Choose a connection identifier C_I and store it for the length of
the protocol.
* Encode message_1 as a sequence of CBOR encoded data items as
specified in Section 5.2.1
5.2.3. Responder Processing of Message 1
The Responder SHALL process message_1 as follows:
* Decode message_1 (see Appendix C.1).
* Verify that the selected cipher suite is supported and that no
prior cipher suite in SUITES_I is supported.
* Pass EAD_1 to the security application.
If any processing step fails, the Responder SHOULD send an EDHOC
error message back, formatted as defined in Section 6, and the
session MUST be discontinued. Sending error messages is essential
for debugging but MAY e.g., be skipped due to denial-of-service
reasons, see Section 8.6. If an error message is sent, the session
MUST be discontinued.
5.3. EDHOC Message 2
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5.3.1. Formatting of Message 2
message_2 SHALL be a CBOR Sequence (see Appendix C.1) as defined
below
message_2 = (
G_Y_CIPHERTEXT_2 : bstr,
C_R : bstr / int,
)
where:
* G_Y_CIPHERTEXT_2 - the concatenation of G_Y, the ephemeral public
key of the Responder, and CIPHERTEXT_2
* C_R - variable length connection identifier
5.3.2. Responder Processing of Message 2
The Responder SHALL compose message_2 as follows:
* Generate an ephemeral ECDH key pair using the curve in the
selected cipher suite and format it as a COSE_Key. Let G_Y be the
'x' parameter of the COSE_Key.
* Choose a connection identifier C_R and store it for the length of
the protocol.
* Compute the transcript hash TH_2 = H( H(message_1), G_Y, C_R )
where H() is the hash function in the selected cipher suite. The
transcript hash TH_2 is a CBOR encoded bstr and the input to the
hash function is a CBOR Sequence. Note that H(message_1) can be
computed and cached already in the processing of message_1.
* Compute MAC_2 = EDHOC-KDF( PRK_3e2m, TH_2, "MAC_2", << ID_CRED_R,
CRED_R, ? EAD_2 >>, mac_length_2 ). If the Responder
authenticates with a static Diffie-Hellman key (method equals 1 or
3), then mac_length_2 is the EDHOC MAC length given by the
selected cipher suite. If the Responder authenticates with a
signature key (method equals 0 or 2), then mac_length_2 is equal
to the output size of the EDHOC hash algorithm given by the
selected cipher suite.
- ID_CRED_R - identifier to facilitate retrieval of CRED_R, see
Section 3.5.4
- CRED_R - CBOR item containing the credential of the Responder,
see Section 3.5.3
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- EAD_2 - unprotected external authorization data, see
Section 3.8
* 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' field of a
COSE_Sign1 object as defined in Section 4.4 of
[I-D.ietf-cose-rfc8152bis-struct] using the signature algorithm of
the selected cipher suite, the private authentication key of the
Responder, and the following parameters as input (see
Appendix C.3):
- protected = << ID_CRED_R >>
- external_aad = << TH_2, CRED_R, ? EAD_2 >>
- payload = MAC_2
* CIPHERTEXT_2 is calculated by using the Expand function as a
binary additive stream cipher.
- plaintext = ( ID_CRED_R / bstr / int, Signature_or_MAC_2, ?
EAD_2 )
o If ID_CRED_R contains a single 'kid' parameter, i.e.,
ID_CRED_R = { 4 : kid_R }, then only the byte string or
integer kid_R is conveyed in the plaintext encoded
accordingly as bstr or int.
- Compute KEYSTREAM_2 = EDHOC-KDF( PRK_2e, TH_2, "KEYSTREAM_2",
h'', plaintext_length ), where plaintext_length is the length
of the plaintext.
- CIPHERTEXT_2 = plaintext XOR KEYSTREAM_2
* Encode message_2 as a sequence of CBOR encoded data items as
specified in Section 5.3.1.
5.3.3. Initiator Processing of Message 2
The Initiator SHALL process message_2 as follows:
* Decode message_2 (see Appendix C.1).
* Retrieve the protocol state using the message correlation provided
by the transport (e.g., the CoAP Token, the 5-tuple, or the
prepended C_I, see Appendix A.3).
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* Decrypt CIPHERTEXT_2, see Section 5.3.2.
* Pass EAD_2 to the security application.
* Verify that the identity of the Responder is an allowed identity
for this connection, see Section 3.5.1.
* Verify Signature_or_MAC_2 using the algorithm in the selected
cipher suite. The verification process depends on the method, see
Section 5.3.2.
If any processing step fails, the Initiator SHOULD send an EDHOC
error message back, formatted as defined in Section 6. Sending error
messages is essential for debugging but MAY e.g., be skipped if a
session cannot be found or due to denial-of-service reasons, see
Section 8.6. If an error message is sent, the session MUST be
discontinued.
5.4. EDHOC Message 3
5.4.1. Formatting of Message 3
message_3 SHALL be a CBOR Sequence (see Appendix C.1) as defined
below
message_3 = (
CIPHERTEXT_3 : bstr,
)
5.4.2. Initiator Processing of Message 3
The Initiator SHALL compose message_3 as follows:
* Compute the transcript hash TH_3 = H(TH_2, CIPHERTEXT_2) where H()
is the hash function in the selected cipher suite. The transcript
hash TH_3 is a CBOR encoded bstr and the input to the hash
function is a CBOR Sequence. Note that H(TH_2, CIPHERTEXT_2) can
be computed and cached already in the processing of message_2.
* Compute MAC_3 = EDHOC-KDF( PRK_4x3m, TH_3, "MAC_3", << ID_CRED_I,
CRED_I, ? EAD_3 >>, mac_length_3 ). If the Initiator
authenticates with a static Diffie-Hellman key (method equals 2 or
3), then mac_length_3 is the EDHOC MAC length given by the
selected cipher suite. If the Initiator authenticates with a
signature key (method equals 0 or 1), then mac_length_3 is equal
to the output size of the EDHOC hash algorithm given by the
selected cipher suite.
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- ID_CRED_I - identifier to facilitate retrieval of CRED_I, see
Section 3.5.4
- CRED_I - CBOR item containing the credential of the Initiator,
see Section 3.5.3
- EAD_3 - protected external authorization data, see Section 3.8
* 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' field of a
COSE_Sign1 object as defined in Section 4.4 of
[I-D.ietf-cose-rfc8152bis-struct] using the signature algorithm of
the selected cipher suite, the private authentication key of the
Initiator, and the following parameters as input (see
Appendix C.3):
- protected = << ID_CRED_I >>
- external_aad = << TH_3, CRED_I, ? EAD_3 >>
- payload = MAC_3
* Compute a COSE_Encrypt0 object as defined in Section 5.3 of
[I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD algorithm
of the selected cipher suite, using the encryption key K_3, the
initialization vector IV_3, the plaintext P, and the following
parameters as input (see Appendix C.3):
- protected = h''
- external_aad = TH_3
where
- K_3 = EDHOC-KDF( PRK_3e2m, TH_3, "K_3", h'', key_length )
o key_length - length of the encryption key of the EDHOC AEAD
algorithm
- IV_3 = EDHOC-KDF( PRK_3e2m, TH_3, "IV_3", h'', iv_length )
o iv_length - length of the intialization vector of the EDHOC
AEAD algorithm
- P = ( ID_CRED_I / bstr / int, Signature_or_MAC_3, ? EAD_3 )
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o If ID_CRED_I contains a single 'kid' parameter, i.e.,
ID_CRED_I = { 4 : kid_I }, only the byte string or integer
kid_I is conveyed in the plaintext encoded accordingly as
bstr or int.
CIPHERTEXT_3 is the 'ciphertext' of COSE_Encrypt0.
* Encode message_3 as a CBOR data item as specified in
Section 5.4.1.
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, see Section 4.3.
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 can securely derive application keys
and send protected application data. However, the Initiator does not
know that the Responder has actually computed the key PRK_4x3m and
therefore the Initiator SHOULD NOT permanently store the keying
material PRK_4x3m and TH_4, or derived application keys, until the
Initiator is assured that the Responder has actually computed the key
PRK_4x3m (explicit key confirmation). This is similar to waiting for
acknowledgement (ACK) in a transport protocol. Explicit key
confirmation is e.g., assured when the Initiator has verified an
OSCORE message or message_4 from the Responder.
5.4.3. Responder Processing of Message 3
The Responder SHALL process message_3 as follows:
* Decode message_3 (see Appendix C.1).
* Retrieve the protocol state using the message correlation provided
by the transport (e.g., the CoAP Token, the 5-tuple, or the
prepended C_I, see Appendix A.3).
* Decrypt and verify the COSE_Encrypt0 as defined in Section 5.3 of
[I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD algorithm
in the selected cipher suite, and the parameters defined in
Section 5.4.2.
* Pass EAD_3 to the security application.
* Verify that the identity of the Initiator is an allowed identity
for this connection, see Section 3.5.1.
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* Verify Signature_or_MAC_3 using the algorithm in the selected
cipher suite. The verification process depends on the method, see
Section 5.4.2.
* Pass the connection identifiers (C_I, C_R), and the application
algorithms in the selected cipher suite to the security
application. The application can now derive application keys
using the EDHOC-Exporter interface.
If any processing step fails, the Responder SHOULD send an EDHOC
error message back, formatted as defined in Section 6. Sending error
messages is essential for debugging but MAY e.g., be skipped if a
session cannot be found or due to denial-of-service reasons, see
Section 8.6. If an error message is sent, the session 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.
5.5. EDHOC Message 4
This section specifies message_4 which is OPTIONAL to support. Key
confirmation is normally provided by sending an application message
from the Responder to the Initiator protected with a key derived with
the EDHOC-Exporter, e.g., using OSCORE (see Appendix A). In
deployments where no protected application message is sent from the
Responder to the Initiator, the Responder MUST send message_4. Two
examples of such deployments:
1. When EDHOC is only used for authentication and no application
data is sent.
2. When application data is only sent from the Initiator to the
Responder.
Further considerations about when to use message_4 are provided in
Section 3.9 and Section 8.1.
5.5.1. Formatting of Message 4
message_4 SHALL be a CBOR Sequence (see Appendix C.1) as defined
below
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message_4 = (
CIPHERTEXT_4 : bstr,
)
5.5.2. Responder Processing of Message 4
The Responder SHALL compose message_4 as follows:
* Compute a COSE_Encrypt0 as defined in Section 5.3 of
[I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD algorithm
of the selected cipher suite, using the encryption key K_4, the
initialization vector IV_4, the plaintext P, and the following
parameters as input (see Appendix C.3):
- protected = h''
- external_aad = TH_4
where
- K_4 = EDHOC-Exporter( "EDHOC_K_4", h'', key_length )
o key_length - length of the encryption key of the EDHOC AEAD
algorithm
- IV_4 = EDHOC-Exporter( "EDHOC_IV_4", h'', iv_length )
o iv_length - length of the intialization vector of the EDHOC
AEAD algorithm
- P = ( ? EAD_4 )
o EAD_4 - protected external authorization data, see
Section 3.8.
CIPHERTEXT_4 is the 'ciphertext' of COSE_Encrypt0.
* Encode message_4 as a CBOR data item as specified in
Section 5.5.1.
5.5.3. Initiator Processing of Message 4
The Initiator SHALL process message_4 as follows:
* Decode message_4 (see Appendix C.1).
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* Retrieve the protocol state using the message correlation provided
by the transport (e.g., the CoAP Token, the 5-tuple, or the
prepended C_I, see Appendix A.3).
* Decrypt and verify the COSE_Encrypt0 as defined in Section 5.3 of
[I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD algorithm
in the selected cipher suite, and the parameters defined in
Section 5.5.2.
* Pass EAD_4 to the security application.
If any processing step fails, the Responder SHOULD send an EDHOC
error message back, formatted as defined in Section 6. Sending error
messages is essential for debugging but MAY e.g., be skipped if a
session cannot be found or due to denial-of-service reasons, see
Section 8.6. If an error message is sent, the session MUST be
discontinued.
6. Error Handling
This section defines the format for error messages.
An EDHOC error message can be sent by either endpoint as a reply to
any non-error EDHOC message. How errors at the EDHOC layer are
transported depends on lower layers, which need to enable error
messages to be sent and processed as intended.
Errors in EDHOC are fatal. After sending an error message, the
sender MUST discontinue the protocol. The receiver SHOULD treat an
error message as an indication that the other party likely has
discontinued the protocol. But as the error message is not
authenticated, a received error message might also have been sent by
an attacker and the receiver MAY therefore try to continue the
protocol.
error SHALL be a CBOR Sequence (see Appendix C.1) as defined below
error = (
ERR_CODE : int,
ERR_INFO : any,
)
Figure 6: EDHOC Error Message
where:
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* ERR_CODE - error code encoded as an integer. The value 0 is used
for success, all other values (negative or positive) indicate
errors.
* ERR_INFO - error information. Content and encoding depend on
error code.
The remainder of this section specifies the currently defined error
codes, see Figure 7. Additional error codes and corresponding error
information may be specified.
+----------+---------------+----------------------------------------+
| ERR_CODE | ERR_INFO Type | Description |
+==========+===============+========================================+
| 0 | any | Success |
+----------+---------------+----------------------------------------+
| 1 | tstr | Unspecified |
+----------+---------------+----------------------------------------+
| 2 | suites | Wrong selected cipher suite |
+----------+---------------+----------------------------------------+
Figure 7: Error Codes and Error Information
6.1. Success
Error code 0 MAY be used internally in an application to indicate
success, e.g., in log files. ERR_INFO can contain any type of CBOR
item. Error code 0 MUST NOT be used as part of the EDHOC message
exchange flow.
6.2. Unspecified
Error code 1 is used for errors that do not have a specific error
code defined. ERR_INFO MUST be a text string containing a human-
readable diagnostic message written in English. The diagnostic text
message is mainly intended for software engineers that during
debugging need to interpret it in the context of the EDHOC
specification. The diagnostic message SHOULD be provided to the
calling application where it SHOULD be logged.
6.3. Wrong Selected Cipher Suite
Error code 2 MUST only be used in a response to message_1 in case the
cipher suite selected by the Initiator is not supported by the
Responder, or if the Responder supports a cipher suite more preferred
by the Initiator than the selected cipher suite, see Section 5.2.3.
ERR_INFO is in this case denoted SUITES_R and is of type suites, see
Section 5.2.1. If the Responder does not support the selected cipher
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suite, then SUITES_R MUST include one or more supported cipher
suites. If the Responder supports a cipher suite in SUITES_I other
than the selected cipher suite (independently of if the selected
cipher suite is supported or not) then SUITES_R MUST include the
supported cipher suite in SUITES_I which is most preferred by the
Initiator. SUITES_R MAY include a single cipher suite, i.e., be
encoded as an int. If the Responder does not support any cipher
suite in SUITES_I, then it SHOULD include all its supported cipher
suites in SUITES_R in any order.
6.3.1. Cipher Suite Negotiation
After receiving SUITES_R, the Initiator can determine which cipher
suite to select (if any) 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 where the Initiator
selects its most preferred and the Responder sends an error with
supported cipher suites. After a successful run of EDHOC, the
Initiator MAY remember the selected cipher suite to use in future
EDHOC sessions. Note that if the Initiator or Responder is updated
with new cipher suite policies, any cached information may be
outdated.
6.3.2. Examples
Assume that the Initiator supports the five cipher suites 5, 6, 7, 8,
and 9 in decreasing order of preference. Figures 8 and 9 show
examples of how the Initiator can format SUITES_I and how SUITES_R is
used by Responders to give the Initiator information about the cipher
suites that the Responder supports.
In the first example (Figure 8), the Responder supports cipher suite
6 but not the initially selected cipher suite 5.
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Initiator Responder
| METHOD, SUITES_I = 5, G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| ERR_CODE = 2, SUITES_R = 6 |
|<------------------------------------------------------------------+
| error |
| |
| METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
Figure 8: Example of Responder supporting suite 6 but not suite 5.
In the second example (Figure 9), the Responder supports cipher
suites 8 and 9 but not the more preferred (by the Initiator) cipher
suites 5, 6 or 7. To illustrate the negotiation mechanics we let the
Initiator first make a guess that the Responder supports suite 6 but
not suite 5. Since the Responder supports neither 5 nor 6, it
responds with SUITES_R containing the supported suites, after which
the Initiator selects its most preferred supported suite. The order
of cipher suites in SUITES_R does not matter. (If the Responder had
supported suite 5, it would have included it in SUITES_R of the
response, and it would in that case have become the selected suite in
the second message_1.)
Initiator Responder
| METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| ERR_CODE = 2, SUITES_R = [9, 8] |
|<------------------------------------------------------------------+
| error |
| |
| METHOD, SUITES_I = [5, 6, 7, 8], G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
Figure 9: Example of Responder supporting suites 8 and 9 but not
5, 6 or 7.
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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 will fail and the
Initiator will discontinue the protocol.
7. Mandatory-to-Implement Compliance Requirements
An implementation may support only Initiator or only Responder.
An implementation may support only a single method. None of the
methods are mandatory-to-implement.
Implementations MUST support 'kid' parameters of type int. None of
the other COSE header parameters are mandatory-to-implement.
An implementation may support only a single credential type (CCS,
CWT, X.509, C509). None of the credential types are mandatory-to-
implement.
Implementations MUST support the EDHOC-Exporter. Implementations
SHOULD support EDHOC-KeyUpdate.
Implementations MAY support message_4. Error codes 1 and 2 MUST be
supported.
Implementations MAY support EAD.
For many constrained IoT devices it is problematic to support more
than one cipher suite. Existing devices can be expected to support
either ECDSA or EdDSA. To enable as much interoperability as we can
reasonably achieve, less constrained devices SHOULD implement both
cipher suite 0 (AES-CCM-16-64-128, SHA-256, 8, X25519, EdDSA, AES-
CCM-16-64-128, SHA-256) and cipher suite 2 (AES-CCM-16-64-128, SHA-
256, 8, P-256, ES256, AES-CCM-16-64-128, SHA-256). Constrained
endpoints SHOULD implement cipher suite 0 or cipher suite 2.
Implementations only need to implement the algorithms needed for
their supported methods.
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8. Security Considerations
8.1. Security Properties
EDHOC inherits its security properties from the theoretical SIGMA-I
protocol [SIGMA]. Using the terminology from [SIGMA], EDHOC provides
forward secrecy, mutual authentication with aliveness, consistency,
and peer awareness. As described in [SIGMA], peer awareness is
provided to the Responder, but not to the Initiator.
As described in [SIGMA], different levels of identity protection is
provided to the Initiator and the Responder. EDHOC protects the
credential identifier of the Initiator against active attacks and the
credential identifier of the Responder against passive attacks. The
roles should be assigned to protect the most sensitive identity/
identifier, typically that which is not possible to infer from
routing information in the lower layers.
Compared to [SIGMA], EDHOC adds an explicit method type and expands
the message authentication coverage to additional elements such as
algorithms, external authorization data, and previous messages. This
protects against an attacker replaying messages or injecting messages
from another session.
EDHOC also adds selection of connection identifiers and downgrade
protected negotiation of cryptographic parameters, i.e., an attacker
cannot affect the negotiated parameters. A single session of EDHOC
does not include negotiation of cipher suites, but it enables the
Responder to verify that the selected cipher suite is the most
preferred cipher suite by the Initiator which is supported by both
the Initiator and the Responder.
As required by [RFC7258], IETF protocols need to mitigate pervasive
monitoring when possible. EDHOC therefore only supports methods with
ephemeral Diffie-Hellman and provides a KeyUpdate function for
lightweight application protocol rekeying with forward secrecy, in
the sense that compromise of the private authentication keys does not
compromise past session keys, and compromise of a session key does
not compromise past session keys.
While the KeyUpdate method can be used to meet cryptographic limits
and provide partial protection against key leakage, it provides
significantly weaker security properties than re-running EDHOC with
ephemeral Diffie-Hellman. Even with frequent use of KeyUpdate,
compromise of one session key compromises all future session keys,
and an attacker therefore only needs to perform static key
exfiltration [RFC7624]. Frequently re-running EDHOC with ephemeral
Diffie-Hellman forces attackers to perform dynamic key exfiltration
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instead of static key exfiltration [RFC7624]. In the dynamic case,
the attacker must have continuous interactions with the collaborator,
which is more complicated and has a higher risk profile than the
static case.
To limit the effect of breaches, it is important to limit the use of
symmetrical group keys for bootstrapping. EDHOC therefore strives to
make the additional cost of using raw public keys and self-signed
certificates as small as possible. Raw public keys and self-signed
certificates are not a replacement for a public key infrastructure
but SHOULD be used instead of symmetrical group keys for
bootstrapping.
Compromise of the long-term keys (private signature or static DH
keys) does not compromise the security of completed EDHOC exchanges.
Compromising the private authentication keys of one party lets an
active attacker impersonate that compromised party in EDHOC exchanges
with other parties but does not let the attacker impersonate other
parties in EDHOC exchanges with the compromised party. Compromise of
the long-term keys does not enable a passive attacker to compromise
future session keys. Compromise of the HDKF input parameters (ECDH
shared secret) leads to compromise of all session keys derived from
that compromised shared secret. Compromise of one session key does
not compromise other session keys. Compromise of PRK_4x3m leads to
compromise of all exported keying material derived after the last
invocation of the EDHOC-KeyUpdate function.
EDHOC provides a minimum of 64-bit security against online brute
force attacks and a minimum of 128-bit security against offline brute
force attacks. This is in line with IPsec, TLS, and COSE. To break
64-bit security against online brute force an attacker would on
average have to send 4.3 billion messages per second for 68 years,
which is infeasible in constrained IoT radio technologies.
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After sending message_3, the Initiator is assured that no other party
than the Responder can compute the key PRK_4x3m (implicit key
authentication). The Initiator does however not know that the
Responder has actually computed the key PRK_4x3m. While the
Initiator can securely send protected application data, the Initiator
SHOULD NOT permanently store the keying material PRK_4x3m and TH_4
until the Initiator is assured that the Responder has actually
computed the key PRK_4x3m (explicit key confirmation). Explicit key
confirmation is e.g., assured when the Initiator has verified an
OSCORE message or message_4 from the Responder. After verifying
message_3, the Responder is assured that the Initiator has calculated
the key PRK_4x3m (explicit key confirmation) and that no other party
than the Responder can compute the key. The Responder can securely
send protected application data and store the keying material
PRK_4x3m and TH_4.
Key compromise impersonation (KCI): In EDHOC authenticated with
signature keys, EDHOC provides KCI protection against an attacker
having access to the long-term key or the ephemeral secret key. With
static Diffie-Hellman key authentication, KCI protection would be
provided against an attacker having access to the long-term Diffie-
Hellman key, but not to an attacker having access to the ephemeral
secret key. Note that the term KCI has typically been used for
compromise of long-term keys, and that an attacker with access to the
ephemeral secret key can only attack that specific session.
Repudiation: In EDHOC authenticated with signature keys, the
Initiator could theoretically prove that the Responder performed a
run of the protocol by presenting the private ephemeral key, and vice
versa. Note that storing the private ephemeral keys violates the
protocol requirements. With static Diffie-Hellman key
authentication, both parties can always deny having participated in
the protocol.
Two earlier versions of EDHOC have been formally analyzed [Norrman20]
[Bruni18] and the specification has been updated based on the
analysis.
8.2. Cryptographic Considerations
The SIGMA protocol requires that the encryption of message_3 provides
confidentiality against active attackers and EDHOC message_4 relies
on the use of authenticated encryption. Hence the message
authenticating functionality of the authenticated encryption in EDHOC
is critical: authenticated encryption MUST NOT be replaced by plain
encryption only, even if authentication is provided at another level
or through a different mechanism.
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To reduce message overhead EDHOC does not use explicit nonces and
instead rely on the ephemeral public keys to provide randomness to
each session. A good amount of randomness is important for the key
generation, to provide liveness, and to protect against interleaving
attacks. For this reason, the ephemeral keys MUST NOT be used in
more than one EDHOC message, and both parties SHALL generate fresh
random ephemeral key pairs. Note that an ephemeral key may be used
to calculate several ECDH shared secrets. When static Diffie-Hellman
authentication is used the same ephemeral key is used in both
ephemeral-ephemeral and ephemeral-static ECDH.
As discussed in [SIGMA], the encryption of message_2 does only need
to protect against passive attacker as active attackers can always
get the Responders identity by sending their own message_1. EDHOC
uses the Expand function (typically HKDF-Expand) as a binary additive
stream cipher. HKDF-Expand provides better confidentiality than AES-
CTR but is not often used as it is slow on long messages, and most
applications require both IND-CCA confidentiality as well as
integrity protection. For the encryption of message_2, any speed
difference is negligible, IND-CCA does not increase security, and
integrity is provided by the inner MAC (and signature depending on
method).
Requirement for how to securely generate, validate, and process the
ephemeral public keys depend on the elliptic curve. For X25519 and
X448, the requirements are defined in [RFC7748]. For secp256r1,
secp384r1, and secp521r1, the requirements are defined in Section 5
of [SP-800-56A]. For secp256r1, secp384r1, and secp521r1, at least
partial public-key validation MUST be done.
8.3. Cipher Suites and Cryptographic Algorithms
When using private cipher suite or registering new cipher suites, the
choice of key length used in the different algorithms needs to be
harmonized, so that a sufficient security level is maintained for
certificates, EDHOC, and the protection of application data. The
Initiator and the Responder should enforce a minimum security level.
The hash algorithms SHA-1 and SHA-256/64 (SHA-256 truncated to
64-bits) SHALL NOT be supported for use in EDHOC except for
certificate identification with x5t and c5t. Note that secp256k1 is
only defined for use with ECDSA and not for ECDH. Note that some
COSE algorithms are marked as not recommended in the COSE IANA
registry.
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8.4. Post-Quantum Considerations
As of the publication of this specification, it is unclear when or
even if a quantum computer of sufficient size and power to exploit
public key cryptography will exist. Deployments that need to
consider risks decades into the future should transition to Post-
Quantum Cryptography (PQC) in the not-too-distant future. Many other
systems should take a slower wait-and-see approach where PQC is
phased in when the quantum threat is more imminent. Current PQC
algorithms have limitations compared to Elliptic Curve Cryptography
(ECC) and the data sizes would be problematic in many constrained IoT
systems.
Symmetric algorithms used in EDHOC such as SHA-256 and AES-CCM-
16-64-128 are practically secure against even large quantum
computers. EDHOC supports all signature algorithms defined by COSE,
including PQC signature algorithms such as HSS-LMS. EDHOC is
currently only specified for use with key exchange algorithms of type
ECDH curves, but any Key Encapsulation Method (KEM), including PQC
KEMs, can be used in method 0. While the key exchange in method 0 is
specified with terms of the Diffie-Hellman protocol, the key exchange
adheres to a KEM interface: G_X is then the public key of the
Initiator, G_Y is the encapsulation, and G_XY is the shared secret.
Use of PQC KEMs to replace static DH authentication would likely
require a specification updating EDHOC with new methods.
8.5. Unprotected Data
The Initiator and the Responder must make sure that unprotected data
and metadata do not reveal any sensitive information. This also
applies for encrypted data sent to an unauthenticated party. In
particular, it applies to EAD_1, ID_CRED_R, EAD_2, and error
messages. Using the same EAD_1 in several EDHOC sessions allows
passive eavesdroppers to correlate the different sessions. Another
consideration is that the list of supported cipher suites may
potentially be used to identify the application.
The Initiator and the Responder must also make sure that
unauthenticated data does not trigger any harmful actions. In
particular, this applies to EAD_1 and error messages.
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8.6. Denial-of-Service
As CoAP provides Denial-of-Service protection in the form of the Echo
option [RFC9175], 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 that forces the initiator
to demonstrate reachability at its apparent network address.
An attacker can also send faked message_2, message_3, message_4, or
error in an attempt to trick the receiving party to send an error
message and discontinue the session. EDHOC implementations MAY
evaluate if a received message is likely to have been forged by an
attacker and ignore it without sending an error message or
discontinuing the session.
8.7. Implementation Considerations
The availability of a secure random number generator is essential for
the security of EDHOC. If no true random number generator is
available, a truly random seed MUST be provided from an external
source and used with a cryptographically secure pseudorandom number
generator. As each pseudorandom number must only be used once, an
implementation needs to get a new truly random seed after reboot, or
continuously store state in nonvolatile memory, see ([RFC8613],
Appendix B.1.1) for issues and solution approaches for writing to
nonvolatile memory. Intentionally or unintentionally weak or
predictable pseudorandom number generators can be abused or exploited
for malicious purposes. [RFC8937] describes a way for security
protocol implementations to augment their (pseudo)random number
generators using a long-term private key and a deterministic
signature function. This improves randomness from broken or
otherwise subverted random number generators. The same idea can be
used with other secrets and functions such as a Diffie-Hellman
function or a symmetric secret and a PRF like HMAC or KMAC. It is
RECOMMENDED to not trust a single source of randomness and to not put
unaugmented random numbers on the wire.
If ECDSA is supported, "deterministic ECDSA" as specified in
[RFC6979] MAY be used. Pure deterministic elliptic-curve signatures
such as deterministic ECDSA and EdDSA have gained popularity over
randomized ECDSA as their security do not depend on a source of high-
quality randomness. Recent research has however found that
implementations of these signature algorithms may be vulnerable to
certain side-channel and fault injection attacks due to their
determinism. See e.g., Section 1 of
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[I-D.mattsson-cfrg-det-sigs-with-noise] for a list of attack papers.
As suggested in Section 6.1.2 of [I-D.ietf-cose-rfc8152bis-algs] this
can be addressed by combining randomness and determinism.
All private keys, symmetric keys, and IVs MUST be secret.
Implementations should provide countermeasures to side-channel
attacks such as timing attacks. Intermediate computed values such as
ephemeral ECDH keys and ECDH shared secrets MUST be deleted after key
derivation is completed.
The Initiator and the Responder are responsible for verifying the
integrity of certificates. The selection of trusted CAs should be
done very carefully and certificate revocation should be supported.
The private authentication keys MUST be kept secret, only the
Responder SHALL have access to the Responder's private authentication
key and only the Initiator SHALL have access to the Initiator's
private authentication key.
The Initiator and the Responder are allowed to select the connection
identifiers C_I and C_R, respectively, for the other party to use in
the ongoing EDHOC protocol as well as in a subsequent application
protocol (e.g., OSCORE [RFC8613]). The choice of connection
identifier is not security critical in EDHOC but intended to simplify
the retrieval of the right security context in combination with using
short identifiers. If the wrong connection identifier of the other
party is used in a protocol message it will result in the receiving
party not being able to retrieve a security context (which will
terminate the protocol) or retrieve the wrong security context (which
also terminates the protocol as the message cannot be verified).
If two nodes unintentionally initiate two simultaneous EDHOC message
exchanges with each other even if they only want to complete a single
EDHOC message exchange, they MAY terminate the exchange with the
lexicographically smallest G_X. If the two G_X values are equal, the
received message_1 MUST be discarded to mitigate reflection attacks.
Note that in the case of two simultaneous EDHOC exchanges where the
nodes only complete one and where the nodes have different preferred
cipher suites, an attacker can affect which of the two nodes'
preferred cipher suites will be used by blocking the other exchange.
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If supported by the device, it is RECOMMENDED that at least the long-
term private keys are stored in a Trusted Execution Environment (TEE)
and that sensitive operations using these keys are performed inside
the TEE. To achieve even higher security it is RECOMMENDED that
additional operations such as ephemeral key generation, all
computations of shared secrets, and storage of the PRK keys can be
done inside the TEE. The use of a TEE enforces that code within that
environment cannot be tampered with, and that any data used by such
code cannot be read or tampered with by code outside that
environment.
9. IANA Considerations
9.1. EDHOC Exporter Label Registry
IANA has created a new registry titled "EDHOC Exporter Label" under
the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The
registration procedure is "Expert Review". The columns of the
registry are Label, Description, and Reference. All columns are text
strings where Label consists only of the printable ASCII characters
0x21 - 0x7e. Labels beginning with "PRIVATE" MAY be used for private
use without registration. All other label values MUST be registered.
The initial contents of the registry are:
Label: EDHOC_K_4
Description: Key used to protect EDHOC message_4
Reference: [[this document]]
Label: EDHOC_IV_4
Description: IV used to protect EDHOC message_4
Reference: [[this document]]
Label: OSCORE_Master_Secret
Description: Derived OSCORE Master Secret
Reference: [[this document]]
Label: OSCORE_Master_Salt
Description: Derived OSCORE Master Salt
Reference: [[this document]]
9.2. EDHOC Cipher Suites Registry
IANA has created a new registry titled "EDHOC Cipher Suites" under
the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The
registration procedure is "Expert Review". The columns of the
registry are Value, Array, Description, and Reference, where Value is
an integer and the other columns are text strings. The initial
contents of the registry are:
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Value: -24
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]
Value: -23
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]
Value: -22
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]
Value: -21
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]
Value: 0
Array: 10, -16, 8, 4, -8, 10, -16
Desc: AES-CCM-16-64-128, SHA-256, 8, X25519, EdDSA,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
Value: 1
Array: 30, -16, 16, 4, -8, 10, -16
Desc: AES-CCM-16-128-128, SHA-256, 16, X25519, EdDSA,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
Value: 2
Array: 10, -16, 8, 1, -7, 10, -16
Desc: AES-CCM-16-64-128, SHA-256, 8, P-256, ES256,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
Value: 3
Array: 30, -16, 16, 1, -7, 10, -16
Desc: AES-CCM-16-128-128, SHA-256, 16, P-256, ES256,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
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Value: 4
Array: 24, -16, 16, 4, -8, 24, -16
Desc: ChaCha20/Poly1305, SHA-256, 16, X25519, EdDSA,
ChaCha20/Poly1305, SHA-256
Reference: [[this document]]
Value: 5
Array: 24, -16, 16, 1, -7, 24, -16
Desc: ChaCha20/Poly1305, SHA-256, 16, P-256, ES256,
ChaCha20/Poly1305, SHA-256
Reference: [[this document]]
Value: 6
Array: 1, -16, 16, 4, -7, 1, -16
Desc: A128GCM, SHA-256, 16, X25519, ES256,
A128GCM, SHA-256
Reference: [[this document]]
Value: 24
Array: 3, -43, 16, 2, -35, 3, -43
Desc: A256GCM, SHA-384, 16, P-384, ES384,
A256GCM, SHA-384
Reference: [[this document]]
Value: 25
Array: 24, -45, 16, 5, -8, 24, -45
Desc: ChaCha20/Poly1305, SHAKE256, 16, X448, EdDSA,
ChaCha20/Poly1305, SHAKE256
Reference: [[this document]]
9.3. EDHOC Method Type Registry
IANA has created a new registry entitled "EDHOC Method Type" under
the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The
registration procedure is "Expert Review". The columns of the
registry are Value, Description, and Reference, where Value is an
integer and the other columns are text strings. The initial contents
of the registry are shown in Figure 4.
9.4. EDHOC Error Codes Registry
IANA has created a new registry entitled "EDHOC Error Codes" under
the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The
registration procedure is "Expert Review". The columns of the
registry are ERR_CODE, ERR_INFO Type and Description, where ERR_CODE
is an integer, ERR_INFO is a CDDL defined type, and Description is a
text string. The initial contents of the registry are shown in
Figure 7.
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9.5. EDHOC External Authorization Data Registry
IANA has created a new registry entitled "EDHOC External
Authorization Data" under the new group name "Ephemeral Diffie-
Hellman Over COSE (EDHOC)". The registration procedure is "Expert
Review". The columns of the registry are Label, Description, Value
Type, and Reference, where Label is an integer and the other columns
are text strings.
9.6. COSE Header Parameters Registry
IANA has registered the following entries in the "COSE Header
Parameters" registry under the group name "CBOR Object Signing and
Encryption (COSE)". The value of the 'kcwt' header parameter is a
COSE Web Token (CWT) [RFC8392], and the value of the 'kccs' header
parameter is an CWT Claims Set (CCS), see Section 1.5. The CWT/CCS
must contain a COSE_Key in a 'cnf' claim [RFC8747]. The Value
Registry for this item is empty and omitted from the table below.
+-----------+-------+----------------+---------------------------+
| Name | Label | Value Type | Description |
+===========+=======+================+===========================+
| kcwt | TBD1 | COSE_Messages | A CBOR Web Token (CWT) |
| | | | containing a COSE_Key in |
| | | | a 'cnf' claim |
+-----------+-------+----------------+---------------------------+
| kccs | TBD2 | map / #6(map) | A CWT Claims Set (CCS) |
| | | | containing a COSE_Key in |
| | | | a 'cnf' claim |
+-----------+-------+----------------+---------------------------+
9.7. COSE Header Parameters Registry
IANA has extended the Value Type of 'kid' in the "COSE Header
Parameters" registry under the group name "CBOR Object Signing and
Encryption (COSE)" to also allow the Value Type int. The resulting
Value Type is bstr / int. The Value Registry for this item is empty
and omitted from the table below.
+------+-------+------------+----------------+-------------------+
| Name | Label | Value Type | Description | Reference |
+------+-------+------------+----------------+-------------------+
| kid | 4 | bstr / int | Key identifier | [[This document]] |
+------+-------+------------+----------------+-------------------+
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9.8. COSE Key Common Parameters Registry
IANA has extended the Value Type of 'kid' in the "COSE Key Common
Parameters" registry under the group name "CBOR Object Signing and
Encryption (COSE)" to also allow the Value Type int. The resulting
Value Type is bstr / int. The Value Registry for this item is empty
and omitted from the table below.
+------+-------+------------+----------------+-------------------+
| Name | Label | Value Type | Description | Reference |
+------+-------+------------+----------------+-------------------+
| kid | 2 | bstr / int | Key identifi- | [[This document]] |
| | | | cation value - | |
| | | | match to kid | |
| | | | in message | |
+------+-------+------------+----------------+-------------------+
9.9. CWT Confirmation Methods Registry
IANA has extended the Value Type of 'kid' in the "CWT Confirmation
Methods" registry under the group name "CBOR Web Token (CWT) Claims"
to also allow the Value Type int. The incorrect term binary string
has been corrected to bstr. The resulting Value Type is bstr / int.
The new updated content for the 'kid' method is shown in the list
below.
* Confirmation Method Name: kid
* Confirmation Method Description: Key Identifier
* JWT Confirmation Method Name: kid
* Confirmation Key: 3
* Confirmation Value Type(s): bstr / int
* Change Controller: IESG
* Specification Document(s): Section 3.4 of RFC 8747 [[This
document]]
9.10. The Well-Known URI Registry
IANA has added the well-known URI "edhoc" to the "Well-Known URIs"
registry under the group name "Well-Known URIs".
* URI suffix: edhoc
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* Change controller: IETF
* Specification document(s): [[this document]]
* Related information: None
9.11. Media Types Registry
IANA has added the media type "application/edhoc" to the "Media
Types" registry.
* Type name: application
* Subtype name: edhoc
* Required parameters: N/A
* Optional parameters: N/A
* Encoding considerations: binary
* Security considerations: See Section 7 of this document.
* Interoperability considerations: N/A
* Published specification: [[this document]] (this document)
* Applications that use this media type: To be identified
* Fragment identifier considerations: N/A
* Additional information:
- Magic number(s): N/A
- File extension(s): N/A
- Macintosh file type code(s): N/A
* Person & email address to contact for further information: See
"Authors' Addresses" section.
* Intended usage: COMMON
* Restrictions on usage: N/A
* Author: See "Authors' Addresses" section.
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* Change Controller: IESG
9.12. CoAP Content-Formats Registry
IANA has added the media type "application/edhoc" to the "CoAP
Content-Formats" registry under the group name "Constrained RESTful
Environments (CoRE) Parameters".
* Media Type: application/edhoc
* Encoding:
* ID: TBD42
* Reference: [[this document]]
9.13. Resource Type (rt=) Link Target Attribute Values Registry
IANA has added the resource type "core.edhoc" to the "Resource Type
(rt=) Link Target Attribute Values" registry under the group name
"Constrained RESTful Environments (CoRE) Parameters".
* Value: "core.edhoc"
* Description: EDHOC resource.
* Reference: [[this document]]
Client applications can use this resource type to discover a server's
resource for EDHOC, where to send a request for executing the EDHOC
protocol.
9.14. Expert Review Instructions
The IANA Registries established in this document is defined as
"Expert Review". This section gives some general guidelines for what
the experts should be looking for, but they are being designated as
experts for a reason so they should be given substantial latitude.
Expert reviewers should take into consideration the following points:
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* Clarity and correctness of registrations. Experts are expected to
check the clarity of purpose and use of the requested entries.
Expert needs to make sure the values of algorithms are taken from
the right registry, when that is required. Expert should consider
requesting an opinion on the correctness of registered parameters
from relevant IETF working groups. Encodings that do not meet
these objective of clarity and completeness should not be
registered.
* Experts should take into account the expected usage of fields when
approving point assignment. The length of the encoded value
should be weighed against how many code points of that length are
left, the size of device it will be used on, and the number of
code points left that encode to that size.
* Specifications are recommended. When specifications are not
provided, the description provided needs to have sufficient
information to verify the points above.
10. References
10.1. Normative References
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-algs-12.txt>.
[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-struct-15.txt>.
[I-D.ietf-cose-x509]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Header parameters for carrying and referencing X.509
certificates", Work in Progress, Internet-Draft, draft-
ietf-cose-x509-08, 14 December 2020,
<https://www.ietf.org/internet-drafts/draft-ietf-cose-
x509-08.txt>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<https://www.rfc-editor.org/info/rfc6090>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <https://www.rfc-editor.org/info/rfc6979>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<https://www.rfc-editor.org/info/rfc7624>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
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[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
[RFC8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
<https://www.rfc-editor.org/info/rfc8742>.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
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[RFC9175] Amsüss, C., Preuß Mattsson, J., and G. Selander,
"Constrained Application Protocol (CoAP): Echo, Request-
Tag, and Token Processing", RFC 9175,
DOI 10.17487/RFC9175, February 2022,
<https://www.rfc-editor.org/info/rfc9175>.
10.2. Informative References
[Bruni18] Bruni, A., Sahl Jørgensen, T., Grønbech Petersen, T., and
C. Schürmann, "Formal Verification of Ephemeral Diffie-
Hellman Over COSE (EDHOC)", November 2018,
<https://www.springerprofessional.de/en/formal-
verification-of-ephemeral-diffie-hellman-over-cose-
edhoc/16284348>.
[CborMe] Bormann, C., "CBOR Playground", May 2018,
<http://cbor.me/>.
[CNSA] (Placeholder), ., "Commercial National Security Algorithm
Suite", August 2015,
<https://apps.nsa.gov/iaarchive/programs/iad-initiatives/
cnsa-suite.cfm>.
[I-D.ietf-core-oscore-edhoc]
Palombini, F., Tiloca, M., Hoeglund, R., Hristozov, S.,
and G. Selander, "Profiling EDHOC for CoAP and OSCORE",
Work in Progress, Internet-Draft, draft-ietf-core-oscore-
edhoc-03, 7 March 2022, <https://www.ietf.org/archive/id/
draft-ietf-core-oscore-edhoc-03.txt>.
[I-D.ietf-core-resource-directory]
Amsüss, C., Shelby, Z., Koster, M., Bormann, C., and P. V.
D. Stok, "CoRE Resource Directory", Work in Progress,
Internet-Draft, draft-ietf-core-resource-directory-28, 7
March 2021, <https://www.ietf.org/archive/id/draft-ietf-
core-resource-directory-28.txt>.
[I-D.ietf-cose-cbor-encoded-cert]
Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", Work in Progress, Internet-Draft, draft-
ietf-cose-cbor-encoded-cert-03, 10 January 2022,
<https://www.ietf.org/archive/id/draft-ietf-cose-cbor-
encoded-cert-03.txt>.
[I-D.ietf-lake-reqs]
Vucinic, M., Selander, G., Mattsson, J. P., and D. Garcia-
Carrillo, "Requirements for a Lightweight AKE for OSCORE",
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Work in Progress, Internet-Draft, draft-ietf-lake-reqs-04,
8 June 2020, <https://www.ietf.org/archive/id/draft-ietf-
lake-reqs-04.txt>.
[I-D.ietf-lwig-security-protocol-comparison]
Mattsson, J. P., Palombini, F., and M. Vucinic,
"Comparison of CoAP Security Protocols", Work in Progress,
Internet-Draft, draft-ietf-lwig-security-protocol-
comparison-05, 2 November 2020,
<https://www.ietf.org/archive/id/draft-ietf-lwig-security-
protocol-comparison-05.txt>.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-43, 30 April 2021, <https://www.ietf.org/internet-
drafts/draft-ietf-tls-dtls13-43.txt>.
[I-D.mattsson-cfrg-det-sigs-with-noise]
Mattsson, J. P., Thormarker, E., and S. Ruohomaa,
"Deterministic ECDSA and EdDSA Signatures with Additional
Randomness", Work in Progress, Internet-Draft, draft-
mattsson-cfrg-det-sigs-with-noise-04, 15 February 2022,
<https://www.ietf.org/archive/id/draft-mattsson-cfrg-det-
sigs-with-noise-04.txt>.
[I-D.selander-ace-ake-authz]
Selander, G., Mattsson, J. P., Vučinić, M., Richardson,
M., and A. Schellenbaum, "Lightweight Authorization for
Authenticated Key Exchange.", Work in Progress, Internet-
Draft, draft-selander-ace-ake-authz-04, 22 October 2021,
<https://www.ietf.org/archive/id/draft-selander-ace-ake-
authz-04.txt>.
[I-D.selander-lake-traces]
Selander, G. and J. P. Mattsson, "Traces of EDHOC", Work
in Progress, Internet-Draft, draft-selander-lake-traces-
02, 20 October 2021, <https://www.ietf.org/archive/id/
draft-selander-lake-traces-02.txt>.
[Norrman20]
Norrman, K., Sundararajan, V., and A. Bruni, "Formal
Analysis of EDHOC Key Establishment for Constrained IoT
Devices", September 2020,
<https://arxiv.org/abs/2007.11427>.
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[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
and C. Wood, "Randomness Improvements for Security
Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
<https://www.rfc-editor.org/info/rfc8937>.
[SECG] "Standards for Efficient Cryptography 1 (SEC 1)", May
2009, <https://www.secg.org/sec1-v2.pdf>.
[SIGMA] Krawczyk, H., "SIGMA - The 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and Its Use in the IKE-
Protocols (Long version)", June 2003,
<http://webee.technion.ac.il/~hugo/sigma-pdf.pdf>.
[SP-800-56A]
Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
Davis, "Recommendation for Pair-Wise Key-Establishment
Schemes Using Discrete Logarithm Cryptography",
NIST Special Publication 800-56A Revision 3, April 2018,
<https://doi.org/10.6028/NIST.SP.800-56Ar3>.
Appendix A. Use with OSCORE and Transfer over CoAP
This appendix describes how to select EDHOC connection identifiers
and derive an OSCORE security context when OSCORE is used with EDHOC,
and how to transfer EDHOC messages over CoAP.
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A.1. Selecting EDHOC Connection Identifier
This section specifies a rule for converting from EDHOC connection
identifier to OSCORE Sender/Recipient ID. (An identifier is Sender
ID or Recipient ID depending on from which endpoint is the point of
view, see Section 3.1 of [RFC8613].)
* If the EDHOC connection identifier is numeric, i.e., encoded as a
CBOR integer on the wire, it is converted to a (naturally byte-
string shaped) OSCORE Sender/Recipient ID equal to its CBOR
encoded form.
For example, a numeric C_R equal to 10 (0x0A in CBOR encoding) is
converted to a (typically client) Sender ID equal to 0x0A, while a
numeric C_I equal to -12 (0x2B in CBOR encoding) is converted to a
(typically client) Sender ID equal to 0x2B.
* If the EDHOC connection identifier is byte-valued, hence encoded
as a CBOR byte string on the wire, it is converted to an OSCORE
Sender/Recipient ID equal to the byte string.
For example, a byte-string valued C_R equal to 0xFF (0x41FF in CBOR
encoding) is converted to a (typically client) Sender ID equal to
0xFF.
Two EDHOC connection identifiers are called "equivalent" if and only
if, by applying the conversion above, they both result in the same
OSCORE Sender/Recipient ID. For example, the two EDHOC connection
identifiers with CBOR encoding 0x0A (numeric) and 0x410A (byte-
valued) are equivalent since they both result in the same OSCORE
Sender/Recipient ID 0x0A.
When EDHOC is used to establish an OSCORE security context, the
connection identifiers C_I and C_R MUST NOT be equivalent.
Furthermore, in case of multiple OSCORE security contexts with
potentially different endpoints, to facilitate retrieval of the
correct OSCORE security context, an endpoint SHOULD select an EDHOC
connection identifier that when converted to OSCORE Recipient ID does
not coincide with its other Recipient IDs.
A.2. Deriving the OSCORE Security Context
This section specifies how to use EDHOC output to derive the OSCORE
security context.
After successful processing of EDHOC message_3, Client and Server
derive Security Context parameters for OSCORE as follows (see
Section 3.2 of [RFC8613]):
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* The Master Secret and Master Salt are derived by using the EDHOC-
Exporter interface, see Section 4.3.
The EDHOC Exporter Labels for deriving the OSCORE Master Secret and
the OSCORE Master Salt, are "OSCORE_Master_Secret" and
"OSCORE_Master_Salt", respectively.
The context parameter is h'' (0x40), the empty CBOR byte string.
By default, key_length is the key length (in bytes) of the
application AEAD Algorithm of the selected cipher suite for the EDHOC
session. Also by default, salt_length has value 8. The Initiator
and Responder MAY agree out-of-band on a longer key_length than the
default and on a different salt_length.
Master Secret = EDHOC-Exporter("OSCORE_Master_Secret", h'', key_length)
Master Salt = EDHOC-Exporter("OSCORE_Master_Salt", h'', salt_length)
* The AEAD Algorithm is the application AEAD algorithm of the
selected cipher suite for the EDHOC session.
* The HKDF Algorithm is the one based on the application hash
algorithm of the selected cipher suite for the EDHOC session. For
example, if SHA-256 is the application hash algorithm of the
selected cipher suite, HKDF SHA-256 is used as HKDF Algorithm in
the OSCORE Security Context.
* In case the Client is Initiator and the Server is Responder, the
Client's OSCORE Sender ID and the Server's OSCORE Sender ID are
determined from the EDHOC connection identifiers C_R and C_I for
the EDHOC session, respectively, by applying the conversion in
Appendix A.1. The reverse applies in case the Client is the
Responder and the Server is the Initiator.
Client and Server use the parameters above to establish an OSCORE
Security Context, as per Section 3.2.1 of [RFC8613].
From then on, Client and Server retrieve the OSCORE protocol state
using the Recipient ID, and optionally other transport information
such as the 5-tuple.
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A.3. Transferring EDHOC over CoAP
This section specifies one instance for how EDHOC can be transferred
as an exchange of CoAP [RFC7252] messages. CoAP provides a reliable
transport that can preserve packet ordering and handle message
duplication. CoAP can also perform fragmentation and protect against
denial-of-service attacks. The underlying CoAP transport should be
used in reliable mode, in particular when fragmentation is used, to
avoid, e.g., situations with hanging endpoints waiting for each
other.
By default, the CoAP client is the Initiator and the CoAP server is
the Responder, but the roles SHOULD be chosen to protect the most
sensitive identity, see Section 8. According to this specification,
EDHOC is transferred in POST requests and 2.04 (Changed) responses to
the Uri-Path: "/.well-known/edhoc". An application may define its
own path that can be discovered, e.g., using resource directory
[I-D.ietf-core-resource-directory].
By default, the message flow is as follows: EDHOC message_1 is sent
in the payload of a POST request from the client to the server's
resource for EDHOC. EDHOC message_2 or the EDHOC error message is
sent from the server to the client in the payload of a 2.04 (Changed)
response. EDHOC message_3 or the EDHOC error message is sent from
the client to the server's resource in the payload of a POST request.
If needed, an EDHOC error message is sent from the server to the
client in the payload of a 2.04 (Changed) response. Alternatively,
if EDHOC message_4 is used, it is sent from the server to the client
in the payload of a 2.04 (Changed) response analogously to message_2.
In order to correlate a message received from a client to a message
previously sent by the server, messages sent by the client are
prepended with the CBOR serialization of the connection identifier
which the server has chosen. This applies independently of if the
CoAP server is Responder or Initiator. For the default case when the
server is Responder, the prepended connection identifier is C_R, and
C_I if the server is Initiator. If message_1 is sent to the server,
the CBOR simple value "true" (0xf5) is sent in its place (given that
the server has not selected C_R yet).
These identifiers are encoded in CBOR and thus self-delimiting. They
are sent in front of the actual EDHOC message, and only the part of
the body following the identifier is used for EDHOC processing.
Consequently, the application/edhoc media type does not apply to
these messages; their media type is unnamed.
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An example of a successful EDHOC exchange using CoAP is shown in
Figure 10. In this case the CoAP Token enables correlation on the
Initiator side, and the prepended C_R enables correlation on the
Responder (server) side.
Client Server
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/edhoc"
| | Payload: true, EDHOC message_1
| |
|<---------+ Header: 2.04 Changed
| 2.04 | Content-Format: application/edhoc
| | Payload: EDHOC message_2
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/edhoc"
| | Payload: C_R, EDHOC message_3
| |
|<---------+ Header: 2.04 Changed
| 2.04 |
| |
Figure 10: Transferring EDHOC in CoAP when the Initiator is CoAP
Client
The exchange in Figure 10 protects the client identity against active
attackers and the server identity against passive attackers.
An alternative exchange that protects the server identity against
active attackers and the client identity against passive attackers is
shown in Figure 11. In this case the CoAP Token enables the
Responder to correlate message_2 and message_3, and the prepended C_I
enables correlation on the Initiator (server) side. If EDHOC
message_4 is used, C_I is prepended, and it is transported with CoAP
in the payload of a POST request with a 2.04 (Changed) response.
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Client Server
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/edhoc"
| |
|<---------+ Header: 2.04 Changed
| 2.04 | Content-Format: application/edhoc
| | Payload: EDHOC message_1
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/edhoc"
| | Payload: C_I, EDHOC message_2
| |
|<---------+ Header: 2.04 Changed
| 2.04 | Content-Format: application/edhoc
| | Payload: EDHOC message_3
| |
Figure 11: Transferring EDHOC in CoAP when the Initiator is CoAP
Server
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 [RFC9175]. This forces the initiator to
demonstrate its reachability at its apparent network address. If
message fragmentation is needed, the EDHOC messages may be fragmented
using the CoAP Block-Wise Transfer mechanism [RFC7959].
EDHOC does not restrict how error messages are transported with CoAP,
as long as the appropriate error message can to be transported in
response to a message that failed (see Section 6). EDHOC error
messages transported with CoAP are carried in the payload.
A.3.1. Transferring EDHOC and OSCORE over CoAP
When using EDHOC over CoAP for establishing an OSCORE Security
Context, EDHOC error messages sent as CoAP responses MUST be sent in
the payload of error responses, i.e., they MUST specify a CoAP error
response code. In particular, it is RECOMMENDED that such error
responses have response code either 4.00 (Bad Request) in case of
client error (e.g., due to a malformed EDHOC message), or 5.00
(Internal Server Error) in case of server error (e.g., due to failure
in deriving EDHOC key material). The Content-Format of the error
response MUST be set to application/edhoc.
A method for combining EDHOC and OSCORE protocols in two round-trips
is specified in [I-D.ietf-core-oscore-edhoc].
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Appendix B. Compact Representation
As described in Section 4.2 of [RFC6090] the x-coordinate of an
elliptic curve public key is a suitable representative for the entire
point whenever scalar multiplication is used as a one-way function.
One example is ECDH with compact output, where only the x-coordinate
of the computed value is used as the shared secret.
This section defines a format for compact representation based on the
Elliptic-Curve-Point-to-Octet-String Conversion defined in
Section 2.3.3 of [SECG]. Using the notation from [SECG], the output
is an octet string of length ceil( (log2 q) / 8 ). See [SECG] for a
definition of q, M, X, xp, and ~yp. The steps in Section 2.3.3 of
[SECG] are replaced by:
1. Convert the field element xp to an octet string X of length ceil(
(log2 q) / 8 ) octets using the conversion routine specified in
Section 2.3.5 of [SECG].
2. Output M = X
The encoding of the point at infinity is not supported. Compact
representation does not change any requirements on validation. If a
y-coordinate is required for validation or compatibily with APIs the
value ~yp SHALL be set to zero. For such use, the compact
representation can be transformed into the SECG point compressed
format by prepending it with the single byte 0x02 (i.e., M = 0x02 ||
X).
Using compact representation have some security benefits. An
implementation does not need to check that the point is not the point
at infinity (the identity element). Similarly, as not even the sign
of the y-coordinate is encoded, compact representation trivially
avoids so called "benign malleability" attacks where an attacker
changes the sign, see [SECG].
Appendix C. Use of CBOR, CDDL and COSE in EDHOC
This Appendix is intended to simplify for implementors not familiar
with CBOR [RFC8949], CDDL [RFC8610], COSE
[I-D.ietf-cose-rfc8152bis-struct], and HKDF [RFC5869].
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C.1. CBOR and CDDL
The Concise Binary Object Representation (CBOR) [RFC8949] is a data
format designed for small code size and small message size. CBOR
builds on the JSON data model but extends it by e.g., encoding binary
data directly without base64 conversion. In addition to the binary
CBOR encoding, CBOR also has a diagnostic notation that is readable
and editable by humans. The Concise Data Definition Language (CDDL)
[RFC8610] provides a way to express structures for protocol messages
and APIs that use CBOR. [RFC8610] also extends the diagnostic
notation.
CBOR data items are encoded to or decoded from byte strings using a
type-length-value encoding scheme, where the three highest order bits
of the initial byte contain information about the major type. CBOR
supports several different types of data items, in addition to
integers (int, uint), simple values, byte strings (bstr), and text
strings (tstr), CBOR also supports arrays [] of data items, maps {}
of pairs of data items, and sequences [RFC8742] of data items. Some
examples are given below.
The EDHOC specification sometimes use CDDL names in CBOR dignostic
notation as in e.g., << ID_CRED_R, ? EAD_2 >>. This means that EAD_2
is optional and that ID_CRED_R and EAD_2 should be substituted with
their values before evaluation. I.e., if ID_CRED_R = { 4 : h'' } and
EAD_2 is omitted then << ID_CRED_R, ? EAD_2 >> = << { 4 : h'' } >>,
which encodes to 0x43a10440.
For a complete specification and more examples, see [RFC8949] and
[RFC8610]. We recommend implementors to get used to CBOR by using
the CBOR playground [CborMe].
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Diagnostic Encoded Type
------------------------------------------------------------------
1 0x01 unsigned integer
24 0x1818 unsigned integer
-24 0x37 negative integer
-25 0x3818 negative integer
true 0xf5 simple value
h'' 0x40 byte string
h'12cd' 0x4212cd byte string
'12cd' 0x4431326364 byte string
"12cd" 0x6431326364 text string
{ 4 : h'cd' } 0xa10441cd map
<< 1, 2, true >> 0x430102f5 byte string
[ 1, 2, true ] 0x830102f5 array
( 1, 2, true ) 0x0102f5 sequence
1, 2, true 0x0102f5 sequence
------------------------------------------------------------------
C.2. CDDL Definitions
This sections compiles the CDDL definitions for ease of reference.
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suites = [ 2* int ] / int
ead = 1* (
ead_label : int,
ead_value : any,
)
message_1 = (
METHOD : int,
SUITES_I : suites,
G_X : bstr,
C_I : bstr / int,
? EAD_1 : ead,
)
message_2 = (
G_Y_CIPHERTEXT_2 : bstr,
C_R : bstr / int,
)
message_3 = (
CIPHERTEXT_3 : bstr,
)
message_4 = (
CIPHERTEXT_4 : bstr,
)
error = (
ERR_CODE : int,
ERR_INFO : any,
)
info = (
transcript_hash : bstr,
label : tstr,
context : bstr,
length : uint,
)
C.3. COSE
CBOR Object Signing and Encryption (COSE)
[I-D.ietf-cose-rfc8152bis-struct] describes how to create and process
signatures, message authentication codes, and encryption using CBOR.
COSE builds on JOSE, but is adapted to allow more efficient
processing in constrained devices. EDHOC makes use of COSE_Key,
COSE_Encrypt0, and COSE_Sign1 objects in the message processing:
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* ECDH ephemeral public keys of type EC2 or OKP in message_1 and
message_2 consist of the COSE_Key parameter named 'x', see
Section 7.1 and 7.2 of [I-D.ietf-cose-rfc8152bis-algs]
* The ciphertexts in message_3 and message_4 consist of a subset of
the single recipient encrypted data object COSE_Encrypt0, which is
described in Sections 5.2-5.3 of
[I-D.ietf-cose-rfc8152bis-struct]. The ciphertext is computed
over the plaintext and associated data, using an encryption key
and an initialization vector. The associated data is an
Enc_structure consisting of protected headers and externally
supplied data (external_aad). COSE constructs the input to the
AEAD [RFC5116] for message_i (i = 3 or 4, see Section 5.4 and
Section 5.5, respectively) as follows:
- Secret key K = K_i
- Nonce N = IV_i
- Plaintext P for message_i
- Associated Data A = [ "Encrypt0", h'', TH_i ]
* Signatures in message_2 of method 0 and 2, and in message_3 of
method 0 and 1, consist of a subset of the single signer data
object COSE_Sign1, which is described in Sections 4.2-4.4 of
[I-D.ietf-cose-rfc8152bis-struct]. The signature is computed over
a Sig_structure containing payload, protected headers and
externally supplied data (external_aad) using a private signature
key and verified using the corresponding public signature key.
For COSE_Sign1, the message to be signed is:
[ "Signature1", protected, external_aad, payload ]
where protected, external_aad and payload are specified in
Section 5.3 and Section 5.4.
Different header parameters to identify X.509 or C509 certificates by
reference are defined in [I-D.ietf-cose-x509] and
[I-D.ietf-cose-cbor-encoded-cert]:
* by a hash value with the 'x5t' or 'c5t' parameters, respectively:
- ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R,
- ID_CRED_x = { TBD3 : COSE_CertHash }, for x = I or R;
* or by a URI with the 'x5u' or 'c5u' parameters, respectively:
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- ID_CRED_x = { 35 : uri }, for x = I or R,
- ID_CRED_x = { TBD4 : uri }, for x = I or R.
When ID_CRED_x does not contain the actual credential, it may be very
short, e.g., if the endpoints have agreed to use a key identifier
parameter 'kid':
* ID_CRED_x = { 4 : key_id_x }, where key_id_x : kid, for x = I or
R.
Note that a COSE header map can contain several header parameters,
for example { x5u, x5t } or { kid, kid_context }.
ID_CRED_x MAY also identify the authentication credential by value.
For example, a certificate chain can be transported in ID_CRED_x with
COSE header parameter c5c or x5chain, defined in
[I-D.ietf-cose-cbor-encoded-cert] and [I-D.ietf-cose-x509] and
credentials of type CWT and CCS can be transported with the COSE
header parameters registered in Section 9.6.
Appendix D. Applicability Template
This appendix contains a rudimentary example of an applicability
statement, see Section 3.9.
For use of EDHOC in the XX protocol, the following assumptions are
made:
1. Transfer in CoAP as specified in Appendix A.3 with requests
expected by the CoAP server (= Responder) at /app1-edh, no
Content-Format needed.
2. METHOD = 1 (I uses signature key, R uses static DH key.)
3. CRED_I is an IEEE 802.1AR IDevID encoded as a C509 certificate of
type 0 [I-D.ietf-cose-cbor-encoded-cert].
* R acquires CRED_I out-of-band, indicated in EAD_1.
* ID_CRED_I = {4: h''} is a 'kid' with value empty CBOR byte
string.
4. CRED_R is a CCS of type OKP as specified in Section 3.5.3.
* The CBOR map has parameters 1 (kty), -1 (crv), and -2
(x-coordinate).
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* ID_CRED_R is {TBD2 : CCS}. Editor's note: TBD2 is the COSE
header parameter value of 'kccs', see Section 9.6
5. External authorization data is defined and processed as specified
in [I-D.selander-ace-ake-authz].
6. EUI-64 used as identity of endpoint.
7. No use of message_4: the application sends protected messages
from R to I.
Appendix E. EDHOC Message Deduplication
EDHOC by default assumes that message duplication is handled by the
transport, in this section exemplified with CoAP.
Deduplication of CoAP messages is described in Section 4.5 of
[RFC7252]. This handles the case when the same Confirmable (CON)
message is received multiple times due to missing acknowledgement on
CoAP messaging layer. The recommended processing in [RFC7252] is
that the duplicate message is acknowledged (ACK), but the received
message is only processed once by the CoAP stack.
Message deduplication is resource demanding and therefore not
supported in all CoAP implementations. Since EDHOC is targeting
constrained environments, it is desirable that EDHOC can optionally
support transport layers which does not handle message duplication.
Special care is needed to avoid issues with duplicate messages, see
Section 5.1.
The guiding principle here is similar to the deduplication processing
on CoAP messaging layer: a received duplicate EDHOC message SHALL NOT
result in a response consisting of another instance of the next EDHOC
message. The result MAY be that a duplicate EDHOC response is sent,
provided it is still relevant with respect the current protocol
state. In any case, the received message MUST NOT be processed more
than once in the same EDHOC session. This is called "EDHOC message
deduplication".
An EDHOC implementation MAY store the previously sent EDHOC message
to be able to resend it. An EDHOC implementation MAY keep the
protocol state to be able to recreate the previously sent EDHOC
message and resend it. The previous message or protocol state MUST
NOT be kept longer than what is required for retransmission, for
example, in the case of CoAP transport, no longer than the
EXCHANGE_LIFETIME (see Section 4.8.2 of [RFC7252]).
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Note that the requirements in Section 5.1 still apply because
duplicate messages are not processed by the EDHOC state machine:
* EDHOC messages SHALL be processed according to the current
protocol state.
* Different instances of the same message MUST NOT be processed in
one session.
Appendix F. Transports Not Natively Providing Correlation
Protocols that do not natively provide full correlation between a
series of messages can send the C_I and C_R identifiers along as
needed.
The transport over CoAP (Appendix A.3) can serve as a blueprint for
other server-client protocols: The client prepends the C_x which the
server selected (or, for message 1, the CBOR simple value 'true'
which is not a valid C_x) to any request message it sends. The
server does not send any such indicator, as responses are matched to
request by the client-server protocol design.
Protocols that do not provide any correlation at all can prescribe
prepending of the peer's chosen C_x to all messages.
Appendix G. Change Log
RFC Editor: Please remove this appendix.
* From -11 to -12:
- Clarified applicability to KEMs
- Clarified use of COSE header parameters
- Updates on MTI
- Updated security considerations
- New section on PQC
- Removed duplicate definition of cipher suites
- Explanations of use of COSE moved to Appendix C.3
- Updated internal references
* From -10 to -11:
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- Restructured section on authentication parameters
- Changed UCCS to CCS
- Changed names and description of COSE header parameters for
CWT/CCS
- Changed several of the KDF and Exporter labels
- Removed edhoc_aead_id from info (already in transcript_hash)
- Added MTI section
- EAD: changed CDDL names and added value type to registry
- Updated Figures 1, 2, and 3
- Some correction and clarifications
- Added core.edhoc to CoRE Resource Type registry
* From -09 to -10:
- SUITES_I simplified to only contain the selected and more
preferred suites
- Info is a CBOR sequence and context is a bstr
- Added kid to UCCS example
- Separate header parameters for CWT and UCCS
- CWT Confirmation Method kid extended to bstr / int
* From -08 to -09:
- G_Y and CIPHERTEXT_2 are now included in one CBOR bstr
- MAC_2 and MAC_3 are now generated with EDHOC-KDF
- Info field "context" is now general and explicit in EDHOC-KDF
- Restructured Section 4, Key Derivation
- Added EDHOC MAC length to cipher suite for use with static DH
- More details on the use of CWT and UCCS
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- Restructured and clarified Section 3.5, Authentication
Parameters
- Replaced 'kid2' with extension of 'kid'
- EAD encoding now supports multiple ead types in one message
- Clarified EAD type
- Updated message sizes
- Replaced "perfect forward secrecy" with "forward secrecy"
- Updated security considerations
- Replaced prepended 'null' with 'true' in the CoAP transport of
message_1
- Updated CDDL definitions
- Expanded on the use of COSE
* From -07 to -08:
- Prepended C_x moved from the EDHOC protocol itself to the
transport mapping
- METHOD_CORR renamed to METHOD, corr removed
- Removed bstr_identifier and use bstr / int instead; C_x can now
be int without any implied bstr semantics
- Defined COSE header parameter 'kid2' with value type bstr / int
for use with ID_CRED_x
- Updated message sizes
- New cipher suites with AES-GCM and ChaCha20 / Poly1305
- Changed from one- to two-byte identifier of CNSA compliant
suite
- Separate sections on transport and connection id with further
sub-structure
- Moved back key derivation for OSCORE from draft-ietf-core-
oscore-edhoc
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- OSCORE and CoAP specific processing moved to new appendix
- Message 4 section moved to message processing section
* From -06 to -07:
- Changed transcript hash definition for TH_2 and TH_3
- Removed "EDHOC signature algorithm curve" from cipher suite
- New IANA registry "EDHOC Exporter Label"
- New application defined parameter "context" in EDHOC-Exporter
- Changed normative language for failure from MUST to SHOULD send
error
- Made error codes non-negative and 0 for success
- Added detail on success error code
- Aligned terminology "protocol instance" -> "session"
- New appendix on compact EC point representation
- Added detail on use of ephemeral public keys
- Moved key derivation for OSCORE to draft-ietf-core-oscore-edhoc
- Additional security considerations
- Renamed "Auxililary Data" as "External Authorization Data"
- Added encrypted EAD_4 to message_4
* From -05 to -06:
- New section 5.2 "Message Processing Outline"
- Optional inital byte C_1 = null in message_1
- New format of error messages, table of error codes, IANA
registry
- Change of recommendation transport of error in CoAP
- Merge of content in 3.7 and appendix C into new section 3.7
"Applicability Statement"
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- Requiring use of deterministic CBOR
- New section on message deduplication
- New appendix containin all CDDL definitions
- New appendix with change log
- Removed section "Other Documents Referencing EDHOC"
- Clarifications based on review comments
* From -04 to -05:
- EDHOC-Rekey-FS -> EDHOC-KeyUpdate
- Clarification of cipher suite negotiation
- Updated security considerations
- Updated test vectors
- Updated applicability statement template
* From -03 to -04:
- Restructure of section 1
- Added references to C509 Certificates
- Change in CIPHERTEXT_2 -> plaintext XOR KEYSTREAM_2 (test
vector not updated)
- "K_2e", "IV_2e" -> KEYSTREAM_2
- Specified optional message 4
- EDHOC-Exporter-FS -> EDHOC-Rekey-FS
- Less constrained devices SHOULD implement both suite 0 and 2
- Clarification of error message
- Added exporter interface test vector
* From -02 to -03:
- Rearrangements of section 3 and beginning of section 4
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- Key derivation new section 4
- Cipher suites 4 and 5 added
- EDHOC-EXPORTER-FS - generate a new PRK_4x3m from an old one
- Change in CIPHERTEXT_2 -> COSE_Encrypt0 without tag (no change
to test vector)
- Clarification of error message
- New appendix C applicability statement
* From -01 to -02:
- New section 1.2 Use of EDHOC
- Clarification of identities
- New section 4.3 clarifying bstr_identifier
- Updated security considerations
- Updated text on cipher suite negotiation and key confirmation
- Test vector for static DH
* From -00 to -01:
- Removed PSK method
- Removed references to certificate by value
Acknowledgments
The authors want to thank Christian Amsuess, Alessandro Bruni,
Karthikeyan Bhargavan, Timothy Claeys, Martin Disch, Loic Ferreira,
Theis Groenbech Petersen, Dan Harkins, Klaus Hartke, Russ Housley,
Stefan Hristozov, 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, Peter van der Stok, Rene
Struik, Vaishnavi Sundararajan, Erik Thormarker, Marco Tiloca, Michel
Veillette, and Malisa Vucinic 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.
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Work on this document has in part been supported by the H2020 project
SIFIS-Home (grant agreement 952652).
Authors' Addresses
Göran Selander
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: goran.selander@ericsson.com
John Preuß Mattsson
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: john.mattsson@ericsson.com
Francesca Palombini
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
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