Network Working Group G. Selander
Internet-Draft J. Mattsson
Intended status: Standards Track F. Palombini
Expires: January 13, 2022 Ericsson AB
July 12, 2021
Ephemeral Diffie-Hellman Over COSE (EDHOC)
draft-ietf-lake-edhoc-08
Abstract
This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
very compact and lightweight authenticated Diffie-Hellman key
exchange with ephemeral keys. EDHOC provides mutual authentication,
perfect forward secrecy, and identity protection. 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Use of EDHOC . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Message Size Examples . . . . . . . . . . . . . . . . . . 5
1.4. Document Structure . . . . . . . . . . . . . . . . . . . 6
1.5. Terminology and Requirements Language . . . . . . . . . . 6
2. EDHOC Outline . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8
3.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Method . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Connection Identifiers . . . . . . . . . . . . . . . . . 9
3.4. Transport . . . . . . . . . . . . . . . . . . . . . . . . 11
3.5. Authentication Parameters . . . . . . . . . . . . . . . . 11
3.6. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 16
3.7. Ephemeral Public Keys . . . . . . . . . . . . . . . . . . 18
3.8. External Authorization Data . . . . . . . . . . . . . . . 18
3.9. Applicability Statement . . . . . . . . . . . . . . . . . 19
4. Key Derivation . . . . . . . . . . . . . . . . . . . . . . . 21
4.1. EDHOC-Exporter Interface . . . . . . . . . . . . . . . . 23
5. Message Formatting and Processing . . . . . . . . . . . . . . 24
5.1. Message Processing Outline . . . . . . . . . . . . . . . 24
5.2. EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . . 25
5.3. EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . . 27
5.4. EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . . 30
5.5. EDHOC Message 4 . . . . . . . . . . . . . . . . . . . . . 33
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 35
6.1. Success . . . . . . . . . . . . . . . . . . . . . . . . . 36
6.2. Unspecified . . . . . . . . . . . . . . . . . . . . . . . 36
6.3. Wrong Selected Cipher Suite . . . . . . . . . . . . . . . 36
7. Security Considerations . . . . . . . . . . . . . . . . . . . 38
7.1. Security Properties . . . . . . . . . . . . . . . . . . . 38
7.2. Cryptographic Considerations . . . . . . . . . . . . . . 40
7.3. Cipher Suites and Cryptographic Algorithms . . . . . . . 41
7.4. Unprotected Data . . . . . . . . . . . . . . . . . . . . 42
7.5. Denial-of-Service . . . . . . . . . . . . . . . . . . . . 42
7.6. Implementation Considerations . . . . . . . . . . . . . . 43
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
8.1. EDHOC Exporter Label . . . . . . . . . . . . . . . . . . 44
8.2. EDHOC Cipher Suites Registry . . . . . . . . . . . . . . 45
8.3. EDHOC Method Type Registry . . . . . . . . . . . . . . . 47
8.4. EDHOC Error Codes Registry . . . . . . . . . . . . . . . 47
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8.5. COSE Header Parameters Registry . . . . . . . . . . . . . 47
8.6. COSE Header Parameters Registry . . . . . . . . . . . . . 47
8.7. COSE Key Common Parameters Registry . . . . . . . . . . . 48
8.8. The Well-Known URI Registry . . . . . . . . . . . . . . . 48
8.9. Media Types Registry . . . . . . . . . . . . . . . . . . 48
8.10. CoAP Content-Formats Registry . . . . . . . . . . . . . . 49
8.11. EDHOC External Authorization Data . . . . . . . . . . . . 49
8.12. Expert Review Instructions . . . . . . . . . . . . . . . 50
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
9.1. Normative References . . . . . . . . . . . . . . . . . . 50
9.2. Informative References . . . . . . . . . . . . . . . . . 53
Appendix A. Use with OSCORE and Transfer over CoAP . . . . . . . 55
A.1. Selecting EDHOC Connection Identifier . . . . . . . . . . 55
A.2. Deriving the OSCORE Security Context . . . . . . . . . . 56
A.3. Transferring EDHOC over CoAP . . . . . . . . . . . . . . 57
Appendix B. Compact Representation . . . . . . . . . . . . . . . 60
Appendix C. Use of CBOR, CDDL and COSE in EDHOC . . . . . . . . 60
C.1. CBOR and CDDL . . . . . . . . . . . . . . . . . . . . . . 60
C.2. CDDL Definitions . . . . . . . . . . . . . . . . . . . . 61
C.3. COSE . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Appendix D. Test Vectors . . . . . . . . . . . . . . . . . . . . 63
D.1. Test Vectors for EDHOC Authenticated with Signature Keys
(x5t) . . . . . . . . . . . . . . . . . . . . . . . . . . 63
D.2. Test Vectors for EDHOC Authenticated with Static Diffie-
Hellman Keys . . . . . . . . . . . . . . . . . . . . . . 81
Appendix E. Applicability Template . . . . . . . . . . . . . . . 96
Appendix F. EDHOC Message Deduplication . . . . . . . . . . . . 96
Appendix G. Transports Not Natively Providing Correlation . . . 97
Appendix H. Change Log . . . . . . . . . . . . . . . . . . . . . 98
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 101
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 101
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
Header Compression (SCHC, [RFC8724]).
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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 perfect 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 makes use of known protocol constructions, such as SIGMA
[SIGMA] and Extract-and-Expand [RFC5869]. COSE also provides crypto
agility and enables the use of future algorithms targeting IoT.
1.2. Use of EDHOC
EDHOC is designed for highly constrained settings making it
especially suitable for low-power wide area networks [RFC8376] such
as Cellular IoT, 6TiSCH, and LoRaWAN. A main objective for EDHOC is
to be a lightweight authenticated key exchange for OSCORE, i.e. to
provide authentication and session key establishment for IoT use
cases such as those built on CoAP [RFC7252]. CoAP is a specialized
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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 7.1.
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.
However, it is recommended to transfer EDHOC messages in CoAP
payloads as 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 two
examples of message sizes for EDHOC with different kinds of
authentication keys and different COSE header parameters for
identification: static Diffie-Hellman keys identified by 'kid'
[I-D.ietf-cose-rfc8152bis-struct], and X.509 signature certificates
identified by a hash value using 'x5t' [I-D.ietf-cose-x509].
=================================
kid x5t
---------------------------------
message_1 37 37
message_2 45 116
message_3 20 91
---------------------------------
Total 103 245
=================================
Figure 1: Example of message sizes in bytes.
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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 message flow, and
formatting of the ephemeral public keys, Section 4 describes the key
derivation, Section 5 specifies EDHOC with authentication based on
signature keys or static Diffie-Hellman keys, Section 6 specifies the
EDHOC error message, and Appendix A describes how EDHOC can be
transferred in CoAP and used to 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 process [I-D.ietf-cose-rfc8152bis-struct], COSE
algorithms [I-D.ietf-cose-rfc8152bis-algs], 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
refer 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].
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), and like IKEv2 [RFC7296], EDHOC
implements the SIGMA-I variant as MAC-then-Sign. The SIGMA-I
protocol using an authenticated encryption algorithm is shown in
Figure 2.
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Initiator Responder
| G_X |
+-------------------------------------------------------->|
| |
| G_Y, AEAD( K_2; ID_CRED_R, Sig(R; CRED_R, G_X, G_Y) ) |
|<--------------------------------------------------------+
| |
| AEAD( K_3; ID_CRED_I, Sig(I; CRED_I, G_Y, G_X) ) |
+-------------------------------------------------------->|
| |
Figure 2: Authenticated encryption 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.
o G_X and G_Y are the ECDH ephemeral public keys of I and R,
respectively.
o CRED_I and CRED_R are the credentials containing the public
authentication keys of I and R, respectively.
o ID_CRED_I and ID_CRED_R are credential identifiers enabling the
recipient party to retrieve the credential of I and R,
respectively.
o Sig(I; . ) and Sig(R; . ) denote signatures made with the private
authentication key of I and R, respectively.
o AEAD(K; . ) denotes authenticated encryption with additional data
using a key K derived from the shared secret.
In order to create a "full-fledged" protocol some additional protocol
elements are needed. EDHOC adds:
o Transcript hashes (hashes of message data) TH_2, TH_3, TH_4 used
for key derivation and as additional authenticated data.
o Computationally independent keys derived from the ECDH shared
secret and used for authenticated encryption of different
messages.
o An optional fourth message giving explicit key confirmation to I
in deployments where no protected application data is sent from R
to I.
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o A key material exporter and a key update function enabling
frequent forward secrecy.
o Verification of a common preferred cipher suite:
* The Initiator lists supported cipher suites in order of
preference
* The Responder verifies that the selected cipher suite is the
first supported cipher suite (or else rejects and states
supported cipher suites).
o Method types and error handling.
o Selection of connection identifiers C_I and C_R which may be used
to identify established keys or protocol state.
o 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 and test vectors including CBOR diagnostic
notation are given in Appendix D.
3. Protocol Elements
3.1. General
An EDHOC message flow consists of three mandatory messages
(message_1, message_2, message_3) between Initiator and Responder, an
optional fourth message (message_4), plus an EDHOC error message.
EDHOC messages are CBOR Sequences [RFC8742], see Figure 3. The
protocol elements in the figure are introduced in the following
sections. Message formatting and processing is specified in
Section 5 and Section 6. An implementation may support only
Initiator or only Responder.
Application data is protected using the agreed application algorithms
(AEAD, hash) in the selected cipher suite (see Section 3.6) 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 8.
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The Initiator can derive symmetric application keys after creating
EDHOC message_3, see Section 4.1. Application protected data can
therefore be sent in parallel or together with EDHOC message_3.
Initiator Responder
| METHOD, SUITES_I, G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| G_Y, C_R, Enc(ID_CRED_R, Signature_or_MAC_2, EAD_2) |
|<------------------------------------------------------------------+
| message_2 |
| |
| AEAD(K_3ae; ID_CRED_I, Signature_or_MAC_3, EAD_3) |
+------------------------------------------------------------------>|
| message_3 |
Figure 3: EDHOC Message Flow
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. (Method 0
corresponds to the case outlined in Section 2 where both Initiator
and Responder authenticate with signature keys.)
An implementation may support only a single method. The Initiator
and the Responder need to have agreed on a single method to be used
for EDHOC, see 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 in the ongoing EDHOC protocol (see
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Section 3.3.2) or in a subsequent application protocol, e.g., OSCORE
(see Section 3.3.3). 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 bytestring 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.
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 3.3.3
for an example.
3.3.2. Use of Connection Identifiers in EDHOC
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, see Section 3.4. For an example when CoAP is used as
transport, see Appendix A.3.
3.3.3. Use of Connection Identifiers in 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.
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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:
o message loss,
o message reordering,
o message duplication,
o fragmentation,
o demultiplex EDHOC messages from other types of messages, and
o denial of service protection.
Besides these common transport oriented properties, EDHOC transport
additionally needs to support the correlation between EDHOC messages,
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. In the absense of
correlation between a message received and a message previously sent
inherent to the transport, the EDHOC connection identifiers may be
added, 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.
The Initiator and the Responder need to have agreed on a transport to
be used for EDHOC, see Section 3.9.
3.5. Authentication Parameters
3.5.1. Authentication Keys
The authentication key MUST be a signature key or static Diffie-
Hellman key. The Initiator and the Responder MAY use different types
of authentication keys, e.g. one uses a signature key and the other
uses a static Diffie-Hellman key. When using a signature key, the
authentication is provided by a signature. When using a static
Diffie-Hellman key the authentication is provided by a Message
Authentication Code (MAC) computed from an ephemeral-static ECDH
shared secret which enables significant reductions in message sizes.
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The MAC is implemented with an AEAD algorithm. 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. The
authentication key algorithm needs to specified with enough
parameters to make it completely determined. Note that for most
signature algorithms, the signature is determined by the signature
algorithm and the authentication key algorithm together. For
example, the curve used in the signature is typically determined by
the authentication key parameters.
o Only the Responder SHALL have access to the Responder's private
authentication key.
o Only the Initiator SHALL have access to the Initiator's private
authentication key.
3.5.2. Identities
EDHOC assumes the existence of mechanisms (certification authority,
trusted third party, manual distribution, etc.) for specifying and
distributing authentication keys and identities. Policies are set
based on the identity of the other party, and parties typically only
allow connections from a specific identity or a small restricted set
of identities. For example, in the case of a device connecting to a
network, the network may only allow connections from devices which
authenticate with certificates having a particular range of serial
numbers in the subject field and signed by a particular CA. On the
other side, the device may only be allowed to connect to a network
which 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 must be able to receive and enforce
information from the application about what is the intended endpoint,
and in particular whether it is a specific identity or a set of
identities.
o When a Public Key Infrastructure (PKI) is used, 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. Before running EDHOC each
party needs at least one CA public key certificate, or just the
public key, and a specific identity or 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
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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.
o When public keys are used but not with a PKI (RPK, self-signed
certificate), the trust anchor is the public authentication key of
the other party. In this case, the identity is typically directly
associated to the public 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 or COSE_Key containing the public
authentication key and the subject name, see Section 3.5.3.
Before running EDHOC, each endpoint needs a specific public
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 proof that the
other party possesses the private authentication key corresponding
to the public authentication key.
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.
The Initiator and the Responder MAY use different types of
credentials, e.g. one uses an RPK and the other uses a public key
certificate.
The credentials CRED_I and CRED_R are signed or MAC:ed (depending on
method) by the Initiator and the Responder, respectively, see
Section 5.4 and Section 5.3.
When the credential is a certificate, CRED_x is an end-entity
certificate (i.e. not the certificate chain) encoded as a CBOR bstr.
In X.509 certificates, signature keys typically have key usage
"digitalSignature" and Diffie-Hellman keys typically have key usage
"keyAgreement".
To prevent misbinding attacks in systems where an attacker can
register public keys without proving knowledge of the private key,
SIGMA [SIGMA] enforces a MAC to be calculated over the "Identity",
which in case of a X.509 certificate would be the 'subject' and
'subjectAltName' fields. EDHOC follows SIGMA by calculating a MAC
over the whole certificate. While the SIGMA paper only focuses on
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the identity, the same principle is true for any information such as
policies connected to the public key.
When the credential is a COSE_Key, CRED_x is a CBOR map only
containing specific fields from the COSE_Key identifying the public
key, and optionally the "Identity". CRED_x needs to be defined such
that it is identical when generated by Initiator or Responder. The
parameters SHALL be encoded in bytewise lexicographic order of their
deterministic encodings as specified in Section 4.2.1 of [RFC8949].
If the parties have agreed on an identity besides the public key, the
identity is included in the CBOR map with the label "subject name",
otherwise the subject name is the empty text string. The public key
parameters depend on key type.
o For COSE_Keys of type OKP the CBOR map SHALL, except for subject
name, only include the parameters 1 (kty), -1 (crv), and -2
(x-coordinate).
o For COSE_Keys of type EC2 the CBOR map SHALL, except for subject
name, only include the parameters 1 (kty), -1 (crv), -2
(x-coordinate), and -3 (y-coordinate).
An example of CRED_x when the RPK contains an X25519 static Diffie-
Hellman key and the parties have agreed on an EUI-64 identity is
shown below:
CRED_x = {
1: 1,
-1: 4,
-2: h'b1a3e89460e88d3a8d54211dc95f0b90
3ff205eb71912d6db8f4af980d2db83a',
"subject name" : "42-50-31-FF-EF-37-32-39"
}
3.5.4. Identification of Credentials
ID_CRED_I and ID_CRED_R are used to identify and optionally transport
the public authentication keys of the Initiator and the Responder,
respectively. ID_CRED_I and ID_CRED_R do not have any cryptographic
purpose in EDHOC.
o ID_CRED_R is intended to facilitate for the Initiator to retrieve
the Responder's public authentication key.
o ID_CRED_I is intended to facilitate for the Responder to retrieve
the Initiator's public authentication key.
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The identifiers ID_CRED_I and ID_CRED_R are COSE header_maps, i.e.
CBOR maps containing Common COSE Header Parameters, see Section 3.1
of [I-D.ietf-cose-rfc8152bis-struct]). In the following we give some
examples of COSE header_maps.
Raw public keys are most optimally stored as COSE_Key objects and
identified with a 'kid2' parameter (see Section 8.6 and Section 8.7):
o ID_CRED_x = { 4 : kid_x }, where kid_x : bstr / int, for x = I or
R.
Note that the integers -24 to 23 and the empty bytestring h'' are
encoded as one byte.
Public key certificates can be identified in different ways. Header
parameters for identifying C509 certificates and X.509 certificates
are defined in [I-D.ietf-cose-cbor-encoded-cert] and
[I-D.ietf-cose-x509], for example:
o by a hash value with the 'c5t' or 'x5t' parameters;
* ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R,
* ID_CRED_x = { TDB3 : COSE_CertHash }, for x = I or R,
o by a URI with the 'c5u' or 'x5u' parameters;
* ID_CRED_x = { 35 : uri }, for x = I or R,
* ID_CRED_x = { TBD4 : uri }, for x = I or R,
o ID_CRED_x MAY contain the actual credential used for
authentication, CRED_x. 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].
It is RECOMMENDED that ID_CRED_x uniquely identify the public
authentication key as the recipient may otherwise have to try several
keys. ID_CRED_I and ID_CRED_R are transported in the 'ciphertext',
see Section 5.4 and Section 5.3.
When ID_CRED_x does not contain the actual credential it may be very
short. One byte credential identifiers are realistic in many
scenarios as most constrained devices only have a few keys. In cases
where a node only has one key, the identifier may even be the empty
byte string.
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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.
Algorithms need to be specified with enough parameters to make them
completely determined. Currently, none of the algorithms require
parameters. EDHOC is only specified for use with key exchange
algorithms of type ECDH curves. Use with other types of key exchange
algorithms would likely require a specification updating EDHOC. Note
that for most signature algorithms, the signature is determined by
the signature algorithm and the authentication key algorithm
together, see Section 3.5.1.
o EDHOC AEAD algorithm
o EDHOC hash algorithm
o EDHOC key exchange algorithm (ECDH curve)
o EDHOC signature algorithm
o Application AEAD algorithm
o Application hash algorithm
Each cipher suite is identified with a pre-defined int label.
EDHOC can be used with all algorithms and curves defined for COSE.
Implementation can either use one of the pre-defined cipher suites
(Section 8.2) or use any combination of COSE algorithms and
parameters to define their own private cipher suite. Private cipher
suites can be identified with any of the four values -24, -23, -22,
-21.
The following CCM cipher suites are for constrained IoT where message
overhead is a very important factor. Cipher suites 1 and 3 use a
larger tag length (128-bit) in the EDHOC AEAD algorithm than the
Application AEAD algorithm (64-bit):
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0. ( 10, -16, 4, -8, 10, -16 )
(AES-CCM-16-64-128, SHA-256, X25519, EdDSA,
AES-CCM-16-64-128, SHA-256)
1. ( 30, -16, 4, -8, 10, -16 )
(AES-CCM-16-128-128, SHA-256, X25519, EdDSA,
AES-CCM-16-64-128, SHA-256)
2. ( 10, -16, 1, -7, 10, -16 )
(AES-CCM-16-64-128, SHA-256, P-256, ES256,
AES-CCM-16-64-128, SHA-256)
3. ( 30, -16, 1, -7, 10, -16 )
(AES-CCM-16-128-128, SHA-256, P-256, ES256,
AES-CCM-16-64-128, SHA-256)
The following ChaCha20 cipher suites are for less constrained
applications and only use 128-bit tag lengths.
4. ( 24, -16, 4, -8, 24, -16 )
(ChaCha20/Poly1305, SHA-256, X25519, EdDSA,
ChaCha20/Poly1305, SHA-256)
5. ( 24, -16, 1, -7, 24, -16 )
(ChaCha20/Poly1305, SHA-256, P-256, ES256,
ChaCha20/Poly1305, SHA-256)
The following GCM cipher suite is for general non-constrained
applications. It uses high performance algorithms that are widely
supported:
6. ( 1, -16, 4, -7, 1, -16 )
(A128GCM, SHA-256, X25519, ES256,
A128GCM, SHA-256)
The following two cipher suites are for high security application
such as government use and financial applications. The two cipher
suites do not share any algorithms. The first of the two cipher
suites is compatible with the CNSA suite [CNSA].
24. ( 3, -43, 2, -35, 3, -43 )
(A256GCM, SHA-384, P-384, ES384,
A256GCM, SHA-384)
25. ( 24, -45, 5, -8, 24, -45 )
(ChaCha20/Poly1305, SHAKE256, X448, EdDSA,
ChaCha20/Poly1305, SHAKE256)
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The different methods use the same cipher suites, but some algorithms
are not used in some methods. The EDHOC signature algorithm is not
used in methods without signature authentication.
The Initiator needs to have a list of cipher suites it supports in
order of preference. The Responder needs to have a list of cipher
suites it supports. SUITES_I is a CBOR array containing cipher
suites that the Initiator supports. SUITES_I is formatted and
processed as detailed in Section 5.2.1 to secure the cipher suite
negotiation. Examples of cipher suite negotiation are given in
Section 6.3.2.
3.7. Ephemeral Public Keys
EDHOC always uses compact representation of elliptic curve points,
see Appendix B. In COSE compact representation is achieved by
formatting the ECDH ephemeral public keys as COSE_Keys of type EC2 or
OKP according to Sections 7.1 and 7.2 of
[I-D.ietf-cose-rfc8152bis-algs], but only including the 'x' parameter
in G_X and G_Y. For Elliptic Curve Keys of type EC2, compact
representation MAY be used also in the COSE_Key. If the COSE
implementation requires an 'y' parameter, the value y = false SHALL
be used. COSE always use compact output for Elliptic Curve Keys of
type EC2.
3.8. External Authorization Data
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 together with the
messages. One example is the transport third-party identity and
authorization information protected out of scope of EDHOC
[I-D.selander-ace-ake-authz]. Another example is the embedding of a
certificate enrolment request or a newly issued certificate.
EDHOC allows opaque 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) must be considered unprotected by EDHOC,
see Section 7.4. External authorization data sent in message_3
(EAD_3) or message_4 (EAD_4) is protected between Initiator and
Responder.
External authorization data is a CBOR sequence (see Appendix C.1) as
defined below:
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EAD = (
type : int,
1* ext_authz_data : any,
)
where type is an int and is followed by one or more ext_authz_data
depending on type as defined in a separate specification.
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 a) it
does not violate security, privacy etc. requirements of the service
which uses this data, and b) it does not violate the security
properties of EDHOC. Security applications making use of the EAD
fields must perform the necessary security analysis.
3.9. Applicability Statement
EDHOC requires certain parameters to be agreed upon between Initiator
and Responder. Some parameters can be agreed through the protocol
execution (specifically cipher suite negotiation, see Section 3.6)
but other parameters may need to be known out-of-band (e.g., which
authentication method is used, see Section 3.2).
The purpose of the applicability statement is 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 COSE_key,
including supported authentication key algorithms (subject public
key algorithm in X.509 certificate).
4. Type used to identify authentication credentials (ID_CRED_I,
ID_CRED_R; see Section 3.5.4).
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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 3.5.2.
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 specified by the protocol, but it
may become simplified by this knowledge.
An example of an applicability statement is shown in Appendix E.
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, but this applies only to the later phases of the
protocol when identities are known. (Initiator does not know
identity of Responder before having verified message_2, and Responder
does not know identity of 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.
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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. The
PRKs are derived using Extract.
PRK = Extract( salt, IKM )
If the EDHOC hash algorithm is SHA-2, then Extract( salt, IKM ) =
HKDF-Extract( salt, IKM ) [RFC5869]. If the EDHOC hash algorithm is
SHAKE128, then Extract( salt, IKM ) = KMAC128( salt, IKM, 256, "" ).
If the EDHOC hash algorithm is SHAKE256, then Extract( salt, IKM ) =
KMAC256( salt, IKM, 512, "" ).
PRK_2e is used to derive a keystream to encrypt message_2. PRK_3e2m
is used to derive keys and IVs to produce a MAC in message_2 and to
encrypt message_3. PRK_4x3m is used to derive keys and IVs to
produce a MAC in message_3 and to derive application specific data.
PRK_2e is derived with the following input:
o The salt SHALL be the empty 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 empty byte string.
o The input keying material (IKM) SHALL be the 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].
Example: Assuming the use of SHA-256 the extract phase of HKDF
produces PRK_2e as follows:
PRK_2e = HMAC-SHA-256( salt, G_XY )
where salt = 0x (the empty byte string).
The pseudorandom keys PRK_3e2m and PRK_4x3m are defined as follow:
o 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.
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o 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.
Example: Assuming the use of curve25519, the ECDH shared secrets
G_XY, G_RX, and G_IY are the outputs of the X25519 function
[RFC7748]:
G_XY = X25519( Y, G_X ) = X25519( X, G_Y )
The keys and IVs used in EDHOC are derived from PRKs using Expand
[RFC5869] where the EDHOC-KDF is instantiated with the EDHOC AEAD
algorithm in the selected cipher suite.
OKM = EDHOC-KDF( PRK, transcript_hash, label, length )
= Expand( PRK, info, length )
where info is the CBOR encoding of
info = [
edhoc_aead_id : int / tstr,
transcript_hash : bstr,
label : tstr,
length : uint
]
where
o edhoc_aead_id is an int or tstr containing the algorithm
identifier of the EDHOC AEAD algorithm in the selected cipher
suite encoded as defined in [I-D.ietf-cose-rfc8152bis-algs]. Note
that a single fixed edhoc_aead_id is used in all invocations of
EDHOC-KDF, including the derivation of KEYSTREAM_2 and invocations
of the EDHOC-Exporter.
o 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 4.1.
o label is a tstr set to the name of the derived key or IV, i.e.
"K_2m", "IV_2m", "KEYSTREAM_2", "K_3m", "IV_3m", "K_3ae", or
"IV_3ae".
o length is the length of output keying material (OKM) in bytes
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(
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PRK, info, L, "" ). If the EDHOC hash algorithm is SHAKE256, then
Expand( PRK, info, length ) = KMAC256( PRK, info, L, "" ).
KEYSTREAM_2 are derived using the transcript hash TH_2 and the
pseudorandom key PRK_2e. K_2m and IV_2m are derived using the
transcript hash TH_2 and the pseudorandom key PRK_3e2m. K_3ae and
IV_3ae are derived using the transcript hash TH_3 and the
pseudorandom key PRK_3e2m. K_3m and IV_3m are derived using the
transcript hash TH_3 and the pseudorandom key PRK_4x3m. IVs are only
used if the EDHOC AEAD algorithm uses IVs.
4.1. EDHOC-Exporter Interface
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)
label_context is a CBOR sequence:
label_context = (
label : tstr,
context : bstr,
)
where label is a registered tstr from the EDHOC Exporter Label
registry (Section 8.1), context is a bstr defined by the application,
and length is a uint defined by the application. The (label,
context) pair must be unique, i.e. a (label, context) MUST NOT be
used for two different purposes. However an application can re-
derive the same key several times as long as it is done in a secure
way. For example, in most encryption algorithms the same (key,
nonce) pair must not be reused.
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.
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
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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 )
5. Message Formatting and Processing
This section specifies formatting of the messages and processing
steps. Error messages are specified in Section 6.
An EDHOC message is encoded as a sequence of CBOR data (CBOR
Sequence, [RFC8742]). Additional optimizations are made to reduce
message overhead.
While EDHOC uses the COSE_Key, COSE_Sign1, and COSE_Encrypt0
structures, only a subset of the parameters is included in the EDHOC
messages. The unprotected COSE header in COSE_Sign1, and
COSE_Encrypt0 (not included in the EDHOC message) MAY contain
parameters (e.g. 'alg').
5.1. Message Processing Outline
This section outlines the message processing of EDHOC.
For each session, the endpoints are assumed to keep an associated
protocol state containing identifiers, keys, etc. used for subsequent
processing of protocol related data. The protocol state is assumed
to be associated to an applicability statement (Section 3.9) which
provides the context for how messages are transported, identified and
processed.
EDHOC messages SHALL be processed according to the current protocol
state. The following steps are expected to be performed at reception
of an EDHOC message:
1. Detect that an EDHOC message has been received, for example by
means of port number, URI, or media type (Section 3.9).
2. Retrieve the protocol state according to the message correlation
provided by the transport, see Section 3.4. If there is no
protocol state, in the case of message_1, a new protocol state is
created. The Responder endpoint needs to make use of available
Denial-of-Service mitigation (Section 7.5).
3. If the message received is an error message then process
according to Section 6, else process as the expected next message
according to the protocol state.
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If the processing fails, then the protocol is discontinued, an error
message sent, and the protocol state erased. Further details are
provided in the following subsections.
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 F.
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 : [ selected : suite, supported : 2* suite ] / suite,
G_X : bstr,
C_I : bstr / int,
? EAD ; EAD_1
)
suite = int
where:
o METHOD = 0, 1, 2, or 3 (see Figure 4).
o SUITES_I - cipher suites which the Initiator supports in order of
(decreasing) preference. The list of supported cipher suites can
be truncated at the end, as is detailed in the processing steps
below and Section 6.3. One of the supported cipher suites is
selected. The selected suite is the first suite in the SUITES_I
CBOR array. If a single supported cipher suite is conveyed then
that cipher suite is selected and SUITES_I is encoded as an int
instead of an array.
o G_X - the ephemeral public key of the Initiator
o C_I - variable length connection identifier
o EAD_1 - unprotected external authorization data, see Section 3.8.
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5.2.2. Initiator Processing of Message 1
The Initiator SHALL compose message_1 as follows:
o The supported cipher suites and the order of preference MUST NOT
be changed based on previous error messages. However, the list
SUITES_I sent to the Responder MAY be truncated such that cipher
suites which are the least preferred are omitted. The amount of
truncation MAY be changed between sessions, e.g. based on previous
error messages (see next bullet), but all cipher suites which are
more preferred than the least preferred cipher suite in the list
MUST be included in the list.
o The Initiator MUST select its most preferred cipher suite,
conditioned on what it can assume to be supported by the
Responder. 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 which also are
supported by the Initiator, then the Initiator SHOULD select the
most preferred cipher suite of those (note that error messages are
not authenticated and may be forged).
o 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.
o Choose a connection identifier C_I and store it for the length of
the protocol.
o 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:
o Decode message_1 (see Appendix C.1).
o Verify that the selected cipher suite is supported and that no
prior cipher suite in SUITES_I is supported.
o 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 7.
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5.3. EDHOC Message 2
5.3.1. Formatting of Message 2
message_2 and data_2 SHALL be CBOR Sequences (see Appendix C.1) as
defined below
message_2 = (
data_2,
CIPHERTEXT_2 : bstr,
)
data_2 = (
G_Y : bstr,
C_R : bstr / int,
)
where:
o G_Y - the ephemeral public key of the Responder
o C_R - variable length connection identifier
5.3.2. Responder Processing of Message 2
The Responder SHALL compose message_2 as follows:
o 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.
o Choose a connection identifier C_R and store it for the length of
the protocol.
o Compute the transcript hash TH_2 = H( H(message_1), data_2 ) 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.
o Compute an inner 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, K_2m, IV_2m, and the following
parameters:
* protected = << ID_CRED_R >>
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+ ID_CRED_R - identifier to facilitate retrieval of CRED_R,
see Section 3.5.4
* external_aad = << TH_2, CRED_R, ? EAD_2 >>
+ CRED_R - bstr containing the credential of the Responder,
see Section 3.5.4
+ EAD_2 = unprotected external authorization data, see
Section 3.8
* plaintext = h''
COSE constructs the input to the AEAD [RFC5116] as follows:
* Key K = EDHOC-KDF( PRK_3e2m, TH_2, "K_2m", length )
* Nonce N = EDHOC-KDF( PRK_3e2m, TH_2, "IV_2m", length )
* Plaintext P = 0x (the empty string)
* Associated data A =
[ "Encrypt0", << ID_CRED_R >>, << TH_2, CRED_R, ? EAD_2 >> ]
MAC_2 is the 'ciphertext' of the inner COSE_Encrypt0.
o If the Responder authenticates with a static Diffie-Hellman key
(method equals 1 or 3), then Signature_or_MAC_2 is MAC_2. If the
Responder authenticates with a signature key (method equals 0 or
2), then Signature_or_MAC_2 is the 'signature' of a COSE_Sign1
object as defined in Section 4.4 of
[I-D.ietf-cose-rfc8152bis-struct] using the signature algorithm in
the selected cipher suite, the private authentication key of the
Responder, and the following parameters:
* protected = << ID_CRED_R >>
* external_aad = << TH_2, CRED_R, ? EAD_2 >>
* payload = MAC_2
COSE constructs the input to the Signature Algorithm as:
* The key is the private authentication key of the Responder.
* The message M to be signed =
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[ "Signature1", << ID_CRED_R >>, << TH_2, CRED_R, ? EAD_2 >>,
MAC_2 ]
o CIPHERTEXT_2 is encrypted by using the Expand function as a binary
additive stream cipher.
* plaintext = ( ID_CRED_R / bstr / int, Signature_or_MAC_2, ?
EAD_2 )
+ Note that if ID_CRED_R contains a single 'kid2' parameter,
i.e., ID_CRED_R = { 4 : kid_R }, only the byte string or
integer kid_R is conveyed in the plaintext encoded as a bstr
/ int.
* CIPHERTEXT_2 = plaintext XOR KEYSTREAM_2
o 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:
o Decode message_2 (see Appendix C.1).
o Retrieve the protocol state using the message correlation provided
by the transport (e.g., the CoAP Token and the 5-tuple as a
client, or the prepended C_I as a server).
o Decrypt CIPHERTEXT_2, see Section 5.3.2.
o Pass EAD_2 to the security application.
o Verify that the identity of the Responder is an allowed identity
for this connection, see Section 3.5.
o 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 7. If an error message is sent, the session MUST be
discontinued.
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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:
o 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.
o Compute an inner 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, K_3m, IV_3m, and the following
parameters:
* protected = << ID_CRED_I >>
+ ID_CRED_I - identifier to facilitate retrieval of CRED_I,
see Section 3.5.4
* external_aad = << TH_3, CRED_I, ? EAD_3 >>
+ CRED_I - bstr containing the credential of the Initiator,
see Section 3.5.4.
+ EAD_3 = protected external authorization data, see
Section 3.8
* plaintext = h''
COSE constructs the input to the AEAD [RFC5116] as follows:
* Key K = EDHOC-KDF( PRK_4x3m, TH_3, "K_3m", length )
* Nonce N = EDHOC-KDF( PRK_4x3m, TH_3, "IV_3m", length )
* Plaintext P = 0x (the empty string)
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* Associated data A =
[ "Encrypt0", << ID_CRED_I >>, << TH_3, CRED_I, ? EAD_3 >> ]
MAC_3 is the 'ciphertext' of the inner COSE_Encrypt0.
o If the Initiator authenticates with a static Diffie-Hellman key
(method equals 2 or 3), then Signature_or_MAC_3 is MAC_3. If the
Initiator authenticates with a signature key (method equals 0 or
1), then Signature_or_MAC_3 is the 'signature' of a COSE_Sign1
object as defined in Section 4.4 of
[I-D.ietf-cose-rfc8152bis-struct] using the signature algorithm in
the selected cipher suite, the private authentication key of the
Initiator, and the following parameters:
* protected = << ID_CRED_I >>
* external_aad = << TH_3, CRED_I, ? EAD_3 >>
* payload = MAC_3
COSE constructs the input to the Signature Algorithm as:
* The key is the private authentication key of the Initiator.
* The message M to be signed =
[ "Signature1", << ID_CRED_I >>, << TH_3, CRED_I, ? EAD_3 >>,
MAC_3 ]
o Compute an outer 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, K_3ae, IV_3ae, and the following
parameters. The protected header SHALL be empty.
* external_aad = TH_3
* plaintext = ( ID_CRED_I / bstr / int, Signature_or_MAC_3, ?
EAD_3 )
+ Note that if ID_CRED_I contains a single 'kid2' parameter,
i.e., ID_CRED_I = { 4 : kid_I }, only the byte string or
integer kid_I is conveyed in the plaintext encoded as a bstr
or int.
COSE constructs the input to the AEAD [RFC5116] as follows:
* Key K = EDHOC-KDF( PRK_3e2m, TH_3, "K_3ae", length )
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* Nonce N = EDHOC-KDF( PRK_3e2m, TH_3, "IV_3ae", length )
* Plaintext P = ( ID_CRED_I / bstr / int, Signature_or_MAC_3, ?
EAD_3 )
* Associated data A = [ "Encrypt0", h'', TH_3 ]
CIPHERTEXT_3 is the 'ciphertext' of the outer COSE_Encrypt0.
o Encode message_3 as a sequence of CBOR encoded data items 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.
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:
o Decode message_3 (see Appendix C.1).
o Retrieve the protocol state using the message correlation provided
by the transport (e.g., the CoAP Token and the 5-tuple as a
client, or the prepended C_R as a server).
o Decrypt and verify the outer 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, K_3ae, and IV_3ae.
o Pass EAD_3 to the security application.
o Verify that the identity of the Initiator is an allowed identity
for this connection, see Section 3.5.
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o 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.
o 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 7. 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 are provided in Section 3.9.
5.5.1. Formatting of Message 4
message_4 SHALL be a CBOR Sequence (see Appendix C.1) as defined
below
message_4 = (
CIPHERTEXT_4 : bstr,
)
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5.5.2. Responder Processing of Message 4
The Responder SHALL compose message_4 as follows:
o Compute a 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 following parameters. The
protected header SHALL be empty.
* protected = h''
* external_aad = TH_4
* plaintext = ( ? EAD_4 )
where EAD_4 is protected external authorization data, see
Section 3.8. COSE constructs the input to the AEAD [RFC5116] as
follows:
* Key K = EDHOC-Exporter( "EDHOC_message_4_Key", h'', length )
* Nonce N = EDHOC-Exporter( "EDHOC_message_4_Nonce", h'', length
)
* Plaintext P = ( ? EAD_4 )
* Associated data A = [ "Encrypt0", h'', TH_4 ]
CIPHERTEXT_4 is the 'ciphertext' of the COSE_Encrypt0.
o Encode message_4 as a sequence of CBOR encoded data items as
specified in Section 5.5.1.
5.5.3. Initiator Processing of Message 4
The Initiator SHALL process message_4 as follows:
o Decode message_4 (see Appendix C.1).
o Retrieve the protocol state using the message correlation provided
by the transport (e.g., the CoAP Token and the 5-tuple as a
client, or the prepended C_I as a server).
o Decrypt and verify the outer 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.
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o Pass EAD_4 to the security application.
If any verification step fails the Initiator MUST send an EDHOC error
message back, formatted as defined in Section 6, and 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 5: EDHOC Error Message
where:
o ERR_CODE - error code encoded as an integer. The value 0 is used
for success, all other values (negative or positive) indicate
errors.
o ERR_INFO - error information. Content and encoding depend on
error code.
The remainder of this section specifies the currently defined error
codes, see Figure 6. Error codes 1 and 2 MUST be supported.
Additional error codes and corresponding error information may be
specified.
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+----------+---------------+----------------------------------------+
| ERR_CODE | ERR_INFO Type | Description |
+==========+===============+========================================+
| 0 | any | Success |
+----------+---------------+----------------------------------------+
| 1 | tstr | Unspecified |
+----------+---------------+----------------------------------------+
| 2 | SUITES_R | Wrong selected cipher suite |
+----------+---------------+----------------------------------------+
Figure 6: 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 of type SUITES_R:
SUITES_R : [ supported : 2* suite ] / suite
If the Responder does not support the selected cipher suite, then
SUITES_R MUST include one or more supported cipher suites. If the
Responder does not support the selected cipher suite, but supports
another cipher suite in SUITES_I, then SUITES_R MUST include the
first supported cipher suite in SUITES_I.
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6.3.1. Cipher Suite Negotiation
After receiving SUITES_R, the Initiator can determine which cipher
suite to select for the next EDHOC run with the Responder.
If the Initiator intends to contact the Responder in the future, the
Initiator SHOULD remember which selected cipher suite to use until
the next message_1 has been sent, otherwise the Initiator and
Responder will likely run into an infinite loop. After a successful
run of EDHOC, the Initiator MAY remember the selected cipher suite to
use in future EDHOC runs. Note that if the Initiator or Responder is
updated with new cipher suite policies, any cached information may be
outdated.
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 7 and 8 show
examples of how the Initiator can truncate 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 7), the Responder supports cipher suite
6 but not the initially selected cipher suite 5.
Initiator Responder
| METHOD, SUITES_I = 5, G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| DIAG_MSG, SUITES_R = 6 |
|<------------------------------------------------------------------+
| error |
| |
| METHOD, SUITES_I = [6, 5, 6], G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
Figure 7: Example of Responder supporting suite 6 but not suite 5.
In the second example (Figure 8), 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 an error and SUITES_R, 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
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suite 5, it would include 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 = [6, 5, 6], G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| DIAG_MSG, SUITES_R = [9, 8] |
|<------------------------------------------------------------------+
| error |
| |
| METHOD, SUITES_I = [8, 5, 6, 7, 8], G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
Figure 8: Example of Responder supporting suites 8 and 9 but not 5, 6
or 7.
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. Security Considerations
7.1. Security Properties
EDHOC inherits its security properties from the theoretical SIGMA-I
protocol [SIGMA]. Using the terminology from [SIGMA], EDHOC provides
perfect forward secrecy, mutual authentication with aliveness,
consistency, and peer awareness. As described in [SIGMA], peer
awareness is provided to the Responder, but not to the Initiator.
EDHOC protects the credential identifier of the Initiator against
active attacks and the credential identifier of the Responder against
passive attacks. The roles should be assigned to protect the most
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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. One way to mitigate pervasive monitoring
is to use a key exchange that provides perfect forward secrecy.
EDHOC therefore only supports methods with perfect forward secrecy.
To limit the effect of breaches, it is important to limit the use of
symmetrical group keys for bootstrapping. EDHOC therefore strives to
make the additional cost of using raw public keys and self-signed
certificates as small as possible. Raw public keys and self-signed
certificates are not a replacement for a public key infrastructure,
but SHOULD be used instead of symmetrical group keys for
bootstrapping.
Compromise of the long-term keys (private signature or static DH
keys) does not compromise the security of completed EDHOC exchanges.
Compromising the private authentication keys of one party lets an
active attacker impersonate that compromised party in EDHOC exchanges
with other parties, but does not let the attacker impersonate other
parties in EDHOC exchanges with the compromised party. Compromise of
the long-term keys does not enable a passive attacker to compromise
future session keys. Compromise of the HDKF input parameters (ECDH
shared secret) leads to compromise of all session keys derived from
that compromised shared secret. Compromise of one session key does
not compromise other session keys. 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
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average have to send 4.3 billion messages per second for 68 years,
which is infeasible in constrained IoT radio technologies.
After sending message_3, the Initiator is assured that no other party
than the Responder can compute the key PRK_4x3m (implicit key
authentication). The Initiator does however not know that the
Responder has actually computed the key PRK_4x3m. While the
Initiator can securely send protected application data, the Initiator
SHOULD NOT permanently store the keying material PRK_4x3m and TH_4
until the Initiator is assured that the Responder has actually
computed the key PRK_4x3m (explicit key confirmation). Explicit key
confirmation is e.g. assured when the Initiator has verified an
OSCORE message or message_4 from the Responder. After verifying
message_3, the Responder is assured that the Initiator has calculated
the key PRK_4x3m (explicit key confirmation) and that no other party
than the Responder can compute the key. The Responder can securely
send protected application data and store the keying material
PRK_4x3m and TH_4.
Key compromise impersonation (KCI): In EDHOC authenticated with
signature keys, EDHOC provides KCI protection against an attacker
having access to the long term key or the ephemeral secret key. With
static Diffie-Hellman key authentication, KCI protection would be
provided against an attacker having access to the long-term Diffie-
Hellman key, but not to an attacker having access to the ephemeral
secret key. Note that the term KCI has typically been used for
compromise of long-term keys, and that an attacker with access to the
ephemeral secret key can only attack that specific protocol run.
Repudiation: In EDHOC authenticated with signature keys, the
Initiator could theoretically prove that the Responder performed a
run of the protocol by presenting the private ephemeral key, and vice
versa. Note that storing the private ephemeral keys violates the
protocol requirements. With static Diffie-Hellman key
authentication, both parties can always deny having participated in
the protocol.
Two earlier versions of EDHOC have been formally analyzed [Norrman20]
[Bruni18] and the specification has been updated based on the
analysis.
7.2. Cryptographic Considerations
The security of the SIGMA protocol requires the MAC to be bound to
the identity of the signer. Hence the message authenticating
functionality of the authenticated encryption in EDHOC is critical:
authenticated encryption MUST NOT be replaced by plain encryption
only, even if authentication is provided at another level or through
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a different mechanism. EDHOC implements SIGMA-I using a MAC-then-
Sign approach.
To reduce message overhead EDHOC does not use explicit nonces and
instead rely on the ephemeral public keys to provide randomness to
each session. A good amount of randomness is important for the key
generation, to provide liveness, and to protect against interleaving
attacks. For this reason, the ephemeral keys MUST NOT be reused, and
both parties SHALL generate fresh random ephemeral key pairs.
As discussed the [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).
The data rates in many IoT deployments are very limited. Given that
the application keys are protected as well as the long-term
authentication keys they can often be used for years or even decades
before the cryptographic limits are reached. If the application keys
established through EDHOC need to be renewed, the communicating
parties can derive application keys with other labels or run EDHOC
again.
Requirement for how to securely generate, validate, and process the
ephermeral 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.
7.3. Cipher Suites and Cryptographic Algorithms
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, X25519, EdDSA, AES-CCM-
16-64-128, SHA-256) and cipher suite 2 (AES-CCM-16-64-128, SHA-256,
P-256, ES256, AES-CCM-16-64-128, SHA-256). Constrained endpoints
SHOULD implement cipher suite 0 or cipher suite 2. Implementations
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only need to implement the algorithms needed for their supported
methods.
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 (256-bit Hash truncated to
64-bits) SHALL NOT be supported for use in EDHOC except for
certificate identification with x5u and c5u. Note that secp256k1 is
only defined for use with ECDSA and not for ECDH.
7.4. Unprotected Data
The Initiator and the Responder must make sure that unprotected data
and metadata do not reveal any sensitive information. This also
applies for encrypted data sent to an unauthenticated party. In
particular, it applies to EAD_1, ID_CRED_R, EAD_2, and error
messages. Using the same EAD_1 in several EDHOC sessions allows
passive eavesdroppers to correlate the different sessions. Another
consideration is that the list of supported cipher suites may
potentially be used to identify the application.
The Initiator and the Responder must also make sure that
unauthenticated data does not trigger any harmful actions. In
particular, this applies to EAD_1 and error messages.
7.5. Denial-of-Service
EDHOC itself does not provide countermeasures against Denial-of-
Service attacks. By sending a number of new or replayed message_1 an
attacker may cause the Responder to allocate state, perform
cryptographic operations, and amplify messages. To mitigate such
attacks, an implementation SHOULD rely on lower layer mechanisms such
as the Echo option in CoAP [I-D.ietf-core-echo-request-tag] that
forces the initiator to demonstrate reachability at its apparent
network address.
An attacker can also send faked message_2, message_3, message_4, or
error in an attempt to trick the receiving party to send an error
message and discontinue the session. EDHOC implementations MAY
evaluate if a received message is likely to have be forged by and
attacker and ignore it without sending an error message or
discontinuing the session.
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7.6. Implementation Considerations
The availability of a secure random number generator is essential for
the security of EDHOC. If no true random number generator is
available, a truly random seed MUST be provided from an external
source and used with a cryptographically secure pseudorandom number
generator. As each pseudorandom number must only be used once, an
implementation need to get a new truly random seed after reboot, or
continuously store state in nonvolatile memory, see ([RFC8613],
Appendix B.1.1) for issues and solution approaches for writing to
nonvolatile memory. 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 keys and a deterministic
signature function. This improves randomness from broken or
otherwise subverted random number generators. The same idea can be
used with other secrets and functions such as a Diffie-Hellman
function or a symmetric secret and a PRF like HMAC or KMAC. It is
RECOMMENDED to not trust a single source of randomness and to not put
unaugmented random numbers on the wire.
If ECDSA is supported, "deterministic ECDSA" as specified in
[RFC6979] MAY be used. Pure deterministic elliptic-curve signatures
such as deterministic ECDSA and EdDSA have gained popularity over
randomized ECDSA as their security do not depend on a source of high-
quality randomness. Recent research has however found that
implementations of these signature algorithms may be vulnerable to
certain side-channel and fault injection attacks due to their
determinism. See e.g. Section 1 of
[I-D.mattsson-cfrg-det-sigs-with-noise] for a list of attack papers.
As suggested in Section 6.1.2 of [I-D.ietf-cose-rfc8152bis-algs] this
can be addressed by combining randomness and determinism.
All private keys, symmetric keys, and IVs MUST be secret.
Implementations should provide countermeasures to side-channel
attacks such as timing attacks. Intermediate computed values such as
ephemeral ECDH keys and ECDH shared secrets MUST be deleted after key
derivation is completed.
The Initiator and the Responder are responsible for verifying the
integrity of certificates. The selection of trusted CAs should be
done very carefully and certificate revocation should be supported.
The private authentication keys MUST be kept secret.
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
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protocol (e.g. OSCORE [RFC8613]). The choice of connection
identifier is not security critical in EDHOC but intended to simplify
the retrieval of the right security context in combination with using
short identifiers. If the wrong connection identifier of the other
party is used in a protocol message it will result in the receiving
party not being able to retrieve a security context (which will
terminate the protocol) or retrieve the wrong security context (which
also terminates the protocol as the message cannot be verified).
If two nodes unintentionally initiate two simultaneous EDHOC message
exchanges with each other even if they only want to complete a single
EDHOC message exchange, they MAY terminate the exchange with the
lexicographically smallest G_X. If the two G_X values are equal, the
received message_1 MUST be discarded to mitigate reflection attacks.
Note that in the case of two simultaneous EDHOC exchanges where the
nodes only complete one and where the nodes have different preferred
cipher suites, an attacker can affect which of the two nodes'
preferred cipher suites will be used by blocking the other exchange.
If supported by the device, it is RECOMMENDED that at least the long-
term private keys are stored in a Trusted Execution Environment (TEE)
and that sensitive operations using these keys are performed inside
the TEE. To achieve even higher security it is RECOMMENDED that in
additional operations such as ephemeral key generation, all
computations of shared secrets, and storage of the pseudorandom keys
(PRK) 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. Note that non-EDHOC code inside the TEE
might still be able to read EDHOC data and tamper with EDHOC code, to
protect against such attacks EDHOC needs to be in its own zone. To
provide better protection against some forms of physical attacks,
sensitive EDHOC data should be stored inside the SoC or encrypted and
integrity protected when sent on a data bus (e.g. between the CPU and
RAM or Flash). Secure boot can be used to increase the security of
code and data in the Rich Execution Environment (REE) by validating
the REE image.
8. IANA Considerations
8.1. EDHOC Exporter Label
IANA has created a new registry titled "EDHOC Exporter Label" under
the new heading "EDHOC". The registration procedure is "Expert
Review". The columns of the registry are Label, Description, and
Reference. All columns are text strings. The initial contents of
the registry are:
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Label: EDHOC_message_4_Key
Description: Key used to protect EDHOC message_4
Reference: [[this document]]
Label: EDHOC_message_4_Nonce
Description: Nonce 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]]
8.2. EDHOC Cipher Suites Registry
IANA has created a new registry titled "EDHOC Cipher Suites" under
the new heading "EDHOC". The registration procedure is "Expert
Review". The columns of the registry are Value, Array, Description,
and Reference, where Value is an integer and the other columns are
text strings. The initial contents of the registry are:
Value: -24
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]
Value: -23
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]
Value: -22
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]
Value: -21
Algorithms: N/A
Desc: Reserved for Private Use
Reference: [[this document]]
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Value: 0
Array: 10, -16, 4, -8, 10, -16
Desc: AES-CCM-16-64-128, SHA-256, X25519, EdDSA,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
Value: 1
Array: 30, -16, 4, -8, 10, -16
Desc: AES-CCM-16-128-128, SHA-256, X25519, EdDSA,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
Value: 2
Array: 10, -16, 1, -7, 10, -16
Desc: AES-CCM-16-64-128, SHA-256, P-256, ES256,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
Value: 3
Array: 30, -16, 1, -7, 10, -16
Desc: AES-CCM-16-128-128, SHA-256, P-256, ES256,
AES-CCM-16-64-128, SHA-256
Reference: [[this document]]
Value: 4
Array: 24, -16, 4, -8, 24, -16
Desc: ChaCha20/Poly1305, SHA-256, X25519, EdDSA,
ChaCha20/Poly1305, SHA-256
Reference: [[this document]]
Value: 5
Array: 24, -16, 1, -7, 24, -16
Desc: ChaCha20/Poly1305, SHA-256, P-256, ES256,
ChaCha20/Poly1305, SHA-256
Reference: [[this document]]
Value: 6
Array: 1, -16, 4, -7, 1, -16
Desc: A128GCM, SHA-256, X25519, ES256,
A128GCM, SHA-256
Reference: [[this document]]
Value: 24
Array: 3, -43, 2, -35, 3, -43
Desc: A256GCM, SHA-384, P-384, ES384,
A256GCM, SHA-384
Reference: [[this document]]
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Value: 25
Array: 24, -45, 5, -8, 24, -45
Desc: ChaCha20/Poly1305, SHAKE256, X448, EdDSA,
ChaCha20/Poly1305, SHAKE256
Reference: [[this document]]
8.3. EDHOC Method Type Registry
IANA has created a new registry entitled "EDHOC Method Type" under
the new heading "EDHOC". The registration procedure is "Expert
Review". The columns of the registry are Value, Description, and
Reference, where Value is an integer and the other columns are text
strings. The initial contents of the registry is shown in Figure 4.
8.4. EDHOC Error Codes Registry
IANA has created a new registry entitled "EDHOC Error Codes" under
the new heading "EDHOC". The registration procedure is
"Specification Required". 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 is shown in Figure 6.
8.5. COSE Header Parameters Registry
This document registers the following entries in the "COSE Header
Parameters" registry under the "CBOR Object Signing and Encryption
(COSE)" heading. The value of the 'cwt' header parameter is a CWT
[RFC8392] or an unprotected CWT Claims Set [I-D.ietf-rats-uccs].
+-----------+-------+----------------+------------------------------+
| Name | Label | Value Type | Description |
+===========+=======+================+==============================+
| cwt | TBD1 | COSE_Messages | A CBOR Web Token (CWT) or an |
| | | / map | unprotected CWT Claims Set |
+-----------+-------+----------------+------------------------------+
8.6. COSE Header Parameters Registry
IANA has added the COSE header parameter 'kid2' to the COSE Header
Parameters registry. The kid2 parameter may point to a COSE key
common parameter 'kid' or 'kid2'. The kid2 parameter can be used to
identify a key stored in a "raw" COSE_Key, in a CWT, or in a
certificate. The Value Reference for this item is empty and omitted
from the table below.
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+------+-------+------------+----------------+-------------------+
| Name | Label | Value Type | Description | Reference |
+------+-------+------------+----------------+-------------------+
| kid2 | TBD | bstr / int | Key identifier | [[This document]] |
+------+-------+------------+----------------+-------------------+
8.7. COSE Key Common Parameters Registry
IANA has added the COSE key common parameter 'kid2' to the COSE Key
Common Parameters registry. The Value Reference for this item is
empty and omitted from the table below.
+------+-------+------------+----------------+-------------------+
| Name | Label | Value Type | Description | Reference |
+------+-------+------------+----------------+-------------------+
| kid2 | TBD | bstr / int | Key identifi- | [[This document]] |
| | | | cation value - | |
| | | | match to kid2 | |
| | | | in message | |
+------+-------+------------+----------------+-------------------+
8.8. The Well-Known URI Registry
IANA has added the well-known URI 'edhoc' to the Well-Known URIs
registry.
o URI suffix: edhoc
o Change controller: IETF
o Specification document(s): [[this document]]
o Related information: None
8.9. Media Types Registry
IANA has added the media type 'application/edhoc' to the Media Types
registry.
o Type name: application
o Subtype name: edhoc
o Required parameters: N/A
o Optional parameters: N/A
o Encoding considerations: binary
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o Security considerations: See Section 7 of this document.
o Interoperability considerations: N/A
o Published specification: [[this document]] (this document)
o Applications that use this media type: To be identified
o Fragment identifier considerations: N/A
o Additional information:
* Magic number(s): N/A
* File extension(s): N/A
* Macintosh file type code(s): N/A
o Person & email address to contact for further information: See
"Authors' Addresses" section.
o Intended usage: COMMON
o Restrictions on usage: N/A
o Author: See "Authors' Addresses" section.
o Change Controller: IESG
8.10. CoAP Content-Formats Registry
IANA has added the media type 'application/edhoc' to the CoAP
Content-Formats registry.
o Media Type: application/edhoc
o Encoding:
o ID: TBD42
o Reference: [[this document]]
8.11. EDHOC External Authorization Data
IANA has created a new registry entitled "EDHOC External
Authorization Data" under the new heading "EDHOC". The registration
procedure is "Expert Review". The columns of the registry are Value,
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Description, and Reference, where Value is an integer and the other
columns are text strings.
8.12. 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:
o 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's 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.
o 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.
o Specifications are recommended. When specifications are not
provided, the description provided needs to have sufficient
information to verify the points above.
9. References
9.1. Normative References
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J. P., and G. Selander, "CoAP:
Echo, Request-Tag, and Token Processing", draft-ietf-core-
echo-request-tag-12 (work in progress), February 2021.
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-12
(work in progress), September 2020.
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[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", draft-ietf-cose-rfc8152bis-
struct-15 (work in progress), February 2021.
[I-D.ietf-cose-x509]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Header parameters for carrying and referencing X.509
certificates", draft-ietf-cose-x509-08 (work in progress),
December 2020.
[I-D.ietf-lake-reqs]
Vucinic, M., Selander, G., Mattsson, J. P., and D. Garcia-
Carrillo, "Requirements for a Lightweight AKE for OSCORE",
draft-ietf-lake-reqs-04 (work in progress), June 2020.
[I-D.ietf-rats-uccs]
Birkholz, H., O'Donoghue, J., Cam-Winget, N., and C.
Bormann, "A CBOR Tag for Unprotected CWT Claims Sets",
draft-ietf-rats-uccs-00 (work in progress), May 2021.
[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>.
[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>.
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[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>.
[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>.
[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>.
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[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>.
9.2. Informative References
[Bruni18] Bruni, A., Sahl Joergensen, T., Groenbech Petersen, T.,
and C. Schuermann, "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, "Combining EDHOC and OSCORE", draft-ietf-
core-oscore-edhoc-00 (work in progress), April 2021.
[I-D.ietf-core-resource-directory]
Amsuess, C., Shelby, Z., Koster, M., Bormann, C., and P.
V. D. Stok, "CoRE Resource Directory", draft-ietf-core-
resource-directory-28 (work in progress), March 2021.
[I-D.ietf-cose-cbor-encoded-cert]
Raza, S., Hoeglund, J., Selander, G., Mattsson, J. P., and
M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", draft-ietf-cose-cbor-encoded-cert-00 (work
in progress), April 2021.
[I-D.ietf-lwig-security-protocol-comparison]
Mattsson, J. P., Palombini, F., and M. Vucinic,
"Comparison of CoAP Security Protocols", draft-ietf-lwig-
security-protocol-comparison-05 (work in progress),
November 2020.
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[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-43 (work in progress), April
2021.
[I-D.mattsson-cfrg-det-sigs-with-noise]
Mattsson, J. P., Thormarker, E., and S. Ruohomaa,
"Deterministic ECDSA and EdDSA Signatures with Additional
Randomness", draft-mattsson-cfrg-det-sigs-with-noise-02
(work in progress), March 2020.
[I-D.selander-ace-ake-authz]
Selander, G., Mattsson, J. P., Vucinic, M., Richardson,
M., and A. Schellenbaum, "Lightweight Authorization for
Authenticated Key Exchange.", draft-selander-ace-ake-
authz-02 (work in progress), November 2020.
[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>.
[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>.
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[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 sppendix 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.
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].)
o 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.
o 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
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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]):
o The Master Secret and Master Salt are derived by using the EDHOC-
Exporter interface, see Section 4.1.
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 )
o The AEAD Algorithm is the application AEAD algorithm of the
selected cipher suite for the EDHOC session.
o 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 ciphersuite, HKDF SHA-256 is used as HKDF Algorithm in
the OSCORE Security Context.
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o In case the Client is Initiator and the Server is Responder, the
Client's OSCORE Sender ID and the Server's OSCORE Sender ID are
determined from the EDHOC connection identifiers C_R and C_I for
the EDHOC session, respectively, by applying the conversion in
Appendix A.1. The reverse applies in case the Client is the
Responder and the Server is the Initiator.
Client and Server use the parameters above to establish an OSCORE
Security Context, as per Section 3.2.1 of [RFC8613].
From then on, Client and Server retrieve the OSCORE protocol state
using the Recipient ID, and optionally other transport information
such as the 5-tuple.
A.3. Transferring EDHOC over CoAP
This section specifies one instance for how EDHOC can be transferred
as an exchange of CoAP [RFC7252] messages. CoAP is a reliable
transport that can preserve packet ordering and handle message
duplication. CoAP can also perform fragmentation and protect against
denial of service attacks. According to this specification, EDHOC
messages are carried in Confirmable messages, which is beneficial
especially if fragmentation is used.
By default, the CoAP client is the Initiator and the CoAP server is
the Responder, but the roles SHOULD be chosen to protect the most
sensitive identity, see Section 7. 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
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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 "null" (0xf6) is sent in its place (given that
the server has not selected C_R yet).
These identifiers are encoded in CBOR and thus self-delimiting. They
are sent in front of the actual EDHOC message, and only the part of
the body following the identifier is used for EDHOC processing.
Consequently, the application/edhoc media type does not apply to
these messages; their media type is unnamed.
An example of a successful EDHOC exchange using CoAP is shown in
Figure 9. 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: null, 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 9: Transferring EDHOC in CoAP when the Initiator is CoAP
Client
The exchange in Figure 9 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 10. 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 10: 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 [I-D.ietf-core-echo-request-tag]. This
forces the initiator to demonstrate its reachability at its apparent
network address. If message fragmentation is needed, the EDHOC
messages may be fragmented using the CoAP Block-Wise Transfer
mechanism [RFC7959]. EDHOC does not restrict how error messages are
transported with CoAP, as long as the appropriate error message can
to be transported in response to a message that failed (see
Section 6).
A.3.1. Transferring EDHOC and OSCORE over CoAP
A method for combining EDHOC and OSCORE protocols in two round-trips
is specified in [I-D.ietf-core-oscore-edhoc].
When using EDHOC over CoAP for establishing an OSCORE Security
Context, EDHOC error messages sent as CoAP responses MUST be 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).
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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].
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
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CBOR encoding, CBOR also has a diagnostic notation that is readable
and editable by humans. The Concise Data Definition Language (CDDL)
[RFC8610] provides a way to express structures for protocol messages
and APIs that use CBOR. [RFC8610] also extends the diagnostic
notation.
CBOR data items are encoded to or decoded from byte strings using a
type-length-value encoding scheme, where the three highest order bits
of the initial byte contain information about the major type. CBOR
supports several different types of data items, in addition to
integers (int, uint), simple values (e.g. null), byte strings (bstr),
and text strings (tstr), CBOR also supports arrays [] of data items,
maps {} of pairs of data items, and sequences [RFC8742] of data
items. Some examples are given below. For a complete specification
and more examples, see [RFC8949] and [RFC8610]. We recommend
implementors to get used to CBOR by using the CBOR playground
[CborMe].
Diagnostic Encoded Type
------------------------------------------------------------------
1 0x01 unsigned integer
24 0x1818 unsigned integer
-24 0x37 negative integer
-25 0x3818 negative integer
null 0xf6 simple value
h'12cd' 0x4212cd byte string
'12cd' 0x4431326364 byte string
"12cd" 0x6431326364 text string
{ 4 : h'cd' } 0xa10441cd map
<< 1, 2, null >> 0x430102f6 byte string
[ 1, 2, null ] 0x830102f6 array
( 1, 2, null ) 0x0102f6 sequence
1, 2, null 0x0102f6 sequence
------------------------------------------------------------------
C.2. CDDL Definitions
This sections compiles the CDDL definitions for ease of reference.
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suite = int
SUITES_R : [ supported : 2* suite ] / suite
message_1 = (
METHOD : int,
SUITES_I : [ selected : suite, supported : 2* suite ] / suite,
G_X : bstr,
C_I : bstr / int,
? EAD ; EAD_1
)
message_2 = (
data_2,
CIPHERTEXT_2 : bstr,
)
data_2 = (
G_Y : bstr,
C_R : bstr / int,
)
message_3 = (
CIPHERTEXT_3 : bstr,
)
message_4 = (
CIPHERTEXT_4 : bstr,
)
error = (
ERR_CODE : int,
ERR_INFO : any
)
info = [
edhoc_aead_id : int / tstr,
transcript_hash : bstr,
label : tstr,
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
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processing in constrained devices. EDHOC makes use of COSE_Key,
COSE_Encrypt0, and COSE_Sign1 objects.
Appendix D. Test Vectors
NOTE 0. These test vectors are compatible with versions -05 and -06
of the specification.
This appendix provides detailed test vectors to ease implementation
and ensure interoperability. In addition to hexadecimal, all CBOR
data items and sequences are given in CBOR diagnostic notation. The
test vectors use the default mapping to CoAP where the Initiator acts
as CoAP client (this means that corr = 1).
A more extensive test vector suite covering more combinations of
authentication method used between Initiator and Responder and
related code to generate them can be found at https://github.com/
lake-wg/edhoc/tree/master/test-vectors-05.
NOTE 1. In the previous and current test vectors the same name is
used for certain byte strings and their CBOR bstr encodings. For
example the transcript hash TH_2 is used to denote both the output of
the hash function and the input into the key derivation function,
whereas the latter is a CBOR bstr encoding of the former. Some
attempts are made to clarify that in this Appendix (e.g. using "CBOR
encoded"/"CBOR unencoded").
NOTE 2. If not clear from the context, remember that CBOR sequences
and CBOR arrays assume CBOR encoded data items as elements.
D.1. Test Vectors for EDHOC Authenticated with Signature Keys (x5t)
EDHOC with signature authentication and X.509 certificates is used.
In this test vector, the hash value 'x5t' is used to identify the
certificate. The optional C_1 in message_1 is omitted. No external
authorization data is sent in the message exchange.
method (Signature Authentication)
0
CoAP is used as transport and the Initiator acts as CoAP client:
corr (the Initiator can correlate message_1 and message_2)
1
From there, METHOD_CORR has the following value:
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METHOD_CORR (4 * method + corr) (int)
1
The Initiator indicates only one cipher suite in the (potentially
truncated) list of cipher suites.
Supported Cipher Suites (1 byte)
00
The Initiator selected the indicated cipher suite.
Selected Cipher Suite (int)
0
Cipher suite 0 is supported by both the Initiator and the Responder,
see Section 3.6.
D.1.1. Message_1
The Initiator generates its ephemeral key pair.
X (Initiator's ephemeral private key) (32 bytes)
8f 78 1a 09 53 72 f8 5b 6d 9f 61 09 ae 42 26 11 73 4d 7d bf a0 06 9a 2d
f2 93 5b b2 e0 53 bf 35
G_X (Initiator's ephemeral public key, CBOR unencoded) (32 bytes)
89 8f f7 9a 02 06 7a 16 ea 1e cc b9 0f a5 22 46 f5 aa 4d d6 ec 07 6b ba
02 59 d9 04 b7 ec 8b 0c
The Initiator chooses a connection identifier C_I:
Connection identifier chosen by Initiator (1 byte)
09
Note that since C_I is a byte string in the interval h'00' to h'2f',
it is encoded as the corresponding integer subtracted by 24. Thus
0x09 = 09, 9 - 24 = -15, and -15 in CBOR encoding is equal to 0x2e.
C_I (1 byte)
2e
Since no external authorization data is sent:
EAD_1 (0 bytes)
The list of supported cipher suites needs to contain the selected
cipher suite. The initiator truncates the list of supported cipher
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suites to one cipher suite only. In this case there is only one
supported cipher suite indicated, 00.
Because one single selected cipher suite is conveyed, it is encoded
as an int instead of an array:
SUITES_I (int)
0
message_1 is constructed as the CBOR Sequence of the data items above
encoded as CBOR. In CBOR diagnostic notation:
message_1 =
(
1,
0,
h'898FF79A02067A16EA1ECCB90FA52246F5AA4DD6EC076BBA0259D904B7EC8B0C',
-15
)
Which as a CBOR encoded data item is:
message_1 (CBOR Sequence) (37 bytes)
01 00 58 20 89 8f f7 9a 02 06 7a 16 ea 1e cc b9 0f a5 22 46 f5 aa 4d d6
ec 07 6b ba 02 59 d9 04 b7 ec 8b 0c 2e
D.1.2. Message_2
Since METHOD_CORR mod 4 equals 1, C_I is omitted from data_2.
The Responder generates the following ephemeral key pair.
Y (Responder's ephemeral private key) (32 bytes)
fd 8c d8 77 c9 ea 38 6e 6a f3 4f f7 e6 06 c4 b6 4c a8 31 c8 ba 33 13 4f
d4 cd 71 67 ca ba ec da
G_Y (Responder's ephemeral public key, CBOR unencoded) (32 bytes)
71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0 19 52
81 75 4c 5e bc af 30 1e
From G_X and Y or from G_Y and X the ECDH shared secret is computed:
G_XY (ECDH shared secret) (32 bytes)
2b b7 fa 6e 13 5b c3 35 d0 22 d6 34 cb fb 14 b3 f5 82 f3 e2 e3 af b2 b3
15 04 91 49 5c 61 78 2b
The key and nonce for calculating the 'ciphertext' are calculated as
follows, as specified in Section 4.
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HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).
PRK_2e = HMAC-SHA-256(salt, G_XY)
Salt is the empty byte string.
salt (0 bytes)
From there, PRK_2e is computed:
PRK_2e (32 bytes)
ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f
d8 2f be b7 99 71 39 4a
The Responder's sign/verify key pair is the following:
SK_R (Responder's private authentication key) (32 bytes)
df 69 27 4d 71 32 96 e2 46 30 63 65 37 2b 46 83 ce d5 38 1b fc ad cd 44
0a 24 c3 91 d2 fe db 94
PK_R (Responder's public authentication key) (32 bytes)
db d9 dc 8c d0 3f b7 c3 91 35 11 46 2b b2 38 16 47 7c 6b d8 d6 6e f5 a1
a0 70 ac 85 4e d7 3f d2
Since neither the Initiator nor the Responder authenticates with a
static Diffie-Hellman key, PRK_3e2m = PRK_2e
PRK_3e2m (32 bytes)
ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f
d8 2f be b7 99 71 39 4a
The Responder chooses a connection identifier C_R.
Connection identifier chosen by Responder (1 byte)
00
Note that since C_R is a byte string in the interval h'00' to h'2f',
it is encoded as the corresponding integer subtracted by 24. Thus
0x00 = 0, 0 - 24 = -24, and -24 in CBOR encoding is equal to 0x37.
C_R (1 byte)
37
Data_2 is constructed as the CBOR Sequence of G_Y and C_R, encoded as
CBOR byte strings. The CBOR diagnostic notation is:
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data_2 =
(
h'71a3d599c21da18902a1aea810b2b6382ccd8d5f9bf0195281754c5ebcaf301e',
-24
)
Which as a CBOR encoded data item is:
data_2 (CBOR Sequence) (35 bytes)
58 20 71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0
19 52 81 75 4c 5e bc af 30 1e 37
From data_2 and message_1, compute the input to the transcript hash
TH_2 = H( H(message_1), data_2 ), as a CBOR Sequence of these 2 data
items.
Input to calculate TH_2 (CBOR Sequence) (72 bytes)
01 00 58 20 89 8f f7 9a 02 06 7a 16 ea 1e cc b9 0f a5 22 46 f5 aa 4d d6
ec 07 6b ba 02 59 d9 04 b7 ec 8b 0c 2e 58 20 71 a3 d5 99 c2 1d a1 89 02
a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0 19 52 81 75 4c 5e bc af 30 1e 37
And from there, compute the transcript hash TH_2 = SHA-256(
H(message_1), data_2 )
TH_2 (CBOR unencoded) (32 bytes)
86 4e 32 b3 6a 7b 5f 21 f1 9e 99 f0 c6 6d 91 1e 0a ce 99 72 d3 76 d2 c2
c1 53 c1 7f 8e 96 29 ff
The Responder's subject name is the empty string:
Responder's subject name (text string)
""
In this version of the test vectors CRED_R is not a DER encoded X.509
certificate, but a string of random bytes.
CRED_R (CBOR unencoded) (100 bytes)
c7 88 37 00 16 b8 96 5b db 20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86
44 2b 87 ec 3f f2 45 b7 0a 47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e c9 d6
b0 53 4b 71 c2 b4 9e 4b f9 03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b 38 1e
98 db 71 41 08 41 5e 5c 50 db 78 97 4c 27 15 79 b0 16 33 a3 ef 62 71 be
5c 22 5e b2
CRED_R is defined to be the CBOR bstr containing the credential of
the Responder.
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CRED_R (102 bytes)
58 64 c7 88 37 00 16 b8 96 5b db 20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40
6e 86 44 2b 87 ec 3f f2 45 b7 0a 47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e
c9 d6 b0 53 4b 71 c2 b4 9e 4b f9 03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b
38 1e 98 db 71 41 08 41 5e 5c 50 db 78 97 4c 27 15 79 b0 16 33 a3 ef 62
71 be 5c 22 5e b2
And because certificates are identified by a hash value with the
'x5t' parameter, ID_CRED_R is the following:
ID_CRED_R = { 34 : COSE_CertHash }. In this example, the hash
algorithm used is SHA-2 256-bit with hash truncated to 64-bits (value
-15). The hash value is calculated over the CBOR unencoded CRED_R.
The CBOR diagnostic notation is:
ID_CRED_R =
{
34: [-15, h'6844078A53F312F5']
}
which when encoded as a CBOR map becomes:
ID_CRED_R (14 bytes)
a1 18 22 82 2e 48 68 44 07 8a 53 f3 12 f5
Since no external authorization data is sent:
EAD_2 (0 bytes)
The plaintext is defined as the empty string:
P_2m (0 bytes)
The Enc_structure is defined as follows: [ "Encrypt0",
<< ID_CRED_R >>, << TH_2, CRED_R >> ], indicating that ID_CRED_R is
encoded as a CBOR byte string, and that the concatenation of the CBOR
byte strings TH_2 and CRED_R is wrapped as a CBOR bstr. The CBOR
diagnostic notation is the following:
A_2m =
[
"Encrypt0",
h'A11822822E486844078A53F312F5',
h'5820864E32B36A7B5F21F19E99F0C66D911E0ACE9972D376D2C2C153C17F8E9629FF
5864C788370016B8965BDB2074BFF82E5A20E09BEC21F8406E86442B87EC3FF245B70A
47624DC9CDC6824B2A4C52E95EC9D6B0534B71C2B49E4BF9031500CEE6869979C297BB
5A8B381E98DB714108415E5C50DB78974C271579B01633A3EF6271BE5C225EB2'
]
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Which encodes to the following byte string to be used as Additional
Authenticated Data:
A_2m (CBOR-encoded) (163 bytes)
83 68 45 6e 63 72 79 70 74 30 4e a1 18 22 82 2e 48 68 44 07 8a 53 f3 12
f5 58 88 58 20 86 4e 32 b3 6a 7b 5f 21 f1 9e 99 f0 c6 6d 91 1e 0a ce 99
72 d3 76 d2 c2 c1 53 c1 7f 8e 96 29 ff 58 64 c7 88 37 00 16 b8 96 5b db
20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44 2b 87 ec 3f f2 45 b7 0a
47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e 4b f9
03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e 5c 50
db 78 97 4c 27 15 79 b0 16 33 a3 ef 62 71 be 5c 22 5e b2
info for K_2m is defined as follows in CBOR diagnostic notation:
info for K_2m =
[
10,
h'864E32B36A7B5F21F19E99F0C66D911E0ACE9972D376D2C2C153C17F8E9629FF',
"K_2m",
16
]
Which as a CBOR encoded data item is:
info for K_2m (CBOR-encoded) (42 bytes)
84 0a 58 20 86 4e 32 b3 6a 7b 5f 21 f1 9e 99 f0 c6 6d 91 1e 0a ce 99 72
d3 76 d2 c2 c1 53 c1 7f 8e 96 29 ff 64 4b 5f 32 6d 10
From these parameters, K_2m is computed. Key K_2m is the output of
HKDF-Expand(PRK_3e2m, info, L), where L is the length of K_2m, so 16
bytes.
K_2m (16 bytes)
80 cc a7 49 ab 58 f5 69 ca 35 da ee 05 be d1 94
info for IV_2m is defined as follows, in CBOR diagnostic notation (10
is the COSE algorithm no. of the AEAD algorithm in the selected
cipher suite 0):
info for IV_2m =
[
10,
h'864E32B36A7B5F21F19E99F0C66D911E0ACE9972D376D2C2C153C17F8E9629FF',
"IV_2m",
13
]
Which as a CBOR encoded data item is:
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info for IV_2m (CBOR-encoded) (43 bytes)
84 0a 58 20 86 4e 32 b3 6a 7b 5f 21 f1 9e 99 f0 c6 6d 91 1e 0a ce 99 72
d3 76 d2 c2 c1 53 c1 7f 8e 96 29 ff 65 49 56 5f 32 6d 0d
From these parameters, IV_2m is computed. IV_2m is the output of
HKDF-Expand(PRK_3e2m, info, L), where L is the length of IV_2m, so 13
bytes.
IV_2m (13 bytes)
c8 1e 1a 95 cc 93 b3 36 69 6e d5 02 55
Finally, COSE_Encrypt0 is computed from the parameters above.
o protected header = CBOR-encoded ID_CRED_R
o external_aad = A_2m
o empty plaintext = P_2m
MAC_2 (CBOR unencoded) (8 bytes)
fa bb a4 7e 56 71 a1 82
To compute the Signature_or_MAC_2, the key is the private
authentication key of the Responder and the message M_2 to be signed
= [ "Signature1", << ID_CRED_R >>, << TH_2, CRED_R, ? EAD_2 >>, MAC_2
]. ID_CRED_R is encoded as a CBOR byte string, the concatenation of
the CBOR byte strings TH_2 and CRED_R is wrapped as a CBOR bstr, and
MAC_2 is encoded as a CBOR bstr.
M_2 =
[
"Signature1",
h'A11822822E486844078A53F312F5',
h'5820864E32B36A7B5F21F19E99F0C66D911E0ACE9972D376D2C2C153C17F8E9629F
F5864C788370016B8965BDB2074BFF82E5A20E09BEC21F8406E86442B87EC3FF245B7
0A47624DC9CDC6824B2A4C52E95EC9D6B0534B71C2B49E4BF9031500CEE6869979C29
7BB5A8B381E98DB714108415E5C50DB78974C271579B01633A3EF6271BE5C225EB2',
h'FABBA47E5671A182'
]
Which as a CBOR encoded data item is:
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M_2 (174 bytes)
84 6a 53 69 67 6e 61 74 75 72 65 31 4e a1 18 22 82 2e 48 68 44 07 8a 53
f3 12 f5 58 88 58 20 86 4e 32 b3 6a 7b 5f 21 f1 9e 99 f0 c6 6d 91 1e 0a
ce 99 72 d3 76 d2 c2 c1 53 c1 7f 8e 96 29 ff 58 64 c7 88 37 00 16 b8 96
5b db 20 74 bf f8 2e 5a 20 e0 9b ec 21 f8 40 6e 86 44 2b 87 ec 3f f2 45
b7 0a 47 62 4d c9 cd c6 82 4b 2a 4c 52 e9 5e c9 d6 b0 53 4b 71 c2 b4 9e
4b f9 03 15 00 ce e6 86 99 79 c2 97 bb 5a 8b 38 1e 98 db 71 41 08 41 5e
5c 50 db 78 97 4c 27 15 79 b0 16 33 a3 ef 62 71 be 5c 22 5e b2 48 fa bb
a4 7e 56 71 a1 82
Since the method = 0, Signature_or_MAC_2 is a signature. The
algorithm with selected cipher suite 0 is Ed25519 and the output is
64 bytes.
Signature_or_MAC_2 (CBOR unencoded) (64 bytes)
1f 17 00 6a 98 48 c9 77 cb bd ca a7 57 b6 fd 46 82 c8 17 39 e1 5c 99 37
c2 1c f5 e9 a0 e6 03 9f 54 fd 2a 6c 3a 11 18 f2 b9 d8 eb cd 48 23 48 b9
9c 3e d7 ed 1b d9 80 6c 93 c8 90 68 e8 36 b4 0f
CIPHERTEXT_2 is the ciphertext resulting from XOR between plaintext
and KEYSTREAM_2 which is derived from TH_2 and the pseudorandom key
PRK_2e.
o plaintext = CBOR Sequence of the items ID_CRED_R and
Signature_or_MAC_2 encoded as CBOR byte strings, in this order
(EAD_2 is empty).
The plaintext is the following:
P_2e (CBOR Sequence) (80 bytes)
a1 18 22 82 2e 48 68 44 07 8a 53 f3 12 f5 58 40 1f 17 00 6a 98 48 c9 77
cb bd ca a7 57 b6 fd 46 82 c8 17 39 e1 5c 99 37 c2 1c f5 e9 a0 e6 03 9f
54 fd 2a 6c 3a 11 18 f2 b9 d8 eb cd 48 23 48 b9 9c 3e d7 ed 1b d9 80 6c
93 c8 90 68 e8 36 b4 0f
KEYSTREAM_2 = HKDF-Expand( PRK_2e, info, length ), where length is
the length of the plaintext, so 80.
info for KEYSTREAM_2 =
[
10,
h'864E32B36A7B5F21F19E99F0C66D911E0ACE9972D376D2C2C153C17F8E9629FF',
"KEYSTREAM_2",
80
]
Which as a CBOR encoded data item is:
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info for KEYSTREAM_2 (CBOR-encoded) (50 bytes)
84 0a 58 20 86 4e 32 b3 6a 7b 5f 21 f1 9e 99 f0 c6 6d 91 1e 0a ce 99 72
d3 76 d2 c2 c1 53 c1 7f 8e 96 29 ff 6b 4b 45 59 53 54 52 45 41 4d 5f 32
18 50
From there, KEYSTREAM_2 is computed:
KEYSTREAM_2 (80 bytes)
ae ea 8e af 50 cf c6 70 09 da e8 2d 8d 85 b0 e7 60 91 bf 0f 07 0b 79 53
6c 83 23 dc 3d 9d 61 13 10 35 94 63 f4 4b 12 4b ea b3 a1 9d 09 93 82 d7
30 80 17 f4 92 62 06 e4 f5 44 9b 9f c9 24 bc b6 bd 78 ec 45 0a 66 83 fb
8a 2f 5f 92 4f c4 40 4f
Using the parameters above, the ciphertext CIPHERTEXT_2 can be
computed:
CIPHERTEXT_2 (CBOR unencoded) (80 bytes)
0f f2 ac 2d 7e 87 ae 34 0e 50 bb de 9f 70 e8 a7 7f 86 bf 65 9f 43 b0 24
a7 3e e9 7b 6a 2b 9c 55 92 fd 83 5a 15 17 8b 7c 28 af 54 74 a9 75 81 48
64 7d 3d 98 a8 73 1e 16 4c 9c 70 52 81 07 f4 0f 21 46 3b a8 11 bf 03 97
19 e7 cf fa a7 f2 f4 40
message_2 is the CBOR Sequence of data_2 and CIPHERTEXT_2, in this
order:
message_2 =
(
data_2,
h'0FF2AC2D7E87AE340E50BBDE9F70E8A77F86BF659F43B024A73EE97B6A2B9C5592FD
835A15178B7C28AF5474A9758148647D3D98A8731E164C9C70528107F40F21463BA811
BF039719E7CFFAA7F2F440'
)
Which as a CBOR encoded data item is:
message_2 (CBOR Sequence) (117 bytes)
58 20 71 a3 d5 99 c2 1d a1 89 02 a1 ae a8 10 b2 b6 38 2c cd 8d 5f 9b f0
19 52 81 75 4c 5e bc af 30 1e 37 58 50 0f f2 ac 2d 7e 87 ae 34 0e 50 bb
de 9f 70 e8 a7 7f 86 bf 65 9f 43 b0 24 a7 3e e9 7b 6a 2b 9c 55 92 fd 83
5a 15 17 8b 7c 28 af 54 74 a9 75 81 48 64 7d 3d 98 a8 73 1e 16 4c 9c 70
52 81 07 f4 0f 21 46 3b a8 11 bf 03 97 19 e7 cf fa a7 f2 f4 40
D.1.3. Message_3
Since corr equals 1, C_R is not omitted from data_3.
The Initiator's sign/verify key pair is the following:
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SK_I (Initiator's private authentication key) (32 bytes)
2f fc e7 a0 b2 b8 25 d3 97 d0 cb 54 f7 46 e3 da 3f 27 59 6e e0 6b 53 71
48 1d c0 e0 12 bc 34 d7
PK_I (Responder's public authentication key) (32 bytes)
38 e5 d5 45 63 c2 b6 a4 ba 26 f3 01 5f 61 bb 70 6e 5c 2e fd b5 56 d2 e1
69 0b 97 fc 3c 6d e1 49
HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).
PRK_4x3m = HMAC-SHA-256 (PRK_3e2m, G_IY)
PRK_4x3m (32 bytes)
ec 62 92 a0 67 f1 37 fc 7f 59 62 9d 22 6f bf c4 e0 68 89 49 f6 62 a9 7f
d8 2f be b7 99 71 39 4a
data 3 is equal to C_R.
data_3 (CBOR Sequence) (1 byte)
37
From data_3, CIPHERTEXT_2, and TH_2, compute the input to the
transcript hash TH_3 = H( H(TH_2 , CIPHERTEXT_2), data_3), as a CBOR
Sequence of 2 data items.
Input to calculate TH_3 (CBOR Sequence) (117 bytes)
58 20 86 4e 32 b3 6a 7b 5f 21 f1 9e 99 f0 c6 6d 91 1e 0a ce 99 72 d3 76
d2 c2 c1 53 c1 7f 8e 96 29 ff 58 50 0f f2 ac 2d 7e 87 ae 34 0e 50 bb de
9f 70 e8 a7 7f 86 bf 65 9f 43 b0 24 a7 3e e9 7b 6a 2b 9c 55 92 fd 83 5a
15 17 8b 7c 28 af 54 74 a9 75 81 48 64 7d 3d 98 a8 73 1e 16 4c 9c 70 52
81 07 f4 0f 21 46 3b a8 11 bf 03 97 19 e7 cf fa a7 f2 f4 40 37
And from there, compute the transcript hash TH_3 = SHA-256( H(TH_2 ,
CIPHERTEXT_2), data_3)
TH_3 (CBOR unencoded) (32 bytes)
f2 4d 18 ca fc e3 74 d4 e3 73 63 29 c1 52 ab 3a ea 9c 7c 0f 65 0c 30 70
b6 f5 1e 68 e2 ae bb 60
The Initiator's subject name is the empty string:
Initiator's subject name (text string)
""
In this version of the test vectors CRED_I is not a DER encoded X.509
certificate, but a string of random bytes.
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CRED_I (CBOR unencoded) (101 bytes)
54 13 20 4c 3e bc 34 28 a6 cf 57 e2 4c 9d ef 59 65 17 70 44 9b ce 7e c6
56 1e 52 43 3a a5 5e 71 f1 fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1 76 60
88 09 84 49 cb 84 8f fc 79 5f 88 af c4 9c be 8a fd d1 ba 00 9f 21 67 5e
8f 6c 77 a4 a2 c3 01 95 60 1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44 48 65
02 ff 7b dd a6
CRED_I is defined to be the CBOR bstr containing the credential of
the Initiator.
CRED_I (103 bytes)
58 65 54 13 20 4c 3e bc 34 28 a6 cf 57 e2 4c 9d ef 59 65 17 70 44 9b ce
7e c6 56 1e 52 43 3a a5 5e 71 f1 fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1
76 60 88 09 84 49 cb 84 8f fc 79 5f 88 af c4 9c be 8a fd d1 ba 00 9f 21
67 5e 8f 6c 77 a4 a2 c3 01 95 60 1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44
48 65 02 ff 7b dd a6
And because certificates are identified by a hash value with the
'x5t' parameter, ID_CRED_I is the following:
ID_CRED_I = { 34 : COSE_CertHash }. In this example, the hash
algorithm used is SHA-2 256-bit with hash truncated to 64-bits (value
-15). The hash value is calculated over the CBOR unencoded CRED_I.
ID_CRED_I =
{
34: [-15, h'705D5845F36FC6A6']
}
which when encoded as a CBOR map becomes:
ID_CRED_I (14 bytes)
a1 18 22 82 2e 48 70 5d 58 45 f3 6f c6 a6
Since no external authorization data is exchanged:
EAD_3 (0 bytes)
The plaintext of the COSE_Encrypt is the empty string:
P_3m (0 bytes)
The associated data is the following: [ "Encrypt0", << ID_CRED_I >>,
<< TH_3, CRED_I, ? EAD_3 >> ].
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A_3m (CBOR-encoded) (164 bytes)
83 68 45 6e 63 72 79 70 74 30 4e a1 18 22 82 2e 48 70 5d 58 45 f3 6f c6
a6 58 89 58 20 f2 4d 18 ca fc e3 74 d4 e3 73 63 29 c1 52 ab 3a ea 9c 7c
0f 65 0c 30 70 b6 f5 1e 68 e2 ae bb 60 58 65 54 13 20 4c 3e bc 34 28 a6
cf 57 e2 4c 9d ef 59 65 17 70 44 9b ce 7e c6 56 1e 52 43 3a a5 5e 71 f1
fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f fc 79
5f 88 af c4 9c be 8a fd d1 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01 95 60
1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44 48 65 02 ff 7b dd a6
Info for K_3m is computed as follows:
info for K_3m =
[
10,
h'F24D18CAFCE374D4E3736329C152AB3AEA9C7C0F650C3070B6F51E68E2AEBB60',
"K_3m",
16
]
Which as a CBOR encoded data item is:
info for K_3m (CBOR-encoded) (42 bytes)
84 0a 58 20 f2 4d 18 ca fc e3 74 d4 e3 73 63 29 c1 52 ab 3a ea 9c 7c 0f
65 0c 30 70 b6 f5 1e 68 e2 ae bb 60 64 4b 5f 33 6d 10
From these parameters, K_3m is computed. Key K_3m is the output of
HKDF-Expand(PRK_4x3m, info, L), where L is the length of K_2m, so 16
bytes.
K_3m (16 bytes)
83 a9 c3 88 02 91 2e 7f 8f 0d 2b 84 14 d1 e5 2c
Nonce IV_3m is the output of HKDF-Expand(PRK_4x3m, info, L), where L
= 13 bytes.
Info for IV_3m is defined as follows:
info for IV_3m =
[
10,
h'F24D18CAFCE374D4E3736329C152AB3AEA9C7C0F650C3070B6F51E68E2AEBB60',
"IV_3m",
13
]
Which as a CBOR encoded data item is:
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info for IV_3m (CBOR-encoded) (43 bytes)
84 0a 58 20 f2 4d 18 ca fc e3 74 d4 e3 73 63 29 c1 52 ab 3a ea 9c 7c 0f
65 0c 30 70 b6 f5 1e 68 e2 ae bb 60 65 49 56 5f 33 6d 0d
From these parameters, IV_3m is computed:
IV_3m (13 bytes)
9c 83 9c 0e e8 36 42 50 5a 8e 1c 9f b2
MAC_3 is the 'ciphertext' of the COSE_Encrypt0:
MAC_3 (CBOR unencoded) (8 bytes)
2f a1 e3 9e ae 7d 5f 8d
Since the method = 0, Signature_or_MAC_3 is a signature. The
algorithm with selected cipher suite 0 is Ed25519.
o The message M_3 to be signed = [ "Signature1", << ID_CRED_I >>,
<< TH_3, CRED_I >>, MAC_3 ], i.e. ID_CRED_I encoded as CBOR bstr,
the concatenation of the CBOR byte strings TH_3 and CRED_I wrapped
as a CBOR bstr, and MAC_3 encoded as a CBOR bstr.
o The signing key is the private authentication key of the
Initiator.
M_3 =
[
"Signature1",
h'A11822822E48705D5845F36FC6A6',
h'5820F24D18CAFCE374D4E3736329C152AB3AEA9C7C0F650C3070B6F51E68E2AEBB6
058655413204C3EBC3428A6CF57E24C9DEF59651770449BCE7EC6561E52433AA55E71
F1FA34B22A9CA4A1E12924EAE1D1766088098449CB848FFC795F88AFC49CBE8AFDD1B
A009F21675E8F6C77A4A2C30195601F6F0A0852978BD43D28207D44486502FF7BDD
A6',
h'2FA1E39EAE7D5F8D']
Which as a CBOR encoded data item is:
M_3 (175 bytes)
84 6a 53 69 67 6e 61 74 75 72 65 31 4e a1 18 22 82 2e 48 70 5d 58 45 f3
6f c6 a6 58 89 58 20 f2 4d 18 ca fc e3 74 d4 e3 73 63 29 c1 52 ab 3a ea
9c 7c 0f 65 0c 30 70 b6 f5 1e 68 e2 ae bb 60 58 65 54 13 20 4c 3e bc 34
28 a6 cf 57 e2 4c 9d ef 59 65 17 70 44 9b ce 7e c6 56 1e 52 43 3a a5 5e
71 f1 fa 34 b2 2a 9c a4 a1 e1 29 24 ea e1 d1 76 60 88 09 84 49 cb 84 8f
fc 79 5f 88 af c4 9c be 8a fd d1 ba 00 9f 21 67 5e 8f 6c 77 a4 a2 c3 01
95 60 1f 6f 0a 08 52 97 8b d4 3d 28 20 7d 44 48 65 02 ff 7b dd a6 48 2f
a1 e3 9e ae 7d 5f 8d
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From there, the 64 byte signature can be computed:
Signature_or_MAC_3 (CBOR unencoded) (64 bytes)
ab 9f 7b bd eb c4 eb f8 a3 d3 04 17 9b cc a3 9d 9c 8a 76 73 65 76 fb 3c
32 d2 fa c7 e2 59 34 e5 33 dc c7 02 2e 4d 68 61 c8 f5 fe cb e9 2d 17 4e
b2 be af 0a 59 a4 15 84 37 2f 43 2e 6b f4 7b 04
Finally, the outer COSE_Encrypt0 is computed.
The plaintext is the CBOR Sequence of the items ID_CRED_I and the
CBOR encoded Signature_or_MAC_3, in this order (EAD_3 is empty).
P_3ae (CBOR Sequence) (80 bytes)
a1 18 22 82 2e 48 70 5d 58 45 f3 6f c6 a6 58 40 ab 9f 7b bd eb c4 eb f8
a3 d3 04 17 9b cc a3 9d 9c 8a 76 73 65 76 fb 3c 32 d2 fa c7 e2 59 34 e5
33 dc c7 02 2e 4d 68 61 c8 f5 fe cb e9 2d 17 4e b2 be af 0a 59 a4 15 84
37 2f 43 2e 6b f4 7b 04
The Associated data A is the following: Associated data A = [
"Encrypt0", h'', TH_3 ]
A_3ae (CBOR-encoded) (45 bytes)
83 68 45 6e 63 72 79 70 74 30 40 58 20 f2 4d 18 ca fc e3 74 d4 e3 73 63
29 c1 52 ab 3a ea 9c 7c 0f 65 0c 30 70 b6 f5 1e 68 e2 ae bb 60
Key K_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).
info is defined as follows:
info for K_3ae =
[
10,
h'F24D18CAFCE374D4E3736329C152AB3AEA9C7C0F650C3070B6F51E68E2AEBB60',
"K_3ae",
16
]
Which as a CBOR encoded data item is:
info for K_3ae (CBOR-encoded) (43 bytes)
84 0a 58 20 f2 4d 18 ca fc e3 74 d4 e3 73 63 29 c1 52 ab 3a ea 9c 7c 0f
65 0c 30 70 b6 f5 1e 68 e2 ae bb 60 65 4b 5f 33 61 65 10
L is the length of K_3ae, so 16 bytes.
From these parameters, K_3ae is computed:
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K_3ae (16 bytes)
b8 79 9f e3 d1 50 4f d8 eb 22 c4 72 62 cd bb 05
Nonce IV_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).
info is defined as follows:
info for IV_3ae =
[
10,
h'F24D18CAFCE374D4E3736329C152AB3AEA9C7C0F650C3070B6F51E68E2AEBB60',
"IV_3ae",
13
]
Which as a CBOR encoded data item is:
info for IV_3ae (CBOR-encoded) (44 bytes)
84 0a 58 20 f2 4d 18 ca fc e3 74 d4 e3 73 63 29 c1 52 ab 3a ea 9c 7c 0f
65 0c 30 70 b6 f5 1e 68 e2 ae bb 60 66 49 56 5f 33 61 65 0d
L is the length of IV_3ae, so 13 bytes.
From these parameters, IV_3ae is computed:
IV_3ae (13 bytes)
74 c7 de 41 b8 4a 5b b7 19 0a 85 98 dc
Using the parameters above, the 'ciphertext' CIPHERTEXT_3 can be
computed:
CIPHERTEXT_3 (CBOR unencoded) (88 bytes)
f5 f6 de bd 82 14 05 1c d5 83 c8 40 96 c4 80 1d eb f3 5b 15 36 3d d1 6e
bd 85 30 df dc fb 34 fc d2 eb 6c ad 1d ac 66 a4 79 fb 38 de aa f1 d3 0a
7e 68 17 a2 2a b0 4f 3d 5b 1e 97 2a 0d 13 ea 86 c6 6b 60 51 4c 96 57 ea
89 c5 7b 04 01 ed c5 aa 8b bc ab 81 3c c5 d6 e7
From the parameter above, message_3 is computed, as the CBOR Sequence
of the following CBOR encoded data items: (C_R, CIPHERTEXT_3).
message_3 =
(
-24,
h'F5F6DEBD8214051CD583C84096C4801DEBF35B15363DD16EBD8530DFDCFB34FCD2EB
6CAD1DAC66A479FB38DEAAF1D30A7E6817A22AB04F3D5B1E972A0D13EA86C66B60514C
9657EA89C57B0401EDC5AA8BBCAB813CC5D6E7'
)
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Which encodes to the following byte string:
message_3 (CBOR Sequence) (91 bytes)
37 58 58 f5 f6 de bd 82 14 05 1c d5 83 c8 40 96 c4 80 1d eb f3 5b 15 36
3d d1 6e bd 85 30 df dc fb 34 fc d2 eb 6c ad 1d ac 66 a4 79 fb 38 de aa
f1 d3 0a 7e 68 17 a2 2a b0 4f 3d 5b 1e 97 2a 0d 13 ea 86 c6 6b 60 51 4c
96 57 ea 89 c5 7b 04 01 ed c5 aa 8b bc ab 81 3c c5 d6 e7
D.1.4. OSCORE Security Context Derivation
From here, the Initiator and the Responder can derive an OSCORE
Security Context, using the EDHOC-Exporter interface.
From TH_3 and CIPHERTEXT_3, compute the input to the transcript hash
TH_4 = H( TH_3, CIPHERTEXT_3 ), as a CBOR Sequence of these 2 data
items.
Input to calculate TH_4 (CBOR Sequence) (124 bytes)
58 20 f2 4d 18 ca fc e3 74 d4 e3 73 63 29 c1 52 ab 3a ea 9c 7c 0f 65 0c
30 70 b6 f5 1e 68 e2 ae bb 60 58 58 f5 f6 de bd 82 14 05 1c d5 83 c8 40
96 c4 80 1d eb f3 5b 15 36 3d d1 6e bd 85 30 df dc fb 34 fc d2 eb 6c ad
1d ac 66 a4 79 fb 38 de aa f1 d3 0a 7e 68 17 a2 2a b0 4f 3d 5b 1e 97 2a
0d 13 ea 86 c6 6b 60 51 4c 96 57 ea 89 c5 7b 04 01 ed c5 aa 8b bc ab 81
3c c5 d6 e7
And from there, compute the transcript hash TH_4 = SHA-256(TH_3 ,
CIPHERTEXT_4)
TH_4 (CBOR unencoded) (32 bytes)
3b 69 a6 7f ec 7e 73 6c c1 a9 52 6c da 00 02 d4 09 f5 b9 ea 0a 2b e9 60
51 a6 e3 0d 93 05 fd 51
The Master Secret and Master Salt are derived as follows:
Master Secret = EDHOC-Exporter( "OSCORE Master Secret", 16 ) = EDHOC-
KDF(PRK_4x3m, TH_4, "OSCORE Master Secret", 16) = HKDF-Expand(
PRK_4x3m, info_ms, 16 )
Master Salt = EDHOC-Exporter( "OSCORE Master Salt", 8 ) = EDHOC-
KDF(PRK_4x3m, TH_4, "OSCORE Master Salt", 8) = HKDF-Expand( PRK_4x3m,
info_salt, 8 )
info_ms for OSCORE Master Secret is defined as follows:
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info_ms = [
10,
h'3B69A67FEC7E736CC1A9526CDA0002D409F5B9EA0A2BE96051A6E30D9305FD51',
"OSCORE Master Secret",
16
]
Which as a CBOR encoded data item is:
info_ms for OSCORE Master Secret (CBOR-encoded) (58 bytes)
84 0a 58 20 3b 69 a6 7f ec 7e 73 6c c1 a9 52 6c da 00 02 d4 09 f5 b9 ea
0a 2b e9 60 51 a6 e3 0d 93 05 fd 51 74 4f 53 43 4f 52 45 20 4d 61 73 74
65 72 20 53 65 63 72 65 74 10
info_salt for OSCORE Master Salt is defined as follows:
info_salt = [
10,
h'3B69A67FEC7E736CC1A9526CDA0002D409F5B9EA0A2BE96051A6E30D9305FD51',
"OSCORE Master Salt",
8
]
Which as a CBOR encoded data item is:
info for OSCORE Master Salt (CBOR-encoded) (56 Bytes)
84 0a 58 20 3b 69 a6 7f ec 7e 73 6c c1 a9 52 6c da 00 02 d4 09 f5 b9 ea
0a 2b e9 60 51 a6 e3 0d 93 05 fd 51 72 4f 53 43 4f 52 45 20 4d 61 73 74
65 72 20 53 61 6c 74 08
From these parameters, OSCORE Master Secret and OSCORE Master Salt
are computed:
OSCORE Master Secret (16 bytes)
96 aa 88 ce 86 5e ba 1f fa f3 89 64 13 2c c4 42
OSCORE Master Salt (8 bytes)
5e c3 ee 41 7c fb ba e9
The client's OSCORE Sender ID is C_R and the server's OSCORE Sender
ID is C_I.
Client's OSCORE Sender ID (1 byte)
00
Server's OSCORE Sender ID (1 byte)
09
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The AEAD Algorithm and the hash algorithm are the application AEAD
and hash algorithms in the selected cipher suite.
OSCORE AEAD Algorithm (int)
10
OSCORE Hash Algorithm (int)
-16
D.2. Test Vectors for EDHOC Authenticated with Static Diffie-Hellman
Keys
EDHOC with static Diffie-Hellman keys and raw public keys is used.
In this test vector, a key identifier is used to identify the raw
public key. The optional C_1 in message_1 is omitted. No external
authorization data is sent in the message exchange.
method (Static DH Based Authentication)
3
CoAP is used as transport and the Initiator acts as CoAP client:
corr (the Initiator can correlate message_1 and message_2)
1
From there, METHOD_CORR has the following value:
METHOD_CORR (4 * method + corr) (int)
13
The Initiator indicates only one cipher suite in the (potentially
truncated) list of cipher suites.
Supported Cipher Suites (1 byte)
00
The Initiator selected the indicated cipher suite.
Selected Cipher Suite (int)
0
Cipher suite 0 is supported by both the Initiator and the Responder,
see Section 3.6.
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D.2.1. Message_1
The Initiator generates its ephemeral key pair.
X (Initiator's ephemeral private key) (32 bytes)
ae 11 a0 db 86 3c 02 27 e5 39 92 fe b8 f5 92 4c 50 d0 a7 ba 6e ea b4 ad
1f f2 45 72 f4 f5 7c fa
G_X (Initiator's ephemeral public key, CBOR unencoded) (32 bytes)
8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80 a5 38 a4 44
ee 9e 2b 57 e2 44 1a 7c
The Initiator chooses a connection identifier C_I:
Connection identifier chosen by Initiator (1 byte)
16
Note that since C_I is a byte string in the interval h'00' to h'2f',
it is encoded as the corresponding integer - 24, i.e. 0x16 = 22, 22 -
24 = -2, and -2 in CBOR encoding is equal to 0x21.
C_I (1 byte)
21
Since no external authorization data is sent:
EAD_1 (0 bytes)
Since the list of supported cipher suites needs to contain the
selected cipher suite, the initiator truncates the list of supported
cipher suites to one cipher suite only, 00.
Because one single selected cipher suite is conveyed, it is encoded
as an int instead of an array:
SUITES_I (int)
0
message_1 is constructed as the CBOR Sequence of the data items above
encoded as CBOR. In CBOR diagnostic notation:
message_1 =
(
13,
0,
h'8D3EF56D1B750A4351D68AC250A0E883790EFC80A538A444EE9E2B57E2441A7C',
-2
)
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Which as a CBOR encoded data item is:
message_1 (CBOR Sequence) (37 bytes)
0d 00 58 20 8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80
a5 38 a4 44 ee 9e 2b 57 e2 44 1a 7c 21
D.2.2. Message_2
Since METHOD_CORR mod 4 equals 1, C_I is omitted from data_2.
The Responder generates the following ephemeral key pair.
Y (Responder's ephemeral private key) (32 bytes)
c6 46 cd dc 58 12 6e 18 10 5f 01 ce 35 05 6e 5e bc 35 f4 d4 cc 51 07 49
a3 a5 e0 69 c1 16 16 9a
G_Y (Responder's ephemeral public key, CBOR unencoded) (32 bytes)
52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db fc 33
01 04 70 69 45 1b af 35
From G_X and Y or from G_Y and X the ECDH shared secret is computed:
G_XY (ECDH shared secret) (32 bytes)
de fc 2f 35 69 10 9b 3d 1f a4 a7 3d c5 e2 fe b9 e1 15 0d 90 c2 5e e2 f0
66 c2 d8 85 f4 f8 ac 4e
The key and nonce for calculating the 'ciphertext' are calculated as
follows, as specified in Section 4.
HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).
PRK_2e = HMAC-SHA-256(salt, G_XY)
Salt is the empty byte string.
salt (0 bytes)
From there, PRK_2e is computed:
PRK_2e (32 bytes)
93 9f cb 05 6d 2e 41 4f 1b ec 61 04 61 99 c2 c7 63 d2 7f 0c 3d 15 fa 16
71 fa 13 4e 0d c5 a0 4d
The Responder's static Diffie-Hellman key pair is the following:
R (Responder's private authentication key) (32 bytes)
bb 50 1a ac 67 b9 a9 5f 97 e0 ed ed 6b 82 a6 62 93 4f bb fc 7a d1 b7 4c
1f ca d6 6a 07 94 22 d0
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G_R (Responder's public authentication key) (32 bytes)
a3 ff 26 35 95 be b3 77 d1 a0 ce 1d 04 da d2 d4 09 66 ac 6b cb 62 20 51
b8 46 59 18 4d 5d 9a 32
Since the Responder authenticates with a static Diffie-Hellman key,
PRK_3e2m = HKDF-Extract( PRK_2e, G_RX ), where G_RX is the ECDH
shared secret calculated from G_R and X, or G_X and R.
From the Responder's authentication key and the Initiator's ephemeral
key (see Appendix D.2.1), the ECDH shared secret G_RX is calculated.
G_RX (ECDH shared secret) (32 bytes)
21 c7 ef f4 fb 69 fa 4b 67 97 d0 58 84 31 5d 84 11 a3 fd a5 4f 6d ad a6
1d 4f cd 85 e7 90 66 68
PRK_3e2m (32 bytes)
75 07 7c 69 1e 35 01 2d 48 bc 24 c8 4f 2b ab 89 f5 2f ac 03 fe dd 81 3e
43 8c 93 b1 0b 39 93 07
The Responder chooses a connection identifier C_R.
Connection identifier chosen by Responder (1 byte)
00
Note that since C_R is a byte string in the interval h'00' to h'2f',
it is encoded as the corresponding integer - 24, i.e. 0x00 = 0, 0 -
24 = -24, and -24 in CBOR encoding is equal to 0x37.
C_R (1 byte)
37
Data_2 is constructed as the CBOR Sequence of G_Y and C_R.
data_2 =
(
h'52FBA0BDC8D953DD86CE1AB2FD7C05A4658C7C30AFDBFC3301047069451BAF35',
-24
)
Which as a CBOR encoded data item is:
data_2 (CBOR Sequence) (35 bytes)
58 20 52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db
fc 33 01 04 70 69 45 1b af 35 37
From data_2 and message_1, compute the input to the transcript hash
TH_2 = H( H(message_1), data_2 ), as a CBOR Sequence of these 2 data
items.
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Input to calculate TH_2 (CBOR Sequence) (72 bytes)
0d 00 58 20 8d 3e f5 6d 1b 75 0a 43 51 d6 8a c2 50 a0 e8 83 79 0e fc 80
a5 38 a4 44 ee 9e 2b 57 e2 44 1a 7c 21 58 20 52 fb a0 bd c8 d9 53 dd 86
ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db fc 33 01 04 70 69 45 1b af 35 37
And from there, compute the transcript hash TH_2 = SHA-256(
H(message_1), data_2 )
TH_2 (CBOR unencoded) (32 bytes)
de cf d6 4a 36 67 64 0a 02 33 b0 4a a8 aa 91 f6 89 56 b8 a5 36 d0 cf 8c
73 a6 e8 a7 c3 62 1e 26
The Responder's subject name is the empty string:
Responder's subject name (text string)
""
ID_CRED_R is the following:
ID_CRED_R =
{
4: h'05'
}
ID_CRED_R (4 bytes)
a1 04 41 05
CRED_R is the following COSE_Key:
{
1: 1,
-1: 4,
-2: h'A3FF263595BEB377D1A0CE1D04DAD2D40966AC6BCB622051B84659184D5D9A32,
"subject name": ""
}
Which encodes to the following byte string:
CRED_R (54 bytes)
a4 01 01 20 04 21 58 20 a3 ff 26 35 95 be b3 77 d1 a0 ce 1d 04 da d2 d4
09 66 ac 6b cb 62 20 51 b8 46 59 18 4d 5d 9a 32 6c 73 75 62 6a 65 63 74
20 6e 61 6d 65 60
Since no external authorization data is sent:
EAD_2 (0 bytes)
The plaintext is defined as the empty string:
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P_2m (0 bytes)
The Enc_structure is defined as follows: [ "Encrypt0",
<< ID_CRED_R >>, << TH_2, CRED_R >> ], so ID_CRED_R is encoded as a
CBOR bstr, and the concatenation of the CBOR byte strings TH_2 and
CRED_R is wrapped in a CBOR bstr.
A_2m =
[
"Encrypt0",
h'A1044105',
h'5820DECFD64A3667640A0233B04AA8AA91F68956B8A536D0CF8C73A6E8A7C3621E2
6A401012004215820A3FF263595BEB377D1A0CE1D04DAD2D40966AC6BCB622051B846
59184D5D9A326C7375626A656374206E616D6560'
]
Which encodes to the following byte string to be used as Additional
Authenticated Data:
A_2m (CBOR-encoded) (105 bytes)
83 68 45 6e 63 72 79 70 74 30 44 a1 04 41 05 58 58 58 20 de cf d6 4a 36
67 64 0a 02 33 b0 4a a8 aa 91 f6 89 56 b8 a5 36 d0 cf 8c 73 a6 e8 a7 c3
62 1e 26 a4 01 01 20 04 21 58 20 a3 ff 26 35 95 be b3 77 d1 a0 ce 1d 04
da d2 d4 09 66 ac 6b cb 62 20 51 b8 46 59 18 4d 5d 9a 32 6c 73 75 62 6a
65 63 74 20 6e 61 6d 65 60
info for K_2m is defined as follows:
info for K_2m =
[
10,
h'DECFD64A3667640A0233B04AA8AA91F68956B8A536D0CF8C73A6E8A7C3621E26',
"K_2m",
16
]
Which as a CBOR encoded data item is:
info for K_2m (CBOR-encoded) (42 bytes)
84 0a 58 20 de cf d6 4a 36 67 64 0a 02 33 b0 4a a8 aa 91 f6 89 56 b8 a5
36 d0 cf 8c 73 a6 e8 a7 c3 62 1e 26 64 4b 5f 32 6d 10
From these parameters, K_2m is computed. Key K_2m is the output of
HKDF-Expand(PRK_3e2m, info, L), where L is the length of K_2m, so 16
bytes.
K_2m (16 bytes)
4e cd ef ba d8 06 81 8b 62 51 b9 d7 86 78 bc 76
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info for IV_2m is defined as follows:
info for IV_2m =
[
10,
h'A51C76463E8AE58FD3B8DC5EDE1E27143CC92D223EACD9E5FF6E3FAC876658A5',
"IV_2m",
13
]
Which as a CBOR encoded data item is:
info for IV_2m (CBOR-encoded) (43 bytes)
84 0a 58 20 de cf d6 4a 36 67 64 0a 02 33 b0 4a a8 aa 91 f6 89 56 b8 a5
36 d0 cf 8c 73 a6 e8 a7 c3 62 1e 26 65 49 56 5f 32 6d 0d
From these parameters, IV_2m is computed. IV_2m is the output of
HKDF-Expand(PRK_3e2m, info, L), where L is the length of IV_2m, so 13
bytes.
IV_2m (13 bytes)
e9 b8 e4 b1 bd 02 f4 9a 82 0d d3 53 4f
Finally, COSE_Encrypt0 is computed from the parameters above.
o protected header = CBOR-encoded ID_CRED_R
o external_aad = A_2m
o empty plaintext = P_2m
MAC_2 is the 'ciphertext' of the COSE_Encrypt0 with empty plaintext.
In case of cipher suite 0 the AEAD is AES-CCM truncated to 8 bytes:
MAC_2 (CBOR unencoded) (8 bytes)
42 e7 99 78 43 1e 6b 8f
Since method = 2, Signature_or_MAC_2 is MAC_2:
Signature_or_MAC_2 (CBOR unencoded) (8 bytes)
42 e7 99 78 43 1e 6b 8f
CIPHERTEXT_2 is the ciphertext resulting from XOR between plaintext
and KEYSTREAM_2 which is derived from TH_2 and the pseudorandom key
PRK_2e.
The plaintext is the CBOR Sequence of the items ID_CRED_R and the
CBOR encoded Signature_or_MAC_2, in this order (EAD_2 is empty).
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Note that since ID_CRED_R contains a single 'kid' parameter, i.e.,
ID_CRED_R = { 4 : kid_R }, only the byte string kid_R is conveyed in
the plaintext encoded as a bstr_identifier. kid_R is encoded as the
corresponding integer - 24, i.e. 0x05 = 5, 5 - 24 = -19, and -19 in
CBOR encoding is equal to 0x32.
The plaintext is the following:
P_2e (CBOR Sequence) (10 bytes)
32 48 42 e7 99 78 43 1e 6b 8f
KEYSTREAM_2 = HKDF-Expand( PRK_2e, info, length ), where length is
the length of the plaintext, so 10.
info for KEYSTREAM_2 =
[
10,
h'DECFD64A3667640A0233B04AA8AA91F68956B8A536D0CF8C73A6E8A7C3621E26',
"KEYSTREAM_2",
10
]
Which as a CBOR encoded data item is:
info for KEYSTREAM_2 (CBOR-encoded) (49 bytes)
84 0a 58 20 de cf d6 4a 36 67 64 0a 02 33 b0 4a a8 aa 91 f6 89 56 b8 a5
36 d0 cf 8c 73 a6 e8 a7 c3 62 1e 26 6b 4b 45 59 53 54 52 45 41 4d 5f 32
0a
From there, KEYSTREAM_2 is computed:
KEYSTREAM_2 (10 bytes)
91 b9 ff ba 9b f5 5a d1 57 16
Using the parameters above, the ciphertext CIPHERTEXT_2 can be
computed:
CIPHERTEXT_2 (CBOR unencoded) (10 bytes)
a3 f1 bd 5d 02 8d 19 cf 3c 99
message_2 is the CBOR Sequence of data_2 and CIPHERTEXT_2, in this
order:
message_2 =
(
data_2,
h'A3F1BD5D028D19CF3C99'
)
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Which as a CBOR encoded data item is:
message_2 (CBOR Sequence) (46 bytes)
58 20 52 fb a0 bd c8 d9 53 dd 86 ce 1a b2 fd 7c 05 a4 65 8c 7c 30 af db
fc 33 01 04 70 69 45 1b af 35 37 4a a3 f1 bd 5d 02 8d 19 cf 3c 99
D.2.3. Message_3
Since corr equals 1, C_R is not omitted from data_3.
The Initiator's static Diffie-Hellman key pair is the following:
I (Initiator's private authentication key) (32 bytes)
2b be a6 55 c2 33 71 c3 29 cf bd 3b 1f 02 c6 c0 62 03 38 37 b8 b5 90 99
a4 43 6f 66 60 81 b0 8e
G_I (Initiator's public authentication key, CBOR unencoded) (32 bytes)
2c 44 0c c1 21 f8 d7 f2 4c 3b 0e 41 ae da fe 9c aa 4f 4e 7a bb 83 5e c3
0f 1d e8 8a db 96 ff 71
HKDF SHA-256 is the HKDF used (as defined by cipher suite 0).
From the Initiator's authentication key and the Responder's ephemeral
key (see Appendix D.2.2), the ECDH shared secret G_IY is calculated.
G_IY (ECDH shared secret) (32 bytes)
cb ff 8c d3 4a 81 df ec 4c b6 5d 9a 57 2e bd 09 64 45 0c 78 56 3d a4 98
1d 80 d3 6c 8b 1a 75 2a
PRK_4x3m = HMAC-SHA-256 (PRK_3e2m, G_IY).
PRK_4x3m (32 bytes)
02 56 2f 1f 01 78 5c 0a a5 f5 94 64 0c 49 cb f6 9f 72 2e 9e 6c 57 83 7d
8e 15 79 ec 45 fe 64 7a
data 3 is equal to C_R.
data_3 (CBOR Sequence) (1 byte)
37
From data_3, CIPHERTEXT_2, and TH_2, compute the input to the
transcript hash TH_3 = H( H(TH_2 , CIPHERTEXT_2), data_3), as a CBOR
Sequence of these 2 data items.
Input to calculate TH_3 (CBOR Sequence) (46 bytes)
58 20 de cf d6 4a 36 67 64 0a 02 33 b0 4a a8 aa 91 f6 89 56 b8 a5 36 d0
cf 8c 73 a6 e8 a7 c3 62 1e 26 4a a3 f1 bd 5d 02 8d 19 cf 3c 99 37
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And from there, compute the transcript hash TH_3 = SHA-256( H(TH_2 ,
CIPHERTEXT_2), data_3)
TH_3 (CBOR unencoded) (32 bytes)
b6 cd 80 4f c4 b9 d7 ca c5 02 ab d7 7c da 74 e4 1c b0 11 82 d7 cb 8b 84
db 03 ff a5 83 a3 5f cb
The initiator's subject name is the empty string:
Initiator's subject name (text string)
""
And its credential is:
ID_CRED_I =
{
4: h'23'
}
ID_CRED_I (4 bytes)
a1 04 41 23
CRED_I is the following COSE_Key:
{
1:1,
-1:4,
-2:h'2C440CC121F8D7F24C3B0E41AEDAFE9CAA4F4E7ABB835EC30F1DE88ADB96FF71',
"subject name":""
}
Which encodes to the following byte string:
CRED_I (54 bytes)
a4 01 01 20 04 21 58 20 2c 44 0c c1 21 f8 d7 f2 4c 3b 0e 41 ae da fe 9c
aa 4f 4e 7a bb 83 5e c3 0f 1d e8 8a db 96 ff 71 6c 73 75 62 6a 65 63 74
20 6e 61 6d 65 60
Since no external authorization data is exchanged:
EAD_3 (0 bytes)
The plaintext of the COSE_Encrypt is the empty string:
P_3m (0 bytes)
The associated data is the following: [ "Encrypt0", << ID_CRED_I >>,
<< TH_3, CRED_I, ? EAD_3 >> ].
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A_3m (CBOR-encoded) (105 bytes)
83 68 45 6e 63 72 79 70 74 30 44 a1 04 41 23 58 58 58 20 b6 cd 80 4f c4
b9 d7 ca c5 02 ab d7 7c da 74 e4 1c b0 11 82 d7 cb 8b 84 db 03 ff a5 83
a3 5f cb a4 01 01 20 04 21 58 20 2c 44 0c c1 21 f8 d7 f2 4c 3b 0e 41 ae
da fe 9c aa 4f 4e 7a bb 83 5e c3 0f 1d e8 8a db 96 ff 71 6c 73 75 62 6a
65 63 74 20 6e 61 6d 65 60
Info for K_3m is computed as follows:
info for K_3m =
[
10,
h'B6CD804FC4B9D7CAC502ABD77CDA74E41CB01182D7CB8B84DB03FFA583A35FCB',
"K_3m",
16
]
Which as a CBOR encoded data item is:
info for K_3m (CBOR-encoded) (42 bytes)
84 0a 58 20 b6 cd 80 4f c4 b9 d7 ca c5 02 ab d7 7c da 74 e4 1c b0 11 82
d7 cb 8b 84 db 03 ff a5 83 a3 5f cb 64 4b 5f 33 6d 10
From these parameters, K_3m is computed. Key K_3m is the output of
HKDF-Expand(PRK_4x3m, info, L), where L is the length of K_2m, so 16
bytes.
K_3m (16 bytes)
02 c7 e7 93 89 9d 90 d1 28 28 10 26 96 94 c9 58
Nonce IV_3m is the output of HKDF-Expand(PRK_4x3m, info, L), where L
= 13 bytes.
Info for IV_3m is defined as follows:
info for IV_3m =
[
10,
h'B6CD804FC4B9D7CAC502ABD77CDA74E41CB01182D7CB8B84DB03FFA583A35FCB',
"IV_3m",
13
]
Which as a CBOR encoded data item is:
info for IV_3m (CBOR-encoded) (43 bytes)
84 0a 58 20 b6 cd 80 4f c4 b9 d7 ca c5 02 ab d7 7c da 74 e4 1c b0 11 82
d7 cb 8b 84 db 03 ff a5 83 a3 5f cb 65 49 56 5f 33 6d 0d
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From these parameters, IV_3m is computed:
IV_3m (13 bytes)
0d a7 cc 3a 6f 9a b2 48 52 ce 8b 37 a6
MAC_3 is the 'ciphertext' of the COSE_Encrypt0 with empty plaintext.
In case of cipher suite 0 the AEAD is AES-CCM truncated to 8 bytes:
MAC_3 (CBOR unencoded) (8 bytes)
ee 59 8e a6 61 17 dc c3
Since method = 3, Signature_or_MAC_3 is MAC_3:
Signature_or_MAC_3 (CBOR unencoded) (8 bytes)
ee 59 8e a6 61 17 dc c3
Finally, the outer COSE_Encrypt0 is computed.
The plaintext is the CBOR Sequence of the items ID_CRED_I and the
CBOR encoded Signature_or_MAC_3, in this order (EAD_3 is empty).
Note that since ID_CRED_I contains a single 'kid' parameter, i.e.,
ID_CRED_I = { 4 : kid_I }, only the byte string kid_I is conveyed in
the plaintext encoded as a bstr_identifier. kid_I is encoded as the
corresponding integer - 24, i.e. 0x23 = 35, 35 - 24 = 11, and 11 in
CBOR encoding is equal to 0x0b.
P_3ae (CBOR Sequence) (10 bytes)
0b 48 ee 59 8e a6 61 17 dc c3
The Associated data A is the following: Associated data A = [
"Encrypt0", h'', TH_3 ]
A_3ae (CBOR-encoded) (45 bytes)
83 68 45 6e 63 72 79 70 74 30 40 58 20 b6 cd 80 4f c4 b9 d7 ca c5 02 ab
d7 7c da 74 e4 1c b0 11 82 d7 cb 8b 84 db 03 ff a5 83 a3 5f cb
Key K_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).
info is defined as follows:
info for K_3ae =
[
10,
h'B6CD804FC4B9D7CAC502ABD77CDA74E41CB01182D7CB8B84DB03FFA583A35FCB',
"K_3ae",
16
]
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Which as a CBOR encoded data item is:
info for K_3ae (CBOR-encoded) (43 bytes)
84 0a 58 20 b6 cd 80 4f c4 b9 d7 ca c5 02 ab d7 7c da 74 e4 1c b0 11 82
d7 cb 8b 84 db 03 ff a5 83 a3 5f cb 65 4b 5f 33 61 65 10
L is the length of K_3ae, so 16 bytes.
From these parameters, K_3ae is computed:
K_3ae (16 bytes)
6b a4 c8 83 1d e3 ae 23 e9 8e f7 35 08 d0 95 86
Nonce IV_3ae is the output of HKDF-Expand(PRK_3e2m, info, L).
info is defined as follows:
info for IV_3ae =
[
10,
h'97D8AD42334833EB25B960A5EB0704505F89671A0168AA1115FAF92D9E67EF04',
"IV_3ae",
13
]
Which as a CBOR encoded data item is:
info for IV_3ae (CBOR-encoded) (44 bytes)
84 0a 58 20 b6 cd 80 4f c4 b9 d7 ca c5 02 ab d7 7c da 74 e4 1c b0 11 82
d7 cb 8b 84 db 03 ff a5 83 a3 5f cb 66 49 56 5f 33 61 65 0d
L is the length of IV_3ae, so 13 bytes.
From these parameters, IV_3ae is computed:
IV_3ae (13 bytes)
6c 6d 0f e1 1e 9a 1a f3 7b 87 84 55 10
Using the parameters above, the 'ciphertext' CIPHERTEXT_3 can be
computed:
CIPHERTEXT_3 (CBOR unencoded) (18 bytes)
d5 53 5f 31 47 e8 5f 1c fa cd 9e 78 ab f9 e0 a8 1b bf
From the parameter above, message_3 is computed, as the CBOR Sequence
of the following items: (C_R, CIPHERTEXT_3).
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message_3 =
(
-24,
h'D5535F3147E85F1CFACD9E78ABF9E0A81BBF'
)
Which encodes to the following byte string:
message_3 (CBOR Sequence) (20 bytes)
37 52 d5 53 5f 31 47 e8 5f 1c fa cd 9e 78 ab f9 e0 a8 1b bf
D.2.4. OSCORE Security Context Derivation
From here, the Initiator and the Responder can derive an OSCORE
Security Context, using the EDHOC-Exporter interface.
From TH_3 and CIPHERTEXT_3, compute the input to the transcript hash
TH_4 = H( TH_3, CIPHERTEXT_3 ), as a CBOR Sequence of these 2 data
items.
Input to calculate TH_4 (CBOR Sequence) (53 bytes)
58 20 b6 cd 80 4f c4 b9 d7 ca c5 02 ab d7 7c da 74 e4 1c b0 11 82 d7 cb
8b 84 db 03 ff a5 83 a3 5f cb 52 d5 53 5f 31 47 e8 5f 1c fa cd 9e 78 ab
f9 e0 a8 1b bf
And from there, compute the transcript hash TH_4 = SHA-256(TH_3 ,
CIPHERTEXT_4)
TH_4 (CBOR unencoded) (32 bytes)
7c cf de dc 2c 10 ca 03 56 e9 57 b9 f6 a5 92 e0 fa 74 db 2a b5 4f 59 24
40 96 f9 a2 ac 56 d2 07
The Master Secret and Master Salt are derived as follows:
Master Secret = EDHOC-Exporter( "OSCORE Master Secret", 16 ) = EDHOC-
KDF(PRK_4x3m, TH_4, "OSCORE Master Secret", 16) = HKDF-Expand(
PRK_4x3m, info_ms, 16 )
Master Salt = EDHOC-Exporter( "OSCORE Master Salt", 8 ) = EDHOC-
KDF(PRK_4x3m, TH_4, "OSCORE Master Salt", 8) = HKDF-Expand( PRK_4x3m,
info_salt, 8 )
info_ms for OSCORE Master Secret is defined as follows:
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info_ms = [
10,
h'7CCFDEDC2C10CA0356E957B9F6A592E0FA74DB2AB54F59244096F9A2AC56D207',
"OSCORE Master Secret",
16
]
Which as a CBOR encoded data item is:
info_ms for OSCORE Master Secret (CBOR-encoded) (58 bytes)
84 0a 58 20 7c cf de dc 2c 10 ca 03 56 e9 57 b9 f6 a5 92 e0 fa 74 db 2a
b5 4f 59 24 40 96 f9 a2 ac 56 d2 07 74 4f 53 43 4f 52 45 20 4d 61 73 74
65 72 20 53 65 63 72 65 74 10
info_salt for OSCORE Master Salt is defined as follows:
info_salt = [
10,
h'7CCFDEDC2C10CA0356E957B9F6A592E0FA74DB2AB54F59244096F9A2AC56D207',
"OSCORE Master Salt",
8
]
Which as a CBOR encoded data item is:
info for OSCORE Master Salt (CBOR-encoded) (56 Bytes)
84 0a 58 20 7c cf de dc 2c 10 ca 03 56 e9 57 b9 f6 a5 92 e0 fa 74 db 2a
b5 4f 59 24 40 96 f9 a2 ac 56 d2 07 72 4f 53 43 4f 52 45 20 4d 61 73 74
65 72 20 53 61 6c 74 08
From these parameters, OSCORE Master Secret and OSCORE Master Salt
are computed:
OSCORE Master Secret (16 bytes)
c3 4a 50 6d 0e bf bd 17 03 04 86 13 5f 9c b3 50
OSCORE Master Salt (8 bytes)
c2 24 34 9d 9b 34 ca 8c
The client's OSCORE Sender ID is C_R and the server's OSCORE Sender
ID is C_I.
Client's OSCORE Sender ID (1 byte)
00
Server's OSCORE Sender ID (1 byte)
16
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The AEAD Algorithm and the hash algorithm are the application AEAD
and hash algorithms in the selected cipher suite.
OSCORE AEAD Algorithm (int)
10
OSCORE Hash Algorithm (int)
-16
Appendix E. Applicability Template
This appendix contains an example of an applicability statement, see
Section 3.9.
For use of EDHOC in the XX protocol, the following assumptions are
made on the parameters:
o METHOD = 1 (I uses signature key, R uses static DH key.)
o EDHOC requests are expected by the server at /app1-edh, no
Content-Format needed.
o CRED_I is an 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 byte string
o CRED_R is a COSE_Key of type OKP as specified in Section 3.5.4.
* The CBOR map has parameters 1 (kty), -1 (crv), and -2
(x-coordinate).
o ID_CRED_R = CRED_R
o No use of message_4: the application sends protected messages from
R to I.
o External authorization data is defined and processed as specified
in [I-D.selander-ace-ake-authz].
Appendix F. EDHOC Message Deduplication
EDHOC by default assumes that message duplication is handled by the
transport, in this section exemplified with CoAP.
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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]).
Note that the requirements in Section 5.1 still apply because
duplicate messages are not processed by the EDHOC state machine:
o EDHOC messages SHALL be processed according to the current
protocol state.
o Different instances of the same message MUST NOT be processed in
one session.
Appendix G. 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
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server selected (or, for message 1, a sentinel null value 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 H. Change Log
Main changes:
o 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
* OSCORE and CoAP specific processing moved to new appendix
* Message 4 section moved to message processing section
o From -06 to -07:
* Changed transcript hash definition for TH_2 and TH_3
* Removed "EDHOC signature algorithm curve" from cipher suite
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* 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
o 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"
* 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"
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* Clarifications based on review comments
o From -04 to -05:
* EDHOC-Rekey-FS -> EDHOC-KeyUpdate
* Clarification of cipher suite negotiation
* Updated security considerations
* Updated test vectors
* Updated applicability statement template
o 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
o From -02 to -03:
* Rearrangements of section 3 and beginning of section 4
* 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)
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* Clarification of error message
* New appendix C applicability statement
o 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
o 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, 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.
Work on this document has in part been supported by the H2020 project
SIFIS-Home (grant agreement 952652).
Authors' Addresses
Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
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John Preuss Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com
Francesca Palombini
Ericsson AB
Email: francesca.palombini@ericsson.com
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