Network Working Group G. Selander
Internet-Draft J. Preuß Mattsson
Intended status: Standards Track F. Palombini
Expires: 19 November 2022 Ericsson
18 May 2022
Ephemeral Diffie-Hellman Over COSE (EDHOC)
draft-ietf-lake-edhoc-14
Abstract
This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
very compact and lightweight authenticated Diffie-Hellman key
exchange with ephemeral keys. EDHOC provides mutual authentication,
forward secrecy, and identity protection. EDHOC is intended for
usage in constrained scenarios and a main use case is to establish an
OSCORE security context. By reusing COSE for cryptography, CBOR for
encoding, and CoAP for transport, the additional code size can be
kept very low.
Status of This Memo
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This Internet-Draft will expire on 19 November 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Message Size Examples . . . . . . . . . . . . . . . . . . 5
1.3. Document Structure . . . . . . . . . . . . . . . . . . . 6
1.4. Terminology and Requirements Language . . . . . . . . . . 6
2. EDHOC Outline . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Protocol Elements . . . . . . . . . . . . . . . . . . . . . . 8
3.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Method . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Connection Identifiers . . . . . . . . . . . . . . . . . 10
3.4. Transport . . . . . . . . . . . . . . . . . . . . . . . . 12
3.5. Authentication Parameters . . . . . . . . . . . . . . . . 13
3.6. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 17
3.7. Ephemeral Public Keys . . . . . . . . . . . . . . . . . . 19
3.8. External Authorization Data (EAD) . . . . . . . . . . . . 19
3.9. Application Profile . . . . . . . . . . . . . . . . . . . 20
4. Key Derivation . . . . . . . . . . . . . . . . . . . . . . . 22
4.1. Keys for EDHOC Message Processing . . . . . . . . . . . . 22
4.2. Keys for EDHOC Applications . . . . . . . . . . . . . . . 25
5. Message Formatting and Processing . . . . . . . . . . . . . . 27
5.1. Message Processing Outline . . . . . . . . . . . . . . . 27
5.2. EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . . 28
5.3. EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . . 30
5.4. EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . . 32
5.5. EDHOC Message 4 . . . . . . . . . . . . . . . . . . . . . 36
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 37
6.1. Success . . . . . . . . . . . . . . . . . . . . . . . . . 39
6.2. Unspecified Error . . . . . . . . . . . . . . . . . . . . 39
6.3. Wrong Selected Cipher Suite . . . . . . . . . . . . . . . 39
7. Compliance Requirements . . . . . . . . . . . . . . . . . . . 42
8. Security Considerations . . . . . . . . . . . . . . . . . . . 43
8.1. Security Properties . . . . . . . . . . . . . . . . . . . 43
8.2. Cryptographic Considerations . . . . . . . . . . . . . . 45
8.3. Cipher Suites and Cryptographic Algorithms . . . . . . . 47
8.4. Post-Quantum Considerations . . . . . . . . . . . . . . . 47
8.5. Unprotected Data and Privacy . . . . . . . . . . . . . . 48
8.6. Updated Internet Threat Model Considerations . . . . . . 48
8.7. Denial-of-Service . . . . . . . . . . . . . . . . . . . . 49
8.8. Implementation Considerations . . . . . . . . . . . . . . 49
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51
9.1. EDHOC Exporter Label Registry . . . . . . . . . . . . . . 52
9.2. EDHOC Cipher Suites Registry . . . . . . . . . . . . . . 52
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9.3. EDHOC Method Type Registry . . . . . . . . . . . . . . . 53
9.4. EDHOC Error Codes Registry . . . . . . . . . . . . . . . 54
9.5. EDHOC External Authorization Data Registry . . . . . . . 54
9.6. COSE Header Parameters Registry . . . . . . . . . . . . . 54
9.7. The Well-Known URI Registry . . . . . . . . . . . . . . . 54
9.8. Media Types Registry . . . . . . . . . . . . . . . . . . 55
9.9. CoAP Content-Formats Registry . . . . . . . . . . . . . . 57
9.10. Resource Type (rt=) Link Target Attribute Values
Registry . . . . . . . . . . . . . . . . . . . . . . . . 57
9.11. Expert Review Instructions . . . . . . . . . . . . . . . 57
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 58
10.1. Normative References . . . . . . . . . . . . . . . . . . 58
10.2. Informative References . . . . . . . . . . . . . . . . . 61
Appendix A. Use with OSCORE and Transfer over CoAP . . . . . . . 65
A.1. Deriving the OSCORE Security Context . . . . . . . . . . 65
A.2. Transferring EDHOC over CoAP . . . . . . . . . . . . . . 66
Appendix B. Compact Representation . . . . . . . . . . . . . . . 70
Appendix C. Use of CBOR, CDDL and COSE in EDHOC . . . . . . . . 70
C.1. CBOR and CDDL . . . . . . . . . . . . . . . . . . . . . . 71
C.2. CDDL Definitions . . . . . . . . . . . . . . . . . . . . 72
C.3. COSE . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Appendix D. Authentication Related Verifications . . . . . . . . 75
D.1. Validating the Authentication Credential . . . . . . . . 76
D.2. Identities . . . . . . . . . . . . . . . . . . . . . . . 76
D.3. Certification Path and Trust Anchors . . . . . . . . . . 77
D.4. Revocation Status . . . . . . . . . . . . . . . . . . . . 78
D.5. Trust-on-first-use . . . . . . . . . . . . . . . . . . . 78
Appendix E. Use of External Authorization Data . . . . . . . . . 78
Appendix F. Application Profile Example . . . . . . . . . . . . 80
Appendix G. EDHOC Message Deduplication . . . . . . . . . . . . 80
Appendix H. Transports Not Natively Providing Correlation . . . 82
Appendix I. Change Log . . . . . . . . . . . . . . . . . . . . . 82
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 90
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 90
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
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Binary Object Representation (CBOR, [RFC8949]), and Static Context
Header Compression (SCHC, [RFC8724]).
The need for special protocols targeting constrained IoT deployments
extends also to the security domain [I-D.ietf-lake-reqs]. Important
characteristics in constrained environments are the number of round
trips and protocol message sizes, which if kept low can contribute to
good performance by enabling transport over a small number of radio
frames, reducing latency due to fragmentation or duty cycles, etc.
Another important criteria is code size, which may be prohibitive for
certain deployments due to device capabilities or network load during
firmware update. Some IoT deployments also need to support a variety
of underlying transport technologies, potentially even with a single
connection.
Some security solutions for such settings exist already. CBOR Object
Signing and Encryption (COSE, [I-D.ietf-cose-rfc8152bis-struct])
specifies basic application-layer security services efficiently
encoded in CBOR. Another example is Object Security for Constrained
RESTful Environments (OSCORE, [RFC8613]) which is a lightweight
communication security extension to CoAP using CBOR and COSE. In
order to establish good quality cryptographic keys for security
protocols such as COSE and OSCORE, the two endpoints may run an
authenticated Diffie-Hellman key exchange protocol, from which shared
secret keying material can be derived. Such a key exchange protocol
should also be lightweight; to prevent bad performance in case of
repeated use, e.g., due to device rebooting or frequent rekeying for
security reasons; or to avoid latencies in a network formation
setting with many devices authenticating at the same time.
This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
lightweight authenticated key exchange protocol providing good
security properties including forward secrecy, identity protection,
and cipher suite negotiation. Authentication can be based on raw
public keys (RPK) or public key certificates and requires the
application to provide input on how to verify that endpoints are
trusted. This specification emphasizes the possibility to reference
rather than to transport credentials in order to reduce message
overhead, but the latter is also supported. EDHOC does not currently
support pre-shared key (PSK) authentication as authentication with
static Diffie-Hellman public keys by reference produces equally small
message sizes but with much simpler key distribution and identity
protection.
EDHOC makes use of known protocol constructions, such as SIGMA
[SIGMA] and Extract-and-Expand [RFC5869]. EDHOC uses COSE for
cryptography and identification of credentials (including COSE_Key,
CBOR Web Token (CWT), CWT Claims Set (CCS), X.509, and CBOR encoded
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X.509 (C509) certificates, see Section 3.5.2). COSE provides crypto
agility and enables the use of future algorithms and credential types
targeting IoT.
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] involving 'things' with
embedded microcontrollers, sensors, and actuators. By reusing the
same lightweight primitives as OSCORE (CBOR, COSE, CoAP) 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.
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). 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.
1.2. Message Size Examples
Compared to the DTLS 1.3 handshake [RFC9147] with ECDHE and
connection ID, the EDHOC message size when transferred in CoAP can be
less than 1/6 when RPK authentication is used, see
[I-D.ietf-lwig-security-protocol-comparison]. Figure 1 shows
examples of EDHOC message sizes based on the assumptions in Section 2
of [I-D.ietf-lwig-security-protocol-comparison], comparing different
kinds of authentication keys and COSE header parameters for
identification: static Diffie-Hellman keys or signature keys, either
in CBOR Web Token (CWT) / CWT Claims Set (CCS) [RFC8392] identified
by a key identifier using 'kid' [I-D.ietf-cose-rfc8152bis-struct], or
in X.509 certificates identified by a hash value using 'x5t'
[I-D.ietf-cose-x509].
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========================================================
Static DH Keys Signature Keys
-------------- --------------
kid x5t kid x5t
--------------------------------------------------------
message_1 37 37 37 37
message_2 45 58 102 115
message_3 19 33 77 90
--------------------------------------------------------
Total 101 128 216 242
========================================================
Figure 1: Examples of EDHOC message sizes in bytes.
1.3. Document Structure
The remainder of the document is organized as follows: Section 2
outlines EDHOC authenticated with signature keys, Section 3 describes
the protocol elements of EDHOC, including formatting of the ephemeral
public keys, Section 4 specifies the key derivation, Section 5
specifies message processing for EDHOC authenticated with signature
keys or static Diffie-Hellman keys, Section 6 describes the error
messages, and Appendix A shows how to transfer EDHOC with CoAP and
establish an OSCORE security context.
1.4. Terminology and Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers are expected to be familiar with the terms and concepts
described in CBOR [RFC8949], CBOR Sequences [RFC8742], COSE
structures and processing [I-D.ietf-cose-rfc8152bis-struct], COSE
algorithms [I-D.ietf-cose-rfc8152bis-algs], CWT and CWT Claims Set
[RFC8392], and the Concise Data Definition Language (CDDL,
[RFC8610]), which is used to express CBOR data structures. Examples
of CBOR and CDDL are provided in Appendix C.1. When referring to
CBOR, this specification always refers to Deterministically Encoded
CBOR as specified in Sections 4.2.1 and 4.2.2 of [RFC8949]. The
single output from authenticated encryption (including the
authentication tag) is called "ciphertext", following [RFC5116].
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2. EDHOC Outline
EDHOC specifies different authentication methods of the ephemeral
Diffie-Hellman key exchange: signature keys and static Diffie-Hellman
keys. This section outlines the signature key 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][RFC9147], EDHOC authenticated with signature keys is
built on a variant of the SIGMA protocol which provides identity
protection of the initiator (SIGMA-I) against active attackers, and
like IKEv2, EDHOC implements the MAC-then-Sign variant of the SIGMA-I
protocol shown in Figure 2.
Initiator Responder
| G_X |
+------------------------------------------------------------------>|
| |
| G_Y, Enc( ID_CRED_R, Sig( R; MAC( CRED_R, G_X, G_Y ) ) ) |
|<------------------------------------------------------------------+
| |
| AEAD( ID_CRED_I, Sig( I; MAC( CRED_I, G_Y, G_X ) ) ) |
+------------------------------------------------------------------>|
| |
Figure 2: MAC-then-Sign variant of the SIGMA-I protocol used by
EDHOC.
The parties exchanging messages are called Initiator (I) and
Responder (R). They exchange ephemeral public keys, compute a shared
secret key PRK_out, and derive symmetric application keys used to
protect application data.
* G_X and G_Y are the ECDH ephemeral public keys of I and R,
respectively.
* CRED_I and CRED_R are the authentication credentials containing
the public authentication keys of I and R, respectively.
* ID_CRED_I and ID_CRED_R are used to identify and optionally
transport the credentials of the Initiator and the Responder,
respectively.
* Sig(I; . ) and Sig(R; . ) denote signatures made with the private
authentication key of I and R, respectively.
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* Enc(), AEAD(), and MAC() denotes encryption, authenticated
encryption with additional data, and message authentication code
using keys derived from the shared secret.
In order to create a "full-fledged" protocol some additional protocol
elements are needed. EDHOC adds:
* Transcript hashes (hashes of message data) TH_2, TH_3, TH_4 used
for key derivation and as additional authenticated data.
* Computationally independent keys derived from the ECDH shared
secret and used for authenticated encryption of different
messages.
* An optional fourth message giving key confirmation to I in
deployments where no protected application data is sent from R to
I.
* A keying material exporter and a key update function with forward
secrecy.
* Verification of the selected cipher suite.
* Method types and error handling.
* Selection of connection identifiers C_I and C_R which may be used
in EDHOC to identify protocol state.
* Transport of external authorization data.
EDHOC is designed to encrypt and integrity protect as much
information as possible, and all symmetric keys are derived using as
much previous information as possible. EDHOC is furthermore designed
to be as compact and lightweight as possible, in terms of message
sizes, processing, and the ability to reuse already existing CBOR,
COSE, and CoAP libraries.
To simplify for implementors, the use of CBOR and COSE in EDHOC is
summarized in Appendix C. Test vectors including CBOR diagnostic
notation are provided in [I-D.ietf-lake-traces].
3. Protocol Elements
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3.1. General
The EDHOC protocol consists of three mandatory messages (message_1,
message_2, message_3) between Initiator and Responder, an optional
fourth message (message_4), and an error message. All EDHOC messages
are CBOR Sequences [RFC8742], and are deterministically encoded.
Figure 3 illustrates an EDHOC message flow with the optional fourth
message as well as the content of each message. The protocol
elements in the figure are introduced in Section 3 and Section 5.
Message formatting and processing are specified in Section 5 and
Section 6.
Application data may be protected using the agreed application
algorithms (AEAD, hash) in the selected cipher suite (see
Section 3.6) and the application can make use of the established
connection identifiers C_I and C_R (see Section 3.3). EDHOC may be
used with the media type application/edhoc+cbor-seq defined in
Section 9.8.
The Initiator can derive symmetric application keys after creating
EDHOC message_3, see Section 4.2.1. Protected application data can
therefore be sent in parallel or together with EDHOC message_3.
EDHOC message_4 is typically not sent.
Initiator Responder
| METHOD, SUITES_I, G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| G_Y, Enc( ID_CRED_R, Signature_or_MAC_2, EAD_2 ), C_R |
|<------------------------------------------------------------------+
| message_2 |
| |
| AEAD( ID_CRED_I, Signature_or_MAC_3, EAD_3 ) |
+------------------------------------------------------------------>|
| message_3 |
| |
| AEAD( EAD_4 ) |
|<- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
| message_4 |
Figure 3: EDHOC message flow including the optional fourth message.
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3.2. Method
The data item METHOD in message_1 (see Section 5.2.1), is an integer
specifying the authentication method. EDHOC supports authentication
with signature or static Diffie-Hellman keys, as defined in the four
authentication methods: 0, 1, 2, and 3, see Figure 4. When using a
static Diffie-Hellman key the authentication is provided by a Message
Authentication Code (MAC) computed from an ephemeral-static ECDH
shared secret which enables significant reductions in message sizes.
The Initiator and the Responder need to have agreed on a single
method to be used for EDHOC, see Section 3.9.
+-------------+--------------------+--------------------+
| Method Type | Initiator | Responder |
| Value | Authentication Key | Authentication Key |
+-------------+--------------------+--------------------+
| 0 | Signature Key | Signature Key |
| 1 | Signature Key | Static DH Key |
| 2 | Static DH Key | Signature Key |
| 3 | Static DH Key | Static DH Key |
+-------------+--------------------+--------------------+
Figure 4: Authentication Keys for Method Types
EDHOC does not have a dedicated message field to indicate protocol
version. Breaking changes to EDHOC can be introduced by specifying
and registering new methods.
3.3. Connection Identifiers
EDHOC includes the selection of connection identifiers (C_I, C_R)
identifying a connection for which keys are agreed.
Connection identifiers may be used to correlate EDHOC messages and
facilitate the retrieval of protocol state during EDHOC execution
(see Section 3.4) or in subsequent applications of EDHOC, e.g., in
OSCORE (see Section 3.3.3). The connection identifiers do not have
any cryptographic purpose in EDHOC except facilitating the retrieval
of security data associated to the protocol state.
Connection identifiers in EDHOC are CBOR byte strings. Since most
constrained devices only have a few connections, short identifiers
are desirable in many cases. However, except for the empty byte
string h'', which encodes as one byte (0x40), all byte strings are
CBOR encoded as two or more bytes. Therefore EDHOC specifies certain
byte strings to be represented as CBOR ints on the wire, see
Section 3.3.2.
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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 it 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. Representation of Byte String Identifiers
To allow identifiers with minimal overhead on the wire, certain byte
strings are defined to have integer representations.
The integers with one-byte CBOR encoding are -24, ..., 23, see
Figure 5. This correspondence between integers and byte strings is a
natural mapping between the byte strings with CBOR diagnostic
notation h'00', h'01', ..., h'37' (except h'18', h'19', ..., h'1F')
and integers which are CBOR encoded as one byte.
Integer: -24 -23 ... -2 -1 0 1 ... 23
CBOR encoding (1 byte): 37 36 ... 21 20 00 01 ... 17
Figure 5: One-Byte CBOR Encoded Integers
The byte strings which coincide with a one-byte CBOR encoding of an
integer MUST be represented by the CBOR encoding of that integer.
Other byte strings are encoded as normal CBOR byte strings.
For example:
* h'21' is represented by 0x21 (CBOR encoding of the integer -2),
not by 0x4121.
* h'0D' is represented by 0x0D (CBOR encoding of the integer 13),
not by 0x410D.
* h'18' is represented by 0x4118.
* h'38' is represented by 0x4138.
* h'ABCD' is represented by 0x42ABCD.
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One way to view this representation of byte strings is as a transport
encoding: A byte string which parses as a CBOR int in the range -24,
..., 23 is just copied directly into the message, a byte string which
doesn't is encoded as a CBOR bstr during transport.
3.3.3. Use of Connection Identifiers with OSCORE
For OSCORE, the choice of 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 connection identifier is a byte string, it
is converted to an OSCORE Recipient ID equal to the byte string.
For example, a C_I equal to 0xFF is converted to a (typically client)
Responder ID equal to 0xFF; a C_R equal to 0x21 is converted to a
(typically server) Responder ID equal to 0x21. Note that the
representation of connection identifiers as CBOR byte strings or CBOR
ints in EDHOC messages as described in Section 3.3.2 has no impact on
this mapping.
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. In addition to transport of
messages including errors, the transport is responsible, where
necessary, to handle:
* message loss,
* message reordering,
* message duplication,
* fragmentation,
* demultiplex EDHOC messages from other types of messages,
* denial-of-service protection,
* message correlation.
The Initiator and the Responder need to have agreed on a transport to
be used for EDHOC, see Section 3.9.
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3.4.1. Use of Connection Identifiers for EDHOC Message Correlation
The transport needs to support the correlation between EDHOC messages
and facilitate the retrieval of protocol state and security context
during EDHOC protocol execution, including an indication of a message
being message_1. The correlation may reuse existing mechanisms in
the transport protocol. For example, the CoAP Token may be used to
correlate EDHOC messages in a CoAP response and an associated CoAP
request.
Connection identifiers may be used to correlate EDHOC messages and
facilitate the retrieval of protocol state/security context during
EDHOC protocol execution. Transports that do not inherently provide
correlation across all EDHOC messages of an exchange can send
connection identifiers along with EDHOC messages to gain that
required capability, e.g., by prepending the appropriate connection
identifier (when available from the EDHOC protocol) to the EDHOC
message. Transport of EDHOC in CoAP payloads is described in
Appendix A.2, which also shows how to use connection identifiers and
message_1 indication with CoAP.
3.5. Authentication Parameters
EDHOC supports various settings for how the other endpoint's
authentication (public) key may be transported, identified, and
trusted.
EDHOC performs the following authentication related operations:
* EDHOC transports information about credentials in ID_CRED_I and
ID_CRED_R (described in Section 3.5.3). Based on this
information, the authentication credentials CRED_I and CRED_R
(described in Section 3.5.2) can be obtained. EDHOC may also
transport certain authentication related information as External
Authorization Data (see Section 3.8).
* EDHOC uses the authentication credentials in two ways (see
Section 5.3.2 and Section 5.4.2):
- The authentication credential is input to the integrity
verification using the MAC fields.
- The authentication key of the authentication credential is used
with the Signature_or_MAC field to verify proof-of-possession
of the private key.
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Other authentication related verifications are out of scope for
EDHOC, and is the responsibility of the application. In particular,
the authentication credential needs to be validated in the context of
the connection for which EDHOC is used, see Appendix D. EDHOC MUST
allow the application to read received information about credential
(ID_CRED_R, ID_CRED_I). EDHOC MUST have access to the authentication
key and the authentication credential.
Note that the type of authentication key, authentication credential,
and the identification of the credential have a large impact on the
message size. For example, the signature_or_MAC field is much
smaller with a static DH key than with a signature key. A CCS is
much smaller than a self-signed certificate/CWT, but if it is
possible to reference the credential with a COSE header like 'kid',
then that is in turn much smaller than a CCS.
3.5.1. Authentication Keys
The authentication key (i.e. the public key used for authentication)
MUST be a signature key or static Diffie-Hellman key. The Initiator
and the Responder MAY use different types of authentication keys,
e.g., one uses a signature key and the other uses a static Diffie-
Hellman key.
The authentication key algorithm needs to be compatible with the
method and the cipher suite (see Section 3.6). The authentication
key algorithm needs to be compatible with the EDHOC key exchange
algorithm when static Diffie-Hellman authentication is used, and
compatible with the EDHOC signature algorithm when signature
authentication is used.
Note that for most signature algorithms, the signature is determined
by the signature algorithm and the authentication key algorithm
together. When using static Diffie-Hellman keys the Initiator's and
Responder's private authentication keys are denoted I and R,
respectively, and the public authentication keys are denoted G_I and
G_R, respectively.
For X.509 certificates the authentication key is represented with a
SubjectPublicKeyInfo field. For CWT and CCS (see Section 3.5.2)) the
authentication key is represented with a 'cnf' claim [RFC8747]
containing a COSE_Key [I-D.ietf-cose-rfc8152bis-struct].
3.5.2. Authentication Credentials
The authentication credentials, CRED_I and CRED_R, contain the public
authentication key of the Initiator and the Responder, respectively.
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EDHOC relies on COSE for identification of credentials (see
Section 3.5.3), for example X.509 certificates [RFC5280], C509
certificates [I-D.ietf-cose-cbor-encoded-cert], CWTs [RFC8392] and
CWT Claims Sets (CCS) [RFC8392]. When the identified credential is a
chain or a bag, the authentication credential CRED_x is just the end
entity X.509 or C509 certificate / CWT.
Since CRED_R is used in the integrity verification, see
Section 5.3.2, it needs to be specified such that it is identical
when used by Initiator or Responder. Similarly for CRED_I, see
Section 5.4.2. The Initiator and Responder are expected to agree on
a specific encoding of the credential, see Section 3.9.
It is RECOMMENDED that the COSE 'kid' parameter, when used to
identify the authentication credential, refers to a specific
encoding. The Initiator and Responder SHOULD use an available
authentication credential (transported in EDHOC or otherwise
provisioned) without re-encoding. If for some reason re-encoding of
the authentication credential may occur, then a potential common
encoding for CBOR based credentials is bytewise lexicographic order
of their deterministic encodings as specified in Section 4.2.1 of
[RFC8949].
* When the authentication credential is an X.509 certificate, CRED_x
SHALL be the DER encoded certificate, encoded as a bstr
[I-D.ietf-cose-x509].
* When the authentication credential is a C509 certificate, CRED_x
SHALL be the C509Certificate [I-D.ietf-cose-cbor-encoded-cert].
* When the authentication credential is a COSE_Key in a CWT, CRED_x
SHALL be the untagged CWT.
* When the authentication credential is a COSE_Key but not in a CWT,
CRED_x SHALL be an untagged CCS.
- Naked COSE_Keys are thus dressed as CCS when used in EDHOC,
which is done by prefixing the COSE_Key with 0xA108A101.
An example of a CRED_x is shown below:
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{ /CCS/
2 : "42-50-31-FF-EF-37-32-39", /sub/
8 : { /cnf/
1 : { /COSE_Key/
1 : 1, /kty/
2 : h'00', /kid/
-1 : 4, /crv/
-2 : h'b1a3e89460e88d3a8d54211dc95f0b90 /x/
3ff205eb71912d6db8f4af980d2db83a'
}
}
}
Figure 6: CWT Claims Set (CCS) containing an X25519 static
Diffie-Hellman key and an EUI-64 identity.
3.5.3. Identification of Credentials
ID_CRED_R and ID_CRED_I are transported in message_2 and message_3,
respectively, see Section 5.3.2 and Section 5.4.2. They are used to
identify and optionally transport credentials:
* ID_CRED_R is intended to facilitate for the Initiator to retrieve
the authentication credential CRED_R and the authentication key of
R.
* ID_CRED_I is intended to facilitate for the Responder to retrieve
the authentication credential CRED_I and the authentication key of
I.
ID_CRED_x may contain the authentication credential CRED_x, but for
many settings it is not necessary to transport the authentication
credential within EDHOC, for example, it may be pre-provisioned or
acquired out-of-band over less constrained links. ID_CRED_I and
ID_CRED_R do not have any cryptographic purpose in EDHOC since the
authentication credentials are integrity protected.
EDHOC relies on COSE for identification of credentials and supports
all credential types for which COSE header parameters are defined
including X.509 certificates ([I-D.ietf-cose-x509]), C509
certificates ([I-D.ietf-cose-cbor-encoded-cert]), CWT (Section 9.6)
and CWT Claims Set (CCS) (Section 9.6).
ID_CRED_I and ID_CRED_R are COSE header maps and contains one or more
COSE header parameters. ID_CRED_I and ID_CRED_R MAY contain
different header parameters. The header parameters typically provide
some information about the format of the credential.
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Note that COSE header parameters in ID_CRED_x are used to identify
the sender's credential. There is therefore no reason to use the
"-sender" header parameters, such as x5t-sender, defined in Section 3
of [I-D.ietf-cose-x509]. Instead, the corresponding parameter
without "-sender", such as x5t, SHOULD be used.
Example: X.509 certificates can be identified by a hash value using
the 'x5t' parameter:
* ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R,
Example: CWT or CCS can be identified by a key identifier using the
'kid' parameter:
* ID_CRED_x = { 4 : key_id_x }, where key_id_x : kid, for x = I or
R.
The value of a COSE 'kid' parameter is a byte string. To allow one-
byte encodings of ID_CRED_x with key identifiers 'kid', which is
useful in scenarios with only a few keys, the integer representation
of identifiers in Section 3.3.2 MUST be applied. For details, see
Section 5.3.2 and Section 5.4.2.
As stated in Section 3.1 of [I-D.ietf-cose-rfc8152bis-struct],
applications MUST NOT assume that 'kid' values are unique and several
keys associated with a 'kid' may need to be checked before the
correct one is found. Applications might use additional information
such as 'kid context' or lower layers to determine which key to try
first. Applications should strive to make ID_CRED_x as unique as
possible, since the recipient may otherwise have to try several keys.
See Appendix C.3 for more examples.
3.6. Cipher Suites
An EDHOC cipher suite consists of an ordered set of algorithms from
the "COSE Algorithms" and "COSE Elliptic Curves" registries as well
as the EDHOC MAC length. All algorithm names and definitions follows
from COSE [I-D.ietf-cose-rfc8152bis-algs]. Note that COSE sometimes
uses peculiar names such as ES256 for ECDSA with SHA-256, A128 for
AES-128, and Ed25519 for the curve edwards25519. Algorithms need to
be specified with enough parameters to make them completely
determined. The MAC length MUST be at least 8 bytes. Any
cryptographic algorithm used in the COSE header parameters in ID_CRED
is selected independently of the cipher suite. EDHOC is currently
only specified for use with key exchange algorithms of type ECDH
curves, but any Key Encapsulation Method (KEM), including Post-
Quantum Cryptography (PQC) KEMs, can be used in method 0, see
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Section 8.4. Use of other types of key exchange algorithms to
replace static DH authentication (method 1,2,3) would likely require
a specification updating EDHOC with new methods.
EDHOC supports all signature algorithms defined by COSE. Just like
in (D)TLS 1.3 [RFC8446][RFC9147] and IKEv2 [RFC7296], a signature in
COSE is determined by the signature algorithm and the authentication
key algorithm together, see Section 3.5.1. The exact details of the
authentication key algorithm depend on the type of authentication
credential. COSE supports different formats for storing the public
authentication keys including COSE_Key and X.509, which use different
names and ways to represent the authentication key and the
authentication key algorithm.
An EDHOC cipher suite consists of the following parameters:
* EDHOC AEAD algorithm
* EDHOC hash algorithm
* EDHOC MAC length in bytes (Static DH)
* EDHOC key exchange algorithm (ECDH curve)
* EDHOC signature algorithm
* Application AEAD algorithm
* Application hash algorithm
Each cipher suite is identified with a pre-defined integer label.
EDHOC can be used with all algorithms and curves defined for COSE.
Implementations can either use any combination of COSE algorithms and
parameters to define their own private cipher suite, or use one of
the pre-defined cipher suites. Private cipher suites can be
identified with any of the four values -24, -23, -22, -21. The pre-
defined cipher suites are listed in the IANA registry (Section 9.2)
with initial content outlined here:
* Cipher suites 0-3, based on AES-CCM, are intended for constrained
IoT where message overhead is a very important factor. Note that
AES-CCM-16-64-128 and AES-CCM-16-64-128 are compatible with the
IEEE CCM* mode.
- Cipher suites 1 and 3 use a larger tag length (128-bit) in
EDHOC than in the Application AEAD algorithm (64-bit).
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* Cipher suites 4 and 5, based on ChaCha20, are intended for less
constrained applications and only use 128-bit tag lengths.
* Cipher suite 6, based on AES-GCM, is for general non-constrained
applications. It consists of high performance algorithms that are
widely used in non-constrained applications.
* Cipher suites 24 and 25 are intended for high security
applications such as government use and financial applications.
These cipher suites do not share any algorithms. Cipher suite 24
consists of algorithms from the CNSA suite [CNSA].
The different methods (Section 3.2) use the same cipher suites, but
some algorithms are not used in some methods. The EDHOC signature
algorithm is not used in methods without signature authentication.
The Initiator needs to have a list of cipher suites it supports in
order of preference. The Responder needs to have a list of cipher
suites it supports. SUITES_I contains cipher suites supported by the
Initiator, formatted and processed as detailed in Section 5.2.1 to
secure the cipher suite negotiation. Examples of cipher suite
negotiation are given in Section 6.3.2.
3.7. Ephemeral Public Keys
The ephemeral public keys in EDHOC (G_X and G_Y) use 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 a 'y' parameter, the value y = false SHALL be
used. COSE always use compact output for Elliptic Curve Keys of type
EC2.
3.8. External Authorization Data (EAD)
In order to reduce round trips and the number of messages or to
simplify processing, external security applications may be integrated
into EDHOC by transporting authorization related data in the
messages.
EDHOC allows opaque external authorization data (EAD) to be sent in
each of the four EDHOC messages (EAD_1, EAD_2, EAD_3, EAD_4).
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External authorization data is a CBOR sequence (see Appendix C.1)
consisting of one or more (ead_label, ead_value) pairs as defined
below:
ead = 1* (
ead_label : int,
ead_value : bstr,
)
A security application using external authorization data need to
register an ead_label, specify the ead_value format for each message
(see Section 9.5), and describe processing and security
considerations.
The EAD fields of EDHOC must not be used for generic application
data. Examples of the use of EAD is provided in Appendix E.
3.9. Application Profile
EDHOC requires certain parameters to be agreed upon between Initiator
and Responder. Some parameters can be negotiated through the
protocol execution (specifically, cipher suite, see Section 3.6) but
other parameters are only communicated and may not be negotiated
(e.g., which authentication method is used, see Section 3.2). Yet
other parameters need to be known out-of-band.
The purpose of an application profile is to describe the intended use
of EDHOC to allow for the relevant processing and verifications to be
made, including things like:
1. How the endpoint detects that an EDHOC message is received. This
includes how EDHOC messages are transported, for example in the
payload of a CoAP message with a certain Uri-Path or Content-
Format; see Appendix A.2.
* 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.2.
2. Authentication method (METHOD; see Section 3.2).
3. Profile for authentication credentials (CRED_I, CRED_R; see
Section 3.5.2), e.g., profile for certificate or CCS, including
supported authentication key algorithms (subject public key
algorithm in X.509 or C509 certificate).
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4. Type used to identify credentials (ID_CRED_I, ID_CRED_R; see
Section 3.5.3).
5. Use and type of external authorization data (EAD_1, EAD_2, EAD_3,
EAD_4; see Section 3.8).
6. Identifier used as the identity of the endpoint; see
Appendix D.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 application profile may also contain information about supported
cipher suites. The procedure for selecting and verifying a cipher
suite is still performed as described in Section 5.2.1 and
Section 6.3, but it may become simplified by this knowledge.
An example of an application profile is shown in Appendix F.
For some parameters, like METHOD, ID_CRED_x, type of EAD, the
receiver is able to verify compliance with the application profile,
and if it needs to fail because of incompliance, to infer the reason
why the protocol failed.
For other parameters, like the profile of CRED_x in the case that it
is not transported, it may not be possible to verify that
incompliance with the application profile was the reason for failure:
Integrity verification in message_2 or message_3 may fail not only
because of wrong 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 application profile may be dependent on the identity of the other
endpoint, or other information carried in an EDHOC message, but it
then applies only to the later phases of the protocol when such
information is known. (The Initiator does not know the identity of
the Responder before having verified message_2, and the Responder
does not know the identity of the Initiator before having verified
message_3.)
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Other conditions may be part of the application profile, 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 application profiles are used, the receiver needs to be able
to determine which is applicable for a given session, for example
based on URI or external authorization data type.
4. Key Derivation
4.1. Keys for EDHOC Message Processing
EDHOC uses Extract-and-Expand [RFC5869] with the EDHOC hash algorithm
in the selected cipher suite to derive keys used in message
processing. This section defines Extract (Section 4.1.1) and Expand
(Section 4.1.2), and how to use them to derive PRK_out
(Section 4.1.3) which is the shared secret key resulting from a
successful EDHOC exchange.
Extract is used to derive fixed-length uniformly pseudorandom keys
(PRK) from ECDH shared secrets. Expand is used to define EDHOC-KDF
for generating MACs and for deriving output keying material (OKM)
from PRKs.
In EDHOC a specific message is protected with a certain pseudorandom
key, but how the key is derived depends on the method as detailed in
Section 5.
4.1.1. Extract
The pseudorandom keys (PRKs) used for EDHOC message processing are
derived using Extract:
PRK = Extract( salt, IKM )
where the input keying material (IKM) and salt are defined for each
PRK below.
The definition of Extract depends on the EDHOC hash algorithm of the
selected cipher suite:
* if the EDHOC hash algorithm is SHA-2, then Extract( salt, IKM ) =
HKDF-Extract( salt, IKM ) [RFC5869]
* if the EDHOC hash algorithm is SHAKE128, then Extract( salt, IKM )
= KMAC128( salt, IKM, 256, "" )
* if the EDHOC hash algorithm is SHAKE256, then Extract( salt, IKM )
= KMAC256( salt, IKM, 512, "" )
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The rest of the section defines the pseudo-random keys PRK_2e,
PRK_3e2m and PRK_4e3m; their use is shown in Figure 7.
4.1.1.1. PRK_2e
The pseudo-random key PRK_2e is derived with the following input:
* The salt SHALL be a zero-length byte string. Note that [RFC5869]
specifies that if the salt is not provided, it is set to a string
of zeros (see Section 2.2 of [RFC5869]). For implementation
purposes, not providing the salt is the same as setting the salt
to the zero-length byte string (0x).
* The IKM SHALL be the ephemeral-ephemeral ECDH shared secret G_XY
(calculated from G_X and Y or G_Y and X) as defined in
Section 6.3.1 of [I-D.ietf-cose-rfc8152bis-algs]. The use of G_XY
gives forward secrecy, in the sense that compromise of the private
authentication keys does not compromise past session keys.
Example: Assuming the use of curve25519, the ECDH shared secret G_XY
is the output of the X25519 function [RFC7748]:
G_XY = X25519( Y, G_X ) = X25519( X, G_Y )
Example: Assuming the use of SHA-256 the extract phase of HKDF
produces PRK_2e as follows:
PRK_2e = HMAC-SHA-256( salt, G_XY )
where salt = 0x (zero-length byte string).
4.1.1.2. PRK_3e2m
The pseudo-random key PRK_3e2m is derived as follows:
If the Responder authenticates with a static Diffie-Hellman key, then
PRK_3e2m = Extract( SALT_3e2m, G_RX ), where
* SALT_3e2m is derived from PRK_2e, see Section 4.1.2, and
* G_RX is the ECDH shared secret calculated from G_R and X, or G_X
and R (the Responder's private authentication key, see
Section 3.5.1),
else PRK_3e2m = PRK_2e.
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4.1.1.3. PRK_4e3m
The pseudo-random key PRK_4e3m is derived as follows:
If the Initiator authenticates with a static Diffie-Hellman key, then
PRK_4e3m = Extract( SALT_4e3m, G_IY ), where
* SALT_4e3m is derived from PRK_3e2m, see Section 4.1.2, and
* G_IY is the ECDH shared secret calculated from G_I and Y, or G_Y
and I (the Initiator's private authentication key, see
Section 3.5.1),
else PRK_4e3m = PRK_3e2m.
4.1.2. Expand and EDHOC-KDF
The output keying material (OKM) - including keys, IVs, and salts -
are derived from the PRKs using the EDHOC-KDF, which is defined
through Expand:
OKM = EDHOC-KDF( PRK, label, context, length )
= Expand( PRK, info, length )
where info is encoded as the CBOR sequence
info = (
label : uint,
context : bstr,
length : uint,
)
where
* label is a uint
* context is a bstr
* length is the length of OKM in bytes
When EDHOC-KDF is used to derive OKM for EDHOC message processing,
then context includes one of the transcript hashes TH_2, TH_3, or
TH_4 defined in Sections 5.3.2 and 5.4.2.
The definition of Expand depends on the EDHOC hash algorithm of the
selected cipher suite:
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* if the EDHOC hash algorithm is SHA-2, then Expand( PRK, info,
length ) = HKDF-Expand( PRK, info, length ) [RFC5869]
* if the EDHOC hash algorithm is SHAKE128, then Expand( PRK, info,
length ) = KMAC128( PRK, info, L, "" )
* if the EDHOC hash algorithm is SHAKE256, then Expand( PRK, info,
length ) = KMAC256( PRK, info, L, "" )
where L = 8*length, the output length in bits.
Figure 7 lists derivations made with EDHOC-KDF during message
processing. How the output keying material is used is specified in
Section 5.
KEYSTREAM_2 = EDHOC-KDF( PRK_2e, 0, TH_2, plaintext_length )
SALT_3e2m = EDHOC-KDF( PRK_2e, 1, TH_2, hash_length )
MAC_2 = EDHOC-KDF( PRK_3e2m, 2, context_2, mac_length_2 )
K_3 = EDHOC-KDF( PRK_3e2m, 3, TH_3, key_length )
IV_3 = EDHOC-KDF( PRK_3e2m, 4, TH_3, iv_length )
SALT_4e3m = EDHOC-KDF( PRK_3e2m, 5, TH_3, hash_length )
MAC_3 = EDHOC-KDF( PRK_4e3m, 6, context_3, mac_length_3 )
PRK_out = EDHOC-KDF( PRK_4e3m, 7, TH_4, hash_length )
K_4 = EDHOC-KDF( PRK_4e3m, 8, TH_4, key_length )
IV_4 = EDHOC-KDF( PRK_4e3m, 9, TH_4, iv_length )
Figure 7: Key derivations using EDHOC-KDF.
4.1.3. PRK_out
The pseudo-random key PRK_out, derived as shown in Figure 7, is the
only secret key shared between Initiator and Responder that needs to
be stored after a successful EDHOC exchange, see Section 5.4. Keys
for applications are derived from PRK_out, see Section 4.2.1.
4.2. Keys for EDHOC Applications
This section defines EDHOC-Exporter and EDHOC-KeyUpdate in terms of
EDHOC-KDF and PRK_out.
4.2.1. EDHOC-Exporter
Keying material for the application can be derived using the EDHOC-
Exporter interface defined as:
EDHOC-Exporter(label, context, length)
= EDHOC-KDF(PRK_exporter, label, context, length)
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where
* label is a registered uint from the EDHOC Exporter Label registry
(Section 9.1)
* context is a bstr defined by the application
* length is a uint defined by the application
* PRK_exporter is derived from PRK_out:
PRK_exporter = EDHOC-KDF( PRK_out, 10, h'', hash_length )
where hash_length denotes the length of the hash function output in
bytes, as specified by the COSE hash algorithm definition.
PRK_exporter MUST be derived anew if PRK_out is updated, in
particular if EDHOC-KeyUpdate is used, see Section 4.2.2.
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
can be reused with different nonces. The context can for example be
the empty CBOR byte string.
Examples of use of the EDHOC-Exporter are given in Appendix A.
4.2.2. EDHOC-KeyUpdate
To provide forward secrecy in an even more efficient way than re-
running EDHOC, EDHOC provides the function EDHOC-KeyUpdate. When
EDHOC-KeyUpdate is called, the old PRK_out is deleted and the new
PRK_out is calculated as a "hash" of the old key using the Expand
function as illustrated by the following pseudocode:
EDHOC-KeyUpdate( context ):
PRK_out = EDHOC-KDF( PRK_out, 11, context, hash_length )
where hash_length denotes the length of the hash function output in
bytes, as specified by the COSE hash algorithm definition.
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The EDHOC-KeyUpdate takes a context as input to enable binding of the
updated PRK_out to some event that triggered the keyUpdate. The
Initiator and the Responder need to agree on the context, which can,
e.g., be a counter or a pseudo-random number such as a hash. The
Initiator and the Responder also need to cache the old PRK_out until
it has verfied that the other endpoint has the correct new PRK_out.
[I-D.ietf-core-oscore-key-update] describes key update for OSCORE
using EDHOC-KeyUpdate.
While this key update method provides forward secrecy it does not
give as strong security properties as re-running EDHOC, see
Section 8.
5. Message Formatting and Processing
This section specifies formatting of the messages and processing
steps. Error messages are specified in Section 6. Annotated traces
of EDHOC protocol runs are provided in [I-D.ietf-lake-traces].
An EDHOC message is encoded as a sequence of CBOR data items (CBOR
Sequence, [RFC8742]). Additional optimizations are made to reduce
message overhead.
While EDHOC uses the COSE_Key, COSE_Sign1, and COSE_Encrypt0
structures, only a subset of the parameters is included in the EDHOC
messages, see Appendix C.3. The unprotected COSE header in
COSE_Sign1, and COSE_Encrypt0 (not included in the EDHOC message) MAY
contain parameters (e.g., 'alg').
5.1. Message Processing Outline
This section outlines the message processing of EDHOC.
For each new/ongoing session, the endpoints are assumed to keep an
associated protocol state containing identifiers, keying material,
etc. used for subsequent processing of protocol related data. The
protocol state is assumed to be associated to an application profile
(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).
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2. Retrieve the protocol state according to the message correlation
provided by the transport, see Section 3.4. If there is no
protocol state, in the case of message_1, a new protocol state is
created. The Responder endpoint needs to make use of available
Denial-of-Service mitigation (Section 8.7).
3. If the message received is an error message, then process it
according to Section 6, else process it as the expected next
message according to the protocol state.
If the processing fails for some reason then, typically, an error
message is sent, the protocol is discontinued, and the protocol state
erased. Further details are provided in the following subsections
and in Section 6.
Different instances of the same message MUST NOT be processed in one
session. Note that processing will fail if the same message appears
a second time for EDHOC processing in the same session 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 G.
5.2. EDHOC Message 1
5.2.1. Formatting of Message 1
message_1 SHALL be a CBOR Sequence (see Appendix C.1) as defined
below
message_1 = (
METHOD : int,
SUITES_I : suites,
G_X : bstr,
C_I : bstr / -24..23,
? EAD_1 : ead,
)
suites = [ 2* int ] / int
where:
* METHOD - authentication method, see Section 3.2.
* SUITES_I - array of cipher suites which the Initiator supports in
order of preference, the first cipher suite in network byte order
is the most preferred by I, the last is the one selected by I for
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this session. If the most preferred cipher suite is selected then
SUITES_I contains only that cipher suite and is encoded as an int.
The processing steps are detailed below and in Section 6.3.
* G_X - the ephemeral public key of the Initiator
* C_I - variable length connection identifier. Note that connection
identifiers are byte strings but certain values are represented as
integers in the message, see Section 3.3.2.
* EAD_1 - external authorization data, see Section 3.8.
5.2.2. Initiator Processing of Message 1
The Initiator SHALL compose message_1 as follows:
* Construct SUITES_I complying with the definition in
Section 5.2.1}, and furthermore:
- The Initiator MUST select its most preferred cipher suite,
conditioned on what it can assume to be supported by the
Responder.
- The selected cipher suite (i.e. the last cipher suite in
SUITES_I) MAY be different between sessions, e.g., based on
previous error messages (see next bullet), but all cipher
suites which are more preferred by I than the selected cipher
suite MUST be included in SUITES_I.
- If the Initiator previously received from the Responder an
error message with error code 2 containing SUITES_R (see
Section 6.3) which indicates cipher suites supported by the
Responder, then the Initiator SHOULD select its most preferred
supported cipher suite among those (bearing in mind that error
messages are not authenticated and may be forged).
- The Initiator MUST NOT change the supported cipher suites and
the order of preference in SUITES_I based on previous error
messages.
* Generate an ephemeral ECDH key pair using the curve in the
selected cipher suite and format it as a COSE_Key. Let G_X be the
'x' parameter of the COSE_Key.
* Choose a connection identifier C_I and store it for the length of
the protocol.
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* Encode message_1 as a sequence of CBOR encoded data items as
specified in Section 5.2.1
5.2.3. Responder Processing of Message 1
The Responder SHALL process message_1 as follows:
* Decode message_1 (see Appendix C.1).
* Verify that the selected cipher suite is supported and that no
prior cipher suite in SUITES_I is supported.
* If EAD_1 is present then make it available to the application for
EAD processing.
If any processing step fails, the Responder MUST send an EDHOC error
message back, formatted as defined in Section 6, and the session MUST
be discontinued.
5.3. EDHOC Message 2
5.3.1. Formatting of Message 2
message_2 SHALL be a CBOR Sequence (see Appendix C.1) as defined
below
message_2 = (
G_Y_CIPHERTEXT_2 : bstr,
C_R : bstr / -24..23,
)
where:
* G_Y_CIPHERTEXT_2 - the concatenation of G_Y (i.e., the ephemeral
public key of the Responder) and CIPHERTEXT_2.
* C_R - variable length connection identifier. Note that connection
identifiers are byte strings but certain values are represented as
integers in the message, see Section 3.3.2.
5.3.2. Responder Processing of Message 2
The Responder SHALL compose message_2 as follows:
* Generate an ephemeral ECDH key pair using the curve in the
selected cipher suite and format it as a COSE_Key. Let G_Y be the
'x' parameter of the COSE_Key.
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* Choose a connection identifier C_R and store it for the length of
the protocol.
* Compute the transcript hash TH_2 = H( G_Y, C_R, H(message_1) )
where H() is the EDHOC hash algorithm of the selected cipher
suite. The transcript hash TH_2 is a CBOR encoded bstr and the
input to the hash function is a CBOR Sequence. Note that
H(message_1) can be computed and cached already in the processing
of message_1.
* Compute MAC_2 as in Section 4.1.2 with context_2 = << ID_CRED_R,
TH_2, CRED_R, ? EAD_2 >>
- If the Responder authenticates with a static Diffie-Hellman key
(method equals 1 or 3), then mac_length_2 is the EDHOC MAC
length given by the selected cipher suite. If the Responder
authenticates with a signature key (method equals 0 or 2), then
mac_length_2 is equal to the output size of the EDHOC hash
algorithm given by the selected cipher suite.
- ID_CRED_R - identifier to facilitate the retrieval of CRED_R,
see Section 3.5.3
- CRED_R - CBOR item containing the authentication credential of
the Responder, see Section 3.5.2
- EAD_2 - external authorization data, see Section 3.8
* If the Responder authenticates with a static Diffie-Hellman key
(method equals 1 or 3), then Signature_or_MAC_2 is MAC_2. If the
Responder authenticates with a signature key (method equals 0 or
2), then Signature_or_MAC_2 is the 'signature' field of a
COSE_Sign1 object, computed as specified in Section 4.4 of
[I-D.ietf-cose-rfc8152bis-struct] using the signature algorithm of
the selected cipher suite, the private authentication key of the
Responder, and the following parameters as input (see Appendix C.3
for an overview of COSE and Appendix C.1 for notation):
- protected = << ID_CRED_R >>
- external_aad = << TH_2, CRED_R, ? EAD_2 >>
- payload = MAC_2
* CIPHERTEXT_2 is calculated by using the Expand function as a
binary additive stream cipher over the following plaintext:
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- PLAINTEXT_2 = ( ? PAD, ID_CRED_R / bstr / -24..23,
Signature_or_MAC_2, ? EAD_2 )
o If ID_CRED_R contains a single 'kid' parameter, i.e.,
ID_CRED_R = { 4 : kid_R }, then only the byte string kid_R
is conveyed in the plaintext, represented as described in
Section 3.3.2.
o PAD = 1*true is padding that may be used to hide the length
of the unpadded plaintext
- Compute KEYSTREAM_2 as in Section 4.1.2, where plaintext_length
is the length of PLAINTEXT_2.
- CIPHERTEXT_2 = PLAINTEXT_2 XOR KEYSTREAM_2
* Encode message_2 as a sequence of CBOR encoded data items as
specified in Section 5.3.1.
5.3.3. Initiator Processing of Message 2
The Initiator SHALL process message_2 as follows:
* Decode message_2 (see Appendix C.1).
* Retrieve the protocol state using the message correlation provided
by the transport (e.g., the CoAP Token, the 5-tuple, or the
prepended C_I, see Appendix A.2).
* Decrypt CIPHERTEXT_2, see Section 5.3.2, and discard padding, if
present.
* Make ID_CRED_R and EAD_2 (if present) available to the application
for authentication- and EAD processing.
* Obtain the authentication credential (CRED_R) and the
authentication key of R from the application (or by other means).
* 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 Responder MUST send an EDHOC error
message back, formatted as defined in Section 6, and the session MUST
be discontinued.
5.4. EDHOC Message 3
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5.4.1. Formatting of Message 3
message_3 SHALL be a CBOR Sequence (see Appendix C.1) as defined
below
message_3 = (
CIPHERTEXT_3 : bstr,
)
5.4.2. Initiator Processing of Message 3
The Initiator SHALL compose message_3 as follows:
* Compute the transcript hash TH_3 = H(TH_2, PLAINTEXT_2) where H()
is the EDHOC hash algorithm of 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, PLAINTEXT_2)
can be computed and cached already in the processing of message_2.
* Compute MAC_3 as in Section 4.1.2, with context_3 = << ID_CRED_I,
TH_3, CRED_I, ? EAD_3 >>
- If the Initiator authenticates with a static Diffie-Hellman key
(method equals 2 or 3), then mac_length_3 is the EDHOC MAC
length given by the selected cipher suite. If the Initiator
authenticates with a signature key (method equals 0 or 1), then
mac_length_3 is equal to the output size of the EDHOC hash
algorithm given by the selected cipher suite.
- ID_CRED_I - identifier to facilitate the retrieval of CRED_I,
see Section 3.5.3
- CRED_I - CBOR item containing the authentication credential of
the Initiator, see Section 3.5.2
- EAD_3 - external authorization data, see Section 3.8
* If the Initiator authenticates with a static Diffie-Hellman key
(method equals 2 or 3), then Signature_or_MAC_3 is MAC_3. If the
Initiator authenticates with a signature key (method equals 0 or
1), then Signature_or_MAC_3 is the 'signature' field of a
COSE_Sign1 object, computed as specified in Section 4.4 of
[I-D.ietf-cose-rfc8152bis-struct] using the signature algorithm of
the selected cipher suite, the private authentication key of the
Initiator, and the following parameters as input (see
Appendix C.3):
- protected = << ID_CRED_I >>
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- external_aad = << TH_3, CRED_I, ? EAD_3 >>
- payload = MAC_3
* Compute a COSE_Encrypt0 object as defined in Sections 5.2 and 5.3
of [I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD
algorithm of the selected cipher suite, using the encryption key
K_3, the initialization vector IV_3 (if used by the AEAD
algorithm), the plaintext PLAINTEXT_3, and the following
parameters as input (see Appendix C.3):
- protected = h''
- external_aad = TH_3
- K_3 and IV_3 are defined in Section 4.1.2, with
o key_length - length of the encryption key of the EDHOC AEAD
algorithm
o iv_length - length of the initialization vector of the EDHOC
AEAD algorithm
- PLAINTEXT_3 = ( ? PAD, ID_CRED_I / bstr / -24..23,
Signature_or_MAC_3, ? EAD_3 )
o If ID_CRED_I contains a single 'kid' parameter, i.e.,
ID_CRED_I = { 4 : kid_I }, then only the byte string kid_I
is conveyed in the plaintext, represented as described in
Section 3.3.2.
o PAD = 1*true is padding that may be used to hide the length
of the unpadded plaintext
CIPHERTEXT_3 is the 'ciphertext' of COSE_Encrypt0.
* Compute the transcript hash TH_4 = H(TH_3, PLAINTEXT_3) where H()
is the EDHOC hash algorithm of the selected cipher suite. The
transcript hash TH_4 is a CBOR encoded bstr and the input to the
hash function is a CBOR Sequence.
* Calculate PRK_out as defined in Figure 7. The Initiator can now
derive application keys using the EDHOC-Exporter interface, see
Section 4.2.1.
* Encode message_3 as a CBOR data item as specified in
Section 5.4.1.
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* Make the connection identifiers (C_I, C_R) and the application
algorithms in the selected cipher suite available to the
application.
The Initiator SHOULD NOT persistently store PRK_out or application
keys until the Initiator has verified message_4 or a message
protected with a derived application key, such as an OSCORE message,
from the Responder. This is similar to waiting for acknowledgement
(ACK) in a transport protocol.
5.4.3. Responder Processing of Message 3
The Responder SHALL process message_3 as follows:
* Decode message_3 (see Appendix C.1).
* Retrieve the protocol state using the message correlation provided
by the transport (e.g., the CoAP Token, the 5-tuple, or the
prepended C_R, see Appendix A.2).
* Decrypt and verify the COSE_Encrypt0 as defined in Sections 5.2
and 5.3 of [I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD
algorithm in the selected cipher suite, and the parameters defined
in Section 5.4.2. Discard padding, if present.
* Make ID_CRED_I and EAD_3 (if present) available to the application
for authentication- and EAD processing.
* Obtain the authentication credential (CRED_I) and the
authentication key of I from the application (or by other means).
* 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.
* Make the connection identifiers (C_I, C_R) and the application
algorithms in the selected cipher suite available to the
application.
After verifying message_3, the Responder can compute PRK_out, see
Section 4.1.3, derive application keys using the EDHOC-Exporter
interface, see Section 4.2.1, persistently store the keying material,
and send protected application data.
If any processing step fails, the Responder MUST send an EDHOC error
message back, formatted as defined in Section 6, and the session MUST
be discontinued.
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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, message_4 MUST be supported and MUST be
used. Two examples of such deployments:
1. When EDHOC is only used for authentication and no application
data is sent.
2. When application data is only sent from the Initiator to the
Responder.
Further considerations about when to use message_4 are provided in
Section 3.9 and Section 8.1.
5.5.1. Formatting of Message 4
message_4 SHALL be a CBOR Sequence (see Appendix C.1) as defined
below
message_4 = (
CIPHERTEXT_4 : bstr,
)
5.5.2. Responder Processing of Message 4
The Responder SHALL compose message_4 as follows:
* Compute a COSE_Encrypt0 as defined in Sections 5.2 and 5.3 of
[I-D.ietf-cose-rfc8152bis-struct], with the EDHOC AEAD algorithm
of the selected cipher suite, using the encryption key K_4, the
initialization vector IV_4 (if used by the AEAD algorithm), the
plaintext PLAINTEXT_4, and the following parameters as input (see
Appendix C.3):
- protected = h''
- external_aad = TH_4
- K_4 and IV_4 are defined in Section 4.1.2, with
o key_length - length of the encryption key of the EDHOC AEAD
algorithm
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o iv_length - length of the initialization vector of the EDHOC
AEAD algorithm
- PLAINTEXT_4 = ( ? PAD, ? EAD_4 )
o PAD = 1*true is padding that may be used to hide the length
of the unpadded plaintext.
o EAD_4 - external authorization data, see Section 3.8.
CIPHERTEXT_4 is the 'ciphertext' of COSE_Encrypt0.
* Encode message_4 as a CBOR data item as specified in
Section 5.5.1.
5.5.3. Initiator Processing of Message 4
The Initiator SHALL process message_4 as follows:
* Decode message_4 (see Appendix C.1).
* Retrieve the protocol state using the message correlation provided
by the transport (e.g., the CoAP Token, the 5-tuple, or the
prepended C_I, see Appendix A.2).
* Decrypt and verify the COSE_Encrypt0 as defined in Sections 5.2
and 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. Discard padding, if present.
* Make EAD_4 (if present) available to the application for EAD
processing.
If any processing step fails, the Responder MUST send an EDHOC error
message back, formatted as defined in Section 6, and the session MUST
be discontinued.
After verifying message_4, the Initiator is assured that the
Responder has calculated the key PRK_out (key confirmation) and that
no other party can derive the key.
6. Error Handling
This section defines the format for error messages, and the
processing associated to the currently defined error codes.
Additional error codes may be registered, see Section 9.4.
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There are many kinds of errors that can occur during EDHOC
processing. As in CoAP, an error can be triggered by errors in the
received message or internal errors in the recieving endpoint.
Except for processing and formatting errors, it is up to the
implementation when to send an error message. Sending error messages
is essential for debugging but MAY be skipped if, for example, a
session cannot be found or due to denial-of-service reasons, see
Section 8.7. Errors messages in EDHOC are always 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.
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.
error SHALL be a CBOR Sequence (see Appendix C.1) as defined below
error = (
ERR_CODE : int,
ERR_INFO : any,
)
Figure 8: EDHOC error message.
where:
* ERR_CODE - error code encoded as an integer. The value 0 is used
for success, all other values (negative or positive) indicate
errors.
* ERR_INFO - error information. Content and encoding depend on
error code.
The remainder of this section specifies the currently defined error
codes, see Figure 9. 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 error |
+----------+---------------+----------------------------------------+
| 2 | suites | Wrong selected cipher suite |
+----------+---------------+----------------------------------------+
Figure 9: Error codes and error information included in the EDHOC
error message.
6.1. Success
Error code 0 MAY be used internally in an application to indicate
success, i.e., as a standard value in case of no error, e.g., in
status reporting or log files. ERR_INFO can contain any type of CBOR
item, the content is out of scope for this specification. Error code
0 MUST NOT be used as part of the EDHOC message exchange flow. If an
endpoint receives an error message with error code 0, then it MUST
discontinue the protocol and MUST NOT send an error message.
6.2. Unspecified Error
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, for example "Method
not supported". 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 when replying to message_1 in case the
cipher suite selected by the Initiator is not supported by the
Responder, or if the Responder supports a cipher suite more preferred
by the Initiator than the selected cipher suite, see Section 5.2.3.
ERR_INFO is in this case denoted SUITES_R and is of type suites, see
Section 5.2.1. If the Responder does not support the selected cipher
suite, then SUITES_R MUST include one or more supported cipher
suites. If the Responder supports a cipher suite in SUITES_I other
than the selected cipher suite (independently of if the selected
cipher suite is supported or not) then SUITES_R MUST include the
supported cipher suite in SUITES_I which is most preferred by the
Initiator. SUITES_R MAY include a single cipher suite, i.e., be
encoded as an int. If the Responder does not support any cipher
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suite in SUITES_I, then it SHOULD include all its supported cipher
suites in SUITES_R.
In contrast to SUITES_I, the order of the cipher suites in SUITES_R
has no significance.
6.3.1. Cipher Suite Negotiation
After receiving SUITES_R, the Initiator can determine which cipher
suite to select (if any) for the next EDHOC run with the Responder.
If the Initiator intends to contact the Responder in the future, the
Initiator SHOULD remember which selected cipher suite to use until
the next message_1 has been sent, otherwise the Initiator and
Responder will likely run into an infinite loop where the Initiator
selects its most preferred and the Responder sends an error with
supported cipher suites. After a successful run of EDHOC, the
Initiator MAY remember the selected cipher suite to use in future
EDHOC sessions. Note that if the Initiator or Responder is updated
with new cipher suite policies, any cached information may be
outdated.
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 the
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.
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 10 and 11 show
examples of how the Initiator can format SUITES_I and how SUITES_R is
used by Responders to give the Initiator information about the cipher
suites that the Responder supports.
In the first example (Figure 10), the Responder supports cipher suite
6 but not the initially selected cipher suite 5.
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Initiator Responder
| METHOD, SUITES_I = 5, G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| ERR_CODE = 2, SUITES_R = 6 |
|<------------------------------------------------------------------+
| error |
| |
| METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
Figure 10: Example of an Initiator supporting suites 5, 6, 7, 8,
and 9 in decreasing order of preference, and a Responder
supporting suite 6 but not suite 5. The Responder rejects the
first message_1 with an error indicating support for suite 6.
The Initiator also supports suite 6, and therefore selects suite
6 in the second message_1. The initiator prepends in SUITES_I
the selected suite 6 with the more preferred suites, in this case
suite 5, to mitigate a potential attack on the cipher suite
negotiation.
In the second example (Figure 11), the Responder supports cipher
suites 8 and 9 but not the more preferred (by the Initiator) cipher
suites 5, 6 or 7. To illustrate the negotiation mechanics we let the
Initiator first make a guess that the Responder supports suite 6 but
not suite 5. Since the Responder supports neither 5 nor 6, it
responds with SUITES_R containing the supported suites, after which
the Initiator selects its most preferred supported suite. (If the
Responder had supported suite 5, it would have included it in
SUITES_R of the response, and it would in that case have become the
selected suite in the second message_1.)
Initiator Responder
| METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| ERR_CODE = 2, SUITES_R = [9, 8] |
|<------------------------------------------------------------------+
| error |
| |
| METHOD, SUITES_I = [5, 6, 7, 8], G_X, C_I, EAD_1 |
+------------------------------------------------------------------>|
| message_1 |
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Figure 11: Example of an Initiator supporting suites 5, 6, 7, 8,
and 9 in decreasing order of preference, and a Responder
supporting suites 8 and 9 but not 5, 6 or 7. The Responder
rejects the first message_1 with an error indicating support for
suites 8 and 9 (in any order). The Initiator also supports
suites 8 and 9, and prefers suite 8, so therefore selects suite 8
in the second message_1. The initiator prepends in SUITES_I the
selected suite 8 with the more preferred suites in order of
preference, in this case suites 5, 6 and 7, to mitigate a
potential attack on the cipher suite negotiation.
7. Compliance Requirements
In the absence of an application profile specifying otherwise:
An implementation MAY support only Initiator or only Responder.
An implementation MAY support only a single method. None of the
methods are mandatory-to-implement.
Implementations MUST support 'kid' parameters. None of the other
COSE header parameters are mandatory-to-implement.
An implementation MAY support only a single credential type (CCS,
CWT, X.509, C509). None of the credential types are mandatory-to-
implement.
Implementations MUST support the EDHOC-Exporter. Implementations
SHOULD support EDHOC-KeyUpdate.
Implementations MAY support message_4. Error codes (ERR_CODE) 1 and
2 MUST be supported.
Implementations MAY support EAD.
Implementations MAY support padding of plaintext when sending
messages. Implementations MUST support padding of plaintext when
receiving messages, i.e. MUST be able to parse padded messages.
Implementations MUST support cipher suite 2 and 3. Cipher suites 2
(AES-CCM-16-64-128, SHA-256, 8, P-256, ES256, AES-CCM-16-64-128, SHA-
256) and 3 (AES-CCM-16-128-128, SHA-256, 16, P-256, ES256, AES-CCM-
16-64-128, SHA-256) only differ in size of the MAC length, so
supporting one or both of these is no essential difference.
Implementations only need to implement the algorithms needed for
their supported methods.
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8. Security Considerations
8.1. Security Properties
EDHOC inherits its security properties from the theoretical SIGMA-I
protocol [SIGMA]. Using the terminology from [SIGMA], EDHOC provides
forward secrecy, mutual authentication with aliveness, consistency,
and peer awareness. As described in [SIGMA], peer awareness is
provided to the Responder, but not to the Initiator.
As described in [SIGMA], different levels of identity protection are
provided to the Initiator and the Responder. EDHOC protects the
credential identifier of the Initiator against active attacks and the
credential identifier of the Responder against passive attacks. An
active attacker can get the credential identifier of the Responder by
eavesdropping on the destination address used for transporting
message_1 and send its own message_1 to the same address. The roles
should be assigned to protect the most sensitive identity/identifier,
typically that which is not possible to infer from routing
information in the lower layers.
Compared to [SIGMA], EDHOC adds an explicit method type and expands
the message authentication coverage to additional elements such as
algorithms, external authorization data, and previous messages. This
protects against an attacker replaying messages or injecting messages
from another session.
EDHOC also adds selection of connection identifiers and downgrade
protected negotiation of cryptographic parameters, i.e., an attacker
cannot affect the negotiated parameters. A single session of EDHOC
does not include negotiation of cipher suites, but it enables the
Responder to verify that the selected cipher suite is the most
preferred cipher suite by the Initiator which is supported by both
the Initiator and the Responder.
As required by [RFC7258], IETF protocols need to mitigate pervasive
monitoring when possible. EDHOC therefore only supports methods with
ephemeral Diffie-Hellman and provides a KeyUpdate function for
lightweight application protocol rekeying with forward secrecy, in
the sense that compromise of the private authentication keys does not
compromise past session keys, and compromise of a session key does
not compromise past session keys.
While the KeyUpdate method can be used to meet cryptographic limits
and provide partial protection against key leakage, it provides
significantly weaker security properties than re-running EDHOC with
ephemeral Diffie-Hellman. Even with frequent use of KeyUpdate,
compromise of one session key compromises all future session keys,
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and an attacker therefore only needs to perform static key
exfiltration [RFC7624]. Frequently re-running EDHOC with ephemeral
Diffie-Hellman forces attackers to perform dynamic key exfiltration
instead of static key exfiltration [RFC7624]. In the dynamic case,
the attacker must have continuous interactions with the collaborator,
which is more complicated and has a higher risk profile than the
static case.
To limit the effect of breaches, it is important to limit the use of
symmetrical group keys for bootstrapping. EDHOC therefore strives to
make the additional cost of using raw public keys and self-signed
certificates as small as possible. Raw public keys and self-signed
certificates are not a replacement for a public key infrastructure
but SHOULD be used instead of symmetrical group keys for
bootstrapping.
Compromise of the long-term keys (private signature or static DH
keys) does not compromise the security of completed EDHOC exchanges.
Compromising the private authentication keys of one party lets an
active attacker impersonate that compromised party in EDHOC exchanges
with other parties but does not let the attacker impersonate other
parties in EDHOC exchanges with the compromised party. Compromise of
the long-term keys does not enable a passive attacker to compromise
future session keys. Compromise of the HDKF input parameters (ECDH
shared secret) leads to compromise of all session keys derived from
that compromised shared secret. Compromise of one session key does
not compromise other session keys. Compromise of PRK_out leads to
compromise of all keying material derived with the EDHOC-Exporter
since the last invocation (if any) of the EDHOC-KeyUpdate function.
Based on the cryptographic algorithms requirements Section 8.3, 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. To break 64-bit security against online brute force an
attacker would on average have to send 4.3 billion messages per
second for 68 years, which is infeasible in constrained IoT radio
technologies. A forgery against a 64-bit MAC in EDHOC breaks the
security of all future application data, while a forgery against a
64-bit MAC in the subsequent application protocol (e.g., OSCORE
[RFC8613]) typically only breaks the security of the data in the
forged packet.
After sending message_3, the Initiator is assured that no other party
than the Responder can compute the key PRK_out. While the Initiator
can securely send protected application data, the Initiator SHOULD
NOT persistently store the keying material PRK_out until the
Initiator has verified an OSCORE message or message_4 from the
Responder. After verifying message_3, the Responder is assured that
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an honest Initiator has computed the key PRK_out. The Responder can
securely derive and store the keying material PRK_out, and send
protected application data.
External authorization data sent in message_1 (EAD_1) or message_2
(EAD_2) should be considered unprotected by EDHOC, see Section 8.5.
EAD_2 is encrypted but the Responder has not yet authenticated the
Initiator. External authorization data sent in message_3 (EAD_3) or
message_4 (EAD_4) is protected between Initiator and Responder by the
protocol, but note that EAD fields may be used by the application
before the message verification is completed, see Section 3.8.
Designing a secure mechanism that uses EAD is not necessarily
straightforward. This document only provides the EAD transport
mechanism, but the problem of agreeing on the surrounding context and
the meaning of the information passed to and from the application
remains. Any new uses of EAD should be subject to careful review.
Key compromise impersonation (KCI): In EDHOC authenticated with
signature keys, EDHOC provides KCI protection against an attacker
having access to the long-term key or the ephemeral secret key. With
static Diffie-Hellman key authentication, KCI protection would be
provided against an attacker having access to the long-term Diffie-
Hellman key, but not to an attacker having access to the ephemeral
secret key. Note that the term KCI has typically been used for
compromise of long-term keys, and that an attacker with access to the
ephemeral secret key can only attack that specific session.
Repudiation: If an endpoint authenticates with a signature, the other
endpoint can prove that the endpoint performed a run of the protocol
by presenting the data being signed as well as the signature itself.
With static Diffie-Hellman key authentication, the authenticating
endpoint can deny having participated in the protocol.
Two earlier versions of EDHOC have been formally analyzed [Norrman20]
[Bruni18] and the specification has been updated based on the
analysis.
8.2. Cryptographic Considerations
The SIGMA protocol requires that the encryption of message_3 provides
confidentiality against active attackers and EDHOC message_4 relies
on the use of authenticated encryption. Hence the message
authenticating functionality of the authenticated encryption in EDHOC
is critical: authenticated encryption MUST NOT be replaced by plain
encryption only, even if authentication is provided at another level
or through a different mechanism.
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To reduce message overhead EDHOC does not use explicit nonces and
instead relies on the ephemeral public keys to provide randomness to
each session. A good amount of randomness is important for the key
generation, to provide liveness, and to protect against interleaving
attacks. For this reason, the ephemeral keys MUST NOT be used in
more than one EDHOC message, and both parties SHALL generate fresh
random ephemeral key pairs. Note that an ephemeral key may be used
to calculate several ECDH shared secrets. When static Diffie-Hellman
authentication is used the same ephemeral key is used in both
ephemeral-ephemeral and ephemeral-static ECDH.
As discussed in [SIGMA], the encryption of message_2 does only need
to protect against passive attacker as active attackers can always
get the Responder's identity by sending their own message_1. EDHOC
uses the Expand function (typically HKDF-Expand) as a binary additive
stream cipher which is proven secure as long as the expand function
is a PRF. HKDF-Expand is not often used as a stream cipher 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).
Requirements for how to securely generate, validate, and process the
ephemeral public keys depend on the elliptic curve. For X25519 and
X448, the requirements are defined in [RFC7748]. For secp256r1,
secp384r1, and secp521r1, the requirements are defined in Section 5
of [SP-800-56A]. For secp256r1, secp384r1, and secp521r1, at least
partial public-key validation MUST be done.
As noted in Section 12 of [I-D.ietf-cose-rfc8152bis-struct] the use
of a single key for multiple algorithms is strongly disencouraged
unless proven secure by a dedicated cryptographic analysis. In
particular this recommendation applies to using the same private key
for static Diffie-Hellman authentication and digital signature
authentication. A preliminary conjecture is that a minor change to
EDHOC may be sufficient to fit the analysis of secure shared
signature and ECDH key usage in [Degabriele11] and [Thormarker21].
So-called selfie attacks are mitigated as long as the Initiator does
not have its own identity in the set of Responder identities it is
allowed to communicate with. In trust on first use (TOFU) use cases
the Initiator should verify that the the Responder's identity is not
equal to its own. Any future EHDOC methods using e.g., pre-shared
keys might need to mitigate this in other ways.
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8.3. Cipher Suites and Cryptographic Algorithms
When using private cipher suite or registering new cipher suites, the
choice of key length used in the different algorithms needs to be
harmonized, so that a sufficient security level is maintained for
certificates, EDHOC, and the protection of application data. The
Initiator and the Responder should enforce a minimum security level.
The output size of the EDHOC hash algorithm MUST be at least
256-bits, i.e., the hash algorithms SHA-1 and SHA-256/64 (SHA-256
truncated to 64-bits) SHALL NOT be supported for use in EDHOC except
for certificate identification with x5t and c5t. For security
considerations of SHA-1, see [RFC6194]. As EDHOC integrity protects
the whole authentication credential, the choice of hash algorithm in
x5t and c5t does not affect security and it is RECOMMENDED to use the
same hash algorithm as in the cipher suite but with as much
truncation as possible, i.e, when the EDHOC hash algorithm is SHA-256
it is RECOMMENDED to use SHA-256/64 in x5t and c5t. The EDHOC MAC
length MUST be at least 8 bytes and the tag length of the EDHOC AEAD
algorithm MUST be at least 64-bits. Note that secp256k1 is only
defined for use with ECDSA and not for ECDH. Note that some COSE
algorithms are marked as not recommended in the COSE IANA registry.
8.4. Post-Quantum Considerations
As of the publication of this specification, it is unclear when or
even if a quantum computer of sufficient size and power to exploit
public key cryptography will exist. Deployments that need to
consider risks decades into the future should transition to Post-
Quantum Cryptography (PQC) in the not-too-distant future. Many other
systems should take a slower wait-and-see approach where PQC is
phased in when the quantum threat is more imminent. Current PQC
algorithms have limitations compared to Elliptic Curve Cryptography
(ECC) and the data sizes would be problematic in many constrained IoT
systems.
Symmetric algorithms used in EDHOC such as SHA-256 and AES-CCM-
16-64-128 are practically secure against even large quantum
computers. EDHOC supports all signature algorithms defined by COSE,
including PQC signature algorithms such as HSS-LMS. EDHOC is
currently only specified for use with key exchange algorithms of type
ECDH curves, but any Key Encapsulation Method (KEM), including PQC
KEMs, can be used in method 0. While the key exchange in method 0 is
specified with terms of the Diffie-Hellman protocol, the key exchange
adheres to a KEM interface: G_X is then the public key of the
Initiator, G_Y is the encapsulation, and G_XY is the shared secret.
Use of PQC KEMs to replace static DH authentication would likely
require a specification updating EDHOC with new methods.
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8.5. Unprotected Data and Privacy
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.
An attacker observing network traffic may use connection identifiers
sent in clear in EDHOC or the subsequent application protocol to
correlate packets sent on different paths or at different times. The
attacker may use this information for traffic flow analysis or to
track an endpoint. Application protocols using connection
identifiers from EDHOC SHOULD provide mechanisms to update the
connection identifier and MAY provide mechanisms to issue several
simultaneously active connection identifiers. See [RFC9000] for a
non-constrained example of such mechanisms.
8.6. Updated Internet Threat Model Considerations
Since the publication of [RFC3552] there has been an increased
awareness of the need to protect against endpoints that are
compromised, malicious, or whose interests simply do not align with
the interests of users
[I-D.arkko-arch-internet-threat-model-guidance]. [RFC7624] describes
an updated threat model for Internet confidentiality, see
Section 8.1. [I-D.arkko-arch-internet-threat-model-guidance] further
expands the threat model. Implementations and users SHOULD consider
these threat models. In particular, even data sent protected to the
other endpoint such as ID_CRED and EAD can be used for tracking, see
Section 2.7 of [I-D.arkko-arch-internet-threat-model-guidance].
The fields ID_CRED_I, ID_CRED_R, EAD_2, EAD_3, and EAD_4 have
variable length and information regarding the length may leak to an
attacker. An passive attacker may e.g., be able to differentiating
endpoints using identifiers of different length. To mitigate this
information leakage an inmplementation may ensure that the fields
have fixed length or use padding. An implementation may e.g., only
use fix length identifiers like 'kid' of length 1. Alternatively
padding may be used to hide the true length of e.g., certificates by
value in 'x5chain' or 'c5c'.
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8.7. Denial-of-Service
EDHOC itself does not provide countermeasures against Denial-of-
Service attacks. In particular, 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. For instance, when EDHOC is transferred as an exchange
of CoAP messages, the CoAP server can use the Echo option defined in
[RFC9175] which forces the CoAP client to demonstrate reachability at
its apparent network address.
An attacker can also send faked message_2, message_3, message_4, or
error in an attempt to trick the receiving party to send an error
message and discontinue the session. EDHOC implementations MAY
evaluate if a received message is likely to have been forged by an
attacker and ignore it without sending an error message or
discontinuing the session.
8.8. 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 random seed must be provided from an external source and
used with a cryptographically secure pseudorandom number generator.
As each pseudorandom number must only be used once, an implementation
needs to get a unique input to the pseudorandom number generator
after reboot, or continuously store state in nonvolatile memory.
Appendix B.1.1 in [RFC8613] describes issues and solution approaches
for writing to nonvolatile memory. Intentionally or unintentionally
weak or predictable pseudorandom number generators can be abused or
exploited for malicious purposes. [RFC8937] describes a way for
security protocol implementations to augment their (pseudo)random
number generators using a long-term private key and a deterministic
signature function. This improves randomness from broken or
otherwise subverted random number generators. The same idea can be
used with other secrets and functions such as a Diffie-Hellman
function or a symmetric secret and a PRF like HMAC or KMAC. It is
RECOMMENDED to not trust a single source of randomness and to not put
unaugmented random numbers on the wire.
Implementations might consider deriving secret and non-secret
randomness from different PNRG/PRF/KDF instances to limit the damage
if the PNRG/PRF/KDF turns out to be fundamentally broken. NIST
generally forbids deriving secret and non-secret randomness from the
same KDF instance, but this decision has been criticized by Krawczyk
[HKDFpaper] and doing so is common practice. In addition to IVs,
other examples are the challenge in EAP-TTLS, the RAND in 3GPP AKAs,
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and the Session-Id in EAP-TLS 1.3. Note that part of KEYSTREAM_2 is
also non-secret randomness as it is known or predictable to an
attacker. As explained by Krawczyk, if any attack is mitigated by
the NIST requirement it would mean that the KDF is fully broken and
would have to be replaced anyway.
For many constrained IoT devices it is problematic to support several
crypto primitives. Existing devices can be expected to support
either ECDSA or EdDSA. 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 2.1.1 of [I-D.ietf-cose-rfc8152bis-algs] this
can be addressed by combining randomness and determinism.
Appendix D of [I-D.ietf-lwig-curve-representations] describes how
Montgomery curves such as X25519 and X448 and (twisted) Edwards
curves as curves such as Ed25519 and Ed448 can mapped to and from
short-Weierstrass form for implementation on platforms that
accelerate elliptic curve group operations in short-Weierstrass form.
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 and validity of certificates. The selection of trusted CAs
should be done very carefully and certificate revocation should be
supported. The choice of revocation mechanism is left to the
application. For example, in case of X.509 certificates, Certificate
Revocation Lists [RFC5280] or OCSP [RFC6960] may be used.
Verification of validity may require the use of a Real-Time Clock
(RTC). The private authentication keys MUST be kept secret, only the
Responder SHALL have access to the Responder's private authentication
key and only the Initiator SHALL have access to the Initiator's
private authentication key.
The Initiator and the Responder are allowed to select its connection
identifiers C_I and C_R, respectively, for the other party to use in
the ongoing EDHOC protocol as well as in a subsequent application
protocol (e.g., OSCORE [RFC8613]). The choice of connection
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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. Note that in cases where several
EDHOC exchanges with different parameter sets (method, COSE headers,
etc.) are used, an attacker can affect which of the parameter sets
that will be used by blocking some of the parameter sets.
If supported by the device, it is RECOMMENDED that at least the long-
term private keys are stored in a Trusted Execution Environment (TEE)
and that sensitive operations using these keys are performed inside
the TEE. To achieve even higher security it is RECOMMENDED that
additional operations such as ephemeral key generation, all
computations of shared secrets, and storage of the PRK keys can be
done inside the TEE. The use of a TEE aims at preventing code within
that environment to be tampered with, and preventing data used by
such code to be read or tampered with by code outside that
environment.
Note that HKDF-Expand has a relativly small maximum output length of
255 * hash_length. This means that when when SHA-256 is used as hash
algorithm, message_2 cannot be longer than 8160 bytes.
The sequence of transcript hashes in EHDOC (TH_2, TH_3, TH_4) do not
make use of a so called running hash, this is a design choice as
running hashes are often not supported on constrained platforms.
When parsing a received EDHOC message, implementations MUST terminate
the protocol if the message does not comply with the CDDL for that
message. It is RECOMMENDED to terminate the protocol if the received
EDHOC message is not deterministic CBOR.
9. IANA Considerations
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9.1. EDHOC Exporter Label Registry
IANA has created a new registry titled "EDHOC Exporter Label" under
the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The
registration procedure is "Expert Review". The columns of the
registry are Label and Description. Label is a uint. Description is
a text string. The initial contents of the registry are:
Label: 0
Description: Derived OSCORE Master Secret
Label: 1
Description: Derived OSCORE Master Salt
9.2. EDHOC Cipher Suites Registry
IANA has created a new registry titled "EDHOC Cipher Suites" under
the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The
registration procedure is "Expert Review". The columns of the
registry are Value, Array and Description, 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
Value: -23
Algorithms: N/A
Desc: Reserved for Private Use
Value: -22
Algorithms: N/A
Desc: Reserved for Private Use
Value: -21
Algorithms: N/A
Desc: Reserved for Private Use
Value: 0
Array: 10, -16, 8, 4, -8, 10, -16
Desc: AES-CCM-16-64-128, SHA-256, 8, X25519, EdDSA,
AES-CCM-16-64-128, SHA-256
Value: 1
Array: 30, -16, 16, 4, -8, 10, -16
Desc: AES-CCM-16-128-128, SHA-256, 16, X25519, EdDSA,
AES-CCM-16-64-128, SHA-256
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Value: 2
Array: 10, -16, 8, 1, -7, 10, -16
Desc: AES-CCM-16-64-128, SHA-256, 8, P-256, ES256,
AES-CCM-16-64-128, SHA-256
Value: 3
Array: 30, -16, 16, 1, -7, 10, -16
Desc: AES-CCM-16-128-128, SHA-256, 16, P-256, ES256,
AES-CCM-16-64-128, SHA-256
Value: 4
Array: 24, -16, 16, 4, -8, 24, -16
Desc: ChaCha20/Poly1305, SHA-256, 16, X25519, EdDSA,
ChaCha20/Poly1305, SHA-256
Value: 5
Array: 24, -16, 16, 1, -7, 24, -16
Desc: ChaCha20/Poly1305, SHA-256, 16, P-256, ES256,
ChaCha20/Poly1305, SHA-256
Value: 6
Array: 1, -16, 16, 4, -7, 1, -16
Desc: A128GCM, SHA-256, 16, X25519, ES256,
A128GCM, SHA-256
Value: 24
Array: 3, -43, 16, 2, -35, 3, -43
Desc: A256GCM, SHA-384, 16, P-384, ES384,
A256GCM, SHA-384
Value: 25
Array: 24, -45, 16, 5, -8, 24, -45
Desc: ChaCha20/Poly1305, SHAKE256, 16, X448, EdDSA,
ChaCha20/Poly1305, SHAKE256
9.3. EDHOC Method Type Registry
IANA has created a new registry entitled "EDHOC Method Type" under
the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The
registration procedure is "Specification Required". The columns of
the registry are Value, Initiator Authentication Key, and Responder
Authentication Key, where Value is an integer and the other columns
are text strings describing the authentication keys. The initial
contents of the registry are shown in Figure 4.
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9.4. EDHOC Error Codes Registry
IANA has created a new registry entitled "EDHOC Error Codes" under
the new group name "Ephemeral Diffie-Hellman Over COSE (EDHOC)". The
registration procedure is "Expert Review". The columns of the
registry are ERR_CODE, ERR_INFO Type and Description, where ERR_CODE
is an integer, ERR_INFO is a CDDL defined type, and Description is a
text string. The initial contents of the registry are shown in
Figure 9.
9.5. EDHOC External Authorization Data Registry
IANA has created a new registry entitled "EDHOC External
Authorization Data" under the new group name "Ephemeral Diffie-
Hellman Over COSE (EDHOC)". The registration procedure is
"Specification Required". The columns of the registry are Label,
Message, Description, and Reference, where Label is an integer and
the other columns are text strings.
9.6. COSE Header Parameters Registry
IANA has registered the following entries in the "COSE Header
Parameters" registry under the group name "CBOR Object Signing and
Encryption (COSE)". The value of the 'kcwt' header parameter is a
COSE Web Token (CWT) [RFC8392], and the value of the 'kccs' header
parameter is a CWT Claims Set (CCS), see Section 1.4. The CWT/CCS
must contain a COSE_Key in a 'cnf' claim [RFC8747]. The Value
Registry for this item is empty and omitted from the table below.
+-----------+-------+----------------+---------------------------+
| Name | Label | Value Type | Description |
+===========+=======+================+===========================+
| kcwt | TBD1 | COSE_Messages | A CBOR Web Token (CWT) |
| | | | containing a COSE_Key in |
| | | | a 'cnf' claim |
+-----------+-------+----------------+---------------------------+
| kccs | TBD2 | map / #6(map) | A CWT Claims Set (CCS) |
| | | | containing a COSE_Key in |
| | | | a 'cnf' claim |
+-----------+-------+----------------+---------------------------+
9.7. The Well-Known URI Registry
IANA has added the well-known URI "edhoc" to the "Well-Known URIs"
registry under the group name "Well-Known URIs".
* URI suffix: edhoc
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* Change controller: IETF
* Specification document(s): [[this document]]
* Related information: None
9.8. Media Types Registry
IANA has added the media types "application/edhoc+cbor-seq" and
"application/cid-edhoc+cbor-seq" to the "Media Types" registry.
9.8.1. application/edhoc+cbor-seq Media Type Registration
* Type name: application
* Subtype name: edhoc+cbor-seq
* Required parameters: N/A
* Optional parameters: N/A
* Encoding considerations: binary
* Security considerations: See Section 7 of this document.
* Interoperability considerations: N/A
* Published specification: [[this document]] (this document)
* Applications that use this media type: To be identified
* Fragment identifier considerations: N/A
* Additional information:
- Magic number(s): N/A
- File extension(s): N/A
- Macintosh file type code(s): N/A
* Person & email address to contact for further information: See
"Authors' Addresses" section.
* Intended usage: COMMON
* Restrictions on usage: N/A
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* Author: See "Authors' Addresses" section.
* Change Controller: IESG
9.8.2. application/cid-edhoc+cbor-seq Media Type Registration
* Type name: application
* Subtype name: cid-edhoc+cbor-seq
* Required parameters: N/A
* Optional parameters: N/A
* Encoding considerations: binary
* Security considerations: See Section 7 of this document.
* Interoperability considerations: N/A
* Published specification: [[this document]] (this document)
* Applications that use this media type: To be identified
* Fragment identifier considerations: N/A
* Additional information:
- Magic number(s): N/A
- File extension(s): N/A
- Macintosh file type code(s): N/A
* Person & email address to contact for further information: See
"Authors' Addresses" section.
* Intended usage: COMMON
* Restrictions on usage: N/A
* Author: See "Authors' Addresses" section.
* Change Controller: IESG
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9.9. CoAP Content-Formats Registry
IANA has added the media types "application/edhoc+cbor-seq" and
"application/cid-edhoc+cbor-seq" to the "CoAP Content-Formats"
registry under the group name "Constrained RESTful Environments
(CoRE) Parameters".
+--------------------------------+----------+------+-------------------+
| Media Type | Encoding | ID | Reference |
+--------------------------------+----------+------+-------------------+
| application/edhoc+cbor-seq | - | TBD5 | [[this document]] |
| application/cid-edhoc+cbor-seq | - | TBD6 | [[this document]] |
+--------------------------------+----------+------+-------------------+
Figure 12: CoAP Content-Format IDs
9.10. Resource Type (rt=) Link Target Attribute Values Registry
IANA has added the resource type "core.edhoc" to the "Resource Type
(rt=) Link Target Attribute Values" registry under the group name
"Constrained RESTful Environments (CoRE) Parameters".
* Value: "core.edhoc"
* Description: EDHOC resource.
* Reference: [[this document]]
9.11. Expert Review Instructions
The IANA Registries established in this document are defined as
"Expert Review". This section gives some general guidelines for what
the experts should be looking for, but they are being designated as
experts for a reason so they should be given substantial latitude.
Expert reviewers should take into consideration the following points:
* Clarity and correctness of registrations. Experts are expected to
check the clarity of purpose and use of the requested entries.
Expert needs to make sure the values of algorithms are taken from
the right registry, when that is required. Expert should consider
requesting an opinion on the correctness of registered parameters
from relevant IETF working groups. Encodings that do not meet
these objective of clarity and completeness should not be
registered.
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* Experts should take into account the expected usage of fields when
approving point assignment. The length of the encoded value
should be weighed against how many code points of that length are
left, the size of device it will be used on, and the number of
code points left that encode to that size.
* Specifications are recommended. When specifications are not
provided, the description provided needs to have sufficient
information to verify the points above.
10. References
10.1. Normative References
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-algs-12.txt>.
[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", Work in Progress, Internet-Draft,
draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-struct-15.txt>.
[I-D.ietf-cose-x509]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Header parameters for carrying and referencing X.509
certificates", Work in Progress, Internet-Draft, draft-
ietf-cose-x509-08, 14 December 2020,
<https://www.ietf.org/internet-drafts/draft-ietf-cose-
x509-08.txt>.
[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>.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April
2002, <https://www.rfc-editor.org/info/rfc3279>.
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[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<https://www.rfc-editor.org/info/rfc6090>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <https://www.rfc-editor.org/info/rfc6979>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<https://www.rfc-editor.org/info/rfc7624>.
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[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>.
[RFC8410] Josefsson, S. and J. Schaad, "Algorithm Identifiers for
Ed25519, Ed448, X25519, and X448 for Use in the Internet
X.509 Public Key Infrastructure", RFC 8410,
DOI 10.17487/RFC8410, August 2018,
<https://www.rfc-editor.org/info/rfc8410>.
[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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[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[RFC9175] Amsüss, C., Preuß Mattsson, J., and G. Selander,
"Constrained Application Protocol (CoAP): Echo, Request-
Tag, and Token Processing", RFC 9175,
DOI 10.17487/RFC9175, February 2022,
<https://www.rfc-editor.org/info/rfc9175>.
10.2. Informative References
[Bruni18] Bruni, A., Sahl Jørgensen, T., Grønbech Petersen, T., and
C. Schürmann, "Formal Verification of Ephemeral Diffie-
Hellman Over COSE (EDHOC)", November 2018,
<https://www.springerprofessional.de/en/formal-
verification-of-ephemeral-diffie-hellman-over-cose-
edhoc/16284348>.
[CborMe] Bormann, C., "CBOR Playground", May 2018,
<http://cbor.me/>.
[CNSA] (Placeholder), ., "Commercial National Security Algorithm
Suite", August 2015,
<https://apps.nsa.gov/iaarchive/programs/iad-initiatives/
cnsa-suite.cfm>.
[Degabriele11]
Degabriele, J.P., Lehmann, A., Paterson, K.G., Smart,
N.P., and M. Strefler, "On the Joint Security of
Encryption and Signature in EMV", December 2011,
<https://eprint.iacr.org/2011/615>.
[HKDFpaper]
Krawczyk, H., "Cryptographic Extraction and Key
Derivation: The HKDF Scheme", May 2010,
<https://eprint.iacr.org/2010/264.pdf>.
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[I-D.arkko-arch-internet-threat-model-guidance]
Arkko, J. and S. Farrell, "Internet Threat Model
Guidance", Work in Progress, Internet-Draft, draft-arkko-
arch-internet-threat-model-guidance-00, 12 July 2021,
<https://www.ietf.org/archive/id/draft-arkko-arch-
internet-threat-model-guidance-00.txt>.
[I-D.ietf-core-oscore-edhoc]
Palombini, F., Tiloca, M., Hoeglund, R., Hristozov, S.,
and G. Selander, "Profiling EDHOC for CoAP and OSCORE",
Work in Progress, Internet-Draft, draft-ietf-core-oscore-
edhoc-03, 7 March 2022, <https://www.ietf.org/archive/id/
draft-ietf-core-oscore-edhoc-03.txt>.
[I-D.ietf-core-oscore-key-update]
Höglund, R. and M. Tiloca, "Key Update for OSCORE
(KUDOS)", Work in Progress, Internet-Draft, draft-ietf-
core-oscore-key-update-01, 7 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-core-oscore-
key-update-01.txt>.
[I-D.ietf-cose-cbor-encoded-cert]
Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", Work in Progress, Internet-Draft, draft-
ietf-cose-cbor-encoded-cert-03, 10 January 2022,
<https://www.ietf.org/archive/id/draft-ietf-cose-cbor-
encoded-cert-03.txt>.
[I-D.ietf-lake-reqs]
Vucinic, M., Selander, G., Mattsson, J. P., and D. Garcia-
Carrillo, "Requirements for a Lightweight AKE for OSCORE",
Work in Progress, Internet-Draft, draft-ietf-lake-reqs-04,
8 June 2020, <https://www.ietf.org/archive/id/draft-ietf-
lake-reqs-04.txt>.
[I-D.ietf-lake-traces]
Selander, G. and J. P. Mattsson, "Traces of EDHOC", Work
in Progress, Internet-Draft, draft-ietf-lake-traces-00, 25
November 2021, <https://www.ietf.org/archive/id/draft-
ietf-lake-traces-00.txt>.
[I-D.ietf-lwig-curve-representations]
Struik, R., "Alternative Elliptic Curve Representations",
Work in Progress, Internet-Draft, draft-ietf-lwig-curve-
representations-23, 21 January 2022,
<https://www.ietf.org/archive/id/draft-ietf-lwig-curve-
representations-23.txt>.
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[I-D.ietf-lwig-security-protocol-comparison]
Mattsson, J. P., Palombini, F., and M. Vucinic,
"Comparison of CoAP Security Protocols", Work in Progress,
Internet-Draft, draft-ietf-lwig-security-protocol-
comparison-05, 2 November 2020,
<https://www.ietf.org/archive/id/draft-ietf-lwig-security-
protocol-comparison-05.txt>.
[I-D.ietf-rats-eat]
Lundblade, L., Mandyam, G., and J. O'Donoghue, "The Entity
Attestation Token (EAT)", Work in Progress, Internet-
Draft, draft-ietf-rats-eat-12, 24 February 2022,
<https://www.ietf.org/archive/id/draft-ietf-rats-eat-
12.txt>.
[I-D.mattsson-cfrg-det-sigs-with-noise]
Mattsson, J. P., Thormarker, E., and S. Ruohomaa,
"Deterministic ECDSA and EdDSA Signatures with Additional
Randomness", Work in Progress, Internet-Draft, draft-
mattsson-cfrg-det-sigs-with-noise-04, 15 February 2022,
<https://www.ietf.org/archive/id/draft-mattsson-cfrg-det-
sigs-with-noise-04.txt>.
[I-D.selander-ace-ake-authz]
Selander, G., Mattsson, J. P., Vučinić, M., Richardson,
M., and A. Schellenbaum, "Lightweight Authorization for
Authenticated Key Exchange.", Work in Progress, Internet-
Draft, draft-selander-ace-ake-authz-05, 18 April 2022,
<https://www.ietf.org/archive/id/draft-selander-ace-ake-
authz-05.txt>.
[Norrman20]
Norrman, K., Sundararajan, V., and A. Bruni, "Formal
Analysis of EDHOC Key Establishment for Constrained IoT
Devices", September 2020,
<https://arxiv.org/abs/2007.11427>.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
DOI 10.17487/RFC2986, November 2000,
<https://www.rfc-editor.org/info/rfc2986>.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<https://www.rfc-editor.org/info/rfc6194>.
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[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC8366] Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
"A Voucher Artifact for Bootstrapping Protocols",
RFC 8366, DOI 10.17487/RFC8366, May 2018,
<https://www.rfc-editor.org/info/rfc8366>.
[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>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
[RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
P. van der Stok, "Constrained RESTful Environments (CoRE)
Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
2022, <https://www.rfc-editor.org/info/rfc9176>.
[SECG] "Standards for Efficient Cryptography 1 (SEC 1)", May
2009, <https://www.secg.org/sec1-v2.pdf>.
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[SIGMA] Krawczyk, H., "SIGMA - The 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and Its Use in the IKE-
Protocols (Long version)", June 2003,
<https://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>.
[Thormarker21]
Thormarker, E., "On using the same key pair for Ed25519
and an X25519 based KEM", April 2021,
<https://eprint.iacr.org/2021/509.pdf>.
Appendix A. Use with OSCORE and Transfer over CoAP
This appendix describes how to derive an OSCORE security context when
OSCORE is used with EDHOC, and how to transfer EDHOC messages over
CoAP.
A.1. Deriving the OSCORE Security Context
This section specifies how to use EDHOC output to derive the OSCORE
security context.
After successful processing of EDHOC message_3, Client and Server
derive Security Context parameters for OSCORE as follows (see
Section 3.2 of [RFC8613]):
* The Master Secret and Master Salt are derived by using the EDHOC-
Exporter interface, see Section 4.2.1.
The EDHOC Exporter Labels for deriving the OSCORE Master Secret and
the OSCORE Master Salt, are the uints 0 and 1, respectively.
The context parameter is h'' (0x40), the empty CBOR byte string.
By default, oscore_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, oscore_salt_length has value 8. The
Initiator and Responder MAY agree out-of-band on a longer
oscore_key_length than the default and on a different
oscore_salt_length.
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Master Secret = EDHOC-Exporter( 0, h'', oscore_key_length )
Master Salt = EDHOC-Exporter( 1, h'', oscore_salt_length )
* The AEAD Algorithm is the application AEAD algorithm of the
selected cipher suite for the EDHOC session.
* The HKDF Algorithm is the one based on the application hash
algorithm of the selected cipher suite for the EDHOC session. For
example, if SHA-256 is the application hash algorithm of the
selected cipher suite, HKDF SHA-256 is used as HKDF Algorithm in
the OSCORE Security Context.
* In case the Client is Initiator and the Server is Responder, the
Client's OSCORE Sender ID and the Server's OSCORE Sender ID are
determined from the EDHOC connection identifiers C_R and C_I for
the EDHOC session, respectively, by applying the conversion in
Section 3.3.3. 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.2. Transferring EDHOC over CoAP
This section specifies one instance for how EDHOC can be transferred
as an exchange of CoAP [RFC7252] messages. CoAP provides a reliable
transport that can preserve packet ordering and handle message
duplication. CoAP can also perform fragmentation and protect against
denial-of-service attacks. The underlying CoAP transport should be
used in reliable mode, in particular when fragmentation is used, to
avoid, e.g., situations with hanging endpoints waiting for each
other.
By default, the CoAP client is the Initiator and the CoAP server is
the Responder, but the roles SHOULD be chosen to protect the most
sensitive identity, see Section 8. Client applications can use the
resource type "core.edhoc" to discover a server's EDHOC resource,
i.e., where to send a request for executing the EDHOC protocol, see
Section 9.10. According to this specification, EDHOC is transferred
in POST requests and 2.04 (Changed) responses to the Uri-Path:
"/.well-known/edhoc", see Section 9.7. An application may define its
own path that can be discovered, e.g., using a resource directory
[RFC9176].
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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 the response, in
the former case with response code 2.04 (Changed), in the latter with
response code as specified in Appendix A.2.1. 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 EDHOC message_4 is used, or in
case of an error message, it is sent from the server to the client in
the payload of the response, with response codes analogously to
message_2. In case of an error message in response to message_4, it
is sent analogously to errors in response to message_2.
In order for the server to correlate a message received from a client
to a message previously sent in the same EDHOC session over CoAP,
messages sent by the client are prepended with the CBOR serialization
of the connection identifier which the server has chosen. This
applies independently of if the CoAP server is Responder or
Initiator.
* For the default case when the server is Responder, message_3 is
sent from the client prepended with the identifier C_R. In this
case message_1 is also sent by the client, and to indicate that
this is a new EDHOC session it is prepended with a dummy
identifier, the CBOR simple value "true" (0xf5), since the server
has not selected C_R yet. See Figure 13.
* In the case when the server is Initiator, message_2 (and
message_4, if present) is sent from the client prepended with the
identifier C_I. See Figure 14.
The prepended identifiers are encoded in CBOR and thus self-
delimiting. The integer representation of identifiers described in
Section 3.3.2 is used, when applicable. They are sent in front of
the actual EDHOC message to keep track of messages in an EDHOC
session, and only the part of the body following the identifier is
used for EDHOC processing. In particular, the connection identifiers
within the EDHOC messages are not impacted by the prepended
identifiers.
The application/edhoc+cbor-seq media type does not apply to these
messages; their media type is application/cid-edhoc+cbor-seq.
An example of a successful EDHOC exchange using CoAP is shown in
Figure 13. In this case the CoAP Token enables correlation on the
Initiator side, and the prepended C_R enables correlation on the
Responder (server) side.
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Client Server
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/edhoc"
| | Content-Format: application/cid-edhoc+cbor-seq
| | Payload: true, EDHOC message_1
| |
|<---------+ Header: 2.04 Changed
| 2.04 | Content-Format: application/edhoc+cbor-seq
| | Payload: EDHOC message_2
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/edhoc"
| | Content-Format: application/cid-edhoc+cbor-seq
| | Payload: C_R, EDHOC message_3
| |
|<---------+ Header: 2.04 Changed
| 2.04 | Content-Format: application/edhoc+cbor-seq
| | Payload: EDHOC message_4
| |
Figure 13: Example of transferring EDHOC in CoAP when the
Initiator is CoAP client. The optional message_4 is included in
this example, without which that message needs no payload.
The exchange in Figure 13 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 14. 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+cbor-seq
| | Payload: EDHOC message_1
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/edhoc"
| | Content-Format: application/cid-edhoc+cbor-seq
| | Payload: C_I, EDHOC message_2
| |
|<---------+ Header: 2.04 Changed
| 2.04 | Content-Format: application/edhoc+cbor-seq
| | Payload: EDHOC message_3
| |
Figure 14: Example of transferring EDHOC in CoAP when the
Initiator is CoAP server.
To protect against denial-of-service attacks, the CoAP server MAY
respond to the first POST request with a 4.01 (Unauthorized)
containing an Echo option [RFC9175]. This forces the Initiator to
demonstrate its reachability at its apparent network address. If
message fragmentation is needed, the EDHOC messages may be fragmented
using the CoAP Block-Wise Transfer mechanism [RFC7959].
EDHOC does not restrict how error messages are transported with CoAP,
as long as the appropriate error message can to be transported in
response to a message that failed (see Section 6). EDHOC error
messages transported with CoAP are carried in the payload.
A.2.1. Transferring EDHOC and OSCORE over CoAP
When using EDHOC over CoAP for establishing an OSCORE Security
Context, EDHOC error messages sent as CoAP responses MUST be sent in
the payload of error responses, i.e., they MUST specify a CoAP error
response code. In particular, it is RECOMMENDED that such error
responses have response code either 4.00 (Bad Request) in case of
client error (e.g., due to a malformed EDHOC message), or 5.00
(Internal Server Error) in case of server error (e.g., due to failure
in deriving EDHOC keying material). The Content-Format of the error
response MUST be set to application/edhoc+cbor-seq, see Section 9.9.
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A method for combining EDHOC and OSCORE protocols in two round-trips
is specified in [I-D.ietf-core-oscore-edhoc]. That specification
also contains conversion from OSCORE Sender/Recipient IDs to EDHOC
connection identifiers, web-linking and target attributes for
discovering of EDHOC resources.
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]. In EDHOC, compact representation is used
for the ephemeral public keys (G_X and G_Y), see Section 3.7. 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 compatibility with APIs
the value ~yp SHALL be set to zero. For such use, the compact
representation can be transformed into the SECG point compressed
format by prepending it with the single byte 0x02 (i.e., M = 0x02 ||
X).
Using compact representation have some security benefits. An
implementation does not need to check that the point is not the point
at infinity (the identity element). Similarly, as not even the sign
of the y-coordinate is encoded, compact representation trivially
avoids so called "benign malleability" attacks where an attacker
changes the sign, see [SECG].
Appendix C. Use of CBOR, CDDL and COSE in EDHOC
This Appendix is intended to simplify for implementors not familiar
with CBOR [RFC8949], CDDL [RFC8610], COSE
[I-D.ietf-cose-rfc8152bis-struct], and HKDF [RFC5869].
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C.1. CBOR and CDDL
The Concise Binary Object Representation (CBOR) [RFC8949] is a data
format designed for small code size and small message size. CBOR
builds on the JSON data model but extends it by e.g., encoding binary
data directly without base64 conversion. In addition to the binary
CBOR encoding, CBOR also has a diagnostic notation that is readable
and editable by humans. The Concise Data Definition Language (CDDL)
[RFC8610] provides a way to express structures for protocol messages
and APIs that use CBOR. [RFC8610] also extends the diagnostic
notation.
CBOR data items are encoded to or decoded from byte strings using a
type-length-value encoding scheme, where the three highest order bits
of the initial byte contain information about the major type. CBOR
supports several different types of data items, in addition to
integers (int, uint), simple values, byte strings (bstr), and text
strings (tstr), CBOR also supports arrays [] of data items, maps {}
of pairs of data items, and sequences [RFC8742] of data items. Some
examples are given below.
The EDHOC specification sometimes use CDDL names in CBOR diagnostic
notation as in e.g., << ID_CRED_R, ? EAD_2 >>. This means that EAD_2
is optional and that ID_CRED_R and EAD_2 should be substituted with
their values before evaluation. I.e., if ID_CRED_R = { 4 : h'' } and
EAD_2 is omitted then << ID_CRED_R, ? EAD_2 >> = << { 4 : h'' } >>,
which encodes to 0x43a10440.
For a complete specification and more examples, see [RFC8949] and
[RFC8610]. We recommend implementors to get used to CBOR by using
the CBOR playground [CborMe].
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Diagnostic Encoded Type
------------------------------------------------------------------
1 0x01 unsigned integer
24 0x1818 unsigned integer
-24 0x37 negative integer
-25 0x3818 negative integer
true 0xf5 simple value
h'' 0x40 byte string
h'12cd' 0x4212cd byte string
'12cd' 0x4431326364 byte string
"12cd" 0x6431326364 text string
{ 4 : h'cd' } 0xa10441cd map
<< 1, 2, true >> 0x430102f5 byte string
[ 1, 2, true ] 0x830102f5 array
( 1, 2, true ) 0x0102f5 sequence
1, 2, true 0x0102f5 sequence
------------------------------------------------------------------
C.2. CDDL Definitions
This sections compiles the CDDL definitions for ease of reference.
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suites = [ 2* int ] / int
ead = 1* (
ead_label : int,
ead_value : bstr,
)
message_1 = (
METHOD : int,
SUITES_I : suites,
G_X : bstr,
C_I : bstr / -24..23,
? EAD_1 : ead,
)
message_2 = (
G_Y_CIPHERTEXT_2 : bstr,
C_R : bstr / -24..23,
)
message_3 = (
CIPHERTEXT_3 : bstr,
)
message_4 = (
CIPHERTEXT_4 : bstr,
)
error = (
ERR_CODE : int,
ERR_INFO : any,
)
info = (
label : tstr,
context : bstr,
length : uint,
)
C.3. COSE
CBOR Object Signing and Encryption (COSE)
[I-D.ietf-cose-rfc8152bis-struct] describes how to create and process
signatures, message authentication codes, and encryption using CBOR.
COSE builds on JOSE, but is adapted to allow more efficient
processing in constrained devices. EDHOC makes use of COSE_Key,
COSE_Encrypt0, and COSE_Sign1 objects in the message processing:
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* ECDH ephemeral public keys of type EC2 or OKP in message_1 and
message_2 consist of the COSE_Key parameter named 'x', see
Section 7.1 and 7.2 of [I-D.ietf-cose-rfc8152bis-algs]
* The ciphertexts in message_3 and message_4 consist of a subset of
the single recipient encrypted data object COSE_Encrypt0, which is
described in Sections 5.2-5.3 of
[I-D.ietf-cose-rfc8152bis-struct]. The ciphertext is computed
over the plaintext and associated data, using an encryption key
and an initialization vector. The associated data is an
Enc_structure consisting of protected headers and externally
supplied data (external_aad). COSE constructs the input to the
AEAD [RFC5116] for message_i (i = 3 or 4, see Section 5.4 and
Section 5.5, respectively) as follows:
- Secret key K = K_i
- Nonce N = IV_i
- Plaintext P for message_i
- Associated Data A = [ "Encrypt0", h'', TH_i ]
* Signatures in message_2 of method 0 and 2, and in message_3 of
method 0 and 1, consist of a subset of the single signer data
object COSE_Sign1, which is described in Sections 4.2-4.4 of
[I-D.ietf-cose-rfc8152bis-struct]. The signature is computed over
a Sig_structure containing payload, protected headers and
externally supplied data (external_aad) using a private signature
key and verified using the corresponding public signature key.
For COSE_Sign1, the message to be signed is:
[ "Signature1", protected, external_aad, payload ]
where protected, external_aad and payload are specified in
Section 5.3 and Section 5.4.
Different header parameters to identify X.509 or C509 certificates by
reference are defined in [I-D.ietf-cose-x509] and
[I-D.ietf-cose-cbor-encoded-cert]:
* by a hash value with the 'x5t' or 'c5t' parameters, respectively:
- ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R,
- ID_CRED_x = { TBD3 : COSE_CertHash }, for x = I or R;
* or by a URI with the 'x5u' or 'c5u' parameters, respectively:
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- ID_CRED_x = { 35 : uri }, for x = I or R,
- ID_CRED_x = { TBD4 : uri }, for x = I or R.
When ID_CRED_x does not contain the actual credential, it may be very
short, e.g., if the endpoints have agreed to use a key identifier
parameter 'kid':
* ID_CRED_x = { 4 : key_id_x }, where key_id_x : kid, for x = I or
R.
Note that a COSE header map can contain several header parameters,
for example { x5u, x5t } or { kid, kid_context }.
ID_CRED_x MAY also identify the credential by value. For example, a
certificate chain can be transported in ID_CRED_x with COSE header
parameter c5c or x5chain, defined in
[I-D.ietf-cose-cbor-encoded-cert] and [I-D.ietf-cose-x509] and
credentials of type CWT and CCS can be transported with the COSE
header parameters registered in Section 9.6.
Appendix D. Authentication Related Verifications
EDHOC performs certain authentication related operations, see
Section 3.5, but in general it is necessary to make additional
verifications beyond EDHOC message processing. What verifications
are needed depend on the deployment, in particular the trust model
and the security policies, but most commonly it can be expressed in
terms of verifications of credential content.
EDHOC assumes the existence of mechanisms (certification authority or
other trusted third party, pre-provisioning, etc.) for generating and
distributing authentication credentials and other credentials, as
well as the existence of trust anchors (CA certificates, trusted
public keys, etc.). For example, a public key certificate or CWT may
rely on a trusted third party whose public key is pre-provisioned,
whereas a CCS or a self-signed certificate/CWT may be used when trust
in the public key can be achieved by other means, or in the case of
trust-on-first-use, see Appendix D.5.
In this section we provide some examples of such verifications.
These verifications are the responsibility of the application but may
be implemented as part of an EDHOC library.
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D.1. Validating the Authentication Credential
The authentication credential may contain, in addition to the
authentication key, other parameters that needs to be verified. For
example:
* In X.509 and C509 certificates, signature keys typically have key
usage "digitalSignature" and Diffie-Hellman public keys typically
have key usage "keyAgreement" [RFC3279][RFC8410].
* In X.509 and C509 certificates validity is expressed using Not
After and Not Before. In CWT and CCS, the "exp" and "nbf" claims
have similar meanings.
D.2. Identities
The application must decide on allowing a connection or not depending
on the intended endpoint, and in particular whether it is a specific
identity or a set of identities. To prevent misbinding attacks, the
identity of the endpoint is included in a MAC verified through the
protocol. More details and examples are provided in this section.
Policies for what connections to allow are typically set based on the
identity of the other endpoint, and endpoints typically only allow
connections from a specific identity or a small restricted set of
identities. For example, in the case of a device connecting to a
network, the network may only allow connections from devices which
authenticate with certificates having a particular range of serial
numbers and signed by a particular CA. Conversely, a device may only
be allowed to connect to a network which authenticates with a
particular public key.
* When a Public Key Infrastructure (PKI) is used with certificates,
the identity is the subject whose unique name, e.g., a domain
name, a Network Access Identifier (NAI), or an Extended Unique
Identifier (EUI), is included in the endpoint's certificate.
* Similarly, when a PKI is used with CWTs, the identity is the
subject identified by the relevant claim(s), such as 'sub'
(subject).
* When PKI is not used (e.g., CCS, self-signed certificate/CWT) the
identity is typically directly associated to the authentication
key of the other party. For example, if identities can be
expressed in the form of unique subject names assigned to public
keys, then a binding to identity is achieved by including both
public key and associated subject name in the authentication
credential: CRED_I or CRED_R may be a self-signed certificate/CWT
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or CCS containing the authentication key and the subject name, see
Section 3.5.2. Each endpoint thus needs to know the specific
authentication key/unique associated subject name, or set of
public authentication keys/unique associated subject names, which
it is allowed to communicate with.
To prevent misbinding attacks in systems where an attacker can
register public keys without proving knowledge of the private key,
SIGMA [SIGMA] enforces a MAC to be calculated over the "identity".
EDHOC follows SIGMA by calculating a MAC over the whole
authentication credential, which in case of an X.509 or C509
certificate includes the "subject" and "subjectAltName" fields, and
in the case of CWT or CCS includes the "sub" claim.
(While the SIGMA paper only focuses on the identity, the same
principle is true for other information such as policies associated
to the public key.)
D.3. Certification Path and Trust Anchors
When a Public Key Infrastructure (PKI) is used with certificates, the
trust anchor is a Certification Authority (CA) certificate. Each
party needs at least one CA public key certificate, or just the CA
public key. The certification path contains proof that the subject
of the certificate owns the public key in the certificate. Only
validated public-key certificates are to be accepted.
Similarly, when a PKI is used with CWTs, each party needs to have at
least one trusted third party public key as trust anchor to verify
the end entity CWTs. The trusted third party public key can, e.g.,
be stored in a self-signed CWT or in a CCS.
The signature of the authentication credential needs to be verified
with the public key of the issuer. X.509 and C509 certificates
includes the "Issuer" field. In CWT and CCS, the "iss" claim has a
similar meaning. The public key is either a trust anchor or the
public key in another valid and trusted credential in a certification
path from trust anchor to authentication credential.
Similar verifications as made with the authentication credential (see
Appendix D.1) are also needed for the other credentials in the
certification path.
When PKI is not used (CCS, self-signed certificate/CWT), the trust
anchor is the authentication key of the other party, in which case
there is no certification path.
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D.4. Revocation Status
The application may need to verify that the credentials are not
revoked, see Section 8.8. Some use cases may be served by short-
lived credentials, for example, where the validity of the credential
is on par with with the interval between revocation checks. But, in
general, credential life time and revokation checking are
complementary measures to control credential status. Revocation
information may be transported as External Authentication Data (EAD),
see Appendix E.
D.5. Trust-on-first-use
TBD
Appendix E. Use of External Authorization Data
In order to reduce the number of messages and round trips, or to
simplify processing, external security applications may be integrated
into EDHOC by transporting external authorization related data (EAD)
in the messages.
The EAD format is specified in Section 3.8, this section contains
examples and further details of how EAD may be used with an
appropriate accompanying specification.
* One example is third-party assisted authorization, requested with
EAD_1, and an authorization artifact ("voucher", cf. [RFC8366])
returned in EAD_2, see [I-D.selander-ace-ake-authz].
* Another example is remote attestation, requested in EAD_2, and an
Entity Attestation Token (EAT, [I-D.ietf-rats-eat]) returned in
EAD_3.
* A third example is certificate enrolment, where a Certificate
Signing Request (CSR, [RFC2986]) is included EAD_3, and the issued
public key certificate (X.509 [RFC5280], C509
[I-D.ietf-cose-cbor-encoded-cert]) or a reference thereof is
returned in EAD_4.
External authorization data should be considered unprotected by
EDHOC, and the protection of EAD is the responsibility of the
security application (third party authorization, remote attestation,
certificate enrolment, etc.). The security properties of the EAD
fields (after EDHOC processing) are discussed in Section 8.1.
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The content of the EAD field may be used in the EDHOC processing of
the message in which they are contained. For example, authentication
related information like assertions and revocation information,
transported in EAD fields may provide input about trust anchors or
validity of credentials relevant to the authentication processing.
The EAD fields (like ID_CRED fields) are therefore made available to
the application before the message is verified, see details of
message processing in Section 5. In the first example above, a
voucher in EAD_2 made available to the application can enable the
Initiator to verify the identity or public key of the Responder
before verifying the signature. An application allowing EAD fields
containing authentication information thus may need to handle
authentication related verifications associated with EAD processing.
Conversely, the security application may need to wait for EDHOC
message verification to complete. In the third example above, the
validation of a CSR carried in EAD_3 is not started by the Responder
before EDHOC has successfully verified message_3 and proven the
possession of the private key of the Initiator.
The security application may reuse EDHOC protocol fields which
therefore need to be available to the application. For example, the
security application may use the same crypto algorithms as in the
EDHOC session and therefore needs access to the selected cipher suite
(or the whole SUITES_I). The application may use the ephemeral
public keys G_X and G_Y, as ephemeral keys or as nonces, see
[I-D.selander-ace-ake-authz].
The processing of (ead_label, ead_value) by the security application
needs to be described in the specification where the ead_label is
registered, see Section 9.5, including the ead_value for each message
and actions in case of errors. An application may support multiple
security applications that make use of EAD, which may result in
multiple (ead_label, ead_value) pairs in one EAD field, see
Section 3.8. Any dependencies on security applications with
previously registered EAD fields needs to be documented, and the
processing needs to consider their simultaneous use.
Since data carried in EAD may not be protected, or be processed by
the application before the EDHOC message is verified, special
considerations need to be made such that it does not violate security
and privacy requirements of the service which uses this data, see
Section 8.5. The content in an EAD field may impact the security
properties provided by EDHOC. Security applications making use of
the EAD fields must perform the necessary security analysis.
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Appendix F. Application Profile Example
This appendix contains a rudimentary example of an application
profile, see Section 3.9.
For use of EDHOC with application X the following assumptions are
made:
1. Transfer in CoAP as specified in Appendix A.2 with requests
expected by the CoAP server (= Responder) at /app1-edh, no
Content-Format needed.
2. METHOD = 1 (I uses signature key, R uses static DH key.)
3. CRED_I is an IEEE 802.1AR IDevID encoded as a C509 certificate of
type 0 [I-D.ietf-cose-cbor-encoded-cert].
* R acquires CRED_I out-of-band, indicated in EAD_1.
* ID_CRED_I = {4: h''} is a 'kid' with value empty CBOR byte
string.
4. CRED_R is a CCS of type OKP as specified in Section 3.5.2.
* The CBOR map has parameters 1 (kty), -1 (crv), and -2
(x-coordinate).
* ID_CRED_R is {TBD2 : CCS}. Editor's note: TBD2 is the COSE
header parameter value of 'kccs', see Section 9.6
5. External authorization data is defined and processed as specified
in [I-D.selander-ace-ake-authz].
6. EUI-64 is used as the identity of the endpoint (see example in
Section 3.5.2).
7. No use of message_4: the application sends protected messages
from R to I.
Appendix G. 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 do 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 another instance of the next EDHOC message. The result MAY
be that a duplicate next EDHOC message is sent, provided it is still
relevant with respect to 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.
In principle, if the EDHOC implementation would deterministically
regenerate the identical EDHOC message previously sent, it would be
possible to instead store the protocol state to be able to recreate
and resend the previously sent EDHOC message. However, even if the
protocol state is fixed, the message generation may introduce
differences which compromises security. For example, in the
generation of message_3, if I is performing a (non-deterministic)
ECDSA signature (say, method 0 or 1, cipher suite 2 or 3) then
PLAINTEXT_3 is randomized, but K_3 and IV_3 are the same, leading to
a key and nonce reuse.
The EDHOC implementation MUST NOT store previous protocol state and
regenerate an EDHOC message if there is a risk that the same key and
IV are used for two (or more) distinct messages.
The previous message or protocol state MUST NOT be kept longer than
what is required for retransmission, for example, in the case of CoAP
transport, no longer than the EXCHANGE_LIFETIME (see Section 4.8.2 of
[RFC7252]).
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Appendix H. 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.2) can serve as a blueprint for
other server-client protocols: The client prepends the C_x which the
server selected (or, for message_1, the CBOR simple value 'true'
which is not a valid C_x) to any request message it sends. The
server does not send any such indicator, as responses are matched to
request by the client-server protocol design.
Protocols that do not provide any correlation at all can prescribe
prepending of the peer's chosen C_x to all messages.
Appendix I. Change Log
RFC Editor: Please remove this appendix.
* From -13 to -14
- Merge of section 1.1 and 1.2
- Connection and key identifiers restricted to be byte strings
- Representation of byte strings as one-byte CBOR ints (-24..23)
- Simplified mapping between EDHOC and OSCORE identifiers
- Rewrite of 3.5
o Clarification of authentication related operations performed
by EDHOC
o Authentication related verifications, including old section
3.5.1, moved to new appendix D
- Rewrite of 3.8
o Move content about use of EAD to new appendix E
o ead_value changed to bstr
- EDHOC-KDF updated
o transcript_hash argument removed
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o TH included in context argument
o label argument is now type uint, all labels replaced
- Key schedule updated
o New salts derived to avoid reuse of same key with expand and
extract
o PRK_4x3m renamed PRK_4e3m
o K_4 and IV_4 derived from PRK_4e3m
o New PRK: PRK_out derived from PRK_4e3m and TH_4
o Clarified main output of EDHOC is the shared secret PRK_out
o Exporter defined by EDHOC-KDF and new PRK PRK_exporter
derived from PRK_out
o Key update defined by Expand instead of Extract
- All applications of EDHOC-KDF in one place
- Update of processing
o EAD and ID_CRED passed to application when available
o identity verification and credential retrieval omitted in
protocol description
o Transcript hash defined by plaintext messages instead of
ciphertext
o Changed order of input to TH_2
o Removed general G_X checking against selfie-attacks
- Support for padding of plaintext
- Updated compliance requirements
- Updated security considerations
o Updated and more clear requirements on MAC length
o Clarification of key confirmation
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o Forbid use of same key for signature and static DH
- Updated appendix on message deduplication
- Clarifications of
o connection identifiers
o cipher suites, including negotiation
o EAD
o Error messages
- Updated media types
- Applicability template renamed application profile
- Editorials
* From -12 to -13
- no changes
* From -12:
- Shortened labels to derive OSCORE key and salt
- ead_value changed to bstr
- Removed general G_X checking against selfie-attacks
- Updated and more clear requirements on MAC length
- Clarifications from Kathleen, Stephen, Marco, Sean, Stefan,
- Authentication Related Verifications moved to appendix
- Updated MTI section and cipher suite
- Updated security considerations
* From -11 to -12:
- Clarified applicability to KEMs
- Clarified use of COSE header parameters
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- Updates on MTI
- Updated security considerations
- New section on PQC
- Removed duplicate definition of cipher suites
- Explanations of use of COSE moved to Appendix C.3
- Updated internal references
* From -10 to -11:
- Restructured section on authentication parameters
- Changed UCCS to CCS
- Changed names and description of COSE header parameters for
CWT/CCS
- Changed several of the KDF and Exporter labels
- Removed edhoc_aead_id from info (already in transcript_hash)
- Added MTI section
- EAD: changed CDDL names and added value type to registry
- Updated Figures 1, 2, and 3
- Some correction and clarifications
- Added core.edhoc to CoRE Resource Type registry
* From -09 to -10:
- SUITES_I simplified to only contain the selected and more
preferred suites
- Info is a CBOR sequence and context is a bstr
- Added kid to UCCS example
- Separate header parameters for CWT and UCCS
- CWT Confirmation Method kid extended to bstr / int
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* From -08 to -09:
- G_Y and CIPHERTEXT_2 are now included in one CBOR bstr
- MAC_2 and MAC_3 are now generated with EDHOC-KDF
- Info field "context" is now general and explicit in EDHOC-KDF
- Restructured Section 4, Key Derivation
- Added EDHOC MAC length to cipher suite for use with static DH
- More details on the use of CWT and UCCS
- Restructured and clarified Section 3.5, Authentication
Parameters
- Replaced 'kid2' with extension of 'kid'
- EAD encoding now supports multiple ead types in one message
- Clarified EAD type
- Updated message sizes
- Replaced "perfect forward secrecy" with "forward secrecy"
- Updated security considerations
- Replaced prepended 'null' with 'true' in the CoAP transport of
message_1
- Updated CDDL definitions
- Expanded on the use of COSE
* From -07 to -08:
- Prepended C_x moved from the EDHOC protocol itself to the
transport mapping
- METHOD_CORR renamed to METHOD, corr removed
- Removed bstr_identifier and use bstr / int instead; C_x can now
be int without any implied bstr semantics
- Defined COSE header parameter 'kid2' with value type bstr / int
for use with ID_CRED_x
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- 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
* From -06 to -07:
- Changed transcript hash definition for TH_2 and TH_3
- Removed "EDHOC signature algorithm curve" from cipher suite
- New IANA registry "EDHOC Exporter Label"
- New application defined parameter "context" in EDHOC-Exporter
- Changed normative language for failure from MUST to SHOULD send
error
- Made error codes non-negative and 0 for success
- Added detail on success error code
- Aligned terminology "protocol instance" -> "session"
- New appendix on compact EC point representation
- Added detail on use of ephemeral public keys
- Moved key derivation for OSCORE to draft-ietf-core-oscore-edhoc
- Additional security considerations
- Renamed "Auxililary Data" as "External Authorization Data"
- Added encrypted EAD_4 to message_4
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* 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"
- Clarifications based on review comments
* From -04 to -05:
- EDHOC-Rekey-FS -> EDHOC-KeyUpdate
- Clarification of cipher suite negotiation
- Updated security considerations
- Updated test vectors
- Updated applicability statement template
* From -03 to -04:
- Restructure of section 1
- Added references to C509 Certificates
- Change in CIPHERTEXT_2 -> plaintext XOR KEYSTREAM_2 (test
vector not updated)
- "K_2e", "IV_2e" -> KEYSTREAM_2
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- Specified optional message 4
- EDHOC-Exporter-FS -> EDHOC-Rekey-FS
- Less constrained devices SHOULD implement both suite 0 and 2
- Clarification of error message
- Added exporter interface test vector
* From -02 to -03:
- Rearrangements of section 3 and beginning of section 4
- Key derivation new section 4
- Cipher suites 4 and 5 added
- EDHOC-EXPORTER-FS - generate a new PRK_4x3m from an old one
- Change in CIPHERTEXT_2 -> COSE_Encrypt0 without tag (no change
to test vector)
- Clarification of error message
- New appendix C applicability statement
* From -01 to -02:
- New section 1.2 Use of EDHOC
- Clarification of identities
- New section 4.3 clarifying bstr_identifier
- Updated security considerations
- Updated text on cipher suite negotiation and key confirmation
- Test vector for static DH
* From -00 to -01:
- Removed PSK method
- Removed references to certificate by value
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Acknowledgments
The authors want to thank Christian Amsuess, Alessandro Bruni,
Karthikeyan Bhargavan, Carsten Bormann, Timothy Claeys, Martin Disch,
Stephen Farrell, Loic Ferreira, Theis Groenbech Petersen, Felix
Guenther, Dan Harkins, Klaus Hartke, Russ Housley, Stefan Hristozov,
Marc Ilunga, Charlie Jacomme, Elise Klein, Steve Kremer, Alexandros
Krontiris, Ilari Liusvaara, Kathleen Moriarty, David Navarro, Karl
Norrman, Salvador Perez, Maiwenn Racouchot, 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, Sean Turner, Michel Veillette, and Malisa Vučinić
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
Göran Selander
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: goran.selander@ericsson.com
John Preuß Mattsson
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: john.mattsson@ericsson.com
Francesca Palombini
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
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