ACE Working Group G. Selander
Internet-Draft J. Mattsson
Intended status: Standards Track F. Palombini
Expires: October 27, 2017 Ericsson AB
April 25, 2017
Ephemeral Diffie-Hellman Over COSE (EDHOC)
draft-selander-ace-cose-ecdhe-06
Abstract
This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
compact, and lightweight authenticated Diffie-Hellman key exchange
with ephemeral keys that can be used over any layer. EDHOC messages
are encoded with CBOR and COSE, allowing reuse of existing libraries.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. EDHOC Overview . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Formatting of the Ephemeral Public Keys . . . . . . . . . 6
3.2. Key Derivation . . . . . . . . . . . . . . . . . . . . . 6
4. EDHOC Authenticated with Asymmetric Keys . . . . . . . . . . 8
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . . 8
4.3. EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . . 11
4.4. EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . . 13
5. EDHOC Authenticated with Symmetric Keys . . . . . . . . . . . 15
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2. EDHOC Message 1 . . . . . . . . . . . . . . . . . . . . . 16
5.3. EDHOC Message 2 . . . . . . . . . . . . . . . . . . . . . 18
5.4. EDHOC Message 3 . . . . . . . . . . . . . . . . . . . . . 19
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 21
6.1. Error Message Format . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
7.1. Media Types Registry . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . 24
10.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Test Vectors . . . . . . . . . . . . . . . . . . . . 26
Appendix B. PSK Chaining . . . . . . . . . . . . . . . . . . . . 26
Appendix C. EDHOC with CoAP and OSCOAP . . . . . . . . . . . . . 26
C.1. Transferring EDHOC in CoAP . . . . . . . . . . . . . . . 26
C.2. Deriving an OSCOAP context from EDHOC . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
Security at the application layer provides an attractive option for
protecting Internet of Things (IoT) deployments, for example where
transport layer security is not sufficient
[I-D.hartke-core-e2e-security-reqs] or where the protocol needs to
work on a variety of underlying protocols. IoT devices may be
constrained in various ways, including memory, storage, processing
capacity, and energy [RFC7228]. A method for protecting individual
messages at the application layer suitable for constrained devices,
is provided by CBOR Object Signing and Encryption (COSE)
[I-D.ietf-cose-msg]), which builds on the Concise Binary Object
Representation (CBOR) [RFC7049].
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In order for a communication session to provide forward secrecy, the
communicating parties can run an Elliptic Curve Diffie-Hellman (ECDH)
key exchange protocol with ephemeral keys, from which shared key
material can be derived. This document specifies Ephemeral Diffie-
Hellman Over COSE (EDHOC), an authenticated ECDH protocol using CBOR
and COSE objects. Authentication is based on credentials established
out of band, e.g. from a trusted third party, such as an
Authorization Server as specified by [I-D.ietf-ace-oauth-authz].
EDHOC supports authentication using pre-shared keys (PSK), raw public
keys (RPK), and certificates (Cert). Note that this document focuses
on authentication and key establishment: for integration with
authorization of resource access, refer to
[I-D.seitz-ace-oscoap-profile]. This document also specifies the
derivation of shared key material.
The ECDH exchange and the key derivation follow [SIGMA], NIST SP-
800-56a [SP-800-56a], and HKDF [RFC5869]. CBOR [RFC7049] and COSE
[I-D.ietf-cose-msg] are used to implement these standards.
1.1. Terminology
This document use the same informational CBOR Data Definition
Language (CDDL) [I-D.greevenbosch-appsawg-cbor-cddl] grammar as COSE
(see Section 1.3 of [I-D.ietf-cose-msg]). A vertical bar | denotes
byte string concatenation.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. These
words may also appear in this document in lowercase, absent their
normative meanings.
2. Protocol Overview
SIGMA (SIGn-and-MAc) is a family of theoretical protocols with a
large number of variants [SIGMA]. Like IKEv2 and TLS 1.3, EDHOC is
built on a variant of the SIGMA protocol which provide identity
protection, and like TLS 1.3, EDHOC implements the SIGMA-I variant as
Sign-then-MAC. The SIGMA-I protocol using an AEAD algorithm is shown
in Figure 1.
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Party U Party V
| E_U |
+------------------------------------------------------>|
| |
| E_V, Enc(K_2; ID_V, Sig(V; E_U, E_V);) |
|<------------------------------------------------------+
| |
| Enc(K_3; ID_U, Sig(U; E_V, E_U);) |
+------------------------------------------------------>|
| |
Figure 1: AEAD variant of the SIGMA-I protocol
The parties exchanging messages are called "U" and "V". They
exchange identities and ephemeral public keys, compute the shared
secret, and derive the keying material. The messages are signed,
MACed, and encrypted.
o E_U and E_V are the ECDH ephemeral public keys of U and V,
respectively.
o ID_U and ID_V are identifiers for the public keys of U and V,
respectively.
o Sig(U; . ) and S(V; . ) denote signatures made with the private
key of U and V, respectively.
o Enc(K; P; A) denotes AEAD encryption of plaintext P and additional
authenticated data A using the key K derived from the shared
secret. The AEAD MUST NOT be replaced by plain encryption, see
Section 8.
As described in Appendix B of [SIGMA], in order to create a "full-
fledge" protocol some additional protocol elements are needed. EDHOC
adds:
o Explicit session identifiers S_U, S_V chosen by U and V,
respectively.
o Explicit nonces N_U, N_V chosen freshly and anew with each session
by U and V, respectively.
o Computationally independent keys derived from the ECDH shared
secret and used for encryption of different messages.
EDHOC also makes the following additions:
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o Negotiation of key derivation, encryption, and signature
algorithms:
* U proposes one or more algorithms of the following kinds:
+ HKDF
+ AEAD
+ Signature verification
+ Signature generation
* V selects one algorithm of each kind
o Verification of common preferred ECDH curve:
* U lists supported ECDH curves in order of preference
* V verifies that the ECDH curve of the ephemeral key is the most
preferred common curve
o Transport of opaque application defined data.
EDHOC is designed to encrypt and integrity protect as much
information as possible, and all symmetric keys are derived using as
much previous information as possible. EDHOC is furthermore designed
to be as compact and lightweight as possible, in terms of message
sizes, processing, and the ability to reuse already existing CBOR and
COSE libraries. EDHOC does not put any requirement on the lower
layers and can therefore be also be used e.g. in environments without
IP.
This paper is organized as follows: Section 3 specifies general
properties of EDHOC, including formatting of the ephemeral public
keys and key derivation, Section 4 specifies EDHOC with asymmetric
key authentication, Section 5 specifies EDHOC with symmetric key
authentication, and Appendix A provides a wealth of test vectors to
ease implementation and ensure interoperability.
3. EDHOC Overview
EDHOC consists of three messages (message_1, message_2, message_3)
that maps directly to the three messages in SIGMA-I, plus an EDHOC
error message. All EDHOC messages consists of a CBOR array where the
first element is an int specifying the message type (MSG_TYPE).
After creating EDHOC message_3, Party U can derive the traffic key
(master secret) and protected application data can therefore be sent
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in parallel with EDHOC message_3. The application data may e.g. be
protected using the negotiated AEAD algorithm. EDHOC may be used
with the media type application/edhoc defined in Section 7.
Party U Party V
| |
| ------------------ EDHOC message_1 -----------------> |
| |
| <----------------- EDHOC message_2 ------------------ |
| |
| ----------------- EDHOC message_3 ------------------> |
| |
| <----------- Protected Application Data ------------> |
| |
Figure 2: EDHOC message flow
The EDHOC message exchange may be authenticated using pre-shared keys
(PSK), raw public keys (RPK), or certificates (Cert). EDHOC assumes
the existence of mechanisms (certification authority, manual
distribution, etc.) for binding identities with authentication keys
(public or pre-shared). EDHOC with symmetric key authentication is
very similar to EDHOC with asymmetric key authentication, the
difference being that information is only MACed, not signed.
EDHOC also allows application data (APP_1, APP_2, APP_3) to be sent
in the respective messages. APP_1 is unprotected, APP_2 is protected
(encrypted and integrity protected), and APP_3 is protected and
mutually authenticated. When EDHOC is used with asymmetric key
authentication APP_2 is sent to an unauthenticated party, but with
symmetric key authentication APP_2 is mutually authenticated.
3.1. Formatting of the Ephemeral Public Keys
The ECDH ephemeral public key SHALL be formatted as a COSE_Key of
type EC2 or OKP according to section 13.1 and 13.2 of
[I-D.ietf-cose-msg]. The curve X25519 is mandatory to implement.
For Elliptic Curve Keys of type EC2, point compression is mandatory
to implement.
3.2. Key Derivation
Key and IV derivation SHALL be done as specified in Section 11.1 of
[I-D.ietf-cose-msg] with the following input:
o The PRF SHALL be the HKDF [RFC5869] in the ECDH-SS w/ HKDF
negotiated during the message exchange (HKDF_V).
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o The secret SHALL be the ECDH shared secret as defined in
Section 12.4.1 of [I-D.ietf-cose-msg].
o salt = PSK / nil
o The context information SHALL be the serialized COSE_KDF_Context
with the following values:
* AlgorithmID = tstr / int
* PartyInfo = ( nil, nil, nil )
* SuppPubInfo SHALL contain:
+ keyDataLength int
+ protected SHALL be a zero length bstr
+ other = aad_2 / aad_3 / exchange_hash
exchange_hash = bstr
where exchange_hash, in diagnostic non-normative notation, is:
exchange_hash = H( H( message_1 | message_2 ) | message_3 )
where H() is the hash function in HKDF_V, and | denotes byte string
concatenation.
The salt SHALL only be present in the symmetric case.
Symmetric keys and IVs SHALL be derived with the negotiated PRF
(HKDF_V) and with the secret set to the ECDH shared secret.
For message_i the key and IV, called K_i and IV_i, SHALL be derived
using other = aad_i, where i = 2 or 3. The key SHALL be derived
using AlgorithmID set to the negotiated AEAD (AEAD_V), and
keyDataLength equal to the key length of AEAD_V. The IV SHALL be
derived using AlgorithmID = "IV-GENERATION" as specified in section
12.1.2. of [I-D.ietf-cose-msg], and keyDataLength equal to the IV
length of AEAD_V.
Application specific traffic keys and other data SHALL be derived
using other = exchange_hash. AlgorithmID is defined by the
application and SHALL be different for different data being derived
(an example is given in Appendix C.2). keyDataLength is set to the
length of the data being derived.
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4. EDHOC Authenticated with Asymmetric Keys
4.1. Overview
EDHOC supports authentication with raw public keys (RPK) and
certificates (Cert) with the requirements that:
o Party U's SHALL be able to identify Party V's public key using
ID_V.
o Party V's SHALL be able to identify Party U's public key using
ID_U.
ID_U and ID_V either enable the other party to retrieve the public
key (kid, x5t, x5u) or they contain the public key (x5c), see
[I-D.schaad-cose-x509]. Party U and party V MAY use different type
of credentials, e.g. one uses RPK and the other Cert. Party U and
party V MAY use different signature algorithms.
EDHOC with asymmetric key authentication is illustrated in Figure 3.
Party U Party V
| S_U, N_U, E_U, ALG_1, APP_1 |
+--------------------------------------------------------------------->|
| message_1 |
| |
|S_U, S_V, N_V, E_V, ALG_2, Enc(K_2; APP_2, ID_V, Sig(V; aad_2); aad_2)|
|<---------------------------------------------------------------------+
| message_2 |
| |
| S_V, Enc(K_3; APP_3, ID_U, Sig(U; aad_3); aad_3) |
+--------------------------------------------------------------------->|
| message_3 |
Figure 3: EDHOC with asymmetric key authentication.
4.1.1. Mandatory to Implement Algorithms
For EDHOC authenticated with asymmetric keys, the COSE algorithms
ECDH-SS + HKDF-256, AES-CCM-64-64-128, and EdDSA are mandatory to
implement.
4.2. EDHOC Message 1
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4.2.1. Formatting of Message 1
message_1 SHALL be a CBOR array as defined below
message_1 = [
MSG_TYPE : int,
S_U : bstr,
N_U : bstr,
E_U : serialized_COSE_Key,
ECDH-Curves_U : alg_array,
HKDFs_U : alg_array,
AEADs_U : alg_array,
SIGs_V : alg_array,
SIGs_U : alg_array,
? APP_1 : bstr
]
serialized_COSE_Key = bstr .cbor COSE_Key
alg_array = [ + alg : int / tstr ]
where:
o MSG_TYPE = 1
o S_U - variable length session identifier
o N_U - 64-bit random nonce
o E_U - the ephemeral public key of Party U
o ECDH-Curves_U - EC curves for ECDH which Party U supports, in the
order of decreasing preference
o HKDFs_U - supported ECDH-SS w/ HKDF algorithms
o AEADs_U - supported AEAD algorithms
o SIGs_V - signature algorithms, with which Party U supports
verification
o SIGs_U - signature algorithms, with which Party U supports signing
o APP_1 - bstr containing application data
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4.2.2. Party U Processing of Message 1
Party U SHALL compose message_1 as follows:
o Determine which ECDH curve to use with Party V. If U previously
received from Party V an error message to message_1 with
diagnostic payload identifying an ECDH curve in ECDH-Curves_U,
then U SHALL retrieve an ephemeral from that curve. Otherwise the
first curve in ECDH-Curves_U MUST be used.
o Retrieve an ephemeral ECDH key pair generated as specified in
Section 5 of [SP-800-56a] and format the ephemeral public key E_U
as a COSE_key as specified in Section 3.1.
o Generate the pseudo-random nonce N_U
o Choose a session identifier S_U and store it for the length of the
protocol.
o Format message_1 as specified in Section 4.2.1.
4.2.3. Party V Processing of Message 1
Party V SHALL process message_1 as follows:
o Verify (OPTIONAL) that N_U has not been received before.
o Verify that at least one of each kind of the proposed algorithms
are supported.
o Verify that the ECDH curve used in E_U is supported, and that no
prior curve in ECDH-Curves_U is supported
If any verification step fails, Party V MUST send an EDHOC error
message back, formatted as defined in Section 6.1, and the protocol
MUST be discontinued. If V does not support the ECDH curve used in
E_U, but supports another ECDH curves in ECDH-Curves_U, then the
error message MUST include the following diagnostic payload
describing the first supported ECDH curve in ECDH-Curves_U:
ERR_MSG = "Curve not supported; X"
where X is the first curve in ECDH-Curves_U that V supports,
encoded as in Table 22 of {{I-D.ietf-cose-msg}}.
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4.3. EDHOC Message 2
4.3.1. Formatting of Message 2
message_2 SHALL be a CBOR array as defined below
message_2 = [
data_2,
COSE_ENC_2 : COSE_Encrypt0
]
data_2 = (
MSG_TYPE : int,
S_U : bstr,
S_V : bstr,
N_V : bstr,
E_V : serialized_COSE_Key,
HKDF_V : int / tstr,
AEAD_V : int / tstr,
SIG_V : int / tstr,
SIG_U : int / tstr
)
aad_2 = bstr
where aad_2, in diagnostic non-normative notation, is:
aad_2 = message_1 | [ data_2 ] | ? Cert_V
where:
o MSG_TYPE = 2
o S_V - variable length session identifier
o N_V - 64-bit random nonce
o E_V - the ephemeral public key of Party V
o HKDF_V - a single chosen algorithm from HKDFs_U
o AEAD_V - a single chosen algorithm from AEADs_U
o SIG_V - a single chosen algorithm from SIGs_V with which Party V
signs
o SIG_U - a single chosen algorithm from SIGs_U with which Party U
signs
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o COSE_ENC_2 has the following fields and values:
* external_aad = aad_2
* plaintext = [ COSE_SIG_V, ? APP_2 ]
o COSE_SIG_V is a COSE_Sign1 object with the following fields and
values:
* unprotected = { xyz: ID_V }
* detached payload = aad_2
o xyz - any COSE map label that can identify a public key, see
Section 4.1
o ID_V - identifier for the public key of Party V
o APP_2 - bstr containing application data
o Cert_V - The end-entity certificate of Party V
o H() - the hash function in HKDF_V
4.3.2. Party V Processing of Message 2
Party V SHALL compose message_2 as follows:
o Retrieve an ephemeral ECDH key pair generated as specified in
Section 5 of [SP-800-56a] using same curve as used in E_U. Format
the ephemeral public key E_V as a COSE_key as specified in
Section 3.1.
o Generate the pseudo-random nonce N_V
o Choose a session identifier S_V and store it for the length of the
protocol.
o Select HKDF_V, AEAD_V, SIG_V, and SIG_U from the algorithms
proposed in HKDFs_U, AEADs_U, SIGs_V, and SIGs_U.
o Format message_2 as specified in Section 4.3.1:
* COSE_Sign1 is computed as defined in section 4.4 of
[I-D.ietf-cose-msg], using algorithm SIG_V and the private key
of Party V.
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* COSE_Encrypt0 is computed as defined in section 5.3 of
[I-D.ietf-cose-msg], with AEAD_V, K_2, and IV_2. The AEAD
algorithm MUST NOT be replaced by plain encryption, see
Section 8.
+ If certificates are used then aad_2 MUST include Cert_V
4.3.3. Party U Processing of Message 2
Party U SHALL process message_2 as follows:
o Use the session identifier S_U to retrieve the protocol state.
o Verify that HKDF_V, AEAD_V, SIG_V, and SIG_U were proposed in
HKDFs_U, AEADs_U, SIGs_V, and SIGs_U.
o Verify (OPTIONAL) that N_V has not been received before.
o Verify message_2 as specified in Section 4.3.1:
* COSE_Encrypt0 is decrypted defined in section 5.3 of
[I-D.ietf-cose-msg], with AEAD_V, K_2, and IV_2.
* COSE_Sign1 is verified as defined in section 4.4 of
[I-D.ietf-cose-msg], using algorithm SIG_V and the public key
of Party V.
If any verification step fails, Party V MUST send an EDHOC error
message back, formatted as defined in Section 6.1, and the protocol
MUST be discontinued.
4.4. EDHOC Message 3
4.4.1. Formatting of Message 3
message_3 SHALL be a CBOR array as defined below
message_3 = [
data_3,
COSE_ENC_3 : COSE_Encrypt0
]
data_3 = (
MSG_TYPE : int,
S_V : bstr
)
aad_3 = bstr
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where aad_3, in diagnostic non-normative notation, is:
aad_3 = H( message_1 | message_2 ) | [ data_3 ] | ? Cert_U
where:
o MSG_TYPE = 3
o COSE_ENC_3 has the following fields and values:
* external_aad = aad_3
* plaintext = [ COSE_SIG_U, ? APP_3 ]
o COSE_SIG_U is a COSE_Sign1 object with the following fields and
values:
* unprotected = { xyz: ID_U }
* detached payload = aad_3
o xyz - any COSE map label that can identify a public key, see
Section 4.1
o ID_U - identifier for the public key of Party U
o APP_3 - bstr containing application data
o Cert_U - The end-entity certificate of Party U
4.4.2. Party U Processing of Message 3
Party U SHALL compose message_3 as follows:
o Format message_3 as specified in Section 4.4.1:
* COSE_Sign1 is computed as defined in section 4.4 of
[I-D.ietf-cose-msg], using algorithm SIG_U and the private key
of Party U.
* COSE_Encrypt0 is computed as defined in section 5.3 of
[I-D.ietf-cose-msg], with AEAD_V, K_3, and IV_3. The AEAD
algorithm MUST NOT be replaced by plain encryption, see
Section 8.
+ If certificates are used then aad_3 MUST include Cert_U
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4.4.3. Party V Processing of Message 3
Party V SHALL process message_3 as follows:
o Use the session identifier S_V to retrieve the protocol state.
o Verify message_3 as specified in Section 4.4.1.
* COSE_Encrypt0 is decrypted as defined in section 5.3 of
[I-D.ietf-cose-msg], with AEAD_V, K_3, and IV_3.
* COSE_Sign1 is verified as defined in section 4.4 of
[I-D.ietf-cose-msg], using algorithm SIG_U and the public key
of Party U;
If any verification step fails, Party V MUST send an EDHOC error
message back, formatted as defined in Section 6.1, and the protocol
MUST be discontinued.
5. EDHOC Authenticated with Symmetric Keys
5.1. Overview
EDHOC supports authentication with pre-shared keys. Party U and V
are assumed to have a pre-shared uniformly random key (PSK) with the
requirement that:
o Party V SHALL be able to identify the PSK using KID.
KID either enable the other party to retrieve the PSK or contain the
PSK (e.g. CBOR Web Token).
EDHOC with symmetric key authentication is illustrated in Figure 4.
Party U Party V
| S_U, N_U, E_U, ALG_1, KID, APP_1 |
+------------------------------------------------------------------>|
| message_1 |
| |
| S_U, S_V, N_V, E_V, ALG_2, Enc(K_2; APP_2; aad_2) |
|<------------------------------------------------------------------+
| message_2 |
| |
| S_V, Enc(K_3; APP_3; aad_3) |
+------------------------------------------------------------------>|
| message_3 |
Figure 4: EDHOC with symmetric key authentication.
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5.1.1. Mandatory to Implement Algorithms
For EDHOC authenticated with symmetric keys, the COSE algorithms
ECDH-SS + HKDF-256 and AES-CCM-64-64-128 are mandatory to implement.
5.2. EDHOC Message 1
5.2.1. Formatting of Message 1
message_1 SHALL be a CBOR array as defined below
message_1 = [
data_1
]
data_1 = (
MSG_TYPE : int,
S_U : bstr,
N_U : bstr,
E_U : serialized_COSE_Key,
ECDH-Curves_U : alg_array,
HKDFs_U : alg_array,
AEADs_U : alg_array,
KID : bstr,
? APP_1 : bstr
)
serialized_COSE_Key = bstr .cbor COSE_Key
alg_array = [ + alg : int / tstr ]
where:
o MSG_TYPE = 4
o S_U - variable length session identifier
o N_U - 64-bit random nonce
o E_U - the ephemeral public key of Party U
o ECDH-Curves_U - EC curves for ECDH which Party U supports, in the
order of decreasing preference
o HKDFs_U - supported ECDH-SS w/ HKDF algorithms
o AEADs_U - supported AEAD algorithms
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o KID - identifier of the pre-shared key
o APP_1 - bstr containing application data
5.2.2. Party U Processing of Message 1
Party U SHALL compose message_1 as follows:
o Determine which ECDH curve to use with Party V. If U previously
received from Party V an error message to message_1 with
diagnostic payload identifying an ECDH curve in ECDH-Curves_U,
then U SHALL retrieve an ephemeral from that curve. Otherwise the
first curve in ECDH-Curves_U MUST be used.
o Retrieve an ephemeral ECDH key pair generated as specified in
Section 5 of [SP-800-56a] and format the ephemeral public key E_U
as a COSE_key as specified in Section 3.1.
o Generate the pseudo-random nonce N_U
o Choose a session identifier S_U and store it for the length of the
protocol.
o Format message_1 as specified in Section 5.2.1.
5.2.3. Party V Processing of Message 1
Party V SHALL process message_1 as follows:
o Verify (OPTIONAL) that N_U has not been received before.
o Verify that at least one of each kind of the proposed algorithms
are supported.
o Verify that the ECDH curve used in E_U is supported, and that no
prior curve in ECDH-Curves_U is supported.
If any verification step fails, Party V MUST send an EDHOC error
message back, formatted as defined in Section 6.1, and the protocol
MUST be discontinued. If V does not support the ECDH curve used in
E_U, but supports another ECDH curves in ECDH-Curves_U, then the
error message SHOULD include a diagnostic payload describing the
first supported ECDH curve in ECDH-Curves_U.
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5.3. EDHOC Message 2
5.3.1. Formatting of Message 2
message_2 SHALL be a CBOR array as defined below
message_2 = [
data_2,
COSE_ENC_2 : COSE_Encrypt0
]
data_2 = (
MSG_TYPE : int,
S_U : bstr,
S_V : bstr,
N_V : bstr,
E_V : serialized_COSE_Key,
HKDF_V : int / tstr,
AEAD_V : int / tstr
)
aad_2, in diagnostic non-normative notation, is:
aad_2 = message_1 | [ data_2 ]
where:
o MSG_TYPE = 5
o S_V - variable length session identifier
o N_V - 64-bit random nonce
o E_V - the ephemeral public key of Party V
o HKDF_V - an single chosen algorithm from HKDFs_U
o AEAD_V - an single chosen algorithm from AEADs_U
o COSE_ENC_2 has the following fields and values:
* external_aad = aad_2
* plaintext = ? APP_2
o APP_2 - bstr containing application data
o H() - the hash function in HKDF_V
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5.3.2. Party V Processing of Message 2
Party V SHALL compose message_2 as follows:
o Retrieve an ephemeral ECDH key pair generated as specified in
Section 5 of [SP-800-56a] using same curve as used in E_U. Format
the ephemeral public key E_V as a COSE_key as specified in
Section 3.1.
o Generate the pseudo-random nonce N_V
o Choose a session identifier S_V and store it for the length of the
protocol.
o Select HKDF_V and AEAD_V from the algorithms proposed in HKDFs_U
and AEADs_U.
o Format message_2 as specified in Section 5.3.1 where COSE_Encrypt0
is computed as defined in section 5.3 of [I-D.ietf-cose-msg], with
AEAD_V, K_2, and IV_2.
5.3.3. Party U Processing of Message 2
Party U SHALL process message_2 as follows:
o Use the session identifier S_U to retrieve the protocol state.
o Verify message_2 as specified in Section 5.3.1 where COSE_Encrypt0
is decrypted defined in section 5.3 of [I-D.ietf-cose-msg], with
AEAD_V, K_2, and IV_2.
If any verification step fails, Party U MUST send an EDHOC error
message back, formatted as defined in Section 6.1, and the protocol
MUST be discontinued.
5.4. EDHOC Message 3
5.4.1. Formatting of Message 3
message_3 SHALL be a CBOR array as defined below
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message_3 = [
data_3,
COSE_ENC_3 : COSE_Encrypt0
]
data_3 = (
MSG_TYPE : int,
S_V : bstr
)
aad_3, in diagnostic non-normative notation, is:
aad_3 = H( message_1 | message_2 ) | [ data_3 ]
where:
o MSG_TYPE = 6
o COSE_ENC_3 has the following fields and values:
* external_aad = aad_3
* plaintext = ? APP_3
o APP_3 - bstr containing application data
5.4.2. Party U Processing of Message 3
Party U SHALL compose message_3 as follows:
o Format message_3 as specified in Section 5.4.1 where COSE_Encrypt0
is computed as defined in section 5.3 of [I-D.ietf-cose-msg], with
AEAD_V, K_3, and IV_3.
5.4.3. Party V Processing of Message 3
Party V SHALL process message_3 as follows:
o Use the session identifier S_V to retrieve the protocol state.
o Verify message_3 as specified in Section 5.4.1 where COSE_Encrypt0
is decrypted and verified as defined in section 5.3 of
[I-D.ietf-cose-msg], with AEAD_V, K_3, and IV_3.
If any verification step fails, Party V MUST send an EDHOC error
message back, formatted as defined in Section 6.1, and the protocol
MUST be discontinued.
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6. Error Handling
6.1. Error Message Format
This section defines a message format for an EDHOC error message,
used during the protocol. This is an error on EDHOC level and is
independent of the transport layer used. An advantage of using such
a construction is to avoid issues created by usage of cross protocol
proxies (e.g. UDP to TCP).
error SHALL be a CBOR array as defined below
error = [
MSG_TYPE : int,
? ERR_MSG : tstr
]
where:
o MSG_TYPE = 0
o ERR_MSG is an optional text string containing the diagnostic
payload, defined in the same way as in Section 5.5.2 of [RFC7252].
7. IANA Considerations
7.1. Media Types Registry
IANA has added the media type 'application/edhoc' to the Media Types
registry:
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Type name: application
Subtype name: edhoc
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See Section 7 of this document.
Interoperability considerations: N/A
Published specification: [[this document]] (this document)
Applications that use this media type: To be identified
Fragment identifier considerations: N/A
Additional information:
* Magic number(s): N/A
* File extension(s): N/A
* Macintosh file type code(s): N/A
Person & email address to contact for further information:
Goeran Selander <goran.selander@ericsson.com>
Intended usage: COMMON
Restrictions on usage: N/A
Author: Goeran Selander <goran.selander@ericsson.com>
Change Controller: IESG
8. Security Considerations
EDHOC builds on the SIGMA-I family of theoretical protocols that
provides perfect forward secrecy and identity protection with a
minimal number of messages. The encryption algorithm of the SIGMA-I
protocol provides identity protection, but the security of the
protocol requires the MAC to cover the identity of the signer. Hence
the message authenticating functionality of the authenticated
encryption in EDHOC is critical: authenticated encryption MUST NOT be
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replaced by plain encryption only, even if authentication is provided
at another level or through a different mechanism.
EDHOC adds an explicit message type and expands the message
authentication coverage to additional elements such as algorithms,
application data, and previous messages. EDHOC uses the same Sign-
then-MAC approach as TLS 1.3.
EDHOC does not include negotiation of parameters related to the
ephemeral key, but it enables Party V to verify that the ECDH curve
used in the protocol is the most preferred curve by U which is
supported by both U and V.
Party U and V must make sure that unprotected data and metadata do
not reveal any sensitive information. This also applies for
encrypted data sent to an unauthenticated party. In particular, it
applies to APP_1 and APP_2 in the asymmetric case, and APP_1 and KID
in the symmetric case. The communicating parties may therefore
anonymize KID.
Using the same KID or unprotected application data in several EDHOC
sessions allows passive eavesdroppers to correlate the different
sessions. Another consideration is that the list of supported
algorithms may be used to identify the application.
Party U and V must make sure that unprotected data does not trigger
any harmful actions. In particular, this applies to APP_1 in the
asymmetric case, and APP_1 and KID in the symmetric case. Party V
should be aware that replays of EDHOC message_1 cannot be detected
unless previous nonces are stored.
The availability of a secure pseudorandom number generator and truly
random seeds are essential for the security of EDHOC. If no true
random number generator is available, a truly random seed must be
provided from an external source. If ECDSA is supported,
"deterministic ECDSA" as specified in RFC6979 is RECOMMENDED.
Nonces MUST NOT be reused, both parties MUST generate fresh random
nonces.
Ephemeral keys SHOULD NOT be reused, both parties SHOULD generate
fresh random ephemeral key pairs. Party V MAY reuse the ephemeral
key to limit the effect of certain DoS attacks. For example, to
reduce processing costs in the case of repeated uncompleted protocol
runs, party V MAY pre-compute its ephemeral key E_V and reuse it for
a small number of concurrent EDHOC executions, for example until a
number of EDHOC protocol instances has been successfully completed,
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which triggers party V to pre-compute a new ephemeral key E_V to use
with subsequent protocol runs.
The referenced processing instructions in [SP-800-56a] must be
complied with, including deleting the intermediate computed values
along with any ephemeral ECDH secrets after the key derivation is
completed.
Party U and V are responsible for verifying the integrity of
certificates. The selection of trusted CAs should be done very
carefully and certificate revocation should be supported.
The choice of key length used in the different algorithms needs to be
harmonized, so that a sufficient security level is maintained for
certificates, EDHOC, and the protection of application data. Party U
and V should enforce a minimum security level.
Note that, depending on the application, the keys established through
the EDHOC protocol will need to be renewed, in which case the
communicating parties need to run the protocol again.
Implementations should provide countermeasures to side-channel
attacks such as timing attacks.
9. Acknowledgments
The authors want to thank Jim Schaad for reviewing intermediate
versions and for contributing many concrete proposals incorporated in
this version. We are also greatful to Ilari Liusvaara and Ludwig
Seitz for reviewing previous versions of the draft.
TODO: This section should be after Appendixes and before Author's
address according to RFC7322.
10. References
10.1. Normative References
[I-D.ietf-cose-msg]
Schaad, J., "CBOR Object Signing and Encryption (COSE)",
draft-ietf-cose-msg-24 (work in progress), November 2016.
[I-D.schaad-cose-x509]
Schaad, J., "CBOR Encoded Message Syntax (COSE): Headers
for carrying and referencing X.509 certificates", draft-
schaad-cose-x509-00 (work in progress), November 2016.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>.
[SIGMA] Krawczyk, H., "SIGMA - The 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and Its Use in the IKE-
Protocols (Long version)", June 2003,
<http://webee.technion.ac.il/~hugo/sigma-pdf.pdf>.
[SP-800-56a]
Barker, E., Chen, L., Roginsky, A., and M. Smid,
"Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography", NIST Special
Publication 800-56A Revision 2, May 2013,
<http://dx.doi.org/10.6028/NIST.SP.800-56Ar2>.
10.2. Informative References
[I-D.greevenbosch-appsawg-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "CBOR data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-greevenbosch-appsawg-
cbor-cddl-10 (work in progress), March 2017.
[I-D.hartke-core-e2e-security-reqs]
Selander, G., Palombini, F., and K. Hartke, "Requirements
for CoAP End-To-End Security", draft-hartke-core-e2e-
security-reqs-02 (work in progress), January 2017.
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE)", draft-ietf-ace-oauth-
authz-06 (work in progress), March 2017.
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", draft-ietf-core-
object-security-02 (work in progress), March 2017.
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[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-10
(work in progress), March 2017.
[I-D.seitz-ace-oscoap-profile]
Seitz, L. and F. Palombini, "OSCOAP profile of ACE",
draft-seitz-ace-oscoap-profile-01 (work in progress),
October 2016.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
Appendix A. Test Vectors
TODO: This section needs to be updated.
Appendix B. PSK Chaining
An application using EDHOC with symmetric keys may have a security
policy to change the PSK as a result of successfully completing the
EDHOC protocol. In this case, the old PSK SHALL be replaced with a
new PSK derived using other = exchange_hash, AlgorithmID = "EDHOC PSK
Chaining" and keyDataLength equal to the key length of AEAD_V, see
Section 3.2.
Appendix C. EDHOC with CoAP and OSCOAP
C.1. Transferring EDHOC in CoAP
EDHOC can be transferred as an exchange of CoAP [RFC7252] messages,
with the CoAP client as party U and the CoAP server as party V. By
default EDHOC is sent to the Uri-Path: "/.well-known/edhoc", but an
application may define its own path that can be discorvered e.g.
using resource directory [I-D.ietf-core-resource-directory].
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In practice, EDHOC message_1 is sent in the payload of a POST request
from the client to the server's resource for EDHOC. EDHOC message_2
or the EDHOC error message is sent from the server to the client in
the payload of a 2.04 Changed response. EDHOC message_3 or the EDHOC
error message is sent from the client to the server's resource in the
payload of a POST request. If needed, an EDHOC error message is sent
from the server to the client in the payload of a 2.04 Changed
response
An example of successful EDHOC exchange using CoAP is shown in
Figure 5.
Client Server
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/edhoc"
| | Content-Type: application/edhoc
| | Payload: EDHOC message_1
| |
|<---------+ Header: 2.04 Changed
| 2.04 | Content-Type: application/edhoc
| | Payload: EDHOC message_2
| |
+--------->| Header: POST (Code=0.02)
| POST | Uri-Path: "/.well-known/edhoc"
| | Content-Type: application/edhoc
| | Payload: EDHOC message_3
| |
|<---------+ Header: 2.04 Changed
| 2.04 |
| |
Figure 5: Transferring EDHOC in CoAP
C.2. Deriving an OSCOAP context from EDHOC
When EDHOC is use to derive parameters for OSCOAP
[I-D.ietf-core-object-security], the parties must make sure that the
EDHOC session identifiers are unique Recipient IDs in OSCOAP. In
case that the CoAP client is party U and the CoAP server is party V:
o The AEAD Algorithm is AEAD_V, as defined in this document
o The KDF algorithm is HKDF_V, as defined in this document
o The Client's Sender ID is S_V, as defined in this document
o The Server's Sender ID is S_U, as defined in this document
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o The Master Secret is derived as specified in Section 3.2 of this
document, with other = exchange_hash, AlgorithmID = "EDHOC OSCOAP
Master Secret" and keyDataLength equal to the key length of
AEAD_V.
o The Master Salt is derived as specified in Section 3.2 of this
document, with other = exchange_hash, AlgorithmID = "EDHOC OSCOAP
Master Salt" and keyDataLength equal to 64 bits.
Authors' Addresses
Goeran Selander
Ericsson AB
Faeroegatan 6
Kista SE-164 80 Stockholm
Sweden
Email: goran.selander@ericsson.com
John Mattsson
Ericsson AB
Faeroegatan 6
Kista SE-164 80 Stockholm
Sweden
Email: john.mattsson@ericsson.com
Francesca Palombini
Ericsson AB
Faeroegatan 6
Kista SE-164 80 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
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