Lightweight Authorization using Ephemeral Diffie-Hellman Over COSE (ELA)
draft-ietf-lake-authz-03
| Document | Type | Active Internet-Draft (lake WG) | |
|---|---|---|---|
| Authors | Göran Selander , John Preuß Mattsson , Mališa Vučinić , Geovane Fedrecheski , Michael Richardson | ||
| Last updated | 2024-10-21 | ||
| Replaces | draft-selander-lake-authz | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-ietf-lake-authz-03
LAKE Working Group G. Selander
Internet-Draft J. Preuß Mattsson
Intended status: Standards Track Ericsson AB
Expires: 24 April 2025 M. Vučinić
G. Fedrecheski
INRIA
M. Richardson
Sandelman Software Works
21 October 2024
Lightweight Authorization using Ephemeral Diffie-Hellman Over COSE (ELA)
draft-ietf-lake-authz-03
Abstract
Ephemeral Diffie-Hellman Over COSE (EDHOC) is a lightweight
authenticated key exchange protocol intended for use in constrained
scenarios. This document specifies Lightweight Authorization using
EDHOC (ELA). The procedure allows authorizing enrollment of new
devices using the extension point defined in EDHOC. ELA is
applicable to zero-touch onboarding of new devices to a constrained
network leveraging trust anchors installed at manufacture time.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://lake-
wg.github.io/authz/draft-ietf-lake-authz.html. Status information
for this document may be found at https://datatracker.ietf.org/doc/
draft-ietf-lake-authz/.
Discussion of this document takes place on the Lightweight
Authenticated Key Exchange Working Group mailing list
(mailto:lake@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/lake/. Subscribe at
https://www.ietf.org/mailman/listinfo/lake/.
Source for this draft and an issue tracker can be found at
https://github.com/lake-wg/authz.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Protocol Outline . . . . . . . . . . . . . . . . . . . . . . 4
3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Device (U) . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Domain Authenticator (V) . . . . . . . . . . . . . . . . 7
3.3. Enrollment Server (W) . . . . . . . . . . . . . . . . . . 8
4. The Protocol . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Reuse of EDHOC . . . . . . . . . . . . . . . . . . . . . 10
4.3. Stateless Operation of V . . . . . . . . . . . . . . . . 12
4.4. Device <-> Enrollment Server (U <-> W) . . . . . . . . . 12
4.5. Device <-> Authenticator (U <-> V) . . . . . . . . . . . 16
4.6. Authenticator <-> Enrollment Server (V <-> W) . . . . . . 17
4.7. Error Handling . . . . . . . . . . . . . . . . . . . . . 20
5. Optimization Strategies . . . . . . . . . . . . . . . . . . . 21
5.1. U anycasts message_1 . . . . . . . . . . . . . . . . . . 22
5.2. V advertises support for ELA . . . . . . . . . . . . . . 22
6. REST Interface at W . . . . . . . . . . . . . . . . . . . . . 23
6.1. Scheme "https" . . . . . . . . . . . . . . . . . . . . . 23
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6.2. Scheme "coaps" . . . . . . . . . . . . . . . . . . . . . 24
6.3. Scheme "coap" . . . . . . . . . . . . . . . . . . . . . . 24
6.4. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
8.1. EDHOC External Authorization Data Registry . . . . . . . 27
8.2. The Well-Known URI Registry . . . . . . . . . . . . . . . 27
8.3. Well-Known Name Under ".arpa" Name Space . . . . . . . . 27
8.4. Media Types Registry . . . . . . . . . . . . . . . . . . 27
8.5. CoAP Content-Formats Registry . . . . . . . . . . . . . . 28
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
9.1. Normative References . . . . . . . . . . . . . . . . . . 29
9.2. Informative References . . . . . . . . . . . . . . . . . 30
Appendix A. Use with EDHOC reverse flow . . . . . . . . . . . . 32
A.1. U is the Initiator . . . . . . . . . . . . . . . . . . . 32
A.2. U is the Responder . . . . . . . . . . . . . . . . . . . 33
Appendix B. Use with Constrained Join Protocol (CoJP) . . . . . 33
B.1. Network Discovery . . . . . . . . . . . . . . . . . . . . 34
B.2. The Enrollment Protocol with Parameter Provisioning . . . 35
Appendix C. Example Advertisement Strategies . . . . . . . . . . 37
C.1. V_INFO in network beacons . . . . . . . . . . . . . . . . 37
C.2. V_INFO in EAD_1 . . . . . . . . . . . . . . . . . . . . . 38
C.3. V_INFO in a CoAP Multicast Packet . . . . . . . . . . . . 39
Appendix D. Examples . . . . . . . . . . . . . . . . . . . . . . 39
D.1. Minimal . . . . . . . . . . . . . . . . . . . . . . . . . 40
D.2. Wrong gateway . . . . . . . . . . . . . . . . . . . . . . 40
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
For constrained IoT deployments [RFC7228] the overhead and processing
contributed by security protocols may be significant, which motivates
the specification of lightweight protocols that are optimizing, in
particular, message overhead (see [I-D.ietf-lake-reqs]). This
document describes Lightweight Authorization using EDHOC (ELA), a
procedure for augmenting the lightweight authenticated Diffie-Hellman
key exchange EDHOC [RFC9528] with third party-assisted authorization.
ELA involves a device, a domain authenticator, and an enrollment
server. The device and domain authenticator perform mutual
authentication and authorization, assisted by the enrollment server
that provides relevant authorization information to the device (a
"voucher") and to the authenticator. The high-level model is similar
to BRSKI [RFC8995].
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In this document, we consider the target interaction for which
authorization is needed to be "enrollment", for example joining a
network for the first time (e.g., [RFC9031]), but it can be applied
to authorize other target interactions.
The enrollment server may represent the manufacturer of the device,
or some other party with information about the device from which a
trust anchor has been pre-provisioned into the device. The (domain)
authenticator may represent the service provider or some other party
controlling access to the network in which the device is enrolling.
ELA assumes that authentication between device and authenticator is
performed with EDHOC [RFC9528], and defines the integration of a
lightweight authorization procedure using the External Authorization
Data (EAD) fields defined in EDHOC.
ELA enables a low message count by performing authorization and
enrollment in parallel with authentication, instead of in sequence,
which is common for network access. It further reuses protocol
elements from EDHOC, leading to reduced message sizes on constrained
links.
This protocol is applicable to a wide variety of settings, and can be
mapped to different authorization architectures.
1.1. Terminology
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 have an understanding of CBOR [RFC8949], CDDL
[RFC8610], and EDHOC [RFC9528]. Appendix C.1 of [RFC9528] contains
some basic info about CBOR.
2. Protocol Outline
The goal of ELA is to enable a (potentially constrained) device (U)
to enroll into a domain over a constrained link. The device
authenticates and enforces authorization of the (non-constrained)
domain authenticator (V) with the help of a voucher conveying
authorization information. The voucher has a similar role as in
[RFC8366] but should be considerably more compact. The domain
authenticator, in turn, authenticates the device and authorizes its
enrollment into the domain.
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The procedure is assisted by a (non-constrained) enrollment server
(W) located in a non-constrained network behind the domain
authenticator, e.g., on the Internet, providing information to the
device (conveyed in the voucher) and to the domain authenticator as
part of the protocol.
The objective of this document is to specify such a protocol that is
lightweight over the constrained link, by reusing elements of EDHOC
[RFC9528] and by shifting message overhead to the non-constrained
side of the network. See illustration in Figure 1.
Note the cardinality of the involved parties. It is expected that
the domain authenticator needs to handle a large unspecified number
of devices, but for a given device type or manufacturer it is
expected that one or a few nodes host enrollment servers.
Voucher
Info
+----------+ | +---------------+ Voucher +---------------+
| | | | | Request | |
| Device +------o---->| Domain +---------->| Enrollment |
| |<---o-------+ Authenticator |<----------+ Server |
| (U) +----+------>| (V) | Voucher | (W) |
| | | | | Response | |
+----------+ | +---------------+ +---------------+
Voucher
Figure 1: Overview of the message flow. EDHOC is used on the
constrained link between U and V. Voucher Info and Voucher are
sent in EDHOC External Authorization Data (EAD). The link
between V and W is not constrained.
3. Assumptions
The protocol is based on the following pre-existing relations between
the device (U), the domain authenticator (V) and the enrollment
server (W), see Figure 2.
* U and W have an explicit relation: U is configured with a public
key of W, see Section 3.1.
* V and W have an implicit relation, e.g., based on web PKI with
trusted CA certificates, see Section 3.2.
* U and V need not have any previous relation. This protocol
establishes a relation between U and V.
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Each of the three parties can gain protected communication with the
other two during the protocol.
V may be able to access credentials over non-constrained networks,
but U may be limited to constrained networks. Implementations
wishing to leverage the zero-touch capabilities of this protocol are
expected to support transmission of credentials from V to U by value
during the EDHOC exchange, which will impact the message size
depending on the type of credential used.
Explicit relation (e.g., from device manufacture)
| <---------------------------------------------------> |
| |
+----+-----+ +---------------+ +-------+-------+
| | | | | |
| Device | con- | Domain | not con- | Enrollment |
| | strained | Authenticator | strained | Server |
| (U) | network | (V) | network | (W) |
| | | | | |
+----+-----+ +-------+-------+ +-------+-------+
| | |
| <----------------------> | <------------------------> |
No previous relation Implicit relation
(e.g., web PKI based)
Figure 2: Overview of pre-existing relations.
3.1. Device (U)
To authenticate to V, the device (U) runs EDHOC in the role of
Initiator with authentication credential CRED_U, for example, an
X.509 certificate [RFC5280] or a CBOR Web Token (CWT, [RFC8392]).
CRED_U may, for example, be carried by value in ID_CRED_I of EDHOC
message_3 or be provisioned to V over a non-constrained network,
leveraging a credential identifier in ID_CRED_I (see bottom of
Figure 3).
U also needs to identify itself to W. The device identifier used for
this is ID_U. The purpose of ID_U is for W to be able to determine
if the device with this identifier is authorized to enroll with V.
ID_U may be a reference to CRED_U, like ID_CRED_I in EDHOC (see
Section 3.5.2 of [RFC9528]), or a device identifier from a different
name space, such as EUI-64 identifiers.
U is also provisioned with information about W:
* A static public DH key of W (G_W) used to establish secure
communication with the enrollment server (see Section 4.4).
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* Location information about the enrollment server (LOC_W) that can
be used by V to reach W. This is typically a URI but may
alternatively be only the domain name.
3.2. Domain Authenticator (V)
To authenticate to U, the domain authenticator (V) runs EDHOC in the
role of Responder with an authentication credential CRED_V containing
a public key of V, see Section 4.5.2.1. This proves to U the
possession of the private key corresponding to the public key of
CRED_V. CRED_V typically needs to be transported to U in EDHOC
(using ID_CRED_R = CRED_V, see Section 3.5.2 of [RFC9528]) since
there is no previous relation between U and V.
V and W need to establish a secure (confidentiality and integrity
protected) connection for the Voucher Request/Response protocol.
Furthermore, W needs to access the same credential CRED_V that V uses
with U (to compute the Voucher), and V needs to prove to W the
possession of the private key corresponding to the public key of
CRED_V. It is RECOMMENDED that V authenticates to W using the same
credential CRED_V as with U.
Note that the choice of protocols may affect which type of credential
and methods should be used by V. For example, in case V and W select
TLS for the secure channel and PoP, then CRED_V is a X.509
certificate, and the EDHOC method used by V is signature-based. Some
of the possible combinations of protocols to secure the connection
between V and W are listed in Table 1 below.
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+=================+=====================+==============+============+
| Secure channel | Proof-of-Possession |Type of |EDHOC method|
| between V and W | from V to W |CRED_V |used by V |
+=================+=====================+==============+============+
| [D]TLS 1.3 with | Provided by [D]TLS. |Restricted to |V MUST |
| mutual | |types that |authenticate|
| authentication, | |are supported |using a |
| where V is the | |by both |signature. |
| client and W is | |[D]TLS and | |
| the server. | |EDHOC, e.g., | |
| | |X.509 | |
| | |certificates. | |
+-----------------+---------------------+--------------+------------+
| [D]TLS 1.3, | Run an EDHOC |Any type |Any method |
| where V is the | session on top of |supported by |may be used.|
| client and W is | the TLS-protected |EDHOC, e.g., | |
| the server. | channel. |X.509, C509, | |
| | |CWT, or CCS. | |
+-----------------+---------------------+--------------+------------+
| EDHOC and | Already provided by |Any type |Any method |
| OSCORE, where V | EDHOC during the |supported by |may be used.|
| is the | setup of the secure |EDHOC. | |
| initiator and W | channel. | | |
| is the | | | |
| responder. | | | |
+-----------------+---------------------+--------------+------------+
Table 1: Examples of how to secure the connection between V and W.
Note also that the secure connection between V and W may be long-
lived and reused for multiple voucher requests/responses.
Other details of proof-of-possession related to CRED_V and transport
of CRED_V are out of scope of this document.
3.3. Enrollment Server (W)
The enrollment server (W) is assumed to have the private DH key
corresponding to G_W, which is used to establish secure communication
with the device (see Section 4.4). W provides to U the authorization
decision for enrollment with V in the form of a voucher (see
Section 4.4.2). Authorization policies are out of scope for this
document.
Authentication credentials and communication security with V is
described in Section 3.2. To calculate the voucher, W needs access
to message_1 and CRED_V as used in the EDHOC session between U and V,
see Section 4.4.2.
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* W MUST verify that CRED_V is bound to the secure connection
between W and V
* W MUST verify that V is in possession of the private key
corresponding to the public key of CRED_V
W needs to be available during the execution of the protocol between
U and V.
4. The Protocol
4.1. Overview
The ELA protocol consist of three security sessions going on in
parallel:
1. The EDHOC session between device (U) and (domain) authenticator
(V)
2. Voucher Request/Response between authenticator (V) and enrollment
server (W)
3. An exchange of voucher-related information, including the voucher
itself, between device (U) and enrollment server (W), mediated by
the authenticator (V).
Figure 3 provides an overview of the message flow detailed in this
section. An outline of EDHOC is given in Section 2 of [RFC9528].
U V W
| | |
| | |
| | Establish secure channel |
| +<--- --- --- --- --- --- --- -->|
| | (e.g., TLS with server cert.) |
| | |
| | Proof-of-possession w.r.t. CRED_V |
| +<--- --- --- --- --- --- --- -->|
| | (e.g., EDHOC) |
| | |
| | |
| | |
------------------------------------------------------------------------
CORE PROTOCOL
| | |
| EDHOC message_1 | |
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+----------------------------->| |
| (EAD_1 = LOC_W, ENC_U_INFO) | |
| | |
| | Voucher Request (VREQ) |
| +-------------------------------------->|
| | (message_1, ?opaque_state) |
| | |
| | Voucher Response (VRES) |
| |<--------------------------------------+
| | (message_1, Voucher, ?opaque_state) |
| | |
| EDHOC message_2 | |
|<-----------------------------+ |
| (EAD_2 = Voucher) | |
| | |
| | |
| EDHOC message_3 | |
+----------------------------->| |
| | |
------------------------------------------------------------------------
| |
| | Credential
| | Database
| | |
| | ID_CRED_I from message_3 |
| +--- --- --- --- --- --- --- -->|
| | |
| | CRED_U |
| |<-- --- --- --- --- --- --- ---+
| | |
| | |
Figure 3: Overview of ELA: W-assisted authorization of U and V to
each other: EDHOC between U and V, and Voucher Request/Response
between V and W. Before the protocol, V and W are assumed to
have established a secure channel and performed proof-of-
possession of relevant keys. Credential lookup of CRED_U may
involve W or other credential database.
4.2. Reuse of EDHOC
The ELA protocol illustrated in Figure 3 reuses several components of
EDHOC:
* G_X, the ephemeral public Diffie-Hellman key of U, is also used in
the protocol between U and W.
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* SUITES_I includes the cipher suite for EDHOC selected by U, and
also defines the algorithms used between U and W (see Section 3.6
of [RFC9528]):
- EDHOC AEAD algorithm: used to encrypt ID_U and to generate
voucher
- EDHOC hash algorithm: used for key derivation
- EDHOC key exchange algorithm: used to calculate the shared
secret between U and W
* EAD_1, EAD_2 are the External Authorization Data message fields of
message_1 and message_2, respectively, see Section 3.8 of
[RFC9528]. This document specifies the EAD items with ead_label =
TBD1, see Section 8.1).
* ID_CRED_I and ID_CRED_R are used to identify the authentication
credentials CRED_U and CRED_V, respectively. As shown at the
bottom of Figure 3, V may use W to obtain CRED_U. CRED_V is
transported in ID_CRED_R in message_2, see Section 4.5.2.1.
The protocol also reuses the EDHOC_Extract and EDHOC_Expand key
derivation from EDHOC (see Section 4 of [RFC9528]).
* The intermediate pseudo-random key PRK is derived using
EDHOC_Extract():
- PRK = EDHOC_Extract(salt, IKM)
o where salt = 0x (the zero-length byte string)
o IKM is computed as an ECDH cofactor Diffie-Hellman shared
secret from the public key of W, G_W, and the private key
corresponding to G_X (or v.v.), see Section 5.7.1.2 of
[NIST-800-56A].
The output keying material OKM is derived from PRK using
EDHOC_Expand(), which is defined in terms of the EDHOC hash algorithm
of the selected cipher suite, see Section 4.1.2 of [RFC9528]:
* OKM = EDHOC_Expand(PRK, info, length)
where
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info = (
info_label : int,
context : bstr,
length : uint,
)
4.3. Stateless Operation of V
V may act statelessly with respect to U: the state of the EDHOC
session started by U may be dropped at V until authorization from W
is received. Once V has received EDHOC message_1 from U and
extracted LOC_W from EAD_1, message_1 is forwarded unmodified to W in
the form of a Voucher Request (see Section 4.6.1). V encapsulates
the internal state that it needs to later respond to U, and sends
that to W together with EDHOC message_1. This state typically
contains addressing information of U (e.g., U's IP address and port
number), together with any other implementation-specific parameter
needed by V to respond to U. At this point, V can drop the EDHOC
session that was initiated by U.
The encapsulated state MUST be protected using a uniformly-
distributed (pseudo-)random key, known only to itself and specific
for the current EDHOC session to prevent replay attacks of old
encapsulated state.
How V serializes and encrypts its internal state is out of scope in
this specification. For example, V may use CBOR and COSE.
Editor's note: Consider to include an example of serialized internal
state.
W sends to V the voucher together with the echoed message_1, as
received from U, and V's internal state, see Section 4.6.2. This
allows V to act as a simple message relay until it has obtained the
authorization from W to enroll U. The reception of a successful
Voucher Response at V from W implies the authorization for V to
enroll U. At this point, V can initialize a new EDHOC session with
U, based on the message and the state retrieved from the Voucher
Response from W.
4.4. Device <-> Enrollment Server (U <-> W)
The protocol between U and W is carried between U and V in message_1
and message_2 (Section 4.5), and between V and W in the Voucher
Request/Response (Section 4.6). The data is protected between the
endpoints using secret keys derived from a Diffie-Hellman shared
secret (see Section 4.2) as further detailed in this section.
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4.4.1. Voucher Info
The external authorization data EAD_1 contains an EAD item with
ead_label = TBD1 and ead_value = Voucher_Info, which is a CBOR byte
string:
Voucher_Info = bstr .cborseq Voucher_Info_Seq
Voucher_Info_Seq = [ ; used as a CBOR sequence, not array
LOC_W: tstr,
ENC_U_INFO: bstr
]
where
* LOC_W is a text string used by V to locate W, e.g., a URI or a
domain name.
* ENC_U_INFO is a byte string containing an encrypted identifier of
U and, optionally, opaque application data prepared by U. It is
calculated as follows:
ENC_U_INFO is encrypted using the EDHOC AEAD algorithm of the
selected cipher suite specified in SUITE_I of EDHOC message_1. It
consists of 'ciphertext' of COSE_Encrypt0 (Section 5.2 of [RFC9052])
computed from the following:
* The encryption key K_1 and nonce IV_1 are derived as specified
below.
* 'protected' is a byte string of size 0
* 'plaintext' and 'external_aad' as below:
plaintext = (
ID_U: bstr,
)
external_aad = (
SS: int,
)
where
* ID_U is an identifier of the device, see Section 3.1.
* SS is the selected cipher suite in SUITES_I of EDHOC message_1,
see Section 4.5.
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The external_aad is wrapped in an enc_structure as defined in
Section 5.3 of [RFC9052].
Editor's note: Add more context to external_aad.
The derivation of K_1 = EDHOC_Expand(PRK, info, length) uses the
following input to the info struct (see OKM in Section 4.2):
* info_label = 0
* context = h'' (the empty CBOR string)
* length is length of the key of the EDHOC AEAD algorithm in bytes
(which is the length of K_1)
The derivation of IV_1 = EDHOC_Expand(PRK, info, length) uses the
following input to the info struct (see OKM in Section 4.2):
* info_label = 1
* context = h'' (the empty CBOR string)
* length is length of the nonce of the EDHOC AEAD algorithm in bytes
(which is the length of IV_1)
4.4.2. Voucher
The voucher is an assertion to U that W has authorized V. It is
encrypted using the EDHOC AEAD algorithm of the selected cipher suite
specified in SUITE_I of EDHOC message_1. It consists of the
'ciphertext' field of a COSE_Encrypt0 object, which is a byte string,
as defined below.
Voucher = bstr
Its corresponding plaintext value consists of an opaque field that
can be used by W to convey information to U, such as a voucher scope.
The authentication tag present in the ciphertext is also bound to
message_1 and the credential of V as described below.
* The encryption key K_2 and nonce IV_2 are derived as specified
below.
* 'protected' is a byte string of size 0
* 'plaintext' and 'external_aad' as below:
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plaintext = (
?OPAQUE_INFO: bstr
)
external_aad = (
H_message_1: bstr,
CRED_V: bstr,
)
where
* OPAQUE_INFO is an opaque field provided by the application. If
present, it will contain application data that W may want to
convey to U, e.g., a voucher scope. Note that OPAQUE_INFO is
opaque when viewed as an information element in EDHOC. It is
opaque to V, while the application in U and W can read its
contents.
* H_message_1 is the hash of EDHOC message_1, calculated from the
associated voucher request, see Section 4.6.1. The hash is
computed by using the EDHOC hash algorithm of the selected cipher
suite specified in SUITE_I of EDHOC message_1.
* CRED_V is the credential used by V to authenticate to U and W, see
Section 4.5.2.1 and Table 1.
The derivation of K_2 = EDHOC_Expand(PRK, info, length) uses the
following input to the info struct (see Section 4.2):
* info_label = 2
* context = h'' (the empty CBOR string)
* length is length of the key of the EDHOC AEAD algorithm in bytes
The derivation of IV_2 = EDHOC_Expand(PRK, info, length) uses the
following input to the info struct (see Section 4.2):
* info_label = 3
* context = h'' (the empty CBOR string)
* length is length of the nonce of the EDHOC AEAD algorithm in bytes
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4.5. Device <-> Authenticator (U <-> V)
This section describes the processing in U and V, which includes the
EDHOC protocol, see Figure 3. Normal EDHOC processing is omitted
here.
4.5.1. Message 1
4.5.1.1. Processing in U
U composes EDHOC message_1 using authentication method, identifiers,
etc. according to an agreed application profile, see Section 3.9 of
[RFC9528]. The selected cipher suite, in this document denoted SS,
applies also to the interaction with W as detailed in Section 4.2, in
particular, with respect to the Diffie-Hellman key agreement
algorithm used between U and W. As part of the normal EDHOC
processing, U generates the ephemeral public key G_X that is reused
in the interaction with W, see Section 4.4.
The device sends EDHOC message_1 with EAD item (-TBD1, Voucher_Info)
included in EAD_1, where Voucher_Info is specified in Section 4.4.
The negative sign indicates that the EAD item is critical, see
Section 3.8 of [RFC9528].
4.5.1.2. Processing in V
V receives EDHOC message_1 from U and processes it as specified in
Section 5.2.3 of [RFC9528], with the additional step of processing
the EAD item in EAD_1. Since the EAD item is critical, if V does not
recognize it or it contains information that V cannot process, then V
MUST abort the EDHOC session, see Section 3.8 of [RFC9528].
Otherwise, the ead_label = TBD1 triggers the voucher request to W as
described in Section 4.6. The exchange between V and W needs to be
completed successfully for the EDHOC session to be continued.
4.5.2. Message 2
4.5.2.1. Processing in V
V receives the voucher response from W as described in Section 4.6.
V sends EDHOC message_2 to U with the critical EAD item (-TBD1,
Voucher) included in EAD_2, i.e., ead_label = TBD1 and ead_value =
Voucher, as specified in Section 4.4.2.
The type of CRED_V may depend on the selected mechanism for the
establishment of a secure channel between V and W, See Table 1.
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In case the network between U and V is constrained, it is recommended
that CRED_V be a CWT Claims Set (CCS) [RFC8392]. The CCS contains
the public authentication key of V encoded as a COSE_Key in the 'cnf'
claim, see Section 3.5.2 of [RFC9528]. ID_CRED_R contains the CWT
Claims Set with 'kccs' as COSE header_map, see Section 10.6 of
[RFC9528].
4.5.2.2. Processing in U
U receives EDHOC message_2 from V and processes it as specified in
Section 5.3.3 of [RFC9528], with the additional step of processing
the EAD item in EAD_2.
If U does not recognize the EAD item or the EAD item contains
information that U cannot process, then U MUST abort the EDHOC
session, see Section 3.8 of [RFC9528]. Otherwise, U MUST verify the
Voucher using H_message_1, CRED_V, and the keys derived as in
Section 4.4.2. If the verification fails then U MUST abort the EDHOC
session.
If OPAQUE_INFO is present, it is made available to the application.
4.5.3. Message 3
4.5.3.1. Processing in U
If all verifications are passed, then U sends EDHOC message_3.
EDHOC message_3 may be combined with an OSCORE-protected application
request, see [I-D.ietf-core-oscore-edhoc].
4.5.3.2. Processing in V
V performs the normal EDHOC verifications of message_3. V may
retrieve CRED_U from a Credential Database, after having learned
ID_CRED_I from U.
4.6. Authenticator <-> Enrollment Server (V <-> W)
It is assumed that V and W have set up a secure connection, W has
accessed the authentication credential CRED_V to be used in the EDHOC
session between V and U, and that W has verified that V is in
possession of the private key corresponding to CRED_V, see
Section 3.2 and Section 3.3. V and W run the Voucher Request/
Response protocol over the secure connection.
4.6.1. Voucher Request
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4.6.1.1. Processing in V
V sends the voucher request to W. The Voucher Request SHALL be a
CBOR array as defined below:
Voucher_Request = [
message_1: bstr,
? opaque_state: bstr
]
where
* message_1 is a CBOR byte string whose value is the byte
serialization of EDHOC message_1 as it was received from U.
* opaque_state is OPTIONAL and represents the serialized and
encrypted opaque state needed by V to statelessly respond to U
after the reception of Voucher_Response.
4.6.1.2. Processing in W
W receives and parses the voucher request received over the secure
connection with V. The voucher request essentially contains EDHOC
message_1 as sent by U to V. W SHALL NOT process message_1 as if it
was an EDHOC message intended for W.
W extracts from message_1:
* SS - the selected cipher suite, which is the (last) integer of
SUITES_I.
* G_X - the ephemeral public key of U
* ENC_U_INFO - the encryption of the device identifier ID_U,
contained in the Voucher_Info field of the EAD item with ead_label
= TBD1 (with minus sign indicating criticality)
W verifies and decrypts ENC_U_INFO using the relevant algorithms of
the selected cipher suite SS (see Section 4.2), and obtains ID_U.
W calculates the hash of message_1 H_message_1, and associates this
session identifier to the device identifier ID_U. Note that
message_1 contains a unique ephemeral key, therefore H_message_1 is
expected to be unique.
If processing fails up until this point, the protocol SHALL be
aborted with an error code signaling a generic issue with the
request, see Section 6.4.1.
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W uses ID_U to look up the associated authorization policies for U
and enforces them. This is out of scope for the specification.
If ID_U is known by W, but authorization fails, the protocol SHALL be
aborted with an error code signaling an access control issue, see
Section 4.7 and Section 6.4.1.
4.6.2. Voucher Response
4.6.2.1. Processing in W
W retrieves CRED_V associated with the secure connection with V, and
constructs the Voucher for the device with identifier ID_U (see
Section 4.4.2).
W generates the voucher response and sends it to V over the secure
connection. The Voucher_Response SHALL be a CBOR array as defined
below:
Voucher_Response = [
message_1: bstr,
Voucher: bstr,
? opaque_state: bstr
]
where
* message_1 is a CBOR byte string whose value is the byte
serialization of EDHOC message_1 as it was received from V.
* The Voucher is defined in Section 4.4.2.
* opaque_state is the echoed byte string opaque_state from
Voucher_Request, if present.
W signals the successful generation of the voucher via a status code
in the REST interface, as defined in Section 6.4.1.
4.6.2.2. Processing in V
V receives the voucher response from W over the secure connection.
If present, V decrypts and verifies opaque_state as received from W.
If that verification fails, then the EDHOC session with U is aborted.
If the voucher response is successfully received from W, then V
responds to U with EDHOC message_2 as described in Section 4.5.2.1.
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4.7. Error Handling
This section specifies a new EDHOC error code and how it is used in
ELA.
4.7.1. EDHOC Error "Access denied"
This section specifies the new EDHOC error "Access denied", see
Figure 4.
+----------+----------------+----------------------------------------+
| ERR_CODE | ERR_INFO Type | Description |
+==========+================+========================================+
| TBD3 | error_content | Access denied |
+----------+----------------+----------------------------------------+
Figure 4: EDHOC error code and error information for ‘Access denied’.
Error code TBD3 is used to indicate to the receiver that access
control has been applied and the sender has aborted the EDHOC
session. The ERR_INFO field contains error_content which is a CBOR
Sequence consisting of an integer and an optional byte string.
error_content = (
REJECT_TYPE : int,
? REJECT_INFO : bstr,
)
The purpose of REJECT_INFO is for the sender to provide verifiable
and actionable information to the receiver about the error, so that
an automated action may be taken to enable access.
+-------------+---------------+--------------------------------------+
| REJECT_TYPE | REJECT_INFO | Description |
+=============+===============+======================================+
| 0 | - | No REJECT_INFO |
+-------------+---------------+--------------------------------------+
| 1 | bstr | REJECT_INFO from trusted third party |
+-------------+---------------+--------------------------------------+
Figure 5: REJECT_TYPE and REJECT_INFO for ‘Access denied’.
4.7.2. Error handling in W, V, and U
ELA uses the EDHOC Error "Access denied" in the following way:
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* W generates error_content and transfers it to V via the secure
connection. If REJECT_TYPE is 1, then REJECT_INFO is encrypted
from W to U using the EDHOC AEAD algorithm. W signals the error
via an appropriate status code in the REST interface, as defined
in Section 6.4.1.
* V receives error_content, prepares an EDHOC "Access denied" error,
and sends it to U.
* U receives the error message and extracts the error_content. If
REJECT_TYPE is 1, then U decrypts REJECT_INFO, based on which it
may retry to gain access.
The encryption of REJECT_INFO follows a procedure analogous to the
one defined in Section 4.4.2, with the following differences:
plaintext = (
OPAQUE_INFO: bstr,
)
external_aad = (
H_message_1: bstr,
)
where
* OPAQUE_INFO is an opaque field that contains actionable
information about the error. It may contain, for example, a list
of suggested Vs through which U should join instead.
* H_message_1 is the hash of EDHOC message_1, calculated from the
associated voucher request, see Section 4.6.1.
5. Optimization Strategies
When ELA is used for zero-touch enrollment, U normally has little to
no knowledge of the available V's. This may lead to situations where
U has to retry several times at different V's until it finds one that
works. This section presents two optimization strategies for such
cases. They were developed to address scenarios where V's are radio
gateways to which U wants to enroll, but may also be applicable to
other use cases.
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5.1. U anycasts message_1
This strategy consists in U disseminating EDHOC message_1 as anycast
(a broadcast from which only one successful message_2 response is
expected). When each of the V's in radio range of U receive
message_1, one of the following can happen:
* V does not implement EDHOC, and drops the message
* V does not implement ELA, and since EAD_1 is critical, it either
responds with an error or drops the message (Editor's note:
dropping actually conflicts with the EAD field being critical)
* V forwards message_1 to W as VREQ, but W does not authorize it,
and error handling is applied
* V forwards message_1 to W as VREQ, W authorizes it, and the
protocol continues normally
U is expected to receive at most one message_2 as response, which
contains the Voucher. In case U receives additional message_2's,
they MUST be silently dropped.
This strategy may increase the number of messages that need to be
processed by V and W, in exchange for reducing resource usage in U.
Security concerns related to this strategy, including potential reuse
of G_X and double processing of message_2, are discussed in
Section 7.
5.2. V advertises support for ELA
In this strategy, V shares some information (V_INFO) with a potential
U, that can help it decide whether to try to enroll with that V.
The exact contents of the V_INFO structure, as well as the mechanism
used to transport it, will depend on the underlying communication
technology and also on application needs. For example, V_INFO may
state that:
* V implements ELA -- similarly to how EAPOL [IEEE802.1X] frames
state support for IEEE 802.1X.
* V is part of a certain domain -- similarly to how Eduroam
[RFC7593] is used in the SSID field of IEEE 802.11 packets
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V_INFO can be sent over a network beacon (see Appendix C.1), which
may require technology specific profiling, e.g., the IEEE 802.15.4
enhanced beacon may be extended according to [RFC8137].
Alternatively, V_INFO can be sent as part of an EAD field, as shown
in Appendix C.2.
As a guideline for implementers, we define the following field that
can be included in a V_INFO structure:
DOMAIN_ID: bstr
The DOMAIN_ID field identifies the domain to which V belongs to, for
example an URL or UUID.
Appendix C presents three examples of how the advertisement strategy
may be applied according to different application needs.
6. REST Interface at W
The interaction between V and W is enabled through a RESTful
interface exposed by W. This RESTful interface MAY be implemented
using either HTTP or CoAP. V SHOULD access the resources exposed by
W through the protocol indicated by the scheme in the LOC_W URI.
6.1. Scheme "https"
In case the scheme indicates "https", V MUST perform a TLS handshake
with W and access the resources defined in Section 6.4 using HTTP.
If the authentication credential CRED_V can be used in a TLS
handshake, e.g., an X.509 certificate of a signature public key, then
V SHOULD use it to authenticate to W as a client. If the
authentication credential CRED_V cannot be used in a TLS handshake,
e.g., if the public key is a static Diffie-Hellman key, then V SHOULD
first perform a TLS handshake with W using available compatible keys.
V MUST then perform an EDHOC session over the TLS connection proving
to W the possession of the private key corresponding to CRED_V.
Performing the EDHOC session is only necessary if V did not
authenticate with CRED_V in the TLS handshake with W.
Editor's note: Clarify that performing TLS handshake is not necessary
for each device request; if there already is a TLS connection between
V and W that should be reused. Similar considerations for 5.2 and
5.3.
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6.2. Scheme "coaps"
In case the scheme indicates "coaps", V SHOULD perform a DTLS
handshake with W and access the resources defined in Section 6.4
using CoAP. The normative requirements in Section 6.1 on performing
the DTLS handshake and EDHOC session remain the same, except that TLS
is replaced with DTLS.
6.3. Scheme "coap"
In case the scheme indicates "coap", V SHOULD perform an EDHOC
session with W, as specified in Appendix A of [RFC9528] and access
the resources defined in Section 6.4 using OSCORE and CoAP. The
authentication credential in this EDHOC session MUST be CRED_V.
6.4. URIs
The URIs defined below are valid for both HTTP and CoAP. W MUST
support the use of the path-prefix "/.well-known/", as defined in
[RFC8615], and the registered name "lake-authz". A valid URI in case
of HTTP thus begins with
* "https://www.example.com/.well-known/lake-authz"
In case of CoAP with DTLS:
* "coaps://example.com/.well-known/lake-authz"
In case of EDHOC and OSCORE:
* "coap://example.com/.well-known/lake-authz"
Each operation specified in the following is indicated by a path-
suffix.
6.4.1. Voucher Request (/voucherrequest)
To request a voucher, V MUST issue a request such that:
* Method is POST
* Payload is the serialization of the Voucher Request object, as
specified in Section 4.6.1.
* Content-Format (Content-Type) is set to "application/lake-authz-
voucherrequest+cbor"
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In case of successful processing at W, W MUST issue a response such
that:
* Status code is 200 OK if using HTTP, or 2.04 Changed if using CoAP
* Payload is the serialized Voucher Response object, as specified in
Section 4.6.2
* Content-Format (Content-Type) is set to "application/lake-authz-
voucherresponse+cbor"
In case of error, two cases should be considered:
* U cannot be identified: this happens either if W fails to process
the Voucher Request, or if it succeeds but ID_U is considered
unknown to W. In this case, W MUST reply with 400 Bad Request if
using HTTP, or 4.00 if using CoAP.
* U is identified but unauthorized: this happens if W is able to
process the Voucher Request, and W recognizes ID_U as a known
device, but the access policies forbid enrollment. For example,
the policy could enforce enrollment within a delimited time
window, via a specific V, etc. In this case, W MUST reply with a
403 Forbidden code if using HTTP, or 4.03 if using CoAP; the
payload is the serialized error_content object, with Content-
Format (Content-Type) set to "application/lake-authz-
vouchererror+cbor". The payload MAY be used by V to prepare an
EDHOC error "Access Denied", see Section 4.7.
6.4.2. Certificate Request (/certrequest)
V requests the public key certificate of U from W through the
"/certrequest" path-suffix. To request U's authentication
credential, V MUST issue a request such that:
* Method is POST
* Payload is the serialization of the ID_CRED_I object, as received
in EDHOC message_3.
* Content-Format (Content-Type) is set to "application/lake-authz-
certrequest+cbor"
In case of a successful lookup of the authentication credential at W,
W MUST issue a response such that:
* Status code is 200 OK if using HTTP, or 2.04 Changed if using CoAP
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* Payload is the serialized CRED_U
* Content-Format (Content-Type) is set to "application/lake-authz-
certresponse+cbor"
7. Security Considerations
This specification builds on and reuses many of the security
constructions of EDHOC, e.g., shared secret calculation and key
derivation. The security considerations of EDHOC [RFC9528] apply
with modifications discussed here.
EDHOC provides identity protection of the Initiator, here the device.
The encryption of the device identifier ID_U in the first message
should consider potential information leaking from the length of
ID_U, either by making all identifiers having the same length or the
use of a padding scheme.
Although W learns about the identity of U after receiving VREQ, this
information must not be disclosed to V, until U has revealed its
identity to V with ID_CRED_I in message_3. W may be used for lookup
of CRED_U from ID_CRED_I, or this credential lookup function may be
separate from the authorization function of W, see Figure 3. The
trust model used here is that U decides to which V it reveals its
identity. In an alternative trust model where U trusts W to decide
to which V it reveals U's identity, CRED_U could be sent in Voucher
Response.
As noted in Section 9.2 of [RFC9528] an ephemeral key may be used to
calculate several ECDH shared secrets. In this specification, the
ephemeral key G_X is also used to calculate G_XW, the shared secret
with the enrollment server.
The private ephemeral key is thus used in the device for calculations
of key material relating to both the authenticator and the enrollment
server. There are different options for where to implement these
calculations. One option is as an addition to EDHOC, i.e., to extend
the EDHOC API in the device, so that EDHOC can import the public key
of W (G_W) and the device identifier of U (ID_U), and then produce
the encryption of ID_U which is included in Voucher_Info in EAD_1.
8. IANA Considerations
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8.1. EDHOC External Authorization Data Registry
IANA has registered the following entry in the "EDHOC External
Authorization Data" registry under the group name "Ephemeral Diffie-
Hellman Over COSE (EDHOC)". The ead_label = TBD1 corresponds to the
ead_value Voucher_Info in EAD_1, and Voucher in EAD_2 with processing
specified in Section 4.5.1 and Section 4.5.2, respectively, of this
document.
+=======+============+=============================+
| Label | Value Type | Description |
+=======+============+=============================+
| TBD1 | bstr | Voucher related information |
+-------+------------+-----------------------------+
Table 2: Addition to the EDHOC EAD registry
8.2. The Well-Known URI Registry
IANA has registered the following entry in "The Well-Known URI
Registry", using the template from [RFC8615]:
* URI suffix: lake-authz
* Change controller: IETF
* Specification document: [[this document]]
* Related information: None
8.3. Well-Known Name Under ".arpa" Name Space
This document allocates a well-known name under the .arpa name space
according to the rules given in [RFC3172] and [RFC6761]. The name
"lake-authz.arpa" is requested. No subdomains are expected, and
addition of any such subdomains requires the publication of an IETF
Standards Track RFC. No A, AAAA, or PTR record is requested.
8.4. Media Types Registry
IANA has added the media types "application/lake-authz-
voucherrequest+cbor" to the "Media Types" registry.
8.4.1. application/lake-authz-voucherrequest+cbor Media Type
Registration
* Type name: application
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* Subtype name: lake-authz-voucherrequest+cbor
* Required parameters: N/A
* Optional parameters: N/A
* Encoding considerations: binary (CBOR)
* Security considerations: See Section 7 of this document.
* Interoperability considerations: N/A
* Published specification: [[this document]] (this document)
* Application 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: IETF
LAKE Working Group (lake@ietf.org)
* Intended usage: COMMON
* Restrictions on usage: N/A
* Author: LAKE WG
* Change Controller: IESG
8.5. CoAP Content-Formats Registry
IANA has added the following Content-Format number in the "CoAP
Content-Formats" registry under the registry group "Constrained
RESTful Environments (CoRE) Parameters".
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+================================+==========+======+============+
| Content Type | Content | ID | Reference |
| | Encoding | | |
+================================+==========+======+============+
| application/lake-authz- | - | TBD2 | [[this |
| voucherrequest+cbor | | | document]] |
+--------------------------------+----------+------+------------+
Table 3: Addition to the CoAP Content-Formats registry
9. References
9.1. Normative References
[NIST-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://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar3.pdf>.
[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>.
[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>.
[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>.
[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>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/info/rfc9052>.
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[RFC9528] Selander, G., Preuß Mattsson, J., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", RFC 9528,
DOI 10.17487/RFC9528, March 2024,
<https://www.rfc-editor.org/info/rfc9528>.
9.2. Informative References
[I-D.amsuess-core-coap-over-gatt]
Amsüss, C., "CoAP over GATT (Bluetooth Low Energy Generic
Attributes)", Work in Progress, Internet-Draft, draft-
amsuess-core-coap-over-gatt-07, 25 September 2024,
<https://datatracker.ietf.org/doc/html/draft-amsuess-core-
coap-over-gatt-07>.
[I-D.ietf-core-oscore-edhoc]
Palombini, F., Tiloca, M., Höglund, R., Hristozov, S., and
G. Selander, "Using Ephemeral Diffie-Hellman Over COSE
(EDHOC) with the Constrained Application Protocol (CoAP)
and Object Security for Constrained RESTful Environments
(OSCORE)", Work in Progress, Internet-Draft, draft-ietf-
core-oscore-edhoc-11, 9 April 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
oscore-edhoc-11>.
[I-D.ietf-lake-reqs]
Vučinić, M., Selander, G., Mattsson, J. P., and D. Garcia-
Carillo, "Requirements for a Lightweight AKE for OSCORE",
Work in Progress, Internet-Draft, draft-ietf-lake-reqs-04,
8 June 2020, <https://datatracker.ietf.org/doc/html/draft-
ietf-lake-reqs-04>.
[IEEE802.15.4]
IEEE standard for Information Technology, "IEEE Std
802.15.4 Standard for Low-Rate Wireless Networks", n.d..
[IEEE802.1X]
IEEE standard for Information Technology, "IEEE Standard
for Local and metropolitan area networks - Port-Based
Network Access Control", February 2010.
[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>.
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[RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/rfc3172>.
[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>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>.
[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>.
[RFC7593] Wierenga, K., Winter, S., and T. Wolniewicz, "The eduroam
Architecture for Network Roaming", RFC 7593,
DOI 10.17487/RFC7593, September 2015,
<https://www.rfc-editor.org/info/rfc7593>.
[RFC8137] Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
Element for the IETF", RFC 8137, DOI 10.17487/RFC8137, May
2017, <https://www.rfc-editor.org/info/rfc8137>.
[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>.
[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>.
[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>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
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[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/info/rfc8995>.
[RFC9031] Vučinić, M., Ed., Simon, J., Pister, K., and M.
Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH",
RFC 9031, DOI 10.17487/RFC9031, May 2021,
<https://www.rfc-editor.org/info/rfc9031>.
Appendix A. Use with EDHOC reverse flow
Editor's note: I am not sure if this should be an appendix or an
actual section in the main document.
This appendix describes how the protocol can be used with the EDHOC
reverse message flow defined in Appendix A.2.2 of [RFC9528], where
the CoAP client is the Responder and the CoAP server is the
initiator.
A.1. U is the Initiator
The reverse flow can be applied when U implements a CoAP server, but
acts as an EDHOC Initiator. In this case, U awaits for a CoAP
request to be sent from V, which will act as a trigger message to
start the EDHOC handshake with ELA. Next, U replies with an EDHOC
message_1, thus executing the protocol normally.
+-------+--------+ +-------+--------+
| Init | Server | | Resp | Client |
+-------+--------+ +----------------+
| U | | V |
+----------------+ +----------------+
| |
| CoAP request |
|<--------------------------------|
| |
| EDHOC message_1 |
+-------------------------------->|
| (EAD_1 = Voucher_Info) |
| |
( ... protocol continues normally ... )
Figure 6: ELA with EDHOC reverse mesasge flow when U is initiator.
One use case is to perform ELA over Bluetooth Low Energy, as
discussed in [I-D.amsuess-core-coap-over-gatt].
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A.2. U is the Responder
The reverse flow can also be used when U implements a CoAP client,
but acts as a Responder, as illustrated in Figure 7. The main
changes in this case are:
* Instead of sending EDHOC message_1, U sends an initial trigger
packet to V, e.g., a CoAP request, which then initiates the
handshake by replying with an EDHOC message_1.
* The Voucher_Info and Voucher structs are sent over EAD_2 and
EAD_3, respectively (instead of over EAD_1 and EAD_2).
+-------+--------+ +-------+--------+
| Resp | Client | | Init | Server |
+-------+--------+ +----------------+
| U | | V |
+----------------+ +----------------+
| |
| CoAP request |
+-------------------------->|
| |
| EDHOC message_1 |
+<--------------------------|
| |
| EDHOC message_2 | W
+-------------------------->| |
| (EAD_2 = Voucher_Info) | |
| +-------------->|
| | VREQ / VRES |
| |<------------->|
| | |
| EDHOC message_3 | |
+<--------------------------|
| (EAD_3 = Voucher) |
Figure 7: ELA with EDHOC reverse mesasge flow when U is responder.
Appendix B. Use with Constrained Join Protocol (CoJP)
This section outlines how ELA is used for network enrollment and
parameter provisioning. An IEEE 802.15.4 network is used as an
example of how a new device (U) can be enrolled into the domain
managed by the domain authenticator (V).
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U V W
| | |
| | |
+ - - - - - - - - - - - - - - - - -->| |
| Optional network solicitation | |
|<-----------------------------------+ |
| Network discovery | |
| | |
+----------------------------------->| |
| EDHOC message_1 | |
| +----------------------------->|
| | Voucher Request (VREQ) |
| |<-----------------------------+
| | Voucher Response (VRES) |
|<-----------------------------------+ |
| EDHOC message_2 | |
| | |
| | |
+----------------------------------->| |
| EDHOC message_3 + CoJP request | |
| | |
+<-----------------------------------| |
| CoJP response | |
|
Figure 8: Use of draft-ietf-lake-authz with CoJP.
B.1. Network Discovery
When a device first boots, it needs to discover the network it
attempts to join. The network discovery procedure is defined by the
link-layer technology in use. In case of Time-slotted Channel
Hopping (TSCH) networks, a mode of [IEEE802.15.4], the device scans
the radio channels for Enhanced Beacon (EB) frames, a procedure known
as passive scan. EBs carry the information about the network, and
particularly the network identifier. Based on the EB, the network
identifier, the information pre-configured into the device, the
device makes the decision on whether it should join the network
advertised by the received EB frame. This process is described in
Section 4.1 of [RFC9031]. In case of other, non-TSCH modes of IEEE
802.15.4, it is possible to use the active scan procedure and send
solicitation frames. These solicitation frames trigger the nearest
network coordinator to respond by emitting a beacon frame. The
network coordinator emitting beacons may be multiple link-layer hops
away from the domain authenticator (V), in which case it plays the
role of a Join Proxy (see [RFC9031]). The Join Proxy does not
participate in the protocol and acts as a transparent router between
the device and the domain authenticator. For simplicity, Figure 8
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illustrates the case when the device and the domain authenticator are
a single hop away and can communicate directly.
B.2. The Enrollment Protocol with Parameter Provisioning
B.2.1. Flight 1
Once the device has discovered the network it wants to join, it
constructs EDHOC message_1, as described in Section 4.5. The device
SHALL map the message to a CoAP request:
* The request method is POST.
* The type is Confirmable (CON).
* The Proxy-Scheme option is set to "coap".
* The Uri-Host option is set to "lake-authz.arpa". This is an
anycast type of identifier of the domain authenticator (V) that is
resolved to its IPv6 address by the Join Proxy.
* By means of Uri-Path options, the Uri-Path is set to ".well-known/
edhoc".
* The payload is the (true, EDHOC message_1) CBOR sequence, where
EDHOC message_1 is constructed as defined in Section 4.5.
B.2.2. Flight 2
The domain authenticator receives message_1 and processes it as
described in Section 4.5. The message triggers the exchange with the
enrollment server, as described in Section 4.6. If the exchange
between V and W completes successfully, the domain authenticator
prepares EDHOC message_2, as described in Section 4.5. The
authenticator SHALL map the message to a CoAP response:
* The response code is 2.04 Changed.
* The payload is the EDHOC message_2, as defined in Section 4.5.
B.2.3. Flight 3
The device receives EDHOC message_2 and processes it as described in
Section 4.5. Upon successful processing of message_2, the device
prepares flight 3, which is an OSCORE-protected CoJP request
containing an EDHOC message_3, as described in
[I-D.ietf-core-oscore-edhoc]. EDHOC message_3 is prepared as
described in Section 4.5. The OSCORE-protected payload is the CoJP
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Join Request object specified in Section 8.4.1 of [RFC9031]. OSCORE
protection leverages the OSCORE Security Context derived from the
EDHOC session, as specified in Appendix A of [RFC9528]. To that end,
[I-D.ietf-core-oscore-edhoc] specifies that the Sender ID of the
client (device) must be set to the connection identifier selected by
the domain authenticator, C_R. OSCORE includes the Sender ID as the
kid in the OSCORE option. The network identifier in the CoJP Join
Request object is set to the network identifier obtained from the
network discovery phase. In case of IEEE 802.15.4 networks, this is
the PAN ID.
The device SHALL map the message to a CoAP request:
* The request method is POST.
* The type is Confirmable (CON).
* The Proxy-Scheme option is set to "coap".
* The Uri-Host option is set to "lake-authz.arpa".
* The Uri-Path option is set to ".well-known/edhoc".
* The EDHOC option [I-D.ietf-core-oscore-edhoc] is set and is empty.
* The payload is prepared as described in Section 3.2 of
[I-D.ietf-core-oscore-edhoc], with EDHOC message_3 and the CoJP
Join Request object as the OSCORE-protected payload.
Note that the OSCORE Sender IDs are derived from the connection
identifiers of the EDHOC session. This is in contrast with [RFC9031]
where ID Context of the OSCORE Security Context is set to the device
identifier (pledge identifier). Since the device identity is
exchanged during the EDHOC session, and the certificate of the device
is communicated to the authenticator as part of the Voucher Response
message, there is no need to transport the device identity in OSCORE
messages. The authenticator playing the role of the [RFC9031] JRC
obtains the device identity from the execution of the authorization
protocol.
B.2.4. Flight 4
Flight 4 is the OSCORE response carrying CoJP response message. The
message is processed as specified in Section 8.4.2 of [RFC9031].
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Appendix C. Example Advertisement Strategies
This appendix presents three example strategies that can be used to
advertise the presence of a V. It includes sending V_INFO in network
beacons, as part of EAD_1 in reverse message flow, or as part of a
periodic CoAP multicast packet. It also presents the advantages,
costs, and security impacts of each strategy.
C.1. V_INFO in network beacons
_PR editor's note: this is approach A1_
This approach allows carrying V_INFO in beacons sent over the network
layer, as shown in Figure 9. It requires that the network layer
offers a mechanism to configure its beacon packets. Depending on the
network type, a solicitation packet may also be needed, as is the
case of non-beaconed IEEE 802.15.4 and BLE with GATT.
+-------+--------+ +-------+--------+
| Init | Client | | Resp | Server |
+-------+--------+ +----------------+
| U | | V |
+----------------+ +----------------+
| |
+ - - - - - - - - - - - - - - - - - - -->|
| Optional network solicitation |
| |
|<---------------------------------------+
| Network discovery (contains V_INFO) |
| |
| EDHOC message_1 |
+--------------------------------------->|
| (?EAD_1 = Voucher_Info) |
| |
( ... protocol continues normally ... )
Figure 9: Advertising ELA using V_INFO in network-layer beacons.
This strategy can be used, for example, in IEEE 802.15.4, where an
Enhanced Beacon [IEEE802.15.4] can be used to transmit V_INFO.
Specifically, a new information element for carrying V_INFO can be
defined according to [RFC8137].
This approach has the advantage of requiring minimal changes to the
default protocol as presented in Section 4.1, i.e., no reverse flow.
It requires, however, some profiling of the lower layer beacons.
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C.2. V_INFO in EAD_1
_PR editor's note: this is approach A2_
ELA with EDHOC in the reverse flow allows implementing advertising
where U first sends a trigger packet, in the format of a CoAP request
that is broadcasted to the newtork. When a suitable V receives the
solicitation, if it implements ELA, it should respond with an EDHOC
message_1 whose EAD_1 has label TBD1 and value V_INFO (see
Section Section 5).
+-------+--------+ +-------+--------+
| Resp | Client | | Init | Server |
+-------+--------+ +----------------+
| U | | V |
+----------------+ +----------------+
| |
+-------------------------------->|
| CoAP discovery/solicitation |
| |
| EDHOC message_1 |
+<--------------------------------|
| (?EAD_1 = V_INFO) |
| |
( ... reverse flow continues normally ... )
Figure 10: Advertising ELA using V_INFO in EAD_1, eploying the
EDHOC reverse flow with U as responder.
Note that V will only reply if it supports ELA, therefore in this
strategy there is no need to transport ELA_ID. V_INFO can then be
structured to contain only the optional domain identifier:
V_INFO = (
?DOMAIN_ID: bstr,
)
This approach enables a simple filtering mechanism, where only V's
that support ELA will reply. It also encrypts Voucher_Info (as part
of EAD_2), wehereas it is sent in the clear in the original flow. In
addition, it may not require layer-two profiling (in case the network
allows transporting data before authorization). Finally, note that
the reverse flow with U as Responder protects the identity of V
(instead of U's as in the forward flow).
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C.3. V_INFO in a CoAP Multicast Packet
_PR editor's note: this is approach A3_
In this approach, V periodically multicasts a CoAP packet containing
V_INFO, see Figure 11. Upon receiving one or more CoAP messages and
processing V_INFO, U can decide whether or not to initiate the ELA
protocol with a given V. Next, the application can either keep U
acting as a server, and thus employ the EDHOC reverse flow, or
implement a CoAP client and use the forward flow.
+--------+---------+ +--------+---------+
| Init | Cli/Ser | | Resp | Cli/Ser |
+--------+---------+ +------------------+
| U | | V |
+------------------+ +------------------+
| |
| POST /ela-advertisement |
|<-----------------------------------------+
| CoAP multicast (contains V_INFO) |
| |
| EDHOC message_1 |
+----------------------------------------->|
| (?EAD_1 = Voucher_Info) |
| |
( ... protocol continues normally ... )
Figure 11: Advertising ELA using the network layer.
The V_INFO structure is sent as part of the CoAP payload. It is
encoded as a CBOR sequence:
V_INFO = (
?DOMAIN_ID: bstr,
)
One advantage of this approach is that, since U is the initiator,
it's identity is protected in the context of the EDHOC handshake. On
the other hand, the periodic multicast may have resource usage
impacts in the network.
Appendix D. Examples
This section presents high level examples of the protocol execution.
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Note: the examples below include samples of access policies used by
W. These are provided for the sake of completeness only, since the
authorization mechanism used by W is out of scope in this document.
D.1. Minimal
This is a simple example that demonstrates a successful execution of
ELA.
Premises:
* device u1 has ID_U = key id = 14
* the access policy in W specifies, via a list of ID_U, that device
u1 can enroll via any domain authenticator, i.e., the list
contains ID_U = 14. In this case, the policy only specifies a
restriction in terms of U, effectively allowing enrollment via any
V.
Execution:
1. device u1 discovers a gateway (v1) and tries to enroll
2. gateway v1 identifies the zero-touch join attempt by checking
that the label of EAD_1 = TBD1, and prepares a Voucher Request
using the information contained in the value of EAD_1
3. upon receiving the request, W obtains ID_U = 14, authorizes the
access, and replies with Voucher Response
D.2. Wrong gateway
In this example, a device u1 tries to enroll a domain via gateway v1,
but W denies the request because the pairing (u1, v1) is not
configured in its access policies.
This example also illustrates how the REJECT_INFO field of the EDHOC
error Access Denied could be used, in this case to suggest that the
device should select another gateway for the join procedure.
Premises:
* devices and gateways communicate via Bluetooth Low Energy (BLE),
therefore their network identifiers are MAC addresses (EUI-48)
* device u1 has ID_U = key id = 14
* there are 3 gateways in the radio range of u1:
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- v1 with MAC address = A2-A1-88-EE-97-75
- v2 with MAC address = 28-0F-70-84-51-E4
- v3 with MAC address = 39-63-C9-D0-5C-62
* the access policy in W specifies, via a mapping of shape (ID_U;
MAC1, MAC2, ...) that device u1 can only join via gateway v3,
i.e., the mapping is: (14; 39-63-C9-D0-5C-62)
* W is able to map the PoP key of the gateways to their respective
MAC addresses
Execution:
1. device u1 tries to join via gateway v1, which forwards the
request to W
2. W determines that MAC address A2-A1-88-EE-97-75 is not in the
access policy mapping, and replies with an error. The
error_content has REJECT_TYPE = 1, and the plaintext OPAQUE_INFO
(used to compute the encrypted REJECT_INFO) specifies a list of
suggested gateways = [h'3963C9D05C62']. The single element in
the list is the 6-byte MAC address of v3, serialized as a bstr.
3. gateway v1 assembles an EDHOC error "Access Denied" with
error_content, and sends it to u1
4. device u1 processes the error, decrypts REJECT_INFO, and retries
the protocol via gateway v3
Acknowledgments
The authors sincerely thank Aurelio Schellenbaum for his contribution
in the initial phase of this work, and Marco Tiloca for extensively
reviewing the document.
Authors' Addresses
Göran Selander
Ericsson AB
Sweden
Email: goran.selander@ericsson.com
John Preuß Mattsson
Ericsson AB
Sweden
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Email: john.mattsson@ericsson.com
Mališa Vučinić
INRIA
France
Email: malisa.vucinic@inria.fr
Geovane Fedrecheski
INRIA
France
Email: geovane.fedrecheski@inria.fr
Michael Richardson
Sandelman Software Works
Canada
Email: mcr+ietf@sandelman.ca
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