Lightweight Authorization for Authenticated Key Exchange.
draft-selander-ace-ake-authz-04
Network Working Group G. Selander
Internet-Draft J. Preuß Mattsson
Intended status: Informational Ericsson AB
Expires: 25 April 2022 M. Vučinić
INRIA
M. Richardson
Sandelman Software Works
A. Schellenbaum
ZHAW
22 October 2021
Lightweight Authorization for Authenticated Key Exchange.
draft-selander-ace-ake-authz-04
Abstract
This document describes a procedure for augmenting the lightweight
authenticated Diffie-Hellman key exchange protocol EDHOC with third
party assisted authorization, targeting constrained IoT deployments
(RFC 7228).
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 25 April 2022.
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Copyright (c) 2021 IETF Trust and the persons identified as the
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Description . . . . . . . . . . . . . . . . . . . . . 3
3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Device . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Domain Authenticator . . . . . . . . . . . . . . . . . . 4
3.3. Authorization Server . . . . . . . . . . . . . . . . . . 5
4. The Protocol . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Reuse of EDHOC . . . . . . . . . . . . . . . . . . . . . 6
4.3. Device <-> Authorization Server . . . . . . . . . . . . . 8
4.4. Device <-> Authenticator . . . . . . . . . . . . . . . . 11
4.5. Authenticator <-> Authorization Server . . . . . . . . . 13
5. ACE Profile . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Protocol Overview . . . . . . . . . . . . . . . . . . . . 16
5.2. AS Request Creation Hints . . . . . . . . . . . . . . . . 17
5.3. Client-to-AS Request . . . . . . . . . . . . . . . . . . 17
5.4. AS-to-Client Response . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8. Informative References . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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 a procedure for augmenting the lightweight
authenticated Diffie-Hellman key exchange EDHOC [I-D.ietf-lake-edhoc]
with third party-assisted authorization.
The procedure involves a device, a domain authenticator and an
authorization server. The device and authenticator perform mutual
authentication and authorization, assisted by the authorization
server which provides relevant authorization information to the
device (a "voucher") and to the authenticator.
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The protocol assumes that authentication between device and
authenticator is performed with EDHOC, and defines the integration of
a lightweight authorization procedure using the External
Authorization Data (EAD) field defined in EDHOC.
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]), or certificate
enrollment (such as [I-D.selander-ace-coap-est-oscore]), but it can
be applied to authorize other target interactions.
The protocol 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. This document
specifies a profile of the ACE framework [I-D.ietf-ace-oauth-authz].
Other settings such as EAP [RFC3748] are out of scope for this
specification.
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 some understanding of CBOR [RFC8949] and
EDHOC [I-D.ietf-lake-edhoc]. Appendix C.1 of [I-D.ietf-lake-edhoc]
contains some basic info about CBOR.
2. Problem Description
The (potentially constrained) device wants to enroll into a domain
over a constrained link. The device authenticates and enforces
authorization of the (non-constrained) domain authenticator with the
help of a voucher, and makes the enrollment request. The domain
authenticator authenticates the device and authorizes its enrollment.
Authentication between device and domain authenticator is made with
the lightweight authenticated Diffie-Hellman key exchange protocol
EDHOC [I-D.ietf-lake-edhoc]. The procedure is assisted by a (non-
constrained) authorization server located in a non-constrained
network behind the domain authenticator providing information to the
device and to the domain authenticator as part of the protocol.
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The objective of this document is to specify such a protocol which is
lightweight over the constrained link and reuses elements of EDHOC.
See illustration in Figure 1.
Voucher
EDHOC Info
+----------+ | | +---------------+ Voucher +---------------+
| | | | | | Request | |
| Device |--|----o-->| Domain |---------->| Authorization |
| |<-|---o----| Authenticator |<----------| Server |
| (U) |--|---|--->| (V) | Voucher | (W) |
| | | | | Response | |
+----------+ | +---------------+ +---------------+
Voucher
Figure 1: Overview of message flow. Link between U anv V is
constrained but link between V and W is not. Voucher Info and
Voucher are sent in EDHOC External Authorization Data.
3. Assumptions
3.1. Device
The device is pre-provisioned with an identity ID_U and asymmetric
key credentials: a private key, a public key (PK_U), and optionally a
public key certificate (Cert_PK_U), issued by a trusted third party
such as e.g. the device manufacturer, used to authenticate to the
domain authenticator. ID_U may be a reference or pointer to the
certificate.
The device is also provisioned with information about its
authorization server:
* At least one static public DH key of the authorization server
(G_W) used to ensure secure communication with the device (see
Section 4.3).
* Location information about the authorization server (LOC_W), e.g.
its domain name. This information may be available in the device
certificate Cert_PK_U.
3.2. Domain Authenticator
The domain authenticator has a private key and a corresponding public
key PK_V used to authenticate to the device.
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The domain authenticator needs to be able to locate the authorization
server of the device for which LOC_W is expected to be sufficient.
The communication between domain authenticator and authorization
server is assumed to be mutually authenticated and protected;
authentication credentials and communication security is out of
scope, except for as specified below in this section.
The domain authenticator may in principle use differents credentials
for authenticating to the authorization server and to the device, for
which PK_V is used. However, the domain authenticator MUST prove
possession of private key of PK_V to the authorization server since
the authorization server is asserting (by means of the voucher to the
device) that this credential belongs to the domain authenticator.
In this version of the draft it is assumed that the domain
authenticator authenticates to the authorization server with PK_V
using some authentication protocol providing proof of possession of
the private key, for example TLS 1.3 [RFC8446]. A future version of
this draft may specify explicit proof of possession of the private
key of PK_V in the voucher request, e.g., by including a signature of
the voucher request with the private key corresponding to PK_V.
3.3. Authorization Server
The authorization server has the private DH key corresponding to G_W,
which is used to secure the communication with the device (see
Section 4.3).
Authentication credentials and communication security used with the
domain authenticator is out of scope, except for the need to verify
the possession of the private key of PK_V as specified in
Section 3.2.
The authorization server provides to the device the authorization
decision for enrollment with the domain authenticator in the form of
a voucher. The authorization server provides information to the
domain authenticator about the device, such as the device's
certificate Cert_PK_U.
The authorization server needs to be available during the execution
of the protocol.
4. The Protocol
4.1. Overview
Three security sessions are going on in parallel:
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1. EDHOC [I-D.ietf-lake-edhoc] between device (U) and (domain)
authenticator (V)
2. Voucher Request/Response between authenticator (V) and
authorization server (W)
3. An exchange of voucher-related information, including the voucher
itself, between device (U) and authorization server (W), mediated
by the authenticator.
Figure 2 provides an overview of the message flow detailed in this
section. Only selected message fields of EDHOC are shown, for more
details see Section 3.1 of [I-D.ietf-lake-edhoc].
U V W
| | |
| SUITES_I, G_X, EAD_1 | |
+----------------------------------->| |
| EDHOC message_1 | SS, G_X, ENC_ID, ?PoP_V |
| +----------------------------->|
| | Voucher Request (VREQ) |
| | |
| | G_X, CERT_PK_U, Voucher |
| |<-----------------------------+
| | Voucher Response (VRES) |
| ID_CRED_R, Sig_or_MAC_2, EAD_2 | |
|<-----------------------------------+ |
| EDHOC message_2 | |
| | |
| ID_CRED_I, Sig_or_MAC_3 | |
+----------------------------------->| |
| EDHOC message_3 | |
where
EAD_1 = (L, Voucher_Info)
Voucher_Info = [LOC_W, ENC_ID]
EAD_2 = (L, Voucher)
Voucher = MAC(V_TYPE, SS, G_X, ID_U, PK_V)
Figure 2: W-assisted authorization of AKE between U and V: EDHOC
between U and V (only selected message fields shown), and Voucher
Request/Response between V and W.
4.2. Reuse of EDHOC
The protocol illustrated in Figure 2 reuses several components of
EDHOC:
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* G_X, the 'x' parameter of the ephemeral public Diffie-Hellman key
of party U, is also used in the protocol between U and W, as
ephemeral key and nonce.
* SUITES_I, the cipher suites relevant to U, which includes the
selected cipher suite - here denoted SS, also defines the
algorithms used between U and W. In particular SS contains
information about (see Section 3.6 of [I-D.ietf-lake-edhoc]):
- EDHOC AEAD algorithm: used to encrypt the identity of U
- EDHOC hash algorithm: used for key derivation and to calculate
the voucher
- EDHOC MAC length in bytes: length of the voucher
- EDHOC key exchange algorithm: used to calculate the shared
secret between U and W
* EAD_1, EAD_2 are the External Authorization Data of message_1 and
message_2, respectively, for which dedicated content is defined in
this document.
* ID_CRED_I and ID_CRED_R are used to identify the public
authentication keys of U and V. In this protocol ID_CRED_I can be
empty since V obtains the certificate of U from W, whereas
ID_CRED_R contains the public authentication key of V.
* Signature_or_MAC_2 and Signature_or_MAC_3 (abbreviated in
Figure 2), containing data generated using the private key of V
and U, respectively, are shown here just to be able to reason
about the use of credentials.
The protocol also reuses the Extract and Expand key derivation from
EDHOC (Section 4 of [I-D.ietf-lake-edhoc]).
* The intermediate pseudo-random key PRK is derived using Extract():
- PRK = Extract(salt, IKM)
o where salt = 0x (the zero-length byte string)
o IKM is the ECDH shared secret G_XW (calculated from G_X and
W or G_W and X) as defined in Section 6.3.1 of [I-D.ietf-
cose-rfc8152bis-algs].
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The shared secret is derived using Expand() which is defined in terms
of the EDHOC hash algorithm of the selected cipher suite, see
Section 4.2. of [I-D.ietf-lake-edhoc]:
* shared secret = Expand(PRK, info, length)
where
info = (
transcript_hash : bstr,
label : tstr,
context : bstr,
length : uint,
)
4.3. Device <-> Authorization Server
The protocol between device and authorization server (U and W in
Figure 2) is carried out via the authenticator (V) with certain data
protected between the endpoints using the equivalent of a hybrid
encryption scheme (see, e.g., [I-D.irtf-cfrg-hpke]). The device uses
the public DH key of the authorization server G_W together with the
private DH key corresponding to ephemeral key G_X in EDHOC message_1,
and vice versa for the authorization server. The endpoints calculate
a shared secret G_XW (see Section 4.2), which is used to derive
secret keys to protect data between U and W, as detailed in this
section.
The data exchanged betweeen U and W is carried between U and V in
EAD_1 and EAD_2 (Section 4.4), and between V and W in Voucher
Request/Response (Section 4.5).
4.3.1. Voucher Info
The external authorization data EAD_1 of EDHOC message_1 includes
Voucher Info, which is the following CBOR sequence:
Voucher_Info = (
LOC_W: tstr,
ENC_ID: bstr
)
where
* LOC_W is location information about the authorization server, used
by the authenticator
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* ENC_ID is the encrypted blob carrying the identity of the device
and an optional identity of the authenticator, passed on from the
authenticator to the authorization server, calculated as follows:
ENC_ID is 'ciphertext' of COSE_Encrypt0 (Section 5.2-5.3 of
[RFC8152]) 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,
? ID_V: bstr,
)
external_aad = (
SS: int,
)
where
* ID_U is the identity of the device, for example a reference or
pointer to the device certificate
* ID_V is the identity of the authenticator as presented to the
authorization server. This may be a name in a name space agreed
out-of-band and managed by a party trusted by the authorization
server, for example a common name of an X.509 certificate signed
by a CA trusted by the authorization server. The value may be
obtained by the device through out-of-band means, possibly through
secure network discovery.
* SS is the selected cipher suite in SUITES_I.
The derivation of K_1 = Expand(PRK, info, length) uses the following
input to the info struct (Section 4.2):
* transcript_hash = h''
* label is "EDHOC_ACE_AKE_AUTHZ_K_1"
* context = h''
* length is length of key of the EDHOC AEAD algorithm
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The derivation of IV_1 = Expand(PRK, info, length) uses the following
input to the info struct (Section 4.2):
* transcript_hash = h''
* label is "EDHOC_ACE_AKE_AUTHZ_IV_1"
* context = h''
* length is length of nonce of the EDHOC AEAD algorithm
4.3.2. Voucher
The voucher is an assertion by the authorization server to the device
that the authorization server has performed the relevant
verifications and that the device is authorized to continue the
protocol with the authenticator. The Voucher is essentially a
message authentication code which binds the identity of the
authenticator to message_1 of EDHOC, integrity protected with the
shared secret context between U and W.
The calculation of Voucher = Expand(PRK, info, length) uses the
following input to the info struct (Section 4.2):
* transcript_hash = h''
* label is "EDHOC_ACE_AKE_AUTHZ_MAC"
* context = bstr .cbor voucher_input
* length is EDHOC MAC length in bytes
where context is a CBOR bstr wrapping of voucher_input:
voucher_input = (
V_TYPE: int,
SS: int,
G_X: bstr,
ID_U: bstr,
PK_V: bstr,
)
where
* V_TYPE indicates the type of voucher used (TBD)
* SS is the selected cipher suite of the EDHOC protocol, see
Section 4.2
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* PK_V is a CWT Claims Set (CCS, [RFC8392]) containing the public
authentication key of the authenticator encoded as a COSE_Key in
the 'cnf' claim, see Section 3.5.3 of [I-D.ietf-lake-edhoc].
* G_X is encoded as in EDHOC message_1, see Section 3.7 of
[I-D.ietf-lake-edhoc]
* ID_U is defined in Section 4.3
Editor's note: With the current definition of EAD as (ead_label,
ead_value), do we need to redefine the voucher to be a CBOR map? Do
we even need the V_TYPE?
4.4. Device <-> Authenticator
The device and authenticator run the EDHOC protocol authenticated
with their public keys (PK_U and PK_V), see Figure 2. Normal EDHOC
processing is omitted here.
4.4.1. Message 1
4.4.1.1. Device processing
The device composes EDHOC message_1 using authentication method,
identifiers, etc. according to the applicability statement, see
Section 3.9 of [I-D.ietf-lake-edhoc]. The selected cipher suite SS
applies also to the interaction with the authorization server as
detailed in Section 4.2, in particular, the key agreement algorithm
which is used with the static public DH key G_W of the authorization
server. As part of the normal EDHOC processing, the device generates
the ephemeral public key G_X which is reused in the interaction with
the authorization server, see Section 4.3.
The device sends EDHOC message_1 with EAD_1 = (L, Voucher_Info) where
L is the External Auxiliary Data Label for this protocol (IANA
registry created in Section 9.5 of [I-D.ietf-lake-edhoc]), and
Voucher_Info is specified in Section 4.3.
4.4.1.2. Authenticator processing
The authenticator receives EDHOC message_1 from the device and
processes as specified in Section 5.2.3 of [I-D.ietf-lake-edhoc],
with the additional step that the presence of EAD with label L
triggers the voucher request to the authorization server as described
in Section 4.5. The exchange with V needs to be completed
successfully for the EDHOC exchange to be continued.
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4.4.2. Message 2
4.4.2.1. Authenticator processing
The authenticator receives the voucher response from the
authorization server as described in Section 4.5.
The authenticator sends EDHOC message_2 to the device with EAD_2 =
(L, Voucher) where L is the External Auxiliary Data Label for this
protocol (IANA registry created in Section 9.5 of
[I-D.ietf-lake-edhoc]) and the Voucher is specified in Section 4.3.
CRED_R is a CWT Claims Set (CCS, [RFC8392]) containing the public
authentication key of the authenticator PK_V encoded as a COSE_Key in
the 'cnf' claim, see Section 3.5.3 of [I-D.ietf-lake-edhoc].
ID_CRED_R contains the CCS with 'kccs' as COSE header_map, see
Section 9.6 of [I-D.ietf-lake-edhoc]. The Sig_or_MAC_2 field
calculated using the private key corresponding to PK_V is either
signature or MAC depending on EDHOC method.
4.4.2.2. Device processing
In addition to normal EDHOC verifications, the device MUST verify the
Voucher by performing the same calculation as in Section 4.3.2 using
the SS, G_X and ID_U carried in message_1 and PK_V received in
message_2. If the voucher calculated in this way is not identical to
what was received in message_2, then the device MUST discontinue the
protocol.
Editor's note: Consider replace SS, G_X, ID_U in Voucher with
H(message_1), since that is already required by EDHOC to be cached by
the initiator. H(message_1) needs to be added to VREQ message in
that case.
4.4.3. Message 3
4.4.3.1. Device processing
If all verifications are passed, then the device sends EDHOC
message_3.
The message field ID_CRED_I contains data enabling the authenticator
to retrieve the public key of the device, PK_U. Since the
authenticator before sending message_2 received a certificate of PK_U
from the authorization server (see Section 4.5), ID_CRED_I SHALL be a
COSE header_map of type 'kid' with the empty byte string as value:
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ID_CRED_I =
{
4 : h''
}
The Sig_or_MAC_3 field calculated using the private key corresponding
to PK_U is either signature or MAC depending on EDHOC method.
EAD_3 MAY contain an enrolment request, see e.g. CSR specified in
[I-D.mattsson-cose-cbor-cert-compress], or other request which the
device is now authorized to make.
EDHOC message_3 may be combined with an OSCORE request, see
[I-D.palombini-core-oscore-edhoc].
4.4.3.2. Authenticator processing
The authenticator performs the normal EDHOC verifications of
message_3, with the exception that the Sig_or_MAC_3 field MUST be
verified using the public key included in Cert_PK_U (see
Section 4.5.2) received from the authorization server. The
authenticator MUST ignore any key related information obtained in
ID_CRED_I.
This enables the authenticator to verify that message_3 was generated
by the device authorized by the authorization server as part of the
associated Voucher Request/Response procedure (see Section 4.5).
4.5. Authenticator <-> Authorization Server
The authenticator and authorization server are assumed to have, or to
be able to, set up a secure connection, for example TLS 1.3
authenticated with certificates. The authenticator is assumed to
authenticate with the public key PK_V, see Section 3.2.
This secure connection protects the Voucher Request/Response Protocol
(see protocol between V and W in Figure 2).
The ephemeral public key G_X sent in EDHOC message_1 from device to
authenticator serves as challenge/response nonce for the Voucher
Request/Response Protocol, and binds together instances of the two
protocols.
4.5.1. Voucher Request
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4.5.1.1. Authenticator processing
Unless already in place, the authenticator and the authorization
server establish a secure connection. The autenticator uses G_X
received from the device as a nonce associated to this connection
with the authorization server. If the same value of the nonce G_X is
already used for a connection with this or other authorization
server, the protocol SHALL be discontinued.
The authenticator sends the voucher request to the authorization
server. The Voucher Request SHALL be a CBOR array as defined below:
Voucher_Request = [
SS: int,
G_X: bstr,
ENC_ID: bstr,
? PoP_V: bstr,
]
where all parameters are defined in Section 4.3, except
* PoP_V is a proof-of-possession of public key PK_V using the
corresponding private key
Editor's note: Define PoP_V (include G_X, ENC_ID in the calculation
for binding to this EDHOC session). One case to study is when V
authenticates to U with static DH and to W with signature.
4.5.1.2. Authorization Server processing
The authorization server receives the voucher request, verifies and
decrypts the identity ID_U of the device, and associates the nonce
G_X to ID_U. If G_X is not unique among nonces associated to this
identity, the protocol SHALL be discontinued. If ENC_ID also
included the identity of V, ID_V, then the authorization server
performs an additional check to verify that the identity of the
authenticator who sent the voucher request over a secure session
between V-W matches the identity of the authenticator as observed by
U. If the identities of V as observed by U, and as observed by W, do
not match, the protocol SHALL be discontinued.
4.5.2. Voucher Response
4.5.2.1. Authorization Server processing
The authorization server uses the identity of the device, ID_U, to
look up the device certificate, Cert_PK_U.
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The authorization server retrieves the public key of V used to
authenticate the secure connection with the authenticator, and
constructs the CWT Claims Set and the Voucher as defined in
Section 4.3.2.
The authorization server generates the voucher response and sends it
to the authenticator over the secure connection. The
Voucher_Response SHALL be a CBOR array as defined below:
Voucher_Response = [
G_X: bstr,
CERT_PK_U: bstr,
Voucher: bstr
]
where
* G_X is copied from the associated voucher request.
* CERT_PK_U is the device certificate of the public key PK_U, issued
by a trusted third party. The format of this certificate is out
of scope.
* The Voucher is defined in Section 4.3.2.
4.5.2.2. Authenticator processing
The authenticator receives the voucher response from the
authorization server over the secure connection. If the received G_X
does not match the value of the nonce associated to the secure
connection, the protocol SHALL be discontinued.
The authenticator verifies the certificate CERT_PK_U and that U is an
admissible device, or else discontinues the protocol.
5. ACE Profile
The messages specified in this document may be carried between the
endpoints in various protocols. This section defines an embedding as
a profile of the ACE framework (see Appendix C of
[I-D.ietf-ace-oauth-authz]).
* U plays the role of the ACE Resource Server (RS).
* V plays the role of the ACE Client (C).
* W plays the role of the ACE Authorization Server (AS).
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Many readers who are used to the diagram having the Client on the
left may be surprised at the cast of characters. The "resource"
which C (V) is trying to access is the "ownership" of U. The AS (W)
is the manufacturer (or previous owner) of RS (U), and is therefore
in a position to grant C (V) ownership of RS (U).
C and RS use EDHOC's EAD to communicate. C and RS use the EDHOC
protocol to protect their communication. EDHOC also provides mutual
authentication of C and RS, assisted by the AS.
5.1. Protocol Overview
RS (U) C (V) AS (W)
| EDHOC message_1 | |
| AD1=AS Request Creation Hints | |
|-------------------------------->| POST /token |
| |-------------------->|
| | |
| | Access Token + |
| EDHOC message_2 | Access Information |
| AD2=Access Token |<--------------------|
|<--------------------------------| |
| EDHOC message_3 | |
|-------------------------------->| |
Figure 3: Overview of the protocol mapping to ACE
1. RS proactively sends the AS Request Creation Hints message to C
to signal the information on where C can reach the AS.
2. RS piggybacks the AS Request Creation Hints message using
Auxiliary Data of EDHOC message_1.
3. Before continuing the EDHOC exchange, based on the AS Request
Creation Hints information, C sends a POST request to the token
endpoint at the AS requesting the access token.
4. The AS issues an assertion to C that is cryptographically
protected based on the secret shared between the AS and RS. In
this profile, the assertion is encoded as a Bearer Token.
5. C presents this token to RS in EAD_2.
6. RS verifies the token based on the possession of the shared
secret with the AS and authenticates C.
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5.2. AS Request Creation Hints
Parameters that can appear in the AS Request Creation Hints message
are specified in Section 5.3 of [I-D.ietf-ace-oauth-authz]. RS MUST
use the "AS" parameter to transport LOC_W, i.e. an absolute URI where
C can reach the AS. RS MUST use the "audience" parameter to
transport the CBOR sequence consisting of two elements: SS, the
selected cipher suite; ENC_ID, the AEAD encrypted blob containing
identities. The "cnonce" parameter MUST be implied to G_X, i.e. the
ephemeral public key of the RS in the underlying EDHOC exchange. The
"cnonce" parameter is not carried in the AS Request Creation Hints
message for byte saving reasons. AS Request Creation Hints MUST be
carried within EAD_1.
An example EAD_1 value in CBOR diagnostic notation is shown below:
EAD_1:
{
"AS" : "coaps://as.example.com/token",
"audience": << h'73',h'737570657273...' >>
}
5.3. Client-to-AS Request
The protocol that provides the secure connection between C and the AS
is out-of-scope. This can, for example, be TLS 1.3. What is
important is that the two peers are mutually authenticated, and that
the secure connection provides message integrity, confidentiality and
freshness. It is also necessary for the AS to be able to extract the
public key of C used in the underlying security handshake.
C sends the POST request to the token endpoint at the AS following
Section 5.8.1. of [I-D.ietf-ace-oauth-authz]. C MUST set the
"audience" parameter to the value received in AS Request Creation
Hints. C MUST set the "cnonce" parameter to G_X, the ephemeral
public key of RS in the EDHOC exchange.
An example exchange using CoAP and CBOR diagnostic notation is shown
below:
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Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"audience" : << h'73',h'737570657273...' >>
"cnonce" : h'756E73686172...'
}
5.4. AS-to-Client Response
Given successful authorization of C at the AS, the AS responds by
issuing a Bearer token and retrieves the certificate of RS on behalf
of C. The access token and the certificate are passed back to C, who
uses it to complete the EDHOC exchange. This document extends the
ACE framework by registering a new Access Information parameter:
rsp_ad: OPTIONAL. Carries additional information from the AS to C
associated with the access token.
The AS-to-Client responsE MUST contain:
* ace_profileparameter set to "edhoc-authz"
* token_type parameter set to "Bearer"
* access_token as specified in Section 4.3.2
* rsp_ad = bstr .cbor cert_gx
cert_gx = (
CERT_PK_U: bstr,
G_X: bstr
)
where:
* CERT_PK_U is the RS's certificate, as discussed in Section 4.5.2.
To be able to retrieve this certificate, the AS first needs to
decrypt the audience value and obtain the RS's identity.
* G_X is the ephemeral key generated by RS in EDHOC message_1.
An example AS response to C is shown below:
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2.01 Created
Content-Format: application/ace+cbor
Max-Age: 3600
Payload:
{
"ace_profile" : "edhoc-authz",
"token_type" : "Bearer",
"access_token" : h'666F726571756172746572...',
"rsp_ad" : h'61726973746F64656D6F637261746963616C...'
}
6. 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
[I-D.ietf-lake-edhoc] apply with modifications discussed here.
EDHOC provides identity protection of the Initiator, disclosed to the
Responder in message_3. The sending of the certificate of U in the
Voucher Response provides information about the identity of the
device already before message_2, which changes the identity
protection properties and thus needs to be validated against a given
use case. The authorization server authenticates the authenticator,
receives the Voucher Request, and can perform potential other
verifications before sending the Voucher Response. This allows the
authorization server to restrict information about the identity of
the device to parties which are authorized to have that. However, if
there are multiple authorized authenticators, the authorization
server may not be able to distinguish between authenticator V which
the device is connecting to and a misbehaving but authorized
authenticator V' constructing a Voucher Request built from an
eavesdropped message_1. A mitigation for this kind of misbehaving
authenticator is that the device discovers the identity of the
authenticator through out-of-bands means before attempting to enroll,
and include the optional ID_V in the ENC_ID encrypted blob. For
example, the network's discovery mechanism can carry asserted
information on the associated identity of the authenticator. The use
of ID_V also changes the identity protection assumptions since it
requires U to know the identity of V before the protocol starts. The
identity of V is still protected against passive adversaries, unless
disclosed by the out-of-band mechanism by which U acquires
information about the identity of V. The privacy considerations
whether the identity of the device or of the authenticator is more
sensitive need to be studied depending on a specific use case.
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For use cases where neither the early disclosure of the device nor of
the authenticator identities are deemed acceptable, the device
certificate must not be sent in the Voucher Response, and the
identity of V must be omitted. Instead, the device certificate could
be retrieved from the authorization server or other certificate
repository after message_3 is received by the authenticator, using
the device identifier provided in ID_CRED_I as lookup. This would
require the device identity to be transported in both message_1 (in
EAD_1) and message_3 but would make the protocol comply with the
default identity protection provided by EDHOC.
The encryption of the device identity in the first message should
consider potential information leaking from the length of the
identifier ID_U, either by making all identifiers having the same
length or the use of a padding scheme.
As noted Section 8.2 of [I-D.ietf-lake-edhoc] 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 authorization server.
The private ephemeral key is thus used in the device for calculations
of key material relating to both the authenticator and the
authorization 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 with input of public key
of W (G_W) and identifier of U (ID_U), and produce the encryption of
ID_U which is included in the external authorization data EAD_1.
7. IANA Considerations
TODO: register rsp_ad ACE parameter
8. Informative References
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
draft-ietf-ace-oauth-authz-45, 29 August 2021,
<https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
authz-45.txt>.
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[I-D.ietf-lake-edhoc]
Selander, G., Mattsson, J. P., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
Progress, Internet-Draft, draft-ietf-lake-edhoc-12, 20
October 2021, <https://www.ietf.org/archive/id/draft-ietf-
lake-edhoc-12.txt>.
[I-D.ietf-lake-reqs]
Vucinic, M., Selander, G., Mattsson, J. P., and D. Garcia-
Carrillo, "Requirements for a Lightweight AKE for OSCORE",
Work in Progress, Internet-Draft, draft-ietf-lake-reqs-04,
8 June 2020, <https://www.ietf.org/archive/id/draft-ietf-
lake-reqs-04.txt>.
[I-D.irtf-cfrg-hpke]
Barnes, R. L., Bhargavan, K., Lipp, B., and C. A. Wood,
"Hybrid Public Key Encryption", Work in Progress,
Internet-Draft, draft-irtf-cfrg-hpke-12, 2 September 2021,
<https://www.ietf.org/archive/id/draft-irtf-cfrg-hpke-
12.txt>.
[I-D.mattsson-cose-cbor-cert-compress]
Raza, S., Höglund, J., Selander, G., Mattsson, J. P., and
M. Furuhed, "CBOR Encoded X.509 Certificates (C509
Certificates)", Work in Progress, Internet-Draft, draft-
mattsson-cose-cbor-cert-compress-08, 22 February 2021,
<https://www.ietf.org/archive/id/draft-mattsson-cose-cbor-
cert-compress-08.txt>.
[I-D.palombini-core-oscore-edhoc]
Palombini, F., Tiloca, M., Hoeglund, R., Hristozov, S.,
and G. Selander, "Combining EDHOC and OSCORE", Work in
Progress, Internet-Draft, draft-palombini-core-oscore-
edhoc-02, 19 February 2021,
<https://www.ietf.org/archive/id/draft-palombini-core-
oscore-edhoc-02.txt>.
[I-D.selander-ace-coap-est-oscore]
Selander, G., Raza, S., Furuhed, M., Vucinic, M., and T.
Claeys, "Protecting EST Payloads with OSCORE", Work in
Progress, Internet-Draft, draft-selander-ace-coap-est-
oscore-05, 5 May 2021, <https://www.ietf.org/archive/id/
draft-selander-ace-coap-est-oscore-05.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<https://www.rfc-editor.org/info/rfc3748>.
[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>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[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>.
[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>.
[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>.
[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>.
[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>.
Authors' Addresses
Göran Selander
Ericsson AB
Sweden
Email: goran.selander@ericsson.com
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John Preuß Mattsson
Ericsson AB
Sweden
Email: john.mattsson@ericsson.com
Mališa Vučinić
INRIA
France
Email: malisa.vucinic@inria.fr
Michael Richardson
Sandelman Software Works
Canada
Email: mcr+ietf@sandelman.ca
Aurelio Schellenbaum
Institute of Embedded Systems, ZHAW
Switzerland
Email: aureliorubendario.schellenbaum@zhaw.ch
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