ACE Working Group S. Gerdes
Internet-Draft O. Bergmann
Intended status: Standards Track C. Bormann
Expires: January 7, 2021 Universitaet Bremen TZI
G. Selander
Ericsson AB
L. Seitz
Combitech
July 06, 2020
Datagram Transport Layer Security (DTLS) Profile for Authentication and
Authorization for Constrained Environments (ACE)
draft-ietf-ace-dtls-authorize-12
Abstract
This specification defines a profile of the ACE framework that allows
constrained servers to delegate client authentication and
authorization. The protocol relies on DTLS version 1.2 for
communication security between entities in a constrained network
using either raw public keys or pre-shared keys. A resource-
constrained server can use this protocol to delegate management of
authorization information to a trusted host with less severe
limitations regarding processing power and memory.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 7, 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Communication Between the Client and the Authorization
Server . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . 6
3.2.1. DTLS Channel Setup Between Client and Resource Server 9
3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 10
3.3.1. DTLS Channel Setup Between Client and Resource Server 14
3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 16
4. Dynamic Update of Authorization Information . . . . . . . . . 17
5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 19
6. Secure Communication with an Authorization Server . . . . . . 19
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7.1. Reuse of Existing Sessions . . . . . . . . . . . . . . . 21
7.2. Multiple Access Tokens . . . . . . . . . . . . . . . . . 21
7.3. Out-of-Band Configuration . . . . . . . . . . . . . . . . 22
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
11.1. Normative References . . . . . . . . . . . . . . . . . . 23
11.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
This specification defines a profile of the ACE framework
[I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource
server use CoAP [RFC7252] over DTLS version 1.2 [RFC6347] to
communicate. The client obtains an access token, bound to a key (the
proof-of-possession key), from an authorization server to prove its
authorization to access protected resources hosted by the resource
server. Also, the client and the resource server are provided by the
authorization server with the necessary keying material to establish
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a DTLS session. The communication between client and authorization
server may also be secured with DTLS. This specification supports
DTLS with Raw Public Keys (RPK) [RFC7250] and with Pre-Shared Keys
(PSK) [RFC4279].
The ACE framework requires that client and server mutually
authenticate each other before any application data is exchanged.
DTLS enables mutual authentication if both client and server prove
their ability to use certain keying material in the DTLS handshake.
The authorization server assists in this process on the server side
by incorporating keying material (or information about keying
material) into the access token, which is considered a "proof of
possession" token.
In the RPK mode, the client proves that it can use the RPK bound to
the token and the server shows that it can use a certain RPK.
The resource server needs access to the token in order to complete
this exchange. For the RPK mode, the client must upload the access
token to the resource server before initiating the handshake, as
described in Section 5.8.1 of the ACE framework
[I-D.ietf-ace-oauth-authz].
In the PSK mode, client and server show with the DTLS handshake that
they can use the keying material that is bound to the access token.
To transfer the access token from the client to the resource server,
the "psk_identity" parameter in the DTLS PSK handshake may be used
instead of uploading the token prior to the handshake.
As recommended in Section 5.8 of [I-D.ietf-ace-oauth-authz], this
specification uses CBOR web tokens to convey claims within an access
token issued by the server. While other formats could be used as
well, those are out of scope for this document.
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 be familiar with the terms and concepts
described in [I-D.ietf-ace-oauth-authz] and in
[I-D.ietf-ace-oauth-params].
The authorization information (authz-info) resource refers to the
authorization information endpoint as specified in
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[I-D.ietf-ace-oauth-authz]. The term "claim" is used in this
document with the same semantics as in [I-D.ietf-ace-oauth-authz],
i.e., it denotes information carried in the access token or returned
from introspection.
2. Protocol Overview
The CoAP-DTLS profile for ACE specifies the transfer of
authentication information and, if necessary, authorization
information between the client (C) and the resource server (RS)
during setup of a DTLS session for CoAP messaging. It also specifies
how the client can use CoAP over DTLS to retrieve an access token
from the authorization server (AS) for a protected resource hosted on
the resource server. As specified in Section 6.7 of
[I-D.ietf-ace-oauth-authz], use of DTLS for one or both of these
interactions is completely independent
This profile requires the client to retrieve an access token for
protected resource(s) it wants to access on the resource server as
specified in [I-D.ietf-ace-oauth-authz]. Figure 1 shows the typical
message flow in this scenario (messages in square brackets are
optional):
C RS AS
| [---- Resource Request ------>]| |
| | |
| [<-AS Request Creation Hints-] | |
| | |
| ------- Token Request ----------------------------> |
| | |
| <---------------------------- Access Token --------- |
| + Access Information |
Figure 1: Retrieving an Access Token
To determine the authorization server in charge of a resource hosted
at the resource server, the client can send an initial Unauthorized
Resource Request message to the resource server. The resource server
then denies the request and sends an AS Request Creation Hints
message containing the address of its authorization server back to
the client as specified in Section 5.1.2 of
[I-D.ietf-ace-oauth-authz].
Once the client knows the authorization server's address, it can send
an access token request to the token endpoint at the authorization
server as specified in [I-D.ietf-ace-oauth-authz]. As the access
token request as well as the response may contain confidential data,
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the communication between the client and the authorization server
must be confidentiality-protected and ensure authenticity. The
client may have been registered at the authorization server via the
OAuth 2.0 client registration mechanism as outlined in Section 5.3 of
[I-D.ietf-ace-oauth-authz].
The access token returned by the authorization server can then be
used by the client to establish a new DTLS session with the resource
server. When the client intends to use an asymmetric proof-of-
possession key in the DTLS handshake with the resource server, the
client MUST upload the access token to the authz-info resource, i.e.
the authz-info endpoint, on the resource server before starting the
DTLS handshake, as described in Section 5.8.1 of
[I-D.ietf-ace-oauth-authz]. In case the client uses a symmetric
proof-of-possession key in the DTLS handshake, the procedure as above
MAY be used, or alternatively, the access token MAY instead be
transferred in the DTLS ClientKeyExchange message (see
Section 3.3.1). In any case, DTLS MUST be used in a mode that
provides replay protection.
Figure 2 depicts the common protocol flow for the DTLS profile after
the client has retrieved the access token from the authorization
server, AS.
C RS AS
| [--- Access Token ------>] | |
| | |
| <== DTLS channel setup ==> | |
| | |
| == Authorized Request ===> | |
| | |
| <=== Protected Resource == | |
Figure 2: Protocol overview
3. Protocol Flow
The following sections specify how CoAP is used to interchange
access-related data between the resource server, the client and the
authorization server so that the authorization server can provide the
client and the resource server with sufficient information to
establish a secure channel, and convey authorization information
specific for this communication relationship to the resource server.
Section 3.1 describes how the communication between the client (C)
and the authorization server (AS) must be secured. Depending on the
used CoAP security mode (see also Section 9 of [RFC7252], the Client-
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to-AS request, AS-to-Client response (see Section 5.6 of
[I-D.ietf-ace-oauth-authz]) and DTLS session establishment carry
slightly different information. Section 3.2 addresses the use of raw
public keys while Section 3.3 defines how pre-shared keys are used in
this profile.
3.1. Communication Between the Client and the Authorization Server
To retrieve an access token for the resource that the client wants to
access, the client requests an access token from the authorization
server. Before the client can request the access token, the client
and the authorization server MUST establish a secure communication
channel. This profile assumes that the keying material to secure
this communication channel has securely been obtained either by
manual configuration or in an automated provisioning process. The
following requirements in alignment with Section 6.5 of
[I-D.ietf-ace-oauth-authz] therefore must be met:
o The client MUST securely have obtained keying material to
communicate with the authorization server.
o Furthermore, the client MUST verify that the authorization server
is authorized to provide access tokens (including authorization
information) about the resource server to the client, and that
this authorization information about the authorization server is
still valid.
o Also, the authorization server MUST securely have obtained keying
material for the client, and obtained authorization rules approved
by the resource owner (RO) concerning the client and the resource
server that relate to this keying material.
The client and the authorization server MUST use their respective
keying material for all exchanged messages. How the security
association between the client and the authorization server is
bootstrapped is not part of this document. The client and the
authorization server must ensure the confidentiality, integrity and
authenticity of all exchanged messages within the ACE protocol.
Section 6 specifies how communication with the authorization server
is secured.
3.2. RawPublicKey Mode
When the client and the resource server use RawPublicKey
authentication, the procedure is as follows: After the client and the
authorization server mutually authenticated each other and validated
each other's authorization, the client sends a token request to the
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authorization server's token endpoint. The client MUST add a
"req_cnf" object carrying either its raw public key or a unique
identifier for a public key that it has previously made known to the
authorization server. It is RECOMMENDED that the client uses DTLS
with the same keying material to secure the communication with the
authorization server, proving possession of the key as part of the
token request. Other mechanisms for proving possession of the key
may be defined in the future.
An example access token request from the client to the authorization
server is depicted in Figure 3.
POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
grant_type : client_credentials,
req_aud : "tempSensor4711",
req_cnf : {
COSE_Key : {
kty : EC2,
crv : P-256,
x : h'e866c35f4c3c81bb96a1...',
y : h'2e25556be097c8778a20...'
}
}
}
Figure 3: Access Token Request Example for RPK Mode
The example shows an access token request for the resource identified
by the string "tempSensor4711" on the authorization server using a
raw public key.
The authorization server MUST check if the client that it
communicates with is associated with the RPK in the "req_cnf"
parameter before issuing an access token to it. If the authorization
server determines that the request is to be authorized according to
the respective authorization rules, it generates an access token
response for the client. The access token MUST be bound to the RPK
of the client by means of the "cnf" claim.
The response MAY contain a "profile" parameter with the value
"coap_dtls" to indicate that this profile MUST be used for
communication between the client and the resource server. The
"profile" may be specified out-of-band, in which case it does not
have to be sent. The response also contains an access token with
information for the resource server about the client's public key.
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The authorization server MUST return in its response the parameter
"rs_cnf" unless it is certain that the client already knows the
public key of the resource server. The authorization server MUST
ascertain that the RPK specified in "rs_cnf" belongs to the resource
server that the client wants to communicate with. The authorization
server MUST protect the integrity of the access token such that the
resource server can detect unauthorized changes. If the access token
contains confidential data, the authorization server MUST also
protect the confidentiality of the access token.
The client MUST ascertain that the access token response belongs to a
certain previously sent access token request, as the request may
specify the resource server with which the client wants to
communicate.
An example access token response from the authorization server to the
client is depicted in Figure 4. Here, the contents of the
"access_token" claim have been truncated to improve readability.
Caching proxies process the Max-Age option in the CoAP response which
has a default value of 60 seconds (Section 5.6.1 of [RFC7252]). The
authorization server SHOULD adjust the Max-Age option such that it
does not exceed the "expires_in" parameter to avoid stale responses.
2.01 Created
Content-Format: application/ace+cbor
Max-Age: 3560
Payload:
{
access_token : b64'SlAV32hkKG...
(remainder of CWT omitted for brevity;
CWT contains the client's RPK in the cnf claim)',
expires_in : 3600,
rs_cnf : {
COSE_Key : {
kty : EC2,
crv : P-256,
x : h'd7cc072de2205bdc1537...',
y : h'f95e1d4b851a2cc80fff...'
}
}
}
Figure 4: Access Token Response Example for RPK Mode
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3.2.1. DTLS Channel Setup Between Client and Resource Server
Before the client initiates the DTLS handshake with the resource
server, the client MUST send a "POST" request containing the obtained
access token to the authz-info resource hosted by the resource
server. After the client receives a confirmation that the resource
server has accepted the access token, it SHOULD proceed to establish
a new DTLS channel with the resource server. The client MUST use its
correct public key in the DTLS handshake. If the authorization
server has specified a "cnf" field in the access token response, the
client MUST use this key. Otherwise, the client MUST use the public
key that it specified in the "req_cnf" of the access token request.
The client MUST specify this public key in the SubjectPublicKeyInfo
structure of the DTLS handshake as described in [RFC7250].
To be consistent with [RFC7252] which allows for shortened MAC tags
in constrained environments, an implementation that supports the RPK
mode of this profile MUST at least support the ciphersuite
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251]. As discussed in
[RFC7748], new ECC curves have been defined recently that are
considered superior to the so-called NIST curves. This specification
therefore mandates implementation support for curve25519 (cf.
[RFC8032], [RFC8422]) as this curve said to be efficient and less
dangerous regarding implementation errors than the secp256r1 curve
mandated in [RFC7252].
The resource server MUST check if the access token is still valid, if
the resource server is the intended destination (i.e., the audience)
of the token, and if the token was issued by an authorized
authorization server. The access token is constructed by the
authorization server such that the resource server can associate the
access token with the Client's public key. The "cnf" claim MUST
contain either the client's RPK or, if the key is already known by
the resource server (e.g., from previous communication), a reference
to this key. If the authorization server has no certain knowledge
that the Client's key is already known to the resource server, the
Client's public key MUST be included in the access token's "cnf"
parameter. If CBOR web tokens [RFC8392] are used (as recommended in
[I-D.ietf-ace-oauth-authz]), keys MUST be encoded as specified in
[RFC8747].
The raw public key used in the DTLS handshake with the client MUST
belong to the resource server. If the resource server has several
raw public keys, it needs to determine which key to use. The
authorization server can help with this decision by including a "cnf"
parameter in the access token that is associated with this
communication. In this case, the resource server MUST use the
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information from the "cnf" field to select the proper keying
material.
Thus, the handshake only finishes if the client and the resource
server are able to use their respective keying material.
3.3. PreSharedKey Mode
To retrieve an access token for the resource that the client wants to
access, the client MAY include a "cnf" object carrying an identifier
for a symmetric key in its access token request to the authorization
server. This identifier can be used by the authorization server to
determine the shared secret to construct the proof-of-possession
token. The authorization server MUST check if the identifier refers
to a symmetric key that was previously generated by the authorization
server as a shared secret for the communication between this client
and the resource server. If no such symmetric key was found, the
authorization server MUST generate a new symmetric key that is
returned in its response to the client.
The authorization server MUST determine the authorization rules for
the client it communicates with as defined by the resource owner and
generate the access token accordingly. If the authorization server
authorizes the client, it returns an AS-to-Client response. If the
profile parameter is present, it is set to "coap_dtls". The
authorization server MUST ascertain that the access token is
generated for the resource server that the client wants to
communicate with. Also, the authorization server MUST protect the
integrity of the access token to ensure that the resource server can
detect unauthorized changes. If the token contains confidential data
such as the symmetric key, the confidentiality of the token MUST also
be protected. Depending on the requested token type and algorithm in
the access token request, the authorization server adds access
Information to the response that provides the client with sufficient
information to setup a DTLS channel with the resource server. The
authorization server adds a "cnf" parameter to the access information
carrying a "COSE_Key" object that informs the client about the shared
secret that is to be used between the client and the resource server.
To convey the same secret to the resource server, the authorization
server can include it directly in the access token by means of the
"cnf" claim or provide sufficient information to enable the resource
server to derive the shared secret from the access token. As an
alternative, the resource server MAY use token introspection to
retrieve the keying material for this access token directly from the
authorization server.
An example access token request for an access token with a symmetric
proof-of-possession key is illustrated in Figure 5.
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POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
audience : "smokeSensor1807",
}
Figure 5: Example Access Token Request, (implicit) symmetric PoP-key
A corresponding example access token response is illustrated in
Figure 6. In this example, the authorization server returns a 2.01
response containing a new access token (truncated to improve
readability) and information for the client, including the symmetric
key in the cnf claim. The information is transferred as a CBOR data
structure as specified in [I-D.ietf-ace-oauth-authz].
2.01 Created
Content-Format: application/ace+cbor
Max-Age: 85800
Payload:
{
access_token : h'd08343a10...
(remainder of CWT omitted for brevity)
token_type : PoP,
expires_in : 86400,
profile : coap_dtls,
cnf : {
COSE_Key : {
kty : symmetric,
kid : h'3d027833fc6267ce',
k : h'73657373696f6e6b6579'
}
}
}
Figure 6: Example Access Token Response, symmetric PoP-key
The access token also comprises a "cnf" claim. This claim usually
contains a "COSE_Key" object that carries either the symmetric key
itself or a key identifier that can be used by the resource server to
determine the secret key it shares with the client. If the access
token carries a symmetric key, the access token MUST be encrypted
using a "COSE_Encrypt0" structure. The authorization server MUST use
the keying material shared with the resource server to encrypt the
token.
The "cnf" structure in the access token is provided in Figure 7.
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cnf : {
COSE_Key : {
kty : symmetric,
kid : h'3d027833fc6267ce'
}
}
Figure 7: Access Token without Keying Material
A response that declines any operation on the requested resource is
constructed according to Section 5.2 of [RFC6749], (cf.
Section 5.6.3. of [I-D.ietf-ace-oauth-authz]). Figure 8 shows an
example for a request that has been rejected due to invalid request
parameters.
4.00 Bad Request
Content-Format: application/ace+cbor
Payload:
{
error : invalid_request
}
Figure 8: Example Access Token Response With Reject
The method for how the resource server determines the symmetric key
from an access token containing only a key identifier is application-
specific; the remainder of this section provides one example.
The authorization server and the resource server are assumed to share
a key derivation key used to derive the symmetric key shared with the
client from the key identifier in the access token. The key
derivation key may be derived from some other secret key shared
between the authorization server and the resource server. This key
needs to be securely stored and processed in the same way as the key
used to protect the communication between the authorization server
and the resource server.
Knowledge of the symmetric key shared with the client must not reveal
any information about the key derivation key or other secret keys
shared between the authorization server and resource server.
In order to generate a new symmetric key to be used by client and
resource server, the authorization server generates a new key
identifier which MUST be unique among all key identifiers used by the
authorization server for this resource server. The authorization
server then uses the key derivation key shared with the resource
server to derive the symmetric key as specified below. Instead of
providing the keying material in the access token, the authorization
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server includes the key identifier in the "kid" parameter, see
Figure 7. This key identifier enables the resource server to
calculate the symmetric key used for the communication with the
client using the key derivation key and a KDF to be defined by the
application, for example HKDF-SHA-256. The key identifier picked by
the authorization server needs to be unique for each access token
where a unique symmetric key is required.
In this example, HKDF consists of the composition of the HKDF-Extract
and HKDF-Expand steps [RFC5869]. The symmetric key is derived from
the key identifier, the key derivation key and other data:
OKM = HKDF(salt, IKM, info, L),
where:
o OKM, the output keying material, is the derived symmetric key
o salt is the empty byte string
o IKM, the input keying material, is the key derivation key as
defined above
o info is the serialization of a CBOR array consisting of
([RFC8610]):
info = [
type : tstr,
L : uint,
access_token: bytes
]
where:
o type is set to the constant text string "ACE-CoAP-DTLS-key-
derivation",
o L is the size of the symmetric key in bytes,
o access_token is the content of the "access_token" field as
transferred from the authorization server to the resource server.
All CBOR data types are encoded in CBOR using preferred serialization
and deterministic encoding as specified in Section 4 of
[I-D.ietf-cbor-7049bis]. This implies in particular that the "type"
and "L" components use the minimum length encoding. The content of
the "access_token" field is treated as opaque data for the purpose of
key derivation.
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Use of a unique (per resource server) "kid" and the use of a key
derivation IKM that is unique per authorization server/resource
server pair as specified above will ensure that the derived key is
not shared across multiple clients. However, to additionally provide
variation in the derived key across different tokens used by the same
client, it is additionally RECOMMENDED to include the "iat" claim and
either the "exp" or "exi" claims in the access token.
3.3.1. DTLS Channel Setup Between Client and Resource Server
When a client receives an access token response from an authorization
server, the client MUST check if the access token response is bound
to a certain previously sent access token request, as the request may
specify the resource server with which the client wants to
communicate.
The client checks if the payload of the access token response
contains an "access_token" parameter and a "cnf" parameter. With
this information the client can initiate the establishment of a new
DTLS channel with a resource server. To use DTLS with pre-shared
keys, the client follows the PSK key exchange algorithm specified in
Section 2 of [RFC4279] using the key conveyed in the "cnf" parameter
of the AS response as PSK when constructing the premaster secret. To
be consistent with the recommendations in [RFC7252] a client is
expected to offer at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8
[RFC6655] to the resource server.
In PreSharedKey mode, the knowledge of the shared secret by the
client and the resource server is used for mutual authentication
between both peers. Therefore, the resource server must be able to
determine the shared secret from the access token. Following the
general ACE authorization framework, the client can upload the access
token to the resource server's authz-info resource before starting
the DTLS handshake. The client then needs to indicate during the
DTLS handshake which previously uploaded access token it intends to
use. To do so, it MUST create a "COSE_Key" structure with the "kid"
that was conveyed in the "rs_cnf" claim in the token response from
the authorization server and the key type "symmetric". This
structure then is included as the only element in the "cnf" structure
that is used as value for "psk_identity" as shown in Figure 9.
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{ cnf : {
COSE_Key : {
kty: symmetric,
kid : h'3d027833fc6267ce'
}
}
}
Figure 9: Access token containing a single kid parameter
As an alternative to the access token upload, the client can provide
the most recent access token in the "psk_identity" field of the
ClientKeyExchange message. To do so, the client MUST treat the
contents of the "access_token" field from the AS-to-Client response
as opaque data as specified in Section 4.2 of [RFC7925] and not
perform any re-coding. This allows the resource server to retrieve
the shared secret directly from the "cnf" claim of the access token.
If a resource server receives a ClientKeyExchange message that
contains a "psk_identity" with a length greater than zero, it MUST
parse the contents of the "psk_identity" field as CBOR data structure
and process the contents as following:
o If the data contains a "cnf" field with a "COSE_Key" structure
with a "kid", the resource server continues the DTLS handshake
with the associated key that corresponds to this kid.
o If the data comprises additional CWT information, this information
must be stored as an access token for this DTLS association before
continuing with the DTLS handshake.
If the contents of the "psk_identity" do not yield sufficient
information to select a valid access token for the requesting client,
the resource server aborts the DTLS handshake with an
"illegal_parameter" alert.
When the resource server receives an access token, it MUST check if
the access token is still valid, if the resource server is the
intended destination (i.e., the audience of the token), and if the
token was issued by an authorized authorization server. This
specification assumes that the access token is a PoP token as
described in [I-D.ietf-ace-oauth-authz] unless specifically stated
otherwise. Therefore, the access token is bound to a symmetric PoP
key that is used as shared secret between the client and the resource
server. The resource server may use token introspection [RFC7662] on
the access token to retrieve more information about the specific
token. The use of introspection is out of scope for this
specification.
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While the client can retrieve the shared secret from the contents of
the "cnf" parameter in the AS-to-Client response, the resource server
uses the information contained in the "cnf" claim of the access token
to determine the actual secret when no explicit "kid" was provided in
the "psk_identity" field. If key derivation is used, the resource
server uses the "COSE_KDF_Context" information as described above.
3.4. Resource Access
Once a DTLS channel has been established as described in Section 3.2
or Section 3.3, respectively, the client is authorized to access
resources covered by the access token it has uploaded to the authz-
info resource hosted by the resource server.
With the successful establishment of the DTLS channel, the client and
the resource server have proven that they can use their respective
keying material. An access token that is bound to the client's
keying material is associated with the channel. According to
Section 5.8.1 of [I-D.ietf-ace-oauth-authz], there should be only one
access token for each client. New access tokens issued by the
authorization server are supposed to replace previously issued access
tokens for the respective client. The resource server therefore must
have a common understanding with the authorization server how access
tokens are ordered.
Any request that the resource server receives on a DTLS channel that
is tied to an access token via its keying material MUST be checked
against the authorization rules that can be determined with the
access token. The resource server MUST check for every request if
the access token is still valid. If the token has expired, the
resource server MUST remove it. Incoming CoAP requests that are not
authorized with respect to any access token that is associated with
the client MUST be rejected by the resource server with 4.01
response. The response SHOULD include AS Request Creation Hints as
described in Section 5.1.1 of [I-D.ietf-ace-oauth-authz].
The resource server MUST only accept an incoming CoAP request as
authorized if the following holds:
1. The message was received on a secure channel that has been
established using the procedure defined in this document.
2. The authorization information tied to the sending client is
valid.
3. The request is destined for the resource server.
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4. The resource URI specified in the request is covered by the
authorization information.
5. The request method is an authorized action on the resource with
respect to the authorization information.
Incoming CoAP requests received on a secure DTLS channel that are not
thus authorized MUST be rejected according to Section 5.8.2 of
[I-D.ietf-ace-oauth-authz]
1. with response code 4.03 (Forbidden) when the resource URI
specified in the request is not covered by the authorization
information, and
2. with response code 4.05 (Method Not Allowed) when the resource
URI specified in the request covered by the authorization
information but not the requested action.
The client MUST ascertain that its keying material is still valid
before sending a request or processing a response. If the client
recently has updated the access token (see Section 4), it must be
prepared that its request is still handled according to the previous
authorization rules as there is no strict ordering between access
token uploads and resource access messages. See also Section 7.2 for
a discussion of access token processing.
If the client gets an error response containing AS Request Creation
Hints (cf. Section 5.1.2 of [I-D.ietf-ace-oauth-authz] as response
to its requests, it SHOULD request a new access token from the
authorization server in order to continue communication with the
resource server.
Unauthorized requests that have been received over a DTLS session
SHOULD be treated as non-fatal by the resource server, i.e., the DTLS
session SHOULD be kept alive until the associated access token has
expired.
4. Dynamic Update of Authorization Information
Resource servers must only use a new access token to update the
authorization information for a DTLS session if the keying material
that is bound to the token is the same that was used in the DTLS
handshake. By associating the access tokens with the identifier of
an existing DTLS session, the authorization information can be
updated without changing the cryptographic keys for the DTLS
communication between the client and the resource server, i.e. an
existing session can be used with updated permissions.
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The client can therefore update the authorization information stored
at the resource server at any time without changing an established
DTLS session. To do so, the client requests a new access token from
the authorization server for the intended action on the respective
resource and uploads this access token to the authz-info resource on
the resource server.
Figure 10 depicts the message flow where the client requests a new
access token after a security association between the client and the
resource server has been established using this protocol. If the
client wants to update the authorization information, the token
request MUST specify the key identifier of the proof-of-possession
key used for the existing DTLS channel between the client and the
resource server in the "kid" parameter of the Client-to-AS request.
The authorization server MUST verify that the specified "kid" denotes
a valid verifier for a proof-of-possession token that has previously
been issued to the requesting client. Otherwise, the Client-to-AS
request MUST be declined with the error code "unsupported_pop_key" as
defined in Section 5.6.3 of [I-D.ietf-ace-oauth-authz].
When the authorization server issues a new access token to update
existing authorization information, it MUST include the specified
"kid" parameter in this access token. A resource server MUST replace
the authorization information of any existing DTLS session that is
identified by this key identifier with the updated authorization
information.
C RS AS
| <===== DTLS channel =====> | |
| + Access Token | |
| | |
| --- Token Request ----------------------------> |
| | |
| <---------------------------- New Access Token - |
| + Access Information |
| | |
| --- Update /authz-info --> | |
| New Access Token | |
| | |
| == Authorized Request ===> | |
| | |
| <=== Protected Resource == | |
Figure 10: Overview of Dynamic Update Operation
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5. Token Expiration
The resource server MUST delete access tokens that are no longer
valid. DTLS associations that have been setup in accordance with
this profile are always tied to specific tokens (which may be
exchanged with a dynamic update as described in Section 4). As
tokens may become invalid at any time (e.g., because they have
expired), the association may become useless at some point. A
resource server therefore MUST terminate existing DTLS association
after the last access token associated with this association has
expired.
As specified in Section 5.8.3 of [I-D.ietf-ace-oauth-authz], the
resource server MUST notify the client with an error response with
code 4.01 (Unauthorized) for any long running request before
terminating the association.
6. Secure Communication with an Authorization Server
As specified in the ACE framework (Sections 5.6 and 5.7 of
[I-D.ietf-ace-oauth-authz]), the requesting entity (the resource
server and/or the client) and the authorization server communicate
via the token endpoint or introspection endpoint. The use of CoAP
and DTLS for this communication is RECOMMENDED in this profile, other
protocols (such as HTTP and TLS, or CoAP and OSCORE [RFC8613]) MAY be
used instead.
How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the
authorization server are established is out of scope for this
profile.
If other means of securing the communication with the authorization
server are used, the communication security requirements from
Section 6.2 of [I-D.ietf-ace-oauth-authz] remain applicable.
7. Security Considerations
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework
[I-D.ietf-ace-oauth-authz]. As it follows this framework's general
approach, the general security considerations from Section 6 of
[I-D.ietf-ace-oauth-authz] also apply to this profile.
The authorization server must ascertain that the keying material for
the client that it provides to the resource server actually is
associated with this client. Malicious clients may hand over access
tokens containing their own access permissions to other entities.
This problem cannot be completely eliminated. Nevertheless, in RPK
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mode it should not be possible for clients to request access tokens
for arbitrary public keys: if the client can cause the authorization
server to issue a token for a public key without proving possession
of the corresponding private key, this allows for identity misbinding
attacks where the issued token is usable by an entity other than the
intended one. The authorization server therefore at some point needs
to validate that the client can actually use the private key
corresponding to the client's public key.
When using pre-shared keys provisioned by the authorization server,
the security level depends on the randomness of PSK, and the security
of the TLS cipher suite and key exchange algorithm. As this
specification targets at constrained environments, message payloads
exchanged between the client and the resource server are expected to
be small and rare. CoAP [RFC7252] mandates the implementation of
cipher suites with abbreviated, 8-byte tags for message integrity
protection. For consistency, this profile requires implementation of
the same cipher suites. For application scenarios where the cost of
full-width authentication tags is low compared to the overall amount
of data being transmitted, the use of cipher suites with 16-byte
integrity protection tags is preferred.
The PSK mode of this profile offers a distribution mechanism to
convey authorization tokens together with a shared secret to a client
and a server. As this specification aims at constrained devices and
uses CoAP [RFC7252] as transfer protocol, at least the ciphersuite
TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported. The access
tokens and the corresponding shared secrets generated by the
authorization server are expected to be sufficiently short-lived to
provide similar forward-secrecy properties to using ephemeral Diffie-
Hellman (DHE) key exchange mechanisms. For longer-lived access
tokens, DHE ciphersuites should be used.
Constrained devices that use DTLS [RFC6347] are inherently vulnerable
to Denial of Service (DoS) attacks as the handshake protocol requires
creation of internal state within the device. This is specifically
of concern where an adversary is able to intercept the initial cookie
exchange and interject forged messages with a valid cookie to
continue with the handshake. A similar issue exists with the
unprotected authorization information endpoint where the resource
server needs to keep valid access tokens until their expiry.
Adversaries can fill up the constrained resource server's internal
storage for a very long time with interjected or otherwise retrieved
valid access tokens. The protection of access tokens that are stored
in the authorization information endpoint depends on the keying
material that is used between the authorization server and the
resource server: The resource server must ensure that it processes
only access tokens that are (encrypted and) integrity-protected by an
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authorization server that is authorized to provide access tokens for
the resource server.
7.1. Reuse of Existing Sessions
To avoid the overhead of a repeated DTLS handshake, [RFC7925]
recommends session resumption [RFC5077] to reuse session state from
an earlier DTLS association and thus requires client side
implementation. In this specification, the DTLS session is subject
to the authorization rules denoted by the access token that was used
for the initial setup of the DTLS association. Enabling session
resumption would require the server to transfer the authorization
information with the session state in an encrypted SessionTicket to
the client. Assuming that the server uses long-lived keying
material, this could open up attacks due to the lack of forward
secrecy. Moreover, using this mechanism, a client can resume a DTLS
session without proving the possession of the PoP key again.
Therefore, the use of session resumption is NOT RECOMMENDED for
resource servers.
Since renegotiation of DTLS associations is prone to attacks as well,
[RFC7925] requires clients to decline any renogiation attempt. A
server that wants to initiate re-keying therefore SHOULD periodically
force a full handshake.
7.2. Multiple Access Tokens
The use of multiple access tokens for a single client increases the
strain on the resource server as it must consider every access token
and calculate the actual permissions of the client. Also, tokens may
contradict each other which may lead the server to enforce wrong
permissions. If one of the access tokens expires earlier than
others, the resulting permissions may offer insufficient protection.
Developers SHOULD avoid using multiple access tokens for a client.
Even when a single access token per client is used, an attacker could
compromise the dynamic update mechanism for existing DTLS connections
by delaying or reordering packets destined for the authz-info
endpoint. Thus, the order in which operations occur at the resource
server (and thus which authorization info is used to process a given
client request) cannot be guaranteed. Especially in the presence of
later-issued access tokens that reduce the client's permissions from
the initial access token, it is impossible to guarantee that the
reduction in authorization will take effect prior to the expiration
of the original token.
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7.3. Out-of-Band Configuration
To communicate securely, the authorization server, the client and the
resource server require certain information that must be exchanged
outside the protocol flow described in this document. The
authorization server must have obtained authorization information
concerning the client and the resource server that is approved by the
resource owner as well as corresponding keying material. The
resource server must have received authorization information approved
by the resource owner concerning its authorization managers and the
respective keying material. The client must have obtained
authorization information concerning the authorization server
approved by its owner as well as the corresponding keying material.
Also, the client's owner must have approved of the client's
communication with the resource server. The client and the
authorization server must have obtained a common understanding how
this resource server is identified to ensure that the client obtains
access token and keying material for the correct resource server. If
the client is provided with a raw public key for the resource server,
it must be ascertained to which resource server (which identifier and
authorization information) the key is associated. All authorization
information and keying material must be kept up to date.
8. Privacy Considerations
This privacy considerations from Section 7 of the
[I-D.ietf-ace-oauth-authz] apply also to this profile.
An unprotected response to an unauthorized request may disclose
information about the resource server and/or its existing
relationship with the client. It is advisable to include as little
information as possible in an unencrypted response. When a DTLS
session between an authenticated client and the resource server
already exists, more detailed information MAY be included with an
error response to provide the client with sufficient information to
react on that particular error.
Also, unprotected requests to the resource server may reveal
information about the client, e.g., which resources the client
attempts to request or the data that the client wants to provide to
the resource server. The client SHOULD NOT send confidential data in
an unprotected request.
Note that some information might still leak after DTLS session is
established, due to observable message sizes, the source, and the
destination addresses.
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9. IANA Considerations
The following registrations are done for the ACE OAuth Profile
Registry following the procedure specified in
[I-D.ietf-ace-oauth-authz].
Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
with the RFC number of this specification and delete this paragraph.
Profile name: coap_dtls
Profile Description: Profile for delegating client authentication and
authorization in a constrained environment by establishing a Datagram
Transport Layer Security (DTLS) channel between resource-constrained
nodes.
Profile ID: TBD (suggested: 1)
Change Controller: IESG
Reference: [RFC-XXXX]
10. Acknowledgments
Thanks to Jim Schaad for his contributions and reviews of this
document. Special thanks to Ben Kaduk for his thorough reviews of
this document.
Ludwig Seitz worked on this document as part of the CelticNext
projects CyberWI, and CRITISEC with funding from Vinnova.
11. References
11.1. Normative 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)", draft-ietf-ace-oauth-authz-35
(work in progress), June 2020.
[I-D.ietf-ace-oauth-params]
Seitz, L., "Additional OAuth Parameters for Authorization
in Constrained Environments (ACE)", draft-ietf-ace-oauth-
params-13 (work in progress), April 2020.
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[I-D.ietf-cbor-7049bis]
Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", draft-ietf-cbor-7049bis-14 (work
in progress), June 2020.
[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>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<https://www.rfc-editor.org/info/rfc4279>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.
[RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
<https://www.rfc-editor.org/info/rfc7251>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
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[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>.
[RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier", RFC 8422,
DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.
11.2. Informative References
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655,
DOI 10.17487/RFC6655, July 2012,
<https://www.rfc-editor.org/info/rfc6655>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.
[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>.
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[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
Authors' Addresses
Stefanie Gerdes
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63906
Email: gerdes@tzi.org
Olaf Bergmann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63904
Email: bergmann@tzi.org
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
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Ludwig Seitz
Combitech
Djaeknegatan 31
Malmoe 211 35
Sweden
Email: ludwig.seitz@combitech.se
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