ACE Working Group S. Gerdes
Internet-Draft O. Bergmann
Intended status: Standards Track C. Bormann
Expires: June 20, 2020 Universitaet Bremen TZI
G. Selander
Ericsson AB
L. Seitz
Combitech
December 18, 2019
Datagram Transport Layer Security (DTLS) Profile for Authentication and
Authorization for Constrained Environments (ACE)
draft-ietf-ace-dtls-authorize-09
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 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 June 20, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
Gerdes, et al. Expires June 20, 2020 [Page 1]
Internet-Draft CoAP-DTLS December 2019
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Communication between C and AS . . . . . . . . . . . . . 5
3.2. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . 6
3.2.1. DTLS Channel Setup Between C and RS . . . . . . . . . 7
3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 8
3.3.1. DTLS Channel Setup Between C and RS . . . . . . . . . 12
3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 13
4. Dynamic Update of Authorization Information . . . . . . . . . 14
5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 16
6. Secure Communication with AS . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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 [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
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].
Gerdes, et al. Expires June 20, 2020 [Page 2]
Internet-Draft CoAP-DTLS December 2019
The DTLS handshake requires the client and server to prove that they
can use certain keying material. In the RPK mode, the client proves
with the DTLS handshake that it can use the RPK bound to the token
and the server shows that it can use a certain RPK. The access token
must be presented to the resource server. For the RPK mode, the
access token needs to be uploaded to the resource server before the
handshake is initiated, 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.
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
[I-D.ietf-ace-oauth-authz].
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 C can use CoAP over DTLS to retrieve an access token from the
authorization server (AS) for a protected resource hosted on the
resource server.
This profile requires the client to retrieve an access token for
protected resource(s) it wants to access on RS 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):
Gerdes, et al. Expires June 20, 2020 [Page 3]
Internet-Draft CoAP-DTLS December 2019
C RS AS
| [---- Resource Request ------>]| |
| | |
| [<-AS Request Creation Hints-] | |
| | |
| ------- Token Request ----------------------------> |
| | |
| <---------------------------- Access Token --------- |
| + Access Information |
Figure 1: Retrieving an Access Token
To determine the AS in charge of a resource hosted at the RS, C MAY
send an initial Unauthorized Resource Request message to the RS. The
RS then denies the request and sends an AS Request Creation Hints
message containing the address of its AS 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 AS as specified
in [I-D.ietf-ace-oauth-authz]. As the access token request as well
as the response may contain confidential data, the communication
between the client and the authorization server MUST be
confidentiality-protected and ensure authenticity. C may have been
registered at the AS 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).
Figure 2 depicts the common protocol flow for the DTLS profile after
the client C has retrieved the access token from the authorization
server AS.
Gerdes, et al. Expires June 20, 2020 [Page 4]
Internet-Draft CoAP-DTLS December 2019
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 C and AS must be
secured. Depending on the used CoAP security mode (see also
Section 9 of [RFC7252], the Client-to-AS request, AS-to-Client
response 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 C and AS
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 C can request the access token, C and AS MUST
establish a secure communication channel. C MUST securely have
obtained keying material to communicate with AS. Furthermore, C MUST
verify that AS is authorized to provide access tokens (including
authorization information) about RS to C. Also, AS MUST securely
have obtained keying material for C, and obtained authorization rules
approved by the resource owner (RO) concerning C and RS that relate
to this keying material. C and AS MUST use their respective keying
material for all exchanged messages. How the security association
between C and AS is bootstrapped is not part of this document. C and
AS MUST ensure the confidentiality, integrity and authenticity of all
exchanged messages.
Section Section 6 specifies how communication with the AS is secured.
Gerdes, et al. Expires June 20, 2020 [Page 5]
Internet-Draft CoAP-DTLS December 2019
3.2. RawPublicKey Mode
After C and AS mutually authenticated each other and validated each
other's authorization, C sends a token request to AS'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. To prove that the
client is in possession of this key, C MUST use the same keying
material that it uses to secure the communication with AS, e.g., the
DTLS session.
An example access token request from the client to the AS 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.
AS MUST check if the client that it communicates with is associated
with the RPK in the cnf object before issuing an access token to it.
If AS determines that the request is to be authorized according to
the respective authorization rules, it generates an access token
response for C. 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 C 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 and an "rs_cnf" parameter containing information
about the public key that is used by the resource server. AS MUST
Gerdes, et al. Expires June 20, 2020 [Page 6]
Internet-Draft CoAP-DTLS December 2019
ascertain that the RPK specified in "rs_cnf" belongs to the resource
server that C wants to communicate with. AS MUST protect the
integrity of the token. If the access token contains confidential
data, AS MUST also protect the confidentiality of the access token.
C 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 C wants to communicate.
An example access token response from the AS to the client is
depicted in Figure 4.
2.01 Created
Content-Format: application/ace+cbor
Max-Age: 3600
Payload:
{
access_token : b64'SlAV32hkKG...
(remainder of CWT omitted for brevity;
CWT contains clients 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
3.2.1. DTLS Channel Setup Between C and RS
Before the client initiates the DTLS handshake with the resource
server, C MUST send a "POST" request containing the new access token
to the authz-info resource hosted by the resource server. After the
client
receives a confirmation that the RS has accepted the access token, it
SHOULD proceed to establish a new DTLS channel with the resource
server. To use the RawPublicKey mode, the client MUST specify the
public key that AS defined in the "cnf" field of the access token
response in the SubjectPublicKeyInfo structure in the DTLS handshake
as specified in [RFC7250].
Gerdes, et al. Expires June 20, 2020 [Page 7]
Internet-Draft CoAP-DTLS December 2019
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] with the ed25519 curve (cf. [RFC8032], [RFC8422]).
Note: According to [RFC7252], CoAP implementations MUST support the
ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251] and the
NIST P-256 curve. As discussed in [RFC7748], new ECC curves have
been defined recently that are considered superior to the so-
called NIST curves. The curve that is mandatory to implement in
this specification is said to be efficient and less dangerous
regarding implementation errors than the secp256r1 curve mandated
in [RFC7252].
RS MUST check if the access token is still valid, if RS is the
intended destination, i.e., the audience, of the token, and if the
token was issued by an authorized AS. 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 C'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 [I-D.ietf-ace-cwt-proof-of-possession]. RS MUST use the
keying material in the handshake that AS specified in the rs_cnf
parameter in the access token. Thus, the handshake only finishes if
C and RS 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. AS MUST check if the identifier refers to a symmetric key
that was previously generated by AS as a shared secret for the
communication between this client and the resource server.
The authorization server MUST determine the authorization rules for
the C it communicates with as defined by RO 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". AS MUST ascertain
that the access token is generated for the resource server that C
wants to communicate with. Also, AS MUST protect the integrity of
Gerdes, et al. Expires June 20, 2020 [Page 8]
Internet-Draft CoAP-DTLS December 2019
the access token. 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. AS
adds a "cnf" parameter to the access information carrying a
"COSE_Key" object that informs the client about the symmetric key
that is to be used between C and the resource server. The access
token MUST be bound to the same symmetric key by means of the cnf
claim.
An example access token request for an access token with a symmetric
proof-of-possession key is illustrated in Figure 5.
POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
audience : "smokeSensor1807",
}
Figure 5: Example Access Token Request, symmetric PoP-key
An 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 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].
Gerdes, et al. Expires June 20, 2020 [Page 9]
Internet-Draft CoAP-DTLS December 2019
2.01 Created
Content-Format: application/ace+cbor
Max-Age: 86400
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 shared with the client. If the access token
carries a symmetric key, the access token MUST be encrypted using a
"COSE_Encrypt0" structure. The AS MUST use the keying material
shared with the RS to encrypt the token.
The "cnf" structure in the access token is provided in Figure 7.
cnf : {
COSE_Key : {
kty : symmetric,
kid : h'6549694f464361396c4f6277'
}
}
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]).
Gerdes, et al. Expires June 20, 2020 [Page 10]
Internet-Draft CoAP-DTLS December 2019
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 AS 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 AS 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
AS and RS.
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 AS and resource server.
In order to generate a new symmetric key to be used by client and
resource server, the AS generates a key identifier and 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 AS includes the key identifier in
the "kid" parameter, see Figure 7. This key identifier enables the
resource server to calculate the keying material for the
communication with the client from the access token using the key
derivation key and following Section 11 of [RFC8152] with parameters
as specified here. The KDF to be used needs to be defined by the
application, for example HKDF-SHA-256. The key identifier picked by
the AS needs to be unique for each access token where a unique
symmetric key is required.
The fields in the context information "COSE_KDF_Context"
(Section 11.2 of [RFC8152]) have the following values:
o AlgorithmID = "ACE-CoAP-DTLS-key-derivation"
o PartyUInfo = PartyVInfo = ( null, null, null )
o keyDataLength needs to be defined by the application
Gerdes, et al. Expires June 20, 2020 [Page 11]
Internet-Draft CoAP-DTLS December 2019
o protected MUST be a zero length bstr
o other is a zero length bstr
o SuppPrivInfo is omitted
3.3.1. DTLS Channel Setup Between C and RS
When a client receives an access token response from an authorization
server, C 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 C wants to communicate.
C 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.
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. Alternatively, the client MAY 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 and not perform any re-coding.
Note: As stated in Section 4.2 of [RFC7925], the PSK identity should
be treated as binary data in the Internet of Things space and not
assumed to have a human-readable form of any sort.
If a resource server receives a ClientKeyExchange message that
contains a "psk_identity" with a length greater zero, it uses the
contents as index for its key store (i.e., treat the contents as key
identifier). The resource server MUST check if it has one or more
access tokens that are associated with the specified key.
If no key with a matching identifier is found, the resource server
MAY process the contents of the "psk_identity" field as access token
that is stored with the authorization information endpoint, before
continuing the DTLS handshake. If the contents of the "psk_identity"
do not yield a valid access token for the requesting client, the DTLS
Gerdes, et al. Expires June 20, 2020 [Page 12]
Internet-Draft CoAP-DTLS December 2019
session setup is terminated with an "illegal_parameter" DTLS alert
message.
Note1: As a resource server cannot provide a client with a
meaningful PSK identity hint in response to the client's
ClientHello message, the resource server SHOULD NOT send a
ServerKeyExchange message.
Note2: According to [RFC7252], CoAP implementations MUST support the
ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655]. A client is
therefore expected to offer at least this ciphersuite to the
resource server.
When RS receives an access token, RS MUST check if the access token
is still valid, if RS is the intended destination, i.e., the audience
of the token, and if the token was issued by an authorized AS. 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.
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 RS 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
and 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, C and RS 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. Any request that the resource server receives on
this channel MUST be checked against these authorization rules. RS
MUST check for every request if the access token is still valid.
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 as described in Section 5.1.1
of [I-D.ietf-ace-oauth-authz].
Gerdes, et al. Expires June 20, 2020 [Page 13]
Internet-Draft CoAP-DTLS December 2019
The resource server SHOULD treat 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.
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 cannot always know a priori if an Authorized Resource
Request will succeed. It MUST check the validity of its keying
material before sending a request or processing a response. If the
client repeatedly gets error responses containing AS 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 RS, i.e., the DTLS session
SHOULD be kept alive until the associated access token has expired.
4. Dynamic Update of Authorization Information
The client can 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
Gerdes, et al. Expires June 20, 2020 [Page 14]
Internet-Draft CoAP-DTLS December 2019
resource and uploads this access token to the authz-info resource on
the resource server.
Figure 9 depicts the message flow where the C 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.
Note: 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.
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 9: Overview of Dynamic Update Operation
Gerdes, et al. Expires June 20, 2020 [Page 15]
Internet-Draft CoAP-DTLS December 2019
5. Token Expiration
DTLS sessions that have been established in accordance with this
profile are always tied to a specific access token. As this token
may become invalid at any time (e.g. because it has expired), the
session may become useless at some point. A resource server
therefore MUST terminate existing DTLS sessions after the access
token for this session has been deleted.
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 session.
6. Secure Communication with AS
As specified in the ACE framework (sections 5.6 and 5.7 of
[I-D.ietf-ace-oauth-authz]), the requesting entity (RS and/or client)
and the AS 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) MAY be used instead.
How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the
AS are established is out of scope for this profile.
If other means of securing the communication with the AS are used,
the security protocol MUST fulfill the communication security
requirements in Section 6.2 of [I-D.ietf-ace-oauth-authz].
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 also
apply to this profile.
When using pre-shared keys provisioned by the AS, the security level
depends on the randomness of PSK, and the security of the TLS cipher
suite and key exchange algorithm.
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
Gerdes, et al. Expires June 20, 2020 [Page 16]
Internet-Draft CoAP-DTLS December 2019
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 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.
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 the 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.
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
Gerdes, et al. Expires June 20, 2020 [Page 17]
Internet-Draft CoAP-DTLS December 2019
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: 1
Change Controller: IESG
Reference: [RFC-XXXX]
10. References
10.1. Normative References
[I-D.ietf-ace-cwt-proof-of-possession]
Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
possession-11 (work in progress), October 2019.
[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-29
(work in progress), December 2019.
[I-D.ietf-ace-oauth-params]
Seitz, L., "Additional OAuth Parameters for Authorization
in Constrained Environments (ACE)", draft-ietf-ace-oauth-
params-07 (work in progress), December 2019.
[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>.
Gerdes, et al. Expires June 20, 2020 [Page 18]
Internet-Draft CoAP-DTLS December 2019
[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>.
[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>.
[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>.
10.2. Informative References
[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>.
[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>.
[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>.
Gerdes, et al. Expires June 20, 2020 [Page 19]
Internet-Draft CoAP-DTLS December 2019
[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>.
[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>.
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
Gerdes, et al. Expires June 20, 2020 [Page 20]
Internet-Draft CoAP-DTLS December 2019
Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
Ludwig Seitz
Combitech
Djaeknegatan 31
Malmoe 211 35
Sweden
Email: ludwig.seitz@combitech.se
Gerdes, et al. Expires June 20, 2020 [Page 21]