wdiff draft-ietf-ace-dtls-authorize-04.txt draft-ietf-ace-dtls-authorize-05.txt

ACE Working Group S. Gerdes
Internet-Draft O. Bergmann
Intended status: Standards Track C. Bormann
Expires: March 10, April 11, 2019 Universitaet Bremen TZI
G. Selander
Ericsson AB
L. Seitz
RISE SICS
September 06,
October 08, 2018

Datagram Transport Layer Security (DTLS) Profile for Authentication and
Authorization for Constrained Environments (ACE)
draft-ietf-ace-dtls-authorize-04
draft-ietf-ace-dtls-authorize-05

Abstract

This specification defines a profile for delegating that allows constrained servers
to delegate client authentication and authorization in a constrained environment by
establishing a Datagram Transport Layer Security (DTLS) channel
between resource-constrained nodes. 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 node 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 March 10, April 11, 2019.

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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Resource Access . . . .
3. Protocol Flow . . . . . . . . . . . . . . . . . 5
2.2. Dynamic Update of Authorization Information . . . . . . . 7
2.3. Token Expiration 5
3.1. Communication between C and AS . . . . . . . . . . . . . 5
3.2. RawPublicKey Mode . . . . . . . 8
3. RawPublicKey Mode . . . . . . . . . . . . . 6
3.2.1. DTLS Channel Setup Between C and RS . . . . . . . . . 9
4. 7
3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . . . 10
4.1. 8
3.3.1. DTLS Channel Setup Between C and RS . . . . . . . . . 10
3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 12
4.2. Updating
4. Dynamic Update of Authorization Information . . . . . . . . . . . 13
5. Security Considerations Token Expiration . . . . . . . . . . . . . . . . . . . . . . 14
6. Privacy Security Considerations . . . . . . . . . . . . . . . . . . . 14 15
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
8. 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
8.2. 16
9.2. Informative References . . . . . . . . . . . . . . . . . 16
8.3. 17
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 17 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 19

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 uses obtains an access token, bound to a key (the proof-of-possession
key) proof-of-
possession key), from an authorization server to authorize prove its access
authorization to access protected resources hosted by the resource
server. DTLS provides communication security, proof of
possession, and server authentication. Optionally Also, the client and the resource server may also use CoAP over DTLS to communicate are provided by the
authorization server with the necessary keying material to establish
a DTLS session. The communication between client and authorization server.
server may also be secured with DTLS. This specification supports the
DTLS handshake with Raw Public Keys (RPK) [RFC7250] and the DTLS handshake with Pre-
Shared Pre-Shared Keys
(PSK) [RFC4279].

The DTLS RPK handshake [RFC7250] requires the client authentication and server to
provide proof-of-possession for prove
that they can use certain keying material. In the key tied RPK mode, the
client proves with the DTLS handshake that it can use the RPK bound
to the access token.
Here token and the server shows that it can use a certain RPK. The
access token needs to must be transferred presented to the resource server
before server. For the handshake is initiated, as RPK
mode, the access token needs to be uploaded to the resource server
before the handshake is initiated, as described in section Section 5.8.1 of
draft-ietf-ace-oauth-authz [1].

The DTLS

In the PSK handshake [RFC4279] provides mode, client and server show with the proof-of-possession for DTLS handshake that
they can use the key tied keying material that is bound to the access token. Furthermore
To transfer the access token from the client to the psk_identity resource server,
the "psk_identity" parameter in the DTLS PSK handshake is may be used to transfer
instead of uploading the access token from the client prior to the resource server. 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]. I-D.ietf-ace-oauth-authz [2].

The authz-info resource refers to the authz-info endpoint as
specified in I-D.ietf-ace-oauth-authz [3].

2. Protocol Overview

The CoAP-DTLS profile for ACE specifies the transfer of
authentication information and, if necessary, authorization
information between the client C (C) and the resource server RS (RS)
during setup of a DTLS session for CoAP messaging. It also specifies
how a Client C can use CoAP over DTLS to retrieve an Access Token access token from the
authorization server AS (AS) for a protected resource hosted on the
resource server RS. server.

This profile requires a Client (C) the client to retrieve an Access Token access token for
the
protected resource(s) it wants to access on a Resource Server (RS) RS as specified in [I-D.ietf-ace-oauth-authz]. I-
D.ietf-ace-oauth-authz [4]. Figure 1 shows the typical message flow
in this scenario (messages in square brackets are optional):

C RS AS
| [-- Resource Request --->] | |
| | |
| [<----- AS Information --] | |
| | |
| --- Token Request ----------------------------> |
| | |
| <---------------------------- Access Token ----- |
| + RS Access Information |

Figure 1: Retrieving an Access Token

To determine the AS in charge of a resource hosted at the RS, the
client C MAY
send an initial Unauthorized Resource Request message to the RS. The
RS then denies the request and sends an AS information message
containing the address of its AS back to the client C as specified in section
Section 5.1.2 of draft-ietf-
ace-oauth-authz [2]. draft-ietf-ace-oauth-authz [5].

Once the client C knows the authorization server's address, it can send
an Access Token access token request to the token endpoint at the AS as specified
in [I-D.ietf-ace-oauth-authz]. I-D.ietf-ace-oauth-authz [6]. As the Access Token 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. How the mutual
authentication between the client and the authorization server is
achieved is out of scope for this document; the client C may have been
configured with a public key of the authorization server and have
been
registered at the AS via the OAuth 2.0 client registration mechanism
as outlined in section Section 5.3 of draft-ietf-ace-oauth-authz [3].

If C wants to use the CoAP RawPublicKey mode as described in
Section 9 of RFC 7252 [4] it MUST provide a key or key identifier
within a "cnf" object in the [7].

The access token request. If returned by the authorization server AS decides that the request is to can then be authorized it generates
an access token response for
used by the client C containing to establish a "profile"
parameter new DTLS session with the value "coap_dtls" to indicate that this profile
MUST be used for communication between the client C and the resource
server.

For RPK mode, the authorization server also adds a "rs_cnf" parameter
containing information about the public that is used by the resource
server (see Section 3).

For PSK mode, the authorization server adds a "cnf" parameter
containing information about the shared secret that C can use to
setup a DTLS session with the resource server (see Section 4).

The Access Token returned by the authorization server then can be
used by the client to establish a new DTLS session with the resource
server. When the client intends resource
server. When the client intends to use asymmetric cryptography in
the DTLS handshake with the resource server, the client MUST upload
the Access Token access token to the authz-info resource resource, i.e. the authz-info
endpoint, on the resource server before starting the DTLS handshake,
as described in section Section 5.8.1 of draft-ietf-ace-oauth-authz [5]. [8]. If
only symmetric cryptography is used between the client and the
resource server, the Access Token access token MAY instead be transferred in the
DTLS ClientKeyExchange message (see Section 4.1). 3.3.1).

Figure 2 depicts the common protocol flow for the DTLS profile after
the client C has retrieved the Access Token access token from the authorization
server AS.

Figure 2: Protocol overview

3. Protocol Flow

The following sections specify how CoAP is used to interchange
access-related data between the resource server 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 desired used CoAP security mode, mode (see also
Section 9 of RFC 7252 [9]), the Client-to-AS request, AS-to-Client
response and DTLS session establishment carry slightly different
information. Section 3 3.2 addresses the use of raw public keys while
Section 4 3.3 defines how pre-shared keys are used in this profile.

2.1. Resource Access

Once a DTLS channel has been established as described in Section 3

3.1. Communication between C and Section 4, respectively, the client is authorized to AS

To retrieve an access
resources covered by the Access Token it has uploaded to the authz-
info resource hosted by the resource server.

On token for the resource server side, successful establishment of the DTLS
channel binds that the client wants to
access, the client requests an access token, functioning as a proof-
of-possession associated key. Any request that token from the resource server
receives on this channel MUST be checked against these authorization
rules that are associated with the identity of
server. Before C can request the client. Incoming
CoAP requests that are not authorized with respect to any Access access token, C and AS must
establish a secure communication channel. C must securely have
obtained keying material to communicate with AS, and C must securely
have received authorization information intended for C that states
that AS is authorized to provide keying material concerning 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 established is not
part of this document. C and AS MUST ensure the confidentiality,
integrity and authenticity of all exchanged messages.

If C is constrained, C and AS should use DTLS to communicate with
each other. But C and AS may also use other means to secure their
communication, e.g., TLS. The used security protocol must provide
confidentiality, integrity and authenticity, and enable the client to
determine if it is the intended recipient of a message, e.g., by
using an AEAD mechanism. C must also be able to determine if a
response from AS belongs to a certain request. Additionally, the
protocol must offer replay protection.

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 "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
{
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 client MUST 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 response SHOULD contain a "profile" parameter
with the value "coap_dtls" to indicate that this profile must be rejected by used
for communication between the client C and the resource server with 4.01 response as described in Section 5.1.1 of
draft-ietf-ace-oauth-authz [6].

Note: server. The identity of
response also contains an access token and an "rs_cnf" parameter
containing information about the client public key that is determined used by the authentication
process
during
resource server. AS MUST ascertain that the DTLS handshake. In RPK specified in
"rs_cnf" belongs to the asymmetric case, resource server that C wants to communicate
with. AS MUST protect the public key
will define integrity of the client's identity, while in token. If the PSK case, access
token contains confidential data, AS MUST also protect the
client's identity is defined by
confidentiality of the shared secret generated by 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
authorization server for this communication.

The
resource server SHOULD treat an incoming CoAP with which C wants to communicate.

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 as
authorized if containing the following holds:

1. The message was received on new access token
to the authz-info resource hosted by the resource server. If this
operation yields a secure positive response, the client SHOULD proceed to
establish a new DTLS channel that has been
established using with the procedure resource server. To use the
RawPublicKey mode, the client MUST specify the public key that AS
defined in this document.

2. The authorization information tied to the sending peer is valid.

3. The request is destined for "cnf" field of the access token response in the resource server.

4. The resource URI specified
SubjectPublicKeyInfo structure in the request is covered by DTLS handshake as specified in
RFC 7250 [10].

An implementation that supports the
authorization information.

5. The request method is an authorized action on RPK mode of this profile MUST at
least support the resource ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
[RFC7251] with
respect to the authorization information.

Incoming ed25519 curve (cf. [RFC8032], [RFC8422]).

Note: According to RFC 7252 [11], CoAP requests received on a secure DTLS channel implementations MUST be
rejected according to [Section 5.1.1 of draft-ietf-ace-oauth-
authz](https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.1.1

1. with response code 4.03 (Forbidden) when support
the resource URI
specified ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251] and
the NIST P-256 curve. As discussed in RFC 7748 [12], new ECC
curves have been defined recently that are considered superior to
the request so-called NIST curves. The curve that is not covered by the authorization
information, mandatory to
implement in this specification is said to be efficient and

2. with response code 4.05 (Method Not Allowed) when less
dangerous regarding implementation errors than the resource
URI specified secp256r1 curve
mandated in RFC 7252 [13].

RS MUST check if the request covered by access token is still valid, if RS is the authorization
information but not
intended destination, i.e., the requested action.

The client cannot always know a priori audience, of the token, and if the
token was issued by an Authorized Resource
Request will succeed. If authorized AS. The access token is
constructed by the client repeatedly gets error responses
containing AS Information (cf. Section 5.1.1 of draft-ietf-ace-
oauth-authz [7] as response to its requests, it SHOULD request a new
Access Token from authorization server such that the authorization resource server in order to continue
communication
can associate the access token with the resource server.

2.2. Dynamic Update of Authorization Information Client's public key. The client can update
"cnf" claim MUST contain either C's RPK or, if the authorization information stored at key is already
known by the resource server at any time without changing an established DTLS
session. To do so, the Client requests (e.g., from previous communication), a
reference to this key. If the authorization server
a new Access Token for the intended action on has no certain
knowledge that the respective resource
and uploads this Access Token Client's key is already known to the authz-info resource on the resource server.

Figure 3 depicts
server, the message flow where Client's public key MUST be included in the client C requests access
token's "cnf" parameter. If CBOR web tokens [RFC8392] are used as
recommended in I-D.ietf-ace-oauth-authz [14], unencrypted keys MUST
be specified using a new
Access Token after "COSE_Key" object, encrypted keys with a security association between the client
"COSE_Encrypt0" structure and references to the
resource server key as "key_id"
parameters in a CBOR map. RS has been established using this protocol. The
token request MUST specify the key identifier of the existing DTLS
channel between use the client and keying material in the resource server
handshake that AS specified in the "kid" rs_cnf parameter of in the Client-to-AS request. The authorization server MUST
verify that access
token. Thus, the specified "kid" denotes a valid verifier for a proof-
of-possession ticket that has previously been issued 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
requesting client. Otherwise, the Client-to-AS request MUST be
declined with a resource that the error code "unsupported_pop_key" as defined in
Section 5.6.3 of draft-ietf-ace-oauth-authz [8].

When client wants to
access, the authorization server issues client MAY include a new "cnf" object carrying an identifier
for a symmetric key in its access token request to update
existing authorization information it MUST include the specified
"kid" parameter in this access token. A resource server MUST
associate the updated authorization information with any existing
DTLS session that is identified
server. This identifier can be used by this key identifier.

Note: By associating the access tokens with authorization server to
determine the identifier of an
existing DTLS session, shared secret to construct the authorization information can be
updated without changing proof-of-possession
token. AS MUST check if the cryptographic keys identifier refers to a symmetric key
that was previously generated by AS as a shared secret for the DTLS
communication between the this client and the resource server, i.e. an
existing session can be used with updated permissions. server.

The authorization server MUST determine the authorization rules for
the C RS AS
| <===== DTLS channel =====> | |
| + Access Token | |
| | |
| --- Token Request ----------------------------> |
| | |
| <---------------------------- New Access Token - |
| + RS Information |
| | |
| --- Update /authz-info --> | |
| New Access Token | |
| | |
| == Authorized Request ===> | |
| | |
| <=== Protected Resource == | |

Figure 3: Overview of Dynamic Update Operation

2.3. Token Expiration

DTLS sessions that have been established in accordance it communicates with this
profile are always tied to a specific set of access tokens. As these
tokens may become invalid at any time (either because as defined by RO and generate the access
token has
expired or accordingly. If the responsible authorization server has revoked authorizes the
token),
client, it returns an AS-to-Client response. If the session may become useless at some point. A resource
server therefore may decide profile
parameter is present, it is set to terminate existing DTLS sessions after "coap_dtls". AS MUST ascertain
that the last valid access token is generated for this session has been deleted.

As specified in section 5.8.3 of draft-ietf-ace-oauth-authz [9], the resource server that C
wants to communicate with. Also, AS MUST notify protect the client with an error response with
code 4.01 (Unauthorized) for any long running request before
terminating integrity of
the session.

The resource server MAY access token. If the token contains confidential data such as
the symmetric key, the confidentiality of the token MUST also keep be
protected. Depending on the session alive for some time requested token type and
respond algorithm in
the access token request, the authorization server adds access
Information to incoming requests the response that provides the client with sufficient
information to setup a 4.01 (Unauthorized) error message
including DTLS channel with the resource server. AS Information
adds a "cnf" parameter to signal the access information carrying a
"COSE_Key" object that informs the client needs about the symmetric key
that is to upload be used between C and the resource server.

An example access token response is illustrated in Figure 4. In this
example, the authorization server returns a 2.01 response containing
a new access token before it can continue using this DTLS session. and information for the client, including the
symmetric key in the cnf claim. The
AS Information information is created transferred as a
CBOR data structure as specified in section 5.1.2 I-D.ietf-ace-oauth-authz [15].

2.01 Created
Content-Format: application/ace+cbor
Max-Age: 86400
{
access_token: h'd08343a10...
(remainder of draft-
ietf-ace-oauth-authz [10]. CWT omitted for brevity)
token_type: pop,
alg: HS256,
expires_in: 86400,
profile: coap_dtls,
cnf: {
COSE_Key: {
kty: symmetric,
k: h'73657373696f6e6b6579'
}
}
}

Figure 4: Example Access Token Response

The resource server SHOULD add access token also comprises a "kid"
parameter to the AS Information denoting the identifier of "cnf" claim. This claim usually
contains a "COSE_Key" object that carries either the symmetric key
that it uses internally for this DTLS session. The client then
includes this "kid" parameter in
itself or or a Client-to-AS request key identifier that can be used by the resource server
to
retrieve determine the shared secret. If the access token carries a new
symmetric key, the access token to MUST be used with this DTLS session. In
case the key identifier is already known by encrypted using a
"COSE_Encrypt0" structure. The AS MUST use the client (e.g. because
it was included in keying material
shared with the RS Information in an AS-to-Client response), to encrypt the "kid" parameter MAY be elided from token.

Instead of providing the keying material, the AS Information.

Table 1 updates Figure 2 MAY include a key
derivation function and a salt in section 5.1.2 of draft-ietf-ace-oauth-
authz [11] with the new "kid" parameter in accordance with [RFC8152].

+----------------+----------+-----------------+
| Parameter name | CBOR Key | Major Type |
+----------------+----------+-----------------+
| kid | 4 | 2 (byte string) |
+----------------+----------+-----------------+

Table 1: Updated AS Information parameters

3. RawPublicKey Mode

To retrieve an access token for the resource that enables the client wants
resource server to
access, calculate the client requests an Access Token keying material for the
communication with C from the authorization
server. The client MUST add access token. In this case, the token
contains a "cnf" object carrying either its raw
public structure that specifies the key or a unique identifier for a public derivation
algorithm and the salt that the AS has used to construct the shared
key. AS and RS MUST use their shared keying material for the key that it has
previously made known to
derivation, and the authorization server. To prove that key derivation MUST follow Section 11 of RFC 8152
[16] with parameters as specified here. The KDF specified in the
client
"alg" parameter SHOULD be HKDF-SHA-256. The salt picked by the AS
must be uniformly random and is carried in possession the "salt" parameter.

The fields in the context information "COSE_KDF_Context"
(Section 11.2 of this key, it RFC 8152 [17]) MUST use have the same public key
as in certificate message that following values:

o AlgorithmID = "ACE-CoAP-DTLS-salt"

o PartyUInfo = PartyVInfo = ( null, null, null )
o keyDataLength is used to establish a uint equal the DTLS session
with length of the authorization server. key shared between
AS and RS in bits

o protected MUST be a zero length bstr

o other is a zero length bstr

o SuppPrivInfo is omitted

An example Access Token request from the client to the resource
server "cnf" structure specifying HMAC-based key derivation of a
symmetric key with SHA-256 as pseudo-random function and a random
salt value is depicted provided in Figure 4.

POST coaps://as.example.com/token
Content-Format: application/cbor
{
grant_type: client_credentials,
aud: "tempSensor4711",
cnf: {
COSE_Key: 5.

cnf : {
kty: EC2,
crv: P-256,
x: h'TODOX',
y: h'TODOY'
}
}
kty : symmetric,
alg : HKDF-SHA-256,
salt : h'eIiOFCa9lObw'
}

Figure 4: Access Token Request Example for RPK Mode

The example shows 5: Key Derivation Specification in an Access Token request for the resource identified
by the audience string "tempSensor4711"

A response that declines any operation on the authorization server
using a raw public key.

When the authorization server authorizes a request, it will return an requested resource is
constructed according to Section 5.2 of RFC 6749 [18], (cf.
Section 5.7.3. of draft-ietf-ace-oauth-authz [19]).

4.00 Bad Request
Content-Format: application/ace+cbor
{
error: invalid_request
}

Figure 6: Example Access Token Response With Reject

3.3.1. DTLS Channel Setup Between C and RS

When a "cnf" object in the AS-to-Client response. Before
the client initiates receives an access token response from an authorization
server, C MUST ascertain that the DTLS handshake with access token response belongs to a
certain previously sent access token request, as the resource server, it
MUST send a "POST" request containing may
specify the new Access Token resource server with which C wants to communicate.

C checks if the
authz-info resource hosted by payload of the resource server. If this operation
yields access token response contains an
"access_token" parameter and a positive response, "cnf" parameter. With this
information the client SHOULD proceed to establish can initiate the establishment of a new DTLS
channel with the a resource server. To use raw public key
mode, DTLS with pre-shared keys,
the client MUST pass follows the same public PSK key that was used for
constructing the Access Token with the SubjectPublicKeyInfo structure
in the DTLS handshake as exchange algorithm specified in [RFC7250].

An implementation that supports the RPK mode
Section 2 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 RFC 4279 [20] using the secp256r1 curve mandated key conveyed in [RFC7252].

The Access Token is constructed by the authorization server such that
the resource server can associate the Access Token with "cnf"
parameter of the Client's
public key. If CBOR web tokens [RFC8392] are used AS response as recommended in
[I-D.ietf-ace-oauth-authz], PSK when constructing the authorization server MUST include a
"COSE_Key" object in premaster
secret.

In PreSharedKey mode, the "cnf" claim knowledge of the Access Token. This
"COSE_Key" object MAY contain a reference to a key for shared secret by the
client and the client
that resource server is already known by used for mutual authentication
between both peers. Therefore, the resource server (e.g., must be able to
determine the shared secret from previous
communication). If the access token. Following the
general ACE authorization server has no certain knowledge
that framework, the Client's key is already known client can upload the access
token to the resource server, server's authz-info resource before starting
the
Client's public key MUST be included in DTLS handshake. Alternatively, the Access Token's "cnf"
parameter.

4. PreSharedKey Mode

To retrieve an client MAY provide the most
recent access token for in the resource that "psk_identity" field of the client wants to
access,
ClientKeyExchange message. To do so, the client MAY include a "cnf" object carrying an identifier
for a symmetric key in its Access Token request to MUST treat the authorization
server. This identifier can be used by
contents of the authorization server to
determine "access_token" field from the shared secret to construct AS-to-Client response
as opaque data and not perform any re-coding.

Note: As stated in Section 4.2 of RFC 7925 [21], the proof-of-possession
token PSK identity
should be treated as binary data in the Internet of Things space
and therefore MUST specify not assumed to have a symmetric key human-readable form of any sort.

If a resource server receives a ClientKeyExchange message that was previously
generated by
contains a "psk_identity" with a length greater zero, it uses the authorization server
contents as a shared secret index for its key store (i.e., treat the
communication between the client and the contents as key
identifier). The resource server.

Depending on the requested token type and algorithm in server MUST check if it has one or more
access tokens that are associated with the Access
Token request, specified key.

If no key with a matching identifier is found, the authorization resource server adds RS Information to
MAY process the
response that provides contents of the client "psk_identity" field as access token
that is stored with sufficient the authorization information to
setup a endpoint, before
continuing the DTLS channel with handshake. If the resource server. For symmetric proof-
of-possession keys (c.f. [I-D.ietf-ace-oauth-authz]), contents of the client
must ensure that "psk_identity"
do not yield a valid access token for the Access Token request requesting client, the DTLS
session setup is sent over terminated with an "illegal_parameter" DTLS alert
message.

Note1: As a secure
channel that guarantees authentication, message integrity and
confidentiality.

When the authorization resource server authorizes the cannot provide a client it returns an AS-
to-Client response with the profile parameter set to "coap_dtls" and
a "cnf" parameter carrying a "COSE_Key" object that contains
meaningful PSK identity hint in response to the
symmetric key client's
ClientHello message, the resource server SHOULD NOT send a
ServerKeyExchange message.

Note2: According to be used between RFC 7252 [22], CoAP implementations MUST support
the ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655]. A client and is
therefore expected to offer at least this ciphersuite to the
resource server
as illustrated in Figure 5.

2.01 Created
Content-Format: application/cbor
Location-Path: /token/asdjbaskd
{
access_token: h'd08343a10...
(remainder 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 CWT omitted for brevity)
token_type: pop,
alg: HS256,
expires_in: 86400,
profile: coap_dtls,
cnf: {
COSE_Key: {
kty: symmetric,
k: h'73657373696f6e6b6579'
}
}
}

Figure 5: Example Access Token response

In this example, the authorization server returns a 2.01 response
containing a new Access Token. The information token, and if the token was issued by an authorized AS. This
specification assumes that the access token is transferred as a
CBOR data structure PoP token as specified
described in [I-D.ietf-ace-oauth-authz].

A response that declines any operation on I-D.ietf-ace-oauth-authz [23] unless specifically stated
otherwise. Therefore, the requested resource access token is
constructed according bound to Section 5.2 of RFC 6749 [12], (cf.
Section 5.7.3 of [I-D.ietf-ace-oauth-authz]).

4.00 Bad Request
Content-Format: application/cbor
{
error: invalid_request
}

Figure 6: Example Access Token response with reject

4.1. DTLS Channel Setup Between C and RS

When a client receives an Access Token from an authorization server,
it checks if symmetric PoP
key that is used as shared secret between the payload contains an "access_token" parameter client and a
"cnf" parameter. With this information the client can initiate
establishment of a new DTLS channel with a resource
server. To use
DTLS with pre-shared keys,

While the client follows can retrieve the PSK key exchange
algorithm specified in Section 2 of [RFC4279] using shared secret from the key conveyed
in contents of
the "cnf" parameter of the AS response as PSK when constructing
the premaster secret.

In PreSharedKey mode, the knowledge of the shared secret by in the
client and AS-to-Client response, the resource server is used for mutual authentication
between both peers. Therefore,
uses the resource server must be able information contained in the "cnf" claim of the access token
to determine the shared actual secret from when no explicit "kid" was provided in
the Access Token. Following "psk_identity" field. If key derivation is used, the
general ACE authorization framework, 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 can upload is authorized to access
resources covered by the Access
Token access token it has uploaded to the authz-
info resource server's authz-info resource before starting
the DTLS handshake. Alternatively, the client MAY provide hosted by the most
recent Access Token in resource server.

With the "psk_identity" field successful establishment of the
ClientKeyExchange message. To do so, DTLS channel, C and RS have
proven that they can use their respective keying material. An access
token that is bound to the client MUST treat client's keying material is associated
with the
contents of channel. Any request that the "access_token" field from resource server receives on
this channel MUST be checked against these authorization rules. RS
MUST check for every request if the AS-to-Client response
as opaque data and access token is still valid.
Incoming CoAP requests that are not perform authorized with respect to any re-coding.

Note: As stated in section 4.2 of [RFC7925],
access token that is associated with the PSK identity should client MUST be treated rejected by
the resource server with 4.01 response as binary data described in the Internet of Things space and not
assumed to have a human-readable form Section 5.1.1
of any sort.

If a draft-ietf-ace-oauth-authz [24].

The resource server receives a ClientKeyExchange SHOULD treat an incoming CoAP request as
authorized if the following holds:

1. The message that
contains a "psk_identity" with was received on a length greater zero, it uses secure channel that has been
established using the
contents as index procedure defined in this document.

2. The authorization information tied to the sending client is
valid.

3. The request is destined for its key store (i.e., treat the contents as key
identifier). 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 server MUST check if it has one or more
Access Tokens with
respect to the authorization information.

Incoming CoAP requests received on a secure DTLS channel that are associated not
thus authorized MUST be rejected according to Section 5.8.2 of draft-
ietf-ace-oauth-authz [25]
1. with response code 4.03 (Forbidden) when the resource URI
specified key. If no
valid Access Token is available for this key, in the DTLS session setup
is terminated with an "illegal_parameter" DTLS alert message.

If no key with a matching identifier request is found not covered by the resource server authorization
information, and

2. with response code 4.05 (Method Not Allowed) when the resource server MAY process the decoded contents of
URI specified in the
"psk_identity" field as access token that is stored with request covered by the authorization
information endpoint before continuing but not the DTLS
handshake. requested action.

The client cannot always know a priori if an Authorized Resource
Request will succeed. If the decoded contents client repeatedly gets error responses
containing AS Information (cf. Section 5.1.2 of the "psk_identity" do not
yield draft-ietf-ace-
oauth-authz [26]) as response to its requests, it SHOULD request a valid
new access token for the requesting client, from the DTLS
session setup is terminated with an "illegal_parameter" DTLS alert
message.

Note1: As a resource authorization server cannot provide a client with a meaningful
PSK identity hint in
response order to continue
communication with the client's ClientHello message, resource server.

4. Dynamic Update of Authorization Information

The client can update the authorization information stored at the
resource server
SHOULD NOT send at any time without changing an established DTLS
session. To do so, the Client requests a ServerKeyExchange message.

Note2: According to [RFC7252], CoAP implementations MUST support new access token from the
authorization server for the
ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655]. A client is
therefore expected to offer at least intended action on the respective
resource and uploads this ciphersuite access token to the authz-info resource on
the resource server.

This specification assumes that

Figure 7 depicts the Access Token is message flow where the C requests a PoP new access
token as
described in [I-D.ietf-ace-oauth-authz] unless specifically stated
otherwise. Therefore, the Access Token is bound to after a symmetric PoP
key that is used as shared secret security association between the client and the
resource
server.

While server has been established using this protocol. If the
client can retrieve wants to update the shared secret from authorization information, the contents token
request MUST specify the key identifier of the "cnf" parameter in existing DTLS channel
between the AS-to-Client response, client and the resource server
uses the information contained in the "cnf" claim "kid" parameter of
the Access Token
to determine Client-to-AS request. The authorization server MUST verify that
the actual secret when no explicit specified "kid" was provided in
the "psk_identity" field. Usually, this is done by including denotes a
"COSE_Key" object carrying either valid verifier for a key proof-of-
possession token that has previously been encrypted 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
draft-ietf-ace-oauth-authz [27].

When the authorization server issues a shared secret new access token to update
existing authorization information, it MUST include the specified
"kid" parameter in this access token. A resource server MUST
associate the updated authorization information with any existing
DTLS session that is identified by this key identifier.

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 authorization server client and the resource server, or a key identifier that i.e. an
existing session can be used by the resource server
to lookup the shared secret.

Instead of the "COSE_Key" object, the authorization server MAY
include a "COSE_Encrypt" structure to enable the resource server to
calculate the shared key from the Access Token. The "COSE_Encrypt"
structure MUST use the _Direct Key with KDF_ method as described in
Section 12.1.2 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 7: Overview of RFC 8152 [13]. The authorization server MUST
include a Context information structure carrying a PartyU "nonce"
parameter carrying the nonce Dynamic Update Operation

5. Token Expiration

DTLS sessions that has have been used by the authorization
server established in accordance with this
profile are always tied to construct the shared key.

This specification mandates that a specific set of access tokens. As these
tokens may become invalid at least any time (either because the key derivation
algorithm "HKDF SHA-256" as defined in [RFC8152] MUST be supported.
This key derivation function is token has
expired or the default when no "alg" field is
included in responsible authorization server has revoked the "COSE_Encrypt" structure for
token), the session may become useless at some point. A resource server.

4.2. Updating Authorization Information

Usually, the authorization information that
server therefore MUST terminate existing DTLS sessions after the resource server keeps last
valid access token for a client is updated by uploading a new Access Token as described this session has been deleted.

As specified in Section 2.2.

The Client MAY also perform a new DTLS handshake according to
Section 4.1 that replaces the existing DTLS session. After
successful completion 5.8.3 of the DTLS handshake draft-ietf-ace-oauth-authz [28], the
resource server
updates MUST notify the existing authorization information client with an error response with
code 4.01 (Unauthorized) for any long running request before
terminating the client
according to session.

Table 1 updates Figure 2 in Section 5.1.2 of draft-ietf-ace-oauth-
authz [29] with the new Access Token.

5. "kid" parameter in accordance with [RFC8152].

Table 1: Updated AS Information parameters

6. 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 and privacy considerations from
section 6 and section 7 also apply to this profile.

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.

[I-D.tiloca-tls-dos-handshake] specifies a TLS extension to prevent
this type of attack which is applicable especially for constrained
environments where the authorization server can act as trust anchor.

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.

7. Privacy Considerations

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.

8. 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: 1

Change Controller: IESG

Reference: [RFC-XXXX]

9. References

8.1.

9.1. 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-13 draft-ietf-ace-oauth-authz-16
(work in progress), July October 2018.

[I-D.tiloca-tls-dos-handshake]
Tiloca, M., Seitz, L., Hoeve, M., and O. Bergmann,
"Extension for protecting (D)TLS handshakes against Denial
of Service", draft-tiloca-tls-dos-handshake-02 (work in
progress), March 2018.

[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>.

[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/info/rfc5746>.

[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>.

[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>.

8.2.

9.2. 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>.

[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>.

8.3.

9.3. URIs

[1] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.8.1
16#section-5.8.1

[2] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.1.2 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

[3] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.3 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

[4] https://tools.ietf.org/html/rfc7252#section-9 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

[5] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.8.1
16#section-5.1.2

[6] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.1.1 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

[7] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.1.1
16#section-5.3

[8] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.6.3
16#section-5.8.1

[9] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.8.3 https://tools.ietf.org/html/rfc7252#section-9

[10] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.1.2 https://tools.ietf.org/html/rfc7250

[11] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
13#section-5.1.2 https://tools.ietf.org/html/rfc7252

[12] https://tools.ietf.org/html/rfc6749#section-5.2 https://tools.ietf.org/html/rfc7748

[13] https://tools.ietf.org/html/rfc8152#section-12.1.2 https://tools.ietf.org/html/rfc7252

[14] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

[15] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

[16] https://tools.ietf.org/html/rfc8152#section-11

[17] https://tools.ietf.org/html/rfc8152#section-11.2

[18] https://tools.ietf.org/html/rfc6749#section-5.2

[19] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz#section-
5.7.3

[20] https://tools.ietf.org/html/rfc4279#section-2

[21] https://tools.ietf.org/html/rfc7925#section-4.2

[22] https://tools.ietf.org/html/rfc7252

[23] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz

[24] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
16#section-5.1.1

[25] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
16#section-5.8.2

[26] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
16#section-5.1.2

[27] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
16#section-5.6.3

[28] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
16#section-5.8.3

[29] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
16#section-5.1.2

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
Faroegatan 6
Kista 164 80
Sweden AB

Email: goran.selander@ericsson.com

Ludwig Seitz
RISE SICS
Scheelevaegen 17
Lund 223 70
Sweden

Email: ludwig.seitz@ri.se