wdiff draft-ietf-ace-dtls-authorize-01.txt draft-ietf-ace-dtls-authorize-02.txt

ACE Working Group S. Gerdes
Internet-Draft O. Bergmann
Intended status: Standards Track C. Bormann
Expires: January 4, May 3, 2018 Universitaet Bremen TZI
G. Selander
Ericsson
L. Seitz
RISE SICS
July 03,
October 30, 2017

Datagram Transport Layer Security (DTLS) Profile Profiles for Authentication and
Authorization for Constrained Environments (ACE)
draft-ietf-ace-dtls-authorize-01
draft-ietf-ace-dtls-authorize-02

Abstract

This specification defines a profile two profiles for delegating client
authentication and authorization in a constrained environment by
establishing a Datagram Transport Layer Security (DTLS) channel
between resource-constrained nodes. The protocol relies on DTLS for
communication security between entities in a constrained network. network
using either raw public keys or pre-shared keys. A
resource-constrained resource-
constrained node 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-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 4, May 3, 2018.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Unauthorized Resource Request Message . . . Access . . . . . . . 5
2.2. AS Information . . . . . . . . . . . . . . 5
2.2. Dynamic Update of Authorization Information . . . . . . . 6
2.3. Resource Access . Token Expiration . . . . . . . . . . . . . . . . . . . . 7
2.4. Dynamic Update of Authorization Information . . . . . . . 8
3. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . . . 9 8
4. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . . . 10
4.1. DTLS Channel Setup Between C and RS . . . . . . . . . . . 11
4.2. Updating Authorization Information . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
5.1. Unprotected AS Information . . . . . . . . . . . . . . . 14
5.2. Use of Nonces for Replay Protection . . . . . . . . . . . 14
5.3.
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7.
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1.
8.1. Normative References . . . . . . . . . . . . . . . . . . 14
7.2.
8.2. Informative References . . . . . . . . . . . . . . . . . 15
7.3.
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16

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 an access token, bound to a key (the proof-of-possession
key) to authorize its access to protected resources hosted by the
resource server. DTLS provides communication security, proof of
possession, and server authentication. Optionally the client and the
resource server may also use CoAP over DTLS to communicate with the
authorization server. This specification supports the DTLS handshake
with Raw Public Keys (RPK) [RFC7250] and the DTLS PSK handshake with Pre-
Shared Keys (PSK) [RFC4279].

The DTLS RPK handshake [RFC7250] requires client authentication to
provide proof-of-possession for the key tied to the access token.
Here the access token needs to be transferred to the resource server
before the handshake is initiated, as described in section 8.1 5.8.1 of
draft-ietf-ace-oauth-authz. [1]
draft-ietf-ace-oauth-authz [1].

The DTLS PSK handshake [RFC4279] provides the proof-of-possession for
the key tied to the access token. Furthermore the psk_identity
parameter in the DTLS PSK handshake is used to transfer the access
token from the client to the resource server.

Note: While the scope of this draft is on client and resource server

communicating using CoAP over DTLS, it is expected that it applies
also to CoAP over TLS, possibly with minor modifications.
However, that is out of scope for this version of the draft.

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 RFC 2119 [RFC2119]. 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].

2. Protocol Overview

The CoAP-DTLS profile for ACE specifies the transfer of
authentication 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 a Client can use
CoAP over DTLS to retrieve an Access Token from the authorization
server AS for a protected resource hosted on the resource server RS.

This profile requires a Client (C) to retrieve an Access Token for
the resource(s) it wants to access on a Resource Server (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):

C RS AS
| [-- Resource Request --->] | |
| | |
| [<----- AS Information --] | |
| | |
| --- Token Request ----------------------------> |
| | |
| <---------------------------- Access Token ----- |
| + RS 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 the address of its
AS back to C.

Instead of the initial Unauthorized Resource Request message, C MAY
look up the desired resource in a resource directory (cf.
[I-D.ietf-core-resource-directory]). client C.

Once the client C knows AS's the authorization server's address, it can
send an Access Token request to the
/token token endpoint at the AS as
specified in [I-D.ietf-ace-oauth-authz]. If C wants to use the CoAP
RawPublicKey mode as described in Section 9 of RFC 7252 [2] it MUST
provide a key or key identifier within a "cnf" object in the token
request. If the authorization server AS decides that the request is
to be authorized it generates an access token response for the client
C containing a "profile" parameter with the value "coap_dtls" to
indicate that this profile MUST be used for communication between the
client C and RS. the resource server. Is also adds a "cnf" parameter
with additional data for the establishment of a secure DTLS channel
between C the client and RS. the resource server. The semantics of the
'cnf' parameter depend on the type of key used between C the client and RS,
the resource server, see Section 3 and Section 4.

The Access Token returned by AS the authorization server then can be
used by C the client to establish a new DTLS session with RS. the resource
server. When C the client intends to use asymmetric cryptography in
the DTLS handshake with RS, C the resource server, the client MUST upload
the Access Token to the "/authz-info" authz-info resource on RS the resource server
before starting the DTLS handshake, as described in section 8.1 5.8.1 of
draft-ietf-ace-oauth-authz [3]. If only symmetric cryptography is
used between C the client and RS, the resource server, the Access Token MAY
instead be transferred in the DTLS ClientKeyExchange message (see
Section 4.1).

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.

Figure 2: Protocol overview

The following sections specify how CoAP is used to interchange
access-related data between RS the resource server and AS the authorization
server so that AS the authorization server can provide C the client and RS
the resource server with sufficient information to establish a secure
channel, and convey authorization information specific for this
communication relationship to RS. the resource server.

Depending on the desired CoAP security mode, the Client-to-AS
request, AS-to-Client response and DTLS session establishment carry
slightly different information. Section 3 addresses the use of raw
public keys while Section 4 defines how pre-shared keys are used in
this profile.

2.1. Unauthorized Resource Request Message

The optional Unauthorized Resource Request message is a request for a
resource hosted by RS for which no proper authorization is granted.
RS MUST treat any CoAP request for a resource other than "/authz-
info" as Unauthorized Resource Request message when any of the
following holds:

o The request has been received on an unprotected channel.

o RS has no valid access token for the sender of the request
regarding the requested action on that resource.

o RS has a valid access token for the sender of the request, but
this does not allow the requested action on the requested
resource.

Note: These conditions ensure that RS can handle requests
autonomously once access was granted and a secure channel has been
established between C and RS. The resource "/authz-info" is publicly
accessible to be able to upload new access tokens to RS (cf.
[I-D.ietf-ace-oauth-authz]).

Unauthorized Resource Request messages MUST be denied with a client
error response. In this response, the Resource Server SHOULD provide
proper AS Information to enable the Client to request an access token
from RS's Authorization Server as described in Section 2.2.

The response code MUST be 4.01 (Unauthorized) in case the sender of
the Unauthorized Resource Request message is not authenticated, or if
RS has no valid access token for C. If RS has an access token for C
but not for the resource that C has requested, RS MUST reject the
request with a 4.03 (Forbidden). If RS has an access token for C but
it does not cover the action C requested on the resource, RS MUST
reject the request with a 4.05 (Method Not Allowed).

Note: The use of the response codes 4.03 and 4.05 is intended to
prevent infinite loops where a dumb Client optimistically tries to
access a requested resource with any access token received from
AS. As malicious clients could pretend to be C to determine C's
privileges, these detailed response codes must be used only when a
certain level of security is already available which can be
achieved only when the Client is authenticated.

2.2. AS Information

The AS Information is sent by RS as a response to an Unauthorized
Resource Request message (see Section 2.1) to point the sender of the
Unauthorized Resource Request message to RS's AS. The AS information
is a set of attributes containing an absolute URI (see Section 4.3 of
[RFC3986]) that specifies the AS in charge of RS.

TBD: We might not want to add more parameters in the AS information
because
this would not only reveal too much information about RS's
capabilities to unauthorized peers but also be of little value as
C cannot really trust that information anyway.

The message MAY also contain a nonce generated by RS to ensure
freshness in case that the RS and AS do not have synchronized clocks.

Figure 3 shows an example for an AS Information message payload using
CBOR [RFC7049] diagnostic notation.

4.01 Unauthorized
Content-Format: application/ace+cbor
{AS: "coaps://as.example.com/token",
nonce: h'e0a156bb3f'}

Figure 3: AS Information payload example

In this example, the attribute AS points the receiver of this message
to the URI "coaps://as.example.com/token" to request access
permissions. The originator of the AS Information payload (i.e., RS)
uses a local clock that is loosely synchronized with a time scale
common between RS and AS (e.g., wall clock time). Therefore, it has
included a parameter "nonce" for replay attack prevention (c.f.
Section 5.2).

Note: There is an ongoing discussion how freshness of access tokens
can be achieved in constrained environments. This specification
for now assumes that RS and AS do not have a common understanding
of time that allows RS to achieve its security objectives without
explicitly adding a nonce.

The examples in this document are written in CBOR diagnostic notation
to improve readability. Figure 4 illustrates the binary encoding of
the message payload shown in Figure 3.

a2 # map(2)
00 # unsigned(0) (=AS)
78 1c # text(28)
636f6170733a2f2f61732e657861
6d706c652e636f6d2f746f6b656e # "coaps://as.example.com/token"
05 # unsigned(5) (=nonce)
45 # bytes(5)
e0a156bb3f

Figure 4: AS Information example encoded in CBOR

2.3. Resource Access

Once Access

Once a DTLS channel has been established as described in Section 3
and Section 4, respectively, C the client is authorized to access
resources covered by the Access Token it has uploaded to the "/authz-info" authz-
info resource hosted by RS. the resource server.

On the resource server side (i.e., RS), side, successful establishment of the DTLS
channel binds C the client to the access token, functioning as a proof-of-
possession proof-
of-possession associated key. Any request that RS the resource server
receives on this channel MUST be checked against these authorization
rules that are associated with the identity of C. the client. Incoming
CoAP requests that are not authorized with respect to any Access
Token that is associated with C the client MUST be rejected by RS the
resource server with 4.01 response as described in Section 2.1. 5.1.1 of
draft-ietf-ace-oauth-authz [4].

Note: The identity of C the client is determined by the authentication
process
during the DTLS handshake. In the asymmetric case, the public key
will define C's the client's identity, while in the PSK case, C's the
client's identity is defined by the session key generated by AS the
authorization server for this communication.

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 peer is valid.

3. The request is destined for RS. 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 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-
08#section-5.1.1

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 a an Authorized Resource
Request will succeed. If C the client repeatedly gets error responses
containing AS Information messages (cf. Section 2.2) 5.1.1 of draft-ietf-ace-
oauth-authz [5] as response to its requests, it SHOULD request a new
Access Token from AS the authorization server in order to continue
communication with RS.

2.4. the resource server.

2.2. Dynamic Update of Authorization Information

The Client client can update the authorization information stored at RS the
resource server at any time. time without changing an established DTLS
session. To do so, the Client requests from AS the authorization server
a new Access Token for the intended action on the respective resource
and uploads this Access Token to the "/authz-info" authz-info resource on RS. the
resource server.

Figure 5 3 depicts the message flow where the client C requests a new
Access Token after a security association between C the client and the
resource server RS has been established using this protocol. The
token request MUST specify the key identifier of 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 ticket that has previously been issued to the
requesting client. Otherwise, the Client-to-AS request MUST be
declined with a the error code "unsupported_pop_key" as defined in
Section 5.6.3 of draft-ietf-ace-oauth-authz [6].

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
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 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 - |
| + RS Information |
| | |
| --- Update /authz-info --> | |
| New Access Token | |
| | |
| == Authorized Request ===> | |
| | |
| <=== Protected Resource == | |

Figure 5: 3: Overview of Dynamic Update Operation

2.3. Token Expiration

DTLS sessions that have been established in accordance with this
profile are always tied to a specific set of access tokens. As these
tokens may become invalid at any time (either because the token has
expired or the responsible authorization server has revoked the
token), the session may become useless at some point. A resource
server therefore may decide to terminate existing DTLS sessions after
the last valid access token for this session has been deleted.

As specified in section 5.8.2 of draft-ietf-ace-oauth-authz [7], 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.

The resource server MAY also keep the session alive for some time and
respond to incoming requests with a 4.01 (Unauthorized) error message
including AS Information to signal that the client needs to upload a
new access token before it can continue using this DTLS session. The
AS Information is created as specified in section 5.1.2 of draft-
ietf-ace-oauth-authz [8]. The resource server SHOULD add a "kid"
parameter to the AS Information denoting the identifier of the key
that it uses internally for this DTLS session. The client then
includes this "kid" parameter in a Client-to-AS request used to
retrieve a new access token to be used with this DTLS session. In
case the key identifier is already known by the client (e.g. because
it was included in the RS Information in an AS-to-Client response),
the "kid" parameter MAY be elided from the AS Information.

Table 1 updates Figure 2 in section 5.1.2 of draft-ietf-ace-oauth-
authz [9] 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 C the client wants to
access,
C the client requests an Access Token from AS. C the authorization
server. 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 AS. the authorization server.

An example Access Token request from C the client to RS the resource
server is depicted in Figure 6. 4.

POST coaps://as.example.com/token
Content-Format: application/cbor
{
grant_type: client_credentials,
aud: "tempSensor4711",
cnf: {
COSE_Key: {
kty: EC2,
crv: P-256,
x: h'TODOX',
y: h'TODOY'
}
}
}

Figure 6: 4: Access Token Request Example for RPK Mode

The example shows an Access Token request for the resource identified
by the audience string "tempSensor4711" on the AS authorization server
using a raw public key.

When AS the authorization server authorizes a request, it will return an
Access Token and a "cnf" object in the AS-to-Client response. Before C
the client initiates the DTLS handshake with RS, the resource server, it
MUST send a "POST" request containing the new Access Token to the "/authz-info"
authz-info resource hosted by RS. the resource server. If this operation
yields a positive response, C the client SHOULD proceed to establish a
new DTLS channel with RS. the resource server. To use raw public key
mode, C the client MUST pass the same public key that was used for
constructing the Access Token with the SubjectPublicKeyInfo structure
in the DTLS handshake as specified in [RFC7250].

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. C the client is therefore expected to offer at
least this ciphersuite to RS. the resource server.

The Access Token is constructed by AS the authorization server such that RS
the resource server can associate the Access Token with the Client's
public key. If CBOR web tokens [I-D.ietf-ace-cbor-web-token] are
used as recommended in [I-D.ietf-ace-oauth-authz], the AS authorization
server MUST include a "COSE_Key" object in the "cnf" claim of the
Access Token. This "COSE_Key" object MAY contain a reference to a
key for C the client that is already known by RS the resource server
(e.g., from previous communication). If the AS authorization server has
no certain knowledge that the Client's key is already known to RS, the
resource server, the Client's public key MUST be included in the
Access Token's "cnf" parameter.

4. PreSharedKey Mode

To retrieve an access token for the resource that C the client wants to
access,
C the client MAY include a "cnf" object carrying an identifier
for a symmetric key in its Access Token request to AS. the authorization
server. This identifier can be used by AS the authorization server to
determine the session key to construct the proof-of-
possession proof-of-possession token
and therefore MUST specify a symmetric key that was previously
generated by AS the authorization server as a session key for the
communication between C the client and RS. the resource server.

Depending on the requested token type and algorithm in the Access
Token request, AS the authorization server adds RS Information to the
response that provides C the client with sufficient information to
setup a DTLS channel with RS. the resource server. For symmetric proof-of-possession proof-
of-possession keys (c.f. [I-D.ietf-ace-oauth-authz]), C the client
must ensure that the Access Token request is sent over a secure
channel that guarantees authentication, message integrity and
confidentiality.

When AS the authorization server authorizes C the client it returns an AS-to-Client AS-
to-Client response with the profile parameter set to "coap_dtls" and
a "cnf" parameter carrying a "COSE_Key" object that contains the
symmetric session key to be used between C the client and RS the resource
server as illustrated in Figure 7. 5.

2.01 Created
Content-Format: application/cbor
Location-Path: /token/asdjbaskd
Max-Age: 86400
{
access_token: b64'SlAV32hkKG ...
(remainder 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 7: 5: Example Access Token response

In this example, AS the authorization server returns a 2.01 response
containing a new Access Token. The information is transferred as a
CBOR data structure as specified in [I-D.ietf-ace-oauth-authz]. The
Max-Age option tells the receiving Client how long this token will be
valid.

A response that declines any operation on the requested resource is
constructed according to Section 5.2 of RFC 6749 [4], [10], (cf.
Section 5.5.3 5.7.3 of [I-D.ietf-ace-oauth-authz]).

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

Figure 8: 6: Example Access Token response with reject

4.1. DTLS Channel Setup Between C and RS

When C a client receives an Access Token from AS, an authorization server,
it checks if the payload contains an "access_token" parameter and a
"cnf" parameter. With this information C the client can initiate
establishment of a new DTLS channel with RS. a resource server. To use
DTLS with pre-shared keys, C 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 session key by C the client
and RS the resource server is used for mutual authentication between
both peers. Therefore, RS the resource server must be able to determine
the session key from the Access Token. Following the general ACE
authorization framework, C the client can upload the Access Token to RS's "/authz-info"
the resource server's authz-info resource before starting the DTLS
handshake. Alternatively, C the client MAY provide the most recent
base64-encoded Access Token in the "psk_identity" field of the
ClientKeyExchange message.

If RS a resource server receives a ClientKeyExchange message that
contains a "psk_identity" with a length greater zero, it MUST
base64-decode its contents and check if the "psk_identity" field contains a key
identifier or Access Token according to the following CDDL
specification:

psk_identity = {
kid => bstr // access_token => bstr
}

The identifiers for the map keys "kid" and "access_token" are used
with use the same meaning resulting byte sequence as in COSE [I-D.ietf-cose-msg] and the ACE
framework [I-D.ietf-ace-oauth-authz] respectively. The identifier
"kid" thus has the value 4 (see [I-D.ietf-cose-msg]), and the
identifier "access_token" has the value 19, respectively (see
[I-D.ietf-ace-oauth-authz]).

If the "psk_identity" field contains a
index for its key identifier, store (i.e., treat the receiver 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 valid Access Token
is available for this key, the DTLS session setup is terminated with
an "illegal_parameter" DTLS alert message.

If instead no key with a matching identifier is found the resource server the
resource server MAY process the decoded contents of the
"psk_identity" field contains an Access Token, it must
processed in the same way as an Access Token access token that has been uploaded
to its "/authz-info" resource. In this case, RS continues processing is stored with the ClientKeyExchange message if
authorization information endpoint before continuing the DTLS
handshake. If the decoded contents of the "psk_identity"
contained do not
yield a valid Access Token. Otherwise, access token for the requesting client, the DTLS
session setup is terminated with an "illegal_parameter" DTLS alert
message.

Note1: As RS a resource server cannot provide C a client with a meaningful
PSK identity hint in
response to C's the client's ClientHello message, RS 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]. C A client is
therefore expected to offer at least this ciphersuite to RS. the
resource server.

This specification assumes that the Access Token is a PoP token as
described in [I-D.ietf-ace-oauth-authz] unless specifically stated
otherwise. Therefore, the Access Token is bound to a symmetric PoP
key that is used as session key between C the client and RS. the resource
server.

While C the client can retrieve the session key from the contents of
the "cnf" parameter in the AS-to-Client response, RS the resource server
uses the information contained in the "cnf" claim of the Access Token
to determine the actual session key when no explicit "kid" was
provided in the "psk_identity" field. Usually, this is done by
including a "COSE_Key" object carrying either a key that has been
encrypted with a shared secret between AS the authorization server and RS,
the resource server, or a key identifier that can be used by RS the
resource server to lookup the session key.

Instead of the "COSE_Key" object, AS the authorization server MAY
include a "COSE_Encrypt" structure to enable RS the resource server to
calculate the session key from the Access Token. The "COSE_Encrypt"
structure MUST use the _Direct Key with KDF_ method as described in
Section 12.1.2 of draft-ietf-cose-msg
[5]. RFC 8152 [11]. The AS authorization server MUST
include a Context information structure carrying a PartyU "nonce"
parameter carrying the nonce that has been used by AS the authorization
server to construct the session key.

This specification mandates that at least the key derivation
algorithm "HKDF SHA-256" as defined in [I-D.ietf-cose-msg] [RFC8152] MUST be supported.
This key derivation function is the default when no "alg" field is
included in the "COSE_Encrypt" structure for RS. the resource server.

4.2. Updating Authorization Information

Usually, the authorization information that RS the resource server keeps
for C a client is updated by uploading a new Access Token as described
in Section 2.4. 2.2.

If the security association with RS the resource server still exists and RS
the resource server has indicated support for session renegotiation
according to [RFC5746], the new Access Token MAY be used to
renegotiate the existing DTLS session. In this case, the Access
Token is used as "psk_identity" as defined in Section 4.1. The
Client MAY also perform a new DTLS handshake according to Section 4.1
that replaces the existing DTLS session.

After successful completion of the DTLS handshake RS the resource server
updates the existing authorization information for C the client
according to the new Access Token.

5. Security Considerations

TODO

5.1. Unprotected AS Information

Initially, no secure channel exists to protect

This document specifies a profile for the communication
between C Authentication and RS. Thus, C cannot determine if the AS information
contained in an unprotected response from RS to an unauthorized
request (c.f. Section 2.2) is authentic. It is therefore advisable
to provide C with a (possibly hard-coded) list of trustworthy
authorization servers. AS information responses referring to a URI
not listed there would be ignored.

5.2. Use of Nonces
Authorization for Replay Protection

RS may add a nonce to Constrained Environments (ACE) framework
[I-D.ietf-ace-oauth-authz]. As it follows this framework's general
approach, the AS Information message sent as a response
to an unauthorized request general security and privacy considerations from
section 6 and section 7 also apply to ensure freshness 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 Access Token
subsequently presented adversary is able to RS. While intercept the initial cookie
exchange and interject forged messages with a timestamp of some granularity
would be sufficient valid cookie to protect against replay attacks, using
randomized nonce is preferred
continue with the handshake.

[I-D.tiloca-tls-dos-handshake] specifies a TLS extension to prevent disclosure
this type of information
about RS's internal clock characteristics.

5.3. attack which is applicable especially for constrained
environments where the authorization server can act as trust anchor.

6. Privacy Considerations

An unprotected response to an unauthorized request (c.f.
Section 2.2) may disclose
information about RS the resource server and/or its existing
relationship with C. the client. It is advisable to include as little
information as possible in an unencrypted response. When a DTLS
session between C the client and RS the resource server already exists,
more detailed information may be included with an error response to
provide C the client with sufficient information to react on that
particular error.

Note that some information might still leak after DTLS session is
established, due to observable message sizes, the source, and the
destination addresses.

7. IANA Considerations

This document has no actions

The following registrations are done for IANA.

7. 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

Specification Document(s): [RFC-XXXX]

8. References

7.1.

8.1. 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)", draft-ietf-ace-oauth-
authz-06
authz-08 (work in progress), March October 2017.

[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-01 (work in
progress), October 2017.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>. <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,
<http://www.rfc-editor.org/info/rfc4279>.
<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,
<http://www.rfc-editor.org/info/rfc5746>.
<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, <http://www.rfc-editor.org/info/rfc6347>. <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,
<http://www.rfc-editor.org/info/rfc7252>.

7.2. <https://www.rfc-
editor.org/info/rfc7252>.

[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. Informative References

[I-D.ietf-ace-cbor-web-token]
Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", draft-ietf-ace-cbor-web-token-07
(work in progress), July 2017.

[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", draft-ietf-core-
object-security-04 (work in progress), July 2017.

[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-10 draft-ietf-ace-cbor-web-token-09
(work in progress), March October 2017.

[I-D.ietf-cose-msg]
Schaad, J., "CBOR Object Signing and Encryption (COSE)",
draft-ietf-cose-msg-24 (work in progress), November 2016.

[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655,
DOI 10.17487/RFC6655, July 2012,
<http://www.rfc-editor.org/info/rfc6655>.

[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>. <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, <http://www.rfc-editor.org/info/rfc7250>. <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,
<http://www.rfc-editor.org/info/rfc7251>.

7.3.
<https://www.rfc-editor.org/info/rfc7251>.

8.3. URIs

[1] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
06#section-5.7.1
08#section-5.8.1

[2] https://tools.ietf.org/html/rfc7252#section-9

[3] https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
03#section-8.1
08#section-5.8.1

[4] https://tools.ietf.org/html/rfc6749#section-5.2 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
08#section-5.5.1

[5] https://tools.ietf.org/html/draft-ietf-cose-msg-23#section-12.1.2 https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
08#section-5.1.1

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

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

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

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

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

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

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

Email: goran.selander@ericsson.com

Ludwig Seitz
RISE SICS
Scheelevaegen 17
Lund 223 70
Sweden

Email: ludwig.seitz@ri.se