wdiff draft-ietf-ace-dtls-authorize-09.txt draft-ietf-ace-dtls-authorize-10.txt

ACE Working Group S. Gerdes
Internet-Draft O. Bergmann
Intended status: Standards Track C. Bormann
Expires: June 20, November 14, 2020 Universitaet Bremen TZI
G. Selander
Ericsson AB
L. Seitz
Combitech
December 18, 2019
May 13, 2020

Datagram Transport Layer Security (DTLS) Profile for Authentication and
Authorization for Constrained Environments (ACE)
draft-ietf-ace-dtls-authorize-09
draft-ietf-ace-dtls-authorize-10

Abstract

This specification defines a profile of the ACE framework that allows
constrained servers to delegate client authentication and
authorization. The protocol relies on DTLS version 1.2 for
communication security between entities in a constrained network
using either raw public keys or pre-shared keys. A resource-constrained 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
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on June 20, November 14, 2020.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3 4
3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Communication between C Between the Client and AS the Authorization
Server . . . . . . . . . . . . . 5 . . . . . . . . . . . . 6
3.2. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . 6
3.2.1. DTLS Channel Setup Between C Client and RS . . . . . . . . . 7 Resource Server 9
3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 8 10
3.3.1. DTLS Channel Setup Between C Client and RS . . . . . . . . . 12 Resource Server 14
3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 13 15
4. Dynamic Update of Authorization Information . . . . . . . . . 14 17
5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 16 18
6. Secure Communication with AS an Authorization Server . . . . . . 18
7. Security Considerations . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . 19
7.1. Reuse of Existing Sessions . . . . . . . . . . . . . . . 20
7.2. Multiple Access Tokens . . . . . . . . . . . . . . . . . 21
7.3. Out-of-Band Configuration . . . . . . . . 16 . . . . . . . . 21
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 22
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. 23
11.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. 23
11.2. Informative References . . . . . . . . . . . . . . . . . 19 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 25

1. Introduction

This specification defines a profile of the ACE framework
[I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource
server use CoAP [RFC7252] over DTLS version 1.2 [RFC6347] to
communicate. The client obtains an access token, bound to a key (the proof-of-
possession
proof-of-possession key), from an authorization server to prove its
authorization to access protected resources hosted by the resource
server. Also, the client and the resource server are provided by the
authorization server with the necessary keying material to establish
a DTLS session. The communication between client and authorization
server may also be secured with DTLS. This specification supports
DTLS with Raw Public Keys (RPK) [RFC7250] and with Pre-Shared Keys
(PSK) [RFC4279].

The DTLS handshake ACE framework requires the that client and server mutually
authenticate each other before any application data is exchanged.
DTLS enables mutual authentication if both client and server to prove that they
can
their ability to use certain keying material. material in the DTLS handshake.
The authorization server assists in this process on the server side
by incorporating keying material (or information about keying
material) into the access token, which is considered a "proof of
possession" token.

In the RPK mode, the client proves
with the DTLS handshake that it can use the RPK bound to
the token and the server shows that it can use a certain RPK.

The resource server needs access token
must be presented to the resource server. token in order to complete
this exchange. For the RPK mode, the client must upload the access
token needs to be uploaded to the resource server before initiating the
handshake is initiated, handshake, as
described in Section 5.8.1 of the ACE framework
[I-D.ietf-ace-oauth-authz].

In the PSK mode, client and server show with the DTLS handshake that
they can use the keying material that is bound to the access token.
To transfer the access token from the client to the resource server,
the "psk_identity" parameter in the DTLS PSK handshake may be used
instead of uploading the token prior to the handshake.

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED",

As recommended in Section 5.8 of [I-D.ietf-ace-oauth-authz], this
specification uses CBOR web tokens to convey claims within an access
token issued by the server. While other formats could be used as
well, those are out of scope for this document.

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

Readers are expected to be familiar with the terms and concepts
described in [I-D.ietf-ace-oauth-authz] and in
[I-D.ietf-ace-oauth-params].

The authorization information (authz-info) resource refers to the
authorization information endpoint as specified in

[I-D.ietf-ace-oauth-authz]. The term "claim" is used in this
document with the same semantics as in [I-D.ietf-ace-oauth-authz],
i.e., it denotes information carried in the access token or returned
from introspection.

2. Protocol Overview

The CoAP-DTLS profile for ACE specifies the transfer of
authentication information and, if necessary, authorization
information between the client (C) and the resource server (RS)
during setup of a DTLS session for CoAP messaging. It also specifies
how C the client can use CoAP over DTLS to retrieve an access token
from the authorization server (AS) for a protected resource hosted on
the resource server. As specified in Section 6.7 of
[I-D.ietf-ace-oauth-authz], use of DTLS for one or both of these
interactions is completely independent

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

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

Figure 1: Retrieving an Access Token

To determine the AS authorization server in charge of a resource hosted
at the RS, C MAY resource server, the client can send an initial Unauthorized
Resource Request message to the RS. resource server. The
RS resource server
then denies the request and sends an AS Request Creation Hints
message containing the address of its AS authorization server back to
the client as specified in Section 5.1.2 of
[I-D.ietf-ace-oauth-authz].

Once the client knows the authorization server's address, it can send
an access token request to the token endpoint at the AS authorization
server as specified in [I-D.ietf-ace-oauth-authz]. As the access
token request as well as the response may contain confidential data,
the communication between the client and the authorization server MUST
must be confidentiality-protected and ensure authenticity. C The
client may have been registered at the AS authorization server via the
OAuth 2.0 client registration mechanism as outlined in Section 5.3 of
[I-D.ietf-ace-oauth-authz].

The access token returned by the authorization server can then be
used by the client to establish a new DTLS session with the resource
server. When the client intends to use an asymmetric proof-of-
possession key in the DTLS handshake with the resource server, the
client MUST upload the access token to the authz-info resource, i.e.
the authz-info endpoint, on the resource server before starting the
DTLS handshake, as described in Section 5.8.1 of
[I-D.ietf-ace-oauth-authz]. In case the client uses a symmetric
proof-of-possession key in the DTLS handshake, the procedure as above
MAY be used, or alternatively, the access token MAY instead be
transferred in the DTLS ClientKeyExchange message (see
Section 3.3.1). In any case, DTLS MUST be used in a mode that
provides replay protection.

Figure 2 depicts the common protocol flow for the DTLS profile after
the client C has retrieved the access token from the authorization
server
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, 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 the client (C)
and AS the authorization server (AS) must be secured. Depending on the
used CoAP security mode (see also Section 9 of [RFC7252], the Client-to-AS Client-
to-AS request, AS-to-Client response (see Section 5.6 of
[I-D.ietf-ace-oauth-authz]) and DTLS session establishment carry
slightly different information. Section 3.2 addresses the use of raw
public keys while Section 3.3 defines how pre-shared keys are used in
this profile.

3.1. Communication between C Between the Client and AS the Authorization Server

To retrieve an access token for the resource that the client wants to
access, the client requests an access token from the authorization
server. Before C the client can request the access token, C the client
and AS the authorization server MUST establish a secure communication
channel. C This profile assumes that the keying material to secure
this communication channel has securely been obtained either by
manual configuration or in an automated provisioning process. The
following requirements in alignment with Section 6.5 of
[I-D.ietf-ace-oauth-authz] therefore must be met:

o The client MUST securely have obtained keying material to
communicate with AS. the authorization server.

o Furthermore, C the client MUST verify that AS the authorization server
is authorized to provide access tokens (including authorization
information) about RS the resource server to C. the client, and that
this authorization information about the authorization server is
still valid.

o Also, AS the authorization server MUST securely have obtained keying
material for C, the client, and obtained authorization rules approved
by the resource owner (RO) concerning C the client and RS the resource
server that relate to this keying material. C

The client and AS the authorization server MUST use their respective
keying material for all exchanged messages. How the security
association between C the client and AS the authorization server is
bootstrapped is not part of this document. C The client and
AS MUST the
authorization server must ensure the confidentiality, integrity and
authenticity of all exchanged messages. messages within the ACE protocol.

Section Section 6 specifies how communication with the AS authorization
server is secured.

3.2. RawPublicKey Mode

When the client and the resource server use RawPublicKey
authentication, the procedure is as follows: After C the client and AS the
authorization server mutually authenticated each other and validated
each other's authorization, C the client sends a token request to AS's the
authorization server's token endpoint. The client MUST add a
"req_cnf" object carrying either its raw public key or a unique
identifier for a public key that it has previously made known to the
authorization server. To prove It is RECOMMENDED that the client is in possession of this key, C MUST use uses DTLS
with the same keying material that it uses to secure the communication with AS, e.g., the
DTLS session.
authorization server, proving possession of the key as part of the
token request. Other mechanisms for proving possession of the key
may be defined in the future.

An example access token request from the client to the AS authorization
server is depicted in Figure 3.

POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
grant_type : client_credentials,
req_aud : "tempSensor4711",
req_cnf : {
COSE_Key : {
kty : EC2,
crv : P-256,
x : h'e866c35f4c3c81bb96a1...',
y : h'2e25556be097c8778a20...'
}
}
}

Figure 3: Access Token Request Example for RPK Mode

The example shows an access token request for the resource identified
by the string "tempSensor4711" on the authorization server using a
raw public key.

The authorization server MUST check if the client that it
communicates with is associated with the RPK in the cnf object "req_cnf"
parameter before issuing an access token to it. If AS the authorization
server determines that the request is to be authorized according to
the respective authorization rules, it generates an access token
response for C. the client. The access token MUST be bound to the RPK
of the client by means of the cnf "cnf" claim.

The response MAY contain a "profile" parameter with the value
"coap_dtls" to indicate that this profile MUST be used for
communication between the client C and the resource server. The
"profile" may be specified out-of-band, in which case it does not
have to be sent. The response also contains an access token and an "rs_cnf" parameter containing with
information for the resource server about the client's public key that key.

The authorization server MUST return in its response the parameter
"rs_cnf" unless it is used by certain that the client already knows the
public key of the resource server. AS The authorization server MUST
ascertain that the RPK specified in "rs_cnf" belongs to the resource
server that C the client wants to communicate with. AS The authorization
server MUST protect the integrity of the token. access token such that the
resource server can detect unauthorized changes. If the access token
contains confidential data, AS the authorization server MUST also
protect the confidentiality of the access token.

The client MUST ascertain that the access token response belongs to a
certain previously sent access token request, as the request may
specify the resource server with which C the client wants to
communicate.

An example access token response from the AS authorization to the client
is depicted in Figure 4. Here, the contents of the "access_token"
claim have been truncated to improve readability. Caching proxies
process the Max-Age option in the CoAP response which has a default
value of 60 seconds (Section 5.6.1 of [RFC7252]). The authorization
server SHOULD adjust the Max-Age option such that it does not exceed
the "expires_in" parameter to avoid stale responses.

2.01 Created
Content-Format: application/ace+cbor
Max-Age: 3600 3560
Payload:
{
access_token : b64'SlAV32hkKG...
(remainder of CWT omitted for brevity;
CWT contains clients the client's RPK in the cnf claim)',
expires_in : 3600,
rs_cnf : {
COSE_Key : {
kty : EC2,
crv : P-256,
x : h'd7cc072de2205bdc1537...',
y : h'f95e1d4b851a2cc80fff...'
}
}
}

Figure 4: Access Token Response Example for RPK Mode

3.2.1. DTLS Channel Setup Between C Client and RS Resource Server

Before the client initiates the DTLS handshake with the resource
server, C the client MUST send a "POST" request containing the new obtained
access token to the authz-info resource hosted by the resource
server. After the client receives a confirmation that the RS resource
server has accepted the access token, it SHOULD proceed to establish
a new DTLS channel with the resource server. To use the RawPublicKey mode, the The client MUST specify the use its
correct public key that AS defined in the DTLS handshake. If the authorization
server has specified a "cnf" field in the access token response, the
client MUST use this key. Otherwise, the client MUST use the public
key that it specified in the "req_cnf" of the access token
response request.
The client MUST specify this public key in the SubjectPublicKeyInfo
structure in of the DTLS handshake as specified described in [RFC7250].

To be consistent with [RFC7252] which allows for shortened MAC tags
in constrained environments, an implementation that supports the RPK
mode of this profile MUST at least support the ciphersuite
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
[RFC7251] 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. [RFC7251]. As discussed in
[RFC7748], new ECC curves have been defined recently that are
considered superior to the so-
called so-called NIST curves. The curve that is mandatory to implement in
this This specification is
therefore mandates implementation support for curve25519 (cf.
[RFC8032], [RFC8422]) as this curve said to be efficient and less
dangerous regarding implementation errors than the secp256r1 curve
mandated in [RFC7252].

The resource server MUST check if the access token is still valid, if RS
the resource server is the intended destination, i.e., destination (i.e., the audience, audience)
of the token, and if the token was issued by an authorized AS.
authorization server. The access token is constructed by the
authorization server such that the resource server can associate the
access token with the Client's public key. The "cnf" claim MUST
contain either C's the client's RPK or, if the key is already known by
the resource server (e.g., from previous communication), a reference
to this key. If the authorization server has no certain knowledge
that the Client's key is already known to the resource server, the
Client's public key MUST be included in the access token's "cnf"
parameter. If CBOR web tokens [RFC8392] are used as recommended in
[I-D.ietf-ace-oauth-authz], keys MUST be encoded as specified in [I-D.ietf-ace-cwt-proof-of-possession]. RS
[RFC8747]. The resource server MUST use the
keying material its own raw public key in
the DTLS handshake with the client. If the resource server has
several raw public keys, it must already know which key it is
supposed to use with this client. How this is achieved is not part
of this profile.

The resource server MUST use the keying material that AS the
authorizations server has specified in the rs_cnf "cnf" parameter in the
access token. token for the DTLS handshake with the client. Thus, the
handshake only finishes if
C the client and RS the resource server are
able to use their respective keying material.

3.3. PreSharedKey Mode

To retrieve an access token for the resource that the client wants to
access, the client MAY include a "cnf" object carrying an identifier
for a symmetric key in its access token request to the authorization
server. This identifier can be used by the authorization server to
determine the shared secret to construct the proof-of-possession
token. AS The authorization server MUST check if the identifier refers
to a symmetric key that was previously generated by AS the authorization
server as a shared secret for the communication between this client
and the resource server. If no such symmetric key was found, the
authorization server MUST generate a new symmetric key that is
returned in its response to the client.

The authorization server MUST determine the authorization rules for
the C client it communicates with as defined by RO the resource owner and
generate the access token accordingly. If the authorization server
authorizes the client, it returns an AS-to-Client response. If the
profile parameter is present, it is set to "coap_dtls". AS The
authorization server MUST ascertain that the access token is
generated for the resource server that C the client wants to
communicate with. Also, AS the authorization server MUST protect the
integrity of the access token. token to ensure that the resource server can
detect unauthorized changes. If the token contains confidential data
such as the symmetric key, the confidentiality of the token MUST also
be protected. Depending on the requested token type and algorithm in
the access token request, the authorization server adds access
Information to the response that provides the client with sufficient
information to setup a DTLS channel with the resource server. AS The
authorization server adds a "cnf" parameter to the access information
carrying a "COSE_Key" object that informs the client about the symmetric key shared
secret that is to be used between C and the client and the resource server. The access
token MUST be bound to
To convey the same symmetric key secret to the resource server, the authorization
server either includes it directly in the access token by means of
the cnf
claim. "cnf" claim or it provides sufficient information to enable the
resource server to derive the key from the access token using key
derivation.

An example access token request for an access token with a symmetric
proof-of-possession key is illustrated in Figure 5.

POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
audience : "smokeSensor1807",
}

Figure 5: Example Access Token Request, (implicit) symmetric PoP-key

A corresponding example access token response is illustrated in
Figure 6. In this example, the authorization server returns a 2.01
response containing a new access token (truncated to improve
readability) and information for the client, including the symmetric
key in the cnf claim. The information is transferred as a CBOR data
structure as specified in [I-D.ietf-ace-oauth-authz].

2.01 Created
Content-Format: application/ace+cbor
Max-Age: 86400 85800
Payload:
{
access_token : h'd08343a10...
(remainder of CWT omitted for brevity)
token_type : pop, PoP,
expires_in : 86400,
profile : coap_dtls,
cnf : {
COSE_Key : {
kty : symmetric,
kid : h'3d027833fc6267ce',
k : h'73657373696f6e6b6579'
}
}
}

Figure 6: Example Access Token Response, symmetric PoP-key

The access token also comprises a "cnf" claim. This claim usually
contains a "COSE_Key" object that carries either the symmetric key
itself or a key identifier that can be used by the resource server to
determine the secret key shared it shares with the client. If the access
token carries a symmetric key, the access token MUST be encrypted
using a "COSE_Encrypt0" structure. The AS authorization server MUST use
the keying material shared with the RS resource server to encrypt the
token.

The "cnf" structure in the access token is provided in Figure 7.

cnf : {
COSE_Key : {
kty : symmetric,
kid : h'6549694f464361396c4f6277' h'3d027833fc6267ce'
}
}

Figure 7: Access Token without Keying Material

A response that declines any operation on the requested resource is
constructed according to Section 5.2 of [RFC6749], (cf.
Section 5.6.3. of [I-D.ietf-ace-oauth-authz]). Figure 8 shows an
example for a request that has been rejected due to invalid request
parameters.

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

Figure 8: Example Access Token Response With Reject

The method for how the resource server determines the symmetric key
from an access token containing only a key identifier is application
specific, application-
specific; the remainder of this section provides one example.

The AS authorization server and the resource server are assumed to share
a key derivation key used to derive the symmetric key shared with the
client from the key identifier in the access token. The key
derivation key may be derived from some other secret key shared
between the AS authorization server and the resource server. This key
needs to be securely stored and processed in the same way as the key
used to protect the communication between
AS the authorization server
and RS. the resource server.

Knowledge of the symmetric key shared with the client must not reveal
any information about the key derivation key or other secret keys
shared between AS the authorization server and resource server.

In order to generate a new symmetric key to be used by client and
resource server, the AS authorization server generates a new key
identifier and which MUST be unique among all key identifiers used by the
authorization server. The authorization server then uses the key
derivation key shared with the resource server to derive the
symmetric key as specified below. Instead of providing the keying
material in the access token, the AS authorization server includes the
key identifier in the "kid" parameter, see Figure 7. This key
identifier enables the resource server to calculate the keying material symmetric key
used for the communication with the client from the access token using the key derivation
key and following Section 11 of [RFC8152] with parameters
as specified here. The a KDF to be used needs to be defined by the application, for example HKDF-SHA-256. HKDF-SHA-
256. The key identifier picked by the AS authorization server needs to
be unique for each access token where a unique symmetric key is
required.

The fields in

In this example, HKDF consists of the context information "COSE_KDF_Context"
(Section 11.2 composition of [RFC8152]) have the following values:

o AlgorithmID HKDF-Extract
and HKDF-Expand steps [RFC5869]. The symmetric key is derived from
the key identifier, the key derivation key and other data:

OKM = "ACE-CoAP-DTLS-key-derivation" HKDF(salt, IKM, info, L),

where:

o PartyUInfo = PartyVInfo = ( null, null, null ) OKM, the output keying material, is the derived symmetric key

o keyDataLength needs to be defined by salt is the application empty byte string

o protected MUST be a zero length bstr IKM, the input keying material, is the key derivation key as
defined above

o other info is the serialization of a zero length bstr CBOR array consisting of
([RFC8610]):

info = [
type : tstr,
L : uint,
access_token: map
]

where:

o SuppPrivInfo type is omitted

3.3.1. DTLS Channel Setup Between C and RS

When a client receives an access token response from an authorization
server, C MUST ascertain that the access token response belongs set to a
certain previously sent access token request, the constant text string "ACE-CoAP-DTLS-key-
derivation",

o L is the size of the symmetric key in bytes,

o access_token is the decrypted access_token as transferred from the
authorization server to the resource server.

To ensure uniqueness of the derived shared secret, the authorization
server SHOULD generate a sufficiently random kid value and include
the claims "iat" and either "exp" or "exi" in the access token.

3.3.1. DTLS Channel Setup Between Client and Resource Server

When a client receives an access token response from an authorization
server, the client MUST check if the access token response is bound
to a certain previously sent access token request, as the request may
specify the resource server with which C the client wants to
communicate.

The client checks if the payload of the access token response
contains an "access_token" parameter and a "cnf" parameter. With
this information the client can initiate the establishment of a new
DTLS channel with a resource server. To use DTLS with pre-shared
keys, the client follows the PSK key exchange algorithm specified in
Section 2 of [RFC4279] using the key conveyed in the "cnf" parameter
of the AS response as PSK when constructing the premaster secret. To
be consistent with the recommendations in [RFC7252] a client is
expected to offer at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8
[RFC6655] to the resource server.

In PreSharedKey mode, the knowledge of the shared secret by the
client and the resource server is used for mutual authentication
between both peers. Therefore, the resource server must be able to
determine the shared secret from the access token. Following the
general ACE authorization framework, the client can upload the access
token to the resource server's authz-info resource before starting
the DTLS handshake. Alternatively, The client then needs to indicate during the
DTLS handshake which previously uploaded access token it intends to
use. To do so, it MUST create a "COSE_Key" structure with the "kid"
that was conveyed in the "rs_cnf" claim in the token response from
the authorization server and the key type "symmetric". This
structure then is included as the only element in the "cnf" structure
that is used as value for "psk_identity" as shown in Figure 9.

{ cnf : {
COSE_Key : {
kty: symmetric,
kid : h'3d027833fc6267ce'
}
}
}

Figure 9: Access token containing a single kid parameter

As an alternative to the access token upload, the client MAY can provide
the most recent access token in the "psk_identity" field of the
ClientKeyExchange message. To do so, the client MUST treat the
contents of the "access_token" field from the AS-to-Client response
as opaque data and not perform any re-coding.

Note: As stated as specified in Section 4.2 of [RFC7925], the PSK identity should
be treated as binary data in the Internet of Things space [RFC7925] and not
assumed to have a human-readable form of
perform any sort.

If a re-coding. This allows the resource server receives a ClientKeyExchange message to retrieve
the shared secret directly from the "cnf" claim of the access token.

If a resource server receives a ClientKeyExchange message that
contains a "psk_identity" with a length greater than zero, it uses the
contents as index for its key store (i.e., treat the contents as key
identifier). The resource server MUST check if it has one or more
access tokens that are associated with the specified key.

If no key with a matching identifier is found, the resource server
MAY
process the contents of the "psk_identity" field as access token that
is stored with the authorization information endpoint, before
continuing the DTLS handshake. If the contents of the "psk_identity"
do not yield a valid access token for the requesting client, the
resource server aborts the DTLS
session setup is terminated handshake with an "illegal_parameter" DTLS alert
message.

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

When the resource server SHOULD NOT send a
ServerKeyExchange message.

Note2: According to [RFC7252], CoAP implementations MUST support the
ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655]. A client is
therefore expected to offer at least this ciphersuite to the
resource server.

When RS receives an access token, RS it MUST check if
the access token is still valid, if RS the resource server is the
intended destination, i.e., destination (i.e., the audience of the token, token), and if the
token was issued by an authorized AS. authorization server. This
specification assumes that the access token is a PoP token as
described in [I-D.ietf-ace-oauth-authz] unless specifically stated
otherwise. Therefore, the access token is bound to a symmetric PoP
key that is used as shared secret between the client and the resource
server. The resource server may use token introspection [RFC7662] on
the access token to retrieve more information about the specific
token. The use of introspection is out of scope for this
specification.

While the client can retrieve the shared secret from the contents of
the "cnf" parameter in the AS-to-Client response, the resource server
uses the information contained in the "cnf" claim of the access token
to determine the actual secret when no explicit "kid" was provided in
the "psk_identity" field. If key derivation is used, the RS resource
server uses the "COSE_KDF_Context" information as described above.

3.4. Resource Access

Once a DTLS channel has been established as described in Section 3.2
and
or Section 3.3, respectively, the client is authorized to access
resources covered by the access token it has uploaded to the authz-
info resource hosted by the resource server.

With the successful establishment of the DTLS channel, C the client and RS
the resource server have proven that they can use their respective
keying material. An access token that is bound to the client's
keying material is associated with the channel. According to section
5.8.1 of [I-D.ietf-ace-oauth-authz], there should be only one access
token for each client. New access tokens issued by the authorization
server are supposed to replace previously issued access tokens for
the respective client. The resource server therefore must have a
common understanding with the authorization server how access tokens
are ordered.

Any request that the resource server receives on
this a DTLS channel that
is tied to an access token via its keying material MUST be checked
against these the authorization rules. RS rules that can be determined with the
access token. The resource server MUST check for every request if
the access token is still valid.
Incoming If the token has expired, the
resource server MUST remove it. Incoming CoAP requests that are not
authorized with respect to any access token that is associated with
the client MUST be rejected by the resource server with 4.01
response. The response MAY include AS Request Creation Hints as
described in Section 5.1.1 of [I-D.ietf-ace-oauth-authz].

The resource server SHOULD treat MUST only accept an incoming CoAP request as
authorized if the following holds:

1. The message was received on a secure channel that has been
established using the procedure defined in this document.

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

3. The request is destined for the resource server.

4. The resource URI specified in the request is covered by the
authorization information.

5. The request method is an authorized action on the resource with
respect to the authorization information.

Incoming CoAP requests received on a secure DTLS channel that are not
thus authorized MUST be rejected according to Section 5.8.2 of
[I-D.ietf-ace-oauth-authz]

1. with response code 4.03 (Forbidden) when the resource URI
specified in the request is not covered by the authorization
information, and

2. with response code 4.05 (Method Not Allowed) when the resource
URI specified in the request covered by the authorization
information but not the requested action.

The client cannot always know a priori if an Authorized Resource
Request will succeed. It MUST check the validity of ascertain that its keying material is still valid
before sending a request or processing a response. If the client repeatedly
gets an error responses response containing AS Request Creation Hints (cf.
Section 5.1.2 of [I-D.ietf-ace-oauth-authz] as response to its
requests, it SHOULD request a new access token from the authorization
server in order to continue communication with the resource server.

Unauthorized requests that have been received over a DTLS session
SHOULD be treated as non-fatal by the RS, resource server, i.e., the DTLS
session SHOULD be kept alive until the associated access token has
expired.

4. Dynamic Update of Authorization Information

Resource servers must only use a new access token to update the
authorization information for a DTLS session if the keying material
that is bound to the token is the same that was used in the DTLS
handshake. By associating the access tokens with the identifier of
an existing DTLS session, the authorization information can be
updated without changing the cryptographic keys for the DTLS
communication between the client and the resource server, i.e. an
existing session can be used with updated permissions.

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

Figure 9 10 depicts the message flow where the C client requests a new
access token after a security association between the client and the
resource server has been established using this protocol. If the
client wants to update the authorization information, the token
request MUST specify the key identifier of the proof-of-possession
key used for the existing DTLS channel between the client and the
resource server in the "kid" parameter of the Client-to-AS request.
The authorization server MUST verify that the specified "kid" denotes
a valid verifier for a proof-of-possession token that has previously
been issued to the requesting client. Otherwise, the Client-to-AS
request MUST be declined with the error code "unsupported_pop_key" as
defined in Section 5.6.3 of [I-D.ietf-ace-oauth-authz].

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

Note: By associating the access tokens with the identifier of an
existing DTLS session, the authorization information can be
updated without changing the cryptographic keys for the DTLS
communication between the client and the resource server, i.e. an
existing session can be used with updated permissions.

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

Figure 9: 10: Overview of Dynamic Update Operation

5. Token Expiration

The resource server MUST delete access tokens that are no longer
valid. DTLS sessions associations that have been established setup in accordance with
this profile are always tied to a specific access token. tokens (which may be
exchanged with a dynamic update as described in Section 4). As this token
tokens may become invalid at any time (e.g. (e.g., because it has they have
expired), the
session association may become useless at some point. A
resource server therefore MUST terminate existing DTLS sessions association
after the last access token for associated with this session association has been deleted.
expired.

As specified in Section 5.8.3 of [I-D.ietf-ace-oauth-authz], the
resource server MUST notify the client with an error response with
code 4.01 (Unauthorized) for any long running request before
terminating the session. association.

6. Secure Communication with AS an Authorization Server

As specified in the ACE framework (sections 5.6 and 5.7 of
[I-D.ietf-ace-oauth-authz]), the requesting entity (RS (the resource
server and/or the client) and the AS authorization server communicate
via the token endpoint or introspection endpoint. The use of CoAP
and DTLS for this communication is RECOMMENDED in this profile, other
protocols (such as HTTP and TLS TLS, or CoAP and OSCORE) OSCORE [RFC8613]) MAY be
used instead.

How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the
AS
authorization server are established is out of scope for this
profile.

If other means of securing the communication with the AS authorization
server are used, the security protocol MUST fulfill the communication security requirements in from
Section 6.2 of [I-D.ietf-ace-oauth-authz]. [I-D.ietf-ace-oauth-authz] remain applicable.

7. Security Considerations

This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework
[I-D.ietf-ace-oauth-authz]. As it follows this framework's general
approach, the general security considerations from section 6 of
[I-D.ietf-ace-oauth-authz] also apply to this profile.

When using

The authorization server must ascertain that the keying material for
the client that it provides to the resource server actually is
associated with this client. Malicious clients may hand over access
tokens containing their own access permissions to other entities.
This problem cannot be completely eliminated. Nevertheless, in RPK
mode it should not be possible for clients to request access tokens
for arbitrary public keys, since that would allow the client to relay
tokens without the need to share its own credentials with others.
The authorization server therefore at some point needs to validate
that the client can actually use the private key corresponding to the
client's public key.

When using pre-shared keys provisioned by the AS, authorization server,
the security level depends on the randomness of PSK, and the security
of the TLS cipher suite and key exchange algorithm. As this
specification targets at constrained environments, message payloads
exchanged between the client and the resource server are expected to
be small and rare. CoAP [RFC7252] mandates the implementation of
cipher suites with abbreviated, 8-byte tags for message integrity
protection. For consistency, this profile requires implementation of
the same cipher suites. For application scenarios where the cost of
full-width authentication tags is low compared to the overall amount
of data being transmitted, the use of cipher suites with 16-byte
integrity protection tags is preferred.

The PSK mode of this profile offers a distribution mechanism to
convey authorization tokens together with a shared secret to a client
and a server. As this specification aims at constrained devices and
uses CoAP [RFC7252] as transfer protocol, at least the ciphersuite
TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported. The access
tokens and the corresponding shared secrets generated by the
authorization server are expected to be sufficiently short-lived to
provide similar forward-secrecy properties to using ephemeral Diffie-
Hellman (DHE) key exchange mechanisms. For longer-lived access
tokens, DHE ciphersuites should be used.

Constrained devices that use DTLS [RFC6347] are inherently vulnerable
to Denial of Service (DoS) attacks as the handshake protocol requires
creation of internal state within the device. This is specifically
of concern where an adversary is able to intercept the initial cookie
exchange and interject forged messages with a valid cookie to
continue with the handshake. A similar issue exists with the
unprotected authorization information endpoint where the resource
server needs to keep valid access tokens until their expiry.
Adversaries can fill up the constrained resource server's internal
storage for a very long time with interjected or otherwise retrieved
valid access tokens. The use protection of multiple access tokens for a single client increases that are stored
in the
strain authorization information endpoint depends on the keying
material that is used between the authorization server and the
resource server: The resource server as it must consider every ensure that it processes
only access token
and tokens that are encrypted and integrity-protected by an
authorization server that is authorized to provide access tokens for
the resource server.

7.1. Reuse of Existing Sessions

To avoid the overhead of a repeated DTLS handshake, [RFC7925]
recommends session resumption [RFC5077] to reuse session state from
an earlier DTLS association and thus requires client side
implementation. In this specification, the DTLS session is subject
to the authorization rules denoted by the access token that was used
for the initial setup of the DTLS association. Enabling session
resumption would require the server to transfer the authorization
information with the session state in an encrypted SessionTicket to
the client. Assuming that the server uses long-lived keying
material, this could open up attacks due to the lack of forward
secrecy. Moreover, using this mechanism, a client can resume a DTLS
session without proving the possession of the PoP key again.
Therefore, the use of session resumption is NOT RECOMMENDED for
resource servers.

Since renogiation of DTLS associations is prone to attacks as well,
[RFC7925] requires clients to decline any renogiation attempt. A
server that wants to initiate re-keying therefore SHOULD periodically
force a full handshake.

7.2. Multiple Access Tokens

The use of multiple access tokens for a single client increases the
strain on the resource server as it must consider every access token
and calculate the actual permissions of the client. Also, tokens may
contradict each other which may lead the server to enforce wrong
permissions. If one of the access tokens expires earlier than
others, the resulting permissions may offer insufficient protection.
Developers SHOULD avoid using multiple access tokens for a client.

Even when a single access token per client is used, an attacker could
compromise the dynamic update mechanism for existing DTLS connections
by delaying or reordering packets destined for the authz-info
endpoint. Thus, the order in which operations occur at the resource
server (and thus which authorization info is used to process a given
client request) cannot be guaranteed. Especially in the presence of
later-issued access tokens that reduce the client's permissions from
the initial access token, it is impossible to guarantee that the
reduction in authorization will take effect prior to the expiration
of the original token.

7.3. Out-of-Band Configuration

To communicate securely, the authorization server, the client and the
resource server require certain information that must be exchanged
outside the protocol flow described in this document. The
authorization server must have obtained authorization information
concerning the client and the resource server that is approved by the
resource owner as well as corresponding keying material. The
resource server must have received authorization information approved
by the resource owner concerning its authorization managers and the
respective keying material. The client must have obtained
authorization information concerning the authorization server
approved by its owner as well as the corresponding keying material.
Also, the client's owner must have approved of the client's
communication with the resource server. The client and the
authorization server must have obtained a common understanding how
this resource server is identified to ensure that the client obtains
access token and keying material for the correct resource server. If
the client is provided with a raw public key for the resource server,
it must be ascertained to which resource server (which identifier and
authorization information) the key is associated. All authorization
information and keying material must be kept up to date.

8. Privacy Considerations

This privacy considerations from section 7 of the
[I-D.ietf-ace-oauth-authz] apply also to this profile.

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

Also, unprotected requests to the resource server may reveal
information about the client, e.g., which resources the client
attempts to request or the data that the client wants to provide to
the resource server. The client SHOULD NOT send confidential data in
an unprotected request.

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

9. IANA Considerations

The following registrations are done for the ACE OAuth Profile
Registry following the procedure specified in
[I-D.ietf-ace-oauth-authz].

Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
with the RFC number of this specification and delete this paragraph.

Profile name: coap_dtls

Profile Description: Profile for delegating client authentication and
authorization in a constrained environment by establishing a Datagram
Transport Layer Security (DTLS) channel between resource-constrained
nodes.

Profile ID: 1 TBD (suggested: 1)

Change Controller: IESG

Reference: [RFC-XXXX]

10. Acknowledgments

Thanks to Jim Schaad for his contributions and reviews of this
document. Special thanks to Ben Kaduk for his thorough review of
this document.

Ludwig Seitz worked on this document as part of the CelticNext
projects CyberWI, and CRITISEC with funding from Vinnova.

11. References

10.1.

11.1. Normative References

[I-D.ietf-ace-cwt-proof-of-possession]
Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
possession-11 (work in progress), October 2019.

[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-29 draft-ietf-ace-oauth-authz-33
(work in progress), December 2019. February 2020.

[I-D.ietf-ace-oauth-params]
Seitz, L., "Additional OAuth Parameters for Authorization
in Constrained Environments (ACE)", draft-ietf-ace-oauth-
params-07
params-13 (work in progress), December 2019. April 2020.

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

[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<https://www.rfc-editor.org/info/rfc4279>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.

[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.

[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.

[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.

[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2.

[RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier", RFC 8422,
DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>.

[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
2020, <https://www.rfc-editor.org/info/rfc8747>.

11.2. Informative References

[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.

[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.

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

[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>.

[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.

[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.

[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<https://www.rfc-editor.org/info/rfc8032>.

[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.

[RFC8422] Nir, Y., Josefsson, S.,

[RFC8610] Birkholz, H., Vigano, C., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and Earlier",
JSON Data Structures", RFC 8422, 8610, DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>. 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.

[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.

Authors' Addresses

Stefanie Gerdes
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany

Phone: +49-421-218-63906
Email: gerdes@tzi.org
Olaf Bergmann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany

Phone: +49-421-218-63904
Email: bergmann@tzi.org

Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany

Phone: +49-421-218-63921
Email: cabo@tzi.org

Goeran Selander
Ericsson AB

Email: goran.selander@ericsson.com

Ludwig Seitz
Combitech
Djaeknegatan 31
Malmoe 211 35
Sweden

Email: ludwig.seitz@combitech.se