draft-ietf-ace-dtls-authorize-06.txt   draft-ietf-ace-dtls-authorize-07.txt 
ACE Working Group S. Gerdes ACE Working Group S. Gerdes
Internet-Draft O. Bergmann Internet-Draft O. Bergmann
Intended status: Standards Track C. Bormann Intended status: Standards Track C. Bormann
Expires: September 1, 2019 Universitaet Bremen TZI Expires: September 12, 2019 Universitaet Bremen TZI
G. Selander G. Selander
Ericsson AB Ericsson AB
L. Seitz L. Seitz
RISE SICS RISE SICS
February 28, 2019 March 11, 2019
Datagram Transport Layer Security (DTLS) Profile for Authentication and Datagram Transport Layer Security (DTLS) Profile for Authentication and
Authorization for Constrained Environments (ACE) Authorization for Constrained Environments (ACE)
draft-ietf-ace-dtls-authorize-06 draft-ietf-ace-dtls-authorize-07
Abstract Abstract
This specification defines a profile of the ACE framework that allows This specification defines a profile of the ACE framework that allows
constrained servers to delegate client authentication and constrained servers to delegate client authentication and
authorization. The protocol relies on DTLS for communication authorization. The protocol relies on DTLS for communication
security between entities in a constrained network using either raw security between entities in a constrained network using either raw
public keys or pre-shared keys. A resource-constrained server can public keys or pre-shared keys. A resource-constrained server can
use this protocol to delegate management of authorization information use this protocol to delegate management of authorization information
to a trusted host with less severe limitations regarding processing to a trusted host with less severe limitations regarding processing
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 1, 2019. This Internet-Draft will expire on September 12, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Communication between C and AS . . . . . . . . . . . . . 5 3.1. Communication between C and AS . . . . . . . . . . . . . 5
3.2. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . 6 3.2. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . 6
3.2.1. DTLS Channel Setup Between C and RS . . . . . . . . . 7 3.2.1. DTLS Channel Setup Between C and RS . . . . . . . . . 7
3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 8 3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 8
3.3.1. DTLS Channel Setup Between C and RS . . . . . . . . . 11 3.3.1. DTLS Channel Setup Between C and RS . . . . . . . . . 12
3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 12 3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 13
4. Dynamic Update of Authorization Information . . . . . . . . . 13 4. Dynamic Update of Authorization Information . . . . . . . . . 14
5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 14 5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15 6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . 16 9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 17 9.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
This specification defines a profile of the ACE framework This specification defines a profile of the ACE framework
[I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource [I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource
server use CoAP [RFC7252] over DTLS [RFC6347] to communicate. The server use CoAP [RFC7252] over DTLS [RFC6347] to communicate. The
client obtains an access token, bound to a key (the proof-of- client obtains an access token, bound to a key (the proof-of-
possession key), from an authorization server to prove its possession key), from an authorization server to prove its
authorization to access protected resources hosted by the resource authorization to access protected resources hosted by the resource
server. Also, the client and the resource server are provided by the server. Also, the client and the resource server are provided by the
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
Readers are expected to be familiar with the terms and concepts Readers are expected to be familiar with the terms and concepts
described in [I-D.ietf-ace-oauth-authz] and in described in [I-D.ietf-ace-oauth-authz] and in
[I-D.ietf-ace-oauth-params]. [I-D.ietf-ace-oauth-params].
The authz-info resource refers to the authz-info endpoint as The authorization information (authz-info) resource refers to the
specified in [I-D.ietf-ace-oauth-authz]. authorization information endpoint as specified in
[I-D.ietf-ace-oauth-authz].
2. Protocol Overview 2. Protocol Overview
The CoAP-DTLS profile for ACE specifies the transfer of The CoAP-DTLS profile for ACE specifies the transfer of
authentication information and, if necessary, authorization authentication information and, if necessary, authorization
information between the client (C) and the resource server (RS) information between the client (C) and the resource server (RS)
during setup of a DTLS session for CoAP messaging. It also specifies during setup of a DTLS session for CoAP messaging. It also specifies
how C can use CoAP over DTLS to retrieve an access token from the how C can use CoAP over DTLS to retrieve an access token from the
authorization server (AS) for a protected resource hosted on the authorization server (AS) for a protected resource hosted on the
resource server. resource server.
This profile requires the client to retrieve an access token for This profile requires the client to retrieve an access token for
protected resource(s) it wants to access on RS as specified in protected resource(s) it wants to access on RS as specified in
[I-D.ietf-ace-oauth-authz]. Figure 1 shows the typical message flow [I-D.ietf-ace-oauth-authz]. Figure 1 shows the typical message flow
in this scenario (messages in square brackets are optional): in this scenario (messages in square brackets are optional):
C RS AS C RS AS
| [-- Resource Request --->] | | | [---- Resource Request ------>]| |
| | | | | |
| [<----- AS Information --] | | | [<-AS Request Creation Hints-] | |
| | | | | |
| --- Token Request ----------------------------> | | ------- Token Request ----------------------------> |
| | | | | |
| <---------------------------- Access Token ----- | | <---------------------------- Access Token --------- |
| + Access Information | | + Access Information |
Figure 1: Retrieving an Access Token Figure 1: Retrieving an Access Token
To determine the AS in charge of a resource hosted at the RS, C MAY To determine the AS in charge of a resource hosted at the RS, C MAY
send an initial Unauthorized Resource Request message to the RS. The send an initial Unauthorized Resource Request message to the RS. The
RS then denies the request and sends an AS information message RS then denies the request and sends an AS Request Creation Hints
containing the address of its AS back to the client as specified in message containing the address of its AS back to the client as
Section 5.1.2 of [I-D.ietf-ace-oauth-authz]. 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 Once the client knows the authorization server's address, it can send
an access token request to the token endpoint at the AS as specified an access token request to the token endpoint at the AS as specified
in [I-D.ietf-ace-oauth-authz]. As the access token request as well in [I-D.ietf-ace-oauth-authz]. As the access token request as well
as the response may contain confidential data, the communication as the response may contain confidential data, the communication
between the client and the authorization server MUST be between the client and the authorization server MUST be
confidentiality-protected and ensure authenticity. C may have been confidentiality-protected and ensure authenticity. C may have been
registered at the AS via the OAuth 2.0 client registration mechanism registered at the AS via the OAuth 2.0 client registration mechanism
as outlined in Section 5.3 of [I-D.ietf-ace-oauth-authz]. as outlined in Section 5.3 of [I-D.ietf-ace-oauth-authz].
The access token returned by the authorization server can then be The access token returned by the authorization server can then be
used by the client to establish a new DTLS session with the resource used by the client to establish a new DTLS session with the resource
server. When the client intends to use asymmetric cryptography in server. When the client intends to use an asymmetric proof-of-
the DTLS handshake with the resource server, the client MUST upload possession key in the DTLS handshake with the resource server, the
the access token to the authz-info resource, i.e. the authz-info client MUST upload the access token to the authz-info resource, i.e.
endpoint, on the resource server before starting the DTLS handshake, the authz-info endpoint, on the resource server before starting the
as described in Section 5.8.1 of [I-D.ietf-ace-oauth-authz]. If only DTLS handshake, as described in Section 5.8.1 of
symmetric cryptography is used between the client and the resource [I-D.ietf-ace-oauth-authz]. In case the client uses a symmetric
server, the access token MAY instead be transferred in the DTLS proof-of-possession key in the DTLS handshake, the procedure as above
ClientKeyExchange message (see Section 3.3.1). MAY be used, or alternatively, the access token MAY instead be
transferred in the DTLS ClientKeyExchange message (see
Section 3.3.1).
Figure 2 depicts the common protocol flow for the DTLS profile after Figure 2 depicts the common protocol flow for the DTLS profile after
the client C has retrieved the access token from the authorization the client C has retrieved the access token from the authorization
server AS. server AS.
C RS AS C RS AS
| [--- Access Token ------>] | | | [--- Access Token ------>] | |
| | | | | |
| <== DTLS channel setup ==> | | | <== DTLS channel setup ==> | |
| | | | | |
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secured. Depending on the used CoAP security mode (see also secured. Depending on the used CoAP security mode (see also
Section 9 of [RFC7252], the Client-to-AS request, AS-to-Client Section 9 of [RFC7252], the Client-to-AS request, AS-to-Client
response and DTLS session establishment carry slightly different response and DTLS session establishment carry slightly different
information. Section 3.2 addresses the use of raw public keys while 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. Section 3.3 defines how pre-shared keys are used in this profile.
3.1. Communication between C and AS 3.1. Communication between C and AS
To retrieve an access token for the resource that the client wants to To retrieve an access token for the resource that the client wants to
access, the client requests an access token from the authorization access, the client requests an access token from the authorization
server. Before C can request the access token, C and AS must server. Before C can request the access token, C and AS MUST
establish a secure communication channel. C must securely have establish a secure communication channel. C MUST securely have
obtained keying material to communicate with AS, and C must securely obtained keying material to communicate with AS. Furthermore, C MUST
have received authorization information intended for C that states verify that AS is authorized to provide access tokens (including
that AS is authorized to provide keying material concerning RS to C. authorization information) about RS to C. Also, AS MUST securely
Also, AS must securely have obtained keying material for C, and have obtained keying material for C, and obtained authorization rules
obtained authorization rules approved by the resource owner (RO) approved by the resource owner (RO) concerning C and RS that relate
concerning C and RS that relate to this keying material. C and AS to this keying material. C and AS MUST use their respective keying
must use their respective keying material for all exchanged messages. material for all exchanged messages. How the security association
How the security association between C and AS is established is not between C and AS is bootstrapped is not part of this document. C and
part of this document. C and AS MUST ensure the confidentiality, AS MUST ensure the confidentiality, integrity and authenticity of all
integrity and authenticity of all exchanged messages. exchanged messages.
If C is constrained, C and AS should use DTLS to communicate with If C is constrained, C and AS should use DTLS to communicate with
each other. But C and AS may also use other means to secure their each other. But C and AS may also use other means to secure their
communication, e.g., TLS. The used security protocol must provide communication, e.g., TLS. The used security protocol MUST fulfill
confidentiality, integrity and authenticity, and enable the client to the communication security requirements in Section 6.2 of
determine if it is the intended recipient of a message, e.g., by [I-D.ietf-ace-oauth-authz].
using an AEAD mechanism. C must also be able to determine if a
response from AS belongs to a certain request. Additionally, the
protocol must offer replay protection.
3.2. RawPublicKey Mode 3.2. RawPublicKey Mode
After C and AS mutually authenticated each other and validated each After C and AS mutually authenticated each other and validated each
other's authorization, C sends a token request to AS's token other's authorization, C sends a token request to AS's token
endpoint. The client MUST add a "req_cnf" object carrying either its 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 raw public key or a unique identifier for a public key that it has
previously made known to the authorization server. To prove that the previously made known to the authorization server. To prove that the
client is in possession of this key, C MUST use the same keying client is in possession of this key, C MUST use the same keying
material that it uses to secure the communication with AS, e.g., the material that it uses to secure the communication with AS, e.g., the
DTLS session. DTLS session.
An example access token request from the client to the AS is depicted An example access token request from the client to the AS is depicted
in Figure 3. in Figure 3.
POST coaps://as.example.com/token POST coaps://as.example.com/token
Content-Format: application/ace+cbor Content-Format: application/ace+cbor
Payload:
{ {
grant_type: client_credentials, "grant_type" : "client_credentials",
req_aud: "tempSensor4711", "req_aud" : "tempSensor4711",
req_cnf: { "req_cnf" : {
COSE_Key: { "COSE_Key" : {
kty: EC2, "kty" : "EC2",
crv: P-256, "crv" : "P-256",
x: h'e866c35f4c3c81bb96a1...', "x" : h'e866c35f4c3c81bb96a1...',
y: h'2e25556be097c8778a20...' "y" : h'2e25556be097c8778a20...'
} }
} }
} }
Figure 3: Access Token Request Example for RPK Mode Figure 3: Access Token Request Example for RPK Mode
The example shows an access token request for the resource identified The example shows an access token request for the resource identified
by the string "tempSensor4711" on the authorization server using a by the string "tempSensor4711" on the authorization server using a
raw public key. raw public key.
AS MUST check if the client that it communicates with is associated AS MUST check if the client that it communicates with is associated
with the RPK in the cnf object before issuing an access token to it. with the RPK in the cnf object before issuing an access token to it.
If AS determines that the request is to be authorized according to If AS determines that the request is to be authorized according to
the respective authorization rules, it generates an access token the respective authorization rules, it generates an access token
response for C. The response SHOULD contain a "profile" parameter response for C. The access token MUST be bound to the RPK of the
with the value "coap_dtls" to indicate that this profile must be used client by means of the cnf claim. The response MAY contain a
for communication between the client C and the resource server. The "profile" parameter with the value "coap_dtls" to indicate that this
response also contains an access token and an "rs_cnf" parameter profile MUST be used for communication between the client C and the
containing information about the public key that is used by the resource server. The "profile" may be specified out-of-band, in
resource server. AS MUST ascertain that the RPK specified in which case it does not have to be sent. The response also contains
"rs_cnf" belongs to the resource server that C wants to communicate an access token and an "rs_cnf" parameter containing information
with. AS MUST protect the integrity of the token. If the access about the public key that is used by the resource server. AS MUST
token contains confidential data, AS MUST also protect the ascertain that the RPK specified in "rs_cnf" belongs to the resource
confidentiality of the access token. server that C wants to communicate with. AS MUST protect the
integrity of the token. If the access token contains confidential
data, AS MUST also protect the confidentiality of the access token.
C MUST ascertain that the access token response belongs to a certain C MUST ascertain that the access token response belongs to a certain
previously sent access token request, as the request may specify the previously sent access token request, as the request may specify the
resource server with which C wants to communicate. resource server with which C wants to communicate.
An example access token response from the AS to the client is
depicted in Figure 4.
2.01 Created
Content-Format: application/ace+cbor
Max-Age: 3600
Payload:
{
"access_token" : "b64'SlAV32hkKG ...
(remainder of CWT omitted for brevity;
CWT contains clients RPK in the "cnf" claim)',
"expires_in" : "3600",
"rs_cnf" : {
"COSE_Key" : {
"kty" : "EC2",
"crv" : "P-256",
"x" : h'd7cc072de2205bdc1537...',
"y" : h'f95e1d4b851a2cc80fff...'
}
}
}
Figure 4: Access Token Response Example for RPK Mode
3.2.1. DTLS Channel Setup Between C and RS 3.2.1. DTLS Channel Setup Between C and RS
Before the client initiates the DTLS handshake with the resource Before the client initiates the DTLS handshake with the resource
server, C MUST send a "POST" request containing the new access token server, C MUST send a "POST" request containing the new access token
to the authz-info resource hosted by the resource server. If this to the authz-info resource hosted by the resource server. After the
operation yields a positive response, the client SHOULD proceed to client
establish a new DTLS channel with the resource server. To use the receives a confirmation that the RS has accepted the access token, it
RawPublicKey mode, the client MUST specify the public key that AS SHOULD proceed to establish a new DTLS channel with the resource
defined in the "cnf" field of the access token response in the server. To use the RawPublicKey mode, the client MUST specify the
SubjectPublicKeyInfo structure in the DTLS handshake as specified in public key that AS defined in the "cnf" field of the access token
[RFC7250]. response in the SubjectPublicKeyInfo structure in the DTLS handshake
as specified in [RFC7250].
An implementation that supports the RPK mode of this profile MUST at An implementation that supports the RPK mode of this profile MUST at
least support the ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 least support the ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
[RFC7251] with the ed25519 curve (cf. [RFC8032], [RFC8422]). [RFC7251] with the ed25519 curve (cf. [RFC8032], [RFC8422]).
Note: According to [RFC7252], CoAP implementations MUST support the Note: According to [RFC7252], CoAP implementations MUST support the
ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251] and the ciphersuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251] and the
NIST P-256 curve. As discussed in [RFC7748], new ECC curves have NIST P-256 curve. As discussed in [RFC7748], new ECC curves have
been defined recently that are considered superior to the so- been defined recently that are considered superior to the so-
called NIST curves. The curve that is mandatory to implement in called NIST curves. The curve that is mandatory to implement in
skipping to change at page 8, line 36 skipping to change at page 9, line 12
that the access token is generated for the resource server that C that the access token is generated for the resource server that C
wants to communicate with. Also, AS MUST protect the integrity of wants to communicate with. Also, AS MUST protect the integrity of
the access token. If the token contains confidential data such as the access token. If the token contains confidential data such as
the symmetric key, the confidentiality of the token MUST also be the symmetric key, the confidentiality of the token MUST also be
protected. Depending on the requested token type and algorithm in protected. Depending on the requested token type and algorithm in
the access token request, the authorization server adds access the access token request, the authorization server adds access
Information to the response that provides the client with sufficient Information to the response that provides the client with sufficient
information to setup a DTLS channel with the resource server. AS information to setup a DTLS channel with the resource server. AS
adds a "cnf" parameter to the access information carrying a adds a "cnf" parameter to the access information carrying a
"COSE_Key" object that informs the client about the symmetric key "COSE_Key" object that informs the client about the symmetric key
that is to be used between C and the resource server. that is to be used between C and the resource server. The access
token MUST be bound to the same symmetric key by means of the cnf
claim.
An example access token response is illustrated in Figure 4. In this An example access token request for an access token with a symmetric
proof-of-possession key is illustrated in Figure 5.
POST coaps://as.example.com/token
Content-Format: application/ace+cbor
Payload:
{
"audience" : "smokeSensor1807",
}
Figure 5: Example Access Token Request, symmetric PoP-key
An example access token response is illustrated in Figure 6. In this
example, the authorization server returns a 2.01 response containing example, the authorization server returns a 2.01 response containing
a new access token and information for the client, including the a new access token and information for the client, including the
symmetric key in the cnf claim. The information is transferred as a symmetric key in the cnf claim. The information is transferred as a
CBOR data structure as specified in [I-D.ietf-ace-oauth-authz]. CBOR data structure as specified in [I-D.ietf-ace-oauth-authz].
2.01 Created 2.01 Created
Content-Format: application/ace+cbor Content-Format: application/ace+cbor
Max-Age: 86400 Max-Age: 86400
Payload:
{ {
access_token: h'd08343a10... "access_token" : h'd08343a10...
(remainder of CWT omitted for brevity) (remainder of CWT omitted for brevity)
token_type: pop, "token_type" : "pop",
expires_in: 86400, "expires_in" : 86400,
profile: coap_dtls, "profile" : "coap_dtls",
cnf: { "cnf" : {
COSE_Key: { "COSE_Key" : {
kty: symmetric, "kty" : "symmetric",
alg: TLS_PSK_WITH_AES_128_CCM_8 "kid" : h'3d027833fc6267ce',
kid: h'3d027833fc6267ce', "k" : h'73657373696f6e6b6579'
k: h'73657373696f6e6b6579'
} }
} }
} }
Figure 4: Example Access Token Response Figure 6: Example Access Token Response, symmetric PoP-key
The access token also comprises a "cnf" claim. This claim usually The access token also comprises a "cnf" claim. This claim usually
contains a "COSE_Key" object that carries either the symmetric key 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 itself or a key identifier that can be used by the resource server to
determine the secret key shared with the client. If the access token determine the secret key shared with the client. If the access token
carries a symmetric key, the access token MUST be encrypted using a carries a symmetric key, the access token MUST be encrypted using a
"COSE_Encrypt0" structure. The AS MUST use the keying material "COSE_Encrypt0" structure. The AS MUST use the keying material
shared with the RS to encrypt the token. shared with the RS to encrypt the token.
The "cnf" structure in the access token is provided in Figure 7.
"cnf" : {
"COSE_Key" : {
"kty" : "symmetric",
"kid" : h'eIiOFCa9lObw'
}
}
Figure 7: Access Token without Keying Material
A response that declines any operation on the requested resource is A response that declines any operation on the requested resource is
constructed according to Section 5.2 of [RFC6749], (cf. constructed according to Section 5.2 of [RFC6749], (cf.
Section 5.6.3. of [I-D.ietf-ace-oauth-authz]). Section 5.6.3. of [I-D.ietf-ace-oauth-authz]).
4.00 Bad Request 4.00 Bad Request
Content-Format: application/ace+cbor Content-Format: application/ace+cbor
Payload:
{ {
error: invalid_request "error" : "invalid_request"
} }
Figure 5: Example Access Token Response With Reject Figure 8: Example Access Token Response With Reject
The method for how the resource server determines the symmetric key The method for how the resource server determines the symmetric key
from an access token containing only a key identifier is application from an access token containing only a key identifier is application
specific, the remainder of this section provides one example. specific, the remainder of this section provides one example.
The AS and the resource server are assumed to share a key derivation The AS and the resource server are assumed to share a key derivation
key used to derive the symmetric key shared with the client from the key used to derive the symmetric key shared with the client from the
key identifier in the access token. The key derivation key may be key identifier in the access token. The key derivation key may be
derived from some other secret key shared between the AS and the derived
resource server. Knowledge of the symmetric key shared with the from some other secret key shared between the AS and the resource
client must not reveal any information about the key derivation key server. This key needs to be securely stored and processed in the
or other secret keys shared between AS and resource server. same way as the key used to protect the communication between AS and
RS.
Knowledge of the symmetric key shared with the client must not reveal
any information about the key derivation key or other secret keys
shared between AS and resource server.
In order to generate a new symmetric key to be used by client and In order to generate a new symmetric key to be used by client and
resource server, the AS generates a key identifier and uses the key resource server, the AS generates a key identifier and uses the key
derivation key shared with the resource server to derive the derivation key shared with the resource server to derive the
symmetric key as specified below. Instead of providing the keying symmetric key as specified below. Instead of providing the keying
material in the access token, the AS includes the key identifier in material in the access token, the AS includes the key identifier in
the "kid" parameter, see Figure 6. This key identifier enables the the "kid" parameter, see Figure 7. This key identifier enables the
resource server to calculate the keying material for the resource server to calculate the keying material for the
communication with the client from the access token using the key communication with the client from the access token using the key
derivation key and following Section 11 of [RFC8152] with parameters derivation key and following Section 11 of [RFC8152] with parameters
as specified here. The KDF to be used needs to be defined by the as specified here. The KDF to be used needs to be defined by the
application, for example HKDF-SHA-256. The key identifier picked by application, for example HKDF-SHA-256. The key identifier picked by
the AS needs to be unique for each access token where a unique the AS needs to be unique for each access token where a unique
symmetric key is required. symmetric key is required.
The fields in the context information "COSE_KDF_Context" The fields in the context information "COSE_KDF_Context"
(Section 11.2 of [RFC8152]) have the following values: (Section 11.2 of [RFC8152]) have the following values:
o AlgorithmID = "ACE-CoAP-DTLS-key-derivation" o AlgorithmID = "ACE-CoAP-DTLS-key-derivation"
o PartyUInfo = PartyVInfo = ( null, null, null ) o PartyUInfo = PartyVInfo = ( null, null, null )
o keyDataLength is a uint equal the length of the symmetric key o keyDataLength needs to be defined by the application
shared between C and RS in bits
o protected MUST be a zero length bstr o protected MUST be a zero length bstr
o other is a zero length bstr o other is a zero length bstr
o SuppPrivInfo is omitted o SuppPrivInfo is omitted
The "cnf" structure in the access token is provided in Figure 6.
cnf : {
COSE_Key : {
kty : symmetric,
alg : TLS_PSK_WITH_AES_128_CCM_8,
kid : h'eIiOFCa9lObw'
}
}
Figure 6: Access Token without Keying Material
3.3.1. DTLS Channel Setup Between C and RS 3.3.1. DTLS Channel Setup Between C and RS
When a client receives an access token response from an authorization When a client receives an access token response from an authorization
server, C MUST ascertain that the access token response belongs to a server, C MUST ascertain that the access token response belongs to a
certain previously sent access token request, as the request may certain previously sent access token request, as the request may
specify the resource server with which C wants to communicate. specify the resource server with which C wants to communicate.
C checks if the payload of the access token response contains an C checks if the payload of the access token response contains an
"access_token" parameter and a "cnf" parameter. With this "access_token" parameter and a "cnf" parameter. With this
information the client can initiate the establishment of a new DTLS information the client can initiate the establishment of a new DTLS
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1. with response code 4.03 (Forbidden) when the resource URI 1. with response code 4.03 (Forbidden) when the resource URI
specified in the request is not covered by the authorization specified in the request is not covered by the authorization
information, and information, and
2. with response code 4.05 (Method Not Allowed) when the resource 2. with response code 4.05 (Method Not Allowed) when the resource
URI specified in the request covered by the authorization URI specified in the request covered by the authorization
information but not the requested action. information but not the requested action.
The client cannot always know a priori if an Authorized Resource The client cannot always know a priori if an Authorized Resource
Request will succeed. It must check the validity of its keying Request will succeed. It MUST check the validity of its keying
material before sending a request or processing a response. If the material before sending a request or processing a response. If the
client repeatedly gets error responses containing AS Creation Hints client repeatedly gets error responses containing AS Creation Hints
(cf. Section 5.1.2 of [I-D.ietf-ace-oauth-authz] as response to its (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 requests, it SHOULD request a new access token from the authorization
server in order to continue communication with the resource server. server in order to continue communication with the resource server.
Unauthorized requests that have been received over a DTLS session Unauthorized requests that have been received over a DTLS session
SHOULD be treated as non-fatal by the RS, i.e., the DTLS session SHOULD be treated as non-fatal by the RS, i.e., the DTLS session
SHOULD be kept alive until the associated access token has expired. SHOULD be kept alive until the associated access token has expired.
4. Dynamic Update of Authorization Information 4. Dynamic Update of Authorization Information
The client can update the authorization information stored at the The client can update the authorization information stored at the
resource server at any time without changing an established DTLS resource server at any time without changing an established DTLS
session. To do so, the Client requests a new access token from the session. To do so, the Client requests a new access token from the
authorization server for the intended action on the respective authorization server for the intended action on the respective
resource and uploads this access token to the authz-info resource on resource and uploads this access token to the authz-info resource on
the resource server. the resource server.
Figure 7 depicts the message flow where the C requests a new access Figure 9 depicts the message flow where the C requests a new access
token after a security association between the client and the token after a security association between the client and the
resource server has been established using this protocol. If the resource server has been established using this protocol. If the
client wants to update the authorization information, the token client wants to update the authorization information, the token
request MUST specify the key identifier of the existing DTLS channel request MUST specify the key identifier of the proof-of-possession
between the client and the resource server in the "kid" parameter of key used for the existing DTLS channel between the client and the
the Client-to-AS request. The authorization server MUST verify that resource server in the "kid" parameter of the Client-to-AS request.
the specified "kid" denotes a valid verifier for a proof-of- The authorization server MUST verify that the specified "kid" denotes
possession token that has previously been issued to the requesting a valid verifier for a proof-of-possession token that has previously
client. Otherwise, the Client-to-AS request MUST be declined with been issued to the requesting client. Otherwise, the Client-to-AS
the error code "unsupported_pop_key" as defined in Section 5.6.3 of request MUST be declined with the error code "unsupported_pop_key" as
[I-D.ietf-ace-oauth-authz]. defined in Section 5.6.3 of [I-D.ietf-ace-oauth-authz].
When the authorization server issues a new access token to update When the authorization server issues a new access token to update
existing authorization information, it MUST include the specified existing authorization information, it MUST include the specified
"kid" parameter in this access token. A resource server MUST replace "kid" parameter in this access token. A resource server MUST replace
the authorization information of any existing DTLS session that is the authorization information of any existing DTLS session that is
identified by this key identifier with the updated authorization identified by this key identifier with the updated authorization
information. information.
Note: By associating the access tokens with the identifier of an Note: By associating the access tokens with the identifier of an
existing DTLS session, the authorization information can be existing DTLS session, the authorization information can be
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| <---------------------------- New Access Token - | | <---------------------------- New Access Token - |
| + Access Information | | + Access Information |
| | | | | |
| --- Update /authz-info --> | | | --- Update /authz-info --> | |
| New Access Token | | | New Access Token | |
| | | | | |
| == Authorized Request ===> | | | == Authorized Request ===> | |
| | | | | |
| <=== Protected Resource == | | | <=== Protected Resource == | |
Figure 7: Overview of Dynamic Update Operation Figure 9: Overview of Dynamic Update Operation
5. Token Expiration 5. Token Expiration
DTLS sessions that have been established in accordance with this DTLS sessions that have been established in accordance with this
profile are always tied to a specific set of access tokens. As these profile are always tied to a specific access token. As this token
tokens may become invalid at any time (either because the token has may become invalid at any time (e.g. because it has expired), the
expired or the responsible authorization server has revoked the session may become useless at some point. A resource server
token), the session may become useless at some point. A resource therefore MUST terminate existing DTLS sessions after the access
server therefore MUST terminate existing DTLS sessions after the last token for this session has been deleted.
valid access token for this session has been deleted.
As specified in Section 5.8.3 of [I-D.ietf-ace-oauth-authz], the 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 resource server MUST notify the client with an error response with
code 4.01 (Unauthorized) for any long running request before code 4.01 (Unauthorized) for any long running request before
terminating the session. terminating the session.
6. Security Considerations 6. Security Considerations
This document specifies a profile for the Authentication and This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework Authorization for Constrained Environments (ACE) framework
[I-D.ietf-ace-oauth-authz]. As it follows this framework's general [I-D.ietf-ace-oauth-authz]. As it follows this framework's general
approach, the general security and privacy considerations from approach, the general security considerations from section 6 also
section 6 and section 7 also apply to this profile. apply to this profile.
When using pre-shared keys provisioned by the AS, the security level
depends on the randomness of PSK, and the security of the TLS cipher
suite and key exchange algorithm.
Constrained devices that use DTLS [RFC6347] are inherently vulnerable Constrained devices that use DTLS [RFC6347] are inherently vulnerable
to Denial of Service (DoS) attacks as the handshake protocol requires to Denial of Service (DoS) attacks as the handshake protocol requires
creation of internal state within the device. This is specifically creation of internal state within the device. This is specifically
of concern where an adversary is able to intercept the initial cookie of concern where an adversary is able to intercept the initial cookie
exchange and interject forged messages with a valid cookie to exchange and interject forged messages with a valid cookie to
continue with the handshake. A similar issue exists with the continue with the handshake. A similar issue exists with the
authorization information endpoint where the resource server needs to authorization information endpoint where the resource server needs to
keep valid access tokens until their expiry. Adversaries can fill up keep valid access tokens until their expiry. Adversaries can fill up
the constrained resource server's internal storage for a very long the constrained resource server's internal storage for a very long
time with interjected or otherwise retrieved valid access tokens. time with interjected or otherwise retrieved valid access tokens.
The use of multiple access tokens for a single client increases the The use of multiple access tokens for a single client increases the
strain on the resource server as it must consider every access token strain on the resource server as it must consider every access token
and calculate the actual permissions of the client. Also, tokens may and calculate the actual permissions of the client. Also, tokens may
contradict each other which may lead the server to enforce wrong contradict each other which may lead the server to enforce wrong
permissions. If one of the access tokens expires earlier than permissions. If one of the access tokens expires earlier than
others, the resulting permissions may offer insufficient protection. others, the resulting permissions may offer insufficient protection.
Developers should avoid using multiple access tokens for a client. Developers SHOULD avoid using multiple access tokens for a client.
7. Privacy Considerations 7. 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 An unprotected response to an unauthorized request may disclose
information about the resource server and/or its existing information about the resource server and/or its existing
relationship with the client. It is advisable to include as little relationship with the client. It is advisable to include as little
information as possible in an unencrypted response. When a DTLS information as possible in an unencrypted response. When a DTLS
session between the client and the resource server already exists, session between the client and the resource server already exists,
more detailed information may be included with an error response to more detailed information MAY be included with an error response to
provide the client with sufficient information to react on that provide the client with sufficient information to react on that
particular error. particular error.
Also, unprotected requests to the resource server may reveal Also, unprotected requests to the resource server may reveal
information about the client, e.g., which resources the client information about the client, e.g., which resources the client
attempts to request or the data that the client wants to provide to attempts to request or the data that the client wants to provide to
the resource server. The client should not send confidential data in the resource server. The client SHOULD NOT send confidential data in
an unprotected request. an unprotected request.
Note that some information might still leak after DTLS session is Note that some information might still leak after DTLS session is
established, due to observable message sizes, the source, and the established, due to observable message sizes, the source, and the
destination addresses. destination addresses.
8. IANA Considerations 8. IANA Considerations
The following registrations are done for the ACE OAuth Profile The following registrations are done for the ACE OAuth Profile
Registry following the procedure specified in Registry following the procedure specified in
skipping to change at page 16, line 48 skipping to change at page 18, line 19
[I-D.ietf-ace-cwt-proof-of-possession] [I-D.ietf-ace-cwt-proof-of-possession]
Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of- Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
possession-06 (work in progress), February 2019. possession-06 (work in progress), February 2019.
[I-D.ietf-ace-oauth-authz] [I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0 Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-21 Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-22
(work in progress), February 2019. (work in progress), March 2019.
[I-D.ietf-ace-oauth-params] [I-D.ietf-ace-oauth-params]
Seitz, L., "Additional OAuth Parameters for Authorization Seitz, L., "Additional OAuth Parameters for Authorization
in Constrained Environments (ACE)", draft-ietf-ace-oauth- in Constrained Environments (ACE)", draft-ietf-ace-oauth-
params-04 (work in progress), February 2019. params-04 (work in progress), February 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
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