ACE Working Group F. Palombini
Internet-Draft Ericsson AB
Intended status: Standards Track L. Seitz
Expires: May 11, 2019 RISE SICS AB
G. Selander
Ericsson AB
M. Gunnarsson
RISE SICS AB
November 7, 2018
OSCORE profile of the Authentication and Authorization for Constrained
Environments Framework
draft-ietf-ace-oscore-profile-05
Abstract
This memo specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework. It
utilizes Object Security for Constrained RESTful Environments
(OSCORE) to provide communication security, server authentication,
and proof-of-possession for a key owned by the client and bound to an
OAuth 2.0 access token.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 11, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. Client-AS Communication . . . . . . . . . . . . . . . . . . . 5
3.1. C-to-AS: POST to token endpoint . . . . . . . . . . . . . 6
3.2. AS-to-C: Access Token . . . . . . . . . . . . . . . . . . 7
3.2.1. OSCORE_Security_Context Object . . . . . . . . . . . 10
4. Client-RS Communication . . . . . . . . . . . . . . . . . . . 13
4.1. C-to-RS: POST to authz-info endpoint . . . . . . . . . . 13
4.2. RS-to-C: 2.01 (Created) . . . . . . . . . . . . . . . . . 14
4.3. OSCORE Setup . . . . . . . . . . . . . . . . . . . . . . 15
4.4. Access rights verification . . . . . . . . . . . . . . . 16
5. Secure Communication with AS . . . . . . . . . . . . . . . . 17
6. Discarding the Security Context . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9.1. ACE OAuth Profile Registry . . . . . . . . . . . . . . . 19
9.2. OSCORE Security Context Parameters Registry . . . . . . . 19
9.3. CWT Confirmation Methods Registry . . . . . . . . . . . . 20
9.4. JWT Confirmation Methods Registry . . . . . . . . . . . . 20
9.5. Expert Review Instructions . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Profile Requirements . . . . . . . . . . . . . . . . 23
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
This memo specifies a profile of the ACE framework
[I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource
server use CoAP [RFC7252] to communicate. The client uses an access
token, bound to a key (the proof-of-possession key) to authorize its
access to the resource server. In order to provide communication
security, proof of possession, and server authentication they use
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Object Security for Constrained RESTful Environments (OSCORE)
[I-D.ietf-core-object-security].
OSCORE specifies how to use CBOR Object Signing and Encryption (COSE)
[RFC8152] to secure CoAP messages. Note that OSCORE can be used to
secure CoAP messages, as well as HTTP and combinations of HTTP and
CoAP; a profile of ACE similar to the one described in this document,
with the difference of using HTTP instead of CoAP as communication
protocol, could be specified analogously to this one.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. These
words may also appear in this document in lowercase, absent their
normative meanings.
Certain security-related terms such as "authentication",
"authorization", "confidentiality", "(data) integrity", "message
authentication code", and "verify" are taken from [RFC4949].
RESTful terminology follows HTTP [RFC7231].
Terminology for entities in the architecture is defined in OAuth 2.0
[RFC6749], such as client (C), resource server (RS), and
authorization server (AS). It is assumed in this document that a
given resource on a specific RS is associated to a unique AS.
Note that the term "endpoint" is used here, as in
[I-D.ietf-ace-oauth-authz], following its OAuth definition, which is
to denote resources such as token and introspect at the AS and authz-
info at the RS. The CoAP [RFC7252] definition, which is "An entity
participating in the CoAP protocol" is not used in this memo.
2. Protocol Overview
This section gives an overview on how to use the ACE Framework
[I-D.ietf-ace-oauth-authz] to secure the communication between a
client and a resource server using OSCORE
[I-D.ietf-core-object-security]. The parameters needed by the client
to negotiate the use of this profile with the authorization server,
as well as OSCORE setup process, are described in detail in the
following sections.
This profile requires a client to retrieve an access token from the
AS for the resource it wants to access on a RS, using the token
endpoint, as specified in section 5.6 of [I-D.ietf-ace-oauth-authz].
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To determine the AS in charge of a resource hosted at the RS, the
client C MAY send an initial Unauthorized Resource Request message to
the RS. The RS then denies the request and sends the address of its
AS back to the client C as specified in section 5.1 of
[I-D.ietf-ace-oauth-authz]. The access token request and response
MUST be confidentiality-protected and ensure authenticity. This
profile RECOMMENDS the use of OSCORE between client and AS, but TLS
or DTLS MAY be used additionally or instead.
Once the client has retrieved the access token, it posts it to the RS
using the authz-info endpoint and mechanisms specified in section 5.8
of [I-D.ietf-ace-oauth-authz]. If the access token is valid, the RS
replies to this request with a 2.01 (Created) response, which
contains a nonce N1.
After receiving the nonce N1, the client generates a nonce N2,
concatenates it with N1 and sets the ID Context in its Security
Context (see section 3 of [I-D.ietf-core-object-security]) to N1
concatenated with N2. The client then derives the complete Security
Context from the ID Context plus the parameters received from the AS.
Finally, the client sends a request protected with OSCORE to the RS.
This message contains the ID Context value. When receiving this
request after the 2.01 (Created) response, the server extract the ID
Context from it, verifies that the first part is equal to the nonce
N1 it previously sent, and if so, sets its own ID Context and derives
the complete Security Context from it plus the parameters received in
the AS, following section 3.2 of [I-D.ietf-core-object-security]. If
the request verifies, then this Security Context is stored in the
server, and used in the response, and in further communications with
the client, until token expiration. Once the client receives a valid
response, it does not continue to include the ID Context value in
further requests.
The use of random nonces during the exchange prevents the reuse of
AEAD nonces and keys with different messages, in case of re-
derivation of the Security Context both for Clients and Resource
Servers from an old non-expired access token, e.g. in case of re-boot
of either the client or RS. In fact, by using random nonces as ID
Context, the request to the authz-info endpoint posting the same
token results in a different Security Context, since Master Secret,
Sender ID and Recipient ID are the same but ID Context is different.
Therefore, the main requirement for the nonces is that they have a
good amount of randomness. If random nonces were not used, a node
re-using a non-expired old token would be susceptible to on-path
attackers provoking the creation of OSCORE messages using old AEAD
keys and nonces.
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An overview of the profile flow for the OSCORE profile is given in
Figure 1.
C RS AS
| [-- Resource Request --->] | |
| | |
| [<----- AS Information --] | |
| | |
| ----- POST /token ----------------------------> |
| | |
| <---------------------------- Access Token ----- |
| + RS Information |
| ---- POST /authz-info ---> | |
| | |
| <--- 2.01 Created (N1) --- | |
| | |
/Sec Context Derivation/ | |
| | |
| ---- OSCORE Request -----> | |
| (N1, N2) | |
| | |
| /Sec Context Derivation/ |
| | |
| <--- OSCORE Response ----- | |
| | |
| ---- OSCORE Request -----> | |
| | |
| <--- OSCORE Response ----- | |
| ... | |
Figure 1: Protocol Overview
3. Client-AS Communication
The following subsections describe the details of the POST request
and response to the token endpoint between client and AS.
Section 3.2 of [I-D.ietf-core-object-security] defines how to derive
a Security Context based on a shared master secret and a set of other
parameters, established between client and server, which the client
receives from the AS in this exchange. The proof-of-possession key
(pop-key) provisioned from the AS MUST be used as master secret in
OSCORE.
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3.1. C-to-AS: POST to token endpoint
The client-to-AS request is specified in Section 5.6.1 of
[I-D.ietf-ace-oauth-authz].
The client MUST send this POST request to the token endpoint over a
secure channel that guarantees authentication, message integrity and
confidentiality (see Section 5).
An example of such a request, in CBOR diagnostic notation without the
tag and value abbreviations is reported in Figure 2
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"grant_type" : "client_credentials",
"client_id" : "myclient",
"req_aud" : "tempSensor4711",
"scope" : "read"
}
Figure 2: Example C-to-AS POST /token request for an access token
bound to a symmetric key.
If the client wants to update its access rights without changing an
existing OSCORE Security Context, it MUST include in its POST request
to the token endpoint a req_cnf object carrying the client's
identifier (that was assigned in section Section 3.2) in the kid
field. This identifier can be used by the AS to determine the shared
secret to construct the proof-of-possession token and therefore MUST
identify a symmetric key that was previously generated by the AS as a
shared secret for the communication between the client and the RS.
The AS MUST verify that the received value identifies a proof-of-
possession key and token that have previously been issued to the
requesting client. If that is not the case, the Client-to-AS request
MUST be declined with the error code 'invalid_request' as defined in
Section 5.6.3 of [I-D.ietf-ace-oauth-authz].
An example of such a request, in CBOR diagnostic notation without the
tag and value abbreviations is reported in Figure 3
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Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"grant_type" : "client_credentials",
"client_id" : "myclient",
"req_aud" : "tempSensor4711",
"scope" : "write",
"req_cnf" : {
"kid" : b64'Qg'
}
Figure 3: Example C-to-AS POST /token request for updating rights to
an access token bound to a symmetric key.
3.2. AS-to-C: Access Token
After verifying the POST request to the token endpoint and that the
client is authorized to obtain an access token corresponding to its
access token request, the AS responds as defined in section 5.6.2 of
[I-D.ietf-ace-oauth-authz]. If the client request was invalid, or
not authorized, the AS returns an error response as described in
section 5.6.3 of [I-D.ietf-ace-oauth-authz].
The AS signals that the use of OSCORE is REQUIRED for a specific
access token by including the "profile" parameter with the value
"coap_oscore" in the access token response. This means that the
client MUST use OSCORE towards all resource servers for which this
access token is valid, and follow Section 4.3 to derive the security
context to run OSCORE.
Moreover, the AS MUST provision the following data:
o a master secret
o a client identifier
o a server identifier
Additionally, the AS MAY provision the following data, in the same
response.
o an AEAD algorithm
o an HKDF algorithm
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o a salt
o a replay window type and size
The master secret MUST be communicated as the 'ms' field in the
OSCORE_Security_Context in the 'cnf' parameter of the access token
response as defined in Section 3.2 of [I-D.ietf-ace-oauth-params].
The OSCORE_Security_Context is a CBOR map object, defined in
Section 3.2.1. The AEAD algorithm MAY be included as the 'alg'
parameter in the OSCORE_Security_Context; the HKDF algorithm MAY be
included as the 'hkdf' parameter of the OSCORE_Security_Context, the
salt MAY be included as the 'salt' parameter of the
COSCORE_Security_Context and the replay window type and size MAY be
included as the 'rpl' of the OSCORE_Security_Context, as defined in
Section 3.2.1.
The same parameters MUST be included as metadata of the access token.
This profile RECOMMENDS the use of CBOR web token (CWT) as specified
in [RFC8392]. If the token is a CWT, the same
OSCORE_Security_Context structure defined above MUST be placed in the
'cnf' claim of this token.
The AS MUST also assign identifiers to both client and RS, which are
then used as Sender ID and Recipient ID in the OSCORE context as
described in section 3.1 of [I-D.ietf-core-object-security]. The
client identifiers MUST be unique in the set of all clients on a
single RS, and RS identifiers MUST be unique in the set of all RS for
any given client. Moreover, these MUST be included in the
OSCORE_Security_Context, as defined in Section 3.2.1.
We assume in this document that a resource is associated to one
single AS, which makes it possible to assume unique identifiers for
each client requesting a particular resource to a RS. If this is not
the case, collisions of identifiers may appear in the RS, in which
case the RS needs to have a mechanism in place to disambiguate
identifiers or mitigate their effect.
Note that in Section 4.3 C sets the Sender ID of its Security Context
to the clientId value received and the Recipient ID to the serverId
value, and RS does the opposite.
Figure 4 shows an example of such an AS response, in CBOR diagnostic
notation without the tag and value abbreviations.
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Header: Created (Code=2.01)
Content-Type: "application/cose+cbor"
Payload:
{
"access_token" : b64'SlAV32hkKG ...
(remainder of access token omitted for brevity)',
"profile" : "coap_oscore",
"expires_in" : "3600",
"cnf" : {
"OSCORE_Security_Context" : {
"alg" : "AES-CCM-16-64-128",
"clientId" : b64'qA',
"serverId" : b64'Qg',
"ms" : b64'+a+Dg2jjU+eIiOFCa9lObw'
}
}
}
Figure 4: Example AS-to-C Access Token response with OSCORE profile.
Figure 5 shows an example CWT, containing the necessary OSCORE
parameters in the 'cnf' claim, in CBOR diagnostic notation without
tag and value abbreviations.
{
"aud" : "tempSensorInLivingRoom",
"iat" : "1360189224",
"exp" : "1360289224",
"scope" : "temperature_g firmware_p",
"cnf" : {
"OSCORE_Security_Context" : {
"alg" : "AES-CCM-16-64-128",
"clientId" : b64'Qg',
"serverId" : b64'qA',
"ms" : b64'+a+Dg2jjU+eIiOFCa9lObw'
}
}
Figure 5: Example CWT with OSCORE parameters.
If the client has requested an update to its access rights using the
same OSCORE Security Context, which is valid and authorized, the AS
MUST omit the 'cnf' parameter in the response, and MUST carry the
client identifier in the 'kid' field in the 'cnf' parameter of the
token. The client identifier needs to be provisioned, in order for
the RS to identify the previously generated Security Context.
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Figure 6 shows an example of such an AS response, in CBOR diagnostic
notation without the tag and value abbreviations.
Header: Created (Code=2.01)
Content-Type: "application/cose+cbor"
Payload:
{
"access_token" : b64'SlAV32hkKG ...
(remainder of access token omitted for brevity)',
"profile" : "coap_oscore",
"expires_in" : "3600"
}
Figure 6: Example AS-to-C Access Token response with OSCORE profile,
for update of access rights.
Figure 7 shows an example CWT, containing the necessary OSCORE
parameters in the 'cnf' claim for update of access rights, in CBOR
diagnostic notation without tag and value abbreviations.
{
"aud" : "tempSensorInLivingRoom",
"iat" : "1360189224",
"exp" : "1360289224",
"scope" : "temperature_h",
"cnf" : {
"kid" : b64'Qg'
}
}
Figure 7: Example CWT with OSCORE parameters for update of access
rights.
3.2.1. OSCORE_Security_Context Object
An OSCORE_Security_Context is an object that represents part or all
of an OSCORE Security Context (Section 3.1 of
[I-D.ietf-core-object-security]). The OSCORE_Security_Context object
can either be encoded as JSON or as CBOR. In both cases, the set of
common parameters that can appear in an OSCORE_Security_Context
object can be found in the IANA "OSCORE Security Context Parameters"
registry (Section Section 9.2) and is defined below. All parameters
are optional. Table 1 provides a summary of the
OSCORE_Security_Context parameters defined in this section.
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+----------+-------+----------------+--------------+----------------+
| name | CBOR | CBOR type | registry | description |
| | label | | | |
+----------+-------+----------------+--------------+----------------+
| ms | 1 | bstr | | OSCORE Master |
| | | | | Secret value |
| | | | | |
| clientId | 2 | bstr | | OSCORE Sender |
| | | | | ID value of |
| | | | | the client, |
| | | | | OSCORE |
| | | | | Recipient ID |
| | | | | value of the |
| | | | | server |
| | | | | |
| serverId | 3 | bstr | | OSCORE Sender |
| | | | | ID value of |
| | | | | the server, |
| | | | | OSCORE |
| | | | | Recipient ID |
| | | | | value of the |
| | | | | client |
| | | | | |
| hkdf | 4 | bstr / int | COSE | OSCORE HKDF |
| | | | Algorithm | value |
| | | | Values | |
| | | | (HMAC-based) | |
| | | | | |
| alg | 5 | tstr / int | COSE | OSCORE AEAD |
| | | | Algorithm | Algorithm |
| | | | Values | value |
| | | | (AEAD) | |
| | | | | |
| salt | 6 | bstr | | OSCORE Master |
| | | | | Salt value |
| | | | | |
| rpl | 7 | bstr / int | | OSCORE Replay |
| | | | | Window Type |
| | | | | and Size |
+----------+-------+----------------+--------------+----------------+
Table 1: OSCORE_Security_Context Parameters
ms: This parameter identifies the OSCORE Master Secret value, which
is a byte string. For more information about this field, see
section 3.1 of [I-D.ietf-core-object-security]. In JSON, the "ms"
value is a Base64 encoded byte string. In CBOR, the "ms" type is
bstr, and has label 1.
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clientId: This parameter identifies a client identifier as a byte
string. This identifier is used as OSCORE Sender ID in the client
and OSCORE Recipient ID in the server. For more information about
this field, see section 3.1 of [I-D.ietf-core-object-security].
In JSON, the "clientID" value is a Base64 encoded byte string. In
CBOR, the "clientID" type is bstr, and has label 2.
serverId: This parameter identifies a server identifier as a byte
string. This identifier is used as OSCORE Sender ID in the client
and OSCORE Recipient ID in the server. For more information about
this field, see section 3.1 of [I-D.ietf-core-object-security].
In JSON, the "serverID" value is a Base64 encoded byte string. In
CBOR, the "serverID" type is bstr, and has label 3.
hkdf: This parameter identifies the OSCORE HKDF Algorithm. For more
information about this field, see section 3.1 of
[I-D.ietf-core-object-security]. The values used MUST be
registered in the IANA "COSE Algorithms" registry and MUST be
HMAC-based HKDF algorithms. The value can either be the integer
or the text string value of the HMAC-based HKDF algorithm in the
"COSE Algorithms" registry. In JSON, the "hkdf" value is a case-
sensitive ASCII string or an integer. In CBOR, the "hkdf" type is
tstr or int, and has label 4.
alg: This parameter identifies the OSCORE AEAD Algorithm. For more
information about this field, see section 3.1 of
[I-D.ietf-core-object-security] The values used MUST be registered
in the IANA "COSE Algorithms" registry and MUST be AEAD
algorithms. The value can either be the integer or the text
string value of the HMAC-based HKDF algorithm in the "COSE
Algorithms" registry. In JSON, the "alg" value is a case-
sensitive ASCII string or an integer. In CBOR, the "alg" type is
tstr or int, and has label 5.
salt: This parameter identifies the OSCORE Master Salt value, which
is a byte string. For more information about this field, see
section 3.1 of [I-D.ietf-core-object-security]. In JSON, the
"salt" value is a Base64 encoded byte string. In CBOR, the "salt"
type is bstr, and has label 6.
repl: This parameter is used to carry the OSCORE value, encoded as a
bstr. This parameter identifies the OSCORE Replay Window Size and
Type value, which is a byte string. For more information about
this field, see section 3.1 of [I-D.ietf-core-object-security].
In JSON, the "repl" value is a Base64 encoded byte string. In
CBOR, the "repl" type is bstr, and has label 7.
An example of JSON OSCORE_Security_Context is given in Figure 8.
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"OSCORE_Security_Context" : {
"alg" : "AES-CCM-16-64-128",
"clientId" : b64'qA',
"serverId" : b64'Qg',
"ms" : b64'+a+Dg2jjU+eIiOFCa9lObw'
}
Figure 8: Example JSON OSCORE_Security_Context object
The CDDL grammar describing the CBOR OSCORE_Security_Context object
is:
OSCORE_Security_Context = {
? 1 => bstr, ; ms
? 2 => bstr, ; clientId
? 3 => bstr, ; serverId
? 4 => tstr / int, ; hkdf
? 5 => tstr / int, ; alg
? 6 => bstr, ; salt
? 7 => bstr / tstr ; rpl
}
4. Client-RS Communication
The following subsections describe the details of the POST request
and response to the authz-info endpoint between client and RS. The
client posts the token that includes the materials provisioned by the
AS to the RS, which can then use Section 3.2 of
[I-D.ietf-core-object-security] to derive a security context based on
a shared master secret and a set of other parameters, established
between client and server.
Note that the proof-of-possession required to bind the access token
to the client is implicitly performed by generating the shared OSCORE
Security Context using the pop-key as master secret, for both client
and RS. An attacker using a stolen token will not be able to
generate a valid OSCORE context and thus not be able to prove
possession of the pop-key.
4.1. C-to-RS: POST to authz-info endpoint
The client MUST use CoAP and the Authorization Information resource
as described in section 5.8.1 of [I-D.ietf-ace-oauth-authz] to
transport the token to the RS.
The authz-info endpoint is not protected, nor are the responses from
this resource.
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The access token MUST be encrypted, since it is transferred from the
client to the RS over an unprotected channel.
Note that a client may be required to re-POST the access token, since
an RS may delete a stored access token, due to lack of memory.
Figure 9 shows an example of the request sent from the client to the
RS.
Header: POST (Code=0.02)
Uri-Host: "rs.example.com"
Uri-Path: "authz-info"
Content-Format: "application/cwt"
Payload:
b64'SlAV32hkKG ...
(remainder of access token omitted for brevity)',
Figure 9: Example C-to-RS POST /authz-info request using CWT
4.2. RS-to-C: 2.01 (Created)
The RS MUST follow the procedures defined in section 5.8.1 of
[I-D.ietf-ace-oauth-authz]: the RS MUST verify the validity of the
token. If the token is valid, the RS MUST respond to the POST
request with 2.01 (Created). If the token is valid but is associated
to claims that the RS cannot process (e.g., an unknown scope) the RS
MUST respond with a response code equivalent to the CoAP code 4.00
(Bad Request). In the latter case the RS MAY provide additional
information in the error response, in order to clarify what went
wrong. The RS MAY make an introspection request to validate the
token before responding to the POST request to the authz-info
endpoint.
Additionally, the RS MUST generate a nonce (N1) with a good amount of
randomness, and include it in the payload of the 2.01 (Created)
response as a CBOR byte string. This profile RECOMMENDS to use a
nonce of 64 bits. The RS MUST store this nonce as long as the access
token related to it is still valid.
Note that, when using this profile, an identifier of the token (e.g.,
the cti for a CWT) is not transported in the payload of this request,
as section 5.8.1 of [I-D.ietf-ace-oauth-authz] allows.
Figure 10 shows an example of the response sent from the RS to the
client.
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Header: Created (Code=2.01)
Content-Format: "application/cbor"
Payload:
h'018a278f7faab55a',
Figure 10: Example RS-to-C 2.01 (Created) response
When receiving an updated access token with updated authorization
information from the client (see section Section 3.1), it is
RECOMMENDED that the RS overwrites the previous token, that is only
the latest authorization information in the token received by the RS
is valid. This simplifies for the RS to keep track of authorization
information for a given client.
As specified in section 5.8.3 of [I-D.ietf-ace-oauth-authz], the RS
MUST notify the client with an error response with code 4.01
(Unauthorized) for any long running request before terminating the
session, when the access token expires.
4.3. OSCORE Setup
Once receiving the 2.01 (Created) response from the RS, following the
POST request to authz-info endpoint, the client MUST extract the
nonce N1 from the CBOR byte string in the payload of the response.
The client MUST generate itself a nonce (N2) with a good amount of
randomness. This profile RECOMMENDS to use a nonce of 64 bits.
Then, the client MUST set the ID Context of the Security Context
created to communicate with the RS to the concatenation of N1 and N2,
in this order: ID Context = N1 | N2, where | denotes byte string
concatenation. The client MUST set the Master Secret, Sender ID and
Recipient ID from the parameters received from the AS in Section 3.2.
The client MUST set the AEAD Algorithm, Master Salt, HKDF and Replay
Window from the parameters received from the AS in Section 3.2, if
present. In case these parameters are omitted, the default values
are used as described in section 3.2 of
[I-D.ietf-core-object-security]. After that, the client MUST derive
the complete Security Context following section 3.2.1 of
[I-D.ietf-core-object-security]. From this point on, the client MUST
use this Security Context to communicate with the RS when accessing
the resources as specified by the authorization information.
The client then uses this Security Context to send requests to RS
using OSCORE. In the first request sent to the RS, the client MUST
include the kid context, with value ID Context, i.e. N1 concatenated
with N2. The client needs to make sure the RS receives the kid
context, possibly adding the kid context to later requests, until it
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receives a valid OSCORE response from the RS using the same Security
Context.
When the RS receives this first OSCORE-protected request, it MUST
extract the kid context from the message first. Then, it needs to
verify that the first part of the kid context corresponds to the
nonce N1 it previously sent, and that it is followed by a non-zero-
length byte string. If that is verified, the RS MUST set the ID
Context to the kid context value. Then, the RS MUST set the Master
Secret, Sender ID and Recipient ID from the parameters received from
the client in the access token in Section 4.1. The RS MUST set the
AEAD Algorithm, Master Salt, HKDF and Replay Window from the
parameters received from the client in the access token in
Section 4.1, if present. In case these parameters are omitted, the
default values are used as described in section 3.2 of
[I-D.ietf-core-object-security]. After that, the RS MUST derive the
complete Security Context following section 3.2.1 of
[I-D.ietf-core-object-security], and MUST associate this Security
Context with the authorization information from the access token.
Then, the RS MUST delete the nonce N1 from memory.
The RS then uses this Security Context to verify the request and send
responses to RS using OSCORE. If OSCORE verification fails, error
responses are used, as specified in section 8 of
[I-D.ietf-core-object-security]. Additionally, if OSCORE
verification succeeds, the verification of access rights is performed
as described in section Section 4.4. The RS MUST NOT use the
Security Context after the related token has expired, and MUST
respond with a unprotected 4.01 (Unauthorized) error message.
4.4. Access rights verification
The RS MUST follow the procedures defined in section 5.8.2 of
[I-D.ietf-ace-oauth-authz]: if an RS receives an OSCORE-protected
request from a client, then it processes according to
[I-D.ietf-core-object-security]. If OSCORE verification succeeds,
and the target resource requires authorization, the RS retrieves the
authorization information from the access token associated to the
Security Context. The RS then MUST verify that the authorization
information covers the resource and the action requested.
The response code MUST be 4.01 (Unauthorized) in case the client has
not used the Security Context associated with the access token, or if
RS has no valid access token for the client. If RS has an access
token for the client but not for the resource that was requested, RS
MUST reject the request with a 4.03 (Forbidden). If RS has an access
token for the client but it does not cover the action that was
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requested on the resource, RS MUST reject the request with a 4.05
(Method Not Allowed).
5. Secure Communication with AS
As specified in the ACE framework (section 5.7 of
[I-D.ietf-ace-oauth-authz]), the requesting entity (RS and/or client)
and the AS communicates via the introspection or token endpoint. The
use of CoAP and OSCORE for this communication is RECOMMENDED in this
profile, other protocols (such as HTTP and DTLS or TLS) MAY be used
instead.
If OSCORE is used, the requesting entity and the AS are expected to
have pre-established security contexts in place. How these security
contexts are established is out of scope for this profile.
Furthermore the requesting entity and the AS communicate using OSCORE
([I-D.ietf-core-object-security]) through the introspection endpoint
as specified in section 5.7 of [I-D.ietf-ace-oauth-authz] and through
the token endpoint as specified in section 5.6 of
[I-D.ietf-ace-oauth-authz].
6. Discarding the Security Context
There are a number of scenarios where a client or RS needs to discard
the OSCORE security context, and acquire a new one.
The client MUST discard the current security context associated with
an RS when:
o the Sequence Number space ends.
o the access token associated with the context expires.
o the client receives a number of 4.01 Unauthorized responses to
OSCORE requests using the same security context. The exact number
needs to be specified by the application.
o the client receives a new nonce in the 2.01 Created response (see
Section 4.2) to a POST request to the authz-info endpoint, when
re-posting a non-expired token associated to the existing context.
The RS MUST discard the current security context associated with a
client when:
o Sequence Number space ends.
o Access token associated with the context expires.
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7. Security Considerations
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework
[I-D.ietf-ace-oauth-authz]. Thus the general security considerations
from the framework also apply to this profile.
Furthermore the general security considerations of OSCORE
[I-D.ietf-core-object-security] also apply to this specific use of
the OSCORE protocol.
OSCORE is designed to secure point-to-point communication, providing
a secure binding between the request and the response(s). Thus the
basic OSCORE protocol is not intended for use in point-to-multipoint
communication (e.g. multicast, publish-subscribe). Implementers of
this profile should make sure that their usecase corresponds to the
expected use of OSCORE, to prevent weakening the security assurances
provided by OSCORE.
Since the use of nonces in the exchange guarantees uniqueness of AEAD
keys and nonces even in case of re-boots, a good amount of randomness
is required. If that is not guaranteed, nodes are still susceptible
to re-using nonces and keys, in case the Security Context is lost,
and on-path attacker replaying messages.
This profiles recommends that the RS maintains a single access token
for a client. The use of multiple access tokens for a single client
increases the strain on the resource server as it must consider every
access token and calculate the actual permissions of the client.
Also, tokens may contradict each other which may lead the server to
enforce wrong permissions. If one of the access tokens expires
earlier than others, the resulting permissions may offer insufficient
protection. Developers should avoid using multiple access tokens for
a client.
8. Privacy Considerations
This document specifies a profile for the Authentication and
Authorization for Constrained Environments (ACE) framework
[I-D.ietf-ace-oauth-authz]. Thus the general privacy considerations
from the framework also apply to this profile.
As this document uses OSCORE, thus the privacy considerations from
[I-D.ietf-core-object-security] apply here as well.
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
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information as possible in an unencrypted response. When an OSCORE
Security Context already exists between the client and the resource
server, more detailed information may be included.
Note that some information might still leak after OSCORE is
established, due to observable message sizes, the source, and the
destination addresses.
9. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "[[this
specification]]" with the RFC number of this specification and delete
this paragraph.
9.1. ACE OAuth Profile Registry
The following registration is done for the ACE OAuth Profile Registry
following the procedure specified in section 8.7 of
[I-D.ietf-ace-oauth-authz]:
o Profile name: coap_oscore
o Profile Description: Profile for using OSCORE to secure
communication between constrained nodes using the Authentication
and Authorization for Constrained Environments framework.
o Profile ID: TBD (value between 1 and 255)
o Change Controller: IESG
o Specification Document(s): [[this specification]]
9.2. OSCORE Security Context Parameters Registry
It is requested that IANA create a new registry entitled "OSCORE
Security Context Parameters" registry. The registry is to be created
as Expert Review Required. Guidelines for the experts is provided
Section 9.5. It should be noted that in additional to the expert
review, some portions of the registry require a specification,
potentially on standards track, be supplied as well.
The columns of the registry are:
name This is a descriptive name that enables easier reference to the
item. It is not used in the CBOR encoding.
CBOR label The value to be used to identify this algorithm. Key map
labels MUST be unique. The label can be a positive integer, a
negative integer or a string. Integer values between 0 and 255
and strings of length 1 are designated as Standards Track Document
required. Integer values from 256 to 65535 and strings of length
2 are designated as Specification Required. Integer values of
greater than 65535 and strings of length greater than 2 are
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designated as expert review. Integer values less than -65536 are
marked as private use.
CBOR Type This field contains the CBOR type for the field.
registry This field denotes the registry that values may come from,
if one exists.
description This field contains a brief description for the field.
specification This contains a pointer to the public specification
for the field if one exists
This registry will be initially populated by the values in Table 1.
The specification column for all of these entries will be this
document.
9.3. CWT Confirmation Methods Registry
The following registration is done for the CWT Confirmation Methods
Registry following the procedure specified in section 7.2.1 of
[I-D.ietf-ace-cwt-proof-of-possession]:
o Confirmation Method Name: "OSCORE_Security_Context"
o Confirmation Method Description: OSCORE_Security_Context carrying
the OSCORE Security Context parameters
o Confirmation Key: TBD (value between 4 and 255)
o Confirmation Value Type(s): map
o Change Controller: IESG
o Specification Document(s): Section 3.2.1 of [[this specification]]
9.4. JWT Confirmation Methods Registry
The following registration is done for the JWT Confirmation Methods
Registry following the procedure specified in section 6.2.1 of
[RFC7800]:
o Confirmation Method Value: "osc"
o Confirmation Method Description: OSCORE_Security_Context carrying
the OSCORE Security Context parameters
o Change Controller: IESG
o Specification Document(s): Section 3.2.1 of [[this specification]]
9.5. Expert Review Instructions
The IANA registry established in this document is defined as expert
review. This section gives some general guidelines for what the
experts should be looking for, but they are being designated as
experts for a reason so they should be given substantial latitude.
Expert reviewers should take into consideration the following points:
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o Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate one that is already
registered and that the point is likely to be used in deployments.
The zones tagged as private use are intended for testing purposes
and closed environments, code points in other ranges should not be
assigned for testing.
o Specifications are required for the standards track range of point
assignment. Specifications should exist for specification
required ranges, but early assignment before a specification is
available is considered to be permissible. Specifications are
needed for the first-come, first-serve range if they are expected
to be used outside of closed environments in an interoperable way.
When specifications are not provided, the description provided
needs to have sufficient information to identify what the point is
being used for.
o Experts should take into account the expected usage of fields when
approving point assignment. The fact that there is a range for
standards track documents does not mean that a standards track
document cannot have points assigned outside of that range. The
length of the encoded value should be weighed against how many
code points of that length are left, the size of device it will be
used on, and the number of code points left that encode to that
size.
10. References
10.1. Normative References
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-16
(work in progress), October 2018.
[I-D.ietf-ace-oauth-params]
Seitz, L., "Additional OAuth Parameters for Authorization
in Constrained Environments (ACE)", draft-ietf-ace-oauth-
params-00 (work in progress), September 2018.
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-15 (work in
progress), August 2018.
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[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>.
[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>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[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>.
10.2. Informative 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-04 (work in progress), November 2018.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[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>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>.
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Appendix A. Profile Requirements
This section lists the specifications on this profile based on the
requirements on the framework, as requested in Appendix C of
[I-D.ietf-ace-oauth-authz].
o (Optional) discovery process of how the client finds the right AS
for an RS it wants to send a request to: Not specified
o communication protocol the client and the RS must use: CoAP
o security protocol the client and RS must use: OSCORE
o how the client and the RS mutually authenticate: Implicitly by
possession of a common OSCORE security context
o Content-format of the protocol messages: "application/cose+cbor"
o proof-of-possession protocol(s) and how to select one; which key
types (e.g. symmetric/asymmetric) supported: OSCORE algorithms;
pre-established symmetric keys
o profile identifier: coap_oscore
o (Optional) how the RS talks to the AS for introspection: HTTP/CoAP
(+ TLS/DTLS/OSCORE)
o how the client talks to the AS for requesting a token: HTTP/CoAP
(+ TLS/DTLS/OSCORE)
o how/if the authz-info endpoint is protected: Security protocol
above
o (Optional)other methods of token transport than the authz-info
endpoint: no
Acknowledgments
The authors wish to thank Jim Schaad and Marco Tiloca for the input
on this memo.
Authors' Addresses
Francesca Palombini
Ericsson AB
Email: francesca.palombini@ericsson.com
Ludwig Seitz
RISE SICS AB
Scheelevagen 17
Lund 22370
Sweden
Email: ludwig.seitz@ri.se
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Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
Martin Gunnarsson
RISE SICS AB
Scheelevagen 17
Lund 22370
Sweden
Email: martin.gunnarsson@ri.se
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