| ACE Working Group | F. Palombini |
| Internet-Draft | Ericsson AB |
| Intended status: Standards Track | L. Seitz |
| Expires: September 3, 2020 | Combitech |
| G. Selander | |
| Ericsson AB | |
| M. Gunnarsson | |
| RISE | |
| March 2, 2020 |
OSCORE profile of the Authentication and Authorization for Constrained Environments Framework
draft-ietf-ace-oscore-profile-09
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.
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This Internet-Draft will expire on September 3, 2020.
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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. Note that this profile uses a symmetric-crypto-based scheme, where the symmetric secret is used as input material for keying material derivation. In order to provide communication security, proof of possession, and server authentication the client and resource server use Object Security for Constrained RESTful Environments (OSCORE) [RFC8613]. Note that the proof of possession is not done by a dedicated protocol element, but rather occurs implicitly, based on knowledge of the security keying material.
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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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.
Concise Data Definition Language (CDDL) [RFC8610] is used in this specification.
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.
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 [RFC8613]. 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.
The RS maintains a collection of OSCORE Security Contexts with associated authorization information for all the clients that it is communicating with. The authorization information is maintained as policy that's used as input to processing requests from those clients.
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]. 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 other protocols (such as TLS or DTLS) can be used as well.
Once the client has retrieved the access token, it generates a nonce N1 and posts both the token and N1 to the RS using the authz-info endpoint and mechanisms specified in section 5.8 of [I-D.ietf-ace-oauth-authz] and Content-Format = application/ace+cbor. Note that, as specified in the ACE framework, the authz-info endpoint is not a protected resource, so there is no cryptographic protection to this request.
If the access token is valid, the RS replies to this request with a 2.01 (Created) response with Content-Format = application/ace+cbor, which contains a nonce N2 in a CBOR map. Moreover, the server concatenates the input salt, N1, and N2 to obtain the Master Salt of the OSCORE Security Context (see section 3 of [RFC8613]). The RS then derives the complete Security Context associated with the received token from it plus the parameters received in the access token from the AS, following section 3.2 of [RFC8613].
After receiving the nonce N2, the client concatenates the input salt, N1 and N2 to obtain the Master Salt of the OSCORE Security Context (see section 3 of [RFC8613]). The client then derives the complete Security Context from the nonces plus the parameters received from the AS.
Finally, the client sends a request protected with OSCORE to the RS. If the request verifies, the server stores the complete Security Context state that is ready for use in protecting messages, and uses it in the response, and in further communications with the client, until token expiration. This Security Context is discarded when a token (whether the same or different) is used to successfully derive a new Security Context for that client.
The use of random nonces during the exchange prevents the reuse of an AEAD nonces/key pair for two different messages. This situation might occur when client and RS derive a new Security Context from an existing (non-expired) access token, as might occur when either party has just rebooted. Instead, by using random nonces as part of the Master Salt, the request to the authz-info endpoint posting the same token results in a different Security Context, by OSCORE construction, since even though the Master Secret, Sender ID and Recipient ID are the same, the Master Salt is different (see Section 3.2.1 of [RFC8613]). 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.
After the whole message exchange has taken place, the client can contact the AS to request an update of its access rights, sending a similar request to the token endpoint that also includes an identifier so that the AS can find the correct OSCORE security material it has previously shared with the Client. This specific identifier, which [I-D.ietf-ace-oauth-authz] encodes as a bstr, is formatted to include two OSCORE identifiers, namely ID context and client ID, that are necessary to determine the correct OSCORE Security Context.
An overview of the profile flow for the OSCORE profile is given in Figure 1.
C RS AS
| [-- Resource Request --->] | |
| | |
| [<---- AS Request ------] | |
| Creation Hints | |
| | |
| ----- POST /token ----------------------------> |
| | |
| <---------------------------- Access Token ----- |
| + Access Information |
| ---- POST /authz-info ---> | |
| (access_token, N1) | |
| | |
| <--- 2.01 Created (N2) --- | |
| | |
/Sec Context /Sec Context |
Derivation/ Derivation/ |
| | |
| ---- OSCORE Request -----> | |
| | |
| <--- OSCORE Response ----- | |
| | |
| ---- OSCORE Request -----> | |
| | |
| <--- OSCORE Response ----- | |
| ... | |
Figure 1: Protocol Overview
The following subsections describe the details of the POST request and response to the token endpoint between client and AS. Section 3.2 of [RFC8613] 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) included in the response from the AS MUST be used as master secret in OSCORE.
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, with payload 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:
{
"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. The req_cnf MUST include a kid field carrying a bstr-wrapped CBOR array object containing the client's identifier (assigned as discussed in Section 3.2) and optionally the context identifier (if assigned as discussed in Section 3.2). The CBOR array is defined in Figure 3, and follows the notation of [RFC8610]. These identifiers, together with other information such as audience, can be used by the AS to determine the shared secret bound to 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 that has 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].
kid_arr = [
clientId,
?IdContext
]
kid = bstr .cbor kid_arr
Figure 3: CDDL Notation of kid for Update of Access Rights
An example of such a request, with payload in CBOR diagnostic notation without the tag and value abbreviations is reported in Figure 4
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"req_aud" : "tempSensor4711",
"scope" : "write",
"req_cnf" : {
"kid" : << ["myclient","contextid1"] >>
}
Figure 4: Example C-to-AS POST /token request for updating rights to an access token bound to a symmetric key.
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 can signal 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. Usually it is assumed that constrained devices will be pre-configured with the necessary profile, so that this kind of profile negotiation can be omitted.
Moreover, the AS MUST send the following data:
Additionally, the AS MAY send the following data, in the same response.
The OSCORE_Security_Context is a CBOR map object, defined in Section 3.2.1. This object is transported in the 'cnf' parameter of the access token response as defined in Section 3.2 of [I-D.ietf-ace-oauth-params], as value of a field named 'osc' registered in Section 9.3 and Section 9.4. The master secret MUST be communicated as the 'ms' field in the 'osc' field in the 'cnf' parameter of the access token response as defined in Section 3.2 of [I-D.ietf-ace-oauth-params]. 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, a salt may be included as the 'salt' parameter of the OSCORE_Security_Context, and the OSCORE version number may be included as the 'version' parameter of the OSCORE_Security_Context.
The same parameters MUST be included as part 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 'osc' field of the 'cnf' claim of this token. The access token MUST be encrypted, since it will be transferred from the client to the RS over an unprotected channel.
The AS MUST also assign an identifier to the RS (serverId), MAY assign an identifier to the client (clientId), and MAY assign an identifier to the context (contextId). These identifiers are then used as Sender ID, Recipient ID and ID Context in the OSCORE context as described in section 3.1 of [RFC8613]. Applications need to consider that these identifiers are sent in the clear and may reveal information about the endpoints, as mentioned in section 12.8 of [RFC8613]. The pair (client identifier, context identifier) MUST be unique in the set of all clients for a single RS. Moreover, clientId, serverId and (when assigned) contextId 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 for the AS to enforce uniqueness of identifiers for each client requesting a particular resource to a RS. If this is not the case, collisions of identifiers may occur at the RS, in which case the RS needs to have a mechanism in place to disambiguate identifiers or mitigate the effect of the collisions.
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 5 shows an example of an AS response, with payload in CBOR diagnostic notation without the tag and value abbreviations. The access token has been truncated for readability.
Header: Created (Code=2.01)
Content-Type: "application/ace+cbor"
Payload:
{
"access_token" : h'a5037674656d7053656e73 ...
(remainder of access token (CWT) omitted for brevity)',
"profile" : "coap_oscore",
"expires_in" : "3600",
"cnf" : {
"osc" : {
"alg" : "AES-CCM-16-64-128",
"clientId" : h'00',
"serverId" : h'01',
"ms" : h'f9af838368e353e78888e1426bd94e6f'
}
}
}
Figure 5: Example AS-to-C Access Token response with OSCORE profile.
Figure 6 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" : {
"osc" : {
"alg" : "AES-CCM-16-64-128",
"clientId" : h'00',
"serverId" : h'01',
"ms" : h'f9af838368e353e78888e1426bd94e6f'
}
}
Figure 6: Example CWT with OSCORE parameters.
The same CWT token as in Figure 6, using the value abbreviations defined in [I-D.ietf-ace-oauth-authz] and [I-D.ietf-ace-cwt-proof-of-possession] and encoded in CBOR is shown in Figure 7.
NOTE TO THE RFC EDITOR: before publishing, it should be checked (and in case fixed) that the values used below (which are not yet registered) are the final values registered in IANA.
A5 # map(5)
03 # unsigned(3)
76 # text(22)
74656D7053656E736F72496E4C6976696E67526F6F6D
# "tempSensorInLivingRoom"
06 # unsigned(6)
1A 5112D728 # unsigned(1360189224)
04 # unsigned(4)
1A 51145DC8 # unsigned(1360289224)
09 # unsigned(9)
78 18 # text(24)
74656D70657261747572655F67206669726D776172655F70
# "temperature_g firmware_p"
08 # unsigned(8)
A1 # map(1)
04 # unsigned(4)
A4 # map(4)
05 # unsigned(5)
0A # unsigned(10)
02 # unsigned(2)
46 # bytes(6)
636C69656E74 # "client"
03 # unsigned(3)
46 # bytes(6)
736572766572 # "server"
01 # unsigned(1)
50 # bytes(16)
F9AF838368E353E78888E1426BD94E6F
# "\xF9\xAF\x83\x83h\xE3S\xE7
\x88\x88\xE1Bk\xD9No"
Figure 7: 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 and the context identifier (if it was set and included in the initial access token response by the AS) in the 'kid' field in the 'cnf' parameter of the token, with the same structure defined in Figure 3. These identifiers need to be included in the response, in order for the RS to identify the previously generated Security Context.
Figure 8 shows an example of such an AS response, with payload in CBOR diagnostic notation without the tag and value abbreviations. The access token has been truncated for readability.
Header: Created (Code=2.01)
Content-Type: "application/ace+cbor"
Payload:
{
"access_token" : h'a5037674656d7053656e73 ...
(remainder of access token (CWT) omitted for brevity)',
"profile" : "coap_oscore",
"expires_in" : "3600"
}
Figure 8: Example AS-to-C Access Token response with OSCORE profile, for update of access rights.
Figure 9 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" : h'43814100'
}
}
Figure 9: Example CWT with OSCORE parameters for update of access rights.
An OSCORE_Security_Context is an object that represents part or all of an OSCORE Security Context, i.e., the local set of information elements necessary to carry out the cryptographic operations in OSCORE (Section 3.1 of [RFC8613]). In particular, the OSCORE_Security_Context object is defined to be serialized and transported between nodes, as specified by this document, but can also be used in this way by other specifications if needed. The OSCORE_Security_Context object can either be encoded as a JSON object or as a CBOR map. 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 9.2), defined for extensibility, and is specified below. All parameters are optional. Table 1 provides a summary of the OSCORE_Security_Context parameters defined in this section.
| name | CBOR label | CBOR type | registry | description |
|---|---|---|---|---|
| version | 0 | int | OSCORE Version | |
| 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 | tstr / int | COSE Algorithm Values (HMAC-based) | OSCORE HKDF value |
| alg | 5 | tstr / int | COSE Algorithm Values (AEAD) | OSCORE AEAD Algorithm value |
| salt | 6 | bstr | OSCORE Master Salt value | |
| contextId | 7 | bstr | OSCORE ID Context value |
An example of JSON OSCORE_Security_Context is given in Figure 10.
"osc" : {
"alg" : "AES-CCM-16-64-128",
"clientId" : b64'AA',
"serverId" : b64'AQ',
"ms" : b64'+a+Dg2jjU+eIiOFCa9lObw'
}
Figure 10: Example JSON OSCORE_Security_Context object
The CDDL grammar describing the CBOR OSCORE_Security_Context object is:
OSCORE_Security_Context = {
? 0 => int, ; version
? 1 => bstr, ; ms
? 2 => bstr, ; clientId
? 3 => bstr, ; serverId
? 4 => tstr / int, ; hkdf
? 5 => tstr / int, ; alg
? 6 => bstr, ; salt
? 7 => bstr, ; contextId
* int / tstr => any
}
The following subsections describe the details of the POST request and response to the authz-info endpoint between client and RS. The client generates a nonce N1, and posts it together with the token that includes the materials (e.g., OSCORE parameters) received from the AS to the RS. The RS then generates a nonce N2, and use Section 3.2 of [RFC8613] to derive a security context based on a shared master secret and the two nonces, established between client and server. The nonces are encoded as CBOR bstr if CBOR is used, and as Base64 string if JSON is used. This security context is used to protect all future communication between client and RS using OSCORE, as long as the access token is valid.
Note that the RS and client authenticates themselves by generating the shared OSCORE Security Context using the pop-key as master secret. An attacker posting a valid token to the RS will not be able to generate a valid OSCORE context and thus not be able to prove possession of the pop-key.
The client MUST generate a nonce value very unlikely to have been previously used with the same input keying material. This profile RECOMMENDS to use a 64-bit long random number as nonce's value. The client MUST store the nonce N1 as long as the response from the RS is not received and the access token related to it is still valid. 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 and N1 to the RS.
Note that the use of the payload and the Content-Format is different from what described in section 5.8.1 of [I-D.ietf-ace-oauth-authz], which only transports the token without any CBOR wrapping. In this profile, the client MUST wrap the token and N1 in a CBOR map. The client MUST use the Content-Format "application/ace+cbor" defined in section 8.14 of [I-D.ietf-ace-oauth-authz]. The client MUST include the access token using the correct CBOR label (e.g., "cwt" for CWT, "jwt" for JWT) and N1 using the 'cnonce' parameter defined in section 5.1.2 of [I-D.ietf-ace-oauth-authz].
The authz-info endpoint is not protected, nor are the responses from this resource.
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 in order to complete a request, since an RS may delete a stored access token at any time, for example due to all storage space being consumed. This situation is detected by the client when it receives an AS Request Creation Hints response.
Figure 11 shows an example of the request sent from the client to the RS, with payload in CBOR diagnostic notation without the tag and value abbreviations. The access token has been truncated for readability.
Header: POST (Code=0.02)
Uri-Host: "rs.example.com"
Uri-Path: "authz-info"
Content-Format: "application/ace+cbor"
Payload:
{
"access_token": h'a5037674656d7053656e73 ...
(remainder of access token (CWT) omitted for brevity)',
"cnonce": h'018a278f7faab55a'
}
Figure 11: Example C-to-RS POST /authz-info request using CWT
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), or if any of the expected parameters in the 'osc' is missing (e.g., any of the mandatory parameters from the AS), or if any parameters received in the 'osc' is unrecognized, the RS must respond with an error response code equivalent to the CoAP code 4.00 (Bad Request). In the latter two cases, 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 N2 very unlikely to have been previously used with the same input keying material, and send it within the 2.01 (Created) response. The payload of the 2.01 (Created) response MUST be a CBOR map containing the 'cnonce' parameter defined in section 5.1.2 of [I-D.ietf-ace-oauth-authz], set to N2. This profile RECOMMENDS to use a 64-bit long random number as nonce's value. Moreover, if the 'osc' field in the token did not contain a 'clientId' parameter, the RS MUST generate an identifier, unique in the set of all its existing client identifiers, and send it in a 'clientId' parameter in the CBOR map as a CBOR bstr. The RS MAY generate and send a 'ClientId' identifier even though the 'osc' field contained such a parameter, in order to guarantee the uniqueness of the client identifier. The RS MUST use the Content-Format "application/ace+cbor" defined in section 8.14 of [I-D.ietf-ace-oauth-authz].
Figure 12 shows an example of the response sent from the RS to the client, with payload in CBOR diagnostic notation without the tag and value abbreviations.
Header: Created (Code=2.01)
Content-Format: "application/ace+cbor"
Payload:
{
"cnonce": h'25a8991cd700ac01'
}
Figure 12: Example RS-to-C 2.01 (Created) response
As specified in section 5.8.1 of [I-D.ietf-ace-oauth-authz], when receiving an updated access token with updated authorization information from the client (see 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.
Once receiving the 2.01 (Created) response from the RS, following the POST request to authz-info endpoint, the client MUST extract the CBOR bstr nonce N2 from the 'cnonce' parameter and the client identifier from the 'clientId' in the CBOR map in the payload of the response. Then, the client MUST set the Master Salt of the Security Context created to communicate with the RS to the concatenation of salt, N1, and N2, in this order: Master Salt = salt | N1 | N2, where | denotes byte string concatenation, where salt was received from the AS in Section 3.2, and where N1 and N2 are the two nonces encoded as CBOR bstr. The client MUST set the Master Secret and Recipient ID from the parameters received from the AS in Section 3.2. The client MUST set the AEAD Algorithm, ID Context, HKDF, and OSCORE Version 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 sections 3.2 and 5.4 of [RFC8613]. The client MUST set the Sender ID from the 'clientId in the 2.01 (Created) response, if present; otherwise, the client MUST set the Sender ID from the parameters received from the AS in Section 3.2. After that, the client MUST derive the complete Security Context following section 3.2.1 of [RFC8613]. 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.
If any of the expected parameters is missing (e.g., any of the mandatory parameters from the AS, or the 'clientId', either received from the AS or in the 2.01 (Created) response from the RS), the client MUST stop the exchange, and MUST NOT derive the Security Context. The client MAY restart the exchange, to get the correct security material.
The client then uses this Security Context to send requests to RS using OSCORE.
After sending the 2.01 (Created) response, the RS MUST set the Master Salt of the Security Context created to communicate with the client to the concatenation of salt, N1, and N2, in this order: Master Salt = salt | N1 | N2, where | denotes byte string concatenation, where salt was received from the AS in Section 4.2, and where N1 and N2 are the two nonces encoded as CBOR bstr. The RS MUST set the Master Secret, Sender ID and Recipient ID from the parameters, received from the AS and forwarded by the client in the access token in Section 4.1 after validation of the token as specified in Section 4.2. The RS MUST set the AEAD Algorithm, ID Context, HKDF, and OSCORE Version from the parameters received from the AS and forwarded by the client in the access token in Section 4.1 after validation of the token as specified in Section 4.2, if present. In case these parameters are omitted, the default values are used as described in sections 3.2 and 5.4 of [RFC8613]. After that, the RS MUST derive the complete Security Context following section 3.2.1 of [RFC8613], and MUST associate this Security Context with the authorization information from the access token.
The RS then uses this Security Context to verify requests and send responses to C using OSCORE. If OSCORE verification fails, error responses are used, as specified in section 8 of [RFC8613]. 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 to requests received that correspond to a Security Context with an expired token.
If the exchange was an update of access rights, i.e., a new Security Context was derived from a client that already had a Security Context in place, the is RECOMMENDED to delete the old Security Context after OSCORE verification and verification of access rights succeed. The RS MUST delete the Security Context if it deletes the access token associated to it.
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 the RS processes it according to [RFC8613]. If OSCORE verification succeeds, and the target resource requires authorization, the RS retrieves the authorization information using 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 a valid token associated with that Security Context, but the Security Context has not been used before, as the proof-of-possession in this profile is performed by both parties verifying that they have established the same Security Context.
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 ([RFC8613]) 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 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].
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:
The RS MUST discard the current security context associated with a client when:
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 [RFC8613] 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, it is REQUIRED that nonces are not reused with the same input keying material even in case of re-boots. This document RECOMMENDS the use of 64 bit random nonces. Considering the birthday paradox, the average collision for each nonce will happen after 2^32 messages, which is considerably more token provisionings than expected for intended applications. If applications use something else, such as a counter, they need to guarantee that reboot and loss of state on either node does not provoke re-use. If that is not guaranteed, nodes are susceptible to re-use of AEAD (nonces, keys) pairs, especially since an on-path attacker can cause the client to use an arbitrary nonce for Security Context establishment by replaying client-to-server messages.
This profile 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 indicating different or disjoint permissions from each other 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.
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 [RFC8613] 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 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.
Although encrypted, the token is sent in the clear to the authz-info endpoint, so if a client uses the same single token from multiple locations with multiple Resource Servers, it can risk being tracked by the token's value.
The nonces exchanged in the request and response to the authz-info endpoint are also sent in the clear, so using random nonces is best for privacy (as opposed to, e.g., a counter, that might leak some information about the client).
The AS is the party tasked of assigning the identifiers used in OSCORE, which are privacy sensitive (see Section 12.8 of [RFC8613]), and which could reveal information about the client, or may be used for correlating requests from one client.
Note that some information might still leak after OSCORE is established, due to observable message sizes, the source, and the destination addresses.
Note to RFC Editor: Please replace all occurrences of "[[this specification]]" with the RFC number of this specification and delete this paragraph.
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]:
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 addition 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:
This registry will be initially populated by the values in Table 1. The specification column for all of these entries will be this document and [RFC8613].
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]:
The following registration is done for the JWT Confirmation Methods Registry following the procedure specified in section 6.2.1 of [RFC7800]:
The IANA registry established in this document is defined to use the Expert Review registration policy. 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:
| [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)", Internet-Draft draft-ietf-ace-cwt-proof-of-possession-11, October 2019. |
| [RFC4949] | Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007. |
| [RFC6749] | Hardt, D., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012. |
| [RFC7231] | Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014. |
| [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. |
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].
The authors wish to thank Jim Schaad and Marco Tiloca for the input on this memo. Special thanks to the responsible area director Ben Kaduk for his extensive review and contributed text.