3.1. OAuth 2.0

The OAuth 2.0 authorization framework enables a client to obtain scoped access to a resource with the permission of a resource owner. Authorization information, or references to it, is passed between the nodes using access tokens. These access tokens are issued to clients by an authorization server with the approval of the resource owner. The client uses the access token to access the protected resources hosted by the resource server.¶

A number of OAuth 2.0 terms are used within this specification:¶

Access Tokens:

Access tokens are credentials needed to access protected resources. An access token is a data structure representing authorization permissions issued by the AS to the client. Access tokens are generated by the AS and consumed by the RS. The access token content is opaque to the client.¶

Access tokens can have different formats and various methods of utilization (e.g., cryptographic properties) based on the security requirements of the given deployment.¶

Introspection:

Introspection is a method for a resource server, or potentially a client, to query the authorization server for the active state and content of a received access token. This is particularly useful in those cases where the authorization decisions are very dynamic and/or where the received access token itself is an opaque reference, rather than a self-contained token. More information about introspection in OAuth 2.0 can be found in [RFC7662].¶

Refresh Tokens:

Refresh tokens are credentials used to obtain access tokens. Refresh tokens are issued to the client by the authorization server and are used to obtain a new access token when the current access token expires or to obtain additional access tokens with identical or narrower scope (such access tokens may have a shorter lifetime and fewer permissions than authorized by the resource owner). Issuing a refresh token is optional at the discretion of the authorization server. If the authorization server issues a refresh token, it is included when issuing an access token (i.e., step (B) in Figure 1).¶

A refresh token in OAuth 2.0 is a string representing the authorization granted to the client by the resource owner. The string is usually opaque to the client. The token denotes an identifier used to retrieve the authorization information. Unlike access tokens, refresh tokens are intended for use only with authorization servers and are never sent to resource servers. In this framework, refresh tokens are encoded in binary instead of strings, if used.¶

Proof-of-Possession Tokens:

A token may be bound to a cryptographic key, which is then used to bind the token to a request authorized by the token. Such tokens are called proof-of-possession tokens (or PoP tokens).¶

The proof-of-possession security concept used here assumes that the AS acts as a trusted third party that binds keys to tokens. In the case of access tokens, these so-called PoP keys are then used by the client to demonstrate the possession of the secret to the RS when accessing the resource. The RS, when receiving an access token, needs to verify that the key used by the client matches the one bound to the access token. When this specification uses the term "access token", it is assumed to be a PoP access token unless specifically stated otherwise.¶

The key bound to the token (the PoP key) may use either symmetric or asymmetric cryptography. The appropriate choice of the kind of cryptography depends on the constraints of the IoT devices as well as on the security requirements of the use case.¶

Symmetric PoP key:

The AS generates a random, symmetric PoP key. The key is either stored to be returned on introspection calls or included in the token. Either the whole token or only the key MUST be encrypted in the latter case. The PoP key is also returned to client together with the token.¶

Asymmetric PoP key:

An asymmetric key pair is generated by the client and the public key is sent to the AS (if it does not already have knowledge of the client's public key). Information about the public key, which is the PoP key in this case, is either stored to be returned on introspection calls or included inside the token and sent back to the client. The resource server consuming the token can identify the public key from the information in the token, which allows the client to use the corresponding private key for the proof of possession.¶

The token is either a simple reference or a structured information object (e.g., CWT [RFC8392]) protected by a cryptographic wrapper (e.g., COSE [RFC8152]). The choice of PoP key does not necessarily imply a specific credential type for the integrity protection of the token.¶

Scopes and Permissions:

In OAuth 2.0, the client specifies the type of permissions it is seeking to obtain (via the scope parameter) in the access token request. In turn, the AS may use the scope response parameter to inform the client of the scope of the access token issued. As the client could be a constrained device as well, this specification defines the use of CBOR encoding (see Section 5) for such requests and responses.¶

The values of the scope parameter in OAuth 2.0 are expressed as a list of space-delimited, case-sensitive strings with a semantic that is well known to the AS and the RS. More details about the concept of scopes are found under Section 3.3 of [RFC6749].¶

Claims:

Information carried in the access token or returned from introspection, called claims, is in the form of name-value pairs. An access token may, for example, include a claim identifying the AS that issued the token (via the "iss" claim) and what audience the access token is intended for (via the "aud" claim). The audience of an access token can be a specific resource, one resource, or many resource servers. The resource owner policies influence what claims are put into the access token by the authorization server.¶

While the structure and encoding of the access token varies throughout deployments, a standardized format has been defined with the JSON Web Token (JWT) [RFC7519], where claims are encoded as a JSON object. In [RFC8392], the CBOR Web Token (CWT) has been defined as an equivalent format using CBOR encoding.¶

Token and Introspection Endpoints:

The AS hosts the token endpoint that allows a client to request access tokens. The client makes a POST request to the token endpoint on the AS and receives the access token in the response (if the request was successful).¶

In some deployments, a token introspection endpoint is provided by the AS, which can be used by the RS and potentially the client, if they need to request additional information regarding a received access token. The requesting entity makes a POST request to the introspection endpoint on the AS and receives information about the access token in the response. (See "Introspection" above.)¶

3.2. CoAP

CoAP is an application-layer protocol similar to HTTP but specifically designed for constrained environments. CoAP typically uses datagram-oriented transport, such as UDP, where reordering and loss of packets can occur. A security solution needs to take the latter aspects into account.¶

While HTTP uses headers and query strings to convey additional information about a request, CoAP encodes such information into header parameters called 'options'.¶

CoAP supports application-layer fragmentation of the CoAP payloads through block-wise transfers [RFC7959]. However, block-wise transfer does not increase the size limits of CoAP options; therefore, data encoded in options has to be kept small.¶

Transport layer security for CoAP can be provided by DTLS or TLS [RFC6347] [RFC8446] [RFC9147]. CoAP defines a number of proxy operations that require transport layer security to be terminated at the proxy. One approach for protecting CoAP communication end-to-end through proxies, and also to support security for CoAP over a different transport in a uniform way, is to provide security at the application layer using an object-based security mechanism, such as COSE [RFC8152].¶

One application of COSE is OSCORE [RFC8613], which provides end-to-end confidentiality, integrity and replay protection, and a secure binding between CoAP request and response messages. In OSCORE, the CoAP messages are wrapped in COSE objects and sent using CoAP.¶

In this framework, the use of CoAP as replacement for HTTP is RECOMMENDED for use in constrained environments. For communication security, this framework does not make an explicit protocol recommendation, since the choice depends on the requirements of the specific application. DTLS [RFC6347] [RFC9147] and OSCORE [RFC8613] are mentioned as examples; other protocols fulfilling the requirements from Section 6.5 are also applicable.¶

5.1. Discovering Authorization Servers

The C must discover the AS in charge of the RS to determine where to request the access token. To do so, the C 1) must find out the AS URI to which the token request message must be sent and 2) MUST validate that the AS with this URI is authorized to provide access tokens for this RS.¶

In order to determine the AS URI, the 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 C (see Section 5.2). How the C validates the AS authorization is not in scope for this document. The C may, for example, ask its owner if this AS is authorized for this RS. The C may also use a mechanism that addresses both problems at once (e.g., by querying a dedicated secure service provided by the client owner) .¶

5.2. Unauthorized Resource Request Message

An Unauthorized Resource Request message is a request for any resource hosted by the RS for which the client does not have authorization granted. The RSs MUST treat any request for a protected resource as an Unauthorized Resource Request message when any of the following hold:¶

• The request has been received on an unsecured channel.¶
• The RS has no valid access token for the sender of the request regarding the requested action on that resource.¶
• The RS has a valid access token for the sender of the request, but that token does not authorize the requested action on the requested resource.¶

Note: These conditions ensure that the RS can handle requests autonomously once access was granted and a secure channel has been established between the C and RS. The authz-info endpoint, as part of the process for authorizing to protected resources, is not itself a protected resource and MUST NOT be protected as specified above (cf. Section 5.10.1).¶

Unauthorized Resource Request messages MUST be denied with an "unauthorized_client" error response. In this response, the resource server SHOULD provide proper "AS Request Creation Hints" to enable the client to request an access token from the RS's AS, as described in Section 5.3.¶

The handling of all client requests (including unauthorized ones) by the RS is described in Section 5.10.2.¶

5.3. AS Request Creation Hints

The "AS Request Creation Hints" message is sent by an RS as a response to an Unauthorized Resource Request message (see Section 5.2) to help the sender of the Unauthorized Resource Request message acquire a valid access token. The "AS Request Creation Hints" message is a CBOR or JSON map, with an OPTIONAL element "AS" specifying an absolute URI (see Section 4.3 of [RFC3986]) that identifies the appropriate AS for the RS.¶

The message can also contain the following OPTIONAL parameters:¶

• An "audience" element contains an identifier the client should request at the AS, as suggested by the RS. With this parameter, when included in the access token request to the AS, the AS is able to restrict the use of the access token to specific RSs. See Section 6.9 for a discussion of this parameter.¶
• A "kid" (key identifier) element contains the key identifier of a key used in an existing security association between the client and the RS. The RS expects the client to request an access token bound to this key in order to avoid having to reestablish the security association.¶
• A "cnonce" element contains a client-nonce. See Section 5.3.1.¶
• A "scope" element contains the suggested scope that the client should request towards the AS.¶

Table 1 summarizes the parameters that may be part of the "AS Request Creation Hints".¶

Note that the schema part of the AS parameter may need to be adapted to the security protocol that is used between the client and the AS. Thus, the example AS value "coap://as.example.com/token" might need to be transformed to "coaps://as.example.com/token". It is assumed that the client can determine the correct schema part on its own depending on the way it communicates with the AS.¶

Figure 2 shows an example for an "AS Request Creation Hints" message payload using CBOR [RFC8949] diagnostic notation, using the parameter names instead of the CBOR keys for better human readability.¶

    4.01 Unauthorized
    Content-Format: application/ace+cbor
    Payload :
    {
     "AS" : "coaps://as.example.com/token",
     "audience" : "coaps://rs.example.com"
     "scope" : "rTempC",
     "cnonce" : h'e0a156bb3f'
    }

In the example above, the response parameter "AS" points the receiver of this message to the URI "coaps://as.example.com/token" to request access tokens. The RS sending this response uses an internal clock that is not synchronized with the clock of the AS. Therefore, it cannot reliably verify the expiration time of access tokens it receives. Nevertheless, to ensure a certain level of access token freshness, the RS has included a cnonce parameter (see Section 5.3.1) in the response. (The hex sequence of the cnonce parameter is encoded in CBOR-based notation in this example.)¶

Figure 3 illustrates the mandatory use of binary encoding of the message payload shown in Figure 2.¶

a4                                   # map(4)
   01                                # unsigned(1) (=AS)
   78 1c                             # text(28)
      636f6170733a2f2f61732e657861
      6d706c652e636f6d2f746f6b656e   # "coaps://as.example.com/token"
   05                                # unsigned(5) (=audience)
   76                                # text(22)
      636f6170733a2f2f72732e657861
      6d706c652e636f6d               # "coaps://rs.example.com"
   09                                # unsigned(9) (=scope)
   66                                # text(6)
      7254656d7043                   # "rTempC"
   18 27                             # unsigned(39) (=cnonce)
   45                                # bytes(5)
      e0a156bb3f                     #

5.4. Authorization Grants

To request an access token, the client obtains authorization from the resource owner or uses its client credentials as a grant. The authorization is expressed in the form of an authorization grant.¶

The OAuth framework [RFC6749] defines four grant types. The grant types can be split up into two groups: those granted on behalf of the resource owner (password, authorization code, implicit) and those for the client (client credentials). Further grant types have been added later, such as an assertion-based authorization grant defined in [RFC7521].¶

The grant type is selected depending on the use case. In cases where the client acts on behalf of the resource owner, the authorization code grant is recommended. If the client acts on behalf of the resource owner but does not have any display or has very limited interaction possibilities, it is recommended to use the device code grant defined in [RFC8628]. In cases where the client acts autonomously, the client credentials grant is recommended.¶

For details on the different grant types, see Section 1.3 of [RFC6749]. The OAuth 2.0 framework provides an extension mechanism for defining additional grant types, so profiles of this framework MAY define additional grant types, if needed.¶

5.5. Client Credentials

Authentication of the client is mandatory independent of the grant type when requesting an access token from the token endpoint. In the case of the client credentials grant type, the authentication and grant coincide.¶

Client registration and provisioning of client credentials to the client is out of scope for this specification.¶

The OAuth framework defines two client credential types in Section 2.3.1 of [RFC6749]: client id and client secret. [OAUTH-RPCC] adds raw public key and pre-shared key to the client credentials types. Profiles of this framework MAY extend with an additional client credentials type using client certificates.¶

5.6. AS Authentication

The client credentials grant does not, by default, authenticate the AS that the client connects to. In classic OAuth, the AS is authenticated with a TLS server certificate.¶

Profiles of this framework MUST specify how clients authenticate the AS and how communication security is implemented. By default, server side TLS certificates, as defined by OAuth 2.0, are required.¶

5.7. The Authorization Endpoint

The OAuth 2.0 authorization endpoint is used to interact with the resource owner and obtain an authorization grant in certain grant flows. The primary use case for the ACE-OAuth framework is for machine-to-machine interactions that do not involve the resource owner in the authorization flow; therefore, this endpoint is out of scope here. Future profiles may define constrained adaptation mechanisms for this endpoint as well. Nonconstrained clients interacting with constrained resource servers can use the specification in Section 3.1 of [RFC6749] and the attack countermeasures suggested in Section 4.2 of [RFC6819].¶

5.8. The Token Endpoint

In standard OAuth 2.0, the AS provides the token endpoint for submitting access token requests. This framework extends the functionality of the token endpoint, giving the AS the possibility to help the client and RS establish shared keys or exchange their public keys. Furthermore, this framework defines encodings using CBOR as a substitute for JSON.¶

The endpoint may also be exposed over HTTPS, as in classical OAuth or even other transports. A profile MUST define the details of the mapping between the fields described below and these transports. If HTTPS with JSON is used, the semantics of Sections 4.1.3 and 4.1.4 of the OAuth 2.0 specification [RFC6749] MUST be followed (with additions as described below). If CBOR is used as the payload format, the semantics described in this section MUST be followed.¶

For the AS to be able to issue a token, the client MUST be authenticated and present a valid grant for the scopes requested. Profiles of this framework MUST specify how the AS authenticates the client and how the communication between the client and AS is protected, fulfilling the requirements specified in Section 5.¶

The default name of this endpoint in a url-path SHOULD be '/token'. However, implementations are not required to use this name and can define their own instead.¶

The figures of this section use CBOR diagnostic notation without the integer abbreviations for the parameters or their values for illustrative purposes. Note that implementations MUST use the integer abbreviations and the binary CBOR encoding if the CBOR encoding is used.¶

Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
  "client_id" : "myclient",
  "audience" : "tempSensor4711"
}

Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
OSCORE: 0x09, 0x05, 0x44, 0x6C
  (h=0, k=1, n=001, partialIV= 0x05, kid=[0x44, 0x6C])
Content-Format: "application/oscore"
Payload:
  0x44025d1 ... (full payload omitted for brevity) ... 68b3825e

Decrypted payload:
{
  "client_id" : "myclient",
  "req_cnf" : {
    "COSE_Key" : {
      "kty" : "EC",
      "kid" : h'11',
      "crv" : "P-256",
      "x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8',
      "y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4'
    }
  }
}

Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
  "client_id" : "myclient",
  "audience" : "valve424",
  "scope" : "read",
  "req_cnf" : {
    "kid" : b64'6kg0dXJM13U'
  }
}

Header: Created (Code=2.01)
Content-Format: "application/ace+cbor"
Payload:
{
  "access_token" : b64'SlAV32hkKG ...
   (remainder of CWT omitted for brevity;
   CWT contains COSE_Key in the "cnf" claim)',
  "ace_profile" : "coap_dtls",
  "expires_in" : "3600",
  "cnf" : {
    "COSE_Key" : {
      "kty" : "Symmetric",
      "kid" : b64'39Gqlw',
      "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
    }
  }
}

5.9. The Introspection Endpoint

Token introspection [RFC7662] MAY be implemented by the AS and the RS. When implemented, it MAY be used by the RS and to query the AS for metadata about a given token, e.g., validity or scope. Analogous to the protocol defined in [RFC7662] for HTTP and JSON, this section defines adaptations to more constrained environments using CBOR and leaving the choice of the application protocol to the profile.¶

Communication between the requesting entity and the introspection endpoint at the AS MUST be integrity protected and encrypted. The communication security protocol MUST also provide a binding between requests and responses. Furthermore, the two interacting parties MUST perform mutual authentication. Finally, the AS SHOULD verify that the requesting entity has the right to access introspection information about the provided token. Profiles of this framework that support introspection MUST specify how authentication and communication security between the requesting entity and the AS is implemented.¶

The default name of this endpoint in a url-path SHOULD be '/introspect'. However, implementations are not required to use this name and can define their own instead.¶

The figures of this section use the CBOR diagnostic notation without the integer abbreviations for the parameters and their values for better readability.¶

Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "introspect"
OSCORE: 0x09, 0x05, 0x25
Content-Format: "application/oscore"
Payload:
... COSE content ...

{
  "token" : b64'7gj0dXJQ43U',
  "token_type_hint" : "PoP"
}

Header: Created (Code=2.01)
Content-Format: "application/ace+cbor"
Payload:
{
  "active" : true,
  "scope" : "read",
  "ace_profile" : "coap_dtls",
  "cnf" : {
    "COSE_Key" : {
      "kty" : "Symmetric",
      "kid" : b64'39Gqlw',
      "k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
    }
  }
}

5.10. The Access Token

In this framework, the use of CBOR Web Token (CWT) as specified in [RFC8392] is RECOMMENDED.¶

In order to facilitate offline processing of access tokens, this document uses the "cnf" claim from [RFC8747] and the "scope" claim from [RFC8693] for JWT- and CWT-encoded tokens. In addition to string encoding specified for the "scope" claim, a binary encoding MAY be used. The syntax of such an encoding is explicitly not specified here and left to profiles or applications, specifically note that a binary encoded scope does not necessarily use the space character '0x20' to delimit scope-tokens.¶

If the AS needs to convey a hint to the RS about which profile it should use to communicate with the client, the AS MAY include an "ace_profile" claim in the access token, with the same syntax and semantics as defined in Section 5.8.4.3.¶

If the client submitted a client-nonce parameter in the access token request (Section 5.8.4.4), the AS MUST include the value of this parameter in the "cnonce" claim specified here. The "cnonce" claim uses binary encoding.¶

6.1. Protecting Tokens

A large range of threats can be mitigated by protecting the contents of the access token by using a digital signature or a keyed message digest, e.g., a Message Authentication Code (MAC) or an Authenticated Encryption with Associated Data (AEAD) algorithm. Consequently, the token integrity protection MUST be applied to prevent the token from being modified, particularly since it contains a reference to the symmetric key or the asymmetric key used for proof of possession. If the access token contains the symmetric key, this symmetric key MUST be encrypted by the authorization server so that only the resource server can decrypt it. Note that using an AEAD algorithm is preferable over using a MAC unless the token needs to be publicly readable.¶

If the token is intended for multiple recipients (i.e., an audience that is a group), integrity protection of the token with a symmetric key, shared between the AS and the recipients, is not sufficient, since any of the recipients could modify the token undetected by the other recipients. Therefore, a token with a multirecipient audience MUST be protected with an asymmetric signature.¶

It is important for the authorization server to include the identity of the intended recipient (the audience), typically a single resource server (or a list of resource servers), in the token. The same shared secret MUST NOT be used as a proof-of-possession key with multiple resource servers, since the benefit from using the proof-of-possession concept is then significantly reduced.¶

If clients are capable of doing so, they should frequently request fresh access tokens, as this allows the AS to keep the lifetime of the tokens short. This allows the AS to use shorter proof-of-possession key sizes, which translate to a performance benefit for the client and for the resource server. Shorter keys also lead to shorter messages (particularly with asymmetric keying material).¶

When authorization servers bind symmetric keys to access tokens, they SHOULD scope these access tokens to a specific permission.¶

In certain situations, it may be necessary to revoke an access token that is still valid. Client-initiated revocation is specified in [RFC7009] for OAuth 2.0. Other revocation mechanisms are currently not specified, as the underlying assumption in OAuth is that access tokens are issued with a relatively short lifetime. This may not hold true for disconnected constrained devices needing access tokens with relatively long lifetimes and would therefore necessitate further standardization work that is out of scope for this document.¶

6.2. Communication Security

Communication with the authorization server MUST use confidentiality protection. This step is extremely important since the client or the RS may obtain the proof-of-possession key from the authorization server for use with a specific access token. Not using confidentiality protection exposes this secret (and the access token) to an eavesdropper, thereby completely negating proof-of-possession security. The requirements for communication security of profiles are specified in Section 5.¶

Additional protection for the access token can be applied by encrypting it, for example, encryption of CWTs is specified in Section 5.1 of [RFC8392]. Such additional protection can be necessary if the token is later transferred over an insecure connection (e.g., when it is sent to the authz-info endpoint).¶

Care must be taken by developers to prevent leakage of the PoP credentials (i.e., the private key or the symmetric key). An adversary in possession of the PoP credentials bound to the access token will be able to impersonate the client. Be aware that this is a real risk with many constrained environments, since adversaries may get physical access to the devices and can therefore use physical extraction techniques to gain access to memory contents. This risk can be mitigated to some extent by making sure that keys are refreshed frequently, by using software isolation techniques, and by using hardware security.¶

6.3. Long-Term Credentials

Both the clients and RSs have long-term credentials that are used to secure communications and authenticate to the AS. These credentials need to be protected against unauthorized access. In constrained devices deployed in publicly accessible places, such protection can be difficult to achieve without specialized hardware (e.g., secure key storage memory).¶

If credentials are lost or compromised, the operator of the affected devices needs to have procedures to invalidate any access these credentials give and needs to revoke tokens linked to such credentials. The loss of a credential linked to a specific device MUST NOT lead to a compromise of other credentials not linked to that device; therefore, secret keys used for authentication MUST NOT be shared between more than two parties.¶

Operators of the clients or RSs SHOULD have procedures in place to replace credentials that are suspected to have been compromised or that have been lost.¶

Operators also SHOULD have procedures for decommissioning devices that include securely erasing credentials and other security-critical material in the devices being decommissioned.¶

6.4. Unprotected AS Request Creation Hints

Initially, no secure channel exists to protect the communication between the C and RS. Thus, the C cannot determine if the "AS Request Creation Hints" contained in an unprotected response from the RS to an unauthorized request (see Section 5.3) are authentic. Therefore, the C MUST determine if an AS is authorized to provide access tokens for a certain RS. How this determination is implemented is out of scope for this document and left to the applications.¶

6.5. Minimal Security Requirements for Communication

This section summarizes the minimal requirements for the communication security of the different protocol interactions.¶

C-AS

All communication between the client and the authorization server MUST be encrypted and integrity and replay protected. Furthermore, responses from the AS to the client MUST be bound to the client's request to avoid attacks where the attacker swaps the intended response for an older one valid for a previous request. This requires that the client and the authorization server have previously exchanged either a shared secret or their public keys in order to negotiate a secure communication. Furthermore, the client MUST be able to determine whether an AS has the authority to issue access tokens for a certain RS. This can, for example, be done through preconfigured lists or through an online lookup mechanism that in turn also must be secured.¶

RS-AS

The communication between the resource server and the authorization server via the introspection endpoint MUST be encrypted and integrity and replay protected. Furthermore, responses from the AS to the RS MUST be bound to the RS's request. This requires that the RS and the authorization server have previously exchanged either a shared secret or their public keys in order to negotiate a secure communication. Furthermore, the RS MUST be able to determine whether an AS has the authority to issue access tokens itself. This is usually configured out of band but could also be performed through an online lookup mechanism, provided that it is also secured in the same way.¶

C-RS

The initial communication between the client and the resource server cannot be secured in general, since the RS is not in possession of on access token for that client, which would carry the necessary parameters. If both parties support DTLS without client authentication, it is RECOMMENDED to use this mechanism for protecting the initial communication. After the client has successfully transmitted the access token to the RS, a secure communication protocol MUST be established between the client and RS for the actual resource request. This protocol MUST provide confidentiality, integrity, and replay protection, as well as a binding between requests and responses. This requires that the client learned either the RS's public key or received a symmetric proof-of-possession key bound to the access token from the AS. The RS must have learned either the client's public key, a shared symmetric key from the claims in the token, or an introspection request. Since ACE does not provide profile negotiation between the C and RS, the client MUST have learned what profile the RS supports (e.g., from the AS or preconfigured) and initiated the communication accordingly.¶

6.6. Token Freshness and Expiration

An RS that is offline faces the problem of clock drift. Since it cannot synchronize its clock with the AS, it may be tricked into accepting old access tokens that are no longer valid or have been compromised. In order to prevent this, an RS may use the nonce-based mechanism (cnonce) defined in Section 5.3 to ensure freshness of an Access Token subsequently presented to this RS.¶

Another problem with clock drift is that evaluating the standard token expiration claim "exp" can give unpredictable results.¶

Acceptable ranges of clock drift are highly dependent on the concrete application. Important factors are how long access tokens are valid and how critical timely expiration of the access token is.¶

The expiration mechanism implemented by the "exi" claim, based on the first time the RS sees the token, was defined to provide a more predictable alternative. The "exi" approach has some drawbacks that need to be considered:¶

• A malicious client may hold back tokens with the "exi" claim in order to prolong their lifespan.¶
• If an RS loses state (e.g., due to an unscheduled reboot), it may lose the current values of counters tracking the "exi" claims of tokens it is storing.¶

The first drawback is inherent to the deployment scenario and the "exi" solution. It can therefore not be mitigated without requiring the RS be online at times. The second drawback can be mitigated by regularly storing the value of "exi" counters to persistent memory.¶

6.7. Combining Profiles

There may be use cases where different transport and security protocols are allowed for the different interactions, and, if that is not explicitly covered by an existing profile, it corresponds to combining profiles into a new one. For example, a new profile could specify that a previously defined MQTT-TLS profile is used between the client and the RS in combination with a previously defined CoAP-DTLS profile for interactions between the client and the AS. The new profile that combines existing profiles MUST specify how the existing profiles' security properties are achieved. Therefore, any profile MUST clearly specify its security requirements and MUST document if its security depends on the combination of various protocol interactions.¶

6.8. Unprotected Information

Communication with the authz-info endpoint, as well as the various error responses defined in this framework, potentially includes sending information over an unprotected channel. These messages may leak information to an adversary or may be manipulated by active attackers to induce incorrect behavior. For example, error responses for requests to the authorization information endpoint can reveal information about an otherwise opaque access token to an adversary who has intercepted this token.¶

As far as error messages are concerned, this framework is written under the assumption that, in general, the benefits of detailed error messages outweigh the risk due to information leakage. For particular use cases where this assessment does not apply, detailed error messages can be replaced by more generic ones.¶

In some scenarios, it may be possible to protect the communication with the authz-info endpoint (e.g., through DTLS with only server-side authentication). In cases where this is not possible, it is RECOMMENDED to use encrypted CWTs or tokens that are opaque references and need to be subjected to introspection by the RS.¶

If the initial Unauthorized Resource Request message (see Section 5.2) is used, the client MUST make sure that it is not sending sensitive content in this request. While GET and DELETE requests only reveal the target URI of the resource, POST and PUT requests would reveal the whole payload of the intended operation.¶

Since the client is not authenticated at the point when it is submitting an access token to the authz-info endpoint, attackers may be pretending to be a client and trying to trick an RS to use an obsolete profile that in turn specifies a vulnerable security mechanism via the authz-info endpoint. Such an attack would require a valid access token containing an "ace_profile" claim requesting the use of said obsolete profile. Resource owners should update the configuration of their RSs to prevent them from using such obsolete profiles.¶

6.9. Identifying Audiences

The audience claim, as defined in [RFC7519], and the equivalent "audience" parameter from [RFC8693] are intentionally vague on how to match the audience value to a specific RS. This is intended to allow application-specific semantics to be used. This section attempts to give some general guidance for the use of audiences in constrained environments.¶

URLs are not a good way of identifying mobile devices that can switch networks and thus be associated with new URLs. If the audience represents a single RS and asymmetric keys are used, the RS can be uniquely identified by a hash of its public key. If this approach is used, it is RECOMMENDED to apply the procedure from Section 3 of [RFC6920].¶

If the audience addresses a group of resource servers, the mapping of a group identifier to an individual RS has to be provisioned to each RS before the group-audience is usable. Managing dynamic groups could be an issue if any RS is not always reachable when the groups' memberships change. Furthermore, issuing access tokens bound to symmetric proof-of-possession keys that apply to a group-audience is problematic, as an RS that is in possession of the access token can impersonate the client towards the other RSs that are part of the group. It is therefore NOT RECOMMENDED to issue access tokens bound to a group audience and symmetric proof-of possession keys.¶

Even the client must be able to determine the correct values to put into the "audience" parameter in order to obtain a token for the intended RS. Errors in this process can lead to the client inadvertently obtaining a token for the wrong RS. The correct values for "audience" can either be provisioned to the client as part of its configuration or dynamically looked up by the client in some directory. In the latter case, the integrity and correctness of the directory data must be assured. Note that the "audience" hint provided by the RS as part of the "AS Request Creation Hints" (Section 5.3) is not typically source authenticated and integrity protected and should therefore not be treated a trusted value.¶

6.10. Denial of Service Against or with Introspection

The optional introspection mechanism provided by OAuth and supported in the ACE framework allows for two types of attacks that need to be considered by implementers.¶

First, an attacker could perform a denial-of-service attack against the introspection endpoint at the AS in order to prevent validation of access tokens. To maintain the security of the system, an RS that is configured to use introspection MUST NOT allow access based on a token for which it couldn't reach the introspection endpoint.¶

Second, an attacker could use the fact that an RS performs introspection to perform a denial-of-service attack against that RS by repeatedly sending tokens to its authz-info endpoint that require an introspection call. The RS can mitigate such attacks by implementing rate limits on how many introspection requests they perform in a given time interval for a certain client IP address submitting tokens to /authz-info. When that limit has been reached, incoming requests from that address are rejected for a certain amount of time. A general rate limit on the introspection requests should also be considered in order to mitigate distributed attacks.¶

8.1. ACE Authorization Server Request Creation Hints

This specification establishes the IANA "ACE Authorization Server Request Creation Hints" registry.¶

The columns of the registry are:¶

Name:

The name of the parameter.¶

CBOR Key:

CBOR map key for the parameter. Different ranges of values use different registration policies [RFC8126]. Integer values from -256 to 255 are designated as Standards Action. Integer values from -65536 to -257 and from 256 to 65535 are designated as Specification Required. Integer values greater than 65535 are designated as Expert Review. Integer values less than -65536 are marked as Private Use.¶

Value Type:

The CBOR data types allowable for the values of this parameter.¶

Reference:

This contains a pointer to the public specification of the Request Creation Hint abbreviation, if one exists.¶

This registry has been initially populated by the values in Table 1. The Reference column for all of these entries is this document.¶

8.2. CoRE Resource Type Registry

IANA has registered a new Resource Type (rt=) Link Target Attribute in the "Resource Type (rt=) Link Target Attribute Values" subregistry under the "Constrained RESTful Environments (CoRE) Parameters" [IANA.CoreParameters] registry:¶

Value:

ace.ai¶

Description:

ACE-OAuth authz-info endpoint resource.¶

Reference:

RFC 9200¶

Specific ACE-OAuth profiles can use this common resource type for defining their profile-specific discovery processes.¶

8.3. OAuth Extensions Error Registration

This specification registers the following error values in the OAuth Extensions Error registry [IANA.OAuthExtensionsErrorRegistry].¶

Error name:

unsupported_pop_key¶

Error usage location:

token error response¶

Related protocol extension:

RFC 9200¶

Change Controller:

IETF¶

Specification document(s):

Section 5.8.3 of RFC 9200¶

Error name:

incompatible_ace_profiles¶

Error usage location:

token error response¶

Related protocol extension:

RFC 9200¶

Change Controller:

IETF¶

Specification document(s):

Section 5.8.3 of RFC 9200¶

8.4. OAuth Error Code CBOR Mappings Registry

This specification establishes the IANA "OAuth Error Code CBOR Mappings" registry.¶

The columns of the registry are:¶

Name:

The OAuth Error Code name, refers to the name in Section 5.2 of [RFC6749], e.g., "invalid_request".¶

CBOR Value:

CBOR abbreviation for this error code. Integer values less than -65536 are marked as Private Use; all other values use the registration policy Expert Review [RFC8126].¶

Reference:

This contains a pointer to the public specification of the error code abbreviation, if one exists.¶

Original Specification:

This contains a pointer to the public specification of the error code, if one exists.¶

This registry has been initially populated by the values in Table 3. The Reference column for all of these entries is this document.¶

8.5. OAuth Grant Type CBOR Mappings

This specification establishes the IANA "OAuth Grant Type CBOR Mappings" registry.¶

The columns of this registry are:¶

Name:

The name of the grant type, as specified in Section 1.3 of [RFC6749].¶

CBOR Value:

CBOR abbreviation for this grant type. Integer values less than -65536 are marked as Private Use; all other values use the registration policy Expert Review [RFC8126].¶

Reference:

This contains a pointer to the public specification of the grant type abbreviation, if one exists.¶

Original Specification:

This contains a pointer to the public specification of the grant type, if one exists.¶

This registry has been initially populated by the values in Table 4. The Reference column for all of these entries is this document.¶

8.6. OAuth Access Token Types

This section registers the following new token type in the "OAuth Access Token Types" registry [IANA.OAuthAccessTokenTypes].¶

Type name:

PoP¶

Additional Token Endpoint Response Parameters:

"cnf", "rs_cnf" (see Section 3.1 of [RFC8747] Section 3.2 of [RFC9201]).¶

HTTP Authentication Scheme(s):

N/A¶

Change Controller:

IETF¶

Specification document(s):

RFC 9200¶

8.7. OAuth Access Token Type CBOR Mappings

This specification establishes the IANA "OAuth Access Token Type CBOR Mappings" registry.¶

The columns of this registry are:¶

Name:

The name of the token type, as registered in the "OAuth Access Token Types" registry, e.g., "Bearer".¶

CBOR Value:

CBOR abbreviation for this token type. Integer values less than -65536 are marked as Private Use; all other values use the registration policy Expert Review [RFC8126].¶

Reference:

This contains a pointer to the public specification of the OAuth token type abbreviation, if one exists.¶

Original Specification:

This contains a pointer to the public specification of the OAuth token type, if one exists.¶

8.8. ACE Profile Registry

This specification establishes the IANA "ACE Profile" registry.¶

The columns of this registry are:¶

Name:

The name of the profile to be used as the value of the profile attribute.¶

Description:

Text giving an overview of the profile and the context it is developed for.¶

CBOR Value:

CBOR abbreviation for this profile name. Different ranges of values use different registration policies [RFC8126]. Integer values from -256 to 255 are designated as Standards Action. Integer values from -65536 to -257 and from 256 to 65535 are designated as Specification Required. Integer values greater than 65535 are designated as Expert Review. Integer values less than -65536 are marked as Private Use.¶

Reference:

This contains a pointer to the public specification of the profile abbreviation, if one exists.¶

8.9. OAuth Parameter Registration

This specification registers the following parameter in the "OAuth Parameters" registry [IANA.OAuthParameters]:¶

Name:

ace_profile¶

Parameter Usage Location:

token response¶

Change Controller:

IETF¶

Reference:

Sections 5.8.2 and 5.8.4.3 of RFC 9200¶

8.10. OAuth Parameters CBOR Mappings Registry

This specification establishes the IANA "OAuth Parameters CBOR Mappings" registry.¶

The columns of this registry are:¶

Name:

The OAuth Parameter name, refers to the name in the OAuth parameter registry, e.g., "client_id".¶

CBOR Key:

CBOR map key for this parameter. Integer values less than -65536 are marked as Private Use; all other values use the registration policy Expert Review [RFC8126].¶

Value Type:

The allowable CBOR data types for values of this parameter.¶

Reference:

This contains a pointer to the public specification of the OAuth parameter abbreviation, if one exists.¶

Original Specification

This contains a pointer to the public specification of the OAuth parameter, if one exists.¶

This registry has been initially populated by the values in Table 5. The Reference column for all of these entries is this document.¶

8.11. OAuth Introspection Response Parameter Registration

This specification registers the following parameters in the OAuth Token Introspection Response registry [IANA.TokenIntrospectionResponse].¶

Name:

ace_profile¶

Description:

The ACE profile used between the client and RS.¶

Change Controller:

IETF¶

Reference:

Section 5.9.2 of RFC 9200¶

Name:

cnonce¶

Description:

"client-nonce". A nonce previously provided to the AS by the RS via the client. Used to verify token freshness when the RS cannot synchronize its clock with the AS.¶

Change Controller:

IETF¶

Reference:

Section 5.9.2 of RFC 9200¶

Name

cti¶

Description

"CWT ID". The identifier of a CWT as defined in [RFC8392].¶

Change Controller

IETF¶

Reference

Section 5.9.2 of RFC 9200¶

Name:

exi¶

Description:

"Expires in". Lifetime of the token in seconds from the time the RS first sees it. Used to implement a weaker form of token expiration for devices that cannot synchronize their internal clocks.¶

Change Controller:

IETF¶

Reference:

Section 5.9.2 of RFC 9200¶

8.12. OAuth Token Introspection Response CBOR Mappings Registry

This specification establishes the IANA "OAuth Token Introspection Response CBOR Mappings" registry.¶

The columns of this registry are:¶

Name:

The OAuth Parameter name, refers to the name in the OAuth parameter registry, e.g., "client_id".¶

CBOR Key:

CBOR map key for this parameter. Integer values less than -65536 are marked as Private Use; all other values use the registration policy Expert Review [RFC8126].¶

Value Type:

The allowable CBOR data types for values of this parameter.¶

Reference:

This contains a pointer to the public specification of the introspection response parameter abbreviation, if one exists.¶

Original Specification

This contains a pointer to the public specification of the OAuth Token Introspection parameter, if one exists.¶

This registry has been initially populated by the values in Table 6. The Reference column for all of these entries is this document.¶

Note that the mappings of parameters corresponding to claim names intentionally coincide with the CWT claim name mappings from [RFC8392].¶

8.13. JSON Web Token Claims

This specification registers the following new claims in the JSON Web Token (JWT) registry of JSON Web Token Claims [IANA.JsonWebTokenClaims]:¶

Claim Name:

ace_profile¶

Claim Description:

The ACE profile a token is supposed to be used with.¶

Change Controller:

IETF¶

Reference:

Section 5.10 of RFC 9200¶

Claim Name:

cnonce¶

Claim Description:

"client-nonce". A nonce previously provided to the AS by the RS via the client. Used to verify token freshness when the RS cannot synchronize its clock with the AS.¶

Change Controller:

IETF¶

Reference:

Section 5.10 of RFC 9200¶

Claim Name:

exi¶

Claim Description:

"Expires in". Lifetime of the token in seconds from the time the RS first sees it. Used to implement a weaker form of token expiration for devices that cannot synchronize their internal clocks.¶

Change Controller:

IETF¶

Reference:

Section 5.10.3 of RFC 9200¶

8.14. CBOR Web Token Claims

This specification registers the following new claims in the "CBOR Web Token (CWT) Claims" registry [IANA.CborWebTokenClaims].¶

Claim Name:

ace_profile¶

Claim Description:

The ACE profile a token is supposed to be used with.¶

JWT Claim Name:

ace_profile¶

Claim Key:

38¶

Claim Value Type(s):

integer¶

Change Controller:

IETF¶

Specification Document(s):

Section 5.10 of RFC 9200¶

Claim Name:

cnonce¶

Claim Description:

The client-nonce sent to the AS by the RS via the client.¶

JWT Claim Name:

cnonce¶

Claim Key:

39¶

Claim Value Type(s):

byte string¶

Change Controller:

IETF¶

Specification Document(s):

Section 5.10 of RFC 9200¶

Claim Name:

exi¶

Claim Description:

The expiration time of a token measured from when it was received at the RS in seconds.¶

JWT Claim Name:

exi¶

Claim Key:

40¶

Claim Value Type(s):

integer¶

Change Controller:

IETF¶

Specification Document(s):

Section 5.10.3 of RFC 9200¶

Claim Name:

scope¶

Claim Description:

The scope of an access token, as defined in [RFC6749].¶

JWT Claim Name:

scope¶

Claim Key:

9¶

Claim Value Type(s):

byte string or text string¶

Change Controller:

IETF¶

Specification Document(s):

Section 4.2 of [RFC8693]¶

8.15. Media Type Registration

This specification registers the "application/ace+cbor" media type for messages of the protocols defined in this document carrying parameters encoded in CBOR. This registration follows the procedures specified in [RFC6838].¶

Type name:

application¶

Subtype name:

ace+cbor¶

Required parameters:

N/A¶

Optional parameters:

N/A¶

Encoding considerations:

Must be encoded as a CBOR map containing the protocol parameters defined in RFC 9200.¶

Security considerations:

See Section 6 of RFC 9200¶

Interoperability considerations:

N/A¶

Published specification:

RFC 9200¶

Applications that use this media type:

The type is used by authorization servers, clients, and resource servers that support the ACE framework with CBOR encoding, as specified in RFC 9200.¶

Fragment identifier considerations:

N/A¶

Additional information:

N/A¶

Person & email address to contact for further information:

IESG <iesg@ietf.org>¶

Intended usage:

COMMON¶

Restrictions on usage:

none¶

Author:

Ludwig Seitz <ludwig.seitz@combitech.se>¶

Change controller:

IETF¶

8.16. CoAP Content-Format Registry

The following entry has been registered in the "CoAP Content-Formats" registry:¶

Media Type:

application/ace+cbor¶

Encoding:

-¶

ID:

19¶

Reference:

RFC 9200¶

8.17. Expert Review Instructions

All of the IANA registries established in this document are defined to use a registration policy of 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:¶

• 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.¶
• 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.¶
• 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, i.e., the size of device it will be used on.¶
• Since a high degree of overlap is expected between these registries and the contents of the OAuth parameters [IANA.OAuthParameters] registries, experts should require new registrations to maintain alignment with parameters from OAuth that have comparable functionality. Deviation from this alignment should only be allowed if there are functional differences that are motivated by the use case and that cannot be easily or efficiently addressed by comparable OAuth parameters.¶

F.1. Local Token Validation

In this scenario, the case where the resource server is offline is considered, i.e., it is not connected to the AS at the time of the access request. This access procedure involves steps (A), (B), (C), and (F) of Figure 1.¶

Since the resource server must be able to verify the access token locally, self-contained access tokens must be used.¶

This example shows the interactions between a client, the authorization server, and a temperature sensor acting as a resource server. Message exchanges A and B are shown in Figure 11.¶

A:

The client first generates a public-private key pair used for communication security with the RS.¶

The client sends a CoAP POST request to the token endpoint at the AS. The security of this request can be transport or application layer. It is up the communication security profile to define. In the example, it is assumed that both the client and AS have performed mutual authentication, e.g., via DTLS. The request contains the public key of the client and the audience parameter set to "tempSensorInLivingRoom", a value that the temperature sensor identifies itself with. The AS evaluates the request and authorizes the client to access the resource.¶

B:

The AS responds with a 2.05 (Content) response containing the Access Information, including the access token. The PoP access token contains the public key of the client, and the Access Information contains the public key of the RS. For communication security, this example uses DTLS RawPublicKey between the client and the RS. The issued token will have a short validity time, i.e., "exp" close to "iat", in order to mitigate attacks using stolen client credentials. The token includes claims, such as "scope", with the authorized access that an owner of the temperature device can enjoy. In this example, the "scope" claim issued by the AS informs the RS that the owner of the token that can prove the possession of a key is authorized to make a GET request against the /temperature resource and a POST request on the /firmware resource. Note that the syntax and semantics of the scope claim are application specific.¶

Note: In this example, it is assumed that the client knows what resource it wants to access and is therefore able to request specific audience and scope claims for the access token.¶

         Authorization
  Client    Server
    |         |
    |<=======>| DTLS Connection Establishment
    |         |   and mutual authentication
    |         |
A:  +-------->| Header: POST (Code=0.02)
    |  POST   | Uri-Path:"token"
    |         | Content-Format: application/ace+cbor
    |         | Payload: <Request-Payload>
    |         |
B:  |<--------+ Header: 2.05 Content
    |  2.05   | Content-Format: application/ace+cbor
    |         | Payload: <Response-Payload>
    |         |

The information contained in the Request-Payload and the Response-Payload is shown in Figure 12. Note that the parameter "rs_cnf" from [RFC9201] is used to inform the client about the resource server's public key.¶

Request-Payload :
{
  "audience" : "tempSensorInLivingRoom",
  "client_id" : "myclient",
  "req_cnf" : {
    "COSE_Key" : {
      "kid" : b64'1Bg8vub9tLe1gHMzV76e8',
      "kty" : "EC",
      "crv" : "P-256",
      "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
      "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
    }
  }
}

Response-Payload :
{
  "access_token" : b64'0INDoQEKoQVNKkXfb7xaWqMTf6 ...',
  "rs_cnf" : {
    "COSE_Key" : {
      "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
      "kty" : "EC",
      "crv" : "P-256",
      "x"   : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4',
      "y"   : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM'
    }
  }
}

The content of the access token is shown in Figure 13.¶

{
  "aud" : "tempSensorInLivingRoom",
  "iat" : "1563451500",
  "exp" : "1563453000",
  "scope" :  "temperature_g firmware_p",
  "cnf" : {
    "COSE_Key" : {
      "kid" : b64'1Bg8vub9tLe1gHMzV76e8',
      "kty" : "EC",
      "crv" : "P-256",
      "x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
      "y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
    }
  }
}

Messages C and F are shown in Figures 14 and 15.¶

C:

The client then sends the PoP access token to the authz-info endpoint at the RS. This is a plain CoAP POST request, i.e., no transport or application-layer security is used between the client and RS since the token is integrity protected between the AS and RS. The RS verifies that the PoP access token was created by a known and trusted AS, which it applies to this RS, and that it is valid. The RS caches the security context together with authorization information about this client contained in the PoP access token.¶

           Resource
 Client     Server
    |         |
C:  +-------->| Header: POST (Code=0.02)
    |  POST   | Uri-Path:"authz-info"
    |         | Payload: 0INDoQEKoQVN ...
    |         |
    |<--------+ Header: 2.04 Changed
    |  2.04   |
    |         |

The client and the RS runs the DTLS handshake using the raw public keys established in steps B and C.¶

The client sends a CoAP GET request to /temperature on the RS over DTLS. The RS verifies that the request is authorized based on previously established security context.¶

F:

The RS responds over the same DTLS channel with a CoAP 2.05 Content response containing a resource representation as payload.¶

           Resource
 Client     Server
    |         |
    |<=======>| DTLS Connection Establishment
    |         |   using Raw Public Keys
    |         |
    +-------->| Header: GET (Code=0.01)
    | GET     | Uri-Path: "temperature"
    |         |
    |         |
    |         |
F:  |<--------+ Header: 2.05 Content
    | 2.05    | Payload: <sensor value>
    |         |

F.2. Introspection Aided Token Validation

In this deployment scenario, it is assumed that a client is not able to access the AS at the time of the access request, whereas the RS is assumed to be connected to the back-end infrastructure. Thus, the RS can make use of token introspection. This access procedure involves steps (A)-(F) of Figure 1 but assumes steps (A) and (B) have been carried out during a phase when the client had connectivity to the AS.¶

Since the client is assumed to be offline, at least for a certain period of time, a preprovisioned access token has to be long lived. Since the client is constrained, the token will not be self-contained (i.e., not a CWT) but instead just a reference. The resource server uses its connectivity to learn about the claims associated to the access token by using introspection, which is shown in the example below.¶

In the example, interactions between an offline client (key fob), an RS (online lock), and an AS is shown. It is assumed that there is a provisioning step where the client has access to the AS. This corresponds to message exchanges A and B, which are shown in Figure 16.¶

Authorization consent from the resource owner can be preconfigured, but it can also be provided via an interactive flow with the resource owner. An example of this for the key fob case could be that the resource owner has a connected car that he buys a generic key for and wants to use with the car. To authorize the key fob, he connects it to his computer that then provides the UI for the device. After that, OAuth 2.0 implicit flow can be used to authorize the key for his car at the car manufacturers AS.¶

Note: In this example, the client does not know the exact door it will be used to access since the token request is not sent at the time of access. So the scope and audience parameters are set quite wide to start with, while tailored values narrowing down the claims to the specific RS being accessed can be provided to that RS during an introspection step.¶

A:

The client sends a CoAP POST request to the token endpoint at the AS. The request contains the audience parameter set to "PACS1337" (Physical Access System (PACS)), a value that identifies the physical access control system to which the individual doors are connected. The AS generates an access token as an opaque string, which it can match to the specific client and the targeted audience. It furthermore generates a symmetric proof-of-possession key. The communication security and authentication between the client and AS is assumed to have been provided at the transport layer (e.g., via DTLS) using a pre-shared security context (pre-shared key (PSK), RPK, or certificate).¶

B:

The AS responds with a CoAP 2.05 Content response, containing as payload the Access Information, including the access token and the symmetric proof-of-possession key. Communication security between the C and RS will be DTLS and PreSharedKey. The PoP key is used as the PreSharedKey.¶

Note: In this example, we are using a symmetric key for a multi-RS audience, which is not recommended normally (see Section 6.9). However, in this case, the risk is deemed to be acceptable, since all the doors are part of the same physical access control system; therefore, the risk of a malicious RS impersonating the client towards another RS is low.¶

         Authorization
 Client     Server
    |         |
    |<=======>| DTLS Connection Establishment
    |         |   and mutual authentication
    |         |
A:  +-------->| Header: POST (Code=0.02)
    |  POST   | Uri-Path:"token"
    |         | Content-Format: application/ace+cbor
    |         | Payload: <Request-Payload>
    |         |
B:  |<--------+ Header: 2.05 Content
    |         | Content-Format: application/ace+cbor
    |  2.05   | Payload: <Response-Payload>
    |         |

The information contained in the Request-Payload and the Response-Payload is shown in Figure 17.¶

Request-Payload:
{
  "client_id" : "keyfob",
  "audience" : "PACS1337"
}

Response-Payload:
{
  "access_token" : b64'VGVzdCB0b2tlbg==',
  "cnf" : {
    "COSE_Key" : {
      "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
      "kty" : "oct",
      "alg" : "HS256",
      "k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE'
    }
  }
}

In this case, the access token is just an opaque byte string referencing the authorization information at the AS.¶

C:

Next, the client POSTs the access token to the authz-info endpoint in the RS. This is a plain CoAP request, i.e., no DTLS between the client and RS. Since the token is an opaque string, the RS cannot verify it on its own, and thus defers to respond to the client with a status code until after step E.¶

D:

The RS sends the token to the introspection endpoint on the AS using a CoAP POST request. In this example, the RS and AS are assumed to have performed mutual authentication using a pre-shared security context (PSK, RPK, or certificate) with the RS acting as the DTLS client.¶

E:

The AS provides the introspection response (2.05 Content) containing parameters about the token. This includes the confirmation key (cnf) parameter that allows the RS to verify the client's proof of possession in step F. Note that our example in Figure 19 assumes a preestablished key (e.g., one used by the client and the RS for a previous token) that is now only referenced by its key identifier 'kid'.¶

After receiving message E, the RS responds to the client's POST in step C with the CoAP response code 2.01 (Created).¶

           Resource
  Client    Server
    |         |
C:  +-------->| Header: POST (T=CON, Code=0.02)
    |  POST   | Uri-Path:"authz-info"
    |         | Payload: b64'VGVzdCB0b2tlbg=='
    |         |
    |         |     Authorization
    |         |       Server
    |         |          |
    |      D: +--------->| Header: POST (Code=0.02)
    |         |  POST    | Uri-Path: "introspect"
    |         |          | Content-Format: "application/ace+cbor"
    |         |          | Payload: <Request-Payload>
    |         |          |
    |      E: |<---------+ Header: 2.05 Content
    |         |  2.05    | Content-Format: "application/ace+cbor"
    |         |          | Payload: <Response-Payload>
    |         |          |
    |         |
    |<--------+ Header: 2.01 Created
    |  2.01   |
    |         |

The information contained in the Request-Payload and the Response-Payload is shown in Figure 19.¶

Request-Payload:
{
  "token" : b64'VGVzdCB0b2tlbg==',
  "client_id" : "FrontDoor",
}

Response-Payload:
{
  "active" : true,
  "aud" : "lockOfDoor4711",
  "scope" : "open, close",
  "iat" : 1563454000,
  "cnf" : {
    "kid" : b64'c29tZSBwdWJsaWMga2V5IGlk'
  }
}

The client uses the symmetric PoP key to establish a DTLS PreSharedKey secure connection to the RS. The CoAP request PUT is sent to the uri-path /state on the RS, changing the state of the door to locked.¶

F:

The RS responds with an appropriate over the secure DTLS channel.¶

           Resource
  Client    Server
    |         |
    |<=======>| DTLS Connection Establishment
    |         |   using Pre Shared Key
    |         |
    +-------->| Header: PUT (Code=0.03)
    | PUT     | Uri-Path: "state"
    |         | Payload: <new state for the lock>
    |         |
F:  |<--------+ Header: 2.04 Changed
    | 2.04    | Payload: <new state for the lock>
    |         |