OSCOAP profile of the Authentication and Authorization for Constrained Environments Framework
draft-seitz-ace-oscoap-profile-04

Abstract

This memo specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Object Security of CoAP (OSCOAP) and Ephemeral Diffie-Hellman over COSE (EDHOC) 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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Table of Contents

• 1. Introduction
• 1.1. Terminology
• 2. Client to Resource Server
• 2.1. Signaling the use of OSCOAP
• 2.2. Key establishment for OSCOAP
• 2.2.1. Using the pop-key with OSCOAP directly (OSCOAP)
• 2.2.2. Using the pop-key with EDHOC (EDHOC+OSCOAP)
• 2.3. Client to Authorization Server
• 3. Resource Server to Authorization Server
• 4. Security Considerations
• 5. Privacy Considerations
• 6. IANA Considerations
• 7. Acknowledgments
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

This memo specifies a profile of the ACE framework [I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource server use CoAP [RFC7252] to communicate. The client uses an access token, bound to a key (the proof-of-possession key) to authorize its access to the resource server. In order to provide communication security, proof of possession, and server authentication they use Object Security of CoAP (OSCOAP) [I-D.ietf-core-object-security] and Ephemeral Diffie-Hellman Over COSE (EDHOC) [I-D.selander-ace-cose-ecdhe]. Optionally the client and the resource server may also use CoAP and OSCOAP to communicate with the authorization server. The use of EDHOC in this profile in addition to OSCOAP, provides perfect forward secrecy (PFS) and the initial proof-of-possession, which ties the proof-of-possession key to an OSCOAP security context.

OSCOAP specifies how to use CBOR Object Signing and Encryption (COSE) [RFC8152] to secure CoAP messages. In order to provide replay and reordering protection OSCOAP also introduces sequence numbers that are used together with COSE. EDHOC specifies an authenticated Diffie-Hellman protocol that allows two parties to use CBOR [RFC7049] and COSE in order to establish a shared secret key with perfect forward secrecy.

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. These words may also appear in this document in lowercase, absent their normative meanings.

Certain security-related terms such as "authentication", "authorization", "confidentiality", "(data) integrity", "message authentication code", and "verify" are taken from [RFC4949].

Since we describe exchanges as RESTful protocol interactions HTTP [RFC7231] offers useful terminology.

Terminology for entities in the architecture is defined in OAuth 2.0 [RFC6749] and [I-D.ietf-ace-actors], such as client (C), resource server (RS), and authorization server (AS). It is assumed in this document that a given resource on a specific RS is associated to a unique AS.

Note that the term "endpoint" is used here following its OAuth definition, which is to denote resources such as /token and /introspect at the AS and /authz-info at the RS. The CoAP [RFC7252] definition, which is "An entity participating in the CoAP protocol" is not used in this memo.

2. Client to Resource Server

The use of OSCOAP for arbitrary CoAP messages is specified in [I-D.ietf-core-object-security]. This section defines the specific uses and their purpose for securing the communication between a client and a resource server, and the parameters needed to negotiate the use of this profile with the token endpoint at the authorization server as specified in section 5.5 of the ACE framework [I-D.ietf-ace-oauth-authz].

2.1. Signaling the use of OSCOAP

A client requests a token at an AS via the /token endpoint. This follows the message formats specified in section 5.5.1 of the ACE framework [I-D.ietf-ace-oauth-authz].

The AS responding to a successful access token request as defined in section 5.5.2 of the ACE framework can signal that the use of OSCOAP is REQUIRED for a specific access token by including the "profile" parameter with the value "coap_oscoap" or "coap_oscoap_edhoc" in the access token response. This means that the client MUST use OSCOAP towards all resource servers for which this access token is valid, and follow Section 2.2.1 to derive the security context to run OSCOAP, if the profile has value "coap_oscoap", or follow Section 2.2.2 to derive the security context to run OSCOAP, if the profile has value "coap_oscoap_edhoc".

The error response procedures defined in section 5.5.3 of the ACE framework are unchanged by this profile.

Note the the client and the authorization server MAY OPTIONALLY use OSCOAP to protect the interaction via the /token endpoint. See section 3 for details.

2.2. Key establishment for OSCOAP

Section 3.2 of OSCOAP [I-D.ietf-core-object-security] defines how to derive a security context based on a shared master secret and a few other parameters, established between client and server. The proof-of-possession key (pop-key) provisioned from the AS MAY, in case of pre-shared keys, be used directly as master secret in OSCOAP. Alternatively the pop-key (symmetric or asymmetric) MAY be used to authenticate the messages in the key exchange protocol EDHOC [I-D.selander-ace-cose-ecdhe], from which a master secret is derived.

2.2.1. Using the pop-key with OSCOAP directly (OSCOAP)

If OSCOAP is used directly with the symmetric pop-key as master secret, then the AS MUST provision the following data, in response to the access token request:

• a master secret
• the sender identifier
• the recipient identifier

Additionally, the AS MAY provision the following data, in the same response. In case these parameters are omitted, the default values are used as described in section 3.2. of [I-D.ietf-core-object-security].

• an AEAD algorithm
• a KDF algorithm
• a salt

The master secret MUST be communicated as COSE_Key in the 'cnf' parameter of the access token response as defined in section 5.5.4.5 of [I-D.ietf-ace-oauth-authz]. The AEAD algorithm MAY be included as the 'alg' parameter in the COSE_Key; the KDF algorithm MAY be included as the 'kdf' parameter of the COSE_Key and the salt MAY be included as the 'slt' parameter of the COSE_Key as defined in table 1. The same parameters MUST be included as metadata of the access token, if the token is a CWT [I-D.ietf-ace-cbor-web-token], the same COSE_Key structure MUST be placed in the 'cnf' claim of this token. The AS MUST also assign identifiers to both client and RS, which are then used as Sender ID and Recipient ID in the OSCOAP context as described in section 3.1. of [I-D.ietf-core-object-security]. These identifiers MUST be unique in the set of all clients and RS identifiers for a certain AS. Moreover, these MUST be included in the COSE_Key as header parameters, as defined in table 1.

We assume in this document that a resource is associated to one single AS, which makes it possible to assume unique identifiers for each client requesting a particular resource to a RS. If this is not the case, collisions of identifiers may appear in the RS, in which case the RS needs to have a mechanism in place to disambiguate identifiers or mitigate their effect.

Note that C should set the Sender ID of its security context to the clientId value received and the Recipient ID to the serverId value, and RS should do the opposite.

+----------+-------+----------------+------------+-------------------+
| name     | label | CBOR type      | registry   | description       |
+----------+-------+----------------+------------+-------------------+
| clientId | TBD   | bstr           |            | Identifies the    |
|          |       |                |            | client in an      |
|          |       |                |            | OSCOAP context    |
|          |       |                |            | using this key    |
|          |       |                |            |                   |
| serverId | TBD   | bstr           |            | Identifies the    |
|          |       |                |            | server in an      |
|          |       |                |            | OSCOAP context    |
|          |       |                |            | using this key    |
|          |       |                |            |                   |
| kdf      | TBD   | bstr           |            | Identifies the    |
|          |       |                |            | KDF algorithm in  |
|          |       |                |            | an OSCOAP context |
|          |       |                |            | using this key    |
|          |       |                |            |                   |
| slt      | TBD   | bstr           |            | Identifies the    |
|          |       |                |            | master salt in    |
|          |       |                |            | an OSCOAP context |
|          |       |                |            | using this key    |
+----------+-------+----------------+------------+-------------------+
        Table 1: Additional common header parameters for COSE_Key

Figure 1 shows an example of such an AS response, in CBOR diagnostic notation without the tag and value abbreviations.

      Header: Created (Code=2.01)
      Content-Type: "application/cose+cbor"
      Payload:
      {
        "access_token" : b64'SlAV32hkKG ...
         (remainder of access token omitted for brevity)',
        "profile" : "coap_oscoap",
        "expires_in" : "3600",
        "cnf" : {
          "COSE_Key" : {
            "kty" : "Symmetric",
            "alg" : "AES-CCM-16-64-128",
            "clientId" : b64'qA',
            "serverId" : b64'Qg',
            "k" : b64'+a+Dg2jjU+eIiOFCa9lObw'
          }
        }
      }

      Figure 1: Example AS response with OSCOAP parameters.

Figure 2 shows an example CWT, containing the necessary OSCOAP 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" : {
        "COSE_Key" : {
          "kty" : "Symmetric",
          "alg" : "AES-CCM-16-64-128",
          "clientId" : b64'Qg',
          "serverId" : b64'qA',
          "k" : b64'+a+Dg2jjU+eIiOFCa9lObw' 
      }
    }

       Figure 2: Example CWT with OSCOAP parameters.

2.2.2. Using the pop-key with EDHOC (EDHOC+OSCOAP)

If EDHOC is used together with OSCOAP, and the pop-key (symmetric or asymmetric) is used to authenticate the messages in EDHOC, then the AS MUST provision the following data, in response to the access token request:

• a symmetric or asymmetric key (associated to the RS)
• a key identifier;

How these parameters are communicated depends on the type of key (asymmetric or symmetric).

Note that in the case described in this section, the 'expires_in' parameter, defined in section 4.2.2. of [RFC6749] defines the lifetime in seconds of both the access token and the shared secret. After expiration, C MUST acquire a new access token from the AS, and run EDHOC again, as specified in this section

2.2.2.1. Using Asymmetric Keys

In case of an asymmetric key, C MUST communicate its own asymmetric key to the AS in the 'cnf' parameter of the access token request, as specified in section 5.5.1 of [I-D.ietf-ace-oauth-authz]; the AS MUST communicate the RS's public key to C in the response, in the 'rs_cnf' parameter, as specified in section 5.5.1 of [I-D.ietf-ace-oauth-authz]. Note that the RS's public key MUST include a 'kid' parameter, and that the value of the 'kid' MUST be included in the access token, to let the RS know which of its public keys C used. If the access token is a CWT [I-D.ietf-ace-cbor-web-token], the key identifier MUST be placed directly in the 'cnf' structure (if the key is only referenced).

Figure 3 shows an example of such a request in CBOR diagnostic notation without tag and value abbreviations.

   Header: POST (Code=0.02)
   Uri-Host: "server.example.com"
   Uri-Path: "token"
   Content-Type: "application/cose+cbor"
   Payload:
   {
     "grant_type" : "client_credentials",
     "cnf" : {
       "COSE_Key" : {
         "kid" : "client_key"
         "kty" : "EC",
         "crv" : "P-256",
         "x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8',
         "y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4'
       }
     }
   }

  Figure 3: Example access token request (OSCOAP+EDHOC, asymmetric).

Figure 4 shows an example of a corresponding response in CBOR diagnostic notation without tag and value abbreviations.

      Header: Created (Code=2.01)
      Content-Type: "application/cose+cbor"
      Payload:
      {
        "access_token" : b64'SlAV32hkKG ...
         (contains "kid" : "server_key")',
        "profile" : "coap_oscoap_edhoc",
        "expires_in" : "3600",
        "rs_cnf" : {
          "COSE_Key" : {
            "kid" : "server_key"
            "kty" : "EC",
            "crv" : "P-256",
            "x" : b64'cGJ90UiglWiGahtagnv8usWxHK2PmfnHKwXPS54m0kT',
            "y" : b64'reASjpkttcsz+1rb7btKLv8EX4IBOL+C3BttVivg+lS'
         }
        }
      }

   Figure 4: Example AS response (EDHOC+OSCOAP, asymmetric).

2.2.2.2. Using Symmetric Keys

In the case of a symmetric key, the AS MUST communicate the key to the client in the 'cnf' parameter of the access token response, as specified in section 5.5.2. of [I-D.ietf-ace-oauth-authz]. AS MUST also select a key identifier, that MUST be included as the 'kid' parameter either directly in the 'cnf' structure, as in figure 4 of [I-D.ietf-ace-oauth-authz], or as the 'kid' parameter of the COSE_key, as in figure 6 of [I-D.ietf-ace-oauth-authz].

Figure 5 shows an example of the necessary parameters in the AS response to the access token request when EDHOC is used. The example uses CBOR diagnostic notation without tag and value abbreviations.

      Header: Created (Code=2.01)
      Content-Type: "application/cose+cbor"
      Payload:
      {
        "access_token" : b64'SlAV32hkKG ...
         (remainder of access token omitted for brevity)',
        "profile" : "coap_oscoap_edhoc",
        "expires_in" : "3600",
        "cnf" : {
          "COSE_Key" : {
            "kty" : "Symmetric",
            "kid" : b64'5tOS+h42dkw',
            "k" : b64'+a+Dg2jjU+eIiOFCa9lObw'
          }
        }
      }

   Figure 5: Example AS response (EDHOC+OSCOAP, symmetric).

In both cases, the AS MUST also include the same key identifier as 'kid' parameter in the access token metadata. If the access token is a CWT [I-D.ietf-ace-cbor-web-token], the key identifier MUST be placed inside the 'cnf' claim as 'kid' parameter of the COSE_Key or directly in the 'cnf' structure (if the key is only referenced).

Figure 6 shows an example CWT containing the necessary EDHOC+OSCOAP 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" : {
        "COSE_Key" : {
          "kty" : "Symmetric",
          "kid" : b64'5tOS+h42dkw',
          "k" : b64'+a+Dg2jjU+eIiOFCa9lObw' 
      }
    }

      Figure 6: Example CWT with EDHOC+OSCOAP, symmetric case.

All other parameters defining OSCOAP security context are derived from EDHOC message exchange, including the master secret (see Appendix C.2 of [I-D.selander-ace-cose-ecdhe]).

2.2.2.3. Processing

To provide forward secrecy and mutual authentication in the case of pre-shared keys, pre-established raw public keys or with X.509 certificates it is RECOMMENDED to use EDHOC [I-D.selander-ace-cose-ecdhe] to generate the keying material. EDHOC MUST be used as defined in Appendix C, with the following additions and modifications.

The first EDHOC message is sent after the access token is posted to the /authz-info endpoint of the RS as specified in section 5.7.1 of, as defined in [I-D.ietf-ace-oauth-authz]. Then the EDHOC message_1 is sent and the EDHOC protocol is initiated [I-D.selander-ace-cose-ecdhe]).

Before the RS continues with the EDHOC protocol and responds to this token submission request, additional verifications on the access token are done: the RS SHALL process the access token according to [I-D.ietf-ace-oauth-authz]. If the token is valid then the RS continues processing EDHOC following Appendix C of [I-D.selander-ace-cose-ecdhe], otherwise it discontinues EDHOC and responds with the error code as specified in [I-D.ietf-ace-oauth-authz].

• In case the EDHOC verification fails, the RS MUST return an error response to the client with code 4.01 (Unauthorized).
• If RS has an access token for C but not for the resource that C has requested, RS MUST reject the request with a 4.03 (Forbidden).
• If RS has an access token for C but it does not cover the action C requested on the resource, RS MUST reject the request with a 4.05 (Method Not Allowed).
• If all verifications above succeeds, further communication between client and RS is protected with OSCOAP, including the RS response to the OSCOAP request.

In the case of EDHOC being used with symmetric keys, the protocol in section 5 of [I-D.selander-ace-cose-ecdhe] MUST be used. If the key is asymmetric, the RS MUST also use an asymmetric key for authentication. This key is known to the client through the access token response (see section 5.5.2 of the ACE framework). In this case the protocol in section 4 of [I-D.selander-ace-cose-ecdhe] MUST be used.

Figure 7 illustrates the message exchanges for using OSCOAP+EDHOC (step C in figure 1 of [I-D.ietf-ace-oauth-authz]).

                     Resource
            Client    Server
            |          |
            |          |
            +--------->| Header: POST (Code=0.02)
            | POST     | Uri-Path:"authz-info"
            |          | Content-Type: application/cbor
            |          | Payload: access token
            |          |
            |          |
            +--------->| Header: POST (Code=0.02)
            |   POST   | Uri-Path: "/.well-known/edhoc"
            |          | Content-Type: application/edhoc
            |          | Payload: EDHOC message_1
            |          |
            |<---------+ Header: 2.04 Changed
            |   2.04   | Content-Type: application/edhoc
            |          | Payload: EDHOC message_2
            |          |
            +--------->| Header: POST (Code=0.02)
            |   POST   | Uri-Path: "/.well-known/edhoc"
            |          | Content-Type: application/edhoc
            |          | Payload: EDHOC message_3
            |          |
            |<---------+ Header: 2.04 Changed
            |   2.04   |
            |          |
     start of protected communication
            |          |
            +--------->| CoAP request +
            |  OSCOAP  | Object-Security option
            | request  | 
            |          |
            |<---------+ CoAP response +
            |  OSCOAP  | Object-Security option
            | response | 
            |          |  

   Figure 7: Access token and key establishment with EDHOC

2.3. Client to Authorization Server

As specified in the ACE framework section 5.5 [I-D.ietf-ace-oauth-authz], the Client and AS can also use CoAP instead of HTTP to communicate via the token endpoint. This section specifies how to use OSCOAP between Client and AS together with CoAP. The use of OSCOAP for this communication is OPTIONAL in this profile, other security protocols (such as DTLS) MAY be used instead.

The client and the AS are expected to have pre-established credentials (e.g. raw public keys). How these credentials are established is out of scope for this profile. Furthermore the client and the AS communicate using CoAP through the token endpoint as specified in section 5.5 of [I-D.ietf-ace-oauth-authz]. At first point of contact, prior to making the token request and response, the client and the AS MAY perform an EDHOC exchange with the pre-established credentials to create forward secret keying material for use with OSCOAP. Subsequent requests and the responses MUST be protected with OSCOAP.

3. Resource Server to Authorization Server

As specified in the ACE framework section 5.6 [I-D.ietf-ace-oauth-authz], the RS and AS can also use CoAP instead of HTTP to communicate via the introspection endpoint. This section specifies how to use OSCOAP between RS and AS together with CoAP. The use of OSCOAP for this communication is OPTIONAL in this profile, other security protocols (such as DTLS) MAY be used instead.

The RS and the AS are expected to have pre-established credentials (e.g. symmetric keys). How these credentials are established is out of scope for this profile. Furthermore the RS and the AS communicate using CoAP through the introspection endpoint as specified in section 5.6 of [I-D.ietf-ace-oauth-authz]. At first point of contact, prior to making the introspection request and response, the RS and the AS MAY perform an EDHOC exchange with the pre-established credentials to create forward secret keying material for use with OSCOAP. Subsequent requests and the responses MUST be protected with OSCOAP

4. Security Considerations

TBD.

5. Privacy Considerations

TBD.

6. IANA Considerations

TBD. 'coap_oscoap' as profile id. Header parameters 'sid', 'rid', 'kdf' and 'slt' for COSE_Key.

7. Acknowledgments

The author wishes to thank Jim Schaad, Goeran Selander and Marco Tiloca for the input on this memo. The error responses specified in section 2.2. were originally specified by Gerdes et al. in [I-D.gerdes-ace-dcaf-authorize].

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses