CoRE Working Group S. Gerdes Internet-Draft O. Bergmann Intended status: Standards Track C. Bormann Expires: April 24, 2014 Universitaet Bremen TZI October 21, 2013 Delegated CoAP Authentication and Authorization Framework (DCAF) draft-gerdes-core-dcaf-authorize-01 Abstract This specification defines a protocol for delegating client authentication and authorization in a constrained environment for establishing a Datagram Transport Layer Security (DTLS) channel between resource-constrained nodes. The protocol relies on DTLS to transfer authorization information and shared secrets for symmetric cryptography between entities in a constrained network. A resource- constrained node can use this protocol to delegate authentication of communication peers and management of authorization information to a trusted host with less severe limitations regarding processing power and memory. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on April 24, 2014. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of Gerdes, et al. Expires April 24, 2014 [Page 1] Internet-Draft DCAF October 2013 publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Gerdes, et al. Expires April 24, 2014 [Page 2] Internet-Draft DCAF October 2013 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Features . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.1. Roles . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2.2. Other terms . . . . . . . . . . . . . . . . . . . . . 5 2. System Overview . . . . . . . . . . . . . . . . . . . . . . . 6 3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2. Unauthorized Resource Request Message . . . . . . . . . . 8 3.3. AS Information Message . . . . . . . . . . . . . . . . . . 9 3.4. Access Request . . . . . . . . . . . . . . . . . . . . . . 10 3.5. Ticket Request Message . . . . . . . . . . . . . . . . . . 11 3.6. Ticket Grant Message . . . . . . . . . . . . . . . . . . . 12 3.7. Ticket Transfer Message . . . . . . . . . . . . . . . . . 13 3.8. DTLS Channel Setup Between C and RS . . . . . . . . . . . 13 3.9. Authorized Resource Request Message . . . . . . . . . . . 14 4. Ticket . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.1. Face . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.2. Verifier . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.3. Revocation . . . . . . . . . . . . . . . . . . . . . . . . 17 4.3.1. Lifetime . . . . . . . . . . . . . . . . . . . . . . . 17 4.3.2. Revocation Messages . . . . . . . . . . . . . . . . . 17 5. Payload Format and Encoding (application/dcaf+json) . . . . . 18 5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 19 6. DTLS PSK Generation Methods . . . . . . . . . . . . . . . . . 22 6.1. DTLS PSK Transfer . . . . . . . . . . . . . . . . . . . . 22 6.2. Distributed Key Derivation . . . . . . . . . . . . . . . . 22 7. Authorization Configuration . . . . . . . . . . . . . . . . . 24 8. Trust Relationships . . . . . . . . . . . . . . . . . . . . . 25 9. Listing Authorization Server Information in a Resource Directory . . . . . . . . . . . . . . . . . . . . . . . . . . 26 10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 10.1. Access Granted . . . . . . . . . . . . . . . . . . . . . . 27 10.2. Access Denied . . . . . . . . . . . . . . . . . . . . . . 29 10.3. Access Restricted . . . . . . . . . . . . . . . . . . . . 29 10.4. Implicit Authorization . . . . . . . . . . . . . . . . . . 30 11. Security Considerations . . . . . . . . . . . . . . . . . . . 31 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 12.1. dcaf+json Media Type Registration . . . . . . . . . . . . 32 12.2. CoAP Content Format Registration . . . . . . . . . . . . . 33 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 13.1. Normative References . . . . . . . . . . . . . . . . . . . 34 13.2. Informative References . . . . . . . . . . . . . . . . . . 34 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36 Gerdes, et al. Expires April 24, 2014 [Page 3] Internet-Draft DCAF October 2013 1. Introduction The Constrained Application Protocol (CoAP) [I-D.ietf-core-coap] is a transfer protocol similar to HTTP which is designed for the special requirements of constrained environments. A serious problem with constrained devices is the realization of secure communication. The devices only have limited resources such as memory, stable storage (such as disk space) and transmission capacity and often lack input/ output devices such as keyboards or displays. Therefore, they are not readily capable of using common protocols. Especially authentication mechanisms are difficult to realize, because the lack of stable storage severely limits the number of keys the system can store. Moreover, CoAP has no mechanism to distinguish access rights for different clients (authorization). The DCAF architecture is designed to relieve the constrained nodes from managing keys for numerous devices by introducing authorization servers which conduct the authentication and authorization for their nodes. To achieve this, access tokens are used. A device which wants to access a constrained node's resource first has to gain permission in the form of a token from the node's authorization server. As fine-grained authorization is not always needed on constrained devices, DCAF supports an implicit authorization mode where no authorization information is exchanged. The main goals of DCAF are the setup of a Datagram Transport Layer Security (DTLS) [RFC6347] channel with symmetric pre-shared keys (PSK) [RFC4279] and to securely transmit authorization tickets. 1.1. Features o Utilize DTLS communication with pre-shared keys. o Authenticated exchange of authorization information. o Simplified authorization mechanism for cases where implicit authorization is sufficient. o Using only symmetric encryption on constrained nodes. 1.2. 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 RFC 2119 [RFC2119]. Gerdes, et al. Expires April 24, 2014 [Page 4] Internet-Draft DCAF October 2013 1.2.1. Roles Resource Server (RS): A constrained device that hosts resources the Client wants to access. Client (C): A device that wants to access resources on the Resource Server. Authorization Server (AS): The node that conducts authentication and authorization for a Resource Server. An Authorization Server can be responsible for a single or multiple devices or even for a whole network. A Resource Server can have multiple Authorization Servers. Authentication Manager (AM): The node that conducts authentication on behalf of the Client. Resource Owner: The principal that owns the resource and controls its access permissions. 1.2.2. Other terms Access ticket: Contains the authentication and, if necessary, the authorization information needed to access a resource. Authorization information: Contains all information needed by RS to decide if C is privileged to access a resource in a specific way. Authentication information: Contains all information needed by RS to decide if the entity claiming to be C is to be trusted. Access information: Contains authentication information and, if necessary, authorization information. Explicit authorization: The Authorization Server informs the Resource Server in detail which privileges are granted to the Client. Implicit authorization: The Authorization Server informs the Resource Server that the Client is authorized to access any resource on RS in any way, without specifying the privileges in detail. Gerdes, et al. Expires April 24, 2014 [Page 5] Internet-Draft DCAF October 2013 2. System Overview Within the DCAF Architecture each Resource Server (RS) has one or more Authorization Servers (AS) which conduct the authentication and authorization for RS. RS and AS share a symmetric key which has to be exchanged initially to provide for a secure channel. The mechanism used for this is not in the scope of this document. To gain access to a specific resource on a Resource Server, a client (C) has to request an access ticket from one of the Authorization Servers serving RS either directly or, if it is a constrained device, using its Authentication Manager (AM). In the following, we always discuss the AM role separately, even if that is co-located within a (more powerful) C. If AS decides that C is allowed to access the resource, it generates a DTLS pre-shared key (PSK) for the communication between C and RS and wraps it into an access ticket. For explicit access control, AS adds the detailed access permissions to the ticket in a way that RS can interpret. After presenting the ticket to RS, C and RS can communicate securely. To be able to provide for the authentication and authorization services, the Authorization Servers have to fulfill several requirements: o An AS must have enough stable storage (such as disk space) to store the necessary number of credentials (matching the number of clients and Resource Servers). o An AS must possess means for user interaction, for example directly or indirectly connected input/output devices like keyboard and display, to allow for configuration of authorization information by the Resource Owner. o An AS must have enough processing power to handle the authorization requests for all RS devices it is responsible for. Gerdes, et al. Expires April 24, 2014 [Page 6] Internet-Draft DCAF October 2013 3. Protocol The DCAF protocol comprises three parts: 1. transfer of authentication and, if necessary, authorization information between C and RS; 2. transfer of access requests and the respective ticket grants between C and AM; and 3. transfer of access requests and the respective ticket grants between AS and AM. 3.1. Overview In Figure 1, a DCAF protocol flow is depicted (messages in square brackets are optional): AM C RS AS | <== DTLS chan. ==> | | <== DTLS chan. ==> | | | [Resource Req.-->] | | | | | | | | [<-- AS Info.] | | | | | | | <-- Access Req. | | | | | | | | <===== TLS/DTLS channel (AM/AS Mutual Authentication) =====> | | | | | | Ticket Request ------------------------------------------> | | | | | | <------------------------------------------ Ticket Grant | | | | | | Ticket Transm. --> | | | | | | | | | <== DTLS chan. ==> | | | | Auth. Res. Req. -> | | Figure 1: Protocol Overview To determine the Authorization Server in charge of a resource hosted at the Resource Server (RS), the Client (C) MAY send an initial Unauthorized Resource Request message to RS. RS then denies the request and sends the address of its Authorization Server (AS) back to the Client. Instead of the initial Unauthorized Resource Request message, C MAY look up the desired resource in a resource directory (cf. Gerdes, et al. Expires April 24, 2014 [Page 7] Internet-Draft DCAF October 2013 [I-D.ietf-core-resource-directory]) that lists RS's resources as discussed in Section 9. Once C knows AS' address, it can send a request for authorization to AS using its own Authentication Manager (AM). AS authenticates AM, who serves as a trusted introducer for C, and decides if C is allowed to communicate with RS and access the requested resource. If it is, AS generates an access ticket for C. The ticket contains keying material for the establishment of a secure channel and, if necessary, a representation of the permissions C has for the resource. C keeps one part of the access ticket and presents the other part to RS to prove its right to access. With their respective parts of the ticket, C and RS are able to establish a secure channel. The following sections specify how CoAP is used to interchange access-related data between RS and AS so that AS can provide C and RS with sufficient information to establish a secure channel, and simultaneously convey authorization information specific for this communication relationship to RS. This document uses JavaScript Object Notation (JSON, [RFC4627]) to express authorization information as set of attributes passed in CoAP payloads. Notation and encoding options are discussed in Section 5. 3.2. Unauthorized Resource Request Message The optional Unauthorized Resource Request message is a request for a resource hosted by RS for which no proper authorization is granted. RS MUST treat any CoAP request as Unauthorized Resource Request message when any of the following holds: o The request has been received on an insecure channel. o RS has no valid access information for the sender of the request regarding the requested action on that resource. o RS has valid access information for the sender of the request, but this does not allow the requested action on the requested resource. Note: These conditions ensure that RS can handle requests autonomously once access was granted and a secure channel has been established between C and RS. Unauthorized Resource Request messages MUST be denied with a client error response. In this response, the Resource Server MUST provide proper AS Information to enable the Client to request an access Gerdes, et al. Expires April 24, 2014 [Page 8] Internet-Draft DCAF October 2013 ticket from RS's Authorization Server as described in Section 3.3. The response code MUST be 4.01 (Unauthorized) in case the sender of the Unauthorized Resource Request message is not authenticated, or if RS has no valid access ticket for C. If RS has authorization information for C but not for the resource that C has requested, RS MUST reject the request with a 4.03 (Forbidden). If RS has authorization information for C but they do not cover the action C requested on the resource, RS MUST reject the request with a 4.05 (Method Not Allowed). Editor's Note: The use of the response codes 4.03 and 4.05 is intended to prevent infinite loops where a dumb Client optimistically tries to access a requested resource with any access token received from the AS. As malicious clients could pretend to be C to determine C's privileges, these detailed response codes must be used only when a certain level of security is already available which can be achieved only when the Client is authenticated. 3.3. AS Information Message The AS Information Message is sent by RS as a response to an Unauthorized Resource Request message (see Section 3.2) to point the sender of the Unauthorized Resource Request message to RS's Authorization Server. The AS information is a set of attributes containing an absolute URI (see Section 4.3 of [RFC3986]) that specifies the Authorization Server in charge of RS. The message MAY also contain a timestamp generated by RS. Figure 2 shows an example for an AS Information message payload using JSON. (Refer to Section 5 for a detailed description of the available attributes and their semantics.) 4.01 Unauthorized Content-Format: application/dcaf+json { "AS": "coaps://as-rs.example.com/authorize", "TS": 168537 } Figure 2: AS Information Payload Example In this example, the attribute AS points the receiver of this message to the URI "coaps://as-rs.example.com/authorize" to request access permissions. The originator of the AS Information payload (i.e. RS) uses a local clock that is loosely synchronized with a time scale Gerdes, et al. Expires April 24, 2014 [Page 9] Internet-Draft DCAF October 2013 common between RS and AS (e.g., wall clock time). Therefore, it has included a time stamp on its own time scale that is used as a nonce for replay attack prevention. Refer to Section 4.1 for more details concerning the usage of time stamps to ensure freshness of access tickets. 3.4. Access Request To retrieve an access ticket for the resource that C wants to access, C sends an Access Request to its authentication manager AM. The Access Request is constructed as follows: 1. The request method is POST. 2. The request URI is set as described below. 3. The message payload contains a data structure that describes the action and resource for which C requests an access ticket. The request URI identifies a resource at AM for handling authorization requests from C. The URI SHOULD be announced by AM in its resource directory as described in Section 9. Note: Where capacity limitations of C do not allow for resource directory lookups, the request URI in Access Requests could be hard-coded during provisioning or set in a specific device configuration profile. The message payload is constructed from the AS information that RS has returned in its AS Information message (see Section 3.3) and information that C provides to describe its intended request(s). The Access Request MUST contain the following attributes: 1. Contact information for the AS to use. 2. An identifier of C that can be used by AS to distinguish Access Requests from different Clients. 3. An absolute URI of the resource that C wants to access. 4. The actions that C wants to perform on the resource. 5. Any time stamp generated by RS. The identifier of C included in the Access Request is used to distinguish the Clients within AM's namespace. To decide if a Client is allowed to access the requested resource, AS must rely on AM's verification of that Client's identity because AS cannot authenticate Gerdes, et al. Expires April 24, 2014 [Page 10] Internet-Draft DCAF October 2013 the Client by itself. (Refer to Section 8 for a detailed discussion of the trust relationship between authentication managers and authorization servers.) Note: Whenever the Resource Owner wants to specify access permissions for an individual client, the Authorization Server must be able to relate those permissions to that respective client using an identifier that is globally unique. An example Access Request from C to AM is depicted in Figure 3. (Refer to Section 5 for a detailed description of the available attributes and their semantics.) POST client-authorize Content-Format: application/dcaf+json { "AS": coaps://as-rs.example.com/authorize", "CI": "node-588", "M": [ "GET", "PUT" ], "R": "coaps://temp451.example.com/s/tempC", "TS": 168537 } Figure 3: Access Request Message Example The example shows an Access Request message payload for the resource "/s/tempC" on the Resource Server "temp451.example.com". Requested operations in attribute M are GET and PUT. The requesting client is identified as "node-588". The attributes AS (that denotes the Authorization Server to use) and TS (a nonce generated by RS) are taken from the AS Information message from RS. The response to an Authorization Request is delivered by AM back to C in a Ticket Transfer message. 3.5. Ticket Request Message When AM receives an Access Request message from C it MAY return a cached response if it is known to be fresh. Otherwise, it checks whether the request payload is of type "application/dcaf+json" and contains at least the fields AS, CI, M, and R. AM MUST respond with 4.00 (Bad Request) if the type is "application/dcaf+json" and any of these fields is missing or does not conform to the format described in Section 5. Content formats other than application/dcaf+json are out of scope of this specification. Gerdes, et al. Expires April 24, 2014 [Page 11] Internet-Draft DCAF October 2013 When the payload is correct, AM creates a Ticket Request message from the Access Request received from C as follows: 1. The destination of the Ticket Request message is derived from the authority information in the URI contained in field "AS" of the Access Request message payload. 2. The request method is POST. 3. The request URI is constructed from the AS field received in the Access Request message payload. 4. The payload is copied from the Access Request sent by C. To send the Ticket Request message to AS a secure channel between AM and AS MUST be used. Depending on the URI scheme used in the AS field of the Access Request message payload, this could be, e.g., a DTLS channel (for "coaps") or a TLS connection (for "https"). AM and AS MUST be able to mutually authenticate each other, e.g. based on a public key infrastructure. (Refer to Section 8 for a detailed discussion of the trust relationship between authentication managers and authorization servers.) 3.6. Ticket Grant Message When AS has received a Ticket Request message it has to evaluate the access request information contained therein. First, it checks whether the request payload is of type "application/dcaf+json" and contains at least the fields AS, CI, M, and R. AS MUST respond with 4.00 (Bad Request) for CoAP (or 400 for HTTP) if the type is "application/dcaf+json" and any of these fields is missing or does not conform to the format described in Section 5. AS decides whether or not access is granted to the requested resource and then creates a Ticket Grant message that reflects the result. To grant access to the requested resource, AS creates an access ticket comprised of a Face and a Verifier as described in Section 4.1. The Ticket Grant message then is constructed as a success response indicating attached content, i.e. 2.05 for CoAP, or 200 for HTTP, respectively. The payload of the Ticket Grant message is a data structure that contains the result of the access request. When access is granted, the data structure contains the ticket's Face, the Verifier and the Session Key Generation Method. The Ticket Grant message MAY provide cache-control options to enable intermediaries to cache the response. The message MAY be cached according to the rules defined in [I-D.ietf-core-coap] to facilitate Gerdes, et al. Expires April 24, 2014 [Page 12] Internet-Draft DCAF October 2013 ticket retrieval when C has crashed and wants to recover the DTLS session with RS. AS sets Max-Age according to the ticket lifetime in its response (Ticket Grant Message). Figure 4 shows an example Ticket Grant message using CoAP. The Face/ Verifier information is transferred as a JSON data structure as specified in Section 5. The Max-Age option tells the receiving AM how long this ticket will be valid. 2.05 Content Content-Format: application/dcaf+json Max-Age: 86400 { "F": { "AI": { "Role" : 3 }, "CI": "2001:db8:ab9:1234:7920:3133:ae5f:87", "TS": "2013-07-10T10:04:12.391", "L": 86400, "G": "hmac_sha256" }, "V": "w+ZeJx5MxIEkt7yBMWjX6ztSYcIBTz+sv4z98m+PUEY=" } Figure 4: Example Ticket Grant Message A Ticket Grant message that declines any operation on the requested resource is illustrated in Figure 5. As no ticket needs to be issued, an empty payload is included with the response. 2.05 Content Content-Format: application/dcaf+json Figure 5: Example Ticket Grant Message With Reject 3.7. Ticket Transfer Message A Ticket Transfer message delivers the access information sent by AS in a Ticket Grant message to the requesting client C. The Ticket Transfer message is the response to the Access Request message sent from C to AM and includes any access information from AS contained in the Ticket Grant message. 3.8. DTLS Channel Setup Between C and RS Using the information contained in a positive response to its Access Request (i.e. a Ticket Transfer message that contains a Face and a Verifier), C can initiate establishment of a new DTLS channel with Gerdes, et al. Expires April 24, 2014 [Page 13] Internet-Draft DCAF October 2013 RS. To use DTLS with pre-shared keys, C follows the PSK key exchange algorithm specified in Section 2 of [RFC4279], with the following additional requirements: 1. C sets the psk_identity field of the ClientKeyExchange message to the ticket Face received in the Ticket Transfer message. 2. C uses the ticket Verifier as PSK when constructing the premaster secret. Note1: As RS cannot provide C with a meaningful PSK identity hint in response to C's ClientHello message, RS SHOULD NOT send a ServerKeyExchange message. Note2: According to [I-D.ietf-core-coap], CoAP implementations MUST support the ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655]. C is therefore expected to offer at least this ciphersuite to RS. Note3: The ticket is constructed by AS such that RS can derive the authorization information as well as the PSK (refer to Section 6 for details). 3.9. Authorized Resource Request Message Successful establishment of the DTLS channel between C and RS ties the authorization information contained in the psk_identity field to this channel. Any request that RS receives on this channel is checked against these authorization rules. Incoming CoAP requests that are not Authorized Resource Requests MUST be rejected by RS with 4.01 response as described in Section 3.2. RS SHOULD treat an incoming CoAP request as Authorized Resource Request if the following holds: 1. The message was received on a secure channel that has been established using the procedure defined in Section 3.8. 2. The authorization information tied to the secure channel is valid. 3. The request is destined for RS. 4. The resource URI specified in the request is covered by the authorization information. 5. The request method is an authorized action on the resource with respect to the authorization information. Gerdes, et al. Expires April 24, 2014 [Page 14] Internet-Draft DCAF October 2013 Note that the authorization information is not restricted to a single resource URI. For example, role-based authorization can be used to authorize a collection of semantically connected resources simultaneously. Implicit authorization also provides access rights to authenticated clients for all actions on all resources that RS offers. As a result, C can use the same DTLS channel not only for subsequent requests for the same resource (e.g. for block-wise transfer as defined in [I-D.ietf-core-block] or refreshing observe- relationships [I-D.ietf-core-observe]) but also for requests to distinct resources. Incoming CoAP requests received on a secure channel according to the procedure defined in Section 3.8 MUST be rejected 1. with response code 4.03 (Forbidden) when the resource URI specified in the request is not covered by the authorization information, and 2. with response code 4.05 (Method Not Allowed) when the resource URI specified in the request covered by the authorization information but not the requested action. Since AS may limit the set of requested actions in its Ticket Grant message, C cannot know a priori if a an Authorized Resource Request will succeed. Gerdes, et al. Expires April 24, 2014 [Page 15] Internet-Draft DCAF October 2013 4. Ticket Access tokens in DCAF are tickets that consist of two parts, namely the Face and the Verifier. 4.1. Face Face is the part of the ticket generated for RS. Face MUST contain all information needed for authorized access to a resource: o Authorization Information o Client Identifier o A timestamp generated by AS Optionally, Face MAY also contain: o A lifetime (optional) o A DTLS pre-shared key (optional) RS MUST verify the integrity of Face, i.e. the information contained in Face stems from AS and was not manipulated by anyone else. Face MUST contain a timestamp to verify that the contained information is fresh. As constrained devices may not have a clock, timestamps MAY be generated using the clock ticks since the last reboot. To circumvent synchronization problems the timestamp MAY be generated by RS and included in the first AS Information message. Alternatively, AS MAY generate the timestamp. In this case, AS and RS MUST use a time synchronization mechanism to make sure that RS interprets the timestamp correctly. Face MAY be encrypted. If Face contains a DTLS PSK, the whole content of Face MUST be encrypted. Note: The integrity of Face can be ensured by various means. Face may be encrypted by AS with a key it shares with RS. Alternatively, RS can use a mechanism to generate the DTLS PSK which includes Face and is only able to calculate the correct key with the correct Face (refer to Section 6 for details). 4.2. Verifier The Verifier part of the ticket is generated for C. It contains the DTLS PSK for C. The Verifier MUST NOT be transmitted over insecure channels. Gerdes, et al. Expires April 24, 2014 [Page 16] Internet-Draft DCAF October 2013 4.3. Revocation The existence of access tickets SHOULD be limited in time. This can be achieved either by explicit Revocation Messages to invalidate a ticket or implicitly by attaching a lifetime to the ticket. 4.3.1. Lifetime Tickets MAY have a lifetime. AS is responsible for defining the ticket lifetime. If AS sets a lifetime for a ticket, AS and RS MUST use a time synchronization method to ensure that RS is able to interpret the lifetime correctly. RS SHOULD end the DTLS connection to C if the lifetime of a ticket has run out and it MUST NOT accept new requests. RS MUST NOT accept tickets with an invalid lifetime. Note: Defining reasonable ticket lifetimes is difficult to accomplish. How long a client needs to access a resource depends heavily on the application scenario and may be difficult to decide for AS. 4.3.2. Revocation Messages AS MAY revoke tickets by sending a ticket revocation message to RS. If RS receives a ticket revocation message, it MUST end the DTLS connection to C and MUST NOT accept any further requests from C. If ticket revocation messages are used, RS MUST check regularly if AS is still available. If RS cannot contact AS, it MUST end all DTLS connections and reject any further requests from C. Note: The loss of the connection between RS and AS prevents all access to RS. This might especially be a severe problem if AS is responsible for several Resource Servers or even a whole network. Gerdes, et al. Expires April 24, 2014 [Page 17] Internet-Draft DCAF October 2013 5. Payload Format and Encoding (application/dcaf+json) Various messages types of the DCAF protocol carry payloads to express authorization information and parameters for generating the DTLS PSK to be used by C and RS. In this section, a representation in JavaScript Object Notation (JSON, [RFC4627]) is defined. The following attributes are defined: AS: Authorization Server. This attribute denotes the authorization server that is in charge of the resource specified in attribute R. The attribute's value is a string that contains an absolute URI according to Section 4.3 of [RFC3986]. AI: Authorization Information. A data structure used to convey authorization information from AS to RS (see below). CI: Client Identity. A string that identifies the initiator of the authorization request. This label MAY be a fully qualified domain name, an IP address, or any other character literal that is used by the Authorization Server to decide whether or not access is granted to the requesting entity. E: Encrypted Ticket Face. A string containing an encrypted ticket Face encoded as base64 according to Section 4 of [RFC4648]. K: Key. A string that identifies the shared key between RS and AS that can be used to decrypt the contents of E. If the attribute E is present and no attribute K has been specified, the default is to use the current session key for the secured channel between RS and AS. M: Methods. The list of actions to be performed on the resource R, encoded as an array of strings. In an authorization request, this list contains the actions that the initiator of the request wants to perform. In an authorization ticket, this attribute denotes the actions that are permitted. R: Resource. A string that denotes the absolute URI of the resource to be accessed. As the access ticket is requested in order to establish a DTLS connection with the server that hosts this resource, the URI scheme typically is "coaps". TS: Time Stamp. An optional time stamp that indicates the instant when the access ticket request was formed. This attribute can be used by the resource server in an AS Information message to convey a time stamp in its local time scale (e.g. when it does not have a real time clock with synchronized global time). When the Gerdes, et al. Expires April 24, 2014 [Page 18] Internet-Draft DCAF October 2013 attribute's value is encoded as a string, it MUST contain a valid UTC timestamp without time zone information. When encoded as integer, TS contains a system timestamp relative to the local time scale of its generator, usually RS. L: Lifetime. A lifetime of the ticket. When encoded as a string, L MUST denote the ticket's expiry time as a valid UTC timestamp without time zone information. When encoded as an integer, L MUST denote the ticket's validity period in seconds relative to TS. G: DTLS PSK Generation Method. A string that identifies the method that RS MUST use to derive the DTLS PSK from the ticket Face. This attribute MUST NOT be used when attribute V is present within the contents of F. F: Ticket Face. An object containing the fields AI, CI, TS, and optionally G, L and V. V: Ticket Verifier. A string containing the shared secret between C and RS, encoded as base64 according to Section 4 of [RFC4648]. The exact specification of AI is out of scope for this document. AI may, e.g., contain a single role identifier known by RS, or an array of pairs (M, R) with M and R defined as above. As AI is used to authorize resource access as defined in Section 3.9, RS MUST be able to interpret the contained information. 5.1. Examples The following example specifies an Authorization Server that will be accessed using HTTP over TLS. The request URI is set to "/a?ep=% 5B2001:DB8::dcaf:1234%5D" (hence denoting the endpoint address to authorize). TS denotes a local timestamp in UTC. POST /a?ep=%5B2001:DB8::dcaf:1234%5D HTTP/1.1 Host: as-rs.example.com Content-Type: application/dcaf+json { "AS": "https://as-rs.example.com/a?ep=%5B2001:DB8::dcaf:1234%5D", "CI": "2001:DB8::dcaf:1234", "M": [ "GET" ], "R": "coaps://temp451.example.com/s/tempC", "TS": "2013-07-14T11:58:22.923" } The following example shows a ticket for the distributed key Gerdes, et al. Expires April 24, 2014 [Page 19] Internet-Draft DCAF October 2013 generation method (cf. Section 6.2), comprised of a Face (F) and a Verifier (V). The Face data structure contains authorization information AI with an application-specific role identifier, a client identifier, a timestamp using the local time scale of RS, and a lifetime relative to RS's time scale. The DTLS PSK Generation Method is set to "hmac_sha256" denoting that the distributed key derivation is used as defined in Section 6.2 with SHA-256 as HMAC function. The Verifier V contains a shared secret to be used as DTLS PSK between C and RS. HTTP/1.1 200 OK Content-Type: application/dcaf+json { "F": { "AI": { "Role" : 3 }, "CI": "2001:db8:ab9:1234:7920:3133:ae5f:87", "TS": 2938749, "L": 3600, "G": "hmac_sha256" }, "V": "zrPOuc6xzr/Pjc+Bz4TOuSDOvM61IM68zq3Ou865Cg==" } The Face may be encrypted as illustrated in the following example. Here, the field E carries an encrypted and base64-encoded Face data structure that contains the same information as the previous example, and an additional Verifier. Encryption was done with a secret shared by AS and RS. (This example uses AES128_CCM with the secret { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f } and RS's timestamp { 0x00, 0x2C, 0xD7, 0x7D } as nonce.) Line breaks have been inserted to improve readability. The attribute K describes the identity of the key to be used by RS to decrypt the contents of attribute E. Here, The value "key0" in this example is used to indicate that the shared session key between RS and AS was used for encrypting E. { "E": "rjtolfjyX9q7Emxgsnz+nf0xTQhe1MjzZBRoIEW4vmSVlyJdW4KDgVtW LyBnQSVX0lmVpxUYbdNuk/5PkCOJBeex0obiEBC1UmKoJfJfjy7bLQhq k9HuJD7cvjHNOVZtNZf5qrxt7xJSoZFe6j/SJuxGNH/72SPDrdMQeXJI pX6vCJB698FcRDOXh/ipi9KT8YWeo/ljUMgJc+LI", "K": "key0", "V": "zrPOuc6xzr/Pjc+Bz4TOuSDOvM61IM68zq3Ou865Cg==" Gerdes, et al. Expires April 24, 2014 [Page 20] Internet-Draft DCAF October 2013 } The decrypted contents of E are depicted below (whitespace has been added to improve readability). The presence of the attribute V indicates that the DTLS PSK Transfer is used to convey the session key (cf. Section 6.1). "F":{"AI":{"Role":3}, "CI":"2001:db8:ab9:1234:7920:3133:ae5f:87", "TS":2938749, "L":3600, "V":"zrPOuc6xzr/Pjc+Bz4TOuSDOvM61IM68zq3Ou865Cg=="} Gerdes, et al. Expires April 24, 2014 [Page 21] Internet-Draft DCAF October 2013 6. DTLS PSK Generation Methods One goal of the DCAF protocol is to provide for a DTLS PSK shared between C and RS. AS and RS MUST negotiate the method for the DTLS PSK generation. 6.1. DTLS PSK Transfer The DTLS PSK is generated by AS and transmitted to C and RS using a secure channel. The DTLS PSK transfer method is defined as follows: o AS generates the DTLS PSK using an algorithm of its choice o AS MUST include a representation of the DTLS PSK in Face and encrypt it together with all other information in Face with a key K(AS,RS) it shares with RS. How AS and RS exchange K(AS,RS) is not in the scope of this document. AS and RS MAY use their preshared key as K(AS,RS). o AS MUST include a representation of the DTLS PSK in the Verifier. o As AS and C do not have a shared secret, the Verifier MUST be transmitted to C using encrypted channels. o RS MUST decrypt Face using K(AS,RS) 6.2. Distributed Key Derivation AS generates a DTLS PSK for C which is transmitted using a secure channel. RS generates its own version of the DTLS PSK using the information contained in Face (see also Section 4.1). The distributed key derivation method is defined as follows: o AS and RS both generate the DTLS PSK using the information. included in Face. They use an HMAC algorithm on Face with a shared key. The result serves as the DTLS PSK. How AS and RS negotiate the used HMAC algorithm is not in the scope of this document. They MAY however use the HMAC algorithm they use for their DTLS connection. o AS MUST include a representation of the DTLS PSK in the Verifier. o As AS and C do not have a shared secret, the Verifier MUST be transmitted to C using encrypted channels. Gerdes, et al. Expires April 24, 2014 [Page 22] Internet-Draft DCAF October 2013 o AS MUST NOT include a representation of the DTLS PSK in Face. o AS MUST NOT encrypt Face. Gerdes, et al. Expires April 24, 2014 [Page 23] Internet-Draft DCAF October 2013 7. Authorization Configuration For the protocol defined in this document, proper configuration of AS is crucial. The principal who owns the resources hosted by RS (i.e. the Resource Owner) needs to define permissions for the resources. The data representation of these permissions are not in the scope of this document. Gerdes, et al. Expires April 24, 2014 [Page 24] Internet-Draft DCAF October 2013 8. Trust Relationships C trusts AM, and RS trusts AS. Obviously, AM trusts C with the specific permissions it hands over to it. How this trust is established, is not in the scope of this document. It may be achieved by using a bootstrapping mechanism similar to [bergmann12]. Additionally, AS and AM need to have a trust relationship established. Its establishment is also not in the scope of this document. It fulfills the following conditions: 1. AS has means to authenticate AM (e.g. it has a certificate of AM or a PKI in which AM is included) and vice versa 2. As far as AS needs to rely on the different clients of AM to receive different permissions, it can be sure that AM correctly identifies these clients towards AS and does not leak tickets that have been generated for a specific client C to another client. AS trusts C indirectly because it trusts AM and AM vouches for C. The DCAF Protocol does not provide any means for AS to validate that a resource requests stems from C. C indirectly trusts AS with some potentially confidential information, and that AS correctly represents RS, because AM trusts AS. AM trusts RS indirectly because it trusts AS and AS vouches for RS. C implicitly trusts RS with some potentially confidential information because it trusts AM and because RS can prove that it shares a key with AS. AM <--------------------> AS /|\ /|\ | | \|/ \|/ C ..................... RS Gerdes, et al. Expires April 24, 2014 [Page 25] Internet-Draft DCAF October 2013 9. Listing Authorization Server Information in a Resource Directory CoAP utilizes the Web Linking format [RFC5988] to facilitate discovery of services in an M2M environment. [RFC6690] defines specific link parameters that can be used to describe resources to be listed in a resource directory [I-D.ietf-core-resource-directory]. This section defines a resource type "auth-request" that can be used by clients to retrieve the request URI for a server's authorization service. When used with the parameter rt in a web link, "auth- request" indicates that the corresponding target URI can be used in a POST message to request authorization for the resource and action that are described in the request payload. The Content-Format "application/dcaf+json" with numeric identifier TBD1 defined in this specification MAY be used to express access requests and their responses. The following example shows the web link used by AM in this document to relay incoming Authorization Request messages to AS. (Whitespace is included only for readability.) ;rt="auth-request";ct=TBD1 ;title="Contact Remote Authorization Server" The resource directory that hosts the resource descriptions of RS could list the following description. In this example, the URI "ep/ node138/a/switch2941" is relative to the resource context "coaps://as-rs.example.com/", i.e. the authorization server AS. ;rt="auth-request";ct=TBD1;ep="node138" ;title="Request Client Authorization" ;anchor="coaps://as-rs.example.com/" Gerdes, et al. Expires April 24, 2014 [Page 26] Internet-Draft DCAF October 2013 10. Examples This section gives a number of short examples with message flows for the initial Unauthorized Resource Request and the subsequent retrieval of a ticket from AS. The notation here follows the role conventions defined in Section 1.2.1. The payload format is encoded as proposed in Section 5. The IP address of AS is 2001:DB8::1, the IP address of RS is 2001:DB8::dcaf:1234, and C's IP address is 2001: DB8::c. 10.1. Access Granted This example shows an Unauthorized PUT request from C to RS that is answered with an AS Information message. C then sends a POST request to AM with a description of its intended request. AM forwards this request to AS using CoAP over a DTLS-secured channel. The response from AS contains an access ticket that is relayed back to AM. C --> RS PUT a/switch2941 [Mid=1234] Content-Format: application/senml+json {e: [{"bv": "1"}]} C <-- RS 4.01 Unauthorized [Mid=1234] Content-Format: application/dcaf+json {"AS": "coaps://[2001:DB8::1]/ep/node138/a/switch2941"} C --> AM POST client-authorize [Mid=1235,Token="tok"] Content-Format: application/dcaf+json {"AS": "coaps://[2001:DB8::1]/ep/node138/a/switch2941", "CI": "2001:DB8::c", "M": [ "PUT" ], "R": "coaps://[2001:DB8::dcaf:1234]/a/switch2941" } AM --> AS [Mid=23146] POST ep/node138/a/switch2941 Content-Format: application/dcaf+json {"AS": "coaps://[2001:DB8::1]/ep/node138/a/switch2941", "CI": "2001:DB8::c", "M": [ "PUT" ], "R": "coaps://[2001:DB8::dcaf:1234]/a/switch2941" } AM <-- AS 2.05 Content [Mid=23146] Gerdes, et al. Expires April 24, 2014 [Page 27] Internet-Draft DCAF October 2013 Content-Format: application/dcaf+json { "F": { "AI": { "R" : "a/switch2941", "M" : [ "GET", "PUT" ] }, "CI": "2001:DB8::c", "TS": "2013-07-04T20:17:38.002, "G": "hmac_sha256" }, "V": "yYVLYZZ5Nssbn0by3fqel9WK6jHdoYyNej2d/kSuBLw=" } C <-- AM 2.05 Content [Mid=1235,Token="tok"] Content-Format: application/dcaf+json { "F": { "AI": { "R" : "a/switch2941", "M" : [ "GET", "PUT" ] }, "CI": "2001:DB8::c", "TS": "2013-07-04T20:17:38.002", "G": "hmac_sha256" }, "V": "MR5TMrNngbSEAkFl0akmsdbmzF0gqxGI/d3KjwT8GxI=" } C --> RS ClientHello (TLS_PSK_WITH_AES_128_CCM_8) C <-- RS ServerHello (TLS_PSK_WITH_AES_128_CCM_8) ServerHelloDone C --> RS ClientKeyExchange psk_identity='"F":{"AI":{"R":"a/switch2941","M":["GET","PUT"]}, "CI": "2001:DB8::c", "TS": "2013-07-04T20:17:38.002", "G": "hmac_sha256"') (C decodes the contents of V and uses the result as PSK) ChangeCipherSpec Finished (RS calculates PSK from AI, CI, TS and its session key HMAC_sha256("{\"R\":\"a/switch2941\",\"M\":[\"GET\",\"PUT\"]}"+ "2001:DB8::c"+"2013-07-04T20:17:38.002", "secret") = 311e5332b36781b484024165d1a926b1d6e6cc5d20ab1188fdddca8f04fc1b12 ) C <-- RS ChangeCipherSpec Finished Gerdes, et al. Expires April 24, 2014 [Page 28] Internet-Draft DCAF October 2013 10.2. Access Denied This example shows a denied Authorization request for the DELETE operation. C --> RS DELETE a/switch2941 C <-- RS 4.01 Unauthorized Content-Format: application/dcaf+json {"AS": "coaps://[2001:DB8::1]/ep/node138/a/switch2941"} C --> AM POST client-authorize Content-Format: application/dcaf+json {"AS": "coaps://[2001:DB8::1]/ep/node138/a/switch2941", "CI": "2001:DB8::c", "M": [ "DELETE" ], "R": "coaps://[2001:DB8::dcaf:1234]/a/switch2941" } AM --> AS POST ep/node138/a/switch2941 Content-Format: application/dcaf+json {"AS": "coaps://[2001:DB8::1]/ep/node138/a/switch2941", "CI": "2001:DB8::c", "M": [ "DELETE" ], "R": "coaps://[2001:DB8::dcaf:1234]/a/switch2941" } AM <-- AS 2.05 Content Content-Format: application/dcaf+json C <-- AM 2.05 Content Content-Format: application/dcaf+json 10.3. Access Restricted This example shows a denied Authorization request for the operations GET, PUT, and DELETE. AS grants access for PUT only. Gerdes, et al. Expires April 24, 2014 [Page 29] Internet-Draft DCAF October 2013 AM --> AS POST ep/node138/a/switch2941 Content-Format: application/dcaf+json {"AS": "coaps://[2001:DB8::1]/ep/node138/a/switch2941", "CI": "2001:DB8::c", "M": [ "GET", "PUT", "DELETE" ], "R": "coaps://[2001:DB8::dcaf:1234]/a/switch2941" } AM <-- AS 2.05 Content Content-Format: application/dcaf+json { "F": { "AI": { "R" : "a/switch2941", "M" : [ "GET", "PUT" ] }, "CI": "2001:DB8::c", "TS": "2013-07-04T21:33:11.930", "G": "hmac_sha256" }, "V": "NZ8Q3o8P4eHOzkoscaUpoRvrn5d74Cscw/aXAiNmC/k=" } 10.4. Implicit Authorization This example shows an Authorization request using implicit authorization. AM initially requests the actions GET and POST on the resource "coaps://[2001:DB8::dcaf:1234]/a/switch2941". AS returns a ticket that has no AI field in its ticket Face, hence implicitly authorizing C. AM --> AS POST ep/node138/a/switch2941 Content-Format: application/dcaf+json {"AS": "coaps://[2001:DB8::1]/ep/node138/a/switch2941", "CI": "2001:DB8::c", "M": [ "GET", "POST" ], "R": "coaps://[2001:DB8::dcaf:1234]/a/switch2941" } AM <-- AS 2.05 Content Content-Format: application/dcaf+json { "F": { "CI": "2001:DB8::c", "TS": "2013-07-16T10:15:43.663", "G": "hmac_sha256" }, "V": "mCYtLG/oZIWki/mCFNJ4nxI0WMPwlDKAnXIo/ir2dtI=" } Gerdes, et al. Expires April 24, 2014 [Page 30] Internet-Draft DCAF October 2013 11. Security Considerations As this protocol builds on transitive trust between authorization servers as mentioned in Section 8, AS has no direct means to validate that a resource request originates from C. It has to trust AM that it correctly vouches for C and that it does not give authorization tickets meant for C to another client nor disclose the contained session key. The Authorization Server also could constitute a single point of failure. If the Authorization Server fails, the resources on all Resource Servers it is responsible for cannot be accessed any more. Thus, it is crucial for large networks to use Authorization Servers in a redundant setup. Gerdes, et al. Expires April 24, 2014 [Page 31] Internet-Draft DCAF October 2013 12. IANA Considerations The following registrations are done following the procedure specified in [RFC6838]. Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]" with the RFC number of this specification. 12.1. dcaf+json Media Type Registration Type name: application Subtype name: dcaf+json Required parameters: none Optional parameters: none Encoding considerations: Must be encoded as using a subset of the encoding allowed in [RFC4627]. Specifically, only the primitive data types String and Number are allowed. The type Number is restricted to unsigned integers (i.e., no negative numbers, fractions or exponents are allowed). Encoding MUST be UTF-8. These restrictions simplify implementations on devices that have very limited memory capacity. Security considerations: TBD Interoperability considerations: TBD Published specification: [RFC-XXXX] Applications that use this media type: TBD Additional information: Magic number(s): none File extension(s): dcaf Macintosh file type code(s): none Person & email address to contact for further information: TBD Intended usage: COMMON Restrictions on usage: None Gerdes, et al. Expires April 24, 2014 [Page 32] Internet-Draft DCAF October 2013 Author: TBD Change controller: IESG 12.2. CoAP Content Format Registration This document specifies a new media type application/dcaf+json (cf. Section 12.1). For use with CoAP, a numeric Content-Format identifier is to be registered in the "CoAP Content-Formats" sub- registry within the "CoRE Parameters" registry. Note to RFC Editor: Please replace all occurrences of "RFC-XXXX" with the RFC number of this specification. +-----------------------+----------+------+------------+ | Media type | Encoding | Id. | Reference | +-----------------------+----------+------+------------+ | application/dcaf+json | - | TBD1 | [RFC-XXXX] | +-----------------------+----------+------+------------+ Gerdes, et al. Expires April 24, 2014 [Page 33] Internet-Draft DCAF October 2013 13. References 13.1. Normative References [I-D.ietf-core-coap] Shelby, Z., Hartke, K., and C. Bormann, "Constrained Application Protocol (CoAP)", draft-ietf-core-coap-18 (work in progress), June 2013. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", RFC 4279, December 2005. [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006. [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012. [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, January 2013. 13.2. Informative References [I-D.ietf-core-block] Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP", draft-ietf-core-block-13 (work in progress), October 2013. [I-D.ietf-core-observe] Hartke, K., "Observing Resources in CoAP", draft-ietf-core-observe-11 (work in progress), October 2013. [I-D.ietf-core-resource-directory] Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource Directory", draft-ietf-core-resource-directory-00 (work in progress), June 2013. [RFC4627] Crockford, D., "The application/json Media Type for JavaScript Object Notation (JSON)", RFC 4627, July 2006. Gerdes, et al. Expires April 24, 2014 [Page 34] Internet-Draft DCAF October 2013 [RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010. [RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for Transport Layer Security (TLS)", RFC 6655, July 2012. [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, August 2012. [bergmann12] Bergmann, O., Gerdes, S., Schaefer, S., Junge, F., and C. Bormann, "Secure Bootstrapping of Nodes in a CoAP Network", IEEE Wireless Communications and Networking Conference Workshops (WCNCW), April 2012. Gerdes, et al. Expires April 24, 2014 [Page 35] Internet-Draft DCAF October 2013 Authors' Addresses Stefanie Gerdes Universitaet Bremen TZI Postfach 330440 Bremen D-28359 Germany Phone: +49-421-218-63906 Email: gerdes@tzi.org Olaf Bergmann Universitaet Bremen TZI Postfach 330440 Bremen D-28359 Germany Phone: +49-421-218-63904 Email: bergmann@tzi.org Carsten Bormann Universitaet Bremen TZI Postfach 330440 Bremen D-28359 Germany Phone: +49-421-218-63921 Email: cabo@tzi.org Gerdes, et al. Expires April 24, 2014 [Page 36]