| ACE Working Group | S. Gerdes |
| Internet-Draft | O. Bergmann |
| Intended status: Standards Track | C. Bormann |
| Expires: September 10, 2015 | Universität Bremen TZI |
| March 09, 2015 |
Delegated CoAP Authentication and Authorization Framework (DCAF)
draft-gerdes-ace-dcaf-authorize-02
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.
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 September 10, 2015.
Copyright (c) 2015 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 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.
The Constrained Application Protocol (CoAP) [RFC7252] 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 system 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 for authorization.
[I-D.gerdes-ace-actors] describes an architecture that is designed to help constrained nodes with authorization-related tasks by introducing less-constrained nodes. These Authorization Managers perform complex security tasks for their nodes such as managing keys for numerous devices, and enable the constrained nodes to enforce the authorization policies of their principals.
DCAF uses access tokens to implement this architecture. A device that wants to access an item of interest on a constrained node first has to gain permission in the form of a token from the node’s Authorization Manager.
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] between two nodes and to securely transmit authorization tickets.
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].
Readers are expected to be familiar with the terms and concepts defined in [I-D.gerdes-ace-actors].
Within the DCAF Architecture each Server (S) has a Server Authorization Manger (SAM) which conducts the authentication and authorization for S. S and SAM 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 S, a Client (C) has to request an access ticket from the SAM serving S either directly or, if it is a constrained device, using its Client Authorization Manager (CAM). In the following, we always discuss the CAM role separately, even if that is co-located within a (more powerful) C (see section Section 11 for details about co-located actors).
CAM decides if S is an authorized source for R according to the policies set by COP and in this case transmits the request to SAM. If SAM decides that C is allowed to access the resource according to the policies set by ROP, it generates a DTLS pre-shared key (PSK) for the communication between C and S and wraps it into an access ticket. For explicit access control, SAM adds the detailed access permissions to the ticket in a way that CAM and S can interpret. CAM checks if the permissions in the access ticket comply with COP’s authorization policies for C, and if this is the case sends it to C. After C presented the ticket to S, C and S can communicate securely.
To be able to provide for the authentication and authorization services, an Authorization Manager has to fulfill several requirements:
The DCAF protocol comprises three parts:
In Figure 1, a DCAF protocol flow is depicted (messages in square brackets are optional):
CAM C S SAM | <== DTLS chan. ==> | | <== DTLS chan. ==> | | | [Resource Req.-->] | | | | | | | | [<-- SAM Info.] | | | | | | | <-- Access Req. | | | | | | | | <==== TLS/DTLS channel (CAM/SAM Mutual Authentication) ====> | | | | | | Ticket Request ------------------------------------------> | | | | | | <------------------------------------------ Ticket Grant | | | | | | Ticket Transf. --> | | | | | | | | | <== DTLS chan. ==> | | | | Auth. Res. Req. -> | |
Figure 1: Protocol Overview
To determine the SAM in charge of a resource hosted at the S, C MAY send an initial Unauthorized Resource Request message to S. S then denies the request and sends the address of its SAM back to C.
Instead of the initial Unauthorized Resource Request message, C MAY look up the desired resource in a resource directory (cf. [I-D.ietf-core-resource-directory]) that lists S’s resources as discussed in Section 9.
Once C knows SAM’s address, it can send a request for authorization to SAM using its own CAM. CAM and SAM authenticate each other and each determine if the request is to be authorized. If it is, SAM 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 S to prove its right to access. With their respective parts of the ticket, C and S are able to establish a secure channel.
The following sections specify how CoAP is used to interchange access-related data between S and SAM so that SAM can provide C and S with sufficient information to establish a secure channel, and simultaneously convey authorization information specific for this communication relationship to S.
This document uses Concise Binary Object Representation (CBOR, [RFC7049]) to express authorization information as set of attributes passed in CoAP payloads. Notation and encoding options are discussed in Section 5.
The optional Unauthorized Resource Request message is a request for a resource hosted by S for which no proper authorization is granted. S MUST treat any CoAP request as Unauthorized Resource Request message when any of the following holds:
Note: These conditions ensure that S can handle requests autonomously once access was granted and a secure channel has been established between C and S.
Unauthorized Resource Request messages MUST be denied with a client error response. In this response, the Server MUST provide proper SAM Information to enable the Client to request an access ticket from S’s SAM 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 S has no valid access ticket for C. If S has an access ticket for C but not for the resource that C has requested, S MUST reject the request with a 4.03 (Forbidden). If S has an access ticket for C but it does not cover the action C requested on the resource, S MUST reject the request with a 4.05 (Method Not Allowed).
The SAM Information Message is sent by S as a response to an Unauthorized Resource Request message (see Section 3.2) to point the sender of the Unauthorized Resource Request message to S’s SAM. The SAM information is a set of attributes containing an absolute URI (see Section 4.3 of [RFC3986]) that specifies the SAM in charge of S.
The message MAY also contain a timestamp generated by S.
Figure 2 shows an example for an SAM Information message payload using CBOR diagnostic notation. (Refer to Section 5 for a detailed description of the available attributes and their semantics.)
4.01 Unauthorized
Content-Format: application/dcaf+cbor
{SAM: "coaps://sam.example.com/authorize", TS: 168537}
Figure 2: SAM Information Payload Example
In this example, the attribute SAM points the receiver of this message to the URI “coaps://sam.example.com/authorize” to request access permissions. The originator of the SAM Information payload (i.e. S) uses a local clock that is loosely synchronized with a time scale common between S and SAM (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.
The examples in this document are written in CBOR diagnostic notation to improve readability. Figure 3 illustrates the binary encoding of the message payload shown in Figure 2.
a2 # map(2)
00 # unsigned(0) (=SAM)
78 21 # text(33)
636f6170733a2f2f73616d2e6578
616d706c652e636f6d2f617574686f72
697a65 # "coaps://sam.example.com/authorize"
05 # unsigned(5) (=TS)
1a 00029259 # unsigned(168537)
Figure 3: SAM Information Payload Example encoded in CBOR
To retrieve an access ticket for the resource that C wants to access, C sends an Access Request to its CAM. The Access Request is constructed as follows:
The request URI identifies a resource at CAM for handling authorization requests from C. The URI SHOULD be announced by CAM in its resource directory as described in Section 9.
The message payload is constructed from the SAM information that S has returned in its SAM 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:
An example Access Request from C to CAM is depicted in Figure 4. (Refer to Section 5 for a detailed description of the available attributes and their semantics.)
POST client-authorize
Content-Format: application/dcaf+cbor
{
SAM: "coaps://sam.example.com/authorize",
SAI: ["coaps://temp451.example.com/s/tempC", 5],
TS: 168537
}
Figure 4: Access Request Message Example
The example shows an Access Request message payload for the resource “/s/tempC” on the Server “temp451.example.com”. Requested operations in attribute SAI are GET and PUT.
The attributes SAM (that denotes the Server Authorization Manager to use) and TS (a nonce generated by S) are taken from the SAM Information message from S.
The response to an Authorization Request is delivered by CAM back to C in a Ticket Transfer message.
When CAM receives an Access Request message from C and COP specified authorization policies for C, CAM MUST check if the requested actions are allowed according to these policies. If this is not the case, CAM MUST send a 4.03 response.
If no authorization policies were specified or the requested action is allowed according to the authorization policies, CAM either returns a cached response or attempts to create a Ticket Request message.
CAM MAY return a cached response if it is known to be fresh according to Max-Age. CAM SHOULD NOT return a cached response if it expires in less than a minute.
If CAM does not send a cached response, it checks whether the request payload is of type “application/dcaf+cbor and contains at least the fields SAM and SAI. CAM MUST respond with 4.00 (Bad Request) if the type is “application/dcaf+cbor and any of these fields is missing or does not conform to the format described in Section 5. Content formats other than application/dcaf+cbor are out of scope of this specification.
If the payload is correct, CAM creates a Ticket Request message from the Access Request received from C as follows:
To send the Ticket Request message to SAM a secure channel between CAM and SAM MUST be used. Depending on the URI scheme used in the SAM field of the Access Request message payload (the less-constrained devices CAM and SAM do not necessarily use CoAP to communicate with each other), this could be, e.g., a DTLS channel (for “coaps”) or a TLS connection (for “https”). CAM and SAM 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 Client Authorization Managers and Server Authorization Managers.)
When SAM 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+cbor” and contains at least the fields SAM and SAI. SAM MUST respond with 4.00 (Bad Request) for CoAP (or 400 for HTTP) if the type is “application/dcaf+cbor” and any of these fields is missing or does not conform to the format described in Section 5.
SAM 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, SAM creates an access ticket comprised of a Face and the Client Information 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 Face, the Client Information, which at this point only consists of 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 [RFC7252] to facilitate ticket retrieval when C has crashed and wants to recover the DTLS session with S.
SAM SHOULD set Max-Age according to the ticket lifetime in its response (Ticket Grant Message).
Figure 5 shows an example Ticket Grant message using CoAP. The Face/Verifier information is transferred as a CBOR data structure as specified in Section 5. The Max-Age option tells the receiving CAM how long this ticket will be valid.
2.05 Content
Content-Format: application/dcaf+cbor
Max-Age: 86400
{ F: {
SAI: [ "/s/tempC", 7 ],
TS: 0("2013-07-10T10:04:12.391"),
L: 86400,
G: hmac_sha256
},
V: h'f89947160c73601c7a65cb5e08812026
6d0f0565160e3ff7d3907441cdf44cc9'
}
Figure 5: Example Ticket Grant Message
A Ticket Grant message that declines any operation on the requested resource is illustrated in Figure 6. As no ticket needs to be issued, an empty payload is included with the response.
2.05 Content
Content-Format: application/dcaf+cbor
Figure 6: Example Ticket Grant Message With Reject
A Ticket Transfer message delivers the access information sent by SAM 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 CAM and includes any access information from SAM contained in the Ticket Grant message.
The Authorization Information provided by SAM in the Ticket Grant Message may grant more permissions than C has requested. The authorization policies of COP and ROP may differ: COP might want restrict the resources C is allowed to access, and the actions that C is allowed to perform on the resource.
If CAM must ensure authorization policies COP configured , CAM MUST add Authorization Information for C (CAI) to the CI. Since C and CAM use a DTLS channel for communication, the autorization information does not need to be encrypted.
CAM includes the Face and Verifier sent by SAM in the Ticket Transfer message. CAM MUST NOT include any other information SAM provided. In particular, CAM MUST NOT include any CAI information provided by SAM.
Figure 7 shows an example Ticket Transfer message that conveys the permissions for actions GET, POST, PUT (but not DELETE) on the resource “/s/tempC” in field SAI. As CAM only wants to permit outbound GET requests, it restricts C’s permissions in the field CAI accordingly.
2.05 Content
Content-Format: application/dcaf+cbor
Max-Age: 86400
{ F: {
SAI: [ "/s/tempC", 7 ],
TS: 0("2013-07-10T10:04:12.391"),
L: 86400,
G: hmac_sha256
},
V: h'f89947160c73601c7a65cb5e08812026
6d0f0565160e3ff7d3907441cdf44cc9'
CAI: [ "/s/tempC", 1 ],
TS: 0("2013-07-10T10:04:12.855"),
L: 86400
}
Figure 7: Example Ticket Transfer Message
Using the information contained in a positive response to its Access Request (i.e. a Ticket Transfer message that contains a Face and a Client Information), C can initiate establishment of a new DTLS channel with S. 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:
Note1: As S cannot provide C with a meaningful PSK identity hint in response to C’s ClientHello message, S SHOULD NOT send a ServerKeyExchange message.
Note2: According to [RFC7252], 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 S.
Note3: The ticket is constructed by SAM such that S can derive the authorization information as well as the PSK (refer to Section 6 for details).
If the Client Information in the Ticket Transfer message contains CAI, C MUST ensure that it only sends requests that according to them are allowed. C therefore MUST check CAI, L and T before every request. If CAI is no longer valid according to L, C MUST terminate the DTLS connection with S and re-request the CAI from CAM using an Access Request Message.
On the Server side, successful establishment of the DTLS channel between C and S ties the SAM authorization information contained in the psk_identity field to this channel. Any request that S receives on this channel is checked against these authorization rules. Incoming CoAP requests that are not Authorized Resource Requests MUST be rejected by S with 4.01 response as described in Section 3.2.
S SHOULD treat an incoming CoAP request as Authorized Resource Request if the following holds:
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 S 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
Since SAM may limit the set of requested actions in its Ticket Grant message, C cannot know a priori if an Authorized Resource Request will succeed.
Once a security association exists between a Client and a Resource Server, the Client can update the Authorization Information stored at the Server at any time. To do so, the Client creates a new Access Request for the intended action on the respective resource and sends this request to its CAM which checks and relays this request to the Server’s SAM as described in Section 3.4.
Figure 8 depicts the message flow where C requests a new Access Tickets after a security association between C and S has been established using this protocol.
CAM C S SAM | <== DTLS chan. ==> | <== DTLS chan. ==> | <== DTLS chan. ==> | | | | | | | [Unauth. R. Req->] | | | |[<- 4.0x+SAM Info.] | | | | | | | <-- Access Req. | | | | | | | | <==== TLS/DTLS channel (CAM/SAM Mutual Authentication) ====> | | | | | | Ticket Request ------------------------------------------> | | | | | | <------------------------------------------ Ticket Grant | | | | | | Ticket Transf. --> | | | | | | | | | <== Update SAI ==> | |
Figure 8: Overview of Dynamic Update Operation
Processing the Ticket Request is done at the SAM as specified in Section 3.6, i.e. the SAM checks whether or not the requested operation is permitted by the Resource Principal’s policy, and then return a Ticket Grant message with the result of this check. If access is granted, the Ticket Grant message contains an Access Ticket comprised of a public Ticket Face and a private Ticket Verifier. This authorization payload is relayed by CAM to the Client in a Ticket Transfer Message as defined in Section 3.7.
The major difference between dynamic update of Authorization Information and the initial handshake is the handling of a Ticket Transfer message by the Client that is described in Section 3.10.1.
If the security association with S still exists and S has indicated support for session renegotiation according to [RFC5746], the ticket Face SHOULD be used to renegotiate the existing DTLS session. In this case, the ticket Face is used as psk_identity as defined in Section 3.8. Otherwise, the Client MUST perform a new DTLS handshake according to Section 3.8 that replaces the existing DTLS session.
After successful completion of the DTLS handshake S updates the existing SAM Authorization Information for C according to the contents of the ticket Face.
Access tokens in DCAF are tickets that consist of two parts, namely the Face and the Client Information. The Face goes to S, the CI goes to the Client. The Face and the CI are parts of the same ticket.
S only needs the information contained in the Ticket Face to authorize the client and make sure that SAM generated the Ticket Face (S cannot make authorization decisions by itself and hence needs SAM to do it). No additional information about the Client is needed. S keeps the Ticket Face as long as it is valid.
Face is the part of the ticket generated for S. Face MUST contain all information needed for authorized access to a resource:
Optionally, Face MAY also contain:
S MUST verify the integrity of Face, i.e. the information contained in Face stems from SAM 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 S and included in the first SAM Information message. Alternatively, SAM MAY generate the timestamp. In this case, SAM and S MUST use a time synchronization mechanism to make sure that S interprets the timestamp correctly.
Face MAY be encrypted. If Face contains a DTLS PSK, the whole content of Face MUST be encrypted.
The integrity of Face can be ensured by various means. Face may be encrypted by SAM with a key it shares with S. Alternatively, S can use a mechanism to generate the DTLS PSK which includes Face. S generates the key from the Face it received. The correct key can only be calculated with the correct Face (refer to Section 6 for details).
The CI part of the ticket is generated for C. It contains the Verifier, i.e. the DTLS PSK for C and MAY contain Client Authorization Information generated by CAM to provide the COP’s authorization policies to C. The Verifier MUST NOT be transmitted over insecure channels.
The existence of access tickets SHOULD be limited in time to avoid stale tickets that waste resources on S and C. This can be achieved either by explicit Revocation Messages to invalidate a ticket or implicitly by attaching a lifetime to the ticket.
Tickets MAY have a lifetime. SAM is responsible for defining the ticket lifetime. If SAM sets a lifetime for a ticket, SAM and S MUST use a time synchronization method to ensure that S is able to interpret the lifetime correctly. S SHOULD end the DTLS connection to C if the lifetime of a ticket has run out and it MUST NOT accept new requests. S MUST NOT accept tickets with an invalid lifetime.
If CAM provides CAI in the CI part of the ticket, CAM MAY add a lifetime for this CAI. If CI contains a lifetime, CAM and C MUST use a time synchronization method to ensure that C is able to interpret the lifetime correctly. C SHOULD end the DTLS connection to S and MUST NOT send new requests if the CAI in the ticket is no longer valid. C 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 SAM.
SAM MAY revoke tickets by sending a ticket revocation message to S. If S 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, S MUST check regularly if SAM is still available. If S cannot contact SAM, it MUST end all DTLS connections and reject any further requests from C.
Note: The loss of the connection between S and SAM prevents all access to S. This might especially be a severe problem if SAM is responsible for several Servers or even a whole network.
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 S. In this section, a representation in Concise Binary Object Representation (CBOR, [RFC7049]) is defined.
DCAF data structures are defined as CBOR maps that contain key value pairs. For efficient encoding, the keys defined in this document are represented as unsigned integers in CBOR, i. e. major type 0. For improved reading, we use symbolic identifiers to represent the corresponding encoded values as defined in Table 1.
| Encoded Value | Key |
|---|---|
| 0b000_00000 | SAM |
| 0b000_00001 | SAI |
| 0b000_00010 | CAI |
| 0b000_00011 | E |
| 0b000_00100 | K |
| 0b000_00101 | TS |
| 0b000_00110 | L |
| 0b000_00111 | G |
| 0b000_01000 | F |
| 0b000_01001 | V |
The following list describes the semantics of the keys defined in DCAF.
| Encoded Value | Mnemonic | Support |
|---|---|---|
| 0b000_00000 | hmac_sha256 | mandatory |
| 0b000_00001 | hmac_sha384 | optional |
| 0b000_00010 | hmac_sha512 | optional |
The following example specifies a SAM 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: sam.example.com
Content-Type: application/dcaf+cbor
{SAM: "https://sam.example.com/a?ep=%5B2001:DB8::dcaf:1234%5D",
SAI: ["coaps://temp451.example.com/s/tempC", 1],
TS: 0("2013-07-14T11:58:22.923")}
The following example shows a ticket for the distributed key generation method (cf. Section 6.2), comprised of a Face (F) and a Verifier (V). The Face data structure contains authorization information SAI, a client descriptor, a timestamp using the local time scale of S, and a lifetime relative to S’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 S.
HTTP/1.1 200 OK
Content-Type: application/dcaf+cbor
{
F: {
SAI: [ "/s/tempC", 1 ],
TS: 2938749,
L: 3600,
G: hmac_sha256
},
V: h'48ae5a81b87241d81618f56cab0b65ec
441202f81faabbe10075b20cb57fa939'
}
The Face may be encrypted as illustrated in the following example. Here, the field E carries an encrypted Face data structure that contains the same information as the previous example, and an additional Verifier. Encryption was done with a secret shared by SAM and S. (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 S’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 S to decrypt the contents of attribute E. Here, The value “key0” in this example is used to indicate that the shared session key between S and SAM was used for encrypting E.
{
E: h'2e75eeae01b831e0b65c2976e06d90f4
82135bec5efef3be3d31520b2fa8c6fb
f572f817203bf7a0940bb6183697567c
e291b03e9fca5e9cbdfa7e560322d4ed
3a659f44a542e55331a1a9f43d7f',
K: "key0",
V: h'48ae5a81b87241d81618f56cab0b65ec
441202f81faabbe10075b20cb57fa939'
}
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: {
SAI: [ "/s/tempC", 1 ],
TS: 2938749,
L: 3600,
G: hmac_sha256
},
V: h'48ae5a81b87241d81618f56cab0b65ec
441202f81faabbe10075b20cb57fa939'
}
One goal of the DCAF protocol is to provide for a DTLS PSK shared between C and S. SAM and S MUST negotiate the method for the DTLS PSK generation.
The DTLS PSK is generated by AS and transmitted to C and S using a secure channel.
The DTLS PSK transfer method is defined as follows:
SAM generates a DTLS PSK for C which is transmitted using a secure channel. S 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:
For the protocol defined in this document, proper configuration of CAM and SAM is crucial. The principals that are in charge of the resource, S and SAM, and the principals that are in charge of C and CAM need to define the respective permissions. The data representation of these permissions are not in the scope of this document.
C has a trust relationship with CAM: C trusts CAM to act in behalf of C’s principal. S has a trust relationship with SAM: S trusts SAM to act in behalf of S’s principal.
Obviously, CAM 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, SAM and CAM need to have a trust relationship established. Its establishment is also not in the scope of this document. It fulfills the following conditions:
SAM trusts C indirectly because it trusts CAM and CAM vouches for C. The DCAF Protocol does not provide any means for SAM to validate that a resource request stems from C.
C indirectly trusts SAM with some potentially confidential information, and that SAM correctly represents S, because CAM trusts SAM.
CAM trusts S indirectly because it trusts SAM and SAM vouches for S.
C implicitly trusts S with some potentially confidential information because it trusts CAM and because S can prove that it shares a key with SAM.
CAM <------------------> SAM
/|\ /|\
| |
\|/ \|/
C ..................... S
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+cbor 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 CAM in this document to relay incoming Authorization Request messages to SAM. (Whitespace is included only for readability.)
<client-authorize>;rt="auth-request";ct=TBD1
;title="Contact Remote Authorization Manager"
The resource directory that hosts the resource descriptions of S could list the following description. In this example, the URI “ep/node138/a/switch2941” is relative to the resource context “coaps://sam.example.com/”, i.e. the Server Authorization Manager SAM.
<ep/node138/a/switch2941>;rt="auth-request";ct=TBD1;ep="node138"
;title="Request Client Authorization"
;anchor="coaps://sam.example.com/"
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 SAM. The notation here follows the actors conventions defined in Section 1.2.1. The payload format is encoded as proposed in Section 5. The IP address of SAM is 2001:DB8::1, the IP address of S is 2001:DB8::dcaf:1234, and C’s IP address is 2001:DB8::c.
This example shows an Unauthorized PUT request from C to S that is answered with a SAM Information message. C then sends a POST request to CAM with a description of its intended request. CAM forwards this request to SAM using CoAP over a DTLS-secured channel. The response from SAM contains an access ticket that is relayed back to CAM.
C --> S
PUT a/switch2941 [Mid=1234]
Content-Format: application/senml+json
{"e": [{"bv": "1"}]}
C <-- S
4.01 Unauthorized [Mid=1234]
Content-Format: application/dcaf+cbor
{SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941"}
C --> CAM
POST client-authorize [Mid=1235,Token="tok"]
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 4]
}
CAM --> SAM [Mid=23146]
POST ep/node138/a/switch2941
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 4]
}
CAM <-- SAM
2.05 Content [Mid=23146]
Content-Format: application/dcaf+cbor
{ F: {
SAI: ["a/switch2941", 5],
TS: 0("2013-07-04T20:17:38.002"),
G: hmac_sha256
},
V: h'7ba4d9e287c8b69dd52fd3498fb8d26d
9503611917b014ee6ec2a570d857987a'
}
C <-- CAM
2.05 Content [Mid=1235,Token="tok"]
Content-Format: application/dcaf+cbor
{ F: {
SAI: ["a/switch2941", 5],
TS: 0("2013-07-04T20:17:38.002"),
G: hmac_sha256
},
V: h'7ba4d9e287c8b69dd52fd3498fb8d26d
9503611917b014ee6ec2a570d857987a'
}
C --> S
ClientHello (TLS_PSK_WITH_AES_128_CCM_8)
C <-- S
ServerHello (TLS_PSK_WITH_AES_128_CCM_8)
ServerHelloDone
C --> S
ClientKeyExchange
psk_identity=0xa301826c612f73776974636832393431
0x0505c077323031332d30372d30345432
0x303a31373a33382e3030320700
(C decodes the contents of V and uses the result as PSK)
ChangeCipherSpec
Finished
(S calculates PSK from SAI, TS and its session key
HMAC_sha256(0xa301826c612f73776974636832393431
0x0505c077323031332d30372d30345432
0x303a31373a33382e3030320700,
0x736563726574)
= 0x7ba4d9e287c8...
)
C <-- S
ChangeCipherSpec
Finished
This example shows a denied Authorization request for the DELETE operation.
C --> S
DELETE a/switch2941
C <-- S
4.01 Unauthorized
Content-Format: application/dcaf+cbor
{SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941"}
C --> CAM
POST client-authorize
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 8]
}
CAM --> SAM
POST ep/node138/a/switch2941
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 8]
}
CAM <-- SAM
2.05 Content
Content-Format: application/dcaf+cbor
C <-- CAM
2.05 Content
Content-Format: application/dcaf+cbor
This example shows a denied Authorization request for the operations GET, PUT, and DELETE. SAM grants access for PUT only.
CAM --> SAM
POST ep/node138/a/switch2941
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 13]
}
CAM <-- SAM
2.05 Content
Content-Format: application/dcaf+cbor
{ F: {
SAI: ["a/switch2941", 5],
TS: 0("2013-07-04T21:33:11.930"),
G: hmac_sha256
},
V: h'c7b5774f2ddcbd548f4ad74b30a1b2e5
b6b04e66a9995edd2545e5a06216c53d'
}
This example shows an Authorization request using implicit authorization. CAM initially requests the actions GET and POST on the resource “coaps://[2001:DB8::dcaf:1234]/a/switch2941”. SAM returns a ticket that has no SAI field in its ticket Face, hence implicitly authorizing C.
CAM --> SAM
POST ep/node138/a/switch2941
Content-Format: application/dcaf+cbor
{
SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 3]
}
CAM <-- SAM
2.05 Content
Content-Format: application/dcaf+cbor
{ F: {
TS: 0("2013-07-16T10:15:43.663"),
G: hmac_sha256
},
V: h'4f7b0e7fdcc498fb2ece648bf6bdf736
61a6067e51278a0078e5b8217147ea06'
}
The general DCAF architure outlined in Section 3.1 illustrates the various actors who participate in the message exchange for authenticated authorization. The message types defined in this document cover the most general case where all four actors are separate entities that may or may not reside on the same device.
Special implementation considerations apply when one single entity takes the role of more than one actor. This section gives advice on the most common usage scenarios where the Client Authorization Manager and Client, the Server Authorization Manager and Server or both Authorization Managers reside on the same (less-constrained) device and have a means of secure communication outside the scope of this document.
When CAM and C reside on the same (less-constrained) device, the Access Request and Ticket Transfer messages can be substituted by other means of secure communication. Figure 9 shows a simplified message exchange for a combined CAM+C device.
CAM+C S SAM | | <== DTLS chan. ==> | | [Resource Req.-->] | | | | | | [<-- SAM Info.] | | | | | | <==== TLS/DTLS chan. (Mutual Auth) ===> | | | | | Ticket Request ---------------------> | | | | | <--------------------- Ticket Grant | | | | | <== DTLS chan. ==> | | | Auth. Res. Req. -> | |
Figure 9: Combined Client Authorization Manager and Client
When CAM+C receives an SAM Information message as a reaction to an Unauthorized Request message, it creates a Ticket Request message as follows:
Based on the Ticket Grant message, CAM+C is able to establish a DTLS channel with S. To do so, CAM+C sets the psk_identity field of the DTLS ClientKeyExchange message to the ticket Face received in the Ticket Grant message and uses the ticket Verifier as PSK when constructing the premaster secret.
In certain scenarios, CAM and SAM may be combined to a single entity that knows both, C and S, and decides if their actions are authorized. Therefore, no explicit communication between CAM and SAM is necessary, resulting in omission of the Ticket Request and Ticket Grant messages. Figure 10 depicts the resulting message sequence in this simplified architecture.
C CAM+SAM S | <== DTLS chan. ==> | <== DTLS chan. ==> | | | | | [Resource Req.----------------------->] | | | | | [<-------------------- SAM Information] | | | | | Access Request --> | | | | | | <-- Ticket Transf. | | | | | | <=========== DTLS channel ===========> | | | | | Authorized Resource Request ----------> |
Figure 10: Combined Client Authorization Manager and Server Authorization Manager
When receiving an Access Request message, CAM+SAM performs the checks specified in Section 3.5 and returns a 4.00 (Bad Request) response in case of failure. Otherwise, if the checks have succeeded, CAM+SAM evaluates the contents of Access Request message as described in Section 3.6.
The decision on the access request is performed by CAM+SAM with respect to the stored policies. When the requested action is permitted on the respective resource, CAM+SAM generates an access ticket as outlined in Section 4.1 and creates a Ticket Transfer message to convey the access ticket to the Client.
A Ticket Transfer message is constructed as a 2.05 response with the access ticket contained in its payload. The response MAY contain a Max-Age option to indicate the ticket’s lifetime to the receiving Client.
This specification defines a CBOR data representation for the access ticket as illustrated in Section 3.6.
If SAM and S are colocated in one entity (SAM+S), the main objective is to allow CAM to delegate access to C. Accordingly, the authorization information could be replaced by a nonce internal to SAM+S. (TBD.)
CAM C SAM+S | <== DTLS chan. ==> | | | | [Resource Req.-->] | | | | | | [<-- SAM Info.] | | | | | <-- Access Req. | | | | | | <========= TLS/DTLS channel =========> | | | | | Ticket Request ---------------------> | | | | | <--------------------- Ticket Grant | | | | | Ticket Transf. --> | | | | | | | <== DTLS chan. ==> | | | Auth. Res. Req. -> |
Figure 11: Combined Server Authorization Manager and Server
As this protocol builds on transitive trust between Authorization Managers as mentioned in Section 8, SAM has no direct means to validate that a resource request originates from C. It has to trust CAM 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 Managers also could constitute a single point of failure. If the Server Authorization Manager fails, the resources on all Servers it is responsible for cannot be accessed any more. If a Client Authorization Manager fails, all clients it is responsible are not able to access resources on a Server. Thus, it is crucial for large networks to use Authorization Managers in a redundant setup.
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.
A sub-registry for the values indicating the PSK key generation method as contents of the field G in a payload of type application/dcaf+cbor is defined. Values in this sub-registry are numeric integers encoded in Concise Binary Object Notation (CBOR, [RFC7049]). This document follows the notation of [RFC7049] for binary values, i.e. a number starts with the prefix “0b”. The major type is separated from the actual numeric value by an underscore to emphasize the value’s internal structure.
Initial entries in this sub-registry are as follows:
| Encoded Value | Name | Reference |
|---|---|---|
| 0b000_00000 | hmac_sha256 | [RFC-XXXX] |
| 0b000_00001 | hmac_sha384 | [RFC-XXXX] |
| 0b000_00010 | hmac_sha512 | [RFC-XXXX] |
New methods can be added to this registry based on designated expert review according to [RFC5226].
(TBD: criteria for expert review.)
Type name: application
Subtype name: dcaf+cbor
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as using a subset of the encoding allowed in [RFC7049]. 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
Author: TBD
Change controller: IESG
This document specifies a new media type application/dcaf+cbor (cf. Section 13.2). 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+cbor | - | TBD1 | [RFC-XXXX] |
| [I-D.bormann-core-ace-aif] | Bormann, C., "An Authorization Information Format (AIF) for ACE", Internet-Draft draft-bormann-core-ace-aif-01, July 2014. |
| [I-D.gerdes-ace-actors] | Gerdes, S., "Actors in the ACE Architecture", Internet-Draft draft-gerdes-ace-actors-02, October 2014. |
| [I-D.ietf-core-block] | Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP", Internet-Draft draft-ietf-core-block-16, October 2014. |
| [I-D.ietf-core-observe] | Hartke, K., "Observing Resources in CoAP", Internet-Draft draft-ietf-core-observe-16, December 2014. |
| [I-D.ietf-core-resource-directory] | Shelby, Z. and C. Bormann, "CoRE Resource Directory", Internet-Draft draft-ietf-core-resource-directory-02, November 2014. |
| [I-D.schmertmann-dice-codtls] | Schmertmann, L., Hartke, K. and C. Bormann, "CoDTLS: DTLS handshakes over CoAP", Internet-Draft draft-schmertmann-dice-codtls-01, August 2014. |
| [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. |