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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ACE Working Group S. Gerdes 3 Internet-Draft O. Bergmann 4 Intended status: Standards Track C. Bormann 5 Expires: December 20, 2020 Universitaet Bremen TZI 6 G. Selander 7 Ericsson AB 8 L. Seitz 9 Combitech 10 June 18, 2020 12 Datagram Transport Layer Security (DTLS) Profile for Authentication and 13 Authorization for Constrained Environments (ACE) 14 draft-ietf-ace-dtls-authorize-11 16 Abstract 18 This specification defines a profile of the ACE framework that allows 19 constrained servers to delegate client authentication and 20 authorization. The protocol relies on DTLS version 1.2 for 21 communication security between entities in a constrained network 22 using either raw public keys or pre-shared keys. A resource- 23 constrained server can use this protocol to delegate management of 24 authorization information to a trusted host with less severe 25 limitations regarding processing power and memory. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at https://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on December 20, 2020. 44 Copyright Notice 46 Copyright (c) 2020 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (https://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 62 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4 64 3. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 5 65 3.1. Communication Between the Client and the Authorization 66 Server . . . . . . . . . . . . . . . . . . . . . . . . . 6 67 3.2. RawPublicKey Mode . . . . . . . . . . . . . . . . . . . . 6 68 3.2.1. DTLS Channel Setup Between Client and Resource Server 9 69 3.3. PreSharedKey Mode . . . . . . . . . . . . . . . . . . . . 10 70 3.3.1. DTLS Channel Setup Between Client and Resource Server 14 71 3.4. Resource Access . . . . . . . . . . . . . . . . . . . . . 16 72 4. Dynamic Update of Authorization Information . . . . . . . . . 17 73 5. Token Expiration . . . . . . . . . . . . . . . . . . . . . . 18 74 6. Secure Communication with an Authorization Server . . . . . . 19 75 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19 76 7.1. Reuse of Existing Sessions . . . . . . . . . . . . . . . 20 77 7.2. Multiple Access Tokens . . . . . . . . . . . . . . . . . 21 78 7.3. Out-of-Band Configuration . . . . . . . . . . . . . . . . 21 79 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22 80 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 81 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23 82 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 83 11.1. Normative References . . . . . . . . . . . . . . . . . . 23 84 11.2. Informative References . . . . . . . . . . . . . . . . . 25 85 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 87 1. Introduction 89 This specification defines a profile of the ACE framework 90 [I-D.ietf-ace-oauth-authz]. In this profile, a client and a resource 91 server use CoAP [RFC7252] over DTLS version 1.2 [RFC6347] to 92 communicate. The client obtains an access token, bound to a key (the 93 proof-of-possession key), from an authorization server to prove its 94 authorization to access protected resources hosted by the resource 95 server. Also, the client and the resource server are provided by the 96 authorization server with the necessary keying material to establish 97 a DTLS session. The communication between client and authorization 98 server may also be secured with DTLS. This specification supports 99 DTLS with Raw Public Keys (RPK) [RFC7250] and with Pre-Shared Keys 100 (PSK) [RFC4279]. 102 The ACE framework requires that client and server mutually 103 authenticate each other before any application data is exchanged. 104 DTLS enables mutual authentication if both client and server prove 105 their ability to use certain keying material in the DTLS handshake. 106 The authorization server assists in this process on the server side 107 by incorporating keying material (or information about keying 108 material) into the access token, which is considered a "proof of 109 possession" token. 111 In the RPK mode, the client proves that it can use the RPK bound to 112 the token and the server shows that it can use a certain RPK. 114 The resource server needs access to the token in order to complete 115 this exchange. For the RPK mode, the client must upload the access 116 token to the resource server before initiating the handshake, as 117 described in Section 5.8.1 of the ACE framework 118 [I-D.ietf-ace-oauth-authz]. 120 In the PSK mode, client and server show with the DTLS handshake that 121 they can use the keying material that is bound to the access token. 122 To transfer the access token from the client to the resource server, 123 the "psk_identity" parameter in the DTLS PSK handshake may be used 124 instead of uploading the token prior to the handshake. 126 As recommended in Section 5.8 of [I-D.ietf-ace-oauth-authz], this 127 specification uses CBOR web tokens to convey claims within an access 128 token issued by the server. While other formats could be used as 129 well, those are out of scope for this document. 131 1.1. Terminology 133 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 134 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 135 "OPTIONAL" in this document are to be interpreted as described in BCP 136 14 [RFC2119] [RFC8174] when, and only when, they appear in all 137 capitals, as shown here. 139 Readers are expected to be familiar with the terms and concepts 140 described in [I-D.ietf-ace-oauth-authz] and in 141 [I-D.ietf-ace-oauth-params]. 143 The authorization information (authz-info) resource refers to the 144 authorization information endpoint as specified in 146 [I-D.ietf-ace-oauth-authz]. The term "claim" is used in this 147 document with the same semantics as in [I-D.ietf-ace-oauth-authz], 148 i.e., it denotes information carried in the access token or returned 149 from introspection. 151 2. Protocol Overview 153 The CoAP-DTLS profile for ACE specifies the transfer of 154 authentication information and, if necessary, authorization 155 information between the client (C) and the resource server (RS) 156 during setup of a DTLS session for CoAP messaging. It also specifies 157 how the client can use CoAP over DTLS to retrieve an access token 158 from the authorization server (AS) for a protected resource hosted on 159 the resource server. As specified in Section 6.7 of 160 [I-D.ietf-ace-oauth-authz], use of DTLS for one or both of these 161 interactions is completely independent 163 This profile requires the client to retrieve an access token for 164 protected resource(s) it wants to access on the resource server as 165 specified in [I-D.ietf-ace-oauth-authz]. Figure 1 shows the typical 166 message flow in this scenario (messages in square brackets are 167 optional): 169 C RS AS 170 | [---- Resource Request ------>]| | 171 | | | 172 | [<-AS Request Creation Hints-] | | 173 | | | 174 | ------- Token Request ----------------------------> | 175 | | | 176 | <---------------------------- Access Token --------- | 177 | + Access Information | 179 Figure 1: Retrieving an Access Token 181 To determine the authorization server in charge of a resource hosted 182 at the resource server, the client can send an initial Unauthorized 183 Resource Request message to the resource server. The resource server 184 then denies the request and sends an AS Request Creation Hints 185 message containing the address of its authorization server back to 186 the client as specified in Section 5.1.2 of 187 [I-D.ietf-ace-oauth-authz]. 189 Once the client knows the authorization server's address, it can send 190 an access token request to the token endpoint at the authorization 191 server as specified in [I-D.ietf-ace-oauth-authz]. As the access 192 token request as well as the response may contain confidential data, 193 the communication between the client and the authorization server 194 must be confidentiality-protected and ensure authenticity. The 195 client may have been registered at the authorization server via the 196 OAuth 2.0 client registration mechanism as outlined in Section 5.3 of 197 [I-D.ietf-ace-oauth-authz]. 199 The access token returned by the authorization server can then be 200 used by the client to establish a new DTLS session with the resource 201 server. When the client intends to use an asymmetric proof-of- 202 possession key in the DTLS handshake with the resource server, the 203 client MUST upload the access token to the authz-info resource, i.e. 204 the authz-info endpoint, on the resource server before starting the 205 DTLS handshake, as described in Section 5.8.1 of 206 [I-D.ietf-ace-oauth-authz]. In case the client uses a symmetric 207 proof-of-possession key in the DTLS handshake, the procedure as above 208 MAY be used, or alternatively, the access token MAY instead be 209 transferred in the DTLS ClientKeyExchange message (see 210 Section 3.3.1). In any case, DTLS MUST be used in a mode that 211 provides replay protection. 213 Figure 2 depicts the common protocol flow for the DTLS profile after 214 the client has retrieved the access token from the authorization 215 server, AS. 217 C RS AS 218 | [--- Access Token ------>] | | 219 | | | 220 | <== DTLS channel setup ==> | | 221 | | | 222 | == Authorized Request ===> | | 223 | | | 224 | <=== Protected Resource == | | 226 Figure 2: Protocol overview 228 3. Protocol Flow 230 The following sections specify how CoAP is used to interchange 231 access-related data between the resource server, the client and the 232 authorization server so that the authorization server can provide the 233 client and the resource server with sufficient information to 234 establish a secure channel, and convey authorization information 235 specific for this communication relationship to the resource server. 237 Section 3.1 describes how the communication between the client (C) 238 and the authorization server (AS) must be secured. Depending on the 239 used CoAP security mode (see also Section 9 of [RFC7252], the Client- 240 to-AS request, AS-to-Client response (see Section 5.6 of 241 [I-D.ietf-ace-oauth-authz]) and DTLS session establishment carry 242 slightly different information. Section 3.2 addresses the use of raw 243 public keys while Section 3.3 defines how pre-shared keys are used in 244 this profile. 246 3.1. Communication Between the Client and the Authorization Server 248 To retrieve an access token for the resource that the client wants to 249 access, the client requests an access token from the authorization 250 server. Before the client can request the access token, the client 251 and the authorization server MUST establish a secure communication 252 channel. This profile assumes that the keying material to secure 253 this communication channel has securely been obtained either by 254 manual configuration or in an automated provisioning process. The 255 following requirements in alignment with Section 6.5 of 256 [I-D.ietf-ace-oauth-authz] therefore must be met: 258 o The client MUST securely have obtained keying material to 259 communicate with the authorization server. 261 o Furthermore, the client MUST verify that the authorization server 262 is authorized to provide access tokens (including authorization 263 information) about the resource server to the client, and that 264 this authorization information about the authorization server is 265 still valid. 267 o Also, the authorization server MUST securely have obtained keying 268 material for the client, and obtained authorization rules approved 269 by the resource owner (RO) concerning the client and the resource 270 server that relate to this keying material. 272 The client and the authorization server MUST use their respective 273 keying material for all exchanged messages. How the security 274 association between the client and the authorization server is 275 bootstrapped is not part of this document. The client and the 276 authorization server must ensure the confidentiality, integrity and 277 authenticity of all exchanged messages within the ACE protocol. 279 Section 6 specifies how communication with the authorization server 280 is secured. 282 3.2. RawPublicKey Mode 284 When the client and the resource server use RawPublicKey 285 authentication, the procedure is as follows: After the client and the 286 authorization server mutually authenticated each other and validated 287 each other's authorization, the client sends a token request to the 288 authorization server's token endpoint. The client MUST add a 289 "req_cnf" object carrying either its raw public key or a unique 290 identifier for a public key that it has previously made known to the 291 authorization server. It is RECOMMENDED that the client uses DTLS 292 with the same keying material to secure the communication with the 293 authorization server, proving possession of the key as part of the 294 token request. Other mechanisms for proving possession of the key 295 may be defined in the future. 297 An example access token request from the client to the authorization 298 server is depicted in Figure 3. 300 POST coaps://as.example.com/token 301 Content-Format: application/ace+cbor 302 Payload: 303 { 304 grant_type : client_credentials, 305 req_aud : "tempSensor4711", 306 req_cnf : { 307 COSE_Key : { 308 kty : EC2, 309 crv : P-256, 310 x : h'e866c35f4c3c81bb96a1...', 311 y : h'2e25556be097c8778a20...' 312 } 313 } 314 } 316 Figure 3: Access Token Request Example for RPK Mode 318 The example shows an access token request for the resource identified 319 by the string "tempSensor4711" on the authorization server using a 320 raw public key. 322 The authorization server MUST check if the client that it 323 communicates with is associated with the RPK in the "req_cnf" 324 parameter before issuing an access token to it. If the authorization 325 server determines that the request is to be authorized according to 326 the respective authorization rules, it generates an access token 327 response for the client. The access token MUST be bound to the RPK 328 of the client by means of the "cnf" claim. 330 The response MAY contain a "profile" parameter with the value 331 "coap_dtls" to indicate that this profile MUST be used for 332 communication between the client and the resource server. The 333 "profile" may be specified out-of-band, in which case it does not 334 have to be sent. The response also contains an access token with 335 information for the resource server about the client's public key. 337 The authorization server MUST return in its response the parameter 338 "rs_cnf" unless it is certain that the client already knows the 339 public key of the resource server. The authorization server MUST 340 ascertain that the RPK specified in "rs_cnf" belongs to the resource 341 server that the client wants to communicate with. The authorization 342 server MUST protect the integrity of the access token such that the 343 resource server can detect unauthorized changes. If the access token 344 contains confidential data, the authorization server MUST also 345 protect the confidentiality of the access token. 347 The client MUST ascertain that the access token response belongs to a 348 certain previously sent access token request, as the request may 349 specify the resource server with which the client wants to 350 communicate. 352 An example access token response from the authorization to the client 353 is depicted in Figure 4. Here, the contents of the "access_token" 354 claim have been truncated to improve readability. Caching proxies 355 process the Max-Age option in the CoAP response which has a default 356 value of 60 seconds (Section 5.6.1 of [RFC7252]). The authorization 357 server SHOULD adjust the Max-Age option such that it does not exceed 358 the "expires_in" parameter to avoid stale responses. 360 2.01 Created 361 Content-Format: application/ace+cbor 362 Max-Age: 3560 363 Payload: 364 { 365 access_token : b64'SlAV32hkKG... 366 (remainder of CWT omitted for brevity; 367 CWT contains the client's RPK in the cnf claim)', 368 expires_in : 3600, 369 rs_cnf : { 370 COSE_Key : { 371 kty : EC2, 372 crv : P-256, 373 x : h'd7cc072de2205bdc1537...', 374 y : h'f95e1d4b851a2cc80fff...' 375 } 376 } 377 } 379 Figure 4: Access Token Response Example for RPK Mode 381 3.2.1. DTLS Channel Setup Between Client and Resource Server 383 Before the client initiates the DTLS handshake with the resource 384 server, the client MUST send a "POST" request containing the obtained 385 access token to the authz-info resource hosted by the resource 386 server. After the client receives a confirmation that the resource 387 server has accepted the access token, it SHOULD proceed to establish 388 a new DTLS channel with the resource server. The client MUST use its 389 correct public key in the DTLS handshake. If the authorization 390 server has specified a "cnf" field in the access token response, the 391 client MUST use this key. Otherwise, the client MUST use the public 392 key that it specified in the "req_cnf" of the access token request. 393 The client MUST specify this public key in the SubjectPublicKeyInfo 394 structure of the DTLS handshake as described in [RFC7250]. 396 To be consistent with [RFC7252] which allows for shortened MAC tags 397 in constrained environments, an implementation that supports the RPK 398 mode of this profile MUST at least support the ciphersuite 399 TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251]. As discussed in 400 [RFC7748], new ECC curves have been defined recently that are 401 considered superior to the so-called NIST curves. This specification 402 therefore mandates implementation support for curve25519 (cf. 403 [RFC8032], [RFC8422]) as this curve said to be efficient and less 404 dangerous regarding implementation errors than the secp256r1 curve 405 mandated in [RFC7252]. 407 The resource server MUST check if the access token is still valid, if 408 the resource server is the intended destination (i.e., the audience) 409 of the token, and if the token was issued by an authorized 410 authorization server. The access token is constructed by the 411 authorization server such that the resource server can associate the 412 access token with the Client's public key. The "cnf" claim MUST 413 contain either the client's RPK or, if the key is already known by 414 the resource server (e.g., from previous communication), a reference 415 to this key. If the authorization server has no certain knowledge 416 that the Client's key is already known to the resource server, the 417 Client's public key MUST be included in the access token's "cnf" 418 parameter. If CBOR web tokens [RFC8392] are used (as recommended in 419 [I-D.ietf-ace-oauth-authz]), keys MUST be encoded as specified in 420 [RFC8747]. 422 The raw public key used in the DTLS handshake with the client MUST 423 belong to the resource server. If the resource server has several 424 raw public keys, it needs to determine which key to use. The 425 authorization server can help with this decision by including a "cnf" 426 parameter in the access token that is associated with this 427 communication. In this case, the resource server MUST use the 428 information from the "cnf" field to select the proper keying 429 material. 431 Thus, the handshake only finishes if the client and the resource 432 server are able to use their respective keying material. 434 3.3. PreSharedKey Mode 436 To retrieve an access token for the resource that the client wants to 437 access, the client MAY include a "cnf" object carrying an identifier 438 for a symmetric key in its access token request to the authorization 439 server. This identifier can be used by the authorization server to 440 determine the shared secret to construct the proof-of-possession 441 token. The authorization server MUST check if the identifier refers 442 to a symmetric key that was previously generated by the authorization 443 server as a shared secret for the communication between this client 444 and the resource server. If no such symmetric key was found, the 445 authorization server MUST generate a new symmetric key that is 446 returned in its response to the client. 448 The authorization server MUST determine the authorization rules for 449 the client it communicates with as defined by the resource owner and 450 generate the access token accordingly. If the authorization server 451 authorizes the client, it returns an AS-to-Client response. If the 452 profile parameter is present, it is set to "coap_dtls". The 453 authorization server MUST ascertain that the access token is 454 generated for the resource server that the client wants to 455 communicate with. Also, the authorization server MUST protect the 456 integrity of the access token to ensure that the resource server can 457 detect unauthorized changes. If the token contains confidential data 458 such as the symmetric key, the confidentiality of the token MUST also 459 be protected. Depending on the requested token type and algorithm in 460 the access token request, the authorization server adds access 461 Information to the response that provides the client with sufficient 462 information to setup a DTLS channel with the resource server. The 463 authorization server adds a "cnf" parameter to the access information 464 carrying a "COSE_Key" object that informs the client about the shared 465 secret that is to be used between the client and the resource server. 466 To convey the same secret to the resource server, the authorization 467 server can include it directly in the access token by means of the 468 "cnf" claim or provide sufficient information to enable the resource 469 server to derive the shared secret from the access token. As an 470 alternative, the resource server MAY use token introspection to 471 retrieve the keying material for this access token directly from the 472 authorization server. 474 An example access token request for an access token with a symmetric 475 proof-of-possession key is illustrated in Figure 5. 477 POST coaps://as.example.com/token 478 Content-Format: application/ace+cbor 479 Payload: 480 { 481 audience : "smokeSensor1807", 482 } 484 Figure 5: Example Access Token Request, (implicit) symmetric PoP-key 486 A corresponding example access token response is illustrated in 487 Figure 6. In this example, the authorization server returns a 2.01 488 response containing a new access token (truncated to improve 489 readability) and information for the client, including the symmetric 490 key in the cnf claim. The information is transferred as a CBOR data 491 structure as specified in [I-D.ietf-ace-oauth-authz]. 493 2.01 Created 494 Content-Format: application/ace+cbor 495 Max-Age: 85800 496 Payload: 497 { 498 access_token : h'd08343a10... 499 (remainder of CWT omitted for brevity) 500 token_type : PoP, 501 expires_in : 86400, 502 profile : coap_dtls, 503 cnf : { 504 COSE_Key : { 505 kty : symmetric, 506 kid : h'3d027833fc6267ce', 507 k : h'73657373696f6e6b6579' 508 } 509 } 510 } 512 Figure 6: Example Access Token Response, symmetric PoP-key 514 The access token also comprises a "cnf" claim. This claim usually 515 contains a "COSE_Key" object that carries either the symmetric key 516 itself or a key identifier that can be used by the resource server to 517 determine the secret key it shares with the client. If the access 518 token carries a symmetric key, the access token MUST be encrypted 519 using a "COSE_Encrypt0" structure. The authorization server MUST use 520 the keying material shared with the resource server to encrypt the 521 token. 523 The "cnf" structure in the access token is provided in Figure 7. 525 cnf : { 526 COSE_Key : { 527 kty : symmetric, 528 kid : h'3d027833fc6267ce' 529 } 530 } 532 Figure 7: Access Token without Keying Material 534 A response that declines any operation on the requested resource is 535 constructed according to Section 5.2 of [RFC6749], (cf. 536 Section 5.6.3. of [I-D.ietf-ace-oauth-authz]). Figure 8 shows an 537 example for a request that has been rejected due to invalid request 538 parameters. 540 4.00 Bad Request 541 Content-Format: application/ace+cbor 542 Payload: 543 { 544 error : invalid_request 545 } 547 Figure 8: Example Access Token Response With Reject 549 The method for how the resource server determines the symmetric key 550 from an access token containing only a key identifier is application- 551 specific; the remainder of this section provides one example. 553 The authorization server and the resource server are assumed to share 554 a key derivation key used to derive the symmetric key shared with the 555 client from the key identifier in the access token. The key 556 derivation key may be derived from some other secret key shared 557 between the authorization server and the resource server. This key 558 needs to be securely stored and processed in the same way as the key 559 used to protect the communication between the authorization server 560 and the resource server. 562 Knowledge of the symmetric key shared with the client must not reveal 563 any information about the key derivation key or other secret keys 564 shared between the authorization server and resource server. 566 In order to generate a new symmetric key to be used by client and 567 resource server, the authorization server generates a new key 568 identifier which MUST be unique among all key identifiers used by the 569 authorization server for this resource server. The authorization 570 server then uses the key derivation key shared with the resource 571 server to derive the symmetric key as specified below. Instead of 572 providing the keying material in the access token, the authorization 573 server includes the key identifier in the "kid" parameter, see 574 Figure 7. This key identifier enables the resource server to 575 calculate the symmetric key used for the communication with the 576 client using the key derivation key and a KDF to be defined by the 577 application, for example HKDF-SHA-256. The key identifier picked by 578 the authorization server needs to be unique for each access token 579 where a unique symmetric key is required. 581 In this example, HKDF consists of the composition of the HKDF-Extract 582 and HKDF-Expand steps [RFC5869]. The symmetric key is derived from 583 the key identifier, the key derivation key and other data: 585 OKM = HKDF(salt, IKM, info, L), 587 where: 589 o OKM, the output keying material, is the derived symmetric key 591 o salt is the empty byte string 593 o IKM, the input keying material, is the key derivation key as 594 defined above 596 o info is the serialization of a CBOR array consisting of 597 ([RFC8610]): 599 info = [ 600 type : tstr, 601 L : uint, 602 access_token: map 603 ] 605 where: 607 o type is set to the constant text string "ACE-CoAP-DTLS-key- 608 derivation", 610 o L is the size of the symmetric key in bytes, 612 o access_token is the decrypted access_token as transferred from the 613 authorization server to the resource server. The decrypted access 614 token usually denotes a CWT claim set represented as CBOR map. 616 Use of a unique (per resource server) "kid" and the use of a key 617 derivation IKM that is unique per authorization server/resource 618 server pair as specified above will ensure that the derived key is 619 not shared across multiple clients. However, to additionally provide 620 variation in the derived key across different tokens used by the same 621 client, it is additionally RECOMMENDED to include the "iat" claim and 622 either the "exp" or "exi" claims in the access token. 624 3.3.1. DTLS Channel Setup Between Client and Resource Server 626 When a client receives an access token response from an authorization 627 server, the client MUST check if the access token response is bound 628 to a certain previously sent access token request, as the request may 629 specify the resource server with which the client wants to 630 communicate. 632 The client checks if the payload of the access token response 633 contains an "access_token" parameter and a "cnf" parameter. With 634 this information the client can initiate the establishment of a new 635 DTLS channel with a resource server. To use DTLS with pre-shared 636 keys, the client follows the PSK key exchange algorithm specified in 637 Section 2 of [RFC4279] using the key conveyed in the "cnf" parameter 638 of the AS response as PSK when constructing the premaster secret. To 639 be consistent with the recommendations in [RFC7252] a client is 640 expected to offer at least the ciphersuite TLS_PSK_WITH_AES_128_CCM_8 641 [RFC6655] to the resource server. 643 In PreSharedKey mode, the knowledge of the shared secret by the 644 client and the resource server is used for mutual authentication 645 between both peers. Therefore, the resource server must be able to 646 determine the shared secret from the access token. Following the 647 general ACE authorization framework, the client can upload the access 648 token to the resource server's authz-info resource before starting 649 the DTLS handshake. The client then needs to indicate during the 650 DTLS handshake which previously uploaded access token it intends to 651 use. To do so, it MUST create a "COSE_Key" structure with the "kid" 652 that was conveyed in the "rs_cnf" claim in the token response from 653 the authorization server and the key type "symmetric". This 654 structure then is included as the only element in the "cnf" structure 655 that is used as value for "psk_identity" as shown in Figure 9. 657 { cnf : { 658 COSE_Key : { 659 kty: symmetric, 660 kid : h'3d027833fc6267ce' 661 } 662 } 663 } 665 Figure 9: Access token containing a single kid parameter 667 As an alternative to the access token upload, the client can provide 668 the most recent access token in the "psk_identity" field of the 669 ClientKeyExchange message. To do so, the client MUST treat the 670 contents of the "access_token" field from the AS-to-Client response 671 as opaque data as specified in Section 4.2 of [RFC7925] and not 672 perform any re-coding. This allows the resource server to retrieve 673 the shared secret directly from the "cnf" claim of the access token. 675 If a resource server receives a ClientKeyExchange message that 676 contains a "psk_identity" with a length greater than zero, it MUST 677 parse the contents of the "psk_identity" field as CBOR data structure 678 and process the contents as following: 680 o If the data contains a "cnf" field with a "COSE_Key" structure 681 with a "kid", the resource server continues the DTLS handshake 682 with the stored key associated with this kid. 684 o If the data comprises additional CWT information, this information 685 must be stored as access token for this DTLS association before 686 continuing with the DTLS handshake. 688 If the contents of the "psk_identity" do not yield sufficient 689 information to select a valid access token for the requesting client, 690 the resource server aborts the DTLS handshake with an 691 "illegal_parameter" alert. 693 When the resource server receives an access token, it MUST check if 694 the access token is still valid, if the resource server is the 695 intended destination (i.e., the audience of the token), and if the 696 token was issued by an authorized authorization server. This 697 specification assumes that the access token is a PoP token as 698 described in [I-D.ietf-ace-oauth-authz] unless specifically stated 699 otherwise. Therefore, the access token is bound to a symmetric PoP 700 key that is used as shared secret between the client and the resource 701 server. The resource server may use token introspection [RFC7662] on 702 the access token to retrieve more information about the specific 703 token. The use of introspection is out of scope for this 704 specification. 706 While the client can retrieve the shared secret from the contents of 707 the "cnf" parameter in the AS-to-Client response, the resource server 708 uses the information contained in the "cnf" claim of the access token 709 to determine the actual secret when no explicit "kid" was provided in 710 the "psk_identity" field. If key derivation is used, the resource 711 server uses the "COSE_KDF_Context" information as described above. 713 3.4. Resource Access 715 Once a DTLS channel has been established as described in Section 3.2 716 or Section 3.3, respectively, the client is authorized to access 717 resources covered by the access token it has uploaded to the authz- 718 info resource hosted by the resource server. 720 With the successful establishment of the DTLS channel, the client and 721 the resource server have proven that they can use their respective 722 keying material. An access token that is bound to the client's 723 keying material is associated with the channel. According to 724 Section 5.8.1 of [I-D.ietf-ace-oauth-authz], there should be only one 725 access token for each client. New access tokens issued by the 726 authorization server are supposed to replace previously issued access 727 tokens for the respective client. The resource server therefore must 728 have a common understanding with the authorization server how access 729 tokens are ordered. 731 Any request that the resource server receives on a DTLS channel that 732 is tied to an access token via its keying material MUST be checked 733 against the authorization rules that can be determined with the 734 access token. The resource server MUST check for every request if 735 the access token is still valid. If the token has expired, the 736 resource server MUST remove it. Incoming CoAP requests that are not 737 authorized with respect to any access token that is associated with 738 the client MUST be rejected by the resource server with 4.01 739 response. The response SHOULD include AS Request Creation Hints as 740 described in Section 5.1.1 of [I-D.ietf-ace-oauth-authz]. 742 The resource server MUST only accept an incoming CoAP request as 743 authorized if the following holds: 745 1. The message was received on a secure channel that has been 746 established using the procedure defined in this document. 748 2. The authorization information tied to the sending client is 749 valid. 751 3. The request is destined for the resource server. 753 4. The resource URI specified in the request is covered by the 754 authorization information. 756 5. The request method is an authorized action on the resource with 757 respect to the authorization information. 759 Incoming CoAP requests received on a secure DTLS channel that are not 760 thus authorized MUST be rejected according to Section 5.8.2 of 761 [I-D.ietf-ace-oauth-authz] 763 1. with response code 4.03 (Forbidden) when the resource URI 764 specified in the request is not covered by the authorization 765 information, and 767 2. with response code 4.05 (Method Not Allowed) when the resource 768 URI specified in the request covered by the authorization 769 information but not the requested action. 771 The client MUST ascertain that its keying material is still valid 772 before sending a request or processing a response. If the client 773 recently has updated the access token (see Section 4), it must be 774 prepared that its request is still handled according to the previous 775 authorization rules as there is no strict ordering between access 776 token uploads and resource access messages. See also Section 7.2 for 777 a discussion of access token processing. 779 If the client gets an error response containing AS Request Creation 780 Hints (cf. Section 5.1.2 of [I-D.ietf-ace-oauth-authz] as response 781 to its requests, it SHOULD request a new access token from the 782 authorization server in order to continue communication with the 783 resource server. 785 Unauthorized requests that have been received over a DTLS session 786 SHOULD be treated as non-fatal by the resource server, i.e., the DTLS 787 session SHOULD be kept alive until the associated access token has 788 expired. 790 4. Dynamic Update of Authorization Information 792 Resource servers must only use a new access token to update the 793 authorization information for a DTLS session if the keying material 794 that is bound to the token is the same that was used in the DTLS 795 handshake. By associating the access tokens with the identifier of 796 an existing DTLS session, the authorization information can be 797 updated without changing the cryptographic keys for the DTLS 798 communication between the client and the resource server, i.e. an 799 existing session can be used with updated permissions. 801 The client can therefore update the authorization information stored 802 at the resource server at any time without changing an established 803 DTLS session. To do so, the client requests a new access token from 804 the authorization server for the intended action on the respective 805 resource and uploads this access token to the authz-info resource on 806 the resource server. 808 Figure 10 depicts the message flow where the client requests a new 809 access token after a security association between the client and the 810 resource server has been established using this protocol. If the 811 client wants to update the authorization information, the token 812 request MUST specify the key identifier of the proof-of-possession 813 key used for the existing DTLS channel between the client and the 814 resource server in the "kid" parameter of the Client-to-AS request. 815 The authorization server MUST verify that the specified "kid" denotes 816 a valid verifier for a proof-of-possession token that has previously 817 been issued to the requesting client. Otherwise, the Client-to-AS 818 request MUST be declined with the error code "unsupported_pop_key" as 819 defined in Section 5.6.3 of [I-D.ietf-ace-oauth-authz]. 821 When the authorization server issues a new access token to update 822 existing authorization information, it MUST include the specified 823 "kid" parameter in this access token. A resource server MUST replace 824 the authorization information of any existing DTLS session that is 825 identified by this key identifier with the updated authorization 826 information. 828 C RS AS 829 | <===== DTLS channel =====> | | 830 | + Access Token | | 831 | | | 832 | --- Token Request ----------------------------> | 833 | | | 834 | <---------------------------- New Access Token - | 835 | + Access Information | 836 | | | 837 | --- Update /authz-info --> | | 838 | New Access Token | | 839 | | | 840 | == Authorized Request ===> | | 841 | | | 842 | <=== Protected Resource == | | 844 Figure 10: Overview of Dynamic Update Operation 846 5. Token Expiration 848 The resource server MUST delete access tokens that are no longer 849 valid. DTLS associations that have been setup in accordance with 850 this profile are always tied to specific tokens (which may be 851 exchanged with a dynamic update as described in Section 4). As 852 tokens may become invalid at any time (e.g., because they have 853 expired), the association may become useless at some point. A 854 resource server therefore MUST terminate existing DTLS association 855 after the last access token associated with this association has 856 expired. 858 As specified in Section 5.8.3 of [I-D.ietf-ace-oauth-authz], the 859 resource server MUST notify the client with an error response with 860 code 4.01 (Unauthorized) for any long running request before 861 terminating the association. 863 6. Secure Communication with an Authorization Server 865 As specified in the ACE framework (Sections 5.6 and 5.7 of 866 [I-D.ietf-ace-oauth-authz]), the requesting entity (the resource 867 server and/or the client) and the authorization server communicate 868 via the token endpoint or introspection endpoint. The use of CoAP 869 and DTLS for this communication is RECOMMENDED in this profile, other 870 protocols (such as HTTP and TLS, or CoAP and OSCORE [RFC8613]) MAY be 871 used instead. 873 How credentials (e.g., PSK, RPK, X.509 cert) for using DTLS with the 874 authorization server are established is out of scope for this 875 profile. 877 If other means of securing the communication with the authorization 878 server are used, the communication security requirements from 879 Section 6.2 of [I-D.ietf-ace-oauth-authz] remain applicable. 881 7. Security Considerations 883 This document specifies a profile for the Authentication and 884 Authorization for Constrained Environments (ACE) framework 885 [I-D.ietf-ace-oauth-authz]. As it follows this framework's general 886 approach, the general security considerations from Section 6 of 887 [I-D.ietf-ace-oauth-authz] also apply to this profile. 889 The authorization server must ascertain that the keying material for 890 the client that it provides to the resource server actually is 891 associated with this client. Malicious clients may hand over access 892 tokens containing their own access permissions to other entities. 893 This problem cannot be completely eliminated. Nevertheless, in RPK 894 mode it should not be possible for clients to request access tokens 895 for arbitrary public keys: if the client can cause the authorization 896 server to issue a token for a public key without proving possession 897 of the corresponding private key, this allows for identity misbinding 898 attacks where the issued token is usable by an entity other than the 899 intended one. The authorization server therefore at some point needs 900 to validate that the client can actually use the private key 901 corresponding to the client's public key. 903 When using pre-shared keys provisioned by the authorization server, 904 the security level depends on the randomness of PSK, and the security 905 of the TLS cipher suite and key exchange algorithm. As this 906 specification targets at constrained environments, message payloads 907 exchanged between the client and the resource server are expected to 908 be small and rare. CoAP [RFC7252] mandates the implementation of 909 cipher suites with abbreviated, 8-byte tags for message integrity 910 protection. For consistency, this profile requires implementation of 911 the same cipher suites. For application scenarios where the cost of 912 full-width authentication tags is low compared to the overall amount 913 of data being transmitted, the use of cipher suites with 16-byte 914 integrity protection tags is preferred. 916 The PSK mode of this profile offers a distribution mechanism to 917 convey authorization tokens together with a shared secret to a client 918 and a server. As this specification aims at constrained devices and 919 uses CoAP [RFC7252] as transfer protocol, at least the ciphersuite 920 TLS_PSK_WITH_AES_128_CCM_8 [RFC6655] should be supported. The access 921 tokens and the corresponding shared secrets generated by the 922 authorization server are expected to be sufficiently short-lived to 923 provide similar forward-secrecy properties to using ephemeral Diffie- 924 Hellman (DHE) key exchange mechanisms. For longer-lived access 925 tokens, DHE ciphersuites should be used. 927 Constrained devices that use DTLS [RFC6347] are inherently vulnerable 928 to Denial of Service (DoS) attacks as the handshake protocol requires 929 creation of internal state within the device. This is specifically 930 of concern where an adversary is able to intercept the initial cookie 931 exchange and interject forged messages with a valid cookie to 932 continue with the handshake. A similar issue exists with the 933 unprotected authorization information endpoint where the resource 934 server needs to keep valid access tokens until their expiry. 935 Adversaries can fill up the constrained resource server's internal 936 storage for a very long time with interjected or otherwise retrieved 937 valid access tokens. The protection of access tokens that are stored 938 in the authorization information endpoint depends on the keying 939 material that is used between the authorization server and the 940 resource server: The resource server must ensure that it processes 941 only access tokens that are encrypted and integrity-protected by an 942 authorization server that is authorized to provide access tokens for 943 the resource server. 945 7.1. Reuse of Existing Sessions 947 To avoid the overhead of a repeated DTLS handshake, [RFC7925] 948 recommends session resumption [RFC5077] to reuse session state from 949 an earlier DTLS association and thus requires client side 950 implementation. In this specification, the DTLS session is subject 951 to the authorization rules denoted by the access token that was used 952 for the initial setup of the DTLS association. Enabling session 953 resumption would require the server to transfer the authorization 954 information with the session state in an encrypted SessionTicket to 955 the client. Assuming that the server uses long-lived keying 956 material, this could open up attacks due to the lack of forward 957 secrecy. Moreover, using this mechanism, a client can resume a DTLS 958 session without proving the possession of the PoP key again. 959 Therefore, the use of session resumption is NOT RECOMMENDED for 960 resource servers. 962 Since renogiation of DTLS associations is prone to attacks as well, 963 [RFC7925] requires clients to decline any renogiation attempt. A 964 server that wants to initiate re-keying therefore SHOULD periodically 965 force a full handshake. 967 7.2. Multiple Access Tokens 969 The use of multiple access tokens for a single client increases the 970 strain on the resource server as it must consider every access token 971 and calculate the actual permissions of the client. Also, tokens may 972 contradict each other which may lead the server to enforce wrong 973 permissions. If one of the access tokens expires earlier than 974 others, the resulting permissions may offer insufficient protection. 975 Developers SHOULD avoid using multiple access tokens for a client. 977 Even when a single access token per client is used, an attacker could 978 compromise the dynamic update mechanism for existing DTLS connections 979 by delaying or reordering packets destined for the authz-info 980 endpoint. Thus, the order in which operations occur at the resource 981 server (and thus which authorization info is used to process a given 982 client request) cannot be guaranteed. Especially in the presence of 983 later-issued access tokens that reduce the client's permissions from 984 the initial access token, it is impossible to guarantee that the 985 reduction in authorization will take effect prior to the expiration 986 of the original token. 988 7.3. Out-of-Band Configuration 990 To communicate securely, the authorization server, the client and the 991 resource server require certain information that must be exchanged 992 outside the protocol flow described in this document. The 993 authorization server must have obtained authorization information 994 concerning the client and the resource server that is approved by the 995 resource owner as well as corresponding keying material. The 996 resource server must have received authorization information approved 997 by the resource owner concerning its authorization managers and the 998 respective keying material. The client must have obtained 999 authorization information concerning the authorization server 1000 approved by its owner as well as the corresponding keying material. 1001 Also, the client's owner must have approved of the client's 1002 communication with the resource server. The client and the 1003 authorization server must have obtained a common understanding how 1004 this resource server is identified to ensure that the client obtains 1005 access token and keying material for the correct resource server. If 1006 the client is provided with a raw public key for the resource server, 1007 it must be ascertained to which resource server (which identifier and 1008 authorization information) the key is associated. All authorization 1009 information and keying material must be kept up to date. 1011 8. Privacy Considerations 1013 This privacy considerations from Section 7 of the 1014 [I-D.ietf-ace-oauth-authz] apply also to this profile. 1016 An unprotected response to an unauthorized request may disclose 1017 information about the resource server and/or its existing 1018 relationship with the client. It is advisable to include as little 1019 information as possible in an unencrypted response. When a DTLS 1020 session between an authenticated client and the resource server 1021 already exists, more detailed information MAY be included with an 1022 error response to provide the client with sufficient information to 1023 react on that particular error. 1025 Also, unprotected requests to the resource server may reveal 1026 information about the client, e.g., which resources the client 1027 attempts to request or the data that the client wants to provide to 1028 the resource server. The client SHOULD NOT send confidential data in 1029 an unprotected request. 1031 Note that some information might still leak after DTLS session is 1032 established, due to observable message sizes, the source, and the 1033 destination addresses. 1035 9. IANA Considerations 1037 The following registrations are done for the ACE OAuth Profile 1038 Registry following the procedure specified in 1039 [I-D.ietf-ace-oauth-authz]. 1041 Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]" 1042 with the RFC number of this specification and delete this paragraph. 1044 Profile name: coap_dtls 1045 Profile Description: Profile for delegating client authentication and 1046 authorization in a constrained environment by establishing a Datagram 1047 Transport Layer Security (DTLS) channel between resource-constrained 1048 nodes. 1050 Profile ID: TBD (suggested: 1) 1052 Change Controller: IESG 1054 Reference: [RFC-XXXX] 1056 10. Acknowledgments 1058 Thanks to Jim Schaad for his contributions and reviews of this 1059 document. Special thanks to Ben Kaduk for his thorough review of 1060 this document. 1062 Ludwig Seitz worked on this document as part of the CelticNext 1063 projects CyberWI, and CRITISEC with funding from Vinnova. 1065 11. References 1067 11.1. Normative References 1069 [I-D.ietf-ace-oauth-authz] 1070 Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and 1071 H. Tschofenig, "Authentication and Authorization for 1072 Constrained Environments (ACE) using the OAuth 2.0 1073 Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-33 1074 (work in progress), February 2020. 1076 [I-D.ietf-ace-oauth-params] 1077 Seitz, L., "Additional OAuth Parameters for Authorization 1078 in Constrained Environments (ACE)", draft-ietf-ace-oauth- 1079 params-13 (work in progress), April 2020. 1081 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1082 Requirement Levels", BCP 14, RFC 2119, 1083 DOI 10.17487/RFC2119, March 1997, 1084 . 1086 [RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key 1087 Ciphersuites for Transport Layer Security (TLS)", 1088 RFC 4279, DOI 10.17487/RFC4279, December 2005, 1089 . 1091 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 1092 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 1093 January 2012, . 1095 [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", 1096 RFC 6749, DOI 10.17487/RFC6749, October 2012, 1097 . 1099 [RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J., 1100 Weiler, S., and T. Kivinen, "Using Raw Public Keys in 1101 Transport Layer Security (TLS) and Datagram Transport 1102 Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, 1103 June 2014, . 1105 [RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES- 1106 CCM Elliptic Curve Cryptography (ECC) Cipher Suites for 1107 TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014, 1108 . 1110 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 1111 Application Protocol (CoAP)", RFC 7252, 1112 DOI 10.17487/RFC7252, June 2014, 1113 . 1115 [RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer 1116 Security (TLS) / Datagram Transport Layer Security (DTLS) 1117 Profiles for the Internet of Things", RFC 7925, 1118 DOI 10.17487/RFC7925, July 2016, 1119 . 1121 [RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)", 1122 RFC 8152, DOI 10.17487/RFC8152, July 2017, 1123 . 1125 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1126 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1127 May 2017, . 1129 [RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic 1130 Curve Cryptography (ECC) Cipher Suites for Transport Layer 1131 Security (TLS) Versions 1.2 and Earlier", RFC 8422, 1132 DOI 10.17487/RFC8422, August 2018, 1133 . 1135 [RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S., and H. 1136 Tschofenig, "Proof-of-Possession Key Semantics for CBOR 1137 Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March 1138 2020, . 1140 11.2. Informative References 1142 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 1143 "Transport Layer Security (TLS) Session Resumption without 1144 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 1145 January 2008, . 1147 [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand 1148 Key Derivation Function (HKDF)", RFC 5869, 1149 DOI 10.17487/RFC5869, May 2010, 1150 . 1152 [RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for 1153 Transport Layer Security (TLS)", RFC 6655, 1154 DOI 10.17487/RFC6655, July 2012, 1155 . 1157 [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", 1158 RFC 7662, DOI 10.17487/RFC7662, October 2015, 1159 . 1161 [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves 1162 for Security", RFC 7748, DOI 10.17487/RFC7748, January 1163 2016, . 1165 [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital 1166 Signature Algorithm (EdDSA)", RFC 8032, 1167 DOI 10.17487/RFC8032, January 2017, 1168 . 1170 [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, 1171 "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, 1172 May 2018, . 1174 [RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data 1175 Definition Language (CDDL): A Notational Convention to 1176 Express Concise Binary Object Representation (CBOR) and 1177 JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, 1178 June 2019, . 1180 [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, 1181 "Object Security for Constrained RESTful Environments 1182 (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, 1183 . 1185 Authors' Addresses 1187 Stefanie Gerdes 1188 Universitaet Bremen TZI 1189 Postfach 330440 1190 Bremen D-28359 1191 Germany 1193 Phone: +49-421-218-63906 1194 Email: gerdes@tzi.org 1196 Olaf Bergmann 1197 Universitaet Bremen TZI 1198 Postfach 330440 1199 Bremen D-28359 1200 Germany 1202 Phone: +49-421-218-63904 1203 Email: bergmann@tzi.org 1205 Carsten Bormann 1206 Universitaet Bremen TZI 1207 Postfach 330440 1208 Bremen D-28359 1209 Germany 1211 Phone: +49-421-218-63921 1212 Email: cabo@tzi.org 1214 Goeran Selander 1215 Ericsson AB 1217 Email: goran.selander@ericsson.com 1219 Ludwig Seitz 1220 Combitech 1221 Djaeknegatan 31 1222 Malmoe 211 35 1223 Sweden 1225 Email: ludwig.seitz@combitech.se