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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-06) exists of draft-birkholz-rats-tuda-02 == Outdated reference: A later version (-17) exists of draft-ietf-teep-architecture-08 Summary: 0 errors (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 RATS Working Group H. Birkholz 3 Internet-Draft Fraunhofer SIT 4 Intended status: Informational D. Thaler 5 Expires: 22 November 2020 Microsoft 6 M. Richardson 7 Sandelman Software Works 8 N. Smith 9 Intel 10 W. Pan 11 Huawei Technologies 12 21 May 2020 14 Remote Attestation Procedures Architecture 15 draft-ietf-rats-architecture-04 17 Abstract 19 In network protocol exchanges, it is often the case that one entity 20 (a Relying Party) requires evidence about a remote peer to assess the 21 peer's trustworthiness, and a way to appraise such evidence. The 22 evidence is typically a set of claims about its software and hardware 23 platform. This document describes an architecture for such remote 24 attestation procedures (RATS). 26 Note to Readers 28 Discussion of this document takes place on the RATS Working Group 29 mailing list (rats@ietf.org), which is archived at 30 https://mailarchive.ietf.org/arch/browse/rats/ 31 (https://mailarchive.ietf.org/arch/browse/rats/). 33 Source for this draft and an issue tracker can be found at 34 https://github.com/ietf-rats-wg/architecture (https://github.com/ 35 ietf-rats-wg/architecture). 37 Status of This Memo 39 This Internet-Draft is submitted in full conformance with the 40 provisions of BCP 78 and BCP 79. 42 Internet-Drafts are working documents of the Internet Engineering 43 Task Force (IETF). Note that other groups may also distribute 44 working documents as Internet-Drafts. The list of current Internet- 45 Drafts is at https://datatracker.ietf.org/drafts/current/. 47 Internet-Drafts are draft documents valid for a maximum of six months 48 and may be updated, replaced, or obsoleted by other documents at any 49 time. It is inappropriate to use Internet-Drafts as reference 50 material or to cite them other than as "work in progress." 52 This Internet-Draft will expire on 22 November 2020. 54 Copyright Notice 56 Copyright (c) 2020 IETF Trust and the persons identified as the 57 document authors. All rights reserved. 59 This document is subject to BCP 78 and the IETF Trust's Legal 60 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 61 license-info) in effect on the date of publication of this document. 62 Please review these documents carefully, as they describe your rights 63 and restrictions with respect to this document. Code Components 64 extracted from this document must include Simplified BSD License text 65 as described in Section 4.e of the Trust Legal Provisions and are 66 provided without warranty as described in the Simplified BSD License. 68 Table of Contents 70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 71 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 72 3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 5 73 3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5 74 3.2. Confidential Machine Learning (ML) Model Protection . . . 6 75 3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 6 76 3.4. Critical Infrastructure Control . . . . . . . . . . . . . 6 77 3.5. Trusted Execution Environment (TEE) Provisioning . . . . 7 78 3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7 79 4. Architectural Overview . . . . . . . . . . . . . . . . . . . 7 80 4.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 9 81 4.2. Two Types of Environments of an Attester . . . . . . . . 9 82 4.3. Layered Attestation Environments . . . . . . . . . . . . 10 83 4.4. Composite Device . . . . . . . . . . . . . . . . . . . . 12 84 5. Topological Models . . . . . . . . . . . . . . . . . . . . . 15 85 5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 15 86 5.2. Background-Check Model . . . . . . . . . . . . . . . . . 16 87 5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 17 88 6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 18 89 7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 19 90 8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 20 91 8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 21 92 8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 21 93 8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 22 94 9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 22 95 10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 24 96 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 25 97 12. Security Considerations . . . . . . . . . . . . . . . . . . . 26 98 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 99 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26 100 15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27 101 16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 27 102 16.1. Example 1: Timestamp-based Passport Model Example . . . 29 103 16.2. Example 2: Nonce-based Passport Model Example . . . . . 30 104 16.3. Example 3: Timestamp-based Background-Check Model 105 Example . . . . . . . . . . . . . . . . . . . . . . . . 31 106 16.4. Example 4: Nonce-based Background-Check Model Example . 31 107 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 32 108 17.1. Normative References . . . . . . . . . . . . . . . . . . 32 109 17.2. Informative References . . . . . . . . . . . . . . . . . 32 110 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 112 1. Introduction 114 In Remote Attestation Procedures (RATS), one peer (the "Attester") 115 produces believable information about itself - Evidence - to enable a 116 remote peer (the "Relying Party") to decide whether to consider that 117 Attester a trustworthy peer or not. RATS are facilitated by an 118 additional vital party, the Verifier. 120 The Verifier appraises Evidence via Appraisal Policies and creates 121 the Attestation Results to support Relying Parties in their decision 122 process. 124 This documents defines a flexible architecture with corresponding 125 roles and their interaction via conceptual messages. Additionally, 126 this document defines a universal set of terms that can be mapped to 127 various existing and emerging Remote Attestation Procedures. Common 128 topological models and the data flows associated with them, such as 129 the "Passport Model" and the "Background-Check Model" are 130 illustrated. The purpose is to enable readers of this document to 131 map their current and emerging solutions to the architecture provided 132 and the corresponding terminology defined. 134 A common terminology that provides a well-understood semantic meaning 135 to the concepts, roles, and models in this document is vital to 136 create semantic interoperability between solutions and across 137 different platforms. 139 Amongst other things, this document is about trust and 140 trustworthiness. Trust is a decision being made. Trustworthiness is 141 a quality that is assessed via evidence created. This is a subtle 142 difference and being familiar with the difference is crucial for 143 using this document. Additionally, the concepts of freshness and 144 trust relationships with respect to RATS are elaborated on to enable 145 implementers in order to choose appropriate solutions to compose 146 their Remote Attestation Procedures. 148 2. Terminology 150 This document uses the following terms. 152 Appraisal Policy for Evidence: A set of rules that direct how a 153 Verifier evaluates the validity of information about an Attester. 154 Compare /security policy/ in [RFC4949] 156 Appraisal Policy for Attestation Result: A set of rules that direct 157 how a Relying Party uses the Attestation Results regarding an 158 Attester generated by the Verifiers. Compare /security policy/ in 159 [RFC4949] 161 Attestation Result: The output generated by a Verifier, typically 162 including information about an Attester, where the Verifier 163 vouches for the validity of the results 165 Attester: An entity whose attributes must be appraised in order to 166 determine whether the entity is considered trustworthy, such as 167 when deciding whether the entity is authorized to perform some 168 operation 170 Claim: A piece of asserted information, often in the form of a name/ 171 value pair. (Compare /claim/ in [RFC7519]) 173 Endorsement: A secure statement that some entity (typically a 174 manufacturer) vouches for the integrity of an Attester's signing 175 capability 177 Endorser: An entity that creates Endorsements that can be used to 178 help to appraise the trustworthiness of Attesters 180 Evidence: A set of information about an Attester that is to be 181 appraised by a Verifier 183 Relying Party: An entity that depends on the validity of information 184 about another entity, typically for purposes of authorization. 185 Compare /relying party/ in [RFC4949] 187 Relying Party Owner: An entity, such as an administrator, that is 188 authorized to configure Appraisal Policy for Attestation Results 189 in a Relying Party. 191 Verifier: An entity that appraises the validity of Evidence about an 192 Attester 194 Verifier Owner: An entity, such as an administrator, that is 195 authorized to configure Appraisal Policy for Evidence in a 196 Verifier 198 3. Reference Use Cases 200 This section covers a number of representative use cases for remote 201 attestation, independent of specific solutions. The purpose is to 202 provide motivation for various aspects of the architecture presented 203 in this draft. Many other use cases exist, and this document does 204 not intend to have a complete list, only to have a set of use cases 205 that collectively cover all the functionality required in the 206 architecture. 208 Each use case includes a description, and a summary of what an 209 Attester and a Relying Party refer to in the use case. 211 3.1. Network Endpoint Assessment 213 Network operators want a trustworthy report of identity and version 214 of information of the hardware and software on the machines attached 215 to their network, for purposes such as inventory, auditing, and/or 216 logging. The network operator may also want a policy by which full 217 access is only granted to devices that meet some definition of 218 health, and so wants to get claims about such information and verify 219 their validity. Remote attestation is desired to prevent vulnerable 220 or compromised devices from getting access to the network and 221 potentially harming others. 223 Typically, solutions start with a specific component (called a "Root 224 of Trust") that provides device identity and protected storage for 225 measurements. These components perform a series of measurements, and 226 express this with Evidence as to the hardware and firmware/software 227 that is running. 229 Attester: A device desiring access to a network 231 Relying Party: A network infrastructure device such as a router, 232 switch, or access point 234 3.2. Confidential Machine Learning (ML) Model Protection 236 A device manufacturer wants to protect its intellectual property in 237 terms of the ML model it developed and that runs in the devices that 238 its customers purchased, and it wants to prevent attackers, 239 potentially including the customer themselves, from seeing the 240 details of the model. 242 This typically works by having some protected environment in the 243 device attest to some manufacturer service. If remote attestation 244 succeeds, then the manufacturer service releases either the model, or 245 a key to decrypt a model the Attester already has in encrypted form, 246 to the requester. 248 Attester: A device desiring to run an ML model to do inferencing 250 Relying Party: A server or service holding ML models it desires to 251 protect 253 3.3. Confidential Data Retrieval 255 This is a generalization of the ML model use case above, where the 256 data can be any highly confidential data, such as health data about 257 customers, payroll data about employees, future business plans, etc. 258 Attestation is desired to prevent leaking data to compromised 259 devices. 261 Attester: An entity desiring to retrieve confidential data 263 Relying Party: An entity that holds confidential data for retrieval 264 by other entities 266 3.4. Critical Infrastructure Control 268 In this use case, potentially dangerous physical equipment (e.g., 269 power grid, traffic control, hazardous chemical processing, etc.) is 270 connected to a network. The organization managing such 271 infrastructure needs to ensure that only authorized code and users 272 can control such processes, and they are protected from malware or 273 other adversaries. When a protocol operation can affect some 274 critical system, the device attached to the critical equipment thus 275 wants some assurance that the requester has not been compromised. As 276 such, remote attestation can be used to only accept commands from 277 requesters that are within policy. 279 Attester: A device or application wishing to control physical 280 equipment 282 Relying Party: A device or application connected to potentially 283 dangerous physical equipment (hazardous chemical processing, 284 traffic control, power grid, etc.) 286 3.5. Trusted Execution Environment (TEE) Provisioning 288 A "Trusted Application Manager (TAM)" server is responsible for 289 managing the applications running in the TEE of a client device. To 290 do this, the TAM wants to assess the state of a TEE, or of 291 applications in the TEE, of a client device. The TEE attests to the 292 TAM, which can then decide whether the TEE is already in compliance 293 with the TAM's latest policy, or if the TAM needs to uninstall, 294 update, or install approved applications in the TEE to bring it back 295 into compliance with the TAM's policy. 297 Attester: A device with a trusted execution environment capable of 298 running trusted applications that can be updated 300 Relying Party: A Trusted Application Manager 302 3.6. Hardware Watchdog 304 One significant problem is malware that holds a device hostage and 305 does not allow it to reboot to prevent updates to be applied. This 306 is a significant problem, because it allows a fleet of devices to be 307 held hostage for ransom. 309 A hardware watchdog can be implemented by forcing a reboot unless 310 remote attestation to a server succeeds within a periodic interval, 311 and having the reboot do remediation by bringing a device into 312 compliance, including installation of patches as needed. 314 Attester: The device that is desired to keep from being held hostage 315 for a long period of time 317 Relying Party: A remote server that will securely grant the Attester 318 permission to continue operating (i.e., not reboot) for a period 319 of time 321 4. Architectural Overview 323 Figure 1 depicts the data that flows between different roles, 324 independent of protocol or use case. 326 ************ ************ **************** 327 * Endorser * * Verifier * * Relying Party* 328 ************ * Owner * * Owner * 329 | ************ **************** 330 | | | 331 Endorsements| | | 332 | |Appraisal | 333 | |Policy | 334 | |for | Appraisal 335 | |Evidence | Policy for 336 | | | Attestation 337 | | | Result 338 v v | 339 .-----------------. | 340 .----->| Verifier |------. | 341 | '-----------------' | | 342 | | | 343 | Attestation| | 344 | Results | | 345 | Evidence | | 346 | | | 347 | v v 348 .----------. .-----------------. 349 | Attester | | Relying Party | 350 '----------' '-----------------' 352 Figure 1: Conceptual Data Flow 354 An Attester creates Evidence that is conveyed to a Verifier. 356 The Verifier uses the Evidence, and any Endorsements from Endorsers, 357 by applying an Evidence Appraisal Policy to assess the 358 trustworthiness of the Attester, and generates Attestation Results 359 for use by Relying Parties. The Evidence Appraisal Policy might be 360 obtained from an Endorser along with the Endorsements, or might be 361 obtained via some other mechanism such as being configured in the 362 Verifier by an administrator. 364 The Relying Party uses Attestation Results by applying its own 365 Appraisal Policy to make application-specific decisions such as 366 authorization decisions. The Attestation Result Appraisal Policy 367 might, for example, be configured in the Relying Party by an 368 administrator. 370 4.1. Appraisal Policies 372 The Verifier, when appraising Evidence, or the Relying Party, when 373 appraising Attestation Results, checks the values of some claims 374 against constraints specified in its Appraisal Policy. Such 375 constraints might involve a comparison for equality against a 376 reference value, or a check for being in a range bounded by reference 377 values, or membership in a set of reference values, or a check 378 against values in other claims, or any other test. 380 Such reference values might be specified as part of the Appraisal 381 Policy itself, or might be obtained from a separate source, such as 382 an Endorsement, and then used by the Appraisal Policy. 384 The actual data format and semantics of any reference values are 385 specific to claims and implementations. This architecture document 386 does not define any general purpose format for them or general means 387 for comparison. 389 4.2. Two Types of Environments of an Attester 391 An Attester consists of at least one Attesting Environment and at 392 least one Target Environment. In some implementations, the Attesting 393 and Target Environments might be combined. Other implementations 394 might have multiple Attesting and Target Environments, such as in the 395 examples described in more detail in Section 4.3 and Section 4.4. 396 Other examples may exist, and the examples discussed could even be 397 combined into even more complex implementations. 399 Claims are collected from Target Environments, as shown in Figure 2. 400 That is, Attesting Environments collect the raw values and the 401 information to be represented in claims, such as by doing some 402 measurement of a Target Environment's code, memory, and/or registers. 403 Attesting Environments then format the claims appropriately, and 404 typically use key material and cryptographic functions, such as 405 signing or cipher algorithms, to create Evidence. Places that 406 Attesting Environments can exist include Trusted Execution 407 Environments (TEE), embedded Secure Elements (eSE), and BIOS 408 firmware. An execution environment may not, by default, be capable 409 of claims collection for a given Target Environment. Attesting 410 Environments are designed specifically with claims collection in 411 mind. 413 .--------------------------------. 414 | | 415 | Verifier | 416 | | 417 '--------------------------------' 418 ^ 419 | 420 .-------------------------|----------. 421 | | | 422 | .----------------. | | 423 | | Target | | | 424 | | Environment | | | 425 | | | | Evidence | 426 | '----------------' | | 427 | | | | 428 | | | | 429 | Collect | | | 430 | Claims | | | 431 | | | | 432 | v | | 433 | .-------------. | 434 | | Attesting | | 435 | | Environment | | 436 | | | | 437 | '-------------' | 438 | Attester | 439 '------------------------------------' 441 Figure 2: Two Types of Environments 443 4.3. Layered Attestation Environments 445 By definition, the Attester role takes on the duty to create 446 Evidence. The fact that an Attester role is composed of environments 447 that can be nested or staged adds complexity to the architectural 448 layout of how an Attester can be composed and therefore has to 449 conduct the Claims collection in order to create believable 450 attestation Evidence. 452 Figure 3 depicts an example of a device that includes (A) a BIOS 453 stored in read-only memory in this example, (B) an updatable 454 bootloader, and (C) an operating system kernel. 456 .----------. .----------. 457 | | | | 458 | Endorser |------------------->| Verifier | 459 | | Endorsements | | 460 '----------' for A, B, and C '----------' 461 ^ 462 .------------------------------------. | 463 | | | 464 | .---------------------------. | | 465 | | Target | | | Layered 466 | | Environment | | | Evidence 467 | | C | | | for 468 | '---------------------------' | | B and C 469 | Collect | | | 470 | claims | | | 471 | .---------------|-----------. | | 472 | | Target v | | | 473 | | Environment .-----------. | | | 474 | | B | Attesting | | | | 475 | | |Environment|-----------' 476 | | | B | | | 477 | | '-----------' | | 478 | | ^ | | 479 | '---------------------|-----' | 480 | Collect | | Evidence | 481 | claims v | for B | 482 | .-----------. | 483 | | Attesting | | 484 | |Environment| | 485 | | A | | 486 | '-----------' | 487 | | 488 '------------------------------------' 490 Figure 3: Layered Attester 492 Attesting Environment A, the read-only BIOS in this example, has to 493 ensure the integrity of the bootloader (Target Environment B). There 494 are potentially multiple kernels to boot, and the decision is up to 495 the bootloader. Only a bootloader with intact integrity will make an 496 appropriate decision. Therefore, these Claims have to be measured 497 securely. At this stage of the boot-cycle of the device, the Claims 498 collected typically cannot be composed into Evidence. 500 After the boot sequence is started, the BIOS conducts the most 501 important and defining feature of layered attestation, which is that 502 the successfully measured Target Environment B now becomes (or 503 contains) an Attesting Environment for the next layer. This 504 procedure in Layered Attestation is sometimes called "staging". It 505 is important that the new Attesting Environment B not be able to 506 alter any Claims about its own Target Environment B. This can be 507 ensured having those Claims be either signed by Attesting Environment 508 A or stored in an untamperable manner by Attesting Environment A. 510 Continuing with this example, the bootloader's Attesting Environment 511 B is now in charge of collecting Claims about Target Environment C, 512 which in this example is the kernel to be booted. The final Evidence 513 thus contains two sets of Claims: one set about the bootloader as 514 measured and signed by the BIOS, plus a set of Claims about the 515 kernel as measured and signed by the bootloader. 517 This example could be extended further by, say, making the kernel 518 become another Attesting Environment for an application as another 519 Target Environment, resulting in a third set of Claims in the 520 Evidence pertaining to that application. 522 The essence of this example is a cascade of staged environments. 523 Each environment has the responsibility of measuring the next 524 environment before the next environment is started. In general, the 525 number of layers may vary by device or implementation, and an 526 Attesting Environment might even have multiple Target Environments 527 that it measures, rather than only one as shown in Figure 3. 529 4.4. Composite Device 531 A Composite Device is an entity composed of multiple sub-entities 532 such that its trustworthiness has to be determined by the appraisal 533 of all these sub-entities. 535 Each sub-entity has at least one Attesting Environment collecting the 536 claims from at least one Target Environment, then this sub-entity 537 generates Evidence about its trustworthiness. Therefore each sub- 538 entity can be called an Attester. Among all the Attesters, there may 539 be only some which have the ability to communicate with the Verifier 540 while others do not. 542 For example, a carrier-grade router consists of a chassis and 543 multiple slots. The trustworthiness of the router depends on all its 544 slots' trustworthiness. Each slot has an Attesting Environment such 545 as a TEE collecting the claims of its boot process, after which it 546 generates Evidence from the claims. Among these slots, only a main 547 slot can communicate with the Verifier while other slots cannot. But 548 other slots can communicate with the main slot by the links between 549 them inside the router. So the main slot collects the Evidence of 550 other slots, produces the final Evidence of the whole router and 551 conveys the final Evidence to the Verifier. Therefore the router is 552 a Composite Device, each slot is an Attester, and the main slot is 553 the lead Attester. 555 Another example is a multi-chassis router composed of multiple single 556 carrier-grade routers. The multi-chassis router provides higher 557 throughput by interconnecting multiple routers and can be logically 558 treated as one router for simpler management. Among these routers, 559 there is only one main router that connects to the Verifier. Other 560 routers are only connected to the main router by the network cables, 561 and therefore they are managed and appraised via this main router's 562 help. So, in this case, the multi-chassis router is the Composite 563 Device, each router is an Attester and the main router is the lead 564 Attester. 566 Figure 4 depicts the conceptual data flow for a Composite Device. 568 .-----------------------------. 569 | Verifier | 570 '-----------------------------' 571 ^ 572 | 573 | Evidence of 574 | Composite Device 575 | 576 .----------------------------------|-------------------------------. 577 | .--------------------------------|-----. .------------. | 578 | | Collect .------------. | | | | 579 | | Claims .--------->| Attesting |<--------| Attester B |-. | 580 | | | |Environment | | '------------. | | 581 | | .----------------. | |<----------| Attester C |-. | 582 | | | Target | | | | '------------' | | 583 | | | Environment(s) | | |<------------| ... | | 584 | | | | '------------' | Evidence '------------' | 585 | | '----------------' | of | 586 | | | Attesters | 587 | | lead Attester A | (via Internal Links or | 588 | '--------------------------------------' Network Connections) | 589 | | 590 | Composite Device | 591 '------------------------------------------------------------------' 593 Figure 4: Conceptual Data Flow for a Composite Device 595 In the Composite Device, each Attester generates its own Evidence by 596 its Attesting Environment(s) collecting the claims from its Target 597 Environment(s). The lead Attester collects the Evidence of all other 598 Attesters and then generates the Evidence of the whole Composite 599 Attester. 601 An entity can take on multiple RATS roles (e.g., Attester, Verifier, 602 Relying Party, etc.) at the same time. The combination of roles can 603 be arbitrary. For example, in this Composite Device scenario, the 604 entity inside the lead Attester can also take on the role of a 605 Verifier, and the outside entity of Verifier can take on the role of 606 a Relying Party. After collecting the Evidence of other Attesters, 607 this inside Verifier verifies them using Endorsements and Appraisal 608 Policies (obtained the same way as any other Verifier), to generate 609 Attestation Results. The inside Verifier then conveys the 610 Attestation Results of other Attesters, whether in the same 611 conveyance protocol as the Evidence or not, to the outside Verifier. 613 In this situation, the trust model described in Section 7 is also 614 suitable for this inside Verifier. 616 5. Topological Models 618 Figure 1 shows a basic model for communication between an Attester, a 619 Verifier, and a Relying Party. The Attester conveys its Evidence to 620 the Verifier for appraisal, and the Relying Party gets the 621 Attestation Results from the Verifier. There are multiple other 622 possible models. This section includes some reference models, but 623 this is not intended to be a restrictive list, and other variations 624 may exist. 626 5.1. Passport Model 628 In this model, an Attester conveys Evidence to a Verifier, which 629 compares the Evidence against its Appraisal Policy. The Verifier 630 then gives back an Attestation Result. If the Attestation Result was 631 a successful one, the Attester can then present the Attestation 632 Result to a Relying Party, which then compares the Attestation Result 633 against its own Appraisal Policy. 635 There are three ways in which the process may fail. First, the 636 Verifier may refuse to issue the Attestation Result due to some error 637 in processing, or some missing input to the Verifier. The second way 638 in which the process may fail is when the resulting Result is 639 examined by the Relying Party, and based upon the Appraisal Policy, 640 the result does not pass the policy. The third way is when the 641 Verifier is unreachable. 643 Since the resource access protocol between the Attester and Relying 644 Party includes an Attestation Result, in this model the details of 645 that protocol constrain the serialization format of the Attestation 646 Result. The format of the Evidence on the other hand is only 647 constrained by the Attester-Verifier remote attestation protocol. 649 +-------------+ 650 | | Compare Evidence 651 | Verifier | against Appraisal Policy 652 | | 653 +-------------+ 654 ^ | 655 Evidence| |Attestation 656 | | Result 657 | v 658 +----------+ +---------+ 659 | |------------->| |Compare Attestation 660 | Attester | Attestation | Relying | Result against 661 | | Result | Party | Appraisal 662 +----------+ +---------+ Policy 663 Figure 5: Passport Model 665 The passport model is so named because of its resemblance to how 666 nations issue passports to their citizens. The nature of the 667 Evidence that an individual needs to provide to its local authority 668 is specific to the country involved. The citizen retains control of 669 the resulting passport document and presents it to other entities 670 when it needs to assert a citizenship or identity claim, such as an 671 airport immigration desk. The passport is considered sufficient 672 because it vouches for the citizenship and identity claims, and it is 673 issued by a trusted authority. Thus, in this immigration desk 674 analogy, the passport issuing agency is a Verifier, the passport is 675 an Attestation Result, and the immigration desk is a Relying Party. 677 5.2. Background-Check Model 679 In this model, an Attester conveys Evidence to a Relying Party, which 680 simply passes it on to a Verifier. The Verifier then compares the 681 Evidence against its Appraisal Policy, and returns an Attestation 682 Result to the Relying Party. The Relying Party then compares the 683 Attestation Result against its own appraisal policy. 685 The resource access protocol between the Attester and Relying Party 686 includes Evidence rather than an Attestation Result, but that 687 Evidence is not processed by the Relying Party. Since the Evidence 688 is merely forwarded on to a trusted Verifier, any serialization 689 format can be used for Evidence because the Relying Party does not 690 need a parser for it. The only requirement is that the Evidence can 691 be _encapsulated in_ the format required by the resource access 692 protocol between the Attester and Relying Party. 694 However, like in the Passport model, an Attestation Result is still 695 consumed by the Relying Party and so the serialization format of the 696 Attestation Result is still important. If the Relying Party is a 697 constrained node whose purpose is to serve a given type resource 698 using a standard resource access protocol, it already needs the 699 parser(s) required by that existing protocol. Hence, the ability to 700 let the Relying Party obtain an Attestation Result in the same 701 serialization format allows minimizing the code footprint and attack 702 surface area of the Relying Party, especially if the Relying Party is 703 a constrained node. 705 +-------------+ 706 | | Compare Evidence 707 | Verifier | against Appraisal 708 | | Policy 709 +-------------+ 710 ^ | 711 Evidence| |Attestation 712 | | Result 713 | v 714 +------------+ +-------------+ 715 | |-------------->| | Compare Attestation 716 | Attester | Evidence | Relying | Result against 717 | | | Party | Appraisal Policy 718 +------------+ +-------------+ 720 Figure 6: Background-Check Model 722 The background-check model is so named because of the resemblance of 723 how employers and volunteer organizations perform background checks. 724 When a prospective employee provides claims about education or 725 previous experience, the employer will contact the respective 726 institutions or former employers to validate the claim. Volunteer 727 organizations often perform police background checks on volunteers in 728 order to determine the volunteer's trustworthiness. Thus, in this 729 analogy, a prospective volunteer is an Attester, the organization is 730 the Relying Party, and a former employer or government agency that 731 issues a report is a Verifier. 733 5.3. Combinations 735 One variation of the background-check model is where the Relying 736 Party and the Verifier on the same machine, and so there is no need 737 for a protocol between the two. 739 It is also worth pointing out that the choice of model is generally 740 up to the Relying Party, and the same device may need to create 741 Evidence for different Relying Parties and different use cases (e.g., 742 a network infrastructure device to gain access to the network, and 743 then a server holding confidential data to get access to that data). 744 As such, both models may simultaneously be in use by the same device. 746 Figure 7 shows another example of a combination where Relying Party 1 747 uses the passport model, whereas Relying Party 2 uses an extension of 748 the background-check model. Specifically, in addition to the basic 749 functionality shown in Figure 6, Relying Party 2 actually provides 750 the Attestation Result back to the Attester, allowing the Attester to 751 use it with other Relying Parties. This is the model that the 752 Trusted Application Manager plans to support in the TEEP architecture 753 [I-D.ietf-teep-architecture]. 755 +-------------+ 756 | | Compare Evidence 757 | Verifier | against Appraisal Policy 758 | | 759 +-------------+ 760 ^ | 761 Evidence| |Attestation 762 | | Result 763 | v 764 +-------------+ 765 | | Compare 766 | Relying | Attestation Result 767 | Party 2 | against Appraisal Policy 768 +-------------+ 769 ^ | 770 Evidence| |Attestation 771 | | Result 772 | v 773 +----------+ +----------+ 774 | |-------------->| | Compare Attestation 775 | Attester | Attestation | Relying | Result against 776 | | Result | Party 1 | Appraisal Policy 777 +----------+ +----------+ 779 Figure 7: Example Combination 781 6. Roles and Entities 783 An entity in the RATS architecture includes at least one of the roles 784 defined in this document. As a result, the entity can participate as 785 a constituent of the RATS architecture. Additionally, an entity can 786 aggregate more than one role into itself. These collapsed roles 787 combine the duties of multiple roles. In these cases, interaction 788 between these roles do not necessarily use the Internet Protocol. 789 They can be using a loopback device or other IP-based communication 790 between separate environments, but they do not have to. Alternative 791 channels to convey conceptual messages include function calls, 792 sockets, GPIO interfaces, local busses, or hypervisor calls. This 793 type of conveyance is typically found in Composite Devices. Most 794 importantly, these conveyance methods are out-of-scope of RATS, but 795 they are presumed to exist in order to convey conceptual messages 796 appropriately between roles. 798 For example, an entity that both connects to a wide-area network and 799 to a system bus is taking on both the Attester and Verifier roles. 800 As a system bus entity, a Verifier consumes Evidence from other 801 devices connected to the system bus that implement Attester roles. 802 As a wide-area network connected entity, it may implement an Attester 803 role. The entity, as a system bus Verifier, may choose to fully 804 isolate its role as a wide-area network Attester. 806 In essence, an entity that combines more than one role also creates 807 and consumes the corresponding conceptual messages as defined in this 808 document. 810 7. Trust Model 812 The scope of this document is scenarios for which a Relying Party 813 trusts a Verifier that can appraise the trustworthiness of 814 information about an Attester. Such trust might come by the Relying 815 Party trusting the Verifier (or its public key) directly, or might 816 come by trusting an entity (e.g., a Certificate Authority) that is in 817 the Verifier's certificate chain. The Relying Party might implicitly 818 trust a Verifier (such as in the Verifying Relying Party 819 combination). Or, for a stronger level of security, the Relying 820 Party might require that the Verifier itself provide information 821 about itself that the Relying Party can use to assess the 822 trustworthiness of the Verifier before accepting its Attestation 823 Results. 825 The Endorser and Verifier Owner may need to trust the Verifier before 826 giving the Endorsement and Appraisal Policy to it. Such trust can 827 also be established directly or indirectly, implicitly or explicitly. 828 One explicit way to establish such trust may be the Verifier first 829 acts as an Attester and creates Evidence about itself to be consumed 830 by the Endorser and/or Verifier Owner as the Relying Parties. If it 831 is accepted as trustworthy, then they can provide Endorsements and 832 Appraisal Policies that enable it to act as a Verifier. 834 The Verifier trusts (or more specifically, the Verifier's security 835 policy is written in a way that configures the Verifier to trust) a 836 manufacturer, or the manufacturer's hardware, so as to be able to 837 appraise the trustworthiness of that manufacturer's devices. In 838 solutions with weaker security, a Verifier might be configured to 839 implicitly trust firmware or even software (e.g., a hypervisor). 840 That is, it might appraise the trustworthiness of an application 841 component, or operating system component or service, under the 842 assumption that information provided about it by the lower-layer 843 hypervisor or firmware is true. A stronger level of security comes 844 when information can be vouched for by hardware or by ROM code, 845 especially if such hardware is physically resistant to hardware 846 tampering. The component that is implicitly trusted is often 847 referred to as a Root of Trust. 849 A conveyance protocol that provides authentication and integrity 850 protection can be used to convey unprotected Evidence, assuming the 851 following properties exists: 853 1. The key material used to authenticate and integrity protect the 854 conveyance channel is trusted by the Verifier to speak for the 855 Attesting Environment(s) that collected claims about the Target 856 Environment(s). 858 2. All unprotected Evidence that is conveyed is supplied exclusively 859 by the Attesting Environment that has the key material that 860 protects the conveyance channel 862 3. The Root of Trust protects both the conveyance channel key 863 material and the Attesting Environment with equivalent strength 864 protections. 866 In some scenarios, Evidence might contain sensitive information such 867 as Personally Identifiable Information. Thus, an Attester must trust 868 entities to which it conveys Evidence, to not reveal sensitive data 869 to unauthorized parties. The Verifier might share this information 870 with other authorized parties, according rules that it controls. In 871 the background-check model, this Evidence may also be revealed to 872 Relying Party(s). 874 8. Conceptual Messages 875 8.1. Evidence 877 Evidence is a set of claims about the target environment that reveal 878 operational status, health, configuration or construction that have 879 security relevance. Evidence is evaluated by a Verifier to establish 880 its relevance, compliance, and timeliness. Claims need to be 881 collected in a manner that is reliable. Evidence needs to be 882 securely associated with the target environment so that the Verifier 883 cannot be tricked into accepting claims originating from a different 884 environment (that may be more trustworthy). Evidence also must be 885 protected from man-in-the-middle attackers who may observe, change or 886 misdirect Evidence as it travels from Attester to Verifier. The 887 timeliness of Evidence can be captured using claims that pinpoint the 888 time or interval when changes in operational status, health, and so 889 forth occur. 891 8.2. Endorsements 893 An Endorsement is a secure statement that some entity (e.g., a 894 manufacturer) vouches for the integrity of the device's signing 895 capability. For example, if the signing capability is in hardware, 896 then an Endorsement might be a manufacturer certificate that signs a 897 public key whose corresponding private key is only known inside the 898 device's hardware. Thus, when Evidence and such an Endorsement are 899 used together, an appraisal procedure can be conducted based on 900 Appraisal Policies that may not be specific to the device instance, 901 but merely specific to the manufacturer providing the Endorsement. 902 For example, an Appraisal Policy might simply check that devices from 903 a given manufacturer have information matching a set of known-good 904 reference values, or an Appraisal Policy might have a set of more 905 complex logic on how to appraise the validity of information. 907 However, while an Appraisal Policy that treats all devices from a 908 given manufacturer the same may be appropriate for some use cases, it 909 would be inappropriate to use such an Appraisal Policy as the sole 910 means of authorization for use cases that wish to constrain _which_ 911 compliant devices are considered authorized for some purpose. For 912 example, an enterprise using remote attestation for Network Endpoint 913 Assessment may not wish to let every healthy laptop from the same 914 manufacturer onto the network, but instead only want to let devices 915 that it legally owns onto the network. Thus, an Endorsement may be 916 helpful information in authenticating information about a device, but 917 is not necessarily sufficient to authorize access to resources which 918 may need device-specific information such as a public key for the 919 device or component or user on the device. 921 8.3. Attestation Results 923 Attestation Results may indicate compliance or non-compliance with a 924 Verifier's Appraisal Policy. A result that indicates non-compliance 925 can be used by an Attester (in the passport model) or a Relying Party 926 (in the background-check model) to indicate that the Attester should 927 not be treated as authorized and may be in need of remediation. In 928 some cases, it may even indicate that the Evidence itself cannot be 929 authenticated as being correct. 931 An Attestation Result that indicates compliance can be used by a 932 Relying Party to make authorization decisions based on the Relying 933 Party's Appraisal Policy. The simplest such policy might be to 934 simply authorize any party supplying a compliant Attestation Result 935 signed by a trusted Verifier. A more complex policy might also 936 entail comparing information provided in the result against known- 937 good reference values, or applying more complex logic on such 938 information. 940 Thus, Attestation Results often need to include detailed information 941 about the Attester, for use by Relying Parties, much like physical 942 passports and drivers licenses include personal information such as 943 name and date of birth. Unlike Evidence, which is often very device- 944 and vendor-specific, Attestation Results can be vendor-neutral if the 945 Verifier has a way to generate vendor-agnostic information based on 946 the appraisal of vendor-specific information in Evidence. This 947 allows a Relying Party's Appraisal Policy to be simpler, potentially 948 based on standard ways of expressing the information, while still 949 allowing interoperability with heterogeneous devices. 951 Finally, whereas Evidence is signed by the device (or indirectly by a 952 manufacturer, if Endorsements are used), Attestation Results are 953 signed by a Verifier, allowing a Relying Party to only need a trust 954 relationship with one entity, rather than a larger set of entities, 955 for purposes of its Appraisal Policy. 957 9. Claims Encoding Formats 959 The following diagram illustrates a relationship to which remote 960 attestation is desired to be added: 962 +-------------+ +------------+ Evaluate 963 | |-------------->| | request 964 | Attester | Access some | Relying | against 965 | | resource | Party | security 966 +-------------+ +------------+ policy 968 Figure 8: Typical Resource Access 970 In this diagram, the protocol between Attester and a Relying Party 971 can be any new or existing protocol (e.g., HTTP(S), COAP(S), 802.1x, 972 OPC UA, etc.), depending on the use case. Such protocols typically 973 already have mechanisms for passing security information for purposes 974 of authentication and authorization. Common formats include JWTs 975 [RFC7519], CWTs [RFC8392], and X.509 certificates. 977 To enable remote attestation to be added to existing protocols, 978 enabling a higher level of assurance against malware for example, it 979 is important that information needed for appraising the Attester be 980 usable with existing protocols that have constraints around what 981 formats they can transport. For example, OPC UA [OPCUA] (probably 982 the most common protocol in industrial IoT environments) is defined 983 to carry X.509 certificates and so security information must be 984 embedded into an X.509 certificate to be passed in the protocol. 985 Thus, remote attestation related information could be natively 986 encoded in X.509 certificate extensions, or could be natively encoded 987 in some other format (e.g., a CWT) which in turn is then encoded in 988 an X.509 certificate extension. 990 Especially for constrained nodes, however, there is a desire to 991 minimize the amount of parsing code needed in a Relying Party, in 992 order to both minimize footprint and to minimize the attack surface 993 area. So while it would be possible to embed a CWT inside a JWT, or 994 a JWT inside an X.509 extension, etc., there is a desire to encode 995 the information natively in the format that is natural for the 996 Relying Party. 998 This motivates having a common "information model" that describes the 999 set of remote attestation related information in an encoding-agnostic 1000 way, and allowing multiple encoding formats (CWT, JWT, X.509, etc.) 1001 that encode the same information into the claims format needed by the 1002 Relying Party. 1004 The following diagram illustrates that Evidence and Attestation 1005 Results might each have multiple possible encoding formats, so that 1006 they can be conveyed by various existing protocols. It also 1007 motivates why the Verifier might also be responsible for accepting 1008 Evidence that encodes claims in one format, while issuing Attestation 1009 Results that encode claims in a different format. 1011 Evidence Attestation Results 1012 .--------------. CWT CWT .-------------------. 1013 | Attester-A |------------. .----------->| Relying Party V | 1014 '--------------' v | `-------------------' 1015 .--------------. JWT .------------. JWT .-------------------. 1016 | Attester-B |-------->| Verifier |-------->| Relying Party W | 1017 '--------------' | | `-------------------' 1018 .--------------. X.509 | | X.509 .-------------------. 1019 | Attester-C |-------->| |-------->| Relying Party X | 1020 '--------------' | | `-------------------' 1021 .--------------. TPM | | TPM .-------------------. 1022 | Attester-D |-------->| |-------->| Relying Party Y | 1023 '--------------' '------------' `-------------------' 1024 .--------------. other ^ | other .-------------------. 1025 | Attester-E |------------' '----------->| Relying Party Z | 1026 '--------------' `-------------------' 1028 Figure 9: Multiple Attesters and Relying Parties with Different 1029 Formats 1031 10. Freshness 1033 It is important to prevent replay attacks where an attacker replays 1034 old Evidence or an old Attestation Result that is no longer correct. 1035 To do so, some mechanism of ensuring that the Evidence and 1036 Attestation Result are fresh, meaning that there is some degree of 1037 assurance that they still reflect the latest state of the Attester, 1038 and that any Attestation Result was generated using the latest 1039 Appraisal Policy for Evidence. There is, however, always a race 1040 condition possible in that the state of the Attester, and the 1041 Appraisal Policy for Evidence, might change immediately after the 1042 Evidence or Attestation Result was generated. The goal is merely to 1043 narrow the time window to something the Verifier (for Evidence) or 1044 Relying Party (for an Attestation Result) is willing to accept. 1046 There are two common approaches to providing some assurance of 1047 freshness. The first approach is that a nonce is generated by a 1048 remote entity (e.g., the Verifier for Evidence, or the Relying Party 1049 for an Attestation Result), and the nonce is then signed and included 1050 along with the claims in the Evidence or Attestation Result, so that 1051 the remote entity knows that the claims were signed after the nonce 1052 was generated. 1054 A second approach is to rely on synchronized clocks, and include a 1055 signed timestamp (e.g., using [I-D.birkholz-rats-tuda]) along with 1056 the claims in the Evidence or Attestation Result, so that the remote 1057 entity knows that the claims were signed at that time, as long as it 1058 has some assurance that the timestamp is correct. This typically 1059 requires additional claims about the signer's time synchronization 1060 mechanism in order to provide such assurance. 1062 In either approach, it is important to note that the actual values in 1063 claims might have been generated long before the claims are signed. 1064 If so, it is the signer's responsibility to ensure that the values 1065 are still correct when they are signed. For example, values might 1066 have been generated at boot, and then used in claims as long as the 1067 signer can guarantee that they cannot have changed since boot. 1069 A more detailed discussion with examples appears in Section 16. 1071 11. Privacy Considerations 1073 The conveyance of Evidence and the resulting Attestation Results 1074 reveal a great deal of information about the internal state of a 1075 device. In many cases, the whole point of the Attestation process is 1076 to provide reliable information about the type of the device and the 1077 firmware/software that the device is running. This information might 1078 be particularly interesting to many attackers. For example, knowing 1079 that a device is running a weak version of firmware provides a way to 1080 aim attacks better. 1082 Evidence and Attestation Results data structures are expected to 1083 support integrity protection encoding (e.g., COSE, JOSE, X.509) and 1084 optionally might support confidentiality protection (e.g., COSE, 1085 JOSE). Therefore, if confidentiality protection is omitted or 1086 unavailable, the protocols that convey Evidence or Attestation 1087 Results are responsible for detailing what kinds of information are 1088 disclosed, and to whom they are exposed. 1090 Furthermore, because Evidence might contain sensitive information, 1091 Attesters are responsible for only sending such Evidence to trusted 1092 Verifiers. Some Attesters might want a stronger level of assurance 1093 of the trustworthiness of a Verifier before sending Evidence to it. 1094 In such cases, an Attester can first act as a Relying Party and ask 1095 for the Verifier's own Attestation Result, and appraising it just as 1096 a Relying Party would appraise an Attestation Result for any other 1097 purpose. 1099 12. Security Considerations 1101 Any solution that conveys information used for security purposes, 1102 whether such information is in the form of Evidence, Attestation 1103 Results, Endorsements, or Appraisal Policy, needs to support end-to- 1104 end integrity protection and replay attack prevention, and often also 1105 needs to support additional security protections. For example, 1106 additional means of authentication, confidentiality, integrity, 1107 replay, denial of service and privacy protection are needed in many 1108 use cases. Section 10 discusses ways in which freshness can be used 1109 in this architecture to protect against replay attacks. 1111 To assess the security provided by a particular Appraisal Policy, it 1112 is important to understand the strength of the Root of Trust, e.g., 1113 whether it is mutable software, or firmware that is read-only after 1114 boot, or immutable hardware/ROM. 1116 It is also important that the Appraisal Policy was itself obtained 1117 securely. As such, if Appraisal Policies for a Relying Party or for 1118 a Verifier can be configured via a network protocol, the ability to 1119 create Evidence about the integrity of the entity providing the 1120 Appraisal Policy needs to be considered. 1122 The security of conveyed information may be applied at different 1123 layers, whether by a conveyance protocol, or an information encoding 1124 format. This architecture expects attestation messages (i.e., 1125 Evidence, Attestation Results, Endorsements and Policies) are end-to- 1126 end protected based on the role interaction context. For example, if 1127 an Attester produces Evidence that is relayed through some other 1128 entity that doesn't implement the Attester or the intended Verifier 1129 roles, then the relaying entity should not expect to have access to 1130 the Evidence. 1132 13. IANA Considerations 1134 This document does not require any actions by IANA. 1136 14. Acknowledgments 1138 Special thanks go to Joerg Borchert, Nancy Cam-Winget, Jessica 1139 Fitzgerald-McKay, Thomas Fossati, Diego Lopez, Laurence Lundblade, 1140 Wei Pan, Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten. 1142 15. Contributors 1144 Thomas Hardjono created older versions of the terminology section in 1145 collaboration with Ned Smith. Eric Voit provided the conceptual 1146 separation between Attestation Provision Flows and Attestation 1147 Evidence Flows. Monty Wisemen created the content structure of the 1148 first three architecture drafts. Carsten Bormann provided many of 1149 the motivational building blocks with respect to the Internet Threat 1150 Model. 1152 16. Appendix A: Time Considerations 1154 The table below defines a number of relevant events, with an ID that 1155 is used in subsequent diagrams. The times of said events might be 1156 defined in terms of an absolute clock time such as Coordinated 1157 Universal Time, or might be defined relative to some other timestamp 1158 or timeticks counter. 1160 +----+------------+-----------------------------------------------+ 1161 | ID | Event | Explanation of event | 1162 +====+============+===============================================+ 1163 | VG | Value | A value to appear in a claim was created | 1164 | | generation | | 1165 +----+------------+-----------------------------------------------+ 1166 | NS | Nonce sent | A random number not predictable to an | 1167 | | | Attester is sent | 1168 +----+------------+-----------------------------------------------+ 1169 | NR | Nonce | The nonce is relayed to an Attester by | 1170 | | relayed | enother entity | 1171 +----+------------+-----------------------------------------------+ 1172 | EG | Evidence | An Attester collects claims and generates | 1173 | | generation | Evidence | 1174 +----+------------+-----------------------------------------------+ 1175 | ER | Evidence | A Relying Party relays Evidence to a Verifier | 1176 | | relayed | | 1177 +----+------------+-----------------------------------------------+ 1178 | RG | Result | A Verifier appraises Evidence and generates | 1179 | | generation | an Attestation Result | 1180 +----+------------+-----------------------------------------------+ 1181 | RR | Result | A Relying Party relays an Attestation Result | 1182 | | relayed | to a Relying Party | 1183 +----+------------+-----------------------------------------------+ 1184 | RA | Result | The Relying Party appraises Attestation | 1185 | | appraised | Results | 1186 +----+------------+-----------------------------------------------+ 1187 | OP | Operation | The Relying Party performs some operation | 1188 | | performed | requested by the Attester. For example, | 1189 | | | acting upon some message just received across | 1190 | | | a session created earlier at time(RA). | 1191 +----+------------+-----------------------------------------------+ 1192 | RX | Result | An Attestation Result should no longer be | 1193 | | expiry | accepted, according to the Verifier that | 1194 | | | generated it | 1195 +----+------------+-----------------------------------------------+ 1197 Table 1 1199 We now walk through a number of hypothetical examples of how a 1200 solution might be built. This list is not intended to be complete, 1201 but is just representative enough to highlight various timing 1202 considerations. 1204 16.1. Example 1: Timestamp-based Passport Model Example 1206 The following example illustrates a hypothetical Passport Model 1207 solution that uses timestamps and requires roughly synchronized 1208 clocks between the Attester, Verifier, and Relying Party, which 1209 depends on using a secure clock synchronization mechanism. 1211 .----------. .----------. .---------------. 1212 | Attester | | Verifier | | Relying Party | 1213 '----------' '----------' '---------------' 1214 time(VG) | | 1215 | | | 1216 ~ ~ ~ 1217 | | | 1218 time(EG) | | 1219 |------Evidence{time(EG)}-------->| | 1220 | time(RG) | 1221 |<-----Attestation Result---------| | 1222 | {time(RG),time(RX)} | | 1223 ~ ~ 1224 | | 1225 |------Attestation Result{time(RG),time(RX)}-->time(RA) 1226 | | 1227 ~ ~ 1228 | | 1229 | time(OP) 1230 | | 1232 The Verifier can check whether the Evidence is fresh when appraising 1233 it at time(RG) by checking "time(RG) - time(EG) < Threshold", where 1234 the Verifier's threshold is large enough to account for the maximum 1235 permitted clock skew between the Verifier and the Attester. 1237 If time(VG) is also included in the Evidence along with the claim 1238 value generated at that time, and the Verifier decides that it can 1239 trust the time(VG) value, the Verifier can also determine whether the 1240 claim value is recent by checking "time(RG) - time(VG) < Threshold", 1241 again where the threshold is large enough to account for the maximum 1242 permitted clock skew between the Verifier and the Attester. 1244 The Relying Party can check whether the Attestation Result is fresh 1245 when appraising it at time(RA) by checking "time(RA) - time(RG) < 1246 Threshold", where the Relying Party's threshold is large enough to 1247 account for the maximum permitted clock skew between the Relying 1248 Party and the Verifier. The result might then be used for some time 1249 (e.g., throughout the lifetime of a connection established at 1250 time(RA)). The Relying Party must be careful, however, to not allow 1251 continued use beyond the period for which it deems the Attestation 1252 Result to remain fresh enough. Thus, it might allow use (at 1253 time(OP)) as long as "time(OP) - time(RG) < Threshold". However, if 1254 the Attestation Result contains an expiry time time(RX) then it could 1255 explicitly check "time(OP) < time(RX)". 1257 16.2. Example 2: Nonce-based Passport Model Example 1259 The following example illustrates a hypothetical Passport Model 1260 solution that uses nonces and thus does not require that any clocks 1261 are synchronized. 1263 .----------. .----------. .---------------. 1264 | Attester | | Verifier | | Relying Party | 1265 '----------' '----------' '---------------' 1266 time(VG) | | 1267 | | | 1268 ~ ~ ~ 1269 | | | 1270 |<---Nonce1--------------------time(NS) | 1271 time(EG) | | 1272 |----Evidence-------------------->| | 1273 | {Nonce1, time(EG)-time(VG)} | | 1274 | time(RG) | 1275 |<---Attestation Result-----------| | 1276 | {time(RX)-time(RG)} | | 1277 ~ ~ 1278 | | 1279 |<---Nonce2------------------------------------time(NS') 1280 time(RR) 1281 |----Attestation Result{time(RX)-time(RG)}---->time(RA) 1282 | Nonce2, time(RR)-time(EG) | 1283 ~ ~ 1284 | | 1285 | time(OP) 1287 In this example solution, the Verifier can check whether the Evidence 1288 is fresh at time(RG) by verifying that "time(RG) - time(NS) < 1289 Threshold". 1291 The Verifier cannot, however, simply rely on a Nonce to determine 1292 whether the value of a claim is recent, since the claim value might 1293 have been generated long before the nonce was sent by the Verifier. 1294 However, if the Verifier decides that the Attester can be trusted to 1295 correctly provide the delta time(EG)-time(VG), then it can determine 1296 recency by checking "time(RG)-time(NS) + time(EG)-time(VG) < 1297 Threshold". 1299 Similarly if, based on an Attestation Result from a Verifier it 1300 trusts, the Relying Party decides that the Attester can be trusted to 1301 correctly provide time deltas, then it can determine whether the 1302 Attestation Result is fresh by checking "time(OP) - time(NS') + 1303 time(RR)-time(EG) < Threshold". Although the Nonce2 and time(RR)- 1304 time(EG) values cannot be inside the Attestation Result, they might 1305 be signed by the Attester such that the Attestation Result vouches 1306 for the Attester's signing capability. 1308 The Relying Party must still be careful, however, to not allow 1309 continued use beyond the period for which it deems the Attestation 1310 Result to remain valid. Thus, if the Attestation Result sends a 1311 validity lifetime in terms of time(RX)-time(RG), then the Relying 1312 Party can check "time(OP) - time(NS') < time(RX)-time(RG)". 1314 16.3. Example 3: Timestamp-based Background-Check Model Example 1316 The following example illustrates a hypothetical Background-Check 1317 Model solution that uses timestamps and requires roughly synchronized 1318 clocks between the Attester, Verifier, and Relying Party. 1320 .----------. .---------------. .----------. 1321 | Attester | | Relying Party | | Verifier | 1322 '----------' '---------------' '----------' 1323 time(VG) | | 1324 | | | 1325 ~ ~ ~ 1326 | | | 1327 time(EG) | | 1328 |----Evidence------->| | 1329 | {time(EG)} time(ER)--Evidence{time(EG)}-->| 1330 | | time(RG) 1331 | time(RA)<-Attestation Result---| 1332 | | {time(RX)} | 1333 ~ ~ ~ 1334 | | | 1335 | time(OP) | 1337 The time considerations in this example are equivalent to those 1338 discussed under Example 1 above. 1340 16.4. Example 4: Nonce-based Background-Check Model Example 1342 The following example illustrates a hypothetical Background-Check 1343 Model solution that uses nonces and thus does not require that any 1344 clocks are synchronized. In this example solution, a nonce is 1345 generated by a Verifier at the request of a Relying Party, when the 1346 Relying Party needs to send one to an Attester. 1348 .----------. .---------------. .----------. 1349 | Attester | | Relying Party | | Verifier | 1350 '----------' '---------------' '----------' 1351 time(VG) | | 1352 | | | 1353 ~ ~ ~ 1354 | | | 1355 | |<-----Nonce-------------time(NS) 1356 |<---Nonce-----------time(NR) | 1357 time(EG) | | 1358 |----Evidence{Nonce}--->| | 1359 | time(ER)--Evidence{Nonce}----->| 1360 | | time(RG) 1361 | time(RA)<-Attestation Result---| 1362 | | {time(RX)-time(RG)} | 1363 ~ ~ ~ 1364 | | | 1365 | time(OP) | 1367 The Verifier can check whether the Evidence is fresh, and whether a 1368 claim value is recent, the same as in Example 2 above. 1370 However, unlike in Example 2, the Relying Party can use the Nonce to 1371 determine whether the Attestation Result is fresh, by verifying that 1372 "time(OP) - time(NR) < Threshold". 1374 The Relying Party must still be careful, however, to not allow 1375 continued use beyond the period for which it deems the Attestation 1376 Result to remain valid. Thus, if the Attestation Result sends a 1377 validity lifetime in terms of time(RX)-time(RG), then the Relying 1378 Party can check "time(OP) - time(ER) < time(RX)-time(RG)". 1380 17. References 1382 17.1. Normative References 1384 [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token 1385 (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, 1386 . 1388 [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, 1389 "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, 1390 May 2018, . 1392 17.2. Informative References 1394 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", 1395 FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, 1396 . 1398 [OPCUA] OPC Foundation, "OPC Unified Architecture Specification, 1399 Part 2: Security Model, Release 1.03", OPC 10000-2 , 25 1400 November 2015, . 1404 [I-D.birkholz-rats-tuda] 1405 Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann, 1406 "Time-Based Uni-Directional Attestation", Work in 1407 Progress, Internet-Draft, draft-birkholz-rats-tuda-02, 9 1408 March 2020, . 1411 [I-D.ietf-teep-architecture] 1412 Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler, 1413 "Trusted Execution Environment Provisioning (TEEP) 1414 Architecture", Work in Progress, Internet-Draft, draft- 1415 ietf-teep-architecture-08, 4 April 2020, 1416 . 1419 Authors' Addresses 1421 Henk Birkholz 1422 Fraunhofer SIT 1423 Rheinstrasse 75 1424 64295 Darmstadt 1425 Germany 1427 Email: henk.birkholz@sit.fraunhofer.de 1429 Dave Thaler 1430 Microsoft 1431 United States of America 1433 Email: dthaler@microsoft.com 1435 Michael Richardson 1436 Sandelman Software Works 1437 Canada 1439 Email: mcr+ietf@sandelman.ca 1440 Ned Smith 1441 Intel Corporation 1442 United States of America 1444 Email: ned.smith@intel.com 1446 Wei Pan 1447 Huawei Technologies 1449 Email: william.panwei@huawei.com