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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. 'GP-TEE-PP' ** Downref: Normative reference to an Informational draft: draft-ietf-rats-architecture (ref. 'I-D.ietf-rats-architecture') -- Possible downref: Non-RFC (?) normative reference: ref. 'OMTP-ATE' Summary: 1 error (**), 0 flaws (~~), 0 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 RATS Working Group E. Voit 3 Internet-Draft Cisco 4 Intended status: Standards Track H. Birkholz 5 Expires: 8 September 2022 Fraunhofer SIT 6 T. Hardjono 7 MIT 8 T. Fossati 9 Arm Limited 10 V. Scarlata 11 Intel 12 7 March 2022 14 Attestation Results for Secure Interactions 15 draft-ietf-rats-ar4si-02 17 Abstract 19 This document defines reusable Attestation Result information 20 elements. When these elements are offered to Relying Parties as 21 Evidence, different aspects of Attester trustworthiness can be 22 evaluated. Additionally, where the Relying Party is interfacing with 23 a heterogeneous mix of Attesting Environment and Verifier types, 24 consistent policies can be applied to subsequent information exchange 25 between each Attester and the Relying Party. 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 8 September 2022. 44 Copyright Notice 46 Copyright (c) 2022 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 (https://trustee.ietf.org/ 51 license-info) in effect on the date of publication of this document. 52 Please review these documents carefully, as they describe your rights 53 and restrictions with respect to this document. Code Components 54 extracted from this document must include Revised BSD License text as 55 described in Section 4.e of the Trust Legal Provisions and are 56 provided without warranty as described in the Revised BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 61 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4 62 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 63 2. Attestation Results for Secure Interactions . . . . . . . . . 5 64 2.1. Information driving a Relying Party Action . . . . . . . 6 65 2.2. Non-repudiable Identity . . . . . . . . . . . . . . . . . 6 66 2.2.1. Attester and Attesting Environment . . . . . . . . . 7 67 2.2.2. Verifier . . . . . . . . . . . . . . . . . . . . . . 10 68 2.2.3. Communicating Identity . . . . . . . . . . . . . . . 10 69 2.3. Trustworthiness Claims . . . . . . . . . . . . . . . . . 11 70 2.3.1. Design Principles . . . . . . . . . . . . . . . . . . 11 71 2.3.2. Enumeration Encoding . . . . . . . . . . . . . . . . 12 72 2.3.3. Assigning a Trustworthiness Claim value . . . . . . . 13 73 2.3.4. Specific Claims . . . . . . . . . . . . . . . . . . . 14 74 2.3.5. Trustworthiness Vector . . . . . . . . . . . . . . . 18 75 2.3.6. Trustworthiness Vector for a type of Attesting 76 Environment . . . . . . . . . . . . . . . . . . . . . 19 77 2.4. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 19 78 3. Secure Interactions Models . . . . . . . . . . . . . . . . . 20 79 3.1. Background-Check . . . . . . . . . . . . . . . . . . . . 20 80 3.1.1. Verifier Retrieval . . . . . . . . . . . . . . . . . 20 81 3.1.2. Co-resident Verifier . . . . . . . . . . . . . . . . 20 82 3.2. Below Zero Trust . . . . . . . . . . . . . . . . . . . . 21 83 3.3. Mutual Attestation . . . . . . . . . . . . . . . . . . . 25 84 3.4. Transport Protocol Integration . . . . . . . . . . . . . 26 85 4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 26 86 5. Security Considerations . . . . . . . . . . . . . . . . . . . 26 87 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 88 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 26 89 7.1. Normative References . . . . . . . . . . . . . . . . . . 26 90 7.2. Informative References . . . . . . . . . . . . . . . . . 27 91 Appendix A. Implementation Guidance . . . . . . . . . . . . . . 28 92 A.1. Supplementing Trustworthiness Claims . . . . . . . . . . 28 93 Appendix B. Supportable Trustworthiness Claims . . . . . . . . . 28 94 B.1. Supportable Trustworthiness Claims for HSM-based CC . . . 29 95 B.2. Supportable Trustworthiness Claims for process-based 96 CC . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 98 B.3. Supportable Trustworthiness Claims for VM-based CC . . . 33 99 Appendix C. Some issues being worked . . . . . . . . . . . . . . 34 100 Appendix D. Contributors . . . . . . . . . . . . . . . . . . . . 34 101 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35 103 1. Introduction 105 The first paragraph of the May 2021 US Presidential Executive Order 106 on Improving the Nation's Cybersecurity [US-Executive-Order] ends 107 with the statement "the trust we place in our digital infrastructure 108 should be proportional to how trustworthy and transparent that 109 infrastructure is." Later this order explores aspects of 110 trustworthiness such as an auditable trust relationship, which it 111 defines as an "agreed-upon relationship between two or more system 112 elements that is governed by criteria for secure interaction, 113 behavior, and outcomes." 115 The Remote ATtestation procedureS (RATS) architecture 116 [I-D.ietf-rats-architecture] provides a useful context for 117 programmatically establishing and maintaining such auditable trust 118 relationships. Specifically, the architecture defines conceptual 119 messages conveyed between architectural subsystems to support 120 trustworthiness appraisal. The RATS conceptual message used to 121 convey evidence of trustworthiness is the Attestation Results. The 122 Attestation Results includes Verifier generated appraisals of an 123 Attester including such information as the identity of the Attester, 124 the security mechanisms employed on this Attester, and the Attester's 125 current state of trustworthiness. 127 Generated Attestation Results are ultimately conveyed to one or more 128 Relying Parties. Reception of an Attestation Result enables a 129 Relying Party to determine what action to take with regards to an 130 Attester. Frequently, this action will be to choose whether to allow 131 the Attester to securely interact with the Relying Party over some 132 connection between the two. 134 When determining whether to allow secure interactions with an 135 Attester, a Relying Party is challenged with a number of difficult 136 problems which it must be able to handle successfully. These 137 problems include: 139 * What Attestation Results (AR) might a Relying Party be willing to 140 trust from a specific Verifier? 142 * What information does a Relying Party need before allowing 143 interactions or choosing policies to apply to a connection? 145 * What are the operating/environmental realities of the Attesting 146 Environment where a Relying Party should only be able to associate 147 a certain confidence regarding Attestation Results out of the 148 Verifier? (In other words, different types of Trusted Execution 149 Environments (TEE) need not be treated as equivalent.) 151 * How to make direct comparisons where there is a heterogeneous mix 152 of Attesting Environments and Verifier types. 154 To address these problems, it is important that specific Attestation 155 Result information elements are framed independently of Attesting 156 Environment specific constraints. If they are not, a Relying Party 157 would be forced to adapt to the syntax and semantics of many vendor 158 specific environments. This is not a reasonable ask as there can be 159 many types of Attesters interacting with or connecting to a Relying 160 Party. 162 The business need therefore is for common Attestation Result 163 information element definitions. With these definitions, consistent 164 interaction or connectivity decisions can be made by a Relying Party 165 where there is a heterogenous mix of Attesting Environment types and 166 Verifier types. 168 This document defines information elements for Attestation Results in 169 a way which normalizes the trustworthiness assertions that can be 170 made from a diverse set of Attesters. 172 1.1. Requirements Notation 174 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 175 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 176 "OPTIONAL" in this document are to be interpreted as described in 177 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 178 capitals, as shown here. 180 1.2. Terminology 182 The following terms are imported from [I-D.ietf-rats-architecture]: 183 Appraisal Policy for Attestation Results, Attester, Attesting 184 Environment, Claims, Evidence, Relying Party, Target Environment and 185 Verifier. 187 [I-D.ietf-rats-architecture] also describes topological patterns that 188 illustrate the need for interoperable conceptual messages. The two 189 patterns called "background-check model" and "passport model" are 190 imported from the RATS architecture and used in this document as a 191 reference to the architectural concepts: Background-Check Model and 192 Passport Model. 194 Newly defined terms for this document: 196 AR-augmented Evidence: a bundle of Evidence which includes at least 197 the following: 199 1. Verifier signed Attestation Results. These Attestation 200 Results must include Identity Evidence for the Attester, a 201 Trustworthiness Vector describing a Verifier's most recent 202 appraisal of an Attester, and some Verifier Proof-of-Freshness 203 (PoF). 205 2. A Relying Party PoF which is bound to the Attestation Results 206 of (1) by the Attester's Attesting Environment signature. 208 3. Sufficient information to determine the elapsed interval 209 between the Verifier PoF and Relying Party PoF. 211 Identity Evidence: Evidence which unambiguously identifies an 212 identity. Identity Evidence could take different forms, such as a 213 certificate, or a signature which can be appraised to have only 214 been generated by a specific private/public key pair. 216 Trustworthiness Claim: a specific quanta of trustworthiness which 217 can be assigned by a Verifier based on its appraisal policy. 219 Trustworthiness Tier: a categorization of the levels of 220 trustworthiness which may be assigned by a Verifier to a specific 221 Trustworthiness Claim. These enumerated categories are: Affirmed, 222 Warning, Contraindicated, and None. 224 Trustworthiness Vector: a set of zero to many Trustworthiness Claims 225 assigned during a single appraisal procedure by a Verifier using 226 Evidence generated by an Attester. The vector is included within 227 Attestation Results. 229 2. Attestation Results for Secure Interactions 231 A Verifier generates the Attestation Results used by a Relying Party. 232 When a Relying Party needs to determine whether to permit 233 communications with an Attester, these Attestation Results must 234 contain a specific set of information elements. This section defines 235 those information elements, and in some cases encodings for 236 information elements. 238 2.1. Information driving a Relying Party Action 240 When the action is a communication establishment attempt with an 241 Attester, there is only a limited set of actions which a Relying 242 Party might take. These actions include: 244 * Allow or deny information exchange with the Attester. When there 245 is a deny, reasons should be returned to the Attester. 247 * Establish a transport connection between an Attester and a 248 specific context within a Relying Party (e.g., a TEE, or Virtual 249 Routing Function (VRF).) 251 * Apply policies on this connection (e.g., rate limits). 253 There are three categories of information which must be conveyed to 254 the Relying Party (which also is integrated with a Verifier) before 255 it determines which of these actions to take. 257 1. Non-repudiable Identity Evidence - Evidence which undoubtably 258 identifies one or more entities involved with a communication. 260 2. Trustworthiness Claims - Specifics a Verifier asserts with 261 regards to its trustworthiness findings about an Attester. 263 3. Claim Freshness - Establishes the time of last update (or 264 refresh) of Trustworthiness Claims. 266 The following sections detail requirements for these three 267 categories. 269 2.2. Non-repudiable Identity 271 Identity Evidence must be conveyed during the establishment of any 272 trust-based relationship. Specific use cases will define the minimum 273 types of identities required by a particular Relying Party as it 274 evaluates Attestation Results, and perhaps additional associated 275 Evidence. At a bare minimum, a Relying Party MUST start with the 276 ability to verify the identity of a Verifier it chooses to trust. 277 Attester identities may then be acquired through signed or encrypted 278 communications with the Verifier identity and/or the pre-provisioning 279 Attester public keys in the Attester. 281 During the Remote Attestation process, the Verifier's identity must 282 be established with a Relying Party, often via a Verifier signature 283 across recent Attestation Results. This Verifier identity could only 284 have come from a key pair maintained by a trusted developer or 285 operator of the Verifier. 287 Additionally, each set of Attestation Results must be provably and 288 non-reputably bound to the identity of the original Attesting 289 Environment which was evaluated by the Verifier. This is 290 accomplished via satisfying two requirements. First the Verifier 291 signed Attestation Results MUST include sufficient Identity Evidence 292 to ensure that this Attesting Environment signature refers to the 293 same Attesting Environment appraised by the Verifier. Second, where 294 the passport model is used as a subsystem, an Attesting Environment 295 signature which spans the Verifier signature MUST also be included. 296 As the Verifier signature already spans the Attester Identity as well 297 as the Attestation Results, this restricts the viability of spoofing 298 attacks. 300 In a subset of use cases, these two pieces of Identity Evidence may 301 be sufficient for a Relying Party to successfully meet the criteria 302 for its Appraisal Policy for Attestation Results. If the use case is 303 a connection request, a Relying Party may simply then establish a 304 transport session with an Attester after a successful appraisal. 305 However an Appraisal Policy for Attestation Results will often be 306 more nuanced, and the Relying Party may need additional information. 307 Some Identity Evidence related policy questions which the Relying 308 Party may consider include: 310 * Does the Relying Party only trust this Verifier to make 311 Trustworthiness Claims on behalf a specific type of Attesting 312 Environment? Might a mix of Verifiers be necessary to cover all 313 mandatory Trustworthiness Claims? 315 * Does the Relying Party only accept connections from a verified- 316 authentic software build from a specific software developer? 318 * Does the Relying Party only accept connections from specific 319 preconfigured list of Attesters? 321 For any of these more nuanced appraisals, additional Identity 322 Evidence or other policy related information must be conveyed or pre- 323 provisioned during the formation of a trust context between the 324 Relying Party, the Attester, the Attester's Attesting Environment, 325 and the Verifier. 327 2.2.1. Attester and Attesting Environment 329 Per [I-D.ietf-rats-architecture] Figure 2, an Attester and a 330 corresponding Attesting Environment might not share common code or 331 even hardware boundaries. Consequently, an Attester implementation 332 needs to ensure that any Evidence which originates from outside the 333 Attesting Environment MUST have been collected and delivered securely 334 before any Attesting Environment signing may occur. After the 335 Verifier performs its appraisal, it will include sufficient 336 information in the Attestation Results to enable a Relying Party to 337 have confidence that the Attester's trustworthiness is represented 338 via Trustworthiness Claims signed by the appropriate Attesting 339 Environment. 341 This document recognizes three general categories of Attesters. 343 1. HSM-based: A Hardware Security Module (HSM) based cryptoprocessor 344 which hashes one or more streams of security measurements from an 345 Attester within the Attesting Environment. Maintenance of this 346 hash enables detection of an Attester which is lying about the 347 set of security measurements taken. An example of a HSM is a 348 TPM2.0 [TPM2.0]. 350 2. Process-based: An individual process which has its runtime memory 351 encrypted by an Attesting Environment in a way that no other 352 processes can read and decrypt that memory (e.g., [SGX] or 353 [I-D.tschofenig-rats-psa-token].) 355 3. VM-based: An entire Guest VM (or a set of containers within a 356 host) have been encrypted as a walled-garden unit by an Attesting 357 Environment. The result is that the host operating system cannot 358 read and decrypt what is executing within that VM (e.g., 359 [SEV-SNP] or [TDX].) 361 Each of these categories of Attesters above will be capable of 362 generating Evidence which is protected using private keys / 363 certificates which are not accessible outside of the corresponding 364 Attesting Environment. The owner of these secrets is the owner of 365 the identity which is bound within the Attesting Environment. 366 Effectively this means that for any Attester identity, there will 367 exist a chain of trust ultimately bound to a hardware-based root of 368 trust in the Attesting Environment. It is upon this root of trust 369 that unique, non-repudiable Attester identities may be founded. 371 There are several types of Attester identities defined in this 372 document. This list is extensible: 374 * chip-vendor: the vendor of the hardware chip used for the 375 Attesting Environment (e.g., a primary Endorsement Key from a TPM) 377 * chip-hardware: specific hardware with specific firmware from an 378 'chip-vendor' 380 * target-environment: a unique instance of a software build running 381 in an Attester (e.g., MRENCLAVE [SGX], an Instance ID 382 [I-D.tschofenig-rats-psa-token], an Identity Block [SEV-SNP], or a 383 hash which represents a set of software loaded since boot (e.g., 384 TPM based integrity verification.)) 386 * target-developer: the organizational unit responsible for a 387 particular 'target-environment' (e.g., MRSIGNER [SGX]) 389 * instance: a unique instantiated instance of an Attesting 390 Environment running on 'chip-hardware' (e.g., an LDevID 391 [IEEE802.1AR]) 393 Based on the category of the Attesting Environment, different types 394 of identities might be exposed by an Attester. 396 +========================+===============+===========+===========+ 397 | Attester Identity type | Process-based | VM-based | HSM-based | 398 +========================+===============+===========+===========+ 399 | chip-vendor | Mandatory | Mandatory | Mandatory | 400 +------------------------+---------------+-----------+-----------+ 401 | chip-hardware | Mandatory | Mandatory | Mandatory | 402 +------------------------+---------------+-----------+-----------+ 403 | target-environment | Mandatory | Mandatory | Optional | 404 +------------------------+---------------+-----------+-----------+ 405 | target-developer | Mandatory | Optional | Optional | 406 +------------------------+---------------+-----------+-----------+ 407 | instance | Optional | Optional | Optional | 408 +------------------------+---------------+-----------+-----------+ 410 Table 1 412 It is expected that drafts subsequent to this specification will 413 provide the definitions and value domains for specific identities, 414 each of which falling within the Attester identity types listed 415 above. In some cases the actual unique identities might encoded as 416 complex structures. An example complex structure might be a 'target- 417 environment' encoded as a Software Bill of Materials (SBOM). 419 With the identity definitions and value domains, a Relying Party will 420 have sufficient information to ensure that the Attester identities 421 and Trustworthiness Claims asserted are actually capable of being 422 supported by the underlying type of Attesting Environment. 423 Consequently, the Relying Party SHOULD require Identity Evidence 424 which indicates of the type of Attesting Environment when it 425 considers its Appraisal Policy for Attestation Results. 427 2.2.2. Verifier 429 For the Verifier identity, it is critical for a Relying Party to 430 review the certificate and chain of trust for that Verifier. 431 Additionally, the Relying Party must have confidence that the 432 Trustworthiness Claims being relied upon from the Verifier considered 433 the chain of trust for the Attesting Environment. 435 There are two categorizations Verifier identities defined in this 436 document. 438 * verifier build: a unique instance of a software build running as a 439 Verifier. 441 * verifier developer: the organizational unit responsible for a 442 particular 'verifier build'. 444 Within each category, communicating the identity can be accomplished 445 via a variety of objects and encodings. 447 2.2.3. Communicating Identity 449 Any of the above identities used by the Appraisal Policy for 450 Attestation Results needed to be pre-established by the Relying Party 451 before, or provided during, the exchange of Attestation Results. 452 When provided during this exchange, the identity may be communicated 453 either implicitly or explicitly. 455 An example of explicit communication would be to include the 456 following Identity Evidence directly within the Attestation Results: 457 a unique identifier for an Attesting Environment, the name of a key 458 which can be provably associated with that unique identifier, and the 459 set of Attestation Results which are signed using that key. As these 460 Attestation Results are signed by the Verifier, it is the Verifier 461 which is explicitly asserting the credentials it believes are 462 trustworthy. 464 An example of implicit communication would be to include Identity 465 Evidence in the form of a signature which has been placed over the 466 Attestation Results asserted by a Verifier. It would be then up to 467 the Relying Party's Appraisal Policy for Attestation Results to 468 extract this signature and confirm that it only could have been 469 generated by an Attesting Environment having access to a specific 470 private key. This implicit identity communication is only viable if 471 the Attesting Environment's public key is already known by the 472 Relying Party. 474 One final step in communicating identity is proving the freshness of 475 the Attestation Results to the degree needed by the Relying Party. A 476 typical way to accomplish this is to include an element of freshness 477 be embedded within a signed portion of the Attestation Results. This 478 element of freshness reduces the identity spoofing risks from a 479 replay attack. For more on this, see Section 2.4. 481 2.3. Trustworthiness Claims 483 2.3.1. Design Principles 485 Trust is not absolute. Trust is a belief in some aspect about an 486 entity (in this case an Attester), and that this aspect is something 487 which can be depended upon (in this case by a Relying Party.) Within 488 the context of Remote Attestation, believability of this aspect is 489 facilitated by a Verifier. This facilitation depends on the 490 Verifier's ability to parse detailed Evidence from an Attester and 491 then to assert conclusions about this aspect in a way interpretable 492 by a Relying Party. 494 Specific aspects for which a Verifier will assert trustworthiness are 495 defined in this section. These are known as Trustworthiness Claims. 496 These claims have been designed to enable a common understanding 497 between a broad array of Attesters, Verifiers, and Relying Parties. 498 The following set of design principles have been applied in the 499 Trustworthiness Claim definitions: 501 1. Expose a small number of Trustworthiness Claims. 503 Reason: a plethora of similar Trustworthiness Claims will result 504 in divergent choices made on which to support between different 505 Verifiers. This would place a lot of complexity in the Relying 506 Party as it would be up to the Relying Party (and its policy 507 language) to enable normalization across rich but incompatible 508 Verifier object definitions. 510 2. Each Trustworthiness Claim enumerates only the specific states 511 that could viably result in a different outcome after the Policy 512 for Attestation Results has been applied. 514 Reason: by explicitly disallowing the standardization of 515 enumerated states which cannot easily be connected to a use case, 516 we avoid forcing implementers from making incompatible guesses on 517 what these states might mean. 519 3. Verifier and RP developers need explicit definitions of each 520 state in order to accomplish the goals of (1) and (2). 522 Reason: without such guidance, the Verifier will append plenty of 523 raw supporting info. This relieves the Verifier of making the 524 hard decisions. Of course, this raw info will be mostly non- 525 interpretable and therefore non-actionable by the Relying Party. 527 4. Support standards and non-standard extensibility for (1) and (2). 529 Reason: standard types of Verifier generated Trustworthiness 530 Claims should be vetted by the full RATS working group, rather 531 than being maintained in a repository which doesn't follow the 532 RFC process. This will keep a tight lid on extensions which must 533 be considered by the Relying Party's policy language. Because 534 this process takes time, non-standard extensions will be needed 535 for implementation speed and flexibility. 537 These design principles are important to keep the number of Verifier 538 generated claims low, and to retain the complexity in the Verifier 539 rather than the Relying Party. 541 2.3.2. Enumeration Encoding 543 Per design principle (2), each Trustworthiness Claim will only expose 544 specific encoded values. To simplify the processing of these 545 enumerations by the Relying Party, the enumeration will be encoded as 546 a single signed 8 bit integer. These value assignments for this 547 integer will be in four Trustworthiness Tiers which follow these 548 guidelines: 550 None: The Verifier makes no assertions regarding this aspect of 551 trustworthiness. 553 * Value 0: The Evidence received is insufficient to make a 554 conclusion. Note: this should always be always treated 555 equivalently by the Relying Party as no claim being made. I.e., 556 the RP's Appraisal Policy for Attestation Results SHOULD NOT make 557 any distinction between a Trustworthiness Claim with enumeration 558 '0', and no Trustworthiness Claim being provided. 560 * Value 1: The Evidence received contains unexpected elements which 561 the Verifier is unable to parse. An example might be that the 562 wrong type of Evidence has been delivered. 564 * Value -1: A verifier malfunction occurred during the Verifier's 565 appraisal processing. 567 Affirming: The Verifier affirms the Attester support for this aspect 568 of trustworthiness. 570 * Values 2 to 31: A standards enumerated reason for affirming. 572 * Values -2 to -32: A non-standard reason for affirming. 574 Warning: The Verifier warns about this aspect of trustworthiness. 576 * Values 32 to 95: A standards enumerated reason for the warning. 578 * Values -33 to -96: A non-standard reason for the warning. 580 Contraindicated: The Verifier asserts the Attester is explicitly 581 untrustworthy in regard to this aspect. 583 * Values 96 to 127: A standards enumerated reason for the 584 contraindication. 586 * Values -97 to -128: A non-standard reason for the 587 contraindication. 589 This enumerated encoding listed above will simplify the Appraisal 590 Policy for Attestation Results. Such a policies may be as simple as 591 saying that a specific Verifier has recently asserted Trustworthiness 592 Claims, all of which are Affirming. 594 2.3.3. Assigning a Trustworthiness Claim value 596 In order to simplify design, only a single encoded value is asserted 597 by a Verifier for any Trustworthiness Claim within a using the 598 following process. 600 1. If applicable, a Verifier MUST assign a standardized value from 601 the Contraindicated tier. 603 2. Else if applicable, a Verifier MUST assign a non-standardized 604 value from the Contraindicated tier. 606 3. Else if applicable, a Verifier MUST assign a standardized value 607 from the Warning tier. 609 4. Else if applicable, a Verifier MUST assign a non-standardized 610 value from the Warning tier. 612 5. Else if applicable, a Verifier MUST assign a standardized value 613 from the Affirming tier. 615 6. Else if applicable, a Verifier MUST assign a non-standardized 616 value from the Affirming tier. 618 7. Else a Verifier MAY assign a 0 or -1. 620 2.3.4. Specific Claims 622 Following are the Trustworthiness Claims and their supported 623 enumerations which may be asserted by a Verifier: 625 configuration: A Verifier has appraised an Attester's configuration, 626 and is able to make conclusions regarding the exposure of known 627 vulnerabilities 629 0: No assertion 631 1: Verifer cannot parse unexpected Evidence. 633 -1: Verifier malfunction 635 2: The configuration is a known and approved config. 637 3: The configuration includes or exposes no known 638 vulnerabilities. 640 32: The configuration includes or exposes known vulnerabilities. 642 96: The configuration is unsupportable as it exposes unacceptable 643 security vulnerabilities. 645 99: Cryptographic validation of the Evidence has failed. 647 executables: A Verifier has appraised and evaluated relevant runtime 648 files, scripts, and/or other objects which have been loaded into 649 the Target environment's memory. 651 0: No assertion 653 1: Verifer cannot parse unexpected Evidence. 655 -1: Verifier malfunction 657 2: Only a recognized genuine set of approved executables, 658 scripts, files, and/or objects have been loaded during and 659 after the boot process. 661 3: Only a recognized genuine set of approved executables have 662 been loaded during the boot process. 664 32: Only a recognized genuine set of executables, scripts, files, 665 and/or objects have been loaded. However the Verifier cannot 666 vouch for a subset of these due to known bugs or other known 667 vulnerabilities. 669 33: Runtime memory includes executables, scripts, files, and/or 670 objects which are not recognized. 672 96: Runtime memory includes executables, scripts, files, and/or 673 object which are contraindicated. 675 99: Cryptographic validation of the Evidence has failed. 677 file-system: A Verifier has evaluated a specific set of directories 678 within the Attester's file system. (Note: the Verifier may or may 679 not indicate what these directory and expected files are via an 680 unspecified management interface.) 682 0: No assertion 684 1: Verifer cannot parse unexpected Evidence. 686 -1: Verifier malfunction 688 2: Only a recognized set of approved files are found. 690 32: The file system includes unrecognized executables, scripts, 691 or files. 693 96: The file system includes contraindicated executables, 694 scripts, or files. 696 99: Cryptographic validation of the Evidence has failed. 698 hardware: A Verifier has appraised any Attester hardware and 699 firmware which are able to expose fingerprints of their identity 700 and running code. 702 0: No assertion 704 1: Verifer cannot parse unexpected Evidence. 706 -1: Verifier malfunction 708 2: An Attester has passed its hardware and/or firmware 709 verifications needed to demonstrate that these are genuine/ 710 supported. 712 32: An Attester contains only genuine/supported hardware and/or 713 firmware, but there are known security vulnerabilities. 715 96: Attester hardware and/or firmware is recognized, but its 716 trustworthiness is contraindicated. 718 97: A Verifier does not recognize an Attester's hardware or 719 firmware, but it should be recognized. 721 99: Cryptographic validation of the Evidence has failed. 723 instance-identity: A Verifier has appraised an Attesting 724 Environment's unique identity based upon private key signed 725 Evidence which can be correlated to a unique instantiated instance 726 of the Attester. (Note: this Trustworthiness Claim should only be 727 generated if the Verifier actually expects to recognize the unique 728 identity of the Attester.) 730 0: No assertion 732 1: Verifer cannot parse unexpected Evidence. 734 -1: Verifier malfunction 736 2: The Attesting Environment is recognized, and the associated 737 instance of the Attester is not known to be compromised. 739 96: The Attesting Environment is recognized, and but its unique 740 private key indicates a device which is not trustworthy. 742 97: The Attesting Environment is not recognized; however the 743 Verifier believes it should be. 745 99: Cryptographic validation of the Evidence has failed. 747 runtime-opaque: A Verifier has appraised the visibility of Attester 748 objects in memory from perspectives outside the Attester. 750 0: No assertion 752 1: Verifer cannot parse unexpected Evidence. 754 -1: Verifier malfunction 756 2: the Attester's executing Target Environment and Attesting 757 Environments are encrypted and within Trusted Execution 758 Environment(s) opaque to the operating system, virtual machine 759 manager, and peer applications. (Note: This value corresponds 760 to the protections asserted by O.RUNTIME_CONFIDENTIALITY from 761 [GP-TEE-PP]) 763 32: the Attester's executing Target Environment and Attesting 764 Environments inaccessible from any other parallel application 765 or Guest VM running on the Attester's physical device. (Note 766 that unlike "1" these environments are not encrypted in a way 767 which restricts the Attester's root operator visibility. See 768 O.TA_ISOLATION from [GP-TEE-PP].) 770 96: The Verifier has concluded that in memory objects are 771 unacceptably visible within the physical host that supports the 772 Attester. 774 99: Cryptographic validation of the Evidence has failed. 776 sourced-data: A Verifier has evaluated of the integrity of data 777 objects from external systems used by the Attester. 779 0: No assertion 781 1: Verifer cannot parse unexpected Evidence. 783 -1: Verifier malfunction 785 2: All essential Attester source data objects have been provided 786 by other Attester(s) whose most recent appraisal(s) had both no 787 Trustworthiness Claims of "0" where the current Trustworthiness 788 Claim is "Affirming", as well as no "Warning" or 789 "Contraindicated" Trustworthiness Claims. 791 32: Attester source data objects come from unattested sources, or 792 attested sources with "Warning" type Trustworthiness Claims. 794 96: Attester source data objects come from contraindicated 795 sources. 797 99: Cryptographic validation of the Evidence has failed. 799 storage-opaque: A Verifier has appraised that an Attester is capable 800 of encrypting persistent storage. (Note: Protections must meet 801 the capabilities of [OMTP-ATE] Section 5, but need not be hardware 802 tamper resistant.) 804 0: No assertion 805 1: Verifer cannot parse unexpected Evidence. 807 -1: Verifier malfunction 809 2: the Attester encrypts all secrets in persistent storage via 810 using keys which are never visible outside an HSM or the 811 Trusted Execution Environment hardware. 813 32: the Attester encrypts all persistently stored secrets, but 814 without using hardware backed keys 816 96: There are persistent secrets which are stored unencrypted in 817 an Attester. 819 99: Cryptographic validation of the Evidence has failed. 821 It is possible for additonal Trustworthiness Claims and enumerated 822 values to be defined in subsequent documents. At the same time, the 823 standardized Trustworthiness Claim values listed above have been 824 designed so there is no overlap within a Trustworthiness Tier. As a 825 result, it is possible to imagine a future where overlapping 826 Trustworthiness Claims within a single Trustworthiness Tier may be 827 defined. Wherever possible, the Verifier SHOULD assign the best 828 fitting standardized value. 830 Where a Relying Party doesn't know how to handle a particular 831 Trustworthiness Claim, it MAY choose an appropriate action based on 832 the Trustworthiness Tier under which the enumerated value fits. 834 It is up to the Verifier to publish the types of evaluations it 835 performs when determining how Trustworthiness Claims are derived for 836 a type of any particular type of Attester. It is out of the scope of 837 this document for the Verifier to provide proof or specific logic on 838 how a particular Trustworthiness Claim which it is asserting was 839 derived. 841 2.3.5. Trustworthiness Vector 843 Multiple Trustworthiness Claims may be asserted about an Attesting 844 Environment at single point in time. The set of Trustworthiness 845 Claims inserted into an instance of Attestation Results by a Verifier 846 is known as a Trustworthiness Vector. The order of Claims in the 847 vector is NOT meaningful. A Trustworthiness Vector with no 848 Trustworthiness Claims (i.e., a null Trustworthiness Vector) is a 849 valid construct. In this case, the Verifier is making no 850 Trustworthiness Claims but is confirming that an appraisal has been 851 made. 853 2.3.6. Trustworthiness Vector for a type of Attesting Environment 855 Some Trustworthiness Claims are implicit based on the underlying type 856 of Attesting Environment. For example, a validated MRSIGNER identity 857 can be present where the underlying [SGX] hardware is 'hw-authentic'. 858 Where such implicit Trustworthiness Claims exist, they do not have to 859 be explicitly included in the Trustworthiness Vector. However, these 860 implicit Trustworthiness Claims SHOULD be considered as being present 861 by the Relying Party. Another way of saying this is if a 862 Trustworthiness Claim is automatically supported as a result of 863 coming from a specific type of TEE, that claim need not be 864 redundantly articulated. Such implicit Trustworthiness Claims can be 865 seen in the tables within Appendix B.2 and Appendix B.3. 867 Additionally, there are some Trustworthiness Claims which cannot be 868 adequately supported by an Attesting Environment. For example, it 869 would be difficult for an Attester that includes only a TPM (and no 870 other TEE) from ever having a Verifier appraise support for 'runtime- 871 opaque'. As such, a Relying Party would be acting properly if it 872 rejects any non-supportable Trustworthiness Claims asserted from a 873 Verifier. 875 As a result, the need for the ability to carry a specific 876 Trustworthiness Claim will vary by the type of Attesting Environment. 877 Example mappings can be seen in Appendix B. 879 2.4. Freshness 881 A Relying Party will care about the recentness of the Attestation 882 Results, and the specific Trustworthiness Claims which are embedded. 883 All freshness mechanisms of [I-D.ietf-rats-architecture], Section 10 884 are supportable by this specification. 886 Additionally, a Relying Party may track when a Verifier expires its 887 confidence for the Trustworthiness Claims or the Trustworthiness 888 Vector as a whole. Mechanisms for such expiry are not defined within 889 this document. 891 There is a subset of secure interactions where the freshness of 892 Trustworthiness Claims may need to be revisited asynchronously. This 893 subset is when trustworthiness depends on the continuous availability 894 of a transport session between the Attester and Relying Party. With 895 such connectivity dependent Attestation Results, if there is a reboot 896 which resets transport connectivity, all established Trustworthiness 897 Claims should be cleared. Subsequent connection re-establishment 898 will allow fresh new Trustworthiness Claims to be delivered. 900 3. Secure Interactions Models 902 There are multiple ways of providing a Trustworthiness Vector to a 903 Relying Party. This section describes two alternatives. 905 3.1. Background-Check 907 3.1.1. Verifier Retrieval 909 It is possible to for a Relying Party to follow the Background-Check 910 Model defined in Section 5.2 of [I-D.ietf-rats-architecture]. In 911 this case, a Relying Party will receive Attestation Results 912 containing the Trustworthiness Vector directly from a Verifier. 913 These Attestation Results can then be used by the Relying Party in 914 determining the appropriate treatment for interactions with the 915 Attester. 917 While applicable in some cases, the utilization of the Background- 918 Check Model without modification has potential drawbacks in other 919 cases. These include: 921 * Verifier scale: if the Attester has many Relying Parties, a 922 Verifier appraising that Attester could be frequently be queried 923 based on the same Evidence. 925 * Information leak: Evidence which the Attester might consider 926 private can be visible to the Relying Party. Hiding that Evidence 927 could devalue any resulting appraisal. 929 * Latency: a Relying Party will need to wait for the Verifier to 930 return Attestation Results before proceeding with secure 931 interactions with the Attester. 933 An implementer should examine these potential drawbacks before 934 selecting this alternative. 936 3.1.2. Co-resident Verifier 938 A simplified Background-Check Model may exist in a very specific 939 case. 940 This is where the Relying Party and Verifier functions are co- 941 resident. This model is appropriate when: 943 * Some hardware-based private key is used by an Attester while 944 proving its identity as part of a mutually authenticated secure 945 channel establishment with the Relying Party, and 947 * this Attester identity is accepted as sufficient proof of Attester 948 integrity. 950 Effectively this means that detailed forensic capabilities of a 951 robust Verifier are unnecessary because it is accepted that the code 952 and operational behavior of the Attester cannot be manipulated after 953 TEE initialization. 955 An example of such a scenario may be when an SGX's MRENCLAVE and 956 MRSIGNER values have been associated with a known QUOTE value. And 957 the code running within the TEE is not modifiable after launch. 959 3.2. Below Zero Trust 961 Zero Trust Architectures are referenced in [US-Executive-Order] 962 eleven times. However despite this high profile, there is an 963 architectural gap with Zero Trust. The credentials used for 964 authentication and admission control can be manipulated on the 965 endpoint. Attestation can fill this gap through the generation of a 966 compound credential called AR-augmented Evidence. 967 This compound credential is rooted in the hardware based Attesting 968 Environment of an endpoint, plus the trustworthiness of a Verifier. 969 The overall solution is known as "Below Zero Trust" as the compound 970 credential cannot be manipulated or spoofed by an administrator of an 971 endpoint with root access. This solution is not adversely impacted 972 by the potential drawbacks with pure background-check described 973 above. 975 To kick-off the "Below Zero Trust" compound credential creation 976 sequence, a Verifier evaluates an Attester and returns signed 977 Attestation Results back to this original Attester no less frequently 978 than a well-known interval. This interval may also be asynchronous, 979 based on the changing of certain Evidence as described in 980 [I-D.ietf-rats-network-device-subscription]. 982 When a Relying Party is to receive information about the Attester's 983 trustworthiness, the Attesting Environment assembles the minimal set 984 of Evidence which can be used to confirm or refute whether the 985 Attester remains in the state of trustworthiness represented by the 986 AR. To this Evidence, the Attesting Environment appends the 987 signature from the most recent AR as well as a Relying Party Proof- 988 of-Freshness. The Attesting Environment then signs the combination. 990 The Attester then assembles AR Augmented Evidence by taking the 991 signed combination and appending the full AR. The assembly now 992 consists of two independent but semantically bound sets of signed 993 Evidence. 995 The AR Augmented Evidence is then sent to the Relying Party. The 996 Relying Party then can appraise these semantically bound sets of 997 signed Evidence by applying an Appraisal Policy for Attestation 998 Results as described below. This policy will consider both the AR as 999 well as additional information about the Attester within the AR 1000 Augmented Evidence the when determining what action to take. 1002 This alternative combines the [I-D.ietf-rats-architecture] Sections 1003 5.1 Passport Model and Section 5.2 Background-Check Model. Figure 1 1004 describes this flow of information. The flows within this combined 1005 model are mapped to [I-D.ietf-rats-architecture] in the following 1006 way. "Verifier A" below corresponds to the "Verifier" Figure 5 1007 within [I-D.ietf-rats-architecture]. And "Relying Party/Verifier B" 1008 below corresponds to the union of the "Relying Party" and "Verifier" 1009 boxes within Figure 6 of [I-D.ietf-rats-architecture]. This union is 1010 possible because Verifier B can be implemented as a simple, self- 1011 contained process. The resulting combined process can appraise the 1012 AR-augmented Evidence to determine whether an Attester qualifies for 1013 secure interactions with the Relying Party. The specific steps of 1014 this process are defined later in this section. 1016 .----------------. 1017 | Attester | 1018 | .-------------.| 1019 | | Attesting || .----------. .---------------. 1020 | | Environment || | Verifier | | Relying Party | 1021 | '-------------'| | A | | / Verifier B | 1022 '----------------' '----------' '---------------' 1023 time(VG) | | 1024 |<------Verifier PoF-------time(NS) | 1025 | | | 1026 time(EG)(1)------Evidence------------>| | 1027 | time(RG) | 1028 |<------Attestation Results-(2) | 1029 ~ ~ ~ 1030 time(VG')? | | 1031 ~ ~ ~ 1032 |<------Relying Party PoF-----------------(3)time(NS') 1033 | | | 1034 time(EG')(4)------AR-augmented Evidence----------------->| 1035 | | time(RG',RA')(5) 1036 (6) 1037 ~ 1038 time(RX') 1040 Figure 1: Below Zero Trust 1042 The interaction model depicted above includes specific time related 1043 events from Appendix A of [I-D.ietf-rats-architecture]. With the 1044 identification of these time related events, time duration/interval 1045 tracking becomes possible. Such duration/interval tracking can 1046 become important if the Relying Party cares if too much time has 1047 elapsed between the Verifier PoF and Relying Party PoF. If too much 1048 time has elapsed, perhaps the Attestation Results themselves are no 1049 longer trustworthy. 1051 Note that while time intervals will often be relevant, there is a 1052 simplified case that does not require a Relying Party's PoF in step 1053 (3). In this simplified case, the Relying Party trusts that the 1054 Attester cannot be meaningfully changed from the outside during any 1055 reportable interval. Based on that assumption, and when this is the 1056 case then the step of the Relying Party PoF can be safely omitted. 1058 In all cases, appraisal policies define the conditions and 1059 prerequisites for when an Attester does qualify for secure 1060 interactions. To qualify, an Attester has to be able to provide all 1061 of the mandatory affirming Trustworthiness Claims and identities 1062 needed by a Relying Party's Appraisal Policy for Attestation Results, 1063 and none of the disqualifying detracting Trustworthiness Claims. 1065 More details on each interaction step of Below Zero Trust are as 1066 follows. The numbers used in this sequence match to the numbered 1067 steps in Figure 1: 1069 1. An Attester sends Evidence which is provably fresh to Verifier A 1070 at time(EG). Freshness from the perspective of Verifier A MAY be 1071 established with Verifier PoF such as a nonce. 1073 2. Verifier A appraises (1), then sends the following items back to 1074 that Attester within Attestation Results: 1076 1. the verified identity of the Attesting Environment, 1078 2. the Verifier A appraised Trustworthiness Vector of an 1079 Attester, 1081 3. a freshness proof associated with the Attestation Results, 1083 4. a Verifier signature across (2.1) though (2.3). 1085 3. At time(EG') a Relying Party PoF (such as a nonce) known to the 1086 Relying Party is sent to the Attester. 1088 4. The Attester generates and sends AR-augmented Evidence to the 1089 Relying Party/Verifier B. This AR-augmented Evidence includes: 1091 1. The Attestation Results from (2) 1093 2. Any (optionally) new incremental Evidence from the Attesting 1094 Environment 1096 3. Attestation Environment signature which spans a hash of the 1097 Attestation Results (such as the signature of (2.4)), the 1098 proof-of-freshness from (3), and (4.2). Note: this construct 1099 allows the delta of time between (2.3) and (3) to be 1100 definitively calculated by the Relying Party. 1102 5. On receipt of (4), the Relying Party applies its Appraisal Policy 1103 for Attestation Results. At minimum, this appraisal policy 1104 process must include the following: 1106 1. Verify that (4.3) includes the nonce from (3). 1108 2. Use a local certificate to validate the signature (4.1). 1110 3. Verify that the hash from (4.3) matches (4.1) 1112 4. Use the identity of (2.1) to validate the signature of (4.3). 1114 5. Failure of any steps (5.1) through (5.4) means the link does 1115 not meet minimum validation criteria, therefore appraise the 1116 link as having a null Verifier B Trustworthiness Vector. 1117 Jump to step (6.1). 1119 6. When there is large or uncertain time gap between time(EG) 1120 and time(EG'), the link should be assigned a null Verifier B 1121 Trustworthiness Vector. Jump to step (6.1). 1123 7. Assemble the Verifier B Trustworthiness Vector 1125 1. Copy Verifier A Trustworthiness Vector to Verifier B 1126 Trustworthiness Vector 1128 2. Add implicit Trustworthiness Claims inherent to the type 1129 of TEE. 1131 3. Prune any Trustworthiness Claims unsupportable by the 1132 Attesting Environment. 1134 4. Prune any Trustworthiness Claims the Relying Party 1135 doesn't accept from this Verifier. 1137 6. The Relying Party takes action based on Verifier B's appraised 1138 Trustworthiness Vector, and applies the Appraisal Policy for 1139 Attestation Results. Following is a reasonable process for such 1140 evaluation: 1142 1. Prune any Trustworthiness Claims from the Trustworthiness 1143 Vector not used in the Appraisal Policy for Attestation 1144 Results. 1146 2. Allow the information exchange from the Attester into a 1147 Relying Party context in the Appraisal Policy for Attestation 1148 Results where the Verifier B appraised Trustworthiness Vector 1149 includes all the mandatory Trustworthiness Claims are in the 1150 "Affirming" value range, and none of the disqualifying 1151 Trustworthiness Claims are in the "Contraindicated" value 1152 range. 1154 3. Disallow any information exchange into a Relying Party 1155 context for which that Verifier B appraised Trustworthiness 1156 Vector is not qualified. 1158 As link layer protocols re-authenticate, steps (1) to (2) and steps 1159 (3) to (6) will independently refresh. This allows the 1160 Trustworthiness of Attester to be continuously re-appraised. There 1161 are only specific event triggers which will drive the refresh of 1162 Evidence generation (1), Attestation Result generation (2), or AR- 1163 augmented Evidence generation (4): 1165 * life-cycle events, e.g. a change to an Authentication Secret of 1166 the Attester or an update of a software component. 1168 * uptime-cycle events, e.g. a hard reset or a re-initialization of 1169 an Attester. 1171 * authentication-cycle events, e.g. a link-layer interface reset 1172 could result in a new (4). 1174 3.3. Mutual Attestation 1176 In the interaction models described above, each device on either side 1177 of a secure interaction may require remote attestation of its peer. 1178 This process is known as mutual-attestation. To support mutual- 1179 attestation, the interaction models listed above may be run 1180 independently on either side of the connection. 1182 3.4. Transport Protocol Integration 1184 Either unidirectional attestation or mutual attestation may be 1185 supported within the protocol interactions needed for the 1186 establishment of a single transport session. While this document 1187 does not mandate specific transport protocols, messages containing 1188 the Attestation Results and AR Augmented Evidence can be passed 1189 within an authentication framework such the EAP protocol [RFC5247] 1190 over TLS [RFC8446]. 1192 4. Privacy Considerations 1194 Privacy Considerations Text 1196 5. Security Considerations 1198 Security Considerations Text 1200 6. IANA Considerations 1202 See Body. 1204 7. References 1206 7.1. Normative References 1208 [GP-TEE-PP] 1209 "Global Platform TEE Protection Profile v1.3", September 1210 2020, . 1213 [I-D.ietf-rats-architecture] 1214 Birkholz, H., Thaler, D., Richardson, M., Smith, N., and 1215 W. Pan, "Remote Attestation Procedures Architecture", Work 1216 in Progress, Internet-Draft, draft-ietf-rats-architecture- 1217 15, 8 February 2022, . 1220 [OMTP-ATE] "Open Mobile Terminal Platform - Advanced Trusted 1221 Environment", May 2009, . 1225 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1226 Requirement Levels", BCP 14, RFC 2119, 1227 DOI 10.17487/RFC2119, March 1997, 1228 . 1230 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1231 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1232 May 2017, . 1234 7.2. Informative References 1236 [I-D.ietf-rats-network-device-subscription] 1237 Birkholz, H., Voit, E., and W. Pan, "Attestation Event 1238 Stream Subscription", Work in Progress, Internet-Draft, 1239 draft-ietf-rats-network-device-subscription-01, 7 March 1240 2022, . 1243 [I-D.tschofenig-rats-psa-token] 1244 Tschofenig, H., Frost, S., Brossard, M., Shaw, A., and T. 1245 Fossati, "Arm's Platform Security Architecture (PSA) 1246 Attestation Token", Work in Progress, Internet-Draft, 1247 draft-tschofenig-rats-psa-token-09, 7 March 2022, 1248 . 1251 [IEEE802.1AR] 1252 "802.1AR: Secure Device Identity", 2 August 2018, 1253 . 1255 [RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible 1256 Authentication Protocol (EAP) Key Management Framework", 1257 RFC 5247, DOI 10.17487/RFC5247, August 2008, 1258 . 1260 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 1261 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 1262 . 1264 [SEV-SNP] "AMD SEV-SNP: Stregthening VM Isolation with Integrity 1265 Protection and More", 2020, 1266 . 1270 [SGX] "Supporting Third Party Attestation for Intel SGX with 1271 Intel Data Center Attestation Primitives", 2017, . 1276 [TDX] "Intel Trust Domain Extensions", 2020, . 1280 [TPM-ID] "TPM Keys for Platform Identity for TPM 1.2", August 2015, 1281 . 1284 [TPM2.0] "Trusted Platform Module Library - Part 1: Architecture", 1285 n.d., . 1289 [US-Executive-Order] 1290 "Executive Order on Improving the Nation's Cybersecurity", 1291 12 May 2021, . 1295 Appendix A. Implementation Guidance 1297 A.1. Supplementing Trustworthiness Claims 1299 What has been encoded into each Trustworthiness Claim is the domain 1300 of integer values which is likely to drive a different programmatic 1301 decision in the Relying Party's Appraisal Policy for Attestation 1302 Results. This will not be the only thing a Relying Party's 1303 Operations team might care to track for measurement or debugging 1304 purposes. 1306 There is also the opportunity for the Verifier to include 1307 supplementary Evidence beyond a set of asserted Trustworthiness 1308 Claims. It is recommended that if supplementary Evidence is provided 1309 by the Verifier within the Attestation Results, that this 1310 supplementary Evidence includes a reference to a specific 1311 Trustworthiness Claim. This will allow a deeper understanding of 1312 some of the reasoning behind the integer value assigned. 1314 Appendix B. Supportable Trustworthiness Claims 1316 The following is a table which shows what Claims are supportable by 1317 different Attesting Environment types. Note that claims MAY BE 1318 implicit to an Attesting Environment type, and therefore do not have 1319 to be included in the Trustworthiness Vector to be considered as set 1320 by the Relying Party. 1322 B.1. Supportable Trustworthiness Claims for HSM-based CC 1324 Following are Trustworthiness Claims which MAY be set for a HSM-based 1325 Confidential Computing Attester. (Such as a TPM [TPM-ID].) 1326 +===================+===========+==================================+ 1327 | Trustworthiness | Required? | Appraisal Method | 1328 | Claim | | | 1329 +===================+===========+==================================+ 1330 | configuration | Optional | Verifier evaluation of Attester | 1331 | | | reveals no configuration lines | 1332 | | | which expose the Attester to | 1333 | | | known security vulnerabilities. | 1334 | | | This may be done with or without | 1335 | | | the involvement of a TPM PCR. | 1336 +-------------------+-----------+----------------------------------+ 1337 | executables | Yes | Checks the TPM PCRs for the | 1338 | | | static operating system, and for | 1339 | | | any tracked files subsequently | 1340 | | | loaded | 1341 +-------------------+-----------+----------------------------------+ 1342 | file-system | No | Can be supported, but TPM | 1343 | | | tracking is unlikely | 1344 +-------------------+-----------+----------------------------------+ 1345 | hardware | Yes | If TPM PCR check ok from BIOS | 1346 | | | checks, through Master Boot | 1347 | | | Record configuration | 1348 +-------------------+-----------+----------------------------------+ 1349 | instance-identity | Optional | Check IDevID | 1350 +-------------------+-----------+----------------------------------+ 1351 | runtime-opaque | n/a | TPMs are not recommended to | 1352 | | | provide a sufficient technology | 1353 | | | base for this Trustworthiness | 1354 | | | Claim. | 1355 +-------------------+-----------+----------------------------------+ 1356 | sourced-data | n/a | TPMs are not recommended to | 1357 | | | provide a sufficient technology | 1358 | | | base for this Trustworthiness | 1359 | | | Claim. | 1360 +-------------------+-----------+----------------------------------+ 1361 | storage-opaque | Minimal | With a TPM, secure storage space | 1362 | | | exists and is writeable by | 1363 | | | external applications. But the | 1364 | | | space is so limited that it | 1365 | | | often is used just be used to | 1366 | | | store keys. | 1367 +-------------------+-----------+----------------------------------+ 1369 Table 2 1371 Setting the Trustworthiness Claims may follow the following logic at 1372 the Verifier A within (2) of Figure 1: 1374 Start: Evidence received starts the generation of a new 1375 Trustworthiness Vector. (e.g., TPM Quote Received, log received, 1376 or appraisal timer expired) 1378 Step 0: set Trustworthiness Vector = Null 1380 Step 1: Is there sufficient fresh signed evidence to appraise? 1381 (yes) - No Action 1382 (no) - Goto Step 6 1384 Step 2: Appraise Hardware Integrity PCRs 1385 if (hardware NOT "0") - push onto vector 1386 if (hardware NOT affirming or warning), go to Step 6 1388 Step 3: Appraise Attesting Environment identity 1389 if (instance-identity <> "0") - push onto vector 1391 Step 4: Appraise executable loaded and filesystem integrity 1392 if (executables NOT "0") - push onto vector 1393 if (executables NOT affirming or warning), go to Step 6 1395 Step 5: Appraise all remaining Trustworthiness Claims 1396 Independently and set as appropriate. 1398 Step 6: Assemble Attestation Results, and push to Attester 1400 End 1402 B.2. Supportable Trustworthiness Claims for process-based CC 1404 Following are Trustworthiness Claims which MAY be set for a process- 1405 based Confidential Computing based Attester. (Such as a SGX Enclaves 1406 and TrustZone.) 1407 +===================+===========+==================================+ 1408 | Trustworthiness | Required? | Appraisal Method | 1409 | Claim | | | 1410 +===================+===========+==================================+ 1411 | instance-identity | Optional | Internally available in TEE. | 1412 | | | But keys might not be known/ | 1413 | | | exposed to the Relying Party by | 1414 | | | the Attesting Environment. | 1415 +-------------------+-----------+----------------------------------+ 1416 | configuration | Optional | If done, this is at the | 1417 | | | Application Layer. Plus each | 1418 | | | process needs it own protection | 1419 | | | mechanism as the protection is | 1420 | | | limited to the process itself. | 1421 +-------------------+-----------+----------------------------------+ 1422 | executables | Optional | Internally available in TEE. | 1423 | | | But keys might not be known/ | 1424 | | | exposed to the Relying Party by | 1425 | | | the Attesting Environment. | 1426 +-------------------+-----------+----------------------------------+ 1427 | file-system | Optional | Can be supported by application, | 1428 | | | but process-based CC is not a | 1429 | | | sufficient technology base for | 1430 | | | this Trustworthiness Claim. | 1431 +-------------------+-----------+----------------------------------+ 1432 | hardware | Implicit | At least the TEE is protected | 1433 | | in | here. Other elements of the | 1434 | | signature | system outside of the TEE might | 1435 | | | need additional protections is | 1436 | | | used by the application process. | 1437 +-------------------+-----------+----------------------------------+ 1438 | runtime-opaque | Implicit | From the TEE | 1439 | | in | | 1440 | | signature | | 1441 +-------------------+-----------+----------------------------------+ 1442 | storage-opaque | Implicit | Although the application must | 1443 | | in | assert that this function is | 1444 | | signature | used by the code itself. | 1445 +-------------------+-----------+----------------------------------+ 1446 | sourced-data | Optional | Will need to be supported by | 1447 | | | application code | 1448 +-------------------+-----------+----------------------------------+ 1450 Table 3 1452 B.3. Supportable Trustworthiness Claims for VM-based CC 1454 Following are Trustworthiness Claims which MAY be set for a VM-based 1455 Confidential Computing based Attester. (Such as SEV, TDX, ACCA, SEV- 1456 SNP.) 1458 +===================+===========+===================================+ 1459 | Trustworthiness | Required? | Appraisal Method | 1460 | Claim | | | 1461 +===================+===========+===================================+ 1462 | instance-identity | Optional | Internally available in TEE. | 1463 | | | But keys might not be known/ | 1464 | | | exposed to the Relying Party by | 1465 | | | the Attesting Environment. | 1466 +-------------------+-----------+-----------------------------------+ 1467 | configuration | Optional | Requires application | 1468 | | | integration. Easier than with | 1469 | | | process-based solution, as the | 1470 | | | whole protected machine can be | 1471 | | | evaluated. | 1472 +-------------------+-----------+-----------------------------------+ 1473 | executables | Optional | Internally available in TEE. | 1474 | | | But keys might not be known/ | 1475 | | | exposed to the Relying Party by | 1476 | | | the Attesting Environment. | 1477 +-------------------+-----------+-----------------------------------+ 1478 | file-system | Optional | Can be supported by application | 1479 +-------------------+-----------+-----------------------------------+ 1480 | hardware | Chip | At least the TEE is protected | 1481 | | dependent | here. Other elements of the | 1482 | | | system outside of the TEE might | 1483 | | | need additional protections is | 1484 | | | used by the application process. | 1485 +-------------------+-----------+-----------------------------------+ 1486 | runtime-opaque | Implicit | From the TEE | 1487 | | in | | 1488 | | signature | | 1489 +-------------------+-----------+-----------------------------------+ 1490 | storage-opaque | Chip | Although the application must | 1491 | | dependent | assert that this function is | 1492 | | | used by the code itself. | 1493 +-------------------+-----------+-----------------------------------+ 1494 | sourced-data | Optional | Will need to be supported by | 1495 | | | application code | 1496 +-------------------+-----------+-----------------------------------+ 1498 Table 4 1500 Appendix C. Some issues being worked 1502 It is possible for a cluster/hierarchy of Verifiers to have aggregate 1503 AR which are perhaps signed/endorsed by a lead Verifier. What should 1504 be the Proof-of-Freshness or Verifier associated with any of the 1505 aggregate set of Trustworthiness Claims? 1507 There will need to be a subsequent document which documents how these 1508 objects which will be translated into a protocol on a wire (e.g. EAP 1509 on TLS). Some breakpoint between what is in this draft, and what is 1510 in specific drafts for wire encoding will need to be determined. 1511 Questions like architecting the cluster/hierarchy of Verifiers fall 1512 into this breakdown. 1514 For some Trustworthiness Claims, there could be value in identifying 1515 a specific Appraisal Policy for Attestation Results applied within 1516 the Attester. One way this could be done would be a URI which 1517 identifies the policy used at Verifier A, and this URI would 1518 reference a specific Trustworthiness Claim. As the URI also could 1519 encode the version of the software, it might also act as a mechanism 1520 to signal the Relying Party to refresh/re-evaluate its view of 1521 Verifier A. Do we need this type of structure to be included here? 1522 Should it be in subsequent documents? 1524 Expand the variant of Figure 1 which requires no Relying Party PoF 1525 into its own picture. 1527 In what document (if any) do we attempt normalization of the identity 1528 claims between different types of TEE. E.g., does MRSIGNER plus 1529 extra loaded software = the sum of TrustZone Signer IDs for loaded 1530 components? 1532 Appendix D. Contributors 1534 Guy Fedorkow 1536 Email: gfedorkow@juniper.net 1538 Dave Thaler 1540 Email: dthaler@microsoft.com 1542 Ned Smith 1544 Email: ned.smith@intel.com 1546 Lawrence Lundblade 1547 Email: lgl@island-resort.com 1549 Authors' Addresses 1551 Eric Voit 1552 Cisco Systems 1553 Email: evoit@cisco.com 1555 Henk Birkholz 1556 Fraunhofer SIT 1557 Rheinstrasse 75 1558 64295 Darmstadt 1559 Germany 1560 Email: henk.birkholz@sit.fraunhofer.de 1562 Thomas Hardjono 1563 MIT 1564 Email: hardjono@mit.edu 1566 Thomas Fossati 1567 Arm Limited 1568 Email: Thomas.Fossati@arm.com 1570 Vincent Scarlata 1571 Intel 1572 Email: vincent.r.scarlata@intel.com