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2 RATS Working Group E. Voit
3 Internet-Draft Cisco
4 Intended status: Standards Track H. Birkholz
5 Expires: March 11, 2022 Fraunhofer SIT
6 T. Hardjono
7 MIT
8 T. Fossati
9 Arm Limited
10 V. Scarlata
11 Intel
12 September 07, 2021
14 Attestation Results for Secure Interactions
15 draft-voit-rats-attestation-results-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 March 11, 2022.
44 Copyright Notice
46 Copyright (c) 2021 IETF Trust and the persons identified as the
47 document authors. All rights reserved.
49 This document is subject to BCP 78 and the IETF Trust's Legal
50 Provisions Relating to IETF Documents
51 (https://trustee.ietf.org/license-info) in effect on the date of
52 publication of this document. Please review these documents
53 carefully, as they describe your rights and restrictions with respect
54 to this document. Code Components extracted from this document must
55 include Simplified BSD License text as described in Section 4.e of
56 the Trust Legal Provisions and are provided without warranty as
57 described in the Simplified BSD License.
59 Table of Contents
61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
62 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4
63 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
64 2. Attestation Results for Secure Interactions . . . . . . . . . 5
65 2.1. Information driving a Relying Party Action . . . . . . . 5
66 2.2. Non-repudiable Identity . . . . . . . . . . . . . . . . . 6
67 2.2.1. Attester and Attesting Environment . . . . . . . . . 7
68 2.2.2. Verifier . . . . . . . . . . . . . . . . . . . . . . 9
69 2.2.3. Communicating Identity . . . . . . . . . . . . . . . 10
70 2.3. Trustworthiness Claims . . . . . . . . . . . . . . . . . 10
71 2.3.1. Design Principles . . . . . . . . . . . . . . . . . . 10
72 2.3.2. Enumeration Encoding . . . . . . . . . . . . . . . . 12
73 2.3.3. Assigning a Trustworthiness Claim value . . . . . . . 13
74 2.3.4. Specific Claims . . . . . . . . . . . . . . . . . . . 13
75 2.3.5. Trustworthiness Vector . . . . . . . . . . . . . . . 17
76 2.3.6. Trustworthiness Vector for a type of Attesting
77 Environment . . . . . . . . . . . . . . . . . . . . . 17
78 2.4. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 18
79 3. Secure Interactions Models . . . . . . . . . . . . . . . . . 18
80 3.1. Pure Background-Check retrieval . . . . . . . . . . . . . 19
81 3.2. Attestation Result Augmented Evidence . . . . . . . . . . 19
82 3.3. Mutual Attestation . . . . . . . . . . . . . . . . . . . 24
83 3.4. Transport Protocol Integration . . . . . . . . . . . . . 24
84 4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 24
85 5. Security Considerations . . . . . . . . . . . . . . . . . . . 24
86 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
87 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
88 7.1. Normative References . . . . . . . . . . . . . . . . . . 24
89 7.2. Informative References . . . . . . . . . . . . . . . . . 25
90 Appendix A. Supportable Trustworthiness Claims . . . . . . . . . 26
91 A.1. Supportable Trustworthiness Claims for HSM-based CC . . . 26
92 A.2. Supportable Trustworthiness Claims for process-based CC . 28
93 A.3. Supportable Trustworthiness Claims for VM-based CC . . . 29
94 Appendix B. Some issues being worked . . . . . . . . . . . . . . 30
95 Appendix C. Contributors . . . . . . . . . . . . . . . . . . . . 31
96 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
98 1. Introduction
100 The first paragraph of the May 2021 US Presidential Executive Order
101 on Improving the Nation's Cybersecurity [US-Executive-Order] ends
102 with the statement "the trust we place in our digital infrastructure
103 should be proportional to how trustworthy and transparent that
104 infrastructure is." Later this order explores aspects of
105 trustworthiness such as an auditable trust relationship, which it
106 defines as an "agreed-upon relationship between two or more system
107 elements that is governed by criteria for secure interaction,
108 behavior, and outcomes."
110 The Remote ATtestation procedureS (RATS) architecture
111 [I-D.ietf-rats-architecture] provides a useful context for
112 programmatically establishing and maintaining such auditable trust
113 relationships. Specifically, the architecture defines conceptual
114 messages conveyed between architectural subsystems to support
115 trustworthiness appraisal. The RATS conceptual message used to
116 convey evidence of trustworthiness is the Attestation Results. The
117 Attestation Results includes Verifier generated appraisals of an
118 Attester including such information as the identity of the Attester,
119 the security mechanisms employed on this Attester, and the Attester's
120 current state of trustworthiness.
122 Generated Attestation Results are ultimately conveyed to one or more
123 Relying Parties. Reception of an Attestation Result enables a
124 Relying Party to determine what action to take with regards to an
125 Attester. Frequently, this action will be to choose whether to allow
126 the Attester to securely interact with the Relying Party over some
127 connection between the two.
129 When determining whether to allow secure interactions with an
130 Attester, a Relying Party is challenged with a number of difficult
131 problems which it must be able to handle successfully. These
132 problems include:
134 o What Attestation Results (AR) might a Relying Party be willing to
135 trust from a specific Verifier?
137 o What information does a Relying Party need before allowing
138 interactions or choosing policies to apply to a connection?
140 o What are the operating/environmental realities of the Attesting
141 Environment where a Relying Party should only be able to associate
142 a certain confidence regarding Attestation Results out of the
143 Verifier? (In other words, different types of Trusted Execution
144 Environments (TEE) need not be treated as equivalent.)
146 o How to make direct comparisons where there is a heterogeneous mix
147 of Attesting Environments and Verifier types.
149 To address these problems, it is important that specific Attestation
150 Result information elements are framed independently of Attesting
151 Environment specific constraints. If they are not, a Relying Party
152 would be forced to adapt to the syntax and semantics of many vendor
153 specific environments. This is not a reasonable ask as there can be
154 many types of Attesters interacting with or connecting to a Relying
155 Party.
157 The business need therefore is for common Attestation Result
158 information element definitions. With these definitions, consistent
159 interaction or connectivity decisions can be made by a Relying Party
160 where there is a heterogenous mix of Attesting Environment types and
161 Verifier types.
163 This document defines information elements for Attestation Results in
164 a way which normalizes the trustworthiness assertions that can be
165 made from a diverse set of Attesters.
167 1.1. Requirements Notation
169 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
170 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
171 "OPTIONAL" in this document are to be interpreted as described in
172 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
173 capitals, as shown here.
175 1.2. Terminology
177 The following terms are imported from [I-D.ietf-rats-architecture]:
178 Appraisal Policy for Attestation Results, Attester, Attesting
179 Environment, Claims, Evidence, Relying Party, Target Environment and
180 Verifier.
182 [I-D.ietf-rats-architecture] also describes topological patterns that
183 illustrate the need for interoperable conceptual messages. The two
184 patterns called "background-check model" and "passport model" are
185 imported from the RATS architecture and used in this document as a
186 reference to the architectural concepts: Background-Check Model and
187 Passport Model.
189 Newly defined terms for this document:
191 AR-augmented Evidence: a bundle of Evidence which includes at least
192 the following:
194 1. Verifier signed Attestation Results. These Attestation
195 Results must include Identity Evidence for the Attester, a
196 Trustworthiness Vector describing a Verifier's most recent
197 appraisal of an Attester, and some Verifier Proof-of-Freshness
198 (PoF).
200 2. A Relying Party PoF which is bound to the Attestation Results
201 of (1) by the Attester's Attesting Environment signature.
203 3. Sufficient information to determine the elapsed interval
204 between the Verifier PoF and Relying Party PoF.
206 Identity Evidence: Evidence which unambiguously identifies an
207 identity. Identity Evidence could take different forms, such as a
208 certificate, or a signature which can be appraised to have only
209 been generated by a specific private/public key pair.
211 Trustworthiness Claim: a specific quanta of trustworthiness which
212 can be assigned by a Verifier based on its appraisal policy.
214 Trustworthiness Tier: a categorization of the levels of
215 trustworthiness which may be assigned by a Verifier to a specific
216 Trustworthiness Claim. These enumerated categories are: Affirmed,
217 Warning, Contraindicated, and None.
219 Trustworthiness Vector: a set of zero to many Trustworthiness Claims
220 assigned during a single appraisal procedure by a Verifier using
221 Evidence generated by an Attester. The vector is included within
222 Attestation Results.
224 2. Attestation Results for Secure Interactions
226 A Verifier generates the Attestation Results used by a Relying Party.
227 When a Relying Party needs to determine whether to permit
228 communications with an Attester, these Attestation Results must
229 contain a specific set of information elements. This section defines
230 those information elements, and in some cases encodings for
231 information elements.
233 2.1. Information driving a Relying Party Action
235 When the action is a communication establishment attempt with an
236 Attester, there is only a limited set of actions which a Relying
237 Party might take. These actions include:
239 o Allow or deny information exchange with the Attester. When there
240 is a deny, reasons should be returned to the Attester.
242 o Establish a transport connection between an Attester and a
243 specific context within a Relying Party (e.g., a TEE, or Virtual
244 Routing Function (VRF).)
246 o Apply policies on this connection (e.g., rate limits).
248 There are three categories of information which must be conveyed to
249 the Relying Party (which also is integrated with a Verifier) before
250 it determines which of these actions to take.
252 1. Non-repudiable Identity Evidence - Evidence which undoubtably
253 identifies one or more entities involved with a connection.
255 2. Trustworthiness Claims - Specifics a Verifier asserts with
256 regards to its trustworthiness findings about an Attester.
258 3. Claim Freshness - Establishes the time of last update (or
259 refresh) of Trustworthiness Claims.
261 The following sections detail requirements for these three
262 categories.
264 2.2. Non-repudiable Identity
266 Identity Evidence must be conveyed during the establishment of any
267 trust-based relationship. Specific use cases will define the minimum
268 types of identities required by a particular Relying Party as it
269 evaluates Attestation Results, and perhaps additional associated
270 Evidence. At a bare minimum, a Relying Party MUST start with the
271 ability to verify the identity of a Verifier it chooses to trust.
272 Attester identities may then be acquired through signed
273 communications with the Verifier identity and/or the pre-provisioning
274 Attester public keys in the Attester.
276 During the Remote Attestation process, the Verifier's identity will
277 be established with a Relying Party via a Verifier signature across
278 recent Attestation Results. This Verifier identity could only have
279 come from a key pair maintained by a trusted developer or operator of
280 the Verifier.
282 Additionally, each set of Attestation Results must be provably and
283 non-reputably bound to the identity of the original Attesting
284 Environment which was evaluated by the Verifier. This will be
285 accomplished via two items.
286 First the Verifier signed Attestation Results MUST include sufficient
287 Identity Evidence to ensure that this Attesting Environment signature
288 refers to the same Attesting Environment appraised by the Verifier.
289 Second, where the passport model is used as a subsystem, an Attesting
290 Environment signature which spans the Verifier signature MUST also be
291 included. As the Verifier signature already spans the Attester
292 Identity as well as the Attestation Results, this restricts the
293 viability of spoofing attacks.
295 In a subset of use cases, these two pieces of Identity Evidence may
296 be sufficient for a Relying Party to successfully meet the criteria
297 for its Appraisal Policy for Attestation Results. If the use case is
298 a connection request, a Relying Party may simply then establish a
299 transport session with an Attester after a successful appraisal.
300 However an Appraisal Policy for Attestation Results will often be
301 more nuanced, and the Relying Party may need additional information.
302 Some Identity Evidence related policy questions which the Relying
303 Party may consider include:
305 o Does the Relying Party only trust this Verifier to make
306 Trustworthiness Claims on behalf a specific type of Attesting
307 Environment? Might a mix of Verifiers be necessary to cover all
308 mandatory Trustworthiness Claims?
310 o Does the Relying Party only accept connections from a verified-
311 authentic software build from a specific software developer?
313 o Does the Relying Party only accept connections from specific
314 preconfigured list of Attesters?
316 For any of these more nuanced appraisals, additional Identity
317 Evidence or other policy related information must be conveyed or pre-
318 provisioned during the formation of a trust context between the
319 Relying Party, the Attester, the Attester's Attesting Environment,
320 and the Verifier.
322 2.2.1. Attester and Attesting Environment
324 Per [I-D.ietf-rats-architecture] Figure 2, an Attester and a
325 corresponding Attesting Environment might not share common code or
326 even hardware boundaries. Consequently, an Attester implementation
327 needs to ensure that any Evidence which originates from outside the
328 Attesting Environment MUST have been collected and delivered securely
329 before any Attesting Environment signing may occur. After the
330 Verifier performs its appraisal, it will include sufficient
331 information in the Attestation Results to enable a Relying Party to
332 have confidence that the Attester's trustworthiness is represented
333 via Trustworthiness Claims signed by the appropriate Attesting
334 Environment.
336 This document recognizes three general categories of Attesters.
338 1. HSM-based: A Hardware Security Module (HSM) based cryptoprocessor
339 which continually hashes security measurements in a way which
340 prevents an Attester from lying about measurements which have
341 been extended into the Attesting Environment (e.g., TPM2.0.)
343 2. Process-based: An individual process which has its runtime memory
344 encrypted by an Attesting Environment in a way that no other
345 processes can read and decrypt that memory (e.g., [SGX] or
346 [I-D.tschofenig-rats-psa-token].)
348 3. VM-based: An entire Guest VM (or a set of containers within a
349 host) have been encrypted as a walled-garden unit by an Attesting
350 Environment. The result is that the host operating system cannot
351 read and decrypt what is executing within that VM (e.g.,
352 [SEV-SNP] or [TDX].)
354 Each of these categories of Attesters above will be capable of
355 generating Evidence which is protected using private keys /
356 certificates which are not accessible outside of the corresponding
357 Attesting Environment. The owner of these secrets is the owner of
358 the identity which is bound within the Attesting Environment.
359 Effectively this means that for any Attester identity, there will
360 exist a chain of trust ultimately bound to a hardware-based root of
361 trust in the Attesting Environment. It is upon this root of trust
362 that unique, non-repudiable Attester identities may be founded.
364 There are several types of Attester identities defined in this
365 document. This list is extensible:
367 o chip-vendor: the vendor of the hardware chip used for the
368 Attesting Environment (e.g., a primary Endorsement Key from a TPM)
370 o chip-hardware: specific hardware with specific firmware from an
371 'ae-vendor'
373 o target-environment: a unique instance of a software build running
374 in an Attester (e.g., MRENCLAVE [SGX], an Instance ID
375 [I-D.tschofenig-rats-psa-token], an Identity Block [SEV-SNP], or a
376 hash which represents a set of software loaded since boot (e.g.,
377 TPM based integrity verification.))
379 o target-developer: the organizational unit responsible for a
380 particular 'target-environment' (e.g., MRSIGNER [SGX])
382 o instance: a unique instantiated instance of an Attesting
383 Environment running on 'chip-hardware' (e.g., an LDevID
384 [IEEE802.1AR])
386 Based on the category of the Attesting Environment, different types
387 of identities might be exposed by an Attester.
389 +------------------------+---------------+-----------+-----------+
390 | Attester Identity type | Process-based | VM-based | HSM-based |
391 +------------------------+---------------+-----------+-----------+
392 | chip-vendor | Mandatory | Mandatory | Mandatory |
393 | | | | |
394 | chip-hardware | Mandatory | Mandatory | Mandatory |
395 | | | | |
396 | target-environment | Mandatory | Mandatory | Optional |
397 | | | | |
398 | target-developer | Mandatory | Optional | Optional |
399 | | | | |
400 | instance | Optional | Optional | Optional |
401 +------------------------+---------------+-----------+-----------+
403 It is expected that drafts subsequent to this specification will
404 provide the definitions and value domains for specific identities,
405 each of which falling within the Attester identity types listed
406 above. In some cases the actual unique identities might encoded as
407 complex structures. An example complex structure might be a 'target-
408 environment' encoded as a Software Bill of Materials (SBOM).
410 With the identity definitions and value domains, a Relying Party will
411 have sufficient information to ensure that the Attester identities
412 and Trustworthiness Claims asserted are actually capable of being
413 supported by the underlying type of Attesting Environment.
414 Consequently, the Relying Party SHOULD require Identity Evidence
415 which indicates of the type of Attesting Environment when it
416 considers its Appraisal Policy for Attestation Results.
418 2.2.2. Verifier
420 For the Verifier identity, it is critical for a Relying Party to
421 review the certificate and chain of trust for that Verifier.
422 Additionally, the Relying Party must have confidence that the
423 Trustworthiness Claims being relied upon from the Verifier considered
424 the chain of trust for the Attesting Environment.
426 There are two categorizations Verifier identities defined in this
427 document.
429 o verifier build: a unique instance of a software build running as a
430 Verifier.
432 o verifier developer: the organizational unit responsible for a
433 particular 'verifier build'.
435 Within each category, communicating the identity can be accomplished
436 via a variety of objects and encodings.
438 2.2.3. Communicating Identity
440 Any of the above identities used by the Appraisal Policy for
441 Attestation Results needed to be pre-established by the Relying Party
442 before, or provided during, the exchange of Attestation Results.
443 When provided during this exchange, the identity may be communicated
444 either implicitly or explicitly.
446 An example of explicit communication would be to include the
447 following Identity Evidence directly within the Attestation Results:
448 a unique identifier for an Attesting Environment, the name of a key
449 which can be provably associated with that unique identifier, and the
450 set of Attestation Results which are signed using that key. As these
451 Attestation Results are signed by the Verifier, it is the Verifier
452 which is explicitly asserting the credentials it believes are
453 trustworthy.
455 An example of implicit communication would be to include Identity
456 Evidence in the form of a signature which has been placed over the
457 Attestation Results asserted by a Verifier. It would be then up to
458 the Relying Party's Appraisal Policy for Attestation Results to
459 extract this signature and confirm that it only could have been
460 generated by an Attesting Environment having access to a specific
461 private key. This implicit identity communication is only viable if
462 the Attesting Environment's public key is already known by the
463 Relying Party.
465 One final step in communicating identity is proving the freshness of
466 the Attestation Results to the degree needed by the Relying Party. A
467 typical way to accomplish this is to include an element of freshness
468 be embedded within a signed portion of the Attestation Results. This
469 element of freshness reduces the identity spoofing risks from a
470 replay attack. For more on this, see Section 2.4.
472 2.3. Trustworthiness Claims
474 2.3.1. Design Principles
476 Trust is not absolute. Trust is a belief in some aspect about an
477 entity (in this case an Attester), and that this aspect is something
478 which can be depended upon (in this case by a Relying Party.) Within
479 the context of Remote Attestation, believability of this aspect is
480 facilitated by a Verifier. This facilitation depends on the
481 Verifier's ability to parse detailed Evidence from an Attester and
482 then to assert conclusions about this aspect in a way interpretable
483 by a Relying Party.
485 Specific aspects for which a Verifier will assert trustworthiness are
486 defined in this section. These are known as Trustworthiness Claims.
487 These claims have been designed to enable a common understanding
488 between a broad array of Attesters, Verifiers, and Relying Parties.
489 The following set of design principles have been applied in the
490 Trustworthiness Claim definitions:
492 1. Expose a small number of Trustworthiness Claims.
494 Reason: a plethora of similar Trustworthiness Claims will result
495 in divergent choices made on which to support between different
496 Verifiers. This would place a lot of complexity in the Relying
497 Party as it would be up to the Relying Party (and its policy
498 language) to enable normalization across rich but incompatible
499 Verifier object definitions.
501 2. Each Trustworthiness Claim enumerates only the specific states
502 that could viably result in a different outcome after the Policy
503 for Attestation Results has been applied.
505 Reason: by explicitly disallowing the standardization of
506 enumerated states which cannot easily be connected to a use case,
507 we avoid forcing implementers from making incompatible guesses on
508 what these states might mean.
510 3. Verifier and RP developers need explicit definitions of each
511 state in order to accomplish the goals of (1) and (2).
513 Reason: without such guidance, the Verifier will append plenty of
514 raw supporting info. This relieves the Verifier of making the
515 hard decisions. Of course, this raw info will be mostly non-
516 interpretable and therefore non-actionable by the Relying Party.
518 4. Support standards and non-standard extensibility for (1) and (2).
520 Reason: standard types of Verifier generated Trustworthiness
521 Claims should be vetted by the full RATS working group, rather
522 than being maintained in a repository which doesn't follow the
523 RFC process. This will keep a tight lid on extensions which must
524 be considered by the Relying Party's policy language. Because
525 this process takes time, non-standard extensions will be needed
526 for implementation speed and flexibility.
528 These design principles are important to keep the number of Verifier
529 generated claims low, and to retain the complexity in the Verifier
530 rather than the Relying Party.
532 2.3.2. Enumeration Encoding
534 Per design principle (2), each Trustworthiness Claim will only expose
535 specific encoded values. To simplify the processing of these
536 enumerations by the Relying Party, the enumeration will be encoded as
537 a single signed 8 bit integer. These value assignments for this
538 integer will be in four Trustworthiness Tiers which follow these
539 guidelines:
541 Affirming: The Verifier affirms the Attester support for this aspect
542 of trustworthiness
544 o Values 1 to 31: A standards enumerated reason for affirming.
546 o Values -2 to -32: A non-standard reason for affirming.
548 Warning: The Verifier warns about this aspect of trustworthiness.
550 o Values 32 to 95: A standards enumerated reason for the warning.
552 o Values -33 to -96: A non-standard reason for the warning.
554 Contraindicated: The Verifier asserts the Attester is explicitly
555 untrustworthy in regard to this aspect.
557 o Values 96 to 127: A standards enumerated reason for the
558 contraindication.
560 o Values -97 to -128: A non-standard reason for the
561 contraindication.
563 None: The Verifier makes no assertions about this Trustworthiness
564 Claim.
566 o Value 0: Note: this should always be always treated equivalently
567 by the Relying Party as no claim being made. I.e., the RP's
568 Appraisal Policy for Attestation Results SHOULD NOT make any
569 distinction between a Trustworthiness Claim with enumeration '0',
570 and no Trustworthiness Claim being provided.
572 o Value -1: An unexpected error occurred during the Verifier's
573 appraisal processing. Note: while no claim is being made, the
574 Relying Party MAY make a distinction between a Trustworthiness
575 Claim with enumeration '-1', and no Trustworthiness Claim being
576 provided.
578 This enumerated encoding listed above will simplify the Appraisal
579 Policy for Attestation Results. Such a policies may be as simple as
580 saying that a specific Verifier has recently asserted Trustworthiness
581 Claims, all of which are Affirming.
583 2.3.3. Assigning a Trustworthiness Claim value
585 In order to simplify design, only a single encoded value is asserted
586 by a Verifier for any Trustworthiness Claim within a using the
587 following process.
589 1. If applicable, a Verifier MUST assign a standardized value from
590 the Contraindicated tier.
592 2. Else if applicable, a Verifier MUST assign a non-standardized
593 value from the Contraindicated tier.
595 3. Else if applicable, a Verifier MUST assign a standardized value
596 from the Warning tier.
598 4. Else if applicable, a Verifier MUST assign a non-standardized
599 value from the Warning tier.
601 5. Else if applicable, a Verifier MUST assign a standardized value
602 from the Affirming tier.
604 6. Else if applicable, a Verifier MUST assign a non-standardized
605 value from the Affirming tier.
607 7. Else a Verifier MAY assign a 0 or -1.
609 2.3.4. Specific Claims
611 Following are the Trustworthiness Claims and their supported
612 enumerations which may be asserted by a Verifier:
614 configuration: A Verifier has appraised an Attester's configuration,
615 and is able to make conclusions regarding the exposure of known
616 vulnerabilities
618 0: No assertion
620 1: The configuration is a known and approved config
622 2: The configuration includes or exposes no known vulnerabilities
623 32: The configuration includes or exposes known vulnerabilities
625 96: The configuration is unsupportable as it exposes unacceptable
626 security vulnerabilities
628 -1: Unexpected error
630 executables: A Verifier has appraised and evaluated relevant runtime
631 files, scripts, and/or other objects which have been loaded into
632 the Target environment's memory.
634 0: No assertion
636 1: Only a recognized genuine set of approved executables, scripts,
637 files, and/or objects have been loaded during and after the
638 boot process.
640 2: Only a recognized genuine set of approved executables have been
641 loaded during the boot process.
643 32: Only a recognized genuine set of executables, scripts, files,
644 and/or objects have been loaded. However the Verifier cannot
645 vouch for a subset of these due to known bugs or other known
646 vulnerabilities.
648 33: Runtime memory includes executables, scripts, files, and/or
649 objects which are not recognized.
651 96: Runtime memory includes executables, scripts, files, and/or
652 object which are contraindicated.
654 -1: Unexpected error
656 file-system: A Verifier has evaluated the Attester's file system.
658 0: No assertion
660 1: Only a recognized set of approved files are found.
662 32: The file system includes unrecognized executables, scripts,
663 or files.
665 96: The file system includes contraindicated executables,
666 scripts, or files
668 -1: Unexpected error
670 hardware: A Verifier has appraised any Attester hardware and
671 firmware which are able to expose fingerprints of their identity
672 and running code.
674 0: No assertion
676 1: An Attester has passed its hardware and/or firmware
677 verifications needed to demonstrate that these are genuine/
678 supported.
680 32: An Attester contains only genuine/supported hardware and/or
681 firmware, but there are known security vulnerabilities.
683 96: Attester hardware and/or firmware is recognized, but its
684 trustworthiness is contraindicated.
686 97: A Verifier does not recognize an Attester's hardware or
687 firmware, but it should be recognized.
689 -1: Unexpected error
691 instance-identity: A Verifier has appraised an Attesting
692 Environment's unique identity based upon private key signed
693 Evidence which can be correlated to a unique instantiated instance
694 of the Attester. (Note: this Trustworthiness Claim should only be
695 generated if the Verifier actually expects to recognize the unique
696 identity of the Attester.)
698 0: No assertion
700 1: The Attesting Environment is recognized, and the associated
701 instance of the Attester is not known to be compromised.
703 96: The Attesting Environment is recognized, and but its unique
704 private key indicates a device which is not trustworthy.
706 97: The Attesting Environment is not recognized; however the
707 Verifier believes it should be.
709 -1: Unexpected error
711 runtime-opaque: A Verifier has appraised the visibility of Attester
712 objects in memory from perspectives outside the Attester.
714 0: No assertion
716 1: the Attester's executing Target Environment and Attesting
717 Environments are encrypted and within Trusted Execution
718 Environment(s) opaque to the operating system, virtual machine
719 manager, and peer applications. (Note: This value corresponds
720 to the protections asserted by O.RUNTIME_CONFIDENTIALITY from
721 [GP-TEE-PP])
723 32: the Attester's executing Target Environment and Attesting
724 Environments inaccessible from any other parallel application
725 or Guest VM running on the Attester's physical device. (Note
726 that unlike "1" these environments are not encrypted in a way
727 which restricts the Attester's root operator visibility. See
728 O.TA_ISOLATION from [GP-TEE-PP].)
730 96: The Verifier has concluded that in memory objects are
731 unacceptably visible within the physical host that supports the
732 Attester.
734 -1: Unexpected error
736 sourced-data: A Verifier has evaluated of the integrity of data
737 objects from external systems used by the Attester.
739 0: No assertion
741 1: All essential Attester source data objects have been provided
742 by other Attester(s) whose most recent appraisal(s) had both no
743 Trustworthiness Claims of "0" where the current Trustworthiness
744 Claim is "Affirming", as well as no "Warning" or
745 "Contraindicated" Trustworthiness Claims.
747 32: Attester source data objects come from unattested sources, or
748 attested sources with "Warning" type Trustworthiness Claims.
750 96: Attester source data objects come from contraindicated
751 sources.
753 -1: Unexpected error
755 storage-opaque: A Verifier has appraised that an Attester is capable
756 of encrypting persistent storage. (Note: Protections must meet
757 the capabilities of [OMTP-ATE] Section 5, but need not be hardware
758 tamper resistant.)
760 0: No assertion
762 1: the Attester encrypts all secrets in persistent storage via
763 using keys which are never visible outside an HSM or the
764 Trusted Execution Environment hardware.
766 32: the Attester encrypts all persistently stored secrets, but
767 without using hardware backed keys
769 96: There are persistent secrets which are stored unencrypted in
770 an Attester.
772 -1: Unexpected error
774 It is possible for additonal Trustworthiness Claims and enumerated
775 values to be defined in subsequent documents. At the same time, the
776 standardized Trustworthiness Claim values listed above have been
777 designed so there is no overlap within a Trustworthiness Tier. As a
778 result, it is possible to imagine a future where overlapping
779 Trustworthiness Claims within a single Trustworthiness Tier may be
780 defined. Wherever possible, the Verifier SHOULD assign the best
781 fitting standardized value.
783 Where a Relying Party doesn't know how to handle a particular
784 Trustworthiness Claim, it MAY choose an appropriate action based on
785 the Trustworthiness Tier under which the enumerated value fits.
787 It is up to the Verifier to publish the types of evaluations it
788 performs when determining how Trustworthiness Claims are derived for
789 a type of any particular type of Attester. It is out of the scope of
790 this document for the Verifier to provide proof or specific logic on
791 how a particular Trustworthiness Claim which it is asserting was
792 derived.
794 2.3.5. Trustworthiness Vector
796 Multiple Trustworthiness Claims may be asserted about an Attesting
797 Environment at single point in time. The set of Trustworthiness
798 Claims inserted into an instance of Attestation Results by a Verifier
799 is known as a Trustworthiness Vector. The order of Claims in the
800 vector is NOT meaningful. A Trustworthiness Vector with no
801 Trustworthiness Claims (i.e., a null Trustworthiness Vector) is a
802 valid construct. In this case, the Verifier is making no
803 Trustworthiness Claims but is confirming that an appraisal has been
804 made.
806 2.3.6. Trustworthiness Vector for a type of Attesting Environment
808 Some Trustworthiness Claims are implicit based on the underlying type
809 of Attesting Environment. For example, a validated MRSIGNER identity
810 can be present where the underlying [SGX] hardware is 'hw-authentic'.
811 Where such implicit Trustworthiness Claims exist, they do not have to
812 be explicitly included in the Trustworthiness Vector. However, these
813 implicit Trustworthiness Claims SHOULD be considered as being present
814 by the Relying Party. Another way of saying this is if a
815 Trustworthiness Claim is automatically supported as a result of
816 coming from a specific type of TEE, that claim need not be
817 redundantly articulated. Such implicit Trustworthiness Claims can be
818 seen in the tables within Appendix A.2 and Appendix A.3.
820 Additionally, there are some Trustworthiness Claims which cannot be
821 adequately supported by an Attesting Environment. For example, it
822 would be difficult for an Attester that includes only a TPM (and no
823 other TEE) from ever having a Verifier appraise support for 'runtime-
824 opaque'. As such, a Relying Party would be acting properly if it
825 rejects any non-supportable Trustworthiness Claims asserted from a
826 Verifier.
828 As a result, the need for the ability to carry a specific
829 Trustworthiness Claim will vary by the type of Attesting Environment.
830 Example mappings can be seen in Appendix A.
832 2.4. Freshness
834 A Relying Party will care about the recentness of the Attestation
835 Results, and the specific Trustworthiness Claims which are embedded.
836 All freshness mechanisms of [I-D.ietf-rats-architecture], Section 10
837 are supportable by this specification.
839 Additionally, a Relying Party may track when a Verifier expires its
840 confidence for the Trustworthiness Claims or the Trustworthiness
841 Vector as a whole. Mechanisms for such expiry are not defined within
842 this document.
844 There is a subset of secure interactions where the freshness of
845 Trustworthiness Claims may need to be revisited asynchronously. This
846 subset is when trustworthiness depends on the continuous availability
847 of a transport session between the Attester and Relying Party. With
848 such connectivity dependent Attestation Results, if there is a reboot
849 which resets transport connectivity, all established Trustworthiness
850 Claims should be cleared. Subsequent connection re-establishment
851 will allow fresh new Trustworthiness Claims to be delivered.
853 3. Secure Interactions Models
855 There are multiple ways of providing a Trustworthiness Vector to a
856 Relying Party. This section describes two alternatives.
858 3.1. Pure Background-Check retrieval
860 It is possible to for a Relying Party to follow the Background-Check
861 Model defined in Section 5.2 of [I-D.ietf-rats-architecture]. In
862 this case, a Relying Party will receive Attestation Results
863 containing the Trustworthiness Vector directly from a Verifier.
864 These Attestation Results can then be used by the Relying Party in
865 determining the appropriate treatment for interactions with the
866 Attester.
868 While applicable in some cases, the utilization of the Background-
869 Check Model without modification has potential drawbacks in other
870 cases. These include:
872 o Verifier scale: if the Attester has many Relying Parties, a
873 Verifier appraising that Attester could be frequently be queried
874 based on the same Evidence.
876 o Information leak: Evidence which the Attester might consider
877 private can be visible to the Relying Party. Hiding that Evidence
878 could devalue any resulting appraisal.
880 o Latency: a Relying Party will need to wait for the Verifier to
881 return Attestation Results before proceeding with secure
882 interactions with the Attester.
884 An implementer should examine these potential drawbacks before
885 selecting this alternative.
887 3.2. Attestation Result Augmented Evidence
889 There is a hybrid alternative for the establishment and maintenance
890 of trustworthiness between an Attester and a Relying Party which is
891 not adversely impacted by the potential drawbacks with pure
892 background-check. In this alternative, a Verifier evaluates an
893 Attester and returns signed Attestation Results back to this original
894 Attester no less frequently than a well-known interval. This
895 interval may also be asynchronous, based on the changing of certain
896 Evidence as described in
897 [I-D.birkholz-rats-network-device-subscription].
899 When a Relying Party is to receive information about the Attester's
900 trustworthiness, the Attesting Environment assembles the minimal set
901 of Evidence which can be used to confirm or refute whether the
902 Attester remains in the state of trustworthiness represented by the
903 AR. To this Evidence, the Attesting Environment appends the
904 signature from the most recent AR as well as a Relying Party Proof-
905 of-Freshness. The Attesting Environment then signs the combination.
907 The Attester then assembles AR Augmented Evidence by taking the
908 signed combination and appending the full AR. The assembly now
909 consists of two independent but semantically bound sets of signed
910 Evidence.
912 The AR Augmented Evidence is then sent to the Relying Party. The
913 Relying Party then can appraise these semantically bound sets of
914 signed Evidence by applying an Appraisal Policy for Attestation
915 Results as described below. This policy will consider both the AR as
916 well as additional information about the Attester within the AR
917 Augmented Evidence the when determining what action to take.
919 This alternative combines the [I-D.ietf-rats-architecture] Sections
920 5.1 Passport Model and Section 5.2 Background-Check Model. Figure 1
921 describes this flow of information. The flows within this combined
922 model are mapped to [I-D.ietf-rats-architecture] in the following
923 way. "Verifier A" below corresponds to the "Verifier" Figure 5
924 within [I-D.ietf-rats-architecture]. And "Relying Party/Verifier B"
925 below corresponds to the union of the "Relying Party" and "Verifier"
926 boxes within Figure 6 of [I-D.ietf-rats-architecture]. This union is
927 possible because Verifier B can be implemented as a simple, self-
928 contained process. The resulting combined process can appraise the
929 AR-augmented Evidence to determine whether an Attester qualifies for
930 secure interactions with the Relying Party. The specific steps of
931 this process are defined later in this section.
933 .----------------.
934 | Attester |
935 | .-------------.|
936 | | Attesting || .----------. .---------------.
937 | | Environment || | Verifier | | Relying Party |
938 | '-------------'| | A | | / Verifier B |
939 '----------------' '----------' '---------------'
940 time(VG) | |
941 |<------Verifier PoF-------time(NS) |
942 | | |
943 time(EG)(1)------Evidence------------>| |
944 | time(RG) |
945 |<------Attestation Results-(2) |
946 ~ ~ ~
947 time(VG')? | |
948 ~ ~ ~
949 |<------Relying Party PoF-----------------(3)time(NS')
950 | | |
951 time(EG')(4)------AR-augmented Evidence----------------->|
952 | | time(RG',RA')(5)
953 (6)
954 ~
955 time(RX')
957 Figure 1: Secure Interactions Model
959 The interaction model depicted above includes specific time related
960 events from Appendix A of [I-D.ietf-rats-architecture]. With the
961 identification of these time related events, time duration/interval
962 tracking becomes possible. Such duration/interval tracking can
963 become important if the Relying Party cares if too much time has
964 elapsed between the Verifier PoF and Relying Party PoF. If too much
965 time has elapsed, perhaps the Attestation Results themselves are no
966 longer trustworthy.
968 Note that while time intervals will often be relevant, there is a
969 simplified case that does not require a Relying Party's PoF in step
970 (3). In this simplified case, the Relying Party trusts that the
971 Attester cannot be meaningfully changed from the outside during any
972 reportable interval. Based on that assumption, and when this is the
973 case then the step of the Relying Party PoF can be safely omitted.
975 In all cases, appraisal policies define the conditions and
976 prerequisites for when an Attester does qualify for secure
977 interactions. To qualify, an Attester has to be able to provide all
978 of the mandatory affirming Trustworthiness Claims and identities
979 needed by a Relying Party's Appraisal Policy for Attestation Results,
980 and none of the disqualifying detracting Trustworthiness Claims.
982 More details on each interaction step are as follows. The numbers
983 used in this sequence match to the numbered steps in Figure 1:
985 1. An Attester sends Evidence which is provably fresh to Verifier A
986 at time(EG). Freshness from the perspective of Verifier A MAY be
987 established with Verifier PoF such as a nonce.
989 2. Verifier A appraises (1), then sends the following items back to
990 that Attester within Attestation Results:
992 1. the verified identity of the Attesting Environment,
994 2. the Verifier A appraised Trustworthiness Vector of an
995 Attester,
997 3. a freshness proof associated with the Attestation Results,
999 4. a Verifier signature across (2.1) though (2.3).
1001 3. At time(EG') a Relying Party PoF (such as a nonce) known to the
1002 Relying Party is sent to the Attester.
1004 4. The Attester generates and sends AR-augmented Evidence to the
1005 Relying Party/Verifier B. This AR-augmented Evidence includes:
1007 1. The Attestation Results from (2)
1009 2. Any (optionally) new incremental Evidence from the Attesting
1010 Environment
1012 3. Attestation Environment signature which spans a hash of the
1013 Attestation Results (such as the signature of (2.4)), the
1014 proof-of-freshness from (3), and (4.2). Note: this construct
1015 allows the delta of time between (2.3) and (3) to be
1016 definitively calculated by the Relying Party.
1018 5. On receipt of (4), the Relying Party applies its Appraisal Policy
1019 for Attestation Results. At minimum, this appraisal policy
1020 process must include the following:
1022 1. Verify that (4.3) includes the nonce from (3).
1024 2. Use a local certificate to validate the signature (4.1).
1026 3. Verify that the hash from (4.3) matches (4.1)
1028 4. Use the identity of (2.1) to validate the signature of (4.3).
1030 5. Failure of any steps (5.1) through (5.4) means the link does
1031 not meet minimum validation criteria, therefore appraise the
1032 link as having a null Verifier B Trustworthiness Vector.
1033 Jump to step (6.1).
1035 6. When there is large or uncertain time gap between time(EG)
1036 and time(EG'), the link should be assigned a null Verifier B
1037 Trustworthiness Vector. Jump to step (6.1).
1039 7. Assemble the Verifier B Trustworthiness Vector
1041 1. Copy Verifier A Trustworthiness Vector to Verifier B
1042 Trustworthiness Vector
1044 2. Add implicit Trustworthiness Claims inherent to the type
1045 of TEE.
1047 3. Prune any Trustworthiness Claims unsupportable by the
1048 Attesting Environment.
1050 4. Prune any Trustworthiness Claims the Relying Party
1051 doesn't accept from this Verifier.
1053 6. The Relying Party takes action based on Verifier B's appraised
1054 Trustworthiness Vector:
1056 1. Prune any Trustworthiness Claims not used in the Appraisal
1057 Policy for Attestation Results.
1059 2. Allow the information exchange from the Attester into a
1060 Relying Party context where the Verifier B appraised
1061 Trustworthiness Vector includes all the mandatory
1062 Trustworthiness Claims of value "1", and none of the
1063 disqualifying Trustworthiness Claims containing values that
1064 are not "0" or "1".
1066 3. Disallow any information exchange into a Relying Party
1067 context for which that Verifier B appraised Trustworthiness
1068 Vector is not qualified.
1070 As link layer protocols re-authenticate, steps (1) to (2) and steps
1071 (3) to (6) will independently refresh. This allows the
1072 Trustworthiness of Attester to be continuously re-appraised. There
1073 are only specific event triggers which will drive the refresh of
1074 Evidence generation (1), Attestation Result generation (2), or AR-
1075 augmented Evidence generation (4):
1077 o life-cycle events, e.g. a change to an Authentication Secret of
1078 the Attester or an update of a software component.
1080 o uptime-cycle events, e.g. a hard reset or a re-initialization of
1081 an Attester.
1083 o authentication-cycle events, e.g. a link-layer interface reset
1084 could result in a new (4).
1086 3.3. Mutual Attestation
1088 In the interaction models described above, each device on either side
1089 of a secure interaction may require remote attestation of its peer.
1090 This process is known as mutual-attestation. To support mutual-
1091 attestation, the interaction models listed above may be run
1092 independently on either side of the connection.
1094 3.4. Transport Protocol Integration
1096 Either unidirectional attestation or mutual attestation may be
1097 supported within the protocol interactions needed for the
1098 establishment of a single transport session. While this document
1099 does not mandate specific transport protocols, messages containing
1100 the Attestation Results and AR Augmented Evidence can be passed
1101 within an authentication framework such the EAP protocol [RFC5247]
1102 over TLS [RFC8446].
1104 4. Privacy Considerations
1106 Privacy Considerations Text
1108 5. Security Considerations
1110 Security Considerations Text
1112 6. IANA Considerations
1114 See Body.
1116 7. References
1118 7.1. Normative References
1120 [GP-TEE-PP]
1121 "Global Platform TEE Protection Profile v1.3", September
1122 2020, .
1125 [I-D.ietf-rats-architecture]
1126 Fraunhofer SIT, Microsoft, Sandelman Software Works, Intel
1127 Corporation, and Huawei Technologies, "Remote Attestation
1128 Procedures Architecture", draft-ietf-rats-architecture-12
1129 (work in progress), April 2021.
1131 [OMTP-ATE]
1132 "Open Mobile Terminal Platform - Advanced Trusted
1133 Environment", May 2009, .
1137 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1138 Requirement Levels", BCP 14, RFC 2119,
1139 DOI 10.17487/RFC2119, March 1997,
1140 .
1142 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
1143 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
1144 May 2017, .
1146 7.2. Informative References
1148 [I-D.birkholz-rats-network-device-subscription]
1149 Fraunhofer SIT, Cisco Systems, Inc., and Huawei
1150 Technologies, "Attestation Event Stream Subscription",
1151 draft-birkholz-rats-network-device-subscription-03 (work
1152 in progress), August 2021.
1154 [I-D.tschofenig-rats-psa-token]
1155 Arm Limited, Arm Limited, Arm Limited, Arm Limited, and
1156 Arm Limited, "Arm's Platform Security Architecture (PSA)
1157 Attestation Token", draft-tschofenig-rats-psa-token-08
1158 (work in progress), March 2021.
1160 [IEEE802.1AR]
1161 "802.1AR: Secure Device Identity", August 2018,
1162 .
1164 [RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
1165 Authentication Protocol (EAP) Key Management Framework",
1166 RFC 5247, DOI 10.17487/RFC5247, August 2008,
1167 .
1169 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
1170 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
1171 .
1173 [SEV-SNP] "AMD SEV-SNP: Stregthening VM Isolation with Integrity
1174 Protection and More", 2020,
1175 .
1179 [SGX] "Supporting Third Party Attestation for Intel SGX with
1180 Intel Data Center Attestation Primitives", 2017, .
1185 [TDX] "Intel Trust Domain Extensions", 2020, .
1189 [TPM-ID] "TPM Keys for Platform Identity for TPM 1.2", August 2015,
1190 .
1193 [US-Executive-Order]
1194 "Executive Order on Improving the Nation's Cybersecurity",
1195 May 2021, .
1199 Appendix A. Supportable Trustworthiness Claims
1201 The following is a table which shows what Claims are supportable by
1202 different Attesting Environment types. Note that claims MAY BE
1203 implicit to an Attesting Environment type, and therefore do not have
1204 to be included in the Trustworthiness Vector to be considered as set
1205 by the Relying Party.
1207 A.1. Supportable Trustworthiness Claims for HSM-based CC
1209 Following are Trustworthiness Claims which MAY be set for a HSM-based
1210 Confidential Computing Attester. (Such as a TPM [TPM-ID].)
1211 +-------------------+-----------+-----------------------------------+
1212 | Trustworthiness | Required? | Appraisal Method |
1213 | Claim | | |
1214 +-------------------+-----------+-----------------------------------+
1215 | configuration | Optional | Verifier evaluation of Attester |
1216 | | | reveals no configuration lines |
1217 | | | which expose the Attester to |
1218 | | | known security vulnerabilities. |
1219 | | | This may be done with or without |
1220 | | | the involvement of a TPM PCR. |
1221 | | | |
1222 | executables | Yes | Checks the TPM PCRs for the |
1223 | | | static operating system, and for |
1224 | | | any tracked files subsequently |
1225 | | | loaded |
1226 | | | |
1227 | file-system | No | Can be supported, but TPM |
1228 | | | tracking is unlikely |
1229 | | | |
1230 | hardware | Yes | If TPM PCR check ok from BIOS |
1231 | | | checks, through Master Boot |
1232 | | | Record configuration |
1233 | | | |
1234 | instance-identity | Optional | Check IDevID |
1235 | | | |
1236 | runtime-opaque | n/a | TPMs are not recommended to |
1237 | | | provide a sufficient technology |
1238 | | | base for this Trustworthiness |
1239 | | | Claim. |
1240 | | | |
1241 | sourced-data | n/a | TPMs are not recommended to |
1242 | | | provide a sufficient technology |
1243 | | | base for this Trustworthiness |
1244 | | | Claim. |
1245 | | | |
1246 | storage-opaque | Minimal | With a TPM, secure storage space |
1247 | | | exists and is writeable by |
1248 | | | external applications. But the |
1249 | | | space is so limited that it often |
1250 | | | is used just be used to store |
1251 | | | keys. |
1252 +-------------------+-----------+-----------------------------------+
1254 Setting the Trustworthiness Claims may follow the following logic at
1255 the Verifier A within (2) of Figure 1:
1257 Start: Evidence received starts the generation of a new
1258 Trustworthiness Vector. (e.g., TPM Quote Received, log received,
1259 or appraisal timer expired)
1261 Step 0: set Trustworthiness Vector = Null
1263 Step 1: Is there sufficient fresh signed evidence to appraise?
1264 (yes) - No Action
1265 (no) - Goto Step 6
1267 Step 2: Appraise Hardware Integrity PCRs
1268 if (hardware NOT "0") - push onto vector
1269 if (hardware NOT affirming or warning), go to Step 6
1271 Step 3: Appraise Attesting Environment identity
1272 if (instance-identity <> "0") - push onto vector
1274 Step 4: Appraise executable loaded and filesystem integrity
1275 if (executables NOT "0") - push onto vector
1276 if (executables NOT affirming or warning), go to Step 6
1278 Step 5: Appraise all remaining Trustworthiness Claims
1279 Independently and set as appropriate.
1281 Step 6: Assemble Attestation Results, and push to Attester
1283 End
1285 A.2. Supportable Trustworthiness Claims for process-based CC
1287 Following are Trustworthiness Claims which MAY be set for a process-
1288 based Confidential Computing based Attester. (Such as a SGX Enclaves
1289 and TrustZone.)
1290 +-------------------+-----------+-----------------------------------+
1291 | Trustworthiness | Required? | Appraisal Method |
1292 | Claim | | |
1293 +-------------------+-----------+-----------------------------------+
1294 | instance-identity | Optional | Internally available in TEE. But |
1295 | | | keys might not be known/exposed |
1296 | | | to the Relying Party by the |
1297 | | | Attesting Environment. |
1298 | | | |
1299 | configuration | Optional | If done, this is at the |
1300 | | | Application Layer. Plus each |
1301 | | | process needs it own protection |
1302 | | | mechanism as the protection is |
1303 | | | limited to the process itself. |
1304 | | | |
1305 | executables | Optional | Internally available in TEE. But |
1306 | | | keys might not be known/exposed |
1307 | | | to the Relying Party by the |
1308 | | | Attesting Environment. |
1309 | | | |
1310 | file-system | Optional | Can be supported by application, |
1311 | | | but process-based CC is not a |
1312 | | | sufficient technology base for |
1313 | | | this Trustworthiness Claim. |
1314 | | | |
1315 | hardware | Implicit | At least the TEE is protected |
1316 | | in | here. Other elements of the |
1317 | | signature | system outside of the TEE might |
1318 | | | need additional protections is |
1319 | | | used by the application process. |
1320 | | | |
1321 | runtime-opaque | Implicit | From the TEE |
1322 | | in | |
1323 | | signature | |
1324 | | | |
1325 | storage-opaque | Implicit | Although the application must |
1326 | | in | assert that this function is used |
1327 | | signature | by the code itself. |
1328 | | | |
1329 | sourced-data | Optional | Will need to be supported by |
1330 | | | application code |
1331 +-------------------+-----------+-----------------------------------+
1333 A.3. Supportable Trustworthiness Claims for VM-based CC
1335 Following are Trustworthiness Claims which MAY be set for a VM-based
1336 Confidential Computing based Attester. (Such as SEV, TDX, ACCA, SEV-
1337 SNP.)
1338 +-------------------+-----------+-----------------------------------+
1339 | Trustworthiness | Required? | Appraisal Method |
1340 | Claim | | |
1341 +-------------------+-----------+-----------------------------------+
1342 | instance-identity | Optional | Internally available in TEE. But |
1343 | | | keys might not be known/exposed |
1344 | | | to the Relying Party by the |
1345 | | | Attesting Environment. |
1346 | | | |
1347 | configuration | Optional | Requires application integration. |
1348 | | | Easier than with process-based |
1349 | | | solution, as the whole protected |
1350 | | | machine can be evaluated. |
1351 | | | |
1352 | executables | Optional | Internally available in TEE. But |
1353 | | | keys might not be known/exposed |
1354 | | | to the Relying Party by the |
1355 | | | Attesting Environment. |
1356 | | | |
1357 | file-system | Optional | Can be supported by application |
1358 | | | |
1359 | hardware | Chip | At least the TEE is protected |
1360 | | dependent | here. Other elements of the |
1361 | | | system outside of the TEE might |
1362 | | | need additional protections is |
1363 | | | used by the application process. |
1364 | | | |
1365 | runtime-opaque | Implicit | From the TEE |
1366 | | in | |
1367 | | signature | |
1368 | | | |
1369 | storage-opaque | Chip | Although the application must |
1370 | | dependent | assert that this function is used |
1371 | | | by the code itself. |
1372 | | | |
1373 | sourced-data | Optional | Will need to be supported by |
1374 | | | application code |
1375 +-------------------+-----------+-----------------------------------+
1377 Appendix B. Some issues being worked
1379 It is possible for a cluster/hierarchy of Verifiers to have aggregate
1380 AR which are perhaps signed/endorsed by a lead Verifier. What should
1381 be the Proof-of-Freshness or Verifier associated with any of the
1382 aggregate set of Trustworthiness Claims?
1384 There will need to be a subsequent document which documents how these
1385 objects which will be translated into a protocol on a wire (e.g. EAP
1386 on TLS). Some breakpoint between what is in this draft, and what is
1387 in specific drafts for wire encoding will need to be determined.
1388 Questions like architecting the cluster/hierarchy of Verifiers fall
1389 into this breakdown.
1391 For some Trustworthiness Claims, there could be value in identifying
1392 a specific Appraisal Policy for Attestation Results applied within
1393 the Attester. One way this could be done would be a URI which
1394 identifies the policy used at Verifier A, and this URI would
1395 reference a specific Trustworthiness Claim. As the URI also could
1396 encode the version of the software, it might also act as a mechanism
1397 to signal the Relying Party to refresh/re-evaluate its view of
1398 Verifier A. Do we need this type of structure to be included here?
1399 Should it be in subsequent documents?
1401 Expand the variant of Figure 1 which requires no Relying Party PoF
1402 into its own picture.
1404 In what document (if any) do we attempt normalization of the identity
1405 claims between different types of TEE. E.g., does MRSIGNER plus
1406 extra loaded software = the sum of TrustZone Signer IDs for loaded
1407 components?
1409 Appendix C. Contributors
1411 Guy Fedorkow
1413 Email: gfedorkow@juniper.net
1415 Dave Thaler
1417 Email: dthaler@microsoft.com
1419 Ned Smith
1421 Email: ned.smith@intel.com
1423 Authors' Addresses
1425 Eric Voit
1426 Cisco Systems
1428 Email: evoit@cisco.com
1429 Henk Birkholz
1430 Fraunhofer SIT
1431 Rheinstrasse 75
1432 Darmstadt 64295
1433 Germany
1435 Email: henk.birkholz@sit.fraunhofer.de
1437 Thomas Hardjono
1438 MIT
1440 Email: hardjono@mit.edu
1442 Thomas Fossati
1443 Arm Limited
1445 Email: Thomas.Fossati@arm.com
1447 Vincent Scarlata
1448 Intel
1450 Email: vincent.r.scarlata@intel.com