RATS Working Group H. Birkholz Internet-Draft Fraunhofer SIT Intended status: Informational D. Thaler Expires: 13 November 2020 Microsoft M. Richardson Sandelman Software Works N. Smith Intel 12 May 2020 Remote Attestation Procedures Architecture draft-ietf-rats-architecture-latest Abstract In network protocol exchanges, it is often the case that one entity (a Relying Party) requires evidence about a remote peer to assess the peer's trustworthiness, and a way to appraise such evidence. The evidence is typically a set of claims about its software and hardware platform. This document describes an architecture for such remote attestation procedures (RATS). Note to Readers Discussion of this document takes place on the RATS Working Group mailing list (rats@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/rats/ (https://mailarchive.ietf.org/arch/browse/rats/). Source for this draft and an issue tracker can be found at https://github.com/ietf-rats-wg/architecture (https://github.com/ ietf-rats-wg/architecture). Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." Birkholz, et al. Expires 13 November 2020 [Page 1] Internet-Draft RATS Arch & Terms May 2020 This Internet-Draft will expire on 13 November 2020. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 4 4. Architectural Overview . . . . . . . . . . . . . . . . . . . 4 5. Topological Models . . . . . . . . . . . . . . . . . . . . . 5 5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 5 5.2. Background-Check Model . . . . . . . . . . . . . . . . . 6 5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 7 6. Two Types of Environments . . . . . . . . . . . . . . . . . . 8 7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 9 8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 9 9. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 10 10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12 11. Security Considerations . . . . . . . . . . . . . . . . . . . 12 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12 14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 13 15. Informative References . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 1. Introduction Remote Attestation, as used in this document, is a process by which one entity (the "Attester") provides evidence about its identity and state to another remote entity (the "Relying Party"), which then assesses the Attester's trustworthiness for the Relying Party's own purposes. Birkholz, et al. Expires 13 November 2020 [Page 2] Internet-Draft RATS Arch & Terms May 2020 2. Terminology This document uses the following terms: * Appraisal Policy for Evidence: A set of rules that direct how a Verifier evaluates the validity of information about an Attester. Compare /security policy/ in [RFC4949]. * Appraisal Policy for Attestation Result: A set of rules that direct how a Relying Party uses the evaluation results about an Attester generated by the Verifiers. Compare /security policy/ in [RFC4949]. * Attestation Result: The evaluation results generated by a Verifier, typically including information about an Attester, where the Verifier vouches for the validity of the results. * Attester: An entity whose attributes must be evaluated in order to determine whether the entity is considered trustworthy, such as when deciding whether the entity is authorized to perform some operation. * Endorsement: A secure statement that some entity (typically a manufacturer) vouches for the integrity of an Attester's signing capability. * Endorser: An entity that creates Endorsements that can be used to help evaluate trustworthiness of Attesters. * Evidence: A set of information about an Attester that is to be evaluated by a Verifier. * Relying Party: An entity that depends on the validity of information about another entity, typically for purposes of authorization. Compare /relying party/ in [RFC4949]. * Relying Party Owner: An entity, such as an administrator, that is authorized to configure Appraisal Policy for Attestation Results in a Relying Party. * Verifier: An entity that evaluates the validity of Evidence about an Attester. * Verifier Owner: An entity, such as an administrator, that is authorized to configure Appraisal Policy for Evidence in a Verifier. Birkholz, et al. Expires 13 November 2020 [Page 3] Internet-Draft RATS Arch & Terms May 2020 3. Reference Use Cases 4. Architectural Overview Figure 1 depicts the data that flows between different roles, independent of protocol or use case. ************ ************ ***************** * Endorser * * Verifier * * Relying Party * ************ * Owner * * Owner * | ************ ***************** | | | Endorsements| | | | |Appraisal | | |Policy for | | |Evidence | Appraisal | | | Policy for | | | Attestation | | | Result v v | .-----------------. | .----->| Verifier |------. | | '-----------------' | | | | | | Attestation| | | Results | | | Evidence | | | | | | v v .----------. .-----------------. | Attester | | Relying Party | '----------' '-----------------' Figure 1: Conceptual Data Flow An Attester creates Evidence that is conveyed to a Verifier. The Verifier uses the Evidence, and any Endorsements from Endorsers, by applying an Evidence Appraisal Policy to assess the trustworthiness of the Attester, and generates Attestation Results for use by Relying Parties. The Evidence Appraisal Policy might be obtained from an Endorser along with the Endorsements, or might be obtained via some other mechanism such as being configured in the Verifier by an administrator. Birkholz, et al. Expires 13 November 2020 [Page 4] Internet-Draft RATS Arch & Terms May 2020 The Relying Party uses Attestation Results by applying its own Appraisal Policy to make application-specific decisions such as authorization decisions. The Attestation Result Appraisal Policy might, for example, be configured in the Relying Party by an administrator. 5. Topological Models There are multiple possible models for communication between an Attester, a Verifier, and a Relying Party. This section includes some reference models, but this is not intended to be a restrictive list, and other variations may exist. 5.1. Passport Model In this model, an Attester sends Evidence to a Verifier, which compares the Evidence against its Appraisal Policy. The Verifier then gives back an Attestation Result. If the Attestation Result was a successful one, the Attester can then present the Attestation Result to a Relying Party, which then compares the Attestation Result against its own Appraisal Policy. Since the resource access protocol between the Attester and Relying Party includes an Attestation Result, in this model the details of that protocol constrain the serialization format of the Attestation Result. The format of the Evidence on the other hand is only constrained by the Attester-Verifier attestation protocol. +-------------+ | | Compare Evidence | Verifier | against Appraisal Policy | | +-------------+ ^ | Evidence| |Attestation | | Result | v +-------------+ +-------------+ | |-------------->| | Compare Attestation | Attester | Attestation | Relying | Result against | | Result | Party | Appraisal Policy +-------------+ +-------------+ Figure 2: Passport Model The passport model is so named because of its resemblance to how nations issue passports to their citizens. The nature of the Evidence that an individual needs to provide to its local authority Birkholz, et al. Expires 13 November 2020 [Page 5] Internet-Draft RATS Arch & Terms May 2020 is specific to the country involved. The citizen retains control of the resulting passport document and presents it to other entities when it needs to assert a citizenship or identity claim, such as an airport immigration desk. The passport is considered sufficient because it vouches for the citizenship and identity claims, and it is issued by a trusted authority. Thus, in this immigration desk analogy, the passport issuing agency is a Verifier, the passport is an Attestation Result, and the immigration desk is a Relying Party. 5.2. Background-Check Model In this model, an Attester sends Evidence to a Relying Party, which simply passes it on to a Verifier. The Verifier then compares the Evidence against its Appraisal Policy, and returns an Attestation Result to the Relying Party. The Relying Party then compares the Attestation Result against its own security policy. The resource access protocol between the Attester and Relying Party includes Evidence rather than an Attestation Result, but that Evidence is not processed by the Relying Party. Since the Evidence is merely forwarded on to a trusted Verifier, any serialization format can be used for Evidence because the Relying Party does not need a parser for it. The only requirement is that the Evidence can be _encapsulated in_ the format required by the resource access protocol between the Attester and Relying Party. However, like in the Passport model, an Attestation Result is still consumed by the Relying Party and so the serialization format of the Attestation Result is still important. If the Relying Party is a constrained node whose purpose is to serve a given type resource using a standard resource access protocol, it already needs the parser(s) required by that existing protocol. Hence, the ability to let the Relying Party obtain an Attestation Result in the same serialization format allows minimizing the code footprint and attack surface area of the Relying Party, especially if the Relying Party is a constrained node. Birkholz, et al. Expires 13 November 2020 [Page 6] Internet-Draft RATS Arch & Terms May 2020 +-------------+ | | Compare Evidence | Verifier | against Appraisal Policy | | +-------------+ ^ | Evidence| |Attestation | | Result | v +-------------+ +-------------+ | |-------------->| | Compare Attestation | Attester | Evidence | Relying | Result against | | | Party | Appraisal Policy +-------------+ +-------------+ Figure 3: Background-Check Model The background-check model is so named because of the resemblance of how employers and volunteer organizations perform background checks. When a prospective employee provides claims about education or previous experience, the employer will contact the respective institutions or former employers to validate the claim. Volunteer organizations often perform police background checks on volunteers in order to determine the volunteer's trustworthiness. Thus, in this analogy, a prospective volunteer is an Attester, the organization is the Relying Party, and a former employer or government agency that issues a report is a Verifier. 5.3. Combinations One variation of the background-check model is where the Relying Party and the Verifier on the same machine, and so there is no need for a protocol between the two. It is also worth pointing out that the choice of model is generally up to the Relying Party, and the same device may need to attest to different Relying Parties for different use cases (e.g., a network infrastructure device to gain access to the network, and then a server holding confidential data to get access to that data). As such, both models may simultaneously be in use by the same device. Birkholz, et al. Expires 13 November 2020 [Page 7] Internet-Draft RATS Arch & Terms May 2020 Figure 4 shows another example of a combination where Relying Party 1 uses the passport model, whereas Relying Party 2 uses an extension of the background-check model. Specifically, in addition to the basic functionality shown in Figure 3, Relying Party 2 actually provides the Attestation Result back to the Attester, allowing the Attester to use it with other Relying Parties. This is the model that the Trusted Application Manager plans to support in the TEEP architecture [I-D.ietf-teep-architecture]. +-------------+ | | Compare Evidence | Verifier | against Appraisal Policy | | +-------------+ ^ | Evidence| |Attestation | | Result | v +-------------+ | | Compare | Relying | Attestation Result | Party 2 | against Appraisal Policy +-------------+ ^ | Evidence| |Attestation | | Result | v +-------------+ +-------------+ | |-------------->| | Compare Attestation | Attester | Attestation | Relying | Result against | | Result | Party 1 | Appraisal Policy +-------------+ +-------------+ Figure 4: Example Combination 6. Two Types of Environments An Attester consists of at least one Attesting Environment and Attested Environment. In some implementations, the Attesting and Attested Environments might be combined. Other implementations might have multiple Attesting and Attested Environments. Birkholz, et al. Expires 13 November 2020 [Page 8] Internet-Draft RATS Arch & Terms May 2020 7. Trust Model The scope of this document is scenarios for which a Relying Party trusts a Verifier that can evaluate the trustworthiness of information about an Attester. Such trust might come by the Relying Party trusting the Verifier (or its public key) directly, or might come by trusting an entity (e.g., a Certificate Authority) that is in the Verifier's certificate chain. The Relying Party might implicitly trust a Verifier (such as in the Verifying Relying Party combination). Or, for a stronger level of security, the Relying Party might require that the Verifier itself provide information about itself that the Relying Party can use to evaluate the trustworthiness of the Verifier before accepting its Attestation Results. In solutions following the background-check model, the Attester is assumed to trust the Verifier (again, whether directly or indirectly via a Certificate Authority that it trusts), since the Attester relies on an Attestation Result it obtains from the Verifier, in order to access resources. The Verifier trusts (or more specifically, the Verifier's security policy is written in a way that configures the Verifier to trust) a manufacturer, or the manufacturer's hardware, so as to be able to evaluate the trustworthiness of that manufacturer's devices. In solutions with weaker security, a Verifier might be configured to implicitly trust firmware or even software (e.g., a hypervisor). That is, it might evaluate the trustworthiness of an application component, or operating system component or service, under the assumption that information provided about it by the lower-layer hypervisor or firmware is true. A stronger level of security comes when information can be vouched for by hardware or by ROM code, especially if such hardware is physically resistant to hardware tampering. The component that is implicitly trusted is often referred to as a Root of Trust. 8. Conceptual Messages Birkholz, et al. Expires 13 November 2020 [Page 9] Internet-Draft RATS Arch & Terms May 2020 Evidence Attestation Results .--------------. CWT CWT .-------------------. | Attester-A |------------. .----------->| Relying Party V | '--------------' v | `-------------------' .--------------. JWT .------------. JWT .-------------------. | Attester-B |-------->| Verifier |-------->| Relying Party W | '--------------' | | `-------------------' .--------------. X.509 | | X.509 .-------------------. | Attester-C |-------->| |-------->| Relying Party X | '--------------' | | `-------------------' .--------------. TPM | | TPM .-------------------. | Attester-D |-------->| |-------->| Relying Party Y | '--------------' '------------' `-------------------' .--------------. other ^ | other .-------------------. | Attester-E |------------' '----------->| Relying Party Z | '--------------' `-------------------' Figure 5: Multiple Attesters and Relying Parties with Different Formats 9. Freshness It is important to prevent replay attacks where an attacker replays old Evidence or an old Attestation Result that is no longer correct. To do so, some mechanism of ensuring that the Evidence and Attestation Results are fresh, meaning that there is some degree of assurance that they still reflect the latest state of the Attester, and that any Attestation Result was generated using the latest Appraisal Policy for Evidence - and latest Evidence, accordingly. There is, however, always a race condition possible in that the state of the Attester, and the Appraisal Policy for Evidence, may change immediately after the Evidence or Attestation Results were generated. The goal is merely to narrow the time window to something the Verifier (for Evidence) or Relying Party (for an Attestation Result) is willing to accept. Effectively, the burden of appraisal with respect to freshness is also a duty of the Verifier. There are two common approaches to providing some assurance of freshness. The first approach is that a nonce is generated by a remote entity (e.g., the Verifier for Evidence, or the Relying Party for an Attestation Result), and the nonce is then signed and included along with the claims in the Evidence or Attestation Result, so that the remote entity knows that the claims were signed after the nonce was generated. Birkholz, et al. Expires 13 November 2020 [Page 10] Internet-Draft RATS Arch & Terms May 2020 A second approach is to rely on synchronized clocks, and include a signed timestamp (e.g., using [I-D.birkholz-rats-tuda]) along with the claims in the Evidence or Attestation Result, so that the remote entity knows that the claims were signed at that time, as long as it has some assurance that the timestamp is correct. This typically requires additional claims about the signer's time synchronization mechanism in order to provide such assurance. In either approach, it is important to note that the actual values in claims might have been generated long before the claims are signed. If so, it is the signer's responsibility to ensure that the values are still correct when they are signed. For example, values might have been generated at boot, and then used in claims as long as the signer can guarantee that they cannot have changed since boot. The term freshness is applied to both Evidence and Attestation Results in this document. Unfortunately, the term freshness is also a bit overloaded and in order to illustrate the application of the term in this document better, two characteristics that can be part of freshness are elaborated on below: recentness and uniqueness: Recentness: a quality of Evidence that is based on the time that has passed between creation of the evidence by an Attester (typically by "sealing" a Claims set via signing) and the reception of that Evidence by a Verifier or (relaying party). Evidence can be not recent enough, if the Evidence created from collected Claims is too old. A nonce supplied during a Challenge/ Response handshake and a time-stamp based proof-of-recentness between time-synchronized peers are both usage scenarios in which recentness characteristics are provided to the Verifier. Uniqueness: a quality of Evidence that is based on state of the Verifier that is tracing Evidence already received (or already requested in some usage scenarios, respectively). Evidence can be non-unique, if the Evidence created from Claims does not include a unique handle (e.g. a UUID used as a nonce). If the Verifier receives the same Evidence payload twice, a list of Evidence queries with respect to an Attester or a list Evidence messages received from an Attester are both usage scenarios in which uniqueness characteristics can be appraised by a Verifier. Consecutively, while a nonce supplied during a Challenge/Response handshake is a usage scenario that does provide a proof-of- uniqueness to a state-less Verifier, a time-stamp based proof-of- recentness between time-synchronized peers is a usage scenario that does not. Birkholz, et al. Expires 13 November 2020 [Page 11] Internet-Draft RATS Arch & Terms May 2020 10. Privacy Considerations The conveyance of Evidence and the resulting Attestation Results reveal a great deal of information about the internal state of a device. In many cases, the whole point of the Attestation process is to provide reliable information about the type of the device and the firmware/software that the device is running. This information is particularly interesting to many attackers. For example, knowing that a device is running a weak version of firmware provides a way to aim attacks better. Protocols that convey Evidence or Attestation Results are responsible for detailing what kinds of information are disclosed, and to whom they are exposed. 11. Security Considerations Any solution that conveys information used for security purposes, whether such information is in the form of Evidence, Attestation Results, or Endorsements, or Appraisal Policy, needs to support end- to-end integrity protection and replay attack prevention, and often also needs to support additional security protections. For example, additional means of authentication, confidentiality, integrity, replay, denial of service and privacy protection are needed in many use cases. Section 9 discusses ways in which freshness can be used in this architecture to protect against replay attacks. To evaluate the security provided by a particular Appraisal Policy, it is important to understand the strength of the Root of Trust, e.g., whether it is mutable software, or firmware that is read-only after boot, or immutable hardware/ROM. It is also important that the Appraisal Policy was itself obtained securely. As such, if Appraisal Policy in a Relying Party or Verifier can be configured via a network protocol, the ability to attest to the health of the client providing the Appraisal Policy needs to be considered. 12. IANA Considerations This document does not require any actions by IANA. 13. Acknowledgments Special thanks go to David Wooten, Joerg Borchert, Hannes Tschofenig, Laurence Lundblade, Diego Lopez, Jessica Fitzgerald-McKay, Frank Xia, and Nancy Cam-Winget. Birkholz, et al. Expires 13 November 2020 [Page 12] Internet-Draft RATS Arch & Terms May 2020 14. Contributors Thomas Hardjono created older versions of the terminology section in collaboration with Ned Smith. Eric Voit provided the conceptual separation between Attestation Provision Flows and Attestation Evidence Flows. Monty Wisemen created the content structure of the first three architecture drafts. Carsten Bormann provided many of the motivational building blocks with respect to the Internet Threat Model. 15. Informative References [I-D.birkholz-rats-tuda] Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann, "Time-Based Uni-Directional Attestation", Work in Progress, Internet-Draft, draft-birkholz-rats-tuda-02, 9 March 2020, . [I-D.ietf-teep-architecture] Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler, "Trusted Execution Environment Provisioning (TEEP) Architecture", Work in Progress, Internet-Draft, draft- ietf-teep-architecture-08, 4 April 2020, . Authors' Addresses Henk Birkholz Fraunhofer SIT Rheinstrasse 75 64295 Darmstadt Germany Email: henk.birkholz@sit.fraunhofer.de Dave Thaler Microsoft United States of America Email: dthaler@microsoft.com Birkholz, et al. Expires 13 November 2020 [Page 13] Internet-Draft RATS Arch & Terms May 2020 Michael Richardson Sandelman Software Works Canada Email: mcr+ietf@sandelman.ca Ned Smith Intel Corporation United States of America Email: ned.smith@intel.com Birkholz, et al. Expires 13 November 2020 [Page 14]