RATS Working Group D. Thaler Internet-Draft Microsoft Intended status: Informational 7 March 2023 Expires: 8 September 2023 RATS Endorsements: CORIM vs EAT draft-dthaler-rats-endorsements-00 Abstract Various formats exist, including standard and vendor-specific formats, for messages in the RATS Architecture. Indeed, one of the purposes of a Verifer is to accept Evidence in a variety of formats and generate Attestation Results in a format needed by a Relying Party. This document discusses considerations around formats for Endorsements, and the suitability of EAT and CORIM as Endorsement formats. 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." This Internet-Draft will expire on 8 September 2023. Copyright Notice Copyright (c) 2023 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 Thaler Expires 8 September 2023 [Page 1] Internet-Draft RATS Endorsements March 2023 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. Current State vs Reference States . . . . . . . . . . . . . . 2 2.1. RATS Conceptual Messages . . . . . . . . . . . . . . . . 3 3. Concrete Formats . . . . . . . . . . . . . . . . . . . . . . 5 3.1. EAT . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2. CORIM . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Scalability . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Normative References . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction Section 3 in the RATS Architecture [RFC9334] gives an overview of the roles and conceptual messages in the IETF Remote Attestation Architecture. As discussed in that document, a Verifier accepts Evidence and Endorsements, and appraises them using Appraisal Policy for Evidence, typically against a set of Reference Values. Various formats exist, including standard and vendor-specific formats, for the conceptual messages shown. Indeed, one of the purposes of a Verifer as depicted in Figure 9 of [RFC9334] is to be able to accept Evidence in a variety of formats and generate Attestation Results in the format needed by a Relying Party. This document discusses considerations around formats for Endorsements, and the suitability of EAT and CORIM as Endorsement formats. 2. Current State vs Reference States Appraisal policies (Appraisal Policy for Evidence, and Appraisal Policy for Attestation Results) involve comparing the current state of an attester against desired or undesired states, in order to determine how trustworthy the attester is for its purposes. Thus, a Verifier needs to receive messages with information about current state, and information about desired/undesired states, and an appraisal policy that controls how the two are compared. Thaler Expires 8 September 2023 [Page 2] Internet-Draft RATS Endorsements March 2023 Current state is a group of claims about the actual state of the attester at a given point in time. Generally speaking, each claim has a name (or other ID) and a singleton value, being the value of that specific attester at a given point in time. (Some claims may inherently have multiple values, such as a list of files in a given location on the device, but for our purposes we will treat such a list as a single unit, meaning one attester at one point in time.) Each attester in general has multiple components (e.g., hardware, firmware, Operating System, etc.), each with their own set of claims (sometimes called a "claimset"), where the current state of the attester is a group of such claimsets, for all the key components of the attester that are essential to determining trustworthiness. Reference state is a group of claims about the desired or undesired state of the attester. Typically, each claim has a name (or other ID) and a set of potential values, being the current values that are allowed/disallowed when determining whether to trust the attester. (In general there may be more gradation than simply "allowed or disallowed" so each value might include some more complex level of gradation in some implementations.) That is, where current state has a single value per claim per component applying to one device at one point in time, reference state has a set of values per claim per component. The appraisal policy then specifies how to match the current value against the set of reference values. Some examples of such matching include: * The current value must be in the set of allowed reference values. * The current value must not be in the set of disallowed reference values. * The current value must be in a range where two reference values are the min and max. 2.1. RATS Conceptual Messages RATS conceptual messages in [RFC9334] fall into the above categories as follows: * Current state: Evidence, Endorsements, Attestation Results * Reference state: Reference Values * Appraisal policy: Appraisal Policy for Evidence, Appraisal Policy for Attestation Results The figure below shows an example of verifier input for a layered attester as discussed in [RFC9334]. Thaler Expires 8 September 2023 [Page 3] Internet-Draft RATS Endorsements March 2023 / .-------------. Appraisal .-----------------. \ | |Current state| Policy | Reference state | | R | | (layer N) | | (layer N) | | e | '-------------' | '-----------------' | f | .-------------. | .-----------------. | e Evidence | |Current state| | | Reference state | | r | | (layer 2) | | | (layer 2) | | e | '-------------' v '-----------------' | n | .-------------. <==========> .-----------------. | c | |Current state| Comparison | Reference state | | e | | (layer 1) | Rules | (layer 1) | | \ '-------------' '-----------------' | V | a / .-------------. .-----------------. | l Endorsement | |Current state| | Reference state | | u | | (layer 0) | | (layer 0) | | e \ '-------------' '-----------------' / s Figure 1: Example Verifier Input While the above example only shows one layer within Endorsements as the typical case, there could be multiple layers within it, such as a chip added to a hardware board potentially from a different vendor. A Trust Anchor Store is a special case of state above, where the Reference State would be the set of trust anchors accepted (or rejected) by the Verifier, and the Current State would be a trust anchor used to sign Evidence or Endorsements. In a DICE-based layered attestation for example, the current state of each layer is signed by a key held by the next lower layer. Thus in the example diagram above, the layer 2 current state (e.g., OS state) is signed by a layer 1 key (e.g., a signing key used by the firmware), the layer 1 current state (e.g., firmware state) is signed by a layer 0 key (e.g., a hardware key stored in ROM), and the layer 0 current state (hardware specs and key ID) is signed by a layer 0 key (e.g., a vendor key) which is matched against the Verifier's trust anchor store, which is part of the layer 0 reference state depicted above. Thaler Expires 8 September 2023 [Page 4] Internet-Draft RATS Endorsements March 2023 3. Concrete Formats The Entity Attestation Token (EAT) [I-D.ietf-rats-eat] format is an IETF work in progress for (at least) Evidence and Attestation Results, being the only two conceptual messages that were originally in score for standardization by the WG. The EAT format is designed to express "singleton" values (meaning one value per component, though a value may be a list of things that exist at the same time on the same device, as noted earlier). The CORIM [I-D.ietf-rats-corim] format is a newly-adopted format that is designed to express multiple values, for use by Reference Values. Trust Anchors, as a type of Reference Values, can also be configured using the CORIM format using [I-D.ietf-rats-concise-ta-stores]. The rest of this section analyzes the applicability of each to Endorsements, while not precluding other (e.g., vendor-proprietary) formats as permitted in the RATS Architecture. 3.1. EAT [I-D.ietf-rats-eat] specifies a format for a group of claimsets for current state, including Evidence and Attestation Results. Thus EAT would be a natural format to use for current state in Endorsements. When all current state (Evidence and Endorsements) about an attester is in the same format (EAT), the Verifier logic to do comparison against desired state is simplest, and adding common claims into EAT allow them to be used across all layers, including Evidence layers generated dynamically by the attester as well as Endorsement layers generated statically by a vendor or owner of a device. 3.2. CORIM [I-D.ietf-rats-corim] specifies a format for multi-valued claims, where no single value is "current" compared to others. As such, CORIM is appropriate for desired state, but is not well suited to current state without duplicating the work of the EAT specification. Some arguments to potentially use CORIM for Endorsements seem to stem from a desire to put Endorsements into the same protocol messages as Reference Values where the latter use CORIM. However, a concrete protocol message could be designed to hold both. Thaler Expires 8 September 2023 [Page 5] Internet-Draft RATS Endorsements March 2023 4. Scalability We currently assume that Reference Value Providers and Endorsers typically provide the same information to a potentially large number of clients (Verifiers, or potentially to other entities for later relay to a Verifier), and are generally on devices that are not constrained nodes, and hence additional scalability, including code size, is not a significant concern. In many scenarios, a Verifiers can also support a variety of different formats, and while code size may not be a huge concern, simplicity and correctness of code is essential to security. "Complexity is the enemy of security" is a popular security mantra and hence to increase security, any decrease in complexity helps. As such, using the same format for both Evidence and Endorsements can reduce complexity and hence increase security. When a format other than EAT (e.g., a vendor-proprietary format) is used for Evidence, the same argument would argue for using the same vendor-proprietary format for both Evidence and Endorsements. Hence unless CORIM is used for Evidence, this argues that using CORIM for Endorsements (as opposed to Reference Values) provides a risk of weaker security. The scenario where scalability in terms of code size is strongest, however, is when a Verifier is embedded into a constrained node. For example, when a constrained node is a Relying Party for most purposes, but still needs a way to establish trust in the Verifier it will use. In such a case, the Relying Party may have a constrained Verifier embedded in it that is only capable of appraising Evidence provided by its desired Verifier. Thus, the Relying Party uses its embedded Verifier for purposes of appraising its desired Verifier which it treats as only an Attester, and once verified, then uses it for verification of all other attesters. In this scenario, the embedded Verifier may have code and data size constraints, and a very simple (by comparison) appraisal policy and desired state (e.g., a required trust anchor that Evidence must be signed with and little else). Using EAT for Evidence, Endorsements, and (later) Attestation Results received from the later Verifier, can provide a code size savings if CORIM is not needed in this limited case. 5. Security Considerations Section 4 argued that, since complexity is the enemy of security, using EAT rather than CORIM as a standard format for Endorsements (leaving CORIM just for use by Reference Values) can provide increased security. Thaler Expires 8 September 2023 [Page 6] Internet-Draft RATS Endorsements March 2023 6. IANA Considerations This document does not require any actions by IANA. 7. Normative References [I-D.ietf-rats-concise-ta-stores] Wallace, C., Housley, R., Fossati, T., and Y. Deshpande, "Concise TA Stores (CoTS)", Work in Progress, Internet- Draft, draft-ietf-rats-concise-ta-stores-00, 6 December 2022, . [I-D.ietf-rats-corim] Birkholz, H., Fossati, T., Deshpande, Y., Smith, N., and W. Pan, "Concise Reference Integrity Manifest", Work in Progress, Internet-Draft, draft-ietf-rats-corim-00, 6 September 2022, . [I-D.ietf-rats-eat] Lundblade, L., Mandyam, G., O'Donoghue, J., and C. Wallace, "The Entity Attestation Token (EAT)", Work in Progress, Internet-Draft, draft-ietf-rats-eat-19, 19 December 2022, . [RFC9334] Birkholz, H., Thaler, D., Richardson, M., Smith, N., and W. Pan, "Remote ATtestation procedureS (RATS) Architecture", RFC 9334, DOI 10.17487/RFC9334, January 2023, . Author's Address Dave Thaler Microsoft United States of America Email: dthaler@microsoft.com Thaler Expires 8 September 2023 [Page 7]