RATS Endorsements

Introduction Section 3 in the RATS Architecture 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 is to be able to accept Evidence in a variety of formats and generate Attestation Results in the format needed by a Relying Party.

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. 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.

RATS Conceptual Messages RATS conceptual messages in 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 .

Example Verifier Input .-----------------. | e | |Current state| Comparison | Reference state | | | | (layer 1) | Rules | (layer 1) | | V \ '-------------' '-----------------' | a | l / .-------------. .-----------------. | u Endorsement | |Current state| | Reference state | | e | | (layer 0) | | (layer 0) | | s \ '-------------' '-----------------' / ]]> 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 layered attestation using DICE 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.

Endorsement Format Considerations This section discusses considerations around formats for Endorsements.

Security Considerations 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.

Scalability Considerations 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. 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 the same message format for Evidence, Endorsements, and (later) Attestation Results received from the later Verifier, can provide a code size savings due to having only a single parser in this limited case.

IANA Considerations This document does not require any actions by IANA.