| Internet-Draft | RATS Arch & Terms | January 2020 |
| Birkholz, et al. | Expires 10 July 2020 | [Page] |
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).¶
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/.¶
Source for this draft and an issue tracker can be found at https://github.com/ietf-rats-wg/architecture.¶
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<more text to be added here>¶
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.¶
This document uses the following terms:¶
<unclear if the WG wants this section in the arch doc>¶
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 |
'----------' '-----------------'
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.¶
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.¶
Many devices are composed of one or more leader units and one or more subordinate units. The leader units are the control or management units and have management interfaces to communicate with outer systems like the Network Management System (NMS). The other subordinate units are for some specific functions and have no management interfaces to communicate outside but can only be managed by the leader units by some kind of inner links. The leader units and subordinate units all have some kind of trusted computing base to measure their trustworthiness. These devices can be called composite devices, and the trustworthiness of the device is therefore only accurately represented by a composite measure that includes all its leader units and subordinate units. A typical example is the carrier-grade switches and routers that have the main control boards as the leader units and the line-processing boards and feature-processing boards as the subordinate units.¶
Figure 2 depicts the data that flows between different roles for the remote attestation of composite devices.¶
************ ************ *****************
* Endorser * * Verifier * * Relying Party *
************ * Owner * * Owner *
| ************ *****************
| | |
|Endorsements |Appraisal |Appraisal
| |Policy for |Policy for
| |Evidence |Attestation
| | |Result
v v |
.-----------------------------. |
| Verifier |------------. |
'-----------------------------' | |
^ | ^ Attestation| |
| | | Results| |
| Endorsements| | | |
Evidence| & Appraisal | |Attestation v v
of | Policy for | |Results of .-----------------.
Leader | Evidence of | |Subordinate | Relying Party |
Unit | Subordinate | |Units '-----------------'
| Units | |
.-------|---------------|-|--------------------------------------.
| | v | |
| .-----------------------------. .-----------------. |
| | .-----------. | | Subordinate | |
| | | Attester | | .------| Unit A | |
| | | Component | | | '-----------------' |
| | '-----------' | | |
| | .-----------. |<--+ Evidence of |
| | | Verifier | | | Subordinate Units |
| | | Component | | | |
| | '-----------' | | .-----------------. |
| | Leader Unit | '------| Subordinate | |
| '-----------------------------' | Unit B | |
| '-----------------' |
| Attester (Composite Device) |
'----------------------------------------------------------------'
As described in the figure, the composite device is the Attester. The leader unit inside the Attester has both the attestation functionality and verification functionality, which is respectively represented as the attester component and verifier component. The leader unit creates the Evidence of itself that is conveyed to the Verifier. Different from the previous overview data flow, in this composite devices data flow the leader unit gets the Endorsements and Evidence Appraisal Policy of the subordinate units from the Verifier, uses that information to verify the trustworthiness of the subordinate units and reports the corresponding verification results to the Verifier.¶
The Verifier receives and verifies the Evidence of the leader unit, and integrates the result with the received Attestation Results of the subordinate units to generate the Attestation Results of the Attester, i.e. the whole composite device.¶
The composite remote attestations also exist in the networks composed of several devices. For some networks, only a few devices can communicate directly with the NMS, and these devices can act as the proxies for other devices to communicate with the NMS. An example is the router cluster where two or more routers can be clustered as a group to provide the larger performance and reduce the management complexity. The routers among the group are interconnected in a network, but only one leader router is selected to directly communicate with the NMS. The NMS manages other routers through the leader router. The NMS can assess the trustworthiness of the leader router, and the leader router can assess other routers and report the results to the NMS.¶
Figure 3 depicts the data that flows between different roles for the remote attestation of networks composed of several devices.¶
************ ************ *****************
* Endorser * * Verifier * * Relying Party *
************ * Owner * * Owner *
| ************ *****************
| | |
|Endorsements |Appraisal |Appraisal
| |Policy for |Policy for
| |Evidence |Attestation
| | |Result
v v |
.-----------------------------. |
| Verifier |------------. |
'-----------------------------' | |
^ | ^ Attestation| |
| | | Results| |
| Endorsements| | | |
Evidence| & Appraisal | |Attestation v v
of | Policy for | |Results of .-----------------.
Leader | Evidence of | |Subordinate | Relying Party |
Device | Subordinate | |Devices '-----------------'
| Devices | |
.-------|---------------|-|--------------------------------------.
| | v | |
| .-----------------------------. .-----------------. |
| | .-----------. | | Subordinate | |
| | | Attester | | .------| Device A | |
| | | Component | | | '-----------------' |
| | '-----------' | | |
| | .-----------. |<--+ Evidence of |
| | | Verifier | | | Subordinate Devices |
| | | Component | | | |
| | Attester '-----------' | | .-----------------. |
| | (Leader Device) | '------| Subordinate | |
| '-----------------------------' | Device B | |
| '-----------------' |
| Composite Network |
'----------------------------------------------------------------'
This data flow is similar with the previous one. But for this composite network, the leader device is the Attester. The leader device creates the Evidence of itself and conveys it to the Verifier. The leader device also gets the Endorsements and Evidence Appraisal Policy of the subordinate devices from the Verifier, uses that information to verify the trustworthiness of the subordinate devices and reports the corresponding verification results to the Verifier.¶
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.¶
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
+-------------+ +-------------+
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 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.¶
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.¶
+-------------+
| | Compare Evidence
| Verifier | against Appraisal Policy
| |
+-------------+
^ |
Evidence| |Attestation
| | Result
| v
+-------------+ +-------------+
| |-------------->| | Compare Attestation
| Attester | Evidence | Relying | Result against
| | | Party | Appraisal Policy
+-------------+ +-------------+
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.¶
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.¶
Figure 6 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 5, 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
+-------------+ +-------------+
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.¶
<this section can include Two Types of Environments content from draft-birkholz-rats-architecture but can we find a better name? also this could be a subsection of something else?>¶
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.¶
<this section can include content from Serialization Formats and Conceptual Messages sections from draft-thaler-rats-architecture, and Role Messages content from draft-birkholz-rats-architecture>¶
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 |
'--------------' `-------------------'
<this section can include some high-level content from draft-birkholz-rats-reference-interaction-model>¶
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.¶
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.¶
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.¶
This document does not require any actions by IANA.¶
Special thanks go to David Wooten, Joerg Borchert, Hannes Tschofenig, Laurence Lundblade, Diego Lopez, Jessica Fitzgerald-McKay, Frank Xia, and Nancy Cam-Winget.¶
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.¶