Remote Attestation Procedures Architecture

3.1. Network Endpoint Assessment

Network operators want a trustworthy report that includes identity and version of information of the hardware and software on the machines attached to their network, for purposes such as inventory, audit, anomaly detection, record maintenance and/or trending reports (logging). The network operator may also want a policy by which full access is only granted to devices that meet some definition of hygiene, and so wants to get claims about such information and verify their validity. Remote attestation is desired to prevent vulnerable or compromised devices from getting access to the network and potentially harming others.¶

Typically, solutions start with a specific component (called a "Root of Trust") that provides device identity and protected storage for measurements. The system components perform a series of measurements that may be signed by the Root of Trust, considered as Evidence about the hardware, firmware, BIOS, software, etc. that is running.¶

Attester:

A device desiring access to a network

Relying Party:

A network infrastructure device such as a router, switch, or access point

3.2. Confidential Machine Learning (ML) Model Protection

A device manufacturer wants to protect its intellectual property. This is primarily the ML model it developed and runs in the devices purchased by its customers. The goals for the protection include preventing attackers, potentially the customer themselves, from seeing the details of the model.¶

This typically works by having some protected environment in the device go through a remote attestation with some manufacturer service that can assess its trustworthiness. If remote attestation succeeds, then the manufacturer service releases either the model, or a key to decrypt a model the Attester already has in encrypted form, to the requester.¶

Attester:

A device desiring to run an ML model to do inferencing

Relying Party:

A server or service holding ML models it desires to protect

3.3. Confidential Data Retrieval

This is a generalization of the ML model use case above, where the data can be any highly confidential data, such as health data about customers, payroll data about employees, future business plans, etc. An assessment of system state is made against a set of policies to evaluate the state of a system using attestations for the system requesting data. Attestation is desired to prevent leaking data to compromised devices.¶

Attester:

An entity desiring to retrieve confidential data

Relying Party:

An entity that holds confidential data for retrieval by other entities

3.4. Critical Infrastructure Control

In this use case, potentially dangerous physical equipment (e.g., power grid, traffic control, hazardous chemical processing, etc.) is connected to a network. The organization managing such infrastructure needs to ensure that only authorized code and users can control such processes, and they are protected from malware or other adversaries. When a protocol operation can affect some critical system, the device attached to the critical equipment thus wants some assurance that the requester has not been compromised. As such, remote attestation can be used to only accept commands from requesters that are within policy.¶

Attester:

A device or application wishing to control physical equipment

Relying Party:

A device or application connected to potentially dangerous physical equipment (hazardous chemical processing, traffic control, power grid, etc.)

3.5. Trusted Execution Environment (TEE) Provisioning

A "Trusted Application Manager (TAM)" server is responsible for managing the applications running in the TEE of a client device. To do this, the TAM wants to assess the state of a TEE, or of applications in the TEE, of a client device. The TEE conducts a remote attestation procedure with the TAM, which can then decide whether the TEE is already in compliance with the TAM's latest policy, or if the TAM needs to uninstall, update, or install approved applications in the TEE to bring it back into compliance with the TAM's policy.¶

Attester:

A device with a trusted execution environment capable of running trusted applications that can be updated

Relying Party:

A Trusted Application Manager

3.6. Hardware Watchdog

One significant problem is malware that holds a device hostage and does not allow it to reboot to prevent updates from being applied. This is a significant problem, because it allows a fleet of devices to be held hostage for ransom.¶

In the case, the Relying Party is the watchdog timer in the TPM/secure enclave itself, as described in [TCGarch] section 43.3. The Attestation Results are returned to the device, and provided to the enclave.¶

If the watchdog does not receive regular, and fresh, Attestation Results as to the systems' health, then it forces a reboot.¶

Attester:

The device that is desired to keep from being held hostage for a long period of time

Relying Party:

A remote server that will securely grant the Attester permission to continue operating (i.e., not reboot) for a period of time

4.1. Appraisal Policies

The Verifier, when appraising Evidence, or the Relying Party, when appraising Attestation Results, checks the values of some claims against constraints specified in its Appraisal Policy. Such constraints might involve a comparison for equality against a reference value, or a check for being in a range bounded by reference values, or membership in a set of reference values, or a check against values in other claims, or any other test.¶

Such reference values might be specified as part of the Appraisal Policy itself, or might be obtained from a separate source, such as an Endorsement, and then used by the Appraisal Policy.¶

The actual data format and semantics of any reference values are specific to claims and implementations. This architecture document does not define any general purpose format for them or general means for comparison.¶

4.2. Two Types of Environments of an Attester

An Attester consists of at least one Attesting Environment and at least one Target Environment. In some implementations, the Attesting and Target Environments might be combined. Other implementations might have multiple Attesting and Target Environments, such as in the examples described in more detail in Section 4.3 and Section 4.4. Other examples may exist, and the examples discussed could even be combined into even more complex implementations.¶

Claims are collected from Target Environments, as shown in Figure 2. That is, Attesting Environments collect the raw values and the information to be represented in claims, such as by doing some measurement of a Target Environment's code, memory, and/or registers. Attesting Environments then format the claims appropriately, and typically use key material and cryptographic functions, such as signing or cipher algorithms, to create Evidence. Places that Attesting Environments can exist include Trusted Execution Environments (TEE), embedded Secure Elements (eSE), and BIOS firmware. An execution environment may not, by default, be capable of claims collection for a given Target Environment. Execution environments that are designed to be capable of claims collection are referred to in this document as Attesting Environments.¶

  .--------------------------------.
  |                                |
  |            Verifier            |
  |                                |
  '--------------------------------'
                          ^
                          | 
.-------------------------|----------.
|                         |          |
|   .----------------.    |          |
|   | Target         |    |          |
|   | Environment    |    |          |
|   |                |    | Evidence |
|   '----------------'    |          |
|                   |     |          |
|                   |     |          |
|          Collect  |     |          | 
|           Claims  |     |          |
|                   |     |          |
|                   v     |          |
|                 .-------------.    |
|                 | Attesting   |    |
|                 | Environment |    |
|                 |             |    |
|                 '-------------'    |
|               Attester             |
'------------------------------------'

4.3. Layered Attestation Environments

By definition, the Attester role creates Evidence. An Attester may consist of one or more nested or staged environments, adding complexity to the architectural structure.  The unifying component is the Root of Trust and the nested, staged, or chained attestation Evidence produced.  The nested or chained structure includes Claims, collected by the Attester to aid in the assurance or believability of the attestation Evidence.¶

Figure 3 depicts an example of a device that includes (A) a BIOS stored in read-only memory in this example, (B) an updatable bootloader, and (C) an operating system kernel.¶

    .----------.                    .----------.
    |          |                    |          |
    | Endorser |------------------->| Verifier |
    |          |    Endorsements    |          |
    '----------'  for A, B, and C   '----------'
                                          ^
.------------------------------------.    | 
|                                    |    |
|   .---------------------------.    |    |
|   | Target                    |    |    | Layered
|   | Environment               |    |    | Evidence
|   | C                         |    |    |   for
|   '---------------------------'    |    | B and C
|           Collect |                |    |
|           claims  |                |    |
|   .---------------|-----------.    |    |
|   | Target        v           |    |    |
|   | Environment .-----------. |    |    |
|   | B           | Attesting | |    |    |
|   |             |Environment|-----------'
|   |             |     B     | |    |
|   |             '-----------' |    |
|   |                     ^     |    |
|   '---------------------|-----'    |
|           Collect |     | Evidence |
|           claims  v     |  for B   |
|                 .-----------.      |
|                 | Attesting |      |
|                 |Environment|      |
|                 |     A     |      |
|                 '-----------'      |
|                                    |
'------------------------------------'

Attesting Environment A, the read-only BIOS in this example, has to ensure the integrity of the bootloader (Target Environment B). There are potentially multiple kernels to boot, and the decision is up to the bootloader. Only a bootloader with intact integrity will make an appropriate decision. Therefore, these Claims have to be measured securely. At this stage of the boot-cycle of the device, the Claims collected typically cannot be composed into Evidence.¶

After the boot sequence is started, the BIOS conducts the most important and defining feature of layered attestation, which is that the successfully measured Target Environment B now becomes (or contains) an Attesting Environment for the next layer. This procedure in Layered Attestation is sometimes called "staging". It is important that the new Attesting Environment B not be able to alter any Claims about its own Target Environment B. This can be ensured having those Claims be either signed by Attesting Environment A or stored in an untamperable manner by Attesting Environment A.¶

Continuing with this example, the bootloader's Attesting Environment B is now in charge of collecting Claims about Target Environment C, which in this example is the kernel to be booted. The final Evidence thus contains two sets of Claims: one set about the bootloader as measured and signed by the BIOS, plus a set of Claims about the kernel as measured and signed by the bootloader.¶

This example could be extended further by making the kernel become another Attesting Environment for an application as another Target Environment. This would result in a third set of Claims in the Evidence pertaining to that application.¶

The essence of this example is a cascade of staged environments. Each environment has the responsibility of measuring the next environment before the next environment is started. In general, the number of layers may vary by device or implementation, and an Attesting Environment might even have multiple Target Environments that it measures, rather than only one as shown in Figure 3.¶

4.4. Composite Device

A Composite Device is an entity composed of multiple sub-entities such that its trustworthiness has to be determined by the appraisal of all these sub-entities.¶

Each sub-entity has at least one Attesting Environment collecting the claims from at least one Target Environment, then this sub-entity generates Evidence about its trustworthiness. Therefore each sub-entity can be called an Attester. Among all the Attesters, there may be only some which have the ability to communicate with the Verifier while others do not.¶

For example, a carrier-grade router consists of a chassis and multiple slots. The trustworthiness of the router depends on all its slots' trustworthiness. Each slot has an Attesting Environment such as a TEE collecting the claims of its boot process, after which it generates Evidence from the claims. Among these slots, only a main slot can communicate with the Verifier while other slots cannot. But other slots can communicate with the main slot by the links between them inside the router. So the main slot collects the Evidence of other slots, produces the final Evidence of the whole router and conveys the final Evidence to the Verifier. Therefore the router is a Composite Device, each slot is an Attester, and the main slot is the lead Attester.¶

Another example is a multi-chassis router composed of multiple single carrier-grade routers. The multi-chassis router provides higher throughput by interconnecting multiple routers and can be logically treated as one router for simpler management. A multi-chassis router provides a management point that connects to the Verifier. Other routers are only connected to the main router by the network cables, and therefore they are managed and appraised via this main router's help. So, in this case, the multi-chassis router is the Composite Device, each router is an Attester and the main router is the lead Attester.¶

Figure 4 depicts the conceptual data flow for a Composite Device.¶

                   .-----------------------------.
                   |           Verifier          |
                   '-----------------------------'
                                   ^
                                   |
                                   | Evidence of
                                   | Composite Device
                                   |
.----------------------------------|-------------------------------.
| .--------------------------------|-----.      .------------.     |
| |  Collect             .------------.  |      |            |     |
| |  Claims   .--------->|  Attesting |<--------| Attester B |-.   |
| |           |          |Environment |  |      '------------. |   |
| |  .----------------.  |            |<----------| Attester C |-. |
| |  |     Target     |  |            |  |        '------------' | |
| |  | Environment(s) |  |            |<------------| ...        | |
| |  |                |  '------------'  | Evidence '------------' |
| |  '----------------'                  |    of                   |
| |                                      | Attesters               |
| | lead Attester A                      | (via Internal Links or  |
| '--------------------------------------' Network Connections)    |
|                                                                  |
|                       Composite Device                           |
'------------------------------------------------------------------'

In the Composite Device, each Attester generates its own Evidence by its Attesting Environment(s) collecting the claims from its Target Environment(s). The lead Attester collects the Evidence of all other Attesters and then generates the Evidence of the whole Composite Attester.¶

An entity can take on multiple RATS roles (e.g., Attester, Verifier, Relying Party, etc.) at the same time. The combination of roles can be arbitrary. For example, in this Composite Device scenario, the entity inside the lead Attester can also take on the role of a Verifier, and the outside entity of Verifier can take on the role of a Relying Party. After collecting the Evidence of other Attesters, this inside Verifier uses Endorsements and Appraisal Policies (obtained the same way as any other Verifier) in the verification process to generate Attestation Results. The inside Verifier then conveys the Attestation Results of other Attesters, whether in the same conveyance protocol as the Evidence or not, to the outside Verifier.¶

In this situation, the trust model described in Section 7 is also suitable for this inside Verifier.¶

5.1. 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 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 conveys 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.¶

There are three ways in which the process may fail. First, the Verifier may refuse to issue the Attestation Result due to some error in processing, or some missing input to the Verifier. The second way in which the process may fail is when the Attestation Result is examined by the Relying Party, and based upon the Appraisal Policy, the result does not pass the policy. The third way is when the Verifier is unreachable.¶

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 remote attestation protocol.¶

      +-------------+
      |             | Compare Evidence
      |   Verifier  | against Appraisal Policy
      |             |
      +-------------+
           ^    |
   Evidence|    |Attestation
           |    |  Result
           |    v
      +----------+              +---------+
      |          |------------->|         |Compare Attestation
      | Attester | Attestation  | Relying | Result against
      |          |    Result    |  Party  | Appraisal
      +----------+              +---------+  Policy

5.2. 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.¶

In this model, an Attester conveys 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 appraisal 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
   +------------+               +-------------+

5.3. Combinations

One variation of the background-check model is where the Relying Party and the Verifier are on the same machine, performing both functions together.  In this case, 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.  The same device may need to create Evidence for different Relying Parties and/or different use cases.  For instance, it would provide Evidence to a network infrastructure device to gain access to the network, and to a server holding confidential data to gain access to that data. As such, both models may simultaneously be in use by the same device.¶

Figure 7 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 6, 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
      +----------+               +----------+

7.1. Relying Party

The scope of this document is scenarios for which a Relying Party trusts a Verifier that can appraise 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 a Verifier/Relying Party combination where the Verifier and Relying Party roles are combined. Or, for a stronger level of security, the Relying Party might require that the Verifier first provide information about itself that the Relying Party can use to assess the trustworthiness of the Verifier before accepting its Attestation Results.¶

For example, one explicit way for a Relying Party "A" to establish such trust in a Verifier "B", would be for B to first act as an Attester where A acts as a combined Verifier/Relying Party. If A then accepts B as trustworthy, it can choose to accept B as a Verifier for other Attesters.¶

Similarly, the Relying Party also needs to trust the Relying Party Owner for providing its Appraisal Policy for Attestation Results, and in some scenarios the Relying Party might even require that the Relying Party Owner go through a remote attestation procedure with it before the Relying Party will accept an updated policy. This can be done similarly to how a Relying Party could establish trust in a Verifier as discussed above.¶

7.2. Attester

In some scenarios, Evidence might contain sensitive information such as Personally Identifiable Information. Thus, an Attester must trust entities to which it conveys Evidence, to not reveal sensitive data to unauthorized parties. The Verifier might share this information with other authorized parties, according to rules that it controls. In the background-check model, this Evidence may also be revealed to Relying Party(s).¶

In some cases where Evidence contains sensitive information, an Attester might even require that a Verifier first go through a remote attestation procedure with it before the Attester will send the sensitive Evidence. This can be done by having the Attester first act as a Verifier/Relying Party, and the Verifier act as its own Attester, as discussed above.¶

7.3. Relying Party Owner

The Relying Party Owner might also require that the Relying Party first act as an Attester, providing Evidence that the Owner can appraise, before the Owner would give the Relying Party an updated policy that might contain sensitive information. In such a case, mutual attestation might be needed, in which case typically one side's Evidence must be considered safe to share with an untrusted entity, in order to bootstrap the sequence.¶

7.4. Verifier

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 appraise 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 appraise the trustworthiness of an application component, 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 assurance 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.¶

A conveyance protocol that provides authentication and integrity protection can be used to convey unprotected Evidence, assuming the following properties exists:¶

1. The key material used to authenticate and integrity protect the conveyance channel is trusted by the Verifier to speak for the Attesting Environment(s) that collected claims about the Target Environment(s).¶
2. All unprotected Evidence that is conveyed is supplied exclusively by the Attesting Environment that has the key material that protects the conveyance channel¶
3. The Root of Trust protects both the conveyance channel key material and the Attesting Environment with equivalent strength protections.¶

7.5. Endorser and Verifier Owner

In some scenarios, the Endorser and Verifier Owner may need to trust the Verifier before giving the Endorsement and Appraisal Policy to it. This can be done similarly to how a Relying Party might establish trust in a Verifier as discussed above, and in such a case, mutual attestation might even be needed as discussed in Section 7.3.¶

8.1. Evidence

Evidence is a set of claims about the target environment that reveal operational status, health, configuration or construction that have security relevance. Evidence is evaluated by a Verifier to establish its relevance, compliance, and timeliness. Claims need to be collected in a manner that is reliable. Evidence needs to be securely associated with the target environment so that the Verifier cannot be tricked into accepting claims originating from a different environment (that may be more trustworthy). Evidence also must be protected from man-in-the-middle attackers who may observe, change or misdirect Evidence as it travels from Attester to Verifier. The timeliness of Evidence can be captured using claims that pinpoint the time or interval when changes in operational status, health, and so forth occur.¶

8.2. Endorsements

An Endorsement is a secure statement that some entity (e.g., a manufacturer) vouches for the integrity of the device's signing capability. For example, if the signing capability is in hardware, then an Endorsement might be a manufacturer certificate that signs a public key whose corresponding private key is only known inside the device's hardware. Thus, when Evidence and such an Endorsement are used together, an appraisal procedure can be conducted based on Appraisal Policies that may not be specific to the device instance, but merely specific to the manufacturer providing the Endorsement. For example, an Appraisal Policy might simply check that devices from a given manufacturer have information matching a set of known-good reference values, or an Appraisal Policy might have a set of more complex logic on how to appraise the validity of information.¶

However, while an Appraisal Policy that treats all devices from a given manufacturer the same may be appropriate for some use cases, it would be inappropriate to use such an Appraisal Policy as the sole means of authorization for use cases that wish to constrain which compliant devices are considered authorized for some purpose. For example, an enterprise using remote attestation for Network Endpoint Assessment may not wish to let every healthy laptop from the same manufacturer onto the network, but instead only want to let devices that it legally owns onto the network. Thus, an Endorsement may be helpful information in authenticating information about a device, but is not necessarily sufficient to authorize access to resources which may need device-specific information such as a public key for the device or component or user on the device.¶

8.3. Attestation Results

Attestation Results may indicate compliance or non-compliance with a Verifier's Appraisal Policy. A result that indicates non-compliance can be used by an Attester (in the passport model) or a Relying Party (in the background-check model) to indicate that the Attester should not be treated as authorized and may be in need of remediation. In some cases, it may even indicate that the Evidence itself cannot be authenticated as being correct.¶

An Attestation Result that indicates compliance can be used by a Relying Party to make authorization decisions based on the Relying Party's Appraisal Policy. The simplest such policy might be to simply authorize any party supplying a compliant Attestation Result signed by a trusted Verifier. A more complex policy might also entail comparing information provided in the result against known-good reference values, or applying more complex logic on such information.¶

Thus, Attestation Results often need to include detailed information about the Attester, for use by Relying Parties, much like physical passports and drivers licenses include personal information such as name and date of birth. Unlike Evidence, which is often very device- and vendor-specific, Attestation Results can be vendor-neutral if the Verifier has a way to generate vendor-agnostic information based on the appraisal of vendor-specific information in Evidence. This allows a Relying Party's Appraisal Policy to be simpler, potentially based on standard ways of expressing the information, while still allowing interoperability with heterogeneous devices.¶

Finally, whereas Evidence is signed by the device (or indirectly by a manufacturer, if Endorsements are used), Attestation Results are signed by a Verifier, allowing a Relying Party to only need a trust relationship with one entity, rather than a larger set of entities, for purposes of its Appraisal Policy.¶

16.1. Example 1: Timestamp-based Passport Model Example

The following example illustrates a hypothetical Passport Model solution that uses timestamps and requires roughly synchronized clocks between the Attester, Verifier, and Relying Party, which depends on using a secure clock synchronization mechanism.¶

   .----------.                     .----------.  .---------------.
   | Attester |                     | Verifier |  | Relying Party |
   '----------'                     '----------'  '---------------'
     time(VG)                             |               |
        |                                 |               |
        ~                                 ~               ~
        |                                 |               |
     time(EG)                             |               |
        |------Evidence{time(EG)}-------->|               |
        |                              time(RG)           |
        |<-----Attestation Result---------|               |
        |      {time(RG),time(RX)}        |               |
        ~                                                 ~
        |                                                 |
        |------Attestation Result{time(RG),time(RX)}-->time(RA)
        |                                                 |
        ~                                                 ~
        |                                                 |
        |                                              time(OP)
        |                                                 |

The Verifier can check whether the Evidence is fresh when appraising it at time(RG) by checking time(RG) - time(EG) < Threshold, where the Verifier's threshold is large enough to account for the maximum permitted clock skew between the Verifier and the Attester.¶

If time(VG) is also included in the Evidence along with the claim value generated at that time, and the Verifier decides that it can trust the time(VG) value, the Verifier can also determine whether the claim value is recent by checking time(RG) - time(VG) < Threshold, again where the threshold is large enough to account for the maximum permitted clock skew between the Verifier and the Attester.¶

The Relying Party can check whether the Attestation Result is fresh when appraising it at time(RA) by checking time(RA) - time(RG) < Threshold, where the Relying Party's threshold is large enough to account for the maximum permitted clock skew between the Relying Party and the Verifier. The result might then be used for some time (e.g., throughout the lifetime of a connection established at time(RA)). The Relying Party must be careful, however, to not allow continued use beyond the period for which it deems the Attestation Result to remain fresh enough. Thus, it might allow use (at time(OP)) as long as time(OP) - time(RG) < Threshold. However, if the Attestation Result contains an expiry time time(RX) then it could explicitly check time(OP) < time(RX).¶

16.2. Example 2: Nonce-based Passport Model Example

The following example illustrates a hypothetical Passport Model solution that uses nonces and thus does not require that any clocks are synchronized.¶

   .----------.                     .----------.  .---------------.
   | Attester |                     | Verifier |  | Relying Party |
   '----------'                     '----------'  '---------------'
     time(VG)                             |               |
        |                                 |               |
        ~                                 ~               ~
        |                                 |               |
        |<---Nonce1--------------------time(NS)           |
     time(EG)                             |               |
        |----Evidence-------------------->|               |
        |     {Nonce1, time(EG)-time(VG)} |               |
        |                              time(RG)           |
        |<---Attestation Result-----------|               |
        |     {time(RX)-time(RG)}         |               |
        ~                                                 ~
        |                                                 |
        |<---Nonce2------------------------------------time(NS')
     time(RR)
        |----Attestation Result{time(RX)-time(RG)}---->time(RA)
        |    Nonce2, time(RR)-time(EG)                    |
        ~                                                 ~
        |                                                 |
        |                                              time(OP)

In this example solution, the Verifier can check whether the Evidence is fresh at time(RG) by verifying that time(RG) - time(NS) < Threshold.¶

The Verifier cannot, however, simply rely on a Nonce to determine whether the value of a claim is recent, since the claim value might have been generated long before the nonce was sent by the Verifier. However, if the Verifier decides that the Attester can be trusted to correctly provide the delta time(EG)-time(VG), then it can determine recency by checking time(RG)-time(NS) + time(EG)-time(VG) < Threshold.¶

Similarly if, based on an Attestation Result from a Verifier it trusts, the Relying Party decides that the Attester can be trusted to correctly provide time deltas, then it can determine whether the Attestation Result is fresh by checking time(OP) - time(NS') + time(RR)-time(EG) < Threshold. Although the Nonce2 and time(RR)-time(EG) values cannot be inside the Attestation Result, they might be signed by the Attester such that the Attestation Result vouches for the Attester's signing capability.¶

The Relying Party must still be careful, however, to not allow continued use beyond the period for which it deems the Attestation Result to remain valid. Thus, if the Attestation Result sends a validity lifetime in terms of time(RX)-time(RG), then the Relying Party can check time(OP) - time(NS') < time(RX)-time(RG).¶

16.3. Example 3: Timestamp-based Background-Check Model Example

The following example illustrates a hypothetical Background-Check Model solution that uses timestamps and requires roughly synchronized clocks between the Attester, Verifier, and Relying Party.¶

.----------.         .---------------.              .----------.
| Attester |         | Relying Party |              | Verifier |
'----------'         '---------------'              '----------'
  time(VG)                   |                           |
        |                    |                           |
        ~                    ~                           ~
        |                    |                           |
  time(EG)                   |                           |
        |----Evidence------->|                           |
        |    {time(EG)}   time(ER)--Evidence{time(EG)}-->|
        |                    |                        time(RG)
        |                 time(RA)<-Attestation Result---|
        |                    |        {time(RX)}         |
        ~                    ~                           ~
        |                    |                           |
        |                 time(OP)                       |

The time considerations in this example are equivalent to those discussed under Example 1 above.¶

16.4. Example 4: Nonce-based Background-Check Model Example

The following example illustrates a hypothetical Background-Check Model solution that uses nonces and thus does not require that any clocks are synchronized. In this example solution, a nonce is generated by a Verifier at the request of a Relying Party, when the Relying Party needs to send one to an Attester.¶

.----------.         .---------------.              .----------.
| Attester |         | Relying Party |              | Verifier |
'----------'         '---------------'              '----------'
  time(VG)                   |                           |
     |                       |                           |
     ~                       ~                           ~
     |                       |                           |
     |                       |<-----Nonce-------------time(NS)
     |<---Nonce-----------time(NR)                       |
  time(EG)                   |                           |
     |----Evidence{Nonce}--->|                           |
     |                    time(ER)--Evidence{Nonce}----->|
     |                       |                        time(RG)
     |                    time(RA)<-Attestation Result---|
     |                       |      {time(RX)-time(RG)}  |
     ~                       ~                           ~
     |                       |                           |
     |                    time(OP)                       |

The Verifier can check whether the Evidence is fresh, and whether a claim value is recent, the same as in Example 2 above.¶

However, unlike in Example 2, the Relying Party can use the Nonce to determine whether the Attestation Result is fresh, by verifying that time(OP) - time(NR) < Threshold.¶

The Relying Party must still be careful, however, to not allow continued use beyond the period for which it deems the Attestation Result to remain valid. Thus, if the Attestation Result sends a validity lifetime in terms of time(RX)-time(RG), then the Relying Party can check time(OP) - time(ER) < time(RX)-time(RG).¶