draft-ietf-rats-architecture-05.txt   draft-ietf-rats-architecture-06.txt 
RATS Working Group H. Birkholz RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT Internet-Draft Fraunhofer SIT
Intended status: Informational D. Thaler Intended status: Informational D. Thaler
Expires: 11 January 2021 Microsoft Expires: 5 March 2021 Microsoft
M. Richardson M. Richardson
Sandelman Software Works Sandelman Software Works
N. Smith N. Smith
Intel Intel
W. Pan W. Pan
Huawei Technologies Huawei Technologies
10 July 2020 1 September 2020
Remote Attestation Procedures Architecture Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-05 draft-ietf-rats-architecture-06
Abstract Abstract
In network protocol exchanges, it is often the case that one entity In network protocol exchanges, it is often the case that one entity
(a Relying Party) requires evidence about a remote peer to assess the (a Relying Party) requires evidence about a remote peer to assess the
peer's trustworthiness, and a way to appraise such evidence. The peer's trustworthiness, and a way to appraise such evidence. The
evidence is typically a set of claims about its software and hardware evidence is typically a set of claims about its software and hardware
platform. This document describes an architecture for such remote platform. This document describes an architecture for such remote
attestation procedures (RATS). attestation procedures (RATS).
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 January 2021. This Internet-Draft will expire on 5 March 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Simplified BSD License. provided without warranty as described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 5 3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 5
3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5 3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5
3.2. Confidential Machine Learning (ML) Model Protection . . . 6 3.2. Confidential Machine Learning (ML) Model Protection . . . 6
3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 6 3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 6
3.4. Critical Infrastructure Control . . . . . . . . . . . . . 6 3.4. Critical Infrastructure Control . . . . . . . . . . . . . 7
3.5. Trusted Execution Environment (TEE) Provisioning . . . . 7 3.5. Trusted Execution Environment (TEE) Provisioning . . . . 7
3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7 3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7
3.7. FIDO Biometric Authentication . . . . . . . . . . . . . . 8
4. Architectural Overview . . . . . . . . . . . . . . . . . . . 8 4. Architectural Overview . . . . . . . . . . . . . . . . . . . 8
4.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 9 4.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 10
4.2. Two Types of Environments of an Attester . . . . . . . . 9 4.2. Two Types of Environments of an Attester . . . . . . . . 10
4.3. Layered Attestation Environments . . . . . . . . . . . . 10 4.3. Layered Attestation Environments . . . . . . . . . . . . 11
4.4. Composite Device . . . . . . . . . . . . . . . . . . . . 12 4.4. Composite Device . . . . . . . . . . . . . . . . . . . . 13
5. Topological Models . . . . . . . . . . . . . . . . . . . . . 15 5. Topological Models . . . . . . . . . . . . . . . . . . . . . 16
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 15 5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 16
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 16 5.2. Background-Check Model . . . . . . . . . . . . . . . . . 17
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 17 5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 18
6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 18 6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 19
7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 19 7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Relying Party . . . . . . . . . . . . . . . . . . . . . . 19 7.1. Relying Party . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Attester . . . . . . . . . . . . . . . . . . . . . . . . 20 7.2. Attester . . . . . . . . . . . . . . . . . . . . . . . . 21
7.3. Relying Party Owner . . . . . . . . . . . . . . . . . . . 20 7.3. Relying Party Owner . . . . . . . . . . . . . . . . . . . 21
7.4. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 20 7.4. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 21
7.5. Endorser and Verifier Owner . . . . . . . . . . . . . . . 21 7.5. Endorser and Verifier Owner . . . . . . . . . . . . . . . 22
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 22
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 21 8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 22
8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 21 8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 22
8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 21 8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 23
8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 22 9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 24
9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 23 10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 26
10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 25 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 28
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 27 12. Security Considerations . . . . . . . . . . . . . . . . . . . 28
12. Security Considerations . . . . . . . . . . . . . . . . . . . 27 12.1. Attester and Attestation Key Protection . . . . . . . . 29
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 12.1.1. On-Device Attester and Key Protection . . . . . . . 29
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28 12.1.2. Attestation Key Provisioning Processes . . . . . . . 30
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 29 12.2. Integrity Protection . . . . . . . . . . . . . . . . . . 30
16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 29 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
16.1. Example 1: Timestamp-based Passport Model Example . . . 30 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
16.2. Example 2: Nonce-based Passport Model Example . . . . . 32 15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 32
16.3. Example 3: Timestamp-based Background-Check Model 16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 32
Example . . . . . . . . . . . . . . . . . . . . . . . . 33 16.1. Example 1: Timestamp-based Passport Model Example . . . 33
16.4. Example 4: Nonce-based Background-Check Model Example . 33 16.2. Example 2: Nonce-based Passport Model Example . . . . . 35
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 34 16.3. Example 3: Handle-based Passport Model Example . . . . . 36
17.1. Normative References . . . . . . . . . . . . . . . . . . 34 16.4. Example 4: Timestamp-based Background-Check Model
17.2. Informative References . . . . . . . . . . . . . . . . . 34 Example . . . . . . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35 16.5. Example 5: Nonce-based Background-Check Model Example . 38
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
17.1. Normative References . . . . . . . . . . . . . . . . . . 39
17.2. Informative References . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction 1. Introduction
In Remote Attestation Procedures (RATS), one peer (the "Attester") In Remote Attestation Procedures (RATS), one peer (the "Attester")
produces believable information about itself - Evidence - to enable a produces believable information about itself - Evidence - to enable a
remote peer (the "Relying Party") to decide whether to consider that remote peer (the "Relying Party") to decide whether to consider that
Attester a trustworthy peer or not. RATS are facilitated by an Attester a trustworthy peer or not. RATS are facilitated by an
additional vital party, the Verifier. additional vital party, the Verifier.
The Verifier appraises Evidence via Appraisal Policies and creates The Verifier appraises Evidence via Appraisal Policies and creates
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If the watchdog does not receive regular, and fresh, Attestation If the watchdog does not receive regular, and fresh, Attestation
Results as to the systems' health, then it forces a reboot. Results as to the systems' health, then it forces a reboot.
Attester: The device that is desired to keep from being held hostage Attester: The device that is desired to keep from being held hostage
for a long period of time for a long period of time
Relying Party: A remote server that will securely grant the Attester Relying Party: A remote server that will securely grant the Attester
permission to continue operating (i.e., not reboot) for a period permission to continue operating (i.e., not reboot) for a period
of time of time
3.7. FIDO Biometric Authentication
In the Fast IDentity Online (FIDO) protocol [WebAuthN], [CTAP], the
device in the user's hand authenticates the human user, whether by
biometrics (such as fingerprints), or by PIN and password. FIDO
authentication puts a large amount of trust in the device compared to
typical password authentication because it is the device that
verifies the biometric, PIN and password inputs from the user, not
the server. For the Relying Party to know that the authentication is
trustworthy, the Relying Party needs to know that the Authenticator
part of the device is trustworthy. The FIDO protocol employs remote
attestation for this.
The FIDO protocol supports several remote attestation protocols and a
mechanism by which new ones can be registered and added. Remote
attestation defined by RATS is thus a candidate for use in the FIDO
protocol.
Other biometric authentication protocols such as the Chinese IFAA
standard and WeChat Pay as well as Google Pay make use of attestation
in one form or another.
Attester: Every FIDO Authenticator contains an Attester.
Relying Party: Any web site, mobile application back end or service
that does biometric authentication.
4. Architectural Overview 4. Architectural Overview
Figure 1 depicts the data that flows between different roles, Figure 1 depicts the data that flows between different roles,
independent of protocol or use case. independent of protocol or use case.
************ ************ **************** ************ ************ ****************
* Endorser * * Verifier * * Relying Party* * Endorser * * Verifier * * Relying Party*
************ * Owner * * Owner * ************ * Owner * * Owner *
| ************ **************** | ************ ****************
| | | | | |
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| v v | v v
.----------. .-----------------. .----------. .-----------------.
| Attester | | Relying Party | | Attester | | Relying Party |
'----------' '-----------------' '----------' '-----------------'
Figure 1: Conceptual Data Flow Figure 1: Conceptual Data Flow
An Attester creates Evidence that is conveyed to a Verifier. An Attester creates Evidence that is conveyed to a Verifier.
The Verifier uses the Evidence, and any Endorsements from Endorsers, The Verifier uses the Evidence, and any Endorsements from Endorsers,
by applying an Evidence Appraisal Policy to assess the by applying an Appraisal Policy for Evidence to assess the
trustworthiness of the Attester, and generates Attestation Results trustworthiness of the Attester, and generates Attestation Results
for use by Relying Parties. The Appraisal Policy for Evidence might for use by Relying Parties. The Appraisal Policy for Evidence might
be obtained from an Endorser along with the Endorsements, or might be be obtained from an Endorser along with the Endorsements, and/or
obtained via some other mechanism such as being configured in the might be obtained via some other mechanism such as being configured
Verifier by an administrator. in the Verifier by the Verifier Owner.
The Relying Party uses Attestation Results by applying its own The Relying Party uses Attestation Results by applying its own
Appraisal Policy to make application-specific decisions such as Appraisal Policy to make application-specific decisions such as
authorization decisions. The Appraisal Policy for Attestation authorization decisions. The Appraisal Policy for Attestation
Results might, for example, be configured in the Relying Party by an Results is configured in the Relying Party by the Relying Party
administrator. Owner, and/or is programmed into the Relying Party.
4.1. Appraisal Policies 4.1. Appraisal Policies
The Verifier, when appraising Evidence, or the Relying Party, when The Verifier, when appraising Evidence, or the Relying Party, when
appraising Attestation Results, checks the values of some claims appraising Attestation Results, checks the values of some claims
against constraints specified in its Appraisal Policy. Such against constraints specified in its Appraisal Policy. Such
constraints might involve a comparison for equality against a constraints might involve a comparison for equality against a
reference value, or a check for being in a range bounded by reference 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 values, or membership in a set of reference values, or a check
against values in other claims, or any other test. against values in other claims, or any other test.
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An Attester consists of at least one Attesting Environment and at An Attester consists of at least one Attesting Environment and at
least one Target Environment. In some implementations, the Attesting least one Target Environment. In some implementations, the Attesting
and Target Environments might be combined. Other implementations and Target Environments might be combined. Other implementations
might have multiple Attesting and Target Environments, such as in the might have multiple Attesting and Target Environments, such as in the
examples described in more detail in Section 4.3 and Section 4.4. examples described in more detail in Section 4.3 and Section 4.4.
Other examples may exist, and the examples discussed could even be Other examples may exist, and the examples discussed could even be
combined into even more complex implementations. combined into even more complex implementations.
Claims are collected from Target Environments, as shown in Figure 2. Claims are collected from Target Environments, as shown in Figure 2.
That is, Attesting Environments collect the raw values and the That is, Attesting Environments collect the values and the
information to be represented in claims, such as by doing some information to be represented in Claims, by reading system registers
measurement of a Target Environment's code, memory, and/or registers. and variables, calling into subsystems, taking measurements on code
Attesting Environments then format the claims appropriately, and or memory and so on of the Target Environment. Attesting
typically use key material and cryptographic functions, such as Environments then format the claims appropriately, and typically use
signing or cipher algorithms, to create Evidence. Places that key material and cryptographic functions, such as signing or cipher
Attesting Environments can exist include Trusted Execution algorithms, to create Evidence. There is no limit to or requirement
Environments (TEE), embedded Secure Elements (eSE), and BIOS on the places that an Attesting Environment can exist, but they
firmware. An execution environment may not, by default, be capable typically are in Trusted Execution Environments (TEE), embedded
of claims collection for a given Target Environment. Execution Secure Elements (eSE), and BIOS firmware. An execution environment
environments that are designed to be capable of claims collection are may not, by default, be capable of claims collection for a given
referred to in this document as Attesting Environments. Target Environment. Execution environments that are designed to be
capable of claims collection are referred to in this document as
Attesting Environments.
.--------------------------------. .--------------------------------.
| | | |
| Verifier | | Verifier |
| | | |
'--------------------------------' '--------------------------------'
^ ^
| |
.-------------------------|----------. .-------------------------|----------.
| | | | | |
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Assessment may not wish to let every healthy laptop from the same Assessment may not wish to let every healthy laptop from the same
manufacturer onto the network, but instead only want to let devices manufacturer onto the network, but instead only want to let devices
that it legally owns onto the network. Thus, an Endorsement may be that it legally owns onto the network. Thus, an Endorsement may be
helpful information in authenticating information about a device, but helpful information in authenticating information about a device, but
is not necessarily sufficient to authorize access to resources which is not necessarily sufficient to authorize access to resources which
may need device-specific information such as a public key for the may need device-specific information such as a public key for the
device or component or user on the device. device or component or user on the device.
8.3. Attestation Results 8.3. Attestation Results
Attestation Results may indicate compliance or non-compliance with a Attestation Results are the input used by the Relying Party to decide
Verifier's Appraisal Policy. A result that indicates non-compliance the extent to which it will trust a particular Attester, and allow it
can be used by an Attester (in the passport model) or a Relying Party to access some data or perform some operation. Attestation Results
(in the background-check model) to indicate that the Attester should may be a Boolean simply indicating compliance or non-compliance with
not be treated as authorized and may be in need of remediation. In a Verifier's Appraisal Policy, or a rich set of Claims about the
some cases, it may even indicate that the Evidence itself cannot be Attester, against which the Relying Party applies its Appraisal
authenticated as being correct. Policy for Attestation Results.
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 An Attestation Result that indicates compliance can be used by a
Relying Party to make authorization decisions based on the Relying Relying Party to make authorization decisions based on the Relying
Party's Appraisal Policy. The simplest such policy might be to Party's Appraisal Policy. The simplest such policy might be to
simply authorize any party supplying a compliant Attestation Result simply authorize any party supplying a compliant Attestation Result
signed by a trusted Verifier. A more complex policy might also signed by a trusted Verifier. A more complex policy might also
entail comparing information provided in the result against known- entail comparing information provided in the result against known-
good reference values, or applying more complex logic on such good reference values, or applying more complex logic on such
information. information.
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Furthermore, because Evidence might contain sensitive information, Furthermore, because Evidence might contain sensitive information,
Attesters are responsible for only sending such Evidence to trusted Attesters are responsible for only sending such Evidence to trusted
Verifiers. Some Attesters might want a stronger level of assurance Verifiers. Some Attesters might want a stronger level of assurance
of the trustworthiness of a Verifier before sending Evidence to it. of the trustworthiness of a Verifier before sending Evidence to it.
In such cases, an Attester can first act as a Relying Party and ask In such cases, an Attester can first act as a Relying Party and ask
for the Verifier's own Attestation Result, and appraising it just as for the Verifier's own Attestation Result, and appraising it just as
a Relying Party would appraise an Attestation Result for any other a Relying Party would appraise an Attestation Result for any other
purpose. purpose.
12. Security Considerations 12. Security Considerations
12.1. Attester and Attestation Key Protection
Implementers need to pay close attention to the isolation and
protection of the Attester and the factory processes for provisioning
the Attestation Key Material. When either of these are compromised,
the remote attestation becomes worthless because the attacker can
forge Evidence.
Remote attestation applies to use cases with a range of security
requirements, so the protections discussed here range from low to
high security where low security may be only application or process
isolation by the device's operating system and high security involves
specialized hardware to defend against physical attacks on a chip.
12.1.1. On-Device Attester and Key Protection
It is assumed that the Attester is located in an isolated environment
of a device like a process, a dedicated chip a TEE or such that
collects the Claims, formats them and signs them with an Attestation
Key. The Attester must be protected from unauthorized modification to
ensure it behaves correctly. There must also be confidentiality so
that the signing key is not captured and used elsewhere to forge
evidence.
In many cases the user or owner of the device must not be able to
modify or exfiltrate keys from the Attesting Environment of the
Attester. For example the owner or user of a mobile phone or FIDO
authenticator is not trusted. The point of remote attestation is for
the Relying Party to be able to trust the Attester even though they
don't trust the user or owner.
Some of the measures for low level security include process or
application isolation by a high-level operating system, and perhaps
restricting access to root or system privilege. For extremely simple
single-use devices that don't use a protected mode operating system,
like a Bluetooth speaker, the isolation might only be the plastic
housing for the device.
At medium level security, a special restricted operating environment
like a Trusted Execution Environment (TEE) might be used. In this
case, only security-oriented software has access to the Attester and
key material.
For high level security, specialized hardware will likely be used
providing protection against chip decapping attacks, power supply and
clock glitching, faulting injection and RF and power side channel
attacks.
12.1.2. Attestation Key Provisioning Processes
Attestation key provisioning is the process that occurs in the
factory or elsewhere that establishes the signing key material on the
device and the verification key material off the device. Sometimes
this is referred to as "personalization".
One way to provision a key is to first generate it external to the
device and then copy the key onto the device. In this case,
confidentiality of the generator, as well as the path over which the
key is provisioned, is necessary. This can be achieved in a number
of ways.
Confidentiality can be achieved entirely with physical provisioning
facility security involving no encryption at all. For low-security
use cases, this might be simply locking doors and limiting personnel
that can enter the facility. For high-security use cases, this might
involve a special area of the facility accessible only to select
security-trained personnel.
Cryptography can also be used to support confidentiality, but keys
that are used to then provision attestation keys must somehow have
been provisioned securely beforehand (a recursive problem).
In many cases both some physical security and some cryptography will
be necessary and useful to establish confidentiality.
Another way to provision the key material is to generate it on the
device and export the verification key. If public key cryptography
is being used, then only integrity is necessary. Confidentiality is
not necessary.
In all cases, the Attestation Key provisioning process must ensure
that only attestation key material that is generated by a valid
Endorser is established in Attesters and then configured correctly.
For many use cases, this will involve physical security at the
facility, to prevent unauthorized devices from being manufactured
that may be counterfeit or incorrectly configured.
12.2. Integrity Protection
Any solution that conveys information used for security purposes, Any solution that conveys information used for security purposes,
whether such information is in the form of Evidence, Attestation whether such information is in the form of Evidence, Attestation
Results, Endorsements, or Appraisal Policy must support end-to-end Results, Endorsements, or Appraisal Policy must support end-to-end
integrity protection and replay attack prevention, and often also integrity protection and replay attack prevention, and often also
needs to support additional security properties, including: needs to support additional security properties, including:
* end-to-end encryption, * end-to-end encryption,
* denial of service protection, * denial of service protection,
* authentication, * authentication,
* auditing, * auditing,
* fine grained access controls, and * fine grained access controls, and
* logging. * logging.
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Model. Model.
16. Appendix A: Time Considerations 16. Appendix A: Time Considerations
The table below defines a number of relevant events, with an ID that The table below defines a number of relevant events, with an ID that
is used in subsequent diagrams. The times of said events might be is used in subsequent diagrams. The times of said events might be
defined in terms of an absolute clock time such as Coordinated defined in terms of an absolute clock time such as Coordinated
Universal Time, or might be defined relative to some other timestamp Universal Time, or might be defined relative to some other timestamp
or timeticks counter. or timeticks counter.
+====+==============+===============================================+ +====+==============+=============================================+
| ID | Event | Explanation of event | | ID | Event | Explanation of event |
+====+==============+===============================================+ +====+==============+=============================================+
| VG | Value | A value to appear in a Claim was | | VG | Value | A value to appear in a Claim was created. |
| | generation | created. | | | generated | In some cases, a value may have technically |
+----+--------------+-----------------------------------------------+ | | | existed before an Attester became aware of |
| AA | Attester | An Attesting Environment starts to | | | | it but the Attester might have no idea how |
| | awareness | be aware of a new/changed Claim | | | | long it has had that value. In such a |
| | | value. | | | | case, the Value created time is the time at |
+----+--------------+-----------------------------------------------+ | | | which the Claim containing the copy of the |
| HD | Handle | A centrally generated identifier for | | | | value was created. |
| | distribution | time-bound recentness across a | +----+--------------+---------------------------------------------+
| | | domain of devices is successfully | | HD | Handle | A centrally generated identifier for time- |
| | | distributed to Attesters. | | | distribution | bound recentness across a domain of devices |
+----+--------------+-----------------------------------------------+ | | | is successfully distributed to Attesters. |
| NS | Nonce sent | A nonce not predictable to an | +----+--------------+---------------------------------------------+
| | | Attester (recentness & uniqueness) | | NS | Nonce sent | A nonce not predictable to an Attester |
| | | is sent to an Attester. | | | | (recentness & uniqueness) is sent to an |
+----+--------------+-----------------------------------------------+ | | | Attester. |
| NR | Nonce | A nonce is relayed to an Attester by | +----+--------------+---------------------------------------------+
| | relayed | another entity. | | NR | Nonce | A nonce is relayed to an Attester by |
+----+--------------+-----------------------------------------------+ | | relayed | another entity. |
| EG | Evidence | An Attester creates Evidence from | +----+--------------+---------------------------------------------+
| | generation | collected Claims. | | HR | Handle | A handle distributed by a Handle |
+----+--------------+-----------------------------------------------+ | | received | Distributor was received. |
| ER | Evidence | A Relying Party relays Evidence to a | +----+--------------+---------------------------------------------+
| | relayed | Verifier. | | EG | Evidence | An Attester creates Evidence from collected |
+----+--------------+-----------------------------------------------+ | | generation | Claims. |
| RG | Result | A Verifier appraises Evidence and | +----+--------------+---------------------------------------------+
| | generation | generates an Attestation Result. | | ER | Evidence | A Relying Party relays Evidence to a |
+----+--------------+-----------------------------------------------+ | | relayed | Verifier. |
| RR | Result | A Relying Party relays an | +----+--------------+---------------------------------------------+
| | relayed | Attestation Result to a Relying | | RG | Result | A Verifier appraises Evidence and generates |
| | | Party. | | | generation | an Attestation Result. |
+----+--------------+-----------------------------------------------+ +----+--------------+---------------------------------------------+
| RA | Result | The Relying Party appraises | | RR | Result | A Relying Party relays an Attestation |
| | appraised | Attestation Results. | | | relayed | Result to a Relying Party. |
+----+--------------+-----------------------------------------------+ +----+--------------+---------------------------------------------+
| OP | Operation | The Relying Party performs some | | RA | Result | The Relying Party appraises Attestation |
| | performed | operation requested by the Attester. | | | appraised | Results. |
| | | For example, acting upon some | +----+--------------+---------------------------------------------+
| | | message just received across a | | OP | Operation | The Relying Party performs some operation |
| | | session created earlier at time(RA). | | | performed | requested by the Attester. For example, |
+----+--------------+-----------------------------------------------+ | | | acting upon some message just received |
| RX | Result | An Attestation Result should no | | | | across a session created earlier at |
| | expiry | longer be accepted, according to the | | | | time(RA). |
| | | Verifier that generated it. | +----+--------------+---------------------------------------------+
+----+--------------+-----------------------------------------------+ | RX | Result | An Attestation Result should no longer be |
| | expiry | accepted, according to the Verifier that |
| | | generated it. |
+----+--------------+---------------------------------------------+
Table 1 Table 1
Using the table above, a number of hypothetical examples of how a Using the table above, a number of hypothetical examples of how a
solution might be built are illustrated below. a solution might be solution might be built are illustrated below. a solution might be
built. This list is not intended to be complete, but is just built. This list is not intended to be complete, but is just
representative enough to highlight various timing considerations. representative enough to highlight various timing considerations.
All times are relative to the local clocks, indicated by an "a"
(Attester), "v" (Verifier), or "r" (Relying Party) suffix.
How and if clocks are synchronized depends upon the model.
16.1. Example 1: Timestamp-based Passport Model Example 16.1. Example 1: Timestamp-based Passport Model Example
The following example illustrates a hypothetical Passport Model The following example illustrates a hypothetical Passport Model
solution that uses timestamps and requires roughly synchronized solution that uses timestamps and requires roughly synchronized
clocks between the Attester, Verifier, and Relying Party, which clocks between the Attester, Verifier, and Relying Party, which
depends on using a secure clock synchronization mechanism. depends on using a secure clock synchronization mechanism. As a
result, the receiver of a conceptual message containing a timestamp
can directly compare it to its own clock and timestamps.
.----------. .----------. .---------------. .----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party | | Attester | | Verifier | | Relying Party |
'----------' '----------' '---------------' '----------' '----------' '---------------'
time(VG) | | time(VG_a) | |
| | | | | |
~ ~ ~ ~ ~ ~
| | | | | |
time(EG) | | time(EG_a) | |
|------Evidence{time(EG)}-------->| | |------Evidence{time(EG_a)}------>| |
| time(RG) | | time(RG_v) |
|<-----Attestation Result---------| | |<-----Attestation Result---------| |
| {time(RG),time(RX)} | | | {time(RG_v),time(RX_v)} | |
~ ~ ~ ~
| | | |
|------Attestation Result{time(RG),time(RX)}-->time(RA) |----Attestation Result{time(RG_v),time(RX_v)}-->time(RA_r)
| | | |
~ ~ ~ ~
| | | |
| time(OP) | time(OP_r)
| | | |
The Verifier can check whether the Evidence is fresh when appraising The Verifier can check whether the Evidence is fresh when appraising
it at time(RG) by checking "time(RG) - time(EG) < Threshold", where it at time(RG_v) by checking "time(RG_v) - time(EG_a) < Threshold",
the Verifier's threshold is large enough to account for the maximum where the Verifier's threshold is large enough to account for the
permitted clock skew between the Verifier and the Attester. maximum permitted clock skew between the Verifier and the Attester.
If time(VG) is also included in the Evidence along with the claim If time(VG_a) is also included in the Evidence along with the claim
value generated at that time, and the Verifier decides that it can value generated at that time, and the Verifier decides that it can
trust the time(VG) value, the Verifier can also determine whether the trust the time(VG_a) value, the Verifier can also determine whether
claim value is recent by checking "time(RG) - time(VG) < Threshold", the claim value is recent by checking "time(RG_v) - time(VG_a) <
again where the threshold is large enough to account for the maximum Threshold", again where the threshold is large enough to account for
permitted clock skew between the Verifier and the Attester. the maximum permitted clock skew between the Verifier and the
Attester.
The Relying Party can check whether the Attestation Result is fresh The Relying Party can check whether the Attestation Result is fresh
when appraising it at time(RA) by checking "time(RA) - time(RG) < when appraising it at time(RA_r) by checking "time(RA_r) - time(RG_v)
Threshold", where the Relying Party's threshold is large enough to < Threshold", where the Relying Party's threshold is large enough to
account for the maximum permitted clock skew between the Relying account for the maximum permitted clock skew between the Relying
Party and the Verifier. The result might then be used for some time Party and the Verifier. The result might then be used for some time
(e.g., throughout the lifetime of a connection established at (e.g., throughout the lifetime of a connection established at
time(RA)). The Relying Party must be careful, however, to not allow time(RA_r)). The Relying Party must be careful, however, to not
continued use beyond the period for which it deems the Attestation allow continued use beyond the period for which it deems the
Result to remain fresh enough. Thus, it might allow use (at Attestation Result to remain fresh enough. Thus, it might allow use
time(OP)) as long as "time(OP) - time(RG) < Threshold". However, if (at time(OP_r)) as long as "time(OP_r) - time(RG_v) < Threshold".
the Attestation Result contains an expiry time time(RX) then it could However, if the Attestation Result contains an expiry time time(RX_v)
explicitly check "time(OP) < time(RX)". then it could explicitly check "time(OP_r) < time(RX_v)".
16.2. Example 2: Nonce-based Passport Model Example 16.2. Example 2: Nonce-based Passport Model Example
The following example illustrates a hypothetical Passport Model The following example illustrates a hypothetical Passport Model
solution that uses nonces and thus does not require that any clocks solution that uses nonces and thus does not require that any clocks
are synchronized. are synchronized.
As a result, the receiver of a conceptual message containing a
timestamp cannot directly compare it to its own clock or timestamps.
Thus we use a suffix ("a" for Attester, "v" for Verifier, and "r" for
Relying Party) on the IDs below indicating which clock generated
them, since times from different clocks cannot be compared. Only the
delta between two events from the sender can be used by the receiver.
.----------. .----------. .---------------. .----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party | | Attester | | Verifier | | Relying Party |
'----------' '----------' '---------------' '----------' '----------' '---------------'
time(VG) | | time(VG_a) | |
| | | | | |
~ ~ ~ ~ ~ ~
| | | | | |
|<---Nonce1--------------------time(NS) | |<--Nonce1---------------------time(NS_v) |
time(EG) | | time(EG_a) | |
|----Evidence-------------------->| | |---Evidence--------------------->| |
| {Nonce1, time(EG)-time(VG)} | | | {Nonce1, time(EG_a)-time(VG_a)} | |
| time(RG) | | time(RG_v) |
|<---Attestation Result-----------| | |<--Attestation Result------------| |
| {time(RX)-time(RG)} | | | {time(RX_v)-time(RG_v)} | |
~ ~ ~ ~
| | | |
|<---Nonce2------------------------------------time(NS') |<--Nonce2-------------------------------------time(NS_r)
time(RR) time(RRa)
|----Attestation Result{time(RX)-time(RG)}---->time(RA) |---Attestation Result{time(RX_v)-time(RG_v)}->time(RA_r)
| Nonce2, time(RR)-time(EG) | | Nonce2, time(RR_a)-time(EG_a) |
~ ~ ~ ~
| | | |
| time(OP) | time(OP_r)
In this example solution, the Verifier can check whether the Evidence In this example solution, the Verifier can check whether the Evidence
is fresh at time(RG) by verifying that "time(RG) - time(NS) < is fresh at "time(RG_v)" by verifying that "time(RG_v)-time(NS_v) <
Threshold". Threshold".
The Verifier cannot, however, simply rely on a Nonce to determine The Verifier cannot, however, simply rely on a Nonce to determine
whether the value of a claim is recent, since the claim value might 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. have been generated long before the nonce was sent by the Verifier.
However, if the Verifier decides that the Attester can be trusted to However, if the Verifier decides that the Attester can be trusted to
correctly provide the delta time(EG)-time(VG), then it can determine correctly provide the delta "time(EG_a)-time(VG_a)", then it can
recency by checking "time(RG)-time(NS) + time(EG)-time(VG) < determine recency by checking "time(RG_v)-time(NS_v) + time(EG_a)-
Threshold". time(VG_a) < Threshold".
Similarly if, based on an Attestation Result from a Verifier it Similarly if, based on an Attestation Result from a Verifier it
trusts, the Relying Party decides that the Attester can be trusted to trusts, the Relying Party decides that the Attester can be trusted to
correctly provide time deltas, then it can determine whether the correctly provide time deltas, then it can determine whether the
Attestation Result is fresh by checking "time(OP) - time(NS') + Attestation Result is fresh by checking "time(OP_r)-time(NS_r) +
time(RR)-time(EG) < Threshold". Although the Nonce2 and time(RR)- time(RR_a)-time(EG_a) < Threshold". Although the Nonce2 and
time(EG) values cannot be inside the Attestation Result, they might "time(RR_a)-time(EG_a)" values cannot be inside the Attestation
be signed by the Attester such that the Attestation Result vouches Result, they might be signed by the Attester such that the
for the Attester's signing capability. Attestation Result vouches for the Attester's signing capability.
The Relying Party must still be careful, however, to not allow The Relying Party must still be careful, however, to not allow
continued use beyond the period for which it deems the Attestation continued use beyond the period for which it deems the Attestation
Result to remain valid. Thus, if the Attestation Result sends a Result to remain valid. Thus, if the Attestation Result sends a
validity lifetime in terms of time(RX)-time(RG), then the Relying validity lifetime in terms of "time(RX_v)-time(RG_v)", then the
Party can check "time(OP) - time(NS') < time(RX)-time(RG)". Relying Party can check "time(OP_r)-time(NS_r) < time(RX_v)-
time(RG_v)".
16.3. Example 3: Timestamp-based Background-Check Model Example 16.3. Example 3: Handle-based Passport Model Example
Handles are a third option to establish time-keeping next to nonces
or timestamps. Handles are opaque data intended to be available to
all RATS roles that interact with each other, such as the Attester or
Verifier, in specified intervals. To enable this availability,
handles are distributed centrally by the Handle Distributor role over
the network. As any other role, the Handle Distributor role can be
taken on by a dedicated entity or collapsed with other roles, such as
a Verifier. The use of handles can compensate for a lack of clocks
or other sources of time on entities taking on RATS roles. The only
entity that requires access to a source of time is the entity taking
on the role of Handle Distributor.
Handles are different from nonces as they can be used more than once
and can be used by more than one entity at the same time. Handles
are different from timestamps as they do not have to convey
information about a point in time, but their reception creates that
information. The reception of a handle is similar to the event that
increments a relative tickcounter. Receipt of a new handle
invalidates a previously received handle.
In this example, Evidence generation based on received handles always
uses the current (most recent) handle. As handles are distributed
over the network, all involved entities receive a fresh handle at
roughly the same time. Due to distribution over the network, there
is some jitter with respect to the time the Handle is received,
time(HR), for each involved entity. To compensate for this jitter,
there is a small period of overlap (a specified offset) in which both
a current handle and corresponding former handle are valid in
Evidence appraisal: "validity-duration = time(HR'_v) + offset -
time(HR_v)". The offset is typically based on a network's round trip
time. Analogously, the generation of valid Evidence is only
possible, if the age of the handle used is lower than the validity-
duration: "time(HR_v) - time(EG_a) < validity-duration".
From the point of view of a Verifier, the generation of valid
Evidence is only possible, if the age of the handle used in the
Evidence generation is younger than the duration of the distribution
interval - "(time(HR'_v)-time(HR_v)) - (time(HR_a)-time(EG_a)) <
validity-duration".
Due to the validity-duration of handles, multiple different pieces of
Evidence can be generated based on the same handle. The resulting
granularity (time resolution) of Evidence freshness is typically
lower than the resolution of clock-based tickcounters.
The following example illustrates a hypothetical Background-Check
Model solution that uses handles and requires a trustworthy time
source available to the Handle Distributor role.
.-------------.
.----------. | Handle | .----------. .---------------.
| Attester | | Distributor | | Verifier | | Relying Party |
'----------' '-------------' '----------' '---------------'
time(VG_a) | | |
| | | |
~ ~ ~ ~
| | | |
time(HR_a)<---------+-------------time(HR_v)------>time(HR_r)
| | | |
time(EG_a) | | |
|----Evidence{time(EG_a)}-------->| |
| {Handle1,time(EG_a)-time(VG_a)}| |
| | time(RG_v) |
|<-----Attestation Result---------| |
| {time(RG_v),time(RX_v)} | |
| | |
~ ~ ~
| | |
time(HR_a')<--------'---------------------------->time(HR_r')
| |
time(RR_a) /
|--Attestation Result{time(RX_v)-time(RG_v)}-->time(RA_r)
| {Handle2, time(RR_a)-time(EG_a)} |
~ ~
| |
| time(OP_r)
| |
16.4. Example 4: Timestamp-based Background-Check Model Example
The following example illustrates a hypothetical Background-Check The following example illustrates a hypothetical Background-Check
Model solution that uses timestamps and requires roughly synchronized Model solution that uses timestamps and requires roughly synchronized
clocks between the Attester, Verifier, and Relying Party. clocks between the Attester, Verifier, and Relying Party.
.----------. .---------------. .----------. .----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier | | Attester | | Relying Party | | Verifier |
'----------' '---------------' '----------' '----------' '---------------' '----------'
time(VG) | | time(VG_a) | |
| | | | | |
~ ~ ~ ~ ~ ~
| | | | | |
time(EG) | | time(EG_a) | |
|----Evidence------->| | |----Evidence------->| |
| {time(EG)} time(ER)--Evidence{time(EG)}-->| | {time(EG_a)} time(ER_r)--Evidence{time(EG_a)}->|
| | time(RG) | | time(RG_v)
| time(RA)<-Attestation Result---| | time(RA_r)<-Attestation Result---|
| | {time(RX)} | | | {time(RX_v)} |
~ ~ ~ ~ ~ ~
| | | | | |
| time(OP) | | time(OP_r) |
The time considerations in this example are equivalent to those The time considerations in this example are equivalent to those
discussed under Example 1 above. discussed under Example 1 above.
16.4. Example 4: Nonce-based Background-Check Model Example 16.5. Example 5: Nonce-based Background-Check Model Example
The following example illustrates a hypothetical Background-Check The following example illustrates a hypothetical Background-Check
Model solution that uses nonces and thus does not require that any Model solution that uses nonces and thus does not require that any
clocks are synchronized. In this example solution, a nonce is clocks are synchronized. In this example solution, a nonce is
generated by a Verifier at the request of a Relying Party, when the generated by a Verifier at the request of a Relying Party, when the
Relying Party needs to send one to an Attester. Relying Party needs to send one to an Attester.
.----------. .---------------. .----------. .----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier | | Attester | | Relying Party | | Verifier |
'----------' '---------------' '----------' '----------' '---------------' '----------'
time(VG) | | time(VG_a) | |
| | | | | |
~ ~ ~ ~ ~ ~
| | | | | |
| |<-----Nonce-------------time(NS) | |<-------Nonce-----------time(NS_v)
|<---Nonce-----------time(NR) | |<---Nonce-----------time(NR_r) |
time(EG) | | time(EG_a) | |
|----Evidence{Nonce}--->| | |----Evidence{Nonce}--->| |
| time(ER)--Evidence{Nonce}----->| | time(ER_r)--Evidence{Nonce}--->|
| | time(RG) | | time(RG_v)
| time(RA)<-Attestation Result---| | time(RA_r)<-Attestation Result-|
| | {time(RX)-time(RG)} | | | {time(RX_v)-time(RG_v)} |
~ ~ ~ ~ ~ ~
| | | | | |
| time(OP) | | time(OP_r) |
The Verifier can check whether the Evidence is fresh, and whether a The Verifier can check whether the Evidence is fresh, and whether a
claim value is recent, the same as in Example 2 above. claim value is recent, the same as in Example 2 above.
However, unlike in Example 2, the Relying Party can use the Nonce to However, unlike in Example 2, the Relying Party can use the Nonce to
determine whether the Attestation Result is fresh, by verifying that determine whether the Attestation Result is fresh, by verifying that
"time(OP) - time(NR) < Threshold". "time(OP_r)-time(NR_r) < Threshold".
The Relying Party must still be careful, however, to not allow The Relying Party must still be careful, however, to not allow
continued use beyond the period for which it deems the Attestation continued use beyond the period for which it deems the Attestation
Result to remain valid. Thus, if the Attestation Result sends a Result to remain valid. Thus, if the Attestation Result sends a
validity lifetime in terms of time(RX)-time(RG), then the Relying validity lifetime in terms of "time(RX_v)-time(RG_v)", then the
Party can check "time(OP) - time(ER) < time(RX)-time(RG)". Relying Party can check "time(OP_r)-time(ER_r) < time(RX_v)-
time(RG_v)".
17. References 17. References
17.1. Normative References 17.1. Normative References
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>. <https://www.rfc-editor.org/info/rfc7519>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>. May 2018, <https://www.rfc-editor.org/info/rfc8392>.
17.2. Informative References 17.2. Informative References
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", [CTAP] FIDO Alliance, "Client to Authenticator Protocol", n.d.,
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, <https://fidoalliance.org/specs/fido-v2.0-id-20180227/
<https://www.rfc-editor.org/info/rfc4949>. fido-client-to-authenticator-protocol-v2.0-id-
20180227.html>.
[RFC8322] Field, J., Banghart, S., and D. Waltermire, "Resource-
Oriented Lightweight Information Exchange (ROLIE)",
RFC 8322, DOI 10.17487/RFC8322, February 2018,
<https://www.rfc-editor.org/info/rfc8322>.
[OPCUA] OPC Foundation, "OPC Unified Architecture Specification,
Part 2: Security Model, Release 1.03", OPC 10000-2 , 25
November 2015, <https://opcfoundation.org/developer-tools/
specifications-unified-architecture/part-2-security-
model/>.
[I-D.birkholz-rats-tuda] [I-D.birkholz-rats-tuda]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann, Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", Work in "Time-Based Uni-Directional Attestation", Work in
Progress, Internet-Draft, draft-birkholz-rats-tuda-02, 9 Progress, Internet-Draft, draft-birkholz-rats-tuda-03, 13
March 2020, <http://www.ietf.org/internet-drafts/draft- July 2020, <http://www.ietf.org/internet-drafts/draft-
birkholz-rats-tuda-02.txt>. birkholz-rats-tuda-03.txt>.
[I-D.ietf-teep-architecture] [I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler, Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP) "Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft- Architecture", Work in Progress, Internet-Draft, draft-
ietf-teep-architecture-11, 2 July 2020, ietf-teep-architecture-12, 13 July 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-teep- <http://www.ietf.org/internet-drafts/draft-ietf-teep-
architecture-11.txt>. architecture-12.txt>.
[OPCUA] OPC Foundation, "OPC Unified Architecture Specification,
Part 2: Security Model, Release 1.03", OPC 10000-2 , 25
November 2015, <https://opcfoundation.org/developer-tools/
specifications-unified-architecture/part-2-security-
model/>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[RFC8322] Field, J., Banghart, S., and D. Waltermire, "Resource-
Oriented Lightweight Information Exchange (ROLIE)",
RFC 8322, DOI 10.17487/RFC8322, February 2018,
<https://www.rfc-editor.org/info/rfc8322>.
[TCGarch] Trusted Computing Group, "Trusted Platform Module Library [TCGarch] Trusted Computing Group, "Trusted Platform Module Library
- Part 1: Architecture", 7 July 2020, - Part 1: Architecture", n.d.,
<https://trustedcomputinggroup.org/wp-content/uploads/ <https://trustedcomputinggroup.org/wp-content/uploads/
TCG_TPM2_r1p62_Part1_Architecture_7july2020.pdf>. TCG_TPM2_r1p62_Part1_Architecture_7july2020.pdf>.
[WebAuthN] W3C, "Web Authentication: An API for accessing Public Key
Credentials", n.d., <https://www.w3.org/TR/webauthn-1/>.
Authors' Addresses Authors' Addresses
Henk Birkholz Henk Birkholz
Fraunhofer SIT Fraunhofer SIT
Rheinstrasse 75 Rheinstrasse 75
64295 Darmstadt 64295 Darmstadt
Germany Germany
Email: henk.birkholz@sit.fraunhofer.de Email: henk.birkholz@sit.fraunhofer.de
Dave Thaler Dave Thaler
Microsoft Microsoft
United States of America United States of America
Email: dthaler@microsoft.com Email: dthaler@microsoft.com
Michael Richardson Michael Richardson
Sandelman Software Works Sandelman Software Works
Canada Canada
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