rfc9397.original   rfc9397.txt 
TEEP M. Pei Internet Engineering Task Force (IETF) M. Pei
Internet-Draft Broadcom Request for Comments: 9397 Broadcom
Intended status: Informational H. Tschofenig Category: Informational H. Tschofenig
Expires: 27 April 2023 Arm Limited ISSN: 2070-1721
D. Thaler D. Thaler
Microsoft Microsoft
D. Wheeler D. Wheeler
Amazon Amazon
24 October 2022 July 2023
Trusted Execution Environment Provisioning (TEEP) Architecture Trusted Execution Environment Provisioning (TEEP) Architecture
draft-ietf-teep-architecture-19
Abstract Abstract
A Trusted Execution Environment (TEE) is an environment that enforces A Trusted Execution Environment (TEE) is an environment that enforces
that any code within that environment cannot be tampered with, and the following: any code within the environment cannot be tampered
that any data used by such code cannot be read or tampered with by with, and any data used by such code cannot be read or tampered with
any code outside that environment. This architecture document by any code outside the environment. This architecture document
motivates the design and standardization of a protocol for managing discusses the motivation for designing and standardizing a protocol
the lifecycle of trusted applications running inside such a TEE. for managing the lifecycle of Trusted Applications running inside
such a TEE.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
This Internet-Draft will expire on 27 April 2023. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9397.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3. Use Cases
3.1. Payment . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Payment
3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 8 3.2. Authentication
3.3. Internet of Things . . . . . . . . . . . . . . . . . . . 8 3.3. Internet of Things
3.4. Confidential Cloud Computing . . . . . . . . . . . . . . 9 3.4. Confidential Cloud Computing
4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 9 4. Architecture
4.1. System Components . . . . . . . . . . . . . . . . . . . . 9 4.1. System Components
4.2. Multiple TEEs in a Device . . . . . . . . . . . . . . . . 12 4.2. Multiple TEEs in a Device
4.3. Multiple TAMs and Relationship to TAs . . . . . . . . . . 14 4.3. Multiple TAMs and Relationship to TAs
4.4. Untrusted Apps, Trusted Apps, and Personalization Data . 16 4.4. Untrusted Apps, Trusted Apps, and Personalization Data
4.4.1. Example: Application Delivery Mechanisms in Intel 4.4.1. Example: Application Delivery Mechanisms in Intel SGX
SGX . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.4.2. Example: Application Delivery Mechanisms in Arm 4.4.2. Example: Application Delivery Mechanisms in Arm
TrustZone . . . . . . . . . . . . . . . . . . . . . . 18 TrustZone
4.5. Entity Relations . . . . . . . . . . . . . . . . . . . . 18 4.5. Entity Relations
5. Keys and Certificate Types . . . . . . . . . . . . . . . . . 20 5. Keys and Certificate Types
5.1. Trust Anchors in a TEEP Agent . . . . . . . . . . . . . . 22 5.1. Trust Anchors in a TEEP Agent
5.2. Trust Anchors in a TEE . . . . . . . . . . . . . . . . . 22 5.2. Trust Anchors in a TEE
5.3. Trust Anchors in a TAM . . . . . . . . . . . . . . . . . 22 5.3. Trust Anchors in a TAM
5.4. Scalability . . . . . . . . . . . . . . . . . . . . . . . 23 5.4. Scalability
5.5. Message Security . . . . . . . . . . . . . . . . . . . . 23 5.5. Message Security
6. TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . . 23 6. TEEP Broker
6.1. Role of the TEEP Broker . . . . . . . . . . . . . . . . . 24 6.1. Role of the TEEP Broker
6.2. TEEP Broker Implementation Consideration . . . . . . . . 24 6.2. TEEP Broker Implementation Consideration
6.2.1. TEEP Broker APIs . . . . . . . . . . . . . . . . . . 25 6.2.1. TEEP Broker APIs
6.2.2. TEEP Broker Distribution . . . . . . . . . . . . . . 26 6.2.2. TEEP Broker Distribution
7. Attestation . . . . . . . . . . . . . . . . . . . . . . . . . 26 7. Attestation
8. Algorithm and Attestation Agility . . . . . . . . . . . . . . 29 8. Algorithm and Attestation Agility
9. Security Considerations . . . . . . . . . . . . . . . . . . . 29 9. Security Considerations
9.1. Broker Trust Model . . . . . . . . . . . . . . . . . . . 29 9.1. Broker Trust Model
9.2. Data Protection . . . . . . . . . . . . . . . . . . . . . 30 9.2. Data Protection
9.3. Compromised REE . . . . . . . . . . . . . . . . . . . . . 31 9.3. Compromised REE
9.4. CA Compromise or Expiry of CA Certificate . . . . . . . . 32 9.4. CA Compromise or Expiry of CA Certificate
9.5. Compromised TAM . . . . . . . . . . . . . . . . . . . . . 32 9.5. Compromised TAM
9.6. Malicious TA Removal . . . . . . . . . . . . . . . . . . 33 9.6. Malicious TA Removal
9.7. TEE Certificate Expiry and Renewal . . . . . . . . . . . 34 9.7. TEE Certificate Expiry and Renewal
9.8. Keeping Secrets from the TAM . . . . . . . . . . . . . . 34 9.8. Keeping Secrets from the TAM
9.9. REE Privacy . . . . . . . . . . . . . . . . . . . . . . . 35 9.9. REE Privacy
10. IANA Considerations
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 11. Informative References
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 35 Acknowledgments
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35 Contributors
13. Informative References . . . . . . . . . . . . . . . . . . . 35 Authors' Addresses
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction 1. Introduction
Applications executing in a device are exposed to many different Applications executing in a device are exposed to many different
attacks intended to compromise the execution of the application or attacks intended to compromise the execution of the application or
reveal the data upon which those applications are operating. These reveal the data upon which those applications are operating. These
attacks increase with the number of other applications on the device, attacks increase with the number of other applications on the device,
with such other applications coming from potentially untrustworthy with such other applications coming from potentially untrustworthy
sources. The potential for attacks further increases with the sources. The potential for attacks further increases with the
complexity of features and applications on devices, and the complexity of features and applications on devices and the unintended
unintended interactions among those features and applications. The interactions among those features and applications. The risk of
risk of attacks on a system increases as the sensitivity of the attacks on a system increases as the sensitivity of the applications
applications or data on the device increases. As an example, or data on the device increases. As an example, exposure of emails
exposure of emails from a mail client is likely to be of concern to from a mail client is likely to be of concern to its owner, but a
its owner, but a compromise of a banking application raises even compromise of a banking application raises even greater concerns.
greater concerns.
The Trusted Execution Environment (TEE) concept is designed to let The Trusted Execution Environment (TEE) concept is designed to let
applications execute in a protected environment that enforces that applications execute in a protected environment that enforces that
any code within that environment cannot be tampered with, and that any code within that environment cannot be tampered with and that any
any data used by such code cannot be read or tampered with by any data used by such code cannot be read or tampered with by any code
code outside that environment, including by a commodity operating outside that environment, including by a commodity operating system
system (if present). In a system with multiple TEEs, this also means (if present). In a system with multiple TEEs, this also means that
that code in one TEE cannot be read or tampered with by code in code in one TEE cannot be read or tampered with by code in another
another TEE. TEE.
This separation reduces the possibility of a successful attack on This separation reduces the possibility of a successful attack on
application components and the data contained inside the TEE. application components and the data contained inside the TEE.
Typically, application components are chosen to execute inside a TEE Typically, application components are chosen to execute inside a TEE
because those application components perform security sensitive because those application components perform security-sensitive
operations or operate on sensitive data. An application component operations or operate on sensitive data. An application component
running inside a TEE is commonly referred to (e.g., in [GPTEE], running inside a TEE is commonly referred to (e.g., in [GPTEE] and
[OP-TEE], etc.) as a Trusted Application (TA), while an application [OP-TEE]) as a Trusted Application (TA), while an application running
running outside any TEE, i.e., in the Rich Execution Environment outside any TEE, i.e., in the Rich Execution Environment (REE), is
(REE), is referred to as an Untrusted Application (UA). In the referred to as an Untrusted Application (UA). In the example of a
example of a banking application, code that relates to the banking application, code that relates to the authentication protocol
authentication protocol could reside in a TA while the application could reside in a TA while the application logic including HTTP
logic including HTTP protocol parsing could be contained in the protocol parsing could be contained in the Untrusted Application. In
Untrusted Application. In addition, processing of credit card addition, processing of credit card numbers or account balances could
numbers or account balances could be done in a TA as it is sensitive be done in a TA as it is sensitive data. The precise code split is
data. The precise code split is ultimately a decision of the ultimately a decision of the developer based on the assets the person
developer based on the assets the person wants to protect according wants to protect according to the threat model.
to the threat model.
TEEs are typically used in cases where software or data assets need TEEs are typically used in cases where software or data assets need
to be protected from unauthorized access where threat actors may have to be protected from unauthorized access where threat actors may have
physical or administrative access to a device. This situation arises physical or administrative access to a device. This situation
for example in gaming consoles where anti-cheat protection is a arises, for example, in gaming consoles where anti-cheat protection
concern, devices such as ATMs or IoT devices placed in locations is a concern, devices such as ATMs or IoT devices placed in locations
where attackers might have physical access, cell phones or other where attackers might have physical access, cell phones or other
devices used for mobile payments, and hosted cloud environments. devices used for mobile payments, and hosted cloud environments.
Such environments can be thought of as hybrid devices where one user Such environments can be thought of as hybrid devices where one user
or administrator controls the REE and a different (remote) user or or administrator controls the REE and a different (remote) user or
administrator controls a TEE in the same physical device. administrator controls a TEE in the same physical device. In some
It may also be the case in some constrained devices that there is no constrained devices, it may also be the case that there is no REE
REE (only a TEE) and there may be no local "user" per se, only a (only a TEE) and no local "user" per se, but only a remote TEE
remote TEE administrator. For further discussion of such administrator. For further discussion of such confidential computing
confidential computing use cases and threat model, see [CC-Overview] use cases and threat model, see [CC-Overview] and
and [CC-Technical-Analysis]. [CC-Technical-Analysis].
TEEs use hardware enforcement combined with software protection to TEEs use hardware enforcement combined with software protection to
secure TAs and their data. TEEs typically offer a more limited set secure TAs and their data. TEEs typically offer a more limited set
of services to TAs than is normally available to Untrusted of services to TAs than what is normally available to Untrusted
Applications. Applications.
Not all TEEs are the same, however, and different vendors may have However, not all TEEs are the same. Different vendors may have
different implementations of TEEs with different security properties, different implementations of TEEs with different security properties,
different features, and different control mechanisms to operate on features, and control mechanisms to operate on TAs. Some vendors may
TAs. Some vendors may themselves market multiple different TEEs with market multiple different TEEs themselves, with different properties
different properties attuned to different markets. A device vendor attuned to different markets. A device vendor may integrate one or
may integrate one or more TEEs into their devices depending on market more TEEs into their devices depending on market needs.
needs.
To simplify the life of TA developers interacting with TAs in a TEE, To simplify the life of TA developers interacting with TAs in a TEE,
an interoperable protocol for managing TAs running in different TEEs an interoperable protocol for managing TAs running in different TEEs
of various devices is needed. This software update protocol needs to of various devices is needed. This software update protocol needs to
make sure that compatible trusted and untrusted components (if any) make sure that compatible trusted and Untrusted Components (if any)
of an application are installed on the correct device. In this TEE of an application are installed on the correct device. In this TEE
ecosystem, there often arises a need for an external trusted party to ecosystem, the need often arises for an external trusted party to
verify the identity, claims, and permissions of TA developers, verify the identity, claims, and permissions of TA developers,
devices, and their TEEs. This external trusted party is the Trusted devices, and their TEEs. This external trusted party is the Trusted
Application Manager (TAM). Application Manager (TAM).
The Trusted Execution Environment Provisioning (TEEP) protocol The Trusted Execution Environment Provisioning (TEEP) protocol
addresses the following problems: addresses the following problems:
* An installer of an Untrusted Application that depends on a given * An installer of an Untrusted Application that depends on a given
TA wants to request installation of that TA in the device's TEE so TA wants to request installation of that TA in the device's TEE so
that the installation of Untrusted Application can complete, but that the installation of the Untrusted Application can complete,
the TEE needs to verify whether such a TA is actually authorized but the TEE needs to verify whether such a TA is actually
to run in the TEE and consume potentially scarce TEE resources. authorized to run in the TEE and consume potentially scarce TEE
resources.
* A TA developer providing a TA whose code itself is considered * A TA developer providing a TA whose code itself is considered
confidential wants to determine security-relevant information of a confidential wants to determine security-relevant information of a
device before allowing their TA to be provisioned to the TEE device before allowing their TA to be provisioned to the TEE
within the device. An example is the verification of the type of within the device. An example is the verification of the type of
TEE included in a device and that it is capable of providing the TEE included in a device and its capability of providing the
security protections required. security protections required.
* A TEE in a device needs to determine whether an entity that wants * A TEE in a device needs to determine whether an entity that wants
to manage a TA in the device is authorized to manage TAs in the to manage a TA in the device is authorized to manage TAs in the
TEE, and what TAs the entity is permitted to manage. TEE and what TAs the entity is permitted to manage.
* A Device Administrator wants to determine if a TA exists (is * A Device Administrator wants to determine if a TA exists on a
installed) on a device (in the TEE), and if not, install the TA in device (i.e., is installed in the TEE) and, if not, install the TA
the TEE. in the TEE.
* A Device Administrator wants to check whether a TA in a device's * A Device Administrator wants to check whether a TA in a device's
TEE is the most up-to-date version, and if not, update the TA in TEE is the most up-to-date version, and if not, update the TA in
the TEE. the TEE.
* A Device Administrator wants to remove a TA from a device's TEE if * A Device Administrator wants to remove a TA from a device's TEE if
the TA developer is no longer maintaining that TA, when the TA has the TA developer is no longer maintaining that TA, when the TA has
been revoked, or is not used for other reasons anymore (e.g., due been revoked, or if the TA is not used for other reasons (e.g.,
to an expired subscription). due to an expired subscription).
For TEEs that simply verify and load signed TA's from an untrusted For TEEs that simply verify and load signed TAs from an untrusted
filesystem, classic application distribution protocols can be used filesystem, classic application distribution protocols can be used
without modification. The problems in the bullets above, on the without modification. On the other hand, the problems listed in the
other hand, require a new protocol, i.e., the TEEP protocol. The bullets above require a new protocol -- the TEEP protocol. The TEEP
TEEP protocol is a solution for TEEs that can install and enumerate protocol is a solution for TEEs that can install and enumerate TAs in
TAs in a TEE-secured location where another domain-specific protocol a TEE-secured location where another domain-specific protocol
standard (e.g., [GSMA], [OTRP]) that meets the needs is not already standard (e.g., [GSMA] and [OTRP]) that meets the needs is not
in use. already in use.
2. Terminology 2. Terminology
The following terms are used: The following terms are used:
* App Store: An online location from which Untrusted Applications App Store: An online location from which Untrusted Applications can
can be downloaded. be downloaded.
* Device: A physical piece of hardware that hosts one or more TEEs, Device: A physical piece of hardware that hosts one or more TEEs,
often along with an REE. often along with an REE.
* Device Administrator: An entity that is responsible for Device Administrator: An entity that is responsible for
administration of a device, which could be the Device Owner. A administration of a device, which could be the Device Owner. A
Device Administrator has privileges on the device to install and Device Administrator has privileges on the device to install and
remove Untrusted Applications and TAs, approve or reject Trust remove Untrusted Applications and TAs, approve or reject Trust
Anchors, and approve or reject TA developers, among possibly other Anchors, and approve or reject TA developers, among other possible
privileges on the device. A Device Administrator can manage the privileges on the device. A Device Administrator can manage the
list of allowed TAMs by modifying the list of Trust Anchors on the list of allowed TAMs by modifying the list of Trust Anchors on the
device. Although a Device Administrator may have privileges and device. Although a Device Administrator may have privileges and
device-specific controls to locally administer a device, the device-specific controls to locally administer a device, the
Device Administrator may choose to remotely administer a device Device Administrator may choose to remotely administer a device
through a TAM. through a TAM.
* Device Owner: A device is always owned by someone. In some cases, Device Owner: A device is always owned by someone. In some cases,
it is common for the (primary) device user to also own the device, it is common for the (primary) device user to also own the device,
making the device user/owner also the Device Administrator. In making the device user/owner also the Device Administrator. In
enterprise environments it is more common for the enterprise to enterprise environments, it is more common for the enterprise to
own the device, and for any device user to have no or limited own the device and for any device user to have no or limited
administration rights. In this case, the enterprise appoints a administration rights. In this case, the enterprise appoints a
Device Administrator that is not the device owner. Device Administrator that is not the Device Owner.
* Device User: A human being that uses a device. Many devices have Device User: A human being that uses a device. Many devices have a
a single device user. Some devices have a primary device user single device user. Some devices have a primary device user with
with other human beings as secondary device users (e.g., a parent other human beings as secondary device users (e.g., a parent
allowing children to use their tablet or laptop). Other devices allowing children to use their tablet or laptop). Other devices
are not used by a human being and hence have no device user. are not used by a human being; hence, they have no device user.
* Personalization Data: A set of configuration data that is specific Personalization Data: A set of configuration data that is specific
to the device or user. The Personalization Data may depend on the to the device or user. The Personalization Data may depend on the
type of TEE, a particular TEE instance, the TA, and even the user type of TEE, a particular TEE instance, the TA, and even the user
of the device; an example of Personalization Data might be a of the device. An example of Personalization Data might be a
secret symmetric key used by a TA to communicate with some secret symmetric key used by a TA to communicate with some
service. service.
* Raw Public Key: A raw public key consists of only the algorithm Raw Public Key: A raw public key consists of only the algorithm
identifier (type) of the key and the cryptographic public key identifier (type) of the key and the cryptographic public key
material, such as the SubjectPublicKeyInfo structure of a PKIX material, such as the SubjectPublicKeyInfo structure of a PKIX
certificate [RFC5280]. Other serialization formats that do not certificate [RFC5280]. Other serialization formats that do not
rely on ASN.1 may also be used. rely on ASN.1 may also be used.
* Rich Execution Environment (REE): An environment that is provided Rich Execution Environment (REE): An environment that is provided
and governed by a typical OS (e.g., Linux, Windows, Android, iOS), and governed by a typical OS (e.g., Linux, Windows, Android, iOS),
potentially in conjunction with other supporting operating systems potentially in conjunction with other supporting operating systems
and hypervisors; it is outside of the TEE(s) managed by the TEEP and hypervisors; it is outside of the TEE(s) managed by the TEEP
protocol. This environment and applications running on it are protocol. This environment and applications running on it are
considered untrusted (or more precisely, less trusted than a TEE). considered untrusted (or more precisely, less trusted than a TEE).
* Trust Anchor: As defined in [RFC6024] and [RFC9019], "A trust Trust Anchor: As defined in [RFC6024] and [RFC9019], a Trust Anchor
anchor represents an authoritative entity via a public key and "represents an authoritative entity via a public key and
associated data. The public key is used to verify digital associated data. The public key is used to verify digital
signatures, and the associated data is used to constrain the types signatures, and the associated data is used to constrain the types
of information for which the trust anchor is authoritative." The of information for which the trust anchor is authoritative." The
Trust Anchor may be a certificate, a raw public key or other Trust Anchor may be a certificate, a raw public key, or other
structure, as appropriate. It can be a non-root certificate when structure, as appropriate. It can be a non-root certificate when
it is a certificate. it is a certificate.
* Trust Anchor Store: As defined in [RFC6024], "A trust anchor store Trust Anchor Store: As defined in [RFC6024], a "trust anchor store
is a set of one or more trust anchors stored in a device... A is a set of one or more trust anchors stored in a device... A
device may have more than one trust anchor store, each of which device may have more than one trust anchor store, each of which
may be used by one or more applications." As noted in [RFC9019], may be used by one or more applications." As noted in [RFC9019],
a Trust Anchor Store must resist modification against unauthorized "a trust anchor store must resist modification against
insertion, deletion, and modification. unauthorized insertion, deletion, and modification."
* Trusted Application (TA): An application (or, in some Trusted Application (TA): An application (or, in some
implementations, an application component) that runs in a TEE. implementations, an application component) that runs in a TEE.
* Trusted Application Manager (TAM): An entity that manages Trusted Trusted Application Manager (TAM): An entity that manages Trusted
Applications and other Trusted Components running in TEEs of Applications and other Trusted Components running in TEEs of
various devices. various devices.
* Trusted Component: A set of code and/or data in a TEE managed as a Trusted Component: A set of code and/or data in a TEE managed as a
unit by a Trusted Application Manager. Trusted Applications and unit by a Trusted Application Manager. Trusted Applications and
Personalization Data are thus managed by being included in Trusted Personalization Data are thus managed by being included in Trusted
Components. Trusted OS code or trusted firmware can also be Components. Trusted OS code or trusted firmware can also be
expressed as Trusted Components that a Trusted Component depends expressed as Trusted Components that a Trusted Component depends
on. on.
* Trusted Component Developer: An entity that develops one or more Trusted Component Developer: An entity that develops one or more
Trusted Components. Trusted Components.
* Trusted Component Signer: An entity that signs a Trusted Component Trusted Component Signer: An entity that signs a Trusted Component
with a key that a TEE will trust. The signer might or might not with a key that a TEE will trust. The signer might or might not
be the same entity as the Trusted Component Developer. For be the same entity as the Trusted Component Developer. For
example, a Trusted Component might be signed (or re-signed) by a example, a Trusted Component might be signed (or re-signed) by a
Device Administrator if the TEE will only trust the Device Device Administrator if the TEE will only trust the Device
Administrator. A Trusted Component might also be encrypted, if Administrator. A Trusted Component might also be encrypted if the
the code is considered confidential, for example, when a developer code is considered confidential, for example, when a developer
wants to provide a TA without revealing its code to others. wants to provide a TA without revealing its code to others.
* Trusted Execution Environment (TEE): An execution environment that Trusted Execution Environment (TEE): An execution environment that
enforces that only authorized code can execute within the TEE, and enforces that only authorized code can execute within the TEE and
data used by that code cannot be read or tampered with by code data used by that code cannot be read or tampered with by code
outside the TEE. A TEE also generally has a device unique outside the TEE. A TEE also generally has a unique device
credential that cannot be cloned. There are multiple technologies credential that cannot be cloned. There are multiple technologies
that can be used to implement a TEE, and the level of security that can be used to implement a TEE, and the level of security
achieved varies accordingly. In addition, TEEs typically use an achieved varies accordingly. In addition, TEEs typically use an
isolation mechanism between Trusted Applications to ensure that isolation mechanism between Trusted Applications to ensure that
one TA cannot read, modify or delete the data and code of another one TA cannot read, modify, or delete the data and code of another
TA. TA.
* Untrusted Application (UA): An application running in an REE. An Untrusted Application (UA): An application running in an REE. An
Untrusted Application might depend on one or more TAs. Untrusted Application might depend on one or more TAs.
3. Use Cases 3. Use Cases
3.1. Payment 3.1. Payment
A payment application in a mobile device requires high security and A payment application in a mobile device requires high security and
trust in the hosting device. Payments initiated from a mobile device trust in the hosting device. Payments initiated from a mobile device
can use a Trusted Application to provide strong identification and can use a Trusted Application to provide strong identification and
proof of transaction. proof of transaction.
For a mobile payment application, some biometric identification For a mobile payment application, some biometric identification
information could also be stored in a TEE. The mobile payment information could also be stored in a TEE. The mobile payment
application can use such information for unlocking the device and for application can use such information for unlocking the device and
local identification of the user. local identification of the user.
A trusted user interface (UI) may be used in a mobile device or A trusted user interface (UI) may be used in a mobile device or
point-of-sale device to prevent malicious software from stealing point-of-sale device to prevent malicious software from stealing
sensitive user input data. Such an implementation often relies on a sensitive user input data. Such an implementation often relies on a
TEE for providing access to peripherals, such as PIN input or a TEE for providing access to peripherals, such as PIN input or a
trusted display, so that the REE cannot observe or tamper with the trusted display, so that the REE cannot observe or tamper with the
user input or output. user input or output.
3.2. Authentication 3.2. Authentication
For better security of authentication, a device may store its keys For better security of authentication, a device may store its keys
and cryptographic libraries inside a TEE limiting access to and cryptographic libraries inside a TEE, limiting access to
cryptographic functions via a well-defined interface and thereby cryptographic functions via a well-defined interface and thereby
reducing access to keying material. reducing access to keying material.
3.3. Internet of Things 3.3. Internet of Things
Weak security in Internet of Things (IoT) devices has been posing Weak security in Internet of Things (IoT) devices has been posing
threats to critical infrastructure, i.e., assets that are essential threats to critical infrastructure, i.e., assets that are essential
for the functioning of a society and economy. It is desirable that for the functioning of a society and economy. It is desirable that
IoT devices can prevent malware from manipulating actuators (e.g., IoT devices can prevent malware from manipulating actuators (e.g.,
unlocking a door), or stealing or modifying sensitive data, such as unlocking a door) or stealing or modifying sensitive data, such as
authentication credentials in the device. A TEE can be one of the authentication credentials in the device. A TEE can be one of the
best ways to implement such IoT security functions. For example, best ways to implement such IoT security functions. For example,
[GPTEE] uses the term "trusted peripheral" to refer to such things [GPTEE] uses the term "trusted peripheral" to refer to such things
being accessible only from the TEE, and this concept is used, and being accessible only from the TEE, and this concept is used in some
this concept is used in some GlobalPlatform-compliant devices today. GlobalPlatform-compliant devices today.
3.4. Confidential Cloud Computing 3.4. Confidential Cloud Computing
A tenant can store sensitive data, such as customer details or credit A tenant can store sensitive data, such as customer details or credit
card numbers, in a TEE in a cloud computing server such that only the card numbers, in a TEE in a cloud computing server such that only the
tenant can access the data, preventing the cloud hosting provider tenant can access the data, which prevents the cloud hosting provider
from accessing the data. A tenant can run TAs inside a server TEE from accessing the data. A tenant can run TAs inside a server TEE
for secure operation and enhanced data security. This provides for secure operation and enhanced data security. This provides
benefits not only to tenants with better data security but also to benefits not only to tenants with better data security but also to
cloud hosting providers for reduced liability and increased cloud cloud hosting providers for reduced liability and increased cloud
adoption. adoption.
4. Architecture 4. Architecture
4.1. System Components 4.1. System Components
skipping to change at page 9, line 46 skipping to change at line 418
| +-->|TA-1| |TA-2| | +-------+ | | | +-------+ | | +-->|TA-1| |TA-2| | +-------+ | | | +-------+ |
| | | | | | |<---------| UA-2 |--+ | | | | | | | | | |<---------| UA-2 |--+ | | |
| | | +----+ +----+ | +-------+ | | | Device | | | +----+ +----+ | +-------+ | | | Device
| | +---------------+ | UA-1 | | | | Administrator | | +---------------+ | UA-1 | | | | Administrator
| | | | | | | | | | | | | |
| +--------------------| |-----+ | | | +--------------------| |-----+ | |
| | |----------+ | | | |----------+ |
| +-------+ | | +-------+ |
+---------------------------------------------+ +---------------------------------------------+
Figure 1: Notional Architecture of TEEP Figure 1: Notional Architecture of TEEP
* Trusted Component Signers and Device Administrators utilize the Trusted Component Signer and Device Administrator: Trusted Component
services of a TAM to manage TAs on devices. Trusted Component Signers and Device Administrators utilize the services of a TAM to
Signers do not directly interact with devices. Device manage TAs on devices. Trusted Component Signers do not directly
Administrators may elect to use a TAM for remote administration of interact with devices. Device Administrators may elect to use a
TAs instead of managing each device directly. TAM for remote administration of TAs instead of managing each
device directly.
* Trusted Application Manager (TAM): A TAM is responsible for Trusted Application Manager (TAM): A TAM is responsible for
performing lifecycle management activity on Trusted Components on performing lifecycle management activity on Trusted Components on
behalf of Trusted Component Signers and Device Administrators. behalf of Trusted Component Signers and Device Administrators.
This includes installation and deletion of Trusted Components, and This includes installation and deletion of Trusted Components and
may include, for example, over-the-air updates to keep Trusted may include, for example, over-the-air updates to keep Trusted
Components up-to-date and clean up when Trusted Components should Components up-to-date and clean up when Trusted Components should
be removed. TAMs may provide services that make it easier for be removed. TAMs may provide services that make it easier for
Trusted Component Signers or Device Administrators to use the Trusted Component Signers or Device Administrators to use the
TAM's service to manage multiple devices, although that is not TAM's service to manage multiple devices, although that is not
required of a TAM. required of a TAM.
The TAM performs its management of Trusted Components on the The TAM performs its management of Trusted Components on the
device through interactions with a device's TEEP Broker, which device through interactions with a device's TEEP Broker, which
relays messages between a TAM and a TEEP Agent running inside the relays messages between a TAM and a TEEP Agent running inside the
TEE. TEEP authentication is performed between a TAM and a TEEP TEE. TEEP authentication is performed between a TAM and a TEEP
Agent. Agent.
When the TEEP Agent runs in a user or enterprise device, network When the TEEP Agent runs in a user or enterprise device, network
and application firewalls normally protect user and enterprise and application firewalls normally protect user and enterprise
devices from arbitrary connections from external network entities. devices from arbitrary connections from external network entities.
In such a deployment, a TAM outside that network might not be able In such a deployment, a TAM outside that network might not be able
to directly contact a TEEP Agent, but needs to wait for the TEEP to directly contact a TEEP Agent but needs to wait for the TEEP
Broker to contact it. The architecture in Figure 1 accommodates Broker to contact it. The architecture in Figure 1 accommodates
this case as well as other less restrictive cases by leaving such this case as well as other less restrictive cases by leaving such
details to an appropriate TEEP transport protocol (e.g., details to an appropriate TEEP transport protocol (e.g.,
[I-D.ietf-teep-otrp-over-http], though other transport protocols [TEEP-HTTP], though other transport protocols can be defined under
can be defined under the TEEP protocol for other cases). the TEEP protocol for other cases).
A TAM may be publicly available for use by many Trusted Component A TAM may be publicly available for use by many Trusted Component
Signers, or a TAM may be private, and accessible by only one or a Signers, or a TAM may be private and accessible by only one or a
limited number of Trusted Component Signers. It is expected that limited number of Trusted Component Signers. It is expected that
many enterprises, manufacturers, and network carriers will run many enterprises, manufacturers, and network carriers will run
their own private TAM. their own private TAM.
A Trusted Component Signer or Device Administrator chooses a A Trusted Component Signer or Device Administrator chooses a
particular TAM based on whether the TAM is trusted by a device or particular TAM based on whether the TAM is trusted by a device or
set of devices. The TAM is trusted by a device if the TAM's set of devices. The TAM is trusted by a device if the TAM's
public key is, or chains up to, an authorized Trust Anchor in the public key is, or chains up to, an authorized Trust Anchor in the
device, and conforms with all constraints defined in the Trust device and conforms with all constraints defined in the Trust
Anchor. A Trusted Component Signer or Device Administrator may Anchor. A Trusted Component Signer or Device Administrator may
run their own TAM, but the devices they wish to manage must run their own TAM, but the devices they wish to manage must
include this TAM's public key or certificate, or a certificate it include this TAM's public key or certificate, or a certificate it
chains up to, in the Trust Anchor Store. chains up to, in the Trust Anchor Store.
A Trusted Component Signer or Device Administrator is free to A Trusted Component Signer or Device Administrator is free to
utilize multiple TAMs. This may be required for managing Trusted utilize multiple TAMs. This may be required for managing Trusted
Components on multiple different types of devices from different Components on multiple different types of devices from different
manufacturers, or mobile devices on different network carriers, manufacturers or mobile devices on different network carriers,
since the Trust Anchor Store on these different devices may since the Trust Anchor Store on these different devices may
contain keys for different TAMs. A Device Administrator may be contain keys for different TAMs. To overcome this limitation,
able to add their own TAM's public key or certificate, or a Device Administrator may be able to add their own TAM's public key
certificate it chains up to, to the Trust Anchor Store on all or certificate, or a certificate it chains up to, to the Trust
their devices, overcoming this limitation. Anchor Store on all their devices.
Any entity is free to operate a TAM. For a TAM to be successful, Any entity is free to operate a TAM. For a TAM to be successful,
it must have its public key or certificate installed in a device's it must have its public key or certificate installed in a device's
Trust Anchor Store. A TAM may set up a relationship with device Trust Anchor Store. A TAM may set up a relationship with device
manufacturers or network carriers to have them install the TAM's manufacturers or network carriers to have them install the TAM's
keys in their device's Trust Anchor Store. Alternatively, a TAM keys in their device's Trust Anchor Store. Alternatively, a TAM
may publish its certificate and allow Device Administrators to may publish its certificate and allow Device Administrators to
install the TAM's certificate in their devices as an after-market install the TAM's certificate in their devices as an aftermarket
action. action.
* TEEP Broker: A TEEP Broker is an application component running in TEEP Broker: A TEEP Broker is an application component running in a
a Rich Execution Environment (REE) that enables the message Rich Execution Environment (REE) that enables the message protocol
protocol exchange between a TAM and a TEE in a device. A TEEP exchange between a TAM and a TEE in a device. A TEEP Broker does
Broker does not process messages on behalf of a TEE, but merely is not process messages on behalf of a TEE but is merely responsible
responsible for relaying messages from the TAM to the TEE, and for for relaying messages from the TAM to the TEE and for returning
returning the TEE's responses to the TAM. In devices with no REE the TEE's responses to the TAM. In devices with no REE (e.g., a
(e.g., a microcontroller where all code runs in an environment microcontroller where all code runs in an environment that meets
that meets the definition of a Trusted Execution Environment in the definition of a Trusted Execution Environment in Section 2),
Section 2), the TEEP Broker would be absent and instead the TEEP the TEEP Broker would be absent, and the TEEP protocol transport
protocol transport would be implemented inside the TEE itself. would be implemented inside the TEE itself.
* TEEP Agent: The TEEP Agent is a processing module running inside a TEEP Agent: The TEEP Agent is a processing module running inside a
TEE that receives TAM requests (typically relayed via a TEEP TEE that receives TAM requests (typically relayed via a TEEP
Broker that runs in an REE). A TEEP Agent in the TEE may parse Broker that runs in an REE). A TEEP Agent in the TEE may parse or
requests or forward requests to other processing modules in a TEE, forward requests to other processing modules in a TEE, which is up
which is up to a TEE provider's implementation. A response to a TEE provider's implementation. A response message
message corresponding to a TAM request is sent back to the TAM, corresponding to a TAM request is sent back to the TAM, again
again typically relayed via a TEEP Broker. typically relayed via a TEEP Broker.
* Certification Authority (CA): A CA is an entity that issues Certification Authority (CA): A CA is an entity that issues digital
digital certificates (especially X.509 certificates) and vouches certificates (especially X.509 certificates) and vouches for the
for the binding between the data items in a certificate [RFC4949]. binding between the data items in a certificate [RFC4949].
Certificates are then used for authenticating a device, a TAM, or Certificates are then used for authenticating a device, a TAM, or
a Trusted Component Signer, as discussed in Section 5. The CAs do a Trusted Component Signer, as discussed in Section 5. The CAs do
not need to be the same; different CAs can be chosen by each TAM, not need to be the same; different CAs can be chosen by each TAM,
and different device CAs can be used by different device and different device CAs can be used by different device
manufacturers. manufacturers.
4.2. Multiple TEEs in a Device 4.2. Multiple TEEs in a Device
Some devices might implement multiple TEEs. In these cases, there Some devices might implement multiple TEEs. In these cases, there
might be one shared TEEP Broker that interacts with all the TEEs in might be one shared TEEP Broker that interacts with all the TEEs in
skipping to change at page 12, line 21 skipping to change at line 534
manager within the TEE. As such, there might be multiple TEEP manager within the TEE. As such, there might be multiple TEEP
Brokers in the REE, where each TEEP Broker communicates with one or Brokers in the REE, where each TEEP Broker communicates with one or
more TEEs associated with it. more TEEs associated with it.
It is up to the REE and the Untrusted Applications how they select It is up to the REE and the Untrusted Applications how they select
the correct TEEP Broker. Verification that the correct TA has been the correct TEEP Broker. Verification that the correct TA has been
reached then becomes a matter of properly verifying TA attestations, reached then becomes a matter of properly verifying TA attestations,
which are unforgeable. which are unforgeable.
The multiple TEEP Broker approach is shown in the diagram below. For The multiple TEEP Broker approach is shown in the diagram below. For
brevity, TEEP Broker 2 is shown interacting with only one TAM and brevity, TEEP Broker 2 is shown interacting with only one TAM,
Untrusted Application and only one TEE, but no such limitations are Untrusted Application, and TEE, but no such limitations are intended
intended to be implied in the architecture. to be implied in the architecture.
+-------------------------------------------+ +-------------------------------------------+
| Device | Trusted Component | Device | Trusted Component
| | Signer | | Signer
| +---------------+ | | | +---------------+ | |
| | TEE-1 | | | | | TEE-1 | | |
| | +-------+ | +--------+ | +--------+ | | | +-------+ | +--------+ | +--------+ |
| | | TEEP | | | TEEP |------------->| |<-+ | | | TEEP | | | TEEP |------------->| |<-+
| | | Agent |<----------| Broker | | | | TA | | | Agent |<----------| Broker | | | | TA
| | | 1 | | | 1 |---------+ | | | | | 1 | | | 1 |---------+ | |
skipping to change at page 13, line 42 skipping to change at line 575
| | | | | | | | | | | | | | | |
| | +----+ | | | | | | | | +----+ | | | | | |
| | |TA-3|<---+ | | +---------+ | | | | | |TA-3|<---+ | | +---------+ | | |
| | | | | | | TEEP |<-+ | | | | | | | | | TEEP |<-+ | |
| | +----+ | +---| Broker | | | | | +----+ | +---| Broker | | |
| | | | 2 |--------------+ | | | | 2 |--------------+
| +---------------+ +---------+ | | +---------------+ +---------+ |
| | | |
+-------------------------------------------+ +-------------------------------------------+
Figure 2: Notional Architecture of TEEP with multiple TEEs Figure 2: Notional Architecture of TEEP with multiple TEEs
In the diagram above, TEEP Broker 1 controls interactions with the In the diagram above, TEEP Broker 1 controls interactions with the
TAs in TEE-1, and TEEP Broker 2 controls interactions with the TAs in TAs in TEE-1, and TEEP Broker 2 controls interactions with the TAs in
TEE-2. This presents some challenges for a TAM in completely TEE-2. This presents some challenges for a TAM in completely
managing the device, since a TAM may not interact with all the TEEP managing the device, since a TAM may not interact with all the TEEP
Brokers on a particular platform. In addition, since TEEs may be Brokers on a particular platform. In addition, since TEEs may be
physically separated, with wholly different resources, there may be physically separated, with wholly different resources, there may be
no need for TEEP Brokers to share information on installed Trusted no need for TEEP Brokers to share information on installed Trusted
Components or resource usage. Components or resource usage.
4.3. Multiple TAMs and Relationship to TAs 4.3. Multiple TAMs and Relationship to TAs
As shown in Figure 2, a TEEP Broker provides communication between As shown in Figure 2, a TEEP Broker provides communication between
one or more TEEP Agents and one or more TAMs. The selection of which one or more TEEP Agents and one or more TAMs. The selection of which
TAM to interact with might be made with or without input from an TAM to interact with might be made with or without input from an
Untrusted Application, but is ultimately the decision of a TEEP Untrusted Application but is ultimately the decision of a TEEP Agent.
Agent.
A TEEP Agent is assumed to be able to determine, for any given For any given Trusted Component, a TEEP Agent is assumed to be able
Trusted Component, whether that Trusted Component is installed (or to determine whether that Trusted Component is installed (or
minimally, is running) in a TEE with which the TEEP Agent is minimally, is running) in a TEE with which the TEEP Agent is
associated. associated.
Each Trusted Component is digitally signed, protecting its integrity, Each Trusted Component is digitally signed, protecting its integrity
and linking the Trusted Component back to the Trusted Component and linking the Trusted Component back to the Trusted Component
Signer. The Trusted Component Signer is often the Trusted Component Signer. The Trusted Component Signer is often the Trusted Component
Developer, but in some cases might be another party such as a Device Developer but, in some cases, might be another party such as a Device
Administrator or other party to whom the code has been licensed (in Administrator or other party to whom the code has been licensed (in
which case the same code might be signed by multiple licensees and which case, the same code might be signed by multiple licensees and
distributed as if it were different TAs). distributed as if it were different TAs).
A Trusted Component Signer selects one or more TAMs and communicates A Trusted Component Signer selects one or more TAMs and communicates
the Trusted Component(s) to the TAM. For example, the Trusted the Trusted Component(s) to the TAM. For example, the Trusted
Component Signer might choose TAMs based upon the markets into which Component Signer might choose TAMs based upon the markets into which
the TAM can provide access. There may be TAMs that provide services the TAM can provide access. There may be TAMs that provide services
to specific types of devices, or device operating systems, or to specific types of devices, device operating systems, specific
specific geographical regions or network carriers. A Trusted geographical regions, or network carriers. A Trusted Component
Component Signer may be motivated to utilize multiple TAMs in order Signer may be motivated to utilize multiple TAMs in order to maximize
to maximize market penetration and availability on multiple types of market penetration and availability on multiple types of devices.
devices. This means that the same Trusted Component will often be This means that the same Trusted Component will often be available
available through multiple TAMs. through multiple TAMs.
When the developer of an Untrusted Application that depends on a When the developer of an Untrusted Application that depends on a
Trusted Component publishes the Untrusted Application to an app store Trusted Component publishes the Untrusted Application to an app store
or other app repository, the developer optionally binds the Untrusted or other app repository, the developer optionally binds the Untrusted
Application with a manifest that identifies what TAMs can be Application with a manifest that identifies what TAMs can be
contacted for the Trusted Component. In some situations, a Trusted contacted for the Trusted Component. In some situations, a Trusted
Component may only be available via a single TAM - this is likely the Component may only be available via a single TAM; this is likely the
case for enterprise applications or Trusted Component Signers serving case for enterprise applications or Trusted Component Signers serving
a closed community. For broad public apps, there will likely be a closed community. For broad public apps, there will likely be
multiple TAMs in the Untrusted Application's manifest - one servicing multiple TAMs in the Untrusted Application's manifest, one servicing
one brand of mobile device and another servicing a different one brand of mobile device and another servicing a different
manufacturer, etc. Because different devices and different manufacturer, etc. Because different devices and manufacturers trust
manufacturers trust different TAMs, the manifest can include multiple different TAMs, the manifest can include multiple TAMs that support
TAMs that support the required Trusted Component. the required Trusted Component.
When a TEEP Broker receives a request (see the RequestTA API in When a TEEP Broker receives a request (see the RequestTA API in
Section 6.2.1) from an Untrusted Application to install a Trusted Section 6.2.1) from an Untrusted Application to install a Trusted
Component, a list of TAM URIs may be provided for that Trusted Component, a list of TAM URIs may be provided for that Trusted
Component, and the request is passed to the TEEP Agent. If the TEEP Component, and the request is passed to the TEEP Agent. If the TEEP
Agent decides that the Trusted Component needs to be installed, the Agent decides that the Trusted Component needs to be installed, the
TEEP Agent selects a single TAM URI that is consistent with the list TEEP Agent selects a single TAM URI that is consistent with the list
of trusted TAMs provisioned in the TEEP Agent, invokes the HTTP of trusted TAMs provisioned in the TEEP Agent, invokes the HTTP
transport for TEEP to connect to the TAM URI, and begins a TEEP transport for TEEP to connect to the TAM URI, and begins a TEEP
protocol exchange. When the TEEP Agent subsequently receives the protocol exchange. When the TEEP Agent subsequently receives the
Trusted Component to install and the Trusted Component's manifest Trusted Component to install and the Trusted Component's manifest
indicates dependencies on any other trusted components, each indicates dependencies on any other Trusted Components, each
dependency can include a list of TAM URIs for the relevant dependency can include a list of TAM URIs for the relevant
dependency. If such dependencies exist that are prerequisites to dependency. If such dependencies exist that are prerequisites to
install the Trusted Component, then the TEEP Agent recursively install the Trusted Component, then the TEEP Agent recursively
follows the same procedure for each dependency that needs to be follows the same procedure for each dependency that needs to be
installed or updated, including selecting a TAM URI that is installed or updated, including selecting a TAM URI that is
consistent with the list of trusted TAMs provisioned on the device, consistent with the list of trusted TAMs provisioned on the device
and beginning a TEEP exchange. If multiple TAM URIs are considered and beginning a TEEP exchange. If multiple TAM URIs are considered
trusted, only one needs to be contacted and they can be attempted in trusted, only one needs to be contacted, and they can be attempted in
some order until one responds. some order until one responds.
Separate from the Untrusted Application's manifest, this framework Separate from the Untrusted Application's manifest, this framework
relies on the use of the manifest format in [I-D.ietf-suit-manifest] relies on the use of the manifest format in [SUIT-MANIFEST] for
for expressing how to install a Trusted Component, as well as any expressing how to install a Trusted Component, as well as any
dependencies on other TEE components and versions. That is, dependencies on other TEE components and versions. That is,
dependencies from Trusted Components on other Trusted Components can dependencies from Trusted Components on other Trusted Components can
be expressed in a SUIT manifest, including dependencies on any other be expressed in a Software Update for the Internet of Things (SUIT)
TAs, trusted OS code (if any), or trusted firmware. Installation manifest, including dependencies on any other TAs, trusted OS code
steps can also be expressed in a SUIT manifest. (if any), or trusted firmware. Installation steps can also be
expressed in a SUIT manifest.
For example, TEEs compliant with GlobalPlatform [GPTEE] may have a For example, TEEs compliant with GlobalPlatform [GPTEE] may have a
notion of a "security domain" (which is a grouping of one or more TAs notion of a "security domain" (which is a grouping of one or more TAs
installed on a device, that can share information within such a installed on a device that can share information within such a group)
group) that must be created and into which one or more TAs can then that must be created and into which one or more TAs can then be
be installed. It is thus up to the SUIT manifest to express a installed. It is thus up to the SUIT manifest to express a
dependency on having such a security domain existing or being created dependency on having such a security domain existing or being created
first, as appropriate. first, as appropriate.
Updating a Trusted Component may cause compatibility issues with any Updating a Trusted Component may cause compatibility issues with any
Untrusted Applications or other components that depend on the updated Untrusted Applications or other components that depend on the updated
Trusted Component, just like updating the OS or a shared library Trusted Component, just like updating the OS or a shared library
could impact an Untrusted Application. Thus, an implementation needs could impact an Untrusted Application. Thus, an implementation needs
to take into account such issues. to take such issues into account.
4.4. Untrusted Apps, Trusted Apps, and Personalization Data 4.4. Untrusted Apps, Trusted Apps, and Personalization Data
In TEEP, there is an explicit relationship and dependence between an In TEEP, there is an explicit relationship and dependence between an
Untrusted Application in an REE and one or more TAs in a TEE, as Untrusted Application in an REE and one or more TAs in a TEE, as
shown in Figure 2. For most purposes, an Untrusted Application that shown in Figure 2. For most purposes, an Untrusted Application that
uses one or more TAs in a TEE appears no different from any other uses one or more TAs in a TEE appears no different from any other
Untrusted Application in the REE. However, the way the Untrusted Untrusted Application in the REE. However, the way the Untrusted
Application and its corresponding TAs are packaged, delivered, and Application and its corresponding TAs are packaged, delivered, and
installed on the device can vary. The variations depend on whether installed on the device can vary. The variations depend on whether
the Untrusted Application and TA are bundled together or are provided the Untrusted Application and TA are bundled together or provided
separately, and this has implications to the management of the TAs in separately, and this has implications to the management of the TAs in
a TEE. In addition to the Untrusted Application and TA(s), the TA(s) a TEE. In addition to the Untrusted Application and TA(s), the TA(s)
and/or TEE may also require additional data to personalize the TA to and/or TEE may also require additional data to personalize the TA to
the device or a user. Implementations of the TEEP protocol must the device or a user. Implementations of the TEEP protocol must
support encryption to preserve the confidentiality of such support encryption to preserve the confidentiality of such
Personalization Data, which may potentially contain sensitive data. Personalization Data, which may potentially contain sensitive data.
The encryption is used to ensure that no personalization data is sent The encryption is used to ensure that no personalization data is sent
in the clear. Implementations must also support mechanisms for in the clear. Implementations must also support mechanisms for
integrity protection of such Personalization Data. Other than the integrity protection of such Personalization Data. Other than the
requirement to support confidentiality and integrity protection, the requirement to support confidentiality and integrity protection, the
TEEP architecture places no limitations or requirements on the TEEP architecture places no limitations or requirements on the
Personalization Data. Personalization Data.
There are multiple possible cases for bundling of an Untrusted There are multiple possible cases for bundling of an Untrusted
Application, TA(s), and Personalization Data. Such cases include Application, TA(s), and Personalization Data. Such cases include
(possibly among others): (possibly among others):
1. The Untrusted Application, TA(s), and Personalization Data are 1. The Untrusted Application, TA(s), and Personalization Data are
all bundled together in a single package by a Trusted Component all bundled together in a single package by a Trusted Component
Signer and either provided to the TEEP Broker through the TAM, or Signer and either provided to the TEEP Broker through the TAM or
provided separately (with encrypted Personalization Data), with provided separately (with encrypted Personalization Data), with
key material needed to decrypt and install the Personalization key material needed to decrypt and install the Personalization
Data and TA provided by a TAM. Data and TA provided by a TAM.
2. The Untrusted Application and the TA(s) are bundled together in a 2. The Untrusted Application and the TA(s) are bundled together in a
single package, which a TAM or a publicly accessible app store single package, which a TAM or a publicly accessible app store
maintains, and the Personalization Data is separately provided by maintains, and the Personalization Data is separately provided by
the Personalization Data provider's TAM. the Personalization Data provider's TAM.
3. All components are independent packages. The Untrusted 3. All components are independent packages. The Untrusted
Application is installed through some independent or device- Application is installed through some independent or device-
specific mechanism, and one or more TAMs provide (directly or specific mechanism, and one or more TAMs provide (directly or
indirectly by reference) the TA(s) and Personalization Data. indirectly by reference) the TA(s) and Personalization Data.
4. The TA(s) and Personalization Data are bundled together into a 4. The TA(s) and Personalization Data are bundled together into a
package provided by a TAM, while the Untrusted Application is package provided by a TAM, while the Untrusted Application is
installed through some independent or device-specific mechanism installed through some independent or device-specific mechanism,
such as an app store. such as an app store.
5. Encrypted Personalization Data is bundled into a package 5. Encrypted Personalization Data is bundled into a package
distributed with the Untrusted Application, while the TA(s) and distributed with the Untrusted Application, while the TA(s) and
key material needed to decrypt and install the Personalization key material needed to decrypt and install the Personalization
Data are in a separate package provided by a TAM. Data are in a separate package provided by a TAM.
Personalization Data is encrypted with a key unique to that Personalization Data is encrypted with a key unique to that
specific TEE, as discussed in Section 5. specific TEE, as discussed in Section 5.
The TEEP protocol can treat each TA, any dependencies the TA has, and The TEEP protocol can treat each TA, any dependencies the TA has, and
Personalization Data as separate Trusted Components with separate Personalization Data as separate Trusted Components with separate
installation steps that are expressed in SUIT manifests, and a SUIT installation steps that are expressed in SUIT manifests, and a SUIT
manifest might contain or reference multiple binaries (see manifest might contain or reference multiple binaries (see
[I-D.ietf-suit-manifest] for more details). The TEEP Agent is [SUIT-MANIFEST] for more details). The TEEP Agent is responsible for
responsible for handling any installation steps that need to be handling any installation steps that need to be performed inside the
performed inside the TEE, such as decryption of private TA binaries TEE, such as decryption of private TA binaries or Personalization
or Personalization Data. Data.
In order to better understand these cases, it is helpful to review In order to better understand these cases, it is helpful to review
actual implementations of TEEs and their application delivery actual implementations of TEEs and their application delivery
mechanisms. mechanisms.
4.4.1. Example: Application Delivery Mechanisms in Intel SGX 4.4.1. Example: Application Delivery Mechanisms in Intel SGX
In Intel Software Guard Extensions (SGX), the Untrusted Application In Intel Software Guard Extensions (SGX), the Untrusted Application
and TA are typically bundled into the same package (Case 2). The TA and TA are typically bundled into the same package (Case 2). The TA
exists in the package as a shared library (.so or .dll). The exists in the package as a shared library (.so or .dll). The
Untrusted Application loads the TA into an SGX enclave when the Untrusted Application loads the TA into an SGX enclave when the
Untrusted Application needs the TA. This organization makes it easy Untrusted Application needs the TA. This organization makes it easy
to maintain compatibility between the Untrusted Application and the to maintain compatibility between the Untrusted Application and the
TA, since they are updated together. It is entirely possible to TA, since they are updated together. It is entirely possible to
create an Untrusted Application that loads an external TA into an SGX create an Untrusted Application that loads an external TA into an SGX
enclave, and use that TA (Cases 3-5). In this case, the Untrusted enclave and use that TA (Cases 3-5). In this case, the Untrusted
Application would require a reference to an external file or download Application would require a reference to an external file or download
such a file dynamically, place the contents of the file into memory, such a file dynamically, place the contents of the file into memory,
and load that as a TA. Obviously, such file or downloaded content and load that as a TA. Obviously, such file or downloaded content
must be properly formatted and signed for it to be accepted by the must be properly formatted and signed for it to be accepted by the
SGX TEE. SGX TEE.
In SGX, any Personalization Data is normally loaded into the SGX In SGX, any Personalization Data is normally loaded into the SGX
enclave (the TA) after the TA has started. Although it is possible enclave (the TA) after the TA has started. Although it is possible
with SGX to include the Untrusted Application in an encrypted package with SGX to include the Untrusted Application in an encrypted package
along with Personalization Data (Cases 1 and 5), there are no along with Personalization Data (Cases 1 and 5), there are currently
instances of this known to be in use at this time, since such a no known instances of this in use, since such a construction would
construction would require a special installation program and SGX TA require a special installation program and SGX TA (which might or
(which might or might not be the TEEP Agent itself based on the might not be the TEEP Agent itself based on the implementation) to
implementation) to receive the encrypted package, decrypt it, receive the encrypted package, decrypt it, separate it into the
separate it into the different elements, and then install each one. different elements, and then install each one. This installation is
This installation is complex because the Untrusted Application complex because the Untrusted Application decrypted inside the TEE
decrypted inside the TEE must be passed out of the TEE to an must be passed out of the TEE to an installer in the REE that would
installer in the REE which would install the Untrusted Application. install the Untrusted Application. Finally, the Personalization Data
Finally, the Personalization Data would need to be sent out of the would need to be sent out of the TEE (encrypted in an SGX enclave-to-
TEE (encrypted in an SGX enclave-to-enclave manner) to the REE's enclave manner) to the REE's installation app, which would pass this
installation app, which would pass this data to the installed data to the installed Untrusted Application, which would in turn send
Untrusted Application, which would in turn send this data to the SGX this data to the SGX enclave (TA). This complexity is due to the
enclave (TA). This complexity is due to the fact that each SGX fact that each SGX enclave is separate and does not have direct
enclave is separate and does not have direct communication to other communication to other SGX enclaves.
SGX enclaves.
As long as signed files (TAs and/or Personalization Data) are As long as signed files (TAs and/or Personalization Data) are
installed into an untrusted filesystem and trust is verified by the installed into an untrusted filesystem and trust is verified by the
TEE at load time, classic distribution mechanisms can be used. Some TEE at load time, classic distribution mechanisms can be used.
uses of SGX, however, allow a model where a TA can be dynamically However, some uses of SGX allow a model where a TA can be dynamically
installed into an SGX enclave that provides a runtime platform. The installed into an SGX enclave that provides a runtime platform. The
TEEP protocol can be used in such cases, where the runtime platform TEEP protocol can be used in such cases, where the runtime platform
could include a TEEP Agent. could include a TEEP Agent.
4.4.2. Example: Application Delivery Mechanisms in Arm TrustZone 4.4.2. Example: Application Delivery Mechanisms in Arm TrustZone
In Arm TrustZone [TrustZone] for A-class devices, the Untrusted In Arm TrustZone [TrustZone] for A-class devices, the Untrusted
Application and TA may or may not be bundled together. This differs Application and TA may or may not be bundled together. This differs
from SGX since in TrustZone the TA lifetime is not inherently tied to from SGX since in TrustZone, the TA lifetime is not inherently tied
a specific Untrusted Application process lifetime as occurs in SGX. to a specific Untrusted Application process lifetime as occurs in
A TA is loaded by a trusted OS running in the TEE such as a SGX. A TA is loaded by a trusted OS running in the TEE, such as a
GlobalPlatform [GPTEE] compliant TEE, where the trusted OS is TEE compliant with GlobalPlatform [GPTEE], where the trusted OS is
separate from the OS in the REE. Thus Cases 2-4 are equally separate from the OS in the REE. Thus, Cases 2-4 are equally
applicable. In addition, it is possible for TAs to communicate with applicable. In addition, it is possible for TAs to communicate with
each other without involving any Untrusted Application, and so the each other without involving any Untrusted Application; thus, the
complexity of Cases 1 and 5 are lower than in the SGX example, though complexity of Cases 1 and 5 are lower than in the SGX example, though
still more complex than Cases 2-4. still more complex than Cases 2-4.
A trusted OS running in the TEE (e.g., OP-TEE [OP-TEE]) that supports A trusted OS running in the TEE (e.g., OP-TEE [OP-TEE]) that supports
loading and verifying signed TAs from an untrusted filesystem can, loading and verifying signed TAs from an untrusted filesystem can,
like SGX, use classic file distribution mechanisms. If secure TA like SGX, use classic file distribution mechanisms. If secure TA
storage is used (e.g., a Replay-Protected Memory Block device) on the storage is used (e.g., a Replay-Protected Memory Block device) on the
other hand, the TEEP protocol can be used to manage such storage. other hand, the TEEP protocol can be used to manage such storage.
4.5. Entity Relations 4.5. Entity Relations
skipping to change at page 19, line 27 skipping to change at line 843
| | | | | | | |
(b) TA -- 2b. Supply ----------> | | | (b) TA -- 2b. Supply ----------> | | |
| | | | | | | |
| --- 3. Install ------> | | | --- 3. Install ------> | |
| | | | | | | |
| | 4. Messaging-->| | | | 4. Messaging-->| |
Figure 3: Example Developer Experience Figure 3: Example Developer Experience
Figure 3 shows an example where the same developer builds and signs Figure 3 shows an example where the same developer builds and signs
two applications: (a) an Untrusted Application; (b) a TA that two applications: (a) an Untrusted Application and (b) a TA that
provides some security functions to be run inside a TEE. This provides some security functions to be run inside a TEE. This
example assumes that the developer, the TEE, and the TAM have example assumes that the developer, the TEE, and the TAM have
previously been provisioned with certificates. previously been provisioned with certificates.
At step 1, the developer authors the two applications. At step 1, the developer authors the two applications.
At step 2, the developer uploads the Untrusted Application (2a) to an At step 2, the developer uploads the Untrusted Application (2a) to an
Application Store. In this example, the developer is also the Application Store. In this example, the developer is also the
Trusted Component Signer, and so generates a signed TA. The Trusted Component Signer and thus generates a signed TA. The
developer can then either bundle the signed TA with the Untrusted developer can then either bundle the signed TA with the Untrusted
Application, or the developer can provide a signed Trusted Component Application or provide a signed Trusted Component containing the TA
containing the TA to a TAM that will be managing the TA in various to a TAM that will be managing the TA in various devices.
devices.
At step 3, a user will go to an Application Store to download the At step 3, a user will go to an Application Store to download the
Untrusted Application (where the arrow indicates the direction of Untrusted Application (where the arrow indicates the direction of
data transfer). data transfer).
At step 4, since the Untrusted Application depends on the TA, At step 4, since the Untrusted Application depends on the TA,
installing the Untrusted Application will trigger TA installation via installing the Untrusted Application will trigger TA installation via
communication with a TAM. The TEEP Agent will interact with the TAM communication with a TAM. The TEEP Agent will interact with the TAM
via a TEEP Broker that facilitates communications between the TAM and via a TEEP Broker that facilitates communications between the TAM and
the TEEP Agent. the TEEP Agent.
Some Trusted Component installation implementations might ask for a Some implementations that install Trusted Components might ask for a
user's consent. In other implementations, a Device Administrator user's consent. In other implementations, a Device Administrator
might choose what Untrusted Applications and related Trusted might choose the Untrusted Applications and related Trusted
Components to be installed. A user consent flow is out of scope of Components to be installed. A user consent flow is out of scope of
the TEEP architecture. the TEEP architecture.
The main components of the TEEP protocol consist of a set of standard The main components of the TEEP protocol consist of a set of standard
messages created by a TAM to deliver Trusted Component management messages created by a TAM to deliver Trusted Component management
commands to a device, and device attestation and response messages commands to a device and device attestation and response messages
created by a TEE that responds to a TAM's message. created by a TEE that responds to a TAM's message.
It should be noted that network communication capability is generally It should be noted that network communication capability is generally
not available in TAs in today's TEE-powered devices. Consequently, not available in TAs in today's TEE-powered devices. Consequently,
Trusted Applications generally rely on a broker in the REE to provide Trusted Applications generally rely on a Broker in the REE to provide
access to network functionality in the REE. A broker does not need access to network functionality in the REE. A Broker does not need
to know the actual content of messages to facilitate such access. to know the actual content of messages to facilitate such access.
Similarly, since the TEEP Agent runs inside a TEE, the TEEP Agent Similarly, since the TEEP Agent runs inside a TEE, the TEEP Agent
generally relies on a TEEP Broker in the REE to provide network generally relies on a TEEP Broker in the REE to provide network
access, and relay TAM requests to the TEEP Agent and relay the access, relay TAM requests to the TEEP Agent, and relay the responses
responses back to the TAM. back to the TAM.
5. Keys and Certificate Types 5. Keys and Certificate Types
This architecture leverages the following credentials, which allow This architecture leverages the following credentials, which allow
achieving end-to-end security between a TAM and a TEEP Agent. achieving end-to-end security between a TAM and a TEEP Agent.
Figure 4 summarizes the relationships between various keys and where Table 1 summarizes the relationships between various keys and where
they are stored. Each public/private key identifies a Trusted they are stored. Each public/private key identifies a Trusted
Component Signer, TAM, or TEE, and gets a certificate that chains up Component Signer, TAM, or TEE and gets a certificate that chains up
to some trust anchor. A list of trusted certificates is used to to some Trust Anchor. A list of trusted certificates is used to
check a presented certificate against. check a presented certificate against.
Different CAs can be used for different types of certificates. TEEP Different CAs can be used for different types of certificates. TEEP
messages are always signed, where the signer key is the message messages are always signed, where the signer key is the message
originator's private key, such as that of a TAM or a TEE. In originator's private key, such as that of a TAM or a TEE. In
addition to the keys shown in Figure 4, there may be additional keys addition to the keys shown in Table 1, there may be additional keys
used for attestation or encryption. Refer to the RATS Architecture used for attestation or encryption. Refer to the RATS Architecture
[I-D.ietf-rats-architecture] for more discussion. [RFC9334] for more discussion.
Cardinality & Location of
Location of Private Key Trust Anchor
Purpose Private Key Signs Store
------------------ ----------- ------------- -------------
Authenticating 1 per TEE TEEP responses TAM
TEEP Agent
Authenticating TAM 1 per TAM TEEP requests TEEP Agent
Code Signing 1 per Trusted TA binary TEE +================+===============+===========+==============+
Component | Purpose | Cardinality & | Private | Location of |
Signer | | Location of | Key Signs | Trust Anchor |
| | Private Key | | Store |
+================+===============+===========+==============+
| Authenticating | 1 per TEE | TEEP | TAM |
| TEEP Agent | | responses | |
+----------------+---------------+-----------+--------------+
| Authenticating | 1 per TAM | TEEP | TEEP Agent |
| TAM | | requests | |
+----------------+---------------+-----------+--------------+
| Code Signing | 1 per Trusted | TA binary | TEE |
| | Component | | |
| | Signer | | |
+----------------+---------------+-----------+--------------+
Figure 4: Signature Keys Table 1: Signature Keys
Note that Personalization Data is not included in the table above. Note that Personalization Data is not included in the table above.
The use of Personalization Data is dependent on how TAs are used and The use of Personalization Data is dependent on how TAs are used and
what their security requirements are. what their security requirements are.
TEEP requests from a TAM to a TEEP Agent are signed with the TAM TEEP requests from a TAM to a TEEP Agent are signed with the TAM
private key (for authentication and integrity protection). private key (for authentication and integrity protection).
Personalization Data and TA binaries can be encrypted with a key Personalization Data and TA binaries can be encrypted with a key
unique to that specific TEE, established with a content-encryption unique to that specific TEE. Conversely, TEEP responses from a TEEP
key established with the TEE public key (to provide confidentiality). Agent to a TAM can be signed with the TEE private key.
Conversely, TEEP responses from a TEEP Agent to a TAM can be signed
with the TEE private key.
The TEE key pair and certificate are thus used for authenticating the The TEE key pair and certificate are thus used for authenticating the
TEE to a remote TAM, and for sending private data to the TEE. Often, TEE to a remote TAM and for sending private data to the TEE. Often,
the key pair is burned into the TEE by the TEE manufacturer and the the key pair is burned into the TEE by the TEE manufacturer, and the
key pair and its certificate are valid for the expected lifetime of key pair and its certificate are valid for the expected lifetime of
the TEE. A TAM provider is responsible for configuring the TAM's the TEE. A TAM provider is responsible for configuring the TAM's
Trust Anchor Store with the manufacturer certificates or CAs that are Trust Anchor Store with the manufacturer certificates or CAs that are
used to sign TEE keys. This is discussed further in Section 5.3 used to sign TEE keys. This is discussed further in Section 5.3.
below. Typically, the same key TEE pair is used for both signing and Typically, the same TEE key pair is used for both signing and
encryption, though separate key pairs might also be used in the encryption, though separate key pairs might also be used in the
future, as the joint security of encryption and signature with a future, as the joint security of encryption and signature with a
single key remains to some extent an open question in academic single key remains, to some extent, an open question in academic
cryptography. cryptography.
The TAM key pair and certificate are used for authenticating a TAM to The TAM key pair and certificate are used for authenticating a TAM to
a remote TEE, and for sending private data to the TAM (separate key a remote TEE and for sending private data to the TAM (separate key
pairs for authentication vs. encryption could also be used in the pairs for authentication vs. encryption could also be used in the
future). A TAM provider is responsible for acquiring a certificate future). A TAM provider is responsible for acquiring a certificate
from a CA that is trusted by the TEEs it manages. This is discussed from a CA that is trusted by the TEEs it manages. This is discussed
further in Section 5.1 below. further in Section 5.1.
The Trusted Component Signer key pair and certificate are used to The Trusted Component Signer key pair and certificate are used to
sign Trusted Components that the TEE will consider authorized to sign Trusted Components that the TEE will consider authorized to
execute. TEEs must be configured with the certificates or keys that execute. TEEs must be configured with the certificates or keys that
it considers authorized to sign TAs that it will execute. This is it considers authorized to sign TAs that it will execute. This is
discussed further in Section 5.2 below. discussed further in Section 5.2.
5.1. Trust Anchors in a TEEP Agent 5.1. Trust Anchors in a TEEP Agent
A TEEP Agent's Trust Anchor Store contains a list of Trust Anchors, A TEEP Agent's Trust Anchor Store contains a list of Trust Anchors,
which are typically CA certificates that sign various TAM which are typically CA certificates that sign various TAM
certificates. The list is typically preloaded at manufacturing time, certificates. The list is usually preloaded at manufacturing time
and can be updated using the TEEP protocol if the TEE has some form and can be updated using the TEEP protocol if the TEE has some form
of "Trust Anchor Manager TA" that has Trust Anchors in its of "Trust Anchor Manager TA" that has Trust Anchors in its
configuration data. Thus, Trust Anchors can be updated similarly to configuration data. Thus, Trust Anchors can be updated similarly to
the Personalization Data for any other TA. the Personalization Data for any other TA.
When Trust Anchor update is carried out, it is imperative that any When a Trust Anchor update is carried out, it is imperative that any
update must maintain integrity where only an authentic Trust Anchor update must maintain integrity where only an authentic Trust Anchor
list from a device manufacturer or a Device Administrator is list from a device manufacturer or a Device Administrator is
accepted. Details are out of scope of the architecture and can be accepted. Details are out of scope of this architecture document and
addressed in a protocol document. can be addressed in a protocol document.
Before a TAM can begin operation in the marketplace to support a Before a TAM can begin operation in the marketplace to support a
device with a particular TEE, it must be able to get its raw public device with a particular TEE, it must be able to get its raw public
key, or its certificate, or a certificate it chains up to, listed in key, its certificate, or a certificate it chains up to listed in the
the Trust Anchor Store of the TEEP Agent. Trust Anchor Store of the TEEP Agent.
5.2. Trust Anchors in a TEE 5.2. Trust Anchors in a TEE
The Trust Anchor Store in a TEE contains a list of Trust Anchors (raw The Trust Anchor Store in a TEE contains a list of Trust Anchors (raw
public keys or certificates) that are used to determine whether TA public keys or certificates) that are used to determine whether TA
binaries are allowed to execute by checking if their signatures can binaries are allowed to execute by checking if their signatures can
be verified. The list is typically preloaded at manufacturing time, be verified. The list is typically preloaded at manufacturing time
and can be updated using the TEEP protocol if the TEE has some form and can be updated using the TEEP protocol if the TEE has some form
of "Trust Anchor Manager TA" that has Trust Anchors in its of "Trust Anchor Manager TA" that has Trust Anchors in its
configuration data. Thus, Trust Anchors can be updated similarly to configuration data. Thus, Trust Anchors can be updated similarly to
the Personalization Data for any other TA, as discussed in the Personalization Data for any other TA, as discussed in
Section 5.1. Section 5.1.
5.3. Trust Anchors in a TAM 5.3. Trust Anchors in a TAM
The Trust Anchor Store in a TAM consists of a list of Trust Anchors, The Trust Anchor Store in a TAM consists of a list of Trust Anchors,
which are certificates that sign various device TEE certificates. A which are certificates that sign various device TEE certificates. A
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When a PKI is used, many intermediate CA certificates can chain to a When a PKI is used, many intermediate CA certificates can chain to a
root certificate, each of which can issue many certificates. This root certificate, each of which can issue many certificates. This
makes the protocol highly scalable. New factories that produce TEEs makes the protocol highly scalable. New factories that produce TEEs
can join the ecosystem. In this case, such a factory can get an can join the ecosystem. In this case, such a factory can get an
intermediate CA certificate from one of the existing roots without intermediate CA certificate from one of the existing roots without
requiring that TAMs are updated with information about the new device requiring that TAMs are updated with information about the new device
factory. Likewise, new TAMs can join the ecosystem, providing they factory. Likewise, new TAMs can join the ecosystem, providing they
are issued a TAM certificate that chains to an existing root whereby are issued a TAM certificate that chains to an existing root whereby
existing TAs in the TEE will be allowed to be personalized by the TAM existing TAs in the TEE will be allowed to be personalized by the TAM
without requiring changes to the TEE itself. This enables the without requiring changes to the TEE itself. This enables the
ecosystem to scale, and avoids the need for centralized databases of ecosystem to scale and avoids the need for centralized databases of
all TEEs produced or all TAMs that exist or all Trusted Component all TEEs produced, all TAMs that exist, or all Trusted Component
Signers that exist. Signers that exist.
5.5. Message Security 5.5. Message Security
Messages created by a TAM are used to deliver Trusted Component Messages created by a TAM are used to deliver Trusted Component
management commands to a device, and device attestation and messages management commands to a device, and device attestation and messages
are created by the device TEE to respond to TAM messages. are created by the device TEE to respond to TAM messages.
These messages are signed end-to-end between a TEEP Agent and a TAM. These messages are signed end-to-end between a TEEP Agent and a TAM.
Confidentiality is provided by encrypting sensitive payloads (such as Confidentiality is provided by encrypting sensitive payloads (such as
skipping to change at page 24, line 6 skipping to change at line 1052
for a software module in the REE world to handle network for a software module in the REE world to handle network
communication with a TAM. communication with a TAM.
A TEEP Broker is an application component running in the REE of the A TEEP Broker is an application component running in the REE of the
device or an SDK that facilitates communication between a TAM and a device or an SDK that facilitates communication between a TAM and a
TEE. It also provides interfaces for Untrusted Applications to query TEE. It also provides interfaces for Untrusted Applications to query
and trigger installation of Trusted Components that the application and trigger installation of Trusted Components that the application
needs to use. needs to use.
An Untrusted Application might communicate with a TEEP Broker at An Untrusted Application might communicate with a TEEP Broker at
runtime to trigger Trusted Component installation itself, or an runtime to trigger Trusted Component installation itself.
Untrusted Application might simply have a metadata file that Alternatively, an Untrusted Application might simply have a metadata
describes the Trusted Components it depends on and the associated file that describes the Trusted Components it depends on and the
TAM(s) for each Trusted Component, and an REE Application Installer associated TAM(s) for each Trusted Component. An REE Application
can inspect this application metadata file and invoke the TEEP Broker Installer can inspect this application metadata file and invoke the
to trigger Trusted Component installation on behalf of the Untrusted TEEP Broker to trigger Trusted Component installation on behalf of
Application without requiring the Untrusted Application to run first. the Untrusted Application without requiring the Untrusted Application
to run first.
6.1. Role of the TEEP Broker 6.1. Role of the TEEP Broker
A TEEP Broker interacts with a TEEP Agent inside a TEE, relaying A TEEP Broker interacts with a TEEP Agent inside a TEE, relaying
messages between the TEEP Agent and the TAM, and may also interact messages between the TEEP Agent and the TAM, and may also interact
with one or more Untrusted Applications (see Section 6.2.1). The with one or more Untrusted Applications (see Section 6.2.1). The
Broker cannot parse encrypted TEEP messages between a TAM and a TEEP Broker cannot parse encrypted TEEP messages exchanged between a TAM
agent but merely relays them. and a TEEP Agent but merely relays them.
When a device has more than one TEE, one TEEP Broker per TEE could be When a device has more than one TEE, one TEEP Broker per TEE could be
present in the REE or a common TEEP Broker could be used by multiple present in the REE, or a common TEEP Broker could be used by multiple
TEEs where the transport protocol (e.g., TEEs where the transport protocol (e.g., [TEEP-HTTP]) allows the TEEP
[I-D.ietf-teep-otrp-over-http]) allows the TEEP Broker to distinguish Broker to distinguish which TEE is relevant for each message from a
which TEE is relevant for each message from a TAM. TAM.
The Broker only needs to return a (transport) error message to the The Broker only needs to return an error message to the TAM if the
TAM if the TEE is not reachable for some reason. Other errors are TEE is not reachable for some reason. Other errors are represented
represented as TEEP response messages returned from the TEE which as TEEP response messages returned from the TEE, which will then be
will then be passed to the TAM. passed to the TAM.
6.2. TEEP Broker Implementation Consideration 6.2. TEEP Broker Implementation Consideration
As depicted in Figure 5, there are multiple ways in which a TEEP As depicted in Figure 4, there are multiple ways in which a TEEP
Broker can be implemented, with more or fewer layers being inside the Broker can be implemented with more or fewer layers being inside the
TEE. For example, in model A, the model with the smallest TEE TEE. For example, in model A (the model with the smallest TEE
footprint, only the TEEP implementation is inside the TEE, whereas footprint), only the TEEP implementation is inside the TEE, whereas
the TEEP/HTTP implementation is in the TEEP Broker outside the TEE. the TEEP/HTTP implementation is in the TEEP Broker outside the TEE.
Model: A B C Model: A B C
TEE TEE TEE TEE TEE TEE
+----------------+ | | | +----------------+ | | |
| TEEP | Agent | | | Agent | TEEP | Agent | | | Agent
| implementation | | | | | implementation | | | |
+----------------+ v | | +----------------+ v | |
| | | | | |
+----------------+ ^ | | +----------------+ ^ | |
| TEEP/HTTP | Broker | | | | TEEP/HTTP | Broker | | |
| implementation | | | | | implementation | | | |
+----------------+ | v | +----------------+ | v |
| | | | | |
+----------------+ | ^ | +----------------+ | ^ |
| HTTP(S) | | | | | HTTP(S) | | | |
| implementation | | | | | implementation | | | |
+----------------+ | | v +----------------+ | | v
| | | | | |
+----------------+ | | ^ +----------------+ | | ^
| TCP or QUIC | | | | Broker | TCP or QUIC | | | | Broker
| implementation | | | | | implementation | | | |
+----------------+ | | | +----------------+ | | |
REE REE REE REE REE REE
Figure 5: TEEP Broker Models Figure 4: TEEP Broker Models
In other models, additional layers are moved into the TEE, increasing In other models, additional layers are moved into the TEE, increasing
the TEE footprint, with the Broker either containing or calling the the TEE footprint, with the Broker either containing or calling the
topmost protocol layer outside of the TEE. An implementation is free topmost protocol layer outside of the TEE. An implementation is free
to choose any of these models. to choose any of these models.
TEEP Broker implementers should consider methods of distribution, TEEP Broker implementers should consider methods of distribution,
scope and concurrency on devices and runtime options. scope, and concurrency on devices and runtime options.
6.2.1. TEEP Broker APIs 6.2.1. TEEP Broker APIs
The following conceptual APIs exist from a TEEP Broker to a TEEP The following conceptual APIs exist from a TEEP Broker to a TEEP
Agent: Agent:
1. RequestTA: A notification from an REE application (e.g., an 1. RequestTA: A notification from an REE application (e.g., an
installer, or an Untrusted Application) that it depends on a installer or an Untrusted Application) that the application
given Trusted Component, which may or may not already be depends on a given Trusted Component, which may or may not
installed in the TEE. already be installed in the TEE.
2. UnrequestTA: A notification from an REE application (e.g., an 2. UnrequestTA: A notification from an REE application (e.g., an
installer, or an Untrusted Application) that it no longer depends installer or an Untrusted Application) that the application no
on a given Trusted Component, which may or may not already be longer depends on a given Trusted Component, which may or may not
installed in the TEE. For example, if the Untrusted Application already be installed in the TEE. For example, if the Untrusted
is uninstalled, the uninstaller might invoke this conceptual API. Application is uninstalled, the uninstaller might invoke this
conceptual API.
3. ProcessTeepMessage: A message arriving from the network, to be 3. ProcessTeepMessage: A message arriving from the network, to be
delivered to the TEEP Agent for processing. delivered to the TEEP Agent for processing.
4. RequestPolicyCheck: A hint (e.g., based on a timer) that the TEEP 4. RequestPolicyCheck: A hint (e.g., based on a timer) that the TEEP
Agent may wish to contact the TAM for any changes, without the Agent may wish to contact the TAM for any changes without the
device itself needing any particular change. device itself needing any particular change.
5. ProcessError: A notification that the TEEP Broker could not 5. ProcessError: A notification that the TEEP Broker could not
deliver an outbound TEEP message to a TAM. deliver an outbound TEEP message to a TAM.
For comparison, similar APIs may exist on the TAM side, where a For comparison, similar APIs may exist on the TAM side, where a
Broker may or may not exist, depending on whether the TAM uses a TEE Broker may or may not exist, depending on whether the TAM uses a TEE
or not: or not:
1. ProcessConnect: A notification that a new TEEP session is being 1. ProcessConnect: A notification that a new TEEP session is being
requested by a TEEP Agent. requested by a TEEP Agent.
2. ProcessTeepMessage: A message arriving at an existing TEEP 2. ProcessTeepMessage: A message arriving at an existing TEEP
session, to be delivered to the TAM for processing. session, to be delivered to the TAM for processing.
For further discussion on these APIs, see For further discussion on these APIs, see [TEEP-HTTP].
[I-D.ietf-teep-otrp-over-http].
6.2.2. TEEP Broker Distribution 6.2.2. TEEP Broker Distribution
The Broker installation is commonly carried out at device The Broker installation is commonly carried out at device
manufacturing time. A user may also dynamically download and install manufacturing time. A user may also dynamically download and install
a Broker on-demand. a Broker on demand.
7. Attestation 7. Attestation
Attestation is the process through which one entity (an Attester) Attestation is the process through which one entity (an Attester)
presents "evidence", in the form of a series of claims, to another presents "evidence" in the form of a series of claims to another
entity (a Verifier), and provides sufficient proof that the claims entity (a Verifier) and provides sufficient proof that the claims are
are true. Different Verifiers may require different degrees of true. Different Verifiers may require different degrees of
confidence in attestation proofs and not all attestations are confidence in attestation proofs, and not all attestations are
acceptable to every Verifier. A third entity (a Relying Party) can acceptable to every Verifier. A third entity (a Relying Party) can
then use "attestation results", in the form of another series of then use "attestation results" in the form of another series of
claims, from a Verifier to make authorization decisions. (See claims from a Verifier to make authorization decisions. (See
[I-D.ietf-rats-architecture] for more discussion.) [RFC9334] for more discussion.)
In TEEP, as depicted in Figure 6, the primary purpose of an
In TEEP, as depicted in Figure 5, the primary purpose of an
attestation is to allow a device (the Attester) to prove to a TAM attestation is to allow a device (the Attester) to prove to a TAM
(the Relying Party) that a TEE in the device has particular (the Relying Party) that a TEE in the device has particular
properties, was built by a particular manufacturer, and/or is properties, was built by a particular manufacturer, and/or is
executing a particular TA. Other claims are possible; TEEP does not executing a particular TA. Other claims are possible; TEEP does not
limit the claims that may appear in evidence or attestation results, limit the claims that may appear in evidence or attestation results,
but defines a minimal set of attestation result claims required for but it defines a minimal set of attestation result claims required
TEEP to operate properly. Extensions to these claims are possible. for TEEP to operate properly. Extensions to these claims are
Other standards or groups may define the format and semantics of possible. Other standards or groups may define the format and
extended claims. semantics of extended claims.
+----------------+ +----------------+
| Device | +----------+ | Device | +----------+
| +------------+ | Evidence | TAM | Evidence +----------+ | +------------+ | Evidence | TAM | Evidence +----------+
| | TEE |------------->| (Relying |-------------->| Verifier | | | TEE |------------->| (Relying |-------------->| Verifier |
| | (Attester) | | | Party) |<--------------| | | | (Attester) | | | Party) |<--------------| |
| +------------+ | +----------+ Attestation +----------+ | +------------+ | +----------+ Attestation +----------+
+----------------+ Result +----------------+ Result
Figure 6: TEEP Attestation Roles Figure 5: TEEP Attestation Roles
As of the writing of this specification, device and TEE attestations At the time of writing this specification, device and TEE
have not been standardized across the market. Different devices, attestations have not been standardized across the market. Different
manufacturers, and TEEs support different attestation protocols. In devices, manufacturers, and TEEs support different attestation
order for TEEP to be inclusive, it is agnostic to the format of protocols. In order for TEEP to be inclusive, it is agnostic to the
evidence, allowing proprietary or standardized formats to be used format of evidence, allowing proprietary or standardized formats to
between a TEE and a Verifier (which may or may not be colocated in be used between a TEE and a Verifier (which may or may not be
the TAM), as long as the format supports encryption of any colocated in the TAM), as long as the format supports encryption of
information that is considered sensitive. any information that is considered sensitive.
However, it should be recognized that not all Verifiers may be able However, it should be recognized that not all Verifiers may be able
to process all proprietary forms of attestation evidence. Similarly, to process all proprietary forms of attestation evidence. Similarly,
the TEEP protocol is agnostic as to the format of attestation the TEEP protocol is agnostic as to the format of attestation results
results, and the protocol (if any) used between the TAM and a and the protocol (if any) used between the TAM and a Verifier, as
Verifier, as long as they convey at least the required set of claims long as they convey at least the required set of claims in some
in some format. Note that the respective attestation algorithms are format. Note that the respective attestation algorithms are not
not defined in the TEEP protocol itself; see defined in the TEEP protocol itself; see [RFC9334] and [TEEP] for
[I-D.ietf-rats-architecture] and [I-D.ietf-teep-protocol] for more more discussion.
discussion.
There are a number of considerations that need to be considered when Considerations when appraising evidence provided by a TEE include the
appraising evidence provided by a TEE, including: following:
* What security measures a manufacturer takes when provisioning keys * What security measures a manufacturer takes when provisioning keys
into devices/TEEs; into devices/TEEs;
* What hardware and software components have access to the * What hardware and software components have access to the
attestation keys of the TEE; attestation keys of the TEE;
* The source or local verification of claims within an attestation * The source or local verification of claims within an attestation
prior to a TEE signing a set of claims; prior to a TEE signing a set of claims;
skipping to change at page 28, line 24 skipping to change at line 1246
* The revocation and recovery process of TEE attestation keys. * The revocation and recovery process of TEE attestation keys.
Some TAMs may require additional claims in order to properly Some TAMs may require additional claims in order to properly
authorize a device or TEE. The specific format for these additional authorize a device or TEE. The specific format for these additional
claims are outside the scope of this specification, but the TEEP claims are outside the scope of this specification, but the TEEP
protocol allows these additional claims to be included in the protocol allows these additional claims to be included in the
attestation messages. attestation messages.
For more discussion of the attestation and appraisal process, see the For more discussion of the attestation and appraisal process, see the
RATS Architecture [I-D.ietf-rats-architecture]. RATS Architecture [RFC9334].
The following information is required for TEEP attestation: The following information is required for TEEP attestation:
* Device Identifying Information: Attestation information may need * Device Identifying Information: Attestation information may need
to uniquely identify a device to the TAM. Unique device to uniquely identify a device to the TAM. Unique device
identification allows the TAM to provide services to the device, identification allows the TAM to provide services to the device,
such as managing installed TAs, and providing subscriptions to such as managing installed TAs, providing subscriptions to
services, and locating device-specific keying material to services, and locating device-specific keying material to
communicate with or authenticate the device. In some use cases it communicate with or authenticate the device. In some use cases,
may be sufficient to identify only the model or class of the it may be sufficient to identify only the model or class of the
device, for example, a DAA Issuer's group public key ID when the device, for example, a DAA Issuer's group public key ID when the
attestation uses DAA, see [I-D.ietf-rats-daa]. Another example of attestation uses DAA; see [RATS-DAA]. Another example of models
models is the hwmodel (Hardware Model) as defined in is the hwmodel (Hardware Model) as defined in [EAT]. The security
[I-D.ietf-rats-eat]. The security and privacy requirements and privacy requirements regarding device identification will vary
regarding device identification will vary with the type of TA with the type of TA provisioned to the TEE.
provisioned to the TEE.
* TEE Identifying Information: The type of TEE that generated this * TEE Identifying Information: The type of TEE that generated this
attestation must be identified. This includes version attestation must be identified. This includes version
identification information for hardware, firmware, and software identification information for hardware, firmware, and software
version of the TEE, as applicable by the TEE type. TEE version of the TEE, as applicable by the TEE type. TEE
manufacturer information for the TEE is required in order to manufacturer information for the TEE is required in order to
disambiguate the same TEE type created by different manufacturers disambiguate the same TEE type created by different manufacturers
and address considerations around manufacturer provisioning, and address considerations around manufacturer provisioning,
keying and support for the TEE. keying, and support for the TEE.
* Freshness Proof: A claim that includes freshness information must * Freshness Proof: A claim that includes freshness information must
be included, such as a nonce or timestamp. be included, such as a nonce or timestamp.
8. Algorithm and Attestation Agility 8. Algorithm and Attestation Agility
[RFC7696] outlines the requirements to migrate from one mandatory-to- [RFC7696] outlines the requirements to migrate from one mandatory-to-
implement cryptographic algorithm suite to another over time. This implement cryptographic algorithm suite to another over time. This
feature is also known as crypto agility. Protocol evolution is feature is also known as "crypto agility". Protocol evolution is
greatly simplified when crypto agility is considered during the greatly simplified when crypto agility is considered during the
design of the protocol. In the case of the TEEP protocol the diverse design of the protocol. In the case of the TEEP protocol, the
range of use cases, from trusted app updates for smartphones and diverse range of use cases (from trusted app updates for smartphones
tablets to updates of code on higher-end IoT devices, creates the and tablets to updates of code on higher-end IoT devices) creates the
need for different mandatory-to-implement algorithms already from the need for different mandatory-to-implement algorithms from the start.
start.
Crypto agility in TEEP concerns the use of symmetric as well as Crypto agility in TEEP concerns the use of symmetric as well as
asymmetric algorithms. In the context of TEEP, symmetric algorithms asymmetric algorithms. In the context of TEEP, symmetric algorithms
are used for encryption and integrity protection of TA binaries and are used for encryption and integrity protection of TA binaries and
Personalization Data whereas the asymmetric algorithms are used for Personalization Data, whereas the asymmetric algorithms are used for
signing messages and managing symmetric keys. signing messages and managing symmetric keys.
In addition to the use of cryptographic algorithms in TEEP, there is In addition to the use of cryptographic algorithms in TEEP, there is
also the need to make use of different attestation technologies. A also the need to make use of different attestation technologies. A
device must provide techniques to inform a TAM about the attestation device must provide techniques to inform a TAM about the attestation
technology it supports. For many deployment cases it is more likely technology it supports. For many deployment cases, it is more likely
for the TAM to support one or more attestation techniques whereas the for the TAM to support one or more attestation techniques, whereas
device may only support one. the device may only support one.
9. Security Considerations 9. Security Considerations
9.1. Broker Trust Model 9.1. Broker Trust Model
The architecture enables the TAM to communicate, via a TEEP Broker, The architecture enables the TAM to communicate, via a TEEP Broker,
with the device's TEE to manage Trusted Components. Since the TEEP with the device's TEE to manage Trusted Components. However, since
Broker runs in a potentially vulnerable REE, the TEEP Broker could, the TEEP Broker runs in a potentially vulnerable REE, the TEEP Broker
however, be (or be infected by) malware. As such, all TAM messages could be malware or be infected by malware. As such, all TAM
are signed and sensitive data is encrypted such that the TEEP Broker messages are signed and sensitive data is encrypted such that the
cannot modify or capture sensitive data, but the TEEP Broker can TEEP Broker cannot modify or capture sensitive data, but the TEEP
still conduct DoS attacks as discussed in Section 9.3. Broker can still conduct DoS attacks as discussed in Section 9.3.
A TEEP Agent in a TEE is responsible for protecting against potential A TEEP Agent in a TEE is responsible for protecting against potential
attacks from a compromised TEEP Broker or rogue malware in the REE. attacks from a compromised TEEP Broker or rogue malware in the REE.
A rogue TEEP Broker might send corrupted data to the TEEP Agent, or A rogue TEEP Broker might send corrupted data to the TEEP Agent,
launch a DoS attack by sending a flood of TEEP protocol requests, or launch a DoS attack by sending a flood of TEEP protocol requests, or
simply drop or delay notifications to a TEE. The TEEP Agent simply drop or delay notifications to a TEE. The TEEP Agent
validates the signature of each TEEP protocol request and checks the validates the signature of each TEEP protocol request and checks the
signing certificate against its Trust Anchors. To mitigate DoS signing certificate against its Trust Anchors. To mitigate DoS
attacks, it might also add some protection scheme such as a threshold attacks, it might also add some protection scheme such as a threshold
on repeated requests or number of TAs that can be installed. on repeated requests or the number of TAs that can be installed.
Some implementations might rely on (due to lack of any available Due to the lack of any available alternative, some implementations
alternative) the use of an untrusted timer or other event to call the might rely on the use of an untrusted timer or other event to call
RequestPolicyCheck API (Section 6.2.1), which means that a the RequestPolicyCheck API (Section 6.2.1), which means that a
compromised REE can cause a TEE to not receive policy changes and compromised REE can cause a TEE to not receive policy changes and
thus be out of date with respect to policy. The same can potentially thus be out of date with respect to policy. The same can potentially
be done by any other manipulator-in-the-middle simply by blocking be done by any other manipulator-in-the-middle simply by blocking
communication with a TAM. Ultimately such outdated compliance could communication with a TAM. Ultimately, such outdated compliance could
be addressed by using attestation in secure communication, where the be addressed by using attestation in secure communication, where the
attestation evidence reveals what state the TEE is in, so that attestation evidence reveals what state the TEE is in, so that
communication (other than remediation such as via TEEP) from an out- communication (other than remediation such as via TEEP) from an out-
of-compliance TEE can be rejected. of-compliance TEE can be rejected.
Similarly, in most implementations the REE is involved in the Similarly, in most implementations, the REE is involved in the
mechanics of installing new TAs. However, the authority for what TAs mechanics of installing new TAs. However, the authority for what TAs
are running in a given TEE is between the TEEP Agent and the TAM. are running in a given TEE is between the TEEP Agent and the TAM.
While a TEEP Broker can in effect make suggestions as discussed in While a TEEP Broker can, in effect, make suggestions as discussed in
Section Section 6.2.1, it cannot decide or enforce what runs where. Section 6.2.1, it cannot decide or enforce what runs where. The TEEP
The TEEP Broker can also control which TEE a given installation Broker can also control which TEE a given installation request is
request is directed at, but a TEEP Agent will only accept TAs that directed at, but a TEEP Agent will only accept TAs that are actually
are actually applicable to it and where installation instructions are applicable to it and where installation instructions are received by
received by a TAM that it trusts. a TAM that it trusts.
The authorization model for the UnrequestTA operation is, however, The authorization model for the UnrequestTA operation is, however,
weaker in that it expresses the removal of a dependency from an weaker in that it expresses the removal of a dependency from an
application that was untrusted to begin with. This means that a application that was untrusted to begin with. This means that a
compromised REE could remove a valid dependency from an Untrusted compromised REE could remove a valid dependency from an Untrusted
Application on a TA. Normal REE security mechanisms should be used Application on a TA. Normal REE security mechanisms should be used
to protect the REE and Untrusted Applications. to protect the REE and Untrusted Applications.
9.2. Data Protection 9.2. Data Protection
It is the responsibility of the TAM to protect data on its servers. It is the responsibility of the TAM to protect data on its servers.
Similarly, it is the responsibility of the TEE implementation to Similarly, it is the responsibility of the TEE implementation to
provide protection of data against integrity and confidentiality provide protection of data against integrity and confidentiality
attacks from outside the TEE. TEEs that provide isolation among TAs attacks from outside the TEE. TEEs that provide isolation among TAs
within the TEE are likewise responsible for protecting TA data within the TEE are likewise responsible for protecting TA data
against the REE and other TAs. For example, this can be used to against the REE and other TAs. For example, this can be used to
protect one user's or tenant's data from compromise by another user protect the data of one user or tenant from compromise by another
or tenant, even if the attacker has TAs. user or tenant, even if the attacker has TAs.
The protocol between TEEP Agents and TAMs similarly is responsible The protocol between TEEP Agents and TAMs is similarly responsible
for securely providing integrity and confidentiality protection for securely providing integrity and confidentiality protection
against adversaries between them. It is a design choice at what against adversaries between them. The layers at which to best
layers to best provide protection against network adversaries. As provide protection against network adversaries is a design choice.
discussed in Section 6, the transport protocol and any security As discussed in Section 6, the transport protocol and any security
mechanism associated with it (e.g., the Transport Layer Security mechanism associated with it (e.g., the Transport Layer Security
protocol) under the TEEP protocol may terminate outside a TEE. If it protocol) under the TEEP protocol may terminate outside a TEE. If it
does, the TEEP protocol itself must provide integrity protection and does, the TEEP protocol itself must provide integrity and
confidentiality protection to secure data end-to-end. For example, confidentiality protection to secure data end-to-end. For example,
confidentiality protection for payloads may be provided by utilizing confidentiality protection for payloads may be provided by utilizing
encrypted TA binaries and encrypted attestation information. See encrypted TA binaries and encrypted attestation information. See
[I-D.ietf-teep-protocol] for how a specific solution addresses the [TEEP] for how a specific solution addresses the design question of
design question of how to provide integrity and confidentiality how to provide integrity and confidentiality protection.
protection.
9.3. Compromised REE 9.3. Compromised REE
It is possible that the REE of a device is compromised. We have It is possible that the REE of a device is compromised. We have
already seen examples of attacks on the public Internet with a large already seen examples of attacks on the public Internet with a large
number of compromised devices being used to mount DDoS attacks. A number of compromised devices being used to mount DDoS attacks. A
compromised REE can be used for such an attack but it cannot tamper compromised REE can be used for such an attack, but it cannot tamper
with the TEE's code or data in doing so. A compromised REE can, with the TEE's code or data in doing so. A compromised REE can,
however, launch DoS attacks against the TEE. however, launch DoS attacks against the TEE.
The compromised REE may terminate the TEEP Broker such that TEEP The compromised REE may terminate the TEEP Broker such that TEEP
transactions cannot reach the TEE, or might drop, replay, or delay transactions cannot reach the TEE or might drop, replay, or delay
messages between a TAM and a TEEP Agent. However, while a DoS attack messages between a TAM and a TEEP Agent. However, while a DoS attack
cannot be prevented, the REE cannot access anything in the TEE if the cannot be prevented, the REE cannot access anything in the TEE if the
TEE is implemented correctly. Some TEEs may have some watchdog TEE is implemented correctly. Some TEEs may have some watchdog
scheme to observe REE state and mitigate DoS attacks against it but scheme to observe REE state and mitigate DoS attacks against it, but
most TEEs don't have such a capability. most TEEs don't have such a capability.
In some other scenarios, the compromised REE may ask a TEEP Broker to In some other scenarios, the compromised REE may ask a TEEP Broker to
make repeated requests to a TEEP Agent in a TEE to install or make repeated requests to a TEEP Agent in a TEE to install or
uninstall a Trusted Component. An installation or uninstallation uninstall a Trusted Component. An installation or uninstallation
request constructed by the TEEP Broker or REE will be rejected by the request constructed by the TEEP Broker or REE will be rejected by the
TEEP Agent because the request won't have the correct signature from TEEP Agent because the request won't have the correct signature from
a TAM to pass the request signature validation. a TAM to pass the request signature validation.
This can become a DoS attack by exhausting resources in a TEE with This can become a DoS attack by exhausting resources in a TEE with
repeated requests. In general, a DoS attack threat exists when the repeated requests. In general, a DoS attack threat exists when the
REE is compromised, and a DoS attack can happen to other resources. REE is compromised and a DoS attack can happen to other resources.
The TEEP architecture doesn't change this. The TEEP architecture doesn't change this.
A compromised REE might also request initiating the full flow of A compromised REE might also request initiating the full flow of
installation of Trusted Components that are not necessary. It may installation of Trusted Components that are not necessary. It may
also repeat a prior legitimate Trusted Component installation also repeat a prior legitimate Trusted Component installation
request. A TEEP Agent implementation is responsible for ensuring request. A TEEP Agent implementation is responsible for ensuring
that it can recognize and decline such repeated requests. It is also that it can recognize and decline such repeated requests. It is also
responsible for protecting the resource usage allocated for Trusted responsible for protecting the resource usage allocated for Trusted
Component management. Component management.
9.4. CA Compromise or Expiry of CA Certificate 9.4. CA Compromise or Expiry of CA Certificate
A root CA for TAM certificates might get compromised, or its A root CA for TAM certificates might get compromised, its certificate
certificate might expire, or a Trust Anchor other than a root CA might expire, or a Trust Anchor other than a root CA certificate may
certificate may also expire or be compromised. TEEs are responsible also expire or be compromised. TEEs are responsible for validating
for validating the entire TAM certificate path, including the TAM the entire TAM certification path, including the TAM certificate and
certificate and any intermediate certificates up to the root any intermediate certificates up to the root certificate. See
certificate. See Section 6 of [RFC5280] for details. Such Section 6 of [RFC5280] for details. Such validation generally
validation generally includes checking for certificate revocation, includes checking for certificate revocation, but certificate status
but certificate status check protocols may not scale down to check protocols may not scale down to constrained devices that use
constrained devices that use TEEP. TEEP.
To address the above issues, a certificate path update mechanism is To address the above issues, a certification path update mechanism is
expected from TAM operators, so that the TAM can get a new expected from TAM operators, so that the TAM can get a new
certificate path that can be validated by a TEEP Agent. In addition, certification path that can be validated by a TEEP Agent. In
the Trust Anchor in the TEEP Agent's Trust Anchor Store may need to addition, the Trust Anchor in the TEEP Agent's Trust Anchor Store may
be updated. To address this, some TEE Trust Anchor update mechanism need to be updated. To address this, a TEE Trust Anchor update
is expected from device OEMs, such as using the TEEP protocol to mechanism is expected from device equipment manufacturers (OEMs),
distribute new Trust Anchors. such as using the TEEP protocol to distribute new Trust Anchors.
Similarly, a root CA for TEE certificates might get compromised, or Similarly, a root CA for TEE certificates might get compromised, its
its certificate might expire, or a Trust Anchor other than a root CA certificate might expire, or a Trust Anchor other than a root CA
certificate may also expire or be compromised. TAMs are responsible certificate may also expire or be compromised. TAMs are responsible
for validating the entire TEE certificate path, including the TEE for validating the entire TEE certification path, including the TEE
certificate and any intermediate certificates up to the root certificate and any intermediate certificates up to the root
certificate. Such validation includes checking for certificate certificate. Such validation includes checking for certificate
revocation. revocation.
If a TEE certificate path validation fails, the TEE might be rejected If a TEE certification path validation fails, the TEE might be
by a TAM, subject to the TAM's policy. To address this, some rejected by a TAM, subject to the TAM's policy. To address this, a
certificate path update mechanism is expected from device OEMs, so certification path update mechanism is expected from device OEMs, so
that the TEE can get a new certificate path that can be validated by that the TEE can get a new certification path that can be validated
a TAM. In addition, the Trust Anchor in the TAM's Trust Anchor Store by a TAM. In addition, the Trust Anchor in the TAM's Trust Anchor
may need to be updated. Store may need to be updated.
9.5. Compromised TAM 9.5. Compromised TAM
Device TEEs are responsible for validating the supplied TAM Device TEEs are responsible for validating the supplied TAM
certificates. A compromised TAM may bring multiple threats and certificates. A compromised TAM may bring multiple threats and
damage to user devices that it can manage and thus to the Device damage to user devices that it can manage and thus to the Device
Owners. Information on devices that the TAM manages may be leaked to Owners. Information on devices that the TAM manages may be leaked to
a bad actor. A compromised TAM can also install many TAs to launch a a bad actor. A compromised TAM can also install many TAs to launch a
DoS attack on devices, for example, by filling up a device's TEE DoS attack on devices, for example, by filling up a device's TEE
resources reserved for TAs such that other TAs may not get resources resources reserved for TAs such that other TAs may not get resources
to be installed or properly function. It may also install malicious to be installed or properly function. It may also install malicious
TAs to potentially many devices under the condition that it also has TAs to potentially many devices under the condition that it also has
a Trusted Component signer key that is trusted by the TEEs. This a Trusted Component signer key that is trusted by the TEEs. This
makes TAMs high value targets. A TAM could be compromised without makes TAMs high-value targets. A TAM could be compromised without
impacting its certificate or raising concern from the TAM's operator. impacting its certificate or raising concern from the TAM's operator.
To mitigate this threat, TEEP Agents and Device Owners have several To mitigate this threat, TEEP Agents and Device Owners have several
options, including but potentially not limited to those listed below, options for detecting and mitigating a compromised TAM, including but
for detecting and mitigating a compromised TAM: potentially not limited to the following:
1. Apply an ACL to the TAM key, limiting which Trusted Components 1. Apply an ACL to the TAM key, limiting which Trusted Components
the TAM is permitted to install or update. the TAM is permitted to install or update.
2. Use a transparency log to expose a TAM compromise: TAMs publish 2. Use a transparency log to expose a TAM compromise. TAMs publish
an out-of-band record of Trusted Component releases, allowing a an out-of-band record of Trusted Component releases, allowing a
TEE to cross-check the Trusted Components delivered against the TEE to cross-check the Trusted Components delivered against the
Trusted Component installs in order to detect a TAM compromise. Trusted Components installed in order to detect a TAM compromise.
3. Use remote attestation of the TAM to prove trustworthiness. 3. Use remote attestation of the TAM to prove trustworthiness.
9.6. Malicious TA Removal 9.6. Malicious TA Removal
It is possible that a rogue developer distributes a malicious It is possible that a rogue developer distributes a malicious
Untrusted Application and intends to get a malicious TA installed. Untrusted Application and intends to have a malicious TA installed.
Such a TA might be able to escape from malware detection by the REE, Such a TA might be able to escape from malware detection by the REE
or access trusted resources within the TEE (but could not access or access trusted resources within the TEE (but could not access
other TEEs, or access other TA's if the TEE provides isolation other TEEs or other TAs if the TEE provides isolation between TAs).
between TAs).
It is the responsibility of the TAM to not install malicious TAs in It is the responsibility of the TAM to not install malicious TAs in
the first place. The TEEP architecture allows a TEEP Agent to decide the first place. The TEEP architecture allows a TEEP Agent to decide
which TAMs it trusts via Trust Anchors, and delegates the TA which TAMs it trusts via Trust Anchors and delegate the TA
authenticity check to the TAMs it trusts. authenticity check to the TAMs it trusts.
It may happen that a TA was previously considered trustworthy but is A TA that was previously considered trustworthy may later be found to
later found to be buggy or compromised. In this case, the TAM can be buggy or compromised. In this case, the TAM can initiate the
initiate the removal of the TA by notifying devices to remove the TA removal of the TA by notifying devices to remove the TA (and
(and potentially the REE or Device Owner to remove any Untrusted potentially notify the REE or Device Owner to remove any Untrusted
Application that depend on the TA). If the TAM does not currently Application that depend on the TA). If the TAM does not currently
have a connection to the TEEP Agent on a device, such a notification have a connection to the TEEP Agent on a device, such a notification
would occur the next time connectivity does exist. That is, to would occur the next time connectivity does exist. That is, to
recover, the TEEP Agent must be able to reach out to the TAM, for recover, the TEEP Agent must be able to reach out to the TAM, for
example whenever the RequestPolicyCheck API (Section 6.2.1) is example, whenever the RequestPolicyCheck API (Section 6.2.1) is
invoked by a timer or other event. invoked by a timer or other event.
Furthermore, the policy in the Verifier in an attestation process can Furthermore, the policy in the Verifier in an attestation process can
be updated so that any evidence that includes the malicious TA would be updated so that any evidence that includes the malicious TA would
result in an attestation failure. There is, however, a time window result in an attestation failure. There is, however, a time window
during which a malicious TA might be able to operate successfully, during which a malicious TA might be able to operate successfully,
which is the validity time of the previous attestation result. For which is the validity time of the previous attestation result. For
example, if the Verifier in Figure 6 is updated to treat a previously example, if the Verifier in Figure 5 is updated to treat a previously
valid TA as no longer trustworthy, any attestation result it valid TA as no longer trustworthy, any attestation result it
previously generated saying that the TA is valid will continue to be previously generated saying that the TA is valid will continue to be
used until the attestation result expires. As such, the TAM's used until the attestation result expires. As such, the TAM's
Verifier should take into account the acceptable time window when Verifier should take into account the acceptable time window when
generating attestation results. See [I-D.ietf-rats-architecture] for generating attestation results. See [RFC9334] for further
further discussion. discussion.
9.7. TEE Certificate Expiry and Renewal 9.7. TEE Certificate Expiry and Renewal
TEE device certificates are expected to be long-lived, longer than TEE device certificates are expected to be long-lived, longer than
the lifetime of a device. A TAM certificate usually has a moderate the lifetime of a device. A TAM certificate usually has a moderate
lifetime of 1 to 5 years. A TAM should get renewed or rekeyed lifetime of 1 to 5 years. A TAM should get renewed or rekeyed
certificates. The root CA certificates for a TAM, which are embedded certificates. The root CA certificates for a TAM, which are embedded
into the Trust Anchor Store in a device, should have long lifetimes into the Trust Anchor Store in a device, should have long lifetimes
that don't require device Trust Anchor updates. On the other hand, that don't require device Trust Anchor updates. On the other hand,
it is imperative that OEMs or device providers plan for support of it is imperative that OEMs or device providers plan for support of a
Trust Anchor update in their shipped devices. Trust Anchor update in their shipped devices.
For those cases where TEE devices are given certificates for which no For those cases where TEE devices are given certificates for which no
good expiration date can be assigned the recommendations in good expiration date can be assigned, the recommendations in
Section 4.1.2.5 of [RFC5280] are applicable. Section 4.1.2.5 of [RFC5280] are applicable.
9.8. Keeping Secrets from the TAM 9.8. Keeping Secrets from the TAM
In some scenarios, it is desirable to protect the TA binary or In some scenarios, it is desirable to protect the TA binary or
Personalization Data from being disclosed to the TAM that distributes Personalization Data from being disclosed to the TAM that distributes
them. In such a scenario, the files can be encrypted end-to-end them. In such a scenario, the files can be encrypted end-to-end
between a Trusted Component Signer and a TEE. However, there must be between a Trusted Component Signer and a TEE. However, there must be
some means of provisioning the decryption key into the TEE and/or some means of provisioning the decryption key into the TEE and/or
some means of the Trusted Component Signer securely learning a public some means of the Trusted Component Signer securely learning a public
key of the TEE that it can use to encrypt. The Trusted Component key of the TEE that it can use to encrypt. The Trusted Component
Signer cannot necessarily even trust the TAM to report the correct Signer cannot necessarily even trust the TAM to report the correct
public key of a TEE for use with encryption, since the TAM might public key of a TEE for use with encryption, since the TAM might
instead provide the public key of a TEE that it controls. instead provide the public key of a TEE that it controls.
One way to solve this is for the Trusted Component Signer to run its One way to solve this is for the Trusted Component Signer to run its
own TAM that is only used to distribute the decryption key via the own TAM that is only used to distribute the decryption key via the
TEEP protocol, and the key file can be a dependency in the manifest TEEP protocol and the key file can be a dependency in the manifest of
of the encrypted TA. Thus, the TEEP Agent would look at the Trusted the encrypted TA. Thus, the TEEP Agent would look at the Trusted
Component manifest, determine there is a dependency with a TAM URI of Component manifest to determine if there is a dependency with a TAM
the Trusted Component Signer's TAM. The Agent would then install the URI of the Trusted Component Signer's TAM. The Agent would then
dependency, and then continue with the Trusted Component installation install the dependency and continue with the Trusted Component
steps, including decrypting the TA binary with the relevant key. installation steps, including decrypting the TA binary with the
relevant key.
9.9. REE Privacy 9.9. REE Privacy
The TEEP architecture is applicable to cases where devices have a TEE The TEEP architecture is applicable to cases where devices have a TEE
that protects data and code from the REE administrator. In such that protects data and code from the REE administrator. In such
cases, the TAM administrator, not the REE administrator, controls the cases, the TAM administrator, not the REE administrator, controls the
TEE in the devices. As some examples: TEE in the devices. Examples include:
* a cloud hoster may be the REE administrator where a customer * A cloud hoster may be the REE administrator where a customer
administrator controls the TEE hosted in the cloud. administrator controls the TEE hosted in the cloud.
* a device manufacturer might control the TEE in a device purchased * A device manufacturer might control the TEE in a device purchased
by a customer by a customer.
The privacy risk is that data in the REE might be susceptible to The privacy risk is that data in the REE might be susceptible to
disclosure to the TEE administrator. This risk is not introduced by disclosure to the TEE administrator. This risk is not introduced by
the TEEP architecture, but is inherent in most uses of TEEs. This the TEEP architecture, but it is inherent in most uses of TEEs. This
risk can be mitigated by making sure the REE administrator is aware risk can be mitigated by making sure the REE administrator explicitly
of and explicitly chooses to have a TEE that is managed by another chooses to have a TEE that is managed by another party. In the cloud
party. In the cloud hoster example, this choice is made by hoster example, this choice is made by explicitly offering a service
explicitly offering a service to customers to provide TEEs for them to customers to provide TEEs for them to administer. In the device
to administer. In the device manufacturer example, this choice is manufacturer example, this choice is made by the customer choosing to
made by the customer choosing to buy a device made by a given buy a device made by a given manufacturer.
manufacturer.
10. IANA Considerations 10. IANA Considerations
This document does not require actions by IANA. This document has no IANA actions.
11. Contributors
* Andrew Atyeo, Intercede (andrew.atyeo@intercede.com)
* Liu Dapeng, Alibaba Group (maxpassion@gmail.com)
12. Acknowledgements
We would like to thank Nick Cook, Minho Yoo, Brian Witten, Tyler Kim,
Alin Mutu, Juergen Schoenwaelder, Nicolae Paladi, Sorin Faibish, Ned
Smith, Russ Housley, Jeremy O'Donoghue, Anders Rundgren, and Brendan
Moran for their feedback.
13. Informative References 11. Informative References
[CC-Overview] [CC-Overview]
Confidential Computing Consortium, "Confidential Confidential Computing Consortium, "Confidential
Computing: Hardware-Based Trusted Execution for Computing: Hardware-Based Trusted Execution for
Applications and Data", January 2021, Applications and Data", November 2022,
<https://confidentialcomputing.io/wp- <https://confidentialcomputing.io/wp-
content/uploads/sites/85/2021/03/ content/uploads/sites/85/2021/03/
confidentialcomputing_outreach_whitepaper-8-5x11-1.pdf>. confidentialcomputing_outreach_whitepaper-8-5x11-1.pdf>.
[CC-Technical-Analysis] [CC-Technical-Analysis]
Confidential Computing Consortium, "A Technical Analysis Confidential Computing Consortium, "A Technical Analysis
of Confidential Computing, v1.2", October 2021, of Confidential Computing", v1.3, November 2022,
<https://confidentialcomputing.io/wp- <https://confidentialcomputing.io/wp-
content/uploads/sites/85/2022/01/CCC-A-Technical-Analysis- content/uploads/sites/10/2023/03/CCC-A-Technical-Analysis-
of-Confidential-Computing-v1.2.pdf>. of-Confidential-Computing-v1.3_unlocked.pdf>.
[GPTEE] GlobalPlatform, "GlobalPlatform Device Technology: TEE
System Architecture, v1.3", GlobalPlatform GPD_SPE_009,
May 2022, <https://globalplatform.org/specs-library/tee-
system-architecture/>.
[GSMA] GSM Association, "GP.22 RSP Technical Specification,
Version 2.2.2", June 2020, <https://www.gsma.com/esim/wp-
content/uploads/2020/06/SGP.22-v2.2.2.pdf>.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture", Work
in Progress, Internet-Draft, draft-ietf-rats-architecture-
22, 28 September 2022, <https://www.ietf.org/archive/id/
draft-ietf-rats-architecture-22.txt>.
[I-D.ietf-rats-daa]
Birkholz, H., Newton, C., Chen, L., and D. Thaler, "Direct
Anonymous Attestation for the Remote Attestation
Procedures Architecture", Work in Progress, Internet-
Draft, draft-ietf-rats-daa-02, 7 September 2022,
<https://www.ietf.org/archive/id/draft-ietf-rats-daa-
02.txt>.
[I-D.ietf-rats-eat] [EAT] Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
Wallace, "The Entity Attestation Token (EAT)", Work in Wallace, "The Entity Attestation Token (EAT)", Work in
Progress, Internet-Draft, draft-ietf-rats-eat-17, 22 Progress, Internet-Draft, draft-ietf-rats-eat-21, 30 June
October 2022, <https://www.ietf.org/archive/id/draft-ietf- 2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
rats-eat-17.txt>. rats-eat-21>.
[I-D.ietf-suit-manifest]
Moran, B., Tschofenig, H., Birkholz, H., Zandberg, K., and
O. Rønningstad, "A Concise Binary Object Representation
(CBOR)-based Serialization Format for the Software Updates
for Internet of Things (SUIT) Manifest", Work in Progress,
Internet-Draft, draft-ietf-suit-manifest-20, 7 October
2022, <https://www.ietf.org/archive/id/draft-ietf-suit-
manifest-20.txt>.
[I-D.ietf-teep-otrp-over-http] [GPTEE] GlobalPlatform, "TEE System Architecture v1.3",
Thaler, D., "HTTP Transport for Trusted Execution GlobalPlatform GPD_SPE_009, May 2022,
Environment Provisioning: Agent Initiated Communication", <https://globalplatform.org/specs-library/tee-system-
Work in Progress, Internet-Draft, draft-ietf-teep-otrp- architecture/>.
over-http-14, 14 October 2022,
<https://www.ietf.org/archive/id/draft-ietf-teep-otrp-
over-http-14.txt>.
[I-D.ietf-teep-protocol] [GSMA] GSM Association, "SGP.22 RSP Technical Specification",
Tschofenig, H., Pei, M., Wheeler, D. M., Thaler, D., and Version 2.2.2, June 2020, <https://www.gsma.com/esim/wp-
A. Tsukamoto, "Trusted Execution Environment Provisioning content/uploads/2020/06/SGP.22-v2.2.2.pdf>.
(TEEP) Protocol", Work in Progress, Internet-Draft, draft-
ietf-teep-protocol-10, 28 July 2022,
<https://www.ietf.org/archive/id/draft-ietf-teep-protocol-
10.txt>.
[OP-TEE] TrustedFirmware.org, "OP-TEE Documentation", 2022, [OP-TEE] TrustedFirmware.org, "OP-TEE Documentation",
<https://optee.readthedocs.io/en/latest/>. <https://optee.readthedocs.io/en/latest/>.
[OTRP] GlobalPlatform, "Open Trust Protocol (OTrP) Profile v1.1", [OTRP] GlobalPlatform, "TEE Management Framework: Open Trust
GlobalPlatform GPD_SPE_123, July 2020, Protocol (OTrP) Profile v1.1", GlobalPlatform GPD_SPE_123,
<https://globalplatform.org/specs-library/tee-management- July 2020, <https://globalplatform.org/specs-library/tee-
framework-open-trust-protocol/>. management-framework-open-trust-protocol/>.
[RATS-DAA] Birkholz, H., Newton, C., Chen, L., and D. Thaler, "Direct
Anonymous Attestation for the Remote Attestation
Procedures Architecture", Work in Progress, Internet-
Draft, draft-ietf-rats-daa-03, 10 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
daa-03>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", [RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>. <https://www.rfc-editor.org/info/rfc4949>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>. <https://www.rfc-editor.org/info/rfc5280>.
skipping to change at page 38, line 15 skipping to change at line 1649
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm [RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms", Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015, BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/info/rfc7696>. <https://www.rfc-editor.org/info/rfc7696>.
[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A [RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things", Firmware Update Architecture for Internet of Things",
RFC 9019, DOI 10.17487/RFC9019, April 2021, RFC 9019, DOI 10.17487/RFC9019, April 2021,
<https://www.rfc-editor.org/info/rfc9019>. <https://www.rfc-editor.org/info/rfc9019>.
[RFC9334] Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote ATtestation procedureS (RATS)
Architecture", RFC 9334, DOI 10.17487/RFC9334, January
2023, <https://www.rfc-editor.org/info/rfc9334>.
[SGX] Intel, "Intel(R) Software Guard Extensions (Intel (R) [SGX] Intel, "Intel(R) Software Guard Extensions (Intel (R)
SGX)", n.d., <https://www.intel.com/content/www/us/en/ SGX)", <https://www.intel.com/content/www/us/en/
architecture-and-technology/software-guard- architecture-and-technology/software-guard-
extensions.html>. extensions.html>.
[SUIT-MANIFEST]
Moran, B., Tschofenig, H., Birkholz, H., Zandberg, K., and
O. Rønningstad, "A Concise Binary Object Representation
(CBOR)-based Serialization Format for the Software Updates
for Internet of Things (SUIT) Manifest", Work in Progress,
Internet-Draft, draft-ietf-suit-manifest-22, 27 February
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
suit-manifest-22>.
[TEEP] Tschofenig, H., Pei, M., Wheeler, D. M., Thaler, D., and
A. Tsukamoto, "Trusted Execution Environment Provisioning
(TEEP) Protocol", Work in Progress, Internet-Draft, draft-
ietf-teep-protocol-15, 3 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teep-
protocol-15>.
[TEEP-HTTP]
Thaler, D., "HTTP Transport for Trusted Execution
Environment Provisioning: Agent Initiated Communication",
Work in Progress, Internet-Draft, draft-ietf-teep-otrp-
over-http-15, 27 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-teep-
otrp-over-http-15>.
[TrustZone] [TrustZone]
Arm, "Arm TrustZone Technology", n.d., Arm, "TrustZone for Cortex-A",
<https://developer.arm.com/ip-products/security-ip/ <https://www.arm.com/technologies/trustzone-for-cortex-a>.
trustzone>.
Acknowledgments
We would like to thank Nick Cook, Minho Yoo, Brian Witten, Tyler Kim,
Alin Mutu, Juergen Schoenwaelder, Nicolae Paladi, Sorin Faibish, Ned
Smith, Russ Housley, Jeremy O'Donoghue, Anders Rundgren, and Brendan
Moran for their feedback.
Contributors
Andrew Atyeo
Intercede
Email: andrew.atyeo@intercede.com
Liu Dapeng
Alibaba Group
Email: maxpassion@gmail.com
Authors' Addresses Authors' Addresses
Mingliang Pei Mingliang Pei
Broadcom Broadcom
Email: mingliang.pei@broadcom.com Email: mingliang.pei@broadcom.com
Hannes Tschofenig Hannes Tschofenig
Arm Limited Email: hannes.tschofenig@gmx.net
Email: hannes.tschofenig@arm.com
Dave Thaler Dave Thaler
Microsoft Microsoft
Email: dthaler@microsoft.com Email: dthaler@microsoft.com
David Wheeler David Wheeler
Amazon Amazon
Email: davewhee@amazon.com Email: davewhee@amazon.com
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