draft-ietf-teep-architecture-05.txt   draft-ietf-teep-architecture-06.txt 
TEEP M. Pei TEEP M. Pei
Internet-Draft Symantec Internet-Draft Symantec
Intended status: Informational H. Tschofenig Intended status: Informational H. Tschofenig
Expires: June 14, 2020 Arm Limited Expires: August 11, 2020 Arm Limited
D. Thaler D. Thaler
Microsoft Microsoft
D. Wheeler D. Wheeler
Intel Intel
December 12, 2019 February 08, 2020
Trusted Execution Environment Provisioning (TEEP) Architecture Trusted Execution Environment Provisioning (TEEP) Architecture
draft-ietf-teep-architecture-05 draft-ietf-teep-architecture-06
Abstract Abstract
A Trusted Execution Environment (TEE) is an environment that enforces A Trusted Execution Environment (TEE) is an environment that enforces
that only authorized code can execute with that environment, and that that only authorized code can execute within that environment, and
any data used by such code cannot be read or tampered with by any that any data used by such code cannot be read or tampered with by
code outside that environment. This architecture document motivates any code outside that environment. This architecture document
the design and standardization of a protocol for managing the motivates the design and standardization of a protocol for managing
lifecycle of trusted applications running inside a TEE. 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 14, 2020. This Internet-Draft will expire on August 11, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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skipping to change at page 2, line 31 skipping to change at page 2, line 31
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Payment . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1. Payment . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 7 3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 7
3.3. Internet of Things . . . . . . . . . . . . . . . . . . . 7 3.3. Internet of Things . . . . . . . . . . . . . . . . . . . 7
3.4. Confidential Cloud Computing . . . . . . . . . . . . . . 7 3.4. Confidential Cloud Computing . . . . . . . . . . . . . . 8
4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. System Components . . . . . . . . . . . . . . . . . . . . 8 4.1. System Components . . . . . . . . . . . . . . . . . . . . 8
4.2. Different Renditions of TEEP Architecture . . . . . . . . 10 4.2. Multiple TEEs in a Device . . . . . . . . . . . . . . . . 10
4.3. Multiple TAMs and Relationship to TAs . . . . . . . . . . 12 4.3. Multiple TAMs and Relationship to TAs . . . . . . . . . . 12
4.4. Untrusted Apps, Trusted Apps, and Personalization Data . 13 4.4. Untrusted Apps, Trusted Apps, and Personalization Data . 13
4.5. Examples of Application Delivery Mechanisms in Existing 4.4.1. Examples of Application Delivery Mechanisms in
TEEs . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Existing TEEs . . . . . . . . . . . . . . . . . . . . 14
4.6. Entity Relations . . . . . . . . . . . . . . . . . . . . 15 4.5. Entity Relations . . . . . . . . . . . . . . . . . . . . 16
5. Keys and Certificate Types . . . . . . . . . . . . . . . . . 17 5. Keys and Certificate Types . . . . . . . . . . . . . . . . . 17
5.1. Trust Anchors in TEE . . . . . . . . . . . . . . . . . . 18 5.1. Trust Anchors in a TEEP Agent . . . . . . . . . . . . . . 18
5.2. Trust Anchors in TAM . . . . . . . . . . . . . . . . . . 19 5.2. Trust Anchors in a TEE . . . . . . . . . . . . . . . . . 19
5.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 19 5.3. Trust Anchors in a TAM . . . . . . . . . . . . . . . . . 19
5.4. Message Security . . . . . . . . . . . . . . . . . . . . 19 5.4. Scalability . . . . . . . . . . . . . . . . . . . . . . . 19
6. TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.5. Message Security . . . . . . . . . . . . . . . . . . . . 20
6. TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Role of the TEEP Broker . . . . . . . . . . . . . . . . . 20 6.1. Role of the TEEP Broker . . . . . . . . . . . . . . . . . 20
6.2. TEEP Broker Implementation Consideration . . . . . . . . 20 6.2. TEEP Broker Implementation Consideration . . . . . . . . 21
6.2.1. TEEP Broker APIs . . . . . . . . . . . . . . . . . . 20 6.2.1. TEEP Broker APIs . . . . . . . . . . . . . . . . . . 21
6.2.2. TEEP Broker Distribution . . . . . . . . . . . . . . 21 6.2.2. TEEP Broker Distribution . . . . . . . . . . . . . . 22
6.2.3. Number of TEEP Brokers . . . . . . . . . . . . . . . 21
7. Attestation . . . . . . . . . . . . . . . . . . . . . . . . . 22 7. Attestation . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1. Information Required in TEEP Claims . . . . . . . . . . . 23 7.1. Information Required in TEEP Claims . . . . . . . . . . . 24
8. Algorithm and Attestation Agility . . . . . . . . . . . . . . 24 8. Algorithm and Attestation Agility . . . . . . . . . . . . . . 24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 25 9. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9.1. TA Trust Check at TEE . . . . . . . . . . . . . . . . . . 25 9.1. Broker Trust Model . . . . . . . . . . . . . . . . . . . 25
9.2. One TA Multiple SP Case . . . . . . . . . . . . . . . . . 25 9.2. Data Protection at TAM and TEE . . . . . . . . . . . . . 25
9.3. Broker Trust Model . . . . . . . . . . . . . . . . . . . 25 9.3. Compromised REE . . . . . . . . . . . . . . . . . . . . . 25
9.4. Data Protection at TAM and TEE . . . . . . . . . . . . . 25 9.4. Compromised CA . . . . . . . . . . . . . . . . . . . . . 26
9.5. Compromised CA . . . . . . . . . . . . . . . . . . . . . 26 9.5. Compromised TAM . . . . . . . . . . . . . . . . . . . . . 26
9.6. Compromised TAM . . . . . . . . . . . . . . . . . . . . . 26 9.6. Malicious TA Removal . . . . . . . . . . . . . . . . . . 26
9.7. Certificate Renewal . . . . . . . . . . . . . . . . . . . 26 9.7. Certificate Renewal . . . . . . . . . . . . . . . . . . . 27
9.8. Keeping Secrets from the TAM . . . . . . . . . . . . . . 26 9.8. Keeping Secrets from the TAM . . . . . . . . . . . . . . 27
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
13. Informative References . . . . . . . . . . . . . . . . . . . 27 13. Informative References . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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 increase 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 interactions among those features and applications. The unintended interactions among those features and applications. The
danger of attacks on a system increases as the sensitivity of the danger of attacks on a system increases as the sensitivity of the
applications or data on the device increases. As an example, applications or data on the device increases. As an example,
exposure of emails from a mail client is likely to be of concern to exposure of emails from a mail client is likely to be of concern to
its owner, but a compromise of a banking application raises even its owner, but a compromise of a banking application raises even
greater concerns. greater concerns.
The Trusted Execution Environment (TEE) concept is designed to The Trusted Execution Environment (TEE) concept is designed to
execute applications in a protected environment that enforces that execute applications in a protected environment that enforces that
only authorized code can execute with that environment, and that any only authorized code can execute within that environment, and that
data used by such code cannot be read or tampered with by any code any data used by such code cannot be read or tampered with by any
outside that environment, including a commodity operating system (if code outside that environment, including by a commodity operating
present). system (if present).
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 referred to as a Trusted Application (TA), running inside a TEE is referred to as a Trusted Application (TA),
while an application running outside any TEE is referred to as an while an application running outside any TEE is referred to as an
Untrusted Application (UA). Untrusted Application.
TEEs use hardware enforcement combined with software protection to TEEs use hardware enforcement combined with software protection to
secure TAs and its data. TEEs typically offer a more limited set of secure TAs and its data. TEEs typically offer a more limited set of
services to TAs than is normally available to Untrusted Applications. services to TAs than is normally available to Untrusted Applications.
But not all TEEs are the same, and different vendors may have Not all TEEs are the same, however, and 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 different features, and different control mechanisms to operate on
TAs. Some vendors may themselves market multiple different TEEs with TAs. Some vendors may themselves market multiple different TEEs with
different properties attuned to different markets. A device vendor different properties attuned to different markets. A device vendor
may integrate one or more TEEs into their devices depending on market may integrate one or more TEEs into their devices depending on market
needs. needs.
To simplify the life of developers and service providers interacting To simplify the life of TA developers interacting with TAs in a TEE,
with TAs in a TEE, an interoperable protocol for managing TAs running an interoperable protocol for managing TAs running in different TEEs
in different TEEs of various devices is needed. In this TEE of various devices is needed. In this TEE ecosystem, there often
ecosystem, there often arises a need for an external trusted party to arises a need for an external trusted party to verify the identity,
verify the identity, claims, and rights of Service Providers (SP), claims, and rights of TA developers, devices, and their TEEs. This
devices, and their TEEs. This trusted third party is the Trusted trusted third party is the Trusted Application Manager (TAM).
Application Manager (TAM).
The Trusted Execution Provisioning (TEEP) protocol addresses the The Trusted Execution Environment Provisioning (TEEP) protocol
following problems: addresses the following problems:
- A Service Provider (SP) intending to provide services through a TA - An installer of an Untrusted Application that depends on a given
to users of a device needs to determine security-relevant TA wants to request installation of that TA in the device's TEE so
information of a device before provisioning their TA to the TEE that the Untrusted Application can complete, but the TEE needs to
verify whether such a TA is actually authorized to run in the TEE
and consume potentially scarce TEE resources.
- A TA developer providing a TA whose code itself is considered
confidential wants to determine security-relevant information of a
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 that it is capable of providing the
security protections required by a particular TA. security protections required.
- A TEE in a device needs to determine whether a Service Provider - A TEE in a device wants to determine whether an entity that wants
(SP) that wants to manage a TA in the device is authorized to to manage a TA in the device is authorized to manage TAs in the
manage TAs in the TEE, and what TAs the SP is permitted to manage. TEE, and what TAs the entity is permitted to manage.
- A Service Provider (SP) must be able to determine if a TA exists - A TAM (e.g., operated by a device administrator) wants to
(is installed) on a device (in the TEE), and if not, install the determine if a TA exists (is installed) on a device (in the TEE),
TA in the TEE. and if not, install the TA in the TEE.
- A Service Provider (SP) must be able to check whether a TA in a - A TAM wants to check whether a TA in a device's TEE is the most
device's TEE is the most up-to-date version, and if not, update up-to-date version, and if not, update the TA in the TEE.
the TA in the TEE.
- A Service Provider (SP) must be able to remove a TA in a device's - A TA developer wants to remove a confidential TA from a device's
TEE if the SP is no longer offering such services or the services TEE if the TA developer is no longer offering such TAs or the TAs
are being revoked from a particular user (or device). For are being revoked from a particular user (or device). For
example, if a subscription or contract for a particular service example, if a subscription or contract for a particular service
has expired, or a payment by the user has not been completed or has expired, or a payment by the user has not been completed or
has been rescinded. has been rescinded.
- A Service Provider (SP) must be able to define the relationship - A TA developer wants to define the relationship between
between cooperating TAs under the SP's control, and specify cooperating TAs under the TA developer's control, and specify
whether the TAs can communicate, share data, and/or share key whether the TAs can communicate, share data, and/or share key
material. material.
Note: The Service Provider requires the help of a TAM to provision Note: The TA developer requires the help of a TAM to provision the
the Trusted Applications to remote devices and the TEEP protocol Trusted Applications to remote devices and the TEEP protocol
exchanges messages between a Trusted Application Manager (TAM) and a exchanges messages between a TAM and a TEEP Agent via a TEEP Broker.
TEEP Agent via a TEEP Broker.
2. Terminology 2. Terminology
The following terms are used: The following terms are used:
- Untrusted Application: An application running in a Rich Execution
Environment, such as an Android, Windows, or iOS application.
- Trusted Application Manager (TAM): An entity that manages Trusted
Applications (TAs) running in different TEEs of various devices.
- 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 a Rich Execution Environment. A Device contains often along with a Rich Execution Environment. A device contains
a default list of Trust Anchors that identify entities (e.g., a default list of Trust Anchors that identify entities (e.g.,
TAMs) that are trusted by the Device. This list is normally set TAMs) that are trusted by the device. This list is normally set
by the Device Manufacturer, and may be governed by the Device's by the device manufacturer, and may be governed by the device's
network carrier. The list of Trust Anchors is normally modifiable network carrier when it is a mobile device. The list of Trust
by the Device's owner or Device Administrator. However the Device Anchors is normally modifiable by the device's owner or Device
manufacturer and network carrier may restrict some modifications, Administrator. However the device manufacturer or network carrier
for example, by not allowing the manufacturer or carrier's Trust (in the mobile device case) may restrict some modifications, for
example, by not allowing the manufacturer or carrier's Trust
Anchor to be removed or disabled. Anchor to be removed or disabled.
- Rich Execution Environment (REE): An environment that is provided - Device Administrator: An entity that is responsible for
and governed by a typical OS (e.g., Linux, Windows, Android, iOS), administration of a device, which could be the device owner. A
potentially in conjunction with other supporting operating systems Device Administrator has privileges on the device to install and
and hypervisors; it is outside of any TEE. This environment and remove Untrusted Applications and TAs, approve or reject Trust
applications running on it are considered untrusted. Anchors, and approve or reject TA developers, among possibly other
privileges on the device. A Device Administrator can manage the
list of allowed TAMs by modifying the list of Trust Anchors on the
device. Although a Device Administrator may have privileges and
device-specific controls to locally administer a device, the
Device Administrator may choose to remotely administer a device
through a TAM.
- Service Provider (SP): An entity that wishes to provide a service - Device Owner: A device is always owned by someone. In some cases,
on Devices that requires the use of one or more Trusted it is common for the (primary) device user to also own the device,
Applications. making the device user/owner also the Device Administrator. In
enterprise environments it is more common for the enterprise to
own the device, and any device user has no or limited
administration rights. In this case, the enterprise appoints a
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 single device user. Some devices have a primary device user a single device user. Some devices have a primary device user
with other human beings as secondary device users (e.g., parent with other human beings as secondary device users (e.g., 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 and hence have no device user.
Relates to Device Owner and Device Administrator. Relates to Device Owner and Device Administrator.
- Device Owner: A device is always owned by someone. In some cases, - Rich Execution Environment (REE): An environment that is provided
it is common for the (primary) device user to also own the device, and governed by a typical OS (e.g., Linux, Windows, Android, iOS),
making the device user/owner also the device administrator. In potentially in conjunction with other supporting operating systems
enterprise environments it is more common for the enterprise to and hypervisors; it is outside of any TEE. This environment and
own the device, and any device user has no or limited applications running on it are considered untrusted (or more
administration rights. In this case, the enterprise appoints a precisely, less trusted than the TEE).
device administrator that is not the device owner.
- Device Administrator (DA): An entity that is responsible for
administration of a Device, which could be the device owner. A
Device Administrator has privileges on the Device to install and
remove applications and TAs, approve or reject Trust Anchors, and
approve or reject Service Providers, among possibly other
privileges on the Device. A Device Administrator can manage the
list of allowed TAMs by modifying the list of Trust Anchors on the
Device. Although a Device Administrator may have privileges and
Device-specific controls to locally administer a device, the
Device Administrator may choose to remotely administrate a device
through a TAM.
- Trust Anchor: As defined in [RFC6024] and - Trust Anchor: As defined in [RFC6024] and
[I-D.ietf-suit-manifest], "A trust anchor represents an [I-D.ietf-suit-manifest], "A trust anchor represents an
authoritative entity via a public key and associated data. The authoritative entity via a public key and associated data. The
public key is used to verify digital signatures, and the public key is used to verify digital signatures, and the
associated data is used to constrain the types of information for associated data is used to constrain the types of information for
which the trust anchor is authoritative." The Trust Anchor may be which the trust anchor is authoritative." The Trust Anchor may be
a certificate or it may be a raw public key along with additional a certificate or it may be a raw public key along with additional
data if necessary such as its public key algorithm and parameters. data if necessary such as its public key algorithm and parameters.
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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 may be used by one or more applications." As noted in
[I-D.ietf-suit-manifest], a trust anchor store must resist [I-D.ietf-suit-manifest], a trust anchor store must resist
modification against unauthorized insertion, deletion, and modification against unauthorized insertion, deletion, and
modification. modification.
- Trusted Application (TA): An application component that runs in a - Trusted Application (TA): An application component that runs in a
TEE. TEE.
- Trusted Application (TA) Developer: An entity that wishes to
provide functionality on devices that requires the use of one or
more Trusted Applications.
- Trusted Application Manager (TAM): An entity that manages Trusted
Applications (TAs) running in TEEs of various devices.
- 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 device unique
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: An application running in a Rich Execution
Environment.
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 about the hosting device. Payments initiated from a mobile trust about the hosting device. Payments initiated from a mobile
device can use a Trusted Application to provide strong identification device can use a Trusted Application to provide strong identification
and proof of transaction. and proof of transaction.
For a mobile payment application, some biometric identification For a mobile payment application, some biometric identification
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such as authentication credentials in the device. A TEE can be the such as authentication credentials in the device. A TEE can be the
best way to implement such IoT security functions. best way to implement such IoT security functions.
3.4. Confidential Cloud Computing 3.4. Confidential Cloud Computing
A tenant can store sensitive data in a TEE in a cloud computing A tenant can store sensitive data in a TEE in a cloud computing
server such that only the tenant can access the data, preventing the server such that only the tenant can access the data, preventing the
cloud hosting provider from accessing the data. A tenant can run TAs cloud hosting provider from accessing the data. A tenant can run TAs
inside a server TEE for secure operation and enhanced data security. inside a server TEE for secure operation and enhanced data security.
This provides benefits not only to tenants with better data security This provides benefits not only to tenants with better data security
but also to cloud hosting provider for reduced liability and but also to cloud hosting providers for reduced liability and
increased cloud adoption. increased cloud adoption.
4. Architecture 4. Architecture
4.1. System Components 4.1. System Components
The following are the main components in the system. Full Figure 1 shows the main components in a typical device with an REE.
descriptions of components not previously defined are provided below. Full descriptions of components not previously defined are provided
Interactions of all components are further explained in the following below. Interactions of all components are further explained in the
paragraphs. following paragraphs.
+-------------------------------------------+ +-------------------------------------------+
| Device | | Device |
| +--------+ | Service Provider | +--------+ | TA Developer
| +-------------+ | |----------+ | | +-------------+ | |-----------+ |
| | TEE-1 | | TEEP |---------+| | | | TEE-1 | | TEEP |---------+ | |
| | +--------+ | +----| Broker | | || +--------+ | | | +--------+ | +----| Broker | | | | +--------+ |
| | | TEEP | | | | |<---+ | |+-->| |<-+ | | | TEEP | | | | |<---+ | | +->| |<-+
| | | Agent |<----+ | | | | | +-| TAM-1 | | | | Agent |<----+ | | | | | +-| TAM-1 |
| | +--------+ | | |<-+ | | +->| | |<-+ | | +--------+ | | |<-+ | | +->| | |<-+
| | | +--------+ | | | | +--------+ | | | | +--------+ | | | | +--------+ |
| | +---+ +---+ | | | | | TAM-2 | | | | +---+ +---+ | | | | | TAM-2 | |
| +-->|TA1| |TA2| | +-------+ | | | +--------+ | | +-->|TA1| |TA2| | +-------+ | | | +--------+ |
| | | | | | |<---------| App-2 |--+ | | | | | | | | | |<---------| App-2 |--+ | | |
| | | +---+ +---+ | +-------+ | | | Device Administrator | | | +---+ +---+ | +-------+ | | | Device Administrator
| | +-------------+ | App-1 | | | | | | +-------------+ | App-1 | | | |
| | | | | | | | | | | | | |
| +--------------------| |---+ | | | +--------------------| |---+ | |
| | |--------+ | | | |--------+ |
| +-------+ | | +-------+ |
+-------------------------------------------+ +-------------------------------------------+
Figure 1: Notional Architecture of TEEP Figure 1: Notional Architecture of TEEP
- Service Providers (SP) and Device Administrators (DA) utilize the - TA developers and Device Administrators utilize the services of a
services of a TAM to manage TAs on Devices. SPs do not directly TAM to manage TAs on devices. TA developers do not directly
interact with devices. DAs may elect to use a TAM for remote interact with devices. Device Administators may elect to use a
administration of 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 TA's on behalf of performing lifecycle management activity on TAs on behalf of TA
Service Providers and Device Administrators. This includes developers and Device Administrators. This includes creation and
creation and deletion of TA's, and may include, for example, over- deletion of TAs, and may include, for example, over-the-air
the-air updates to keep an SP's TAs up-to-date and clean up when a updates to keep TAs up-to-date and clean up when a version should
version should be removed. TAMs may provide services that make it be removed. TAMs may provide services that make it easier for TA
easier for SPs or DAs to use the TAM's service to manage multiple developers or Device Administators to use the TAM's service to
devices, although that is not required of a TAM. manage multiple devices, although that is not required of a TAM.
The TAM performs its management of TA's through an interaction The TAM performs its management of TAs on the device through
with a Device's TEEP Broker. As shown in Figure 1, the TAM cannot interactions with a device's TEEP Broker, which relays messages
directly contact a TEEP Agent, but must wait for the TEEP Broker between a TAM and a TEEP Agent running inside the TEE. As shown
to contact the TAM requesting a particular service. This in Figure 1, the TAM cannot directly contact a TEEP Agent, but
architecture is intentional in order to accommodate network and must wait for the TEEP Broker to contact the TAM requesting a
application firewalls that normally protect user and enterprise particular service. This architecture is intentional in order to
devices from arbitrary connections from external network entities. accommodate network and application firewalls that normally
protect user and enterprise devices from arbitrary connections
from external network entities.
A TAM may be publicly available for use by many SPs, or a TAM may A TAM may be publicly available for use by many TA developers, or
be private, and accessible by only one or a limited number of SPs. a TAM may be private, and accessible by only one or a limited
It is expected that manufacturers and carriers will run their own number of TA developers. It is expected that many manufacturers
private TAM. Another example of a private TAM is a TAM running as and network carriers will run their own private TAM.
a Software-as-a-Service (SaaS) within an SP.
A SP or Device Administrator chooses a particular TAM based on A TA developer or Device Administrator chooses a particular TAM
whether the TAM is trusted by a Device or set of Devices. The TAM based on whether the TAM is trusted by a device or set of devices.
is trusted by a device if the TAM's public key is an authorized The TAM is trusted by a device if the TAM's public key is, or
Trust Anchor in the Device. A SP or Device Administrator may run chains up to, an authorized Trust Anchor in the device. A TA
their own TAM, however the Devices they wish to manage must developer or Device Administrator may run their own TAM, but the
include this TAM's pubic key in the Trust Anchor list. devices they wish to manage must include this TAM's public key/
certificate, or a certificate it chains up to, in the Trust Anchor
list.
A SP or Device Administrator is free to utilize multiple TAMs. A TA developer or Device Administrator is free to utilize multiple
This may be required for a SP to manage multiple different types TAMs. This may be required for a TA developer to manage multiple
of devices from different manufacturers, or devices on different different types of devices from different manufacturers, or to
carriers, since the Trust Anchor list on these different devices manage mobile devices on different network carriers, since the
may contain different TAMs. A Device Administrator may be able to Trust Anchor list on these different devices may contain different
add their own TAM's public key or certificate to the Trust Anchor TAMs. A Device Administrator may be able to add their own TAM's
list on all their devices, overcoming this limitation. public key or certificate to the Trust Anchor list on all their
devices, overcoming this limitation.
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 Devices it must have its public key or certificate installed in a device's
Trust Anchor list. A TAM may set up a relationship with device Trust Anchor list. A TAM may set up a relationship with device
manufacturers or carriers to have them install the TAM's keys in manufacturers or network carriers to have them install the TAM's
their device's Trust Anchor list. Alternatively, a TAM may keys in their device's Trust Anchor list. Alternatively, a TAM
publish its certificate and allow Device Administrators to install may publish its certificate and allow Device Administrators to
the TAM's certificate in their devices as an after-market-action. install the TAM's certificate in their devices as an after-market-
action.
- TEEP Broker: The TEEP Broker is an application component running - TEEP Broker: A TEEP Broker is an application component running in
in a Rich Execution Environment (REE) that enables the message a Rich Execution Environment (REE) that enables the message
protocol exchange between a TAM and a TEE in a device. The TEEP protocol exchange between a TAM and a TEE in a device. A TEEP
Broker does not process messages on behalf of a TEE, but merely is Broker does not process messages on behalf of a TEE, but merely is
responsible for relaying messages from the TAM to the TEE, and for responsible for relaying messages from the TAM to the TEE, and for
returning the TEE's responses to the TAM. returning the TEE's responses to the TAM. In devices with no REE,
the TEEP Broker would be absent and instead the TEEP protocol
transport 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 that are relayed via a TEEP Broker TEE that receives TAM requests (typically relayed via a TEEP
that runs in an REE. A TEEP Agent in the TEE may parse requests Broker that runs in an REE). A TEEP Agent in the TEE may parse
or forward requests to other processing modules in a TEE, which is requests or forward requests to other processing modules in a TEE,
up to a TEE provider's implementation. A response message which is up to a TEE provider's implementation. A response
corresponding to a TAM request is sent by a TEEP Agent back to a message corresponding to a TAM request is sent back to the TAM,
TEEP Broker. again typically relayed via a TEEP Broker.
- Certification Authority (CA): Certificate-based credentials used - Certification Authority (CA): Certificate-based credentials used
for authenticating a device, a TAM and an SP. A device embeds a for authenticating a device, a TAM and a TA developer. A device
list of root certificates (Trust Anchors), from trusted CAs that a embeds a list of root certificates (Trust Anchors), from trusted
TAM will be validated against. A TAM will remotely attest a CAs that a TAM will be validated against. A TAM will remotely
device by checking whether a device comes with a certificate from attest a device by checking whether a device comes with a
a CA that the TAM trusts. The CAs do not need to be the same; certificate from a CA that the TAM trusts. The CAs do not need to
different CAs can be chosen by each TAM, and different device CAs be the same; different CAs can be chosen by each TAM, and
can be used by different device manufacturers. different device CAs can be used by different device
manufacturers.
4.2. Different Renditions of TEEP Architecture 4.2. Multiple TEEs in a Device
There is nothing prohibiting a device from implementing multiple Some devices might implement multiple TEEs. In these cases, there
TEEs. In addition, some TEEs (for example, SGX) present themselves might be one shared TEEP Broker that interacts with all the TEEs in
the device. However, some TEEs (for example, SGX) present themselves
as separate containers within memory without a controlling manager as separate containers within memory without a controlling manager
within the TEE. In these cases, the Rich Execution Environment hosts within the TEE. As such, there might be multiple TEEP Brokers in the
multiple TEEP brokers, where each Broker manages a particular TEE or Rich Execution Environment, where each TEEP Broker communicates with
set of TEEs. Enumeration and access to the appropriate TEEP Broker one or more TEEs associated with it.
is up to the Rich Execution Environment and the Untrusted
Applications. Verification that the correct TA has been reached then It is up to the Rich Execution Environment and the Untrusted
becomes a matter of properly verifying TA attestations, which are Applications how they select the correct TEEP Broker. Verification
unforgeable. The multiple TEEP Broker approach is shown in the that the correct TA has been reached then becomes a matter of
diagram below. For brevity, TEEP Broker 2 is shown interacting with properly verifying TA attestations, which are unforgeable.
only one TAM and UA, but no such limitation is intended to be implied
in the architecture. The multiple TEEP Broker approach is shown in the diagram below. For
brevity, TEEP Broker 2 is shown interacting with only one TAM and
Untrusted Application and only one TEE, but no such limitations are
intended to be implied in the architecture.
+-------------------------------------------+ +-------------------------------------------+
| Device | | Device |
| +--------+ | Service Provider | | TA Developer
| | |----------+ | | +-------------+ | |
| +-------------+ | TEEP |---------+| | | | TEE-1 | | |
| | TEE-1 | +---| Broker | | || +--------+ | | | +-------+ | +--------+ | +--------+ |
| | | | | 1 |<---+ | |+-->| |<-+ | | | TEEP | | | TEEP |------------->| |<-+
| | +-------+ | | | | | | | | | | | | Agent |<----------| Broker | | | |
| | | TEEP | | | | | | | | | | | | | 1 | | | 1 |---------+ | |
| | | Agent |<------+ | | | | | | | | | +-------+ | | | | | | |
| | | 1 | | | | | | | | | | | | | |<---+ | | | |
| | +-------+ | | | | | | | |
| | | | | | | | | |
| | +---+ +---+ | | | | | | +-| TAM-1 | | | +---+ +---+ | | | | | | +-| TAM-1 |
| | |TA1| |TA2| | | |<-+ | | +->| | |<-+ | | |TA1| |TA2| | | |<-+ | | +->| | |<-+
| +-->| | | |<---+ +--------+ | | | | +--------+ | | +-->| | | |<---+ +--------+ | | | | +--------+ |
| | | +---+ +---+ | | | | | | TAM-2 | | | | | +---+ +---+ | | | | | | TAM-2 | |
| | | | | +-------+ | | | +--------+ | | | | | | +-------+ | | | +--------+ |
| | +-------------+ +-----| App-2 |--+ | | ^ | | | +-------------+ +-----| App-2 |--+ | | ^ |
| | +-------+ | | | | Device | | +-------+ | | | | Device
| +--------------------| App-1 | | | | | Administrator | +--------------------| App-1 | | | | | Administrator
| +------| | | | | | | +------| | | | | |
| +-----------|-+ | |---+ | | | | +-----------|-+ | |---+ | | |
| | TEE-2 | | | |--------+ | | | | TEE-2 | | | |--------+ | |
| | +------+ | | | |------+ | | | | +------+ | | | |------+ | |
| | | TEEP | | | +-------+ | | | | | | TEEP | | | +-------+ | | |
| | | Agent|<-----+ | | | | | | Agent|<-----+ | | |
| | | 2 | | | | | | | | | | 2 | | | | | | |
| | +------+ | | | | | | | | +------+ | | | | | |
| | | | | | | | | | | | | | | |
| | +---+ | | | | | | | | +---+ | | | | | |
| | |TA3|<----+ | | +----------+ | | | | | |TA3|<----+ | | +----------+ | | |
| | | | | | | 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
TA's in TEE-1, and TEEP Broker 2 controls interactions with the TA's TAs in TEE-1, and TEEP Broker 2 controls interactions with the TAs in
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 TEE's 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 TAs or no need for TEEP Brokers to share information on installed TAs or
resource usage. However, the architecture guarantees that the TAM resource usage.
will receive all the relevant information from the TEEP Broker to
which it communicates.
4.3. Multiple TAMs and Relationship to TAs 4.3. Multiple TAMs and Relationship to TAs
As shown in Figure 2, the TEEP Broker provides connections from the As shown in Figure 2, a TEEP Broker provides communication between
TEE and the Untrusted Application to one or more TAMs. The selection one or more TEEP Agents and one or more TAMs. The selection of which
of which TAM to communicate with is dependent on information from the TAM to communicate with might be made with or without input from an
Untrusted Application and is directly related to the TA. Untrusted Application, but is ultimately the decision of a TEEP
Agent.
When a SP offers a service which requires a TA, the SP associates Each TA is digitally signed, protecting its integrity, and linking
that service with a specific TA. The TA itself is digitally signed, the TA back to the signer. The signer is usually the TA software
protecting its integrity, but the signature also links the TA back to author, but in some cases might be another party that the TA software
the signer. The signer is usually the SP, but in some cases may be author trusts, or a party to whom the code has been licensed (in
another party that the SP trusts. The SP selects one or more TAMs which case the same code might be signed by multiple licensees and
through which to offer their service, and communicates the distributed as if it were different TAs).
information of the service and the specific Untrusted Applications
and TAs to the TAM.
The SP chooses TAMs based upon the markets into which the TAM can A TA author or signer selects one or more TAMs through which to offer
provide access. There may be TAMs that provide services to specific their TA(s), and communicates the TA(s) to the TAM. In this
types of mobile devices, or mobile device operating systems, or document, we use the term "TA developer" to refer to the entity that
specific geographical regions or network carriers. A SP may be selects a TAM and publishes a signed TA to it, independent of whether
the publishing entity is the TA software author or the signer or
both.
The TA developer chooses TAMs based upon the markets into which the
TAM can provide access. There may be TAMs that provide services to
specific types of devices, or device operating systems, or specific
geographical regions or network carriers. A TA developer may be
motivated to utilize multiple TAMs for its service in order to motivated to utilize multiple TAMs for its service in order to
maximize market penetration and availability on multiple types of maximize market penetration and availability on multiple types of
devices. This likely means that the same service will be available devices. This likely means that the same TA will be available
through multiple TAMs. through multiple TAMs.
When the SP publishes the Untrusted Application to an app store or When the developer of an Untrusted Application that depends on a TA
other app repositories, the SP binds the Untrusted Application with a publishes the Untrusted Application to an app store or other app
manifest that identifies what TAMs can be contacted for the TA. In repository, the developer optionally binds the Untrusted Application
some situations, an SP may use only a single TAM - this is likely the with a manifest that identifies what TAMs can be contacted for the
case for enterprise applications or SPs serving a closed community. TA. In some situations, a TA may only be available via a single TAM
For broad public apps, there will likely be multiple TAMs in the - this is likely the case for enterprise applications or TA
manifest - one servicing one brand of mobile device and another developers serving a closed community. For broad public apps, there
servicing a different manufacturer, etc. Because different devices will likely be multiple TAMs in the manifest - one servicing one
and different manufacturers trust different TAMs, the manifest will brand of mobile device and another servicing a different
include different TAMs that support this SP's Untrusted Application manufacturer, etc. Because different devices and different
and TA. Multiple TAMs allow the SP to provide their service and this manufacturers trust different TAMs, the manifest can include multiple
app (and TA) to multiple different devices. TAMs that support the required TA.
When a TEEP Broker receives a request from an Untrusted Application When a TEEP Broker receives a request from an Untrusted Application
to install a TA, a list of TAM URIs may be provided for that TA, and to install a TA, a list of TAM URIs may be provided for that TA, and
the request is passed to the TEEP Agent. If the TEEP Agent decides the request is passed to the TEEP Agent. If the TEEP Agent decides
that the TA needs to be installed, the TEEP Agent selects a single that the TA needs to be installed, the TEEP Agent selects a single
TAM URI that is consistent with the list of trusted TAMs provisioned TAM URI that is consistent with the list of trusted TAMs provisioned
on the device invokes the HTTP transport for TEEP to connect to the on the device, invokes the HTTP transport for TEEP to connect to the
TAM URI and begins a TEEP protocol exchange. When the TEEP Agent TAM URI, and begins a TEEP protocol exchange. When the TEEP Agent
subsequently receives the TA to install and the TA's manifest subsequently receives the TA to install and the TA'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 TA, then the TEEP Agent recursively follows the same install the TA, then the TEEP Agent recursively follows the same
procedure for each dependency that needs to be installed or updated, procedure for each dependency that needs to be installed or updated,
including selecting a TAM URI that is consistent with the list of including selecting a TAM URI that is consistent with the list of
trusted TAMs provisioned on the device, and beginning a TEEP trusted TAMs provisioned on the device, and beginning a TEEP
exchange. If multiple TAM URIs are considered trusted, only one exchange. If multiple TAM URIs are considered trusted, only one
needs to be contacted and they can be attempted in some order until needs to be contacted and they can be attempted in some order until
one responds. 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 [I-D.ietf-suit-manifest]
for expressing how to install the TA as well as dependencies on other for expressing how to install a TA, as well as any dependencies on
TEE components and versions. That is, dependencies from TAs on other other TEE components and versions. That is, dependencies from TAs on
TEE components can be expressed in a SUIT manifest, including other TEE components can be expressed in a SUIT manifest, including
dependencies on any other TAs, or trusted OS code (if any), or dependencies on any other TAs, or trusted OS code (if any), or
trusted firmware. Installation steps can also be expressed in a SUIT trusted firmware. Installation steps can also be expressed in a SUIT
manifest. manifest.
For example, TEE's compliant with Global Platform may have a notion For example, TEEs compliant with GlobalPlatform may have a notion of
of a "security domain" (which is a grouping of one or more TAs a "security domain" (which is a grouping of one or more TAs installed
installed on a device, that can share information within such a on a device, that can share information within such a group) that
group) that must be created and into which one or more TAs can then must be created and into which one or more TAs can then be installed.
be installed. It is thus up to the SUIT manifest to express a It is thus up to the SUIT manifest to express a dependency on having
dependency on having such a security domain existing or being created such a security domain existing or being created first, as
first, as appropriate. appropriate.
Updating a TA may cause compatibility issues with any Untrusted Updating a TA may cause compatibility issues with any Untrusted
Applications or other components that depend on the updated TA, just Applications or other components that depend on the updated TA, just
like updating the OS or a shared library could impact an Untrusted like updating the OS or a shared library could impact an Untrusted
Application. Thus, an implementation needs to take into account such Application. Thus, an implementation needs to take into account such
issues. issues.
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 the In TEEP, there is an explicit relationship and dependence between an
Untrusted Application in the REE and one or more TAs in the TEE, as Untrusted Application in a REE and one or more TAs in a TEE, as shown
shown in Figure 2. For most purposes, an Untrusted Application that in Figure 2. For most purposes, an Untrusted Application that uses
uses one or more TA's in a TEE appears no different from any other 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 TA's 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 are 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
the TEE. In addition to the Untrusted Application and TA, the TA a TEE. In addition to the Untrusted Application and TA(s), the TA(s)
and/or TEE may require some additional data to personalize the TA to and/or TEE may require some additional data to personalize the TA to
the service provider or the device or a user. This personalization the TA developer or the device or a user. This personalization data
data is dependent on the TEE, the TA and the SP; an example of is dependent on the TEE, the TA, and the TA developer; an example of
personalization data might be a secret symmetric key used by the TA personalization data might be a secret symmetric key used by the TA
to communicate with the SP. The personalization data must be to communicate with the TA developer. The personalization data must
encrypted to preserve the confidentiality of potentially sensitive be encrypted to preserve the confidentiality of potentially sensitive
data contained within it. Other than this requirement to support data contained within it. Other than this requirement to support
confidentiality, TEEP place no limitations or requirements on the confidentiality, the TEEP architecture places no limitations or
personalization data. requirements on the personalization data.
There are three possible cases for bundling of the Untrusted There are three possible cases for bundling of an Untrusted
Application, TA, and personalization data: Application, TA(s), and personalization data:
1. The Untrusted Application, TA, and personalization data are all 1. The Untrusted Application, TA(s), and personalization data are
bundled together in a single package by the SP and provided to all bundled together in a single package by a TA developer and
the TEEP Broker through the TAM. provided to the TEEP Broker through the TAM.
2. The Untrusted Application and the TA 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 SP's TAM. the TA developer's TAM.
3. All components are independent. The Untrusted Application is 3. All components are independent. The Untrusted Application is
installed through some independent or device-specific mechanism, installed through some independent or device-specific mechanism,
and the TAM provides the TA and personalization data from the SP. and the TAM provides the TA and personalization data from the TA
Delivery of the TA and personalization data may be combined or developer. Delivery of the TA and personalization data may be
separate. combined or separate.
The TEEP protocol treats the TA, any dependencies the TA has, and The TEEP protocol treats each TA, any dependencies the TA has, and
personalization data as separate components with separate personalization data as separate 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 {{I- manifest might contain or reference multiple binaries (see
D.ietf-suit-manifest} for more details). The TEEP Agent is [I-D.ietf-suit-manifest] for more details). The TEEP Agent is
responsible for handling any installation steps that need to be responsible for handling any installation steps that need to be
performed inside the TEE, such as decryption of private TA bianries performed inside the TEE, such as decryption of private TA binaries
or personalization data. or personalization data.
4.5. Examples of Application Delivery Mechanisms in Existing TEEs 4.4.1. Examples of Application Delivery Mechanisms in Existing TEEs
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.
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 (Case 3). In this case, the Untrusted enclave, and use that TA (Case 3). 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. In SGX, for Case 2 and Case 3, the personalization data is SGX TEE. In SGX, for Case 2 and Case 3, the personalization data is
normally loaded into the SGX enclave (the TA) after the TA has normally loaded into the SGX enclave (the TA) after the TA has
started. Although Case 1 is possible with SGX, there are no started. Although Case 1 is possible with SGX, there are no
instances of this known to be in use at this time, since such a instances of this known to be in use at this time, since such a
construction would require a special installation program and SGX TA construction would require a special installation program and SGX TA
to receive the encrypted binary, decrypt it, separate it into the to receive the encrypted binary, decrypt it, separate it into the
three different elements, and then install all three. This three different elements, and then install all three. This
installation is complex, because the Untrusted Application decrypted installation is complex because the Untrusted Application decrypted
inside the TEE must be passed out of the TEE to an installer in the inside the TEE must be passed out of the TEE to an installer in the
REE which would install the Untrusted Application; this assumes that REE which would install the Untrusted Application; this assumes that
the Untrusted Application package includes the TA code also, since the Untrusted Application package includes the TA code also, since
otherwise there is a significant problem in getting the SGX enclave otherwise there is a significant problem in getting the SGX enclave
code (the TA) from the TEE, through the installer and into the code (the TA) from the TEE, through the installer, and into the
Untrusted Application in a trusted fashion. Finally, the Untrusted Application in a trusted fashion. Finally, the
personalization data would need to be sent out of the TEE (encrypted personalization data would need to be sent out of the TEE (encrypted
in an SGX encalve-to-enclave manner) to the REE's installation app, in an SGX enclave-to-enclave manner) to the REE's installation app,
which would pass this data to the installed Untrusted Application, which would pass this data to the installed Untrusted Application,
which would in turn send this data to the SGX enclave (TA). This which would in turn send this data to the SGX enclave (TA). This
complexity is due to the fact that each SGX enclave is separate and complexity is due to the fact that each SGX enclave is separate and
does not have direct communication to other SGX enclaves. does not have direct communication to other SGX enclaves.
In Arm TrustZone for A- and R-class devices, the Untrusted In Arm TrustZone for A- and R-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 to
a specific Untrused Application process lifetime as occurs in SGX. A a specific Untrused Application process lifetime as occurs in SGX. A
TA is loaded by a trusted OS running in the TEE, where the trusted OS TA is loaded by a trusted OS running in the TEE, where the trusted OS
is separate from the OS in the REE. Thus Cases 2 and 3 are equally is separate from the OS in the REE. Thus Cases 2 and 3 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 the Untrusted Application, and so the each other without involving any Untrusted Application, and so the
complexity of Case 1 is lower than in the SGX example, and so Case 1 complexity of Case 1 is lower than in the SGX example. Thus, Case 1
is possible as well though still more complex than Cases 2 and 3. is possible as well, though still more complex than Cases 2 and 3.
4.6. Entity Relations 4.5. Entity Relations
This architecture leverages asymmetric cryptography to authenticate a This architecture leverages asymmetric cryptography to authenticate a
device to a TAM. Additionally, a TEE in a device authenticates a TAM device to a TAM. Additionally, a TEEP Agent in a device
and TA signer. The provisioning of Trust Anchors to a device may be authenticates a TAM. The provisioning of Trust Anchors to a device
different from one use case to the other. A device administrator may may be different from one use case to the other. A Device
want to have the capability to control what TAs are allowed. A Administrator may want to have the capability to control what TAs are
device manufacturer enables verification of the TA signers and TAM allowed. A device manufacturer enables verification of the TAM
providers; it may embed a list of default Trust Anchors that the providers and TA binary signers; it may embed a list of default Trust
signer of an allowed TA's signer certificate should chain to. A Anchors into the TEEP Agent and TEE for TAM trust verification and TA
device administrator may choose to accept a subset of the allowed TAs signer verification.
via consent or action of downloading.
(App Developer) (App Store) (TAM) (Device with TEE) (CAs) (App Developers) (App Store) (TAM) (Device with TEE) (CAs)
| | | | | | |
| --> (Embedded TEE cert) <-- | | | (Embedded TEE cert) <--|
| | | | | | |
| <------------------------------ Get an app cert ----- | | <--- Get an app cert -----------------------------------|
| | <-- Get a TAM cert ------ | | | | | |
| | | | <-- Get a TAM cert ---------|
1. Build two apps: | | | | |
Untrusted Application 1. Build two apps: | | | |
TA | | | |
| (a) Untrusted | | | |
| App - 2a. Supply --> | --- 3. Install ------> | |
Untrusted Application -- 2a. --> | ----- 3. Install -------> | | | | |
TA ----------------- 2b. Supply ------> | 4. Messaging-->| (b) TA -- 2b. Supply ----------> | 4. Messaging-->| |
| | | | | | | |
Figure 3: Developer Experience Figure 3: Developer Experience
Note that Figure 3 shows the app developer as a TA signer and not the Note that Figure 3 shows the TA developer as a TA signer. The TA
SP. However, the App Developer is either closely associated with the signer is either the same as the TA developer, or is a related entity
SP or the SP delegates the signing authority to the app developer. trusted to sign the developer's TAs.
For the purpose of this document there is no difference between the
SP and the app developer.
Figure 3 shows an application developer building two applications: 1) Figure 3 shows an example where the same developer builds two
an Untrusted Application; 2) a TA that provides some security applications: 1) an Untrusted Application; 2) a TA that provides some
functions to be run inside a TEE. At step 2, the application security functions to be run inside a TEE. At step 2, the TA
developer uploads the Untrusted Application (2a) to an Application developer uploads the Untrusted Application (2a) to an Application
Store. The Untrusted Application may optionally bundle the TA Store. The Untrusted Application may optionally bundle the TA
binary. Meanwhile, the application developer may provide its TA to a binary. Meanwhile, the TA developer may provide its TA to a TAM that
TAM provider that will be managing the TA in various devices. 3. A will be managing the TA in various devices. At step 3, a user will
user will go to an Application Store to download the Untrusted go to an Application Store to download the Untrusted Application.
Application. The Untrusted Application will trigger TA installation Since the Untrusted Application depends on the TA, installing the
by initiating communication with a TAM. This is the step 4. The Untrusted Application will trigger TA installation by initiating
Untrusted Application will get messages from TAM, and interacts with communication with a TAM. This is step 4. The TEEP Agent will
device TEE via an Agent. interact with TAM via a TEEP Broker that faciliates communications
between a TAM and the TEEP Agent in TEE.
Some TA installation implementations might ask for a user's consent.
In other implementations, a Device Administrator might choose what
Untrusted Applications and related TAs to be installed. A user
consent flow is out of scope of the TEEP architecture.
The main components consist of a set of standard messages created by The main components consist of a set of standard messages created by
a TAM to deliver TA management commands to a device, and device a TAM to deliver TA management commands to a device, and device
attestation and response messages created by a TEE that responds to a attestation and response messages created by a TEE that responds to a
TAM's message. 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. Trusted not available in TAs in today's TEE-powered devices. Consequently,
Applications need to rely on a broker in the REE to interact with a Trusted Applications generally rely on broker in the REE to provide
TEE for network message exchanges. Consequently, a TAM generally access to nnetwork functionality in the REE. A broker does not need
communicates with an Untrusted Application about how it gets messages to know the actual content of messages to facilitate such access.
that originate from a TEE inside a device. Similarly, a TA or TEE
generally gets messages from a TAM via a TEEP Broker in this protocol
architecture, not directly from the network.
It is imperative to have an interoperable protocol to communicate Similarly, since the TEEP Agent runs inside a TEE, the TEEP Agent
with different TAMs and different TEEs in different devices. This is generally relies on a TEEP Broker in the REE to provide network
the role of the Broker, which is a software component that bridges access, and relay TAM requests to the TEEP Agent and relay the
communication between a TAM and a TEE. Furthermore the Broker responses back to the TAM.
communicates with a Agent inside a TEE that is responsible to process
TAM requests. The Broker in REE does not need to know the actual
content of messages except for the TEE routing information.
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
delivering end-to-end security between a TAM and a TEEP Agent. delivering end-to-end security between a TAM and a TEEP Agent.
Figure 4 summarizes the relationships between various keys and where Figure 4 summarizes the relationships between various keys and where
they are stored. Each public/private key identifies an SP, TAM, or they are stored. Each public/private key identifies a TA developer,
TEE, and gets a certificate that chains up to some CA. A list of TAM, or TEE, and gets a certificate that chains up to some CA. A
trusted certificates is then used to check a presented certificate list of trusted certificates is then used to check a presented
against. 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's private originator's private key, such as that of a TAM or a TEE. In
key. In addition to the keys shown in Figure 4, there may be addition to the keys shown in Figure 4, there may be additional keys
additional keys used for attestation. Refer to the RATS Architecture used for attestation. Refer to the RATS Architecture
for more discussion. [I-D.ietf-rats-architecture] for more discussion.
Cardinality & Location of Cardinality & Location of
Location of Private Key Corresponding Location of Private Key Trust Anchor
Purpose Private Key Signs CA Certs Purpose Private Key Signs Store
------------------ ----------- ------------- ------------- ------------------ ----------- ------------- -------------
Authenticating TEE 1 per TEE TEEP responses TAM Authenticating TEE 1 per TEE TEEP responses TAM
Authenticating TAM 1 per TAM TEEP requests TEEP Agent Authenticating TAM 1 per TAM TEEP requests TEEP Agent
Code Signing 1 per SP TA binary TEE Code Signing 1 per TA TA binary TEE
developer
Figure 4: Keys Figure 4: 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.
The TEE key pair and certificate are used for authenticating the TEE The TEE key pair and certificate are used for authenticating the TEE
to a remote TAM. Often, the key pair is burned into the TEE by the to a remote TAM. Often, the key pair is burned into the TEE by the
TEE manufacturer and the key pair and its certificate are valid for TEE manufacturer and the key pair and its certificate are valid for
the expected lifetime of the TEE. A TAM provider is responsible for the expected lifetime of the TEE. A TAM provider is responsible for
configuring its TAM with the manufacturer certificates or CAs that configuring the TAM's Trust Anchor Store with the manufacturer
are used to sign TEE keys. certificates or CAs that are used to sign TEE keys. This is
discussed further in Section 5.3 below.
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. A TAM provider is responsible for acquiring a a remote TEE. A TAM provider is responsible for acquiring a
certificate from a CA that is trusted by the TEEs it manages. certificate from a CA that is trusted by the TEEs it manages. This
is discussed further in Section 5.1 below.
The SP key pair and certificate are used to sign TAs that the TEE The TA developer key pair and certificate are used to sign TAs that
will consider authorized to execute. TEEs must be configured with the TEE will consider authorized to execute. TEEs must be configured
the CAs that it considers authorized to sign TAs that it will with the certificates or keys that it considers authorized to sign
execute. TAs that it will execute. This is discussed further in Section 5.2
below.
5.1. Trust Anchors in TEE 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 CA certificates that sign various TAM certificates. The which are CA certificates that sign various TAM certificates. The
list is typically preloaded at manufacturing time, and can be updated list is typically preloaded at manufacturing time, and can be updated
using the TEEP protocol if the TEE has some form of "Trust Anchor using the TEEP protocol if the TEE has some form of "Trust Anchor
Manager TA" that has Trust Anchors in its configuration data. Thus, Manager TA" that has Trust Anchors in its configuration data. Thus,
Trust Anchors can be updated similar to updating the configuration Trust Anchors can be updated similar to updating the configuration
data for any other TA. data for any other TA.
When Trust Anchor update is carried out, it is imperative that any When Trust Anchor update is carried out, it is imperative that any
update must maintain integrity where only authentic Trust Anchor list update must maintain integrity where only an authentic Trust Anchor
from a device manufacturer or a Device Administrator is accepted. list from a device manufacturer or a Device Administrator is
This calls for a complete lifecycle flow in authorizing who can make accepted. Details are out of scope of the architecture and can be
Trust Anchor update and whether a given Trust Anchor list are non- addressed in a protocol document.
tampered from the original provider. The signing of a Trust Anchor
list for integrity check and update authorization methods are
desirable to be developed. This can be addressed outside of this
architecture 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 obtain a TAM certificate from a device with a particular TEE, it must obtain a TAM certificate from a
CA that is listed in the Trust Anchor store of the TEE. CA that is listed in the Trust Anchor Store of the TEEP Agent.
5.2. Trust Anchors in TAM 5.2. Trust Anchors in a TEE
The Trust Anchor store in a TAM consists of a list of Trust Anchors, A TEE determines whether TA binaries are allowed to execute by
which are CA certificates that sign various device TEE certificates. verifying whether the TA's signer chains up to a certificate in the
A TAM will accept a device for TA management if the TEE in the device TEE's Trust Anchor Store. The list is typically preloaded at
uses a TEE certificate that is chained to a CA that the TAM trusts. manufacturing time, 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 configuration data. Thus, Trust Anchors can be updated
similar to updating the configuration data for any other TA, as
discussed in Section 5.1.
5.3. Scalability 5.3. Trust Anchors in a TAM
This architecture uses a PKI. Trust Anchors exist on the devices to The Trust Anchor Store in a TAM consists of a list of Trust Anchors,
enable the TEE to authenticate TAMs, and TAMs use Trust Anchors to which are certificates that sign various device TEE certificates. A
authenticate TEEs. Since a PKI is used, many intermediate CA TAM will accept a device for TA management if the TEE in the device
uses a TEE certificate that is chained to a certificate that the TAM
trusts.
5.4. Scalability
This architecture uses a PKI, although self-signed certificates are
also permitted. Trust Anchors exist on the devices to enable the TEE
to authenticate TAMs and TA signers, and TAMs use Trust Anchors to
authenticate TEEs. When a PKI is used, many intermediate CA
certificates can chain to a root certificate, each of which can issue certificates can chain to a root certificate, each of which can issue
many certificates. This makes the protocol highly scalable. New many certificates. This makes the protocol highly scalable. New
factories that produce TEEs can join the ecosystem. In this case, factories that produce TEEs can join the ecosystem. In this case,
such a factory can get an intermediate CA certificate from one of the such a factory can get an intermediate CA certificate from one of the
existing roots without requiring that TAMs are updated with existing roots without requiring that TAMs are updated with
information about the new device factory. Likewise, new TAMs can information about the new device factory. Likewise, new TAMs can
join the ecosystem, providing they are issued a TAM certificate that join the ecosystem, providing they are issued a TAM certificate that
chains to an existing root whereby existing TEEs will be allowed to chains to an existing root whereby existing TEEs will be allowed to
be personalized by the TAM without requiring changes to the TEE be personalized by the TAM without requiring changes to the TEE
itself. This enables the ecosystem to scale, and avoids the need for itself. This enables the ecosystem to scale, and avoids the need for
centralized databases of all TEEs produced or all TAMs that exist. centralized databases of all TEEs produced or all TAMs that exist or
all TA developers that exist.
5.4. Message Security 5.5. Message Security
Messages created by a TAM are used to deliver TA management commands Messages created by a TAM are used to deliver TA management commands
to a device, and device attestation and messages created by the to a device, and device attestation and messages created by the
device TEE to respond to TAM messages. 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,
and are typically encrypted such that only the targeted device TEE or and are typically encrypted such that only the targeted device TEE or
TAM is able to decrypt and view the actual content. TAM is able to decrypt and view the actual content.
6. TEEP Broker 6. TEEP Broker
skipping to change at page 20, line 5 skipping to change at page 20, line 28
communicate outside of the hosting device. For example, communicate outside of the hosting device. For example,
GlobalPlatform [GPTEE] specifies one such architecture. This calls GlobalPlatform [GPTEE] specifies one such architecture. This calls
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 TA installation that the application needs to use. and trigger TA installation that the application needs to use.
An Untrusted Application might communicate with the TEEP Broker at An Untrusted Application might communicate with a TEEP Broker at
runtime to trigger TA installation itself. Or an Untrusted runtime to trigger TA installation itself, or an Untrusted
Application might simply have a metadata file that describes the TAs Application might simply have a metadata file that describes the TAs
it depends on and the associated TAM(s) for each TA, and an REE it depends on and the associated TAM(s) for each TA, and an REE
Application Installer can inspect this application metadata file and Application Installer can inspect this application metadata file and
invoke the TEEP Broker to trigger TA installation on behalf of the invoke the TEEP Broker to trigger TA installation on behalf of the
Untrusted Application without requiring the Untrusted Application to Untrusted Application without requiring the Untrusted Application to
run first. run first.
6.1. Role of the TEEP Broker 6.1. Role of the TEEP Broker
A TEEP Broker abstracts the message exchanges with a TEE in a device. A TEEP Broker abstracts the message exchanges with a TEE in a device.
The input data is originated from a TAM or the first initialization The input data is originated from a TAM or the first initialization
call to trigger a TA installation. call to trigger a TA installation.
The Broker doesn't need to parse a message content received from a The Broker doesn't need to parse a message content received from a
TAM that should be processed by a TEE. When a device has more than TAM that should be processed by a TEE. When a device has more than
one TEE, one TEEP Broker per TEE could be present in REE. A TEEP one TEE, one TEEP Broker per TEE could be present in the REE. A TEEP
Broker interacts with a TEEP Agent inside a TEE. Broker interacts with a TEEP Agent inside a TEE.
A TAM message may indicate the target TEE where a TA should be A TAM message may indicate the target TEE where a TA should be
installed. A compliant TEEP protocol should include a target TEE installed. A compliant TEEP protocol should include a target TEE
identifier for a TEEP Broker when multiple TEEs are present. identifier for a TEEP Broker when multiple TEEs are present.
The Broker relays the response messages generated from a TEEP Agent The Broker relays the response messages generated from a TEEP Agent
in a TEE to the TAM. The Broker is not expected to handle any in a TEE to the TAM.
network connection with an application or TAM.
The Broker only needs to return an error message if the TEE is not The Broker only needs to return a (transport) error message if the
reachable for some reason. Other errors are represented as response TEE is not reachable for some reason. Other errors are represented
messages returned from the TEE which will then be passed to the TAM. as response messages returned from the TEE which will then be passed
to the TAM.
6.2. TEEP Broker Implementation Consideration 6.2. TEEP Broker Implementation Consideration
A Provider should consider methods of distribution, scope and TEEP Broker implementers should consider methods of distribution,
concurrency on devices and runtime options when implementing a TEEP scope and concurrency on devices and runtime options. Several non-
Broker. Several non-exhaustive options are discussed below. exhaustive options are discussed below.
Providers are encouraged to take advantage of the latest
communication and platform capabilities to offer the best user
experience.
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 a normal application) that it depends on a given installer, or an Untrusted Application) that it depends on a
TA, which may or may not already be installed in the TEE. given TA, which may or may not already be installed in the TEE.
2. ProcessTeepMessage: A message arriving from the network, to be 2. ProcessTeepMessage: A message arriving from the network, to be
delivered to the TEEP Agent for processing. delivered to the TEEP Agent for processing.
3. RequestPolicyCheck: A hint (e.g., based on a timer) that the TEEP 3. 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.
4. ProcessError: A notification that the TEEP Broker could not 4. 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 an incoming TEEP session is 1. ProcessConnect: A notification that an incoming TEEP session is
being requested by a TEEP Agent. being requested by a TEEP Agent.
2. ProcessTeepMessage: A message arriving from the network, to be 2. ProcessTeepMessage: A message arriving from the network, to be
delivered to the TAM for processing. delivered to the TAM for processing.
For further discussion on these APIs, see For further discussion on these APIs, see
[I-D.ietf-teep-otrp-over-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 OEM time. A user The Broker installation is commonly carried out at OEM time. A user
can dynamically download and install a Broker on-demand. can dynamically download and install a Broker on-demand.
6.2.3. Number of TEEP Brokers
There should be generally only one shared TEEP Broker in a device.
The device's TEE vendor will most probably supply one Broker. When
multiple TEEs are present in a device, one TEEP Broker per TEE may be
used.
When only one Broker is used per device, the Broker provider is
responsible to allow multiple TAMs and TEE providers to achieve
interoperability. With a standard Broker interface, each TAM can
implement its own SDK for its SP Untrusted Applications to work with
this Broker.
Multiple independent Broker providers can be used as long as they
have standard interface to an Untrusted Application or TAM SDK. Only
one Broker is generally expected in a device.
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 true. Different verifiers may have different standards for are true. Different Verifiers may have different standards for
attestation proofs and not all attestations are acceptable to every attestation proofs and not all attestations are acceptable to every
verifier. A third entity (a Relying Party) can then use "attestation verifier. A third entity (a Relying Party) can then use "attestation
results", in the form of another series of claims, from a Verifier to results", in the form of another series of claims, from a Verifier to
make authorization decisions. make authorization decisions. (See [I-D.ietf-rats-architecture] for
more discussion.)
In TEEP, as depicted in Figure 5, 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 TAMs (the attestation is to allow a device (the Attester) to prove to a TAM
Relying Parties) that a TEE in the device has particular properties, (the Relying Party) that a TEE in the device has particular
was built by a particular manufacturer, or is executing a particular properties, was built by a particular manufacturer, and/or is
TA. Other claims are possible; TEEP does not limit the claims that executing a particular TA. Other claims are possible; TEEP does not
may appear in evidence or attestation results, but defines a minimal limit the claims that may appear in evidence or attestation results,
set of attestation result claims required for TEEP to operate but defines a minimal set of attestation result claims required for
properly. Extensions to these claims are possible. Other standards TEEP to operate properly. Extensions to these claims are possible.
or groups may define the format and semantics of extended claims. Other standards or groups may define the format and 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 5: TEEP Attestation Roles Figure 5: TEEP Attestation Roles
As of the writing of this specification, device and TEE attestations As of the writing of this specification, device and TEE attestations
have not been standardized across the market. Different devices, have not been standardized across the market. Different devices,
manufacturers, and TEEs support different attestation algorithms and manufacturers, and TEEs support different attestation algorithms and
mechanisms. In order for TEEP to be inclusive, it is agnostic to the mechanisms. In order for TEEP to be inclusive, it is agnostic to the
format of evidence, allowing proprietary or standardized formats to format of evidence, allowing proprietary or standardized formats to
be used between a TEE and a verifier (which may or may not be be used between a TEE and a verifier (which may or may not be
colocated in the TAM). However, it should be recognized that not all colocated in the TAM). However, it should be recognized that not all
verifiers may be able to process all proprietary forms of attestation Verifiers may be able to process all proprietary forms of attestation
evidence. Similarly, the TEEP protocol is agnostic as to the format evidence. Similarly, the TEEP protocol is agnostic as to the format
of attestation results, and the protocol (if any) used between the of attestation results, and the protocol (if any) used between the
TAM and a verifier, as long as they convey at least the required set TAM and a verifier, as long as they convey at least the required set
of claims in some format. of claims in some format.
The assumptions which may apply to an attestation have to do with the The assumptions that may apply to an attestation have to do with the
quality of the attestation and the quality and security provided by quality of the attestation and the quality and security provided by
the TEE, the device, the manufacturer, or others involved in the the TEE, the device, the manufacturer, or others involved in the
device or TEE ecosystem. Some of the assumptions that might apply to device or TEE ecosystem. Some of the assumptions that might apply to
an attestations include (this may not be a comprehensive list): an attestations include (this may not be a comprehensive list):
- Assumptions regarding the security measures a manufacturer takes - Assumptions regarding the security measures a manufacturer takes
when provisioning keys into devices/TEEs; when provisioning keys into devices/TEEs;
- Assumptions regarding what hardware and software components have - Assumptions regarding what hardware and software components have
access to the Attestation keys of the TEE; access to the attestation keys of the TEE;
- Assumptions related to the source or local verification of claims - Assumptions related to the source or local verification of claims
within an attestation prior to a TEE signing a set of claims; within an attestation prior to a TEE signing a set of claims;
- Assumptions regarding the level of protection afforded to - Assumptions regarding the level of protection afforded to
attestation keys against exfiltration, modification, and side attestation keys against exfiltration, modification, and side
channel attacks; channel attacks;
- Assumptions regarding the limitations of use applied to TEE - Assumptions regarding the limitations of use applied to TEE
Attestation keys; attestation keys;
- Assumptions regarding the processes in place to discover or detect - Assumptions regarding the processes in place to discover or detect
TEE breeches; and TEE breeches; and
- Assumptions regarding the revocation and recovery process of TEE - Assumptions regarding the revocation and recovery process of TEE
attestation keys. attestation keys.
TAMs must be comfortable with the assumptions that are inherently TAMs must be comfortable with the assumptions that are inherently
part of any attestation result they accept. Alternatively, any TAM part of any attestation result they accept. Alternatively, any TAM
may choose not to accept an attestation result generated using may choose not to accept an attestation result generated using
skipping to change at page 23, line 46 skipping to change at page 24, line 8
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. These additional claims may help clear up authorize a device or TEE. These additional claims may help clear up
any assumptions for which the TAM wants to alleviate. The specific any assumptions for which the TAM wants to alleviate. The specific
format for these additional claims are outside the scope of this format for these additional claims are outside the scope of this
specification, but the TEEP protocol allows these additional claims specification, but the TEEP protocol allows these additional claims
to be included in the attestation messages. to be included in the attestation messages.
7.1. Information Required in TEEP Claims 7.1. Information Required in TEEP Claims
- Device Identifying Info: TEEP attestations may need to uniquely - Device Identifying Info: TEEP attestations may need to uniquely
identify a device to the TAM and SP. Unique device identification identify a device to the TAM and TA developer. Unique device
allows the TAM to provide services to the device, such as managing identification allows the TAM to provide services to the device,
installed TAs, and providing subscriptions to services, and such as managing installed TAs, and providing subscriptions to
locating device-specific keying material to communicate with or services, and locating device-specific keying material to
authenticate the device. In some use cases it may be sufficient communicate with or authenticate the device. In some use cases it
to identify only the class of the device. The security and may be sufficient to identify only the class of the device. The
privacy requirements regarding device identification will vary security and privacy requirements regarding device identification
with the type of TA provisioned to the TEE. will vary with the type of TA provisioned to the TEE.
- TEE Identifying info: The type of TEE that generated this - TEE Identifying info: The type of TEE that generated this
attestation must be identified. Standard TEE types are identified attestation must be identified, including version identification
by an IANA number, but also must include version identification
information such as the hardware, firmware, and software version information such as the hardware, firmware, and software version
of the TEE, as applicable by the TEE type. TEE manufacturer of the TEE, as applicable by the TEE type. TEE manufacturer
information for the TEE is required in order to disambiguate the information for the TEE is required in order to disambiguate the
same TEE type created by different manufacturers and resolve same TEE type created by different manufacturers and resolve
potential assumptions around manufacturer provisioning, keying and potential assumptions around manufacturer provisioning, keying and
support for the TEE. support for the TEE.
- Liveness Proof: A claim that includes liveness information must be - Freshness Proof: A claim that includes freshness information must
included, such as a nonce or timestamp. be included, such as a nonce or timestamp.
- Requested Components: A list of zero or more components (TAs or - Requested Components: A list of zero or more components (TAs or
other dependencies needed by a TEE) that are requested by some other dependencies needed by a TEE) that are requested by some
depending app, but which are not currently installed in the TEE. depending app, but which are not currently installed in the TEE.
8. Algorithm and Attestation Agility 8. Algorithm and Attestation Agility
RFC 7696 [RFC7696] outlines the requirements to migrate from one RFC 7696 [RFC7696] outlines the requirements to migrate from one
mandatory-to-implement algorithm suite to another over time. This mandatory-to-implement 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 already considered during greatly simplified when crypto agility is considered during the
the design of the protocol. In the case of the Trusted Execution design of the protocol. In the case of the TEEP protocol the diverse
Provisioning (TEEP) Protocol the diverse range of use cases, from range of use cases, from trusted app updates for smart phones and
trusted app updates for smart phones and tablets to updates of code tablets to updates of code on higher-end IoT devices, creates the
on higher-end IoT devices, creates the need for different mandatory- need for different mandatory-to-implement algorithms already from the
to-implement algorithms already 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. Symmetric algorithms are used for encryption asymmetric algorithms. Symmetric algorithms are used for encryption
of content whereas the asymmetric algorithms are mostly used for of content whereas the asymmetric algorithms are mostly used for
signing messages. signing messages.
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 the
Device may only support one. device may only support one.
9. Security Considerations 9. Security Considerations
9.1. TA Trust Check at TEE 9.1. Broker Trust Model
A TA binary is signed by a TA signer certificate. This TA signing
certificate/private key belongs to the SP, and may be self-signed
(i.e., it need not participate in a trust hierarchy). It is the
responsibility of the TAM to only allow verified TAs from trusted SPs
into the system. Delivery of that TA to the TEE is then the
responsibility of the TEE, using the security mechanisms provided by
the protocol.
We allow a way for an Untrusted Application to check the The architecture enables the TAM to communicate, via a TEEP Broker,
trustworthiness of a TA. A TEEP Broker has a function to allow an with the device's TEE to manage TAs. Since the TEEP Broker runs in a
application to query the information about a TA. potentially vulnerable REE, the TEEP Broker could, however, be (or be
infected by) malware. As such, all TAM messages are signed and
sensitive data is encrypted such that the TEEP Broker cannot modify
or capture sensitive data.
An Untrusted Application may perform verification of the TA by A TEEP Agent in a TEE is responsible for protecting against potential
verifying the signature of the TA. An application can do additional attacks from a compromised TEEP Broker or rogue malware in the REE.
trust checks on the certificate returned for this TA. It might trust A rogue TEEP Broker might send corrupted data to the TEEP Agent, or
the TAM, or require additional SP signer trust chaining. launch a DoS attack by sending a flood of TEEP protocol requests.
The TEEP Agent validates the signature of each TEEP protocol request
and checks the signing certificate against its Trust Anchors. To
mitigate DoS attacks, it might also add some protection scheme such
as a threshold on repeated requests or number of TAs that can be
installed.
9.2. One TA Multiple SP Case 9.2. Data Protection at TAM and TEE
A TA for multiple SPs must have a different identifier per SP. They The TEE implementation provides protection of data on the device. It
should appear as different TAs when they are installed in the same is the responsibility of the TAM to protect data on its servers.
device.
9.3. Broker Trust Model 9.3. Compromised REE
A TEEP Broker could be malware in the vulnerable REE. An Untrusted It is possible that the REE of a device is compromised. If the REE
Application will connect its TAM provider for required TA is compromised, several DoS attacks may be launched. The compromised
installation. It gets command messages from the TAM, and passes the REE may terminate the TEEP Broker such that TEEP transactions cannot
message to the Broker. reach the TEE. However, while a DoS attack cannot be prevented, the
REE cannot access anything in the TEE if it is implemented correctly.
Some TEEs may have some watchdog scheme to observe REE state and
mitigate DoS attacks against it but most TEEs don't have have such
capability.
The architecture enables the TAM to communicate with the device's TEE In some other scenarios, the compromised REE may ask a TEEP Broker to
to manage TAs. All TAM messages are signed and sensitive data is make repeated requests to a TEEP Agent in a TEE to install or
encrypted such that the TEEP Broker cannot modify or capture uninstall a TA. A TA installation or uninstallation request
sensitive data. constructed by the TEEP Broker or REE will be rejected by the TEEP
Agent because the request won't have the correct signature from a TAM
to pass the request signature validation.
9.4. Data Protection at TAM and TEE This can become a DoS attack by exhausting resources in a TEE with
repeated requests. In general, a DoS attack threat exists when the
REE is compromised, and a DoS attack can happen to other resources.
The TEEP architecture doesn't change this.
The TEE implementation provides protection of data on the device. It A compromised REE might also request initiating the full flow of
is the responsibility of the TAM to protect data on its servers. installation of TAs that are not necessary. It may also repeat a
prior legitimate TA installation request. A TEEP Agent
implementation is responsible for ensuring that it can recognize and
decline such repeated requests. It is also responsible for
protecting the resource usage allocated for TA management.
9.5. Compromised CA 9.4. Compromised CA
A root CA for TAM certificates might get compromised. Some TEE Trust A root CA for TAM certificates might get compromised. Some TEE Trust
Anchor update mechanism is expected from device OEMs. TEEs are Anchor update mechanism is expected from device OEMs. TEEs are
responsible for validating certificate revocation about a TAM responsible for validating certificate revocation about a TAM
certificate chain. certificate chain.
If the root CA of some TEE device certificates is compromised, these If the root CA of some TEE device certificates is compromised, these
devices might be rejected by a TAM, which is a decision of the TAM devices might be rejected by a TAM, which is a decision of the TAM
implementation and policy choice. TAMs are responsible for implementation and policy choice. TAMs are responsible for
validating any intermediate CA for TEE device certificates. validating any intermediate CA for TEE device certificates.
9.6. 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 to determine that the TAM is trustworthy. certificates to determine that the TAM is trustworthy.
9.6. Malicious TA Removal
It is possible that a rogue developer distributes a malicious
Untrusted Application and intends to get a malicious TA installed.
It's the responsibility of the TAM to not install malicious trusted
apps in the first place. The TEEP architecture allows a TEEP Agent
to decide which TAMs it trusts via Trust Anchors, and delegates the
TA authenticity check to the TAMs it trusts.
It may happen that a TA was previously considered trustworthy but is
later found to be buggy or compromised. In this case, the TAM can
initiate the removal of the TA by notifying devices to remove the TA
(and potentially the REE or device owner to remove any Untrusted
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
would occur the next time connectivity does exist.
Furthermore the policy in the Verifier in an attestation process can
be updated so that any evidence that includes the malicious TA would
result in an attestation failure.
9.7. Certificate Renewal 9.7. Certificate 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 2 to 5 years. A TAM should get renewed or rekeyed lifetime of 2 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 update. On the other hand, it that don't require device Trust Anchor update. On the other hand, it
is imperative that OEMs or device providers plan for support of Trust is imperative that OEMs or device providers plan for support of Trust
Anchor update in their shipped devices. Anchor update in their shipped devices.
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
configuration from being disclosed to the TAM that distributes them. configuration from being disclosed to the TAM that distributes them.
In such a scenario, the files can be encrypted end-to-end between an In such a scenario, the files can be encrypted end-to-end between a
SP and a TEE. However, there must be some means of provisioning the TA developer and a TEE. However, there must be some means of
decryption key into the TEE and/or some means of the SP securely provisioning the decryption key into the TEE and/or some means of the
learning a public key of the TEE that it can use to encrypt. One way TA developer securely learning a public key of the TEE that it can
to do this is for the SP to run its own TAM, merely to distribute the use to encrypt. One way to do this is for the TA developer to run
decryption key via the TEEP protocol, and the key file can be a its own TAM so that it can distribute the decryption key via the TEEP
dependency in the manifest of the encrypted TA. Thus, the TEEP Agent protocol, and the key file can be a dependency in the manifest of the
would look at the TA manifest, determine there is a dependency with a encrypted TA. Thus, the TEEP Agent would look at the TA manifest,
TAM URI of the SP's TAM. The Agent would then install the determine there is a dependency with a TAM URI of the TA developer's
dependency, and then continue with the TA installation steps, TAM. The Agent would then install the dependency, and then continue
including decrypting the TA binary with the relevant key. with the TA installation steps, including decrypting the TA binary
with the relevant key.
10. IANA Considerations 10. IANA Considerations
This document does not require actions by IANA. This document does not require actions by IANA.
11. Contributors 11. Contributors
- Andrew Atyeo - Andrew Atyeo, Intercede (andrew.atyeo@intercede.com)
- Intercede
- andrew.atyeo@intercede.com
- Liu Dapeng
- Alibaba Group
- maxpassion@gmail.com - Liu Dapeng, Alibaba Group (maxpassion@gmail.com)
12. Acknowledgements 12. Acknowledgements
We would like to thank Nick Cook, Minho Yoo, Brian Witten, Tyler Kim, We would like to thank Nick Cook, Minho Yoo, Brian Witten, Tyler Kim,
Alin Mutu, Juergen Schoenwaelder, Nicolae Paladi, Sorin Faibish, Ned Alin Mutu, Juergen Schoenwaelder, Nicolae Paladi, Sorin Faibish, Ned
Smith, Russ Housley, Jeremy O'Donoghue, and Anders Rundgren for their Smith, Russ Housley, Jeremy O'Donoghue, and Anders Rundgren for their
feedback. feedback.
13. Informative References 13. Informative References
[GPTEE] Global Platform, "GlobalPlatform Device Technology: TEE [GPTEE] GlobalPlatform, "GlobalPlatform Device Technology: TEE
System Architecture, v1.1", Global Platform GPD_SPE_009, System Architecture, v1.1", GlobalPlatform GPD_SPE_009,
January 2017, <https://globalplatform.org/specs-library/ January 2017, <https://globalplatform.org/specs-library/
tee-system-architecture-v1-1/>. tee-system-architecture-v1-1/>.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., and N. Smith,
"Remote Attestation Procedures Architecture", draft-ietf-
rats-architecture-01 (work in progress), February 2020.
[I-D.ietf-suit-manifest] [I-D.ietf-suit-manifest]
Moran, B., Tschofenig, H., and H. Birkholz, "A Concise Moran, B., Tschofenig, H., and H. Birkholz, "A Concise
Binary Object Representation (CBOR)-based Serialization Binary Object Representation (CBOR)-based Serialization
Format for the Software Updates for Internet of Things Format for the Software Updates for Internet of Things
(SUIT) Manifest", draft-ietf-suit-manifest-02 (work in (SUIT) Manifest", draft-ietf-suit-manifest-03 (work in
progress), November 2019. progress), February 2020.
[I-D.ietf-teep-otrp-over-http] [I-D.ietf-teep-otrp-over-http]
Thaler, D., "HTTP Transport for Trusted Execution Thaler, D., "HTTP Transport for Trusted Execution
Environment Provisioning: Agent-to- TAM Communication", Environment Provisioning: Agent-to- TAM Communication",
draft-ietf-teep-otrp-over-http-03 (work in progress), draft-ietf-teep-otrp-over-http-03 (work in progress),
November 2019. November 2019.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management [RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <https://www.rfc-editor.org/info/rfc6024>. 2010, <https://www.rfc-editor.org/info/rfc6024>.
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