TEEP M. Pei
Internet-Draft Symantec
Intended status: Informational H. Tschofenig
Expires: June 8, 2020 Arm Limited
D. Wheeler
Intel
A. Atyeo
Intercede
L. Dapeng
Alibaba Group
December 06, 2019
Trusted Execution Environment Provisioning (TEEP) Architecture
draft-ietf-teep-architecture-04
Abstract
A Trusted Execution Environment (TEE) is an environment that enforces
that only authorized code can execute with that environment, and that
any data used by such code cannot be read or tampered with by any
code outside that environment. This architecture document motivates
the design and standardization of a protocol for managing the
lifecycle of trusted applications running inside a TEE.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on June 8, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Payment . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 7
3.3. Internet of Things . . . . . . . . . . . . . . . . . . . 7
3.4. Confidential Cloud Computing . . . . . . . . . . . . . . 8
4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. System Components . . . . . . . . . . . . . . . . . . . . 8
4.2. Different Renditions of TEEP Architecture . . . . . . . . 10
4.3. Multiple TAMs and Relationship to TAs . . . . . . . . . . 12
4.4. Untrusted Apps, Trusted Apps, and Personalization Data . 13
4.5. Examples of Application Delivery Mechanisms in Existing
TEEs . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.6. Entity Relations . . . . . . . . . . . . . . . . . . . . 15
5. Keys and Certificate Types . . . . . . . . . . . . . . . . . 17
5.1. Trust Anchors in TEE . . . . . . . . . . . . . . . . . . 18
5.2. Trust Anchors in TAM . . . . . . . . . . . . . . . . . . 18
5.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 18
5.4. Message Security . . . . . . . . . . . . . . . . . . . . 19
6. TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1. Role of the TEEP Broker . . . . . . . . . . . . . . . . . 20
6.2. TEEP Broker Implementation Consideration . . . . . . . . 20
6.2.1. TEEP Broker APIs . . . . . . . . . . . . . . . . . . 20
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6.2.2. TEEP Broker Distribution . . . . . . . . . . . . . . 21
6.2.3. Number of TEEP Brokers . . . . . . . . . . . . . . . 21
7. Attestation . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Information Required in TEEP Claims . . . . . . . . . . . 23
8. Algorithm and Attestation Agility . . . . . . . . . . . . . . 24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9.1. TA Trust Check at TEE . . . . . . . . . . . . . . . . . . 24
9.2. One TA Multiple SP Case . . . . . . . . . . . . . . . . . 25
9.3. Broker Trust Model . . . . . . . . . . . . . . . . . . . 25
9.4. Data Protection at TAM and TEE . . . . . . . . . . . . . 25
9.5. Compromised CA . . . . . . . . . . . . . . . . . . . . . 25
9.6. Compromised TAM . . . . . . . . . . . . . . . . . . . . . 25
9.7. Certificate Renewal . . . . . . . . . . . . . . . . . . . 26
9.8. Keeping Secrets from the TAM . . . . . . . . . . . . . . 26
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
12. Informative References . . . . . . . . . . . . . . . . . . . 26
Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
Applications executing in a device are exposed to many different
attacks intended to compromise the execution of the application, or
reveal the data upon which those applications are operating. These
attacks increase with the number of other applications on the device,
with such other applications coming from potentially untrustworthy
sources. The potential for attacks further increase with the
complexity of features and applications on devices, and the
unintended interactions among those features and applications. The
danger of attacks on a system increases as the sensitivity of the
applications or data on the device increases. As an example,
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
greater concerns.
The Trusted Execution Environment (TEE) concept is designed to
execute applications in a protected environment that enforces that
only authorized code can execute with that environment, and that any
data used by such code cannot be read or tampered with by any code
outside that environment, including a commodity operating system (if
present).
This separation reduces the possibility of a successful attack on
application components and the data contained inside the TEE.
Typically, application components are chosen to execute inside a TEE
because those application components perform security sensitive
operations or operate on sensitive data. An application component
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running inside a TEE is referred to as a Trusted Application (TA),
while an application running outside any TEE is referred to as an
Untrusted Application (UA).
The TEE typically uses hardware to enforce protections on the TA and
its data, but also presents a more limited set of services to
applications inside the TEE than is normally available to Untrusted
Applications.
But not all TEEs are the same, and different vendors may have
different implementations of TEEs with different security properties,
different features, and different control mechanisms to operate on
TAs. Some vendors may themselves market multiple different TEEs with
different properties attuned to different markets. A device vendor
may integrate one or more TEEs into their devices depending on market
needs.
To simplify the life of developers and service providers interacting
with TAs in a TEE, an interoperable protocol for managing TAs running
in different TEEs of various devices is needed. In this TEE
ecosystem, there often arises a need for an external trusted party to
verify the identity, claims, and rights of Service Providers (SP),
devices, and their TEEs. This trusted third party is the Trusted
Application Manager (TAM).
The Trusted Execution Provisioning (TEEP) protocol addresses the
following problems:
- A Service Provider (SP) intending to provide services through a TA
to users of a device needs to determine security-relevant
information of a device before provisioning their TA to the TEE
within the device. An example is the verification of the type of
TEE included in a device.
- A TEE in a device needs to determine whether a Service Provider
(SP) that wants to manage a TA in the device is authorized to
manage TAs in the TEE, and what TAs the SP is permitted to manage.
- The parties involved in the protocol must be able to attest that a
TEE is genuine and capable of providing the security protections
required by a particular TA.
- A Service Provider (SP) must be able to determine if a TA exists
(is installed) on a device (in the TEE), and if not, install the
TA in the TEE.
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- A Service Provider (SP) must be able to check whether a TA in a
device's TEE is the most up-to-date version, and if not, update
the TA in the TEE.
- A Service Provider (SP) must be able to remove a TA in a device's
TEE if the SP is no longer offering such services or the services
are being revoked from a particular user (or device). For
example, if a subscription or contract for a particular service
has expired, or a payment by the user has not been completed or
has been rescinded.
- A Service Provider (SP) must be able to define the relationship
between cooperating TAs under the SP's control, and specify
whether the TAs can communicate, share data, and/or share key
material.
2. Terminology
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,
often along with a Rich Execution Environment. A Device contains
a default list of Trust Anchors that identify entities (e.g.,
TAMs) that are trusted by the Device. This list is normally set
by the Device Manufacturer, and may be governed by the Device's
network carrier. The list of Trust Anchors is normally modifiable
by the Device's owner or Device Administrator. However the Device
manufacturer and network carrier may restrict some modifications,
for example, by not allowing the manufacturer or carrier's Trust
Anchor to be removed or disabled.
- Rich Execution Environment (REE): An environment that is provided
and governed by a typical OS (e.g., Linux, Windows, Android, iOS),
potentially in conjunction with other supporting operating systems
and hypervisors; it is outside of any TEE. This environment and
applications running on it are considered untrusted.
- Service Provider (SP): An entity that wishes to provide a service
on Devices that requires the use of one or more Trusted
Applications. A Service Provider requires the help of a TAM in
order to provision the Trusted Applications to remote devices.
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- Device User: A human being that uses a device. Many devices have
a single device user. Some devices have a primary device user
with other human beings as secondary device users (e.g., parent
allowing children to use their tablet or laptop). Other devices
are not used by a human being and hence have no device user.
Relates to Device Owner and Device Administrator.
- Device Owner: A device is always owned by someone. In some cases,
it is common for the (primary) device user to also own the device,
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 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
[I-D.ietf-suit-manifest], "A trust anchor represents an
authoritative entity via a public key and associated data. The
public key is used to verify digital signatures, and the
associated data is used to constrain the types of information for
which the trust anchor is authoritative." The Trust Anchor may be
a certificate or it may be a raw public key along with additional
data if necessary such as its public key algorithm and parameters.
- Trust Anchor Store: As defined in [RFC6024], "A trust anchor store
is a set of one or more trust anchors stored in a device. A
device may have more than one trust anchor store, each of which
may be used by one or more applications." As noted in
[I-D.ietf-suit-manifest], a trust anchor store must resist
modification against unauthorized insertion, deletion, and
modification.
- Trusted Application (TA): An application component that runs in a
TEE.
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- Trusted Execution Environment (TEE): An execution environment that
enforces that only authorized code can execute within the TEE, and
data used by that code cannot be read or tampered with by code
outside the TEE. A TEE also generally has a device unique
credential that cannot be cloned. There are multiple technologies
that can be used to implement a TEE, and the level of security
achieved varies accordingly. In addition, TEEs typically use an
isolation mechanism between Trusted Applications to ensure that
one TA cannot read, modify or delete the data and code of another
TA.
3. Use Cases
3.1. Payment
A payment application in a mobile device requires high security and
trust about the hosting device. Payments initiated from a mobile
device can use a Trusted Application to provide strong identification
and proof of transaction.
For a mobile payment application, some biometric identification
information could also be stored in a TEE. The mobile payment
application can use such information for authentication.
A secure user interface (UI) may be used in a mobile device to
prevent malicious software from stealing sensitive user input data.
Such an application implementation often relies on a TEE for user
input protection.
3.2. Authentication
For better security of authentication, a device may store its
sensitive authentication keys inside a TEE, providing TEE-protected
security key strength and trusted code execution.
3.3. Internet of Things
The Internet of Things (IoT) has been posing threats to networks and
national infrastructures because of existing weak security in
devices. It is very desirable that IoT devices can prevent malware
from manipulating actuators (e.g., unlocking a door), or stealing or
modifying sensitive data such as authentication credentials in the
device. A TEE can be the best way to implement such IoT security
functions.
TEEs could be used to store variety of sensitive data for IoT
devices. For example, a TEE could be used in smart door locks to
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store a user's biometric information for identification, and for
protecting access the locking mechanism.
3.4. Confidential 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
cloud hosting provider from accessing the data. A tenant can run TAs
inside a server TEE for secure operation and enhanced data security.
This provides benefits not only to tenants with better data security
but also to cloud hosting provider for reduced liability and
increased cloud adoption.
4. Architecture
4.1. System Components
The following are the main components in the system. Full
descriptions of components not previously defined are provided below.
Interactions of all components are further explained in the following
paragraphs.
+-------------------------------------------+
| Device |
| +--------+ | Service Provider
| +-------------+ | |----------+ |
| | TEE-1 | | TEEP |---------+| |
| | +--------+ | +----| Broker | | || +--------+ |
| | | TEEP | | | | |<---+ | |+-->| |<-+
| | | Agent |<----+ | | | | | +-| TAM-1 |
| | +--------+ | | |<-+ | | +->| | |<-+
| | | +--------+ | | | | +--------+ |
| | +---+ +---+ | | | | | TAM-2 | |
| +-->|TA1| |TA2| | +-------+ | | | +--------+ |
| | | | | | |<---------| App-2 |--+ | | |
| | | +---+ +---+ | +-------+ | | | Device Administrator
| | +-------------+ | App-1 | | | |
| | | | | | |
| +--------------------| |---+ | |
| | |--------+ |
| +-------+ |
+-------------------------------------------+
Figure 1: Notional Architecture of TEEP
- Service Providers (SP) and Device Administrators (DA) utilize the
services of a TAM to manage TAs on Devices. SPs do not directly
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interact with devices. DAs may elect to use a TAM for remote
administration of TAs instead of managing each device directly.
- Trusted Application Manager (TAM): A TAM is responsible for
performing lifecycle management activity on TA's on behalf of
Service Providers and Device Administrators. This includes
creation and deletion of TA's, and may include, for example, over-
the-air updates to keep an SP's TAs up-to-date and clean up when a
version should be removed. TAMs may provide services that make it
easier for SPs or DAs to use the TAM's service to manage multiple
devices, although that is not required of a TAM.
The TAM performs its management of TA's through an interaction
with a Device's TEEP Broker. As shown in Figure 1, the TAM cannot
directly contact a Device, but must wait for the TEEP Broker to
contact the TAM requesting a particular service. This
architecture is intentional in order to 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
be private, and accessible by only one or a limited number of SPs.
It is expected that manufacturers and carriers will run their own
private TAM. Another example of a private TAM is a TAM running as
a Software-as-a-Service (SaaS) within an SP.
A SP or Device Administrator chooses a particular TAM based on
whether the TAM is trusted by a Device or set of Devices. The TAM
is trusted by a device if the TAM's public key is an authorized
Trust Anchor in the Device. A SP or Device Administrator may run
their own TAM, however the Devices they wish to manage must
include this TAM's pubic key in the Trust Anchor list.
A SP or Device Administrator is free to utilize multiple TAMs.
This may be required for a SP to manage multiple different types
of devices from different manufacturers, or devices on different
carriers, since the Trust Anchor list on these different devices
may contain different TAMs. A Device Administrator may be able to
add their own TAM's 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,
it must have its public key or certificate installed in Devices
Trust Anchor list. A TAM may set up a relationship with device
manufacturers or carriers to have them install the TAM's keys in
their device's Trust Anchor list. Alternatively, a TAM may
publish its certificate and allow Device Administrators to install
the TAM's certificate in their devices as an after-market-action.
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- TEEP Broker: The TEEP Broker is an application component running
in a Rich Execution Environment (REE) that enables the message
protocol exchange between a TAM and a TEE in a device. The TEEP
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
returning the TEE's responses to the TAM.
- TEEP Agent: the TEEP Agent is a processing module running inside a
TEE that receives TAM requests that are relayed via a TEEP Broker
that runs in an REE. A TEEP Agent in the TEE may parse requests
or forward requests to other processing modules in a TEE, which is
up to a TEE provider's implementation. A response message
corresponding to a TAM request is sent by a TEEP Agent back to a
TEEP Broker.
- Certification Authority (CA): Certificate-based credentials used
for authenticating a device, a TAM and an SP. A device embeds a
list of root certificates (Trust Anchors), from trusted CAs that a
TAM will be validated against. A TAM will remotely attest a
device by checking whether a device comes with a certificate from
a CA that the TAM trusts. The CAs do not need to be the same;
different CAs can be chosen by each TAM, and different device CAs
can be used by different device manufacturers.
4.2. Different Renditions of TEEP Architecture
There is nothing prohibiting a device from implementing multiple
TEEs. In addition, some TEEs (for example, SGX) present themselves
as separate containers within memory without a controlling manager
within the TEE. In these cases, the Rich Execution Environment hosts
multiple TEEP brokers, where each Broker manages a particular TEE or
set of TEEs. Enumeration and access to the appropriate TEEP Broker
is up to the Rich Execution Environment and the Untrusted
Applications. Verification that the correct TA has been reached then
becomes a matter of properly verifying TA attestations, which are
unforgeable. The multiple TEE approach is shown in the diagram
below. For brevity, TEEP Broker 2 is shown interacting with only one
TAM and UA, but no such limitation is intended to be implied in the
architecture.
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+-------------------------------------------+
| Device |
| +--------+ | Service Provider
| | |----------+ |
| +-------------+ | TEEP |---------+| |
| | TEE-1 | +---| Broker | | || +--------+ |
| | | | | 1 |<---+ | |+-->| |<-+
| | +-------+ | | | | | | | | |
| | | TEEP | | | | | | | | | |
| | | Agent |<------+ | | | | | | |
| | | 1 | | | | | | | | |
| | +-------+ | | | | | | | |
| | | | | | | | | |
| | +---+ +---+ | | | | | | +-| TAM-1 |
| | |TA1| |TA2| | | |<-+ | | +->| | |<-+
| +-->| | | |<---+ +--------+ | | | | +--------+ |
| | | +---+ +---+ | | | | | | TAM-2 | |
| | | | | +-------+ | | | +--------+ |
| | +-------------+ +-----| App-2 |--+ | | ^ |
| | +-------+ | | | | Device
| +--------------------| App-1 | | | | | Administrator
| +------| | | | | |
| +-----------|-+ | |---+ | | |
| | TEE-2 | | | |--------+ | |
| | +------+ | | | |------+ | |
| | | TEEP | | | +-------+ | | |
| | | Agent|<-----+ | | |
| | | 2 | | | | | | |
| | +------+ | | | | | |
| | | | | | | |
| | +---+ | | | | | |
| | |TA3|<----+ | | +----------+ | | |
| | | | | | | TEEP |<--+ | |
| | +---+ | +--| Broker |----------------+
| | | | 2 | |
| +-------------+ +----------+ |
| |
+-------------------------------------------+
Figure 2: Notional Architecture of TEEP with multiple TEEs
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
in TEE-2. This presents some challenges for a TAM in completely
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
physically separated, with wholly different resources, there may be
no need for TEEP Brokers to share information on installed TAs or
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resource usage. However, the architecture guarantees that the TAM
will receive all the relevant information from the TEEP Broker to
which it communicates.
4.3. Multiple TAMs and Relationship to TAs
As shown in Figure 2, the TEEP Broker provides connections from the
TEE and the Untrusted Application to one or more TAMs. The selection
of which TAM to communicate with is dependent on information from the
Untrusted Application and is directly related to the TA.
When a SP offers a service which requires a TA, the SP associates
that service with a specific TA. The TA itself is digitally signed,
protecting its integrity, but the signature also links the TA back to
the signer. The signer is usually the SP, but in some cases may be
another party that the SP trusts. The SP selects one or more TAMs
through which to offer their service, and communicates the
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
provide access. There may be TAMs that provide services to specific
types of mobile devices, or mobile device operating systems, or
specific geographical regions or network carriers. A SP may be
motivated to utilize multiple TAMs for its service in order to
maximize market penetration and availability on multiple types of
devices. This likely means that the same service will be available
through multiple TAMs.
When the SP publishes the Untrusted Application to an app store or
other app repositories, the SP binds the Untrusted Application with a
manifest that identifies what TAMs can be contacted for the TA. In
some situations, an SP may use only a single TAM - this is likely the
case for enterprise applications or SPs serving a closed community.
For broad public apps, there will likely be multiple TAMs in the
manifest - one servicing one brand of mobile device and another
servicing a different manufacturer, etc. Because different devices
and different manufacturers trust different TAMs, the manifest will
include different TAMs that support this SP's Untrusted Application
and TA. Multiple TAMs allow the SP to provide their service and this
app (and TA) to multiple different devices.
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
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
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
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TAM URI and begins a TEEP protocol exchange. When the TEEP Agent
subsequently receives the TA to install and the TA's manifest
indicates dependencies on any other trusted components, each
dependency can include a list of TAM URIs for the relevant
dependency. If such dependencies exist that are prerequisites to
install the TA, then the TEEP Agent recursively follows the same
procedure for each dependency that needs to be installed or updated,
including selecting a TAM URI that is consistent with the list of
trusted TAMs provisioned on the device, and beginning a TEEP
exchange. If multiple TAM URIs are considered trusted, only one
needs to be contacted and they can be attempted in some order until
one responds.
Separate from the Untrusted Application's manifest, this framework
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
TEE components and versions. That is, dependencies from TAs on other
TEE components can be expressed in a SUIT manifest, including
dependencies on any other TAs, or trusted OS code (if any), or
trusted firmware. Installation steps can also be expressed in a SUIT
manifest.
For example, TEE's compliant with Global Platform may have a notion
of a "security domain" (which is a grouping of one or more TAs
installed on a device, that can share information within such a
group) that must be created and into which one or more TAs can then
be installed. It is thus up to the SUIT manifest to express a
dependency on having such a security domain existing or being created
first, as appropriate.
Updating a TA may cause compatibility issues with any Untrusted
Applications or other components that depend on the updated TA, just
like updating the OS or a shared library could impact an Untrusted
Application. Thus, an implementation needs to take into account such
issues.
4.4. Untrusted Apps, Trusted Apps, and Personalization Data
In TEEP, there is an explicit relationship and dependence between the
Untrusted Application in the REE and one or more TAs in the TEE, as
shown in Figure 2. For most purposes, an Untrusted Application that
uses one or more TA's in a TEE appears no different from any other
Untrusted Application in the REE. However, the way the Untrusted
Application and its corresponding TA's are packaged, delivered, and
installed on the device can vary. The variations depend on whether
the Untrusted Application and TA are bundled together or are provided
separately, and this has implications to the management of the TAs in
the TEE. In addition to the Untrusted Application and TA, the TA
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and/or TEE may require some additional data to personalize the TA to
the service provider or the device or a user. This personalization
data is dependent on the TEE, the TA and the SP; an example of
personalization data might be username and password of an account
with the SP, or a secret symmetric key used by the TA to communicate
with the SP. The personalization data must be encrypted to preserve
the confidentiality of potentially sensitive data contained within
it. Other than this requirement to support confidentiality, TEEP
place no limitations or requirements on the personalization data.
There are three possible cases for bundling of the Untrusted
Application, TA, and personalization data:
1. The Untrusted Application, TA, and personalization data are all
bundled together in a single package by the SP and provided to
the TEEP Broker through the TAM.
2. The Untrusted Application and the TA are bundled together in a
single package, which a TAM or a publicly accessible app store
maintains, and the personalization data is separately provided by
the SP's TAM.
3. All components are independent. The Untrusted Application is
installed through some independent or device-specific mechanism,
and the TAM provides the TA and personalization data from the SP.
Delivery of the TA and personalization data may be combined or
separate.
The TEEP protocol treats the TA, any dependencies the TA has, and
personalization data as separate components with separate
installation steps that are expressed in SUIT manifests, and a SUIT
manifest might contain or reference multiple binaries (see {{I-
D.ietf-suit-manifest} for more details). The TEEP Agent is
responsible for handling any installation steps that need to be
performed inside the TEE, such as decryption of private TA bianries
or personalization data.
4.5. Examples of Application Delivery Mechanisms in Existing TEEs
In order to better understand these cases, it is helpful to review
actual implementations of TEEs and their application delivery
mechanisms.
In Intel Software Guard Extensions (SGX), the Untrusted Application
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
Untrusted Application loads the TA into an SGX enclave when the
Untrusted Application needs the TA. This organization makes it easy
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to maintain compatibility between the Untrusted Application and the
TA, since they are updated together. It is entirely possible to
create an Untrusted Application that loads an external TA into an SGX
enclave and use that TA (Case 3). In this case, the Untrusted
Application would require a reference to an external file or download
such a file dynamically, place the contents of the file into memory,
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
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
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
construction would require a special installation program and SGX TA
to receive the encrypted binary, decrypt it, separate it into the
three different elements, and then install all three. This
installation is complex, because the Untrusted Application decrypted
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
the Untrusted Application package includes the TA code also, since
otherwise there is a significant problem in getting the SGX enclave
code (the TA) from the TEE, through the installer and into the
Untrusted Application in a trusted fashion. Finally, the
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,
which would pass this data to the installed Untrusted Application,
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
does not have direct communication to other SGX enclaves.
In ARM TrustZone based environments, the Untrusted 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 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 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
each other without involving the Untrusted Application, and so the
complexity of Case 1 is lower than in the SGX example, and so Case 1
is possible as well though still more complex than Cases 2 and 3.
4.6. Entity Relations
This architecture leverages asymmetric cryptography to authenticate a
device to a TAM. Additionally, a TEE in a device authenticates a TAM
and TA signer. The provisioning of Trust Anchors to a device may be
different from one use case to the other. A device administrator may
want to have the capability to control what TAs are allowed. A
device manufacturer enables verification of the TA signers and TAM
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providers; it may embed a list of default Trust Anchors that the
signer of an allowed TA's signer certificate should chain to. A
device administrator may choose to accept a subset of the allowed TAs
via consent or action of downloading.
(App Developer) (App Store) (TAM) (Device with TEE) (CAs)
| |
| --> (Embedded TEE cert) <--
| |
| <------------------------------ Get an app cert ----- |
| | <-- Get a TAM cert ------ |
|
1. Build two apps:
Untrusted Application
TA
|
|
Untrusted Application -- 2a. --> | ----- 3. Install -------> |
TA ----------------- 2b. Supply ------> | 4. Messaging-->|
| | | |
Figure 3: Developer Experience
Figure 3 shows an application developer building two applications: 1)
an Untrusted Application; 2) a TA that provides some security
functions to be run inside a TEE. At step 2, the application
developer uploads the Untrusted Application (2a) to an Application
Store. The Untrusted Application may optionally bundle the TA
binary. Meanwhile, the application developer may provide its TA to a
TAM provider that will be managing the TA in various devices. 3. A
user will go to an Application Store to download the Untrusted
Application. The Untrusted Application will trigger TA installation
by initiating communication with a TAM. This is the step 4. The
Untrusted Application will get messages from TAM, and interacts with
device TEE via an Agent.
The main components consist of a set of standard messages created by
a TAM to deliver TA management commands to a device, and device
attestation and response messages created by a TEE that responds to a
TAM's message.
It should be noted that network communication capability is generally
not available in TAs in today's TEE-powered devices. Trusted
Applications need to rely on a broker in the REE to interact with a
TEE for network message exchanges. Consequently, a TAM generally
communicates with an Untrusted Application about how it gets messages
that originate from a TEE inside a device. Similarly, a TA or TEE
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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
with different TAMs and different TEEs in different devices. This is
the role of the Broker, which is a software component that bridges
communication between a TAM and a TEE. Furthermore the Broker
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
This architecture leverages the following credentials, which allow
delivering end-to-end security between a TAM and a TEEP Agent,
without relying on any transport security.
Figure 4 summarizes the relationships between various keys and where
they are stored. Each public/private key identifies an SP, TAM, or
TEE, and gets a certificate that chains up to some CA. A list of
trusted certificates is then used to check a presented certificate
against.
Different CAs can be used for different types of certificates. TEEP
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
key. In addition to the keys shown in Figure 4, there may be
additional keys used for attestation. Refer to the RATS Architecture
for more discussion.
Cardinality & Location of
Location of Private Key Corresponding
Purpose Private Key Signs CA Certs
------------------ ----------- ------------- -------------
Authenticating TEE 1 per TEE TEEP responses TAM
Authenticating TAM 1 per TAM TEEP requests TEEP Agent
Code Signing 1 per SP TA binary TEE
Figure 4: Keys
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
TEE manufacturer and the key pair and its certificate are valid for
the expected lifetime of the TEE. A TAM provider is responsible for
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configuring its TAM with the manufacturer certificates or CAs that
are used to sign TEE keys.
The TAM key pair and certificate are used for authenticating a TAM to
a remote TEE. A TAM provider is responsible for acquiring a
certificate from a CA that is trusted by the TEEs it manages.
The SP key pair and certificate are used to sign TAs that the TEE
will consider authorized to execute. TEEs must be configured with
the CAs that it considers authorized to sign TAs that it will
execute.
5.1. Trust Anchors in TEE
A TEEP Agent's Trust Anchor store contains a list of Trust Anchors,
which are CA certificates that sign various TAM certificates. The
list is typically preloaded at 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.
When Trust Anchor update is carried out, it is imperative that any
update must maintain integrity where only authentic Trust Anchor list
from a device manufacturer or a Device Administrator is accepted.
This calls for a complete lifecycle flow in authorizing who can make
Trust Anchor update and whether a given Trust Anchor list are non-
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
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.
5.2. Trust Anchors in TAM
The Trust Anchor store in a TAM consists of a list of Trust Anchors,
which are CA certificates that sign various device TEE certificates.
A TAM will accept a device for TA management if the TEE in the device
uses a TEE certificate that is chained to a CA that the TAM trusts.
5.3. Scalability
This architecture uses a PKI. Trust Anchors exist on the devices to
enable the TEE to authenticate TAMs, and TAMs use Trust Anchors to
authenticate TEEs. Since a PKI is used, many intermediate CA
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certificates can chain to a root certificate, each of which can issue
many certificates. This makes the protocol highly scalable. New
factories that produce TEEs can join the ecosystem. In this case,
such a factory can get an intermediate CA certificate from one of the
existing roots without requiring that TAMs are updated with
information about the new device factory. Likewise, new TAMs can
join the ecosystem, providing they are issued a TAM certificate that
chains to an existing root whereby existing TEEs will be allowed to
be personalized by the TAM without requiring changes to the TEE
itself. This enables the ecosystem to scale, and avoids the need for
centralized databases of all TEEs produced or all TAMs that exist.
5.4. Message Security
Messages created by a TAM are used to deliver TA management commands
to a device, and device attestation and messages created by the
device TEE to respond to TAM messages.
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
TAM is able to decrypt and view the actual content.
6. TEEP Broker
A TEE and TAs often do not have the capability to directly
communicate outside of the hosting device. For example,
GlobalPlatform [GPTEE] specifies one such architecture. This calls
for a software module in the REE world to handle network
communication with a TAM.
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
TEE. It also provides interfaces for Untrusted Applications to query
and trigger TA installation that the application needs to use.
An Untrusted Application might communicate with the TEEP Broker at
runtime to trigger TA installation itself. Or an Untrusted
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
Application Installer can inspect this application metadata file and
invoke the TEEP Broker to trigger TA installation on behalf of the
Untrusted Application without requiring the Untrusted Application to
run first.
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6.1. Role of the TEEP Broker
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
call to trigger a TA installation.
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
one TEE, one TEEP Broker per TEE could be present in REE. A TEEP
Broker interacts with a TEEP Agent inside a TEE.
A TAM message may indicate the target TEE where a TA should be
installed. A compliant TEEP protocol should include a target TEE
identifier for a TEEP Broker when multiple TEEs are present.
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
network connection with an application or TAM.
The Broker only needs to return an error message if the TEE is not
reachable for some reason. Other errors are represented as response
messages returned from the TEE which will then be passed to the TAM.
6.2. TEEP Broker Implementation Consideration
A Provider should consider methods of distribution, scope and
concurrency on devices and runtime options when implementing a TEEP
Broker. Several non-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
The following conceptual APIs exist from a TEEP Broker to a TEEP
Agent:
1. RequestTA: A notification from an REE application (e.g., an
installer, or a normal application) that it depends on a given
TA, which may or may not already be installed in the TEE.
2. ProcessTeepMessage: A message arriving from the network, to be
delivered to the TEEP Agent for processing.
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
device itself needing any particular change.
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4. ProcessError: A notification that the TEEP Broker could not
deliver an outbound TEEP message to a TAM.
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
or not):
1. ProcessConnect: A notification that an incoming TEEP session is
being requested by a TEEP Agent.
2. ProcessTeepMessage: A message arriving from the network, to be
delivered to the TAM for processing.
For further discussion on these APIs, see
[I-D.ietf-teep-otrp-over-http].
6.2.2. TEEP Broker Distribution
The Broker installation is commonly carried out at OEM time. A user
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
Attestation is the process through which one entity (an Attester)
presents "evidence", in the form of a series of claims, to another
entity (a Verifier), and provides sufficient proof that the claims
are true. Different verifiers may have different standards for
attestation proofs and not all attestations are acceptable to every
verifier. A third entity (a Relying Party) can then use "attestation
results", in the form of another series of claims, from a Verifier to
make authorization decisions.
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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
Relying Parties) that a TEE in the device has particular properties,
was built by a particular manufacturer, or is executing a particular
TA. Other claims are possible; TEEP does not limit the claims that
may appear in evidence or attestation results, but defines a minimal
set of attestation result claims required for TEEP to operate
properly. Extensions to these claims are possible. Other standards
or groups may define the format and semantics of extended claims.
+----------------+
| Device | +----------+
| +------------+ | Evidence | TAM | Evidence +----------+
| | TEE |------------->| (Relying |-------------->| Verifier |
| | (Attester) | | | Party) |<--------------| |
| +------------+ | +----------+ Attestation +----------+
+----------------+ Result
Figure 5: TEEP Attestation Roles
As of the writing of this specification, device and TEE attestations
have not been standardized across the market. Different devices,
manufacturers, and TEEs support different attestation algorithms and
mechanisms. In order for TEEP to be inclusive, it is agnostic to the
format of evidence, allowing proprietary or standardized formats to
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
verifiers may be able to process all proprietary forms of attestation
evidence. Similarly, the TEEP protocol is agnostic as to the format
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
of claims in some format.
The assumptions which may apply to an attestation have to do with the
quality of the attestation and the quality and security provided by
the TEE, the device, the manufacturer, or others involved in the
device or TEE ecosystem. Some of the assumptions that might apply to
an attestations include (this may not be a comprehensive list):
- Assumptions regarding the security measures a manufacturer takes
when provisioning keys into devices/TEEs;
- Assumptions regarding what hardware and software components have
access to the Attestation keys of the TEE;
- Assumptions related to the source or local verification of claims
within an attestation prior to a TEE signing a set of claims;
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- Assumptions regarding the level of protection afforded to
attestation keys against exfiltration, modification, and side
channel attacks;
- Assumptions regarding the limitations of use applied to TEE
Attestation keys;
- Assumptions regarding the processes in place to discover or detect
TEE breeches; and
- Assumptions regarding the revocation and recovery process of TEE
attestation keys.
TAMs must be comfortable with the assumptions that are inherently
part of any attestation result they accept. Alternatively, any TAM
may choose not to accept an attestation result generated using
evidence from a particular manufacturer or device's TEE based on the
inherent assumptions. The choice and policy decisions are left up to
the particular TAM.
Some TAMs may require additional claims in order to properly
authorize a device or TEE. These additional claims may help clear up
any assumptions for which the TAM wants to alleviate. The specific
format for these additional claims are outside the scope of this
specification, but the TEEP protocol allows these additional claims
to be included in the attestation messages.
7.1. Information Required in TEEP Claims
- Device Identifying Info: TEEP attestations must uniquely identify
a device to the TAM and SP. This identifier allows the TAM to
provide services unique to the device, such as managing installed
TAs, and providing subscriptions to services, and locating device-
specific keying material to communicate with or authenticate the
device. Additionally, device manufacturer information must be
provided to provide better universal uniqueness qualities without
requiring globally unique identifiers for all devices.
- TEE Identifying info: The type of TEE that generated this
attestation must be identified. Standard TEE types are identified
by an IANA number, but also must include version identification
information such as the hardware, firmware, and software version
of the TEE, as applicable by the TEE type. TEE manufacturer
information for the TEE is required in order to disambiguate the
same TEE type created by different manufacturers and resolve
potential assumptions around manufacturer provisioning, keying and
support for the TEE.
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- Liveness Proof: A claim that includes liveness information must be
included, such as a nonce or timestamp.
- Requested Components: A list of zero or more components (TAs or
other dependencies needed by a TEE) that are requested by some
depending app, but which are not currently installed in the TEE.
8. Algorithm and Attestation Agility
RFC 7696 [RFC7696] outlines the requirements to migrate from one
mandatory-to-implement algorithm suite to another over time. This
feature is also known as crypto agility. Protocol evolution is
greatly simplified when crypto agility is already considered during
the design of the protocol. In the case of the Trusted Execution
Provisioning (TEEP) Protocol the diverse range of use cases, from
trusted app updates for smart phones and tablets to updates of code
on higher-end IoT devices, creates the need for different mandatory-
to-implement algorithms already from the start.
Crypto agility in TEEP concerns the use of symmetric as well as
asymmetric algorithms. Symmetric algorithms are used for encryption
of content whereas the asymmetric algorithms are mostly used for
signing messages.
In addition to the use of cryptographic algorithms in TEEP there is
also the need to make use of different attestation technologies. A
Device must provide techniques to inform a TAM about the attestation
technology it supports. For many deployment cases it is more likely
for the TAM to support one or more attestation techniques whereas the
Device may only support one.
9. Security Considerations
9.1. TA Trust Check at TEE
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
trustworthiness of a TA. A TEEP Broker has a function to allow an
application to query the information about a TA.
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An Untrusted Application may perform verification of the TA by
verifying the signature of the TA. An application can do additional
trust checks on the certificate returned for this TA. It might trust
the TAM, or require additional SP signer trust chaining.
9.2. One TA Multiple SP Case
A TA for multiple SPs must have a different identifier per SP. They
should appear as different TAs when they are installed in the same
device.
9.3. Broker Trust Model
A TEEP Broker could be malware in the vulnerable REE. An Untrusted
Application will connect its TAM provider for required TA
installation. It gets command messages from the TAM, and passes the
message to the Broker.
The architecture enables the TAM to communicate with the device's TEE
to manage TAs. All TAM messages are signed and sensitive data is
encrypted such that the TEEP Broker cannot modify or capture
sensitive data.
9.4. Data Protection at TAM and TEE
The TEE implementation provides protection of data on the device. It
is the responsibility of the TAM to protect data on its servers.
9.5. Compromised CA
A root CA for TAM certificates might get compromised. Some TEE Trust
Anchor update mechanism is expected from device OEMs. TEEs are
responsible for validating certificate revocation about a TAM
certificate chain.
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
implementation and policy choice. TAMs are responsible for
validating any intermediate CA for TEE device certificates.
9.6. Compromised TAM
Device TEEs are responsible for validating the supplied TAM
certificates to determine that the TAM is trustworthy.
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9.7. Certificate Renewal
TEE device certificates are expected to be long lived, longer than
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
certificates. The root CA certificates for a TAM, which are embedded
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
is imperative that OEMs or device providers plan for support of Trust
Anchor update in their shipped devices.
9.8. Keeping Secrets from the TAM
In some scenarios, it is desirable to protect the TA binary or
configuration from being disclosed to the TAM that distributes them.
In such a scenario, the files can be encrypted end-to-end between an
SP and a TEE. However, there must be some means of provisioning the
decryption key into the TEE and/or some means of the SP securely
learning a public key of the TEE that it can use to encrypt. One way
to do this is for the SP to run its own TAM, merely to distribute the
decryption key via the TEEP protocol, and the key file can be a
dependency in the manifest of the encrypted TA. Thus, the TEEP Agent
would look at the TA manifest, determine there is a dependency with a
TAM URI of the SP's TAM. The Agent would then install the
dependency, and then continue with the TA installation steps,
including decrypting the TA binary with the relevant key.
10. IANA Considerations
This document does not require actions by IANA.
11. Acknowledgements
Some content of this document is based on text in a previous OTrP
protocol document [I-D.ietf-teep-opentrustprotocol]. We thank the
former co-authors Nick Cook and Minho Yoo for the initial document
content, and contributors Brian Witten, Tyler Kim, and Alin Mutu.
12. Informative References
[GPTEE] Global Platform, "GlobalPlatform Device Technology: TEE
System Architecture, v1.1", Global Platform GPD_SPE_009,
January 2017, <https://globalplatform.org/specs-library/
tee-system-architecture-v1-1/>.
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[I-D.ietf-suit-manifest]
Moran, B., Tschofenig, H., and H. Birkholz, "A Concise
Binary Object Representation (CBOR)-based Serialization
Format for the Software Updates for Internet of Things
(SUIT) Manifest", draft-ietf-suit-manifest-02 (work in
progress), November 2019.
[I-D.ietf-teep-opentrustprotocol]
Pei, M., Atyeo, A., Cook, N., Yoo, M., and H. Tschofenig,
"The Open Trust Protocol (OTrP)", draft-ietf-teep-
opentrustprotocol-03 (work in progress), May 2019.
[I-D.ietf-teep-otrp-over-http]
Thaler, D., "HTTP Transport for Trusted Execution
Environment Provisioning: Agent-to- TAM Communication",
draft-ietf-teep-otrp-over-http-03 (work in progress),
November 2019.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <https://www.rfc-editor.org/info/rfc6024>.
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
<https://www.rfc-editor.org/info/rfc7696>.
Pei, et al. Expires June 8, 2020 [Page 27]
Internet-Draft TEEP Architecture December 2019
Appendix A. History
RFC EDITOR: PLEASE REMOVE THIS SECTION
IETF Drafts
draft-00: - Initial working group document
Authors' Addresses
Mingliang Pei
Symantec
EMail: mingliang_pei@symantec.com
Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@arm.com
David Wheeler
Intel
EMail: david.m.wheeler@intel.com
Andrew Atyeo
Intercede
EMail: andrew.atyeo@intercede.com
Liu Dapeng
Alibaba Group
EMail: maxpassion@gmail.com
Pei, et al. Expires June 8, 2020 [Page 28]
