TEEP M. Pei
Internet-Draft Broadcom
Intended status: Informational H. Tschofenig
Expires: January 3, 2021 Arm Limited
D. Thaler
Microsoft
D. Wheeler
Intel
July 02, 2020
Trusted Execution Environment Provisioning (TEEP) Architecture
draft-ietf-teep-architecture-11
Abstract
A Trusted Execution Environment (TEE) is an environment that enforces
that any code within that environment cannot be tampered with, 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 such a TEE.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 3, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Payment . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 8
3.3. Internet of Things . . . . . . . . . . . . . . . . . . . 8
3.4. Confidential Cloud Computing . . . . . . . . . . . . . . 8
4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. System Components . . . . . . . . . . . . . . . . . . . . 8
4.2. Multiple TEEs in a Device . . . . . . . . . . . . . . . . 11
4.3. Multiple TAMs and Relationship to TAs . . . . . . . . . . 13
4.4. Untrusted Apps, Trusted Apps, and Personalization Data . 14
4.4.1. Example: Application Delivery Mechanisms in Intel SGX 15
4.4.2. Example: Application Delivery Mechanisms in Arm
TrustZone . . . . . . . . . . . . . . . . . . . . . . 16
4.5. Entity Relations . . . . . . . . . . . . . . . . . . . . 17
5. Keys and Certificate Types . . . . . . . . . . . . . . . . . 18
5.1. Trust Anchors in a TEEP Agent . . . . . . . . . . . . . . 20
5.2. Trust Anchors in a TEE . . . . . . . . . . . . . . . . . 20
5.3. Trust Anchors in a TAM . . . . . . . . . . . . . . . . . 20
5.4. Scalability . . . . . . . . . . . . . . . . . . . . . . . 20
5.5. Message Security . . . . . . . . . . . . . . . . . . . . 21
6. TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.1. Role of the TEEP Broker . . . . . . . . . . . . . . . . . 22
6.2. TEEP Broker Implementation Consideration . . . . . . . . 22
6.2.1. TEEP Broker APIs . . . . . . . . . . . . . . . . . . 22
6.2.2. TEEP Broker Distribution . . . . . . . . . . . . . . 23
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7. Attestation . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Information Required in TEEP Claims . . . . . . . . . . . 25
8. Algorithm and Attestation Agility . . . . . . . . . . . . . . 25
9. Security Considerations . . . . . . . . . . . . . . . . . . . 26
9.1. Broker Trust Model . . . . . . . . . . . . . . . . . . . 26
9.2. Data Protection . . . . . . . . . . . . . . . . . . . . . 26
9.3. Compromised REE . . . . . . . . . . . . . . . . . . . . . 27
9.4. Compromised CA . . . . . . . . . . . . . . . . . . . . . 27
9.5. Compromised TAM . . . . . . . . . . . . . . . . . . . . . 28
9.6. Malicious TA Removal . . . . . . . . . . . . . . . . . . 28
9.7. Certificate Expiry and Renewal . . . . . . . . . . . . . 28
9.8. Keeping Secrets from the TAM . . . . . . . . . . . . . . 29
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 29
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
13. Informative References . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
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 increases 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
any code within that environment cannot be tampered with, and that
any data used by such code cannot be read or tampered with by any
code outside that environment, including by a commodity operating
system (if present). In a system with multiple TEEs, this also means
that code in one TEE cannot be read or tampered with by code in the
other TEE.
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, i.e., in the Rich
Execution Environment (REE), is referred to as an Untrusted
Application. In the example of a banking application, code that
relates to the authentication protocol could reside in a TA while the
application logic including HTTP protocol parsing could be contained
in the Untrusted Application. In addition, processing of credit card
numbers or account balances could be done in a TA as it is sensitive
data. The precise code split is ultimately a decision of the
developer based on the assets he or she wants to protect according to
the threat model.
TEEs use hardware enforcement combined with software protection to
secure TAs and its data. TEEs typically offer a more limited set of
services to TAs than is normally available to Untrusted Applications.
Not all TEEs are the same, however, 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 TA developers interacting with TAs in a TEE,
an interoperable protocol for managing TAs running in different TEEs
of various devices is needed. This software update protocol needs to
make sure that compatible trusted and untrusted components (if any)
of an application are installed on the correct device. In this TEE
ecosystem, there often arises a need for an external trusted party to
verify the identity, claims, and rights of TA developers, devices,
and their TEEs. This trusted third party is the Trusted Application
Manager (TAM).
The Trusted Execution Environment Provisioning (TEEP) protocol
addresses the following problems:
- An installer of an Untrusted Application that depends on a given
TA wants to request installation of that TA in the device's TEE so
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
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TEE included in a device and that it is capable of providing the
security protections required.
- A TEE in a device wants to determine whether an entity that wants
to manage a TA in the device is authorized to manage TAs in the
TEE, and what TAs the entity is permitted to manage.
- A TAM (e.g., operated by a device administrator) wants to
determine if a TA exists (is installed) on a device (in the TEE),
and if not, install the TA in the TEE.
- A TAM wants to check whether a TA in a device's TEE is the most
up-to-date version, and if not, update the TA in the TEE.
- A Device Administrator wants to remove a TA from a device's TEE if
the TA developer is no longer maintaining that TA, when the TA has
been revoked or is not used for other reasons anymore (e.g., due
to an expired subscription).
- A TA developer wants to define the relationship between
cooperating TAs under the TA developer's control, and specify
whether the TAs can communicate, share data, and/or share key
material.
2. Terminology
The following terms are used:
- Device: A physical piece of hardware that hosts one or more TEEs,
often along with a REE. 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 when it is a
mobile device. The list of Trust Anchors is normally modifiable
by the device's owner or Device Administrator. However the device
manufacturer or network carrier (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.
- Device Administrator: 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 Untrusted Applications and TAs, approve or reject Trust
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
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Device Administrator may choose to remotely administer a device
through a TAM.
- 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 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.
- Raw Public Key (RPK): The RPK only consists of the
SubjectPublicKeyInfo structure of a PKIX certificate that carries
the parameters necessary to describe the public key. Other
serialization formats that do not rely on ASN.1 may also be used.
- 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 (or more
precisely, less trusted than a TEE).
- 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.
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- Trusted Application (TA): An application (or, in some
implementations, an application component) that runs in a TEE.
- Trusted Application (TA) Developer: An entity that develops one or
more TAs.
- Trusted Application (TA) Signer: An entity that signs a TA with a
key that a TEE will trust. The signer might or might not be the
same entity as the TA Developer. For example, a TA might be
signed (or re-signed) by a Device Administrator if the TEE will
only trust the Device Administrator. A TA might also be
encrypted, if the code is considered confidential.
- 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
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.
- Untrusted Application: An application running in an REE. An
Untrusted Application might depend on one or more TAs.
3. Use Cases
3.1. Payment
A payment application in a mobile device requires high security and
trust in 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 unlocking the device and for
local identification of the user.
A trusted user interface (UI) may be used in a mobile device to
prevent malicious software from stealing sensitive user input data.
Such an implementation often relies on a TEE for providing access to
peripherals, such as PIN input.
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3.2. Authentication
For better security of authentication, a device may store its keys
and cryptographic libraries inside a TEE limiting access to
cryptographic functions via a well-defined interface and thereby
reducing access to keying material.
3.3. Internet of Things
The Internet of Things (IoT) has been posing threats to critical
infrastructure because of weak security in devices. It is 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.
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 providers for reduced liability and
increased cloud adoption.
4. Architecture
4.1. System Components
Figure 1 shows the main components in a typical device with an REE.
Full descriptions of components not previously defined are provided
below. Interactions of all components are further explained in the
following paragraphs.
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+-------------------------------------------+
| Device |
| +--------+ | TA Developer
| +-------------+ | |-----------+ |
| | TEE-1 | | TEEP |---------+ | |
| | +--------+ | +----| Broker | | | | +--------+ |
| | | TEEP | | | | |<---+ | | +->| |<-+
| | | Agent |<----+ | | | | | +-| TAM-1 |
| | +--------+ | | |<-+ | | +->| | |<-+
| | | +--------+ | | | | +--------+ |
| | +---+ +---+ | | | | | TAM-2 | |
| +-->|TA1| |TA2| | +-------+ | | | +--------+ |
| | | | | | |<---------| App-2 |--+ | | |
| | | +---+ +---+ | +-------+ | | | Device Administrator
| | +-------------+ | App-1 | | | |
| | | | | | |
| +--------------------| |---+ | |
| | |--------+ |
| +-------+ |
+-------------------------------------------+
Figure 1: Notional Architecture of TEEP
- TA Signers and Device Administrators utilize the services of a TAM
to manage TAs on devices. TA Signers do not directly interact
with devices. Device Administators 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 TAs on behalf of TA
Signers and Device Administrators. This includes creation and
deletion of TAs, and may include, for example, over-the-air
updates to keep TAs up-to-date and clean up when a version should
be removed. TAMs may provide services that make it easier for TA
Signers or Device Administators to use the TAM's service to manage
multiple devices, although that is not required of a TAM.
The TAM performs its management of TAs on the device through
interactions with a device's TEEP Broker, which relays messages
between a TAM and a TEEP Agent running inside the TEE. As shown
in Figure 1, the TAM cannot directly contact a TEEP Agent, 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.
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A TAM may be publicly available for use by many TA Signers, or a
TAM may be private, and accessible by only one or a limited number
of TA Signers. It is expected that many manufacturers and network
carriers will run their own private TAM.
A TA Signer 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, or chains
up to, an authorized Trust Anchor in the device. A TA Signer or
Device Administrator may run their own TAM, but the devices they
wish to manage must include this TAM's public key/certificate
[RFC5280], or a certificate it chains up to, in the Trust Anchor
Store.
A TA Signer or Device Administrator is free to utilize multiple
TAMs. This may be required for managing TAs on multiple different
types of devices from different manufacturers, or mobile devices
on different network carriers, since the Trust Anchor Store 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 Store 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 a device's
Trust Anchor Store. A TAM may set up a relationship with device
manufacturers or network carriers to have them install the TAM's
keys in their device's Trust Anchor Store. 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.
- TEEP Broker: A 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. A 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. In devices with no REE
(e.g., a microcontroller where all code runs in an environment
that meets the definition of a Trusted Execution Environment in
Section 2), 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
TEE that receives TAM requests (typically 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
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message corresponding to a TAM request is sent back to the TAM,
again typically relayed via a TEEP Broker.
- Certification Authority (CA): A CA is an entity that issues
digital certificates (especially X.509 certificates) and vouches
for the binding between the data items in a certificate [RFC4949].
Certificates are then used for authenticating a device, a TAM and
a TA Signer. 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. Multiple TEEs in a Device
Some devices might implement multiple TEEs. In these cases, there
might be one shared TEEP Broker that interacts with all the TEEs in
the device. However, some TEEs (for example, SGX [SGX]) present
themselves as separate containers within memory without a controlling
manager within the TEE. As such, there might be multiple TEEP
Brokers in the REE, where each TEEP Broker communicates with one or
more TEEs associated with it.
It is up to the REE and the Untrusted Applications how they select
the correct TEEP Broker. Verification that the correct TA has been
reached then becomes a matter of properly verifying TA attestations,
which are unforgeable.
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.
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+-------------------------------------------+
| Device |
| | TA Signer
| +-------------+ | |
| | TEE-1 | | |
| | +-------+ | +--------+ | +--------+ |
| | | TEEP | | | TEEP |------------->| |<-+
| | | Agent |<----------| Broker | | | | TA
| | | 1 | | | 1 |---------+ | |
| | +-------+ | | | | | | |
| | | | |<---+ | | | |
| | +---+ +---+ | | | | | | +-| TAM-1 |Policy
| | |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
TAs in TEE-1, and TEEP Broker 2 controls interactions with the TAs in
TEE-2. This presents some challenges for a TAM in completely
managing the device, since a TAM may not interact with all the TEEP
Brokers on a particular platform. In addition, since TEEs may be
physically separated, with wholly different resources, there may be
no need for TEEP Brokers to share information on installed TAs or
resource usage.
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4.3. Multiple TAMs and Relationship to TAs
As shown in Figure 2, a TEEP Broker provides communication between
one or more TEEP Agents and one or more TAMs. The selection of which
TAM to communicate with might be made with or without input from an
Untrusted Application, but is ultimately the decision of a TEEP
Agent.
A TEEP Agent is assumed to be able to determine, for any given TA,
whether that TA is installed (or minimally, is running) in a TEE with
which the TEEP Agent is associated.
Each TA is digitally signed, protecting its integrity, and linking
the TA back to the TA Signer. The TA Signer is often the TA
Developer, but in some cases might be another party such as a Device
Administrator or other party to whom the code has been licensed (in
which case the same code might be signed by multiple licensees and
distributed as if it were different TAs).
A TA Signer selects one or more TAMs and communicates the TA(s) to
the TAM. For example, the TA Signer might choose 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 Signer may be motivated to utilize multiple TAMs in
order to maximize market penetration and availability on multiple
types of devices. This means that the same TA will often be
available through multiple TAMs.
When the developer of an Untrusted Application that depends on a TA
publishes the Untrusted Application to an app store or other app
repository, the developer optionally binds the Untrusted Application
with a manifest that identifies what TAMs can be contacted for the
TA. In some situations, a TA may only be available via a single TAM
- this is likely the case for enterprise applications or TA Signers
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 can include multiple TAMs that support the required TA.
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
TAM URI, and begins a TEEP protocol exchange. When the TEEP Agent
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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 a TA, as well as any 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, TEEs compliant with GlobalPlatform 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 an
Untrusted Application in a REE and one or more TAs in a TEE, as shown
in Figure 2. For most purposes, an Untrusted Application that uses
one or more TAs in a TEE appears no different from any other
Untrusted Application in the REE. However, the way the Untrusted
Application and its corresponding TAs 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
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
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the device or a user. This personalization data may depend on the
type of TEE, a particular TEE instance, the TA, and even the user of
the device; an example of personalization data might be a secret
symmetric key used by the TA to communicate with some service.
Implementations must support encryption of personalization data to
preserve the confidentiality of potentially sensitive data contained
within it and support integrity protection of the personalization
data. Other than the requirement to support confidentiality and
integrity protection, the TEEP architecture places no limitations or
requirements on the personalization data.
There are three possible cases for bundling of an Untrusted
Application, TA(s), and personalization data:
1. The Untrusted Application, TA(s), and personalization data are
all bundled together in a single package by a TA Signer and
provided to the TEEP Broker through the TAM.
2. The Untrusted Application and the TA(s) 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 TA Signer'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 TA
Signer. Delivery of the TA and personalization data may be
combined or separate.
The TEEP protocol treats each 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 binaries
or personalization data.
In order to better understand these cases, it is helpful to review
actual implementations of TEEs and their application delivery
mechanisms.
4.4.1. Example: Application Delivery Mechanisms in Intel SGX
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
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Untrusted Application needs the TA. This organization makes it easy
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 enclave-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.
4.4.2. Example: Application Delivery Mechanisms in Arm TrustZone
In Arm TrustZone [TrustZone] for A-class devices, 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 such as a
GlobalPlatform compliant 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 any Untrusted Application, and so the 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.
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4.5. Entity Relations
This architecture leverages asymmetric cryptography to authenticate a
device to a TAM. Additionally, a TEEP Agent in a device
authenticates a TAM. 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 by one or more
TAMs and by TA Signers; it may embed a list of default Trust Anchors
into the TEEP Agent and TEE for TAM trust verification and TA
signature verification.
(App Developers) (App Store) (TAM) (Device with TEE) (CAs)
| | | | |
| | | (Embedded TEE cert) <--|
| | | | |
| <--- Get an app cert -----------------------------------|
| | | | |
| | | <-- Get a TAM cert ---------|
| | | | |
1. Build two apps: | | | |
| | | |
(a) Untrusted | | | |
App - 2a. Supply --> | --- 3. Install ------> | |
| | | |
(b) TA -- 2b. Supply ----------> | 4. Messaging-->| |
| | | |
Figure 3: Example Developer Experience
Figure 3 shows an example where the same developer builds and signs
two applications: 1) an Untrusted Application; 2) a TA that provides
some security functions to be run inside a TEE.
At step 2, the developer uploads the Untrusted Application (2a) to an
Application Store. In this example, the developer is also the TA
Signer, and so generates a signed TA. The developer can then either
bundle the signed TA with the Untrusted Application, or the developer
can provide the signed TA to a TAM that will be managing the TA in
various devices.
At step 3, a user will go to an Application Store to download the
Untrusted Application. Since the Untrusted Application depends on
the TA, installing the Untrusted Application will trigger TA
installation by initiating communication with a TAM. This is step 4.
The TEEP Agent will interact with TAM via a TEEP Broker that
faciliates communications between a TAM and the TEEP Agent in TEE.
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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
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. Consequently,
Trusted Applications generally rely on broker in the REE to provide
access to network functionality in the REE. A broker does not need
to know the actual content of messages to facilitate such access.
Similarly, since the TEEP Agent runs inside a TEE, the TEEP Agent
generally relies on a TEEP Broker in the REE to provide network
access, and relay TAM requests to the TEEP Agent and relay the
responses back to the TAM.
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.
Figure 4 summarizes the relationships between various keys and where
they are stored. Each public/private key identifies a TA Signer,
TAM, or TEE, and gets a certificate that chains up to some trust
anchor. 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. In
addition to the keys shown in Figure 4, there may be additional keys
used for attestation. Refer to the RATS Architecture
[I-D.ietf-rats-architecture] for more discussion.
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Cardinality & Location of
Location of Private Key Trust Anchor
Purpose Private Key Signs Store
------------------ ----------- ------------- -------------
Authenticating TEE 1 per TEE TEEP responses TAM
Authenticating TAM 1 per TAM TEEP requests TEEP Agent
Code Signing 1 per TA TA binary TEE
Signer
Figure 4: Signature Keys
Note that personalization data is not included in the table above.
The use of personalization data is dependent on how TAs are used and
what their security requirements are.
TEEP requests from a TAM to a TEEP Agent are signed with the TAM
private key (for authentication and integrity protection).
Personalization data and TA binaries can be encrypted with a key that
is established with a content encryption key established with the TEE
public key (to provide confidentiality). Conversely, TEEP responses
from a TEEP Agent to a TAM can be signed with the TEE private key.
The TEE key pair and certificate are thus used for authenticating the
TEE to a remote TAM, and for sending private data to the TEE. Often,
the key pair is burned into the TEE by the TEE manufacturer and the
key pair and its certificate are valid for the expected lifetime of
the TEE. A TAM provider is responsible for configuring the TAM's
Trust Anchor Store with the manufacturer 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
a remote TEE, and for sending private data to the TAM. A TAM
provider is responsible for acquiring a certificate from a CA that is
trusted by the TEEs it manages. This is discussed further in
Section 5.1 below.
The TA Signer key pair and certificate are used to sign TAs that the
TEE will consider authorized to execute. TEEs must be configured
with the certificates or keys that it considers authorized to sign
TAs that it will execute. This is discussed further in Section 5.2
below.
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5.1. Trust Anchors in a TEEP Agent
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 an authentic Trust Anchor
list from a device manufacturer or a Device Administrator is
accepted. Details are out of scope of the architecture and can be
addressed in a protocol 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 or the raw public key of a TAM that is listed in the Trust Anchor
Store of the TEEP Agent.
5.2. Trust Anchors in a TEE
A TEE determines whether TA binaries are allowed to execute by
verifying whether their signature can be verified using
certificate(s) or raw public key(s) in the TEE's Trust Anchor Store.
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, as discussed in Section 5.1.
5.3. Trust Anchors in a TAM
The Trust Anchor Store in a TAM consists of a list of Trust Anchors,
which are 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 certificate or raw public
key that the TAM trusts, is contained in an allow list, is not found
on a block list, and/or fulfills any other policy criteria.
5.4. Scalability
This architecture uses a PKI (including self-signed certificates).
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 many certificates. This
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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 or all TA Signers that exist.
5.5. 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.
Confidentiality is provided by encrypting sensitive payloads (such as
personalization data and attestation evidence), rather than
encrypting the messages themselves. Using encrypted payloads is
important to ensure 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 a 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 the 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 only needs to return a (transport) 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
TEEP Broker implementers should consider methods of distribution,
scope and concurrency on devices and runtime options. Several non-
exhaustive options are discussed below.
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 an Untrusted 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.
4. ProcessError: A notification that the TEEP Broker could not
deliver an outbound TEEP message to a TAM.
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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.
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 require different degrees of
confidence in 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. (See
[I-D.ietf-rats-architecture] for more discussion.)
In TEEP, as depicted in Figure 5, the primary purpose of an
attestation is to allow a device (the Attester) to prove to a TAM
(the Relying Party) that a TEE in the device has particular
properties, was built by a particular manufacturer, and/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.
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+----------------+
| 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 protocols. 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), as long as the format supports encryption of any
information that is considered sensitive.
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. Note that the respective attestation algorithms are
not defined in the TEEP protocol itself; see
[I-D.ietf-rats-architecture] and [I-D.ietf-teep-protocol] for more
discussion.
There are a number of considerations that need to be considered when
appraising evidence provided by a TEE, including:
- What security measures a manufacturer takes when provisioning keys
into devices/TEEs;
- What hardware and software components have access to the
attestation keys of the TEE;
- The source or local verification of claims within an attestation
prior to a TEE signing a set of claims;
- The level of protection afforded to attestation keys against
exfiltration, modification, and side channel attacks;
- The limitations of use applied to TEE attestation keys;
- The processes in place to discover or detect TEE breaches; and
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- The revocation and recovery process of TEE attestation keys.
Some TAMs may require additional claims in order to properly
authorize a device or TEE. 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.
For more discussion of the attestation and appraisal process, see the
RATS Architecture [I-D.ietf-rats-architecture].
7.1. Information Required in TEEP Claims
- Device Identifying Information: TEEP attestations may need to
uniquely identify a device to the TAM. Unique device
identification allows the TAM to provide services 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. In some use cases it
may be sufficient to identify only the class of the device. The
security and privacy requirements regarding device identification
will vary with the type of TA provisioned to the TEE.
- TEE Identifying Information: The type of TEE that generated this
attestation must be identified, including 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 address
considerations around manufacturer provisioning, keying and
support for the TEE.
- Freshness Proof: A claim that includes freshness 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 cryptographic algorithm suite to another over
time. This feature is also known as crypto agility. Protocol
evolution is greatly simplified when crypto agility is considered
during the design of the protocol. In the case of the 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,
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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. In the context of TEEP symmetric algorithms
are used for encryption of TA binaries and personalization data
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. Broker Trust Model
The architecture enables the TAM to communicate, via a TEEP Broker,
with the device's TEE to manage TAs. Since the TEEP Broker runs in a
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, but the TEEP Broker can still conduct DoS
attacks as discussed in Section 9.3.
A TEEP Agent in a TEE is responsible for protecting against potential
attacks from a compromised TEEP Broker or rogue malware in the REE.
A rogue TEEP Broker might send corrupted data to the TEEP Agent, or
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. Data Protection
The TEE implementation provides protection of data on the device. It
is the responsibility of the TAM to protect data on its servers.
The protocol between TEEP Agents and TAMs similarly is responsible
for securely providing integrity and confidentiality protection
against adversaries between them. Since the transport protocol under
the TEEP protocol might be implemented outside a TEE, as discussed in
Section 6, it cannot be relied upon for sufficient protection. The
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TEEP protocol provides integrity protection, but confidentiality must
be provided by payload security, i.e., using encrypted TA binaries
and encrypted attestation information. See [I-D.ietf-teep-protocol]
for more discussion.
9.3. Compromised REE
It is possible that the REE of a device is compromised. If the REE
is compromised, several DoS attacks may be launched. The compromised
REE may terminate the TEEP Broker such that TEEP transactions cannot
reach the TEE, or might drop or delay messages between a TAM and a
TEEP Agent. However, while a DoS attack cannot be prevented, the REE
cannot access anything in the TEE if 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 such a
capability.
In some other scenarios, the compromised REE may ask a TEEP Broker to
make repeated requests to a TEEP Agent in a TEE to install or
uninstall a TA. A TA installation or uninstallation request
constructed by the TEEP Broker or REE will be rejected by the TEEP
Agent because the request won't have the correct signature from a TAM
to pass the request signature validation.
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.
A compromised REE might also request initiating the full flow of
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.
9.4. Compromised CA
A root CA for TAM certificates might get compromised. A Trust Anchor
other than a root CA certificate may also be compromised. Some TEE
Trust Anchor update mechanism is expected from device OEMs.
TEEs are responsible for validating certificate revocation about a
TAM certificate chain, including the TAM certificate and the
intermediate CA certificates up to the root certificate. This will
detect a compromised TAM certificate and also any compromised
intermediate CA certificate.
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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.
9.5. Compromised TAM
Device TEEs are responsible for validating the supplied TAM
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. That is, to
recover, the TEEP Agent must be able to reach out to the TAM, for
example whenever the RequestPolicyCheck API (Section 6.2.1) is
invoked by a timer or other event.
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. There is, however, a time window
during which a malicious TA might be able to operate successfully,
which is the validity time of the previous attestation result. For
example, if the Verifier in Figure 5 is updated to treat a previously
valid TA as no longer trustworthy, any attestation result it
previously generated saying that the TA is valid will continue to be
used until the attestation result expires. As such, the TAM's
Verifier should take into account the acceptable time window when
generating attestation results. See [I-D.ietf-rats-architecture] for
further discussion.
9.7. Certificate Expiry and 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
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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 updates. On the other hand,
it is imperative that OEMs or device providers plan for support of
Trust Anchor update in their shipped devices.
For those cases where TEE devices are given certificates for which no
good expiration date can be assigned the recommendations in
Section 4.1.2.5 of RFC 5280 [RFC5280] are applicable.
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 a
TA Signer and a TEE. However, there must be some means of
provisioning the decryption key into the TEE and/or some means of the
TA Signer securely learning a public key of the TEE that it can use
to encrypt. One way to do this is for the TA Signer to run its own
TAM so that it can 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 TA Signer'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. Contributors
- Andrew Atyeo, Intercede (andrew.atyeo@intercede.com)
- Liu Dapeng, Alibaba Group (maxpassion@gmail.com)
12. Acknowledgements
We would like to thank Nick Cook, Minho Yoo, Brian Witten, Tyler Kim,
Alin Mutu, Juergen Schoenwaelder, Nicolae Paladi, Sorin Faibish, Ned
Smith, Russ Housley, Jeremy O'Donoghue, and Anders Rundgren for their
feedback.
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13. Informative References
[GPTEE] GlobalPlatform, "GlobalPlatform Device Technology: TEE
System Architecture, v1.1", GlobalPlatform GPD_SPE_009,
January 2017, .
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture",
draft-ietf-rats-architecture-04 (work in progress), May
2020.
[I-D.ietf-suit-manifest]
Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
"A Concise Binary Object Representation (CBOR)-based
Serialization Format for the Software Updates for Internet
of Things (SUIT) Manifest", draft-ietf-suit-manifest-07
(work in progress), June 2020.
[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-06 (work in progress),
April 2020.
[I-D.ietf-teep-protocol]
Tschofenig, H., Pei, M., Wheeler, D., Thaler, D., and A.
Tsukamoto, "Trusted Execution Environment Provisioning
(TEEP) Protocol", draft-ietf-teep-protocol-02 (work in
progress), April 2020.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, .
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[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
.
[SGX] Intel, "Intel(R) Software Guard Extensions (Intel (R)
SGX)", n.d., .
[TrustZone]
Arm, "Arm TrustZone Technology", n.d.,
.
Authors' Addresses
Mingliang Pei
Broadcom
EMail: mingliang.pei@broadcom.com
Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@arm.com
Dave Thaler
Microsoft
EMail: dthaler@microsoft.com
David Wheeler
Intel
EMail: david.m.wheeler@intel.com
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