TEEP                                                              M. Pei
Internet-Draft                                                  Symantec
Intended status: Informational                             H. Tschofenig
Expires: August 11, 2020                                     Arm Limited
                                                               D. Thaler
                                                              D. Wheeler
                                                       February 08, 2020

     Trusted Execution Environment Provisioning (TEEP) Architecture


   A Trusted Execution Environment (TEE) is an environment that enforces
   that only authorized code can execute within 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 such a TEE.

Status of This Memo

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   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 August 11, 2020.

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   (http://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
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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.  Multiple TEEs in a Device . . . . . . . . . . . . . . . .  10
     4.3.  Multiple TAMs and Relationship to TAs . . . . . . . . . .  12
     4.4.  Untrusted Apps, Trusted Apps, and Personalization Data  .  13
       4.4.1.  Examples of Application Delivery Mechanisms in
               Existing TEEs . . . . . . . . . . . . . . . . . . . .  14
     4.5.  Entity Relations  . . . . . . . . . . . . . . . . . . . .  16
   5.  Keys and Certificate Types  . . . . . . . . . . . . . . . . .  17
     5.1.  Trust Anchors in a TEEP Agent . . . . . . . . . . . . . .  18
     5.2.  Trust Anchors in a TEE  . . . . . . . . . . . . . . . . .  19
     5.3.  Trust Anchors in a TAM  . . . . . . . . . . . . . . . . .  19
     5.4.  Scalability . . . . . . . . . . . . . . . . . . . . . . .  19
     5.5.  Message Security  . . . . . . . . . . . . . . . . . . . .  20
   6.  TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . .  20
     6.1.  Role of the TEEP Broker . . . . . . . . . . . . . . . . .  20
     6.2.  TEEP Broker Implementation Consideration  . . . . . . . .  21
       6.2.1.  TEEP Broker APIs  . . . . . . . . . . . . . . . . . .  21
       6.2.2.  TEEP Broker Distribution  . . . . . . . . . . . . . .  22
   7.  Attestation . . . . . . . . . . . . . . . . . . . . . . . . .  22

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     7.1.  Information Required in TEEP Claims . . . . . . . . . . .  24
   8.  Algorithm and Attestation Agility . . . . . . . . . . . . . .  24
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
     9.1.  Broker Trust Model  . . . . . . . . . . . . . . . . . . .  25
     9.2.  Data Protection at TAM and TEE  . . . . . . . . . . . . .  25
     9.3.  Compromised REE . . . . . . . . . . . . . . . . . . . . .  25
     9.4.  Compromised CA  . . . . . . . . . . . . . . . . . . . . .  26
     9.5.  Compromised TAM . . . . . . . . . . . . . . . . . . . . .  26
     9.6.  Malicious TA Removal  . . . . . . . . . . . . . . . . . .  26
     9.7.  Certificate Renewal . . . . . . . . . . . . . . . . . . .  27
     9.8.  Keeping Secrets from the TAM  . . . . . . . . . . . . . .  27
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  27
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   13. Informative References  . . . . . . . . . . . . . . . . . . .  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 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
   only authorized code can execute within that environment, 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).

   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
   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.

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   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

   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.  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
      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 TA developer wants to remove a confidential TA from a device's
      TEE if the TA developer is no longer offering such TAs or the TAs

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      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 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

   Note: The TA developer requires the help of a TAM to provision the
   Trusted Applications to remote devices and the TEEP protocol
   exchanges messages between a TAM and a TEEP Agent via a TEEP Broker.

2.  Terminology

   The following terms are used:

   -  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 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
      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

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      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.

   -  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 the 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

   -  Trusted Application (TA): An application component that runs in a

   -  Trusted Application (TA) Developer: An entity that wishes to
      provide functionality on devices that requires the use of one or
      more Trusted Applications.

   -  Trusted Application Manager (TAM): An entity that manages Trusted
      Applications (TAs) running in TEEs of various devices.

   -  Trusted Execution Environment (TEE): An execution environment that
      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

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      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

   -  Untrusted Application: An application running in a Rich Execution

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 unlocking the phone 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.

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.

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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.

   | 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 developers and Device Administrators utilize the services of a
      TAM to manage TAs on devices.  TA developers 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.

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   -  Trusted Application Manager (TAM): A TAM is responsible for
      performing lifecycle management activity on TAs on behalf of TA
      developers 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
      developers 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.

      A TAM may be publicly available for use by many TA developers, or
      a TAM may be private, and accessible by only one or a limited
      number of TA developers.  It is expected that many manufacturers
      and network carriers will run their own private TAM.

      A TA developer 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
      developer or Device Administrator may run their own TAM, but the
      devices they wish to manage must include this TAM's public key/
      certificate, or a certificate it chains up to, in the Trust Anchor

      A TA developer or Device Administrator is free to utilize multiple
      TAMs.  This may be required for a TA developer to manage multiple
      different types of devices from different manufacturers, or to
      manage mobile devices on different network 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 a device's
      Trust Anchor list.  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 list.  Alternatively, a TAM
      may publish its certificate and allow Device Administrators to

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      install the TAM's certificate in their devices as an after-market-

   -  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,
      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
      message corresponding to a TAM request is sent back to the TAM,
      again typically relayed via a TEEP Broker.

   -  Certification Authority (CA): Certificate-based credentials used
      for authenticating a device, a TAM and a TA developer.  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

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) present themselves
   as separate containers within memory without a controlling manager
   within the TEE.  As such, there might be multiple TEEP Brokers in the
   Rich Execution Environment, where each TEEP Broker communicates with
   one or more TEEs associated with it.

   It is up to the Rich Execution Environment 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

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   Untrusted Application and only one TEE, but no such limitations are
   intended to be implied in the architecture.

   | Device                                    |
   |                                           |        TA Developer
   |    +-------------+                        |                  |
   |    | TEE-1       |                        |                  |
   |    | +-------+   |       +--------+       |      +--------+  |
   |    | | TEEP  |   |       | TEEP   |------------->|        |<-+
   |    | | Agent |<----------| Broker |       |      |        |
   |    | | 1     |   |       | 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
   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

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   no need for TEEP Brokers to share information on installed TAs or
   resource usage.

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

   Each TA is digitally signed, protecting its integrity, and linking
   the TA back to the signer.  The signer is usually the TA software
   author, but in some cases might be another party that the TA software
   author trusts, or a 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 author or signer selects one or more TAMs through which to offer
   their TA(s), and communicates the TA(s) to the TAM.  In this
   document, we use the term "TA developer" to refer to the entity that
   selects a TAM and publishes a signed TA to it, independent of whether
   the publishing entity is the TA software author or the signer or

   The TA developer chooses TAMs based upon the markets into which the
   TAM can provide access.  There may be TAMs that provide services to
   specific types of devices, or device operating systems, or specific
   geographical regions or network carriers.  A TA developer may be
   motivated to utilize multiple TAMs for its service in order to
   maximize market penetration and availability on multiple types of
   devices.  This likely means that the same TA will 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
   developers 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.

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   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
   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

   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

   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

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

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   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
   the TA developer or the device or a user.  This personalization data
   is dependent on the TEE, the TA, and the TA developer; an example of
   personalization data might be a secret symmetric key used by the TA
   to communicate with the TA developer.  The personalization data must
   be encrypted to preserve the confidentiality of potentially sensitive
   data contained within it.  Other than this requirement to support
   confidentiality, 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 developer 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 developer'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
       developer.  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.

4.4.1.  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

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   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
   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.

   In Arm TrustZone for A- and R-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, 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 of the TAM
   providers and TA binary signers; it may embed a list of default Trust
   Anchors into the TEEP Agent and TEE for TAM trust verification and TA
   signer 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: Developer Experience

   Note that Figure 3 shows the TA developer as a TA signer.  The TA
   signer is either the same as the TA developer, or is a related entity
   trusted to sign the developer's TAs.

   Figure 3 shows an example where the same developer builds two
   applications: 1) an Untrusted Application; 2) a TA that provides some
   security functions to be run inside a TEE.  At step 2, the TA
   developer uploads the Untrusted Application (2a) to an Application
   Store.  The Untrusted Application may optionally bundle the TA
   binary.  Meanwhile, the TA developer may provide its 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 nnetwork 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 developer,
   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.  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

                              Figure 4: 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.

   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
   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.  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 developer 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

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

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   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 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 the TA's signer chains up to a certificate 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 that the TAM

5.4.  Scalability

   This architecture uses a PKI, although self-signed certificates are
   also permitted.  Trust Anchors exist on the devices to enable the TEE
   to authenticate TAMs and TA signers, and TAMs use Trust Anchors to
   authenticate TEEs.  When a PKI is used, many intermediate CA
   certificates can chain to a root certificate, each of which can issue
   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 or
   all TA developers that exist.

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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,
   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 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.

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.

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   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

   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.

   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

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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 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.  (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.

   | 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

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   TAM and a verifier, as long as they convey at least the required set
   of claims in some format.

   The assumptions that 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;

   -  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.

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7.1.  Information Required in TEEP Claims

   -  Device Identifying Info: TEEP attestations may need to uniquely
      identify a device to the TAM and TA developer.  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 info: 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 resolve
      potential assumptions 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 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, creates the
   need for different mandatory-to-implement algorithms already from the

   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

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   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.

   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

9.2.  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.

9.3.  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.  However, while a DoS attack cannot be prevented, the
   REE cannot access anything in the TEE if it is implemented correctly.
   Some TEEs may have some watchdog scheme to observe REE state and
   mitigate DoS attacks against it but most TEEs don't have have such

   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.

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   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.  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.

9.5.  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.

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   Furthermore the policy in the Verifier in an attestation process can
   be updated so that any evidence that includes the malicious TA would
   result in an attestation failure.

9.7.  Certificate Renewal

   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 a
   TA developer and a TEE.  However, there must be some means of
   provisioning the decryption key into the TEE and/or some means of the
   TA developer securely learning a public key of the TEE that it can
   use to encrypt.  One way to do this is for the TA developer 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 developer'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

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13.  Informative References

   [GPTEE]    GlobalPlatform, "GlobalPlatform Device Technology: TEE
              System Architecture, v1.1", GlobalPlatform GPD_SPE_009,
              January 2017, <https://globalplatform.org/specs-library/

              Birkholz, H., Thaler, D., Richardson, M., and N. Smith,
              "Remote Attestation Procedures Architecture", draft-ietf-
              rats-architecture-01 (work in progress), February 2020.

              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-03 (work in
              progress), February 2020.

              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,

Authors' Addresses

   Mingliang Pei

   EMail: mingliang_pei@symantec.com

   Hannes Tschofenig
   Arm Limited

   EMail: hannes.tschofenig@arm.com

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   Dave Thaler

   EMail: dthaler@microsoft.com

   David Wheeler

   EMail: david.m.wheeler@intel.com

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