Trusted Execution Environment Provisioning (TEEP) Architecture
draft-ietf-teep-architecture-15

Document Type Active Internet-Draft (teep WG)
Authors Mingliang Pei  , Hannes Tschofenig  , Dave Thaler  , Dave Wheeler 
Last updated 2021-07-28 (latest revision 2021-07-12)
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TEEP                                                              M. Pei
Internet-Draft                                                  Broadcom
Intended status: Informational                             H. Tschofenig
Expires: January 13, 2022                                    Arm Limited
                                                               D. Thaler
                                                               Microsoft
                                                              D. Wheeler
                                                                  Amazon
                                                           July 12, 2021

     Trusted Execution Environment Provisioning (TEEP) Architecture
                    draft-ietf-teep-architecture-15

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 13, 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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  16
       4.4.2.  Example: Application Delivery Mechanisms in Arm
               TrustZone . . . . . . . . . . . . . . . . . . . . . .  17
     4.5.  Entity Relations  . . . . . . . . . . . . . . . . . . . .  17
   5.  Keys and Certificate Types  . . . . . . . . . . . . . . . . .  19
     5.1.  Trust Anchors in a TEEP Agent . . . . . . . . . . . . . .  21
     5.2.  Trust Anchors in a TEE  . . . . . . . . . . . . . . . . .  21
     5.3.  Trust Anchors in a TAM  . . . . . . . . . . . . . . . . .  21
     5.4.  Scalability . . . . . . . . . . . . . . . . . . . . . . .  21
     5.5.  Message Security  . . . . . . . . . . . . . . . . . . . .  22
   6.  TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . .  22
     6.1.  Role of the TEEP Broker . . . . . . . . . . . . . . . . .  23
     6.2.  TEEP Broker Implementation Consideration  . . . . . . . .  23
       6.2.1.  TEEP Broker APIs  . . . . . . . . . . . . . . . . . .  24
       6.2.2.  TEEP Broker Distribution  . . . . . . . . . . . . . .  25
   7.  Attestation . . . . . . . . . . . . . . . . . . . . . . . . .  25
   8.  Algorithm and Attestation Agility . . . . . . . . . . . . . .  27
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
     9.1.  Broker Trust Model  . . . . . . . . . . . . . . . . . . .  28
     9.2.  Data Protection . . . . . . . . . . . . . . . . . . . . .  28
     9.3.  Compromised REE . . . . . . . . . . . . . . . . . . . . .  29
     9.4.  CA Compromise or Expiry of CA Certificate . . . . . . . .  30
     9.5.  Compromised TAM . . . . . . . . . . . . . . . . . . . . .  30
     9.6.  Malicious TA Removal  . . . . . . . . . . . . . . . . . .  31
     9.7.  Certificate Expiry and Renewal  . . . . . . . . . . . . .  31
     9.8.  Keeping Secrets from the TAM  . . . . . . . . . . . . . .  32
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32

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   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  32
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32
   13. Informative References  . . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

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
   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.

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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
   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
      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 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.

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   -  A Device Administrator wants to check whether a TA in a device's
      TEE is the most up-to-date version, and if not, update the TA in
      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).

   For TEEs that simply verify and load signed TA's from an untrusted
   filesystem, classic application distribution protocols can be used
   without modification.  The problems in the bullets above, on the
   other hand, require a new protocol, i.e., the TEEP protocol, for TEEs
   that can install and enumerate TAs in a TEE-secured location and
   where another domain-specific protocol standard (e.g., [GSMA],
   [OTRP]) that meets the needs is not already in use.

2.  Terminology

   The following terms are used:

   -  Device: A physical piece of hardware that hosts one or more TEEs,
      often along with an REE.

   -  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
      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

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      are not used by a human being and hence have no device user.
      Relates to Device Owner and Device Administrator.

   -  Personalization Data: A set of configuration data that is specific
      to the device or user.  The Personalization Data may depend on the
      type of TEE, a particular TEE instance, the TA, and even the user
      of the device; an example of Personalization Data might be a
      secret symmetric key used by a TA to communicate with some
      service.

   -  Raw Public Key: The raw public key only consists of the
      SubjectPublicKeyInfo structure of a PKIX certificate [RFC5280]
      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.

   -  Trusted Application (TA): An application (or, in some
      implementations, an application component) that runs in a TEE.

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

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   -  Trusted Component: A set of code and/or data in a TEE managed as a
      unit by a Trusted Application Manager.  Trusted Applications and
      Personalization Data are thus managed by being included in Trusted
      Components.  Trusted OS code or trusted firmware can also be
      expressed as Trusted Components that a Trusted Component depends
      on.

   -  Trusted Component Developer: An entity that develops one or more
      Trusted Components.

   -  Trusted Component Signer: An entity that signs a Trusted Component
      with a key that a TEE will trust.  The signer might or might not
      be the same entity as the Trusted Component Developer.  For
      example, a Trusted Component might be signed (or re-signed) by a
      Device Administrator if the TEE will only trust the Device
      Administrator.  A Trusted Component might also be encrypted, if
      the code is considered confidential.

   -  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.

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   Such an implementation often relies on a TEE for providing access to
   peripherals, such as PIN input or a trusted display, so that the REE
   cannot observe or tamper with the user input or output.

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, such as customer details or credit
   card numbers, 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                                    |    Trusted Component
   |                          +--------+       |              Signer
   |    +-------------+       |        |-----------+              |
   |    | TEE-1       |       | TEEP   |---------+ |              |
   |    | +--------+  |  +----| Broker |       | | |  +--------+  |
   |    | | TEEP   |  |  |    |        |<---+  | | +->|        |<-+
   |    | | Agent  |<----+    |        |    |  | |  +-|  TAM-1 |
   |    | +--------+  |       |        |<-+ |  | +->| |        |<-+
   |    |             |       +--------+  | |  |    | +--------+  |
   |    | +---+ +---+ |                   | |  |    | TAM-2  |    |
   |  +-->|TA1| |TA2| |        +-------+  | |  |    +--------+    |
   |  | | |   | |   |<---------| App-2 |--+ |  |                  |
   |  | | +---+ +---+ |    +-------+   |    |  |    Device Administrator
   |  | +-------------+    | App-1 |   |    |  |
   |  |                    |       |   |    |  |
   |  +--------------------|       |---+    |  |
   |                       |       |--------+  |
   |                       +-------+           |
   +-------------------------------------------+

                  Figure 1: Notional Architecture of TEEP

   -  Trusted Component Signers and Device Administrators utilize the
      services of a TAM to manage TAs on devices.  Trusted Component
      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 Trusted Components on
      behalf of Trusted Component Signers and Device Administrators.
      This includes installation and deletion of Trusted Components, and
      may include, for example, over-the-air updates to keep Trusted
      Components up-to-date and clean up when one should be removed.
      TAMs may provide services that make it easier for Trusted
      Component 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 Trusted Components 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.  TEEP authentication is performed between a TAM and a TEEP
      Agent.

      As shown in Figure 1, the TAM cannot directly contact a TEEP
      Agent, but must wait for the TEEP Broker to contact the TAM

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      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 Trusted Component
      Signers, or a TAM may be private, and accessible by only one or a
      limited number of Trusted Component Signers.  It is expected that
      many manufacturers and network carriers will run their own private
      TAM.

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

      A Trusted Component Signer or Device Administrator is free to
      utilize multiple TAMs.  This may be required for managing Trusted
      Components 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.

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   -  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): 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, or
      a Trusted Component Signer, as discussed in Section 5.  The CAs do
      not need to be the same; different CAs can be chosen by each TAM,
      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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   +-------------------------------------------+            Trusted
   | Device                                    |          Component
   |                                           |             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 Trusted
   Components 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
   Trusted Component, whether that Trusted Component is installed (or
   minimally, is running) in a TEE with which the TEEP Agent is
   associated.

   Each Trusted Component is digitally signed, protecting its integrity,
   and linking the Trusted Component back to the Trusted Component
   Signer.  The Trusted Component Signer is often the Trusted Component
   Developer, but in some cases might be another party such as a Device
   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 Trusted Component Signer selects one or more TAMs and communicates
   the Trusted Component(s) to the TAM.  For example, the Trusted
   Component 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 Trusted
   Component 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 Trusted Component will often be
   available through multiple TAMs.

   When the developer of an Untrusted Application that depends on a
   Trusted Component 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 Trusted Component.  In some situations, a Trusted
   Component may only be available via a single TAM - this is likely the
   case for enterprise applications or Trusted Component Signers serving
   a closed community.  For broad public apps, there will likely be
   multiple TAMs in the Untrusted Application's 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 Trusted Component.

   When a TEEP Broker receives a request (see the RequestTA API in
   Section 6.2.1) from an Untrusted Application to install a Trusted

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   Component, a list of TAM URIs may be provided for that Trusted
   Component, and the request is passed to the TEEP Agent.  If the TEEP
   Agent decides that the Trusted Component needs to be installed, the
   TEEP Agent selects a single TAM URI that is consistent with the list
   of trusted TAMs provisioned in the TEEP Agent, 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
   Trusted Component to install and the Trusted Component'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 Trusted Component, 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 Trusted Component, as well as any
   dependencies on other TEE components and versions.  That is,
   dependencies from Trusted Components on other Trusted 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 Trusted Component may cause compatibility issues with any
   Untrusted Applications or other components that depend on the updated
   Trusted Component, 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 an 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 also require additional data to personalize the TA to
   the device or a user.  Implementations must support encryption of
   such 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 Trusted Component
       Signer and either provided to the TEEP Broker through the TAM, or
       provided separately (with encrypted Personalization Data), with
       key material needed to decrypt and install the Personalization
       Data and TA provided by a 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 Trusted Component 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
       Trusted Component Signer.  Delivery of the TA and Personalization
       Data may be combined or separate.

   The TEEP protocol can treat each TA, any dependencies the TA has, and
   Personalization Data as separate Trusted 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.

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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
   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.

   As long as signed files (TAs and/or Personalization Data) are
   installed into an untrusted filesystem and trust is verified by the
   TEE at load time, classic distribution mechanisms can be used.  Some
   uses of SGX, however, allow a model where a TA can be dynamically
   installed into an SGX enclave that provides a runtime platform.  The
   TEEP protocol can be used in such cases, where the runtime platform
   could include a TEEP Agent.

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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.

   A trusted OS running in the TEE (e.g., OP-TEE) that supports loading
   and verifying signed TAs from an untrusted filesystem can, like SGX,
   use classic file distribution mechanisms.  If secure TA storage is
   used (e.g., a Replay-Protected Memory Block device) on the other
   hand, the TEEP protocol can be used to manage such storage.

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 Trusted Component Signers; it may embed a list of default
   Trust Anchors into the TEEP Agent and TEE for TAM trust verification
   and TA signature verification.

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    (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: (a) an Untrusted Application; (b) a TA that
   provides some security functions to be run inside a TEE.  This
   example assumes that the developer, the TEE, and the TAM have
   previously been provisioned with certificates.

   At step 1, the developer authors the two applications.

   At step 2, the developer uploads the Untrusted Application (2a) to an
   Application Store.  In this example, the developer is also the
   Trusted Component 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 a signed Trusted Component
   containing the 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 (where the arrow indicates the direction of
   data transfer).

   At step 4, since the Untrusted Application depends on the TA,
   installing the Untrusted Application will trigger TA installation by
   initiating communication with a TAM.  The TEEP Agent will interact
   with TAM via a TEEP Broker that faciliates communications between a
   TAM and the TEEP Agent in TEE.

   Some Trusted Component installation implementations might ask for a
   user's consent.  In other implementations, a Device Administrator
   might choose what Untrusted Applications and related Trusted

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   Components 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 Trusted Component 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 Trusted
   Component 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        1 per TEE    TEEP responses       TAM
   TEEP Agent

   Authenticating TAM    1 per TAM    TEEP requests     TEEP Agent

   Code Signing          1 per Trusted  TA binary          TEE
                         Component
                         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 Trusted Component Signer key pair and certificate are used to
   sign Trusted Components 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 Personalization
   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
   Personalization 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 Trusted Component 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 Trusted Component Signers, and TAMs use Trust Anchors to
   authenticate TEEs.  When a PKI is used, many intermediate CA

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   certificates can chain to a root certificate, each of which can issue
   many certificates.  This makes the protocol highly scalable.  New
   factories that produce TEEs can join the ecosystem.  In this case,
   such a factory can get an intermediate CA certificate from one of the
   existing roots without requiring that TAMs are updated with
   information about the new device factory.  Likewise, new TAMs can
   join the ecosystem, providing they are issued a TAM certificate that
   chains to an existing root whereby existing TEEs will be allowed to
   be personalized by the TAM without requiring changes to the TEE
   itself.  This enables the ecosystem to scale, and avoids the need for
   centralized databases of all TEEs produced or all TAMs that exist or
   all Trusted Component Signers that exist.

5.5.  Message Security

   Messages created by a TAM are used to deliver Trusted Component
   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 installation of Trusted Components that the application
   needs to use.

   An Untrusted Application might communicate with a TEEP Broker at
   runtime to trigger Trusted Component installation itself, or an
   Untrusted Application might simply have a metadata file that
   describes the Trusted Components it depends on and the associated
   TAM(s) for each Trusted Component, and an REE Application Installer
   can inspect this application metadata file and invoke the TEEP Broker
   to trigger Trusted Component 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 Trusted Component installation.

   The Broker doesn't need to parse a message content received from a
   TAM that should be processed by a TEE (see the ProcessTeepMessage API
   in Section 6.2.1).  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 Trusted Component
   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

   As depicted in Figure 5, there are multiple ways in which a TEEP
   Broker can be implemented, with more or fewer layers being inside the
   TEE.  For example, in model A, the model with the smallest TEE
   footprint, only the TEEP implementation is inside the TEE, whereas
   the TEEP/HTTP implementation is in the TEEP Broker outside the TEE.

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                           Model:    A      B      C     ...

                                    TEE    TEE    TEE
        +----------------+           |      |      |
        |      TEEP      |     Agent |      |      | Agent
        | implementation |           |      |      |
        +----------------+           v      |      |
                 |                          |      |
        +----------------+           ^      |      |
        |    TEEP/HTTP   |    Broker |      |      |
        | implementation |           |      |      |
        +----------------+           |      v      |
                 |                   |             |
        +----------------+           |      ^      |
        |     HTTP(S)    |           |      |      |
        | implementation |           |      |      |
        +----------------+           |      |      v
                 |                   |      |
        +----------------+           |      |      ^
        |   TCP or QUIC  |           |      |      | Broker
        | implementation |           |      |      |
        +----------------+           |      |      |
                                    REE    REE    REE

                       Figure 5: TEEP Broker Models

   In other models, additional layers are moved into the TEE, increasing
   the TEE footprint, with the Broker either containing or calling the
   topmost protocol layer outside of the TEE.  An implementation is free
   to choose any of these models.

   TEEP Broker implementers should consider methods of distribution,
   scope and concurrency on devices and runtime options.

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 Trusted Component, which may or may not already be
       installed in the TEE.

   2.  UnrequestTA: A notification from an REE application (e.g., an
       installer, or an Untrusted Application) that it no longer depends
       on a given Trusted Component, which may or may not already be

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       installed in the TEE.  For example, if the Untrusted Application
       is uninstalled, the uninstaller might invoke this conceptual API.

   3.  ProcessTeepMessage: A message arriving from the network, to be
       delivered to the TEEP Agent for processing.

   4.  RequestPolicyCheck: A hint (e.g., based on a timer) that the TEEP
       Agent may wish to contact the TAM for any changes, without the
       device itself needing any particular change.

   5.  ProcessError: A notification that the TEEP Broker could not
       deliver an outbound TEEP message to a TAM.

   For comparison, similar APIs may exist on the TAM side, where a
   Broker may or may not exist, depending on whether the TAM uses a TEE
   or not:

   1.  ProcessConnect: A notification that an incoming TEEP session is
       being requested by a TEEP Agent.

   2.  ProcessTeepMessage: A message arriving from the network, to be
       delivered to the TAM for processing.

   For further discussion on these APIs, see
   [I-D.ietf-teep-otrp-over-http].

6.2.2.  TEEP Broker Distribution

   The Broker installation is commonly carried out at OEM time.  A user
   can dynamically download and install a Broker on-demand.

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 6, 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

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   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 6: 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;

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   -  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

   -  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].

   The following information is required for TEEP attestation:

   -  Device Identifying Information: Attestation information 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.  This includes version
      identification information for 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.

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

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   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 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 and integrity protection of TA binaries and
   Personalization Data whereas the asymmetric algorithms are used for
   signing messages and managing symmetric keys.

   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 Trusted Components.  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

   It is the responsibility of the TAM to protect data on its servers.
   Similarly, it is the responsibility of the TEE implementation to
   provide protection of data against integrity and confidentiality
   attacks from outside the TEE.  TEEs that provide isolation among TAs
   within the TEE are likewise responsible for protecting TA data

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   against the REE and other TAs.  For example, this can be used to
   protect one user's or tenant's data from compromise by another user
   or tenant, even if the attacker has TAs.

   The protocol between TEEP Agents and TAMs similarly is responsible
   for securely providing integrity and confidentiality protection
   against adversaries between them.  It is a design choice at what
   layers to best provide protection against network adversaries.  As
   discussed in Section 6, the transport protocol and any security
   mechanism associated with it (e.g., the Transport Layer Security
   protocol) under the TEEP protocol may terminate outside a TEE.  If it
   does, the TEEP protocol itself must provide integrity protection and
   confidentiality protection to secure data end-to-end.  For example,
   confidentiality protection for payloads may be provided by utilizing
   encrypted TA binaries and encrypted attestation information.  See
   [I-D.ietf-teep-protocol] for how a specific solution addresses the
   design question of how to provide integrity and confidentiality
   protection.

9.3.  Compromised REE

   It is possible that the REE of a device is compromised.  We have
   already seen examples of attacks on the public Internet of billions
   of compromised devices being used to mount DDoS attacks.  A
   compromised REE can be used for such an attack but it cannot tamper
   with the TEE's code or data in doing so.  A compromised REE can,
   however, launch DoS attacks against the TEE.

   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 Trusted Component.  An 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.

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   A compromised REE might also request initiating the full flow of
   installation of Trusted Components that are not necessary.  It may
   also repeat a prior legitimate Trusted Component 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 Trusted
   Component management.

9.4.  CA Compromise or Expiry of CA Certificate

   A root CA for TAM certificates might get compromised or its
   certificate might expire, or a Trust Anchor other than a root CA
   certificate may also expire or be compromised.  TEEs are responsible
   for validating the entire TAM certificate path, including the TAM
   certificate and any intermediate certificates up to the root
   certificate.  Such validation includes checking for certificate
   revocation.  See Section 6 of [RFC5280] for details.

   If a TAM certificate path validation fails, the TAM might be rejected
   by a TEEP Agent.  To address this, some certificate path update
   mechanism is expected from TAM operators, so that the TAM can get a
   new certificate path that can be validated by a TEEP Agent.  In
   addition, the Trust Anchor in the TEEP Agent's Trust Anchor Store may
   need to be updated.  To address this, some TEE Trust Anchor update
   mechanism is expected from device OEMs.

   Similarly, a root CA for TEE certificates might get compromised or
   its certificate might expire, or a Trust Anchor other than a root CA
   certificate may also expire or be compromised.  TAMs are responsible
   for validating the entire TEE certificate path, including the TEE
   certificate and any intermediate certificates up to the root
   certificate.  Such validation includes checking for certificate
   revocation.

   If a TEE certificate path validation fails, the TEE might be rejected
   by a TAM, subject to the TAM's policy.  To address this, some
   certificate path update mechanism is expected from device OEMs, so
   that the TEE can get a new certificate path that can be validated by
   a TAM.  In addition, the Trust Anchor in the TAM's Trust Anchor Store
   may need to be updated.

9.5.  Compromised TAM

   Device TEEs are responsible for validating the supplied TAM
   certificates to determine that the TAM is trustworthy.

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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.
   Such a TA might be able to escape from malware detection by the REE,
   or access trusted resources within the TEE (but could not access
   other TEEs, or access other TA's if the TEE provides isolation
   between TAs).

   It is the responsibility of the TAM to not install malicious TAs in
   the first place.  The TEEP architecture allows a TEEP Agent to decide
   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 6 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
   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.

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   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 [RFC5280] are applicable.

9.8.  Keeping Secrets from the TAM

   In some scenarios, it is desirable to protect the TA binary or
   Personalization Data from being disclosed to the TAM that distributes
   them.  In such a scenario, the files can be encrypted end-to-end
   between a Trusted Component Signer and a TEE.  However, there must be
   some means of provisioning the decryption key into the TEE and/or
   some means of the Trusted Component Signer securely learning a public
   key of the TEE that it can use to encrypt.  One way to do this is for
   the Trusted Component 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 Trusted Component manifest, determine
   there is a dependency with a TAM URI of the Trusted Component
   Signer's TAM.  The Agent would then install the dependency, and then
   continue with the Trusted Component 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.

13.  Informative References

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

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   [GSMA]     GSM Association, "GP.22 RSP Technical Specification,
              Version 2.2.2", June 2020, <https://www.gsma.com/esim/wp-
              content/uploads/2020/06/SGP.22-v2.2.2.pdf>.

   [I-D.ietf-rats-architecture]
              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture",
              draft-ietf-rats-architecture-12 (work in progress), April
              2021.

   [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-14
              (work in progress), July 2021.

   [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-11 (work in progress), July
              2021.

   [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-05 (work in
              progress), February 2021.

   [OTRP]     GlobalPlatform, "Open Trust Protocol (OTrP) Profile v1.1",
              GlobalPlatform GPD_SPE_123, July 2020,
              <https://globalplatform.org/specs-library/tee-management-
              framework-open-trust-protocol/>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [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>.

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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,
              <https://www.rfc-editor.org/info/rfc7696>.

   [SGX]      Intel, "Intel(R) Software Guard Extensions (Intel (R)
              SGX)", n.d., <https://www.intel.com/content/www/us/en/
              architecture-and-technology/software-guard-
              extensions.html>.

   [TrustZone]
              Arm, "Arm TrustZone Technology", n.d.,
              <https://developer.arm.com/ip-products/security-ip/
              trustzone>.

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
   Amazon

   EMail: davewhee@amazon.com

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