TEEP                                                              M. Pei
Internet-Draft                                                  Symantec
Intended status: Informational                             H. Tschofenig
Expires: January 9, 2020                                     Arm Limited
                                                              D. Wheeler
                                                                A. Atyeo
                                                               L. Dapeng
                                                           Alibaba Group
                                                           July 08, 2019

     Trusted Execution Environment Provisioning (TEEP) Architecture


   A Trusted Execution Environment (TEE) is designed to provide a
   hardware-isolation mechanism to separate a regular operating system
   from security-sensitive application components.

   This architecture document motivates the design and standardization
   of a protocol for managing the lifecycle of trusted applications
   running inside a TEE.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 9, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Payment . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Authentication  . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Internet of Things  . . . . . . . . . . . . . . . . . . .   9
     4.4.  Confidential Cloud Computing  . . . . . . . . . . . . . .   9
   5.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  System Components . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Different Renditions of TEEP Architecture . . . . . . . .  12
     5.3.  Multiple TAMs and Relationship to TAs . . . . . . . . . .  14
     5.4.  Client Apps, Trusted Apps, and Personalization Data . . .  15
     5.5.  Examples of Application Delivery Mechanisms in Existing
           TEEs  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     5.6.  TEEP Architectural Support for Client App, TA, and
           Personalization Data Delivery . . . . . . . . . . . . . .  17
     5.7.  Entity Relations  . . . . . . . . . . . . . . . . . . . .  17
     5.8.  Trust Anchors in TEE  . . . . . . . . . . . . . . . . . .  20
     5.9.  Trust Anchors in TAM  . . . . . . . . . . . . . . . . . .  20
     5.10. Keys and Certificate Types  . . . . . . . . . . . . . . .  21
     5.11. Scalability . . . . . . . . . . . . . . . . . . . . . . .  23
     5.12. Message Security  . . . . . . . . . . . . . . . . . . . .  23
     5.13. Security Domain . . . . . . . . . . . . . . . . . . . . .  23

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     5.14. A Sample Device Setup Flow  . . . . . . . . . . . . . . .  23
   6.  TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . .  24
     6.1.  Role of the TEEP Broker . . . . . . . . . . . . . . . . .  25
     6.2.  TEEP Broker Implementation Consideration  . . . . . . . .  25
       6.2.1.  TEEP Broker Distribution  . . . . . . . . . . . . . .  26
       6.2.2.  Number of TEEP Brokers  . . . . . . . . . . . . . . .  26
   7.  Attestation . . . . . . . . . . . . . . . . . . . . . . . . .  26
     7.1.  Attestation Cryptographic Properties  . . . . . . . . . .  28
     7.2.  TEEP Attestation Structure  . . . . . . . . . . . . . . .  29
     7.3.  TEEP Attestation Claims . . . . . . . . . . . . . . . . .  31
     7.4.  TEEP Attestation Flow . . . . . . . . . . . . . . . . . .  31
     7.5.  Attestation Key Example . . . . . . . . . . . . . . . . .  31
       7.5.1.  Attestation Hierarchy Establishment: Manufacture  . .  32
       7.5.2.  Attestation Hierarchy Establishment: Device Boot  . .  32
       7.5.3.  Attestation Hierarchy Establishment: TAM  . . . . . .  32
   8.  Algorithm and Attestation Agility . . . . . . . . . . . . . .  32
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  33
     9.1.  TA Trust Check at TEE . . . . . . . . . . . . . . . . . .  33
     9.2.  One TA Multiple SP Case . . . . . . . . . . . . . . . . .  33
     9.3.  Broker Trust Model  . . . . . . . . . . . . . . . . . . .  34
     9.4.  Data Protection at TAM and TEE  . . . . . . . . . . . . .  34
     9.5.  Compromised CA  . . . . . . . . . . . . . . . . . . . . .  34
     9.6.  Compromised TAM . . . . . . . . . . . . . . . . . . . . .  34
     9.7.  Certificate Renewal . . . . . . . . . . . . . . . . . . .  34
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  35
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  35
     12.2.  Informative References . . . . . . . . . . . . . . . . .  35
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

   Applications executing in a device are exposed to many different
   attacks intended to compromise the execution of the application, or
   reveal the data upon which those applications are operating.  These
   attacks increase with the number of other applications on the device,
   with such other applications coming from potentially untrustworthy
   sources.  The potential for attacks further increase with the
   complexity of features and applications on devices, and the
   unintended interactions among those features and applications.  The
   danger of attacks on a system increases as the sensitivity of the
   applications or data on the device increases.  As an example,
   exposure of emails from a mail client is likely to be of concern to
   its owner, but a compromise of a banking application raises even
   greater concerns.

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   The Trusted Execution Environment (TEE) concept is designed to
   execute applications in a protected environment that separates
   applications inside the TEE from the regular operating system and
   from other applications on the device.  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 a normal application running in the
   regular operating system is referred to as an Untrusted Application

   The TEE uses hardware to enforce protections on the TA and its data,
   but also presents a more limited set of services to applications
   inside the TEE than is normally available to UA's running in the
   normal operating system.

   But not all TEEs are the same, and different vendors may have
   different implementations of TEEs with different security properties,
   different features, and different control mechanisms to operate on
   TAs.  Some vendors may themselves market multiple different TEEs with
   different properties attuned to different markets.  A device vendor
   may integrate one or more TEEs into their devices depending on market

   To simplify the life of developers and service providers interacting
   with TAs in a TEE, an interoperable protocol for managing TAs running
   in different TEEs of various devices is needed.  In this TEE
   ecosystem, there often arises a need for an external trusted party to
   verify the identity, claims, and rights of Service Providers(SP),
   devices, and their TEEs.  This trusted third party is the Trusted
   Application Manager (TAM).

   This protocol addresses the following problems:

   -  A Service Provider (SP) intending to provide services through a TA
      to users of a device needs to determine security-relevant
      information of a device before provisioning their TA to the TEE
      within the device.  Examples include the verification of the
      device 'root of trust' and the type of TEE included in a device.

   -  A TEE in a device needs to determine whether a Service Provider
      (SP) that wants to manage a TA in the device is authorized to
      manage TAs in the TEE, and what TAs the SP is permitted to manage.

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   -  The parties involved in the protocol must be able to attest that a
      TEE is genuine and capable of providing the security protections
      required by a particular TA.

   -  A Service Provider (SP) must be able to determine if a TA exists
      (is installed) on a device (in the TEE), and if not, install the
      TA in the TEE.

   -  A Service Provider (SP) must be able to check whether a TA in a
      device's TEE is the most up-to-date version, and if not, update
      the TA in the TEE.

   -  A Service Provider (SP) must be able to remove a TA in a device's
      TEE if the SP is no longer offering such services or the services
      are being revoked from a particular user (or device).  For
      example, if a subscription or contract for a particular service
      has expired, or a payment by the user has not been completed or
      has been rescinded.

   -  A Service Provider (SP) must be able to define the relationship
      between cooperating TAs under the SP's control, and specify
      whether the TAs can communicate, share data, and/or share key

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are used:

   -  Client Application: An application running in a Rich Execution
      Environment, such as an Android, Windows, or iOS application.  We
      sometimes refer to this as the 'Client App'.

   -  Device: A physical piece of hardware that hosts a TEE along with a
      Rich Execution Environment.  A Device contains a default list of
      Trust Anchors that identify entities (e.g., TAMs) that are trusted
      by the Device.  This list is normally set by the Device
      Manufacturer, and may be governed by the Device's network carrier.
      The list of Trust Anchors is normally modifiable by the Device's
      owner or Device Administrator.  However the Device manufacturer
      and network carrier may restrict some modifications, for example,
      by not allowing the manufacturer or carrier's Trust Anchor to be
      removed or disabled.

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   -  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 the TEE.  This environment and
      applications running on it are considered un-trusted.

   -  Service Provider (SP): An entity that wishes to provide a service
      on Devices that requires the use of one or more Trusted
      Applications.  A Service Provider requires the help of a TAM in
      order to provision the Trusted Applications to remote devices.

   -  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).  Relates to
      Device Owner and Device Administrator.

   -  Device Owner: A device is always owned by someone.  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 device users have no or limited administration rights.
      In this case, the enterprise appoints a device administrator that
      is not the device owner.

   -  Device Administrator (DA): An entity that is responsible for
      administration of a Device, which could be the device owner.  A
      Device Administrator has privileges on the Device to install and
      remove applications and TAs, approve or reject Trust Anchors, and
      approve or reject Service Providers, among possibly other
      privileges on the Device.  A Device Administrator can manage the
      list of allowed TAMs by modifying the list of Trust Anchors on the
      Device.  Although a Device Administrator may have privileges and
      Device-specific controls to locally administer a device, the
      Device Administrator may choose to remotely administrate a device
      through a TAM.

   -  Trust Anchor: A public key in a device whose corresponding private
      key is held by an entity implicitly trusted by the device.  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.  The Trust Anchor is normally stored in
      a location that resists unauthorized modification, insertion, or
      replacement.  The digital fingerprint of a Trust Anchor may be
      stored along with the Trust Anchor certificate or public key.  A
      device can use the fingerprint to uniquely identify a Trust
      Anchor.  The Trust Anchor private key owner can sign certificates
      of other public keys, which conveys trust about those keys to the

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      device.  A certificate signed by the Trust Anchor communicates
      that the private key holder of the signed certificate is trusted
      by the Trust Anchor holder, and can therefore be trusted by the
      device.  Trust Anchors in a device may be updated by an authorized
      party when a Trust Anchor should be deprecated or a new Trust
      Anchor should be added.

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

   -  Trusted Execution Environment (TEE): An execution environment that
      runs alongside of, but is isolated from, an REE.  A TEE has
      security capabilities and meets certain security-related
      requirements.  It protects TEE assets from general software
      attacks, defines rigid safeguards as to data and functions that a
      program can access, and resists a set of defined threats.  It
      should have at least the following three properties:

      (a) A device unique credential that cannot be cloned;

      (b) Assurance that only authorized code can run in the TEE;

      (c) Memory that cannot be read by code outside the TEE.

      There are multiple technologies that can be used to implement a
      TEE, and the level of security achieved varies accordingly.

   -  Root-of-Trust (RoT): A hardware or software component in a device
      that is inherently trusted to perform a certain security-critical
      function.  A RoT should be secure by design, small, and protected
      by hardware against modification or interference.  Examples of
      RoTs include software/firmware measurement and verification using
      a Trust Anchor (RoT for Verification), provide signed assertions
      using a protected attestation key (RoT for Reporting), or protect
      the storage and/or use of cryptographic keys (RoT for Storage).
      Other RoTs are possible, including RoT for Integrity, and RoT for
      Measurement.  Reference: NIST SP800-164 (Draft).

   -  Trusted Firmware (TFW): A firmware in a device that can be
      verified with a Trust Anchor by RoT for Verification.

   -  Bootloader key: This symmetric key is protected by
      electronic fuse (eFUSE) technology.  In this context it is used to
      decrypt a
      TFW private key, which belongs to a device-unique private/public
      key pair.  Not every device is equipped with a bootloader key.

   This document uses the following abbreviations:

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   -  CA: Certificate Authority

   -  REE: Rich Execution Environment

   -  RoT: Root of Trust

   -  SD: Security Domain

   -  SP: Service Provider

   -  TA: Trusted Application

   -  TAM: Trusted Application Manager

   -  TEE: Trusted Execution Environment

   -  TFW: Trusted Firmware

3.  Assumptions

   This specification assumes that an applicable device is equipped with
   one or more TEEs and each TEE is pre-provisioned with a device-unique
   public/private key pair, which is securely stored.

   A TEE uses an isolation mechanism between Trusted Applications to
   ensure that one TA cannot read, modify or delete the data and code of
   another TA.

4.  Use Cases

4.1.  Payment

   A payment application in a mobile device requires high security and
   trust about the hosting device.  Payments initiated from a mobile
   device can use a Trusted Application to provide strong identification
   and proof of transaction.

   For a mobile payment application, some biometric identification
   information could also be stored in a TEE.  The mobile payment
   application can use such information for authentication.

   A secure user interface (UI) may be used in a mobile device to
   prevent malicious software from stealing sensitive user input data.
   Such an application implementation often relies on a TEE for user
   input protection.

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4.2.  Authentication

   For better security of authentication, a device may store its
   sensitive authentication keys inside a TEE, providing hardware-
   protected security key strength and trusted code execution.

4.3.  Internet of Things

   The Internet of Things (IoT) has been posing threats to networks and
   national infrastructures because of existing weak security in
   devices.  It is very desirable that IoT devices can prevent malware
   from manipulating actuators (e.g., unlocking a door), or stealing or
   modifying sensitive data such as authentication credentials in the
   device.  A TEE can be the best way to implement such IoT security

   TEEs could be used to store variety of sensitive data for IoT
   devices.  For example, a TEE could be used in smart door locks to
   store a user's biometric information for identification, and for
   protecting access the locking mechanism.

4.4.  Confidential Cloud Computing

   A tenant can store sensitive data in a TEE in a cloud computing
   server such that only the tenant can access the data, preventing the
   cloud hosting provider from accessing the data.  A tenant can run TAs
   inside a server TEE for secure operation and enhanced data security.
   This provides benefits not only to tenants with better data security
   but also to cloud hosting provider for reduced liability and
   increased cloud adoption.

5.  Architecture

5.1.  System Components

   The following are the main components in the system.  Full
   descriptions of components not previously defined are provided below.
   Interactions of all components are further explained in the following

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   | Device                                    |
   |                          +--------+       |        Service Provider
   |    +-------------+       |        |----------+               |
   |    | TEE-1       |       | TEEP   |---------+|               |
   |    | +--------+  |  +----| Broker |       | ||   +--------+  |
   |    | | TEEP   |  |  |    |        |<---+  | |+-->|        |<-+
   |    | | Agent  |<----+    |        |    |  | |  +-|  TAM-1 |
   |    | +--------+  |       |        |<-+ |  | +->| |        |<-+
   |    |             |       +--------+  | |  |    | +--------+  |
   |    | +---+ +---+ |                   | |  |    | TAM-2  |    |
   |  +-->|TA1| |TA2| |        +-------+  | |  |    +--------+    |
   |  | | |   | |   |<---------| App-2 |--+ |  |                  |
   |  | | +---+ +---+ |    +-------+   |    |  |    Device Administrator
   |  | +-------------+    | App-1 |   |    |  |
   |  |                    |       |   |    |  |
   |  +--------------------|       |---+    |  |
   |                       |       |--------+  |
   |                       +-------+           |

                  Figure 1: Notional Architecture of TEEP

   -  Service Providers (SP) and Device Administrators (DA) utilize the
      services of a TAM to manage TAs on Devices.  SPs do not directly
      interact with devices.  DAs may elect to use a TAM for remote
      administration of TAs instead of managing each device directly.

   -  TAM: A TAM is responsible for performing lifecycle management
      activity on TA's on behalf of Service Providers and Device
      Administrators.  This includes creation and deletion of TA's, and
      may include, for example, over-the-air updates to keep an SP's TAs
      up-to-date and clean up when a version should be removed.  TAMs
      may provide services that make it easier for SPs or DAs to use the
      TAM's service to manage multiple devices, although that is not
      required of a TAM.

      The TAM performs its management of TA's through an interaction
      with a Device's TEEP Broker.  As shown in #notionalarch, the TAM
      cannot directly contact a Device, but must wait for a the TEEP
      Broker or a Client Application to contact the TAM requesting a
      particular service.  This architecture is intentional in order to
      accommodate network and application firewalls that normally
      protect user and enterprise devices from arbitrary connections
      from external network entities.

      A TAM may be publicly available for use by many SPs, or a TAM may
      be private, and accessible by only one or a limited number of SPs.

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      It is expected that manufacturers and carriers will run their own
      private TAM.  Another example of a private TAM is a TAM running as
      a Software-as-a-Service (SaaS) within an SP.

      A SP or Device Administrator chooses a particular TAM based on
      whether the TAM is trusted by a Device or set of Devices.  The TAM
      is trusted by a device if the TAM's public key is an authorized
      Trust Anchor in the Device.  A SP or Device Administrator may run
      their own TAM, however the Devices they wish to manage must
      include this TAM's pubic key in the Trust Anchor list.

      A SP or Device Administrator is free to utilize multiple TAMs.
      This may be required for a SP to manage multiple different types
      of devices from different manufacturers, or devices on different
      carriers, since the Trust Anchor list on these different devices
      may contain different TAMs.  A Device Administrator may be able to
      add their own TAM's public key or certificate to the Trust Anchor
      list on all their devices, overcoming this limitation.

      Any entity is free to operate a TAM.  For a TAM to be successful,
      it must have its public key or certificate installed in Devices
      Trust Anchor list.  A TAM may set up a relationship with device
      manufacturers or carriers to have them install the TAM's keys in
      their device's Trust Anchor list.  Alternatively, a TAM may
      publish its certificate and allow Device Administrators to install
      the TAM's certificate in their devices as an after-market-action.

   -  TEEP Broker: The TEEP Broker is an application running in a Rich
      Execution Environment (REE) that enables the message protocol
      exchange between a TAM and a TEE in a device.  The TEEP Broker
      does not process messages on behalf of a TEE, but merely is
      responsible for relaying messages from the TAM to the TEE, and for
      returning the TEE's responses to the TAM.

      A Client Application is expected to communicate with a TAM to
      request TAs that it needs to use.  The Client Application needs to
      pass the messages from the TAM to TEEs in the device.  This calls
      for a component in the REE that Client Applications can use to
      pass messages to TEEs.  The TEEP Broker is thus an application in
      the REE or software library that can relay messages from a Client
      Application to a TEE in the device.  A device usually comes with
      only one active TEE.  A TEE may provide such a Broker to the
      device manufacturer to be bundled in devices.  Such a TEE must
      also include a Broker counterpart, namely, a TEEP Agent inside the
      TEE, to parse TAM messages sent through the Broker.  A TEEP Broker
      is generally acting as a dummy relaying box with just the TEE
      interacting capability; it doesn't need and shouldn't parse
      protocol messages.

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   -  TEEP Agent: the TEEP Agent is a processing module running inside a
      TEE that receives TAM requests that are relayed via a TEEP Broker
      that runs in an REE.  A TEEP Agent in the TEE may parse requests
      or forward requests to other processing modules in a TEE, which is
      up to a TEE provider's implementation.  A response message
      corresponding to a TAM request is sent by a TEEP Agent back to a
      TEEP Broker.

   -  Certification Authority (CA): Certificate-based credentials used
      for authenticating a device, a TAM and an SP.  A device embeds a
      list of root certificates (Trust Anchors), from trusted CAs that a
      TAM will be validated against.  A TAM will remotely attest a
      device by checking whether a device comes with a certificate from
      a CA that the TAM trusts.  The CAs do not need to be the same;
      different CAs can be chosen by each TAM, and different device CAs
      can be used by different device manufacturers.

5.2.  Different Renditions of TEEP Architecture

   There is nothing prohibiting a device from implementing multiple
   TEEs.  In addition, some TEEs (for example, SGX) present themselves
   as separate containers within memory without a controlling manager
   within the TEE.  In these cases, the rich operating system hosts
   multiple TEEP brokers, where each broker manages a particular TEE or
   set of TEEs.  Enumeration and access to the appropriate broker is up
   to the rich OS and the applications.  Verification that the correct
   TA has been reached then becomes a matter of properly verifying TA
   attestations, which are unforgeable.  The multiple TEE approach is
   shown in the diagram below.  For brevity, TEEP Broker 2 is shown
   interacting with only one TAM and UA, but no such limitation is
   intended to be implied in the architecture.

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   | Device                                    |
   |                          +--------+       |        Service Provider
   |                          |        |----------+               |
   |    +-------------+       | TEEP   |---------+|               |
   |    | TEE-1       |   +---| Broker |       | ||   +--------+  |
   |    |             |   |   | 1      |<---+  | |+-->|        |<-+
   |    | +-------+   |   |   |        |    |  | |    |        |
   |    | | TEEP  |   |   |   |        |    |  | |    |        |
   |    | | Agent |<------+   |        |    |  | |    |        |
   |    | | 1     |   |       |        |    |  | |    |        |
   |    | +-------+   |       |        |    |  | |    |        |
   |    |             |       |        |    |  | |    |        |
   |    | +---+ +---+ |       |        |    |  | |  +-|  TAM-1 |
   |    | |TA1| |TA2| |       |        |<-+ |  | +->| |        |<-+
   |  +-->|   | |   |<---+    +--------+  | |  |    | +--------+  |
   |  | | +---+ +---+ |  |                | |  |    | TAM-2  |    |
   |  | |             |  |     +-------+  | |  |    +--------+    |
   |  | +-------------+  +-----| App-2 |--+ |  |       ^          |
   |  |                    +-------+   |    |  |       |        Device
   |  +--------------------| App-1 |   |    |  |       |   Administrator
   |                +------|       |   |    |  |       |
   |    +-----------|-+    |       |---+    |  |       |
   |    | TEE-2     | |    |       |--------+  |       |
   |    | +------+  | |    |       |------+    |       |
   |    | | TEEP |  | |    +-------+      |    |       |
   |    | | Agent|<-----+                |    |       |
   |    | | 2    |  | | |                 |    |       |
   |    | +------+  | | |                 |    |       |
   |    |           | | |                 |    |       |
   |    | +---+     | | |                 |    |       |
   |    | |TA3|<----+ | |  +----------+   |    |       |
   |    | |   |       | |  | TEEP     |<--+    |       |
   |    | +---+       | +--| Broker   |----------------+
   |    |             |    | 2        |        |
   |    +-------------+    +----------+        |
   |                                           |

        Figure 2: Notional Architecture of TEEP wtih multiple TEEs

   In the diagram above, TEEP Broker 1 controls interactions with the
   TA's in TEE-1, and TEEP Broker 2 controls interactions with the TA's
   in TEE-2.  This presents some challenges for a TAM in completely
   managing the device, since a TAM may not interact with all the TEEP
   Brokers on a particular platform.  In addition, since TEE's may be
   physically separated, with wholly different resources, there may be
   no need for TEEP Brokers to share information on installed TAs or

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   resource usage.  However, the architecture guarantees that the TAM
   will receive all the relevant information from the TEEP Broker to
   which it communicates.

5.3.  Multiple TAMs and Relationship to TAs

   As shown in Figure 2, the TEEP Broker provides connections from the
   TEE and the Client App to one or more TAMs.  The selection of which
   TAM to communicate with is dependent on information from the Client
   App and is directly related to the TA.

   When a SP offers a service which requires a TA, the SP associates
   that service with a specific TA.  The TA itself is digitally signed,
   protecting its integrity, but the signature also links the TA back to
   the signer.  The signer is usually the SP, but in some cases may be
   another party that the SP trusts.  The SP selects one or more TAMs
   through which to offer their service, and communicates the
   information of the service and the specific client apps and TAs to
   the TAM.

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

   When the SP publishes the Client App to an app store or other app
   repositories, the SP binds the Client App with a manifest that
   identifies what TAMs can be contacted for the TA.  In some
   situations, an SP may use only a single TAM - this is likely the case
   for enterprise applications or SPs serving a closed community.  For
   broad public apps, there will likely be multiple TAMs in the manifest
   - one servicing one brand of mobile device and another servicing a
   different manufacturer, etc.  Because different devices and different
   manufacturers trust different TAMs, the manifest will include
   different TAMs that support this SP's client app and TA.  Multiple
   TAMs allow the SP to provide thier service and this app (and TA) to
   multiple different devices.

   When the TEEP Broker receives a request to contact the TAM for a
   Client App in order to install a TA, a list of TAMs may be provided.
   The TEEP Broker selects a single TAM that is consistent with the list
   of trusted TAMs (trust anchors) provisioned on the device.  For any
   client app, there should be only a single TAM for the TEEP Broker to
   contact.  This is also the case when a Client App uses multiple TAs,

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   or when one TA depends on anther TA in a software dependency (see
   section TBD).  The reason is that the SP should provide each TAM that
   it places in the Client App's manifest all the TAs that the app
   requires.  There is no benefit to going to multiple different TAMs,
   and there is no need for a special TAM to be contacted for a specific

   [Note: This should always be the case.  When a particular device or
   TEE supports only a special proprietary attestation mechanism, then a
   specific TAM will be needed that supports that attestation scheme.
   The TAM should also support standard atttestation signatures as well.
   It is highly unlikely that a set of TAs would use different
   proprietary attestation mechanisms since a TEE is likley to support
   only one such proprietary scheme.]

   [Note: This situation gets more complex in situations where a Client
   App expects another application or a device to already have a
   specific TA installed.  This situation does not occur with SGX, but
   could occur in situations where the secure world maintains an trusted
   operating system and runs an entire trusted system with multiple TAs
   running.  This requires more discussion.]

5.4.  Client Apps, Trusted Apps, and Personalization Data

   In TEEP, there is an explicit relationship and dependence between the
   client app in the REE and one or more TAs in the TEE, as shown in
   Figure 2.  From the perspective of a device user, a client app that
   uses one or more TA's in a TEE appears no different from any other
   untrusted application in the REE.  However, the way the client app
   and its corresponding TA's are packaged, delivered, and installed on
   the device can vary.  The variations depend on whether the client app
   and TA are bundled together or are provided separately, and this has
   implications to the management of the TAs in the TEE.  In addition to
   the client app and TA, the TA and/or TEE may require some additional
   data to personalize the TA to the service provider or the device
   user.  This personalization data is dependent on the TEE, the TA and
   the SP; an example of personalization data might be username and
   password of the device user's account with the SP, or a secret
   symmetric key used to by the TA to communicate with the SP.  The
   personalization data must be encrypted to preserve the
   confidentiality of potentially sensitive data contained within it.
   Other than this requirement to support confidentiality, TEEP place no
   limitations or requirements on the personalization data.

   There are three possible cases for bundling of the Client App, TA,
   and personalization data:

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   1.  The Client App, TA, and personalization data are all bundled
       together in a single package by the SP and provided to the TEEP
       Broker through the TAM.

   2.  The Client App and the TA are bundled together in a single
       binary, which the TAM or a publicly accessible app store
       maintains in repository, and the personalization data is
       separately provided by the SP.  In this case, the personalization
       data is collected by the TAM and included in the InstallTA
       message to the TEEP Broker.

   3.  All components are independent.  The device user installs the
       Client App through some independent or device-specific mechanism,
       and the TAM provides the TA and personalization data from the SP.
       Delivery of the TA and personalization data may be combined or

5.5.  Examples of Application Delivery Mechanisms in Existing TEEs

   In order to better understand these cases, it is helpful to review
   actual implementations of TEEs and their application delivery

   In Intel Software Guard Extensions (SGX), the Client App and TA are
   typically bound into the same binary (Case 2).  The TA is compiled
   into the Client App binary using SGX tools, and exists in the binary
   as a shared library (.so or .dll).  The Client App loads the TA into
   an SGX enclave when the client needs the TA.  This organization makes
   it easy to maintain compatibility between the Client App and the TA,
   since they are updated together.  It is entirely possible to create a
   Client App that loads an external TA into an SGX enclave and use that
   TA (Case 3).  In this case, the Client App 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 required a special installation program and
   SGX TA to recieve the encrypted binary, decrypt it, separate it into
   the three different elements, and then install all three.  This
   installation is complex, because the Client App decrypted inside the
   TEE must be passed out of the TEE to an installer in the REE which
   would install the Client App; this assumes that the Client App binary
   includes the TA code also, otherwise there is a significant problem
   in getting the SGX encalve code (the TA) from the TEE, through the
   installer and into the Client App in a trusted fashion.  Finally, the

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   personalization data would need to be sent out of the TEE (encrypted
   in an SGX encalve-to-enclave manner) to the REE's installation app,
   which would pass this data to the installed Client App, 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 one another.

   [NOTE: Need to add an equivalent discussion for an ARM/TZ

5.6.  TEEP Architectural Support for Client App, TA, and Personalization
      Data Delivery

   This section defines TEEP support for the three different cases for
   delivery of the Client App, TA, and personalization data.

   [Note: discussion of format of this single binary, and who/what is
   responsible for splitting these things apart, and installing the
   client app into the REE, the TA into the TEE, and the personalization
   data into the TEE or TA.  Obviously the decryption must be done by
   the TEE but this may not be supported by all TAs.]

5.7.  Entity Relations

   This architecture leverages asymmetric cryptography to authenticate a
   device to a TAM.  Additionally, a TEE in a device authenticates a TAM
   and TA signer.  The provisioning of Trust Anchors to a device may
   different from one use case to the other.  A device administrator may
   want to have the capability to control what TAs are allowed.  A
   device manufacturer enables verification of the TA signers and TAM
   providers; it may embed a list of default Trust Anchors that the
   signer of an allowed TA's signer certificate should chain to.  A
   device administrator may choose to accept a subset of the allowed TAs
   via consent or action of downloading.

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   PKI    CA    -- CA                                 CA --
           |    |                                         |
           |    |                                         |
           |    |                                         |
   Device  |    |   ---    Agent / Client App   ---       |
   SW      |    |   |                             |       |
           |    |   |                             |       |
           |    |   |                             |       |
           |    -- TEE                           TAM-------

                            Figure 3: Entities

    (App Developer)    (App Store)    (TAM)     (Device with TEE)  (CAs)
           |                                            |
           |                               --> (Embedded TEE cert) <--
           |                                            |
           | <------------------------------  Get an app cert ----- |
           |                           | <--  Get a TAM cert ------ |
   1. Build two apps:
       Client App
      Client App -- 2a. --> | ----- 3. Install -------> |
         TA ------- 2b. Supply ------> | 4. Messaging-->|
           |                |          |                |

                      Figure 4: Developer Experience

   Figure 4 shows an application developer building two applications: 1)
   a rich Client Application; 2) a TA that provides some security
   functions to be run inside a TEE.  At step 2, the application
   developer uploads the Client Application (2a) to an Application
   Store.  The Client Application may optionally bundle the TA binary.
   Meanwhile, the application developer may provide its TA to a TAM
   provider that will be managing the TA in various devices. 3.  A user
   will go to an Application Store to download the Client Application.
   The Client Application will trigger TA installation by initiating
   communication with a TAM.  This is the step 4.  The Client
   Application will get messages from TAM, and interacts with device TEE
   via an Agent.

   The following diagram shows a system diagram about the entity
   relationships between CAs, TAMs, SPs and devices.

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           ------- Message Protocol  -----
           |                             |
           |                             |
    --------------------           ---------------   ----------
    |  REE   |  TEE    |           |    TAM      |   |  SP    |
    |  ---   |  ---    |           |    ---      |   |  --    |
    |        |         |           |             |   |        |
    | Client | TEEP    |           |      TA     |   |  TA    |
    |  Apps  | Agent   |           |     Mgmt    |   |        |
    |   |    |         |           |             |   |        |
    |   |    |  TAs    |           |             |   |        |
    |  TEEP  |         |           |             |   |        |
    | Broker | List of |           |  List of    |   |        |
    |        | Trusted |           |  Trusted    |   |        |
    |        |  TAM/SP |           |   FW/TEE    |   |        |
    |        |   CAs   |           |    CAs      |   |        |
    |        |         |           |             |   |        |
    |        |TEE Key/ |           |  TAM Key/   |   |SP Key/ |
    |        |  Cert   |           |    Cert     |   | Cert   |
    |        | FW Key/ |           |             |   |        |
    |        |  Cert   |           |             |   |        |
    --------------------           ---------------   ----------
                 |                        |              |
                 |                        |              |
           -------------              ----------      ---------
           | TEE CA    |              | TAM CA |      | SP CA |
           -------------              ----------      ---------

                              Figure 5: Keys

   In the previous diagram, different CAs can be used for different
   types of certificates.  Messages are always signed, where the signer
   key is the message originator's private key such as that of a TAM,
   the private key of trusted firmware (TFW), or a TEE's private key.

   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.  The networking
   functionality must be delegated to a rich Client Application.  Client
   Applications will need to rely on an agent in the REE to interact
   with a TEE for message exchanges.  Consequently, a TAM generally
   communicates with a Client Application about how it gets messages
   that originate from a TEE inside a device.  Similarly, a TA or TEE
   generally gets messages from a TAM via some Client Application,

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   namely, a TEEP Broker in this protocol architecture, not directly
   from the network.

   It is imperative to have an interoperable protocol to communicate
   with different TAMs and different TEEs in different devices.  This is
   the role of the Broker, which is a software component that bridges
   communication between a TAM and a TEE.  Furthermore the Broker
   communicates with a Agent inside a TEE that is responsible to process
   TAM requests.  The Broker in REE does not need to know the actual
   content of messages except for the TEE routing information.

5.8.  Trust Anchors in TEE

   Each TEE comes with a trust store that contains a whitelist of Trust
   Anchors that are used to validate a TAM's certificate.  A TEE will
   accept a TAM to create new Security Domains and install new TAs on
   behalf of an SP only if the TAM's certificate is chained to one of
   the root CA certificates in the TEE's trust store.

   A TEE's trust store is typically preloaded at manufacturing time.  It
   is out of the scope in this document to specify how the trust anchors
   should be updated when a new root certificate should be added or
   existing one should be updated or removed.  A device manufacturer is
   expected to provide its TEE trust anchors live update or out-of-band
   update to Device Administrators.

   When trust anchor update is carried out, it is imperative that any
   update must maintain integrity where only authentic trust anchor list
   from a device manufacturer or a Device Administrator is accepted.
   This calls for a complete lifecycle flow in authorizing who can make
   trust anchor update and whether a given trust anchor list are non-
   tampered from the original provider.  The signing of a trust anchor
   list for integrity check and update authorization methods are
   desirable to be developed.  This can be addressed outside of this
   architecture document.

   Before a TAM can begin operation in the marketplace to support a
   device with a particular TEE, it must obtain a TAM certificate from a
   CA that is listed in the trust store of the TEE.

5.9.  Trust Anchors in TAM

   The Trust Anchor store in a TAM consists of a list of CA certificates
   that sign various device TEE certificates.  A TAM will accept a
   device for TA management if the TEE in the device uses a TEE
   certificate that is chained to a CA that the TAM trusts.

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5.10.  Keys and Certificate Types

   This architecture leverages the following credentials, which allow
   delivering end-to-end security without relying on any transport

   | Key Entity  | Location | Issuer | Checked Against   | Cardinality |
   | Name        |          |        |                   |             |
   | 1. TFW key  | Device   | FW CA  | A whitelist of    | 1 per       |
   | pair and    | secure   |        | FW root CA        | device      |
   | certificate | storage  |        | trusted by TAMs   |             |
   |             |          |        |                   |             |
   | 2. TEE key  | Device   | TEE CA | A whitelist of    | 1 per       |
   | pair and    | TEE      | under  | TEE root CA       | device      |
   | certificate |          | a root | trusted by TAMs   |             |
   |             |          | CA     |                   |             |
   |             |          |        |                   |             |
   | 3. TAM key  | TAM      | TAM CA | A whitelist of    | 1 or        |
   | pair and    | provider | under  | TAM root CA       | multiple    |
   | certificate |          | a root | embedded in TEE   | can be used |
   |             |          | CA     |                   | by a TAM    |
   |             |          |        |                   |             |
   | 4. SP key   | SP       | SP     | A SP uses a TAM.  | 1 or        |
   | pair and    |          | signer | TA is signed by a | multiple    |
   | certificate |          | CA     | SP signer. TEE    | can be used |
   |             |          |        | delegates trust   | by a TAM    |
   |             |          |        | of TA to TAM. SP  |             |
   |             |          |        | signer is         |             |
   |             |          |        | associated with a |             |
   |             |          |        | TA as the owner.  |             |

                    Figure 6: Key and Certificate Types

   1.  TFW key pair and certificate: A key pair and certificate for
       evidence of trustworthy firmware in a device.  This key pair is
       optional for TEEP architecture.  Some TEE may present its trusted
       attributes to a TAM using signed attestation with a TFW key.  For
       example, a platform that uses a hardware based TEE can have
       attestation data signed by a hardware protected TFW key.

       o  Location: Device secure storage

       o  Supported Key Type: RSA and ECC

       o  Issuer: OEM CA

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       o  Checked Against: A whitelist of FW root CA trusted by TAMs

       o  Cardinality: One per device

   2.  TEE key pair and certificate: It is used for device attestation
       to a remote TAM and SP.

       o  This key pair is burned into the device by the device
          manufacturer.  The key pair and its certificate are valid for
          the expected lifetime of the device.

       o  Location: Device TEE

       o  Supported Key Type: RSA and ECC

       o  Issuer: A CA that chains to a TEE root CA

       o  Checked Against: A whitelist of TEE root CAs trusted by TAMs

       o  Cardinality: One per device

   3.  TAM key pair and certificate: A TAM provider acquires a
       certificate from a CA that a TEE trusts.

       o  Location: TAM provider

       o  Supported Key Type: RSA and ECC.

       o  Supported Key Size: RSA 2048-bit, ECC P-256 and P-384.  Other
          sizes should be anticipated in future.

       o  Issuer: TAM CA that chains to a root CA

       o  Checked Against: A whitelist of TAM root CAs embedded in a TEE

       o  Cardinality: One or multiple can be used by a TAM

   4.  SP key pair and certificate: An SP uses its own key pair and
       certificate to sign a TA.

       o  Location: SP

       o  Supported Key Type: RSA and ECC

       o  Supported Key Size: RSA 2048-bit, ECC P-256 and P-384.  Other
          sizes should be anticipated in future.

       o  Issuer: An SP signer CA that chains to a root CA

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       o  Checked Against: An SP uses a TAM.  A TEE trusts an SP by
          validating trust against a TAM that the SP uses.  A TEE trusts
          a TAM to ensure that a TA is trustworthy.

       o  Cardinality: One or multiple can be used by an SP

5.11.  Scalability

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

5.12.  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 and are typically encrypted such
   that only the targeted device TEE or TAM is able to decrypt and view
   the actual content.

5.13.  Security Domain

   No security domain (SD) is explicitly assumed in a TEE for TA
   management.  Some TEE, for example, some TEE compliant with Global
   Platform (GP), may continue to choose to use SD to organize resource
   partition and security boundaries.  It is up to a TEE implementation
   to decide how a SD is attached to a TA installation, for example, one
   SD could be created per TA.

5.14.  A Sample Device Setup Flow

   Step 1: Prepare Images for Devices

   1.  [TEE vendor] Deliver TEE Image (CODE Binary) to device OEM

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   2.  [CA] Deliver root CA Whitelist

   3.  [Soc] Deliver TFW Image

   Step 2: Inject Key Pairs and Images to Devices

   1.  [OEM] Generate TFW Key Pair (May be shared among multiple

   2.  [OEM] Flash signed TFW Image and signed TEE Image onto devices
       (signed by TFW Key)

   Step 3: Set up attestation key pairs in devices

   1.  [OEM] Flash TFW Public Key and a bootloader key.

   2.  [TFW/TEE] Generate a unique attestation key pair and get a
       certificate for the device.

   Step 4: Set up Trust Anchors in devices

   1.  [TFW/TEE] Store the key and certificate encrypted with the
       bootloader key

   2.  [TEE vendor or OEM] Store trusted CA certificate list into

6.  TEEP Broker

   A TEE and TAs do not generally have the capability to communicate to
   the 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 the network communication.  Each
   Client Application in the REE might carry this communication
   functionality but such functionality must also interact with the TEE
   for the message exchange.
   The TEE interaction will vary according to different TEEs.  In order
   for a Client Application to transparently support different TEEs, it
   is imperative to have a common interface for a Client Application to
   invoke for exchanging messages with TEEs.

   A shared module in REE comes to meet this need.  A TEEP broker is an
   application running in the REE of the device or an SDK that
   facilitates communication between a TAM and a TEE.  It also provides
   interfaces for Client Applications to query and trigger TA
   installation that the application needs to use.

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   It isn't always that a Client Application directly calls such a
   Broker to interact with a TEE.  A REE Application Installer might
   carry out TEE and TAM interaction to install all required TAs that a
   Client Application depends.  A Client Application may have a metadata
   file that describes the TAs it depends on and the associated TAM that
   each TA installation goes to use.  The REE Application Installer can
   inspect the application metadata file and installs TAs on behalf of
   the Client Application without requiring the Client Application to
   run first.

   This interface for Client Applications or Application Installers may
   be commonly in a form of an OS service call for an REE OS.  A Client
   Application or an Application Installer interacts with the device TEE
   and the TAMs.

6.1.  Role of the TEEP Broker

   A TEEP Broker abstracts the message exchanges with a TEE in a device.
   The input data is originated from a TAM or the first initialization
   call to trigger a TA installation.

   The Broker doesn't need to parse a message content received from a
   TAM that should be processed by a TEE.  When a device has more than
   one TEE, one TEEP Broker per TEE could be present in REE.  A TEEP
   Broker interacts with a TEEP Agent inside a TEE.

   A TAM message may indicate the target TEE where a TA should be
   installed.  A compliant TEEP protocol should include a target TEE
   identifier for a TEEP Broker when multiple TEEs are present.

   The Broker relays the response messages generated from a TEEP Agent
   in a TEE to the TAM.  The Broker is not expected to handle any
   network connection with an application or TAM.

   The Broker only needs to return an error message if the TEE is not
   reachable for some reason.  Other errors are represented as response
   messages returned from the TEE which will then be passed to the TAM.

6.2.  TEEP Broker Implementation Consideration

   A Provider should consider methods of distribution, scope and
   concurrency on devices and runtime options when implementing a TEEP
   Broker.  Several non-exhaustive options are discussed below.
   Providers are encouraged to take advantage of the latest
   communication and platform capabilities to offer the best user

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6.2.1.  TEEP Broker Distribution

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

6.2.2.  Number of TEEP Brokers

   There should be generally only one shared TEEP Broker in a device.
   The device's TEE vendor will most probably supply one Broker.  When
   multiple TEEs are present in a device, one TEEP Broker per TEE may be

   When only one Broker is used per device, the Broker provider is
   responsible to allow multiple TAMs and TEE providers to achieve
   interoperability.  With a standard Broker interface, each TAM can
   implement its own SDK for its SP Client Applications to work with
   this Broker.

   Multiple independent Broker providers can be used as long as they
   have standard interface to a Client Application or TAM SDK.  Only one
   Broker is generally expected in a device.

7.  Attestation

   Attestation is the process through which one entity (an attestor)
   presents 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.  TEEP attestations
   are based upon the use of an asymmetric key pair under the control of
   the TEE to create digital signatures across a well-defined claim set.

   In TEEP, the primary purpose of an attestation is to allow a device
   to prove to TAMs and SPs that a TEE in the device has particular
   properties, was built by a particular manufacturer, or is executing a
   particular TA.  Other claims are possible; this architecture
   specification does not limit the attestation claims, but defines a
   minimal set of claims required for TEEP to operate properly.
   Extensions to these claims are possible, but are not defined in the
   TEEP specifications.  Other standards or groups may define the format
   and semantics of extended claims.  The TEEP specification defines the
   claims format such that these extended claims may be easily included
   in a TEEP attestation message.

   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, the attestation

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   format shall allow for both proprietary attestation signatures, as
   well as a standardized form of attestation signature.  Either form of
   attestation signature may be applied to a set of TEEP claims, and
   both forms of attestation shall be considered conformant with TEEP.
   However, it should be recognized that not all TAMs or SPs may be able
   to process all proprietary forms of attestations.  All TAMs and SPs
   MUST be able to process the TEEP standard attestation format and
   attached signature.

   The attestation formats and mechanisms described and mandated by TEEP
   shall convey a particular set of cryptographic properties based on
   minimal assumptions.  The cryptographic properties are conveyed by
   the attestation; however the assumptions are not conveyed within the
   attestation itself.

   The assumptions which may apply to an attestation have to do with the
   quality of the attestation and the quality and security provided by
   the TEE, the device, the manufacturer, or others involved in the
   device or TEE ecosystem.  Some of the assumptions that might apply to
   an attestations include (this may not be a comprehensive list):

   -  Assumptions regarding the security measures a manufacturer takes
      when provisioning keys into devices/TEEs;

   -  Assumptions regarding what hardware and software components have
      access to the Attestation keys of the TEE;

   -  Assumptions related to the source or local verification of claims
      within an attestation prior to a TEE signing a set of claims;

   -  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 and SPs must be comfortable with the assumptions that are
   inherently part of any attestation they accept.  Alternatively, any
   TAM or SP may choose not to accept an attestation generated from a
   particular manufacturer or device's TEE based on the inherent

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   assumptions.  The choice and policy decisions are left up to the
   particular TAM/SP.

   Some TAMs or SPs 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/SP wants to alleviate.  The
   specific format for these additional claims are outside the scope of
   this specification, but the OTrP protocol SHALL allow these
   additional claims to be included in the attestation messages.

   The following sub-sections define the cryptographic properties
   conveyed by the TEEP attestation, the basic set of TEEP claims
   required in a TEEP attestation, the TEEP attestation flow between the
   TAM the device TEE, and some implementation examples of how an
   attestation key may be realized in a real TEEP device.

7.1.  Attestation Cryptographic Properties

   The attestation constructed by TEEP must convey certain cryptographic
   properties from the attestor to the verifier; in the case of TEEP,
   the attestation must convey properties from the device to the TAM
   and/or SP.  The properties required by TEEP include:

   -  Non-repudiation, Unique Proof of Source - the cryptographic
      digital signature across the attestation, and optionally along
      with information in the attestion itself SHALL uniquely identify a
      specific TEE in a specific device.

   -  Integrity of claims - the cryptographic digital signature across
      the attestation SHALL cover the entire attestation including all
      meta data and all the claims in the attestation, ensuring that the
      attestation has not be modified since the TEE signed the

   Standard public key algorithms such as RSA and ECDSA digital
   signatures convey these properties.  Group public key algorithms such
   as EPID can also convey these properties, if the attestation includes
   a unique device identifier and an identifier for the TEE.  Other
   cryptographic operations used in other attestation schemes may also
   convey these properties.

   The TEEP standard attestation format SHALL use one of the following
   digital signature formats:

   -  RSA-2048 with SHA-256 or SHA-384 in RSASSA-PKCS1-v1_5 or PSS

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   -  RSA-3072 with SHA-256 or SHA-384 in RSASSA-PKCS1-v1_5 or PSS

   -  ECDSA-256 using NIST P256 curve using SHA-256

   -  ECDSA-384 using NIST P384 curve using SHA-384

   -  HashEdDSA using Ed25519 with SHA-512 (Ed25519ph in RFC8032) and
      context="TEEP Attestation"

   -  EdDSA using Ed448 with SHAK256 (Ed448ph in RFC8032) and
      context="TEEP Attestation"

   All TAMs and SPs MUST be able to accept attestations using these
   algorithms, contingent on their acceptance of the assumptions implied
   by the attestations.

7.2.  TEEP Attestation Structure

   For a TEEP attestation to be useful, it must contain an information
   set allowing the TAM and/or SP to assess the attestation and make a
   related security policy decision.  The structure of the TEEP
   attestation is shown in the diagram below.

                      +------(Signed By)-----------+
                      |                            |
        /--------------------------\               V
      | Attestation   | The         | The                      |
      | Header        | Claims      | Attestation Signature(s) |
             |            |                  |                 |              |
             V            V                  V                 V              V
      +-------------+  +-------------+  +----------+   +-----------------+  +------------+
      | Device      |  | TEE         |  |          |   | Action or       |  | Additional |
      | Identifying |  | Identifying |  | Liveness |   | Operation       |  | or optional|
      | Info        |  | Info        |  | Proof    |   | Specific claims |  | Claims     |
      +-------------+  +-------------+  +----------+   +-----------------+  +------------+

                  Figure 7: Structure of TEEP Attestation

   The Attestation Header SHALL identify the "Attestation Type" and the
   "Attestation Signature Type" along with an "Attestation Format
   Version Number."  The "Attestation Type" identifies the minimal set
   of claims that MUST be included in the attestation; this is an

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   identifier for a profile that defines the claims that should be
   included in the attestation as part of the "Action or Operation
   Specific Claims."  The "Attestation Signature Type" identifies the
   type of attestation signature that is attached.  The type of
   attestation signature SHALL be one of the standard signatures types
   identified by an IANA number, a proprietary signature type identified
   by an IANA number, or the generic "Proprietary Signature" with an
   accompanying proprietary identifier.  Not all TAMs may be able to
   process proprietary signatures.

   The claims in the attestation are set of mandatory and optional
   claims.  The claims themselves SHALL be defined in an attestation
   claims dictionary.  See the next section on TEEP Attestation Claims.
   Claims are grouped in profiles under an identifier (Attestation
   Type), however all attestations require a minimal set of claims which

   -  Device Identifying Info: TEEP attestations must uniquely identify
      a device to the TAM and SP.  This identifier allows the TAM/SP to
      provide services unique to the device, such as managing installed
      TAs, and providing subscriptions to services, and locating device-
      specific keying material to communicate wiht or authenticate the
      device.  Additionally, device manufacturer information must be
      provided to provide better universal uniqueness qualities without
      requiring globally unique identifiers for all devices.

   -  TEE Identifying info: The type of TEE that generated this
      attestation must be identified.  Standard TEE types are identified
      by an IANA number, but also must include version identification
      information such as the hardware, firmware, and software version
      of the TEE, as applicable by the TEE type.  Structure to the
      version number is required.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.

   -  Liveness Proof: a claim that includes liveness information SHALL
      be included which may be a large nonce or may be a timestamp and
      short nonce.

   -  Action Specific Claims: Certain attestation types shall include
      specific claims.  For example an attestation from a specific TA
      shall include a measurement, version and signing public key for
      the TA.

   -  Additional Claims: (Optional - May be empty set) A TAM or SP may
      require specific additional claims in order to address potential
      assumptions, such as the requirement that a device's REE performed

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      a secure boot, or that the device is not currenlty in a debug or
      non-productions state.  A TAM may require a device to provide a
      device health attestation that may include some claims or
      measurements about the REE.  These claims are TAM specific.

7.3.  TEEP Attestation Claims

   TEEP requires a set of attestation claims that provide sufficient
   evidence to the TAM and/or SP that the device and its TEE meet
   certain minimal requirements.  Because attestation formats are not
   yet broadly standardized across the industry, standardization work is
   currently ongoing, and it is expected that extensions to the
   attestation claims will be required as new TEEs and devices are
   created, the set of attestation claims required by TEEP SHALL be
   defined in an IANA registry.  That registry SHALL be defined in the
   OTrP protocol with sufficient elements to address basic TEEP claims,
   expected new standard claims (for example from
   https://www.ietf.org/id/draft-mandyam-eat-01.txt), and proprietary
   claim sets.

7.4.  TEEP Attestation Flow

   Attesations are required in TEEP under the following flows:

   -  When a TEE responds with device state information (dsi) to the TAM
      or SP, including a "GetDeviceState" response, "InstallTA"
      response, etc.

   -  When a new key pair is generated for a TA-to-TAM or TA-to-SP
      communication, the keypair must be covered by an attestation,
      including "CreateSecurityDomain" response, "UpdateSecurityDomain"
      response, etc.

7.5.  Attestation Key Example

   The attestation hierarchy and seed required for TAM protocol
   operation must be built into the device at manufacture.  Additional
   TEEs can be added post-manufacture using the scheme proposed, but it
   is outside of the current scope of this document to detail that.

   It should be noted that the attestation scheme described is based on
   signatures.  The only decryption that may take place is through the
   use of a bootloader key.

   A boot module generated attestation can be optional where the
   starting point of device attestation can be at TEE certificates.  A
   TAM can define its policies on what kinds of TEE it trusts if TFW
   attestation is not included during the TEE attestation.

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7.5.1.  Attestation Hierarchy Establishment: Manufacture

   During manufacture the following steps are required:

   1.  A device-specific TFW key pair and certificate are burnt into the
       device.  This key pair will be used for signing operations
       performed by the boot module.

   2.  TEE images are loaded and include a TEE instance-specific key
       pair and certificate.  The key pair and certificate are included
       in the image and covered by the code signing hash.

   3.  The process for TEE images is repeated for any subordinate TEEs,
       which are additional TEEs after the root TEE that some devices

7.5.2.  Attestation Hierarchy Establishment: Device Boot

   During device boot the following steps are required:

   1.  The boot module releases the TFW private key by decrypting it
       with the bootloader key.

   2.  The boot module verifies the code-signing signature of the active
       TEE and places its TEE public key into a signing buffer, along
       with its identifier for later access.  For a TEE non-compliant to
       this architecture, the boot module leaves the TEE public key
       field blank.

   3.  The boot module signs the signing buffer with the TFW private

   4.  Each active TEE performs the same operation as the boot module,
       building up their own signed buffer containing subordinate TEE

7.5.3.  Attestation Hierarchy Establishment: TAM

   Before a TAM can begin operation in the marketplace, it must obtain a
   TAM certificate from a CA that is registered in the trust store of
   devices.  In this way, the TEE can check the intermediate and root CA
   and verify that it trusts this TAM to perform operations on 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

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   greatly simplified when crypto agility is already considered during
   the design of the protocol.  In the case of the Open Trust Protocol
   (OTrP) 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 the OTrP 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 OTrP there is
   also the need to make use of different attestation technologies.  A
   Device must provide techniques to inform a TAM about the attestation
   technology it supports.  For many deployment cases it is more likely
   for the TAM to support one or more attestation techniques whereas the
   Device may only support one.

9.  Security Considerations

9.1.  TA Trust Check at TEE

   A TA binary is signed by a TA signer certificate.  This TA signing
   certificate/private key belongs to the SP, and may be self-signed
   (i.e., it need not participate in a trust hierarchy).  It is the
   responsibility of the TAM to only allow verified TAs from trusted SPs
   into the system.  Delivery of that TA to the TEE is then the
   responsibility of the TEE, using the security mechanisms provided by
   the protocol.

   We allow a way for an (untrusted) application to check the
   trustworthiness of a TA.  A TEEP Broker has a function to allow an
   application to query the information about a TA.

   An application in the Rich O/S may perform verification of the TA by
   verifying the signature of the TA.  The GetTAInformation function is
   available to return the TEE supplied TA signer and TAM signer
   information to the application.  An application can do additional
   trust checks on the certificate returned for this TA.  It might trust
   the TAM, or require additional SP signer trust chaining.

9.2.  One TA Multiple SP Case

   A TA for multiple SPs must have a different identifier per SP.  They
   should appear as different TAs when they are installed in the same

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9.3.  Broker Trust Model

   A TEEP Broker could be malware in the vulnerable REE.  A Client
   Application will connect its TAM provider for required TA
   installation.  It gets command messages from the TAM, and passes the
   message to the Broker.

   The architecture enables the TAM to communicate with the device's TEE
   to manage TAs.  All TAM messages are signed and sensitive data is
   encrypted such that the TEEP Broker cannot modify or capture
   sensitive data.

9.4.  Data Protection at TAM and TEE

   The TEE implementation provides protection of data on the device.  It
   is the responsibility of the TAM to protect data on its servers.

9.5.  Compromised CA

   A root CA for TAM certificates might get compromised.  Some TEE trust
   anchor update mechanism is expected from device OEMs.  A compromised
   intermediate CA is covered by OCSP stapling and OCSP validation check
   in the protocol.  A TEE should validate 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.  Any intermediate CA for TEE device
   certificates SHOULD be validated by TAM with a Certificate Revocation
   List (CRL) or Online Certificate Status Protocol (OCSP) method.

9.6.  Compromised TAM

   The TEE SHOULD use validation of the supplied TAM certificates and
   OCSP stapled data to validate that the TAM is trustworthy.

   Since PKI is used, the integrity of the clock within the TEE
   determines the ability of the TEE to reject an expired TAM
   certificate, or revoked TAM certificate.  Since OCSP stapling
   includes signature generation time, certificate validity dates are
   compared to the current time.

9.7.  Certificate Renewal

   TFW and 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

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   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.

10.  IANA Considerations

   This document does not require actions by IANA.

11.  Acknowledgements

   The authors thank Dave Thaler for his very thorough review and many
   important suggestions.  Most content of this document is split from a
   previously combined OTrP protocol document
   [I-D.ietf-teep-opentrustprotocol].  We thank the former co-authors
   Nick Cook and Minho Yoo for the initial document content, and
   contributors Brian Witten, Tyler Kim, and Alin Mutu.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.2.  Informative References

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

              Pei, M., Atyeo, A., Cook, N., Yoo, M., and H. Tschofenig,
              "The Open Trust Protocol (OTrP)", draft-ietf-teep-
              opentrustprotocol-03 (work in progress), May 2019.

   [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,

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Appendix A.  History


   IETF Drafts

   draft-00: - Initial working group document

Authors' Addresses

   Mingliang Pei

   EMail: mingliang_pei@symantec.com

   Hannes Tschofenig
   Arm Limited

   EMail: hannes.tschofenig@arm.com

   David Wheeler

   EMail: david.m.wheeler@intel.com

   Andrew Atyeo

   EMail: andrew.atyeo@intercede.com

   Liu Dapeng
   Alibaba Group

   EMail: maxpassion@gmail.com

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