TEEP                                                              M. Pei
Internet-Draft                                                  Symantec
Intended status: Informational                             H. Tschofenig
Expires: April 26, September 12, 2019                                  Arm Limited
                                                              D. Wheeler
                                                                   Intel
                                                                A. Atyeo
                                                               Intercede
                                                               L. Dapeng
                                                           Alibaba Group
                                                        October 23, 2018
                                                          March 11, 2019

     Trusted Execution Environment Provisioning (TEEP) Architecture
                    draft-ietf-teep-architecture-01
                    draft-ietf-teep-architecture-02

Abstract

   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 April 26, September 12, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Scope and  Assumptions . . . . . . . . . . . . . . . . . . . .   7 . . . . .   8
   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Payment . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Authentication  . . . . . . . . . . . . . . . . . . . . .   8   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 . . . . . . . . . .  13
     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  . . . . . . . . . . . . . . . . . . . .  12
     5.4.  17
     5.8.  Trust Anchors in TEE  . . . . . . . . . . . . . . . . . .  15
     5.5.  20
     5.9.  Trust Anchors in TAM  . . . . . . . . . . . . . . . . . .  15
     5.6.  20
     5.10. Keys and Certificate Types  . . . . . . . . . . . . . . .  15
     5.7.  20
     5.11. Scalability . . . . . . . . . . . . . . . . . . . . . . .  18
     5.8.  23
     5.12. Message Security  . . . . . . . . . . . . . . . . . . . .  18
     5.9.  23
     5.13. Security Domain Hierarchy and Ownership . . . . . . . . .  18
     5.10.  23
     5.14. SD Owner Identification and TAM Certificate Requirements   19
     5.11.   24
     5.15. Service Provider Container  . . . . . . . . . . . . . . .  20
     5.12.  25
     5.16. A Sample Device Setup Flow  . . . . . . . . . . . . . . .  20  25
   6.  TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . .  21  26
     6.1.  Role of the Agent . . . . . . . . . . . . . . . . . . . .  22  27
     6.2.  Agent Implementation Consideration  . . . . . . . . . . .  22  27
       6.2.1.  Agent Distribution  . . . . . . . . . . . . . . . . .  22  27
       6.2.2.  Number of Agents  . . . . . . . . . . . . . . . . . .  23  27
   7.  Attestation . . . . . . . . . . . . . . . . . . . . . . . . .  23  28
     7.1.  Attestation Hierarchy Cryptographic Properties  . . . . . . . . . .  30
     7.2.  TEEP Attestation Structure  . . . . . . . . . .  23
       7.1.1. . . . . .  31
     7.3.  TEEP Attestation Claims . . . . . . . . . . . . . . . . .  32
     7.4.  TEEP Attestation Flow . . . . . . . . . . . . . . . . . .  33
     7.5.  Attestation Key Example . . . . . . . . . . . . . . . . .  33
       7.5.1.  Attestation Hierarchy Establishment: Manufacture  . .  23
       7.1.2.  33
       7.5.2.  Attestation Hierarchy Establishment: Device Boot  . .  24
       7.1.3.  34
       7.5.3.  Attestation Hierarchy Establishment: TAM  . . . . . .  24  34
   8.  Algorithm and Attestation Agility . . . . . . . . . . . . . .  24  34
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  25  35
     9.1.  TA Trust Check at TEE . . . . . . . . . . . . . . . . . .  25  35
     9.2.  One TA Multiple SP Case . . . . . . . . . . . . . . . . .  25  35
     9.3.  Agent Trust Model . . . . . . . . . . . . . . . . . . . .  25  35
     9.4.  Data Protection at TAM and TEE  . . . . . . . . . . . . .  26  36
     9.5.  Compromised CA  . . . . . . . . . . . . . . . . . . . . .  26  36
     9.6.  Compromised TAM . . . . . . . . . . . . . . . . . . . . .  26  36
     9.7.  Certificate Renewal . . . . . . . . . . . . . . . . . . .  26  36
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26  36
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27  37
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27  37
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  27  37
     12.2.  Informative References . . . . . . . . . . . . . . . . .  27  37
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  28  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28  38

1.  Introduction

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

   The Trusted Execution Environment (TEE) concept is designed to
   execute applications in a protected environment that 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
   (UA).

   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
   needs.

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

   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.

   -  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 deterine 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 recinded.

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

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.

   -  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 Administrator: An entity User: A human being that owns or is responsible for
      administration of uses a Device.  A Device Administrator has privileges
      on the Device to install and remove applications and TAs, approve
      or reject Trust Anchors, and 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 owner 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. key
      along with additional data if necessary such as its public key
      algorithm and parameters.  The trust anchor Trust Anchor is normally stored in
      a location that resists unauthorized modification, insertion, or
      replacement.  The trust anchor 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
      device.  A certificate signed by the trust anchor Trust Anchor communicates
      that the private key holder of the signed certificate is trusted
      by the
      trust anchor 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
      TEE.

   -  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 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 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:

   -  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.  Scope and  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.  This key pair is
   referred

   A TEE uses an isolation mechanism between Trusted Applications to as
   ensure that one TA cannot read, modify or delete the 'root of trust' for remote attestation data and code of the
   associated TEE in a device by an TAM.

   New note: SD is for managing keys for TAs
   A Security Domain (SD) concept is used as the security boundary
   inside a TEE for trusted applications.  Each SD is typically
   associated with one TA provider as the owner, which is a logical
   space that contains an SP's TAs.  One TA provider may request to have
   multiple SDs in a TEE.  One SD may contain multiple TAs.  Each
   Security Domain requires the management operations of TAs in the form
   of installation, update and deletion.

   Each TA binary and configuration data can be from either of two
   sources:

   1.  A TAM supplies the signed and encrypted TA binary and any
       required configuration data

   2.  A Client Application supplies the TA binary

   The architecture covers the first case where the TA binary and
   configuration data are delivered from a TAM.  The second case calls
   for an extension when a TAM is absent.
   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.

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
   functions.

   TEEs could be used to store variety of sensitive data for IoT
   devices.  For example, a TEE could be used in smart door locks to
   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
   paragraphs.

   +-------------------------------------------+
   | Device                                    |
   |                          +--------+       |        Service Provider
   |                          |        |----------+               |
   |    +-------------+       | TEEP   |---------+|               |
   |    | TEE-1       |<------| Broker |       | ||   +--------+  |
   |    |             |       |        |<---+  | |+-->|        |<-+
   |    |             |       |        |    |  | |  +-|  TAM-1 |
   |    |             |       |        |<-+ |  | +->| |        |<-+
   |    | +---+ +---+ |       +--------+  | |  |    | +--------+  |
   |    | |TA1| |TA2| |                   | |  |    | TAM-2  |    |
   |  +-->|   | |   | |        +-------+  | |  |    +--------+    |
   |  | | |   | |   |<---------| 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 and SD's on behalf of Service Providers and
      Device Administrators.  This includes creation and deletion of
      TA's and SD'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 and SD'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 publically publicly available for use by many SPs, or a TAM may
      be private, and accessible by only one or a limited number of SPs.

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

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

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

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

   -  TEEP Broker: The TEEP Broker is an application running in a Rich
      Execution Environment 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.  An Agent 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 an Agent to the
      device manufacturer to be bundled in devices.  Such a TEE must
      also include an Agent counterpart, namely, a processing module
      inside the TEE, to parse TAM messages sent through the Agent.  An
      Agent is generally acting as a dummy relaying box with just the
      TEE interacting capability; it doesn't need and shouldn't parse
      protocol messages.

   -  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), (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

5.3.  Entity Relations

   This architecture leverages asymmetric cryptography to authenticate

   There is nothing prohibiting a device to from implementing multiple
   TEEs.  In addition, some TEEs ( for example, SGX) present themselves
   as separate containers within memory without a TAM.  Additionally, controlling manager
   within the TEE.  In these cases, the rich operating system hosts
   multiple TEEP brokers, where each broker manages a particular TEE in a device authenticates a TAM
   and TA signer.  The provisioning or
   set of trust anchors to a device may
   different from one use case TEEs.  Enumeration and access to the other.  A device administrator may
   want appropriate broker is up
   to have the capability to control what TAs are allowed.  A
   device manufacturer enables verification of rich OS and the applications.  Verification that the correct
   TA signers and TAM
   providers; it may embed has been reached then becomes a list matter of default trust anchors that properly verifying TA
   attestations, which are unforgeable.  The multiple TEE approach is
   shown in the
   signer of an allowed TA's signer certificate should chain to.  A
   device administrator may choose diagram below.  For brevity, TEEP Broker 2 is shown
   interacting with only one TAM and UA, but no such limitation is
   intended to accept a subset of be implied in the allowed TAs
   via consent or action of downloading.

   PKI    CA    -- CA                                 CA -- architecture.

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

                            Figure 2: 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
          TA    | +---+     |
      Client App -- 2a. --> | ----- 3. Install ------->    +-------+      |
         TA ------- 2b. Supply ------>    | 4. Messaging-->|       |
   |    | |TA3|<----+ |

                      Figure 3: Developer Experience

   Figure 3 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.

           ------- Message Protocol  -----
           |                             |
           |                             |
    --------------------           ---------------   ----------
    |  REE   |  TEE    |           |    TAM      |   |  SP    |
    |  ---   |  ---    |           |    ---      |   |  --    |
    |    +----------+   |    |       |
   |    | |   | Client | SD (TAs)|           |   SD / TA   |       |  TA    | TEEP     |<--+    |  Apps       |
   |    |     Mgmt +---+       |<---| Broker   |----------------+
   |    |             |    | 2        |        |
   |    |             |    |          |        |
   |    +-------------+    +----------+        | List of
   |                                           |  List
   +-------------------------------------------+

        Figure 2: Notional Architecture of    |   |        |
    |        | Trusted |           |  Trusted    |   |        |
    | Agent  |  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 4: Keys TEEP wtih multiple TEEs

   In the previous diagram, 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 CAs can resources, there may be used
   no need for different
   types of certificates.  Messages are always signed, where the signer
   key is TEEP Brokers to share information on installed TAs or
   resource usage.  However, the message originator's private key such as architecture guarantees that of a TAM, the private key of trusted firmware (TFW), 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 a TEE's private key. more TAMs.  The main components consist of a set selection of standard messages created by
   a which
   TAM to deliver device SD communicate with is dependent on information from the Client
   App and TA management commands is directly related to the TA.

   When a device,
   and device attestation and response messages created by SP offers a TEE that
   responds to service which requires a TAM's message.

   It should be noted TA, the SP associates
   that network communication capability is generally
   not available in TAs in today's TEE-powered devices. service with a specific TA.  The networking
   functionality must be delegated to a rich Client Application.  Client
   Applications will need to rely on an agent in TA itself is digitally signed,
   protecting its integrity, but the signature also links 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,
   namely, an agent in this protocol architecture, not directly from back to
   the
   network.

   It signer.  The signer is imperative to have an interoperable protocol to communicate
   with different 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 different TEEs in different devices.  This is communicates the role
   information of the agent, which is a software component that bridges
   communication between a TAM service and a TEE.  The agent does not need the specific client apps and TAs to
   know
   the actual content of messages except for TAM.

   The SP chooses TAMs based upon the TEE routing
   information.

5.4.  Trust Anchors in TEE

   Each TEE comes with a trust store that contains a whitelist of root
   CA certificates markets into which the TAM can
   provide access.  There may be TAMs that are used provide services to validate a TAM's certificate. specific
   types of mobile devices, or mobile device operating systems, or
   specific geographical regions or network carriers.  A TEE
   will accept a TAM SP may be
   motivated to create new Security Domains utilize multiple TAMs for its service in order to
   maximize market penetration and install new TAs availability on behalf multiple types of an SP only if
   devices.  This likely means that the TAM's certificate is chained to one of same service will be available
   through multiple TAMs.

   When the root CA certificates in SP publishes the TEE's trust store.

   A TEE's trust Client App to an app store is typically preloaded at manufacturing time.  It
   is out of or other app
   repositories, the scope in this document to specify how SP binds the trust store
   should be updated when Client App with a new root certificate should manifest that
   identifies what TAMs can be added contacted for the TA.  In some
   situations, an SP may use only a single TAM - this is likely the case
   for enterprise applications or
   existing one should SPs serving a closed community.  For
   broad public apps, there will likely be updated or removed.  A multiple TAMs in the manifest
   - one servicing one brand of mobile device manufacturer is
   expected 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 its TEE trust store live update or out-of-band
   update thier service and this app (and TA) to
   multiple different devices.

   Before

   When the TEEP Broker recieves a request to contact the TAM can begin operation for a
   Client App in the marketplace order to support install a
   device with TA, a particular TEE, it must obtain list of TAMs may be provided.
   The TEEP Broker selects a single TAM certificate from a
   CA that is listed in consistent with the trust store list
   of trusted TAMs (trust anchors) provisioned on the TEE.

5.5.  Trust Anchors in TAM

   The trust anchor store in device.  For any
   client app, there should be only a single TAM consists of for the TEEP Broker to
   contact.  This is also the case when a list of CA certificates
   that sign various device TEE certificates.  A Client App uses multiple TAs,
   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 decides what
   devices that
   it will trust places in the TEE in.

5.6.  Keys Client App's manifest all the TAs that the app
   requires.  There is no benefit to going to multiple different TAMs,
   and Certificate Types there is no need for a special TAM to be contacted for a specific
   TA.

   [Note: This architecture leverages should always be the following credentials, which allow
   delivering end-to-end security without relying on any transport
   security.

   +-------------+----------+--------+-------------------+-------------+
   | 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        | case.  When a particular device      |
   | certificate | storage  |        | trusted by TAMs   |             |
   |             |          |        |                   |             |
   | 2. TEE key  | Device   | or
   TEE CA | A whitelist 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    | 1 per       |
   | pair and    | TEE      | under  | TAs would use different
   proprietary attestation mechanisms since a TEE root CA       | 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      |
   | certificate |          | 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 personalizaiton data:

   1.  The Client App, TA, and personnalization 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
       separate.

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
   mechanisms.

   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
   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
   implementation]

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 suported 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.

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

                            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
          TA
           |
           |
      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.

           ------- Message Protocol  -----
           |                             |
           |                             |
    --------------------           ---------------   ----------
    |  REE   |  TEE    |           |    TAM      |   |  SP    |
    |  ---   |  ---    |           |    ---      |   |  --    |
    |        |         |           |             |   |        |
    | Client | SD (TAs)|           |   SD / TA   |   |  TA    |
    |  Apps  |         |           |     Mgmt    |   |        |
    |   |    |         |           |             |   |        |
    |   |    | List of |           |  List of    |   |        |
    |        | Trusted |           |  Trusted    |   |        |
    | Agent  |  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 device SD and 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,
   namely, an agent 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 agent, which is a software component that bridges
   communication between a TAM and a TEE.  The agent 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.

5.10.  Keys and Certificate Types

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

   +-------------+----------+--------+-------------------+-------------+
   | 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 |             |
   |             |          |        | SD 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

       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

       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 device SD and 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 Hierarchy and Ownership

   The primary job of a TAM is to help an SP to manage its trusted
   applications.  A TA is typically installed in an SD.  An SD is
   commonly created for an SP.

   When an SP delegates its SD and TA management to a TAM, an SD is
   created on behalf of a TAM in a TEE and the owner of the SD is
   assigned to the TAM.  An SD may be associated with an SP but the TAM
   has full privilege to manage the SD for the SP.

   Each SD for an SP is associated with only one TAM.  When an SP
   changes TAM, a new SP SD must be created to associate with the new
   TAM.  The TEE will maintain a registry of TAM ID and SP SD ID
   mapping.

   From an SD ownership perspective, the SD tree is flat and there is
   only one level.  An SD is associated with its owner.  It is up to the
   TEE implementation how it maintains SD binding information for a root | trusted by TAMs   |             |
   |             |          | CA     |                   |             |
   |             |          |        |                   |             |
   | 3. TAM key  | TAM      | TAM CA | A whitelist of    | 1 or        |
   | pair
   and    | provider | different SPs under  | TAM root CA       | multiple    |
   | certificate |          | a root | embedded the same TAM.

   It is an important decision in this architecture that a TEE   | can be used |
   |             |          | CA     |                   | by doesn't
   need to know whether a TAM    |
   |             |          |        |                   |             |
   | 4. SP key   | is authorized to manage the SD for an SP.
   This authorization is implicitly triggered by an SP       | Client
   Application, which instructs what TAM it wants to use.  An SD is
   always associated with a TAM in addition to its SP     | ID.  A SP uses a TAM.  | 1 or        |
   | pair and    |          | signer | TA is signed rogue TAM
   isn't able to do anything on an unauthorized SP's SD managed by
   another TAM.

   Since a | TAM may support multiple    |
   | certificate |          | CA     | SP signer. TEE    | SPs, sharing the same SD name for
   different SPs creates a dependency in deleting an SD.  An SD can be used |
   |             |          |        | delegates trust   | by
   deleted only after all TAs associated with the SD are deleted.  An SP
   cannot delete a Security Domain on its own with a TAM if a TAM
   decides to introduce such sharing.  There are cases where multiple
   virtual SPs belong to the same organization, and a TAM    |
   |             |          |        | of TA chooses to TAM. SP  |             |
   |             |          |        | signer is         |             |
   |             |          |        | associated with a |             |
   |             |          |        | use
   the same SD as name for those SPs.  This is totally up to the owner.  |             |
   +-------------+----------+--------+-------------------+-------------+

                    Figure 5: Key and Certificate Types

   1.  TFW key pair TAM
   implementation and certificate: A key pair out of scope of this specification.

5.14.  SD Owner Identification and certificate for
       evidence TAM Certificate Requirements

   There is a need of trustworthy firmware cryptographically binding proof about the owner of
   an SD in a device.  This key pair  When an SD is
       optional for TEEP architecture.  Some TEE may present its trusted
       attributes to created on behalf of a TAM using signed attestation with TAM, a TFW key.  For
       example,
   future request from the TAM must present itself as a platform way that uses a hardware based the TEE
   can have
       attestation data signed by verify it is the true owner.  The certificate itself cannot
   reliably used as the owner because TAM may change its certificate.

   ** need to handle the normal key roll-over case, as well as the less
   frequent key compromise case

   To this end, each TAM will be associated with a hardware protected TFW key.

       o  Location: Device secure storage

       o  Supported Key Type: RSA and ECC

       o  Issuer: OEM CA

       o  Checked Against: A whitelist of FW root trusted identifier
   defined as an attribute in the TAM certificate.  This field is kept
   the same when the TAM renew its certificates.  A TAM CA trusted by TAMs

       o  Cardinality: One per device

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

       o attribute value.

   This key pair is burned into identifier value must not collide among different TAM providers,
   and one TAM shouldn't be able to claim the device identifier used by the device
          manufacturer. another
   TAM provider.

   The key pair and its certificate are valid for extension name to carry the expected lifetime identifier can initially
   use SubjectAltName:registeredID.  A dedicated new extension name may
   be registered later.

   One common choice of the device.

       o  Location: Device TEE

       o  Supported Key Type: RSA and ECC

       o  Issuer: identifier value is the TAM's service URL.
   A CA that chains to a TEE root CA

       o  Checked Against: A whitelist can verify the domain ownership of the URL with the TAM in the
   certificate enrollment process.

   A TEE root CAs trusted can assign this certificate attribute value as the TAM owner ID
   for the SDs that are created for the TAM.

   An alternative way to represent an SD ownership by TAMs

       o  Cardinality: One per device

   3. a TAM is to have a
   unique secret key pair upon SD creation such that only the creator TAM is
   able to produce a proof-of-possession (PoP) data with the secret.

5.15.  Service Provider Container

   A sample Security Domain hierarchy for the TEE is shown in Figure 7.

          ----------
          |  TEE   |
          ----------
              |
              |          ----------
              |----------| SP1 SD1 |
              |          ----------
              |          ----------
              |----------| SP1 SD2 |
              |          ----------
              |          ----------
              |----------| SP2 SD1 |
                         ----------

                    Figure 7: Security Domain Hierarchy

   The architecture separates SDs and certificate: A TAM provider acquires a
       certificate from a CA TAs such 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 can only
   manage or retrieve data for SDs and P-384.  Other
          sizes should be anticipated in future.

       o  Issuer: TAM CA TAs that chains to a root CA

       o  Checked Against: it previously created
   for the SPs it represents.

5.16.  A whitelist of TAM root CAs embedded in a Sample Device Setup Flow

   Step 1: Prepare Images for Devices

   1.  [TEE vendor] Deliver 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 Image (CODE Binary) to sign a TA.

       o  Location: SP

       o  Supported device OEM

   2.  [CA] Deliver root CA Whitelist

   3.  [Soc] Deliver TFW Image

   Step 2: Inject Key Type: RSA Pairs and ECC

       o  Supported Images to Devices

   1.  [OEM] Generate TFW Key Size: RSA 2048-bit, ECC P-256 and P-384.  Other
          sizes should Pair (May be anticipated in future.

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

       o  Checked Against: An SP uses a TAM.  A shared among multiple
       devices)

   2.  [OEM] Flash signed TFW Image and signed TEE trusts an SP Image onto devices
       (signed by
          validating trust against a TAM that the SP uses.  A TEE trusts TFW Key)

   Step 3: Set up attestation key pairs in devices

   1.  [OEM] Flash TFW Public Key and a TAM to ensure that bootloader key.

   2.  [TFW/TEE] Generate a TA is trustworthy.

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

5.7.  Scalability

   This architecture uses unique attestation key pair and get a PKI.  Trust anchors exist on
       certificate for the device.

   Step 4: Set up Trust Anchors in devices to
   enable

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

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

6.  TEEP Broker

   A TEE to authenticate TAMs, and TAMs use trust anchors TAs do not generally have the capability to
   authenticate TEEs.  Since a PKI is used, many intermediate CA
   certificates can chain communicate to a root certificate, each
   the outside of which can issue
   many certificates. the hosting device.  For example, GlobalPlatform
   [GPTEE] specifies one such architecture.  This makes calls for a software
   module in the protocol highly scalable.  New
   factories that produce TEEs can join REE world to handle the ecosystem.  In network communication.  Each
   Client Application in the REE might carry this case, communication
   functionality but such a factory can get an intermediate CA certificate from one of the
   existing roots without requiring that TAMs are updated functionality must also interact with
   information about the new device factory.  Likewise, new TAMs can
   join TEE
   for the ecosystem, providing they are issued a TAM certificate that
   chains to an existing root whereby existing TEEs message exchange.
   The TEE interaction will be allowed vary according to
   be personalized by the TAM without requiring changes different TEEs.  In order
   for a Client Application to the TEE
   itself.  This enables the ecosystem transparently support different TEEs, it
   is imperative to have a common interface for a Client Application to scale, and avoids the need
   invoke for
   centralized databases exchanging messages with TEEs.

   A shared agent comes to meet this need.  An agent is an application
   running in the REE of all TEEs produced the device or all TAMs an SDK that exist.

5.8.  Message Security

   Messages created by facilitates
   communication between a TAM are used to deliver device SD and TA
   management commands to a device, and device attestation TEE.  It also provides interfaces
   for Client Applications to query and messages
   created by trigger TA installation that the device TEE
   application needs to respond use.

   It isn't always that a Client Application directly calls such an
   agent to TAM messages.

   These messages are signed end-to-end interact with a TEE.  A REE Application Installer might
   carry out TEE and are typically encrypted such TAM interaction to install all required TAs that only a
   Client Application depends.  A Client Application may have a metadata
   file that describes the targeted device TEE or TAs it depends on and the associated TAM is able that
   each TA installation goes to decrypt and view use.  The REE Application Installer can
   inspect the actual content.

5.9.  Security Domain Hierarchy application metadata file and Ownership

   The primary job installs TAs on behalf of a TAM is to help an SP
   the Client Application without requiring the Client Application to manage its trusted
   applications.  A TA is typically installed in an SD.  An SD is
   commonly created
   run first.

   This interface for Client Applications or Application Installers may
   be commonly an SP.

   When OS service call for an SP delegates its SD and TA management to a TAM, REE OS.  A Client Application
   or an SD is
   created on behalf of a TAM in a Application Installer interacts with the device TEE and the owner
   TAMs.

6.1.  Role of the SD is
   assigned to the TAM. Agent

   An SD may be associated agent abstracts the message exchanges with an SP but the TEE in a device.
   The input data is originated from a TAM
   has full privilege to manage or the SD for first initialization
   call to trigger a TA installation.

   The agent may internally process a message from a TAM.  At least, it
   needs to know where to route a message, e.g., TEE instance.  It does
   not need to process or verify message content.

   The agent returns TEE / TFW generated response messages to the SP.

   Each SD for an SP
   caller.  The agent is associated not expected to handle any network connection
   with only one an application or TAM.  When

   The agent only needs to return an SP
   changes TAM, a new SP SD must agent 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 created passed to associate with
   the new TAM.  The TEE will maintain a registry

6.2.  Agent Implementation Consideration

   A Provider should consider methods of TAM ID distribution, scope and SP SD ID
   mapping.

   From
   concurrency on devices and runtime options when implementing an SD ownership perspective,
   agent.  Several non-exhaustive options are discussed below.
   Providers are encouraged to take advantage of the SD tree is flat latest
   communication and there is
   only one level.  An SD platform capabilities to offer the best user
   experience.

6.2.1.  Agent Distribution

   The agent installation is associated with its owner. commonly carried out at OEM time.  A user
   can dynamically download and install an agent on-demand.

   It is up important to the
   TEE implementation how it maintains SD binding information for ensure a TAM
   and different SPs under the same TAM.

   It legitimate agent is installed and used.
   If an important decision agent is compromised it may drop messages and thereby introduce
   a denial of service.

6.2.2.  Number of Agents

   We anticipate only one shared agent instance in this architecture that a device.  The
   device's TEE doesn't
   need vendor will most probably supply one agent.

   With one shared agent, the agent provider is responsible to know whether allow
   multiple TAMs and TEE providers to achieve interoperability.  With a
   standard agent interface, each TAM is authorized to manage the SD can implement its own SDK for an SP.
   This authorization is implicitly triggered by an its
   SP Client
   Application, which instructs what TAM it wants Applications to use.  An SD is
   always associated work with this agent.

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

   TAM providers are generally expected to its provide an SDK for SP ID.  A rogue TAM
   isn't able
   applications to do anything on interact with an unauthorized SP's SD managed by
   another TAM.

   Since a agent for the TAM and TEE
   interaction.

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 support multiple SPs, sharing 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 same SD name for
   different primary purpose of an attestation is to allow a device
   to prove to TAMs and SPs creates that a dependency TEE in deleting an SD.  An SD can be
   deleted only after all TAs associated with the SD are deleted.  An SP
   cannot delete device has particular
   properities, was built by a Security Domain on its own with particular manufacturer, or is executing
   a TAM if particular TA.  Other claims are possible; this architecture
   specification does not limit the attestation claims, but defines a TAM
   decides
   minimal set of claims required for TEEP to introduce such sharing.  There are cases where multiple
   virtual SPs belong operate properly.
   Extensions to these claims are possible, but are not defined in the same organization,
   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 TAM chooses to use TEEP attestation message.

   As of the same SD name 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 those SPs.  This is totally up TEEP to be inclusive, the TAM
   implementation and out attestation
   format shall allow for both proprietary attestation signatures, as
   well as a standardized form of scope attestation signature.  Either form of this specification.

5.10.  SD Owner Identification and TAM Certificate Requirements

   There is
   attesation signature may be applied to a need set of cryptographically binding proof about the owner TEEP claims, and both
   forms of
   an SD in 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 device.  When an SD is created on behalf particular set of a TAM, a
   future request from cryptographic properties based on
   minimal assumptions.  The cryptographic properties are conveyed by
   the TAM must present itself as a way that attestation; however the TEE
   can verify it is assumptions are not conveyed within the true owner.
   attestation itself.

   The certificate itself cannot
   reliably used as the owner because TAM assumptions which may change its certificate.

   ** need apply to handle an attestation have to do with the normal key roll-over case, as well as
   quality of the less
   frequent key compromise case

   To this end, each TAM will be associated with a trusted identifier
   defined as an attribute in attestation and the TAM certificate.  This field is kept quality and security provided by
   the same when TEE, the TAM renew its certificates.  A TAM CA is
   responsible to vet device, the requested TAM attribute value.

   This identifier value must manufacturer, or others involved in the
   device or TEE ecosystem.  Some of the assumptions that might apply to
   an attestations include (this may not collide among different TAM providers,
   and one TAM shouldn't be able to claim a comprehensive list):

   -  Assumptions regarding the identifier used by another
   TAM provider.

   The certificate extension name security measures a manufacturer takes
      when provisioning keys into devices/TEEs;

   -  Assumptions regarding what hardware and software components have
      access to carry the identifier can initially
   use SubjectAltName:registeredID.  A dedicated new extension name may
   be registered later.

   One common choice Attestation keys of the identifier value is TEE;

   -  Assumptions related to the TAM's service URL.
   A CA can verify source or local verification of claims
      within an attestation prior to a TEE signing a set of claims;

   -  Assumptions regarding the domain ownership level of protection afforded to
      attestation keys against exfiltration, modification, and side
      channel attacks;

   -  Assumptions regarding the URL with limitations of use applied to TEE
      Attestation keys;

   -  Assumptions regarding the TAM processes in the
   certificate enrollment process.

   A place to discover or detect
      TEE can assign this certificate attribute value as breeches; and

   -  Assumptions regarding the TAM owner ID
   for revocation and recovery process of TEE
      attestation keys.

   TAMs and SPs must be comfortable with the SDs assumptions that are created for the TAM.

   An alternative way
   inherently part of any attestation they accept.  Alternatively, any
   TAM or SP may choose not to represent accept an SD ownership by attestation generated from a TAM is
   particular manufacturer or device's TEE based on the inherent
   assumptions.  The choice and policy decisions are left up to have a
   unique secret key upon SD creation such that only the creator TAM is
   able
   particular TAM/SP.

   Some TAMs or SPs may require additional claims in order to produce properly
   authorize a proof-of-possession (PoP) data with device or TEE.  These additional claims may help clear up
   any assumptions for which the secret.

5.11.  Service Provider Container

   A sample Security Domain hierarchy TAM/SP wants to alleviate.  The
   specific format for these additional claims are outside the TEE is shown scope of
   this specification, but the OTrP protocol SHALL allow these
   additional claims to be included in Figure 6.

          ----------
          |  TEE   |
          ----------
              |
              |          ----------
              |----------| SP1 SD1 |
              |          ----------
              |          ----------
              |----------| SP1 SD2 |
              |          ----------
              |          ----------
              |----------| SP2 SD1 |
                         ----------

                    Figure 6: Security Domain Hierarchy the attestation messages.

   The architecture separates SDs and TAs such that 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 can only
   manage or retrieve data for SDs the device TEE, and TAs that it previously created
   for 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 SPs it represents.

5.12.  A Sample Device Setup Flow

   Step 1: Prepare Images for Devices

   -

      1.  [TEE vendor] Deliver TEE Image (CODE Binary) attestor to the verifier; in the case of TEEP,
   the attestation must convey properties from the device OEM

   -

      1.  [CA]  Deliver root CA Whitelist

   -

      1.  [Soc]  Deliver TFW Image

   Step 2: Inject Key Pairs and Images to Devices the TAM
   and/or SP.  The properties required by TEEP include:

   -

      1.  [OEM] Generate TFW Key Pair (May be shared among multiple
          devices)  Non-repudiation, Unique Proof of Source -

      1.  [OEM] Flash signed TFW Image the cryptographic
      digital signature across the attestation, and signed optionally along
      with information in the attestion itself SHALL uniquely identify a
      specific 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. specific device.

   -

      1.  [TFW/TEE] Generate a unique  Integrity of claims - the cryptographic digital signature across
      the attestation SHALL cover the entire attesation including all
      meta data and all the claims in the attestation, ensuring that the
      attestation has not be modified since the TEE signed the
      attestation.

   Standard public key pair algorithms such as RSA and get ECDSA digital
   signatures convey these properties.  Group public key algorithms such
   as EPID can also convey these properties, if the attestation includes
   a
          certificate unique device identifier and an identifier for the device.

   Step 4: Set up trust anchors TEE.  Other
   cryptographic operations used in devices

   -

      1.  [TFW/TEE] Store other attestation schemes may also
   convey these properties.

   The TEEP standard attestation format SHALL use one of the key and certificate encrypted following
   digital signature formats:

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

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

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

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

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

   -

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

6.  TEEP Broker

   A TEE  EdDSA using Ed448 with SHAK256 (Ed448ph in RFC8032) and TAs do not generally have the capability to communicate
      context="TEEP Attestation"

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

7.2.  TEEP Attestation Structure

   For example, GlobalPlatform
   [GPTEE] specifies one such architecture.  This calls for a software
   module in the REE world TEEP attestation to handle be useful, it must contain an information
   set allowing the network communication.  Each
   Client Application in TAM and/or SP to assess the REE might carry this communication
   functionality but such functionality must also interact with attestation and make a
   related security policy decision.  The structure of the TEE
   for TEEP
   attestation is shown in the message exchange. diagram below.

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

                  Figure 8: Structure of TEEP Attestation

   The Attestation Header SHALL identify the "Attestation Type" and the
   "Attestation Signature Type" along with TEEs.

   A shared agent comes to meet an "Attestation Format
   Version Number."  The "Attestation Type" identifies the minimal set
   of claims that MUST be included in the attestation; this need.  An agent is an application
   running
   identifier for a profile that defines the claims that should be
   included in the REE attestation as part of the device or an SDK that facilitates
   communication between a TAM and a TEE.  It also provides interfaces
   for TAM SDK "Action or Client Applications to query and trigger TA
   installation that Operation
   Specific Claims."  The "Attestation Signature Type" identifies the application needs to use.

   This interface for Client Applications may
   type of attestation signature that is attached.  The type of
   attestation signature SHALL be commonly one of the standard signatures types
   identified by an OS service
   call for IANA number, a proprietary signature type identified
   by an REE OS.  A Client Application interacts IANA number, or the generic "Proprietary Signature" with a TAM, and
   turns around to pass messages received from TAM to agent.

   In an
   accompanying proprietary identifier.  Not all cases, a Client Application needs to TAMs may be able to identify an
   agent that it can use.

6.1.  Role of the Agent

   An agent abstracts
   process proprietary signatures.

   The claims in the message exchanges with attestation are set of mandatory and optional
   claims.  The claims themselves SHALL be defined in an attestation
   claims dictionary.  See the TEE next section on TEEP Attestation Claims.

   Claims are grouped in profiles under an identifier (Attestation
   Type), however all attestations require a device.
   The input data is originated from minimal set of claims which
   includes:

   -  Device Identifying Info: TEEP attestations must uniquely identify
      a device to the TAM and SP.  This identifier allows the TAM/SP to which a Client Application
   connects.  A Client Application may also directly call an Agent for
   some TA query functions.

   The agent may internally process a message from a TAM.  At least, it
   needs
      provide services unique to know where the device, such as managing installed
      TAs, and providing subscriptions to route a message, e.g., TEE instance.  It does
   not need services, and locating device-
      specific keying material to process communicate wiht or verify message content. 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 agent returns type of TEE / TFW that generated response messages to the
   caller.  The agent is not expected to handle any network connection
   with an application or TAM.

   The agent only needs to return this
      attestation must be identified.  Standard TEE types are identified
      by an agent error message if 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
   not reachable required.TEE manufacturer information for some reason.  Other errors are represented as
   response messages returned from the
      TEE which will then be passed is required in order to disambiguate the TAM.

6.2.  Agent Implementation Consideration

   A Provider should consider methods of distribution, scope and
   concurrency on devices same TEE type created
      by different manufacturers and runtime options when implementing an
   agent.  Several non-exhaustive options are discussed below.
   Providers are encouraged to take advantage of the latest
   communication resolve potential assumptions
      around manufacturer provisioning, keying and platform capabilities to offer support for the best user
   experience.

6.2.1.  Agent Distribution

   The agent installation is commonly carried out at OEM time.  A user
   can dynamically download TEE.

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

   -  Action Specific Claims: Certain attestation types shall include
      specific claims.  For example an agent on-demand.

   It is important to ensure attestation from a legitimate agent is installed specific TA
      shall include a measurement, version and used.
   If an agent is compromised it signing public key for
      the TA.

   -  Additional Claims: (Optional - May be empty set) A TAM or SP may drop messages and thereby introduce
   a denial of service.

6.2.2.  Number of Agents

   We anticipate only one shared agent instance
      require specific additional claims in order to address potential
      assumptions, such as the requirement that a device.  The device's TEE vendor will most probably supply one agent.

   With one shared agent, REE performed
      a secure boot, or that the agent provider device is responsible to allow
   multiple TAMs and TEE providers to achieve interoperability.  With not currenlty in a
   standard agent interface, each debug or
      non-productions state.  A TAM can implement its own SDK for its
   SP Client Applications to work with this agent.

   Multiple independent agent providers can be used as long as they have
   standard interface may require a device to provide a Client Application
      device health attestation that may include some claims or
      measurements about the REE.  These claims are TAM SDK.  Only one
   agent is expected in specific.

7.3.  TEEP Attestation Claims

   TEEP requires a device. set of attestation claims that provide sufficient
   evidence to the TAM providers and/or SP that the device and its TEE meet
   certain minimal requirements.  Because attestation formats are generally not
   yet broadly standardized across the industry, standardization work is
   currently ongoing, and it is expected that extensions to provide 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 SDK for SP
   applications IANA registry.  That registry SHALL be defined in the
   OTrP protocol with sufficient elements to interact 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 an agent for device state information (dsi) to the TAM and TEE
   interaction.

7.  Attestation

7.1.
      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 Hierarchy 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.

7.1.1.

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
       have.

7.1.2.

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
       key.

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

7.1.3.

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
   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.  An agent 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.  A TA
   will be installed in a different SD for each respective SP.

9.3.  Agent Trust Model

   An agent 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 agent.

   The architecture enables the TAM to communicate with the device's TEE
   to manage SDs and TAs.  All TAM messages are signed and sensitive
   data is encrypted such that the agent 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
   embedded into the trust anchor Trust Anchor store in a device, should have long
   lifetimes that don't require device trust anchor Trust Anchor update.  On the
   other hand, it is imperative that OEMs or device providers plan for
   support of trust anchor 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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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/
              tee-system-architecture-v1-1/>.

   [I-D.ietf-teep-opentrustprotocol]
              Pei, M., Atyeo, A., Cook, N., Yoo, M., and H. Tschofenig,
              "The Open Trust Protocol (OTrP)", draft-ietf-teep-
              opentrustprotocol-01
              opentrustprotocol-02 (work in progress), July October 2018.

   [RFC6024]  Reddy, R. and C. Wallace, "Trust Anchor Management
              Requirements", RFC 6024, DOI 10.17487/RFC6024, October
              2010, <https://www.rfc-editor.org/info/rfc6024>.

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <https://www.rfc-editor.org/info/rfc7696>.

Appendix A.  History

   RFC EDITOR: PLEASE REMOVE THIS SECTION

   IETF Drafts

   draft-00: - Initial working group document

Authors' Addresses

   Mingliang Pei
   Symantec

   EMail: mingliang_pei@symantec.com

   Hannes Tschofenig
   Arm Limited

   EMail: hannes.tschofenig@arm.com

   David Wheeler
   Intel

   EMail: david.m.wheeler@intel.com

   Andrew Atyeo
   Intercede

   EMail: andrew.atyeo@intercede.com

   Liu Dapeng
   Alibaba Group

   EMail: maxpassion@gmail.com