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

     Trusted Execution Environment Provisioning (TEEP) Architecture


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

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

Status of This Memo

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

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   This Internet-Draft will expire on April 26, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Scope and Assumptions . . . . . . . . . . . . . . . . . . . .   7
   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Payment . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Authentication  . . . . . . . . . . . . . . . . . . . . .   8
     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.  Entity Relations  . . . . . . . . . . . . . . . . . . . .  12
     5.4.  Trust Anchors in TEE  . . . . . . . . . . . . . . . . . .  15
     5.5.  Trust Anchors in TAM  . . . . . . . . . . . . . . . . . .  15
     5.6.  Keys and Certificate Types  . . . . . . . . . . . . . . .  15
     5.7.  Scalability . . . . . . . . . . . . . . . . . . . . . . .  18
     5.8.  Message Security  . . . . . . . . . . . . . . . . . . . .  18
     5.9.  Security Domain Hierarchy and Ownership . . . . . . . . .  18
     5.10. SD Owner Identification and TAM Certificate Requirements   19
     5.11. Service Provider Container  . . . . . . . . . . . . . . .  20
     5.12. A Sample Device Setup Flow  . . . . . . . . . . . . . . .  20
   6.  TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . .  21
     6.1.  Role of the Agent . . . . . . . . . . . . . . . . . . . .  22
     6.2.  Agent Implementation Consideration  . . . . . . . . . . .  22

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       6.2.1.  Agent Distribution  . . . . . . . . . . . . . . . . .  22
       6.2.2.  Number of Agents  . . . . . . . . . . . . . . . . . .  23
   7.  Attestation . . . . . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  Attestation Hierarchy . . . . . . . . . . . . . . . . . .  23
       7.1.1.  Attestation Hierarchy Establishment: Manufacture  . .  23
       7.1.2.  Attestation Hierarchy Establishment: Device Boot  . .  24
       7.1.3.  Attestation Hierarchy Establishment: TAM  . . . . . .  24
   8.  Algorithm and Attestation Agility . . . . . . . . . . . . . .  24
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
     9.1.  TA Trust Check at TEE . . . . . . . . . . . . . . . . . .  25
     9.2.  One TA Multiple SP Case . . . . . . . . . . . . . . . . .  25
     9.3.  Agent Trust Model . . . . . . . . . . . . . . . . . . . .  25
     9.4.  Data Protection at TAM and TEE  . . . . . . . . . . . . .  26
     9.5.  Compromised CA  . . . . . . . . . . . . . . . . . . . . .  26
     9.6.  Compromised TAM . . . . . . . . . . . . . . . . . . . . .  26
     9.7.  Certificate Renewal . . . . . . . . . . . . . . . . . . .  26
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  27
     12.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  History  . . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   Applications executing in a device are exposed to many different
   attacks intended to compromise the execution of the application, or
   reveal the data upon which those applications are operating.  These
   attacks increase with the number of other applications on the device,
   with such other applications coming from potentially untrustworthy
   sources.  The potential for attacks further 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.

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   An application component running inside a TEE is referred to as a
   Trusted Application (TA), while a normal application running in the
   regular operating system is referred to as an Untrusted Application

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

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

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

   This protocol addresses the following problems:

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

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

   -  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.

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   -  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

2.  Terminology

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

   The following terms are used:

   -  Client Application: An application running in a Rich Execution
      Environment, such as an Android, Windows, or iOS application.

   -  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

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      Applications.  A Service Provider requires the help of a TAM in
      order to provision the Trusted Applications to remote devices.

   -  Device Administrator: An entity that owns or is responsible for
      administration of a Device.  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 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.
      The trust anchor is normally stored in a location that resists
      unauthorized modification, insertion, or replacement.
      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 communicates that the
      private key holder of the signed certificate is trusted by the
      trust anchor holder, and can therefore be trusted by the device.

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

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

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

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

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

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

   -  Root-of-Trust (RoT): A hardware or software component in a device
      that is inherently trusted to perform a certain security-critical
      function.  A RoT should be secure by design, small, and protected

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      by hardware against modification or interference.  Examples of
      RoTs include software/firmware measurement and verification using
      a trust anchor (RoT for Verification), provide signed assertions
      using a protected attestation key (RoT for Reporting), or protect
      the storage and/or use of cryptographic keys (RoT for Storage).
      Other RoTs are possible, including RoT for Integrity, and RoT for
      Measurement.  Reference: NIST SP800-164 (Draft).

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

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

   This document uses the following abbreviations:

   -  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 to as the 'root of trust' for remote attestation of the
   associated TEE in a device by an TAM.

   New note: SD is for managing keys for TAs

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   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

   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.

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.

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4.3.  Internet of Things

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

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

4.4.  Confidential Cloud Computing

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

5.  Architecture

5.1.  System Components

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

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

                  Figure 1: Notional Architecture of TEEP

   -  Service Providers and Device Administrators 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 available for use by many SPs, or a TAM
      may be private, and accessible by only one or a limited number of

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      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.

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

5.2.  Different Renditions of TEEP Architecture

5.3.  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-------

                            Figure 2: Entities

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

                      Figure 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.

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           ------- 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 4: 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

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   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

5.4.  Trust Anchors in TEE

   Each TEE comes with a trust store that contains a whitelist of root
   CA certificates 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 store
   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 store live update or out-of-band
   update to devices.

   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.

5.5.  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 decides what
   devices it will trust the TEE in.

5.6.  Keys and Certificate Types

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

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   | 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 5: 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

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   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.

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

5.7.  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.

5.8.  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.

5.9.  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

   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

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   TEE implementation how it maintains SD binding information for a TAM
   and different SPs under the same TAM.

   It is an important decision in this architecture that a TEE doesn't
   need to know whether a TAM 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 rogue TAM
   isn't able to do anything on an unauthorized SP's SD managed by
   another TAM.

   Since a TAM may support multiple SPs, sharing the same SD name for
   different SPs creates a dependency in deleting an SD.  An SD can be
   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 chooses to use
   the same SD name for those SPs.  This is totally up to the TAM
   implementation and out of scope of this specification.

5.10.  SD Owner Identification and TAM Certificate Requirements

   There is a need of cryptographically binding proof about the owner of
   an SD in a device.  When an SD is created on behalf of a TAM, a
   future request from the TAM must present itself as a way that the TEE
   can 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 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 is
   responsible to vet the requested TAM attribute value.

   This identifier value must not collide among different TAM providers,
   and one TAM shouldn't be able to claim the identifier used by another
   TAM provider.

   The certificate extension name to carry the identifier can initially
   use SubjectAltName:registeredID.  A dedicated new extension name may
   be registered later.

   One common choice of the identifier value is the TAM's service URL.
   A CA can verify the domain ownership of the URL with the TAM in the
   certificate enrollment process.

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

5.11.  Service Provider Container

   A sample Security Domain hierarchy for the TEE is shown in Figure 6.

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

                    Figure 6: Security Domain Hierarchy

   The architecture separates SDs and TAs such that a TAM can only
   manage or retrieve data for SDs and TAs that it previously created
   for 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) to device OEM


      1.  [CA]  Deliver root CA Whitelist


      1.  [Soc]  Deliver TFW Image

   Step 2: Inject Key Pairs and Images to Devices

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


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

   Step 3: Set up attestation key pairs in devices


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


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

   Step 4: Set up trust anchors in devices


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


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

6.  TEEP Broker

   A TEE and TAs do not generally have the capability to communicate to
   the outside of the hosting device.  For example, GlobalPlatform
   [GPTEE] specifies one such architecture.  This calls for a software
   module in the REE world to handle the network communication.  Each
   Client Application in the REE might carry this communication
   functionality but such functionality must also interact with the TEE
   for the message exchange.  The TEE interaction will vary according to
   different TEEs.  In order for a Client Application to transparently
   support different TEEs, it is imperative to have a common interface
   for a Client Application to invoke for exchanging messages with TEEs.

   A shared agent comes to meet this need.  An agent is an application
   running in the REE of the device or an SDK that facilitates

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   communication between a TAM and a TEE.  It also provides interfaces
   for TAM SDK or Client Applications to query and trigger TA
   installation that the application needs to use.

   This interface for Client Applications may be commonly an OS service
   call for an REE OS.  A Client Application interacts with a TAM, and
   turns around to pass messages received from TAM to agent.

   In all cases, a Client Application needs to be able to identify an
   agent that it can use.

6.1.  Role of the Agent

   An agent abstracts the message exchanges with the TEE in a device.
   The input data is originated from a TAM 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 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
   caller.  The agent is not expected to handle any network connection
   with an application or TAM.

   The agent only needs to return an 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 passed to
   the TAM.

6.2.  Agent Implementation Consideration

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

6.2.1.  Agent Distribution

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

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

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6.2.2.  Number of Agents

   We anticipate only one shared agent instance in a device.  The
   device's TEE vendor will most probably supply one agent.

   With one shared agent, the agent provider is responsible to allow
   multiple TAMs and TEE providers to achieve interoperability.  With a
   standard agent interface, each 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 to a Client Application or TAM SDK.  Only one
   agent is expected in a device.

   TAM providers are generally expected to provide an SDK for SP
   applications to interact with an agent for the TAM and TEE

7.  Attestation

7.1.  Attestation Hierarchy

   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.  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.

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

7.1.2.  Attestation Hierarchy Establishment: Device Boot

   During device boot the following steps are required:

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

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

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

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

7.1.3.  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 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.

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   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.

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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 store in a device, should have long
   lifetimes that don't require device trust anchor update.  On the
   other hand, it is imperative that OEMs or device providers plan for
   support of trust anchor update in their shipped devices.

10.  IANA Considerations

   This document does not require actions by IANA.

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11.  Acknowledgements

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

12.  References

12.1.  Normative References

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

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

12.2.  Informative References

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

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

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,

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Appendix A.  History


   IETF Drafts

   draft-00: - Initial working group document

Authors' Addresses

   Mingliang Pei

   EMail: mingliang_pei@symantec.com

   Hannes Tschofenig
   Arm Limited

   EMail: hannes.tschofenig@arm.com

   David Wheeler

   EMail: david.m.wheeler@intel.com

   Andrew Atyeo

   EMail: andrew.atyeo@intercede.com

   Liu Dapeng
   Alibaba Group

   EMail: maxpassion@gmail.com

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