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Versions: 00

TEEP                                                              M. Pei
Internet-Draft                                                  Symantec
Intended status: Informational                             H. Tschofenig
Expires: January 3, 2019                                        Arm Ltd.
                                                                A. Atyeo
                                                               Intercede
                                                                  D. Liu
                                                           Alibaba Group
                                                            July 2, 2018


     Trusted Execution Environment Provisioning (TEEP) Architecture
                  draft-ietf-teep-architecture-00.txt

Abstract

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

   This architecture document motivates the design and standardization
   of a protocol for managing the lifecyle 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on January 3, 2019.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Scope and Assumptions . . . . . . . . . . . . . . . . . . . .   6
   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Payment . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Authentication  . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Internet of Things  . . . . . . . . . . . . . . . . . . .   7
     4.4.  Confidential Cloud Computing  . . . . . . . . . . . . . .   7
   5.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  System Components . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Entity Relations  . . . . . . . . . . . . . . . . . . . .   9
     5.3.  Trust Anchors in TEE  . . . . . . . . . . . . . . . . . .  12
     5.4.  Trust Anchors in TAM  . . . . . . . . . . . . . . . . . .  12
     5.5.  Keys and Certificate Types  . . . . . . . . . . . . . . .  12
     5.6.  Scalability . . . . . . . . . . . . . . . . . . . . . . .  15
     5.7.  Message Security  . . . . . . . . . . . . . . . . . . . .  15
     5.8.  Security Domain Hierarchy and Ownership . . . . . . . . .  15
     5.9.  SD Owner Identification and TAM Certificate Requirements   16
     5.10. Service Provider Container  . . . . . . . . . . . . . . .  17
     5.11. A Sample Device Setup Flow  . . . . . . . . . . . . . . .  17
   6.  Agent . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Role of the Agent . . . . . . . . . . . . . . . . . . . .  18
     6.2.  Agent Implementation Consideration  . . . . . . . . . . .  19
       6.2.1.  Agent Distribution  . . . . . . . . . . . . . . . . .  19
       6.2.2.  Number of Agents  . . . . . . . . . . . . . . . . . .  19
   7.  Attestation . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Attestation Hierarchy . . . . . . . . . . . . . . . . . .  20
       7.1.1.  Attestation Hierarchy Establishment: Manufacture  . .  20
       7.1.2.  Attestation Hierarchy Establishment: Device Boot  . .  20
       7.1.3.  Attestation Hierarchy Establishment: TAM  . . . . . .  21
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   9.  Security Consideration  . . . . . . . . . . . . . . . . . . .  21
     9.1.  TA Trust Check at TEE . . . . . . . . . . . . . . . . . .  21
     9.2.  One TA Multiple SP Case . . . . . . . . . . . . . . . . .  22
     9.3.  Agent Trust Model . . . . . . . . . . . . . . . . . . . .  22
     9.4.  Data Protection at TAM and TEE  . . . . . . . . . . . . .  22
     9.5.  Compromised CA  . . . . . . . . . . . . . . . . . . . . .  22
     9.6.  Compromised TAM . . . . . . . . . . . . . . . . . . . . .  22



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     9.7.  Certificate Renewal . . . . . . . . . . . . . . . . . . .  23
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     10.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   The Trusted Execution Environment (TEE) concept has been designed to
   separate a regular operating system, also referred as a Rich
   Execution Environment (REE), from security- sensitive applications.
   A TEE provides hardware-enforcement so that any data inside the TEE
   cannot be read by code outside of the TEE.  Compromising a REE and
   normal applications in the REE do not affect code inside the TEE,
   which is called a Trusted Application (TA), running inside the TEE.

   In an TEE ecosystem, a Trusted Application Manager (TAM) is commonly
   used to manage keys and TAs that run in a device.  Different device
   vendors may use different TEE implementations.  Different application
   providers or device administrators may choose to use different TAM
   providers.

   To simplify the life of developers an interoperable protocol for
   managing TAs running in different TEEs of various devices is needed.

   The protocol addresses the following problems.

   1.  A Device Administrator (DA) or Service Provider (SP) of the
       device users needs to determine security-relevant information of
       a device before provisioning the TA to the device with a TEE.
       Examples include the verification of the device 'root of trust'
       and the type of TEE included in a device.

   2.  A TEE in a device needs to determine whether a Device
       Administrator (DA) or a Service Provider (SP) that wants to
       manage an TA in the device is authorized to manage applications
       in the TEE.

   3.  Attestation must be able to ensure a TEE is genuine.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Client Application:  An application running on a rich OS, such as an
       Android, Windows, or iOS application.



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   Device:  A physical piece of hardware that hosts a TEE along with a
       rich OS.



   Agent:  An application running in the rich OS allowing the message
       protocol exchange between a TAM and a TEE in a device.  A TEE is
       responsible to processing relayed messages and for returning an
       appropriate reponse.



   Rich Execution Environment (REE)  An environment that is provided and
       governed by a typical OS (Linux, Windows, Android, iOS, etc.),
       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.



   Secure Boot Module (SBM):  A firmware in a device that delivers
       secure boot functionality.  It is generally signed and can be
       verified whether it can be trusted.



   Service Provider (SP):  An entity that wishes to supply Trusted
       Applications to remote devices.  A Service Provider requires the
       help of a TAM in order to provision the Trusted Applications to
       the devices.



   Trust Anchor:  A root certificate that can be used to validate its
       children certificates.  It is usually embedded in a device or
       configured by a TAM for validating the trust of a remote entity's
       certificate.



   Trusted Application (TA):  An Application 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



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       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 of TEE.


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



   Trusted Firmware (TFW):  A signed SBM firmware that can be verified
       and is trusted by a TEE in a device.



   This document uses the following abbreviations:

   CA      Certificate Authority

   REE     Rich Execution Environment

   SD      Security Domain

   SP      Service Provider

   SBM     Secure Boot Module

   TA      Trusted Application

   TEE     Trusted Execution Environment

   TFW     Trusted Firmware

   TAM     Trusted Application Manager








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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.

   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 a 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.

   A TA binary and configuration data can be from two sources:

   1.  A TAM supplies the signed and encrypted TA binary

   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.

   Messages exchange with a TAM require some transport and HTTPS is one
   commonly used transport.

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 running inside TEE in the device
   to provide strong identification and proof of transaction.

   For a mobile payment application, some biometric identification
   information could also be stored in the 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 TEE for user input
   protection.





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4.2.  Authentication

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

4.3.  Internet of Things

   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 a malware
   from 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.  Bike-sharing is another
   example that shares a similar usage scenario.

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 host 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 host provider for reduced liability and increased
   cloud adoption.

5.  Architecture

5.1.  System Components

   The following are the main components in the system.

   TAM:  A TAM is responsible for originating and coordinating lifecycle
       management activity on a particular TEE on behalf of a Service
       Provider or a Device Administrator.  For example, a payment
       application provider, which also provides payment service as a
       Service Provider using its payment TA, may choose to use a TAM
       that it runs or a third party TAM service to distribute and
       update its payment TA application in payment user devices.  The
       payment SP isn't a device administrator of the user devices.  A
       user who chooses to download the payment TA into its devices acts
       as the device administrator, authorizing the TA installation via
       the downloading consent.  The device manufacturer is typically



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       responsible for embedding the TAM trust verification capability
       in its device TEE.

       A TAM may be used by one SP or many SPs where a TAM may run as a
       Software-as-a-Service (SaaS).  A TAM may provide Security Domain
       management and TA management in a device for the SD and TAs that
       a SP owns.  In particular, a TAM typically offers over-the-air
       update to keep a SP's TAs up-to-date and clean up when a version
       should be removed.  A TEE administrator or device administrator
       may decide TAMs that it trusts to manage its devices.

   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.

   TEE:  A TEE in a device is responsible for protecting applications
       from attack, enabling the application to perform secure
       operations.

   REE:  The REE in a device is responsible for enabling off-device
       communications to be established between a TEE and TAM.  The
       architecture does not assume or require that the REE or Client
       Applications is secure.

   Agent:  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 REE that the Client Application can use
       to pass messages to TEEs.  An Agent is this component to fill the
       role.  In other words, an Agent is an application in the REE or
       software library that can simply relays messages from a Client
       Application to a TEE in the device.  A device usually comes with
       only one active TEE.  A TEE that supports may provide such an
       Agent to the device manufacturer to be bundled in devices.  Such
       a compliant 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.

   Device Administrator:  A device owner or administrator may want to
       manage what TAs allowed to run in its devices.  A default list of
       allowed TA trust root CA certificates is included in a device by



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       the device's manufacturer, which may be governed by the device
       carriers sometimes.  There may be needs to expose overriding
       capability for a device owner to decide the list of allowed TAs
       by updating the list of trusted CA certificates.

   Secure Boot:  Secure boot must enable authenticity checking of TEEs
       by the TAM.  Note that some TEE implementations do not require
       secure boot functionality.

5.2.  Entity Relations

   This architecture leverages asymmetric cryptography to authenticate a
   device towards a TAM.  Additionally, a TEE in a device authenticates
   a TAM provider and TA signer.  The provisioning of trust anchors to a
   device may different from one use case to the other.  The 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 1: 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
          TA
           |
           |
      Client App -- 2a. --> | ----- 3. Install -------> |
         TA ------- 2b. Supply ------> | 4. Messaging-->|
           |                |          |                |

                      Figure 2: Developer Experience

   Figure 2 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 calling 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 will show a system diagram about the entity
   relationships between CAs, TAM, SP and devices.



















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           ------- Message Protocol  -----
           |                             |
           |                             |
    --------------------           ---------------   ----------
    |  REE   |  TEE    |           |    TAM      |   |  SP    |
    |  ---   |  ---    |           |    ---      |   |  --    |
    |        |         |           |             |   |        |
    | Client | SD (TAs)|           |   SD / TA   |   |  TA    |
    |  Apps  |         |           |     Mgmt    |   |        |
    |   |    |         |           |             |   |        |
    |   |    |         |           |             |   |        |
    |        | 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 3: 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 a 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 originates from 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
   internet.



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   It is imperative to have an interoperable protocol to communicate
   with different TEEs in different devices that a Client Application
   needs to run and access a TA inside a TEE.  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.

5.3.  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 a 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 TEE-
   powered devices with a particular TEE, it must obtain a TAM
   certificate from a CA that is listed in the trust store of the TEE.

5.4.  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.

5.5.  Keys and Certificate Types

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














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   +-------------+----------+--------+-------------------+-------------+
   | Key Entity  | Location | Issuer | Checked Against   | Cardinality |
   | Name        |          |        |                   |             |
   +-------------+----------+--------+-------------------+-------------+
   | 1. TFW key  | Device   | FW CA  | A white list of   | 1 per       |
   | pair and    | secure   |        | FW root CA        | device      |
   | certificate | storage  |        | trusted by TAMs   |             |
   |             |          |        |                   |             |
   | 2. TEE key  | Device   | TEE CA | A white list 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 white list 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.  |             |
   +-------------+----------+--------+-------------------+-------------+

                    Table 1: Key and Certificate Types

   1.  TFW key pair and certificate:  A key pair and certificate for
       evidence of secure boot and trustworthy firmware in a device.

       Location:   Device secure storage

       Supported Key Type:   RSA and ECC

       Issuer:   OEM CA

       Checked Against:   A white list of FW root CA trusted by TAMs

       Cardinality:   One per device

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






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

       Location:   Device TEE

       Supported Key Type:   RSA and ECC

       Issuer:   A CA that chains to a TEE root CA

       Checked Against:   A white list of TEE root CA trusted by TAMs

       Cardinality:   One per device

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

       Location:   TAM provider

       Supported Key Type:   RSA and ECC.

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

       Issuer:   TAM CA that chains to a root CA

       Checked Against:   A white list of TAM root CA embedded in TEE

       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.

       Location:   SP

       Supported Key Type:   RSA and ECC

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

       Issuer:   an SP signer CA that chains to a root CA

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

       Cardinality:   One or multiple can be used by an SP




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5.6.  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 CAs
   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.

5.7.  Message Security

   Messages created by a TAM are used to deliver device SD and TA
   management commands to a device, and device attestation and response
   messages created by the 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.

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




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   It is an important decision in this protocol specification 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 this SD is 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.

5.9.  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.

   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.

   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.



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5.10.  Service Provider Container

   A sample Security Domain hierarchy for the TEE is shown in Figure 4.

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

                    Figure 4: Security Domain Hiearchy

   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.

5.11.  A Sample Device Setup Flow

   Step 1: Prepare Images for Devices

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

   2.  [CA]  Deliver root CA Whitelist

   3.  [Soc]  Deliver TFW Image

   Step 2: Inject Key Pairs and Images to Devices

   1.  [OEM] Generate Secure Boot Key Pair (May be shared among multiple
       devices)

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

   Step 3: Setup attestation key pairs in devices

   1.  [OEM]  Flash Secure Boot Public Key and eFuse Key (eFuse key is
       unique per device)





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

   Step 4: Setup trust anchors in devices

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

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

6.  Agent

   A TEE and TAs do not generally have capability to communicate to the
   outside of the hosting device.  For example, the Global Platform
   [GPTEE] specifies one such architecture.  This calls for a software
   module in the REE world to handle the network communication.  Each
   Client Application in REE may carry this communication functionality
   but it 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 meed this need.  An agent is an application
   running in the REE of the device or a SDK that facilitates
   communication between a TAM and 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 Android
   service call for an Android powered device.  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 that a Client Application
   connects.  A Client Application may also directly call Agent for some
   TA query functions.

   The agent may internally process a request from 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.



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   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 device 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 experience.

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
   introducing a denial of service.

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 aent.

   With one shared agent, the agent provider is responsible to allow
   multiple TAMs and TEE providers to achieve interoperability.  With a
   standard agent interface, 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 SDK for SP
   applications to interact with an agent for the TAM and TEE
   interaction.








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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 encryption that takes place may be the use of a
   so-called eFuse to release the SBM signing key and later during the
   protocol lifecycle management interchange with the TAM.

   SBM attestation can be optional in TEEP architecture where the
   starting point of device attestion can be at TEE certfificates.  TAM
   can define its policies on what kind of TEE it trusts if TFW
   attestation isn't 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, encrypted by eFuse.  This key pair will be used for
       signing operations performed by the SBM.

   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.  Attestation Hierarchy Establishment: Device Boot

   During device boot the following steps are required:

   1.  Secure boot releases the TFW private key by decrypting it with
       eFuse.

   2.  The SBM 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 SBM leaves the TEE public key field blank.

   3.  The SBM signs the signing buffer with the TFW private key.



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

7.1.3.  Attestation Hierarchy Establishment: TAM

   Before a TAM can begin operation in the marketplace to support
   devices of a given TEE, it must obtain a TAM certificate from a CA
   that is registered in the trust store of devices with that TEE.  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.  Acknowledgements

   The authors thank Dave Thaler for his very thorough review and many
   important suggestions.  Most content of this document are 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.

9.  Security Consideration

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.








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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 Rich OS.  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 OEM.  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.





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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.  References

10.1.  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>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <https://www.rfc-editor.org/info/rfc7516>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015, <https://www.rfc-
              editor.org/info/rfc7517>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015, <https://www.rfc-
              editor.org/info/rfc7518>.

10.2.  Informative References

   [GPTEE]    Global Platform, "Global Platform, GlobalPlatform Device
              Technology: TEE System Architecture, v1.0", 2013.

   [GPTEECLAPI]
              Global Platform, "Global Platform, GlobalPlatform Device
              Technology: TEE Client API Specification, v1.0", 2013.



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   [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-00 (work in progress), May 2018.

Authors' Addresses

   Mingliang Pei
   Symantec
   350 Ellis St
   Mountain View, CA  94043
   USA

   Email: mingliang_pei@symantec.com


   Hannes Tschofenig
   Arm Ltd.
   Absam, Tirol  6067
   Austria

   Email: Hannes.Tschofenig@arm.com


   Andrew Atyeo
   Intercede
   St. Mary's Road, Lutterworth
   Leicestershire, LE17  4PS
   Great Britain

   Email: andrew.atyeo@intercede.com


   Dapeng
   Alibaba Group
   Wangjing East Garden 4th Area,Chaoyang District
   Beijing  100102
   China

   Email: maxpassion@gmail.com











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