Trusted Execution Environment Provisioning (TEEP) Architecture
draft-ietf-teep-architecture-00
The information below is for an old version of the document.
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| Authors | Mingliang Pei , Hannes Tschofenig , Andrew Atyeo , Dapeng Liu | ||
| Last updated | 2018-07-04 | ||
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draft-ietf-teep-architecture-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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