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2 TEEP M. Pei
3 Internet-Draft Symantec
4 Intended status: Informational H. Tschofenig
5 Expires: December 14, 2020 Arm Limited
6 D. Thaler
7 Microsoft
8 D. Wheeler
9 Intel
10 June 12, 2020
12 Trusted Execution Environment Provisioning (TEEP) Architecture
13 draft-ietf-teep-architecture-09
15 Abstract
17 A Trusted Execution Environment (TEE) is an environment that enforces
18 that any code within that environment cannot be tampered with, and
19 that any data used by such code cannot be read or tampered with by
20 any code outside that environment. This architecture document
21 motivates the design and standardization of a protocol for managing
22 the lifecycle of trusted applications running inside such a TEE.
24 Status of This Memo
26 This Internet-Draft is submitted in full conformance with the
27 provisions of BCP 78 and BCP 79.
29 Internet-Drafts are working documents of the Internet Engineering
30 Task Force (IETF). Note that other groups may also distribute
31 working documents as Internet-Drafts. The list of current Internet-
32 Drafts is at http://datatracker.ietf.org/drafts/current/.
34 Internet-Drafts are draft documents valid for a maximum of six months
35 and may be updated, replaced, or obsoleted by other documents at any
36 time. It is inappropriate to use Internet-Drafts as reference
37 material or to cite them other than as "work in progress."
39 This Internet-Draft will expire on December 14, 2020.
41 Copyright Notice
43 Copyright (c) 2020 IETF Trust and the persons identified as the
44 document authors. All rights reserved.
46 This document is subject to BCP 78 and the IETF Trust's Legal
47 Provisions Relating to IETF Documents
48 (http://trustee.ietf.org/license-info) in effect on the date of
49 publication of this document. Please review these documents
50 carefully, as they describe your rights and restrictions with respect
51 to this document. Code Components extracted from this document must
52 include Simplified BSD License text as described in Section 4.e of
53 the Trust Legal Provisions and are provided without warranty as
54 described in the Simplified BSD License.
56 This document may contain material from IETF Documents or IETF
57 Contributions published or made publicly available before November
58 10, 2008. The person(s) controlling the copyright in some of this
59 material may not have granted the IETF Trust the right to allow
60 modifications of such material outside the IETF Standards Process.
61 Without obtaining an adequate license from the person(s) controlling
62 the copyright in such materials, this document may not be modified
63 outside the IETF Standards Process, and derivative works of it may
64 not be created outside the IETF Standards Process, except to format
65 it for publication as an RFC or to translate it into languages other
66 than English.
68 Table of Contents
70 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
71 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
72 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7
73 3.1. Payment . . . . . . . . . . . . . . . . . . . . . . . . . 7
74 3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 8
75 3.3. Internet of Things . . . . . . . . . . . . . . . . . . . 8
76 3.4. Confidential Cloud Computing . . . . . . . . . . . . . . 8
77 4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8
78 4.1. System Components . . . . . . . . . . . . . . . . . . . . 8
79 4.2. Multiple TEEs in a Device . . . . . . . . . . . . . . . . 11
80 4.3. Multiple TAMs and Relationship to TAs . . . . . . . . . . 13
81 4.4. Untrusted Apps, Trusted Apps, and Personalization Data . 14
82 4.4.1. Example: Application Delivery Mechanisms in Intel SGX 16
83 4.4.2. Example: Application Delivery Mechanisms in Arm
84 TrustZone . . . . . . . . . . . . . . . . . . . . . . 16
85 4.5. Entity Relations . . . . . . . . . . . . . . . . . . . . 17
86 5. Keys and Certificate Types . . . . . . . . . . . . . . . . . 18
87 5.1. Trust Anchors in a TEEP Agent . . . . . . . . . . . . . . 20
88 5.2. Trust Anchors in a TEE . . . . . . . . . . . . . . . . . 20
89 5.3. Trust Anchors in a TAM . . . . . . . . . . . . . . . . . 20
90 5.4. Scalability . . . . . . . . . . . . . . . . . . . . . . . 20
91 5.5. Message Security . . . . . . . . . . . . . . . . . . . . 21
92 6. TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . . 21
93 6.1. Role of the TEEP Broker . . . . . . . . . . . . . . . . . 21
94 6.2. TEEP Broker Implementation Consideration . . . . . . . . 22
95 6.2.1. TEEP Broker APIs . . . . . . . . . . . . . . . . . . 22
96 6.2.2. TEEP Broker Distribution . . . . . . . . . . . . . . 23
98 7. Attestation . . . . . . . . . . . . . . . . . . . . . . . . . 23
99 7.1. Information Required in TEEP Claims . . . . . . . . . . . 24
100 8. Algorithm and Attestation Agility . . . . . . . . . . . . . . 25
101 9. Security Considerations . . . . . . . . . . . . . . . . . . . 26
102 9.1. Broker Trust Model . . . . . . . . . . . . . . . . . . . 26
103 9.2. Data Protection at TAM and TEE . . . . . . . . . . . . . 26
104 9.3. Compromised REE . . . . . . . . . . . . . . . . . . . . . 26
105 9.4. Compromised CA . . . . . . . . . . . . . . . . . . . . . 27
106 9.5. Compromised TAM . . . . . . . . . . . . . . . . . . . . . 27
107 9.6. Malicious TA Removal . . . . . . . . . . . . . . . . . . 27
108 9.7. Certificate Expiry and Renewal . . . . . . . . . . . . . 28
109 9.8. Keeping Secrets from the TAM . . . . . . . . . . . . . . 28
110 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
111 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 29
112 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
113 13. Informative References . . . . . . . . . . . . . . . . . . . 29
114 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
116 1. Introduction
118 Applications executing in a device are exposed to many different
119 attacks intended to compromise the execution of the application or
120 reveal the data upon which those applications are operating. These
121 attacks increase with the number of other applications on the device,
122 with such other applications coming from potentially untrustworthy
123 sources. The potential for attacks further increases with the
124 complexity of features and applications on devices, and the
125 unintended interactions among those features and applications. The
126 danger of attacks on a system increases as the sensitivity of the
127 applications or data on the device increases. As an example,
128 exposure of emails from a mail client is likely to be of concern to
129 its owner, but a compromise of a banking application raises even
130 greater concerns.
132 The Trusted Execution Environment (TEE) concept is designed to
133 execute applications in a protected environment that enforces that
134 any code within that environment cannot be tampered with, and that
135 any data used by such code cannot be read or tampered with by any
136 code outside that environment, including by a commodity operating
137 system (if present).
139 This separation reduces the possibility of a successful attack on
140 application components and the data contained inside the TEE.
141 Typically, application components are chosen to execute inside a TEE
142 because those application components perform security sensitive
143 operations or operate on sensitive data. An application component
144 running inside a TEE is referred to as a Trusted Application (TA),
145 while an application running outside any TEE is referred to as an
146 Untrusted Application. In the example of a banking application, code
147 that relates to the authentication protocol could reside in a TA
148 while the application logic including HTTP protocol parsing could be
149 contained in the Untrusted Application. In addition, processing of
150 credit card numbers or account balances could be done in a TA as it
151 is sensitive data. The precise code split is ultimately a decision
152 of the developer based on the assets he or she wants to protect
153 according to the threat model.
155 TEEs use hardware enforcement combined with software protection to
156 secure TAs and its data. TEEs typically offer a more limited set of
157 services to TAs than is normally available to Untrusted Applications.
159 Not all TEEs are the same, however, and different vendors may have
160 different implementations of TEEs with different security properties,
161 different features, and different control mechanisms to operate on
162 TAs. Some vendors may themselves market multiple different TEEs with
163 different properties attuned to different markets. A device vendor
164 may integrate one or more TEEs into their devices depending on market
165 needs.
167 To simplify the life of TA developers interacting with TAs in a TEE,
168 an interoperable protocol for managing TAs running in different TEEs
169 of various devices is needed. This software update protocol needs to
170 make sure that compatible trusted and untrusted components (if any)
171 of an application are installed on the correct device. In this TEE
172 ecosystem, there often arises a need for an external trusted party to
173 verify the identity, claims, and rights of TA developers, devices,
174 and their TEEs. This trusted third party is the Trusted Application
175 Manager (TAM).
177 The Trusted Execution Environment Provisioning (TEEP) protocol
178 addresses the following problems:
180 - An installer of an Untrusted Application that depends on a given
181 TA wants to request installation of that TA in the device's TEE so
182 that the Untrusted Application can complete, but the TEE needs to
183 verify whether such a TA is actually authorized to run in the TEE
184 and consume potentially scarce TEE resources.
186 - A TA developer providing a TA whose code itself is considered
187 confidential wants to determine security-relevant information of a
188 device before allowing their TA to be provisioned to the TEE
189 within the device. An example is the verification of the type of
190 TEE included in a device and that it is capable of providing the
191 security protections required.
193 - A TEE in a device wants to determine whether an entity that wants
194 to manage a TA in the device is authorized to manage TAs in the
195 TEE, and what TAs the entity is permitted to manage.
197 - A TAM (e.g., operated by a device administrator) wants to
198 determine if a TA exists (is installed) on a device (in the TEE),
199 and if not, install the TA in the TEE.
201 - A TAM wants to check whether a TA in a device's TEE is the most
202 up-to-date version, and if not, update the TA in the TEE.
204 - A Device Administrator wants to remove a TA from a device's TEE if
205 the TA developer is no longer maintaining that TA, when the TA has
206 been revoked or is not used for other reasons anymore (e.g., due
207 to an expired subscription).
209 - A TA developer wants to define the relationship between
210 cooperating TAs under the TA developer's control, and specify
211 whether the TAs can communicate, share data, and/or share key
212 material.
214 Note: The TA developer requires the help of a TAM and most likely the
215 Device Administrator to provision the Trusted Applications to remote
216 devices and the TEEP protocol exchanges messages between a TAM and a
217 TEEP Agent via a TEEP Broker.
219 2. Terminology
221 The following terms are used:
223 - Device: A physical piece of hardware that hosts one or more TEEs,
224 often along with a Rich Execution Environment. A device contains
225 a default list of Trust Anchors that identify entities (e.g.,
226 TAMs) that are trusted by the device. This list is normally set
227 by the device manufacturer, and may be governed by the device's
228 network carrier when it is a mobile device. The list of Trust
229 Anchors is normally modifiable by the device's owner or Device
230 Administrator. However the device manufacturer or network carrier
231 (in the mobile device case) may restrict some modifications, for
232 example, by not allowing the manufacturer or carrier's Trust
233 Anchor to be removed or disabled.
235 - Device Administrator: An entity that is responsible for
236 administration of a device, which could be the device owner. A
237 Device Administrator has privileges on the device to install and
238 remove Untrusted Applications and TAs, approve or reject Trust
239 Anchors, and approve or reject TA developers, among possibly other
240 privileges on the device. A Device Administrator can manage the
241 list of allowed TAMs by modifying the list of Trust Anchors on the
242 device. Although a Device Administrator may have privileges and
243 device-specific controls to locally administer a device, the
244 Device Administrator may choose to remotely administer a device
245 through a TAM.
247 - Device Owner: A device is always owned by someone. In some cases,
248 it is common for the (primary) device user to also own the device,
249 making the device user/owner also the Device Administrator. In
250 enterprise environments it is more common for the enterprise to
251 own the device, and any device user has no or limited
252 administration rights. In this case, the enterprise appoints a
253 Device Administrator that is not the device owner.
255 - Device User: A human being that uses a device. Many devices have
256 a single device user. Some devices have a primary device user
257 with other human beings as secondary device users (e.g., parent
258 allowing children to use their tablet or laptop). Other devices
259 are not used by a human being and hence have no device user.
260 Relates to Device Owner and Device Administrator.
262 - Rich Execution Environment (REE): An environment that is provided
263 and governed by a typical OS (e.g., Linux, Windows, Android, iOS),
264 potentially in conjunction with other supporting operating systems
265 and hypervisors; it is outside of any TEE. This environment and
266 applications running on it are considered untrusted (or more
267 precisely, less trusted than a TEE).
269 - Trust Anchor: As defined in [RFC6024] and
270 [I-D.ietf-suit-manifest], "A trust anchor represents an
271 authoritative entity via a public key and associated data. The
272 public key is used to verify digital signatures, and the
273 associated data is used to constrain the types of information for
274 which the trust anchor is authoritative." The Trust Anchor may be
275 a certificate or it may be a raw public key along with additional
276 data if necessary such as its public key algorithm and parameters.
278 - Trust Anchor Store: As defined in [RFC6024], "A trust anchor store
279 is a set of one or more trust anchors stored in a device. A
280 device may have more than one trust anchor store, each of which
281 may be used by one or more applications." As noted in
282 [I-D.ietf-suit-manifest], a trust anchor store must resist
283 modification against unauthorized insertion, deletion, and
284 modification.
286 - Trusted Application (TA): An application component that runs in a
287 TEE.
289 - Trusted Application (TA) Developer: An entity that wishes to
290 provide functionality on devices that requires the use of one or
291 more Trusted Applications. The TA developer signs the TA binary
292 (or more precisely the manifest associated with the TA binary) or
293 uses another entity on his or her behalf to get the TA binary
294 signed. (A TA binary may also be encrypted by the developer or by
295 some third party service.) For editorial reasons, we assume that
296 the TA developer signs the TA binary ignoring the distinction
297 between the binary and the manifest and by simplifying the case
298 where the TA developer outsources signing and encryption to a
299 third party entity or service.
301 - Trusted Application Manager (TAM): An entity that manages Trusted
302 Applications (TAs) running in TEEs of various devices.
304 - Trusted Execution Environment (TEE): An execution environment that
305 enforces that only authorized code can execute within the TEE, and
306 data used by that code cannot be read or tampered with by code
307 outside the TEE. A TEE also generally has a device unique
308 credential that cannot be cloned. There are multiple technologies
309 that can be used to implement a TEE, and the level of security
310 achieved varies accordingly. In addition, TEEs typically use an
311 isolation mechanism between Trusted Applications to ensure that
312 one TA cannot read, modify or delete the data and code of another
313 TA.
315 - Untrusted Application: An application running in a Rich Execution
316 Environment.
318 3. Use Cases
320 3.1. Payment
322 A payment application in a mobile device requires high security and
323 trust about the hosting device. Payments initiated from a mobile
324 device can use a Trusted Application to provide strong identification
325 and proof of transaction.
327 For a mobile payment application, some biometric identification
328 information could also be stored in a TEE. The mobile payment
329 application can use such information for unlocking the phone and for
330 local identification of the user.
332 A trusted user interface (UI) may be used in a mobile device to
333 prevent malicious software from stealing sensitive user input data.
334 Such an implementation often relies on a TEE for providing access to
335 peripherals, such as PIN input.
337 3.2. Authentication
339 For better security of authentication, a device may store its keys
340 and cryptographic libraries inside a TEE limiting access to
341 cryptographic functions via a well-defined interface and thereby
342 reducing access to keying material.
344 3.3. Internet of Things
346 The Internet of Things (IoT) has been posing threats to critical
347 infrastructure because of weak security in devices. It is desirable
348 that IoT devices can prevent malware from manipulating actuators
349 (e.g., unlocking a door), or stealing or modifying sensitive data,
350 such as authentication credentials in the device. A TEE can be the
351 best way to implement such IoT security functions.
353 3.4. Confidential Cloud Computing
355 A tenant can store sensitive data in a TEE in a cloud computing
356 server such that only the tenant can access the data, preventing the
357 cloud hosting provider from accessing the data. A tenant can run TAs
358 inside a server TEE for secure operation and enhanced data security.
359 This provides benefits not only to tenants with better data security
360 but also to cloud hosting providers for reduced liability and
361 increased cloud adoption.
363 4. Architecture
365 4.1. System Components
367 Figure 1 shows the main components in a typical device with an REE.
368 Full descriptions of components not previously defined are provided
369 below. Interactions of all components are further explained in the
370 following paragraphs.
372 +-------------------------------------------+
373 | Device |
374 | +--------+ | TA Developer
375 | +-------------+ | |-----------+ |
376 | | TEE-1 | | TEEP |---------+ | |
377 | | +--------+ | +----| Broker | | | | +--------+ |
378 | | | TEEP | | | | |<---+ | | +->| |<-+
379 | | | Agent |<----+ | | | | | +-| TAM-1 |
380 | | +--------+ | | |<-+ | | +->| | |<-+
381 | | | +--------+ | | | | +--------+ |
382 | | +---+ +---+ | | | | | TAM-2 | |
383 | +-->|TA1| |TA2| | +-------+ | | | +--------+ |
384 | | | | | | |<---------| App-2 |--+ | | |
385 | | | +---+ +---+ | +-------+ | | | Device Administrator
386 | | +-------------+ | App-1 | | | |
387 | | | | | | |
388 | +--------------------| |---+ | |
389 | | |--------+ |
390 | +-------+ |
391 +-------------------------------------------+
393 Figure 1: Notional Architecture of TEEP
395 - TA developers and Device Administrators utilize the services of a
396 TAM to manage TAs on devices. TA developers do not directly
397 interact with devices. Device Administators may elect to use a
398 TAM for remote administration of TAs instead of managing each
399 device directly.
401 - Trusted Application Manager (TAM): A TAM is responsible for
402 performing lifecycle management activity on TAs on behalf of TA
403 developers and Device Administrators. This includes creation and
404 deletion of TAs, and may include, for example, over-the-air
405 updates to keep TAs up-to-date and clean up when a version should
406 be removed. TAMs may provide services that make it easier for TA
407 developers or Device Administators to use the TAM's service to
408 manage multiple devices, although that is not required of a TAM.
410 The TAM performs its management of TAs on the device through
411 interactions with a device's TEEP Broker, which relays messages
412 between a TAM and a TEEP Agent running inside the TEE. As shown
413 in Figure 1, the TAM cannot directly contact a TEEP Agent, but
414 must wait for the TEEP Broker to contact the TAM requesting a
415 particular service. This architecture is intentional in order to
416 accommodate network and application firewalls that normally
417 protect user and enterprise devices from arbitrary connections
418 from external network entities.
420 A TAM may be publicly available for use by many TA developers, or
421 a TAM may be private, and accessible by only one or a limited
422 number of TA developers. It is expected that many manufacturers
423 and network carriers will run their own private TAM.
425 A TA developer or Device Administrator chooses a particular TAM
426 based on whether the TAM is trusted by a device or set of devices.
427 The TAM is trusted by a device if the TAM's public key is, or
428 chains up to, an authorized Trust Anchor in the device. A TA
429 developer or Device Administrator may run their own TAM, but the
430 devices they wish to manage must include this TAM's public key/
431 certificate, or a certificate it chains up to, in the Trust Anchor
432 list.
434 A TA developer or Device Administrator is free to utilize multiple
435 TAMs. This may be required for a TA developer to manage multiple
436 different types of devices from different manufacturers, or to
437 manage mobile devices on different network carriers, since the
438 Trust Anchor list on these different devices may contain different
439 TAMs. A Device Administrator may be able to add their own TAM's
440 public key or certificate to the Trust Anchor list on all their
441 devices, overcoming this limitation.
443 Any entity is free to operate a TAM. For a TAM to be successful,
444 it must have its public key or certificate installed in a device's
445 Trust Anchor list. A TAM may set up a relationship with device
446 manufacturers or network carriers to have them install the TAM's
447 keys in their device's Trust Anchor list. Alternatively, a TAM
448 may publish its certificate and allow Device Administrators to
449 install the TAM's certificate in their devices as an after-market-
450 action.
452 - TEEP Broker: A TEEP Broker is an application component running in
453 a Rich Execution Environment (REE) that enables the message
454 protocol exchange between a TAM and a TEE in a device. A TEEP
455 Broker does not process messages on behalf of a TEE, but merely is
456 responsible for relaying messages from the TAM to the TEE, and for
457 returning the TEE's responses to the TAM. In devices with no REE
458 (e.g., a microcontroller where all code runs in an environment
459 that meets the definition of a Trusted Execution Environment in
460 Section 2), the TEEP Broker would be absent and instead the TEEP
461 protocol transport would be implemented inside the TEE itself.
463 - TEEP Agent: The TEEP Agent is a processing module running inside a
464 TEE that receives TAM requests (typically relayed via a TEEP
465 Broker that runs in an REE). A TEEP Agent in the TEE may parse
466 requests or forward requests to other processing modules in a TEE,
467 which is up to a TEE provider's implementation. A response
468 message corresponding to a TAM request is sent back to the TAM,
469 again typically relayed via a TEEP Broker.
471 - Certification Authority (CA): A CA is an entity that issues
472 digital certificates (especially X.509 certificates) and vouches
473 for the binding between the data items in a certificate [RFC4949].
474 Certificates are then used for authenticating a device, a TAM and
475 a TA developer. A device embeds a list of root certificates
476 (Trust Anchors), from trusted CAs that a TAM will be validated
477 against. A TAM will remotely attest a device by checking whether
478 a device comes with a certificate from a CA that the TAM trusts.
479 The CAs do not need to be the same; different CAs can be chosen by
480 each TAM, and different device CAs can be used by different device
481 manufacturers.
483 4.2. Multiple TEEs in a Device
485 Some devices might implement multiple TEEs. In these cases, there
486 might be one shared TEEP Broker that interacts with all the TEEs in
487 the device. However, some TEEs (for example, SGX [SGX]) present
488 themselves as separate containers within memory without a controlling
489 manager within the TEE. As such, there might be multiple TEEP
490 Brokers in the Rich Execution Environment, where each TEEP Broker
491 communicates with one or more TEEs associated with it.
493 It is up to the Rich Execution Environment and the Untrusted
494 Applications how they select the correct TEEP Broker. Verification
495 that the correct TA has been reached then becomes a matter of
496 properly verifying TA attestations, which are unforgeable.
498 The multiple TEEP Broker approach is shown in the diagram below. For
499 brevity, TEEP Broker 2 is shown interacting with only one TAM and
500 Untrusted Application and only one TEE, but no such limitations are
501 intended to be implied in the architecture.
503 +-------------------------------------------+
504 | Device |
505 | | TA Developer
506 | +-------------+ | |
507 | | TEE-1 | | |
508 | | +-------+ | +--------+ | +--------+ |
509 | | | TEEP | | | TEEP |------------->| |<-+
510 | | | Agent |<----------| Broker | | | |
511 | | | 1 | | | 1 |---------+ | |
512 | | +-------+ | | | | | | |
513 | | | | |<---+ | | | |
514 | | +---+ +---+ | | | | | | +-| TAM-1 |
515 | | |TA1| |TA2| | | |<-+ | | +->| | |<-+
516 | +-->| | | |<---+ +--------+ | | | | +--------+ |
517 | | | +---+ +---+ | | | | | | TAM-2 | |
518 | | | | | +-------+ | | | +--------+ |
519 | | +-------------+ +-----| App-2 |--+ | | ^ |
520 | | +-------+ | | | | Device
521 | +--------------------| App-1 | | | | | Administrator
522 | +------| | | | | |
523 | +-----------|-+ | |---+ | | |
524 | | TEE-2 | | | |--------+ | |
525 | | +------+ | | | |------+ | |
526 | | | TEEP | | | +-------+ | | |
527 | | | Agent|<-----+ | | |
528 | | | 2 | | | | | | |
529 | | +------+ | | | | | |
530 | | | | | | | |
531 | | +---+ | | | | | |
532 | | |TA3|<----+ | | +----------+ | | |
533 | | | | | | | TEEP |<--+ | |
534 | | +---+ | +--| Broker | | |
535 | | | | 2 |----------------+
536 | +-------------+ +----------+ |
537 | |
538 +-------------------------------------------+
540 Figure 2: Notional Architecture of TEEP with multiple TEEs
542 In the diagram above, TEEP Broker 1 controls interactions with the
543 TAs in TEE-1, and TEEP Broker 2 controls interactions with the TAs in
544 TEE-2. This presents some challenges for a TAM in completely
545 managing the device, since a TAM may not interact with all the TEEP
546 Brokers on a particular platform. In addition, since TEEs may be
547 physically separated, with wholly different resources, there may be
548 no need for TEEP Brokers to share information on installed TAs or
549 resource usage.
551 4.3. Multiple TAMs and Relationship to TAs
553 As shown in Figure 2, a TEEP Broker provides communication between
554 one or more TEEP Agents and one or more TAMs. The selection of which
555 TAM to communicate with might be made with or without input from an
556 Untrusted Application, but is ultimately the decision of a TEEP
557 Agent.
559 A TEEP Agent is assumed to be able to determine, for any given TA,
560 whether that TA is installed (or minimally, is running) in a TEE with
561 which the TEEP Agent is associated.
563 Each TA is digitally signed, protecting its integrity, and linking
564 the TA back to the signer. The signer is usually the TA developer,
565 but in some cases might be another party that the TA developer
566 trusts, or a party to whom the code has been licensed (in which case
567 the same code might be signed by multiple licensees and distributed
568 as if it were different TAs).
570 A TA author or signer selects one or more TAMs through which to offer
571 their TA(s), and communicates the TA(s) to the TAM. In this
572 document, we use the term "TA developer" to refer to the entity that
573 selects a TAM and publishes a signed TA to it, independent of whether
574 the publishing entity is the TA developer or the signer or both.
576 The TA developer chooses TAMs based upon the markets into which the
577 TAM can provide access. There may be TAMs that provide services to
578 specific types of devices, or device operating systems, or specific
579 geographical regions or network carriers. A TA developer may be
580 motivated to utilize multiple TAMs for its service in order to
581 maximize market penetration and availability on multiple types of
582 devices. This likely means that the same TA will be available
583 through multiple TAMs.
585 When the developer of an Untrusted Application that depends on a TA
586 publishes the Untrusted Application to an app store or other app
587 repository, the developer optionally binds the Untrusted Application
588 with a manifest that identifies what TAMs can be contacted for the
589 TA. In some situations, a TA may only be available via a single TAM
590 - this is likely the case for enterprise applications or TA
591 developers serving a closed community. For broad public apps, there
592 will likely be multiple TAMs in the manifest - one servicing one
593 brand of mobile device and another servicing a different
594 manufacturer, etc. Because different devices and different
595 manufacturers trust different TAMs, the manifest can include multiple
596 TAMs that support the required TA.
598 When a TEEP Broker receives a request from an Untrusted Application
599 to install a TA, a list of TAM URIs may be provided for that TA, and
600 the request is passed to the TEEP Agent. If the TEEP Agent decides
601 that the TA needs to be installed, the TEEP Agent selects a single
602 TAM URI that is consistent with the list of trusted TAMs provisioned
603 on the device, invokes the HTTP transport for TEEP to connect to the
604 TAM URI, and begins a TEEP protocol exchange. When the TEEP Agent
605 subsequently receives the TA to install and the TA's manifest
606 indicates dependencies on any other trusted components, each
607 dependency can include a list of TAM URIs for the relevant
608 dependency. If such dependencies exist that are prerequisites to
609 install the TA, then the TEEP Agent recursively follows the same
610 procedure for each dependency that needs to be installed or updated,
611 including selecting a TAM URI that is consistent with the list of
612 trusted TAMs provisioned on the device, and beginning a TEEP
613 exchange. If multiple TAM URIs are considered trusted, only one
614 needs to be contacted and they can be attempted in some order until
615 one responds.
617 Separate from the Untrusted Application's manifest, this framework
618 relies on the use of the manifest format in [I-D.ietf-suit-manifest]
619 for expressing how to install a TA, as well as any dependencies on
620 other TEE components and versions. That is, dependencies from TAs on
621 other TEE components can be expressed in a SUIT manifest, including
622 dependencies on any other TAs, or trusted OS code (if any), or
623 trusted firmware. Installation steps can also be expressed in a SUIT
624 manifest.
626 For example, TEEs compliant with GlobalPlatform may have a notion of
627 a "security domain" (which is a grouping of one or more TAs installed
628 on a device, that can share information within such a group) that
629 must be created and into which one or more TAs can then be installed.
630 It is thus up to the SUIT manifest to express a dependency on having
631 such a security domain existing or being created first, as
632 appropriate.
634 Updating a TA may cause compatibility issues with any Untrusted
635 Applications or other components that depend on the updated TA, just
636 like updating the OS or a shared library could impact an Untrusted
637 Application. Thus, an implementation needs to take into account such
638 issues.
640 4.4. Untrusted Apps, Trusted Apps, and Personalization Data
642 In TEEP, there is an explicit relationship and dependence between an
643 Untrusted Application in a REE and one or more TAs in a TEE, as shown
644 in Figure 2. For most purposes, an Untrusted Application that uses
645 one or more TAs in a TEE appears no different from any other
646 Untrusted Application in the REE. However, the way the Untrusted
647 Application and its corresponding TAs are packaged, delivered, and
648 installed on the device can vary. The variations depend on whether
649 the Untrusted Application and TA are bundled together or are provided
650 separately, and this has implications to the management of the TAs in
651 a TEE. In addition to the Untrusted Application and TA(s), the TA(s)
652 and/or TEE may require some additional data to personalize the TA to
653 the TA developer or the device or a user. This personalization data
654 may dependent on the type of TEE, a particular TEE instance, the TA,
655 the TA developer and even the user of the device; an example of
656 personalization data might be a secret symmetric key used by the TA
657 to communicate with some service. Implementations must support
658 encryption of personalization data to preserve the confidentiality of
659 potentially sensitive data contained within it and support integrity
660 protection of the personalization data. Other than the requirement
661 to support confidentiality and integrity protection, the TEEP
662 architecture places no limitations or requirements on the
663 personalization data.
665 There are three possible cases for bundling of an Untrusted
666 Application, TA(s), and personalization data:
668 1. The Untrusted Application, TA(s), and personalization data are
669 all bundled together in a single package by a TA developer and
670 provided to the TEEP Broker through the TAM.
672 2. The Untrusted Application and the TA(s) are bundled together in a
673 single package, which a TAM or a publicly accessible app store
674 maintains, and the personalization data is separately provided by
675 the TA developer's TAM.
677 3. All components are independent. The Untrusted Application is
678 installed through some independent or device-specific mechanism,
679 and the TAM provides the TA and personalization data from the TA
680 developer. Delivery of the TA and personalization data may be
681 combined or separate.
683 The TEEP protocol treats each TA, any dependencies the TA has, and
684 personalization data as separate components with separate
685 installation steps that are expressed in SUIT manifests, and a SUIT
686 manifest might contain or reference multiple binaries (see
687 [I-D.ietf-suit-manifest] for more details). The TEEP Agent is
688 responsible for handling any installation steps that need to be
689 performed inside the TEE, such as decryption of private TA binaries
690 or personalization data.
692 In order to better understand these cases, it is helpful to review
693 actual implementations of TEEs and their application delivery
694 mechanisms.
696 4.4.1. Example: Application Delivery Mechanisms in Intel SGX
698 In Intel Software Guard Extensions (SGX), the Untrusted Application
699 and TA are typically bundled into the same package (Case 2). The TA
700 exists in the package as a shared library (.so or .dll). The
701 Untrusted Application loads the TA into an SGX enclave when the
702 Untrusted Application needs the TA. This organization makes it easy
703 to maintain compatibility between the Untrusted Application and the
704 TA, since they are updated together. It is entirely possible to
705 create an Untrusted Application that loads an external TA into an SGX
706 enclave, and use that TA (Case 3). In this case, the Untrusted
707 Application would require a reference to an external file or download
708 such a file dynamically, place the contents of the file into memory,
709 and load that as a TA. Obviously, such file or downloaded content
710 must be properly formatted and signed for it to be accepted by the
711 SGX TEE. In SGX, for Case 2 and Case 3, the personalization data is
712 normally loaded into the SGX enclave (the TA) after the TA has
713 started. Although Case 1 is possible with SGX, there are no
714 instances of this known to be in use at this time, since such a
715 construction would require a special installation program and SGX TA
716 to receive the encrypted binary, decrypt it, separate it into the
717 three different elements, and then install all three. This
718 installation is complex because the Untrusted Application decrypted
719 inside the TEE must be passed out of the TEE to an installer in the
720 REE which would install the Untrusted Application; this assumes that
721 the Untrusted Application package includes the TA code also, since
722 otherwise there is a significant problem in getting the SGX enclave
723 code (the TA) from the TEE, through the installer, and into the
724 Untrusted Application in a trusted fashion. Finally, the
725 personalization data would need to be sent out of the TEE (encrypted
726 in an SGX enclave-to-enclave manner) to the REE's installation app,
727 which would pass this data to the installed Untrusted Application,
728 which would in turn send this data to the SGX enclave (TA). This
729 complexity is due to the fact that each SGX enclave is separate and
730 does not have direct communication to other SGX enclaves.
732 4.4.2. Example: Application Delivery Mechanisms in Arm TrustZone
734 In Arm TrustZone [TrustZone] for A-class devices, the Untrusted
735 Application and TA may or may not be bundled together. This differs
736 from SGX since in TrustZone the TA lifetime is not inherently tied to
737 a specific Untrused Application process lifetime as occurs in SGX. A
738 TA is loaded by a trusted OS running in the TEE, where the trusted OS
739 is separate from the OS in the REE. Thus Cases 2 and 3 are equally
740 applicable. In addition, it is possible for TAs to communicate with
741 each other without involving any Untrusted Application, and so the
742 complexity of Case 1 is lower than in the SGX example. Thus, Case 1
743 is possible as well, though still more complex than Cases 2 and 3.
745 4.5. Entity Relations
747 This architecture leverages asymmetric cryptography to authenticate a
748 device to a TAM. Additionally, a TEEP Agent in a device
749 authenticates a TAM. The provisioning of Trust Anchors to a device
750 may be different from one use case to the other. A Device
751 Administrator may want to have the capability to control what TAs are
752 allowed. A device manufacturer enables verification by one or more
753 TAMs and by TA developers; it may embed a list of default Trust
754 Anchors into the TEEP Agent and TEE for TAM and TA trust
755 verification.
757 (App Developers) (App Store) (TAM) (Device with TEE) (CAs)
758 | | | | |
759 | | | (Embedded TEE cert) <--|
760 | | | | |
761 | <--- Get an app cert -----------------------------------|
762 | | | | |
763 | | | <-- Get a TAM cert ---------|
764 | | | | |
765 1. Build two apps: | | | |
766 | | | |
767 (a) Untrusted | | | |
768 App - 2a. Supply --> | --- 3. Install ------> | |
769 | | | |
770 (b) TA -- 2b. Supply ----------> | 4. Messaging-->| |
771 | | | |
773 Figure 3: Developer Experience
775 Note that Figure 3 shows the view from a TA developer point of view.
776 The TA developer signs the TA or is a related entity trusted to sign
777 the developer-created TAs.
779 Figure 3 shows an example where the same developer builds two
780 applications: 1) an Untrusted Application; 2) a TA that provides some
781 security functions to be run inside a TEE. At step 2, the TA
782 developer uploads the Untrusted Application (2a) to an Application
783 Store. The Untrusted Application may optionally bundle the TA
784 binary. Meanwhile, the TA developer may provide its TA to a TAM that
785 will be managing the TA in various devices. At step 3, a user will
786 go to an Application Store to download the Untrusted Application.
787 Since the Untrusted Application depends on the TA, installing the
788 Untrusted Application will trigger TA installation by initiating
789 communication with a TAM. This is step 4. The TEEP Agent will
790 interact with TAM via a TEEP Broker that faciliates communications
791 between a TAM and the TEEP Agent in TEE.
793 Some TA installation implementations might ask for a user's consent.
794 In other implementations, a Device Administrator might choose what
795 Untrusted Applications and related TAs to be installed. A user
796 consent flow is out of scope of the TEEP architecture.
798 The main components consist of a set of standard messages created by
799 a TAM to deliver TA management commands to a device, and device
800 attestation and response messages created by a TEE that responds to a
801 TAM's message.
803 It should be noted that network communication capability is generally
804 not available in TAs in today's TEE-powered devices. Consequently,
805 Trusted Applications generally rely on broker in the REE to provide
806 access to nnetwork functionality in the REE. A broker does not need
807 to know the actual content of messages to facilitate such access.
809 Similarly, since the TEEP Agent runs inside a TEE, the TEEP Agent
810 generally relies on a TEEP Broker in the REE to provide network
811 access, and relay TAM requests to the TEEP Agent and relay the
812 responses back to the TAM.
814 5. Keys and Certificate Types
816 This architecture leverages the following credentials, which allow
817 delivering end-to-end security between a TAM and a TEEP Agent.
819 Figure 4 summarizes the relationships between various keys and where
820 they are stored. Each public/private key identifies a TA developer,
821 TAM, or TEE, and gets a certificate that chains up to some CA. A
822 list of trusted certificates is then used to check a presented
823 certificate against.
825 Different CAs can be used for different types of certificates. TEEP
826 messages are always signed, where the signer key is the message
827 originator's private key, such as that of a TAM or a TEE. In
828 addition to the keys shown in Figure 4, there may be additional keys
829 used for attestation. Refer to the RATS Architecture
830 [I-D.ietf-rats-architecture] for more discussion.
832 Cardinality & Location of
833 Location of Private Key Trust Anchor
834 Purpose Private Key Signs Store
835 ------------------ ----------- ------------- -------------
836 Authenticating TEE 1 per TEE TEEP responses TAM
838 Authenticating TAM 1 per TAM TEEP requests TEEP Agent
840 Code Signing 1 per TA TA binary TEE
841 developer
843 Figure 4: Keys
845 Note that personalization data is not included in the table above.
846 The use of personalization data is dependent on how TAs are used and
847 what their security requirements are.
849 TEEP requests from a TAM to a TEEP Agent can be encrypted with the
850 TEE public key (to provide confidentiality), and are then signed with
851 the TAM private key (for authentication and integrity protection).
852 Conversely, TEEP responses from a TEEP Agent to a TAM can be
853 encrypted with the TAM public key, and are then signed with the TEE
854 private key.
856 The TEE key pair and certificate are thus used for authenticating the
857 TEE to a remote TAM, and for sending private data to the TEE. Often,
858 the key pair is burned into the TEE by the TEE manufacturer and the
859 key pair and its certificate are valid for the expected lifetime of
860 the TEE. A TAM provider is responsible for configuring the TAM's
861 Trust Anchor Store with the manufacturer certificates or CAs that are
862 used to sign TEE keys. This is discussed further in Section 5.3
863 below.
865 The TAM key pair and certificate are used for authenticating a TAM to
866 a remote TEE, and for sending private data to the TAM. A TAM
867 provider is responsible for acquiring a certificate from a CA that is
868 trusted by the TEEs it manages. This is discussed further in
869 Section 5.1 below.
871 The TA developer key pair and certificate are used to sign TAs that
872 the TEE will consider authorized to execute. TEEs must be configured
873 with the certificates or keys that it considers authorized to sign
874 TAs that it will execute. This is discussed further in Section 5.2
875 below.
877 5.1. Trust Anchors in a TEEP Agent
879 A TEEP Agent's Trust Anchor Store contains a list of Trust Anchors,
880 which are CA certificates that sign various TAM certificates. The
881 list is typically preloaded at manufacturing time, and can be updated
882 using the TEEP protocol if the TEE has some form of "Trust Anchor
883 Manager TA" that has Trust Anchors in its configuration data. Thus,
884 Trust Anchors can be updated similar to updating the configuration
885 data for any other TA.
887 When Trust Anchor update is carried out, it is imperative that any
888 update must maintain integrity where only an authentic Trust Anchor
889 list from a device manufacturer or a Device Administrator is
890 accepted. Details are out of scope of the architecture and can be
891 addressed in a protocol document.
893 Before a TAM can begin operation in the marketplace to support a
894 device with a particular TEE, it must obtain a TAM certificate from a
895 CA that is listed in the Trust Anchor Store of the TEEP Agent.
897 5.2. Trust Anchors in a TEE
899 A TEE determines whether TA binaries are allowed to execute by
900 verifying whether the TA's signer chains up to a certificate in the
901 TEE's Trust Anchor Store. The list is typically preloaded at
902 manufacturing time, and can be updated using the TEEP protocol if the
903 TEE has some form of "Trust Anchor Manager TA" that has Trust Anchors
904 in its configuration data. Thus, Trust Anchors can be updated
905 similar to updating the configuration data for any other TA, as
906 discussed in Section 5.1.
908 5.3. Trust Anchors in a TAM
910 The Trust Anchor Store in a TAM consists of a list of Trust Anchors,
911 which are certificates that sign various device TEE certificates. A
912 TAM will accept a device for TA management if the TEE in the device
913 uses a TEE certificate that is chained to a certificate that the TAM
914 trusts.
916 5.4. Scalability
918 This architecture uses a PKI, although self-signed certificates are
919 also permitted. Trust Anchors exist on the devices to enable the TEE
920 to authenticate TAMs and TA developer, and TAMs use Trust Anchors to
921 authenticate TEEs. When a PKI is used, many intermediate CA
922 certificates can chain to a root certificate, each of which can issue
923 many certificates. This makes the protocol highly scalable. New
924 factories that produce TEEs can join the ecosystem. In this case,
925 such a factory can get an intermediate CA certificate from one of the
926 existing roots without requiring that TAMs are updated with
927 information about the new device factory. Likewise, new TAMs can
928 join the ecosystem, providing they are issued a TAM certificate that
929 chains to an existing root whereby existing TEEs will be allowed to
930 be personalized by the TAM without requiring changes to the TEE
931 itself. This enables the ecosystem to scale, and avoids the need for
932 centralized databases of all TEEs produced or all TAMs that exist or
933 all TA developers that exist.
935 5.5. Message Security
937 Messages created by a TAM are used to deliver TA management commands
938 to a device, and device attestation and messages created by the
939 device TEE to respond to TAM messages.
941 These messages are signed end-to-end between a TEEP Agent and a TAM,
942 and are typically encrypted such that only the targeted device TEE or
943 TAM is able to decrypt and view the actual content.
945 6. TEEP Broker
947 A TEE and TAs often do not have the capability to directly
948 communicate outside of the hosting device. For example,
949 GlobalPlatform [GPTEE] specifies one such architecture. This calls
950 for a software module in the REE world to handle network
951 communication with a TAM.
953 A TEEP Broker is an application component running in the REE of the
954 device or an SDK that facilitates communication between a TAM and a
955 TEE. It also provides interfaces for Untrusted Applications to query
956 and trigger TA installation that the application needs to use.
958 An Untrusted Application might communicate with a TEEP Broker at
959 runtime to trigger TA installation itself, or an Untrusted
960 Application might simply have a metadata file that describes the TAs
961 it depends on and the associated TAM(s) for each TA, and an REE
962 Application Installer can inspect this application metadata file and
963 invoke the TEEP Broker to trigger TA installation on behalf of the
964 Untrusted Application without requiring the Untrusted Application to
965 run first.
967 6.1. Role of the TEEP Broker
969 A TEEP Broker abstracts the message exchanges with a TEE in a device.
970 The input data is originated from a TAM or the first initialization
971 call to trigger a TA installation.
973 The Broker doesn't need to parse a message content received from a
974 TAM that should be processed by a TEE. When a device has more than
975 one TEE, one TEEP Broker per TEE could be present in the REE. A TEEP
976 Broker interacts with a TEEP Agent inside a TEE.
978 A TAM message may indicate the target TEE where a TA should be
979 installed. A compliant TEEP protocol should include a target TEE
980 identifier for a TEEP Broker when multiple TEEs are present.
982 The Broker relays the response messages generated from a TEEP Agent
983 in a TEE to the TAM.
985 The Broker only needs to return a (transport) error message if the
986 TEE is not reachable for some reason. Other errors are represented
987 as response messages returned from the TEE which will then be passed
988 to the TAM.
990 6.2. TEEP Broker Implementation Consideration
992 TEEP Broker implementers should consider methods of distribution,
993 scope and concurrency on devices and runtime options. Several non-
994 exhaustive options are discussed below.
996 6.2.1. TEEP Broker APIs
998 The following conceptual APIs exist from a TEEP Broker to a TEEP
999 Agent:
1001 1. RequestTA: A notification from an REE application (e.g., an
1002 installer, or an Untrusted Application) that it depends on a
1003 given TA, which may or may not already be installed in the TEE.
1005 2. ProcessTeepMessage: A message arriving from the network, to be
1006 delivered to the TEEP Agent for processing.
1008 3. RequestPolicyCheck: A hint (e.g., based on a timer) that the TEEP
1009 Agent may wish to contact the TAM for any changes, without the
1010 device itself needing any particular change.
1012 4. ProcessError: A notification that the TEEP Broker could not
1013 deliver an outbound TEEP message to a TAM.
1015 For comparison, similar APIs may exist on the TAM side, where a
1016 Broker may or may not exist, depending on whether the TAM uses a TEE
1017 or not:
1019 1. ProcessConnect: A notification that an incoming TEEP session is
1020 being requested by a TEEP Agent.
1022 2. ProcessTeepMessage: A message arriving from the network, to be
1023 delivered to the TAM for processing.
1025 For further discussion on these APIs, see
1026 [I-D.ietf-teep-otrp-over-http].
1028 6.2.2. TEEP Broker Distribution
1030 The Broker installation is commonly carried out at OEM time. A user
1031 can dynamically download and install a Broker on-demand.
1033 7. Attestation
1035 Attestation is the process through which one entity (an Attester)
1036 presents "evidence", in the form of a series of claims, to another
1037 entity (a Verifier), and provides sufficient proof that the claims
1038 are true. Different Verifiers may have different standards for
1039 attestation proofs and not all attestations are acceptable to every
1040 verifier. A third entity (a Relying Party) can then use "attestation
1041 results", in the form of another series of claims, from a Verifier to
1042 make authorization decisions. (See [I-D.ietf-rats-architecture] for
1043 more discussion.)
1045 In TEEP, as depicted in Figure 5, the primary purpose of an
1046 attestation is to allow a device (the Attester) to prove to a TAM
1047 (the Relying Party) that a TEE in the device has particular
1048 properties, was built by a particular manufacturer, and/or is
1049 executing a particular TA. Other claims are possible; TEEP does not
1050 limit the claims that may appear in evidence or attestation results,
1051 but defines a minimal set of attestation result claims required for
1052 TEEP to operate properly. Extensions to these claims are possible.
1053 Other standards or groups may define the format and semantics of
1054 extended claims.
1056 +----------------+
1057 | Device | +----------+
1058 | +------------+ | Evidence | TAM | Evidence +----------+
1059 | | TEE |------------->| (Relying |-------------->| Verifier |
1060 | | (Attester) | | | Party) |<--------------| |
1061 | +------------+ | +----------+ Attestation +----------+
1062 +----------------+ Result
1064 Figure 5: TEEP Attestation Roles
1066 As of the writing of this specification, device and TEE attestations
1067 have not been standardized across the market. Different devices,
1068 manufacturers, and TEEs support different attestation protocols. In
1069 order for TEEP to be inclusive, it is agnostic to the format of
1070 evidence, allowing proprietary or standardized formats to be used
1071 between a TEE and a verifier (which may or may not be colocated in
1072 the TAM). However, it should be recognized that not all Verifiers
1073 may be able to process all proprietary forms of attestation evidence.
1074 Similarly, the TEEP protocol is agnostic as to the format of
1075 attestation results, and the protocol (if any) used between the TAM
1076 and a verifier, as long as they convey at least the required set of
1077 claims in some format. Note that the respective attestation
1078 algorithms are not defined in the TEEP protocol itself; see
1079 [I-D.ietf-rats-architecture] and [I-D.ietf-teep-protocol] for more
1080 discussion.
1082 There are a number of considerations that need to be considered when
1083 appraising evidence provided by a TEE, including:
1085 - What security measures a manufacturer takes when provisioning keys
1086 into devices/TEEs;
1088 - What hardware and software components have access to the
1089 attestation keys of the TEE;
1091 - The source or local verification of claims within an attestation
1092 prior to a TEE signing a set of claims;
1094 - The level of protection afforded to attestation keys against
1095 exfiltration, modification, and side channel attacks;
1097 - The limitations of use applied to TEE attestation keys;
1099 - The processes in place to discover or detect TEE breeches; and
1101 - The revocation and recovery process of TEE attestation keys.
1103 Some TAMs may require additional claims in order to properly
1104 authorize a device or TEE. The specific format for these additional
1105 claims are outside the scope of this specification, but the TEEP
1106 protocol allows these additional claims to be included in the
1107 attestation messages.
1109 For more discussion of the attestation and appraisal process, see the
1110 RATS Architecture [I-D.ietf-rats-architecture].
1112 7.1. Information Required in TEEP Claims
1114 - Device Identifying Info: TEEP attestations may need to uniquely
1115 identify a device to the TAM and TA developer. Unique device
1116 identification allows the TAM to provide services to the device,
1117 such as managing installed TAs, and providing subscriptions to
1118 services, and locating device-specific keying material to
1119 communicate with or authenticate the device. In some use cases it
1120 may be sufficient to identify only the class of the device. The
1121 security and privacy requirements regarding device identification
1122 will vary with the type of TA provisioned to the TEE.
1124 - TEE Identifying info: The type of TEE that generated this
1125 attestation must be identified, including version identification
1126 information such as the hardware, firmware, and software version
1127 of the TEE, as applicable by the TEE type. TEE manufacturer
1128 information for the TEE is required in order to disambiguate the
1129 same TEE type created by different manufacturers and address
1130 considerations around manufacturer provisioning, keying and
1131 support for the TEE.
1133 - Freshness Proof: A claim that includes freshness information must
1134 be included, such as a nonce or timestamp.
1136 - Requested Components: A list of zero or more components (TAs or
1137 other dependencies needed by a TEE) that are requested by some
1138 depending app, but which are not currently installed in the TEE.
1140 8. Algorithm and Attestation Agility
1142 RFC 7696 [RFC7696] outlines the requirements to migrate from one
1143 mandatory-to-implement algorithm suite to another over time. This
1144 feature is also known as crypto agility. Protocol evolution is
1145 greatly simplified when crypto agility is considered during the
1146 design of the protocol. In the case of the TEEP protocol the diverse
1147 range of use cases, from trusted app updates for smart phones and
1148 tablets to updates of code on higher-end IoT devices, creates the
1149 need for different mandatory-to-implement algorithms already from the
1150 start.
1152 Crypto agility in TEEP concerns the use of symmetric as well as
1153 asymmetric algorithms. In the context of TEEP symmetric algorithms
1154 are used for encryption of TA binaries and personalization data
1155 whereas the asymmetric algorithms are mostly used for signing
1156 messages.
1158 In addition to the use of cryptographic algorithms in TEEP, there is
1159 also the need to make use of different attestation technologies. A
1160 device must provide techniques to inform a TAM about the attestation
1161 technology it supports. For many deployment cases it is more likely
1162 for the TAM to support one or more attestation techniques whereas the
1163 device may only support one.
1165 9. Security Considerations
1167 9.1. Broker Trust Model
1169 The architecture enables the TAM to communicate, via a TEEP Broker,
1170 with the device's TEE to manage TAs. Since the TEEP Broker runs in a
1171 potentially vulnerable REE, the TEEP Broker could, however, be (or be
1172 infected by) malware. As such, all TAM messages are signed and
1173 sensitive data is encrypted such that the TEEP Broker cannot modify
1174 or capture sensitive data, but the TEEP Broker can still conduct DoS
1175 attacks as discussed in Section 9.3.
1177 A TEEP Agent in a TEE is responsible for protecting against potential
1178 attacks from a compromised TEEP Broker or rogue malware in the REE.
1179 A rogue TEEP Broker might send corrupted data to the TEEP Agent, or
1180 launch a DoS attack by sending a flood of TEEP protocol requests.
1181 The TEEP Agent validates the signature of each TEEP protocol request
1182 and checks the signing certificate against its Trust Anchors. To
1183 mitigate DoS attacks, it might also add some protection scheme such
1184 as a threshold on repeated requests or number of TAs that can be
1185 installed.
1187 9.2. Data Protection at TAM and TEE
1189 The TEE implementation provides protection of data on the device. It
1190 is the responsibility of the TAM to protect data on its servers.
1192 9.3. Compromised REE
1194 It is possible that the REE of a device is compromised. If the REE
1195 is compromised, several DoS attacks may be launched. The compromised
1196 REE may terminate the TEEP Broker such that TEEP transactions cannot
1197 reach the TEE, or might drop or delay messages between a TAM and a
1198 TEEP Agent. However, while a DoS attack cannot be prevented, the REE
1199 cannot access anything in the TEE if it is implemented correctly.
1200 Some TEEs may have some watchdog scheme to observe REE state and
1201 mitigate DoS attacks against it but most TEEs don't have such a
1202 capability.
1204 In some other scenarios, the compromised REE may ask a TEEP Broker to
1205 make repeated requests to a TEEP Agent in a TEE to install or
1206 uninstall a TA. A TA installation or uninstallation request
1207 constructed by the TEEP Broker or REE will be rejected by the TEEP
1208 Agent because the request won't have the correct signature from a TAM
1209 to pass the request signature validation.
1211 This can become a DoS attack by exhausting resources in a TEE with
1212 repeated requests. In general, a DoS attack threat exists when the
1213 REE is compromised, and a DoS attack can happen to other resources.
1214 The TEEP architecture doesn't change this.
1216 A compromised REE might also request initiating the full flow of
1217 installation of TAs that are not necessary. It may also repeat a
1218 prior legitimate TA installation request. A TEEP Agent
1219 implementation is responsible for ensuring that it can recognize and
1220 decline such repeated requests. It is also responsible for
1221 protecting the resource usage allocated for TA management.
1223 9.4. Compromised CA
1225 A root CA for TAM certificates might get compromised. A Trust Anchor
1226 other than a root CA certificate may also be compromised. Some TEE
1227 Trust Anchor update mechanism is expected from device OEMs.
1229 TEEs are responsible for validating certificate revocation about a
1230 TAM certificate chain, including the TAM certificate and the
1231 intermediate CA certificates up to the root certificate. This will
1232 detect a compromised TAM certificate and also any compromised
1233 intermediate CA certificate.
1235 If the root CA of some TEE device certificates is compromised, these
1236 devices might be rejected by a TAM, which is a decision of the TAM
1237 implementation and policy choice. TAMs are responsible for
1238 validating any intermediate CA for TEE device certificates.
1240 9.5. Compromised TAM
1242 Device TEEs are responsible for validating the supplied TAM
1243 certificates to determine that the TAM is trustworthy.
1245 9.6. Malicious TA Removal
1247 It is possible that a rogue developer distributes a malicious
1248 Untrusted Application and intends to get a malicious TA installed.
1249 It's the responsibility of the TAM to not install malicious trusted
1250 apps in the first place. The TEEP architecture allows a TEEP Agent
1251 to decide which TAMs it trusts via Trust Anchors, and delegates the
1252 TA authenticity check to the TAMs it trusts.
1254 It may happen that a TA was previously considered trustworthy but is
1255 later found to be buggy or compromised. In this case, the TAM can
1256 initiate the removal of the TA by notifying devices to remove the TA
1257 (and potentially the REE or device owner to remove any Untrusted
1258 Application that depend on the TA). If the TAM does not currently
1259 have a connection to the TEEP Agent on a device, such a notification
1260 would occur the next time connectivity does exist. That is, to
1261 recover, the TEEP Agent must be able to reach out to the TAM, for
1262 example whenever the RequestPolicyCheck API (Section 6.2.1) is
1263 invoked by a timer or other event.
1265 Furthermore the policy in the Verifier in an attestation process can
1266 be updated so that any evidence that includes the malicious TA would
1267 result in an attestation failure. There is, however, a time window
1268 during which a malicious TA might be able to operate successfully,
1269 which is the validity time of the previous attestation result. For
1270 example, if the Verifier in Figure 5 is updated to treat a previously
1271 valid TA as no longer trustworthy, any attestation result it
1272 previously generated saying that the TA is valid will continue to be
1273 used until the attestation result expires. As such, the TAM's
1274 Verifier should take into account the acceptable time window when
1275 generating attestation results. See [I-D.ietf-rats-architecture] for
1276 further discussion.
1278 9.7. Certificate Expiry and Renewal
1280 TEE device certificates are expected to be long lived, longer than
1281 the lifetime of a device. A TAM certificate usually has a moderate
1282 lifetime of 2 to 5 years. A TAM should get renewed or rekeyed
1283 certificates. The root CA certificates for a TAM, which are embedded
1284 into the Trust Anchor store in a device, should have long lifetimes
1285 that don't require device Trust Anchor update. On the other hand, it
1286 is imperative that OEMs or device providers plan for support of Trust
1287 Anchor update in their shipped devices.
1289 For those cases where TEE devices are given certificates for which no
1290 good expiration date can be assigned the recommendations in
1291 Section 4.1.2.5 of RFC 5280 [RFC5280] are applicable.
1293 9.8. Keeping Secrets from the TAM
1295 In some scenarios, it is desirable to protect the TA binary or
1296 configuration from being disclosed to the TAM that distributes them.
1297 In such a scenario, the files can be encrypted end-to-end between a
1298 TA developer and a TEE. However, there must be some means of
1299 provisioning the decryption key into the TEE and/or some means of the
1300 TA developer securely learning a public key of the TEE that it can
1301 use to encrypt. One way to do this is for the TA developer to run
1302 its own TAM so that it can distribute the decryption key via the TEEP
1303 protocol, and the key file can be a dependency in the manifest of the
1304 encrypted TA. Thus, the TEEP Agent would look at the TA manifest,
1305 determine there is a dependency with a TAM URI of the TA developer's
1306 TAM. The Agent would then install the dependency, and then continue
1307 with the TA installation steps, including decrypting the TA binary
1308 with the relevant key.
1310 10. IANA Considerations
1312 This document does not require actions by IANA.
1314 11. Contributors
1316 - Andrew Atyeo, Intercede (andrew.atyeo@intercede.com)
1318 - Liu Dapeng, Alibaba Group (maxpassion@gmail.com)
1320 12. Acknowledgements
1322 We would like to thank Nick Cook, Minho Yoo, Brian Witten, Tyler Kim,
1323 Alin Mutu, Juergen Schoenwaelder, Nicolae Paladi, Sorin Faibish, Ned
1324 Smith, Russ Housley, Jeremy O'Donoghue, and Anders Rundgren for their
1325 feedback.
1327 13. Informative References
1329 [GPTEE] GlobalPlatform, "GlobalPlatform Device Technology: TEE
1330 System Architecture, v1.1", GlobalPlatform GPD_SPE_009,
1331 January 2017, .
1334 [I-D.ietf-rats-architecture]
1335 Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
1336 W. Pan, "Remote Attestation Procedures Architecture",
1337 draft-ietf-rats-architecture-04 (work in progress), May
1338 2020.
1340 [I-D.ietf-suit-manifest]
1341 Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
1342 "A Concise Binary Object Representation (CBOR)-based
1343 Serialization Format for the Software Updates for Internet
1344 of Things (SUIT) Manifest", draft-ietf-suit-manifest-07
1345 (work in progress), June 2020.
1347 [I-D.ietf-teep-otrp-over-http]
1348 Thaler, D., "HTTP Transport for Trusted Execution
1349 Environment Provisioning: Agent-to- TAM Communication",
1350 draft-ietf-teep-otrp-over-http-06 (work in progress),
1351 April 2020.
1353 [I-D.ietf-teep-protocol]
1354 Tschofenig, H., Pei, M., Wheeler, D., Thaler, D., and A.
1355 Tsukamoto, "Trusted Execution Environment Provisioning
1356 (TEEP) Protocol", draft-ietf-teep-protocol-02 (work in
1357 progress), April 2020.
1359 [RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
1360 FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
1361 .
1363 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
1364 Housley, R., and W. Polk, "Internet X.509 Public Key
1365 Infrastructure Certificate and Certificate Revocation List
1366 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
1367 .
1369 [RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
1370 Requirements", RFC 6024, DOI 10.17487/RFC6024, October
1371 2010, .
1373 [RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
1374 Agility and Selecting Mandatory-to-Implement Algorithms",
1375 BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
1376 .
1378 [SGX] Intel, "Intel(R) Software Guard Extensions (Intel (R)
1379 SGX)", n.d., .
1383 [TrustZone]
1384 Arm, "Arm TrustZone Technology", n.d.,
1385 .
1388 Authors' Addresses
1390 Mingliang Pei
1391 Symantec
1393 EMail: mingliang_pei@symantec.com
1395 Hannes Tschofenig
1396 Arm Limited
1398 EMail: hannes.tschofenig@arm.com
1400 Dave Thaler
1401 Microsoft
1403 EMail: dthaler@microsoft.com
1404 David Wheeler
1405 Intel
1407 EMail: david.m.wheeler@intel.com