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2 TEEP M. Pei
3 Internet-Draft Symantec
4 Intended status: Informational H. Tschofenig
5 Expires: October 6, 2020 Arm Limited
6 D. Thaler
7 Microsoft
8 D. Wheeler
9 Intel
10 April 04, 2020
12 Trusted Execution Environment Provisioning (TEEP) Architecture
13 draft-ietf-teep-architecture-08
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 October 6, 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 . . . . . . . . . . . . . . . . . . . . . 7
75 3.3. Internet of Things . . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . 10
80 4.3. Multiple TAMs and Relationship to TAs . . . . . . . . . . 12
81 4.4. Untrusted Apps, Trusted Apps, and Personalization Data . 14
82 4.4.1. Examples of Application Delivery Mechanisms in
83 Existing TEEs . . . . . . . . . . . . . . . . . . . . 15
84 4.5. Entity Relations . . . . . . . . . . . . . . . . . . . . 16
85 5. Keys and Certificate Types . . . . . . . . . . . . . . . . . 17
86 5.1. Trust Anchors in a TEEP Agent . . . . . . . . . . . . . . 19
87 5.2. Trust Anchors in a TEE . . . . . . . . . . . . . . . . . 19
88 5.3. Trust Anchors in a TAM . . . . . . . . . . . . . . . . . 19
89 5.4. Scalability . . . . . . . . . . . . . . . . . . . . . . . 19
90 5.5. Message Security . . . . . . . . . . . . . . . . . . . . 20
91 6. TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . . 20
92 6.1. Role of the TEEP Broker . . . . . . . . . . . . . . . . . 20
93 6.2. TEEP Broker Implementation Consideration . . . . . . . . 21
94 6.2.1. TEEP Broker APIs . . . . . . . . . . . . . . . . . . 21
95 6.2.2. TEEP Broker Distribution . . . . . . . . . . . . . . 22
96 7. Attestation . . . . . . . . . . . . . . . . . . . . . . . . . 22
97 7.1. Information Required in TEEP Claims . . . . . . . . . . . 24
98 8. Algorithm and Attestation Agility . . . . . . . . . . . . . . 24
99 9. Security Considerations . . . . . . . . . . . . . . . . . . . 25
100 9.1. Broker Trust Model . . . . . . . . . . . . . . . . . . . 25
101 9.2. Data Protection at TAM and TEE . . . . . . . . . . . . . 25
102 9.3. Compromised REE . . . . . . . . . . . . . . . . . . . . . 25
103 9.4. Compromised CA . . . . . . . . . . . . . . . . . . . . . 26
104 9.5. Compromised TAM . . . . . . . . . . . . . . . . . . . . . 26
105 9.6. Malicious TA Removal . . . . . . . . . . . . . . . . . . 26
106 9.7. Certificate Renewal . . . . . . . . . . . . . . . . . . . 27
107 9.8. Keeping Secrets from the TAM . . . . . . . . . . . . . . 27
108 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
109 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 28
110 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
111 13. Informative References . . . . . . . . . . . . . . . . . . . 28
112 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
114 1. Introduction
116 Applications executing in a device are exposed to many different
117 attacks intended to compromise the execution of the application or
118 reveal the data upon which those applications are operating. These
119 attacks increase with the number of other applications on the device,
120 with such other applications coming from potentially untrustworthy
121 sources. The potential for attacks further increases with the
122 complexity of features and applications on devices, and the
123 unintended interactions among those features and applications. The
124 danger of attacks on a system increases as the sensitivity of the
125 applications or data on the device increases. As an example,
126 exposure of emails from a mail client is likely to be of concern to
127 its owner, but a compromise of a banking application raises even
128 greater concerns.
130 The Trusted Execution Environment (TEE) concept is designed to
131 execute applications in a protected environment that enforces that
132 any code within that environment cannot be tampered with, and that
133 any data used by such code cannot be read or tampered with by any
134 code outside that environment, including by a commodity operating
135 system (if present).
137 This separation reduces the possibility of a successful attack on
138 application components and the data contained inside the TEE.
139 Typically, application components are chosen to execute inside a TEE
140 because those application components perform security sensitive
141 operations or operate on sensitive data. An application component
142 running inside a TEE is referred to as a Trusted Application (TA),
143 while an application running outside any TEE is referred to as an
144 Untrusted Application.
146 TEEs use hardware enforcement combined with software protection to
147 secure TAs and its data. TEEs typically offer a more limited set of
148 services to TAs than is normally available to Untrusted Applications.
150 Not all TEEs are the same, however, and different vendors may have
151 different implementations of TEEs with different security properties,
152 different features, and different control mechanisms to operate on
153 TAs. Some vendors may themselves market multiple different TEEs with
154 different properties attuned to different markets. A device vendor
155 may integrate one or more TEEs into their devices depending on market
156 needs.
158 To simplify the life of TA developers interacting with TAs in a TEE,
159 an interoperable protocol for managing TAs running in different TEEs
160 of various devices is needed. In this TEE ecosystem, there often
161 arises a need for an external trusted party to verify the identity,
162 claims, and rights of TA developers, devices, and their TEEs. This
163 trusted third party is the Trusted Application Manager (TAM).
165 The Trusted Execution Environment Provisioning (TEEP) protocol
166 addresses the following problems:
168 - An installer of an Untrusted Application that depends on a given
169 TA wants to request installation of that TA in the device's TEE so
170 that the Untrusted Application can complete, but the TEE needs to
171 verify whether such a TA is actually authorized to run in the TEE
172 and consume potentially scarce TEE resources.
174 - A TA developer providing a TA whose code itself is considered
175 confidential wants to determine security-relevant information of a
176 device before allowing their TA to be provisioned to the TEE
177 within the device. An example is the verification of the type of
178 TEE included in a device and that it is capable of providing the
179 security protections required.
181 - A TEE in a device wants to determine whether an entity that wants
182 to manage a TA in the device is authorized to manage TAs in the
183 TEE, and what TAs the entity is permitted to manage.
185 - A TAM (e.g., operated by a device administrator) wants to
186 determine if a TA exists (is installed) on a device (in the TEE),
187 and if not, install the TA in the TEE.
189 - A TAM wants to check whether a TA in a device's TEE is the most
190 up-to-date version, and if not, update the TA in the TEE.
192 - A TA developer wants to remove a confidential TA from a device's
193 TEE if the TA developer is no longer offering such TAs or the TAs
194 are being revoked from a particular user (or device). For
195 example, if a subscription or contract for a particular service
196 has expired, or a payment by the user has not been completed or
197 has been rescinded.
199 - A TA developer wants to define the relationship between
200 cooperating TAs under the TA developer's control, and specify
201 whether the TAs can communicate, share data, and/or share key
202 material.
204 Note: The TA developer requires the help of a TAM to provision the
205 Trusted Applications to remote devices and the TEEP protocol
206 exchanges messages between a TAM and a TEEP Agent via a TEEP Broker.
208 2. Terminology
210 The following terms are used:
212 - Device: A physical piece of hardware that hosts one or more TEEs,
213 often along with a Rich Execution Environment. A device contains
214 a default list of Trust Anchors that identify entities (e.g.,
215 TAMs) that are trusted by the device. This list is normally set
216 by the device manufacturer, and may be governed by the device's
217 network carrier when it is a mobile device. The list of Trust
218 Anchors is normally modifiable by the device's owner or Device
219 Administrator. However the device manufacturer or network carrier
220 (in the mobile device case) may restrict some modifications, for
221 example, by not allowing the manufacturer or carrier's Trust
222 Anchor to be removed or disabled.
224 - Device Administrator: An entity that is responsible for
225 administration of a device, which could be the device owner. A
226 Device Administrator has privileges on the device to install and
227 remove Untrusted Applications and TAs, approve or reject Trust
228 Anchors, and approve or reject TA developers, among possibly other
229 privileges on the device. A Device Administrator can manage the
230 list of allowed TAMs by modifying the list of Trust Anchors on the
231 device. Although a Device Administrator may have privileges and
232 device-specific controls to locally administer a device, the
233 Device Administrator may choose to remotely administer a device
234 through a TAM.
236 - Device Owner: A device is always owned by someone. In some cases,
237 it is common for the (primary) device user to also own the device,
238 making the device user/owner also the Device Administrator. In
239 enterprise environments it is more common for the enterprise to
240 own the device, and any device user has no or limited
241 administration rights. In this case, the enterprise appoints a
242 Device Administrator that is not the device owner.
244 - Device User: A human being that uses a device. Many devices have
245 a single device user. Some devices have a primary device user
246 with other human beings as secondary device users (e.g., parent
247 allowing children to use their tablet or laptop). Other devices
248 are not used by a human being and hence have no device user.
249 Relates to Device Owner and Device Administrator.
251 - Rich Execution Environment (REE): An environment that is provided
252 and governed by a typical OS (e.g., Linux, Windows, Android, iOS),
253 potentially in conjunction with other supporting operating systems
254 and hypervisors; it is outside of any TEE. This environment and
255 applications running on it are considered untrusted (or more
256 precisely, less trusted than a TEE).
258 - Trust Anchor: As defined in [RFC6024] and
259 [I-D.ietf-suit-manifest], "A trust anchor represents an
260 authoritative entity via a public key and associated data. The
261 public key is used to verify digital signatures, and the
262 associated data is used to constrain the types of information for
263 which the trust anchor is authoritative." The Trust Anchor may be
264 a certificate or it may be a raw public key along with additional
265 data if necessary such as its public key algorithm and parameters.
267 - Trust Anchor Store: As defined in [RFC6024], "A trust anchor store
268 is a set of one or more trust anchors stored in a device. A
269 device may have more than one trust anchor store, each of which
270 may be used by one or more applications." As noted in
271 [I-D.ietf-suit-manifest], a trust anchor store must resist
272 modification against unauthorized insertion, deletion, and
273 modification.
275 - Trusted Application (TA): An application component that runs in a
276 TEE.
278 - Trusted Application (TA) Developer: An entity that wishes to
279 provide functionality on devices that requires the use of one or
280 more Trusted Applications.
282 - Trusted Application Manager (TAM): An entity that manages Trusted
283 Applications (TAs) running in TEEs of various devices.
285 - Trusted Execution Environment (TEE): An execution environment that
286 enforces that only authorized code can execute within the TEE, and
287 data used by that code cannot be read or tampered with by code
288 outside the TEE. A TEE also generally has a device unique
289 credential that cannot be cloned. There are multiple technologies
290 that can be used to implement a TEE, and the level of security
291 achieved varies accordingly. In addition, TEEs typically use an
292 isolation mechanism between Trusted Applications to ensure that
293 one TA cannot read, modify or delete the data and code of another
294 TA.
296 - Untrusted Application: An application running in a Rich Execution
297 Environment.
299 3. Use Cases
301 3.1. Payment
303 A payment application in a mobile device requires high security and
304 trust about the hosting device. Payments initiated from a mobile
305 device can use a Trusted Application to provide strong identification
306 and proof of transaction.
308 For a mobile payment application, some biometric identification
309 information could also be stored in a TEE. The mobile payment
310 application can use such information for unlocking the phone and for
311 local identification of the user.
313 A trusted user interface (UI) may be used in a mobile device to
314 prevent malicious software from stealing sensitive user input data.
315 Such an implementation often relies on a TEE for providing access to
316 peripherals, such as PIN input.
318 3.2. Authentication
320 For better security of authentication, a device may store its keys
321 and cryptographic libraries inside a TEE limiting access to
322 cryptographic functions via a well-defined interface and thereby
323 reducing access to keying material.
325 3.3. Internet of Things
327 The Internet of Things (IoT) has been posing threats to critical
328 infrastructure because of weak security in devices. It is desirable
329 that IoT devices can prevent malware from manipulating actuators
330 (e.g., unlocking a door), or stealing or modifying sensitive data,
331 such as authentication credentials in the device. A TEE can be the
332 best way to implement such IoT security functions.
334 3.4. Confidential Cloud Computing
336 A tenant can store sensitive data in a TEE in a cloud computing
337 server such that only the tenant can access the data, preventing the
338 cloud hosting provider from accessing the data. A tenant can run TAs
339 inside a server TEE for secure operation and enhanced data security.
340 This provides benefits not only to tenants with better data security
341 but also to cloud hosting providers for reduced liability and
342 increased cloud adoption.
344 4. Architecture
346 4.1. System Components
348 Figure 1 shows the main components in a typical device with an REE.
349 Full descriptions of components not previously defined are provided
350 below. Interactions of all components are further explained in the
351 following paragraphs.
353 +-------------------------------------------+
354 | Device |
355 | +--------+ | TA Developer
356 | +-------------+ | |-----------+ |
357 | | TEE-1 | | TEEP |---------+ | |
358 | | +--------+ | +----| Broker | | | | +--------+ |
359 | | | TEEP | | | | |<---+ | | +->| |<-+
360 | | | Agent |<----+ | | | | | +-| TAM-1 |
361 | | +--------+ | | |<-+ | | +->| | |<-+
362 | | | +--------+ | | | | +--------+ |
363 | | +---+ +---+ | | | | | TAM-2 | |
364 | +-->|TA1| |TA2| | +-------+ | | | +--------+ |
365 | | | | | | |<---------| App-2 |--+ | | |
366 | | | +---+ +---+ | +-------+ | | | Device Administrator
367 | | +-------------+ | App-1 | | | |
368 | | | | | | |
369 | +--------------------| |---+ | |
370 | | |--------+ |
371 | +-------+ |
372 +-------------------------------------------+
374 Figure 1: Notional Architecture of TEEP
376 - TA developers and Device Administrators utilize the services of a
377 TAM to manage TAs on devices. TA developers do not directly
378 interact with devices. Device Administators may elect to use a
379 TAM for remote administration of TAs instead of managing each
380 device directly.
382 - Trusted Application Manager (TAM): A TAM is responsible for
383 performing lifecycle management activity on TAs on behalf of TA
384 developers and Device Administrators. This includes creation and
385 deletion of TAs, and may include, for example, over-the-air
386 updates to keep TAs up-to-date and clean up when a version should
387 be removed. TAMs may provide services that make it easier for TA
388 developers or Device Administators to use the TAM's service to
389 manage multiple devices, although that is not required of a TAM.
391 The TAM performs its management of TAs on the device through
392 interactions with a device's TEEP Broker, which relays messages
393 between a TAM and a TEEP Agent running inside the TEE. As shown
394 in Figure 1, the TAM cannot directly contact a TEEP Agent, but
395 must wait for the TEEP Broker to contact the TAM requesting a
396 particular service. This architecture is intentional in order to
397 accommodate network and application firewalls that normally
398 protect user and enterprise devices from arbitrary connections
399 from external network entities.
401 A TAM may be publicly available for use by many TA developers, or
402 a TAM may be private, and accessible by only one or a limited
403 number of TA developers. It is expected that many manufacturers
404 and network carriers will run their own private TAM.
406 A TA developer or Device Administrator chooses a particular TAM
407 based on whether the TAM is trusted by a device or set of devices.
408 The TAM is trusted by a device if the TAM's public key is, or
409 chains up to, an authorized Trust Anchor in the device. A TA
410 developer or Device Administrator may run their own TAM, but the
411 devices they wish to manage must include this TAM's public key/
412 certificate, or a certificate it chains up to, in the Trust Anchor
413 list.
415 A TA developer or Device Administrator is free to utilize multiple
416 TAMs. This may be required for a TA developer to manage multiple
417 different types of devices from different manufacturers, or to
418 manage mobile devices on different network carriers, since the
419 Trust Anchor list on these different devices may contain different
420 TAMs. A Device Administrator may be able to add their own TAM's
421 public key or certificate to the Trust Anchor list on all their
422 devices, overcoming this limitation.
424 Any entity is free to operate a TAM. For a TAM to be successful,
425 it must have its public key or certificate installed in a device's
426 Trust Anchor list. A TAM may set up a relationship with device
427 manufacturers or network carriers to have them install the TAM's
428 keys in their device's Trust Anchor list. Alternatively, a TAM
429 may publish its certificate and allow Device Administrators to
430 install the TAM's certificate in their devices as an after-market-
431 action.
433 - TEEP Broker: A TEEP Broker is an application component running in
434 a Rich Execution Environment (REE) that enables the message
435 protocol exchange between a TAM and a TEE in a device. A TEEP
436 Broker does not process messages on behalf of a TEE, but merely is
437 responsible for relaying messages from the TAM to the TEE, and for
438 returning the TEE's responses to the TAM. In devices with no REE
439 (e.g., a microcontroller where all code runs in an environment
440 that meets the definition of a Trusted Execution Environment in
441 Section 2), the TEEP Broker would be absent and instead the TEEP
442 protocol transport would be implemented inside the TEE itself.
444 - TEEP Agent: The TEEP Agent is a processing module running inside a
445 TEE that receives TAM requests (typically relayed via a TEEP
446 Broker that runs in an REE). A TEEP Agent in the TEE may parse
447 requests or forward requests to other processing modules in a TEE,
448 which is up to a TEE provider's implementation. A response
449 message corresponding to a TAM request is sent back to the TAM,
450 again typically relayed via a TEEP Broker.
452 - Certification Authority (CA): Certificate-based credentials used
453 for authenticating a device, a TAM and a TA developer. A device
454 embeds a list of root certificates (Trust Anchors), from trusted
455 CAs that a TAM will be validated against. A TAM will remotely
456 attest a device by checking whether a device comes with a
457 certificate from a CA that the TAM trusts. The CAs do not need to
458 be the same; different CAs can be chosen by each TAM, and
459 different device CAs can be used by different device
460 manufacturers.
462 4.2. Multiple TEEs in a Device
464 Some devices might implement multiple TEEs. In these cases, there
465 might be one shared TEEP Broker that interacts with all the TEEs in
466 the device. However, some TEEs (for example, SGX) present themselves
467 as separate containers within memory without a controlling manager
468 within the TEE. As such, there might be multiple TEEP Brokers in the
469 Rich Execution Environment, where each TEEP Broker communicates with
470 one or more TEEs associated with it.
472 It is up to the Rich Execution Environment and the Untrusted
473 Applications how they select the correct TEEP Broker. Verification
474 that the correct TA has been reached then becomes a matter of
475 properly verifying TA attestations, which are unforgeable.
477 The multiple TEEP Broker approach is shown in the diagram below. For
478 brevity, TEEP Broker 2 is shown interacting with only one TAM and
479 Untrusted Application and only one TEE, but no such limitations are
480 intended to be implied in the architecture.
482 +-------------------------------------------+
483 | Device |
484 | | TA Developer
485 | +-------------+ | |
486 | | TEE-1 | | |
487 | | +-------+ | +--------+ | +--------+ |
488 | | | TEEP | | | TEEP |------------->| |<-+
489 | | | Agent |<----------| Broker | | | |
490 | | | 1 | | | 1 |---------+ | |
491 | | +-------+ | | | | | | |
492 | | | | |<---+ | | | |
493 | | +---+ +---+ | | | | | | +-| TAM-1 |
494 | | |TA1| |TA2| | | |<-+ | | +->| | |<-+
495 | +-->| | | |<---+ +--------+ | | | | +--------+ |
496 | | | +---+ +---+ | | | | | | TAM-2 | |
497 | | | | | +-------+ | | | +--------+ |
498 | | +-------------+ +-----| App-2 |--+ | | ^ |
499 | | +-------+ | | | | Device
500 | +--------------------| App-1 | | | | | Administrator
501 | +------| | | | | |
502 | +-----------|-+ | |---+ | | |
503 | | TEE-2 | | | |--------+ | |
504 | | +------+ | | | |------+ | |
505 | | | TEEP | | | +-------+ | | |
506 | | | Agent|<-----+ | | |
507 | | | 2 | | | | | | |
508 | | +------+ | | | | | |
509 | | | | | | | |
510 | | +---+ | | | | | |
511 | | |TA3|<----+ | | +----------+ | | |
512 | | | | | | | TEEP |<--+ | |
513 | | +---+ | +--| Broker | | |
514 | | | | 2 |----------------+
515 | +-------------+ +----------+ |
516 | |
517 +-------------------------------------------+
519 Figure 2: Notional Architecture of TEEP with multiple TEEs
521 In the diagram above, TEEP Broker 1 controls interactions with the
522 TAs in TEE-1, and TEEP Broker 2 controls interactions with the TAs in
523 TEE-2. This presents some challenges for a TAM in completely
524 managing the device, since a TAM may not interact with all the TEEP
525 Brokers on a particular platform. In addition, since TEEs may be
526 physically separated, with wholly different resources, there may be
527 no need for TEEP Brokers to share information on installed TAs or
528 resource usage.
530 4.3. Multiple TAMs and Relationship to TAs
532 As shown in Figure 2, a TEEP Broker provides communication between
533 one or more TEEP Agents and one or more TAMs. The selection of which
534 TAM to communicate with might be made with or without input from an
535 Untrusted Application, but is ultimately the decision of a TEEP
536 Agent.
538 A TEEP Agent is assumed to be able to determine, for any given TA,
539 whether that TA is installed (or minimally, is running) in a TEE with
540 which the TEEP Agent is associated.
542 Each TA is digitally signed, protecting its integrity, and linking
543 the TA back to the signer. The signer is usually the TA software
544 author, but in some cases might be another party that the TA software
545 author trusts, or a party to whom the code has been licensed (in
546 which case the same code might be signed by multiple licensees and
547 distributed as if it were different TAs).
549 A TA author or signer selects one or more TAMs through which to offer
550 their TA(s), and communicates the TA(s) to the TAM. In this
551 document, we use the term "TA developer" to refer to the entity that
552 selects a TAM and publishes a signed TA to it, independent of whether
553 the publishing entity is the TA software author or the signer or
554 both.
556 The TA developer chooses TAMs based upon the markets into which the
557 TAM can provide access. There may be TAMs that provide services to
558 specific types of devices, or device operating systems, or specific
559 geographical regions or network carriers. A TA developer may be
560 motivated to utilize multiple TAMs for its service in order to
561 maximize market penetration and availability on multiple types of
562 devices. This likely means that the same TA will be available
563 through multiple TAMs.
565 When the developer of an Untrusted Application that depends on a TA
566 publishes the Untrusted Application to an app store or other app
567 repository, the developer optionally binds the Untrusted Application
568 with a manifest that identifies what TAMs can be contacted for the
569 TA. In some situations, a TA may only be available via a single TAM
570 - this is likely the case for enterprise applications or TA
571 developers serving a closed community. For broad public apps, there
572 will likely be multiple TAMs in the manifest - one servicing one
573 brand of mobile device and another servicing a different
574 manufacturer, etc. Because different devices and different
575 manufacturers trust different TAMs, the manifest can include multiple
576 TAMs that support the required TA.
578 When a TEEP Broker receives a request from an Untrusted Application
579 to install a TA, a list of TAM URIs may be provided for that TA, and
580 the request is passed to the TEEP Agent. If the TEEP Agent decides
581 that the TA needs to be installed, the TEEP Agent selects a single
582 TAM URI that is consistent with the list of trusted TAMs provisioned
583 on the device, invokes the HTTP transport for TEEP to connect to the
584 TAM URI, and begins a TEEP protocol exchange. When the TEEP Agent
585 subsequently receives the TA to install and the TA's manifest
586 indicates dependencies on any other trusted components, each
587 dependency can include a list of TAM URIs for the relevant
588 dependency. If such dependencies exist that are prerequisites to
589 install the TA, then the TEEP Agent recursively follows the same
590 procedure for each dependency that needs to be installed or updated,
591 including selecting a TAM URI that is consistent with the list of
592 trusted TAMs provisioned on the device, and beginning a TEEP
593 exchange. If multiple TAM URIs are considered trusted, only one
594 needs to be contacted and they can be attempted in some order until
595 one responds.
597 Separate from the Untrusted Application's manifest, this framework
598 relies on the use of the manifest format in [I-D.ietf-suit-manifest]
599 for expressing how to install a TA, as well as any dependencies on
600 other TEE components and versions. That is, dependencies from TAs on
601 other TEE components can be expressed in a SUIT manifest, including
602 dependencies on any other TAs, or trusted OS code (if any), or
603 trusted firmware. Installation steps can also be expressed in a SUIT
604 manifest.
606 For example, TEEs compliant with GlobalPlatform may have a notion of
607 a "security domain" (which is a grouping of one or more TAs installed
608 on a device, that can share information within such a group) that
609 must be created and into which one or more TAs can then be installed.
610 It is thus up to the SUIT manifest to express a dependency on having
611 such a security domain existing or being created first, as
612 appropriate.
614 Updating a TA may cause compatibility issues with any Untrusted
615 Applications or other components that depend on the updated TA, just
616 like updating the OS or a shared library could impact an Untrusted
617 Application. Thus, an implementation needs to take into account such
618 issues.
620 4.4. Untrusted Apps, Trusted Apps, and Personalization Data
622 In TEEP, there is an explicit relationship and dependence between an
623 Untrusted Application in a REE and one or more TAs in a TEE, as shown
624 in Figure 2. For most purposes, an Untrusted Application that uses
625 one or more TAs in a TEE appears no different from any other
626 Untrusted Application in the REE. However, the way the Untrusted
627 Application and its corresponding TAs are packaged, delivered, and
628 installed on the device can vary. The variations depend on whether
629 the Untrusted Application and TA are bundled together or are provided
630 separately, and this has implications to the management of the TAs in
631 a TEE. In addition to the Untrusted Application and TA(s), the TA(s)
632 and/or TEE may require some additional data to personalize the TA to
633 the TA developer or the device or a user. This personalization data
634 is dependent on the TEE, the TA, and the TA developer; an example of
635 personalization data might be a secret symmetric key used by the TA
636 to communicate with the TA developer. Implementations must support
637 encryption of personalization data to preserve the confidentiality of
638 potentially sensitive data contained within it. Other than this
639 requirement to support confidentiality, the TEEP architecture places
640 no limitations or requirements on the personalization data.
642 There are three possible cases for bundling of an Untrusted
643 Application, TA(s), and personalization data:
645 1. The Untrusted Application, TA(s), and personalization data are
646 all bundled together in a single package by a TA developer and
647 provided to the TEEP Broker through the TAM.
649 2. The Untrusted Application and the TA(s) are bundled together in a
650 single package, which a TAM or a publicly accessible app store
651 maintains, and the personalization data is separately provided by
652 the TA developer's TAM.
654 3. All components are independent. The Untrusted Application is
655 installed through some independent or device-specific mechanism,
656 and the TAM provides the TA and personalization data from the TA
657 developer. Delivery of the TA and personalization data may be
658 combined or separate.
660 The TEEP protocol treats each TA, any dependencies the TA has, and
661 personalization data as separate components with separate
662 installation steps that are expressed in SUIT manifests, and a SUIT
663 manifest might contain or reference multiple binaries (see
664 [I-D.ietf-suit-manifest] for more details). The TEEP Agent is
665 responsible for handling any installation steps that need to be
666 performed inside the TEE, such as decryption of private TA binaries
667 or personalization data.
669 4.4.1. Examples of Application Delivery Mechanisms in Existing TEEs
671 In order to better understand these cases, it is helpful to review
672 actual implementations of TEEs and their application delivery
673 mechanisms.
675 In Intel Software Guard Extensions (SGX), the Untrusted Application
676 and TA are typically bundled into the same package (Case 2). The TA
677 exists in the package as a shared library (.so or .dll). The
678 Untrusted Application loads the TA into an SGX enclave when the
679 Untrusted Application needs the TA. This organization makes it easy
680 to maintain compatibility between the Untrusted Application and the
681 TA, since they are updated together. It is entirely possible to
682 create an Untrusted Application that loads an external TA into an SGX
683 enclave, and use that TA (Case 3). In this case, the Untrusted
684 Application would require a reference to an external file or download
685 such a file dynamically, place the contents of the file into memory,
686 and load that as a TA. Obviously, such file or downloaded content
687 must be properly formatted and signed for it to be accepted by the
688 SGX TEE. In SGX, for Case 2 and Case 3, the personalization data is
689 normally loaded into the SGX enclave (the TA) after the TA has
690 started. Although Case 1 is possible with SGX, there are no
691 instances of this known to be in use at this time, since such a
692 construction would require a special installation program and SGX TA
693 to receive the encrypted binary, decrypt it, separate it into the
694 three different elements, and then install all three. This
695 installation is complex because the Untrusted Application decrypted
696 inside the TEE must be passed out of the TEE to an installer in the
697 REE which would install the Untrusted Application; this assumes that
698 the Untrusted Application package includes the TA code also, since
699 otherwise there is a significant problem in getting the SGX enclave
700 code (the TA) from the TEE, through the installer, and into the
701 Untrusted Application in a trusted fashion. Finally, the
702 personalization data would need to be sent out of the TEE (encrypted
703 in an SGX enclave-to-enclave manner) to the REE's installation app,
704 which would pass this data to the installed Untrusted Application,
705 which would in turn send this data to the SGX enclave (TA). This
706 complexity is due to the fact that each SGX enclave is separate and
707 does not have direct communication to other SGX enclaves.
709 In Arm TrustZone for A- and R-class devices, the Untrusted
710 Application and TA may or may not be bundled together. This differs
711 from SGX since in TrustZone the TA lifetime is not inherently tied to
712 a specific Untrused Application process lifetime as occurs in SGX. A
713 TA is loaded by a trusted OS running in the TEE, where the trusted OS
714 is separate from the OS in the REE. Thus Cases 2 and 3 are equally
715 applicable. In addition, it is possible for TAs to communicate with
716 each other without involving any Untrusted Application, and so the
717 complexity of Case 1 is lower than in the SGX example. Thus, Case 1
718 is possible as well, though still more complex than Cases 2 and 3.
720 4.5. Entity Relations
722 This architecture leverages asymmetric cryptography to authenticate a
723 device to a TAM. Additionally, a TEEP Agent in a device
724 authenticates a TAM. The provisioning of Trust Anchors to a device
725 may be different from one use case to the other. A Device
726 Administrator may want to have the capability to control what TAs are
727 allowed. A device manufacturer enables verification of the TAM
728 providers and TA binary signers; it may embed a list of default Trust
729 Anchors into the TEEP Agent and TEE for TAM trust verification and TA
730 signer verification.
732 (App Developers) (App Store) (TAM) (Device with TEE) (CAs)
733 | | | | |
734 | | | (Embedded TEE cert) <--|
735 | | | | |
736 | <--- Get an app cert -----------------------------------|
737 | | | | |
738 | | | <-- Get a TAM cert ---------|
739 | | | | |
740 1. Build two apps: | | | |
741 | | | |
742 (a) Untrusted | | | |
743 App - 2a. Supply --> | --- 3. Install ------> | |
744 | | | |
745 (b) TA -- 2b. Supply ----------> | 4. Messaging-->| |
746 | | | |
748 Figure 3: Developer Experience
750 Note that Figure 3 shows the TA developer as a TA signer. The TA
751 signer is either the same as the TA developer, or is a related entity
752 trusted to sign the developer's TAs.
754 Figure 3 shows an example where the same developer builds two
755 applications: 1) an Untrusted Application; 2) a TA that provides some
756 security functions to be run inside a TEE. At step 2, the TA
757 developer uploads the Untrusted Application (2a) to an Application
758 Store. The Untrusted Application may optionally bundle the TA
759 binary. Meanwhile, the TA developer may provide its TA to a TAM that
760 will be managing the TA in various devices. At step 3, a user will
761 go to an Application Store to download the Untrusted Application.
762 Since the Untrusted Application depends on the TA, installing the
763 Untrusted Application will trigger TA installation by initiating
764 communication with a TAM. This is step 4. The TEEP Agent will
765 interact with TAM via a TEEP Broker that faciliates communications
766 between a TAM and the TEEP Agent in TEE.
768 Some TA installation implementations might ask for a user's consent.
769 In other implementations, a Device Administrator might choose what
770 Untrusted Applications and related TAs to be installed. A user
771 consent flow is out of scope of the TEEP architecture.
773 The main components consist of a set of standard messages created by
774 a TAM to deliver TA management commands to a device, and device
775 attestation and response messages created by a TEE that responds to a
776 TAM's message.
778 It should be noted that network communication capability is generally
779 not available in TAs in today's TEE-powered devices. Consequently,
780 Trusted Applications generally rely on broker in the REE to provide
781 access to nnetwork functionality in the REE. A broker does not need
782 to know the actual content of messages to facilitate such access.
784 Similarly, since the TEEP Agent runs inside a TEE, the TEEP Agent
785 generally relies on a TEEP Broker in the REE to provide network
786 access, and relay TAM requests to the TEEP Agent and relay the
787 responses back to the TAM.
789 5. Keys and Certificate Types
791 This architecture leverages the following credentials, which allow
792 delivering end-to-end security between a TAM and a TEEP Agent.
794 Figure 4 summarizes the relationships between various keys and where
795 they are stored. Each public/private key identifies a TA developer,
796 TAM, or TEE, and gets a certificate that chains up to some CA. A
797 list of trusted certificates is then used to check a presented
798 certificate against.
800 Different CAs can be used for different types of certificates. TEEP
801 messages are always signed, where the signer key is the message
802 originator's private key, such as that of a TAM or a TEE. In
803 addition to the keys shown in Figure 4, there may be additional keys
804 used for attestation. Refer to the RATS Architecture
805 [I-D.ietf-rats-architecture] for more discussion.
807 Cardinality & Location of
808 Location of Private Key Trust Anchor
809 Purpose Private Key Signs Store
810 ------------------ ----------- ------------- -------------
811 Authenticating TEE 1 per TEE TEEP responses TAM
813 Authenticating TAM 1 per TAM TEEP requests TEEP Agent
815 Code Signing 1 per TA TA binary TEE
816 developer
818 Figure 4: Keys
820 Note that personalization data is not included in the table above.
821 The use of personalization data is dependent on how TAs are used and
822 what their security requirements are.
824 TEEP requests from a TAM to a TEEP Agent can be encrypted with the
825 TEE public key (to provide confidentiality), and are then signed with
826 the TAM private key (for authentication and integrity protection).
827 Conversely, TEEP responses from a TEEP Agent to a TAM can be
828 encrypted with the TAM public key, and are then signed with the TEE
829 private key.
831 The TEE key pair and certificate are thus used for authenticating the
832 TEE to a remote TAM, and for sending private data to the TEE. Often,
833 the key pair is burned into the TEE by the TEE manufacturer and the
834 key pair and its certificate are valid for the expected lifetime of
835 the TEE. A TAM provider is responsible for configuring the TAM's
836 Trust Anchor Store with the manufacturer certificates or CAs that are
837 used to sign TEE keys. This is discussed further in Section 5.3
838 below.
840 The TAM key pair and certificate are used for authenticating a TAM to
841 a remote TEE, and for sending private data to the TAM. A TAM
842 provider is responsible for acquiring a certificate from a CA that is
843 trusted by the TEEs it manages. This is discussed further in
844 Section 5.1 below.
846 The TA developer key pair and certificate are used to sign TAs that
847 the TEE will consider authorized to execute. TEEs must be configured
848 with the certificates or keys that it considers authorized to sign
849 TAs that it will execute. This is discussed further in Section 5.2
850 below.
852 5.1. Trust Anchors in a TEEP Agent
854 A TEEP Agent's Trust Anchor Store contains a list of Trust Anchors,
855 which are CA certificates that sign various TAM certificates. The
856 list is typically preloaded at manufacturing time, and can be updated
857 using the TEEP protocol if the TEE has some form of "Trust Anchor
858 Manager TA" that has Trust Anchors in its configuration data. Thus,
859 Trust Anchors can be updated similar to updating the configuration
860 data for any other TA.
862 When Trust Anchor update is carried out, it is imperative that any
863 update must maintain integrity where only an authentic Trust Anchor
864 list from a device manufacturer or a Device Administrator is
865 accepted. Details are out of scope of the architecture and can be
866 addressed in a protocol document.
868 Before a TAM can begin operation in the marketplace to support a
869 device with a particular TEE, it must obtain a TAM certificate from a
870 CA that is listed in the Trust Anchor Store of the TEEP Agent.
872 5.2. Trust Anchors in a TEE
874 A TEE determines whether TA binaries are allowed to execute by
875 verifying whether the TA's signer chains up to a certificate in the
876 TEE's Trust Anchor Store. The list is typically preloaded at
877 manufacturing time, and can be updated using the TEEP protocol if the
878 TEE has some form of "Trust Anchor Manager TA" that has Trust Anchors
879 in its configuration data. Thus, Trust Anchors can be updated
880 similar to updating the configuration data for any other TA, as
881 discussed in Section 5.1.
883 5.3. Trust Anchors in a TAM
885 The Trust Anchor Store in a TAM consists of a list of Trust Anchors,
886 which are certificates that sign various device TEE certificates. A
887 TAM will accept a device for TA management if the TEE in the device
888 uses a TEE certificate that is chained to a certificate that the TAM
889 trusts.
891 5.4. Scalability
893 This architecture uses a PKI, although self-signed certificates are
894 also permitted. Trust Anchors exist on the devices to enable the TEE
895 to authenticate TAMs and TA signers, and TAMs use Trust Anchors to
896 authenticate TEEs. When a PKI is used, many intermediate CA
897 certificates can chain to a root certificate, each of which can issue
898 many certificates. This makes the protocol highly scalable. New
899 factories that produce TEEs can join the ecosystem. In this case,
900 such a factory can get an intermediate CA certificate from one of the
901 existing roots without requiring that TAMs are updated with
902 information about the new device factory. Likewise, new TAMs can
903 join the ecosystem, providing they are issued a TAM certificate that
904 chains to an existing root whereby existing TEEs will be allowed to
905 be personalized by the TAM without requiring changes to the TEE
906 itself. This enables the ecosystem to scale, and avoids the need for
907 centralized databases of all TEEs produced or all TAMs that exist or
908 all TA developers that exist.
910 5.5. Message Security
912 Messages created by a TAM are used to deliver TA management commands
913 to a device, and device attestation and messages created by the
914 device TEE to respond to TAM messages.
916 These messages are signed end-to-end between a TEEP Agent and a TAM,
917 and are typically encrypted such that only the targeted device TEE or
918 TAM is able to decrypt and view the actual content.
920 6. TEEP Broker
922 A TEE and TAs often do not have the capability to directly
923 communicate outside of the hosting device. For example,
924 GlobalPlatform [GPTEE] specifies one such architecture. This calls
925 for a software module in the REE world to handle network
926 communication with a TAM.
928 A TEEP Broker is an application component running in the REE of the
929 device or an SDK that facilitates communication between a TAM and a
930 TEE. It also provides interfaces for Untrusted Applications to query
931 and trigger TA installation that the application needs to use.
933 An Untrusted Application might communicate with a TEEP Broker at
934 runtime to trigger TA installation itself, or an Untrusted
935 Application might simply have a metadata file that describes the TAs
936 it depends on and the associated TAM(s) for each TA, and an REE
937 Application Installer can inspect this application metadata file and
938 invoke the TEEP Broker to trigger TA installation on behalf of the
939 Untrusted Application without requiring the Untrusted Application to
940 run first.
942 6.1. Role of the TEEP Broker
944 A TEEP Broker abstracts the message exchanges with a TEE in a device.
945 The input data is originated from a TAM or the first initialization
946 call to trigger a TA installation.
948 The Broker doesn't need to parse a message content received from a
949 TAM that should be processed by a TEE. When a device has more than
950 one TEE, one TEEP Broker per TEE could be present in the REE. A TEEP
951 Broker interacts with a TEEP Agent inside a TEE.
953 A TAM message may indicate the target TEE where a TA should be
954 installed. A compliant TEEP protocol should include a target TEE
955 identifier for a TEEP Broker when multiple TEEs are present.
957 The Broker relays the response messages generated from a TEEP Agent
958 in a TEE to the TAM.
960 The Broker only needs to return a (transport) error message if the
961 TEE is not reachable for some reason. Other errors are represented
962 as response messages returned from the TEE which will then be passed
963 to the TAM.
965 6.2. TEEP Broker Implementation Consideration
967 TEEP Broker implementers should consider methods of distribution,
968 scope and concurrency on devices and runtime options. Several non-
969 exhaustive options are discussed below.
971 6.2.1. TEEP Broker APIs
973 The following conceptual APIs exist from a TEEP Broker to a TEEP
974 Agent:
976 1. RequestTA: A notification from an REE application (e.g., an
977 installer, or an Untrusted Application) that it depends on a
978 given TA, which may or may not already be installed in the TEE.
980 2. ProcessTeepMessage: A message arriving from the network, to be
981 delivered to the TEEP Agent for processing.
983 3. RequestPolicyCheck: A hint (e.g., based on a timer) that the TEEP
984 Agent may wish to contact the TAM for any changes, without the
985 device itself needing any particular change.
987 4. ProcessError: A notification that the TEEP Broker could not
988 deliver an outbound TEEP message to a TAM.
990 For comparison, similar APIs may exist on the TAM side, where a
991 Broker may or may not exist, depending on whether the TAM uses a TEE
992 or not:
994 1. ProcessConnect: A notification that an incoming TEEP session is
995 being requested by a TEEP Agent.
997 2. ProcessTeepMessage: A message arriving from the network, to be
998 delivered to the TAM for processing.
1000 For further discussion on these APIs, see
1001 [I-D.ietf-teep-otrp-over-http].
1003 6.2.2. TEEP Broker Distribution
1005 The Broker installation is commonly carried out at OEM time. A user
1006 can dynamically download and install a Broker on-demand.
1008 7. Attestation
1010 Attestation is the process through which one entity (an Attester)
1011 presents "evidence", in the form of a series of claims, to another
1012 entity (a Verifier), and provides sufficient proof that the claims
1013 are true. Different Verifiers may have different standards for
1014 attestation proofs and not all attestations are acceptable to every
1015 verifier. A third entity (a Relying Party) can then use "attestation
1016 results", in the form of another series of claims, from a Verifier to
1017 make authorization decisions. (See [I-D.ietf-rats-architecture] for
1018 more discussion.)
1020 In TEEP, as depicted in Figure 5, the primary purpose of an
1021 attestation is to allow a device (the Attester) to prove to a TAM
1022 (the Relying Party) that a TEE in the device has particular
1023 properties, was built by a particular manufacturer, and/or is
1024 executing a particular TA. Other claims are possible; TEEP does not
1025 limit the claims that may appear in evidence or attestation results,
1026 but defines a minimal set of attestation result claims required for
1027 TEEP to operate properly. Extensions to these claims are possible.
1028 Other standards or groups may define the format and semantics of
1029 extended claims.
1031 +----------------+
1032 | Device | +----------+
1033 | +------------+ | Evidence | TAM | Evidence +----------+
1034 | | TEE |------------->| (Relying |-------------->| Verifier |
1035 | | (Attester) | | | Party) |<--------------| |
1036 | +------------+ | +----------+ Attestation +----------+
1037 +----------------+ Result
1039 Figure 5: TEEP Attestation Roles
1041 As of the writing of this specification, device and TEE attestations
1042 have not been standardized across the market. Different devices,
1043 manufacturers, and TEEs support different attestation algorithms and
1044 mechanisms. In order for TEEP to be inclusive, it is agnostic to the
1045 format of evidence, allowing proprietary or standardized formats to
1046 be used between a TEE and a verifier (which may or may not be
1047 colocated in the TAM). However, it should be recognized that not all
1048 Verifiers may be able to process all proprietary forms of attestation
1049 evidence. Similarly, the TEEP protocol is agnostic as to the format
1050 of attestation results, and the protocol (if any) used between the
1051 TAM and a verifier, as long as they convey at least the required set
1052 of claims in some format.
1054 The assumptions that may apply to an attestation have to do with the
1055 quality of the attestation and the quality and security provided by
1056 the TEE, the device, the manufacturer, or others involved in the
1057 device or TEE ecosystem. Some of the assumptions that might apply to
1058 an attestations include (this may not be a comprehensive list):
1060 - Assumptions regarding the security measures a manufacturer takes
1061 when provisioning keys into devices/TEEs;
1063 - Assumptions regarding what hardware and software components have
1064 access to the attestation keys of the TEE;
1066 - Assumptions related to the source or local verification of claims
1067 within an attestation prior to a TEE signing a set of claims;
1069 - Assumptions regarding the level of protection afforded to
1070 attestation keys against exfiltration, modification, and side
1071 channel attacks;
1073 - Assumptions regarding the limitations of use applied to TEE
1074 attestation keys;
1076 - Assumptions regarding the processes in place to discover or detect
1077 TEE breeches; and
1079 - Assumptions regarding the revocation and recovery process of TEE
1080 attestation keys.
1082 TAMs must be comfortable with the assumptions that are inherently
1083 part of any attestation result they accept. Alternatively, any TAM
1084 may choose not to accept an attestation result generated using
1085 evidence from a particular manufacturer or device's TEE based on the
1086 inherent assumptions. The choice and policy decisions are left up to
1087 the particular TAM.
1089 Some TAMs may require additional claims in order to properly
1090 authorize a device or TEE. These additional claims may help clear up
1091 any assumptions for which the TAM wants to alleviate. The specific
1092 format for these additional claims are outside the scope of this
1093 specification, but the TEEP protocol allows these additional claims
1094 to be included in the attestation messages.
1096 7.1. Information Required in TEEP Claims
1098 - Device Identifying Info: TEEP attestations may need to uniquely
1099 identify a device to the TAM and TA developer. Unique device
1100 identification allows the TAM to provide services to the device,
1101 such as managing installed TAs, and providing subscriptions to
1102 services, and locating device-specific keying material to
1103 communicate with or authenticate the device. In some use cases it
1104 may be sufficient to identify only the class of the device. The
1105 security and privacy requirements regarding device identification
1106 will vary with the type of TA provisioned to the TEE.
1108 - TEE Identifying info: The type of TEE that generated this
1109 attestation must be identified, including version identification
1110 information such as the hardware, firmware, and software version
1111 of the TEE, as applicable by the TEE type. TEE manufacturer
1112 information for the TEE is required in order to disambiguate the
1113 same TEE type created by different manufacturers and resolve
1114 potential assumptions around manufacturer provisioning, keying and
1115 support for the TEE.
1117 - Freshness Proof: A claim that includes freshness information must
1118 be included, such as a nonce or timestamp.
1120 - Requested Components: A list of zero or more components (TAs or
1121 other dependencies needed by a TEE) that are requested by some
1122 depending app, but which are not currently installed in the TEE.
1124 8. Algorithm and Attestation Agility
1126 RFC 7696 [RFC7696] outlines the requirements to migrate from one
1127 mandatory-to-implement algorithm suite to another over time. This
1128 feature is also known as crypto agility. Protocol evolution is
1129 greatly simplified when crypto agility is considered during the
1130 design of the protocol. In the case of the TEEP protocol the diverse
1131 range of use cases, from trusted app updates for smart phones and
1132 tablets to updates of code on higher-end IoT devices, creates the
1133 need for different mandatory-to-implement algorithms already from the
1134 start.
1136 Crypto agility in TEEP concerns the use of symmetric as well as
1137 asymmetric algorithms. Symmetric algorithms are used for encryption
1138 of content whereas the asymmetric algorithms are mostly used for
1139 signing messages.
1141 In addition to the use of cryptographic algorithms in TEEP, there is
1142 also the need to make use of different attestation technologies. A
1143 device must provide techniques to inform a TAM about the attestation
1144 technology it supports. For many deployment cases it is more likely
1145 for the TAM to support one or more attestation techniques whereas the
1146 device may only support one.
1148 9. Security Considerations
1150 9.1. Broker Trust Model
1152 The architecture enables the TAM to communicate, via a TEEP Broker,
1153 with the device's TEE to manage TAs. Since the TEEP Broker runs in a
1154 potentially vulnerable REE, the TEEP Broker could, however, be (or be
1155 infected by) malware. As such, all TAM messages are signed and
1156 sensitive data is encrypted such that the TEEP Broker cannot modify
1157 or capture sensitive data, but the TEEP Broker can still conduct DoS
1158 attacks as discussed in Section 9.3.
1160 A TEEP Agent in a TEE is responsible for protecting against potential
1161 attacks from a compromised TEEP Broker or rogue malware in the REE.
1162 A rogue TEEP Broker might send corrupted data to the TEEP Agent, or
1163 launch a DoS attack by sending a flood of TEEP protocol requests.
1164 The TEEP Agent validates the signature of each TEEP protocol request
1165 and checks the signing certificate against its Trust Anchors. To
1166 mitigate DoS attacks, it might also add some protection scheme such
1167 as a threshold on repeated requests or number of TAs that can be
1168 installed.
1170 9.2. Data Protection at TAM and TEE
1172 The TEE implementation provides protection of data on the device. It
1173 is the responsibility of the TAM to protect data on its servers.
1175 9.3. Compromised REE
1177 It is possible that the REE of a device is compromised. If the REE
1178 is compromised, several DoS attacks may be launched. The compromised
1179 REE may terminate the TEEP Broker such that TEEP transactions cannot
1180 reach the TEE, or might drop or delay messages between a TAM and a
1181 TEEP Agent. However, while a DoS attack cannot be prevented, the REE
1182 cannot access anything in the TEE if it is implemented correctly.
1183 Some TEEs may have some watchdog scheme to observe REE state and
1184 mitigate DoS attacks against it but most TEEs don't have such a
1185 capability.
1187 In some other scenarios, the compromised REE may ask a TEEP Broker to
1188 make repeated requests to a TEEP Agent in a TEE to install or
1189 uninstall a TA. A TA installation or uninstallation request
1190 constructed by the TEEP Broker or REE will be rejected by the TEEP
1191 Agent because the request won't have the correct signature from a TAM
1192 to pass the request signature validation.
1194 This can become a DoS attack by exhausting resources in a TEE with
1195 repeated requests. In general, a DoS attack threat exists when the
1196 REE is compromised, and a DoS attack can happen to other resources.
1197 The TEEP architecture doesn't change this.
1199 A compromised REE might also request initiating the full flow of
1200 installation of TAs that are not necessary. It may also repeat a
1201 prior legitimate TA installation request. A TEEP Agent
1202 implementation is responsible for ensuring that it can recognize and
1203 decline such repeated requests. It is also responsible for
1204 protecting the resource usage allocated for TA management.
1206 9.4. Compromised CA
1208 A root CA for TAM certificates might get compromised. Some TEE Trust
1209 Anchor update mechanism is expected from device OEMs. TEEs are
1210 responsible for validating certificate revocation about a TAM
1211 certificate chain.
1213 If the root CA of some TEE device certificates is compromised, these
1214 devices might be rejected by a TAM, which is a decision of the TAM
1215 implementation and policy choice. TAMs are responsible for
1216 validating any intermediate CA for TEE device certificates.
1218 9.5. Compromised TAM
1220 Device TEEs are responsible for validating the supplied TAM
1221 certificates to determine that the TAM is trustworthy.
1223 9.6. Malicious TA Removal
1225 It is possible that a rogue developer distributes a malicious
1226 Untrusted Application and intends to get a malicious TA installed.
1227 It's the responsibility of the TAM to not install malicious trusted
1228 apps in the first place. The TEEP architecture allows a TEEP Agent
1229 to decide which TAMs it trusts via Trust Anchors, and delegates the
1230 TA authenticity check to the TAMs it trusts.
1232 It may happen that a TA was previously considered trustworthy but is
1233 later found to be buggy or compromised. In this case, the TAM can
1234 initiate the removal of the TA by notifying devices to remove the TA
1235 (and potentially the REE or device owner to remove any Untrusted
1236 Application that depend on the TA). If the TAM does not currently
1237 have a connection to the TEEP Agent on a device, such a notification
1238 would occur the next time connectivity does exist. That is, to
1239 recover, the TEEP Agent must be able to reach out to the TAM, for
1240 example whenever the RequestPolicyCheck API (Section 6.2.1) is
1241 invoked by a timer or other event.
1243 Furthermore the policy in the Verifier in an attestation process can
1244 be updated so that any evidence that includes the malicious TA would
1245 result in an attestation failure. There is, however, a time window
1246 during which a malicious TA might be able to operate successfully,
1247 which is the validity time of the previous attestation result. For
1248 example, if the Verifier in Figure 5 is updated to treat a previously
1249 valid TA as no longer trustworthy, any attestation result it
1250 previously generated saying that the TA is valid will continue to be
1251 used until the attestation result expires. As such, the TAM's
1252 Verifier should take into account the acceptable time window when
1253 generating attestation results. See [I-D.ietf-rats-architecture] for
1254 further discussion.
1256 9.7. Certificate Renewal
1258 TEE device certificates are expected to be long lived, longer than
1259 the lifetime of a device. A TAM certificate usually has a moderate
1260 lifetime of 2 to 5 years. A TAM should get renewed or rekeyed
1261 certificates. The root CA certificates for a TAM, which are embedded
1262 into the Trust Anchor store in a device, should have long lifetimes
1263 that don't require device Trust Anchor update. On the other hand, it
1264 is imperative that OEMs or device providers plan for support of Trust
1265 Anchor update in their shipped devices.
1267 9.8. Keeping Secrets from the TAM
1269 In some scenarios, it is desirable to protect the TA binary or
1270 configuration from being disclosed to the TAM that distributes them.
1271 In such a scenario, the files can be encrypted end-to-end between a
1272 TA developer and a TEE. However, there must be some means of
1273 provisioning the decryption key into the TEE and/or some means of the
1274 TA developer securely learning a public key of the TEE that it can
1275 use to encrypt. One way to do this is for the TA developer to run
1276 its own TAM so that it can distribute the decryption key via the TEEP
1277 protocol, and the key file can be a dependency in the manifest of the
1278 encrypted TA. Thus, the TEEP Agent would look at the TA manifest,
1279 determine there is a dependency with a TAM URI of the TA developer's
1280 TAM. The Agent would then install the dependency, and then continue
1281 with the TA installation steps, including decrypting the TA binary
1282 with the relevant key.
1284 10. IANA Considerations
1286 This document does not require actions by IANA.
1288 11. Contributors
1290 - Andrew Atyeo, Intercede (andrew.atyeo@intercede.com)
1292 - Liu Dapeng, Alibaba Group (maxpassion@gmail.com)
1294 12. Acknowledgements
1296 We would like to thank Nick Cook, Minho Yoo, Brian Witten, Tyler Kim,
1297 Alin Mutu, Juergen Schoenwaelder, Nicolae Paladi, Sorin Faibish, Ned
1298 Smith, Russ Housley, Jeremy O'Donoghue, and Anders Rundgren for their
1299 feedback.
1301 13. Informative References
1303 [GPTEE] GlobalPlatform, "GlobalPlatform Device Technology: TEE
1304 System Architecture, v1.1", GlobalPlatform GPD_SPE_009,
1305 January 2017, .
1308 [I-D.ietf-rats-architecture]
1309 Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
1310 W. Pan, "Remote Attestation Procedures Architecture",
1311 draft-ietf-rats-architecture-02 (work in progress), March
1312 2020.
1314 [I-D.ietf-suit-manifest]
1315 Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
1316 "A Concise Binary Object Representation (CBOR)-based
1317 Serialization Format for the Software Updates for Internet
1318 of Things (SUIT) Manifest", draft-ietf-suit-manifest-04
1319 (work in progress), March 2020.
1321 [I-D.ietf-teep-otrp-over-http]
1322 Thaler, D., "HTTP Transport for Trusted Execution
1323 Environment Provisioning: Agent-to- TAM Communication",
1324 draft-ietf-teep-otrp-over-http-05 (work in progress),
1325 March 2020.
1327 [RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
1328 Requirements", RFC 6024, DOI 10.17487/RFC6024, October
1329 2010, .
1331 [RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
1332 Agility and Selecting Mandatory-to-Implement Algorithms",
1333 BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
1334 .
1336 Authors' Addresses
1338 Mingliang Pei
1339 Symantec
1341 EMail: mingliang_pei@symantec.com
1343 Hannes Tschofenig
1344 Arm Limited
1346 EMail: hannes.tschofenig@arm.com
1348 Dave Thaler
1349 Microsoft
1351 EMail: dthaler@microsoft.com
1353 David Wheeler
1354 Intel
1356 EMail: david.m.wheeler@intel.com