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The disclaimer is usually necessary only for documents that revise or obsolete older RFCs, and that take significant amounts of text from those RFCs. If you can contact all authors of the source material and they are willing to grant the BCP78 rights to the IETF Trust, you can and should remove the disclaimer. Otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (June 12, 2020) is 1413 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- == Outdated reference: A later version (-22) exists of draft-ietf-rats-architecture-04 == Outdated reference: A later version (-25) exists of draft-ietf-suit-manifest-07 == Outdated reference: A later version (-15) exists of draft-ietf-teep-otrp-over-http-06 == Outdated reference: A later version (-18) exists of draft-ietf-teep-protocol-02 Summary: 0 errors (**), 0 flaws (~~), 6 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 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