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Dapeng 11 Alibaba Group 12 October 23, 2018 14 Trusted Execution Environment Provisioning (TEEP) Architecture 15 draft-ietf-teep-architecture-01 17 Abstract 19 A Trusted Execution Environment (TEE) is designed to provide a 20 hardware-isolation mechanism to separate a regular operating system 21 from security-sensitive application components. 23 This architecture document motivates the design and standardization 24 of a protocol for managing the lifecycle of trusted applications 25 running inside a TEE. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at https://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on April 26, 2019. 44 Copyright Notice 46 Copyright (c) 2018 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (https://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 This document may contain material from IETF Documents or IETF 60 Contributions published or made publicly available before November 61 10, 2008. The person(s) controlling the copyright in some of this 62 material may not have granted the IETF Trust the right to allow 63 modifications of such material outside the IETF Standards Process. 64 Without obtaining an adequate license from the person(s) controlling 65 the copyright in such materials, this document may not be modified 66 outside the IETF Standards Process, and derivative works of it may 67 not be created outside the IETF Standards Process, except to format 68 it for publication as an RFC or to translate it into languages other 69 than English. 71 Table of Contents 73 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 74 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 75 3. Scope and Assumptions . . . . . . . . . . . . . . . . . . . . 7 76 4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 8 77 4.1. Payment . . . . . . . . . . . . . . . . . . . . . . . . . 8 78 4.2. Authentication . . . . . . . . . . . . . . . . . . . . . 8 79 4.3. Internet of Things . . . . . . . . . . . . . . . . . . . 9 80 4.4. Confidential Cloud Computing . . . . . . . . . . . . . . 9 81 5. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 9 82 5.1. System Components . . . . . . . . . . . . . . . . . . . . 9 83 5.2. Different Renditions of TEEP Architecture . . . . . . . . 12 84 5.3. Entity Relations . . . . . . . . . . . . . . . . . . . . 12 85 5.4. Trust Anchors in TEE . . . . . . . . . . . . . . . . . . 15 86 5.5. Trust Anchors in TAM . . . . . . . . . . . . . . . . . . 15 87 5.6. Keys and Certificate Types . . . . . . . . . . . . . . . 15 88 5.7. Scalability . . . . . . . . . . . . . . . . . . . . . . . 18 89 5.8. Message Security . . . . . . . . . . . . . . . . . . . . 18 90 5.9. Security Domain Hierarchy and Ownership . . . . . . . . . 18 91 5.10. SD Owner Identification and TAM Certificate Requirements 19 92 5.11. Service Provider Container . . . . . . . . . . . . . . . 20 93 5.12. A Sample Device Setup Flow . . . . . . . . . . . . . . . 20 94 6. TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . . 21 95 6.1. Role of the Agent . . . . . . . . . . . . . . . . . . . . 22 96 6.2. Agent Implementation Consideration . . . . . . . . . . . 22 97 6.2.1. Agent Distribution . . . . . . . . . . . . . . . . . 22 98 6.2.2. Number of Agents . . . . . . . . . . . . . . . . . . 23 99 7. Attestation . . . . . . . . . . . . . . . . . . . . . . . . . 23 100 7.1. Attestation Hierarchy . . . . . . . . . . . . . . . . . . 23 101 7.1.1. Attestation Hierarchy Establishment: Manufacture . . 23 102 7.1.2. Attestation Hierarchy Establishment: Device Boot . . 24 103 7.1.3. Attestation Hierarchy Establishment: TAM . . . . . . 24 104 8. Algorithm and Attestation Agility . . . . . . . . . . . . . . 24 105 9. Security Considerations . . . . . . . . . . . . . . . . . . . 25 106 9.1. TA Trust Check at TEE . . . . . . . . . . . . . . . . . . 25 107 9.2. One TA Multiple SP Case . . . . . . . . . . . . . . . . . 25 108 9.3. Agent Trust Model . . . . . . . . . . . . . . . . . . . . 25 109 9.4. Data Protection at TAM and TEE . . . . . . . . . . . . . 26 110 9.5. Compromised CA . . . . . . . . . . . . . . . . . . . . . 26 111 9.6. Compromised TAM . . . . . . . . . . . . . . . . . . . . . 26 112 9.7. Certificate Renewal . . . . . . . . . . . . . . . . . . . 26 113 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 114 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27 115 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 27 116 12.1. Normative References . . . . . . . . . . . . . . . . . . 27 117 12.2. Informative References . . . . . . . . . . . . . . . . . 27 118 Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 28 119 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 121 1. Introduction 123 Applications executing in a device are exposed to many different 124 attacks intended to compromise the execution of the application, or 125 reveal the data upon which those applications are operating. These 126 attacks increase with the number of other applications on the device, 127 with such other applications coming from potentially untrustworthy 128 sources. The potential for attacks further increase with the 129 complexity of features and applications on devices, and the 130 unintended interactions among those features and applications. The 131 danger of attacks on a system increases as the sensitivity of the 132 applications or data on the device increases. As an example, 133 exposure of emails from a mail client is likely to be of concern to 134 its owner, but a compromise of a banking application raises even 135 greater concerns. 137 The Trusted Execution Environment (TEE) concept is designed to 138 execute applications in a protected environment that separates 139 applications inside the TEE from the regular operating system and 140 from other applications on the device. This separation reduces the 141 possibility of a successful attack on application components and the 142 data contained inside the TEE. Typically, application components are 143 chosen to execute inside a TEE because those application components 144 perform security sensitive operations or operate on sensitive data. 146 An application component running inside a TEE is referred to as a 147 Trusted Application (TA), while a normal application running in the 148 regular operating system is referred to as an Untrusted Application 149 (UA). 151 The TEE uses hardware to enforce protections on the TA and its data, 152 but also presents a more limited set of services to applications 153 inside the TEE than is normally available to UA's running in the 154 normal operating system. 156 But not all TEEs are the same, and different vendors may have 157 different implementations of TEEs with different security properties, 158 different features, and different control mechanisms to operate on 159 TAs. Some vendors may themselves market multiple different TEEs with 160 different properties attuned to different markets. A device vendor 161 may integrate one or more TEEs into their devices depending on market 162 needs. 164 To simplify the life of developers and service providers interacting 165 with TAs in a TEE, an interoperable protocol for managing TAs running 166 in different TEEs of various devices is needed. In this TEE 167 ecosystem, there often arises a need for an external trusted party to 168 verify the identity, claims, and rights of Service Providers(SP), 169 devices, and their TEEs. This trusted third party is the Trusted 170 Application Manager (TAM). 172 This protocol addresses the following problems: 174 - A Service Provider (SP) intending to provide services through a TA 175 to users of a device needs to determine security-relevant 176 information of a device before provisioning their TA to the TEE 177 within the device. Examples include the verification of the 178 device 'root of trust' and the type of TEE included in a device. 180 - A TEE in a device needs to determine whether a Service Provider 181 (SP) that wants to manage a TA in the device is authorized to 182 manage TAs in the TEE, and what TAs the SP is permitted to manage. 184 - The parties involved in the protocol must be able to attest that a 185 TEE is genuine and capable of providing the security protections 186 required by a particular TA. 188 - A Service Provider (SP) must be able to deterine if a TA exists 189 (is installed) on a device (in the TEE), and if not, install the 190 TA in the TEE. 192 - A Service Provider (SP) must be able to check whether a TA in a 193 device's TEE is the most up-to-date version, and if not, update 194 the TA in the TEE. 196 - A Service Provider (SP) must be able to remove a TA in a device's 197 TEE if the SP is no longer offering such services or the services 198 are being revoked from a particular user (or device). For 199 example, if a subscription or contract for a particular service 200 has expired, or a payment by the user has not been completed or 201 has been recinded. 203 - A Service Provider (SP) must be able to define the relationship 204 between cooperating TAs under the SP's control, and specify 205 whether the TAs can communicate, share data, and/or share key 206 material. 208 2. Terminology 210 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 211 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 212 "OPTIONAL" in this document are to be interpreted as described in BCP 213 14 [RFC2119] [RFC8174] when, and only when, they appear in all 214 capitals, as shown here. 216 The following terms are used: 218 - Client Application: An application running in a Rich Execution 219 Environment, such as an Android, Windows, or iOS application. 221 - Device: A physical piece of hardware that hosts a TEE along with a 222 Rich Execution Environment. A Device contains a default list of 223 Trust Anchors that identify entities (e.g., TAMs) that are trusted 224 by the Device. This list is normally set by the Device 225 Manufacturer, and may be governed by the Device's network carrier. 226 The list of Trust Anchors is normally modifiable by the Device's 227 owner or Device Administrator. However the Device manufacturer 228 and network carrier may restrict some modifications, for example, 229 by not allowing the manufacturer or carrier's Trust Anchor to be 230 removed or disabled. 232 - Rich Execution Environment (REE): An environment that is provided 233 and governed by a typical OS (e.g., Linux, Windows, Android, iOS), 234 potentially in conjunction with other supporting operating systems 235 and hypervisors; it is outside of the TEE. This environment and 236 applications running on it are considered un-trusted. 238 - Service Provider (SP): An entity that wishes to provide a service 239 on Devices that requires the use of one or more Trusted 240 Applications. A Service Provider requires the help of a TAM in 241 order to provision the Trusted Applications to remote devices. 243 - Device Administrator: An entity that owns or is responsible for 244 administration of a Device. A Device Administrator has privileges 245 on the Device to install and remove applications and TAs, approve 246 or reject Trust Anchors, and approve or reject Service Providers, 247 among possibly other privileges on the Device. A device owner can 248 manage the list of allowed TAMs by modifying the list of Trust 249 Anchors on the Device. Although a Device Administrator may have 250 privileges and Device-specific controls to locally administer a 251 device, the Device Administrator may choose to remotely 252 administrate a device through a TAM. 254 - Trust Anchor: A public key in a device whose corresponding private 255 key is held by an entity implicitly trusted by the device. The 256 Trust Anchor may be a certificate or it may be a raw public key. 257 The trust anchor is normally stored in a location that resists 258 unauthorized modification, insertion, or replacement. 259 The trust anchor private key owner can sign certificates of other 260 public keys, which conveys trust about those keys to the device. 261 A certificate signed by the trust anchor communicates that the 262 private key holder of the signed certificate is trusted by the 263 trust anchor holder, and can therefore be trusted by the device. 265 - Trusted Application (TA): An application component that runs in a 266 TEE. 268 - Trusted Execution Environment (TEE): An execution environment that 269 runs alongside of, but is isolated from, an REE. A TEE has 270 security capabilities and meets certain security-related 271 requirements. It protects TEE assets from general software 272 attacks, defines rigid safeguards as to data and functions that a 273 program can access, and resists a set of defined threats. It 274 should have at least the following three properties: 276 (a) A device unique credential that cannot be cloned; 278 (b) Assurance that only authorized code can run in the TEE; 280 (c) Memory that cannot be read by code outside the TEE. 282 There are multiple technologies that can be used to implement a 283 TEE, and the level of security achieved varies accordingly. 285 - Root-of-Trust (RoT): A hardware or software component in a device 286 that is inherently trusted to perform a certain security-critical 287 function. A RoT should be secure by design, small, and protected 288 by hardware against modification or interference. Examples of 289 RoTs include software/firmware measurement and verification using 290 a trust anchor (RoT for Verification), provide signed assertions 291 using a protected attestation key (RoT for Reporting), or protect 292 the storage and/or use of cryptographic keys (RoT for Storage). 293 Other RoTs are possible, including RoT for Integrity, and RoT for 294 Measurement. Reference: NIST SP800-164 (Draft). 296 - Trusted Firmware (TFW): A firmware in a device that can be 297 verified with a trust anchor by RoT for Verification. 299 - Bootloader key: This symmetric key is protected by 300 electronic fuse (eFUSE) technology. In this context it is used to 301 decrypt a 302 TFW private key, which belongs to a device-unique private/public 303 key pair. Not every device is equipped with a bootloader key. 305 This document uses the following abbreviations: 307 - CA: Certificate Authority 309 - REE: Rich Execution Environment 311 - RoT: Root of Trust 313 - SD: Security Domain 315 - SP: Service Provider 317 - TA: Trusted Application 319 - TAM: Trusted Application Manager 321 - TEE: Trusted Execution Environment 323 - TFW: Trusted Firmware 325 3. Scope and Assumptions 327 This specification assumes that an applicable device is equipped with 328 one or more TEEs and each TEE is pre-provisioned with a device-unique 329 public/private key pair, which is securely stored. This key pair is 330 referred to as the 'root of trust' for remote attestation of the 331 associated TEE in a device by an TAM. 333 New note: SD is for managing keys for TAs 334 A Security Domain (SD) concept is used as the security boundary 335 inside a TEE for trusted applications. Each SD is typically 336 associated with one TA provider as the owner, which is a logical 337 space that contains an SP's TAs. One TA provider may request to have 338 multiple SDs in a TEE. One SD may contain multiple TAs. Each 339 Security Domain requires the management operations of TAs in the form 340 of installation, update and deletion. 342 Each TA binary and configuration data can be from either of two 343 sources: 345 1. A TAM supplies the signed and encrypted TA binary and any 346 required configuration data 348 2. A Client Application supplies the TA binary 350 The architecture covers the first case where the TA binary and 351 configuration data are delivered from a TAM. The second case calls 352 for an extension when a TAM is absent. 354 4. Use Cases 356 4.1. Payment 358 A payment application in a mobile device requires high security and 359 trust about the hosting device. Payments initiated from a mobile 360 device can use a Trusted Application to provide strong identification 361 and proof of transaction. 363 For a mobile payment application, some biometric identification 364 information could also be stored in a TEE. The mobile payment 365 application can use such information for authentication. 367 A secure user interface (UI) may be used in a mobile device to 368 prevent malicious software from stealing sensitive user input data. 369 Such an application implementation often relies on a TEE for user 370 input protection. 372 4.2. Authentication 374 For better security of authentication, a device may store its 375 sensitive authentication keys inside a TEE, providing hardware- 376 protected security key strength and trusted code execution. 378 4.3. Internet of Things 380 The Internet of Things (IoT) has been posing threats to networks and 381 national infrastructures because of existing weak security in 382 devices. It is very desirable that IoT devices can prevent malware 383 from manipulating actuators (e.g., unlocking a door), or stealing or 384 modifying sensitive data such as authentication credentials in the 385 device. A TEE can be the best way to implement such IoT security 386 functions. 388 TEEs could be used to store variety of sensitive data for IoT 389 devices. For example, a TEE could be used in smart door locks to 390 store a user's biometric information for identification, and for 391 protecting access the locking mechanism. 393 4.4. Confidential Cloud Computing 395 A tenant can store sensitive data in a TEE in a cloud computing 396 server such that only the tenant can access the data, preventing the 397 cloud hosting provider from accessing the data. A tenant can run TAs 398 inside a server TEE for secure operation and enhanced data security. 399 This provides benefits not only to tenants with better data security 400 but also to cloud hosting provider for reduced liability and 401 increased cloud adoption. 403 5. Architecture 405 5.1. System Components 407 The following are the main components in the system. Full 408 descriptions of components not previously defined are provided below. 409 Interactions of all components are further explained in the following 410 paragraphs. 412 +-------------------------------------------+ 413 | Device | 414 | +--------+ | Service Provider 415 | | |----------+ | 416 | +-------------+ | TEEP |---------+| | 417 | | TEE-1 |<------| Broker | | || +--------+ | 418 | | | | |<---+ | |+-->| |<-+ 419 | | | | | | | | +-| TAM-1 | 420 | | | | |<-+ | | +->| | |<-+ 421 | | +---+ +---+ | +--------+ | | | | +--------+ | 422 | | |TA1| |TA2| | | | | | TAM-2 | | 423 | +-->| | | | | +-------+ | | | +--------+ | 424 | | | | | | |<---------| App-2 |--+ | | | 425 | | | +---+ +---+ | +-------+ | | | Device Administrator 426 | | +-------------+ | App-1 | | | | 427 | | | | | | | 428 | +--------------------| |---+ | | 429 | | |--------+ | 430 | +-------+ | 431 +-------------------------------------------+ 433 Figure 1: Notional Architecture of TEEP 435 - Service Providers and Device Administrators utilize the services 436 of a TAM to manage TAs on Devices. SPs do not directly interact 437 with devices. DAs may elect to use a TAM for remote 438 administration of TAs instead of managing each device directly. 440 - TAM: A TAM is responsible for performing lifecycle management 441 activity on TA's and SD's on behalf of Service Providers and 442 Device Administrators. This includes creation and deletion of 443 TA's and SD's, and may include, for example, over-the-air updates 444 to keep an SP's TAs up-to-date and clean up when a version should 445 be removed. TAMs may provide services that make it easier for SPs 446 or DAs to use the TAM's service to manage multiple devices, 447 although that is not required of a TAM. 449 The TAM performs its management of TA's and SD's through an 450 interaction with a Device's TEEP Broker. As shown in 451 #notionalarch, the TAM cannot directly contact a Device, but must 452 wait for a the TEEP Broker or a Client Application to contact the 453 TAM requesting a particular service. This architecture is 454 intentional in order to accommodate network and application 455 firewalls that normally protect user and enterprise devices from 456 arbitrary connections from external network entities. 458 A TAM may be publically available for use by many SPs, or a TAM 459 may be private, and accessible by only one or a limited number of 460 SPs. It is expected that manufacturers and carriers will run 461 their own private TAM. Another example of a private TAM is a TAM 462 running as a Software-as-a-Service (SaaS) within an SP. 464 A SP or Device Administrator chooses a particular TAM based on 465 whether the TAM is trusted by a Device or set of Devices. The TAM 466 is trusted by a device if the TAM's public key is an authorized 467 Trust Anchor in the Device. A SP or Device Administrator may run 468 their own TAM, however the Devices they wish to manage must 469 include this TAM's pubic key in the Trust Anchor list. 471 A SP or Device Administrator is free to utilize multiple TAMs. 472 This may be required for a SP to manage multiple different types 473 of devices from different manufacturers, or devices on different 474 carriers, since the Trust Anchor list on these different devices 475 may contain different TAMs. A Device Administrator may be able to 476 add their own TAM's public key or certificate to the Trust Anchor 477 list on all their devices, overcoming this limitation. 479 Any entity is free to operate a TAM. For a TAM to be successful, 480 it must have its public key or certificate installed in Devices 481 Trust Anchor list. A TAM may set up a relationship with device 482 manufacturers or carriers to have them install the TAM's keys in 483 their device's Trust Anchor list. Alternatively, a TAM may 484 publish its certificate and allow Device Administrators to install 485 the TAM's certificate in their devices as an after-market-action. 487 - TEEP Broker: The TEEP Broker is an application running in a Rich 488 Execution Environment that enables the message protocol exchange 489 between a TAM and a TEE in a device. The TEEP Broker does not 490 process messages on behalf of a TEE, but merely is responsible for 491 relaying messages from the TAM to the TEE, and for returning the 492 TEE's responses to the TAM. 494 A Client Application is expected to communicate with a TAM to 495 request TAs that it needs to use. The Client Application needs to 496 pass the messages from the TAM to TEEs in the device. This calls 497 for a component in the REE that Client Applications can use to 498 pass messages to TEEs. An Agent is thus an application in the REE 499 or software library that can relay messages from a Client 500 Application to a TEE in the device. A device usually comes with 501 only one active TEE. A TEE may provide such an Agent to the 502 device manufacturer to be bundled in devices. Such a TEE must 503 also include an Agent counterpart, namely, a processing module 504 inside the TEE, to parse TAM messages sent through the Agent. An 505 Agent is generally acting as a dummy relaying box with just the 506 TEE interacting capability; it doesn't need and shouldn't parse 507 protocol messages. 509 - Certification Authority (CA): Certificate-based credentials used 510 for authenticating a device, a TAM and an SP. A device embeds a 511 list of root certificates (trust anchors), from trusted CAs that a 512 TAM will be validated against. A TAM will remotely attest a 513 device by checking whether a device comes with a certificate from 514 a CA that the TAM trusts. The CAs do not need to be the same; 515 different CAs can be chosen by each TAM, and different device CAs 516 can be used by different device manufacturers. 518 5.2. Different Renditions of TEEP Architecture 520 5.3. Entity Relations 522 This architecture leverages asymmetric cryptography to authenticate a 523 device to a TAM. Additionally, a TEE in a device authenticates a TAM 524 and TA signer. The provisioning of trust anchors to a device may 525 different from one use case to the other. A device administrator may 526 want to have the capability to control what TAs are allowed. A 527 device manufacturer enables verification of the TA signers and TAM 528 providers; it may embed a list of default trust anchors that the 529 signer of an allowed TA's signer certificate should chain to. A 530 device administrator may choose to accept a subset of the allowed TAs 531 via consent or action of downloading. 533 PKI CA -- CA CA -- 534 | | | 535 | | | 536 | | | 537 Device | | --- Agent / Client App --- | 538 SW | | | | | 539 | | | | | 540 | | | | | 541 | -- TEE TAM------- 542 | 543 | 544 FW 546 Figure 2: Entities 548 (App Developer) (App Store) (TAM) (Device with TEE) (CAs) 549 | | 550 | --> (Embedded TEE cert) <-- 551 | | 552 | <------------------------------ Get an app cert ----- | 553 | | <-- Get a TAM cert ------ | 554 | 555 1. Build two apps: 556 Client App 557 TA 558 | 559 | 560 Client App -- 2a. --> | ----- 3. Install -------> | 561 TA ------- 2b. Supply ------> | 4. Messaging-->| 562 | | | | 564 Figure 3: Developer Experience 566 Figure 3 shows an application developer building two applications: 1) 567 a rich Client Application; 2) a TA that provides some security 568 functions to be run inside a TEE. At step 2, the application 569 developer uploads the Client Application (2a) to an Application 570 Store. The Client Application may optionally bundle the TA binary. 571 Meanwhile, the application developer may provide its TA to a TAM 572 provider that will be managing the TA in various devices. 3. A user 573 will go to an Application Store to download the Client Application. 574 The Client Application will trigger TA installation by initiating 575 communication with a TAM. This is the step 4. The Client 576 Application will get messages from TAM, and interacts with device TEE 577 via an Agent. 579 The following diagram shows a system diagram about the entity 580 relationships between CAs, TAMs, SPs and devices. 582 ------- Message Protocol ----- 583 | | 584 | | 585 -------------------- --------------- ---------- 586 | REE | TEE | | TAM | | SP | 587 | --- | --- | | --- | | -- | 588 | | | | | | | 589 | Client | SD (TAs)| | SD / TA | | TA | 590 | Apps | | | Mgmt | | | 591 | | | | | | | | 592 | | | List of | | List of | | | 593 | | Trusted | | Trusted | | | 594 | Agent | TAM/SP | | FW/TEE | | | 595 | | CAs | | CAs | | | 596 | | | | | | | 597 | |TEE Key/ | | TAM Key/ | |SP Key/ | 598 | | Cert | | Cert | | Cert | 599 | | FW Key/ | | | | | 600 | | Cert | | | | | 601 -------------------- --------------- ---------- 602 | | | 603 | | | 604 ------------- ---------- --------- 605 | TEE CA | | TAM CA | | SP CA | 606 ------------- ---------- --------- 608 Figure 4: Keys 610 In the previous diagram, different CAs can be used for different 611 types of certificates. Messages are always signed, where the signer 612 key is the message originator's private key such as that of a TAM, 613 the private key of trusted firmware (TFW), or a TEE's private key. 615 The main components consist of a set of standard messages created by 616 a TAM to deliver device SD and TA management commands to a device, 617 and device attestation and response messages created by a TEE that 618 responds to a TAM's message. 620 It should be noted that network communication capability is generally 621 not available in TAs in today's TEE-powered devices. The networking 622 functionality must be delegated to a rich Client Application. Client 623 Applications will need to rely on an agent in the REE to interact 624 with a TEE for message exchanges. Consequently, a TAM generally 625 communicates with a Client Application about how it gets messages 626 that originate from a TEE inside a device. Similarly, a TA or TEE 627 generally gets messages from a TAM via some Client Application, 628 namely, an agent in this protocol architecture, not directly from the 629 network. 631 It is imperative to have an interoperable protocol to communicate 632 with different TAMs and different TEEs in different devices. This is 633 the role of the agent, which is a software component that bridges 634 communication between a TAM and a TEE. The agent does not need to 635 know the actual content of messages except for the TEE routing 636 information. 638 5.4. Trust Anchors in TEE 640 Each TEE comes with a trust store that contains a whitelist of root 641 CA certificates that are used to validate a TAM's certificate. A TEE 642 will accept a TAM to create new Security Domains and install new TAs 643 on behalf of an SP only if the TAM's certificate is chained to one of 644 the root CA certificates in the TEE's trust store. 646 A TEE's trust store is typically preloaded at manufacturing time. It 647 is out of the scope in this document to specify how the trust store 648 should be updated when a new root certificate should be added or 649 existing one should be updated or removed. A device manufacturer is 650 expected to provide its TEE trust store live update or out-of-band 651 update to devices. 653 Before a TAM can begin operation in the marketplace to support a 654 device with a particular TEE, it must obtain a TAM certificate from a 655 CA that is listed in the trust store of the TEE. 657 5.5. Trust Anchors in TAM 659 The trust anchor store in a TAM consists of a list of CA certificates 660 that sign various device TEE certificates. A TAM decides what 661 devices it will trust the TEE in. 663 5.6. Keys and Certificate Types 665 This architecture leverages the following credentials, which allow 666 delivering end-to-end security without relying on any transport 667 security. 669 +-------------+----------+--------+-------------------+-------------+ 670 | Key Entity | Location | Issuer | Checked Against | Cardinality | 671 | Name | | | | | 672 +-------------+----------+--------+-------------------+-------------+ 673 | 1. TFW key | Device | FW CA | A whitelist of | 1 per | 674 | pair and | secure | | FW root CA | device | 675 | certificate | storage | | trusted by TAMs | | 676 | | | | | | 677 | 2. TEE key | Device | TEE CA | A whitelist of | 1 per | 678 | pair and | TEE | under | TEE root CA | device | 679 | certificate | | a root | trusted by TAMs | | 680 | | | CA | | | 681 | | | | | | 682 | 3. TAM key | TAM | TAM CA | A whitelist of | 1 or | 683 | pair and | provider | under | TAM root CA | multiple | 684 | certificate | | a root | embedded in TEE | can be used | 685 | | | CA | | by a TAM | 686 | | | | | | 687 | 4. SP key | SP | SP | A SP uses a TAM. | 1 or | 688 | pair and | | signer | TA is signed by a | multiple | 689 | certificate | | CA | SP signer. TEE | can be used | 690 | | | | delegates trust | by a TAM | 691 | | | | of TA to TAM. SP | | 692 | | | | signer is | | 693 | | | | associated with a | | 694 | | | | SD as the owner. | | 695 +-------------+----------+--------+-------------------+-------------+ 697 Figure 5: Key and Certificate Types 699 1. TFW key pair and certificate: A key pair and certificate for 700 evidence of trustworthy firmware in a device. This key pair is 701 optional for TEEP architecture. Some TEE may present its trusted 702 attributes to a TAM using signed attestation with a TFW key. For 703 example, a platform that uses a hardware based TEE can have 704 attestation data signed by a hardware protected TFW key. 706 o Location: Device secure storage 708 o Supported Key Type: RSA and ECC 710 o Issuer: OEM CA 712 o Checked Against: A whitelist of FW root CA trusted by TAMs 714 o Cardinality: One per device 716 2. TEE key pair and certificate: It is used for device attestation 717 to a remote TAM and SP. 719 o This key pair is burned into the device by the device 720 manufacturer. The key pair and its certificate are valid for 721 the expected lifetime of the device. 723 o Location: Device TEE 725 o Supported Key Type: RSA and ECC 727 o Issuer: A CA that chains to a TEE root CA 729 o Checked Against: A whitelist of TEE root CAs trusted by TAMs 731 o Cardinality: One per device 733 3. TAM key pair and certificate: A TAM provider acquires a 734 certificate from a CA that a TEE trusts. 736 o Location: TAM provider 738 o Supported Key Type: RSA and ECC. 740 o Supported Key Size: RSA 2048-bit, ECC P-256 and P-384. Other 741 sizes should be anticipated in future. 743 o Issuer: TAM CA that chains to a root CA 745 o Checked Against: A whitelist of TAM root CAs embedded in a TEE 747 o Cardinality: One or multiple can be used by a TAM 749 4. SP key pair and certificate: An SP uses its own key pair and 750 certificate to sign a TA. 752 o Location: SP 754 o Supported Key Type: RSA and ECC 756 o Supported Key Size: RSA 2048-bit, ECC P-256 and P-384. Other 757 sizes should be anticipated in future. 759 o Issuer: An SP signer CA that chains to a root CA 761 o Checked Against: An SP uses a TAM. A TEE trusts an SP by 762 validating trust against a TAM that the SP uses. A TEE trusts 763 a TAM to ensure that a TA is trustworthy. 765 o Cardinality: One or multiple can be used by an SP 767 5.7. Scalability 769 This architecture uses a PKI. Trust anchors exist on the devices to 770 enable the TEE to authenticate TAMs, and TAMs use trust anchors to 771 authenticate TEEs. Since a PKI is used, many intermediate CA 772 certificates can chain to a root certificate, each of which can issue 773 many certificates. This makes the protocol highly scalable. New 774 factories that produce TEEs can join the ecosystem. In this case, 775 such a factory can get an intermediate CA certificate from one of the 776 existing roots without requiring that TAMs are updated with 777 information about the new device factory. Likewise, new TAMs can 778 join the ecosystem, providing they are issued a TAM certificate that 779 chains to an existing root whereby existing TEEs will be allowed to 780 be personalized by the TAM without requiring changes to the TEE 781 itself. This enables the ecosystem to scale, and avoids the need for 782 centralized databases of all TEEs produced or all TAMs that exist. 784 5.8. Message Security 786 Messages created by a TAM are used to deliver device SD and TA 787 management commands to a device, and device attestation and messages 788 created by the device TEE to respond to TAM messages. 790 These messages are signed end-to-end and are typically encrypted such 791 that only the targeted device TEE or TAM is able to decrypt and view 792 the actual content. 794 5.9. Security Domain Hierarchy and Ownership 796 The primary job of a TAM is to help an SP to manage its trusted 797 applications. A TA is typically installed in an SD. An SD is 798 commonly created for an SP. 800 When an SP delegates its SD and TA management to a TAM, an SD is 801 created on behalf of a TAM in a TEE and the owner of the SD is 802 assigned to the TAM. An SD may be associated with an SP but the TAM 803 has full privilege to manage the SD for the SP. 805 Each SD for an SP is associated with only one TAM. When an SP 806 changes TAM, a new SP SD must be created to associate with the new 807 TAM. The TEE will maintain a registry of TAM ID and SP SD ID 808 mapping. 810 From an SD ownership perspective, the SD tree is flat and there is 811 only one level. An SD is associated with its owner. It is up to the 812 TEE implementation how it maintains SD binding information for a TAM 813 and different SPs under the same TAM. 815 It is an important decision in this architecture that a TEE doesn't 816 need to know whether a TAM is authorized to manage the SD for an SP. 817 This authorization is implicitly triggered by an SP Client 818 Application, which instructs what TAM it wants to use. An SD is 819 always associated with a TAM in addition to its SP ID. A rogue TAM 820 isn't able to do anything on an unauthorized SP's SD managed by 821 another TAM. 823 Since a TAM may support multiple SPs, sharing the same SD name for 824 different SPs creates a dependency in deleting an SD. An SD can be 825 deleted only after all TAs associated with the SD are deleted. An SP 826 cannot delete a Security Domain on its own with a TAM if a TAM 827 decides to introduce such sharing. There are cases where multiple 828 virtual SPs belong to the same organization, and a TAM chooses to use 829 the same SD name for those SPs. This is totally up to the TAM 830 implementation and out of scope of this specification. 832 5.10. SD Owner Identification and TAM Certificate Requirements 834 There is a need of cryptographically binding proof about the owner of 835 an SD in a device. When an SD is created on behalf of a TAM, a 836 future request from the TAM must present itself as a way that the TEE 837 can verify it is the true owner. The certificate itself cannot 838 reliably used as the owner because TAM may change its certificate. 840 ** need to handle the normal key roll-over case, as well as the less 841 frequent key compromise case 843 To this end, each TAM will be associated with a trusted identifier 844 defined as an attribute in the TAM certificate. This field is kept 845 the same when the TAM renew its certificates. A TAM CA is 846 responsible to vet the requested TAM attribute value. 848 This identifier value must not collide among different TAM providers, 849 and one TAM shouldn't be able to claim the identifier used by another 850 TAM provider. 852 The certificate extension name to carry the identifier can initially 853 use SubjectAltName:registeredID. A dedicated new extension name may 854 be registered later. 856 One common choice of the identifier value is the TAM's service URL. 857 A CA can verify the domain ownership of the URL with the TAM in the 858 certificate enrollment process. 860 A TEE can assign this certificate attribute value as the TAM owner ID 861 for the SDs that are created for the TAM. 863 An alternative way to represent an SD ownership by a TAM is to have a 864 unique secret key upon SD creation such that only the creator TAM is 865 able to produce a proof-of-possession (PoP) data with the secret. 867 5.11. Service Provider Container 869 A sample Security Domain hierarchy for the TEE is shown in Figure 6. 871 ---------- 872 | TEE | 873 ---------- 874 | 875 | ---------- 876 |----------| SP1 SD1 | 877 | ---------- 878 | ---------- 879 |----------| SP1 SD2 | 880 | ---------- 881 | ---------- 882 |----------| SP2 SD1 | 883 ---------- 885 Figure 6: Security Domain Hierarchy 887 The architecture separates SDs and TAs such that a TAM can only 888 manage or retrieve data for SDs and TAs that it previously created 889 for the SPs it represents. 891 5.12. A Sample Device Setup Flow 893 Step 1: Prepare Images for Devices 895 - 897 1. [TEE vendor] Deliver TEE Image (CODE Binary) to device OEM 899 - 901 1. [CA] Deliver root CA Whitelist 903 - 905 1. [Soc] Deliver TFW Image 907 Step 2: Inject Key Pairs and Images to Devices 908 - 910 1. [OEM] Generate TFW Key Pair (May be shared among multiple 911 devices) 913 - 915 1. [OEM] Flash signed TFW Image and signed TEE Image onto devices 916 (signed by TFW Key) 918 Step 3: Set up attestation key pairs in devices 920 - 922 1. [OEM] Flash TFW Public Key and a bootloader key. 924 - 926 1. [TFW/TEE] Generate a unique attestation key pair and get a 927 certificate for the device. 929 Step 4: Set up trust anchors in devices 931 - 933 1. [TFW/TEE] Store the key and certificate encrypted with the 934 bootloader key 936 - 938 1. [TEE vendor or OEM] Store trusted CA certificate list into 939 devices 941 6. TEEP Broker 943 A TEE and TAs do not generally have the capability to communicate to 944 the outside of the hosting device. For example, GlobalPlatform 945 [GPTEE] specifies one such architecture. This calls for a software 946 module in the REE world to handle the network communication. Each 947 Client Application in the REE might carry this communication 948 functionality but such functionality must also interact with the TEE 949 for the message exchange. The TEE interaction will vary according to 950 different TEEs. In order for a Client Application to transparently 951 support different TEEs, it is imperative to have a common interface 952 for a Client Application to invoke for exchanging messages with TEEs. 954 A shared agent comes to meet this need. An agent is an application 955 running in the REE of the device or an SDK that facilitates 956 communication between a TAM and a TEE. It also provides interfaces 957 for TAM SDK or Client Applications to query and trigger TA 958 installation that the application needs to use. 960 This interface for Client Applications may be commonly an OS service 961 call for an REE OS. A Client Application interacts with a TAM, and 962 turns around to pass messages received from TAM to agent. 964 In all cases, a Client Application needs to be able to identify an 965 agent that it can use. 967 6.1. Role of the Agent 969 An agent abstracts the message exchanges with the TEE in a device. 970 The input data is originated from a TAM to which a Client Application 971 connects. A Client Application may also directly call an Agent for 972 some TA query functions. 974 The agent may internally process a message from a TAM. At least, it 975 needs to know where to route a message, e.g., TEE instance. It does 976 not need to process or verify message content. 978 The agent returns TEE / TFW generated response messages to the 979 caller. The agent is not expected to handle any network connection 980 with an application or TAM. 982 The agent only needs to return an agent error message if the TEE is 983 not reachable for some reason. Other errors are represented as 984 response messages returned from the TEE which will then be passed to 985 the TAM. 987 6.2. Agent Implementation Consideration 989 A Provider should consider methods of distribution, scope and 990 concurrency on devices and runtime options when implementing an 991 agent. Several non-exhaustive options are discussed below. 992 Providers are encouraged to take advantage of the latest 993 communication and platform capabilities to offer the best user 994 experience. 996 6.2.1. Agent Distribution 998 The agent installation is commonly carried out at OEM time. A user 999 can dynamically download and install an agent on-demand. 1001 It is important to ensure a legitimate agent is installed and used. 1002 If an agent is compromised it may drop messages and thereby introduce 1003 a denial of service. 1005 6.2.2. Number of Agents 1007 We anticipate only one shared agent instance in a device. The 1008 device's TEE vendor will most probably supply one agent. 1010 With one shared agent, the agent provider is responsible to allow 1011 multiple TAMs and TEE providers to achieve interoperability. With a 1012 standard agent interface, each TAM can implement its own SDK for its 1013 SP Client Applications to work with this agent. 1015 Multiple independent agent providers can be used as long as they have 1016 standard interface to a Client Application or TAM SDK. Only one 1017 agent is expected in a device. 1019 TAM providers are generally expected to provide an SDK for SP 1020 applications to interact with an agent for the TAM and TEE 1021 interaction. 1023 7. Attestation 1025 7.1. Attestation Hierarchy 1027 The attestation hierarchy and seed required for TAM protocol 1028 operation must be built into the device at manufacture. Additional 1029 TEEs can be added post-manufacture using the scheme proposed, but it 1030 is outside of the current scope of this document to detail that. 1032 It should be noted that the attestation scheme described is based on 1033 signatures. The only decryption that may take place is through the 1034 use of a bootloader key. 1036 A boot module generated attestation can be optional where the 1037 starting point of device attestation can be at TEE certificates. A 1038 TAM can define its policies on what kinds of TEE it trusts if TFW 1039 attestation is not included during the TEE attestation. 1041 7.1.1. Attestation Hierarchy Establishment: Manufacture 1043 During manufacture the following steps are required: 1045 1. A device-specific TFW key pair and certificate are burnt into the 1046 device. This key pair will be used for signing operations 1047 performed by the boot module. 1049 2. TEE images are loaded and include a TEE instance-specific key 1050 pair and certificate. The key pair and certificate are included 1051 in the image and covered by the code signing hash. 1053 3. The process for TEE images is repeated for any subordinate TEEs, 1054 which are additional TEEs after the root TEE that some devices 1055 have. 1057 7.1.2. Attestation Hierarchy Establishment: Device Boot 1059 During device boot the following steps are required: 1061 1. The boot module releases the TFW private key by decrypting it 1062 with the bootloader key. 1064 2. The boot module verifies the code-signing signature of the active 1065 TEE and places its TEE public key into a signing buffer, along 1066 with its identifier for later access. For a TEE non-compliant to 1067 this architecture, the boot module leaves the TEE public key 1068 field blank. 1070 3. The boot module signs the signing buffer with the TFW private 1071 key. 1073 4. Each active TEE performs the same operation as the boot module, 1074 building up their own signed buffer containing subordinate TEE 1075 information. 1077 7.1.3. Attestation Hierarchy Establishment: TAM 1079 Before a TAM can begin operation in the marketplace, it must obtain a 1080 TAM certificate from a CA that is registered in the trust store of 1081 devices. In this way, the TEE can check the intermediate and root CA 1082 and verify that it trusts this TAM to perform operations on the TEE. 1084 8. Algorithm and Attestation Agility 1086 RFC 7696 [RFC7696] outlines the requirements to migrate from one 1087 mandatory-to-implement algorithm suite to another over time. This 1088 feature is also known as crypto agility. Protocol evolution is 1089 greatly simplified when crypto agility is already considered during 1090 the design of the protocol. In the case of Open Trust Protocol 1091 (OTrP) the diverse range of use cases, from trusted app updates for 1092 smart phones and tablets to updates of code on higher-end IoT 1093 devices, creates the need for different mandatory-to-implement 1094 algorithms already from the start. 1096 Crypto agility in the OTrP concerns the use of symmetric as well as 1097 asymmetric algorithms. Symmetric algorithms are used for encryption 1098 of content whereas the asymmetric algorithms are mostly used for 1099 signing messages. 1101 In addition to the use of cryptographic algorithms in OTrP there is 1102 also the need to make use of different attestation technologies. A 1103 Device must provide techniques to inform a TAM about the attestation 1104 technology it supports. For many deployment cases it is more likely 1105 for the TAM to support one or more attestation techniques whereas the 1106 Device may only support one. 1108 9. Security Considerations 1110 9.1. TA Trust Check at TEE 1112 A TA binary is signed by a TA signer certificate. This TA signing 1113 certificate/private key belongs to the SP, and may be self-signed 1114 (i.e., it need not participate in a trust hierarchy). It is the 1115 responsibility of the TAM to only allow verified TAs from trusted SPs 1116 into the system. Delivery of that TA to the TEE is then the 1117 responsibility of the TEE, using the security mechanisms provided by 1118 the protocol. 1120 We allow a way for an (untrusted) application to check the 1121 trustworthiness of a TA. An agent has a function to allow an 1122 application to query the information about a TA. 1124 An application in the Rich O/S may perform verification of the TA by 1125 verifying the signature of the TA. The GetTAInformation function is 1126 available to return the TEE supplied TA signer and TAM signer 1127 information to the application. An application can do additional 1128 trust checks on the certificate returned for this TA. It might trust 1129 the TAM, or require additional SP signer trust chaining. 1131 9.2. One TA Multiple SP Case 1133 A TA for multiple SPs must have a different identifier per SP. A TA 1134 will be installed in a different SD for each respective SP. 1136 9.3. Agent Trust Model 1138 An agent could be malware in the vulnerable REE. A Client 1139 Application will connect its TAM provider for required TA 1140 installation. It gets command messages from the TAM, and passes the 1141 message to the agent. 1143 The architecture enables the TAM to communicate with the device's TEE 1144 to manage SDs and TAs. All TAM messages are signed and sensitive 1145 data is encrypted such that the agent cannot modify or capture 1146 sensitive data. 1148 9.4. Data Protection at TAM and TEE 1150 The TEE implementation provides protection of data on the device. It 1151 is the responsibility of the TAM to protect data on its servers. 1153 9.5. Compromised CA 1155 A root CA for TAM certificates might get compromised. Some TEE trust 1156 anchor update mechanism is expected from device OEMs. A compromised 1157 intermediate CA is covered by OCSP stapling and OCSP validation check 1158 in the protocol. A TEE should validate certificate revocation about 1159 a TAM certificate chain. 1161 If the root CA of some TEE device certificates is compromised, these 1162 devices might be rejected by a TAM, which is a decision of the TAM 1163 implementation and policy choice. Any intermediate CA for TEE device 1164 certificates SHOULD be validated by TAM with a Certificate Revocation 1165 List (CRL) or Online Certificate Status Protocol (OCSP) method. 1167 9.6. Compromised TAM 1169 The TEE SHOULD use validation of the supplied TAM certificates and 1170 OCSP stapled data to validate that the TAM is trustworthy. 1172 Since PKI is used, the integrity of the clock within the TEE 1173 determines the ability of the TEE to reject an expired TAM 1174 certificate, or revoked TAM certificate. Since OCSP stapling 1175 includes signature generation time, certificate validity dates are 1176 compared to the current time. 1178 9.7. Certificate Renewal 1180 TFW and TEE device certificates are expected to be long lived, longer 1181 than the lifetime of a device. A TAM certificate usually has a 1182 moderate lifetime of 2 to 5 years. A TAM should get renewed or 1183 rekeyed certificates. The root CA certificates for a TAM, which are 1184 embedded into the trust anchor store in a device, should have long 1185 lifetimes that don't require device trust anchor update. On the 1186 other hand, it is imperative that OEMs or device providers plan for 1187 support of trust anchor update in their shipped devices. 1189 10. IANA Considerations 1191 This document does not require actions by IANA. 1193 11. Acknowledgements 1195 The authors thank Dave Thaler for his very thorough review and many 1196 important suggestions. Most content of this document is split from a 1197 previously combined OTrP protocol document 1198 [I-D.ietf-teep-opentrustprotocol]. We thank the former co-authors 1199 Nick Cook and Minho Yoo for the initial document content, and 1200 contributors Brian Witten, Tyler Kim, and Alin Mutu. 1202 12. References 1204 12.1. Normative References 1206 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1207 Requirement Levels", BCP 14, RFC 2119, 1208 DOI 10.17487/RFC2119, March 1997, 1209 . 1211 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1212 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1213 May 2017, . 1215 12.2. Informative References 1217 [GPTEE] Global Platform, "GlobalPlatform Device Technology: TEE 1218 System Architecture, v1.1", Global Platform GPD_SPE_009, 1219 January 2017, . 1222 [I-D.ietf-teep-opentrustprotocol] 1223 Pei, M., Atyeo, A., Cook, N., Yoo, M., and H. Tschofenig, 1224 "The Open Trust Protocol (OTrP)", draft-ietf-teep- 1225 opentrustprotocol-01 (work in progress), July 2018. 1227 [RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm 1228 Agility and Selecting Mandatory-to-Implement Algorithms", 1229 BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015, 1230 . 1232 Appendix A. History 1234 RFC EDITOR: PLEASE REMOVE THIS SECTION 1236 IETF Drafts 1238 draft-00: - Initial working group document 1240 Authors' Addresses 1242 Mingliang Pei 1243 Symantec 1245 EMail: mingliang_pei@symantec.com 1247 Hannes Tschofenig 1248 Arm Limited 1250 EMail: hannes.tschofenig@arm.com 1252 David Wheeler 1253 Intel 1255 EMail: david.m.wheeler@intel.com 1257 Andrew Atyeo 1258 Intercede 1260 EMail: andrew.atyeo@intercede.com 1262 Liu Dapeng 1263 Alibaba Group 1265 EMail: maxpassion@gmail.com