--- 1/draft-ietf-teep-architecture-09.txt 2020-06-19 17:13:10.962075243 -0700 +++ 2/draft-ietf-teep-architecture-10.txt 2020-06-19 17:13:11.030076984 -0700 @@ -1,23 +1,23 @@ TEEP M. Pei -Internet-Draft Symantec +Internet-Draft Broadcom Intended status: Informational H. Tschofenig -Expires: December 14, 2020 Arm Limited +Expires: December 21, 2020 Arm Limited D. Thaler Microsoft D. Wheeler Intel - June 12, 2020 + June 19, 2020 Trusted Execution Environment Provisioning (TEEP) Architecture - draft-ietf-teep-architecture-09 + draft-ietf-teep-architecture-10 Abstract A Trusted Execution Environment (TEE) is an environment that enforces that any code within that environment cannot be tampered with, and that any data used by such code cannot be read or tampered with by any code outside that environment. This architecture document motivates the design and standardization of a protocol for managing the lifecycle of trusted applications running inside such a TEE. @@ -29,21 +29,21 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on December 14, 2020. + This Internet-Draft will expire on December 21, 2020. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents @@ -72,42 +72,42 @@ 3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1. Payment . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2. Authentication . . . . . . . . . . . . . . . . . . . . . 8 3.3. Internet of Things . . . . . . . . . . . . . . . . . . . 8 3.4. Confidential Cloud Computing . . . . . . . . . . . . . . 8 4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. System Components . . . . . . . . . . . . . . . . . . . . 8 4.2. Multiple TEEs in a Device . . . . . . . . . . . . . . . . 11 4.3. Multiple TAMs and Relationship to TAs . . . . . . . . . . 13 4.4. Untrusted Apps, Trusted Apps, and Personalization Data . 14 - 4.4.1. Example: Application Delivery Mechanisms in Intel SGX 16 + 4.4.1. Example: Application Delivery Mechanisms in Intel SGX 15 4.4.2. Example: Application Delivery Mechanisms in Arm TrustZone . . . . . . . . . . . . . . . . . . . . . . 16 - 4.5. Entity Relations . . . . . . . . . . . . . . . . . . . . 17 + 4.5. Entity Relations . . . . . . . . . . . . . . . . . . . . 16 5. Keys and Certificate Types . . . . . . . . . . . . . . . . . 18 - 5.1. Trust Anchors in a TEEP Agent . . . . . . . . . . . . . . 20 + 5.1. Trust Anchors in a TEEP Agent . . . . . . . . . . . . . . 19 5.2. Trust Anchors in a TEE . . . . . . . . . . . . . . . . . 20 5.3. Trust Anchors in a TAM . . . . . . . . . . . . . . . . . 20 5.4. Scalability . . . . . . . . . . . . . . . . . . . . . . . 20 - 5.5. Message Security . . . . . . . . . . . . . . . . . . . . 21 + 5.5. Message Security . . . . . . . . . . . . . . . . . . . . 20 6. TEEP Broker . . . . . . . . . . . . . . . . . . . . . . . . . 21 6.1. Role of the TEEP Broker . . . . . . . . . . . . . . . . . 21 6.2. TEEP Broker Implementation Consideration . . . . . . . . 22 6.2.1. TEEP Broker APIs . . . . . . . . . . . . . . . . . . 22 - 6.2.2. TEEP Broker Distribution . . . . . . . . . . . . . . 23 + 6.2.2. TEEP Broker Distribution . . . . . . . . . . . . . . 22 7. Attestation . . . . . . . . . . . . . . . . . . . . . . . . . 23 7.1. Information Required in TEEP Claims . . . . . . . . . . . 24 8. Algorithm and Attestation Agility . . . . . . . . . . . . . . 25 - 9. Security Considerations . . . . . . . . . . . . . . . . . . . 26 - 9.1. Broker Trust Model . . . . . . . . . . . . . . . . . . . 26 - 9.2. Data Protection at TAM and TEE . . . . . . . . . . . . . 26 + 9. Security Considerations . . . . . . . . . . . . . . . . . . . 25 + 9.1. Broker Trust Model . . . . . . . . . . . . . . . . . . . 25 + 9.2. Data Protection . . . . . . . . . . . . . . . . . . . . . 26 9.3. Compromised REE . . . . . . . . . . . . . . . . . . . . . 26 9.4. Compromised CA . . . . . . . . . . . . . . . . . . . . . 27 9.5. Compromised TAM . . . . . . . . . . . . . . . . . . . . . 27 9.6. Malicious TA Removal . . . . . . . . . . . . . . . . . . 27 9.7. Certificate Expiry and Renewal . . . . . . . . . . . . . 28 9.8. Keeping Secrets from the TAM . . . . . . . . . . . . . . 28 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 29 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29 13. Informative References . . . . . . . . . . . . . . . . . . . 29 @@ -127,37 +127,40 @@ applications or data on the device increases. As an example, exposure of emails from a mail client is likely to be of concern to its owner, but a compromise of a banking application raises even greater concerns. The Trusted Execution Environment (TEE) concept is designed to execute applications in a protected environment that enforces that any code within that environment cannot be tampered with, and that any data used by such code cannot be read or tampered with by any code outside that environment, including by a commodity operating - system (if present). + system (if present). In a system with multiple TEEs, this also means + that code in one TEE cannot be read or tampered with by code in the + other TEE. This separation reduces the possibility of a successful attack on application components and the data contained inside the TEE. Typically, application components are chosen to execute inside a TEE because those application components perform security sensitive operations or operate on sensitive data. An application component running inside a TEE is referred to as a Trusted Application (TA), - while an application running outside any TEE is referred to as an - Untrusted Application. In the example of a banking application, code - that relates to the authentication protocol could reside in a TA - while the application logic including HTTP protocol parsing could be - contained in the Untrusted Application. In addition, processing of - credit card numbers or account balances could be done in a TA as it - is sensitive data. The precise code split is ultimately a decision - of the developer based on the assets he or she wants to protect - according to the threat model. + while an application running outside any TEE, i.e., in the Rich + Execution Environment (REE), is referred to as an Untrusted + Application. In the example of a banking application, code that + relates to the authentication protocol could reside in a TA while the + application logic including HTTP protocol parsing could be contained + in the Untrusted Application. In addition, processing of credit card + numbers or account balances could be done in a TA as it is sensitive + data. The precise code split is ultimately a decision of the + developer based on the assets he or she wants to protect according to + the threat model. TEEs use hardware enforcement combined with software protection to secure TAs and its data. TEEs typically offer a more limited set of services to TAs than is normally available to Untrusted Applications. Not all TEEs are the same, however, and different vendors may have different implementations of TEEs with different security properties, different features, and different control mechanisms to operate on TAs. Some vendors may themselves market multiple different TEEs with different properties attuned to different markets. A device vendor @@ -204,43 +207,37 @@ - A Device Administrator wants to remove a TA from a device's TEE if the TA developer is no longer maintaining that TA, when the TA has been revoked or is not used for other reasons anymore (e.g., due to an expired subscription). - A TA developer wants to define the relationship between cooperating TAs under the TA developer's control, and specify whether the TAs can communicate, share data, and/or share key material. - Note: The TA developer requires the help of a TAM and most likely the - Device Administrator to provision the Trusted Applications to remote - devices and the TEEP protocol exchanges messages between a TAM and a - TEEP Agent via a TEEP Broker. - 2. Terminology The following terms are used: - Device: A physical piece of hardware that hosts one or more TEEs, - often along with a Rich Execution Environment. A device contains - a default list of Trust Anchors that identify entities (e.g., - TAMs) that are trusted by the device. This list is normally set - by the device manufacturer, and may be governed by the device's - network carrier when it is a mobile device. The list of Trust - Anchors is normally modifiable by the device's owner or Device - Administrator. However the device manufacturer or network carrier - (in the mobile device case) may restrict some modifications, for - example, by not allowing the manufacturer or carrier's Trust - Anchor to be removed or disabled. + often along with a REE. A device contains a default list of Trust + Anchors that identify entities (e.g., TAMs) that are trusted by + the device. This list is normally set by the device manufacturer, + and may be governed by the device's network carrier when it is a + mobile device. The list of Trust Anchors is normally modifiable + by the device's owner or Device Administrator. However the device + manufacturer or network carrier (in the mobile device case) may + restrict some modifications, for example, by not allowing the + manufacturer or carrier's Trust Anchor to be removed or disabled. - Device Administrator: An entity that is responsible for - administration of a device, which could be the device owner. A + administration of a device, which could be the Device Owner. A Device Administrator has privileges on the device to install and remove Untrusted Applications and TAs, approve or reject Trust Anchors, and approve or reject TA developers, among possibly other privileges on the device. A Device Administrator can manage the list of allowed TAMs by modifying the list of Trust Anchors on the device. Although a Device Administrator may have privileges and device-specific controls to locally administer a device, the Device Administrator may choose to remotely administer a device through a TAM. @@ -252,88 +249,91 @@ administration rights. In this case, the enterprise appoints a Device Administrator that is not the device owner. - Device User: A human being that uses a device. Many devices have a single device user. Some devices have a primary device user with other human beings as secondary device users (e.g., parent allowing children to use their tablet or laptop). Other devices are not used by a human being and hence have no device user. Relates to Device Owner and Device Administrator. + - Raw Public Key (RPK): The RPK only consists of the + SubjectPublicKeyInfo structure of a PKIX certificate that carries + the parameters necessary to describe the public key. Other + serialization formats that do not rely on ASN.1 may also be used. + - Rich Execution Environment (REE): An environment that is provided and governed by a typical OS (e.g., Linux, Windows, Android, iOS), potentially in conjunction with other supporting operating systems and hypervisors; it is outside of any TEE. This environment and applications running on it are considered untrusted (or more precisely, less trusted than a TEE). - Trust Anchor: As defined in [RFC6024] and [I-D.ietf-suit-manifest], "A trust anchor represents an authoritative entity via a public key and associated data. The public key is used to verify digital signatures, and the associated data is used to constrain the types of information for which the trust anchor is authoritative." The Trust Anchor may be a certificate or it may be a raw public key along with additional data if necessary such as its public key algorithm and parameters. - Trust Anchor Store: As defined in [RFC6024], "A trust anchor store is a set of one or more trust anchors stored in a device. A device may have more than one trust anchor store, each of which may be used by one or more applications." As noted in - [I-D.ietf-suit-manifest], a trust anchor store must resist + [I-D.ietf-suit-manifest], a Trust Anchor Store must resist modification against unauthorized insertion, deletion, and modification. - - Trusted Application (TA): An application component that runs in a - TEE. + - Trusted Application (TA): An application (or, in some + implementations, an application component) that runs in a TEE. - - Trusted Application (TA) Developer: An entity that wishes to - provide functionality on devices that requires the use of one or - more Trusted Applications. The TA developer signs the TA binary - (or more precisely the manifest associated with the TA binary) or - uses another entity on his or her behalf to get the TA binary - signed. (A TA binary may also be encrypted by the developer or by - some third party service.) For editorial reasons, we assume that - the TA developer signs the TA binary ignoring the distinction - between the binary and the manifest and by simplifying the case - where the TA developer outsources signing and encryption to a - third party entity or service. + - Trusted Application (TA) Developer: An entity that develops one or + more TAs. + + - Trusted Application (TA) Signer: An entity that signs a TA with a + key that a TEE will trust. The signer might or might not be the + same entity as the TA Developer. For example, a TA might be + signed (or re-signed) by a Device Administrator if the TEE will + only trust the Device Administrator. A TA might also be + encrypted, if the code is considered confidential. - Trusted Application Manager (TAM): An entity that manages Trusted Applications (TAs) running in TEEs of various devices. - Trusted Execution Environment (TEE): An execution environment that enforces that only authorized code can execute within the TEE, and data used by that code cannot be read or tampered with by code outside the TEE. A TEE also generally has a device unique credential that cannot be cloned. There are multiple technologies that can be used to implement a TEE, and the level of security achieved varies accordingly. In addition, TEEs typically use an isolation mechanism between Trusted Applications to ensure that one TA cannot read, modify or delete the data and code of another TA. - - Untrusted Application: An application running in a Rich Execution - Environment. + - Untrusted Application: An application running in an REE. An + Untrusted Application might depend on one or more TAs. 3. Use Cases 3.1. Payment A payment application in a mobile device requires high security and - trust about the hosting device. Payments initiated from a mobile - device can use a Trusted Application to provide strong identification - and proof of transaction. + trust in the hosting device. Payments initiated from a mobile device + can use a Trusted Application to provide strong identification and + proof of transaction. For a mobile payment application, some biometric identification information could also be stored in a TEE. The mobile payment - application can use such information for unlocking the phone and for + application can use such information for unlocking the device and for local identification of the user. A trusted user interface (UI) may be used in a mobile device to prevent malicious software from stealing sensitive user input data. Such an implementation often relies on a TEE for providing access to peripherals, such as PIN input. 3.2. Authentication For better security of authentication, a device may store its keys @@ -385,73 +385,73 @@ | | | +---+ +---+ | +-------+ | | | Device Administrator | | +-------------+ | App-1 | | | | | | | | | | | | +--------------------| |---+ | | | | |--------+ | | +-------+ | +-------------------------------------------+ Figure 1: Notional Architecture of TEEP - - TA developers and Device Administrators utilize the services of a - TAM to manage TAs on devices. TA developers do not directly - interact with devices. Device Administators may elect to use a - TAM for remote administration of TAs instead of managing each - device directly. + - TA Signers and Device Administrators utilize the services of a TAM + to manage TAs on devices. TA Signers do not directly interact + with devices. Device Administators may elect to use a TAM for + remote administration of TAs instead of managing each device + directly. - Trusted Application Manager (TAM): A TAM is responsible for performing lifecycle management activity on TAs on behalf of TA - developers and Device Administrators. This includes creation and + Signers and Device Administrators. This includes creation and deletion of TAs, and may include, for example, over-the-air updates to keep TAs up-to-date and clean up when a version should be removed. TAMs may provide services that make it easier for TA - developers or Device Administators to use the TAM's service to - manage multiple devices, although that is not required of a TAM. + Signers or Device Administators to use the TAM's service to manage + multiple devices, although that is not required of a TAM. The TAM performs its management of TAs on the device through interactions with a device's TEEP Broker, which relays messages between a TAM and a TEEP Agent running inside the TEE. As shown in Figure 1, the TAM cannot directly contact a TEEP Agent, but must wait for the TEEP Broker to contact the TAM requesting a particular service. This architecture is intentional in order to accommodate network and application firewalls that normally protect user and enterprise devices from arbitrary connections from external network entities. - A TAM may be publicly available for use by many TA developers, or - a TAM may be private, and accessible by only one or a limited - number of TA developers. It is expected that many manufacturers - and network carriers will run their own private TAM. + A TAM may be publicly available for use by many TA Signers, or a + TAM may be private, and accessible by only one or a limited number + of TA Signers. It is expected that many manufacturers and network + carriers will run their own private TAM. - A TA developer or Device Administrator chooses a particular TAM - based on whether the TAM is trusted by a device or set of devices. - The TAM is trusted by a device if the TAM's public key is, or - chains up to, an authorized Trust Anchor in the device. A TA - developer or Device Administrator may run their own TAM, but the - devices they wish to manage must include this TAM's public key/ - certificate, or a certificate it chains up to, in the Trust Anchor - list. + A TA Signer or Device Administrator chooses a particular TAM based + on whether the TAM is trusted by a device or set of devices. The + TAM is trusted by a device if the TAM's public key is, or chains + up to, an authorized Trust Anchor in the device. A TA Signer or + Device Administrator may run their own TAM, but the devices they + wish to manage must include this TAM's public key/certificate + [RFC5280], or a certificate it chains up to, in the Trust Anchor + Store. - A TA developer or Device Administrator is free to utilize multiple - TAMs. This may be required for a TA developer to manage multiple - different types of devices from different manufacturers, or to - manage mobile devices on different network carriers, since the - Trust Anchor list on these different devices may contain different - TAMs. A Device Administrator may be able to add their own TAM's - public key or certificate to the Trust Anchor list on all their - devices, overcoming this limitation. + A TA Signer or Device Administrator is free to utilize multiple + TAMs. This may be required for managing TAs on multiple different + types of devices from different manufacturers, or mobile devices + on different network carriers, since the Trust Anchor Store on + these different devices may contain different TAMs. A Device + Administrator may be able to add their own TAM's public key or + certificate to the Trust Anchor Store on all their devices, + overcoming this limitation. Any entity is free to operate a TAM. For a TAM to be successful, it must have its public key or certificate installed in a device's - Trust Anchor list. A TAM may set up a relationship with device + Trust Anchor Store. A TAM may set up a relationship with device manufacturers or network carriers to have them install the TAM's - keys in their device's Trust Anchor list. Alternatively, a TAM + keys in their device's Trust Anchor Store. Alternatively, a TAM may publish its certificate and allow Device Administrators to install the TAM's certificate in their devices as an after-market- action. - TEEP Broker: A TEEP Broker is an application component running in a Rich Execution Environment (REE) that enables the message protocol exchange between a TAM and a TEE in a device. A TEEP Broker does not process messages on behalf of a TEE, but merely is responsible for relaying messages from the TAM to the TEE, and for returning the TEE's responses to the TAM. In devices with no REE @@ -465,60 +465,60 @@ Broker that runs in an REE). A TEEP Agent in the TEE may parse requests or forward requests to other processing modules in a TEE, which is up to a TEE provider's implementation. A response message corresponding to a TAM request is sent back to the TAM, again typically relayed via a TEEP Broker. - Certification Authority (CA): A CA is an entity that issues digital certificates (especially X.509 certificates) and vouches for the binding between the data items in a certificate [RFC4949]. Certificates are then used for authenticating a device, a TAM and - a TA developer. A device embeds a list of root certificates - (Trust Anchors), from trusted CAs that a TAM will be validated - against. A TAM will remotely attest a device by checking whether - a device comes with a certificate from a CA that the TAM trusts. - The CAs do not need to be the same; different CAs can be chosen by - each TAM, and different device CAs can be used by different device + a TA Signer. A device embeds a list of root certificates (Trust + Anchors), from trusted CAs that a TAM will be validated against. + A TAM will remotely attest a device by checking whether a device + comes with a certificate from a CA that the TAM trusts. The CAs + do not need to be the same; different CAs can be chosen by each + TAM, and different device CAs can be used by different device manufacturers. 4.2. Multiple TEEs in a Device Some devices might implement multiple TEEs. In these cases, there might be one shared TEEP Broker that interacts with all the TEEs in the device. However, some TEEs (for example, SGX [SGX]) present themselves as separate containers within memory without a controlling manager within the TEE. As such, there might be multiple TEEP - Brokers in the Rich Execution Environment, where each TEEP Broker - communicates with one or more TEEs associated with it. + Brokers in the REE, where each TEEP Broker communicates with one or + more TEEs associated with it. - It is up to the Rich Execution Environment and the Untrusted - Applications how they select the correct TEEP Broker. Verification - that the correct TA has been reached then becomes a matter of - properly verifying TA attestations, which are unforgeable. + It is up to the REE and the Untrusted Applications how they select + the correct TEEP Broker. Verification that the correct TA has been + reached then becomes a matter of properly verifying TA attestations, + which are unforgeable. The multiple TEEP Broker approach is shown in the diagram below. For brevity, TEEP Broker 2 is shown interacting with only one TAM and Untrusted Application and only one TEE, but no such limitations are intended to be implied in the architecture. +-------------------------------------------+ | Device | - | | TA Developer + | | TA Signer | +-------------+ | | | | TEE-1 | | | | | +-------+ | +--------+ | +--------+ | | | | TEEP | | | TEEP |------------->| |<-+ - | | | Agent |<----------| Broker | | | | + | | | Agent |<----------| Broker | | | | TA | | | 1 | | | 1 |---------+ | | | | +-------+ | | | | | | | | | | | |<---+ | | | | - | | +---+ +---+ | | | | | | +-| TAM-1 | + | | +---+ +---+ | | | | | | +-| TAM-1 |Policy | | |TA1| |TA2| | | |<-+ | | +->| | |<-+ | +-->| | | |<---+ +--------+ | | | | +--------+ | | | | +---+ +---+ | | | | | | TAM-2 | | | | | | | +-------+ | | | +--------+ | | | +-------------+ +-----| App-2 |--+ | | ^ | | | +-------+ | | | | Device | +--------------------| App-1 | | | | | Administrator | +------| | | | | | | +-----------|-+ | |---+ | | | | | TEE-2 | | | |--------+ | | @@ -554,53 +554,47 @@ one or more TEEP Agents and one or more TAMs. The selection of which TAM to communicate with might be made with or without input from an Untrusted Application, but is ultimately the decision of a TEEP Agent. A TEEP Agent is assumed to be able to determine, for any given TA, whether that TA is installed (or minimally, is running) in a TEE with which the TEEP Agent is associated. Each TA is digitally signed, protecting its integrity, and linking - the TA back to the signer. The signer is usually the TA developer, - but in some cases might be another party that the TA developer - trusts, or a party to whom the code has been licensed (in which case - the same code might be signed by multiple licensees and distributed - as if it were different TAs). - - A TA author or signer selects one or more TAMs through which to offer - their TA(s), and communicates the TA(s) to the TAM. In this - document, we use the term "TA developer" to refer to the entity that - selects a TAM and publishes a signed TA to it, independent of whether - the publishing entity is the TA developer or the signer or both. + the TA back to the TA Signer. The TA Signer is often the TA + Developer, but in some cases might be another party such as a Device + Administrator or other party to whom the code has been licensed (in + which case the same code might be signed by multiple licensees and + distributed as if it were different TAs). - The TA developer chooses TAMs based upon the markets into which the - TAM can provide access. There may be TAMs that provide services to - specific types of devices, or device operating systems, or specific - geographical regions or network carriers. A TA developer may be - motivated to utilize multiple TAMs for its service in order to - maximize market penetration and availability on multiple types of - devices. This likely means that the same TA will be available - through multiple TAMs. + A TA Signer selects one or more TAMs and communicates the TA(s) to + the TAM. For example, the TA Signer might choose TAMs based upon the + markets into which the TAM can provide access. There may be TAMs + that provide services to specific types of devices, or device + operating systems, or specific geographical regions or network + carriers. A TA Signer may be motivated to utilize multiple TAMs in + order to maximize market penetration and availability on multiple + types of devices. This means that the same TA will often be + available through multiple TAMs. When the developer of an Untrusted Application that depends on a TA publishes the Untrusted Application to an app store or other app repository, the developer optionally binds the Untrusted Application with a manifest that identifies what TAMs can be contacted for the TA. In some situations, a TA may only be available via a single TAM - - this is likely the case for enterprise applications or TA - developers serving a closed community. For broad public apps, there - will likely be multiple TAMs in the manifest - one servicing one - brand of mobile device and another servicing a different - manufacturer, etc. Because different devices and different - manufacturers trust different TAMs, the manifest can include multiple - TAMs that support the required TA. + - this is likely the case for enterprise applications or TA Signers + serving a closed community. For broad public apps, there will likely + be multiple TAMs in the manifest - one servicing one brand of mobile + device and another servicing a different manufacturer, etc. Because + different devices and different manufacturers trust different TAMs, + the manifest can include multiple TAMs that support the required TA. When a TEEP Broker receives a request from an Untrusted Application to install a TA, a list of TAM URIs may be provided for that TA, and the request is passed to the TEEP Agent. If the TEEP Agent decides that the TA needs to be installed, the TEEP Agent selects a single TAM URI that is consistent with the list of trusted TAMs provisioned on the device, invokes the HTTP transport for TEEP to connect to the TAM URI, and begins a TEEP protocol exchange. When the TEEP Agent subsequently receives the TA to install and the TA's manifest indicates dependencies on any other trusted components, each @@ -643,48 +637,47 @@ Untrusted Application in a REE and one or more TAs in a TEE, as shown in Figure 2. For most purposes, an Untrusted Application that uses one or more TAs in a TEE appears no different from any other Untrusted Application in the REE. However, the way the Untrusted Application and its corresponding TAs are packaged, delivered, and installed on the device can vary. The variations depend on whether the Untrusted Application and TA are bundled together or are provided separately, and this has implications to the management of the TAs in a TEE. In addition to the Untrusted Application and TA(s), the TA(s) and/or TEE may require some additional data to personalize the TA to - the TA developer or the device or a user. This personalization data - may dependent on the type of TEE, a particular TEE instance, the TA, - the TA developer and even the user of the device; an example of - personalization data might be a secret symmetric key used by the TA - to communicate with some service. Implementations must support - encryption of personalization data to preserve the confidentiality of - potentially sensitive data contained within it and support integrity - protection of the personalization data. Other than the requirement - to support confidentiality and integrity protection, the TEEP - architecture places no limitations or requirements on the - personalization data. + the device or a user. This personalization data may depend on the + type of TEE, a particular TEE instance, the TA, and even the user of + the device; an example of personalization data might be a secret + symmetric key used by the TA to communicate with some service. + Implementations must support encryption of personalization data to + preserve the confidentiality of potentially sensitive data contained + within it and support integrity protection of the personalization + data. Other than the requirement to support confidentiality and + integrity protection, the TEEP architecture places no limitations or + requirements on the personalization data. There are three possible cases for bundling of an Untrusted Application, TA(s), and personalization data: 1. The Untrusted Application, TA(s), and personalization data are - all bundled together in a single package by a TA developer and + all bundled together in a single package by a TA Signer and provided to the TEEP Broker through the TAM. 2. The Untrusted Application and the TA(s) are bundled together in a single package, which a TAM or a publicly accessible app store maintains, and the personalization data is separately provided by - the TA developer's TAM. + the TA Signer's TAM. 3. All components are independent. The Untrusted Application is installed through some independent or device-specific mechanism, and the TAM provides the TA and personalization data from the TA - developer. Delivery of the TA and personalization data may be + Signer. Delivery of the TA and personalization data may be combined or separate. The TEEP protocol treats each TA, any dependencies the TA has, and personalization data as separate components with separate installation steps that are expressed in SUIT manifests, and a SUIT manifest might contain or reference multiple binaries (see [I-D.ietf-suit-manifest] for more details). The TEEP Agent is responsible for handling any installation steps that need to be performed inside the TEE, such as decryption of private TA binaries or personalization data. @@ -743,140 +736,140 @@ is possible as well, though still more complex than Cases 2 and 3. 4.5. Entity Relations This architecture leverages asymmetric cryptography to authenticate a device to a TAM. Additionally, a TEEP Agent in a device authenticates a TAM. The provisioning of Trust Anchors to a device may be different from one use case to the other. A Device Administrator may want to have the capability to control what TAs are allowed. A device manufacturer enables verification by one or more - TAMs and by TA developers; it may embed a list of default Trust - Anchors into the TEEP Agent and TEE for TAM and TA trust - verification. + TAMs and by TA Signers; it may embed a list of default Trust Anchors + into the TEEP Agent and TEE for TAM trust verification and TA + signature verification. (App Developers) (App Store) (TAM) (Device with TEE) (CAs) | | | | | | | | (Embedded TEE cert) <--| | | | | | | <--- Get an app cert -----------------------------------| | | | | | | | | <-- Get a TAM cert ---------| | | | | | 1. Build two apps: | | | | | | | | (a) Untrusted | | | | App - 2a. Supply --> | --- 3. Install ------> | | | | | | (b) TA -- 2b. Supply ----------> | 4. Messaging-->| | | | | | - Figure 3: Developer Experience + Figure 3: Example Developer Experience - Note that Figure 3 shows the view from a TA developer point of view. - The TA developer signs the TA or is a related entity trusted to sign - the developer-created TAs. + Figure 3 shows an example where the same developer builds and signs + two applications: 1) an Untrusted Application; 2) a TA that provides + some security functions to be run inside a TEE. - Figure 3 shows an example where the same developer builds two - applications: 1) an Untrusted Application; 2) a TA that provides some - security functions to be run inside a TEE. At step 2, the TA - developer uploads the Untrusted Application (2a) to an Application - Store. The Untrusted Application may optionally bundle the TA - binary. Meanwhile, the TA developer may provide its TA to a TAM that - will be managing the TA in various devices. At step 3, a user will - go to an Application Store to download the Untrusted Application. - Since the Untrusted Application depends on the TA, installing the - Untrusted Application will trigger TA installation by initiating - communication with a TAM. This is step 4. The TEEP Agent will - interact with TAM via a TEEP Broker that faciliates communications - between a TAM and the TEEP Agent in TEE. + At step 2, the developer uploads the Untrusted Application (2a) to an + Application Store. In this example, the developer is also the TA + Signer, and so generates a signed TA. The developer can then either + bundle the signed TA with the Untrusted Application, or the developer + can provide the signed TA to a TAM that will be managing the TA in + various devices. + + At step 3, a user will go to an Application Store to download the + Untrusted Application. Since the Untrusted Application depends on + the TA, installing the Untrusted Application will trigger TA + installation by initiating communication with a TAM. This is step 4. + The TEEP Agent will interact with TAM via a TEEP Broker that + faciliates communications between a TAM and the TEEP Agent in TEE. Some TA installation implementations might ask for a user's consent. In other implementations, a Device Administrator might choose what Untrusted Applications and related TAs to be installed. A user consent flow is out of scope of the TEEP architecture. The main components consist of a set of standard messages created by a TAM to deliver TA management commands to a device, and device attestation and response messages created by a TEE that responds to a TAM's message. It should be noted that network communication capability is generally not available in TAs in today's TEE-powered devices. Consequently, Trusted Applications generally rely on broker in the REE to provide - access to nnetwork functionality in the REE. A broker does not need + access to network functionality in the REE. A broker does not need to know the actual content of messages to facilitate such access. Similarly, since the TEEP Agent runs inside a TEE, the TEEP Agent generally relies on a TEEP Broker in the REE to provide network access, and relay TAM requests to the TEEP Agent and relay the responses back to the TAM. 5. Keys and Certificate Types This architecture leverages the following credentials, which allow delivering end-to-end security between a TAM and a TEEP Agent. Figure 4 summarizes the relationships between various keys and where - they are stored. Each public/private key identifies a TA developer, - TAM, or TEE, and gets a certificate that chains up to some CA. A - list of trusted certificates is then used to check a presented - certificate against. + they are stored. Each public/private key identifies a TA Signer, + TAM, or TEE, and gets a certificate that chains up to some trust + anchor. A list of trusted certificates is then used to check a + presented certificate against. Different CAs can be used for different types of certificates. TEEP messages are always signed, where the signer key is the message originator's private key, such as that of a TAM or a TEE. In addition to the keys shown in Figure 4, there may be additional keys used for attestation. Refer to the RATS Architecture [I-D.ietf-rats-architecture] for more discussion. Cardinality & Location of Location of Private Key Trust Anchor Purpose Private Key Signs Store ------------------ ----------- ------------- ------------- Authenticating TEE 1 per TEE TEEP responses TAM Authenticating TAM 1 per TAM TEEP requests TEEP Agent Code Signing 1 per TA TA binary TEE - developer + Signer - Figure 4: Keys + Figure 4: Signature Keys Note that personalization data is not included in the table above. The use of personalization data is dependent on how TAs are used and what their security requirements are. - TEEP requests from a TAM to a TEEP Agent can be encrypted with the - TEE public key (to provide confidentiality), and are then signed with - the TAM private key (for authentication and integrity protection). - Conversely, TEEP responses from a TEEP Agent to a TAM can be - encrypted with the TAM public key, and are then signed with the TEE - private key. + TEEP requests from a TAM to a TEEP Agent are signed with the TAM + private key (for authentication and integrity protection). + Personalization data and TA binaries can be encrypted with a key that + is established with a content encryption key established with the TEE + public key (to provide confidentiality). Conversely, TEEP responses + from a TEEP Agent to a TAM can be signed with the TEE private key. The TEE key pair and certificate are thus used for authenticating the TEE to a remote TAM, and for sending private data to the TEE. Often, the key pair is burned into the TEE by the TEE manufacturer and the key pair and its certificate are valid for the expected lifetime of the TEE. A TAM provider is responsible for configuring the TAM's Trust Anchor Store with the manufacturer certificates or CAs that are used to sign TEE keys. This is discussed further in Section 5.3 below. The TAM key pair and certificate are used for authenticating a TAM to a remote TEE, and for sending private data to the TAM. A TAM provider is responsible for acquiring a certificate from a CA that is trusted by the TEEs it manages. This is discussed further in Section 5.1 below. - The TA developer key pair and certificate are used to sign TAs that - the TEE will consider authorized to execute. TEEs must be configured + The TA Signer key pair and certificate are used to sign TAs that the + TEE will consider authorized to execute. TEEs must be configured with the certificates or keys that it considers authorized to sign TAs that it will execute. This is discussed further in Section 5.2 below. 5.1. Trust Anchors in a TEEP Agent A TEEP Agent's Trust Anchor Store contains a list of Trust Anchors, which are CA certificates that sign various TAM certificates. The list is typically preloaded at manufacturing time, and can be updated using the TEEP protocol if the TEE has some form of "Trust Anchor @@ -885,69 +878,73 @@ data for any other TA. When Trust Anchor update is carried out, it is imperative that any update must maintain integrity where only an authentic Trust Anchor list from a device manufacturer or a Device Administrator is accepted. Details are out of scope of the architecture and can be addressed in a protocol document. Before a TAM can begin operation in the marketplace to support a device with a particular TEE, it must obtain a TAM certificate from a - CA that is listed in the Trust Anchor Store of the TEEP Agent. + CA or the raw public key of a TAM that is listed in the Trust Anchor + Store of the TEEP Agent. 5.2. Trust Anchors in a TEE A TEE determines whether TA binaries are allowed to execute by - verifying whether the TA's signer chains up to a certificate in the - TEE's Trust Anchor Store. The list is typically preloaded at - manufacturing time, and can be updated using the TEEP protocol if the - TEE has some form of "Trust Anchor Manager TA" that has Trust Anchors - in its configuration data. Thus, Trust Anchors can be updated - similar to updating the configuration data for any other TA, as - discussed in Section 5.1. + verifying whether their signature can be verified using + certificate(s) or raw public key(s) in the TEE's Trust Anchor Store. + The list is typically preloaded at manufacturing time, and can be + updated using the TEEP protocol if the TEE has some form of "Trust + Anchor Manager TA" that has Trust Anchors in its configuration data. + Thus, Trust Anchors can be updated similar to updating the + configuration data for any other TA, as discussed in Section 5.1. 5.3. Trust Anchors in a TAM The Trust Anchor Store in a TAM consists of a list of Trust Anchors, which are certificates that sign various device TEE certificates. A TAM will accept a device for TA management if the TEE in the device - uses a TEE certificate that is chained to a certificate that the TAM - trusts. + uses a TEE certificate that is chained to a certificate or raw public + key that the TAM trusts, is contained in an allow list, is not found + on a block list, and/or fulfills any other policy criteria. 5.4. Scalability - This architecture uses a PKI, although self-signed certificates are - also permitted. Trust Anchors exist on the devices to enable the TEE - to authenticate TAMs and TA developer, and TAMs use Trust Anchors to - authenticate TEEs. When a PKI is used, many intermediate CA - certificates can chain to a root certificate, each of which can issue - many certificates. This makes the protocol highly scalable. New - factories that produce TEEs can join the ecosystem. In this case, - such a factory can get an intermediate CA certificate from one of the - existing roots without requiring that TAMs are updated with - information about the new device factory. Likewise, new TAMs can - join the ecosystem, providing they are issued a TAM certificate that - chains to an existing root whereby existing TEEs will be allowed to - be personalized by the TAM without requiring changes to the TEE - itself. This enables the ecosystem to scale, and avoids the need for - centralized databases of all TEEs produced or all TAMs that exist or - all TA developers that exist. + This architecture uses a PKI (including self-signed certificates). + Trust Anchors exist on the devices to enable the TEE to authenticate + TAMs and TA Signers, and TAMs use Trust Anchors to authenticate TEEs. + When a PKI is used, many intermediate CA certificates can chain to a + root certificate, each of which can issue many certificates. This + makes the protocol highly scalable. New factories that produce TEEs + can join the ecosystem. In this case, such a factory can get an + intermediate CA certificate from one of the existing roots without + requiring that TAMs are updated with information about the new device + factory. Likewise, new TAMs can join the ecosystem, providing they + are issued a TAM certificate that chains to an existing root whereby + existing TEEs will be allowed to be personalized by the TAM without + requiring changes to the TEE itself. This enables the ecosystem to + scale, and avoids the need for centralized databases of all TEEs + produced or all TAMs that exist or all TA Signers that exist. 5.5. Message Security Messages created by a TAM are used to deliver TA management commands to a device, and device attestation and messages created by the device TEE to respond to TAM messages. - These messages are signed end-to-end between a TEEP Agent and a TAM, - and are typically encrypted such that only the targeted device TEE or - TAM is able to decrypt and view the actual content. + These messages are signed end-to-end between a TEEP Agent and a TAM. + Confidentiality is provided by encrypting sensitive payloads (such as + personalization data and attestation evidence), rather than + encrypting the messages themselves. Using encrypted payloads is + important to ensure that only the targeted device TEE or TAM is able + to decrypt and view the actual content. 6. TEEP Broker A TEE and TAs often do not have the capability to directly communicate outside of the hosting device. For example, GlobalPlatform [GPTEE] specifies one such architecture. This calls for a software module in the REE world to handle network communication with a TAM. A TEEP Broker is an application component running in the REE of the @@ -1028,26 +1025,26 @@ 6.2.2. TEEP Broker Distribution The Broker installation is commonly carried out at OEM time. A user can dynamically download and install a Broker on-demand. 7. Attestation Attestation is the process through which one entity (an Attester) presents "evidence", in the form of a series of claims, to another entity (a Verifier), and provides sufficient proof that the claims - are true. Different Verifiers may have different standards for - attestation proofs and not all attestations are acceptable to every - verifier. A third entity (a Relying Party) can then use "attestation - results", in the form of another series of claims, from a Verifier to - make authorization decisions. (See [I-D.ietf-rats-architecture] for - more discussion.) + are true. Different Verifiers may require different degrees of + confidence in attestation proofs and not all attestations are + acceptable to every verifier. A third entity (a Relying Party) can + then use "attestation results", in the form of another series of + claims, from a Verifier to make authorization decisions. (See + [I-D.ietf-rats-architecture] for more discussion.) In TEEP, as depicted in Figure 5, the primary purpose of an attestation is to allow a device (the Attester) to prove to a TAM (the Relying Party) that a TEE in the device has particular properties, was built by a particular manufacturer, and/or is executing a particular TA. Other claims are possible; TEEP does not limit the claims that may appear in evidence or attestation results, but defines a minimal set of attestation result claims required for TEEP to operate properly. Extensions to these claims are possible. Other standards or groups may define the format and semantics of @@ -1062,27 +1059,30 @@ +----------------+ Result Figure 5: TEEP Attestation Roles As of the writing of this specification, device and TEE attestations have not been standardized across the market. Different devices, manufacturers, and TEEs support different attestation protocols. In order for TEEP to be inclusive, it is agnostic to the format of evidence, allowing proprietary or standardized formats to be used between a TEE and a verifier (which may or may not be colocated in - the TAM). However, it should be recognized that not all Verifiers - may be able to process all proprietary forms of attestation evidence. - Similarly, the TEEP protocol is agnostic as to the format of - attestation results, and the protocol (if any) used between the TAM - and a verifier, as long as they convey at least the required set of - claims in some format. Note that the respective attestation - algorithms are not defined in the TEEP protocol itself; see + the TAM), as long as the format supports encryption of any + information that is considered sensitive. + + However, it should be recognized that not all Verifiers may be able + to process all proprietary forms of attestation evidence. Similarly, + the TEEP protocol is agnostic as to the format of attestation + results, and the protocol (if any) used between the TAM and a + verifier, as long as they convey at least the required set of claims + in some format. Note that the respective attestation algorithms are + not defined in the TEEP protocol itself; see [I-D.ietf-rats-architecture] and [I-D.ietf-teep-protocol] for more discussion. There are a number of considerations that need to be considered when appraising evidence provided by a TEE, including: - What security measures a manufacturer takes when provisioning keys into devices/TEEs; - What hardware and software components have access to the @@ -1105,56 +1105,56 @@ claims are outside the scope of this specification, but the TEEP protocol allows these additional claims to be included in the attestation messages. For more discussion of the attestation and appraisal process, see the RATS Architecture [I-D.ietf-rats-architecture]. 7.1. Information Required in TEEP Claims - Device Identifying Info: TEEP attestations may need to uniquely - identify a device to the TAM and TA developer. Unique device - identification allows the TAM to provide services to the device, - such as managing installed TAs, and providing subscriptions to - services, and locating device-specific keying material to - communicate with or authenticate the device. In some use cases it - may be sufficient to identify only the class of the device. The - security and privacy requirements regarding device identification - will vary with the type of TA provisioned to the TEE. + identify a device to the TAM. Unique device identification allows + the TAM to provide services to the device, such as managing + installed TAs, and providing subscriptions to services, and + locating device-specific keying material to communicate with or + authenticate the device. In some use cases it may be sufficient + to identify only the class of the device. The security and + privacy requirements regarding device identification will vary + with the type of TA provisioned to the TEE. - TEE Identifying info: The type of TEE that generated this attestation must be identified, including version identification information such as the hardware, firmware, and software version of the TEE, as applicable by the TEE type. TEE manufacturer information for the TEE is required in order to disambiguate the same TEE type created by different manufacturers and address considerations around manufacturer provisioning, keying and support for the TEE. - Freshness Proof: A claim that includes freshness information must be included, such as a nonce or timestamp. - Requested Components: A list of zero or more components (TAs or other dependencies needed by a TEE) that are requested by some depending app, but which are not currently installed in the TEE. 8. Algorithm and Attestation Agility RFC 7696 [RFC7696] outlines the requirements to migrate from one - mandatory-to-implement algorithm suite to another over time. This - feature is also known as crypto agility. Protocol evolution is - greatly simplified when crypto agility is considered during the - design of the protocol. In the case of the TEEP protocol the diverse - range of use cases, from trusted app updates for smart phones and - tablets to updates of code on higher-end IoT devices, creates the - need for different mandatory-to-implement algorithms already from the - start. + mandatory-to-implement cryptographic algorithm suite to another over + time. This feature is also known as crypto agility. Protocol + evolution is greatly simplified when crypto agility is considered + during the design of the protocol. In the case of the TEEP protocol + the diverse range of use cases, from trusted app updates for smart + phones and tablets to updates of code on higher-end IoT devices, + creates the need for different mandatory-to-implement algorithms + already from the start. Crypto agility in TEEP concerns the use of symmetric as well as asymmetric algorithms. In the context of TEEP symmetric algorithms are used for encryption of TA binaries and personalization data whereas the asymmetric algorithms are mostly used for signing messages. In addition to the use of cryptographic algorithms in TEEP, there is also the need to make use of different attestation technologies. A device must provide techniques to inform a TAM about the attestation @@ -1177,25 +1177,35 @@ A TEEP Agent in a TEE is responsible for protecting against potential attacks from a compromised TEEP Broker or rogue malware in the REE. A rogue TEEP Broker might send corrupted data to the TEEP Agent, or launch a DoS attack by sending a flood of TEEP protocol requests. The TEEP Agent validates the signature of each TEEP protocol request and checks the signing certificate against its Trust Anchors. To mitigate DoS attacks, it might also add some protection scheme such as a threshold on repeated requests or number of TAs that can be installed. -9.2. Data Protection at TAM and TEE +9.2. Data Protection The TEE implementation provides protection of data on the device. It is the responsibility of the TAM to protect data on its servers. + The protocol between TEEP Agents and TAMs similarly is responsible + for securely providing integrity and confidentiality protection + against adversaries between them. Since the transport protocol under + the TEEP protocol might be implemented outside a TEE, as discussed in + Section 6, it cannot be relied upon for sufficient protection. The + TEEP protocol provides integrity protection, but confidentiality must + be provided by payload security, i.e., using encrypted TA binaries + and encrypted attestation information. See [I-D.ietf-teep-protocol] + for more discussion. + 9.3. Compromised REE It is possible that the REE of a device is compromised. If the REE is compromised, several DoS attacks may be launched. The compromised REE may terminate the TEEP Broker such that TEEP transactions cannot reach the TEE, or might drop or delay messages between a TAM and a TEEP Agent. However, while a DoS attack cannot be prevented, the REE cannot access anything in the TEE if it is implemented correctly. Some TEEs may have some watchdog scheme to observe REE state and mitigate DoS attacks against it but most TEEs don't have such a @@ -1247,21 +1257,21 @@ It is possible that a rogue developer distributes a malicious Untrusted Application and intends to get a malicious TA installed. It's the responsibility of the TAM to not install malicious trusted apps in the first place. The TEEP architecture allows a TEEP Agent to decide which TAMs it trusts via Trust Anchors, and delegates the TA authenticity check to the TAMs it trusts. It may happen that a TA was previously considered trustworthy but is later found to be buggy or compromised. In this case, the TAM can initiate the removal of the TA by notifying devices to remove the TA - (and potentially the REE or device owner to remove any Untrusted + (and potentially the REE or Device Owner to remove any Untrusted Application that depend on the TA). If the TAM does not currently have a connection to the TEEP Agent on a device, such a notification would occur the next time connectivity does exist. That is, to recover, the TEEP Agent must be able to reach out to the TAM, for example whenever the RequestPolicyCheck API (Section 6.2.1) is invoked by a timer or other event. Furthermore the policy in the Verifier in an attestation process can be updated so that any evidence that includes the malicious TA would result in an attestation failure. There is, however, a time window @@ -1274,42 +1284,42 @@ Verifier should take into account the acceptable time window when generating attestation results. See [I-D.ietf-rats-architecture] for further discussion. 9.7. Certificate Expiry and Renewal TEE device certificates are expected to be long lived, longer than the lifetime of a device. A TAM certificate usually has a moderate lifetime of 2 to 5 years. A TAM should get renewed or rekeyed certificates. The root CA certificates for a TAM, which are embedded - into the Trust Anchor store in a device, should have long lifetimes - that don't require device Trust Anchor update. On the other hand, it - is imperative that OEMs or device providers plan for support of Trust - Anchor update in their shipped devices. + into the Trust Anchor Store in a device, should have long lifetimes + that don't require device Trust Anchor updates. On the other hand, + it is imperative that OEMs or device providers plan for support of + Trust Anchor update in their shipped devices. For those cases where TEE devices are given certificates for which no good expiration date can be assigned the recommendations in Section 4.1.2.5 of RFC 5280 [RFC5280] are applicable. 9.8. Keeping Secrets from the TAM In some scenarios, it is desirable to protect the TA binary or configuration from being disclosed to the TAM that distributes them. In such a scenario, the files can be encrypted end-to-end between a - TA developer and a TEE. However, there must be some means of + TA Signer and a TEE. However, there must be some means of provisioning the decryption key into the TEE and/or some means of the - TA developer securely learning a public key of the TEE that it can - use to encrypt. One way to do this is for the TA developer to run - its own TAM so that it can distribute the decryption key via the TEEP + TA Signer securely learning a public key of the TEE that it can use + to encrypt. One way to do this is for the TA Signer to run its own + TAM so that it can distribute the decryption key via the TEEP protocol, and the key file can be a dependency in the manifest of the encrypted TA. Thus, the TEEP Agent would look at the TA manifest, - determine there is a dependency with a TAM URI of the TA developer's + determine there is a dependency with a TAM URI of the TA Signer's TAM. The Agent would then install the dependency, and then continue with the TA installation steps, including decrypting the TA binary with the relevant key. 10. IANA Considerations This document does not require actions by IANA. 11. Contributors @@ -1381,27 +1391,27 @@ extensions.html>. [TrustZone] Arm, "Arm TrustZone Technology", n.d., . Authors' Addresses Mingliang Pei - Symantec + Broadcom - EMail: mingliang_pei@symantec.com + EMail: mingliang.pei@broadcom.com Hannes Tschofenig Arm Limited EMail: hannes.tschofenig@arm.com - Dave Thaler Microsoft EMail: dthaler@microsoft.com + David Wheeler Intel EMail: david.m.wheeler@intel.com