Trusted execution environment

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A trusted execution environment (TEE) is a secure area of a main processor. It helps code and data loaded inside it to be protected with respect to confidentiality and integrity. Data confidentiality prevents unauthorized entities from outside the TEE from reading data, while code integrity prevents code in the TEE from being replaced or modified by unauthorized entities, which may also be the computer owner itself as in certain DRM schemes described in SGX. This is done by implementing unique, immutable, and confidential architectural security such as Intel Software Guard Extensions (Intel SGX) which offers hardware-based memory encryption that isolates specific application code and data in memory. Intel SGX allows user-level code to allocate private regions of memory, called enclaves, which are designed to be protected from processes running at higher privilege levels. [1] [2] [3] A TEE as an isolated execution environment provides security features such as isolated execution, integrity of applications executing with the TEE, along with confidentiality of their assets. [4] In general terms, the TEE offers an execution space that provides a higher level of security for trusted applications running on the device than a rich operating system (OS) and more functionality than a 'secure element' (SE).

Contents

History

The Open Mobile Terminal Platform (OMTP) first defined TEE in their "Advanced Trusted Environment:OMTP TR1" standard, defining it as a "set of hardware and software components providing facilities necessary to support Applications" which had to meet the requirements of one of two defined security levels. The first security level, Profile 1, was targeted against only software attacks and while Profile 2, was targeted against both software and hardware attacks. [5]

Commercial TEE solutions based on ARM TrustZone technology, conforming to the TR1 standard, were later launched, such as Trusted Foundations developed by Trusted Logic. [6]

Work on the OMTP standards ended in mid 2010 when the group transitioned into the Wholesale Applications Community (WAC). [7]

The OMTP standards, including those defining a TEE, are hosted by GSMA. [8]

Details

The TEE typically consists of a hardware isolation mechanism, plus a secure operating system running on top of that isolation mechanism – although the term has been used more generally to mean a protected solution. [9] [10] [11] [12] Whilst a GlobalPlatform TEE requires hardware isolation, others such as EMVCo use the term TEE to refer to both hardware/software and only software-based solutions. [13] FIDO uses the concept of TEE in the restricted operating environment for TEEs based on hardware isolation. [14] Only trusted applications running in a TEE have access to the full power of a device's main processor, peripherals and memory, while hardware isolation protects these from user installed apps running in a main operating system. Software and cryptographic isolation inside the TEE protect the trusted applications contained within from each other. [15]

Service providers, mobile network operators (MNO), operating system developers, application developers, device manufacturers, platform providers and silicon vendors are the main stakeholders contributing to the standardization efforts around the TEE.

To prevent simulation of hardware with user-controlled software, a so-called "hardware root of trust" is used. This is a set of private keys that are embedded directly into the chip during manufacturing; one-time programmable memory such as eFuses are usually used on mobile devices. These cannot be changed, even after device resets, and whose public counterparts reside in a manufacturer database, together with a non-secret hash of a public key belonging to the trusted party (usually a chip vendor) which is used to sign trusted firmware alongside the circuits doing cryptographic operations and controlling access. The hardware is designed in a way which prevents all software not signed by the trusted party's key from accessing the privileged features. The public key of the vendor is provided at runtime and hashed; this hash is then compared to the one embedded in the chip. If the hash matches, the public key is used to verify a digital signature of trusted vendor-controlled firmware (such as a chain of bootloaders on Android devices or 'architectural enclaves' in SGX). The trusted firmware is then used to implement remote attestation. [16]

When an application is attested, its untrusted component loads its trusted component into memory; the trusted application is protected from modification by untrusted components with hardware. A nonce is requested by the untrusted party from verifier's server, and is used as a part of a cryptographic authentication protocol, proving integrity of the trusted application. The proof is passed to the verifier, which verifies it. A valid proof cannot be computed in a simulated hardware (i.e. QEMU) because in order to construct it, access to the keys baked into hardware is required; only trusted firmware has access to these keys and/or the keys derived from them or obtained using them. Because only the platform owner is meant to have access to the data recorded in the foundry, the verifying party must interact with the service set up by the vendor. If the scheme is implemented improperly, the chip vendor can track which applications are used on which chip and selectively deny service by returning a message indicating that authentication has not passed. [17]

To simulate hardware in a way which enables it to pass remote authentication, an attacker would have to extract keys from the hardware, which is costly because of the equipment and technical skill required to execute it. For example, using focused ion beams, scanning electron microscopes, microprobing, and chip decapsulation [18] [19] [20] [21] [22] [23] is difficult, or even impossible, if the hardware is designed in such a way that reverse-engineering destroys the keys. In most cases, the keys are unique for each piece of hardware, so that a key extracted from one chip cannot be used by others (for example physically unclonable functions [24] [25] ).

Though deprivation of ownership is not an inherent property of TEEs (it is possible to design the system in a way that allows only the user who has obtained ownership of the device first to control the system, by burning a hash of an own key into e-fuses), in practice all such systems in consumer electronics are intentionally designed so as to allow chip manufacturers to control access to attestation and its algorithms. It allows manufacturers to grant access to TEEs only to software developers who have a (usually commercial) business agreement with the manufacturer, this way monetizing the user base of the hardware, to enable such use cases as tivoization and DRM and to allow certain hardware features to be used only with vendor-supplied software, forcing users to use it despite of its antifeatures, like ads, tracking and use case restriction for market segmentation.

Uses

There are a number of use cases for the TEE. Though not all possible use cases exploit the deprivation of ownership, TEE is usually used exactly for this.

Premium Content Protection/Digital Rights Management

Note: Much TEE literature covers this topic under the definition "premium content protection" which is the preferred nomenclature of many copyright holders. Premium content protection is a specific use case of digital rights management (DRM), and is controversial among some communities, such as the Free Software Foundation. [26] It is widely used by copyrights holders to restrict the ways in which end users can consume content such as 4K high definition films.

The TEE is a suitable environment for protecting digitally encoded information (for example, HD films or audio) on connected devices such as smart phones, tablets and HD televisions. This suitability comes from the ability of the TEE to deprive the owner of the device from reading stored secrets, and the fact that there is often a protected hardware path between the TEE and the display and/or subsystems on devices.

The TEE is used to protect the content once it is on the device: while the content is protected during transmission or streaming by the use of encryption, the TEE protects the content once it has been decrypted on the device by ensuring that decrypted content is not exposed to the environment not approved by the app developer or platform vendor.

Mobile financial services

Mobile commerce applications such as: mobile wallets, peer-to-peer payments, contactless payments or using a mobile device as a point of sale (POS) terminal often have well-defined security requirements. TEEs can be used, often in conjunction with near-field communication (NFC), SEs and trusted backend systems to provide the security required to enable financial transactions to take place

In some scenarios, interaction with the end user is required, and this may require the user to expose sensitive information such as a PIN, password or biometric identifier to the mobile OS as a means of authenticating the user. The TEE optionally offers a trusted user interface which can be used to construct user authentication on a mobile device.

With the rise of cryptocurrency, TEEs are increasingly used to implement crypto-wallets, as they offer the ability to store tokens more securely than regular operating systems, and can provide the necessary computation and authentication applications. [27]

Authentication

The TEE is well-suited for supporting biometric identification methods (facial recognition, fingerprint sensor and voice authorization), which may be easier to use and harder to steal than PINs and passwords. The authentication process is generally split into three main stages:

A TEE is a good area within a mobile device to house the matching engine and the associated processing required to authenticate the user. The environment is designed to protect the data and establish a buffer against the non-secure apps located in mobile OSes. This additional security may help to satisfy the security needs of service providers in addition to keeping the costs low for handset developers.

Enterprise, government, and cloud

The TEE can be used by governments, enterprises, and cloud service providers to enable the secure handling of confidential information on mobile devices and on server infrastructure. The TEE offers a level of protection against software attacks generated in the mobile OS and assists in the control of access rights. It achieves this by housing sensitive, ‘trusted’ applications that need to be isolated and protected from the mobile OS and any malicious malware that may be present. Through utilizing the functionality and security levels offered by the TEE, governments and enterprises can be assured that employees using their own devices are doing so in a secure and trusted manner. Likewise, server-based TEEs help defend against internal and external attacks against backend infrastructure.

Secure modular programming

With the rise of software assets and reuses, modular programming is the most productive process to design software architecture, by decoupling the functionalities into small independent modules. As each module contains everything necessary to execute its desired functionality, the TEE allows to organize the complete system featuring a high level of reliability and security, while preventing each module from vulnerabilities of the others.

In order for the modules to communicate and share data, TEE provide means to securely have payloads sent/received between the modules, using mechanisms such as objects serialization, in conjunction with proxies.

See Component-based software engineering

TEE operating systems

CompanyProductHardware UsedAPI StandardCertification typeReferences
Alibaba Cloud Link TEEGlobalPlatformFull [28]
Apple Secure EnclaveSeparate processorProprietary [29]
BeanPodARM TrustZoneGlobalPlatform [30]
Huawei iTrusteeARM TrustZoneGlobalPlatformFull [31]
Google TrustyARM / IntelProprietary [32]
Linaro OPTEEARM TrustZoneGlobalPlatform [33]
Qualcomm QTEEARM TrustZoneGlobalPlatform + Proprietary [34]
Samsung TEEgrisARM TrustZoneGlobalPlatformFull [35]
TrustKernelT6Arm / IntelGlobalPlatform [36]
TrustonicKinibiARM TrustZoneGlobalPlatformFull [37]
TrustonicSW TEESW TEE onGlobalPlatform [37]
uberSparkuberXMHFARM TrustZone / IntelFormal Mechanized Proof [38]
WatchdataWatchTrustARM TrustZoneGlobalPlatformFull [39]

Hardware support

The following hardware technologies can be used to support TEE implementations:

See also

Related Research Articles

An authenticator is a means used to confirm a user's identity, that is, to perform digital authentication. A person authenticates to a computer system or application by demonstrating that he or she has possession and control of an authenticator. In the simplest case, the authenticator is a common password.

Trusted Computing (TC) is a technology developed and promoted by the Trusted Computing Group. The term is taken from the field of trusted systems and has a specialized meaning that is distinct from the field of confidential computing. With Trusted Computing, the computer will consistently behave in expected ways, and those behaviors will be enforced by computer hardware and software. Enforcing this behavior is achieved by loading the hardware with a unique encryption key that is inaccessible to the rest of the system and the owner.

<span class="mw-page-title-main">Next-Generation Secure Computing Base</span> Software architecture by Microsoft

The Next-Generation Secure Computing Base is a software architecture designed by Microsoft which claimed to provide users of the Windows operating system with better privacy, security, and system integrity. NGSCB was the result of years of research and development within Microsoft to create a secure computing solution that equaled the security of closed platforms such as set-top boxes while simultaneously preserving the backward compatibility, flexibility, and openness of the Windows operating system. Microsoft's primary stated objective with NGSCB was to "protect software from software."

<span class="mw-page-title-main">Secure cryptoprocessor</span> Device used for encryption

A secure cryptoprocessor is a dedicated computer-on-a-chip or microprocessor for carrying out cryptographic operations, embedded in a packaging with multiple physical security measures, which give it a degree of tamper resistance. Unlike cryptographic processors that output decrypted data onto a bus in a secure environment, a secure cryptoprocessor does not output decrypted data or decrypted program instructions in an environment where security cannot always be maintained.

Key management refers to management of cryptographic keys in a cryptosystem. This includes dealing with the generation, exchange, storage, use, crypto-shredding (destruction) and replacement of keys. It includes cryptographic protocol design, key servers, user procedures, and other relevant protocols.

<span class="mw-page-title-main">Trusted Computing Group</span> American-based computer technology consortium

The Trusted Computing Group is a group formed in 2003 as the successor to the Trusted Computing Platform Alliance which was previously formed in 1999 to implement Trusted Computing concepts across personal computers. Members include Intel, AMD, IBM, Microsoft, and Cisco.

<span class="mw-page-title-main">Trusted Platform Module</span> Standard for secure cryptoprocessors

Trusted Platform Module (TPM) is an international standard for a secure cryptoprocessor, a dedicated microcontroller designed to secure hardware through integrated cryptographic keys. The term can also refer to a chip conforming to the standard ISO/IEC 11889.

<span class="mw-page-title-main">Protection ring</span> Layer of protection in computer systems

In computer science, hierarchical protection domains, often called protection rings, are mechanisms to protect data and functionality from faults and malicious behavior.

<span class="mw-page-title-main">Hardware security module</span> Physical computing device

A hardware security module (HSM) is a physical computing device that safeguards and manages secrets, performs encryption and decryption functions for digital signatures, strong authentication and other cryptographic functions. These modules traditionally come in the form of a plug-in card or an external device that attaches directly to a computer or network server. A hardware security module contains one or more secure cryptoprocessor chips.

<span class="mw-page-title-main">Intel vPro</span> Umbrella marketing term by Intel

Intel vPro technology is an umbrella marketing term used by Intel for a large collection of computer hardware technologies, including VT-x, VT-d, Trusted Execution Technology (TXT), and Intel Active Management Technology (AMT). When the vPro brand was launched, it was identified primarily with AMT, thus some journalists still consider AMT to be the essence of vPro.

<span class="mw-page-title-main">Full virtualization</span> Computing technique involving instances of an environment

In computer science, full virtualization (fv) is a modern virtualization technique developed in late 1990s. It is different from simulation and emulation. Virtualization employs techniques that can create instances of a virtual environment, as opposed to simulation, which models the environment; and emulation, which replicates the target environment with certain kinds of virtual environments called emulation environments for virtual machines. Full virtualization requires that every salient feature of the hardware be reflected into one of several virtual machines – including the full instruction set, input/output operations, interrupts, memory access, and whatever other elements are used by the software that runs on the bare machine, and that is intended to run in a virtual machine. In such an environment, any software capable of execution on the raw hardware can be run in the virtual machine and, in particular, any operating systems. The obvious test of full virtualization is whether an operating system intended for stand-alone use can successfully run inside a virtual machine.

<span class="mw-page-title-main">Intel Active Management Technology</span> Out-of-band management platform by Intel

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An embedded hypervisor is a hypervisor that supports the requirements of embedded systems.

<span class="mw-page-title-main">Multi-factor authentication</span> Method of computer access control

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Intel Software Guard Extensions (SGX) is a set of instruction codes implementing trusted execution environment that are built into some Intel central processing units (CPUs). They allow user-level and operating system code to define protected private regions of memory, called enclaves. SGX is designed to be useful for implementing secure remote computation, secure web browsing, and digital rights management (DRM). Other applications include concealment of proprietary algorithms and of encryption keys.

Biometric tokenization is the process of substituting a stored biometric template with a non-sensitive equivalent, called a token, that lacks extrinsic or exploitable meaning or value. The process combines the biometrics with public-key cryptography to enable the use of a stored biometric template for secure or strong authentication to applications or other systems without presenting the template in its original, replicable form.

<span class="mw-page-title-main">Hardware-based encryption</span> Use of computer hardware to assist software in the process of data encryption

Hardware-based encryption is the use of computer hardware to assist software, or sometimes replace software, in the process of data encryption. Typically, this is implemented as part of the processor's instruction set. For example, the AES encryption algorithm can be implemented using the AES instruction set on the ubiquitous x86 architecture. Such instructions also exist on the ARM architecture. However, more unusual systems exist where the cryptography module is separate from the central processor, instead being implemented as a coprocessor, in particular a secure cryptoprocessor or cryptographic accelerator, of which an example is the IBM 4758, or its successor, the IBM 4764. Hardware implementations can be faster and less prone to exploitation than traditional software implementations, and furthermore can be protected against tampering.

<span class="mw-page-title-main">Foreshadow</span> Hardware vulnerability for Intel processors

Foreshadow, known as L1 Terminal Fault (L1TF) by Intel, is a vulnerability that affects modern microprocessors that was first discovered by two independent teams of researchers in January 2018, but was first disclosed to the public on 14 August 2018. The vulnerability is a speculative execution attack on Intel processors that may result in the disclosure of sensitive information stored in personal computers and third-party clouds. There are two versions: the first version (original/Foreshadow) targets data from SGX enclaves; and the second version (next-generation/Foreshadow-NG) targets virtual machines (VMs), hypervisors (VMM), operating systems (OS) kernel memory, and System Management Mode (SMM) memory. A listing of affected Intel hardware has been posted.

Confidential computing is a security and privacy-enhancing computational technique focused on protecting data in use. Confidential computing can be used in conjunction with storage and network encryption, which protect data at rest and data in transit respectively. It is designed to address software, protocol, cryptographic, and basic physical and supply-chain attacks, although some critics have demonstrated architectural and side-channel attacks effective against the technology.

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