Trusted Platform Module

Trusted Platform Module (TPM) was conceived by a computer industry consortium called Trusted Computing Group (TCG). It evolved into TPM Main Specification Version 1.2 which was standardized by International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC) in 2009 as ISO/IEC 11889:2009.[2] TPM Main Specification Version 1.2 was finalized on March 3, 2011, completing its revision.[3][4]

On April 9, 2014, the Trusted Computing Group announced a major upgrade to their specification entitled TPM Library Specification 2.0.[5] The group continues work on the standard incorporating errata, algorithmic additions and new commands, with its most recent edition published as 2.0 in November 2019.[6] This version became ISO/IEC 11889:2015.

When a new revision is released it is divided into multiple parts by the Trusted Computing Group. Each part consists of a document that makes up the whole of the new TPM specification.

• Part 1 – Architecture (renamed from Design Principles)
• Part 2 – Structures of the TPM
• Part 3 – Commands
• Part 4 – Supporting Routines (added in TPM 2.0)

Trusted Platform Module provides

• A hardware random number generator[7][8]
• Facilities for the secure generation of cryptographic keys for limited uses.
• Remote attestation: Creates a nearly unforgeable hash key summary of the hardware and software configuration. One could use the hash to verify that the hardware and software have not been changed. The software in charge of hashing the setup determines the extent of the summary.
• Binding: Encrypts data using the TPM bind key, a unique RSA key descended from a storage key. Computers that incorporate a TPM can create cryptographic keys and encrypt them so that they can only be decrypted by the TPM. This process, often called wrapping or binding a key, can help protect the key from disclosure. Each TPM has a master wrapping key, called the storage root key, which is stored within the TPM itself. User-level RSA key containers are stored with the Windows user profile for a particular user and can be used to encrypt and decrypt information for applications that run under that specific user identity.[9][10]
• Sealing: Similar to binding, but in addition, specifies the TPM state[11] for the data to be decrypted (unsealed).[12]
• Other Trusted Computing functions for the data to be decrypted (unsealed).[13]

Computer programs can use a TPM to authenticate hardware devices, since each TPM chip has a unique and secret Endorsement Key (EK) burned in as it is produced. Security embedded in hardware provides more protection than a software-only solution.[14] Its use is restricted in some countries.[15]

Trusted Platform Module installed on a motherboard

In 2006, new laptops began being sold with a built-in TPM chip. In the future, this concept could be co-located on an existing motherboard chip in computers, or any other device where the TPM facilities could be employed, such as a cellphone. On a PC, either the LPC bus or the SPI bus is used to connect to the TPM chip.

The Trusted Computing Group (TCG) has certified TPM chips manufactured by Infineon Technologies, Nuvoton, and STMicroelectronics,[25] having assigned TPM vendor IDs to Advanced Micro Devices, Atmel, Broadcom, IBM, Infineon, Intel, Lenovo, National Semiconductor, Nationz Technologies, Nuvoton, Qualcomm, Rockchip, Standard Microsystems Corporation, STMicroelectronics, Samsung, Sinosun, Texas Instruments, and Winbond.[26]

There are five different types of TPM 2.0 implementations (listed in order from most to least secure):[27][28]

• Discrete TPMs are dedicated chips that implement TPM functionality in their own tamper resistant semiconductor package. They are the most secure, certified to FIPS-140 with level 3 physical security[29] resistance to attack versus routines implemented in software, and their packages are required to implement some tamper resistance. For example, the TPM for the brake controller in a car is protected from hacking by sophisticated methods.[30]
• Integrated TPMs are part of another chip. While they use hardware that resists software bugs, they are not required to implement tamper resistance. Intel has integrated TPMs in some of its chipsets.
• Firmware TPMs (fTPMs) are firmware-based (e.g. UEFI) solutions that run in a CPU's trusted execution environment. Intel, AMD and Qualcomm have implemented firmware TPMs.
• Hypervisor TPMs (vTPMs) are virtual TPMs provided by and rely on hypervisors, in an isolated execution environment that is hidden from the software running inside virtual machines to secure their code from the software in the virtual machines. They can provide a security level comparable to a firmware TPM. Google Cloud Platform has implemented vTPM.[31]
• Software TPMs are software emulators of TPMs that run with no more protection than a regular program gets within an operating system. They depend entirely on the environment that they run in, so they provide no more security than what can be provided by the normal execution environment. They are useful for development purposes.

The official TCG reference implementation of the TPM 2.0 Specification has been developed by Microsoft. It is licensed under BSD License and the source code is available on GitHub.[32] Microsoft provides a Visual Studio solution and Linux autotools build scripts.

In 2018, Intel open-sourced its Trusted Platform Module 2.0 (TPM2) software stack with support for Linux and Microsoft Windows.[33] The source code is hosted on GitHub and licensed under BSD License.[34][35]

Infineon funded the development of an open source TPM middleware that complies with the Software Stack (TSS) Enhanced System API (ESAPI) specification of the TCG.[36] It was developed by Fraunhofer Institute for Secure Information Technology (SIT).[37]

IBM's Software TPM 2.0 is an implementation of the TCG TPM 2.0 specification. It is based on the TPM specification Parts 3 and 4 and source code donated by Microsoft. It contains additional files to complete the implementation. The source code is hosted on SourceForge[38] and GitHub[39] and licensed under BSD License.

In 2022, AMD announced that under certain circumstances their fTPM implementation causes performance problems. A fix is available in form of a BIOS-Update.[40][41]

While TPM 2.0 addresses many of the same use cases and has similar features, the details are different. TPM 2.0 is not backward compatible with TPM 1.2.[42][43][44]

The TPM 2.0 policy authorization includes the 1.2 HMAC, locality, physical presence, and PCR. It adds authorization based on an asymmetric digital signature, indirection to another authorization secret, counters and time limits, NVRAM values, a particular command or command parameters, and physical presence. It permits the ANDing and ORing of these authorization primitives to construct complex authorization policies.[57]

TCG has faced resistance to the deployment of this technology in some areas, where some authors see possible uses not specifically related to Trusted Computing, which may raise privacy concerns. The concerns include the abuse of remote validation of software (where the manufacturer‍—‌and not the user who owns the computer system‍—‌decides what software is allowed to run) and possible ways to follow actions taken by the user being recorded in a database, in a manner that is completely undetectable to the user.[58]

The TrueCrypt disk encryption utility, as well as its derivative VeraCrypt, do not support TPM. The original TrueCrypt developers were of the opinion that the exclusive purpose of the TPM is "to protect against attacks that require the attacker to have administrator privileges, or physical access to the computer". The attacker who has physical or administrative access to a computer can circumvent TPM, e.g., by installing a hardware keystroke logger, by resetting TPM, or by capturing memory contents and retrieving TPM-issued keys. The condemning text goes so far as to claim that TPM is entirely redundant.[59] The VeraCrypt publisher has reproduced the original allegation with no changes other than replacing "TrueCrypt" with "VeraCrypt".[60] The author is right that, after achieving either unrestricted physical access or administrative privileges, it is only a matter of time before other security measures in place are bypassed.[61][62] However, stopping an attacker in possession of administrative privileges has never been one of the goals of TPM (see § Uses for details), and TPM can stop some physical tampering.[16][18][21][22][23]

In 2015, Richard Stallman suggested to replace the term "Trusted computing" with the term "Treacherous computing" due to the danger that the computer can be made to systematically disobey its owner if the cryptographical keys are kept secret from them. He also considers that TPMs available for PCs in 2015 are not currently dangerous and that there is no reason not to include one in a computer or support it in software due to failed attempts from the industry to use that technology for DRM, but that the TPM2 released in 2022 is precisely the "treacherous computing" threat he had warned of.[63]

In 2010, Christopher Tarnovsky presented an attack against TPMs at Black Hat Briefings, where he claimed to be able to extract secrets from a single TPM. He was able to do this after 6 months of work by inserting a probe and spying on an internal bus for the Infineon SLE 66 CL PC.[64][65]

In 2015, as part of the Snowden revelations, it was revealed that in 2010 a US CIA team claimed at an internal conference to have carried out a differential power analysis attack against TPMs that was able to extract secrets.[66][67]

In 2018, a design flaw in the TPM 2.0 specification for the static root of trust for measurement (SRTM) was reported (CVE-2018-6622). It allows an adversary to reset and forge platform configuration registers which are designed to securely hold measurements of software that are used for bootstrapping a computer.[68] Fixing it requires hardware-specific firmware patches.[68] An attacker abuses power interrupts and TPM state restores to trick TPM into thinking that it is running on non-tampered components.[69]

Main Trusted Boot (tboot) distributions before November 2017 are affected by a dynamic root of trust for measurement (DRTM) attack CVE-2017-16837, which affects computers running on Intel's Trusted eXecution Technology (TXT) for the boot-up routine.[69]

In case of physical access, computers with TPM are vulnerable to cold boot attacks as long as the system is on or can be booted without a passphrase from shutdown or hibernation, which is the default setup for Windows computers with BitLocker full disk encryption.[70]

In 2021, the Dolos Group showed an attack on a discrete TPM, where the TPM chip itself had some tamper resistance, but the other endpoints of its communication bus did not. They read a full-disk-encryption key as it was transmitted across the motherboard, and used it to decrypt the laptop's SSD.[71]

Currently, a TPM is provided by nearly all PC and notebook manufacturers in their products.

TPM endorsement keys (EKs) are asymmetric key pairs unique to each TPM. They use the RSA and ECC algorithms. The TPM manufacturer usually provisions endorsement key certificates in TPM non-volatile memory. The certificates assert that the TPM is authentic. Starting with TPM 2.0, the certificates are in X.509 DER format.

These manufacturers typically provide their certificate authority root (and sometimes intermediate) certificates on their web sites.

• AMD[108][109][110][111]
• Infineon[112]
• Intel[113][114]
• NationZ [115][116][117][118]
• Nuvoton[119][120][121][122][123]
• ST Micro[124][125][126][127][128][129][130][131][132][133]

To utilize a TPM, the user needs a software library that communicates with the TPM and provides a friendlier API than the raw TPM communication. Currently, there are several such open-source TPM 2.0 libraries. Some of them also support TPM 1.2, but mostly TPM 1.2 chips are now deprecated and modern development is focused on TPM 2.0.

Typically, a TPM library provides an API with one-to-one mappings to TPM commands. The TCG specification calls this layer the System API(SAPI). This way the user has more control over the TPM operations, however the complexity is high. To hide some of the complexity most libraries also offer simpler ways to invoke complex TPM operations. The TCG specification call these two layers Enhanced System API(ESAPI) and Feature API(FAPI).

There is currently only one stack that follows the TCG specification. All the other available open-source TPM libraries use their own form of richer API.

(*) There is a separate project called "CHARRA" by Fraunhofer[143] that uses the tpm2-tss library for Remote Attestation. The other stacks have accompanying attestation servers or directly include examples for attestation. IBM offer their open-source Remote Attestation Server called "IBM ACS" on SourceForge and Google have "Go-Attestation" available on GitHub, while "wolfTPM" offers time and local attestation examples directly in its open-source code, also on GitHub.

(**) There is an application note[144] about an example project for the AURIX 32-bit SoC using the tpm2-tss library.

(***) Requires additional libraries (dotnet) to run on Linux.

These TPM libraries are sometimes also called TPM stacks, because they provide the interface for the developer or user to interact with the TPM. As seen from the table, the TPM stacks abstract the operating system and transport layer, so the user could migrate one application between platforms. For example, by using TPM stack API the user would interact the same way with a TPM, regardless if the physical chip is connected over SPI, I2C or LPC interface to the Host system.

• AMD Platform Security Processor
• ARM TrustZone
• Crypto-shredding
• Hardware security
• Hardware security module
• Hengzhi chip
• Intel Management Engine
• Microsoft Pluton
• Next-Generation Secure Computing Base
• Secure Enclave
• Threat model