Trusted Execution Technology

Intel Trusted Execution Technology (Intel TXT, formerly known as LaGrande Technology) is a computer hardware technology whose primary goals are:

Intel TXT uses a Trusted Platform Module (TPM) and cryptographic techniques to provide measurements of software and platform components so that system software as well as local and remote management applications may use those measurements to make trust decisions. This technology is based on an industry initiative by the Trusted Computing Group (TCG) to promote safer computing. It defends against software-based attacks aimed at stealing sensitive information by corrupting system or BIOS code, or modifying the platform's configuration.

The Trusted Platform Module (TPM) as specified by the TCG provides many security functions including special registers (called Platform Configuration Registers – PCRs) which hold various measurements in a shielded location in a manner that prevents spoofing. Measurements consist of a cryptographic hash using a Secure Hashing Algorithm (SHA); the TPM v1.0 specification uses the SHA-1 hashing algorithm. More recent TPM versions (v2.0+) call for SHA-2.

A desired characteristic of a cryptographic hash algorithm is that (for all practical purposes) the hash result (referred to as a hash digest or a hash) of any two modules will produce the same hash value only if the modules are identical.

Measurements can be of code, data structures, configuration, information, or anything that can be loaded into memory. TCG requires that code not be executed until after it has been measured. To further protect the integrity of the measurements, hash measurements are not written to PCRs, but rather a PCR is "extended" with a measurement. This means that the TPM takes the current value of the PCR and the measurement to be extended, hashes them together, and replaces the content of the PCR with that hash result. The effect is that the only way to arrive at a particular measurement in a PCR is to extend exactly the same measurements in exactly the same order. Therefore, if any module being measured has been modified, the resulting PCR measurement will be different and thus it is easy to detect if any code, configuration, data, etc. that has been measured had been altered or corrupted.

The technology supports both a static chain of trust and a dynamic chain of trust. The static chain of trust starts when the platform powers on (or the platform is reset), which resets all PCRs to their default value. For server platforms, the first measurement is made by hardware (i.e., the processor) to measure a digitally signed module (called an Authenticated Code Module or ACM) provided by the chipset manufacturer. The processor validates the signature and integrity of the signed module before executing it. The ACM then measures the first BIOS code module, which can make additional measurements.

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