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Platform Hierarchy

The platform hierarchy is intended to be under the control of the platform manufacturer, represented by the early boot code shipped with the platform. [1] The platform hierarchy is new for TPM 2.0. In TPM 1.2, the platform firmware could not be assured that the TPM was enabled. Thus, platform firmware developers could not include tasks that relied on the TPM.

the platform firmware must verify an rsa digital signature to authenticate software as part of the Unified extensible Firmware interface (UeFi) secure boot process. the platform OeM stores a public key, or a digest of a list of trusted public keys, in a tpM NV index. the controls on the index permit only the platform OeM to update it. During boot, the platform firmware uses this trusted public key to verify a signature.

the tpM provides two benefits. First, it provides a secure location to store the public key. second, it offers the rsa algorithm, so it need not be implemented in software. here are the steps:

1. TPM_NV_Read

2. TPM2_LoadExternal

3. TPM2_VerifySignature

Unique among the hierarchies, at reboot, the platform hierarchy is enabled, the platform authorization value is set to a zero-length password, and the policy is set to one that can't be satisfied. The intent is that the platform firmware will generate a strong platform authorization value (and optionally install its policy). Unlike the other hierarchies, which may have a human enter an authorization value, the platform authorization is entered by the platform firmware. Therefore, there is no reason to have the authorization persist (and to find a secure place to store it) rather than regenerate it each time.

Because the platform hierarchy has its own enable flag, the platform firmware decides when to enable or disable the hierarchy. The intent is that it should always be enabled and available for use by the platform firmware and the operating system.

Storage Hierarchy

The storage hierarchy is intended to be used by the platform owner: either the enterprise IT department or the end user. The storage hierarchy is equivalent to the TPM 1.2 storage hierarchy. It has an owner policy and an authorization value, both of which persist through reboots. The intent is that they be set and rarely changed.

The hierarchy can be disabled by the owner without affecting the platform hierarchy. This permits the platform software to use the TPM even if the owner disables its hierarchy. In TPM 1.2, turning off the single storage hierarchy disabled the TPM. Similarly, this hierarchy can be cleared (by changing the primary seed and deleting persistent objects) independent of the other hierarchies.

The storage hierarchy is intended for non-privacy-sensitive operations, whereas the endorsement hierarchy, with separate controls, addresses privacy.

Endorsement Hierarchy

The endorsement hierarchy is the privacy-sensitive tree and is the hierarchy of choice when the user has privacy concerns. TPM and platform vendors certify that primary keys in this hierarchy are constrained to an authentic TPM attached to an authentic platform. As with TPM 1.2, a primary key can be an encryption key; and certificates can be created using TPM2_ActivateCredential, equivalent to the TPM 1.2 activate identity command. Unlike with TPM 1.2, a primary key can also be a signing key. Creating and certifying such a key is privacy sensitive because it permits correlation of keys back to a single TPM.

Because the endorsement hierarchy is intended for privacy-sensitive operations, its enable flag, policy, and authorization value are independent of the other hierarchies.

They're under the control of a privacy administrator, who may be the end user. A user with privacy concerns can disable the endorsement hierarchy while still using the storage hierarchy for TPM applications and permitting the platform software to use the TPM.

  • [1] In an x86 PC platform, this early boot code was called BIOS. More recently, it's called UEFI firmware.
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