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Platform OEM Provisioning

Platform OEM provisioning has two concerns:

• Authenticity

• Control

As with the TPM manufacturer, a platform manufacturer certificate[1] (typically in X.509 format) asserts that the hardware is authentic. It asserts that the TPM is attached to the OEM's platform. It further asserts that the platform software meets certain

TCG recommended standards. For a PC client, this includes a CRTM that performs measurements of software and extends those measurements to PCRs.

Although an attacker could physically remove a TPM from the OEM platform and put it in a counterfeit, the TCG technology doesn't defend against physical attacks. Further, this would compromise only one platform per attack. On the other hand, a successful attack that extracts a primary seed from a TPM would permit the attacker to manufacture an unlimited number of counterfeits.

Platform certification typically begins with an endorsement primary key generated by the TPM at the TPM manufacturer. The OEM wants to verify that the TPM is authentic before asserting that its platform is authentic. It reads the TPM certificate and validates it; it may go further, reading the public key and verifying that it matches, or even use the private key to prove that the key pair is present.

As with the TPM vendor endorsement key certificate, there is no security-related reason to store the platform certificates on the TPM before shipping the platform.

Practical considerations will likely drive the OEM to include a certificate in the TPM's persistent memory, corresponding to the vendor TPM certificate.

TPM 2.0 specifies a platform hierarchy. The platform OEM may optionally provision that hierarchy with a platform policy. As explained earlier, the TPM initializes this policy to empty: a policy that can never be satisfied. The OEM must provision it at startup using TPM2_SetPrimaryPolicy.

Where does the platform policy value (a hash) come from? We expect that the value will be embedded in early platform boot software, protected by the same OEM mechanism that protects the CRTM. So, this value is provisioned during platform manufacturing not into the TPM, but into the platform CRTM. It's inserted into the TPM at startup.

End User Provisioning

The term end user is used loosely here. For a home computer, the end user is typically the literal user of the computer. For an enterprise, centrally managed platform, the end user may be the actual user or support personnel.

The end user must provision the endorsement and storage hierarchies. This is a case where an IT organization may decide to provision the endorsement hierarchy and/or the dictionary-attack reset authorizations, and leave the provisioning of the storage hierarchy to the person who is actually using the platform.

The first consideration is whether to disable the hierarchies (always enabled at startup) using TPM2_HierarchyControl. The method of disabling a hierarchy is platform specific. We expect something equivalent to a BIOS screen, with a means of disabling a hierarchy for the remainder of a boot cycle or having it persist through boot cycles. (We hope to present enough valuable use cases that you will never dream of disabling the TPM.)

Next, the end user must provision the endorsement and storage hierarchy, and the dictionary-attack protection policies and authorization values. Owner authorization is initially empty (no authorization required), as is owner policy (no policy is present).

TPM2_HierarchyChangeAuth and TPM2_SetPrimaryPolicy change these values, which persist until cleared. The endorsement authorization and policy are set and changed using the same commands. The owner authorization and endorsement authorization should be set to high-entropy values, because they aren't guarded by the dictionary-attack protection.

The dictionary-attack logic has both a policy and an authorization value. The same party may provision all policies and authorization values, but they may choose different logic and values. It's particularly important to provision the dictionary-attack reset policy. If triggered, the dictionary-attack password can be used only once before a large wait is enforced. However, the policy can be used to reset the dictionary-attack counter even when the dictionary-attack password has been locked out.

The endorsement primary seed is generated during TPM vendor manufacturing. The end user doesn't typically change this value.

The storage primary seed is similarly generated during TPM vendor manufacturing.

The end user can either use this value or generate a new one. Generating a new one invalidates all objects in the storage and endorsement hierarchies except the endorsement hierarchy primary keys. Thus, the endorsement primary key certificate is still useful.

  • [1] We know of no OEM that currently is provisioning platform certificates.
 
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