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Home arrow Computer Science arrow A Practical Guide to TPM 2.0
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Quick Key Loading (new in 2.0)

In the TPM 1.2 specification, when a key was initially loaded, it had to go through a time-consuming private-key decryption using the key's parent's private key. To avoid having to do this multiple times during a session, it was possible to cache loaded keys by encrypting them with a symmetric key that only the TPM knew. During that power cycle, the TPM could reload the key using a symmetric-key operation, which was faster even if the parent no longer resided in the TPM. Once the TPM was turned off, the symmetric key was erased: the next time the key was loaded, it again required a private key operation.

In 2.0, except for the case of a key being imported into a TPM's key structure from outside, keys stored by the TPM using external memory are encrypted using a symmetric-key operation. As a result, the keys are loaded quickly. There is little reason to cache keys out to disk (unless a parent key becomes unavailable), because loading them is usually as fast as recovering them from a cached file.

This quicker loading enables multiple users to use a TPM without noticing a long delay. This in turn makes it easier to design a system on which multiple applications appear to have unfettered access to a TPM.

Non-Brittle PCRs (New in 2.0)

Fragility of PCR values was one of the most annoying problems with the 1.0 family of TPMs. PCR values typically represent the state of the machine, with lower-numbered PCRs representing the process of booting of the system and higher-numbered ones representing events after the kernel has booted. Both keys and data can be locked to certain PCRs having particular values, an action called sealing. But if keys or data are locked to a PCR that represents the BIOS of a system, it's tricky to upgrade the BIOS. This is PCR fragility. Typically, before a BIOS upgrade was performed on TPM 1.2 systems, all secrets locked to PCR 0, for example (which represents the BIOS), had to be unsealed and then resealed after the upgrade was done. This is both a manageability nightmare and a nuisance to users.

In the TPM 2.0 specification, you can seal things to a PCR value approved by a particular signer instead of to a particular PCR value (although this can still be done if you wish). That is, you can have the TPM release a secret only if PCRs are in a state approved (via a digital signature) by a particular authority.

In typical usage, an IT organization may approve BIOS versions for PCs and then provide signatures of the PCRs that would result from approved BIOS versions being installed on PC clients. Values that formerly could be recovered in only one of those states become recoverable in any of them.

This is done via the TPM2_PolicyAuthorize command, which you can also use many

other ways. It's a general-purpose way of making any policy flexible.

This new capability enables a number of different use cases, such as these:

• Locking resources to be used on machines that have any BIOS signed by the OEM

• Locking resources to be used on machines that have any kernels signed by an OEM

• Locking resources to be used on machines that have any set of values for PCRs that are approved by the IT organization

 
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