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A TPM hardware device has very limited non-volatile protected memory just sufficient to store the EK (Endorsement Key) and SRK (Storage Root Key). How does a TPM allow nearly unlimited number of symmetric keys to be safely stored on an otherwise very vulnerable HDD?

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up vote 7 down vote accepted

The Storage Root Key (SRK) is used to wrap TPM protected keys which can be stored outside the TPM. That data stored outside the TPM can be decrypted by passing it back through the TPM again for a decryption operation.

Keys wrapped by the SRK can themselves be used to wrap other keys, too. This method of wrapping can be used to create a key hierarchy of parent key and child keys. To load a child, first load its parent. Once the child is loaded, the parent key can be unloaded from the TPM to free up TPM chip resources.

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The safety of the TPM storage is based on two factors:

  • There is no hardware API for retrieving the private keys.
    Under no circumstances will the key be transmitted to the host
  • Tamper-resistent hardware.
    Countermeasures such as permanently sealing the chip in resin help prevent physical hacking. In most cases, there is a high probability that you will break the chip if you attempt to gain direct physical access.

In the end, though, no storage medium is 100% secure. This is about increasing the cost and decreasing the probability of a physical breach.

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How does this answer the question? –  SquareRootOfTwentyThree Sep 11 '12 at 19:32
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