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I am interested in 'adding' preexisting keys to a TPM's storage hierarchy.

An example of a desired key hierarchy can be shown as:

(SRK)----->(User Storage Key)-------> User Working Key 1
                             |
                             |------> User Working Key 2
                             |
                             |------> User Working Key 3

Where:

  • SRK is the storage root key created and managed by TPM administrator
  • User Storage Key is created as a (public, private) key pair outside TPM. For example by using the OpenSSL command line tools.
  • User Working Keys are also created outside TPM using OpenSSL.

How can I insert "User Storage Key" and "User Working Keys" in the above hierarchy?

The TPM allows me to create (public, private) key pairs internally inside TPM (and to export public key along with a (wrapped form of) private key thereof). But it is not clear to me if TPM commands like TPM_CreateWrapKey and TPM_LoadKey2 allow this option.

This is warranted by situations where user's signing/encryption keys are already created elsewhere and we are interested in using the TPM as a secure repository of keys on the operating platform. (Binding/sealing to create opaque data inside TPM is not useful here, since bound/sealed data is incapable of extending the key hierarchy).

  • The TPM is not a key repository. The key hierarchy is stored entirely on the hard-disk or other main media, and managed entirely by the TPM software stack (TSS) you have installed. So, you are really just using the TSS, i.e. TrouSerS. In this case, you might as well leverage other key management software. – Wilbur Whateley May 2 '16 at 23:43
  • Just for interest, I tried to force keys to remain in the TPM by creating "owner evict" keys. I found that the TPM had room for <5 keys before it complained, thus demonstrating that it really isn't a very good repository. – Wilbur Whateley May 2 '16 at 23:45
  • True that the secure NVRAM in TPM hardware has very limited storage for storing persistent secrets. The original question was about bring keys originally not created in the TPM to the TPM's storage hierarchy. Understandably, they will be stored in encrypted format on the hard drive when the TPM is not in active state & the relevant subtree of the key hierarchy is not loaded. – user13311 Jan 25 '18 at 19:51
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I wouldn't have thought this was a good idea. The TPM can only provide "trust" for keys that were created within the TPM. Otherwise, who knows what happened to the keys before they were added to the TPM? If the private key ever exists outside the TPM, all bets are off.

  • 1
    Assume that the external key to be imported to TPM is previously administered in a secure key server or an HSM. – user13311 Jan 25 '18 at 19:52

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