On Mon, 25 Jun 2007, Hal Finney wrote:
> The idea of putting a TPM on a smart card or other removable device is
> even more questionable from this perspective.  A TPM which communicates
> via an easily accessible and tamperable bus is almost useless for the
> security concepts behind the Trusted Computing Group architecture.

Even if a TPM is soldered to the motherboard it does not mean
that unsoldering is an esoteric art. There is a difference
between what media hypes about TPM and what TCG technical
documents say [1]:

   It is not expected that a TPM will be able to defeat
   sophisticated physical attacks.

> The exception might be if there were additional hardware to encrypt
> the bus, but that is not part of the standard spec.

Encrypted bus requires encryption cores on both ends and key
distribution resistant to MitM attacks. I suspect that if you
system already has so many crypto blocks in it, it would be
cheaper to implement TPM inside.

> So this would allow a removable TPM but it has to be "logically" bound
> to the motherboard via cryptography, presumably something like an
> encrypted bus.

To logically bound TPM to the motherboard it is enough for BIOS
`loader' that hashes the rest of the BIOS, to include unique ID of the
motherboard into the hash.

[1] <https://www.trustedcomputinggroup.org/groups/tpm/TPM_1_2_Changes_final.pdf>


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