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I have a laptop with TPM 2.0, dual-booting Windows 10 and Linux. By default, Windows automatically takes ownership of the TPM with a throwaway password, but can be configured to store it in the Registry.

However, Linux seems to have access to the TPM even without the owner password – at least, I'm able to successfully seal a LUKS key against some PCRs, unseal it later, and not able to unseal it when the PCRs change, just as expected.

This, and in fact the whole "throwaway owner password" thing by Win10, leaves me confused about what exactly the purpose of an owner password is – what operations require it?

Also: if I have something that is probably the current owner password, how do I check/verify it using Linux TPM 2.0 tools, preferrably without making any changes to the TPM contents? (I have both the IBM and Intel userspace tools installed.)

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Simply, 'taking ownership' creates a new Storage Root Key. The password insures the user doing the 'owning' is the same person again should person wish to perform options associated with the ownership password, in command line mode. The password is referred to as the 'shared secret' in the TPM docs and is required in the syntax of the command, required if a vendor wishes to conform to the TPM specs, whether it necessary or not in their implementation of TPM. In Win 7 acommand was, which allows control of bitlocker as well as TPM, which bitlocker depends on:

manage-bde -tpm [-turnon] [-takeownership !Owner Password!] [-computername ] [{-?|/?}] [{-help|-h}]

A problem with TPM implementations is while the standard exist, the implementation by vendors varies and even how the commands behave. I can only speak from Windows experience, it began with making procedure calls like you are doing now around Win7, to fully automated in Win10 or using less error prone management consoles, like tpm.msc. When a user is personally present to take ownership, Admin privilege is enough in Win 10 to execute TPM.msc and use the Win10 generated password, that is unknown to the user, as this password is coordinated by various Windows maintenance consoles. If the user PC is part of a network, its assumed the system Admin is the only one allowed to manage the security features and setting TPM. If a PC user alters their TPM as a local or even as a system Admin and has trouble later, they simply repeat the whole ownership process; given subsequent keys wrapped it will take a while for the process to complete. So in toto, in Win10, Admin can perform the tasks needed for ownership without knowing the previous password used for ownership.

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