Re: [Tpm2] Better understanding auth/object hierarchies

(P.S despite my best efforts, some of the above code samples may have been
garbled a bit by HTML email... a formatted version of all the same steps
can be found at
https://unix.stackexchange.com/questions/749355/why-can-tpm2-0-sealing-objects-created-under-password-protected-primary-keys-be
)

Cheers,
Stewart Webb


On Thu, 22 Jun 2023 at 18:06, Stewart Webb <[email protected]> wrote:

> Hi there,
>
> I have been investigating using the TPM2 for secure persistence of
> secrets. It seems sealing objects are the way to go for this kind of thing.
>
> I am hoping someone here might be able to better explain how sealing
> objects are related to the parent objects they are created and loaded
> under, especially when password auth values are involved. I've been a bit
> confused by some examples I've put together that allow various operations
> without supplying the passwords I would have thought were required.
>
> (note: many of the examples below require calls to tpm2_flushcontext to
> clear out space for intermediate object loading, but I have excluded them
> for brevity)
>
> As part of my evaluation I have learnt that sealing objects must always be
> created underneath some other 'primary object'/primary key that must be
> created first.
>
>     # make a new primary key
>     $ tpm2_createprimary -C o -c testprim.ctx
>     # make a new sealing object under this primary key
>     $ tpm2_create -u testseal.pub -r testseal.priv -i hunter2.txt -C testprim.ctx
>
>
> These primary objects can optionally be created with password auth values
> on them. As you'd expect, trying to create a sealing object via a
> password-protected primary object without supplying its password doesn't
> work:
>
>     $ tpm2_createprimary -C o -p str:testpwd -c testprim-wauth.ctx
>     $ tpm2_create -u testseal-wpauth.pub -r testseal-wpauth.priv -i hunter2.txt -C testprim-wauth.ctx    WARNING:esys:src/tss2-esys/api/Esys_Create.c:398:Esys_Create_Finish() Received TPM Error    ERROR:esys:src/tss2-esys/api/Esys_Create.c:134:Esys_Create() Esys Finish ErrorCode (0x0000098e)    ERROR: Esys_Create(0x98E) - tpm:session(1):the authorization HMAC check failed and DA counter incremented    ERROR: Unable to run tpm2_create    $ tpm2_create -u testseal-wpauth.pub -r testseal-wpauth.priv -i hunter2.txt -C testprim-wauth.ctx -P str:testpwd    # OK
>
>
> What confuses me a bit is that loading and unsealing one of these
> created-under-the-auth-protected-primary-key objects works fine if the
> loading is done under a non-authed primary key:
>
>     # no password required here
>     $ tpm2_load -C testprim.ctx -u testseal-wpauth.pub -r testseal-wpauth.priv -n testseal-loadedwnopauth.name -c testseal-loadedwnopauth.ctx
>     $ tpm2_unseal -c testseal-loadedwnopauth.ctx
>     hunter2
>
>
> However, if loading under the auth-value-protected primary key, the
> password is required as expected for the loading...
>
>     # Try load without password
>     $ tpm2_load -C testprim-wauth.ctx -u testseal-wpauth.pub -r testseal-wpauth.priv -n testseal-loadedwpauth.name -c testseal-loadedwpauth.ctx    WARNING:esys:src/tss2-esys/api/Esys_Load.c:324:Esys_Load_Finish() Received TPM Error    ERROR:esys:src/tss2-esys/api/Esys_Load.c:112:Esys_Load() Esys Finish ErrorCode (0x0000098e)    ERROR: Eys_Load(0x98E) - tpm:session(1):the authorization HMAC check failed and DA counter incremented    ERROR: Unable to run tpm2_load    # with password:    $ tpm2_load -C testprim-wauth.ctx -u testseal-wpauth.pub -r testseal-wpauth.priv -n testseal-loadedwpauth.name -c testseal-loadedwpauth.ctx -P str:testpwd    # OK
>
>
> ...but the unsealing still works without a password too. I'm assuming this
> is because the sealing object itself has no password on it.
>
> $ tpm2_unseal -c testseal-loadedwpauth.ctx
> hunter2
>
> If the sealing object itself is given a password auth value (-p flag in
> tpm2_create) then it can never be unsealed without that password, which is
> expected behaviour, so that at least works fine.
>
> I am just confused as to why things created under password-protected
> primary keys can still be seemingly later used without needing that
> password? Is the password purely for the ability to create subsequent child
> objects under the primary key? Or am I missing something here?
>
> Cheers,
> Stewart Webb
>