Can't read from NV memory about wolftpm

[dgarske]

Hi @horver ,

This is because the auth is not set in the case when wolfTPM2_NVCreateAuth is called and the handle already exists.

The right solution would be to call wolfTPM2_NVReadPublic first to see if it exists. If it does then you'll need to set the NV handle like this:

nv.handle.hndl = (TPM_HANDLE) 0x01800000;
nv.handle.auth.size = sizeof(gNvAuth) - 1;
XMEMCPY(nv.handle.auth.buffer, gNvAuth, nvhandle.auth.size);

Then call wolfTPM2_NVReadAuth.

You do bring up a good improvement that could be made to the wrapper, so I've pushed a PR here:
#99

Thanks,
David Garske, wolfSSL

[horver]

Hello @tomoveu @dgarske,

Thank you very much for your answer and the quick fix!
Now I understand the mechanism and it works! :)

[tomoveu]

Hi @horver,

This also means, you should handle the return code TPM_RC_NV_DEFINED in your application not as error case. After #99 the wrapper now sets the auth and you can perform the NVReadAuth.

Of course, you are left with the challenge to know that you are the owner of this NV field. This is why you should check the outcome of NV_ReadAuth to figure if the NV field written matches your auth, thus verifying "aha , this is my field and the value inside is the one i stored" :)

ps: @dgarske nice fix! Simple and quick.