tpm2_policynvwritten - Man Page

Restrict TPM object authorization to the written state of an NV index.


tpm2_policynvwritten [Options] [ARGUMENT]


tpm2_policynvwritten(1) - Restricts TPM object authorization to the written state of an NV index. Useful when creating write once NV indexes.

As an [ARGUMENT] it takes the expected written state of the NV index. It can be specified as s|c|0|1.



Common Options

This collection of options are common to many programs and provide information that many users may expect.

TCTI Configuration

The TCTI or "Transmission Interface" is the communication mechanism with the TPM. TCTIs can be changed for communication with TPMs across different mediums.

To control the TCTI, the tools respect:

  1. The command line option -T or --tcti
  2. The environment variable: TPM2TOOLS_TCTI.

Note: The command line option always overrides the environment variable.

The current known TCTIs are:

The arguments to either the command line option or the environment variable are in the form:


Specifying an empty string for either the <tcti-name> or <tcti-option-config> results in the default being used for that portion respectively.

TCTI Defaults

When a TCTI is not specified, the default TCTI is searched for using dlopen(3) semantics. The tools will search for tabrmd, device and mssim TCTIs IN THAT ORDER and USE THE FIRST ONE FOUND. You can query what TCTI will be chosen as the default by using the -v option to print the version information. The "default-tcti" key-value pair will indicate which of the aforementioned TCTIs is the default.

Custom TCTIs

Any TCTI that implements the dynamic TCTI interface can be loaded. The tools internally use dlopen(3), and the raw tcti-name value is used for the lookup. Thus, this could be a path to the shared library, or a library name as understood by dlopen(3) semantics.

Tcti Options

This collection of options are used to configure the various known TCTI modules available:


Create a write once NV index. To do this the NV index is defined with a write policy that is valid only if the NV index attribute "TPMA_NV_WRITTEN" was never set.

Define the NV index write policy

tpm2_startauthsession -S session.dat
tpm2_policycommandcode -S session.dat TPM2_CC_NV_Write
tpm2_policynvwritten -S session.dat -L nvwrite.policy c
tpm2_flushcontext session.dat

Define the NV index with the policy

 tpm2_nvdefine -s 1 -a "authread|policywrite" -p nvrdpass -L nvwrite.policy

Write the NV index by satisfying the policy

tpm2_startauthsession -S session.dat --policy-session
tpm2_policycommandcode -S session.dat TPM2_CC_NV_Write
tpm2_policynvwritten -S session.dat c
echo 0xAA | xxd -r -p | tpm2_nvwrite 0x01000000 -i- -P session:session.dat
tpm2_flushcontext session.dat


Tools can return any of the following codes:


It expects a session to be already established via tpm2_startauthsession(1) and requires one of the following:

Without it, most resource managers will not save session state between command invocations.


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tpm2-tools General Commands Manual