tpm2_policyauthorize man page

tpm2_policyauthorize(1) — Allows for mutable policies by tethering to a signing authority.

Synopsis

tpm2_policyauthorize [Options]

Description

tpm2_policyauthorize(1) - This command allows for policies to change by associating the policy to a signing authority and allowing the policy contents to change.

  1. If the input session is a trial session this tool generates a policy digest that associates a signing authority's public key name with the policy being authorized.
  2. If the input session is real policy session tpm2_policyauthorize(1) looks for a verification ticket from the TPM to attest that the TPM has verified the signature on the policy digest before authorizing the policy in the policy digest.

Options

References

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:

<tcti-name>:<tcti-option-config>

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:

Examples

Starts a trial session, builds a PCR policy. This PCR policy digest is then an input to the tpm2_policyauthorize(1) along with policy qualifier data and a signer public. The resultant policy digest is then used in creation of objects.

Subsequently when the PCR change and so does the PCR policy digest, the actual policy digest from the tpm2_policyauthorize(1) used in creation of the object will not change. At runtime the new PCR policy needs to be satisfied along with verification of the signature on the PCR policy digest using tpm2_policyauthorize(1)

Create a signing authority

openssl genrsa -out signing_key_private.pem 2048

openssl rsa -in signing_key_private.pem -out signing_key_public.pem -pubout

tpm2_loadexternal -G rsa -C o -u signing_key_public.pem -c signing_key.ctx -n signing_key.name

Create a policy to be authorized like a PCR policy

tpm2_pcrread -opcr0.sha256 sha256:0

tpm2_startauthsession -S session.ctx

tpm2_policypcr -S session.ctx -l sha256:0 -f pcr0.sha256 -L pcr.policy

tpm2_flushcontext session.ctx

Sign the policy

openssl dgst -sha256 -sign signing_key_private.pem -out pcr.signature pcr.policy

Authorize the policy in the policy digest

tpm2_startauthsession -S session.ctx

tpm2_policyauthorize -S session.ctx -L authorized.policy -i pcr.policy -n signing_key.name

tpm2_flushcontext session.ctx

Create a TPM object like a sealing object with the authorized policy

based authentication

tpm2_createprimary -C o -g sha256 -G rsa -c prim.ctx

tpm2_create -g sha256 -u sealing_pubkey.pub -r sealing_prikey.pub -i- -C prim.ctx -L authorized.policy <<< "secret to seal"

Satisfy policy and unseal the secret

tpm2_verifysignature -c signing_key.ctx -g sha256 -m pcr.policy -s pcr.signature -t verification.tkt -f rsassa

tpm2_startauthsession \--policy-session -S session.ctx

tpm2_policypcr -S session.ctx -l sha256:0 -L pcr.policy

tpm2_policyauthorize -S session.ctx -L authorized.policy -i pcr.policy -n signing_key.name -t verification.tkt

tpm2_load -C prim.ctx -u sealing_pubkey.pub -r sealing_prikey.pub -c sealing_key.ctx

unsealed=$(tpm2_unseal -p"session:session.ctx" -c sealing_key.ctx)

echo $unsealed

tpm2_flushcontext session.ctx

Returns

Tools can return any of the following codes:

Limitations

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.

Bugs

Github Issues (https://github.com/tpm2-software/tpm2-tools/issues)

Help

See the Mailing List (https://lists.01.org/mailman/listinfo/tpm2)

Referenced By

tpm2_policypcr(1).

tpm2-tools General Commands Manual