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tpm2_policyauthorize.1.md

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% tpm2_policyauthorize(1) tpm2-tools | General Commands Manual

NAME

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

  • -L, --policy=FILE:

    File to save the policy digest.

  • -S, --session=FILE:

    The policy session file generated via the -S option to tpm2_startauthsession(1).

  • -i, --input=FILE:

    The policy digest that has to be authorized.

  • -q, --qualification=FILE_OR_HEX:

    The policy qualifier data signed in conjunction with the input policy digest. This is unique data that the signer can choose to include in the signature and can either be a path or hex string.

  • -n, --name=FILE:

    File containing the name of the verifying public key. This ties the final policy digest with a signer. This can be retrieved with tpm2_readpublic(1)

  • -t, --ticket=FILE:

    The ticket file to record the validation structure. This is generated with tpm2_verifysignature(1).

  • --cphash=FILE

    File path to record the hash of the command parameters. This is commonly termed as cpHash. NOTE: When this option is selected, The tool will not actually execute the command, it simply returns a cpHash.

References

common options collection of common options that provide information many users may expect.

common tcti options collection of options used to configure the various known TCTI modules.

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 the authorize policy digest

tpm2_startauthsession -S session.ctx

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

tpm2_flushcontext session.ctx

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_desired

tpm2_flushcontext session.ctx

Sign the policy

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

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"

Verify the desired policy digest comes from the signing authority, read the actual value of PCR and check that read policy and desired policy are equal.

tpm2_verifysignature -c signing_key.ctx -g sha256 -m  pcr.policy_desired -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_read

tpm2_policyauthorize -S session.ctx -L authorized.policy -i pcr.policy_desired -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

limitations

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