tpm2_policysigned(1)

🎀 NAME 🎀

Okay so bestie, welcome to tpm2_policysigned(1) from the General Commands Manual 💖 This icon enables policy authorization by verifying the signature of optional TPM2 parameters, and that signature is generated by a signing authority. Slay.

✨ SYNOPSIS ✨

tpm2_policysigned [OPTIONS]

🦋 DESCRIPTION 🦋

tpm2_policysigned(1) enables policy authorization by verifying the signature of optional TPM2 parameters. The signature is generated by a signing authority. The optional TPM2 parameters in question are cpHashA, nonceTPM, policyRef and expiration. No cap, that's the whole vibe check.

💅 OPTIONS 💅

• -L, --policy=FILE — File to save the compounded policy digest.
• -S, --session=FILE — The policy session file generated via the -S option to tpm2_startauthsession(1).
• -c, --key-context=OBJECT — Context object for the key context used for the operation. Either a file or a handle number. See section “Context Object Format”.
• -g, --hash-algorithm=ALGORITHM — The hash algorithm used to digest the message.
• -s, --signature=FILE — The input signature file of the signature to be validated.
• -f, --format=FORMAT — Set the input signature file to a specified format. The default is the tpm2.0 TPMT_SIGNATURE data format, however different schemes can be selected if the data came from an external source like OpenSSL. The tool currently supports rsassa and ecdsa.
• -t, --expiration=NATURAL_NUMBER — Set the expiration time of the policy in seconds. In absence of nonceTPM the expiration time is the policy timeout value. If expiration is a negative value an authorization ticket is additionally returned. If expiration value is 0 then the policy does not have a time limit on the authorization.
• --cphash-input=FILE — The command parameter hash (cpHash), enforcing the TPM command to be authorized as well as its handle and parameter values.
• --raw-data=FILE — The raw data, generated based on the selected parameters, serves as the input for computing the hash that must be signed.
• --ticket=FILE — The ticket file to record the authorization ticket structure.
• --timeout=FILE — The file path to record the timeout structure returned.
• -q, --qualification=FILE_OR_HEX_STR — Optional, the policy qualifier data that the signer can choose to include in the signature. Can be either a hex string or path.
• -x, --nonce-tpm — Enable the comparison of the current session’s nonceTPM to ensure the validity of the policy authorization is limited to the current session.

References

🌈 COMMON OPTIONS 🌈

This collection of options are common to many programs and provide information that many users may expect. Bestie, these are the everyday icons:

• -h, --help=[man|no-man] — Display the tools manpage. By default, it attempts to invoke the manpager for the tool, however, on failure will output a short tool summary. This is the same behavior if the “man” option argument is specified, however if explicit “man” is requested, the tool will provide errors from man on stderr. If the “no-man” option if specified, or the manpager fails, the short options will be output to stdout.

To successfully use the manpages feature requires the manpages to be installed or on MANPATH, See man(1) for more details.

• -v, --version — Display version information for this tool, supported tctis and exit.
• -V, --verbose — Increase the information that the tool prints to the console during its execution. When using this option the file and line number are printed.
• -Q, --quiet — Silence normal tool output to stdout.
• -Z, --enable-errata — Enable the application of errata fixups. Useful if an errata fixup needs to be applied to commands sent to the TPM. Defining the environment TPM2TOOLS_ENABLE_ERRATA is equivalent.
• -z, --pwd-session — Use password session instead of a HMAC session for authentication. A clear text password is passed to the TPM to authorize the action. This option can be used to avoid problems when unsalted sessions are used in OpenSSL FIPS mode. If auth values are used a salted session should be used for authentication.
• -R, --autoflush — Enable autoflush for transient objects created by the command. If a parent object is loaded from a context file also the transient parent object will be flushed. Autoflush can also be activated if the environment variable TPM2TOOLS_AUTOFLUSH is is set to yes or true. information 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:

• tabrmd — The resource manager, called tabrmd. Note that tabrmd and abrmd as a tcti name are synonymous.
• mssim — Typically used for communicating to the TPM software simulator.
• device — Used when talking directly to a TPM device file.
• none — Do not initalize a connection with the TPM. Some tools allow for off-tpm options and thus support not using a TCTI. Tools that do not support it will error when attempted to be used without a TCTI connection. Does not support ANY options and MUST BE presented as the exact text of “none”.

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:

• device — For the device TCTI, the TPM character device file for use by the device TCTI can be specified. The default is /dev/tpm0.

Example: -T device:/dev/tpm0 or export TPM2TOOLS_TCTI=“device:/dev/tpm0”

• mssim — For the mssim TCTI, the domain name or IP address and port number used by the simulator can be specified. The default are 127.0.0.1 and 2321.

Example: -T mssim:host=localhost,port=2321 or export TPM2TOOLS_TCTI=“mssim:host=localhost,port=2321”

• abrmd — For the abrmd TCTI, the configuration string format is a series of simple key value pairs separated by a , character. Each key and value string are separated by a = character.
  • TCTI abrmd supports two keys:
    1. bus_name : The name of the tabrmd service on the bus (a string).
    2. bus_type : The type of the dbus instance (a string) limited to session and system.

Specify the tabrmd tcti name and a config string of bus_name=com.example.FooBar:

\--tcti=tabrmd:bus_name=com.example.FooBar

Specify the default (abrmd) tcti and a config string of bus_type=session:

\--tcti:bus_type=session

NOTE: abrmd and tabrmd are synonymous. the various known TCTI modules.

🔥 EXAMPLES 🔥

Authorize a TPM operation on an object whose authorization is bound to specific signing authority.

Create the signing authority

openssl genrsa -out private.pem 2048

openssl rsa -in private.pem -outform PEM -pubout -out public.pem

Generate signature with nonceTPM, cpHashA, policyRef and expiration set to 0

echo "00 00 00 00" | xxd -r -p | \
openssl dgst -sha256 -sign private.pem -out signature.dat

Load the verification key and Create the policysigned policy

tpm2_loadexternal -C o -G rsa -u public.pem -c signing_key.ctx

tpm2_startauthsession -S session.ctx

tpm2_policysigned -S session.ctx -g sha256 -s signature.dat -f rsassa \
-c signing_key.ctx -L policy.signed

tpm2_flushcontext session.ctx

Create a sealing object to use the policysigned

echo "plaintext" > secret.data

tpm2_createprimary -C o -c prim.ctx

tpm2_create -u key.pub -r sealing_key.priv -c sealing_key.ctx -C prim.ctx \
-i secret.data -L policy.signed

Satisfy the policy and unseal secret

tpm2_startauthsession -S session.ctx --policy-session

tpm2_policysigned -S session.ctx -g sha256 -s signature.dat -f rsassa \
-c signing_key.ctx -L policy.signed

tpm2_unseal -p session:session.ctx -c sealing_key.ctx

tpm2_flushcontext session.ctx

👑 Returns 👑

Tools can return any of the following codes:

• 0 — Success.
• 1 — General non-specific error.
• 2 — Options handling error.
• 3 — Authentication error.
• 4 — TCTI related error.
• 5 — Non supported scheme. Applicable to tpm2_testparams.

😭 Limitations 😭

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

• direct device access
• extended session support with tpm2-abrmd.

Without it, most resource managers will not save session state between command invocations. That's the tea, bestie.

💀 BUGS 💀

Github Issues

🫶 HELP 🫶

See the Mailing List

⭐ License ⭐

This work is licensed under the Creative Commons Attribution 4.0 International License (CC BY 4.0).

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Signing off with source/version tpm2-tools, page tpm2_policysigned(1), and date: the footer didn't show one, so we're just gonna vibe 💖💅✨