tpm2_policyauthvalue(1)

👑 NAME 👑

okay bestie, this is tpm2_policyauthvalue(1) from the General Commands Manual — the tool that enables binding a policy to the authorization value of the authorized TPM object. 💅✨

✨ SYNOPSIS ✨

tpm2_policyauthvalue [OPTIONS]

🎀 DESCRIPTION 🎀

So tpm2_policyauthvalue(1) enables a policy that requires the object’s authentication passphrase be provided. It’s literally equivalent to authenticating using the object passphrase in plaintext or HMAC, except it enforces it as a policy. It gives you a mechanism to allow for password authentication when an object only allows policy based authorization, aka the object attribute “userwithauth” is 0. 🫶

🦋 OPTIONS 🦋

• -L, --policy=FILE — the file to save the compounded policy digest. 💾
• -S, --session=FILE — the policy session file generated via the -S option to tpm2_startauthsession(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 🧚

yo, the original has this heading right here but no bullets under it; it’s giving empty section, no cap 💀.

😭 COMMON OPTIONS 😭

This collection of options are common to many programs and provide information that many users may expect. (okay so that’s the original’s own words, and yes, the autoflush bullet below also tacks on “information many users may expect.” at the end — not me, bestie 💀)

• -h, --help=[man|no-man] — display the tool’s 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. periodt.

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 (yes, original spelling, bestie 💀). 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. (yes, that trailing bit is in the original too 💀)

🌸 EXAMPLES 🌸

We want to authenticate using the TPM objects plaintext authentication value. While we could authenticate with an ephemeral password session, in this example we will authenticate with the plaintext passphrase in a policy session instead using the tpm2_policyauthvalue(1) tool. 💅

🍓 Create the password policy 🍓

tpm2_startauthsession -S session.dat

tpm2_policyauthvalue -S session.dat -L policy.dat

tpm2_flushcontext session.dat

🍓 Create the object with a passphrase and the password policy 🍓

tpm2_createprimary -C o -c prim.ctx

tpm2_create -g sha256 -G aes -u key.pub -r key.priv -C prim.ctx -L policy.dat \
  -p testpswd

🍓 Authenticate with plaintext passphrase input 🍓

tpm2_load -C prim.ctx -u key.pub -r key.priv -n key.name -c key.ctx

echo "plaintext" > plain.txt
tpm2_encryptdecrypt -c key.ctx -o encrypt.out plain.txt -p testpswd

🍓 Authenticate with password and the policy 🍓

tpm2_startauthsession --policy-session -S session.dat

tpm2_policyauthvalue -S session.dat

tpm2_encryptdecrypt -c key.ctx -o encrypt.out -p session:session.dat+testpswd \
plain.txt

tpm2_flushcontext session.dat

💀 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.

🙈 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).

mwah 💖 okay that’s the whole page. source/version: tpm2-tools; page: tpm2_policyauthvalue(1). The provided footer doesn’t include a date string, so I’m not inventing one, bestie 💀✨