tpm2_policysecret(1)

📦
version 5.8 · 10 versions
, in arch rolling

🌸 NAME 🌸

okay so tpm2_policysecret(1) from the General Commands Manual is literally the tool that couples the authorization of an object to that of an existing object. iconic matching-credentials energy, no cap.

🌈 SYNOPSIS 🌈

tpm2_policysecret [OPTIONS] [ARGUMENT]

💖 DESCRIPTION 💖

tpm2_policysecret(1) couples the authorization of an object to that of an existing object without requiring exposing the existing secret until time of object use. the secret stays tucked away until it is actually needed — we love a discreet queen 🫶

✨ OPTIONS ✨

  • -c, --object-context=OBJECT — A context object specifier of a transient/permanent/persistent object. Either a file path of a object context blob or a loaded/persistent/permanent handle id. See the “Context Object Format” section. As an argument, it takes the auth value of the associated TPM object; a single dash - can be used to read the auth value from stdin. The argument follows the “authorization formatting standards”, see the “Authorization Formatting” section.
  • -S, --session=FILE — The policy session file generated via the -S option to tpm2_startauthsession(1).
  • -L, --policy=FILE — File to save the policy digest.
  • -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 value is 0 then the policy does not have a time limit on the authorization.
  • --ticket=FILE — The ticket file to record the authorization ticket structure.
  • --timeout=FILE — The file path to record the timeout structure returned.
  • -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.
  • -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.
  • --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 to be used in an audit or a policycphash.
  • ARGUMENT — the command line argument specifies the AUTH to be set for the object specified with -c.

📚 References 📚

🧸 Context Object Format 🧸

The type of a context object, whether it is a handle or file name, is determined according to the following logic in-order:

  • If the argument is a file path, then the file is loaded as a restored TPM transient object.
  • If the argument is a prefix match on one of:
    • owner: the owner hierarchy
    • platform: the platform hierarchy
    • endorsement: the endorsement hierarchy
    • lockout: the lockout control persistent object
  • If the argument can be loaded as a number it will be treated as a handle, e.g. 0x81010013 and used directly. _OBJECT_ (the original really has that trailing bit, so we keep it — don't come for me 💀)

🔐 Authorization Formatting 🔐

Authorization for use of an object in TPM2.0 can come in 3 different forms:

  1. Password
  2. HMAC
  3. Sessions

NOTE: “Authorizations default to the EMPTY PASSWORD when not specified”.

For authentication with auth values, the tool commands implicitly generate an HMAC session. Thus passwords are not transmitted unencrypted. This does not apply to passwords defined for objects. In this case, an HMAC session should be used (see tpm2_startauthsession(1)).

🔑 Passwords 🔑

Passwords are interpreted in the following forms below using prefix identifiers.

Note: By default passwords are assumed to be in the string form when they do not have a prefix.

String

A string password, specified by prefix str: or its absence (raw string without prefix) is not interpreted, and is directly used for authorization.

Examples

foobar
str:foobar

Hex-string

A hex-string password, specified by prefix hex: is converted from a hexadecimal form into a byte array form, thus allowing passwords with non-printable and/or terminal un-friendly characters.

Example

hex:1122334455667788

File

A file based password, specified by prefix file: should be the path of a file containing the password to be read by the tool or a - to use stdin. Storing passwords in files prevents information leakage; passwords passed as options can be read from the process list or common shell history features.

Examples

# to use stdin and be prompted
file:-

# to use a file from a path
file:path/to/password/file

# to echo a password via stdin:
echo foobar | tpm2_tool -p file:-

# to use a bash here-string via stdin:

tpm2_tool -p file:- <<< foobar

🫧 Sessions 🫧

When using a policy session to authorize the use of an object, prefix the option argument with the session keyword. Then indicate a path to a session file that was created with tpm2_startauthsession(1). Optionally, if the session requires an auth value to be sent with the session handle (e.g. policy password), then append a + and a string as described in the Passwords section.

Examples

To use a session context file called session.ctx.

session:session.ctx

To use a session context file called session.ctx AND send the authvalue mypassword.

session:session.ctx+mypassword

To use a session context file called session.ctx AND send the HEX authvalue 0x11223344.

session:session.ctx+hex:11223344

🧬 PCR Authorizations 🧬

You can satisfy a PCR policy using the pcr: prefix and the PCR minilanguage. The PCR minilanguage is as follows:

<pcr-spec>=<raw-pcr-file>

The PCR spec is documented in in the section “PCR bank specifiers”. (yes, “in in” — the page said it, not me 😭)

The raw-pcr-file is an optional argument that contains the output of the raw PCR contents as returned by tpm2_pcrread(1).

PCR bank specifiers

Examples

To satisfy a PCR policy of sha256 on banks 0, 1, 2 and 3 use a specifier of:

pcr:sha256:0,1,2,3

specifying AUTH.

🌟 COMMON OPTIONS 🌟

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

  • -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 is 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 initialize 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 🔌

Okay bestie, this collection of options is used to configure the various known TCTI modules available. Lowkey important stuff 💅

  • 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 defaults 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 💖

Associate auth value of a sealing object to the owner hierarchy password. Here's the plan the page lays out:

  • Start a trial auth session and run tpm2_policysecret(1) to create policy that can only be satisfied if owner hierarchy auth value is supplied.
  • Start a real policy session and provide the owner hierarchy auth value.
  • Provide the session input where in the policysecret for owner hierarchy auth was satisfied to the unseal tool.
  • If the policy was satisfied unsealing should succeed.

✨ Generate a policy that binds to the secret of the owner hiearchy ✨

tpm2_startauthsession -S session.ctx

tpm2_policysecret -S session.ctx -c o -L secret.policy

tpm2_flushcontext session.ctx

🧸 Create a TPM object using the policy 🧸

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

tpm2_create -Q -g sha256 -u sealing_key.pub -r sealing_key.priv -i- \
  -C prim.ctx -L secret.policy <<< "SEALED-SECRET"

tpm2_load -C prim.ctx -u sealing_key.pub -r sealing_key.priv \
  -c sealing_key.ctx

🦋 Satisfy the policy and unseal the secret 🦋

tpm2_startauthsession --policy-session -S session.ctx

tpm2_policysecret -S session.ctx -c o -L secret.policy

tpm2_unseal -p "session:session.ctx" -c sealing_key.ctx
SEALED-SECRET

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.

💀 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).

Source/version: tpm2-tools · page: tpm2_policysecret(1) · date: not shown in the footer text I got, so I'm not inventing one — mwah 💋💖 periodt.

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