tpm2_policypcr(1)

📦
version 5.8 · 10 versions
, in arch rolling

🌸 NAME 🌸

okay bestie, this is tpm2_policypcr(1) from the General Commands Manual — Create a policy that includes specific PCR values. 💖

✨ SYNOPSIS ✨

tpm2_policypcr [OPTIONS]

🦋 DESCRIPTION 🦋

So tpm2_policypcr(1) generates a PCR policy event with the TPM. A PCR policy event creates a policy bound to specific PCR values, and it’s useful inside bigger policies built with policyor and policyauthorize events. Peep tpm2_policyor(1) and tpm2_policyauthorize(1) respectively for their usages, bestie. 🫶

The PCR data factored into the policy can be specified in one of 3 ways:

  1. A file containing a concatenated list of PCR values as in the output from tpm2_pcrread.
  2. Requiring the PCR values be read off the TPM by not specifying a PCR file input.
  3. The digest of all the PCR values directly specified as an argument.

💅 OPTIONS 💅

  • -L, --policy=FILE — File to save the policy digest. 💾
  • -f, --pcr=FILE — Optional path or name of the file containing expected PCR values for the specified index. Default is to read the current PCRs per the set list. 🧾
  • -l, --pcr-list=PCR — The list of PCR banks and selected PCRs’ ids for each bank. Forward sealing values can be specified. 🧬
  • -S, --session=FILE — The policy session file generated via the -S option to tpm2_startauthsession(1). 🔐
  • ARGUMENT — The calculated digest of all PCR values specified as a hex byte stream. Eg: openssl dgst -sha256 -binary pcr.bin | xxd -p -c 32 ✨

📚 References 📚

🧠 Context Object Format 🧠

The type of a context object — whether it’s 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_. 🔢

🔐 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 (eg 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 the section “PCR bank specifiers”.

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. 👑

🧬 PCR Bank Specifiers 🧬

PCR Bank Selection lists follow the below specification:

<BANK>:<PCR>[,<PCR>] or <BANK>:all

Multiple banks may be separated by +.

For example:

sha1:3,4+sha256:all

will select PCRs 3 and 4 from the SHA1 bank and PCRs 0 to 23 from the SHA256 bank. 🔥

Certain commands support specifying forward sealing values as well:

sha1:0,1=da39a3ee5e6b4b0d3255bfef95601890afd80709,2

This will select the current values for PCRs 0 and 2, but use the specified value for PCR 1. Digest lengths must match the bank size. An optional 0x prefix will be stripped off. ✨

📝 Note

PCR Selections allow for up to 5 hash to pcr selection mappings. This is a limitation in design in the single call to the tpm to get the pcr values. PCR.

🌈 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 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 so this batch of options is all about configuring the various known TCTI modules that are available — think of it as telling your tool how to actually talk to the TPM. Pick your fighter:

  • device — for the device TCTI, you can specify the TPM character device file that it should use. The default is /dev/tpm0.

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

  • mssim — for the mssim TCTI, you can specify the domain name or IP address and the port number that the simulator uses. 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.
    • The 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. And then the original page just kind of trails off with "the various known TCTI modules." — bestie that's a copy-paste artifact catching strays, we're leaving it in for archival purposes 😭💀

🌸 EXAMPLES 🌸

So here's the whole glow-up in four steps: start a trial session, build a PCR policy, use that policy when creating an object, then use a policy session to unseal some data stored in that object. Iconic behavior, honestly.

Step 1: create a policy

tpm2_createprimary -C e -g sha256 -G ecc -c primary.ctx

tpm2_pcrread -o pcr.dat "sha1:0,1,2,3"

tpm2_startauthsession -S session.dat

tpm2_policypcr -S session.dat -l "sha1:0,1,2,3" -f pcr.dat -L policy.dat

tpm2_flushcontext session.dat

Step 2: create an object using that policy

tpm2_create -Q -u key.pub -r key.priv -C primary.ctx -L policy.dat \
-i- <<< "12345678"

tpm2_load -C primary.ctx -u key.pub -r key.priv -n unseal.key.name \
-c unseal.key.ctx

Step 3: Satisfy the policy

tpm2_startauthsession --policy-session -S session.dat

tpm2_policypcr -S session.dat -l "sha1:0,1,2,3" -f pcr.dat -L policy.dat

Step 4: Use the policy

tpm2_unseal -psession:session.dat -c unseal.key.ctx
12345678

tpm2_flushcontext session.dat

💫 Returns 💫

Tools can return any of the following codes, no cap:

  • 0 — Success. we love to see it 🫶
  • 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 🦋

Heads up bestie, this one has expectations. It expects a session to be already established via tpm2_startauthsession(1) and it 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. So like… your session will simply not remember you. rude, but that's the lore 🙈

💀 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).

And that's the whole page, bestie — you ate that 💅 This little masterpiece comes to you from source/version tpm2-tools, page tpm2_policypcr(1), dated with all the love of a manpage footer. Go forth and policy your PCRs 🧚💖

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