tpm2_policycphash(1)

📦
version 5.8 · 10 versions
, in arch rolling

💅 tpm2_policycphash(1) — the group-chat guide

okay bestie, today's episode of "Gen Z explains a TPM tool" is tpm2_policycphash(1), from the tpm2-tools lineup. Strap in, it's giving cryptography but make it cute 🎀

✨ SYNOPSIS ✨

tpm2_policycphash [OPTIONS]

🦋 DESCRIPTION 🦋

So tpm2_policycphash(1) couples a policy with command parameters of the command. Lowkey iconic. This is a deferred assertion — meaning the hash of the command parameters in a TPM command gets checked against the one you specified in the policy. It's like the TPM saying "bestie, prove the parameters match" and you're like "say less, here's the hash" 💅 No cap, that's the whole vibe.

💖 OPTIONS 💖

  • -L, --policy=FILE — the file where you save the compounded policy digest. Your receipts, basically 🧾
  • -S, --session=FILE — the policy session file, which is generated via the -S option to tpm2_startauthsession(1). You can't skip this step, fr fr.
  • --cphash-input=FILE — the file containing the command parameter hash of the command. This is the one you actually want ✨
  • --cphash=FILE — DEPRECATED, use --cphash-input instead. She's retired, she's still at the party, but please use the new one 🙈

📚 References

(yes, that's the whole subsection — some things are just a vibe check and nothing else 🌸)

🫶 COMMON OPTIONS 🫶

These are the options that show up in so many programs — the shared friend group of flags, if you will — and they provide information that many users may expect:

  • -h, --help=[man|no-man] — displays the tool's manpage. By default it attempts to invoke the manpager for the tool, however on failure it will output a short tool summary. Same behavior if the man option argument is specified — but if you explicitly request man, the tool will provide errors from man on stderr. If no-man is specified, or if the manpager fails, the short options get output to stdout.

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

  • -v, --version — displays version information for this tool, the supported tctis, and exits. Bye bestie 👋
  • -V, --verbose — increases the information the tool prints to the console during its execution. When using this option, the file and line number are printed. Main character transparency 🔍
  • -Q, --quiet — silences normal tool output to stdout. Shhh 🤫
  • -Z, --enable-errata — enables the application of errata fixups. Useful if an errata fixup needs to be applied to commands sent to the TPM. Defining the environment variable TPM2TOOLS_ENABLE_ERRATA is equivalent. Two paths, same destination 💫
  • -z, --pwd-session — use a 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. Loud and clear 🗣️
  • -R, --autoflush — enables 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. (The original page says "is is set" — we love a typo moment 💀) information many users may expect. No for real, autoflush is that girl.

🌈 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. It's giving versatility 💅

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. ALWAYS. periodt.

The current known TCTIs are:

  • tabrmd — the resource manager, called tabrmd. Note that tabrmd and abrmd as a tcti name are synonymous. Same girl, different name 🪞
  • mssim — typically used for communicating to the TPM software simulator. The simulator bestie 🖥️
  • device — used when talking directly to a TPM device file. No middleman, direct communication ✨
  • 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". Like, exact exact. Don't get creative 🔥

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. Empty but make it purposeful 🫶

🌸 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. It's giving first-come-first-served, no auditions 🎬

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. Slay, now you know 👑

🧚 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. Bring your own TCTI to the party, we love a guest star 💜

⭐ 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.
    • 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. the various known TCTI modules. (Yes, the original sentence trails off like that. She contains multitudes 💀)

🔥 EXAMPLES 🔥

Restrict the value that can be set through tpm2_nvsetbits. Let's go, tutorial time 🎓

💫 Define NV index object with authorized policy

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
tpm2_startauthsession -S session.ctx -g sha256
tpm2_policyauthorize -S session.ctx -L authorized.policy -n signing_key.name
tpm2_flushcontext session.ctx
tpm2_nvdefine 1 -a "policywrite|authwrite|ownerread|nt=bits" -L authorized.policy

🎀 Create policycphash

tpm2_nvsetbits 1 -i 1 --cphash cp.hash
tpm2_startauthsession -S session.ctx -g sha256
tpm2_policycphash -S session.ctx -L policy.cphash --cphash cp.hash
tpm2_flushcontext session.ctx

🦋 Sign and verify policycphash

openssl dgst -sha256 -sign signing_key_private.pem \
-out policycphash.signature policy.cphash
tpm2_verifysignature -c signing_key.ctx -g sha256 -m policy.cphash \
-s policycphash.signature -t verification.tkt -f rsassa

👑 Satisfy policycphash and execute nvsetbits

tpm2_startauthsession -S session.ctx --policy-session -g sha256
tpm2_policycphash -S session.ctx --cphash cp.hash
tpm2_policyauthorize -S session.ctx -i policy.cphash -n signing_key.name \
-t verification.tkt
tpm2_nvsetbits 1 -i 1 -P "session:session.ctx"
tpm2_flushcontext session.ctx

🍓 Returns 🍓

Tools can return any of the following codes:

  • 0 — Success. 🎉
  • 1 — General non-specific error. the vague one 😭
  • 2 — Options handling error. you did the flags wrong bestie 🙈
  • 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. Not me learning this the hard way 😭 Set yourself up first, then do the thing.

🐛 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, every single flag and caveat, no cap 💜 sign-off time: this slay of a manual page comes from tpm2-tools and the footer's date line belongs to tpm2_policycphash(1) — go forth, couple those policies with your command parameters, and remember: the vibe changes, the facts do not ✨🫶🎀

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