tpm2_getse...uditdigest(1)

📦
version 5.8 · 10 versions
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💅 NAME 💅

okay so this page is from the General Commands Manual, and the star of the show is tpm2_getsessionauditdigest(1) — it grabs the command audit attestation data from the TPM. periodt. 🎀

✨ SYNOPSIS ✨

tpm2_getsessionauditdigest [OPTIONS]

🦋 DESCRIPTION 🦋

tpm2_getsessionauditdigest(1) pulls the session audit digest attestation data out of the TPM, no cap. 📜

That attestation data is a two-for-one deal: it includes the session audit digest and a signature over that session audit digest. The way this works is the session itself gets started with the tpm2_startauthsession command first — so that's your prerequisite, bestie, don't skip it. 💖

🔥 OPTIONS 🔥

  • -P, --hierarchy-auth=AUTH — sets the authorization value for the endorsement hierarchy.
  • -c, --key-context=OBJECT — the context object for the signing key that signs the attestation data. iykyk 🔑
  • -p, --auth=AUTH — the authorization value for the key you specified with option -c.
  • -q, --qualification=HEX_STRING_OR_PATH — data given as a hex string or a binary file, used to qualify the quote. optional! this is typically how you add a nonce against replay attacks. 🛡️
  • -s, --signature=FILE — signature output file, and it records the signature in the format specified via the -f option.
  • -m, --message=FILE — message output file. records the quote message that makes up the data that is signed by the TPM — which is the command audit digest attestation data. 📨
  • -f, --format=FORMAT — format selection for the signature output file.
  • -g, --hash-algorithm — hash algorithm for the signature. defaults to sha256. 🧮
  • --scheme=ALGORITHM — the signing scheme used to sign the message. optional. signing schemes should follow the “formatting standards”, see section “Algorithm Specifiers”. also, see section “Supported Signing Schemes” for a list of supported signature schemes. if you specify it, the signature scheme must match the key type; if you leave it unspecified, a default signature scheme for the key type gets used. 🎯
  • -S, --session=FILE — the path of the session that enables and records the audit digests.

📎 References 📎

🗂️ Context Object Format 🗂️

the type of a context object — handle or file name, pick your fighter — is figured out according to this 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 argument can be loaded as a number it will be treat 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 that 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 don't have a prefix.

🍓 String 🍓

a string password, specified by prefix str: or it's 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 hexidecimal form into a byte array form, thus allowing passwords with non-printable and/or terminal un-friendly characters. lowkey genius 🙈

Example

hex:1122334455667788

🎀 File 🎀

a file based password, specified be 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. protect urself bestie 🫶

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 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.

⭐ Signature Format Specifiers ⭐

format selection for the signature output file. tss (the default) will output a binary blob according to the TPM 2.0 specification and any potential compiler padding. the option plain will output the plain signature data as defined by the used cryptographic algorithm. signature FORMAT. ✍️

👑 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 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 🎀

okay so these are the little config knobs for the TCTI modules you might actually have, one bullet per module bestie 💅

  • device — for the device TCTI, you can tell it which TPM character device file to 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 plus the port number the simulator is using. 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.

Here's how you specify the tabrmd tcti name plus a config string of bus_name=com.example.FooBar:

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

And here's how you specify the default (abrmd) tcti plus 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 fragment is literally hanging out there in the original, she said what she said 💀)

✨ EXAMPLES ✨

like, here's the whole little ritual start to finish, copy it, it slays:

tpm2_createprimary -Q -C e -c prim.ctx

tpm2_create -Q -C prim.ctx -c signing_key.ctx -u signing_key.pub \
-r signing_key.priv

tpm2_startauthsession -S session.ctx --audit-session

tpm2_getrandom 8 -S session.ctx

tpm2_getsessionauditdigest -c signing_key.ctx -m att.data -s att.sig \
-S session.ctx

💖 Returns

tools can hand you back any of these codes, so vibe check your exit status bestie:

  • 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.

💀 BUGS

Github Issues — that's where the drama goes, not my DMs 😭

🫶 HELP

See the Mailing List 🦋

📜 License

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

aaaand that's a wrap on this one, bestie 🎀 this page ships from tpm2-tools (that's the source/version, it's right there in the footer) with its date stamped in the footer too — go forth, audit those sessions, and stay iconic ✨💜

📦 where this page comes from