tpm2_rsadecrypt(1)

📦
version 5.8 · 10 versions
, in arch rolling

🎀 tpm2_rsadecrypt(1) 🎀

okay so picture this: it's a General Commands Manual page, it's called tpm2_rsadecrypt(1), and it's here to help you do RSA decryption with your TPM. she's iconic, she's focused, and she has a lot of options. buckle up bestie 💖

✨ NAME ✨

tpm2_rsadecrypt(1) — Performs an RSA decryption operation using the TPM. that's it, that's the whole vibe. 💅

🌸 SYNOPSIS 🌸

tpm2_rsadecrypt [OPTIONS] [ARGUMENT]

🦋 DESCRIPTION 🦋

tpm2_rsadecrypt(1) does RSA decryption on the contents of a file, using whatever padding scheme you point at, according to IETF RFC 3447 (PKCS#1). and if you don't give it a command line argument? it defaults to stdin. lowkey convenient, highkey iconic. ✨

the key you reference with key-context has got to be, no exceptions:

  1. An RSA key
  2. Have the attribute decrypt SET in its attributes

like... if it doesn't have decrypt set, it's giving "not my job" energy and the whole thing falls apart. 💀

💖 OPTIONS 💖

  • -c, --key-context=*OBJECT* — the context object pointing at the public portion of the RSA key you want to use for decryption. can be either a file or a handle number. see the section “Context Object Format” below, iykyk 🫶
  • -p, --auth=*AUTH* — optional authorization value to use with the key you named in -c. only if it needs one, bestie 🔑
  • -o, --output=*FILE* — optional output file path where the decrypted data gets recorded. default behaviour is to print the binary encrypted data to STDOUT, which is... a choice, but it's the documented one 🍓
  • -s, --scheme=*FORMAT* — optional, sets the padding scheme. defaults to rsaes. the options are giving:
    • null — TPM_ALG_NULL, which uses the key's scheme if one is set
    • rsaes — TPM_ALG_RSAES, which is RSAES_PKCSV1.5
    • oaep — TPM_ALG_OAEP, which is RSAES_OAEP

like, pick your fighter 💅

  • -l, --label=*FILE OR STRING* — optional, sets the label data. important caveat incoming: the TPM requires the last byte of the label to be zero, and this is handled internally to the tool. but no other embedded 0 bytes can exist, or the TPM will truncate your label. so like... don't sneak extra zeros in there, she will clip it 😭
  • --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. so it's a little preview era, not the real thing 🙈
  • ARGUMENT — the command line argument specifies the file containing the data to be decrypted. that's the whole job 🎀

🔥 References 🔥

🌈 Context Object Format 🌈

the type of a context object — whether it's a handle or a file name — gets 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 these:
    • 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”. like, no auth given? empty password. that's the default, periodt.

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 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. genuinely so useful 💜

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. so like, protect your secrets 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 you're 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. okay that's actually so clean ✨

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 goes like this:

<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.

⭐ COMMON OPTIONS ⭐

this collection of options is common to many programs and provides information that many users may expect. they're the little guys that show up everywhere 🫶

  • -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. cute little info dump 💅
  • -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. she's telling you everything 🗣️
  • -Q, --quiet — silence normal tool output to stdout. shhh 🤫
  • -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. 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, two names 💅
  • 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. literally the exact text, no notes 🙅‍♀️

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. she's decisive, okay 💖

🧚 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. go off, custom library 🎀

okay so bestie, this next bit is literally just the ✨receipts✨ for pointing at the various known TCTI modules — this collection of options is what configures all of them, and you're gonna want to know these fr fr:

  • device — for the device TCTI, you get to pick which TPM character device file it uses. 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 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 config string format is a series of simple key value pairs separated by a , character. each key and value string is 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.

Wanna specify the tabrmd tcti name and a config string of bus_name=com.example.FooBar? do this:

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

Wanna specify the default (abrmd) tcti and a config string of bus_type=session? do this instead:

\--tcti:bus_type=session
NOTE: abrmd and tabrmd are synonymous. the various known TCTI modules. (yes that last bit is verbatim, it's giving mystery fragment but we respect the source 💅)

🎀 EXAMPLES 🎀

bestie this is the part where it all comes together, no cap. here's the whole little journey from "no key" to "decrypted message, slay".

🔑 Create an RSA key and load it

tpm2_createprimary -c primary.ctx
tpm2_create -C primary.ctx -Grsa2048 -u key.pub -r key.priv
tpm2_load -C primary.ctx -u key.pub -r key.priv -c key.ctx

🔒 Encrypt using RSA

echo "my message" > msg.dat
tpm2_rsaencrypt -c key.ctx -o msg.enc msg.dat

🦋 Decrypt using RSA

tpm2_rsadecrypt -c key.ctx -o msg.ptext msg.enc
cat msg.ptext
my message

and that's it — my message comes right back out, main character energy ✨

🌈 Returns 🌈

tools can return any of the following codes, so keep an eye on that 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 (yes, that's the whole section — go file one if something's cursed 💀)

🆘 HELP 🆘

See the Mailing List (the whole section, again, she's concise 😭)

📜 License 📜

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

okay that's the whole page, you made it!! this is tpm2-tools 🌸 you were reading tpm2_rsadecrypt(1) — go forth and decrypt, ily, bye 🫶

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