tpm2_startauthsession - Man Page

Start a session with the TPM.

Synopsis

tpm2_startauthsession [Options]

Description

tpm2_startauthsession(1) - Starts a session with the TPM. The default is to start a trial session unless the -a option is specified. Saves the policy session data to a file. This file can then be used in subsequent tools that can use a policy file for authorization or policy events.

This will not work with resource managers (RMs) outside of tpm2-abrmd (https://github.com/tpm2-software/tpm2-abrmd), as most RMs will flush session handles when a client disconnects from the IPC channel. However, when using a RM without the session gapping feature, one can use the command TCTI to keep the connection open.

The first step is to create a socket listener that uses tpm2_send:

mknod "$HOME/backpipe" p
while [ 1 ]; do tpm2_send 0<"$HOME/backpipe" | nc -lU "$HOME/sock" 1>"$HOME/backpipe"; done;

The next step is to use the command TCTI and netcat (nc) to send data to the socket.

tpm2_startauthsession --tcti="cmd:nc -q 0 -U $HOME/sock" <options>

When finishing ensure to kill the listener. For commands executed with the command tcti against the listener, one will need to manage transient handles. The simplest way is to add a flush after each command: tpm2_flushcontext --tcti="cmd:nc -q 0 -U $HOME/sock" -t

Note: This example uses UNIX sockets, since the socket is controlled with Linux access controls. Using a port is not recommended as it’s either open to any user on the system (localhost) or bound to a network card and exposed to the network.

This will work with direct TPM access, but note that internally this calls a ContextSave and a ContextLoad on the session handle, thus the session cannot be saved/loaded again.

Options

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:

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”.

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 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.

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.

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.

Common Options

This collection of options are common to many programs and provide information that 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:

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

This collection of options are used to configure the various known TCTI modules available:

Start a trial session and save the session data to a file

tpm2_startauthsession -S mysession.ctx

Start a policy session and save the session data to a file

tpm2_startauthsession --policy-session -S mysession.ctx

Start an encrypted and bound policy session and save the session data to a file

tpm2_createprimary -c primary.ctx
tpm2_startauthsession --policy-session -c primary.ctx -S mysession.ctx

Returns

Tools can return any of the following codes:

Bugs

Github Issues (https://github.com/tpm2-software/tpm2-tools/issues)

Help

See the Mailing List (https://lists.linuxfoundation.org/mailman/listinfo/tpm2)

Referenced By

tpm2_activatecredential(1), tpm2_certify(1), tpm2_certifycreation(1), tpm2_changeauth(1), tpm2_changeeps(1), tpm2_changepps(1), tpm2_clear(1), tpm2_clearcontrol(1), tpm2_clockrateadjust(1), tpm2_create(1), tpm2_createak(1), tpm2_createek(1), tpm2_createprimary(1), tpm2_dictionarylockout(1), tpm2_duplicate(1), tpm2_encodeobject(1), tpm2_encryptdecrypt(1), tpm2_evictcontrol(1), tpm2_flushcontext(1), tpm2_getcommandauditdigest(1), tpm2_getsessionauditdigest(1), tpm2_gettime(1), tpm2_hash(1), tpm2_hierarchycontrol(1), tpm2_hmac(1), tpm2_import(1), tpm2_load(1), tpm2_loadexternal(1), tpm2_nvcertify(1), tpm2_nvdefine(1), tpm2_nvextend(1), tpm2_nvincrement(1), tpm2_nvread(1), tpm2_nvreadlock(1), tpm2_nvsetbits(1), tpm2_nvundefine(1), tpm2_nvwrite(1), tpm2_nvwritelock(1), tpm2_pcrallocate(1), tpm2_pcrevent(1), tpm2_policyauthorize(1), tpm2_policyauthorizenv(1), tpm2_policyauthvalue(1), tpm2_policycommandcode(1), tpm2_policycountertimer(1), tpm2_policycphash(1), tpm2_policyduplicationselect(1), tpm2_policylocality(1), tpm2_policynamehash(1), tpm2_policynv(1), tpm2_policynvwritten(1), tpm2_policyor(1), tpm2_policypassword(1), tpm2_policypcr(1), tpm2_policyrestart(1), tpm2_policysecret(1), tpm2_policysigned(1), tpm2_policytemplate(1), tpm2_policyticket(1), tpm2_print(1), tpm2_quote(1), tpm2_rsadecrypt(1), tpm2_rsaencrypt(1), tpm2_setclock(1), tpm2_setcommandauditstatus(1), tpm2_setprimarypolicy(1), tpm2_sign(1), tpm2_unseal(1).

tpm2-tools General Commands Manual