Trusted Applications

This document tells how to implement a Trusted Application for OP-TEE, using OP-TEE’s so called TA-devkit to both build and sign the Trusted Application binary. In this document, a Trusted Application running in the OP-TEE os is referred to as a TA. Note that in the default setup a private test key is distributed along with the optee_os source is used for signing Trusted Applications. See TASign for more details, including offline signing of TAs.

TA Mandatory files

The Makefile for a Trusted Application must be written to rely on OP-TEE TA-devkit resources in order to successfully build the target application. TA-devkit is built when one builds optee_os.

To build a TA, one must provide:

  • Makefile, a make file that should set some configuration variables and include the TA-devkit make file.
  •, a make file that lists the sources to build (local source files, subdirectories to parse, source file specific build directives).
  • user_ta_header_defines.h, a specific ANSI-C header file to define most of the TA properties.
  • An implementation of at least the TA entry points, as extern functions: TA_CreateEntryPoint(), TA_DestroyEntryPoint(), TA_OpenSessionEntryPoint(), TA_CloseSessionEntryPoint(), TA_InvokeCommandEntryPoint()

TA file layout example

As an example, hello_world looks like this:

├── ...
└── ta
    ├── Makefile                  BINARY=<uuid>
    ├──                Android way to invoke the Makefile
    ├──                    srcs-y += hello_world_ta.c
    ├── include
    │   └── hello_world_ta.h      Header exported to non-secure: TA commands API
    ├── hello_world_ta.c          Implementation of TA entry points
    └── user_ta_header_defines.h  TA_UUID, TA_FLAGS, TA_DATA/STACK_SIZE, ...

TA Makefile Basics

Required variables

The main TA-devkit make file is located in optee_os at ta/mk/ The make file supports make targets such as all and clean to build a TA or a library and clean the built objects.

The make file expects a couple of configuration variables:

Base directory of the TA-devkit. Used by the TA-devkit itself to locate its tools.
These are exclusive, meaning that you cannot use both at the same time. If building a TA, BINARY shall provide the TA filename used to load the TA. The built and signed TA binary file will be named ${BINARY}.ta. In native OP-TEE, it is the TA UUID, used by tee-supplicant to identify TAs. If one is building a static library (that will be later linked by a TA), then LIBNAME shall provide the name of the library. The generated library binary file will be named lib${LIBNAME}.a
Cross compiler for the TA or the library source files. CROSS_COMPILE32 is optional. It allows to target AArch32 builds on AArch64 capable systems. On AArch32 systems, CROSS_COMPILE32 defaults to CROSS_COMPILE.

Optional variables

Some optional configuration variables can be supported, for example:

Base directory for build objects filetree. If not set, TA-devkit defaults to ./out from the TA source tree base directory.

Example Makefile

A typical Makefile for a TA looks something like this

# Append specific configuration to the C source build (here log=info)
# The UUID for the Trusted Application

# Source the TA-devkit make file
include $(TA_DEV_KIT_DIR)/mk/ directives

The make file expects that current directory contains a file that is the entry point for listing the source files to build and other specific build directives. Here are a couple of examples of directives one can implement in a make file:

# Adds /hello_world_ta.c from current directory to the list of the source
# file to build and link.
srcs-y += hello_world_ta.c

# Includes path **./include/** from the current directory to the include
# path.
global-incdirs-y += include/

# Adds directive -Wno-strict-prototypes only to the file hello_world_ta.c
cflags-hello_world_ta.c-y += -Wno-strict-prototypes

# Removes directive -Wno-strict-prototypes from the build directives for
# hello_world_ta.c only.
cflags-remove-hello_world_ta.c-y += -Wno-strict-prototypes

# Adds the static library foo to the list of the linker directive -lfoo.
libnames += foo

# Adds the directory path to the libraries pathes list. Archive file
# libfoo.a is expected in this directory.
libdirs += path/to/libfoo/install/directory

# Adds the static library binary to the TA build dependencies.
libdeps += path/to/greatlib/libgreatlib.a

Android Build Environment

OP-TEE’s TA-devkit supports building in an Android build environment. One can write an file for the TA (stored side by side with the Makefile). Android’s build system will parse the file for the TA which in turn will parse a TA-devkit Android make file to locate TA build resources. Then the Android build will execute a make command to built the TA through its generic Makefile file.

A typical file for a TA looks like this ( for hello_world is used as an example here).

# Define base path for the TA sources filetree
LOCAL_PATH := $(call my-dir)

# Define the module name as the signed TA binary filename.
local_module := 8aaaf200-2450-11e4-abe2-0002a5d5c51b.ta

# Include the devkit Android make script
include $(OPTEE_OS_DIR)/mk/

TA Mandatory Entry Points

A TA must implement a couple of mandatory entry points, these are:

TEE_Result TA_CreateEntryPoint(void)
    /* Allocate some resources, init something, ... */

    /* Return with a status */
    return TEE_SUCCESS;

void TA_DestroyEntryPoint(void)
    /* Release resources if required before TA destruction */

TEE_Result TA_OpenSessionEntryPoint(uint32_t ptype,
                                    TEE_Param param[4],
                                    void **session_id_ptr)
    /* Check client identity, and alloc/init some session resources if any */

    /* Return with a status */
    return TEE_SUCCESS;

void TA_CloseSessionEntryPoint(void *sess_ptr)
    /* check client and handle session resource release, if any */

TEE_Result TA_InvokeCommandEntryPoint(void *session_id,
                                      uint32_t command_id,
                                      uint32_t parameters_type,
                                      TEE_Param parameters[4])
    /* Decode the command and process execution of the target service */

    /* Return with a status */
    return TEE_SUCCESS;

TA Properties

Trusted Application properties shall be defined in a header file named user_ta_header_defines.h, which should contain:

  • TA_UUID defines the TA uuid value
  • TA_FLAGS define some of the TA properties
  • TA_STACK_SIZE defines the RAM size to be reserved for TA stack
  • TA_DATA_SIZE defines the RAM size to be reserved for TA heap (TEE_Malloc() pool)

Refer to TA Properties to understand how to configure these macros.


UUIDs can be generated using python

python -c 'import uuid; print(uuid.uuid4())'

or in most Linux systems using either

cat /proc/sys/kernel/random/uuid # Linux only
uuidgen # available from the util-linux package in most distributions

Example of a property header file


#define TA_UUID
    { 0x8aaaf200, 0x2450, 0x11e4, \
        { 0xab, 0xe2, 0x00, 0x02, 0xa5, 0xd5, 0xc5, 0x1b} }

#define TA_FLAGS                    (TA_FLAG_EXEC_DDR | \
                        TA_FLAG_SINGLE_INSTANCE | \
#define TA_STACK_SIZE                       (2 * 1024)
#define TA_DATA_SIZE                        (32 * 1024)

    { "gp.ta.description", USER_TA_PROP_TYPE_STRING, "Foo TA for some purpose." }, \
    { "gp.ta.version", USER_TA_PROP_TYPE_U32, &(const uint32_t){ 0x0100 } }



It is recommended to use the TA_CURRENT_TA_EXT_PROPERTIES as above to define extra properties of the TA.


Generating a fresh UUID with suitable formatting for the header file can be done using:

python -c "import uuid; u=uuid.uuid4(); print(u); \
   n = [', 0x'] * 11; \
   n[::2] = ['{:12x}'.format(u.node)[i:i + 2] for i in range(0, 12, 2)]; \
   print('\n' + '#define TA_UUID\n\t{ ' + \
         '0x{:08x}'.format(u.time_low) + ', ' + \
         '0x{:04x}'.format(u.time_mid) + ', ' + \
         '0x{:04x}'.format(u.time_hi_version) + ', \\ \n\n\t\t{ ' + \
         '0x{:02x}'.format(u.clock_seq_hi_variant) + ', ' + \
         '0x{:02x}'.format(u.clock_seq_low) + ', ' + \
         '0x' + ''.join(n) + '} }')"

Checking TA parameters

GlobalPlatforms TEE Client APIs TEEC_InvokeCommand() and TEE_OpenSession() allow clients to invoke a TA with some invocation parameters: values or references to memory buffers. It is mandatory that TA’s verify the parameters types before using the parameters themselves. For this a TA can rely on the macro TEE_PARAM_TYPE_GET(param_type, param_index) to get the type of a parameter and check its value according to the expected parameter.

For example, if a TA expects that command ID 0 comes with params[0] being a input value, params[1] being a output value, and params[2] being a in/out memory reference (buffer), then the TA should implemented the following sequence:

TEE_Result handle_command_0(void *session, uint32_t cmd_id,
                            uint32_t param_types, TEE_Param params[4])
    if ((TEE_PARAM_TYPE_GET(param_types, 0) != TEE_PARAM_TYPE_VALUE_IN) ||
        (TEE_PARAM_TYPE_GET(param_types, 1) != TEE_PARAM_TYPE_VALUE_OUT) ||
        (TEE_PARAM_TYPE_GET(param_types, 2) != TEE_PARAM_TYPE_MEMREF_INOUT) ||
        (TEE_PARAM_TYPE_GET(param_types, 3) != TEE_PARAM_TYPE_NONE)) {

    /* process command */

TEE_Result TA_InvokeCommandEntryPoint(void *session, uint32_t command_id,
                      uint32_t param_types, TEE_Param params[4])
    switch (command_id) {
    case 0:
        return handle_command_0(session, param_types, params);


Signing of TAs

All REE Filesystem Trusted Applications need to be signed. The signature is verified by optee_os upon loading of the TA. Within the optee_os source is a directory keys. The public part of keys/default_ta.pem will be compiled into the optee_os binary and the signature of each TA will be verified against this key upon loading. Currently keys/default_ta.pem must contain an RSA key.


optee_os comes with a default private key in its source to facilitate easy development, testing, debugging and QA. Never deploy an optee_os binary with this key in production. Instead replace this key as soon as possible with a public key and keep the private part of the key offline, preferably on an HSM.


Currently only a single key for signing TAs is supported by optee_os.

TAs are signed using the script referenced from ta/mk/ in optee_os. Its default behaviour is to sign a compiled TA binary and attach the signature to form a complete TA for deployment. For offline signing, a three-step process is required: In a first step a digest of the compiled binary has to be generated, in the second step this digest is signed offline using the private key and finally in the third step the binary and its signature are stitched together into the full TA.

Offline Signing of TAs

The TA dev kit does sign an application as last step of the linking process. For example, the file ta/arch/arm/ in the optee_os source tree contains the statement

$(q)$(SIGN) --key $(TA_SIGN_KEY) --uuid $(user-ta-uuid) \
        --in $$< --out $$@

To avoid build errors when signing offline, this make script needs to be adopted. The signing script can be found at $(TA_DEV_KIT_DIR)/../scripts/

Overall, offline signing is done with the following sequence of steps:

0. (Preparation) Generate a 2048 bit RSA key for signing in a secure, offline environment. Extract the public key and copy it to the keys directory in the optee_os source tree. Adjust TA_SIGN_KEY for different file/path names. (Copy and) modify the file for the default linking step to produce a digest of the TA binary instead of the full TA.

1. Manually (or with the modified linking script) generate a digest of the TA binary using digest --key $(TA_SIGN_KEY) --uuid $(user-ta-uuid)
  1. Sign this digest offline, e.g. with OpenSSL
base64 --decode digestfile | \
openssl pkeyutl -sign -inkey $TA_SIGN_KEY \
    -pkeyopt digest:sha256 -pkeyopt rsa_padding_mode:pss \
    -pkeyopt rsa_pss_saltlen:digest -pkeyopt rsa_mgf1_md:sha256 | \
base64 > sigfile

or with pkcs11-tool using a Nitrokey HSM

echo "0000: 3031300D 06096086 48016503 04020105 000420" | \
  xxd -c 19 -r > /tmp/sighdr
cat /tmp/sighdr $(base64 --decode digestfile) > /tmp/hashtosign
pkcs11-tool --id $key_id -s --login -m RSA-PKCS-PSS --hash-algorithm SHA256 --mgf MGF1-SHA256 \
  --input-file /tmp/hashtosign | \
  base64 > sigfile
  1. Manually (or with an extra make target) stitch the TA together using stitch --key $(TA_SIGN_KEY) --uuid $(user-ta-uuid)

By default the UUID is taken as the base file name for all files. Different file names and paths can be set through additional options to Consult --help for a full list of options and parameters.