Secure communication with TPM

This document describes the project ws2022_team_4_tpm which implements a Secure Communication component and TPM support.

With this component it is possible to connect with a socket interface to other devices and communicate over an encrypted connection.

For testing purposes a TestApp component is also present.

Overview of software based implementation:

Overview of tpm based implementation:

Built With

• TRENTOS SDK
• SeL4 microkernel
• wolfTPM
• wolfSSL
• mbedtls

Compilation

The project folder has to be placed into the TRENTOS SDK at

./sdk/demos/ws2022_team_4_tpm

Several patches have to be added to the TRENTOS SDK with patch -p1 < {PATCH}.diff

• Patch KeyStoreRamFV
• Patch RSA 1
• Patch RSA 2

Adjust the default value of key sizes in OS_Keystore:

• ../libs/os_keystore/os_keystore_ram_fv/include/OS_KeystoreRamFV.h: change the value of OS_KeystoreRamFV_MAX_KEY_SIZE in line 36 from 2048 to 2080
• ../libs/os_keystore/os_keystore_ram_fv/keystore_ram_fv/KeystoreRamFV.h: change the value of KeystoreRamFV_KEY_DATA_SIZE in line 12 from 2052 to 2084

(Optional) For a more clear debug log, comment out the ZF_lOGE macros at:

./sdk/sdk-sel4-camkes/libs/sel4_util_libs/libplatsupport/src/plat/bcm2837/system_timer.c

Build the project with the following command:

bash sdk/scripts/open_trentos_build_env.sh sdk/build-system.sh sdk/demos/ws2022_team_4_tpm rpi3 build-rpi3-Debug-demo_tpm -DCMAKE_BUILD_TYPE=Debug -DHW_TPM=ON

-DHW_TPM=ON adds hardware support for the TPM. To run the component without it use -DHW_TPM=OFF.

Compilation Flags

Add flags with -D{FLAG}={VALUE} to toggle functionality.

Flag	Values	Default Value	Description
HW_TPM	ON, OFF	OFF	Toggles hardware support for the SLB9670 TPM
CLEAR_TPM	ON, OFF	OFF	Clears TPM on boot
WOLF_DEBUG	ON, OFF	OFF	Toggles wolfTPM and wolfSSL debug info
BENCHMARK	ON, OFF	OFF	Toggles rudimentary benchmark of encrypt, decrypt and keygen functionality

Note: The filesystem functionality is not supported if HW_TPM=ON is used.

Hardware Setup

This project is tailored to the RaspberryPi 3b+ platform. For more information and a detailed setup on the RPI3 see the TRENTOS Handbook V1.3 on page 256.

Connect the UART Controller to the RPi according to the following picture and connect it with your developement machine. Connect the RPi and your developement machine via ethernet. Connect the TPM module according to the picture.

Configure the network interface for the wired connection between the RPi and your developement machine (e.g. with ip addr add or NetworkManager) to be assigned with the ip address 10.0.0.10.

Format an sd card with a FAT filesystem and copy all files from the folder ./sdk/resources/rpi3_sd_card onto the sd card. After compilation copy the image ../build-rpi3-Debug-demo_tpm/images/os_image.elf onto the sd card. Ensure files are written to sd card.

In the file config.txt in the boot partition of the RPi add the line enable_jtag_gpio=0.

Insert the sd card into the RPI.

Test

Launch the UART interface with the following command:

sudo picocom -b 115200 /dev/<ttyUSBX>

Hereby, acts as a placeholder for the specific device representing the USB-to-UART adapter, e.g. ttyUSB0.

Launch the test server ./sdk/demos/ws2022_team_4_tpm/Project Assignments - Resources-20221220/srv.py (Note: If the test server is not able to listen on the wired connection, you may have to power on the RPI first before starting the test server)

Power on the RPI.

Key loading and generation with -DHW_TPM=ON

If no client key is found in TPM_Keystore, the program will try to import one from system_config.h. If CLIENT_PRIVATE_KEY in system_config.h is set to an invalid value it will generate a new key. In any case the new key will be saved to the keystore for future reboots. On each boot cycle the client public client key will be printed to the debug console.

The server public key is not stored in the TPM_Keystore. This would require a different if_Keystore implementation, since wolfTPM does not support storing only the public key without it's private counter-part with wolfTPM2_NVStoreKey(). In my opinion the use-case doesn't justify a different implementation since the public key is configured on the server and is by definition not a secret, so I decided against implementing one. For that reason, the server public key will always be imported from the SERVER_PUBLIC_KEY constant defined in system_config.h.

The CLIENT_PUBLIC_KEY constant will not be used in TPM mode, since the client public key will be generated from it's private counterpart.

The TPM can be cleared on boot with the -DCLEAR_TPM=ON build flag. This will clear the entire TPM, not just the keystore. Use with care!

Implementation Details

Custom Interfaces

The TPM component provides two custom interfaces:

• if_Crypto
• if_Keystore

For convenience two wrapper libraries are provided:

• TPM_Crypto
• TPM_Keystore

The interface and libraries are declared in components/RPi_SPI_TPM/interfaces. See the library header files for implementation details.

Demo Video

In this demo HW_TPM=ON is used and a TPM is used which already has created a root key and has a client key stored. The server key is imported from the system_config.h file.

Target in demo was build with:

bash sdk/scripts/open_trentos_build_env.sh sdk/build-system.sh sdk/demos/ws2022_team_4_tpm rpi3 build-rpi3-Debug-demo_tpm -DCMAKE_BUILD_TYPE=Debug -DHW_TPM=ON

Presentation Slides

Vortrag_TRENTOS.pdf