Trusted Firmware-M Profile Medium Design

Author

David Hu

Organization

Arm Limited

Contact

david.hu@arm.com

Introduction

Compared with Profile Small, Profile Medium aims to securely connect devices to Cloud services with asymmetric cipher support. Profile Medium target devices need more resources for more cipher algorithms and higher isolation levels.

For more descriptions and background of TF-M Profile, please refer to Profile Small design document [PROFILE-S].

Overall design

TF-M Profile Medium defines the following feature set:

  • Firmware Framework

  • Internal Trusted Storage (ITS)

  • Crypto

    • Support both symmetric cryptography and asymmetric cryptography

    • Asymmetric key based cipher suite suggested in TLS/DTLS profiles for IoT [RFC7925] and CoAP [RFC7252], including

      • Authenticated Encryption with Associated Data (AEAD) algorithm

      • Asymmetric key algorithm based signature and verification

      • Public-key cryptography based key exchange

      • Hash function

      • HMAC for default Pseudorandom Function (PRF)

    • Asymmetric digital signature and verification for Initial Attestation Token (IAT)

  • Initial Attestation

    • Asymmetric key algorithm based Initial Attestation

  • Lightweight boot

    • Anti-rollback protection

    • Multiple image boot

  • Protected Storage (PS) if off-chip storage device is integrated

    • Data confidentiality

    • Data integrity

    • Rollback protection

Design details

More details of TF-M Profile Medium design are described in following sections.

Firmware framework

Profile Medium with IPC model and isolation level 2 aims to support usage scenarios which require more complicated secure service model and additional protection to PSA RoT.

Level 2 isolation

Profile Medium selects isolation level 2 by default. In addition to isolation level 1, the PSA Root of Trust (PSA RoT) is also protected from access by the Application Root of Trust (App RoT) in level 2 isolation.

IPC model

Profile Medium enables IPC model by default. IPC model can achieve a more flexible framework and higher levels of isolation, but may require more memory footprint and bring in longer latency, compared to Library model.

TF-M IPC model implementation follows the PSA Firmware Framework for M (PSA-FF-M) [PSA-FF-M].

Crypto service

Compared to Profile Small, Profile Medium includes asymmetric cryptography to support direct connection to Cloud services via common protocols, such as TLS/DTLS 1.2.

As suggested in CoAP [RFC7252] and [RFC7925], TF-M Profile Medium by default selects TLS_ECDHE_ECDSA_WITH_AES_128_CCM as reference, which requires:

  • ECDHE_ECDSA as key exchange algorithm.

  • AES-128-CCM (AES CCM mode with 128-bit key) as AEAD algorithm. Platforms can implement AES-128-CCM with truncated authentication tag to achieve less network bandwidth [RFC7925].

  • SHA256 as Hash function.

  • HMAC as Message Authentication Code algorithm.

Applications can also support TLS PSK [RFC4279] cipher suites, such as TLS_PSK_WITH_AES_128_CCM [RFC7925].

Note

Implementation note

Developers can replace default algorithms with others or implement more algorithms according to actual usage scenarios and device capabilities.

If a Crypto hardware accelerator is integrated, the cipher suites and algorithms also depend on those accelerator features.

More details of cipher suite are described below.

Digital signature and verification

ECDSA is selected by default in Profile Medium. ECDSA requires much shorter keys compared with RSA at the same security level. Therefore, ECDSA can cost less storage area for assets and less network bandwidth to setup a TLS connection. ECDSA is also preferred for forward compatibility of future TLS versions.

As requested in [RFC7251], ECC curve secp256r1 should be supported. More ECC curves can be added based on the requirements in production.

If usage scenarios require RSA algorithm for backward compatibility and legacy applications, platforms can add RSA support or replace ECDSA with RSA. The cipher suite should be switched accordingly.

AEAD algorithm

If Protected Storage (PS) is implemented, it is recommended to select the same AEAD algorithm for PS service as the one used by TLS/DTLS cipher suite.

Internal Trusted Storage

The configuration of ITS is the same as those in Profile Small [PROFILE-S].

Lightweight boot

BL2 implementation can be device specific. Devices may implement diverse boot processes with different features and configurations. However, the boot loader must support anti-rollback protection. Boot loader must be able to prevent unauthorized rollback, to protect devices from being downgraded to earlier versions with known vulnerabilities.

MCUBoot in TF-M is configured as multiple image boot by default in Profile Medium. In multiple image boot, secure and non-secure images can be signed independently with different keys and they can be updated separately. It can support multiple vendors scenarios, in which non-secure and secure images are generated and updated by different vendors. Multiple image boot may require more storage area compared with single image boot.

Protected Storage

PS service is required if an off-chip storage device is integrated and used on the platform.

TF-M PS service relies on an AEAD algorithm to ensure data confidentiality and integrity. It is recommended to select the same AEAD algorithm as the one used for TLS/DTLS cipher suite.

Anti-rollback protection in PS relies on non-volatile counter(s) provided by TF-M Platform Secure Partition (SP).

TF-M audit logging service

TF-M audit logging service allows secure services in the system to log critical system events and information.

TF-M audit logging service is not enabled in Profile Medium since its IPC model dedicated interface is not ready yet.

Note

Implementation note

Please note that there is no dedicated PSA specification for Audit Logging yet. The design, interfaces and implementation of TF-M audit logging service may change.

Implementation

Overview

The basic idea is to add dedicated profile CMake configuration files under folder config/profile for TF-M Profile Medium default configuration, the same as Profile Small does.

The top-level Profile Medium config file collects all the necessary configuration flags and set them to default values, to explicitly enable the features required in Profile Medium and disable the unnecessary ones, during TF-M build.

A platform/use case can provide a configuration extension file to overwrite Profile Medium default setting and append other configurations. This configuration extension file can be added via parameter TFM_EXTRA_CONFIG_PATH in build command line.

The behaviour of the Profile Medium build flow (particularly the order of configuration loading and overriding) can be found at Cmake configuration

The details of configurations will be covered in each module in Implementation details.

Implementation details

This section discusses the details of Profile Medium implementation.

Top-level configuration files

The firmware framework configurations in config/profile/profile_medium are shown below.

Table 7: Config flags in Profile Medium top-level CMake config file

Configs

Default value

Descriptions

TFM_ISOLATION_LEVEL

2

Select level 2 isolation

TFM_LIB_MODEL

OFF | Select IPC model

TFM_PARTITION_INTERNAL_TRUSTED_STORAGE

ON | Enable ITS SP

ITS_BUF_SIZE

32

ITS internal transient buffer size

TFM_PARTITION_CRYPTO

ON

Enable Crypto service

CRYPTO_ASYM_ENCRYPT_MODULE_DISABLED

ON

Disable Crypto asymmetric encryption operations

TFM_MBEDCRYPTO_CONFIG_PATH

${CMAKE_SOURCE_DIR}/lib/ext/mbedcrypto/mbedcrypto_config/tfm_mbedcrypto_config_profile_medium.h

Mbed Crypto config file path

TFM_PARTITION_INITIAL_ATTESTATION

ON

Enable Initial Attestation service

TFM_PARTITION_PROTECTED_STORAGE 1

ON

Enable PS service

TFM_PARTITION_PLATFORM

ON

Enable TF-M Platform SP

TFM_PARTITION_AUDIT_LOG

OFF

Disable TF-M audit logging service

1

PS service is enabled by default. Platforms without off-chip storage devices can turn off TFM_PARTITION_PROTECTED_STORAGE to disable PS service. See Protected Storage Secure Partition for details.

Note

Where a configuration is the same as the default in config/config_default.cmake, it is omitted from the profile configuration file.

Test configuration

Standard regression test configuration applies. This means that enabling regression testing via

-DTEST_S=ON -DTEST_NS=ON

Will enable testing for all enabled partitions. See above for details of enabled partitions. Because Profile Medium enables IPC mode, the IPC tests are also enabled.

Some cryptography tests are disabled due to the reduced Mbed Crypto config.

Table 8: TFM options in Profile Medium top-level CMake config file

Configs

Default value

Descriptions

TFM_CRYPTO_TEST_ALG_CBC

OFF

Disable CBC mode test

TFM_CRYPTO_TEST_ALG_CCM

ON

Enable CCM mode test

TFM_CRYPTO_TEST_ALG_CFB

OFF

Disable CFB mode test

TFM_CRYPTO_TEST_ALG_CTR

OFF

Disable CTR mode test

TFM_CRYPTO_TEST_ALG_GCM

OFF

Disable GCM mode test

TFM_CRYPTO_TEST_ALG_SHA_512

OFF

Disable SHA-512 algorithm test

TFM_CRYPTO_TEST_HKDF

OFF

Disable HKDF algorithm test

TFM_CRYPTO_TEST_ECDH

ON

Enable ECDH key agreement test

Device configuration extension

To change default configurations and add platform specific configurations, a platform can add a platform configuration file at platform/ext<TFM_PLATFORM>/config.cmake

Crypto service configurations

Crypto Secure Partition

TF-M Profile Medium enables Crypto SP in top-level CMake config file. The following PSA Crypto operationts are enabled by default.

  • Hash operations

  • Message authentication codes

  • Symmetric ciphers

  • AEAD operations

  • Asymmetric key algorithm based signature and verification

  • Key derivation

  • Key management

Mbed Crypto configurations

TF-M Profile Medium adds a dedicated Mbed Crypto config file tfm_mbedcrypto_config_profile_medium.h at /lib/ext/mbedcrypto/mbedcrypto_config folder, instead of the common one tfm_mbedcrypto_config_default.h [CRYPTO-DESIGN].

Major Mbed Crypto configurations are set as listed below:

  • Enable SHA256

  • Enable generic message digest wrappers

  • Enable AES

  • Enable CCM mode for symmetric ciphers

  • Disable other modes for symmetric ciphers

  • Enable ECDH

  • Enable ECDSA

  • Select ECC curve secp256r1

  • Other configurations required by selected option above

Other configurations can be selected to optimize the memory footprint of Crypto module.

A device/use case can append an extra config header to the Profile Medium default Mbed Crypto config file. This can be done by setting the TFM_MBEDCRYPTO_PLATFORM_EXTRA_CONFIG_PATH cmake variable in the platform config file platform/ext<TFM_PLATFORM>/config.cmake. This cmake variable is a wrapper around the MBEDTLS_USER_CONFIG_FILE options, but is preferred as it keeps all configuration in cmake.

Internal Trusted Storage configurations

ITS service is enabled in top-level Profile Medium CMake config file by default.

The internal transient buffer size ITS_BUF_SIZE [ITS-INTEGRATE] is set to 32 bytes by default. A platform/use case can overwrite the buffer size in its specific configuration extension according to its actual requirement of assets and Flash attributes.

Profile Medium CMake config file won’t touch the configurations of device specific Flash hardware attributes [ITS-INTEGRATE].

Protected Storage Secure Partition

Data confidentiality, integrity and anti-rollback protection are enabled by default in PS.

If PS is selected, AES-CCM is used as AEAD algorithm by default. It requires to enable PS implementation to select diverse AEAD algorithm.

If platforms don’t integrate any off-chip storage device, platforms can disable PS in platform specific configuration extension file via platform/ext<TFM_PLATFORM>/config.cmake.

BL2 setting

Profile Medium enables MCUBoot provided by TF-M by default. A platform can overwrite this configuration by disabling MCUBoot in its configuration extension file platform/ext<TFM_PLATFORM>/config.cmake.

If MCUBoot provided by TF-M is enabled, multiple image boot is selected by default in TF-M Profile Medium top-level CMake config file.

If a device implements its own boot loader, the configurations are implementation defined.

Platform support

To enable Profile Medium on a platform, the platform specific CMake file should be added into the platform support list in top-level Profile Medium CMake config file.

Building Profile Medium

To build Profile Medium, argument TFM_PROFILE in build command line should be set to profile_medium.

Take AN521 as an example:

The following commands build Profile Medium without test cases on AN521 with build type MinSizeRel, built by Armclang.

cd <TFM root dir>
mkdir build && cd build
cmake -DTFM_PLATFORM=arm/mps2/an521 \
      -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
      -DTFM_PROFILE=profile_medium \
      -DCMAKE_BUILD_TYPE=MinSizeRel \
      ../
cmake --build ./ -- install

The following commands build Profile Medium with regression test cases on AN521 with build type MinSizeRel, built by Armclang.

cd <TFM root dir>
mkdir build && cd build
cmake -DTFM_PLATFORM=arm/mps2/an521 \
      -DTFM_TOOLCHAIN_FILE=../toolchain_ARMCLANG.cmake \
      -DTFM_PROFILE=profile_medium \
      -DCMAKE_BUILD_TYPE=MinSizeRel \
      -DTEST_S=ON -DTEST_NS=ON \
      ../
cmake --build ./ -- install

Note

  • For devices with more contrained memory and flash requirements, it is possible to build with either only TEST_S enabled or only TEST_NS enabled. This will decrease the size of the test images. Note that both test suites must still be run to ensure correct operation.

More details of building instructions and parameters can be found TF-M build instruction guide [TFM-BUILD].