Trusted Firmware-M Profile Small Design

Author

David Hu

Organization

Arm Limited

Contact

david.hu@arm.com

Introduction

The capabilities and resources may dramatically vary on different IoT devices. Some IoT devices may have very limited memory resource. The program on those devices should keep small memory footprint and basic functionalities. On the other hand, some devices may consist of more memory and extended storage, to support stronger software capabilities.

Diverse IoT use cases also require different levels of security and requirements on device resource. For example, use cases require different cipher capabilities. Selecting cipher suites can be sensitive to memory footprint on devices with constrained resource.

Trusted Firmware-M (TF-M) defines 3 general profiles, Profile Small, Profile Medium and Profile Large, to provide different levels of security to fit diverse device capabilities and use cases. Each profile specifies a predefined list of features, targeting typical use cases with specific hardware constraints. Profiles can serve as reference designs, based on which developers can continue further development and configurations, according to use case.

As one of the TF-M Profiles, TF-M Profile Small (Profile S) consists of lightweight TF-M framework and basic Secure Services to keep smallest memory footprint, supporting fundamental security features on devices with ultra constrained resource.

This profile enables connecting with Edge Gateways and IoT Cloud Services supporting secure connection based solely on symmetric cryptography.

This document summarizes and discusses the features specified in TF-M Profile Small.

Overall design

TF-M Profile Small defines the following features:

  • Lightweight framework

    • Library model or Secure Function (SFN) model 2

    • Level 1 isolation

    • Buffer sharing allowed

    • Single secure context

  • Crypto

    • Symmetric cipher only

    • Cipher suite for symmetric-key algorithms based protocols, such as cipher suites defined in TLS pre-shared key (TLS-PSK) 1.

      • Advanced Encryption Standard (AES) as symmetric crypto algorithm

      • SHA256 as Hash function

      • HMAC as Message Authentication Code algorithm

  • Internal Trusted Storage (ITS)

    • No encryption

    • No rollback protection

    • Decrease internal transient buffer size

  • Initial Attestation

    • Based on symmetric key algorithms

  • Lightweight boot

    • Single image boot

    • Anti-rollback protection is enabled

Protected Storage, audit logging and other Secure Services provided by TF-M are disabled by default.

Design details

More details of TF-M Profile Small design are discussed in following sections.

Lightweight framework

TF-M framework model

Library model is selected by default in Profile Small implementation. Library model implements secure function calls, via which clients directly call secure services. It provides a more simple implementation of TF-M framework and may reduce memory footprint, compared with Inter-Process Communication (IPC) model 3.

As Library model is TF-M specific implementation, please check some of its dedicated implementation details as described in Appendix, before adopting Library model on your platforms.

You can select SFN model instead of Library model in Profile Small. SFN model is defined in FF-M 1.1 extensions 2. It is a more simple implementation of TF-M framework and may also reduce memory footprint, compared with Inter-Process Communication (IPC) model 3.

Level 1 isolation

PSA Security Model 4 defines 3 levels of isolation.

  • Level 1 isolation isolates Secure Processing Environment (SPE) from Non-secure Processing Environment (NSPE).

  • PSA Root of Trust (PSA RoT) and Application Root of Trust (ARoT) are isolated from each other in level 2 isolation.

  • Individual secure partitions are isolated from each other even within a particular security domain (PSA RoT, ARoT), in level 3 isolation.

Profile Small dedicated use cases with simple service model may not require level 2 or level 3 isolation. Devices which Profile Small aims at may be unable to implement stricter isolation, limited by hardware capabilities.

Level 1 isolation reduces requirements enforced by hardware isolation and cost of software for management.

Note

Security note

If a device or a use case enforces level 2 or level 3 isolation, it is suggested to apply other configurations, other than TF-M Profile Small.

Crypto service

TF-M Profile Small only requires symmetric crypto since symmetric algorithms require shorter keys and less computational burden, compared with asymmetric crypto.

By default, TF-M Profile Small requires the same capabilities as defined in TLS-PSK, to support symmetric key algorithms based protocols.

Note

Implementation note

Please note that TF-M Profile Small doesn’t require that TLS-PSK is mandatory in applications. Instead, Profile Small only requires the same capabilities as defined in TLS-PSK, such as one symmetric cipher algorithm and one hash function.

TF-M Profile Small selects TLS-PSK cipher suite TLS_PSK_WITH_AES_128_CCM 6 as reference, which requires:

  • AES-128-CCM (AES CCM mode with 128-bit key) as symmetric crypto algorithm

  • SHA256 as Hash function

  • HMAC as Message Authentication Code algorithm

TLS_PSK_WITH_AES_128_CCM is selected since it requires small key length and less hardware capabilities, while keeping enough level of security.

Note

Implementation note

Developers can replace default algorithms with others or implement more algorithms.

Proper symmetric key algorithms and cipher suites should be selected according to device capabilities, the use case and the requirement of peers in connection.

Refer to Crypto service configuration for implementation details of configuring algorithms and cipher suites.

Note

Security note

It is recommended not to use MD5 or SHA-1 for message digests as they are subject to collision attacks 7 8.

Secure Storage

TF-M Profile Small assumes that extremely constrained devices only contain basic on-chip storage, without external or removable storage. As a result, TF-M Profile Small includes ITS service and disables Protected Storage service.

Encryption and rollback protection

Neither encryption nor rollback protection is enabled in current ITS implementation.

It is expected that ITS relies solely on the physical inaccessibility property of on-chip storage, together with PSA isolation, without requiring additional cryptographic protection.

Internal transient buffer

ITS implements a internal transient buffer 9 to hold the data read from/written to storage, especially for flash, to solve the alignment and security issues.

The internal transient buffer is aligned to the flash device’s program unit. Copying data to it from the caller can align all write requests to the flash device’s program unit. The internal transient buffer can help protect Flash access from some attacks, such as TOCTOU attack.

Although removing this internal buffer can save some memory consumption, typically 512 bytes, it may bring alignment or security issues. Therefore, to achieve a better trade-off between memory footprint and security, TF-M Profile Small optimizes the internal buffer size to 32 bytes by default.

As discussed in Crypto service, TF-M Profile Small requires AES-128 and SHA-256, which use 128-bit key and 256-bit key respectively. Besides, either long public/private keys or PKI-based certificates should be very rare as asymmetric crypto is not supported in Profile Small. Therefore, a 32-byte internal buffer should cover the assets in TF-M Profile Small use cases.

The buffer size can be adjusted according to use case and device Flash attributes. Refer to Internal Trusted Storage configurations for more details.

Initial Attestation

Profile Small requires an Initial Attestation secure service based on symmetric key algorithms. Refer to PSA Attestation API document 10 for details of Initial Attestation based on symmetric key algorithms.

It can heavily increase memory footprint to support Initial Attestation based on asymmetric key algorithms, due to asymmetric ciphers and related PKI modules.

Note

Implementation note

As pointed out by PSA Attestation API document 10, the use cases of Initial Attestation based on symmetric key algorithms can be limited due to the associated infrastructure costs for key management and operational complexities. It may also restrict the ability to interoperate with scenarios that involve third parties.

If asymmetric key algorithms based Initial Attestation is required in use scenarios, it is recommended to select other TF-M Profiles which support asymmetric key algorithms.

Note

Implementation note

It is recommended to utilize the same MAC algorithm supported in Crypto service to complete the signing in COSE_Mac0, to minimize memory footprint.

Lightweight boot

If MCUBoot provided by TF-M is enabled, single image boot 11 is selected by default in Profile Small. In case of single image boot, secure and non-secure images are handled as a single blob and signed together during image generation.

However, secure and non-secure images must be updated together in single image boot. It may decrease the flexibility of image update and cost longer update process. Since the image sizes should usually be small with limited functionalities in Profile Small dedicated use case, the cost may still be reasonable.

BL2 implementation can be device specific. Devices may implement diverse boot processes with different features and configurations. However, anti-rollback protection is required as a mandatory feature of boot loader. Boot loader should be able to prevent unauthorized rollback, to protect devices from being downgraded to earlier versions with known vulnerabilities.

Implementation

Overview

The basic idea is to add dedicated profile CMake configuration files under folder config/profile for TF-M Profile Small default configuration.

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

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

The behavior of the Profile Small 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 Small implementation.

Top-level configuration files

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

Table 9: TFM options in Profile Small top-level CMake config file

Configs

Default value

Descriptions

TFM_ISOLATION_LEVEL

1

Select level 2 isolation

TFM_LIB_MODEL

ON

Select Library 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

TFM_MBEDCRYPTO_CONFIG_PATH

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

Mbed Crypto config file path

CRYPTO_ASYM_SIGN_MODULE_DISABLED

ON

Disable asymmetric signature

CRYPTO_ASYM_ENCRYPT_MODULE_DISABLED

ON

Disable asymmetric encryption

TFM_PARTITION_INITIAL_ATTESTATION

ON

Enable Initial Attestation service

SYMMETRIC_INITIAL_ATTESTATION

ON

Enable symmetric attestation

TFM_PARTITION_PROTECTED_STORAGE

OFF

Enable PS service

TFM_PARTITION_PLATFORM

OFF

Enable TF-M Platform SP

TFM_PARTITION_AUDIT_LOG

OFF

Disable TF-M audit logging service

Note

Implementation note

The following sections focus on the feature selection via configuration setting. Dedicated optimization on memory footprint is not covered in this document.

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

TF-M framework setting

The top-level Profile Small CMake config file selects Library model and level 1 isolation.

Users can set -DCONFIG_TFM_SPM_BACKEND=SFN in build command to select SFN model instead.

Crypto service configuration

Crypto Secure Partition

TF-M Profile Small enables Crypto Secure Partition (SP) in its top-level CMake config file. Crypto SP modules not supported in TF-M Profile Small are disabled. The disabled modules are shown below.

  • Disable asymmetric cipher

Other modules and configurations 12 are kept as default values.

Additional configuration flags with more fine granularity can be added to control building of specific crypto algorithms and corresponding test cases.

Mbed Crypto configurations

TF-M Profile Small adds a dedicated Mbed Crypto config file tfm_mbedcrypto_config_profile_small.h at /lib/ext/mbedcrypto/mbedcrypto_config file, instead of the common one tfm_mbedcrypto_config_default.h 12.

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

  • Disable asymmetric ciphers

  • Disable HMAC-based key derivation function (HKDF)

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 Small 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 Small CMake config file.

The internal transient buffer size ITS_BUF_SIZE 9 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 Small CMake config file won’t touch the configurations of device specific Flash hardware attributes 9.

Initial Attestation secure service

TF-M Profile Small provides a reference implementation of symmetric key algorithms based Initial Attestation, using HMAC SHA-256 as MAC algorithm in COSE_Mac0 structure. The implementation follows PSA Attestation API document 10.

Profile Small top-level config file enables Initial Attestation secure service and selects symmetric key algorithms based Initial Attestation by default.

  • Set TFM_PARTITION_INITIAL_ATTESTATION to ON

  • Set SYMMETRIC_INITIAL_ATTESTATION to ON

Symmetric and asymmetric key algorithms based Initial Attestation can share the same generations of token claims, except Instance ID claim.

Profile Small may implement the procedure or rely on a 3rd-party tool to construct and sign COSE_Mac0 structure.

Details of symmetric key algorithms based Initial Attestation design will be covered in a dedicated document.

Disabled secure services

Audit logging, Protected Storage, and Platform Service are disabled by default in Profile Small top-level CMake config file.

Test configuration

Some cryptography tests are disabled due to the reduced Mbed Crypto config. Some of them are shown in the table below.

Table 10: TFM options in Profile Small top-level CMake config file

Configs

Default value

Descriptions

TFM_CRYPTO_TEST_ALG_CBC

OFF

Test CBC cryptography mode

TFM_CRYPTO_TEST_ALG_CCM

ON

Test CCM cryptography mode

TFM_CRYPTO_TEST_ALG_CFB

OFF

Test CFB cryptography mode

TFM_CRYPTO_TEST_ALG_CTR

OFF

Test CTR cryptography mode

TFM_CRYPTO_TEST_ALG_GCM

OFF

Test GCM cryptography mode

TFM_CRYPTO_TEST_ALG_SHA_512

OFF

Test SHA-512 cryptography algorithm

TFM_CRYPTO_TEST_HKDF

OFF

Test HKDF key derivation algorithm

TFM_CRYPTO_TEST_ECDH

OFF

Test ECDH key agreement algorithm

BL2 setting

Profile Small 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, single image boot is selected in TF-M Profile Small top-level CMake config file.

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

Table 11: BL2 options in Profile Small top-level CMake config file

Configs

Default value

Descriptions

BL2

ON

Enable MCUBoot bootloader

MCUBOOT_IMAGE_NUMBER

1

Combine S and NS images

Platform support

Building Profile Small

To build Profile Small, argument TFM_PROFILE in build command line should be set to profile_small.

Take AN521 as an example.

The following commands build Profile Small without test cases on AN521 with build type MinSizeRel, built by Armclang. Library model is selected by default.

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

The following commands build Profile Small with regression test cases on AN521 with build type MinSizeRel, built by Armclang. Library model is selected by default.

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

Note

  • For devices with more constrained 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.

The following commands build Profile Small with SFN model on AN521 with build type MinSizeRel, built by GNU Arm compiler.

cd <TFM root dir>
mkdir build && cd build
cmake -DTFM_PLATFORM=arm/mps2/an521 \
      -DTFM_PROFILE=profile_small \
      -DCMAKE_BUILD_TYPE=MinSizeRel \
      -DCONFIG_TFM_SPM_BACKEND=SFN \
      ../
cmake --build ./ -- install

More details of building instructions and parameters can be found TF-M build instruction guide 13.

Appendix

TF-M Library model implementation details

Note

Implementation note

Please note that there is no public dedicated specification for Library model. The design, interfaces and implementation of Library model in TF-M may change.

Buffer sharing allowed

To simplify interface and reduce memory footprint, TF-M Library model directly handles client call input vectors from non-secure client buffers and later writes results back to those buffers, without keeping a copy in a transient buffer inside TF-M.

Note

Security note

There can be security vulnerabilities if non-secure client buffers are directly shared between NSPE and SPE, such as Time-of-check to time-of-use (TOCTOU) attack.

Developers need to check if this can meet the Security Functional Requirements (SFR) of the integration of their devices. Some SFRs are listed in a set of example Threat Models and Security Analyses (TMSA) offered by PSA for common IoT use cases. 5

Single secure context

TF-M Library model only supports single secure context.

It cannot support multiple contexts or the scheduling implemented in IPC model. It neither can support multiple outstanding PSA client calls.

But correspondingly, it can save memory footprint and runtime complexity in context management and scheduling.

Note

Security note

Non-secure software should prevent triggering multiple outstanding PSA client calls concurrently. Otherwise, it may crash current running secure context.