Trusted Firmware-M Profile Small Design
Introduction
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
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
Only enable multi-part functions in hash, symmetric ciphers, Message Authentication Code (MAC) and Authenticated Encryption with Associated Data (AEAD) operations.
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, firmware update 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
SFN model is selected by default in Profile Small implementation. 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 [5] 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 [6] [7].
By default, Profile Small only enables multi-part functions defined in PSA Cryptography API [13] in hash, symmetric ciphers, MAC and AEAD operations. Disabling single-part functions optimizes the code size of TF-M crypto service. Multi-part operations allows the message data to be processed in fragments instead of all at once. In static memory allocation, single-part operation may require to allocate a large memory space to support long message with unknown length. Therefore single-part operations can help users optimize memory footprint, especially while dealing with streaming data on IoT devices.
It may slightly increase the code size in applications to replace single-part implementation with mulit-part implementation. Althgouth the code size increment can be qaulified, if users are concerned about the code size increment, they can enable single-part operations by toggling Profile Small default configuration.
It may increase latency and overall time cost to implement cryptography functionality with single-part operations, compared to with multi-part ones. Users can enable single-part operations if the usage scenario requires single-part opreations to meet its perfermance metrics.
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 [8] 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 [9] 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 [9], 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 [10] 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 Build 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.
Configs |
Default value |
Descriptions |
|---|---|---|
|
|
Select level 2 isolation |
|
|
Enable ITS SP |
|
|
ITS internal transient buffer size |
|
|
Enable Crypto service |
|
|
Mbed Crypto config file path |
|
|
Mbed Crypto PSA config file path |
|
|
Enable asymmetric signature |
|
|
Enable asymmetric encryption |
|
|
Enable Initial Attestation service |
|
|
Enable symmetric attestation |
|
|
Enable PS service |
|
|
Enable TF-M Platform SP |
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 SFN model and level 1 isolation.
In SFN model, -DPSA_FRAMEWORK_HAS_MM_IOVEC is enabled by default.
It reduces memory footprint by avoiding the transient copy from input
vectors and copy to output vectors.
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/features are shown below.
Disable asymmetric cipher
Disable single-part operations in Hash, MAC, AEAD and symmetric ciphers via selecting
CRYPTO_SINGLE_PART_FUNCS_DISABLED
Other modules and configurations [11] 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 and Mbed Crypto PSA config file
crypto_config_profile_small.h at /lib/ext/mbedcrypto/mbedcrypto_config
folder, instead of the common one tfm_mbedcrypto_config_default.h and
crypto_config_default.h [11].
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 [8] 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 [8].
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
[9].
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_ATTESTATIONtoONSet
SYMMETRIC_INITIAL_ATTESTATIONtoON
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
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.
Configs |
Default value |
Descriptions |
|---|---|---|
|
|
Test CBC cryptography mode |
|
|
Test CCM cryptography mode |
|
|
Test CFB cryptography mode |
|
|
Test ECB cryptography mode |
|
|
Test CTR cryptography mode |
|
|
Test OFB cryptography mode |
|
|
Test GCM cryptography mode |
|
|
Test SHA-384 cryptography algorithm |
|
|
Test SHA-512 cryptography algorithm |
|
|
Test HKDF key derivation algorithm |
|
|
Test ECDH key agreement algorithm |
|
|
Test ChaCha20 stream cipher |
|
|
Test ChaCha20-Poly1305 AEAD algorithm |
|
|
Test single-part operations in hash, MAC, AEAD and symmetric ciphers |
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.
Configs |
Default value |
Descriptions |
|---|---|---|
|
|
Enable MCUBoot bootloader |
|
|
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. SFN 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. SFN 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.
More details of building instructions and parameters can be found TF-M build instruction guide [12].
Reference
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