Docs: Profile Medium design document
Add a design document of TF-M Profile Medium.
Change-Id: Ic479eef9516b5c6771faee5c88a48303a75bced3
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/docs/design_documents/tfm_profile_medium.rst b/docs/design_documents/tfm_profile_medium.rst
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+########################################
+Trusted Firmware-M Profile Medium Design
+########################################
+
+:Authors: 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
+
+ - Inter-Process Communication (IPC) model [PSA-FF-M]_
+ - Isolation level 2 [PSA-FF-M]_
+
+ - Internal Trusted Storage (ITS)
+
+ - Crypto
+
+ - Support both symmetric ciphers and asymmetric ciphers
+ - 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 cipher 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 top-level CMake configuration files under
+folder ``configs`` for TF-M Profile Medium default configuration, as 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_PROFILE_CONFIG_EXT`` in build command line. The top-level config file will
+include the device configuration extension file to load platform/use case
+specific configurations.
+
+The overall build flow of Profile Medium is shown as the flowchart below.
+
+.. uml::
+
+ @startuml
+
+ title Overall build flow
+
+ start
+
+ :Profile Medium CMake file;
+ note left
+ Top-level CMake config file under ""configs"".
+ Set configurations to default values.
+ endnote
+
+ if (Platform config\nextension specified?) then (Yes)
+ :Include platform specific\nconfig extension file;
+ note left
+ Platform specific configuration extension file
+ is provided via ""TFM_PROFILE_CONFIG_EXT"" in
+ build command line.
+ endnote
+
+ :Overwrite default configurations;
+ else (No)
+ endif
+
+ :CommonConfig.cmake;
+ note left
+ Normal building sequence
+ endnote
+
+ stop
+
+ @enduml
+
+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
+-----------------------------
+
+Profile Medium provides ``ConfigDefaultProfileM`` as a default top-level
+configuration file without test cases.
+
+The firmware framework configurations in ``ConfigDefaultProfileM`` are shown
+below.
+
+.. table:: Config flags in Profile Medium top-level CMake config file
+ :widths: auto
+ :align: center
+
+ +--------------------------------------------+-------------------------------------+-------------------------------------+
+ | Configs | Default value | Descriptions |
+ +============================================+=====================================+=====================================+
+ | ``TFM_LVL`` | ``2`` | Select level 2 isolation |
+ +--------------------------------------------+-------------------------------------+-------------------------------------+
+ | ``CORE_IPC`` | ``True`` | 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 |
+ +--------------------------------------------+-------------------------------------+-------------------------------------+
+ | ``MBEDTLS_CONFIG_FILE`` | ``tfm_profile_m_mbedcrypto_config`` | Default Mbed Crypto config file for |
+ | | | Profile Medium under |
+ | | | ``platform/ext/common`` |
+ +--------------------------------------------+-------------------------------------+-------------------------------------+
+ | ``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.
+
+Profile Medium provides ``ConfigRegressionProfileM`` as the default top-level
+configuration file with regression tests.
+
+The table below collects ``ConfigRegressionProfileM`` major configurations
+which are not covered or different from those in ``ConfigDefaultProfileM``.
+
+.. table:: Major config flags in Profile Medium top-level CMake config file with tests
+ :widths: auto
+ :align: center
+
+ +------------------+---------------+------------------------+
+ | Configs | Default value | Descriptions |
+ +==================+===============+========================+
+ | ``REGRESSION`` | ``ON`` | Enable Regression test |
+ +------------------+---------------+------------------------+
+ | ``CORE_TEST`` | ``ON`` | Enable Core test |
+ +------------------+---------------+------------------------+
+ | ``IPC_TEST`` | ``ON`` | Enable IPC test |
+ +------------------+---------------+------------------------+
+ | ``PSA_API_TEST`` | ``OFF`` | Disable PSA API test |
+ +------------------+---------------+------------------------+
+
+The test cases should be disabled in ``ConfigRegressionProfileM`` if the
+corresponding feature is not selected in Profile Medium by default.
+
+Device configuration extension
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To change default configurations and add platform specific configurations,
+a platform can set the path to its own configuration extension file in parameter
+``TFM_PROFILE_CONFIG_EXT`` in command line.
+
+A platform can also add its device specific configurations into its specific
+CMake file under ``platform/ext/`` folder.
+
+Crypto service configurations
+-----------------------------
+
+Crypto Secure Partition
+^^^^^^^^^^^^^^^^^^^^^^^
+
+TF-M Profile Medium enables Crypto SP in top-level CMake config file and selects
+all the Crypto modules.
+
+Mbed Crypto configurations
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+TF-M Profile Medium adds a dedicated Mbed Crypto config file
+``tfm_profile_m_mbedcrypto_config.h`` under ``platform/ext/common``.
+TF-M Profile Medium specifies ``tfm_profile_m_mbedcrypto_config.h`` as the
+default Mbed Crypto config in ``MBEDTLS_CONFIG_FILE`` in top-level CMake config
+file, instead of the common one ``tfm_mbedcrypto_config.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 replace Profile Medium default Mbed Crypto config file
+with its specific one to overwrite the default configurations. Alternatively, a
+device can overwrite the configurations by appending a config file via
+``MBEDTLS_USER_CONFIG_FILE``.
+
+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
+``TFM_PROFILE_CONFIG_EXT``.
+Profile Medium provides a configuration extension file example
+``profile_m_config_ext_ps_disabled.cmake`` which disables PS service.
+
+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 or in its specific CMake file under ``platform/ext/`` folder.
+
+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 ``PROJ_CONFIG`` in build command line should
+be set to following config files
+
+ - ``ConfigDefaultProfileM.cmake``
+ - ``ConfigRegressionProfileM.cmake``.
+
+Take AN521 as an example:
+
+The following commands build Profile Medium without test cases on **AN521** with
+build type **MinSizeRel**, built by **Armclang**.
+
+.. code-block:: bash
+
+ cmake -G"Unix Makefiles" -DPROJ_CONFIG=`readlink -f ../configs/ConfigDefaultProfileM.cmake` \
+ -DTARGET_PLATFORM=AN521 \
+ -DCMAKE_BUILD_TYPE=MinSizeRel \
+ -DCOMPILER=ARMCLANG ../
+ cmake --build ./ -- install
+
+The following commands build Profile Medium with regression test cases on
+**AN521** with build type **MinSizeRel**, built by **Armclang**.
+
+.. code-block:: bash
+
+ cmake -G"Unix Makefiles" -DPROJ_CONFIG=`readlink -f ../configs/ConfigRegressionProfileM.cmake` \
+ -DTARGET_PLATFORM=AN521 \
+ -DCMAKE_BUILD_TYPE=MinSizeRel \
+ -DCOMPILER=ARMCLANG ../
+ cmake --build ./ -- install
+
+More details of building instructions and parameters can be found TF-M build
+instruction guide [TFM-BUILD]_.
+
+The following commands include platform specific configuration extension file
+via ``TFM_PROFILE_CONFIG_EXT`` in command line. ``TFM_PROFILE_CONFIG_EXT`` can
+be an absolute path or a relative one to TF-M code root directory.
+The configuration extension file ``profile_m_config_ext_ps_disabled.cmake`` in
+the example below overwrites and disables PS service.
+
+.. code-block:: bash
+
+ cmake -G"Unix Makefiles" -DPROJ_CONFIG=`readlink -f ../configs/ConfigDefaultProfileM.cmake` \
+ -DTARGET_PLATFORM=AN521 \
+ -DCMAKE_BUILD_TYPE=MinSizeRel \
+ -DCOMPILER=ARMCLANG \
+ -DTFM_PROFILE_CONFIG_EXT=configs/profile_m_config_ext_ps_disabled.cmake ../
+ cmake --build ./ -- install
+
+
+*********
+Reference
+*********
+
+.. [PSA-FF-M] `Arm Platform Security Architecture Firmware Framework 1.0 <https://developer.arm.com/-/media/Files/pdf/PlatformSecurityArchitecture/Architect/DEN0063-PSA_Firmware_Framework-1.0.0-2.pdf?revision=2d1429fa-4b5b-461a-a60e-4ef3d8f7f4b4>`_
+
+.. [RFC7925] `Transport Layer Security (TLS) / Datagram Transport Layer Security (DTLS) Profiles for the Internet of Things <https://tools.ietf.org/html/rfc7925>`_
+
+.. [PROFILE-S] :doc:`Trusted Firmware-M Profile Small Design </docs/design_documents/tfm_profile_small>`
+
+.. [RFC7252] `The Constrained Application Protocol (CoAP) <https://tools.ietf.org/html/rfc7252>`_
+
+.. [RFC4279] `Pre-Shared Key Ciphersuites for Transport Layer Security (TLS) <https://tools.ietf.org/html/rfc4279>`_
+
+.. [RFC7251] `AES-CCM Elliptic Curve Cryptography (ECC) Cipher Suites for TLS <https://tools.ietf.org/html/rfc7251>`_
+
+.. [CRYPTO-DESIGN] :doc:`Crypto design </docs/design_documents/tfm_crypto_design>`
+
+.. [ITS-INTEGRATE] :doc:`ITS integration guide </docs/reference/services/tfm_its_integration_guide>`
+
+.. [TFM-BUILD] :doc:`TF-M build instruction </docs/getting_started/tfm_build_instruction>`
+
+--------------
+
+*Copyright (c) 2020, Arm Limited. All rights reserved.*