docs: Update docs with firmware encryption feature

Update documentation with optional firmware encryption feature.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: I26691b18e1ee52a73090954260f26f2865c4e05a

docs/change-log-upcoming.rst

@@ -46,6 +46,7 @@ New Features
 
 - Security
    - Example: "UBSAN support and handlers"
+   - Add support for optional firmware encryption feature (experimental).
 
 - Tools
    - Example: "fiptool: Add support to build fiptool on Windows."

docs/design/auth-framework.rst

@@ -934,7 +934,7 @@ i.e. verify a hash or a digital signature. Arm platforms will use a library
 based on mbed TLS, which can be found in
 ``drivers/auth/mbedtls/mbedtls_crypto.c``. This library is registered in the
 authentication framework using the macro ``REGISTER_CRYPTO_LIB()`` and exports
-three functions:
+four functions:
 
 .. code:: c
 
@@ -945,6 +945,11 @@ three functions:
                          void *pk_ptr, unsigned int pk_len);
     int verify_hash(void *data_ptr, unsigned int data_len,
                     void *digest_info_ptr, unsigned int digest_info_len);
+    int auth_decrypt(enum crypto_dec_algo dec_algo, void *data_ptr,
+                     size_t len, const void *key, unsigned int key_len,
+                     unsigned int key_flags, const void *iv,
+                     unsigned int iv_len, const void *tag,
+                     unsigned int tag_len)
 
 The mbedTLS library algorithm support is configured by both the
 ``TF_MBEDTLS_KEY_ALG`` and ``TF_MBEDTLS_KEY_SIZE`` variables.
@@ -957,6 +962,9 @@ The mbedTLS library algorithm support is configured by both the
 -  ``TF_MBEDTLS_KEY_SIZE`` sets the supported RSA key size for TFA. Valid values
    include 1024, 2048, 3072 and 4096.
 
+-  ``TF_MBEDTLS_USE_AES_GCM`` enables the authenticated decryption support based
+   on AES-GCM algorithm. Valid values are 0 and 1.
+
 .. note::
    If code size is a concern, the build option ``MBEDTLS_SHA256_SMALLER`` can
    be defined in the platform Makefile. It will make mbed TLS use an

docs/design/trusted-board-boot.rst

@@ -229,6 +229,34 @@ library that is required is given in the :ref:`Prerequisites` document.
 Instructions for building and using the tool can be found at
 :ref:`tools_build_cert_create`.
 
+Authenticated Encryption Framework
+----------------------------------
+
+The authenticated encryption framework included in TF-A provides support to
+implement the optional firmware encryption feature. This feature can be
+optionally enabled on platforms to implement the optional requirement:
+R060_TBBR_FUNCTION as specified in the `Trusted Board Boot Requirements (TBBR)`_
+document.
+
+Note that due to security considerations and complexity of this feature, it is
+marked as experimental.
+
+Firmware Encryption Tool
+------------------------
+
+The ``encrypt_fw`` tool is built and runs on the host machine as part of the
+TF-A build process when ``DECRYPTION_SUPPORT != none``. It takes the plain
+firmware image as input and generates the encrypted firmware image which can
+then be passed as input to the ``fiptool`` utility for creating the FIP.
+
+The encrypted firmwares are also stored individually in the output build
+directory.
+
+The tool resides in the ``tools/encrypt_fw`` directory. It uses OpenSSL SSL
+library version 1.0.1 or later to do authenticated encryption operation.
+Instructions for building and using the tool can be found in the
+:ref:`tools_build_enctool`.
+
 --------------
 
 *Copyright (c) 2015-2019, Arm Limited and Contributors. All rights reserved.*

docs/getting_started/build-options.rst

@@ -263,6 +263,22 @@ Common build options
    platform hook needs to be implemented. The value is passed as the last
    component of the option ``-fstack-protector-$ENABLE_STACK_PROTECTOR``.
 
+-  ``ENCRYPT_BL31``: Binary flag to enable encryption of BL31 firmware. This
+   flag depends on ``DECRYPTION_SUPPORT`` build flag which is marked as
+   experimental.
+
+-  ``ENCRYPT_BL32``: Binary flag to enable encryption of Secure BL32 payload.
+   This flag depends on ``DECRYPTION_SUPPORT`` build flag which is marked as
+   experimental.
+
+-  ``ENC_KEY``: A 32-byte (256-bit) symmetric key in hex string format. It could
+   either be SSK or BSSK depending on ``FW_ENC_STATUS`` flag. This value depends
+   on ``DECRYPTION_SUPPORT`` build flag which is marked as experimental.
+
+-  ``ENC_NONCE``: A 12-byte (96-bit) encryption nonce or Initialization Vector
+   (IV) in hex string format. This value depends on ``DECRYPTION_SUPPORT``
+   build flag which is marked as experimental.
+
 -  ``ERROR_DEPRECATED``: This option decides whether to treat the usage of
    deprecated platform APIs, helper functions or drivers within Trusted
    Firmware as error. It can take the value 1 (flag the use of deprecated
@@ -287,6 +303,18 @@ Common build options
 -  ``FWU_FIP_NAME``: This is an optional build option which specifies the FWU
    FIP filename for the ``fwu_fip`` target. Default is ``fwu_fip.bin``.
 
+-  ``FW_ENC_STATUS``: Top level firmware's encryption numeric flag, values:
+
+   ::
+
+     0: Encryption is done with Secret Symmetric Key (SSK) which is common
+        for a class of devices.
+     1: Encryption is done with Binding Secret Symmetric Key (BSSK) which is
+        unique per device.
+
+   This flag depends on ``DECRYPTION_SUPPORT`` build flag which is marked as
+   experimental.
+
 -  ``GENERATE_COT``: Boolean flag used to build and execute the ``cert_create``
    tool to create certificates as per the Chain of Trust described in
    :ref:`Trusted Board Boot`. The build system then calls ``fiptool`` to

docs/getting_started/porting-guide.rst

@@ -872,6 +872,35 @@ twice.
 
 On success the function should return 0 and a negative error code otherwise.
 
+Function : plat_get_enc_key_info() [when FW_ENC_STATUS == 0 or 1]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Arguments : enum fw_enc_status_t fw_enc_status, uint8_t *key,
+                size_t *key_len, unsigned int *flags, const uint8_t *img_id,
+                size_t img_id_len
+    Return    : int
+
+This function provides a symmetric key (either SSK or BSSK depending on
+fw_enc_status) which is invoked during runtime decryption of encrypted
+firmware images. `plat/common/plat_bl_common.c` provides a dummy weak
+implementation for testing purposes which must be overridden by the platform
+trying to implement a real world firmware encryption use-case.
+
+It also allows the platform to pass symmetric key identifier rather than
+actual symmetric key which is useful in cases where the crypto backend provides
+secure storage for the symmetric key. So in this case ``ENC_KEY_IS_IDENTIFIER``
+flag must be set in ``flags``.
+
+In addition to above a platform may also choose to provide an image specific
+symmetric key/identifier using img_id.
+
+On success the function should return 0 and a negative error code otherwise.
+
+Note that this API depends on ``DECRYPTION_SUPPORT`` build flag which is
+marked as experimental.
+
 Common optional modifications
 -----------------------------
 

docs/getting_started/tools-build.rst

@@ -135,6 +135,33 @@ verbose. The following command should be used to obtain help about the tool:
 
     ./tools/cert_create/cert_create -h
 
+.. _tools_build_enctool:
+
+Building the Firmware Encryption Tool
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``encrypt_fw`` tool is built as part of the TF-A build process when the
+``fip`` make target is specified, DECRYPTION_SUPPORT and TBB are enabled, but
+it can also be built separately with the following command:
+
+.. code:: shell
+
+    make PLAT=<platform> [DEBUG=1] [V=1] enctool
+
+``DEBUG=1`` builds the tool in debug mode. ``V=1`` makes the build process more
+verbose. The following command should be used to obtain help about the tool:
+
+.. code:: shell
+
+    ./tools/encrypt_fw/encrypt_fw -h
+
+Note that the enctool in its current implementation only supports encryption
+key to be provided in plain format. A typical implementation can very well
+extend this tool to support custom techniques to protect encryption key.
+
+Also, a user may choose to provide encryption key or nonce as an input file
+via using ``cat <filename>`` instead of a hex string.
+
 --------------
 
 *Copyright (c) 2019, Arm Limited. All rights reserved.*