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doc: add support for GATT_DATABASE_HASH

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mringwal committed Jan 12, 2020
1 parent 1be6721 commit 44b583f51937d5afe897592f97c101928f668cf5
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@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](

### Added
- att_db_util: provide GATT Database Hash via att_db_util_hash_calc
- GATT Compiler: provide GATT Database Hash via GATT_DATABASE_HASH Characteristic

### Changed
- btstack_crypto: update AES-CMAC implementation to access all message bytes sequentially
@@ -563,3 +563,29 @@ In addition to the attribute value handle, the handle for the Client Characteris
Finally, in order to send Notifications and Indications independently from the main application, *att_server_register_can_send_now_callback* can be used to request a callback when it's possible to send a Notification or Indication.

To see how this works together, please check out the Battery Service Server in *src/ble/battery_service_server.c*.

### GATT Database Hash

When a GATT Client connects to a GATT Server, it cannot know if the GATT Database has changed
and has to discover the provided GATT Services and Characteristics after each connect.

To speed this up, the Bluetooth
specification defines a GATT Service Changed Characteristic, with the idea that a GATT Server would notify
a bonded GATT Client if its database changed. However, this is quite fragile and it is not clear how it can be implemented
in a robust way.

The Bluetooth Core Spec 5.1 introduced the GATT Database Hash Characteristic, which allows for a simple
robust mechanism to cache a remote GATT Database. The GATT Database Hash is a 16-byte value that is calculated
over the list of Services and Characteristics. If there is any change to the database, the hash will change as well.

To support this on the GATT Server, you only need to add a GATT Service with the GATT Database Characteristic to your .gatt file.
The hash value is then calculated by the GATT compiler.


Note: make sure to install the PyCryptodome python package as the hash is calculated using AES-CMAC,
e.g. with:

pip install pycryptodomex

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