Decrypting the Apple Accessory Protocol
The AAP (Apple Accessory Protocol) is a L2CAP based protocol used by apple to allow communication between e.g. their headphones and their iOS or macOS line. The fun in parsing the protocol is that it includes the magic pairing key, in-ear detection and accurate battery information, configuring the headphones, ...
The decryption of their protocol would be the first step in creating a linux bluez protocol defintion for it, enabling linux users to use the Apple with all their advanced configurations.
The protocol defintion
The protocol is defined in the Kaitai Language. This can either be edited in the browser, or using a combination of your favorite text editor and the ksv in the terminal.
How to decode the protocol
An incredible resource is the PacketLogger included in the additional developer tools. Sadly Apple removed the parsing of the AAP language in systems above macOS Mojave (10.14) - probably with their introduction of their upgraded bluetooth architecture.
I'm personally using macOS Mojave together with the PacketLogger 6.0.9. The parsing is incomplete, but it provides a good foundation.
Creating examples
I found it helpful to split the whole communication into tiny packages containing a few informations.See can be found in the examples folder. The .data file is the raw stream and the md the corresponding description.
These files can be created using create-examples.py by following:
- enter the file name (root is in
examplesfolder). Tab completion works - the raw data as a hex stream (e.g.
020004000000) - the description - edited using your
editorcommand