If I pair my Android phone and my laptop, I can share files over Bluetooth from the phone to the laptop. I’ve started finding this a really convenient method for me to send files to a Linux laptop without needing to install a separate app on either the phone or my laptop. Especially when I’m away from my home network (I use SFTP at home).
How secure is this? Is there encryption by default and could someone else nearby with a receiver potentially decode the file you’re sending?
Bluetooth is very insecure. Almost all the security comes from needing devices to be close together. And even the latest standards/devices won’t be that secure because the goal of the tech is simple easy connections with low latency. Encrypting your music stream would piss away battery power while you listen to Trump ASMR.
As others have said, there are plenty of downgrade attacks unless your devices are basically the latest and the protocol is set to encrypted file sharing.
You mention an android phone and a Linux laptop. If you’re worried about security then use a usb cable as the gold standard, or transfer encrypted files only. If you like convenience then just look around you for anybody suspicious.