the Linux (BlueZ) API.
- L2CAP or RFCOMM connections can require the baseband radio link
mode be any of:
authenticated (devices are paired)
encrypted (implies authentication)
secured (encryption, plus generate new link key)
- for sockets, the mode is set using setsockopt(2) and the socket
connection will be aborted if the mode change fails.
- mode settings will be applied during connection establishment, and
for safety, we enter a wait state and will only proceed when the mode
settings are successfuly set.
- It is possible to change the mode on already open connections, but
not possible to guarantee that data already queued (from either end)
will not be delivered. (this is a feature, not a bug)
- bthidev(4) and rfcomm_sppd(1) support "auth", "encrypt" and
"secure" options
- btdevctl(8) by default enables "auth" for HIDs, and "encrypt" for
keyboards (which are required to support it)
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