Bluetooth is a short-range wireless communications protocol used for peripherals, operating on the 2.4 GHz band. Bluetooth host support is mostly found in laptops, tablets and cell phones but increasingly in desktop computers: especially in small form factors with fewer USB ports.
The wireless standard had been developed at Swedish telecom company Ericsson for use with mobile phones. The name refers to the historic Nordic king "Harald Bluetooth", which according to legend would have had bad dental hygiene. (The Norse languages did not have the word "black", using "blue" to mean the same thing.) The standard has since had many revisions with different properties.
Although the protocol is encrypted, the encryption considered very weak. The vulnerability for eavesdropping is reason for Bluetooth being banned in many security-conscious environments.
The "KNOB" attack, published in August 2019, could be used to force devices to pair with one-another using an encryption key of only eight bits in length — which can be cracked very quickly. Mitigations exist for Windows, Apple devices, Android and Cisco hosts, and both host and device need to be vulnerable for the attack to work.
Bluetooth keyboards and mice typically use the higher levels of the HID protocol borrowed from the USB standard. Because of the limited bandwidth of Bluetooth, only equivalent to low-speed USB is possible: Bluetooth keyboards are therefore in effect limited to 6-key rollover. [This may have been overcome in later versions of the protocol]
In theory, up to seven devices can be connected to a host-side receiver at once, but in practice only three to four active devices are considered usable at once because of them sharing bandwidth. There are many other devices and protocols on the 2.4 GHz band, and interference from those could lower the practical use even more.
Hosts may also limit the number of simultaneous connected devices with the same profile: for instance allow only one headset at once but allow multiple keyboards.
The Bluetooth standard allows a host receiver to pair up to 255 devices, but the actual limit may be smaller depending on the receiver. If the peripheral supports it, it could be paired to up to five hosts but be connected to only one at a time, unless it has "multipoint functionality".
- Ars Technica—New Attack exploiting serious Bluetooth weakness can intercept sensitive data. By Dan Goodin. Published 2019-08-17. Retrieved 2019-08-18.
- Apple—Using a Bluetooth mouse, keyboard, or trackpad with your Mac. Dated 2018-11-26. Retrieved 2019-01-13.