An overview of Bluetooth Low Energy with emphasis on security.

1. Using Bluetooth LE
(For Good)

2. Who?
• Donald Ness
• Software Developer from
Boulder, CO
• Learned Bluetooth building
wireless controls for robots
• See Anarchy as a giant
engineering project
• IANAC

3. What is this talk about?
• What is Bluetooth Low Energy?
• Why does it matter?
• How can we use it for good?

4. What is BLE?
• New wireless scheme for low power devices
• small size, small cost
• same spectrum as WiFi, Bluetooth Classic
• a.k.a. Bluetooth SMART
• Subset of Bluetooth 4.0 spec since 2010

5. BLE vs Bluetooth Classic
BLE Classic
security broken key exchange*
secure pairing protocol
(ECDH)
throughput 0.2 Mbit/s 2-3 Mbit/s
range 10 - 30m 50 - 300m
power consumption 0.01 to 0.5W 1W
faster connection 0.1s 5s
smaller size very small small
lower cost ~$2 @ 5000 ~$7 @ 5000
* unless done out-of-band. more on that soon.

6. Why does it matter?
It is everywhere!

7. Why does it matter?
• Low energy and small size
➡ Convenient devices that can fit on a keychain
• Low cost and flexible development
➡ Good solution for DIY open source hardware

8. A Safe Haven
For Private Keys
Problem:
• 0-day exploits
• backdoors in proprietary hardware
• web wallets require third party trust
• cold storage wallets are cumbersome
Hardware wallets are best current solution, but
• expensive! ~$120
• don’t work with mobile
• require USB cables

9. Technical Overview

10. Protocol Stack
GATT/GAP
ATT
L2CAP
Link Layer
PHY
• generic attribute/access protocol
• groups attributes into services
• attribute protocol
• bound to L2CAP
• similar to SDP in BT Classic
• logical link control and adaptation protocol
• segments and assembles packets
• optional QoS via retransmission + CRC
• 3 advertising channels
• 37 data channels
• hop increment + interval negotiated
• 2.4GHz ISM spectrum
• GFSK RF modulation

11. Security
• 128-bit AES block cipher at link layer. Great! Right…?
• PROBLEM: in-band key exchange is broken
• Passive attack can snoop for LTK during pairing.
• Active attack can force new LTK exchange and then snoop.
• SOLUTION: use out-of-band key exchange
• Simple Secure Pairing at application layer
• Uses ECDH: ~5 sec on 8-bit CPU
• For more info: http://lacklustre.net/bluetooth/

12. Device Roles
• Peripheral as Server
• ex. a controller for an Aquaponics system
• It advertises to Centrals
• Central as Client
• ex. a Smartphone
• It connects to Peripherals

13. Services and Characteristics
• A Service is a collection of data and functions, or
characteristics, associated with a peripheral.
• ex. A service representing aquaponics sensors
• A Characteristic represents a single aspect of a
peripheral. It has properties which determine how
it can be used (i.e. read, write)
• ex. Temperature, pH level, oxygen level

14. UUID
• Services and Characteristics are identified by
UUIDs
• 16-bit defined by Bluetooth SIG
• http://developer.bluetooth.org
• 128-bit user defined

15. Advertising
Central Scan Scan
Peripheral Adv Adv Adv Adv Adv Adv Adv Adv Adv
• A peripheral broadcasts advertising packets to tell the world about itself:
• A name
• Some primary Service UUIDs
• Transmit Power
• Manufacturer Data
• A central scans for advertising packets to find something to connect to.

16. Characteristic Properties
• Read
• Write
PDU
C C C C
• With Response (guaranteed write)
• Without Response
• Notify
• Subscribe to a value and get events when value changes
• Limitations
• 20 byte limit for characteristics
PDU
C
PDU
C C C

17. A Note on Beacons
• Beacons are just specially formatted advertising packets,
usually emitted at a fast interval (~20ms)
• It allows proximity-based micro location using RSSI to
determine distance.
• iBeacons are Apple’s version:
UUID (16 bytes) Best Buy
Major ID (2 bytes) Store 101
Minor ID (2 bytes) TV Aisle

18. Example Code

19. Using Bluetooth LE Peripherals
Sign Bitcoin Transactions
• Store a private key (or a password-protected
private key) safely in the flash memory of a
microcontroller.
• Send a transaction input via Bluetooth LE to the
microcontroller.
• Hash the input with our private key, and send it
back to the central to complete the transaction.

20. Other Uses
• Signing messages with GPG key
• Exchanging public keys in person
• Replacing LED screens with BLE radios for
hardware projects
• Drop Safes
• Many other things…?

21. Software Stacks
• Linux — BlueZ
• Mac & iOS — CoreBluetooth
• Chrome APIs
• Node.js — noble/bleno

22. Hardware
• Popular Manufacturers
• Nordic Semiconductor NRF51
• Packed with a ARM M0-cortex
• Great development kit
• Texas Instruments CC2640
• BlueGiga
• BlueRadios
• DIY
• RedBearLab BLE Mini — $25
• RFDuino — $25
• Intel Edison SoC — $50

23. Firmware
• micro-ecc
• https://github.com/kmackay/micro-ecc
• ECDH and ECDSA for 8-bit, 32-bit, and 64-bit
processors.
• nano-ecc
• https://github.com/iSECPartners/nano-ecc
• A very small ECC implementation for 8-bit
microcontrollers

24. Questions?
Twitter: @programmarchy