Security of communication protocols

Question 1

ZigBee

Answer 1

• Communication standard for sensor wireless personal area networks
• Topologies: star, mesh, tree
• low cost
• low energy
• low bandwidth
• Profiles: home automation, light link, green power
• Device roles: Coordinator, Router, End Device

Question 2

ZigBee layers

Answer 2

• Network layer: Network setup, device dis-/connection, routing, device configuration, peer-to-peer multi hop
• Application support layer: Provides services and addressing objects
• Security Service Provider: Key management, generation
• ZigBee Device Object: Device discovery, network management

Question 3

Three roles in ZigBee

Answer 3

• Coordinator: exactly one, sets up network, scan and join processing, keeps track of devices, defines TrustCenter, always awake
• Router: forwards packets, can include/exclude devices, can double as End Device, always awake
• End Device: Applications runs on the device, can sleep

Question 4

ZigBee Topologies

Answer 4

• Star: Only coordinator and end devices
• Tree: Routers include more routers or end devices
• Mesh: self healing, multiple connections between routers

Question 5

ZigBee Security

Answer 5

• Profiles have different security specifications:
  -> Green Power for ultra low power devices
  -> Light Link for devices in vicinity
  -> Home Automation for devices in 10-100m range
• Low energy consumption - security trade off
• Ultra low power devices cannot receive packets, no challenge response or key agreement possible

Question 6

ZigBee Key Hierarchy

Answer 6

• Different profiles have different types of keys to encrypt communication
• Only some keys are interoperable
• Network Key is used between profiles

Question 7

Key Usage: Green Power

Answer 7

• Green Power devices can be connected to each other (using a GP key) or to an existing ZigBee Pro Network using a Network Key
• One type of GP key is chosen by the device: derived, pre installed, shared

Question 8

Key Types: Green Power

Answer 8

• NWK: all devices in the ZigBee Pro are using it. It is sent to the GP device in plaintext
• GP Key: Key for securing communication between GP devices
  -> Derived: Key is derived from the NWK and sent in plaintext
  -> Pre-installed GP key: GP devices are pre-installed by an (individual) key, every device without a pre-installed key will get this key in plaintext
  -> GP key derived from pre-installed key: Pre-installed key and MAC is used to derive an individual GP key per device, every device without a pre-installed key will get this key in plaintext

Question 9

Key Usage: Light Link

Answer 9

• Can be connected to each other or to an existing Pro network with Light Link Network Key or NWK
• Coordinator sends NWK to new Light Link device or another Light Link device is sending the NWK using Touch Link

Question 10

Key Types: Light Link

Answer 10

• NWK: used for broadcast to all Light Link and ZigBee Pro devices
• Pre-installed Link Key: Used to derive Key-Transport Key
• Key-Transport Key: Used to transport the NWK

Question 11

Key Usage: Home Automation

Answer 11

• If a new device joins, the NWK has to be shared, encrypted by a Key Transport Key
• Transport Keys are derived from Link Keys
• Link Keys can be unique per device, pre-installed or shared
• For transporting link keys between devices, Key-Load Keys are used
• Coordinator generates random NWK on a network startup
• NWK is used to encrypt all communication

Question 12

Key Types: Home Automation

Answer 12

• NWK: used for broadcasts to all devices
• Link keys:
  -> Application/Trust Center Link Key: Used for unicast between devices (may be unique per device)
  -> Centralized/Distributed global link key: all devices have to know this key prior to joining the network
  -> Install code link key: Key derived from secret (printed) on joining device (unique)
• Key-Transport Key: Derived from a Link Key using keyed-hash with key “0x00”
• Key-Load Key: Derived from a Link Key using keyed-hash with key “0x02”

Question 13

Problem with pre-shared keys

Answer 13

• When using key-exchange the pre-shared key is a single point of failure
• Touch Link is trying to mitigate this by exchanging keys only to devices in proximity
• If the pre-shared Link Key of any device is leaked, an attacker can listen to every key exchange

Question 14

Attack scenarios

Answer 14

• Malicious firmware updates
  -> non-integrity protected OTA
• Exploiting the use of default keys
• Hardware attacks
  -> e.g. memory-protection-bit is not set (access through the chip’s external debug interface)
• Side-channel attacks

Question 15

Bluetooth Pairing and Bonding Security

Answer 15

• Provides passive eavesdropping and MITM protection using asymmetric keys and PIN codes
• Updates algorithms in pairing