u8.15 Bluetooth Link Layer services

u8.15 Bluetooth Link Layer services

1. 2 Services

   • Authentication of Peer Devices
   • Confidentiality for transmitted data

2. Each link uses link keys to drive the authentication mechanisim and occasionally for encryption

3. Link keys are secret

   • 1 device cant ask another device what the link key is
     • t has to be exchanged

4. Unit link key

   • At manufacture unit link keys are built in
     • exists for the lifetime of the device

5. Initialisation link key

   • First key that is used
     • protects the initialisation parameters when they're passed between 2 devices
   • Derived from 3 values
     • BD_ADDR
     • random number issued by 1 device
     • a PIN
       • Usually arbitratly selected and entered into both units
   • Process
     • PIN code entered into claimant device
       • PIN code entered into verifier device
         • Claimant sends its BD_ADDR to the verifier
           • Verifier sends random number to claimant
             • Claimant derives Kinit using BD_ADDR, random number and PIN as input to the E0 cipher
               • Verifier derives Kinit using BD_ADDR, random number and PIN as input to the E0 cipher
                 • Both devices now have the initialisation link key.
                   • Each device can now authenticate each other, using a challenge–response scheme.
   • Not used for encryption
     • encryption keys are derived from the link key

6. Combination Master & Link Key

   • After auth devices decide on which key they will use
     • 3 choices
       • either devices unit key
       • a combination key (KeyAb) derived jointly
       • One device creates a link key known as the master key
     • Devices that have authorised once are known as trusted devices

7. 4 types of key

   • Unit Key
     • Initialisation link key
       • a combination key
         • Master key