10.5 GSM Encryption

10.5 GSM Encryption

1. Data on the radio path encrypted between mobile and the Base Station

   • protects user traffic and sensitive signalling data against eavesdropping
     • Makes it difficult to hijack calls
   • Extends the influence of authentication to entire duration of the call
     • using encryption key KC makes it difficult to for an attacker use NW to make calls
   • Scope of encryption is limited
     • Especially in view of microwave links tfrom Base Station to Mobile switching centre
       • Could recover information by intercepting traffic on these links

2. Mechanism

   • Performed by applying a stream cipher called A5 to the GSM TDMA (time division multiple access) frames
     • Choice of stream cipher influendec by
       • Handset limitations
       • Speech coder
       • Error propagation
         • Block ciphers would propagate errors
       • low tolerance of delay
       • Handover issues
     • TDMA frames repeat every 3.5 hours, so the keystream will repeat.
       • In theory by XORing some cipher texts you may learn something about the underlying plaintext
         • Not a serious weakness because the phone is reauthenticated less than every 3.5 hours
     • KC and an IV (derived from frame number) used as a key for the A5 Algorithm
       • Different keystream for every frame

3. Managing the encryption

   • Base Station instructs the Mobile to start cipher using the cipher command
     • Base Station starts decrypting
       • Base Station starts encryption when cipher command is acknowledges
     • Phone starts encryption and decrypting when it receives the cipher command

4. Strength of Kc

   • Originally 64bits with 10 bits forced to zero in SIM and AUC = 54 bits
     • Now pure 64 bit

5. A5 has 3 standardised versions so networks can operate globaly

   • A5/1
     • Most widely used
   • A5/2
     • secondaty use
   • A5/3
     • New and being phased in
   • All phones support 1 & 2