Mobile Telephone System And GSM Security

The Mobile Telephone System First-Generation Mobile Phones First-Generation Mobile Phones Analog Voice Analog Voice Second-Generation Mobile Phones Second-Generation Mobile Phones Digital Voice Digital Voice Third-Generation Mobile Phones Third-Generation Mobile Phones Digital Voice and Data Digital Voice and Data

Advanced Mobile Phone System (a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used.

Channel Categories 832 channels in four categories: 832 channels in four categories: Control (base to mobile) to manage the system Control (base to mobile) to manage the system Paging (base to mobile) to alert users to calls for them Paging (base to mobile) to alert users to calls for them Access (bidirectional) for call setup and channel assignment Access (bidirectional) for call setup and channel assignment Data (bidirectional) for voice, fax, or data Data (bidirectional) for voice, fax, or data

D-AMPS Digital Advanced Mobile Phone System (a) A D-AMPS channel with three users. (b) A D-AMPS channel with six users.

GSM (1) GSM uses 124 frequency channels, each of which uses an eight-slot TDM system

GSM (2) A portion of the GSM framing structure.

GSM Security

GSM System Architecture IMSI: International Mobile Subscriber Identity BSC: Base Station Controller BTS: Base Transceiver Station MSC: Mobile Switching Center HLR: Home Location Register VLR: Visitor Location Register AuC: Authentication Center

GSM Authentication Protocol SRES = A3(Ki, RAND)

GSM Encryption A5 Kc(64 bit), Frame Number(22 bit) 114 bit key stream Frame Cipher text A5 Kc(64 bit), Frame Number(22 bit) 114 bit key stream Frame MS BTS Kc = A8(Ki, RAND)

GSM Security Issues Limited Encryption Scope Limited Encryption Scope Only the over-the-air traffic is encrypted Only the over-the-air traffic is encrypted Message on backbone are plain text Message on backbone are plain text Active Attacks (Impersonating) Active Attacks (Impersonating) Unilateral Authentication Unilateral Authentication Only user authentication, No BTS authentication Only user authentication, No BTS authentication Potential for false base station attack Potential for false base station attack Key Transmission Key Transmission IMSI, RAND, SRES, Kc are transmitted without encryption IMSI, RAND, SRES, Kc are transmitted without encryption

GSM Security Issues Weak Encryption Algorithms Weak Encryption Algorithms Both A3 and A8 use COMP 128 which has been broken Both A3 and A8 use COMP 128 which has been broken A5/0 has no security at all A5/0 has no security at all A5/1 and A5/2 has been broken A5/1 and A5/2 has been broken SIM card can be cloned by now SIM card can be cloned by now Channel Hijack Channel Hijack Protection against radio channel hijack relies on encryption Protection against radio channel hijack relies on encryption Encryption is not used in some networks. Encryption is not used in some networks. Inflexibility Inflexibility Inadequate flexibility to upgrade and improve security functionality over time Inadequate flexibility to upgrade and improve security functionality over time

Questions? Mahdi Hasheminezhad