Computer Security and Cryptography

8.12 IRREGULAR CLOCKING

Nonlinearity may also be introduced by irregular clocking, XORing several LFSRs but shifting the LFSRs in a state- and key-dependent manner. One such scheme is described by Günther [1987]. The Global System for Mobile Communication (GSM) Users Association is a consortium providing mobile communication services. GSM has established an elaborate key exchange and encryption protocol to provide both secrecy (privacy) and authentication. Each mobile (telephone) contains a SIM (Subscriber Identity Module) card, and a processor with memory containing

• The caller's telephone number, International Mobile Subscribers Identification Number (MISDN) of up to 15 (BCD) coded decimal digits

  – MCC, Mobile Country Code;

  – MNC, Mobile Network Code;

  – MSIN, Mobile Subscriber Number.

• Implementation of two algorithms – A38 and A5.
• A user-unique 128-bit secret key K_U.

It is assumed that the SIM may not be probed to reveal K_U and that cloning is very difficult.

When a user wants to make a call, the mobile requests service from the network providing its MISDN. The authentication process consists of several steps (Fig. 8.13).

8.12.1 Authentication

A1.	The GSM Mobile Services Switching Center (MSC) generates and transmits to the mobile a 128-bit random number RAND.
A2.	The mobile's SIM uses RAND and KU with the A38 one-way function to derive a 32-bit response SRES = A38{KU, RAND}[0…3], which is returned to MSC. (Note, the GSM standard allows GSM networks to implement different ...