Ciphering in GSM Mobile Stations Base Station Subsystem Exchange System Network Management Subscriber and terminal equipment databases BSC MSC VLR HLR EIR AUC OMC BTS A5 Encryption 1

Ciphering in GSM A5 CIPHERING SEQUENCE PLAINTEXT SEQUENCE KcKc FRAME NUMBER Sender (Mobile Station or Network) Receiver (Network or Mobile Station) CIPHERTEXT SEQUENCE A5 CIPHERING SEQUENCE KcKc FRAME NUMBER PLAINTEXT SEQUENCE 2

A5/1 Overview A5/1 is a stream cipher, which is initialized all over again for every frame sent. Consists of 3 LFSRs of 19, 22, 23 bits length. The 3 registers are clocked in a stop/go fashion using the majority rule. “Cryptography is a mixture of mathematics and muddle, and without the muddle the mathematics can be used against you.” - Ian Cassells, a former Bletchly Park cryptanalyst. 3 Register Number Length In bits Primitive Polynomial Clock-Controlling Bit (LSB is 0) Bits that Are XORed 119 x 19 + x 18 + x 17 + x 16 + x ,17,16, x 22 + x 21 + x , x 23 + x 22 + x 21 + x 20 + x ,21,20,7 Parameters of the A5/1 Registers

clock control C3 C2 C1 R2 R1 R

A5/1 : Operation All 3 registers are zeroed 64 cycles (without the stop/go clock) : ◦ Each bit of K C (lsb to msb) is XOR'ed in parallel into the lsb's of the registers 22 cycles (without the stop/go clock) : ◦ Each bit of F n (lsb to msb) is XOR'ed in parallel into the lsb's of the registers 100 cycles with the stop/go clock control, discarding the output 228 cycles with the stop/go clock control which produce the output bit sequence. 5