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Ki. A5 GSM Encryption. Kingdom Special Operations SJS-KW. Ki. Encryption key words. Symmetric algorithms Block ciphers Stream ciphers Public key algorithms One-way hash functions. MSC. BS. MS. SIM. GSM Security Features. Distribution of security features SIM=

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A5 gsm encryption l.jpg

Ki

A5 GSM Encryption

Kingdom Special Operations

SJS-KW


Encryption key words l.jpg

Ki

Encryption key words

  • Symmetric algorithms

    • Block ciphers

    • Stream ciphers

  • Public key algorithms

  • One-way hash functions


Gsm security features l.jpg

MSC

BS

MS

SIM

GSM Security Features

  • Distribution of security features

    • SIM=

      • A3 & A8 IMSI Ki TMSI/LAI Kc/CKSN

    • MS=

      • A5

    • BS=

      • A5 Kc [TMSI/IMSI = VLR]


Gsm security features4 l.jpg

Ki

MS

BS

GSM Security Features

  • Authentication

    • MS and network compute SRES from RAND

    • Network [VLR] compares results

RAND

SRES = A3(RAND , Ki)

SRES = A3(RAND , Ki)

=?

Fail

Authentication

Successful


Gsm security features5 l.jpg

Ki

MS

BS

GSM Security Features

  • Key generation

    • Kc is computed from RAND

RAND

Kc = A8(RAND , Ki)

Kc = A8(RAND , Ki)

Kc

Kc

A5

A5

DATA

TDMA

frame

TDMA

frame


How a5 gsm encryption run l.jpg

Ki

How A5 GSM Encryption run

  • Three clock controlled LFSRs

  • Feedback polynomials have degree 19,23, and 22 (their sum is 64)

  • The 64-bit cipher key initializes the LFSRs

  • 22-bit TDMA frame number is fed in

  • Two 114-bit key streams (uplink and downlink)


A5 gsm encryption of data l.jpg

Ki

REG_FILE

9x16 Bit

LFSR

CONTROL

CIP_SEQ_RAM

30x8 Bit

cip_data

A5 GSM Encryption of data

data

cip_key:86

A5 mode

start

done

ld_key

clk_en

cip_bit

sel

clk

AT MS*

AT BTS*

reset

data

wr

add

sel

cip_add


A5 gsm x3 register encryption l.jpg

Ki

A5 GSM x3 register Encryption

  • LFSRs

R1 : 19

Out

Func

R2 : 23

R3 : 22

Clock Control

Kc : 64

Frame: 22


A5 gsm encryption fsm l.jpg

Ki

A5 GSM Encryption FSM

  • CONTROL

idle

load

key

roll

100

blk_1

blk_2

done


A5 gsm encryption10 l.jpg

Ki

A5 GSM Encryption

  • Registers

    • TMDA Frame number 22Bits

  • Kc Cipher Key 64


A5 1 vs a5 2 l.jpg

Ki

A5.1 vs. A5.2

  • Export restrictions

    • A5.1 algorithm is not exportable to designated countries [Only In Europe]

    • A5.2 algorithm modified for export

  • Implementation Differences A5.1 A5.2

    • Output function

    • Clock control

    • Control unit