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A5 GSM Encryption

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

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  1. Ki A5 GSM Encryption Kingdom Special Operations SJS-KW

  2. Ki Encryption key words • Symmetric algorithms • Block ciphers • Stream ciphers • Public key algorithms • One-way hash functions

  3. 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]

  4. 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

  5. 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

  6. 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)

  7. 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

  8. Ki A5 GSM x3 register Encryption • LFSRs R1 : 19 Out Func R2 : 23 R3 : 22 Clock Control Kc : 64 Frame: 22

  9. Ki A5 GSM Encryption FSM • CONTROL idle load key roll 100 blk_1 blk_2 done

  10. Ki A5 GSM Encryption • Registers • TMDA Frame number 22Bits • Kc Cipher Key 64

  11. 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

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