1. Prof. Saravanan Vijayakumaran
A5/1 Stream Cipher
A5/1 is a stream cipher used to provide over-the- air communication privacy in the GSM cellular telephone standard
Course Name: Cryptography Level: UG/PG
Authors
Phani Swathi Chitta
Mentor
Prof. Saravanan Vijayakumaran

2. Learning Objectives
After interacting with this Learning Object, the learner will be able to:
Explain the regular operation of A5/1
Explain the operation of initial state generation using session key and publicly known frame number

3. 1 2 3 4 5 Definitions of the components/Keywords:
A5/1 is built from three short linear feedback shift registers (LFSR) of lengths 19, 22, and 23 bits, which are denoted by R1, R2 and R3 respectively. The rightmost bit in each register is labeled as bit zero. The taps of R1 are at bit positions 13,16,17,18; the taps of R2 are at bit positions 20,21 and the taps of R3 are at bit positions 7, 20,21,22.
When a register is clocked, its taps are XORed together and the result is stored in the rightmost bit of the left-shifted register.
They are clocked in a stop/go fashion using the following majority rule: Each register has a single "clocking" tap (bit 8 for R1, bit 10 for R2, and bit 10 for for R3); each clock cycle, the majority function of the clocking taps is calculated and only those registers whose clocking taps agree with the majority bit are actually clocked. At each step either two or three registers are clocked. 2 3 4
LFSR number
Length in bits
Feedback polynomial
Clocking bit
Tapped bits 1 19
x19 + x18 + x17 + x14 + 1 8
13,16,17,18 2 22
x22 + x21 + 1 10
20,21 3 23
x23 + x22 + x21 + x8 + 1
7,20,21,22 5 3

4. 1 Master Layout 1 2 3 4 5 1 1 1 clock control 18 17 16 13 R1 C1 21 20
Part 1 - Regular operation of A5/1
Part 2 - Operation of initial state generation using session key and publicly known frame number 18 17 16 13 1 R1 2 C1
clock
control 21 20 R2 1 3 C2 4 21 22 20 7 1 R3 C3 5
The master layout figure should appear first
R1, R2, R3 are three linear feedback shift registers
C1,C2 and C3 are clocking bits

5. 3 Step 1: 1 2 4 5 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R2 C21 1 R1
LFSR – Linear Feedback Shift Register
MSB – Most Significant Bit
LSB – Least Significant Bit C1
clock
control 2 21 20 1 R2 1 C2 3 21 22 20 7 1 R3 4 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The first sentence in DT should appear with master layout figure.
The first blue circle should blink once and red 1 should appear.
The second sentence in DT should appear with step 1 figure.
The contents of the three LFSRs represent the state of the A5/1 stream cipher
The MSBs of all the registers are XORed to generate the output of the stream cipher 5

6. 3 Step 2: 1 2 4 5 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R2 C21 R1 C1 2
clock
control 21 20 1 R2 1 3 C2 21 22 20 7 4 1 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The orange lines should blink once.
The registers are clocked in a stop/go fashion using the majority rule.
A register is clocked if the clocking bit agrees with the majority bit. 5

7. 3 Step 3: 1 2 4 5 1 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R21 1 R1 1 C1
clock
control 2 21 20 1 R2 1 C2 3 21 22 20 7 1 R3 4 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The 3 violet circles in first register and 3 violet circles in third register should blink once.
The text in DT should be displayed.
After the text in DT red 1 at R1 and red 0 at R3 should appear
The LFSRs R1 and R3 are clocked as their clocking bits C1 and C3 agree with the majority of C1, C2 and C3.
LFSR R2 is not clocked.
When a register is clocked, its taps are XORed together to generate the new LSB. 5

8. 3 Step 4: 1 2 4 5 1 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R21 R1 1 C1 2
clock
control 21 20 1 R2 1 3 C2 21 22 20 7 4 1 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The figure in step4 should be shown such that the bits (1s and 0s ) are moved to left in R1 and R3.
The contents of LFSRs R1 and R3 are left shifted. 5

9. 3 Step 5: 1 2 4 5 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R2 C21 R1 C1 2
clock
control 21 20 1 R2 1 3 C2 21 22 20 7 4 1 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The figure in step 5 should appear such that the red 1 and 0 are placed in R1 and R3.
The new LSBs which were generated prior to left shifting of the registers is stored in the rightmost bit of the left -shifted registers. 5

10. 3 Step 6: 1 2 4 5 1 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R21 1 R1 C1
clock
control 2 21 20
1 1 R2 1 C2 3 21 22 20 7 1 R3 4 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The first blue circle should blink once and green 1 should appear.
The sentence in DT should appear with step 6 figure.
The MSBs of all the registers are XORed to generate the output of the stream cipher 5

11. 3 Step 7: 1 2 4 5 1 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R21 1 R1 C1
clock
control 2 21 20
1 1 R2 1 C2 3 21 22 20 7 1 R3 4 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The orange lines should blink once.
The registers are clocked in a stop/go fashion using the majority rule.
A register is clocked if the clocking bit agrees with the majority bit. 5

12. 3 Step 8: 1 2 4 5 1 1 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R21 1 R1 C1
clock
control 2 21 20
1 1 R2 1 1 C2 3 21 22 20 7 1 R3 4 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The 3 violet circles in first register and 1 violet circle in second register should blink once.
The text in DT should be displayed.
After the text in DT green 0 at R1 and green 1 at R2 should appear
The LFSRs R1 and R2 are clocked as their clocking bits C1 and C2 agree with the majority of C1, C2 and C3.
LFSR R3 is not clocked.
When a register is clocked, its taps are XORed together to generate the new LSB. 5

13. 3 Step 9: 1 2 4 5 1 1 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R21 1 R1 C1
clock
control 2 21 20
1 1 R2 1 1 C2 3 21 22 20 7 1 R3 4 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The figure in step9 should be shown such that the bits (1s and 0s ) are moved to left in R1 and R2.
The contents of LFSRs R1 and R2 are left shifted. 5

14. 3 Step 10: 1 2 4 5 1 1 1 1 1 clock control 18 17 16 13 R1 C1 21 20 R21 1 R1 C1
clock
control 2 21 20
1 1 R2 1 C2 3 21 22 20 7 1 R3 4 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The figure in step 10 should appear such that the green 1 and 0 are placed in R1 and R2.
The new LSBs which were generated prior to left shifting of the registers is stored in the rightmost bit of the left -shifted registers. 5

15. Master Layout 2 1
Part 1 - Regular operation of A5/1
Part 2 - Operation of initial state generation using session key and publicly known frame number 18 17 16 13 R1 C1 2 21 20 R2 3 C2 21 22 20 7 4 R3 C3 5
The master layout figure should appear first
R1, R2, R3 are three linear feedback shift registers
Initially all the registers are zeros

16. Session key + Frame no bits 1 18 17 16 13
Step 1: 1 R1 C1 2 21 20 R2 1 3 C2 21 22 20 7 1 4 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The red 1 in the top bit sequence is placed at all three registers
After the 1s are shown blink the 0s in the last boxes of all registers and red 1s
The LSB of session key is XORed with the LSB of R1, R2, R3.
During the initial state generation the output of the stream cipher is ignored. 5

17. Session key + Frame no bits 1 18 17 16 13
Step 2: 1 R1 C1 2 21 20 R2 1 3 C2 21 22 20 7 4 1 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The red 1s should be placed in the last boxes of all the registers.
The result of XORing is stored in LSBs of all the registers 5

18. Session key + Frame no bits 1 18 17 16 13
Step 3: 1 R1 C1 2 21 20 R2 1 3 C2 21 22 20 7 4 1 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The small violet circles of first register should blink and the black 0 should appear
Then the same for the second register and after for the third register
The clocking is done i.e., the XOR of taps of all the registers is done together 5

19. Session key + Frame no bits 1 18 17 16 13
Step 4: 1 R1 C1 2 21 20 R2 1 3 C2 21 22 20 7 4 1 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The bits should be left shifted and black 0s should be placed in last boxes
The bits are left shifted and the new LSB from XORing taps is stored in LSB 5

20. Session key + Frame no bits 1 18 17 16 13
Step 5: 1 1 R1 C1 2 21 20 R2 1 1 3 C2 21 22 20 7 1 4 1 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The green 1 in the top bit sequence is placed at all three registers
When the 1s are shown blink the 0s in the last boxes and green 1s
The next bit of session key is XORed with the LSB of R1, R2, R3. 5

21. Session key + Frame no bits 1 18 17 16 13
Step 6: 1 R1 C1 2 21 20 R2 1 C2 21 22 20 7 4 1 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The green 1s should be placed in the last boxes of all the registers.
The result of XOR is stored in LSBs of all the registers 5

22. Session key + Frame no bits 1 18 17 16 13
Step 7: 1 R1 C1 2 21 20 R2 1 3 C2 21 22 20 7 4 1 R3 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The small violet circles should blink and the black 0s should appear
The clocking is done i.e., the XOR of taps of all the registers is done together 5

23. Session key + Frame no bits 18 17 16 13 1
Step 8: 1 R1 C1 2 21 20 R2 1 3 C2 21 22 20 7 1 R3 4 C3
Instruction for the animator
Text to be displayed in the working area (DT)
The bits should be left shifted and black 0s should be placed in last boxes
There should be a next button so that the user can complete the process for 86 times.
The bits are left shifted and the result of XORed taps is stored in LSB
This process is continued for 86 cycles with regular clocking
The contents of the registers after 86 cycles is called the initial state of the frame
After this, the registers are clocked for 100cycles with irregular clocking 5

24. Questionnaire 1
1. If the registers R1, R2 and R3 have clocking bits as 1,1,1, then which registers will be clocked?
Answers: a) R b) R2 c) R d)‏R1, R2, R3
2. If the registers R1, R2 and R3 have clocking bits as 1,0,0, then which registers will be clocked?
Answers:
a)R b)R1, R c) R2, R d)‏ R1, R2, R3
3. The minimum number of registers which are clocked at a time _______
Answers: a) b) c) 2 d)‏ 3
The answers are given in red 2 3 4 5

25. Links for further reading
Reference websites:
Books:
Mobile Communication Systems and Security – Man Young Rhee, John Wiley & Sons(April 2009, Wiley-ieee Press)
Research papers:

26. Summary
A5/1 is a stream cipher used to provide over-the-air communication privacy in the GSM cellular telephone standard
A5/1 is built from three short linear feedback shift registers (LFSR) of lengths 19, 22, and 23 bits, which are denoted by R1, R2 and R3
When a register is clocked, its taps are XORed together and the result is stored in the rightmost bit of the left-shifted register
They are clocked in a stop/go fashion using the majority rule
At each step either two or three registers are clocked