1. A5/1 A5/1 consists of 3 shift registers X: 19 bits (x18,x17,x16, …,x0)
Y: 22 bits (y21,y20,y19, …,y0)
Z: 23 bits (z22,z21,z20, …,z0)
Part 1  Cryptography

2. A5/1 At each step: m = maj(x8, y10, z10) If x8 = m then X steps
Examples: maj(0,1,0) = 0 and maj(1,1,0) = 1
If x8 = m then X steps
t = x18  x17  x16  x13
xi = xi1 for i = 18,17,…,1 and x0 = t
If y10 = m then Y steps
t = y21  y20
yi = yi1 for i = 21,20,…,1 and y0 = t
If z10 = m then Z steps
t = z22  z21  z20  z7
zi = zi1 for i = 22,21,…,1 and z0 = t
Keystream bit is x18  y21  z22
Part 1  Cryptography

3. A5/1 X Y  Z Each value is a single bit
Key is initial fill of register
Each register steps or not, based on (x8, y10, z10)
Keystream bit is XOR of left bit of each register
Part 1  Cryptography

4. A5/1 Example X Y  Z Each register element is a single bit1  Y  1  1 Z 
Each register element is a single bit
Key is initial fill of register
An example of register fills is given above
See animation on next slide
Part 1  Cryptography

5. A5/1 Example X Y  Z We have m = maj(0,1,1) = 1 X 1   Y 1  1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1  0 = 1   1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Z
1  1  0  1 = 1 
We have m = maj(0,1,1) = 1
Register X does nothing
Registers Y and Z step
Keystream bit is 0  0  1 = 1
Part 1  Cryptography