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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
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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 = xi1 for i = 18,17,…,1 and x0 = t If y10 = m then Y steps t = y21 y20 yi = yi1 for i = 21,20,…,1 and y0 = t If z10 = m then Z steps t = z22 z21 z20 z7 zi = zi1 for i = 22,21,…,1 and z0 = t Keystream bit is x18 y21 z22 Part 1 Cryptography
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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
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A5/1 Example X Y Z Each register element is a single bit
1 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
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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
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