# Known Plaintext Attack A5/2

I am trying to understand the non-optimized known plaintext attack on A5/2 from Barkan, Bikan and Keller.

I don't understand the construction of the equation system: $$S_{R4_1} * S_1 = k$$

Where $S$ is the system's matrix and $k=k_1||k_2||k_3||k_4$.

How do I create the system's matrix $S_{R4_1}$ and $S_1$?

• Formatting hint: you can use \$\$ prefix / appendix instead of a single \\$ to create a centralized block formula. – Maarten Bodewes Dec 1 '17 at 22:05