Decrypting a Stream Cipher Based on J–K Flip-Flops

Pless has proposed a stream cipher based on J–K flip-flops that uses 8 linear shift registers with feedback, having a combined length of 97 bits, 4 J–K flip-flops, and a 4-stage cycling counter. The cipher has 2.54 × 10
⁵¹
initial states (keys), and generates a presumably pseudorandom stream whose period is 1.52 × 10
²⁹
bits. Despite these impressive statistics, it is computationally feasible to solve such a cipher with a known-plaintext attack, using as few as 15 characters