Consider a stream cipher as follows. The outputs of three LFSRs having length 29, 37 and

41 are combined by a Boolean function f(x1; x2; x3) = x3 x1x2 to generate the key stream.

Then the plaintext bits are XORed with the key stream to generate the cipher.

(a) Describe a known plaintext attack on this scheme that requires lesser time complexity

than the exhaustive search.

(b) Revise the above attack to mount a ciphertext only attack on this cipher.