Let $k_{1}k_{2}k_{3}\ldots$ be the keystrem generated by the LFSR. The bits in the stream satisfy

k_n = k_{n-16} \oplus k_{n-17}, \quad n = 18, 19 \ldots \tag{*}

Using his knowledge of the plaintext bits, the adversary first computes $k_i$ for $i = 1001, 1002, \ldots, 1100$ as $k_i = c_i \oplus m_i$ . He can then compute the rest of the keystream using (). He can also compute the first 1000 bits of the keystream by rewriting ( as $k_{n-17} = k_{n-16} \oplus k_n$ . When he knows the entire keystream, he can decrypt and find $m = c \oplus k$ .