1-In cryptography , why it's a bad idea to have all bits as 11111111 , 0101010 ? in stream cipher

2- Take a linear (A,B) two variables,

S0 = seed

F(Si) = Si + 1 = ASi + B

if Oscar (Oscar is an attacker) knows x1, x2, x3 and y1, y2, y3 (he knows three cipher text and three plain texts with the same size)

then Oscar can discover

1- S1, S2, S3

2- A, B

Tell that it's not good to use leaner numbers, why its security is not good and how Osacr can discover S1, S2,S3