线性反馈移位寄存器问题，以查找可能的重复发生：
The keys:
1 1 1 0 0 1 0 1 1 1 ...
I started like this:
x1 = 1, x2 = 1, x3 = 1, x4 = 0 ...
with the length of 2
now n =1 and n = 2;
xn+2 = c0 xn + c1 xn+1
1 = c0.1 + c1.1
1 = c0 + c1
0 = c.1 + c1. 1
如何找到密钥的可能重复关系？