L3LFSR keystream generator for cipher from ctf.moscow task "line"

cipher.py

class L3LFSR:
    def __init__(self, key):
        self.r1 = key[:67]
        self.r2 = key[67:67+59]
        self.r3 = key[-2:]
        self.b1 = [67, 5, 2, 1]
        self.b2 = [59, 6, 5, 4, 3, 1]
        self.b3 = [2, 1]

    def _clock_r(self, reg, branches):
        ret = reg[len(reg) - 1]
        new = 0
        for i in branches:
            new ^= reg[i - 1]
        reg = [new] + reg[:-1]

        return reg, ret

    def next_bit(self):
        self.r1, x1 = self._clock_r(self.r1, self.b1)
        self.r2, x2 = self._clock_r(self.r2, self.b2)
        self.r3, x3 = self._clock_r(self.r3, self.b3)
        f = x1 ^ x2 ^ x3 ^ 1

        return f