shape shifter - nahamcon

from itertools import permutations, product
from Crypto.Util.number import bytes_to_long as b2l

FLAG = open('flag.txt', 'r').read().strip().encode()


class LFSR():
    def __init__(self, iv):
        self.state = [int(c) for c in iv]
        # self.state = self.iv

    def shift(self):
        s = self.state
        newbit = s[15] ^ s[13] ^ s[12] ^ s[10]  # ^ s[0]
        s.pop()
        self.state = [newbit] + s


if __name__ == "__main__":
    arr = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c',
           'd', 'e', 'f', 'l', 'g', '{', '}']
    r = 2
    chars = [f'1000110001010111', '1111001000000010','1110110010100010','0111111111011101','1000010111110110','0011111000100000','0100001000010110','0001111000001010','0111000100101010','0101100111100011',
             '0000000101011010','1010111100001010','0010101111110000','1110100111110111','1001101000100010','0010001111000001','0111101111111100','0011110000010101','0110110010001100'
             ]
    flagdec = ''
    mychars = [p for p in product(arr, repeat=2)]
    for i, char in enumerate(chars):
        for flag in mychars:
            strg = ''.join(flag)
            curr = bytes(strg, 'utf-8')
            chars = f'{b2l(curr):016b}'
            assert len(chars) == 16
            lfsr = LFSR(chars)
            for _ in range(31337):
                lfsr.shift()
            finalstate = ''.join([str(c) for c in lfsr.state])
            if (f'{finalstate}' == f'{char}'):
                flagdec = flagdec + strg
                break
    print(flagdec)