初遇流密码

在一次比赛中遇到一题叫prng，大概意思是伪随机数。后来才知道是流密码。线性反馈移位寄存器 - LFSR。没全懂，先记录在这。

题目如下

from Crypto.Util.number import *
import uuid
def PRNG(R,mask):
    nextR = (R << 1) & 0xffffffff
    i=(R&mask)&0xffffffff
    nextbit=0
    while i!=0:
        nextbit^=(i%2)
        i=i//2
    nextR^=nextbit 
    return (nextR,nextbit)

R=str(uuid.uuid4())
flag='hgame{'+R+'}'
print(flag)
R=R.replace('-','')
Rlist=[int(R[i*8:i*8+8],16) for i in range(4)]

mask=0b10001001000010000100010010001001
output=[]
for i in range(4):
    R=Rlist[i]
    out=''
    for _ in range(1000):
        (R,nextbit)=PRNG(R,mask)
        out+=str(nextbit)
    output.append(out)

print(f'output={output}')
#output=['1111110110111011110000101011010001000111111001111110100101000011110111111100010000111110110111100001001000101101011110111100010010100000011111101101110101011010111000000011110000100011101111011011000100101100110100101110001010001101101110000010001000111100101010010110110111101110011011001011111011010101011000011011000111011011111001101010111100101100110001011010010101110011101001100111000011110111000001101110000001111100000100000101111100010110111001110011010000011011110110011000001101011111111010110011010111010101001000010011110110011110110101011110111010011010010110111111010011101000110101111101111000110011111110010110000100100100101101010101110010101001101010101011110111010011101110000100101111010110101111110001111111110010000000001110011100100001011111110100111011000101001101001110010010001100011000001101000111010010000101101111101011000000101000001110001011001010010001000011000000100010010010010111010011111111011100100100100101111111001110000111110110001111001111100101001001100010', '0010000000001010111100001100011101111101111000100100111010101110010110011001011110101100011101010000001100000110000000011000000110101111111011100100110111011010000100011111000111001000101001110010110010001000110010101011110011101000011111101101011000011110001101011111000110111000011000110011100100101100111100000100100101111001011101110001011011111111011010100010111011000010010101110110100000110100000100010101000010111101001000011000000000111010010101010111101101011111011001000101000100011001100101010110110001010010001010110111011011111101011100111001101111111111010011101111010010011110011111110100110011111110110001000111100010111000101111000011011011111101110101110100111000011100001010110111100011001011010011010111000110101100110100011101101011101000111011000100110110001100110101010110010011011110000111110100111101110000100010000111100010111000010000010001111110110100001000110110100100110110010110111010011111101011110000011101010100110101011110000110101110111011010110110000010000110001', '1110110110010001011100111110111110111001111101010011001111100100001000111001101011010100010111110101110101111010111100101100010011001001011101000101011000110111000010000101001000100111010110001010000111110110111000011001100010001101000010001111111100000101111000100101000000001001001001101110000100111001110001001011010111111010111101101101001110111010111110110011001000010001010100010010110110101011100000101111100100110011110001001001111100101111001111011011010111001001111010001100110001100001100000110000011111010100101111000000101011111010000111110000101111100010000010010111010110100101010101001111100101011100011001001011000101010101001101100010110000010001110011110011100111000110101010111010011010000001100001011000011101101000000011111000101111101011110011000011011000100100110111010011001111101100101100011000101001110101111001000010110010111101110110010101101000000101001011000000001110001110000100000001001111100011010011000000011011101111101001111110001011101100000010001001010011000001', '0001101010101010100001001001100010000101010100001010001000100011101100110001001100001001110000110100010101111010110111001101011011101110000011001000100100101000011011101000111001001010011100010001010110111011100100111110111001010010111010100000100111110101110010010110100001000010010001101111001110100010001011101100111011101011101100100101011010101000101001000101110011011111110110011111111100000000011100000010011000110001000110101010001011000010101000110000101001110101010111011010010111011001010011100010101001100110000110101100010000100110101110100001101001011011110011100110011001010110100101010111110110111100000111010001111101110000000000111011011101000011001010010111001110111000100111011110100101000100011011101100011111000101110110110111111001111000000011100011000010000101001011001101110101000010101001000100110010000101001111100101000001011011010011110001101000001101111010100101001100010100000111000011110101010100011011001110001011110111010111011010101101100000110000001010010101111011']

1.首先生成一个随机的 UUID 作为种子，这里的 R 是一个全局变量，用于后续 PRNG 函数中生成随机序列。
2.将 UUID 中的 - 符号替换为空，然后将其分割成四个16进制数（每段32位），这里将 UUID 转换为一个包含四个32位整数的列表 Rlist
3.定义一个掩码 mask，这是一个确定的二进制位模式，用于影响 PRNG 的输出
4.对于 Rlist 中的每个32位整数，调用 PRNG 函数进行迭代，生成一个由连续的 nextbit 组成的字符串

链接lazzzaro，crypto-流密码

https://www.cnblogs.com/mumuhhh/p/18012024

#对mask提取出相与有效位1
mask = '10001001000010000100010010001001'
for i in range(len(mask)):
    if int(mask[i]) == 1: 
        print(i, end=' ')
#0 4 7 12 17 21 24 28 31

#恢复R
output=['1111110110111011110000101011010001000111111001111110100101000011110111111100010000111110110111100001001000101101011110111100010010100000011111101101110101011010111000000011110000100011101111011011000100101100110100101110001010001101101110000010001000111100101010010110110111101110011011001011111011010101011000011011000111011011111001101010111100101100110001011010010101110011101001100111000011110111000001101110000001111100000100000101111100010110111001110011010000011011110110011000001101011111111010110011010111010101001000010011110110011110110101011110111010011010010110111111010011101000110101111101111000110011111110010110000100100100101101010101110010101001101010101011110111010011101110000100101111010110101111110001111111110010000000001110011100100001011111110100111011000101001101001110010010001100011000001101000111010010000101101111101011000000101000001110001011001010010001000011000000100010010010010111010011111111011100100100100101111111001110000111110110001111001111100101001001100010', '0010000000001010111100001100011101111101111000100100111010101110010110011001011110101100011101010000001100000110000000011000000110101111111011100100110111011010000100011111000111001000101001110010110010001000110010101011110011101000011111101101011000011110001101011111000110111000011000110011100100101100111100000100100101111001011101110001011011111111011010100010111011000010010101110110100000110100000100010101000010111101001000011000000000111010010101010111101101011111011001000101000100011001100101010110110001010010001010110111011011111101011100111001101111111111010011101111010010011110011111110100110011111110110001000111100010111000101111000011011011111101110101110100111000011100001010110111100011001011010011010111000110101100110100011101101011101000111011000100110110001100110101010110010011011110000111110100111101110000100010000111100010111000010000010001111110110100001000110110100100110110010110111010011111101011110000011101010100110101011110000110101110111011010110110000010000110001', '1110110110010001011100111110111110111001111101010011001111100100001000111001101011010100010111110101110101111010111100101100010011001001011101000101011000110111000010000101001000100111010110001010000111110110111000011001100010001101000010001111111100000101111000100101000000001001001001101110000100111001110001001011010111111010111101101101001110111010111110110011001000010001010100010010110110101011100000101111100100110011110001001001111100101111001111011011010111001001111010001100110001100001100000110000011111010100101111000000101011111010000111110000101111100010000010010111010110100101010101001111100101011100011001001011000101010101001101100010110000010001110011110011100111000110101010111010011010000001100001011000011101101000000011111000101111101011110011000011011000100100110111010011001111101100101100011000101001110101111001000010110010111101110110010101101000000101001011000000001110001110000100000001001111100011010011000000011011101111101001111110001011101100000010001001010011000001', '0001101010101010100001001001100010000101010100001010001000100011101100110001001100001001110000110100010101111010110111001101011011101110000011001000100100101000011011101000111001001010011100010001010110111011100100111110111001010010111010100000100111110101110010010110100001000010010001101111001110100010001011101100111011101011101100100101011010101000101001000101110011011111110110011111111100000000011100000010011000110001000110101010001011000010101000110000101001110101010111011010010111011001010011100010101001100110000110101100010000100110101110100001101001011011110011100110011001010110100101010111110110111100000111010001111101110000000000111011011101000011001010010111001110111000100111011110100101000100011011101100011111000101110110110111111001111000000011100011000010000101001011001101110101000010101001000100110010000101001111100101000001011011010011110001101000001101111010100101001100010100000111000011110101010100011011001110001011110111010111011010101101100000110000001010010101111011']
flag = ''
for i in range(4):
    nextbits = output[i]

    R = []
    for _ in range(32):   #每次恢复1bit 一共是32bit 因为与0xffffffff 起到限制位数的作用 
        temp = '0' + ''.join(R) + nextbits[:(32-1-len(R))]   #补全32位 第一个为猜测位0 第二个为已知R位 第三个为nextbit填充位
        print(temp)
        #进行猜测校验判断
        if(int(temp[0]) ^ int(temp[4]) ^ int(temp[7]) ^ int(temp[12]) ^ int(temp[17]) ^ int(temp[21]) ^ int(temp[24]) ^ int(temp[28]) ^ int(temp[31]) == int(nextbits[32-1-len(R)])):
            #猜测成功则填充0
            R.insert(0, '0')  #在第0位插入0
        else:
            R.insert(0, '1')
    R = ''.join(R)
    R = hex(int(R,2))[2:]  #进制转换
    flag += R
print(flag)
#fbbbee823f434f919337907880e4191a
#hgame{fbbbee82-3f43-4f91-9337-907880e4191a}