100行代码手把手带你实现Feisitel加密算法

plain_text = 'Hello world!'
# 为了便于取整，选择8bit
block_size = 8
rounds = 16
keys = []

def rand_key(n):
  res = ''
  for i in range(n):
    bit = random.randint(0, 1)
    res += str(bit)
  return res
# length 是key的长度，可以根据需要调整
def generate_keys(length, rounds):
  for i in range (rounds):
    keys.append(rand_key(length))
  return res

def exor(a, b):
  n = len(a)
  res = ''
  for i in range(n):
    if a[i] == b[i]: res += '0'
    else: res += '1'
  return res
def round(str, key):
  n = len(str) // 2
  L1 = str[0:n]
  R1 = str[n:]
  # 这里的f函数选择exor操作
  # 可以替换为其他的加密函数
  f1 = exor(R1, key)
  R2 = exor(f1, L1)
  L2 = R1
  return L2 + R2

def feistel_encode(s, rounds):
  res = ''
  # 将密文转为8bit二进制表示
  str_ascii = [ord(x) for x in s]
  str_bin = [format(x, '08b') for x in str_ascii]
  str_bin = ''.join(str_bin)
  # 选择block size的一半最为key length是因为将f函数定义为xor操作
  # 保证长度和block size的一半相等，在执行时就不需要补padding
  # 实际的key length会更长，一般不会少于128bit避免被快速爆破
  key_length = block_size // 2
  generate_keys(key_length, rounds)
  res_bin = ''
  for n in range(0, len(str_bin), block_size):
    temp = str_bin[n:n + block_size]
    for i in range(rounds):
      temp = round(temp, keys[i])
    # 最后一轮循环后交换左右部分
    temp = temp[key_length::] + temp[0:key_length]
    res_bin += temp
  # 将密文转为ASCII字符表示
  for i in range(0, len(res_bin), 8):
    bin_data = res_bin[i: i + 8]
    decimal = int(bin_data, 2)
    res += chr(decimal)
  return res

def feistel_decode(s):
  res = ''
  str_ascii = [ord(x) for x in s]
  str_bin = [format(x, '08b') for x in str_ascii]
  str_bin = ''.join(str_bin)
  res_bin = ''
  for n in range(0, len(str_bin), block_size):
    temp = str_bin[n:n + block_size]
    for key in reversed(keys):  # 确保使用反向密钥
      temp = round(temp, key)
    key_length = len(keys[0])
    # 最后一轮循环后交换左右部分
    temp = temp[key_length::] + temp[0:key_length]
    res_bin += temp
  # 转为ASCII字符
  for i in range(0, len(res_bin), 8):
    bin_data = res_bin[i: i + 8]
    decimal = int(bin_data, 2)
    res += chr(decimal)
  return res

def main():
  print('plain_text: ', plain_text)
  # plain_text:  Hello world!
  cipher_text = feistel_encode(plain_text, rounds)
  print('cipher_text: ', cipher_text)
  # cipher_text:  yààÓlKÓàh}
  decode_text = feistel_decode(cipher_text)
  print('decode_text: ', decode_text)
  # decode_text:  Hello world!
if __name__ == "__main__":
  main()