Question

我编写了以下代码来使用Python代码置换位：

iptable=[56,48,40,32,24,16,8,0,57,49, 41,33,25,17,9,1, 58, 50, 42, 34, 26,
     18, 10,  2, 59, 51, 43, 35,
     62, 54, 46, 38, 30, 22, 14,
      6, 61, 53, 45, 37, 29, 21,
     13,  5, 60, 52, 44, 36, 28,
     20, 12,  4, 27, 19, 11, 3]
msg= b'AABBCCDD'
enc = ''
i = 0
while i <= 63:
    x = iptable[i]
    enc = enc + msg[x]
    i = i + 1
print(enc)

它返回了

  enc = enc + msg[x]
IndexError: index out of range

那么错误在哪里？并且msg变量中的每个字符是否转换为8位或更大，更少？

Answer 1

您的方法存在很多问题

您使用错误的工具进行交易。您可能应该寻找bitarray
iptable（56）的长度不符合迭代的长度（64）

使用bitarray的示例更正代码可能如下所示

>>> iptable=[56,48,40,32,24,16,8,0,57,49, 41,33,25,17,9,1, 58, 50, 42, 34, 26,
     18, 10,  2, 59, 51, 43, 35,
     62, 54, 46, 38, 30, 22, 14,
      6, 61, 53, 45, 37, 29, 21,
     13,  5, 60, 52, 44, 36, 28,
     20, 12,  4, 27, 19, 11, 3]
>>> from bitarray import bitarray
>>> msg = bitarray(endian='little')
>>> msg.frombytes('AABBCCDD')
>>> enc = bitarray(endian='little')
>>> for i in iptable:
    enc.append(msg[i])


>>> enc
bitarray('00110011001111001100000000001111111100000000000000000000')
>>> msg
bitarray('1000001010000010010000100100001011000010110000100010001000100010')

Answer 2

您可以通过将消息转换为整数来置换位：

def tobin(n):
    return bin(n)[2:].zfill(len(msg)*8)

msgint = int.from_bytes(msg, byteorder='big')
print(tobin(msgint))
print(msgint.to_bytes(len(msg), byteorder='big', signed=False))
# -> 0100000101000001010000100100001001000011010000110100010001000100
# -> b'AABBCCDD'

然后你可以直接执行位操作：

result = 0
for i, j in enumerate(iptable):
    if msgint & (1 << j):  # if j-th bit set in msg
        result |= (1 << i) # set i-th bit in the result

print(tobin(result))
print(result.to_bytes(len(msg), byteorder='big', signed=False))
# -> 0000000000000000000000000000111111110000110000000011110000110011
# -> b'\x00\x00\x00\x0f\xf0\xc0<3'

如何在Python中置换位？

2 个答案: