Vigenere密码破坏了一些

Question

import sys
if not len(sys.argv) == 4:
    print "Not enough arguments."
    print "Usage: python vigener.py <encrypt/decrypt> 'ciphertext' 'key'"
    sys.exit()
mode = sys.argv[1]
ctext = sys.argv[2]
key = sys.argv[3]
if mode == "encrypt":
    print "Encrypting using vigener cipher..."
elif mode == "decrypt":
    print "Decrypting using vigener cipher..."
else:
    print "Unknown function '"+str(mode)+"'."
    print "Usage: python vigener.py <encrypt/decrypt> 'ciphertext' 'key'"
MAGIC_NUMBER = 96
ctext = ctext.lower()
repeated_key = ( key * (1+len(ctext)/len(key)) )[:len(ctext)]
if mode == "encrypt":
    ctext = ctext.replace(" ", "{")
    nums = [ord(ltr)-MAGIC_NUMBER for ltr in ctext]
    rk_nums = [ord(ltr)-MAGIC_NUMBER for ltr in repeated_key]
    enc_nums = [(num+rk_nums[ nums.index(num) ]) % 27 for num in nums]
    enc_ltrs = [chr(num+MAGIC_NUMBER) for num in enc_nums]
    print "".join(enc_ltrs)
elif mode == "decrypt":
    enc_nums = [ord(ltr)-MAGIC_NUMBER for ltr in ctext]
    rk_nums = [ord(ltr)-MAGIC_NUMBER for ltr in repeated_key]
    dec_nums = [(num-rk_nums[ enc_nums.index(num) ]) for num in enc_nums]
    dec_nums2 = [ (num + 27 if num < 1 else num) for num in dec_nums]
    dec_ltrs = [chr(num+MAGIC_NUMBER) for num in dec_nums2]
    dec_str = "".join(dec_ltrs)
    dec_str = dec_str.replace("{", " ")
    print "".join(dec_str)

下面是我的终端输出：

$ python vigener.py encrypt 'this is confidential' 'secretkey'
Encrypting using vigener cipher...
lmljeljeagsiliyslltq
$ python vigener.py decrypt 'lmljeljeagsiliyslltq' 'secretkey'
Decrypting using vigener cipher...
thts ts conftfenttal
$

它似乎只对某些字母进行了不正确的加密/解密。到底是怎么回事？

Answer 1

简短的回答--这两行需要修正：

enc_nums = [(x + y) % 27 for (x, y) in zip(nums, rk_nums)]
...
dec_nums = [(x - y) % 27 for (x, y) in zip(enc_nums, rk_nums)]

长答案：您的代码使用list.index()的方式是一个逻辑错误。

假设我们正在进行加密，明文是'banana'，key是'secretkey'。在旧代码中，发生的情况如下：

nums = [2, 1, 14, 1, 14, 1]      # 'banana'
rk_nums = [19, 5, 3, 18, 5, 20]  # 'secret'
enc_nums = [
  ( 2 + rk_nums[nums.index( 2)]) % 27,
  ( 1 + rk_nums[nums.index( 1)]) % 27,
  (14 + rk_nums[nums.index(14)]) % 27,
  ( 1 + rk_nums[nums.index( 1)]) % 27,
  (14 + rk_nums[nums.index(14)]) % 27,
  ( 1 + rk_nums[nums.index( 1)]) % 27 ]

enc_nums = [
  ( 2 + rk_nums[0]) % 27,
  ( 1 + rk_nums[1]) % 27,
  (14 + rk_nums[2]) % 27,
  ( 1 + rk_nums[1]) % 27,
  (14 + rk_nums[2]) % 27,
  ( 1 + rk_nums[1]) % 27 ]