Vigenere cipher corrupts some

Question

I am making a simple vigenere cipher encrypter/decrypter in python, and it works for the most part. I'm not getting any errors, but some letters aren't encrypted or decrypted (or both?) properly. Here is my code:

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)

and here is my terminal output:

$ 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
$

It seems to only encrypt/decrypt some letters incorrectly. What the heck is going on?

Answer 1

Short answer - these two lines need to be fixed:

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)]

---

Long answer: The way your code uses list.index() is a logical error.

Suppose we're doing encryption, and the plaintext is 'banana', key is 'secretkey'. In the old code, this is what happens:

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 ]

We can elaborate on what happens with the list enc_nums:

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 ]

The problem occurs when a letter occurs in the plaintext more than once. The method list.index() returns the index of the first occurrence. Thus the wrong index of the key (rk_nums) is used for encrypting the letters.

The easy solution is to use the zip() function, which pairs up elements from both lists at the same index. For example, zip([9, 8, 7, 6], [0, 1, 2, 3]) will return the list [(9,0), (8,1), (7,2), (6,3)]. This way, you can ensure that the plaintext numbers and key numbers are used in sync all the time.