CODEWITHSUNDEEP
pythonencryption
MissSprezzatura
MissSprezzatura

Reputation: 313

Vignere Cipher Decryption function not return correct decrypt python 2.7

I created a Encrypter and Decrypter for the Vignere cipher. The encrypter works it turned "THE EAGLE HAS LANDED" into "WHZ RCOOE PNU OAILRF". However when I try to decrypt "WHZ RCOOE PNU OAILRF" using DAVINCI, it doesn't return THE EAGLE HAS LANDED. Instead it returns a random jumble "ZHU EEWRE XAW RADTEH". Is the issue in the decryptVignere or is it the previous functions? I know encryptVignere works perfectly.

"""
This program will decrypt/encrypt a Vignere with the Keyword: DaVinci
"""

# helper functions
# letter to index
def letterToIndex(ch):
   alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ "
   idx = alphabet.find(ch)
   if idx < 0:
      print ("error: letter not in the alphabet", ch)
   return idx

# index to letter
def indexToLetter(idx):
   alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ "
   if idx > 25:
      print ('error: ', idx, ' is too large')
      letter = ' '
   elif idx < 0:
      print ('error: ', idx, ' is less than 0')
      letter = ' '
   else:
      letter = alphabet[idx]
   return letter

# looking up a letter in the Vignere square
def vignereIndex(keyLetter, plainTextLetter):
   keyIndex = letterToIndex(keyLetter)
   ptIndex = letterToIndex(plainTextLetter)
   newIdx = (ptIndex + keyIndex) % 26
   return indexToLetter(newIdx)

def decryptVignere(key, cipherText):
   plainText = ""
   keyLen = len(key)
   for i in range (len(cipherText)):
      ch = cipherText[i]
      if ch == ' ':
         plainText = plainText + ch
      else:
         plainText = plainText + vignereIndex(key[i%keyLen], ch)
   return plainText

#encrypting a message using the Vignere cipher
def encryptVignere(key, plainText):
   cipherText = ""
   keyLen = len(key)
   for i in range (len(plainText)):
      ch = plainText[i]
      if ch == ' ':
         cipherText = cipherText + ch
      else:
         cipherText = cipherText + vignereIndex(key[i%keyLen], ch)
   return cipherText

# decrypt
messageOne = "WHZ RCOOE PNU OAILRF" # raw_input("Enter your string: ")?
keyOne = "DAVINCI"

deStr = decryptVignere(keyOne, messageOne)
print deStr

Upvotes: 0

Views: 135

Answers (1)

wvdz
wvdz

Reputation: 16651

Your decrypt method is exactly identical to your encrypt method! The decrypt method should do the opposite, i.e. subtract the key instead of adding it.

A possible solution is to add a parameter to your vignereIndex method to toggle between encrypting and decrypting.

def vignereIndex(keyLetter, plainTextLetter, encrypt):
    keyIndex = letterToIndex(keyLetter)
    ptIndex = letterToIndex(plainTextLetter)
    if not encrypt:
        keyIndex *= -1
    newIdx = (ptIndex + keyIndex) % 26
    return indexToLetter(newIdx)

Upvotes: 1

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