def __init__(self):
self.letCounter = LetterCounts()
self.ic = { "english" : 0.065, "equiprobable" : 0.038}
# ic of english language
self.engIC = 0.065
# ic of perfectly random message (all letters equiprobable)
self.ranIC = 0.038
self.vigenereSquare = self.constructVigenereSquare()
class VigenereCipher:
def __init__(self):
self.letCounter = LetterCounts()
self.ic = { "english" : 0.065, "equiprobable" : 0.038}
# ic of english language
self.engIC = 0.065
# ic of perfectly random message (all letters equiprobable)
self.ranIC = 0.038
self.vigenereSquare = self.constructVigenereSquare()
## clearMsg
#
# cleans up the ciphertext so that it only contains
# A-Z characterws
#
def clearMsg(self, msg):
return msg.replace(' ', '')
## Index of Coincidence (IC)
# The IC of the English Language is 0.065
# The IC of a text written where all letters are
# equiprobable is 0.038
#
# IC is equivalent to the sum from i=1 to 26
# of c**2/n**2 where n is the length of the
# string and c is the count of the letter
# (corresponding letter of the alphabet) in
# the message
#
def calcIC(self, msg):
# Determine the letter counts in the message
counts = self.letCounter.getCounts(msg)
# Determine the squared length of the message
n = len(msg)**2
# The IC (results from a summation)
ic = 0
for letter,num in counts:
# Since some of the counts could be 0, skip those
if num == 0:
continue
ic += num**2/float(n)
return ic
## isMonoalphabetic
#
# determines if the IC is closer to the English Language
# IC or to the IC of text with equiprobable letter occurrence
#
# return: whether the IC is closer to english IC or not
def isMonoalphabetic(self, ic):
deltaEng = abs(self.ic["english"] - ic)
deltaEqui = abs(self.ic["equiprobable"] - ic)
return deltaEng > deltaEqui
## constructVigenereSquare
#
# The Vigenere Square is a mapping of the simple shifts performed
# when encrypting a message using a Vigenere cipher keyword
#
# In other words, keyword 'a' shifts letters by 0
# so vigengereSquare['a']['c'] = 'C'
def constructVigenereSquare(self):
alphabet = list(string.ascii_uppercase)
lowerbet = list(string.ascii_lowercase)
vigenereSquare = {}
for shift, rowLet in enumerate(lowerbet):
vigenereSquare[rowLet] = {}
for idx, colLet in enumerate(lowerbet):
vigenereSquare[rowLet][colLet] = alphabet[(idx+shift)%len(alphabet)]
return vigenereSquare
## vigenereSquareDecrypt
#
# c is the cipher letter (i.e. 'N')
# p is the plaintext letter that the cipher letter
# is expected to correspond to (i.e. 'e')
#
def vigenereSquareDecrypt(self, c, p='e'):
for key, value in self.vigenereSquare[p].items():
if value == c:
return key
return None
## vigenereSquareLookup
#
# k is the keyword letter (i.e. 'j')
# p is the plaintext letter to encrypt (shift)
# c is the ciphertext letter found via table lookup
#
def vigenereSquareLookup(self, k, p, printTable=False):
return self.vigenereSquare[k][p]
def encrypt(self, msg, key, printTable=False):
keyLen = len(key)
encryptedMsg = ""
if printTable:
print " __________________________________"
print "| Keyword | Plaintext | Ciphertext |"
for i in range(0, len(msg), keyLen):
chunk = msg[i:i+keyLen]
# encrypt using the vigenere square
for chunkIdx in range(0, len(chunk)):
keyLet = key[chunkIdx]
msgLet = chunk[chunkIdx]
cipherLet = self.vigenereSquareLookup(keyLet, msgLet)
if printTable:
print "| %s | %s | %s | " % (keyLet, msgLet, cipherLet)
encryptedMsg += cipherLet
if printTable:
print "|__________________________________|"
return encryptedMsg
## decrypt
#
# Decrypts a message using the key and the Vigenere Square
#
# msg is the message to decrypt
# key is the key used to encrypt the message
#
def decrypt(self, msg, key, printTable=False):
keyLen = len(key)
decryptedMsg = ""
if printTable:
print " __________________________________"
print "| Keyword | Ciphertext | Plaintext |"
for i in range(0, len(msg), keyLen):
# slice out a chunk of the message (len <= size of key)
chunk = msg[i:i+keyLen]
# decrypt the chunk using the key
for chunkIdx in range(0, len(chunk)):
keyLet = key[chunkIdx]
msgLet = chunk[chunkIdx]
plainLet = self.vigenereSquareDecrypt(msgLet, keyLet)
if printTable:
print "| %s | %s | %s | " % (keyLet, msgLet, plainLet)
decryptedMsg += plainLet
if printTable:
print "|__________________________________|"
return decryptedMsg