playfair_cipher_decrypt.py

# source : https://www.geeksforgeeks.org/playfair-cipher-with-examples/
def toLowerCase(plain):
	# Convert all the characters of a string to lowercase
	return plain.lower()


def removeSpaces(plain):
	# Remove all spaces in a string
	# can be extended to remove punctuation
	return ''.join(plain.split())


def generateKeyTable(key):
	# generates the 5x5 key square
	keyT = [['' for i in range(5)] for j in range(5)]
	dicty = {chr(i + 97): 0 for i in range(26)}

	for i in range(len(key)):
		if key[i] != 'j':
			dicty[key[i]] = 2
	dicty['j'] = 1

	i, j, k = 0, 0, 0
	while k < len(key):
		if dicty[key[k]] == 2:
			dicty[key[k]] -= 1
			keyT[i][j] = key[k]
			j += 1
			if j == 5:
				i += 1
				j = 0
		k += 1

	for k in dicty.keys():
		if dicty[k] == 0:
			keyT[i][j] = k
			j += 1
			if j == 5:
				i += 1
				j = 0

	return keyT


def search(keyT, a, b):
	# Search for the characters of a digraph in the key square and return their position
	arr = [0, 0, 0, 0]

	if a == 'j':
		a = 'i'
	elif b == 'j':
		b = 'i'

	for i in range(5):
		for j in range(5):
			if keyT[i][j] == a:
				arr[0], arr[1] = i, j
			elif keyT[i][j] == b:
				arr[2], arr[3] = i, j

	return arr


def mod5(a):
	# Function to find the modulus with 5
	if a < 0:
		a += 5
	return a % 5


def decrypt(str, keyT):
	# Function to decrypt
	ps = len(str)
	i = 0
	while i < ps:
		a = search(keyT, str[i], str[i+1])
		if a[0] == a[2]:
			str = str[:i] + keyT[a[0]
								][mod5(a[1]-1)] + keyT[a[0]][mod5(a[3]-1)] + str[i+2:]
		elif a[1] == a[3]:
			str = str[:i] + keyT[mod5(a[0]-1)][a[1]] + \
				keyT[mod5(a[2]-1)][a[1]] + str[i+2:]
		else:
			str = str[:i] + keyT[a[0]][a[3]] + keyT[a[2]][a[1]] + str[i+2:]
		i += 2

	return str


def decryptByPlayfairCipher(str, key):
	# Function to call decrypt
	ks = len(key)
	key = removeSpaces(toLowerCase(key))
	str = removeSpaces(toLowerCase(str))
	keyT = generateKeyTable(key)
	return decrypt(str, keyT)

def final(key, text):
	return decryptByPlayfairCipher(text, key)