math3302.py

# Globals
alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']


### Basic Conversion Functions

def letterToNumber(letter):
	"""
	Returns the number (0-25) equivalent of a letter.
	"""
	return alphabet.index(letter)
	
def numberToLetter(num):
	"""
	Returns the letter (a-z) equivalent of a number.
	"""
	return alphabet[num%26]
	
def textAsNumbers(string):
	"""
	Converts a string into a list of numbers, one for each letter.
	"""
	nums = []
	for char in string:
		nums.append(letterToNumber(char))
	return nums

def numbersAsText(list):
	"""
	Converts a list of numbers into a string.
	"""
	string = ""
	for n in list:
		string += numberToLetter(n)
	return string


def shiftDecrypt(ciphertext, key):
	"""
	Uses key to decrypt ciphertext with a shift cipher.
	"""
	numbers = textAsNumbers(ciphertext)
	plainNumbers = []
	for n in numbers:
		n = (n - key) % 26
		plainNumbers.append(n)

	plaintext = numbersAsText(plainNumbers)
	return plaintext



ciphertext = "uqslziolqnmirijtsoagqulkmxwkmhvfkzjbqhvmagqzdysmyckyallnzyvr"

keys = textAsNumbers("must")
groups = ["","","",""]


# Sort the ciphertext into the four groups.
for i in range(0, len(ciphertext)):
	groups[i%4] += ciphertext[i]


# Decrypt each group with a key of one letter of the keyword.
for i in range(0,4):
	groups[i] = shiftDecrypt(groups[i], keys[i])


#Put the four groups back together to form the plaintext.
plaintext = ""
for i in range(0,15):
	plaintext += groups[0][i] + groups[1][i] + groups[2][i] + groups[3][i]


print plaintext