smart_caeser_decrypt.py

msg = """Fgdzqp uf gb etagxp za hmxxqk oageuz tq. Ebqmwuzs zgyqdage mew pup taddunxq bmowmsqe eqf. Metmyqp tqdeqxr tme puefmzf omz efgpuqp yde. Xqp ftqdqradq ufe yuppxqfaz bqdbqfgmx rgxruxxqp bdahueuaz rdmzwzqee. Eymxx tq pdmiz mrfqd myazs qhqdk ftdqq za. Mxx tmhuzs ngf kag qpimdp sqzuge ftagst dqymdw azq. 

Mpyudmfuaz iq egddagzpqp baeeqeeuaz rdqcgqzfxk tq. Dqymdwmnxk pup uzodqmeuzs aoomeuazmx faa ufe purruogxfk rmd qebqoumxxk. Wzaiz fuxqp ngf eaddk vak nmxxe. Nqp egppqz ymzzqd uzpqqp rmf zai rqqnxk. Rmoq pa iuft uz zqqp ar iurq bmup ftmf nq. Za yq mbbxmgpqp ad rmhagdufq pmetiaape ftqdqradq gb puefdgefe qjbxmuzqp. 
"""
# list of a-z lowercase
letters = [chr(b) for b in range(ord('a'), (ord('z')+1))]

# import matplotlib.pyplot as plt

# returns decrypted caeser ciphers; anything thats not a letter is ignored
def decrypt(msg,offset):
	shift = 7-offset
	msg = msg.split(" ")

	dec = r""
	# Goes through each char; handles capitaliisation and ignores char if not part of letters list
	for eachWord in msg:
		for eachChar in eachWord:
			if eachChar.lower() in letters:
				if eachChar.isupper():
					eachChar = eachChar.lower()
					# Modulus used for wraparound on a list
					newL = (letters[(ord(eachChar)+shift)%len(letters)])
					newL = newL.capitalize()
					dec += newL
				else:

					# newL = chr(ord(eachChar)-shift)
					(ord(eachChar)-shift)%len(letters)
					newL = (letters[(ord(eachChar)+shift)%len(letters)])
					dec += newL
			else:
				dec += eachChar
		# space after each word
		dec += " "
	return dec

# Decrypts caeser ciphers using simple frequency analysis
def autodecrypt(msg):

	freq = []
	# Go through all letters and store their frequency in the message in a list
	for eachL in letters:
		letterCount = msg.count(eachL)
		freq.append(letterCount)

	# Calculate how much the shift is
	highestIndex = freq.index(max(freq))
	shift = highestIndex-4 # 'e' is the 4th element in letters list
	print(decrypt(msg,shift))
	# print()
	# print(letters[highestIndex])
	# plt.plot(letters,freq)
	# plt.show()


# decrypt(msg,9)
autodecrypt(msg)