#This code provides methods to encrypt and decrypt text using a cipher
#based on Z_40, the group of integers under addition modulo 40. Below
#Z_40 is represented as a sequence of letters.
#
#David Faden
#dfaden@iastate.edu
#February 16, 2004
#May 23, 2004

Z_40 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmn"

encoder = {}
for i in range(0, 27):
	encoder[chr(ord('A') + i)] = Z_40[(3 * i) % 40] + Z_40[(3 * i + 1) % 40] + Z_40[(3 * i + 2) % 40]

decoder = dict(map(lambda (k, v): (v, k), encoder.items()))

def encrypt(text):
	text = text.upper()
	text = filter(lambda c: 'A' <= c <= 'Z', text)
	return "".join(map(lambda c:encoder[c], text))
	
def decrypt(text):
	plain_text = ""
	for i in range(0, len(text)/3):
		plain_text += decoder[text[3*i : 3*i + 3]]
	return plain_text

if __name__ == '__main__':
    print encrypt("THISTEXTISPLAINAGAIN")
    print decrypt(encrypt("THISTEXTISPLAINAGAIN"))