decrypt.py

def decrypt( ciphertext, key ):
	""" given ciphertext and key, return decoded plaintext"""
 
	# Represent each plaintext letter as a corresponding number 
	#  for example a=>0, b=>1, ... y=>25, z=>26, '<space>'=>27
	ciphertext_as_numbers = []	
	for l in ciphertext.lower():
		ciphertext_as_numbers.append( letters.index(l) )
 
	# encryption... Ceaser cipher shift
	#  add the key to each number and mod by 27
	decrypted = []
	for n in ciphertext_as_numbers:
		decrypted.append( (n - key) % 27 )
 
	# convert the shifted/encrypted numbers back to letters
	plaintext = ""
	for n in decrypted:
		plaintext += letters[n]
 
	return plaintext