# Jay Summet
# 
# ROT 13 example (for lowercase letters only)
#

plaintext = "this is a stringz"

def rot13( aString):
   newStr = ""
   for letter in aString:
      num = ord(letter)
      if 97 <= num <= 122:
         num = num + 13
         if num > 122:
            num = num -26
      newStr = newStr + chr(num)
   return newStr




ct = rot13(plaintext)
print(ct)
pt = rot13(ct)
print(pt)