1 -> 5
2 -> 4
3 -> 3
4 -> 2
5 -> 1

3 + (3 - N)

def cipher(char): 
   if 65 <= ord(char) <= 90: 
      return chr(77.5 + (77.5 - ord(char)))
   elif 97 <= ord(char) <= 122: 
      return chr(109.5 + (109.5 - ord(char)))
   else: return char

$ python atb.py attest.txt
Gsrh rh gsv urihg ormv.
Zmw gsrh rh gsv hvxlmw.

$ python atbash.py attest.txt
Gsrh rh gsv urihg ormv.
Zmw gsrh rh gsv hvxlmw.