substitution cypher with DSL

subcypher.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import string

literals = string.printable

HOP_FORWARD = 1
HOP_BACKWARD = -1

def get_nth_replacement_literal(literal, n, hop_direction):
  if hop_direction not in (HOP_FORWARD, HOP_BACKWARD): return None
  start = literals.index(literal)
  hops = n * hop_direction
  return literals[(start + hops) % len(literals)]

def fibo_sub(text, hop_direction):
  if hop_direction not in (HOP_FORWARD, HOP_BACKWARD): return None
  x,y = 0,1
  processed_text = []
  for ch in text:
    n = x+y
    processed_text.append(get_nth_replacement_literal(ch, n, hop_direction))
    x,y = y,n
  return ''.join(processed_text)

def simple_sub(text, hops, hop_direction):
  if hop_direction not in (HOP_FORWARD, HOP_BACKWARD): return None
  processed_text = ''.join(get_nth_replacement_literal(ch, hops, hop_direction) for ch in text)
  return processed_text

cypher = lambda s : fibo_sub(simple_sub(s, 5, HOP_FORWARD), HOP_FORWARD)
decypher = lambda s : simple_sub(fibo_sub(s, HOP_BACKWARD), 5, HOP_BACKWARD)

raw_text = '''Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.'''

cyphered_text = cypher(raw_text)
decyphered_text = decypher(cyphered_text)

print decyphered_text == raw_text