transposition die roll example

transp.py

#!/usr/bin/env python3
import random

def encode(plain):
  plain = list(plain)
  cypher = ''
  while len(plain) > 0:
    # Roll a die, add 3 to the result
    step = random.randint(4,9)
    # start at the beginning of the remaining string
    offset = 0
    # Add the offset to step through the cypher
    cypher += str(step)

    # while there's remaining characters
    while len(plain) > offset:
      # take the x*step character
      cypher += plain[offset]
      # clear it out from our plaintext
      plain[offset] = None
      # and move forward
      offset += step

    # reduce our plaintext to remove those transposed characters
    plain = list(filter(None, plain))

  return cypher
  
def decode(cypher):
  # start with an empty grid
  plain = [None] * len(cypher)

  for i in cypher:
    # If we find a number then it's a transposition instruction
    if i.isdigit():     
      step = int(i)
      # reset all our pointers and go to the next character
      pointer = 0
      # we start at offset "0" of the first available slot each time
      countdown = 0
      continue

    # We find the next available slot 
    # and make sure our destination slot is free
    while countdown > 0 or plain[pointer] is not None:
      if plain[pointer] is None:
        countdown -= 1
      pointer += 1
  
    # putting our letter in place
    plain[pointer] = i

    # And start our counting again
    countdown = step - 1
    
  return ''.join( filter(None, plain) )

plain = 'Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me; thy rod and thy staff they comfort me.'
cypher = encode(plain)
maybe = decode(cypher)

print("\n".join([plain, cypher, maybe]))