A space efficient implementation of the Railway Cipher.

railway_cipher.py

#!/usr/bin/env python3
# Railway Cipher, without a multidimensional array.
# Author: Alan Morgan

def get_rail_len(h): return 2*(h+1)-2

def get_level_len(level, fence_height, s):
    rail_length = get_rail_len(fence_height)
    rem = len(s) % rail_length - 1
    level_len = len(s) // rail_length
    if level != 0 and level != fence_height:
        level_len *= 2
        if rem >= 0+level:              level_len+=1
        if rem >= 2*fence_height-level: level_len+=1
    elif rem >= level:
        level_len += 1

    return level_len

def get_fence_height(s, p, minimum_height=2):
    p_val, divider = 0, 1 if len(s) < 2 else len(s)//2
    for x in p: p_val += ord(x)
    return (p_val % divider) + minimum_height

def get_final_index(level, level_ind, fence_height):
    rail_length = get_rail_len(fence_height)
    offset, level_divisor = level, 1
    if level != 0 and level != fence_height:
        level_divisor = 2
        if level_ind % 2 == 1:
            offset = rail_length - level

    return offset+(level_ind//level_divisor)*(rail_length)

def base_algorithm(s, p, cipher, cipher_func):
    # Fence Height
    fence_height = get_fence_height(s, p)

    # Next scramble the password.
    level, level_ind = 0, 0
    level_len = get_level_len(level, fence_height, s)

    for i in range(len(s)):
        final_ind = get_final_index(level, level_ind, fence_height)
        cipher_func(cipher, s, i, final_ind)

        level_ind = (level_ind + 1) % level_len
        if level_ind == 0:
            level += 1
            level_len = get_level_len(level, fence_height, s)

    return ''.join(x for x in cipher)

def encrypt_helper(c, s, i, f): c.append(s[f])
def decrypt_helper(c, s, i, f): c[f] = s[i]
encrypt = lambda s, p: base_algorithm(s, p, list(),  encrypt_helper)
decrypt = lambda s, p: base_algorithm(s, p, list(s), decrypt_helper)

m = input("Enter your message: ")
k = input("Enter your key: ")
s = encrypt(m, k)
d = decrypt(s, k)
print("-------------"+"-"*len(s))
print("Encrypted: '{}'".format(s))
print("Decrypted: '{}'".format(d))