Some basic ciphers for infosec.

cipher.py

#python 2.7
from __future__ import division

padding = '|'
cipher_key = 'abcdefghijklmnopqrstuvwxyz '
cipher_key_length = len(cipher_key)

# A shift cipher. Can also use a key, turning it into a vernam cipher.
# text = the text
# key = the key. Can be a number or a string.
# shift_forward = whether the shift goes forward or back. Set to False to decrypt.
def shift(text, key, shift_forward=True):

    # Shift direction
    forward = 1 if shift_forward else -1
    
    key_length = len(key)
    
    # Convert numerical keys to tiny shift key
    if type(key) is int: key = cipher_key[key]

    ciphertext = ''
    
    # Perform the shift
    for decode_index, c in enumerate(text):
        shifted_pos = cipher_key.index(c) + \
                      (forward * cipher_key.index(key[decode_index % key_length]))
        ciphertext += cipher_key[shifted_pos % cipher_key_length]

    return ciphertext

# A columnar transposition cipher. Encrypt only.
# plaintext = the text to encrypt
# keyphrase = the keyphrase. Make sure it has unique letters! Lowercase only!
def transposition_encode(plaintext, keyphrase):

    # Length of keyphrase
    row_length = len(keyphrase)
    
    # Fragment the plaintext into rows
    rows = [plaintext[i:i + row_length] for i in range(0, len(plaintext), row_length)]
    
    # Pad the last row if needed
    rows[-1] += padding * (row_length - len(rows[-1]))
    
    # Arrange keyphrase in abc order
    abc_order = sorted(keyphrase, key = lambda c : cipher_key.index(c))
    
    ciphertext = ''

    # Read in the characters by column, in abc order
    for col in [keyphrase.index(c) for c in abc_order]:
        for row in rows:
            ciphertext += row[col]
            
    return ciphertext

# A columnar transposition cipher. Decryption only.
# ciphertext = the text to decrypt.
# keyphrase = the keyphrase. Make sure it has unique letters! Lowercase only!
def transposition_decode(ciphertext, keyphrase):
    
    # Get the column lengths
    col_length = len(ciphertext) // len(keyphrase)
    
    # Break the ciphertext into column groups
    cols = [ciphertext[i:i + col_length] for i in range(0, len(ciphertext), col_length)]
    
    # Arrange keyphrase in abc order
    abc_order = sorted(keyphrase, key = lambda c : cipher_key.index(c))
    
    # Re-order the columns according to the keyphrase
    ordered_cols = [None] * len(cols)
    for i, key_char in enumerate(abc_order):
        ordered_cols[keyphrase.index(key_char)] = cols[i]

    plaintext = ''
    
    # Read in the characters down each row
    for i in range(col_length):
        for col in ordered_cols:
            plaintext += col[i]

    # Strip the padding
    plaintext = plaintext.replace(padding, '')
    
    return plaintext
    
def test():
    print shift(shift('this should be readable', 'abacab'), 'abacab', False)
    s = 'we are discovered flee at once'
    key = 'zebras'
    coded = transposition_encode(s, key)
    print s, '->', coded, '->', transposition_decode(coded, key)

subcipher.py

from random import shuffle

# Set up the cipher's key
_cipher_key = list('abcdefghijklmnopqrstuvwxyz ')
_cipher_subs = _cipher_key[:]
shuffle(_cipher_subs)

def encrypt(plaintext):
	ciphertext = ''
	for c in plaintext:
		ciphertext += _cipher_subs[_cipher_key.index(c)]
	return ciphertext

def decrypt(plaintext):
	ciphertext = ''
	for c in plaintext:
		ciphertext += _cipher_key[_cipher_subs.index(c)]
	return ciphertext