vigen.py

#!/usr/bin/env python2
# -*- coding: utf-8 -*-
# vim: set fileencoding=UTF-8 :
'''
Vigenère Cipher implementation by Michael Yanovich (aka: yano)
Copyright 2013 - Licensed under the Eiffel Forum License v2

USAGE:
./vigen.py KEY
Enter text to be mangled: I LIKE CHEESE
Offset: 0
Encrypted: S PGUI ARICCI
'''

import math
import sys
import string

alpha = string.lowercase


try:
    KEY = sys.argv[1]
except:
    print 'Please run as ./vigen.py KEY_HERE'
    sys.exit(1)

incoming = raw_input('Enter text to be mangled: ')

try:
    offset = int(raw_input('Offset: '))
except:
    print 'The offset was not a number!'
    sys.exit(1)

## default to an offset of 1 if none provided
if not offset:
    offset = 1

## subtract 1 so it works with python's 0's index for lists
## this part actually broke it, and ended up making it a ROT12
## when it should have been a ROT13
#offset -= 1

## generate the Vigenère square shading
chart = dict()
k = 0
for letter in alpha:
    if k + offset >= len(alpha):
        ## rotation won't go beyond end of row
        sanit = k + offset - len(alpha)
        chart[letter] = alpha[sanit:] + alpha[:sanit]
    else:
        ## rotation goes beyond the end of the row
        chart[letter] = alpha[k + offset:] + alpha[:k + offset]
    k += 1

## display the chart to stdout
ordered_rows = chart.keys()
ordered_rows.sort()
for x in ordered_rows:
    print x, chart[x]
print ''

## pad the key with the length of the incoming text
multi = math.ceil(len(incoming) / float(len(KEY)))
long_key = KEY * int(multi)


## encrypted the incoming text
j = 0
output_encrypted = str()
for x in incoming:
    y = x.lower()
    if y in chart:
        row = chart[y]
        idx = (string.lowercase).find(long_key[j].lower())
        output_encrypted += chart[y][idx]
        j += 1
    else:
        output_encrypted += y

## decrypt the incoming text
p = 0
output_decrypted = str()
for x in incoming:
    y = x.lower()
    stop = False
    before = len(output_decrypted)
    for letter in chart:
        row = chart[letter]
        if not stop and y in chart:
            idx = (string.lowercase).find(long_key[p].lower())
            if row[idx] == y:
                output_decrypted += letter
                p += 1
                stop = True
    after = len(output_decrypted)
    if before == after:
        output_decrypted += y

print 'Encrypted:', output_encrypted.upper()
print 'Decrypted:', output_decrypted.upper()