Very old script of mine implementing Vigenere's cipher encrypt/decrypt.

vigenere.py

#!/usr/bin/python
#
# Simple encoder/decoder of Vigenere cipher
#
#-----------------------------------------

from string import *


g_Tabula 	= []
g_Alls		= []


#-----------------------------------------

def getindex( char):
	for i in g_Tabula:
		if i[0] == char:
			return g_Tabula.index(i)
	return -1

def encode(psk, txt):
	enc = ''
	i = 0

	for a in txt:	
		i += 1
		if a in g_Alls:
			id = getindex(psk[i % len(psk)])
			if id == -1:
				enc += a
				continue
			enc += g_Tabula[id][g_Alls.find(a)]
		else:
			enc += a

	return enc


def decode(psk, txt):
	raw = ''
	enc = ''
	i = 0

	for a in txt:	
		i += 1
		if a in g_Alls:
			id = getindex(psk[i % len(psk)])
			if id == -1:
				enc += a
				continue
			raw += g_Alls[g_Tabula[id].find(txt[i-1])]
		else:
			raw += a
	return raw


#-----------------------------------------

if __name__ == '__main__':

	# Generating Trithemius Tabula Recta
	g_Alls = ascii_letters + digits
	for i in range( len( g_Alls)):
		g_Tabula.append( g_Alls[i:] + g_Alls[0:i] )

	print '\n\t\tVigenere\'s cipher encoder/decoder'
	print 'Used alphabet:\n' + g_Alls
	
	opt = 0
	while 1:
		o = raw_input( '\nType:\t0 for encode\t1 for decode\t')
		if o == '1':
			opt = 1
			break
		elif o == '0':
			break

	psk = ''
	while len(psk) == 0:
		psk = raw_input( 'Input passphrase:\t')

	txt = ''
	file = ''

	txt = raw_input('\nInput text to encode or "file" and filename' \
			'to specify file:\n')
	if 'file ' in lower(txt) :
		file = txt[txt.find(' ')+1:]
		f = open( file, 'r')
		txt = ''
		txt = f.read()
		f.close()

	enc = ''
	if opt == 1:
		enc = decode(psk, txt)
	else:
		enc = encode(psk, txt)

	if len( file) > 0:
		s = 'encoded'
		if opt == 1: s = 'decoded'
		f = open( file[:file.rfind('.')]+'_' + s + \
				file[file.rfind('.'):] )
		f.write( enc)
		f.close()
	else:
		s = 'Encoded'
		if opt == 1: s = 'Decoded'
		print '\n%s:\t***********************\n' % s
		print enc + '\n'