Generates a string from a randomly generated tabula recta that is hopefully secure.

tabula_recta.py

#! /usr/bin/env python

import sys
import json
import argparse

from string import uppercase, digits, letters, punctuation
from random import SystemRandom

FULL_MAP = letters + digits + punctuation
ALPHA_MAP = letters + digits

parser = argparse.ArgumentParser(description='Generate a string, 16 chars, from a tabula recta.')
parser.add_argument('-r', '--row', help='The row to use, select `A`-`Z`')
parser.add_argument('-c', '--col', help='The column to use, select `A`-`Z`')
parser.add_argument('-s', '--show', default=False, help='Set `True` to display current table.')
parser.add_argument('-a', '--alpha', default=False, help='Set `True` to use an alphanumeric table.')
args = parser.parse_args()

r = SystemRandom()

if args.row:
    row = args.row.upper()
    if row in uppercase and len(row) == 1:
        row = uppercase.index(row)
    else:
        print('Invalid row.')
        sys.exit()

if args.col:
    col = args.col.upper()
    if col in uppercase and len(col) == 1:
        col = uppercase.index(col)
    else:
        print('Invalid column.')
        sys.exit()

if not args.alpha:
    try:
        table = json.load(open('tabula.json'))
    except IOError:
        table = [[r.choice(FULL_MAP) for i in xrange(len(uppercase))] for i in xrange(len(uppercase))]
        json.dump(table, open('tabula.json', 'w'))
else:
    try:
        table = json.load(open('alpha_tabula.json'))
    except IOError:
        table = [[r.choice(ALPHA_MAP) for i in xrange(len(uppercase))] for i in xrange(len(uppercase))]
        json.dump(table, open('alpha_tabula.json', 'w'))


def traverse(row,col):
    s = ''
    LEN = 16
    WIDTH = 25    
    edge = False

    while len(s) < LEN:
    
        if not edge:
            s += table[row][col]

            if row == WIDTH and col == WIDTH and len(s) < LEN:
                edge = True
                continue
            
            if row < WIDTH: 
                row += 1
            if col < WIDTH:
                col += 1

        else:
            if row == 0 and col == 0 and len(s) < LEN:
                edge = False
                continue

            if row > (WIDTH / 2):
                col -= 1
                s += table[row][col]

    return s


def display():
    printr = [' '.join(row) for row in [[c for c in uppercase]] + table]

    output = ''
    i = 0
    for row in printr:
        add = ''
        if i == 1:
            add += '  ' + '+' + ''.join(['-' for c in xrange(len(uppercase + uppercase))]) + '\n' + uppercase[i-1] + ' | '
        if i == 0:
            add += '    ' + row + '\n' 
        elif i != len(uppercase):
            add += row + '\n' + uppercase[i] + ' | '
        else:
            add += row
        output += add
        i += 1
    
    return output


if __name__ == '__main__':
    
    if args.row and args.col:
        print('\n' + 'Row `{0}` and Column `{1}` returned: '.format(args.row.upper(), args.col.upper()) + traverse(row,col) + '\n')

    if args.show:
        print(display())