otp.py

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##
#  
# This module provides an Otp object that can be used to get one time
# pad strings.  It should be adequate for situations that don't require
# high security.
# 
# You can call the module directly as a script; it will want the
# number of otp strings to print out and an optional seed number.
# For example, 'python otp.py 20' will print out 20 otp strings.
# 
# You may pass in a function to the constructor.  This function takes
# an integer parameter (defaults to 0) and must return a string.
# This string is hashed with the MD5 algorithm and the hex
# representation of the hash is returned.  If you do not pass in a
# string generating function, an internal function is used that is
# based on the whrandom module.
# 
# Once you have constructed an Otp object, call the Get() method to
# return an OTP string.  The Get() method can have an integer
# parameter that is passed to the str_function() function.  For the
# default function (GenerateString()), if the seed is nonzero, the
# Wichmann-Hill generator is started over and initialized from the
# seed.
# 
# Each call to Get generates a new random string, sends it to
# md5.update(), (which appends it to its own internal copy of all the
# strings it's been sent), and then a new md5 hash is gotten, which is
# converted to the hex representation with 32 hex characters.
# 
# This module could be made to provide cryptographically secure
# one time pads by substituting a cryptographic quality random
# number generator for the whrandom object.  You can go out on
# the web and search for "random number" and find some hardware
# devices to do this.
# 
# Copyright (C) 2002 GDS Software
# 
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License as
# published by the Free Software Foundation; either version 2 of
# the License, or (at your option) any later version.
# 
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
# 
# You should have received a copy of the GNU General Public
# License along with this program; if not, write to the Free
# Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
# MA  02111-1307  USA
# 
# See http://www.gnu.org/licenses/licenses.html for more details.
# 
import md5, whrandom
__version__ = "$Id: otp.py,v 1.3 2002/08/21 12:41:49 donp Exp $"

whG = whrandom.whrandom()

##
# Generate a string from a four byte integer.  The string is the
#     4 bytes of the integer, each converted to a character.
#     
def GenerateString(seed = 0):
    global whG
    if seed:      # seed != 0 means to restart; whrandom seeds from time.
        whG.seed(seed & 0xff, 
                 (seed & 0xff00) >> 8, 
                 (seed & 0xff0000) >> 16)
    n = whG.randint(0, 2**30-1)
    str = ""
    str = str + chr((n & 0xFF000000) >> 24)
    str = str + chr((n & 0x00FF0000) >> 16)
    str = str + chr((n & 0x0000FF00) >>  8)
    str = str + chr((n & 0x000000FF) >>  0)
    return str

class Otp:
    def __init__(self, str_function = GenerateString, seed = 0):
        str_function(seed) # Initialize random number generator
        self.m = md5.new()

    ##
    # Return an OTP.
    #         
    def Get(self, seed=0):
        self.m.update(GenerateString(seed))
        string = self.m.digest()
        str = ""
        for ix in xrange(len(string)):
            str = str + "%02X" % ord(string[ix])
        return str

if __name__ == "__main__":
    import sys
    num = 1
    seed = 0
    if len(sys.argv) < 2:
        print "Usage:  otp num_times [seed]"
        sys.exit(1)
    num = int(sys.argv[1])
    if len(sys.argv) == 3:
        seed = int(sys.argv[2])
    o = Otp(seed=seed)
    for ix in xrange(num):
        print o.Get()