Source Code for Module common.crypto

  1  # common crypto functions (mostly specific to XEP-0116, but useful elsewhere) 
  2  # -*- coding:utf-8 -*- 
  3  ## src/common/crypto.py 
  4  ## 
  5  ## Copyright (C) 2007 Brendan Taylor <whateley AT gmail.com> 
  6  ## 
  7  ## This file is part of Gajim. 
  8  ## 
  9  ## Gajim is free software; you can redistribute it and/or modify 
 10  ## it under the terms of the GNU General Public License as published 
 11  ## by the Free Software Foundation; version 3 only. 
 12  ## 
 13  ## Gajim is distributed in the hope that it will be useful, 
 14  ## but WITHOUT ANY WARRANTY; without even the implied warranty of 
 15  ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 
 16  ## GNU General Public License for more details. 
 17  ## 
 18  ## You should have received a copy of the GNU General Public License 
 19  ## along with Gajim. If not, see <http://www.gnu.org/licenses/>. 
 20  ## 
 21   
 22  import os 
 23  import math 
 24   
 25  from hashlib import sha256 as SHA256 
 26   
 27  # convert a large integer to a big-endian bitstring 
 28 -def encode_mpi(n): 
 29      if n >= 256: 
 30          return encode_mpi(n / 256) + chr(n % 256) 
 31      else: 
 32          return chr(n) 
 33   
 34  # convert a large integer to a big-endian bitstring, padded with \x00s to 
 35  # a multiple of 16 bytes 
 36 -def encode_mpi_with_padding(n): 
 37      return pad_to_multiple(encode_mpi(n), 16, '\x00', True) 
 38   
 39  # pad 'string' to a multiple of 'multiple_of' with 'char'. 
 40  # pad on the left if 'left', otherwise pad on the right. 
 41 -def pad_to_multiple(string, multiple_of, char, left): 
 42      mod = len(string) % multiple_of 
 43      if mod == 0: 
 44          return string 
 45      else: 
 46          padding = (multiple_of - mod) * char 
 47   
 48      if left: 
 49          return padding + string 
 50      else: 
 51          return string + padding 
 52   
 53  # convert a big-endian bitstring to an integer 
 54 -def decode_mpi(s): 
 55      if len(s) == 0: 
 56          return 0 
 57      else: 
 58          return 256 * decode_mpi(s[:-1]) + ord(s[-1]) 
 59   
 60 -def sha256(string): 
 61      sh = SHA256() 
 62      sh.update(string) 
 63      return sh.digest() 
 64   
 65  base28_chr = "acdefghikmopqruvwxy123456789" 
 66   
 67 -def sas_28x5(m_a, form_b): 
 68      sha = sha256(m_a + form_b + 'Short Authentication String') 
 69      lsb24 = decode_mpi(sha[-3:]) 
 70      return base28(lsb24) 
 71   
 72 -def base28(n): 
 73      if n >= 28: 
 74          return base28(n / 28) + base28_chr[n % 28] 
 75      else: 
 76          return base28_chr[n] 
 77   
 78 -def random_bytes(bytes_): 
 79      return os.urandom(bytes_) 
 80   
 81 -def generate_nonce(): 
 82      return random_bytes(8) 
 83   
 84  # generate a random number between 'bottom' and 'top' 
 85 -def srand(bottom, top): 
 86      # minimum number of bytes needed to represent that range 
 87      bytes = int(math.ceil(math.log(top - bottom, 256))) 
 88   
 89      # in retrospect, this is horribly inadequate. 
 90      return (decode_mpi(random_bytes(bytes)) % (top - bottom)) + bottom 
 91   
 92  # a faster version of (base ** exp) % mod 
 93  #               taken from <http://lists.danga.com/pipermail/yadis/2005-September/001445.html> 
 94 -def powmod(base, exp, mod): 
 95      square = base % mod 
 96      result = 1 
 97   
 98      while exp > 0: 
 99          if exp & 1: # exponent is odd 
100              result = (result * square) % mod 
101   
102          square = (square * square) % mod 
103          exp /= 2 
104   
105      return result 
106