src/common/crypto.py

   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 Crypto.Hash import 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.new()
  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
 107 # vim: se ts=3: