rsa_source/rsa/common.py

   1 # -*- coding: utf-8 -*-
   2 #
   3 #  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
   4 #
   5 #  Licensed under the Apache License, Version 2.0 (the "License");
   6 #  you may not use this file except in compliance with the License.
   7 #  You may obtain a copy of the License at
   8 #
   9 #      http://www.apache.org/licenses/LICENSE-2.0
  10 #
  11 #  Unless required by applicable law or agreed to in writing, software
  12 #  distributed under the License is distributed on an "AS IS" BASIS,
  13 #  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14 #  See the License for the specific language governing permissions and
  15 #  limitations under the License.
  16
  17 '''Common functionality shared by several modules.'''
  18
  19
  20 import math
  21
  22 def bit_size(number):
  23     '''Returns the number of bits required to hold a specific long number.
  24
  25     >>> bit_size(1023)
  26     10
  27     >>> bit_size(1024)
  28     11
  29     >>> bit_size(1025)
  30     11
  31
  32     >>> bit_size(1 << 1024)
  33     1025
  34     >>> bit_size((1 << 1024) + 1)
  35     1025
  36     >>> bit_size((1 << 1024) - 1)
  37     1024
  38
  39     '''
  40
  41     if number < 0:
  42         raise ValueError('Only nonnegative numbers possible: %s' % number)
  43
  44     if number == 0:
  45         return 1
  46
  47     # This works, even with very large numbers. When using math.log(number, 2),
  48     # you'll get rounding errors and it'll fail.
  49     bits = 0
  50     while number:
  51         bits += 1
  52         number >>= 1
  53
  54     return bits
  55
  56
  57 def byte_size(number):
  58     """Returns the number of bytes required to hold a specific long number.
  59
  60     The number of bytes is rounded up.
  61
  62     >>> byte_size(1 << 1023)
  63     128
  64     >>> byte_size((1 << 1024) - 1)
  65     128
  66     >>> byte_size(1 << 1024)
  67     129
  68     """
  69
  70     return int(math.ceil(bit_size(number) / 8.0))
  71
  72 if __name__ == '__main__':
  73     import doctest
  74     doctest.testmod()
  75