86 lines
2.4 KiB
Python
86 lines
2.4 KiB
Python
# * coding: utf8 *


#


# Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>


#


# Licensed under the Apache License, Version 2.0 (the "License");


# you may not use this file except in compliance with the License.


# You may obtain a copy of the License at


#


# http://www.apache.org/licenses/LICENSE2.0


#


# Unless required by applicable law or agreed to in writing, software


# distributed under the License is distributed on an "AS IS" BASIS,


# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.


# See the License for the specific language governing permissions and


# limitations under the License.




'''Functions for generating random numbers.'''




# Source inspired by code by Yesudeep Mangalapilly <yesudeep@gmail.com>




import os




from rsa import common, transform


from rsa._compat import byte




def read_random_bits(nbits):


'''Reads 'nbits' random bits.




If nbits isn't a whole number of bytes, an extra byte will be appended with


only the lower bits set.


'''




nbytes, rbits = divmod(nbits, 8)




# Get the random bytes


randomdata = os.urandom(nbytes)




# Add the remaining random bits


if rbits > 0:


randomvalue = ord(os.urandom(1))


randomvalue >>= (8  rbits)


randomdata = byte(randomvalue) + randomdata




return randomdata






def read_random_int(nbits):


'''Reads a random integer of approximately nbits bits.


'''




randomdata = read_random_bits(nbits)


value = transform.bytes2int(randomdata)




# Ensure that the number is large enough to just fill out the required


# number of bits.


value = 1 << (nbits  1)




return value




def randint(maxvalue):


'''Returns a random integer x with 1 <= x <= maxvalue




May take a very long time in specific situations. If maxvalue needs N bits


to store, the closer maxvalue is to (2 ** N)  1, the faster this function


is.


'''




bit_size = common.bit_size(maxvalue)




tries = 0


while True:


value = read_random_int(bit_size)


if value <= maxvalue:


break




if tries and tries % 10 == 0:


# After a lot of tries to get the right number of bits but still


# smaller than maxvalue, decrease the number of bits by 1. That'll


# dramatically increase the chances to get a large enough number.


bit_size = 1


tries += 1




return value




