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'''Functions for generating random numbers.'''
# Source inspired by code by Yesudeep Mangalapilly <yesudeep@gmail.com>
import os
from rsa import common, transform
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 = chr(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
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