| # -*- coding: utf-8 -*- |
| # |
| # 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/LICENSE-2.0 |
| # |
| # 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 |
| |
| |