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# https://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"""Functions for generating random numbers.""" 18 19# Source inspired by code by Yesudeep Mangalapilly <yesudeep@gmail.com> 20 21import os 22 23from rsa import common, transform 24from rsa._compat import byte 25 26 27def read_random_bits(nbits): 28 """Reads 'nbits' random bits. 29 30 If nbits isn't a whole number of bytes, an extra byte will be appended with 31 only the lower bits set. 32 """ 33 34 nbytes, rbits = divmod(nbits, 8) 35 36 # Get the random bytes 37 randomdata = os.urandom(nbytes) 38 39 # Add the remaining random bits 40 if rbits > 0: 41 randomvalue = ord(os.urandom(1)) 42 randomvalue >>= (8 - rbits) 43 randomdata = byte(randomvalue) + randomdata 44 45 return randomdata 46 47 48def read_random_int(nbits): 49 """Reads a random integer of approximately nbits bits. 50 """ 51 52 randomdata = read_random_bits(nbits) 53 value = transform.bytes2int(randomdata) 54 55 # Ensure that the number is large enough to just fill out the required 56 # number of bits. 57 value |= 1 << (nbits - 1) 58 59 return value 60 61 62def read_random_odd_int(nbits): 63 """Reads a random odd integer of approximately nbits bits. 64 65 >>> read_random_odd_int(512) & 1 66 1 67 """ 68 69 value = read_random_int(nbits) 70 71 # Make sure it's odd 72 return value | 1 73 74 75def randint(maxvalue): 76 """Returns a random integer x with 1 <= x <= maxvalue 77 78 May take a very long time in specific situations. If maxvalue needs N bits 79 to store, the closer maxvalue is to (2 ** N) - 1, the faster this function 80 is. 81 """ 82 83 bit_size = common.bit_size(maxvalue) 84 85 tries = 0 86 while True: 87 value = read_random_int(bit_size) 88 if value <= maxvalue: 89 break 90 91 if tries % 10 == 0 and tries: 92 # After a lot of tries to get the right number of bits but still 93 # smaller than maxvalue, decrease the number of bits by 1. That'll 94 # dramatically increase the chances to get a large enough number. 95 bit_size -= 1 96 tries += 1 97 98 return value 99