86 lines
2.4 KiB
Python
86 lines
2.4 KiB
Python
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# -*- coding: utf-8 -*-
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#
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# Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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'''Functions for generating random numbers.'''
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# Source inspired by code by Yesudeep Mangalapilly <yesudeep@gmail.com>
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import os
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from rsa import common, transform
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from rsa._compat import byte
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def read_random_bits(nbits):
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'''Reads 'nbits' random bits.
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If nbits isn't a whole number of bytes, an extra byte will be appended with
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only the lower bits set.
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'''
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nbytes, rbits = divmod(nbits, 8)
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# Get the random bytes
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randomdata = os.urandom(nbytes)
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# Add the remaining random bits
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if rbits > 0:
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randomvalue = ord(os.urandom(1))
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randomvalue >>= (8 - rbits)
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randomdata = byte(randomvalue) + randomdata
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return randomdata
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def read_random_int(nbits):
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'''Reads a random integer of approximately nbits bits.
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'''
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randomdata = read_random_bits(nbits)
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value = transform.bytes2int(randomdata)
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# Ensure that the number is large enough to just fill out the required
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# number of bits.
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value |= 1 << (nbits - 1)
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return value
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def randint(maxvalue):
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'''Returns a random integer x with 1 <= x <= maxvalue
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May take a very long time in specific situations. If maxvalue needs N bits
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to store, the closer maxvalue is to (2 ** N) - 1, the faster this function
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is.
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'''
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bit_size = common.bit_size(maxvalue)
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tries = 0
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while True:
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value = read_random_int(bit_size)
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if value <= maxvalue:
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break
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if tries and tries % 10 == 0:
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# After a lot of tries to get the right number of bits but still
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# smaller than maxvalue, decrease the number of bits by 1. That'll
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# dramatically increase the chances to get a large enough number.
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bit_size -= 1
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tries += 1
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return value
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