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author | herbertxue <herbertxue@google.com> | 2018-06-04 02:50:00 +0000 |
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committer | herbertxue <herbertxue@google.com> | 2018-06-12 06:38:20 +0000 |
commit | d8ac8301abc7ccb1a099ceb7e4d435ea55bf4a46 (patch) | |
tree | 75545e9a8e9a5fce6df78d519c65628720ec47b5 /rsa/randnum.py | |
parent | dbe61a7ce8c6b3a1ffbf632590ea05c61231b687 (diff) | |
parent | fd70d79610ac1af8b072aa27fadf660b4a64797c (diff) | |
download | rsa-d8ac8301abc7ccb1a099ceb7e4d435ea55bf4a46.tar.gz |
Merge commit 'fd70d79' into rsa_3.4.2android-p-preview-5android-p-preview-4
Inital commit of rsa 3.4.2 with history
Added:
- Android.bp
- MODULE_LICENSE_APACHE2
- NOTICE
- METADATA
Bug: b/80314772
Test: Complied acloud with rsa and was able to import rsa.
Change-Id: Iec6de78cc16267eac9d5f999728a1797c20eb8e2
Diffstat (limited to 'rsa/randnum.py')
-rw-r--r-- | rsa/randnum.py | 98 |
1 files changed, 98 insertions, 0 deletions
diff --git a/rsa/randnum.py b/rsa/randnum.py new file mode 100644 index 0000000..3c788a5 --- /dev/null +++ b/rsa/randnum.py @@ -0,0 +1,98 @@ +# -*- 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 +# +# https://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 read_random_odd_int(nbits): + """Reads a random odd integer of approximately nbits bits. + + >>> read_random_odd_int(512) & 1 + 1 + """ + + value = read_random_int(nbits) + + # Make sure it's odd + return value | 1 + + +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 |