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Adjust RNG algorithm

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This commit is contained in:
RunasSudo 2021-05-23 01:31:54 +10:00
parent 86695aa67e
commit 5c185b386c
Signed by: RunasSudo
GPG Key ID: 7234E476BF21C61A
3 changed files with 7 additions and 5 deletions
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4
docs/rng.md
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@ -6,6 +6,8 @@ The algorithm takes a *seed* value, which is an arbitrary character string. The
In order to generate a value between 0 (inclusive) and *n* (exclusive), to the state is appended a comma (",") followed by the value of the counter, and a SHA-256 hash *H* is computed of the resulting string encoded using UTF-8. The hash *H* is represented as an unsigned hexadecimal integer, *k*. The counter is incremented by 1. In order to generate a value between 0 (inclusive) and *n* (exclusive), to the state is appended a comma (",") followed by the value of the counter, and a SHA-256 hash *H* is computed of the resulting string encoded using UTF-8. The hash *H* is represented as an unsigned hexadecimal integer, *k*. The counter is incremented by 1.
In order to avoid modulo bias, if *k* ≥ ⌊*M*/*n*⌋ × *n* (where *M* is the maximum possible value of *k*, 2^256 - 1), *k* is discarded and the algorithm is repeated. In order to avoid modulo bias, if *k* ≥ ⌊*M*/*n*⌋ × *n* (where *M* = 2^256), *k* is discarded and the algorithm is repeated.
Otherwise, the result is *k* modulo *n*. Otherwise, the result is *k* modulo *n*.
In revision [86695aa](https://yingtongli.me/git/pyRCV2/commit/?id=86695aa67e2af936a84d78cb7a7aed9844f61ca9) and older, a slightly different formula was in use to avoid modulo bias – see the documentation of previous revisions for details. It is theoretically possible, though overwhelmingly unlikely, that this may result in different outcomes.

4
pyRCV2/random/sharandom_js.py
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@ -1,5 +1,5 @@
# pyRCV2: Preferential vote counting # pyRCV2: Preferential vote counting
# Copyright © 2020 Lee Yingtong Li (RunasSudo) # Copyright © 2020–2021 Lee Yingtong Li (RunasSudo)
# #
# This program is free software: you can redistribute it and/or modify # This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as published by # it under the terms of the GNU Affero General Public License as published by
@ -15,7 +15,7 @@
# along with this program. If not, see <https://www.gnu.org/licenses/>. # along with this program. If not, see <https://www.gnu.org/licenses/>.
if __pragma__('js', '{}', 'typeof(bigInt)') != 'undefined': if __pragma__('js', '{}', 'typeof(bigInt)') != 'undefined':
_MAX_VAL = bigInt(2).pow(256).subtract(1) _MAX_VAL = bigInt(2).pow(256)
else: else:
# Fail gracefully if dependencies not present # Fail gracefully if dependencies not present
_MAX_VAL = None _MAX_VAL = None

4
pyRCV2/random/sharandom_py.py
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@ -1,5 +1,5 @@
# pyRCV2: Preferential vote counting # pyRCV2: Preferential vote counting
# Copyright © 2020 Lee Yingtong Li (RunasSudo) # Copyright © 2020–2021 Lee Yingtong Li (RunasSudo)
# #
# This program is free software: you can redistribute it and/or modify # This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as published by # it under the terms of the GNU Affero General Public License as published by
@ -17,7 +17,7 @@
import hashlib import hashlib
class SHARandom: class SHARandom:
MAX_VAL = 2 ** 256 - 1 MAX_VAL = 2 ** 256
def __init__(self, seed): def __init__(self, seed):
self.seed = seed self.seed = seed