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choose_rounds.py
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"""cli helper for selecting appropriate <rounds> value for a given hash"""
#=============================================================================
# imports
#=============================================================================
# core
import math
import logging; log = logging.getLogger(__name__)
import sys
# site
# pkg
from passlib.registry import get_crypt_handler
from passlib.utils import tick
# local
__all__ = [
"main",
]
#=============================================================================
# main
#=============================================================================
_usage = "usage: python choose_rounds.py <hash_name> [<target_milliseconds>] [<backend>]\n"
def main(*args):
#---------------------------------------------------------------
# parse args
#---------------------------------------------------------------
args = list(args)
def print_error(msg):
print("error: %s\n" % msg)
# parse hasher
if args:
name = args.pop(0)
if name == "-h" or name == "--help":
print(_usage)
return 1
try:
hasher = get_crypt_handler(name)
except KeyError:
print_error("unknown hash %r" % name)
return 1
if 'rounds' not in hasher.setting_kwds:
print_error("%s does not support variable rounds" % name)
return 1
else:
print_error("hash name not specified")
print(_usage)
return 1
# parse target time
if args:
try:
target = int(args.pop(0))*.001
if target <= 0:
raise ValueError
except ValueError:
print_error("target time must be integer milliseconds > 0")
return 1
else:
target = .350
# parse backend
if args:
backend = args.pop(0)
if hasattr(hasher, "set_backend"):
hasher.set_backend(backend)
else:
print_error("%s does not support multiple backends")
return 1
#---------------------------------------------------------------
# setup some helper functions
#---------------------------------------------------------------
if hasher.rounds_cost == "log2":
# time cost varies logarithmically with rounds parameter,
# so speed = (2**rounds) / elapsed
def rounds_to_cost(rounds):
return 2 ** rounds
def cost_to_rounds(cost):
return math.log(cost, 2)
else:
# time cost varies linearly with rounds parameter,
# so speed = rounds / elapsed
assert hasher.rounds_cost == "linear"
rounds_to_cost = cost_to_rounds = lambda value: value
def clamp_rounds(rounds):
"""convert float rounds to int value, clamped to hasher's limits"""
if hasher.max_rounds and rounds > hasher.max_rounds:
rounds = hasher.max_rounds
rounds = int(rounds)
if getattr(hasher, "_avoid_even_rounds", False):
rounds |= 1
return max(hasher.min_rounds, rounds)
def average(seq):
if not hasattr(seq, "__length__"):
seq = tuple(seq)
return sum(seq) / len(seq)
def estimate_speed(rounds):
"""estimate speed using specified # of rounds"""
# time a single verify() call
secret = "S0m3-S3Kr1T"
hash = hasher.using(rounds=rounds).hash(secret)
def helper():
start = tick()
hasher.verify(secret, hash)
return tick() - start
# try to get average time over a few samples
# XXX: way too much variability between sampling runs,
# would like to improve this bit
elapsed = min(average(helper() for _ in range(4)) for _ in range(4))
return rounds_to_cost(rounds) / elapsed
#---------------------------------------------------------------
# get rough estimate of speed using fraction of default_rounds
# (so we don't take crazy long amounts of time on slow systems)
#---------------------------------------------------------------
rounds = clamp_rounds(cost_to_rounds(.5 * rounds_to_cost(hasher.default_rounds)))
speed = estimate_speed(rounds)
#---------------------------------------------------------------
# re-do estimate using previous result,
# to get more accurate sample using a larger number of rounds.
#---------------------------------------------------------------
for _ in range(2):
rounds = clamp_rounds(cost_to_rounds(speed * target))
speed = estimate_speed(rounds)
#---------------------------------------------------------------
# using final estimate, calc desired number of rounds for target time
#---------------------------------------------------------------
if hasattr(hasher, "backends"):
name = "%s (using %s backend)" % (name, hasher.get_backend())
print("hash............: %s" % name)
if speed < 1000:
speedstr = "%.2f" % speed
else:
speedstr = int(speed)
print("speed...........: %s iterations/second" % speedstr)
print("target time.....: %d ms" % (target*1000,))
rounds = cost_to_rounds(speed * target)
if hasher.rounds_cost == "log2":
# for log2 rounds parameter, target time will usually fall
# somewhere between two integer values, which will have large gulf
# between them. if target is within <tolerance> percent of
# one of two ends, report it, otherwise list both and let user decide.
tolerance = .05
lower = clamp_rounds(rounds)
upper = clamp_rounds(math.ceil(rounds))
lower_elapsed = rounds_to_cost(lower) / speed
upper_elapsed = rounds_to_cost(upper) / speed
if (target-lower_elapsed)/target < tolerance:
print("target rounds...: %d" % lower)
elif (upper_elapsed-target)/target < tolerance:
print("target rounds...: %d" % upper)
else:
faster = (target - lower_elapsed)
prin("target rounds...: %d (%dms -- %dms/%d%% faster than requested)" % \
(lower, lower_elapsed*1000, faster * 1000, round(100 * faster / target)))
slower = (upper_elapsed - target)
print("target rounds...: %d (%dms -- %dms/%d%% slower than requested)" % \
(upper, upper_elapsed*1000, slower * 1000, round(100 * slower / target)))
else:
# for linear rounds parameter, just use nearest integer value
rounds = clamp_rounds(round(rounds))
print("target rounds...: %d" % (rounds,))
print()
if __name__ == "__main__":
sys.exit(main(*sys.argv[1:]))
#=============================================================================
# eof
#=============================================================================