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Filename : entropy.py
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# Copyright (C) Dnspython Contributors, see LICENSE for text of ISC license

# Copyright (C) 2009-2017 Nominum, Inc.
#
# Permission to use, copy, modify, and distribute this software and its
# documentation for any purpose with or without fee is hereby granted,
# provided that the above copyright notice and this permission notice
# appear in all copies.
#
# THE SOFTWARE IS PROVIDED "AS IS" AND NOMINUM DISCLAIMS ALL WARRANTIES
# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL NOMINUM BE LIABLE FOR
# ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
# OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

import hashlib
import os
import random
import threading
import time
from typing import Any, Optional


class EntropyPool:
    # This is an entropy pool for Python implementations that do not
    # have a working SystemRandom.  I'm not sure there are any, but
    # leaving this code doesn't hurt anything as the library code
    # is used if present.

    def __init__(self, seed: Optional[bytes] = None):
        self.pool_index = 0
        self.digest: Optional[bytearray] = None
        self.next_byte = 0
        self.lock = threading.Lock()
        self.hash = hashlib.sha1()
        self.hash_len = 20
        self.pool = bytearray(b"\0" * self.hash_len)
        if seed is not None:
            self._stir(seed)
            self.seeded = True
            self.seed_pid = os.getpid()
        else:
            self.seeded = False
            self.seed_pid = 0

    def _stir(self, entropy: bytes) -> None:
        for c in entropy:
            if self.pool_index == self.hash_len:
                self.pool_index = 0
            b = c & 0xFF
            self.pool[self.pool_index] ^= b
            self.pool_index += 1

    def stir(self, entropy: bytes) -> None:
        with self.lock:
            self._stir(entropy)

    def _maybe_seed(self) -> None:
        if not self.seeded or self.seed_pid != os.getpid():
            try:
                seed = os.urandom(16)
            except Exception:  # pragma: no cover
                try:
                    with open("/dev/urandom", "rb", 0) as r:
                        seed = r.read(16)
                except Exception:
                    seed = str(time.time()).encode()
            self.seeded = True
            self.seed_pid = os.getpid()
            self.digest = None
            seed = bytearray(seed)
            self._stir(seed)

    def random_8(self) -> int:
        with self.lock:
            self._maybe_seed()
            if self.digest is None or self.next_byte == self.hash_len:
                self.hash.update(bytes(self.pool))
                self.digest = bytearray(self.hash.digest())
                self._stir(self.digest)
                self.next_byte = 0
            value = self.digest[self.next_byte]
            self.next_byte += 1
        return value

    def random_16(self) -> int:
        return self.random_8() * 256 + self.random_8()

    def random_32(self) -> int:
        return self.random_16() * 65536 + self.random_16()

    def random_between(self, first: int, last: int) -> int:
        size = last - first + 1
        if size > 4294967296:
            raise ValueError("too big")
        if size > 65536:
            rand = self.random_32
            max = 4294967295
        elif size > 256:
            rand = self.random_16
            max = 65535
        else:
            rand = self.random_8
            max = 255
        return first + size * rand() // (max + 1)


pool = EntropyPool()

system_random: Optional[Any]
try:
    system_random = random.SystemRandom()
except Exception:  # pragma: no cover
    system_random = None


def random_16() -> int:
    if system_random is not None:
        return system_random.randrange(0, 65536)
    else:
        return pool.random_16()


def between(first: int, last: int) -> int:
    if system_random is not None:
        return system_random.randrange(first, last + 1)
    else:
        return pool.random_between(first, last)