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[EDIT] FILE: _IntegerCustom.py

# ===================================================================
#
# Copyright (c) 2018, Helder Eijs <helderijs@gmail.com>
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions
# are met:
#
# 1. Redistributions of source code must retain the above copyright
#    notice, this list of conditions and the following disclaimer.
# 2. Redistributions in binary form must reproduce the above copyright
#    notice, this list of conditions and the following disclaimer in
#    the documentation and/or other materials provided with the
#    distribution.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
# COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
# BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
# ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
# POSSIBILITY OF SUCH DAMAGE.
# ===================================================================

from ._IntegerNative import IntegerNative

from Crypto.Util.number import long_to_bytes, bytes_to_long

from Crypto.Util._raw_api import (load_pycryptodome_raw_lib,
                                  create_string_buffer,
                                  get_raw_buffer, backend,
                                  c_size_t, c_ulonglong)

from Crypto.Random.random import getrandbits

c_defs = """
int monty_pow(const uint8_t *base,
               const uint8_t *exp,
               const uint8_t *modulus,
               uint8_t       *out,
               size_t len,
               uint64_t seed);
"""

_raw_montgomery = load_pycryptodome_raw_lib("Crypto.Math._modexp", c_defs)
implementation = {"library": "custom", "api": backend}

class IntegerCustom(IntegerNative):

@staticmethod
    def from_bytes(byte_string, byteorder='big'):
        if byteorder == 'big':
            pass
        elif byteorder == 'little':
            byte_string = bytearray(byte_string)
            byte_string.reverse()
        else:
            raise ValueError("Incorrect byteorder")
        return IntegerCustom(bytes_to_long(byte_string))

def inplace_pow(self, exponent, modulus=None):
        exp_value = int(exponent)
        if exp_value < 0:
            raise ValueError("Exponent must not be negative")

# No modular reduction
        if modulus is None:
            self._value = pow(self._value, exp_value)
            return self

# With modular reduction
        mod_value = int(modulus)
        if mod_value < 0:
            raise ValueError("Modulus must be positive")
        if mod_value == 0:
            raise ZeroDivisionError("Modulus cannot be zero")

# C extension only works with odd moduli
        if (mod_value & 1) == 0:
            self._value = pow(self._value, exp_value, mod_value)
            return self

# C extension only works with bases smaller than modulus
        if self._value >= mod_value:
            self._value %= mod_value

max_len = len(long_to_bytes(max(self._value, exp_value, mod_value)))

base_b = long_to_bytes(self._value, max_len)
        exp_b = long_to_bytes(exp_value, max_len)
        modulus_b = long_to_bytes(mod_value, max_len)

out = create_string_buffer(max_len)

error = _raw_montgomery.monty_pow(
                    out,
                    base_b,
                    exp_b,
                    modulus_b,
                    c_size_t(max_len),
                    c_ulonglong(getrandbits(64))
                    )

if error:
            raise ValueError("monty_pow failed with error: %d" % error)

result = bytes_to_long(get_raw_buffer(out))
        self._value = result
        return self

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