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Filename : cSHAKE128.py
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# ===================================================================
#
# Copyright (c) 2021, Legrandin <helderijs@gmail.com>
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# modification, are permitted provided that the following conditions
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# 1. Redistributions of source code must retain the above copyright
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from Crypto.Util.py3compat import bchr

from Crypto.Util._raw_api import (VoidPointer, SmartPointer,
                                  create_string_buffer,
                                  get_raw_buffer, c_size_t,
                                  c_uint8_ptr, c_ubyte)

from Crypto.Util.number import long_to_bytes

from Crypto.Hash.keccak import _raw_keccak_lib


def _left_encode(x):
    """Left encode function as defined in NIST SP 800-185"""

    assert (x < (1 << 2040) and x >= 0)

    # Get number of bytes needed to represent this integer.
    num = 1 if x == 0 else (x.bit_length() + 7) // 8

    return bchr(num) + long_to_bytes(x)


def _right_encode(x):
    """Right encode function as defined in NIST SP 800-185"""

    assert (x < (1 << 2040) and x >= 0)

    # Get number of bytes needed to represent this integer.
    num = 1 if x == 0 else (x.bit_length() + 7) // 8

    return long_to_bytes(x) + bchr(num)


def _encode_str(x):
    """Encode string function as defined in NIST SP 800-185"""

    bitlen = len(x) * 8
    if bitlen >= (1 << 2040):
        raise ValueError("String too large to encode in cSHAKE")

    return _left_encode(bitlen) + x


def _bytepad(x, length):
    """Zero pad byte string as defined in NIST SP 800-185"""

    to_pad = _left_encode(length) + x

    # Note: this implementation works with byte aligned strings,
    # hence no additional bit padding is needed at this point.
    npad = (length - len(to_pad) % length) % length

    return to_pad + b'\x00' * npad


class cSHAKE_XOF(object):
    """A cSHAKE hash object.
    Do not instantiate directly.
    Use the :func:`new` function.
    """

    def __init__(self, data, custom, capacity, function):
        state = VoidPointer()

        if custom or function:
            prefix_unpad = _encode_str(function) + _encode_str(custom)
            prefix = _bytepad(prefix_unpad, (1600 - capacity)//8)
            self._padding = 0x04
        else:
            prefix = None
            self._padding = 0x1F  # for SHAKE

        result = _raw_keccak_lib.keccak_init(state.address_of(),
                                             c_size_t(capacity//8),
                                             c_ubyte(24))
        if result:
            raise ValueError("Error %d while instantiating cSHAKE"
                             % result)
        self._state = SmartPointer(state.get(),
                                   _raw_keccak_lib.keccak_destroy)
        self._is_squeezing = False

        if prefix:
            self.update(prefix)

        if data:
            self.update(data)

    def update(self, data):
        """Continue hashing of a message by consuming the next chunk of data.

        Args:
            data (byte string/byte array/memoryview): The next chunk of the message being hashed.
        """

        if self._is_squeezing:
            raise TypeError("You cannot call 'update' after the first 'read'")

        result = _raw_keccak_lib.keccak_absorb(self._state.get(),
                                               c_uint8_ptr(data),
                                               c_size_t(len(data)))
        if result:
            raise ValueError("Error %d while updating %s state"
                             % (result, self.name))
        return self

    def read(self, length):
        """
        Compute the next piece of XOF output.

        .. note::
            You cannot use :meth:`update` anymore after the first call to
            :meth:`read`.

        Args:
            length (integer): the amount of bytes this method must return

        :return: the next piece of XOF output (of the given length)
        :rtype: byte string
        """

        self._is_squeezing = True
        bfr = create_string_buffer(length)
        result = _raw_keccak_lib.keccak_squeeze(self._state.get(),
                                                bfr,
                                                c_size_t(length),
                                                c_ubyte(self._padding))
        if result:
            raise ValueError("Error %d while extracting from %s"
                             % (result, self.name))

        return get_raw_buffer(bfr)


def _new(data, custom, function):
    # Use Keccak[256]
    return cSHAKE_XOF(data, custom, 256, function)


def new(data=None, custom=None):
    """Return a fresh instance of a cSHAKE128 object.

    Args:
       data (bytes/bytearray/memoryview):
        Optional.
        The very first chunk of the message to hash.
        It is equivalent to an early call to :meth:`update`.
       custom (bytes):
        Optional.
        A customization bytestring (``S`` in SP 800-185).

    :Return: A :class:`cSHAKE_XOF` object
    """

    # Use Keccak[256]
    return cSHAKE_XOF(data, custom, 256, b'')