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# Copyright (c) 2014, Legrandin <helderijs@gmail.com>
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from Cryptodome.Random import get_random_bytes

from Cryptodome.Util.py3compat import _copy_bytes
from Cryptodome.Util._raw_api import (load_pycryptodome_raw_lib,
                                  create_string_buffer,
                                  get_raw_buffer, VoidPointer,
                                  SmartPointer, c_size_t,
                                  c_uint8_ptr, c_ulong,
                                  is_writeable_buffer)

_raw_chacha20_lib = load_pycryptodome_raw_lib("Cryptodome.Cipher._chacha20",
                    """
                    int chacha20_init(void **pState,
                                      const uint8_t *key,
                                      size_t keySize,
                                      const uint8_t *nonce,
                                      size_t nonceSize);

                    int chacha20_destroy(void *state);

                    int chacha20_encrypt(void *state,
                                         const uint8_t in[],
                                         uint8_t out[],
                                         size_t len);

                    int chacha20_seek(void *state,
                                      unsigned long block_high,
                                      unsigned long block_low,
                                      unsigned offset);
                    int hchacha20(  const uint8_t key[32],
                                    const uint8_t nonce16[16],
                                    uint8_t subkey[32]);
                    """)


def _HChaCha20(key, nonce):

    assert(len(key) == 32)
    assert(len(nonce) == 16)

    subkey = bytearray(32)
    result = _raw_chacha20_lib.hchacha20(
                c_uint8_ptr(key),
                c_uint8_ptr(nonce),
                c_uint8_ptr(subkey))
    if result:
        raise ValueError("Error %d when deriving subkey with HChaCha20" % result)

    return subkey


class ChaCha20Cipher(object):
    """ChaCha20 (or XChaCha20) cipher object.
    Do not create it directly. Use :py:func:`new` instead.

    :var nonce: The nonce with length 8, 12 or 24 bytes
    :vartype nonce: bytes
    """

    block_size = 1

    def __init__(self, key, nonce):
        """Initialize a ChaCha20/XChaCha20 cipher object

        See also `new()` at the module level."""

        # XChaCha20 requires a key derivation with HChaCha20
        # See 2.3 in https://tools.ietf.org/html/draft-arciszewski-xchacha-03
        if len(nonce) == 24:
            key = _HChaCha20(key, nonce[:16])
            nonce = b'\x00' * 4 + nonce[16:]
            self._name = "XChaCha20"
        else:
            self._name = "ChaCha20"

        self.nonce = _copy_bytes(None, None, nonce)

        self._next = ( self.encrypt, self.decrypt )

        self._state = VoidPointer()
        result = _raw_chacha20_lib.chacha20_init(
                        self._state.address_of(),
                        c_uint8_ptr(key),
                        c_size_t(len(key)),
                        self.nonce,
                        c_size_t(len(nonce)))
        if result:
            raise ValueError("Error %d instantiating a %s cipher" % (result,
                                                                     self._name))
        self._state = SmartPointer(self._state.get(),
                                   _raw_chacha20_lib.chacha20_destroy)

    def encrypt(self, plaintext, output=None):
        """Encrypt a piece of data.

        Args:
          plaintext(bytes/bytearray/memoryview): The data to encrypt, of any size.
        Keyword Args:
          output(bytes/bytearray/memoryview): The location where the ciphertext
            is written to. If ``None``, the ciphertext is returned.
        Returns:
          If ``output`` is ``None``, the ciphertext is returned as ``bytes``.
          Otherwise, ``None``.
        """

        if self.encrypt not in self._next:
            raise TypeError("Cipher object can only be used for decryption")
        self._next = ( self.encrypt, )
        return self._encrypt(plaintext, output)

    def _encrypt(self, plaintext, output):
        """Encrypt without FSM checks"""

        if output is None:
            ciphertext = create_string_buffer(len(plaintext))
        else:
            ciphertext = output

            if not is_writeable_buffer(output):
                raise TypeError("output must be a bytearray or a writeable memoryview")

            if len(plaintext) != len(output):
                raise ValueError("output must have the same length as the input"
                                 "  (%d bytes)" % len(plaintext))

        result = _raw_chacha20_lib.chacha20_encrypt(
                                         self._state.get(),
                                         c_uint8_ptr(plaintext),
                                         c_uint8_ptr(ciphertext),
                                         c_size_t(len(plaintext)))
        if result:
            raise ValueError("Error %d while encrypting with %s" % (result, self._name))

        if output is None:
            return get_raw_buffer(ciphertext)
        else:
            return None

    def decrypt(self, ciphertext, output=None):