#!/usr/bin/env python3

# SAT plugin for handling AES-GCM file encryption
# Copyright (C) 2009-2020 Jérôme Poisson (goffi@goffi.org)

# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.

# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Affero General Public License for more details.

# You should have received a copy of the GNU Affero General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

from textwrap import dedent
from functools import partial
import secrets
from cryptography.hazmat.primitives import ciphers
from cryptography.hazmat.primitives.ciphers import modes
from cryptography.hazmat import backends
from cryptography.exceptions import AlreadyFinalized
from urllib import parse
import treq
from sat.core.i18n import _
from sat.core.constants import Const as C
from sat.core import exceptions
from sat.tools import stream
from sat.core.log import getLogger

log = getLogger(__name__)

PLUGIN_INFO = {
    C.PI_NAME: "AES-GCM",
    C.PI_IMPORT_NAME: "AES-GCM",
    C.PI_TYPE: "SEC",
    C.PI_PROTOCOLS: ["OMEMO Media sharing"],
    C.PI_DEPENDENCIES: ["XEP-0363", "XEP-0384", "DOWNLOAD"],
    C.PI_MAIN: "AESGCM",
    C.PI_HANDLER: "no",
    C.PI_DESCRIPTION: dedent(_("""\
    Implementation of AES-GCM scheme, a way to encrypt files (not official XMPP standard).
    See https://xmpp.org/extensions/inbox/omemo-media-sharing.html for details
    """)),
}


class AESGCM(object):

    def __init__(self, host):
        self.host = host
        log.info(_("AESGCM plugin initialization"))
        host.plugins["DOWNLOAD"].registerScheme(
            "aesgcm", self.download
        )
        host.trigger.add("XEP-0363_upload_size", self._uploadSizeTrigger)
        host.trigger.add("XEP-0363_upload", self._uploadTrigger)

    async def download(self, client, uri_parsed, dest_path, options):
        fragment = bytes.fromhex(uri_parsed.fragment)

        # legacy method use 16 bits IV, but OMEMO media sharing published spec indicates
        # which is 12 bits IV (AES-GCM spec recommandation), so we have to determine
        # which size has been used.
        if len(fragment) == 48:
            iv_size = 16
        elif len(fragment) == 44:
            iv_size = 12
        else:
            raise ValueError(
                f"Invalid URL fragment, can't decrypt file at {uri_parsed.get_url()}")

        iv, key = fragment[:iv_size], fragment[iv_size:]

        decryptor = ciphers.Cipher(
            ciphers.algorithms.AES(key),
            modes.GCM(iv),
            backend=backends.default_backend(),
        ).decryptor()

        download_url = parse.urlunparse(
            ('https', uri_parsed.netloc, uri_parsed.path, '', '', ''))

        head_data = await treq.head(download_url)
        content_length = int(head_data.headers.getRawHeaders('content-length')[0])
        # the 128 bits tag is put at the end
        file_size = content_length - 16

        file_obj = stream.SatFile(
            self.host,
            client,
            dest_path,
            mode="wb",
            size = file_size,
        )

        progress_id = file_obj.uid

        resp = await treq.get(download_url, unbuffered=True)
        d = treq.collect(resp, partial(
            self.onDataDownload,
            client=client,
            file_obj=file_obj,
            decryptor=decryptor))
        return progress_id, d

    def onDataDownload(self, data, client, file_obj, decryptor):
        if file_obj.tell() + len(data) > file_obj.size:
            # we're reaching end of file with this bunch of data
            # we may still have a last bunch if the tag is incomplete
            bytes_left = file_obj.size - file_obj.tell()
            if bytes_left > 0:
                decrypted = decryptor.update(data[:bytes_left])
                file_obj.write(decrypted)
                tag = data[bytes_left:]
            else:
                tag = data
            if len(tag) < 16:
                # the tag is incomplete, either we'll get the rest in next data bunch
                # or we have already the other part from last bunch of data
                try:
                    # we store partial tag in decryptor._sat_tag
                    tag = decryptor._sat_tag + tag
                except AttributeError:
                    # no other part, we'll get the rest at next bunch
                    decryptor.sat_tag = tag
                else:
                    # we have the complete tag, it must be 128 bits
                    if len(tag) != 16:
                        raise ValueError(f"Invalid tag: {tag}")
            remain = decryptor.finalize_with_tag(tag)
            file_obj.write(remain)
            file_obj.close()
        else:
            decrypted = decryptor.update(data)
            file_obj.write(decrypted)

    def _uploadSizeTrigger(self, client, options, file_path, size, size_adjust):
        if options.get('encryption') != C.ENC_AES_GCM:
            return True
        # the tag is appended to the file
        size_adjust.append(16)
        return True

    def _encrypt(self, data, encryptor):
        if data:
            return encryptor.update(data)
        else:
            try:
                # end of file is reached, me must finalize
                ret = encryptor.finalize()
                tag = encryptor.tag
                return ret + tag
            except AlreadyFinalized:
                # as we have already finalized, we can now send EOF
                return b''

    def _uploadTrigger(self, client, options, sat_file, file_producer, slot):
        if options.get('encryption') != C.ENC_AES_GCM:
            return True
        log.debug("encrypting file with AES-GCM")
        # specification talks about 12 bytes IV, but in practice and for legacy reasons
        # 16 bytes are used by most clients (and also in the specification example).
        # It seems that some clients don't handle 12 bytes IV (apparently,
        # that's the case for ChatSecure).
        # So we have to follow the de-facto standard and use 16 bytes to be sure
        # to be compatible with a maximum of clients.
        iv = secrets.token_bytes(16)
        key = secrets.token_bytes(32)
        fragment = f'{iv.hex()}{key.hex()}'
        ori_url = parse.urlparse(slot.get)
        # we change the get URL with the one with aesgcm scheme and containing the
        # encoded key + iv
        slot.get = parse.urlunparse(['aesgcm', *ori_url[1:5], fragment])

        # encrypted data size will be bigger than original file size
        # so we need to check with final data length to avoid a warning on close()
        sat_file.check_size_with_read = True

        # file_producer get length directly from file, and this cause trouble has
        # we have to change the size because of encryption. So we adapt it here,
        # else the producer would stop reading prematurely
        file_producer.length = sat_file.size

        encryptor = ciphers.Cipher(
            ciphers.algorithms.AES(key),
            modes.GCM(iv),
            backend=backends.default_backend(),
        ).encryptor()

        if sat_file.data_cb is not None:
            raise exceptions.InternalError(
                f"data_cb was expected to be None, it is set to {sat_file.data_cb}")

        # with data_cb we encrypt the file on the fly
        sat_file.data_cb = partial(self._encrypt, encryptor=encryptor)
        return True