Telethon/telethon/_crypto/cdndecrypter.py

"""
This module holds the CdnDecrypter utility class.
"""
from hashlib import sha256

from .. import _tl
from ..crypto import AESModeCTR
from ..errors import CdnFileTamperedError


class CdnDecrypter:
    """
    Used when downloading a file results in a 'FileCdnRedirect' to
    both prepare the redirect, decrypt the file as it downloads, and
    ensure the file hasn't been tampered. https://core.telegram.org/cdn
    """
    def __init__(self, cdn_client, file_token, cdn_aes, cdn_file_hashes):
        """
        Initializes the CDN decrypter.

        :param cdn_client: a client connected to a CDN.
        :param file_token: the token of the file to be used.
        :param cdn_aes: the AES CTR used to decrypt the file.
        :param cdn_file_hashes: the hashes the decrypted file must match.
        """
        self.client = cdn_client
        self.file_token = file_token
        self.cdn_aes = cdn_aes
        self.cdn_file_hashes = cdn_file_hashes

    @staticmethod
    async def prepare_decrypter(client, cdn_client, cdn_redirect):
        """
        Prepares a new CDN decrypter.

        :param client: a TelegramClient connected to the main servers.
        :param cdn_client: a new client connected to the CDN.
        :param cdn_redirect: the redirect file object that caused this call.
        :return: (CdnDecrypter, first chunk file data)
        """
        cdn_aes = AESModeCTR(
            key=cdn_redirect.encryption_key,
            # 12 first bytes of the IV..4 bytes of the offset (0, big endian)
            iv=cdn_redirect.encryption_iv[:12] + bytes(4)
        )

        # We assume that cdn_redirect.cdn_file_hashes are ordered by offset,
        # and that there will be enough of these to retrieve the whole file.
        decrypter = CdnDecrypter(
            cdn_client, cdn_redirect.file_token,
            cdn_aes, cdn_redirect.cdn_file_hashes
        )

        cdn_file = await cdn_client(_tl.fn.upload.GetCdnFile(
            file_token=cdn_redirect.file_token,
            offset=cdn_redirect.cdn_file_hashes[0].offset,
            limit=cdn_redirect.cdn_file_hashes[0].limit
        ))
        if isinstance(cdn_file, _tl.upload.CdnFileReuploadNeeded):
            # We need to use the original client here
            await client(_tl.fn.upload.ReuploadCdnFile(
                file_token=cdn_redirect.file_token,
                request_token=cdn_file.request_token
            ))

            # We want to always return a valid upload.CdnFile
            cdn_file = decrypter.get_file()
        else:
            cdn_file.bytes = decrypter.cdn_aes.encrypt(cdn_file.bytes)
            cdn_hash = decrypter.cdn_file_hashes.pop(0)
            decrypter.check(cdn_file.bytes, cdn_hash)

        return decrypter, cdn_file

    def get_file(self):
        """
        Calls GetCdnFileRequest and decrypts its bytes.
        Also ensures that the file hasn't been tampered.

        :return: the CdnFile result.
        """
        if self.cdn_file_hashes:
            cdn_hash = self.cdn_file_hashes.pop(0)
            cdn_file = self.client(_tl.fn.upload.GetCdnFile(
                self.file_token, cdn_hash.offset, cdn_hash.limit
            ))
            cdn_file.bytes = self.cdn_aes.encrypt(cdn_file.bytes)
            self.check(cdn_file.bytes, cdn_hash)
        else:
            cdn_file = _tl.upload.CdnFile(bytes(0))

        return cdn_file

    @staticmethod
    def check(data, cdn_hash):
        """
        Checks the integrity of the given data.
        Raises CdnFileTamperedError if the integrity check fails.

        :param data: the data to be hashed.
        :param cdn_hash: the expected hash.
        """
        if sha256(data).digest() != cdn_hash.hash:
            raise CdnFileTamperedError()