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105 lines
3.7 KiB
Python
105 lines
3.7 KiB
Python
"""
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This module holds the CdnDecrypter utility class.
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"""
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from hashlib import sha256
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from .. import _tl
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from .._crypto import AESModeCTR
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from ..errors import CdnFileTamperedError
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class CdnDecrypter:
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"""
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Used when downloading a file results in a 'FileCdnRedirect' to
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both prepare the redirect, decrypt the file as it downloads, and
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ensure the file hasn't been tampered. https://core.telegram.org/cdn
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"""
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def __init__(self, cdn_client, file_token, cdn_aes, cdn_file_hashes):
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"""
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Initializes the CDN decrypter.
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:param cdn_client: a client connected to a CDN.
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:param file_token: the token of the file to be used.
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:param cdn_aes: the AES CTR used to decrypt the file.
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:param cdn_file_hashes: the hashes the decrypted file must match.
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"""
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self.client = cdn_client
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self.file_token = file_token
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self.cdn_aes = cdn_aes
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self.cdn_file_hashes = cdn_file_hashes
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@staticmethod
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async def prepare_decrypter(client, cdn_client, cdn_redirect):
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"""
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Prepares a new CDN decrypter.
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:param client: a TelegramClient connected to the main servers.
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:param cdn_client: a new client connected to the CDN.
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:param cdn_redirect: the redirect file object that caused this call.
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:return: (CdnDecrypter, first chunk file data)
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"""
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cdn_aes = AESModeCTR(
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key=cdn_redirect.encryption_key,
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# 12 first bytes of the IV..4 bytes of the offset (0, big endian)
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iv=cdn_redirect.encryption_iv[:12] + bytes(4)
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)
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# We assume that cdn_redirect.cdn_file_hashes are ordered by offset,
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# and that there will be enough of these to retrieve the whole file.
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decrypter = CdnDecrypter(
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cdn_client, cdn_redirect.file_token,
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cdn_aes, cdn_redirect.cdn_file_hashes
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)
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cdn_file = await cdn_client(_tl.fn.upload.GetCdnFile(
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file_token=cdn_redirect.file_token,
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offset=cdn_redirect.cdn_file_hashes[0].offset,
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limit=cdn_redirect.cdn_file_hashes[0].limit
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))
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if isinstance(cdn_file, _tl.upload.CdnFileReuploadNeeded):
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# We need to use the original client here
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await client(_tl.fn.upload.ReuploadCdnFile(
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file_token=cdn_redirect.file_token,
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request_token=cdn_file.request_token
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))
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# We want to always return a valid upload.CdnFile
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cdn_file = decrypter.get_file()
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else:
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cdn_file.bytes = decrypter.cdn_aes.encrypt(cdn_file.bytes)
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cdn_hash = decrypter.cdn_file_hashes.pop(0)
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decrypter.check(cdn_file.bytes, cdn_hash)
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return decrypter, cdn_file
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def get_file(self):
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"""
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Calls GetCdnFile and decrypts its bytes.
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Also ensures that the file hasn't been tampered.
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:return: the CdnFile result.
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"""
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if self.cdn_file_hashes:
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cdn_hash = self.cdn_file_hashes.pop(0)
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cdn_file = self.client(_tl.fn.upload.GetCdnFile(
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self.file_token, cdn_hash.offset, cdn_hash.limit
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))
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cdn_file.bytes = self.cdn_aes.encrypt(cdn_file.bytes)
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self.check(cdn_file.bytes, cdn_hash)
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else:
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cdn_file = _tl.upload.CdnFile(bytes(0))
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return cdn_file
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@staticmethod
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def check(data, cdn_hash):
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"""
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Checks the integrity of the given data.
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Raises CdnFileTamperedError if the integrity check fails.
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:param data: the data to be hashed.
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:param cdn_hash: the expected hash.
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"""
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if sha256(data).digest() != cdn_hash.hash:
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raise CdnFileTamperedError()
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