Turn HashChecker into CdnDecrypter to abstract CDN-specific aspects

telethon/crypto/cdn_decrypter.py

@@ -0,0 +1,116 @@
+from hashlib import sha256
+import pyaes
+
+from ..tl import JsonSession
+from ..tl.functions.upload import GetCdnFileRequest, ReuploadCdnFileRequest
+from ..tl.types.upload import CdnFileReuploadNeeded
+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.
+    """
+    def __init__(self, cdn_client, file_token, cdn_aes, cdn_file_hashes):
+        self.client = cdn_client
+        self.file_token = file_token
+        self.cdn_aes = cdn_aes
+        self.cdn_file_hashes = cdn_file_hashes
+        self.shaes = [sha256() for _ in range(len(cdn_file_hashes))]
+
+    @staticmethod
+    def prepare_decrypter(client, client_cls, cdn_redirect, offset, part_size):
+        """Prepares a CDN decrypter, returning (decrypter, file data).
+           'client' should be the original TelegramBareClient that
+           tried to download the file.
+
+           'client_cls' should be the class of the TelegramBareClient.
+        """
+        # TODO Avoid the need for 'client_cls=TelegramBareClient'
+        # https://core.telegram.org/cdn
+        # TODO Use libssl if available
+        cdn_aes = pyaes.AESModeOfOperationCTR(cdn_redirect.encryption_key)
+
+        # The returned IV is the counter used on CTR
+        cdn_aes._counter._counter = list(
+            cdn_redirect.encryption_iv[:12] +
+            (offset >> 4).to_bytes(4, 'big')
+        )
+
+        # Create a new client on said CDN
+        dc = client._get_dc(cdn_redirect.dc_id, cdn=True)
+        session = JsonSession(client.session)
+        session.server_address = dc.ip_address
+        session.port = dc.port
+        cdn_client = client_cls(  # Avoid importing TelegramBareClient
+            session, client.api_id, client.api_hash,
+            timeout=client._timeout
+        )
+        # This will make use of the new RSA keys for this specific CDN
+        cdn_file = cdn_client.connect(initial_query=GetCdnFileRequest(
+            cdn_redirect.file_token, offset, part_size
+        ))
+
+        # CDN client is ready, create the resulting CdnDecrypter
+        decrypter = CdnDecrypter(
+            cdn_client, cdn_redirect.file_token,
+            cdn_aes, cdn_redirect.cdn_file_hashes
+        )
+
+        if isinstance(cdn_file, CdnFileReuploadNeeded):
+            # We need to use the original client here
+            client(ReuploadCdnFileRequest(
+                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(offset, part_size)
+        else:
+            cdn_file.bytes = decrypter.cdn_aes.encrypt(cdn_file.bytes)
+            decrypter.check(offset, cdn_file.bytes)
+
+        return decrypter, cdn_file
+
+    def get_file(self, offset, limit):
+        """Calls GetCdnFileRequest and decrypts its bytes.
+           Also ensures that the file hasn't been tampered.
+        """
+        result = self.client(GetCdnFileRequest(self.file_token, offset, limit))
+        result.bytes = self.cdn_aes.encrypt(result.bytes)
+        self.check(offset, result.bytes)
+        return result
+
+    def check(self, offset, data):
+        """Checks the integrity of the given data"""
+        for cdn_hash, sha in zip(self.cdn_file_hashes, self.shaes):
+            inter = self.intersect(
+                cdn_hash.offset, cdn_hash.offset + cdn_hash.limit,
+                offset, offset + len(data)
+            )
+            if inter:
+                x1, x2 = inter[0] - offset, inter[1] - offset
+                sha.update(data[x1:x2])
+            elif offset > cdn_hash.offset:
+                if cdn_hash.hash == sha.digest():
+                    self.cdn_file_hashes.remove(cdn_hash)
+                    self.shaes.remove(sha)
+                else:
+                    raise CdnFileTamperedError()
+
+    def finish_check(self):
+        """Similar to the check method, but for all unchecked hashes"""
+        for cdn_hash, sha in zip(self.cdn_file_hashes, self.shaes):
+            if cdn_hash.hash != sha.digest():
+                raise CdnFileTamperedError()
+
+        self.cdn_file_hashes.clear()
+        self.shaes.clear()
+
+    @staticmethod
+    def intersect(x1, x2, z1, z2):
+        if x1 > z1:
+            return None if x1 > z2 else (x1, min(x2, z2))
+        else:
+            return (z1, min(x2, z2)) if x2 > z1 else None