Verify the files downloaded from CDNs and raise on sha256 mismatch

telethon/errors/__init__.py

@@ -3,7 +3,7 @@ import re
 
 from .common import (
     ReadCancelledError, InvalidParameterError, TypeNotFoundError,
-    InvalidChecksumError
+    InvalidChecksumError, CdnFileTamperedError
 )
 
 from .rpc_errors import (

telethon/errors/common.py

@@ -35,3 +35,11 @@ class InvalidChecksumError(Exception):
 
         self.checksum = checksum
         self.valid_checksum = valid_checksum
+
+
+class CdnFileTamperedError(Exception):
+    def __init__(self):
+        super().__init__(
+            self,
+            'The CDN file has been altered and its download cancelled.'
+        )