Implement MtProto 2.0 (closes #484, thanks @delivrance!)

Huge shoutout to @delivrance's pyrogram, specially this commit:
pyrogram/pyrogram/commit/42f9a2d6994baaf9ecad590d1ff4d175a8c56454

telethon/helpers.py

@@ -1,6 +1,11 @@
 """Various helpers not related to the Telegram API itself"""
-from hashlib import sha1, sha256
 import os
+import struct
+from hashlib import sha1, sha256
+
+from telethon.crypto import AES
+from telethon.extensions import BinaryReader
+
 
 # region Multiple utilities
 
@@ -21,9 +26,48 @@ def ensure_parent_dir_exists(file_path):
 # region Cryptographic related utils
 
 
+def pack_message(session, message):
+    """Packs a message following MtProto 2.0 guidelines"""
+    # See https://core.telegram.org/mtproto/description
+    data = struct.pack('<qq', session.salt, session.id) + bytes(message)
+    padding = os.urandom(-(len(data) + 12) % 16 + 12)
+
+    # Being substr(what, offset, length); x = 0 for client
+    # "msg_key_large = SHA256(substr(auth_key, 88+x, 32) + pt + padding)"
+    msg_key_large = sha256(
+        session.auth_key.key[88:88 + 32] + data + padding).digest()
+
+    # "msg_key = substr (msg_key_large, 8, 16)"
+    msg_key = msg_key_large[8:24]
+    aes_key, aes_iv = calc_key_2(session.auth_key.key, msg_key, True)
+
+    key_id = struct.pack('<Q', session.auth_key.key_id)
+    return key_id + msg_key + AES.encrypt_ige(data + padding, aes_key, aes_iv)
+
+
+def unpack_message(session, reader):
+    """Unpacks a message following MtProto 2.0 guidelines"""
+    # See https://core.telegram.org/mtproto/description
+    reader.read_long(signed=False)  # remote_auth_key_id
+
+    msg_key = reader.read(16)
+    aes_key, aes_iv = calc_key_2(session.auth_key.key, msg_key, False)
+    data = BinaryReader(AES.decrypt_ige(reader.read(), aes_key, aes_iv))
+
+    data.read_long()  # remote_salt
+    data.read_long()  # remote_session_id
+    remote_msg_id = data.read_long()
+    remote_sequence = data.read_int()
+    msg_len = data.read_int()
+    message = data.read(msg_len)
+
+    return message, remote_msg_id, remote_sequence
+
+
 def calc_key(shared_key, msg_key, client):
-    """Calculate the key based on Telegram guidelines,
-       specifying whether it's the client or not
+    """
+    Calculate the key based on Telegram guidelines,
+    specifying whether it's the client or not.
     """
     x = 0 if client else 8
 
@@ -40,6 +84,23 @@ def calc_key(shared_key, msg_key, client):
     return key, iv
 
 
+def calc_key_2(auth_key, msg_key, client):
+    """
+    Calculate the key based on Telegram guidelines
+    for MtProto 2, specifying whether it's the client or not.
+    """
+    # https://core.telegram.org/mtproto/description#defining-aes-key-and-initialization-vector
+    x = 0 if client else 8
+
+    sha256a = sha256(msg_key + auth_key[x: x + 36]).digest()
+    sha256b = sha256(auth_key[x + 40:x + 76] + msg_key).digest()
+
+    aes_key = sha256a[:8] + sha256b[8:24] + sha256a[24:32]
+    aes_iv = sha256b[:8] + sha256a[8:24] + sha256b[24:32]
+
+    return aes_key, aes_iv
+
+
 def calc_msg_key(data):
     """Calculates the message key from the given data"""
     return sha1(data).digest()[4:20]