diff --git a/telethon/client/telegrambaseclient.py b/telethon/client/telegrambaseclient.py
index 83648af9..56325312 100644
--- a/telethon/client/telegrambaseclient.py
+++ b/telethon/client/telegrambaseclient.py
@@ -2,7 +2,6 @@ import abc
 import asyncio
 import logging
 import platform
-import sys
 import time
 from datetime import datetime, timezone
 
@@ -13,6 +12,7 @@ from ..network import MTProtoSender, ConnectionTcpFull, TcpMTProxy
 from ..sessions import Session, SQLiteSession, MemorySession
 from ..tl import TLObject, functions, types
 from ..tl.alltlobjects import LAYER
+from ..entitycache import EntityCache
 
 DEFAULT_DC_ID = 4
 DEFAULT_IPV4_IP = '149.154.167.51'
@@ -229,14 +229,17 @@ class TelegramBaseClient(abc.ABC):
 
         self.flood_sleep_threshold = flood_sleep_threshold
 
-        # TODO Figure out how to use AsyncClassWrapper(session)
-        # The problem is that ChatGetter and SenderGetter rely
-        # on synchronous calls to session.get_entity precisely
-        # to avoid network access and the need for await.
+        # TODO Use AsyncClassWrapper(session)
+        # ChatGetter and SenderGetter can use the in-memory _entity_cache
+        # to avoid network access and the need for await in session files.
         #
-        # With asynchronous sessions, it would need await,
-        # and defeats the purpose of properties.
+        # The session files only wants the entities to persist
+        # them to disk, and to save additional useful information.
+        # TODO Make use of _entity_cache
+        # TODO Session should probably return all cached
+        #      info of entities, not just the input versions
         self.session = session
+        self._entity_cache = EntityCache()
         self.api_id = int(api_id)
         self.api_hash = api_hash
 
diff --git a/telethon/client/updates.py b/telethon/client/updates.py
index 2c409735..b15853b7 100644
--- a/telethon/client/updates.py
+++ b/telethon/client/updates.py
@@ -179,6 +179,8 @@ class UpdateMethods(UserMethods):
 
     async def _handle_update(self, update):
         self.session.process_entities(update)
+        self._entity_cache.add(update)
+
         if isinstance(update, (types.Updates, types.UpdatesCombined)):
             entities = {utils.get_peer_id(x): x for x in
                         itertools.chain(update.users, update.chats)}
diff --git a/telethon/client/users.py b/telethon/client/users.py
index 8fc4b141..6bbf86ab 100644
--- a/telethon/client/users.py
+++ b/telethon/client/users.py
@@ -49,6 +49,7 @@ class UserMethods(TelegramBaseClient):
                             results.append(None)
                             continue
                         self.session.process_entities(result)
+                        self._entity_cache.add(result)
                         exceptions.append(None)
                         results.append(result)
                         request_index += 1
@@ -59,6 +60,7 @@ class UserMethods(TelegramBaseClient):
                 else:
                     result = await future
                     self.session.process_entities(result)
+                    self._entity_cache.add(result)
                     return result
             except (errors.ServerError, errors.RpcCallFailError,
                     errors.RpcMcgetFailError) as e:
diff --git a/telethon/entitycache.py b/telethon/entitycache.py
new file mode 100644
index 00000000..e74af793
--- /dev/null
+++ b/telethon/entitycache.py
@@ -0,0 +1,45 @@
+import itertools
+from . import utils
+from .tl import types
+
+
+class EntityCache:
+    """
+    In-memory input entity cache, defaultdict-like behaviour.
+    """
+    def add(self, entities):
+        """
+        Adds the given entities to the cache, if they weren't saved before.
+        """
+        if not utils.is_list_like(entities):
+            # Invariant: all "chats" and "users" are always iterables
+            entities = itertools.chain(
+                [getattr(entities, 'user', None)],
+                getattr(entities, 'chats', []),
+                getattr(entities, 'users', [])
+            )
+
+        for entity in entities:
+            try:
+                pid = utils.get_peer_id(entity)
+                if pid not in self.__dict__:
+                    # Note: `get_input_peer` already checks for `access_hash`
+                    self.__dict__[pid] = utils.get_input_peer(entity)
+            except TypeError:
+                pass
+
+    def __getitem__(self, item):
+        """
+        Gets the corresponding :tl:`InputPeer` for the given ID or peer,
+        or returns `None` on error/not found.
+        """
+        if not isinstance(item, int) or item < 0:
+            try:
+                return self.__dict__.get(utils.get_peer_id(item))
+            except TypeError:
+                return None
+
+        for cls in (types.PeerUser, types.PeerChat, types.PeerChannel):
+            result = self.__dict__.get(cls(item))
+            if result:
+                return result
diff --git a/telethon/sessions/memory.py b/telethon/sessions/memory.py
index cdc144f2..1e5db226 100644
--- a/telethon/sessions/memory.py
+++ b/telethon/sessions/memory.py
@@ -99,18 +99,12 @@ class MemorySession(Session):
             p = utils.get_input_peer(e, allow_self=False)
             marked_id = utils.get_peer_id(p)
         except TypeError:
+            # Note: `get_input_peer` already checks for
+            # non-zero `access_hash`. See issues #354 and #392.
             return
 
         if isinstance(p, (InputPeerUser, InputPeerChannel)):
-            if not p.access_hash:
-                # Some users and channels seem to be returned without
-                # an 'access_hash', meaning Telegram doesn't want you
-                # to access them. This is the reason behind ensuring
-                # that the 'access_hash' is non-zero. See issue #354.
-                # Note that this checks for zero or None, see #392.
-                return
-            else:
-                p_hash = p.access_hash
+            p_hash = p.access_hash
         elif isinstance(p, InputPeerChat):
             p_hash = 0
         else: