diff --git a/src/Client.cs b/src/Client.cs
index c8b6791..144ef92 100644
--- a/src/Client.cs
+++ b/src/Client.cs
@@ -307,7 +307,7 @@ namespace WTelegram
 				throw new ApplicationException($"Received a packet encrypted with unexpected key {authKeyId:X}");
 			if (authKeyId == 0) // Unencrypted message
 			{
-				using var reader = new BinaryReader(new MemoryStream(data, 8, data.Length - 8));
+				using var reader = new TL.BinaryReader(new MemoryStream(data, 8, data.Length - 8), this);
 				long msgId = _lastRecvMsgId = reader.ReadInt64();
 				if ((msgId & 1) == 0) throw new ApplicationException($"Invalid server msgId {msgId}");
 				int length = reader.ReadInt32();
@@ -327,7 +327,7 @@ namespace WTelegram
 				byte[] decrypted_data = EncryptDecryptMessage(data.AsSpan(24), false, _session.AuthKey, msgKeyLarge);
 				if (decrypted_data.Length < 36) // header below+ctorNb
 					throw new ApplicationException($"Decrypted packet too small: {decrypted_data.Length}");
-				using var reader = new BinaryReader(new MemoryStream(decrypted_data));
+				using var reader = new TL.BinaryReader(new MemoryStream(decrypted_data), this);
 				var serverSalt = reader.ReadInt64();			// int64 salt
 				var sessionId = reader.ReadInt64();				// int64 session_id
 				var msgId = _lastRecvMsgId = reader.ReadInt64();// int64 message_id
@@ -385,7 +385,7 @@ namespace WTelegram
 			};
 		}
 
-		internal MsgContainer ReadMsgContainer(BinaryReader reader)
+		internal MsgContainer ReadMsgContainer(TL.BinaryReader reader)
 		{
 			int count = reader.ReadInt32();
 			var array = new _Message[count];
@@ -422,7 +422,7 @@ namespace WTelegram
 			return new MsgContainer { messages = array };
 		}
 
-		private RpcResult ReadRpcResult(BinaryReader reader)
+		private RpcResult ReadRpcResult(TL.BinaryReader reader)
 		{
 			long msgId = reader.ReadInt64();
 			var (type, tcs) = PullPendingRequest(msgId);
@@ -838,5 +838,34 @@ namespace WTelegram
 					reply_to_msg_id: reply_to_msg_id, entities: entities, schedule_date: schedule_date);
 		}
 		#endregion
+
+		/// <summary>Enable the collection of id/access_hash pairs (experimental)</summary>
+		public bool CollectAccessHash { get; set; }
+		readonly Dictionary<Type, Dictionary<long, long>> _accessHashes = new();
+		public IEnumerable<KeyValuePair<long, long>> AllAccessHashesFor<T>() where T : ITLObject
+			=> _accessHashes.GetValueOrDefault(typeof(T));
+		/// <summary>Retrieve the access_hash associated with this id (for a TL class)</summary>
+		/// <typeparam name="T">a TL object class. For example User, Channel or Photo</typeparam>
+		public long? GetAccessHashFor<T>(long id) where T : ITLObject
+		{
+			lock (_accessHashes)
+				return _accessHashes.GetOrCreate(typeof(T)).TryGetValue(id, out var access_hash) ? access_hash : null;
+		}
+		public void SetAccessHashFor<T>(long id, long access_hash) where T : ITLObject
+		{
+			lock (_accessHashes)
+				_accessHashes.GetOrCreate(typeof(T))[id] = access_hash;
+		}
+		internal void UpdateAccessHash(object obj, Type type, object access_hash)
+		{
+			if (!CollectAccessHash) return;
+			if (access_hash is not long accessHash) return;
+			if (type.GetField("id") is not FieldInfo idField) return;
+			if (idField.GetValue(obj) is not long id)
+				if (idField.GetValue(obj) is not int idInt) return;
+				else id = idInt;
+			lock (_accessHashes)
+				_accessHashes.GetOrCreate(type)[id] = accessHash;
+		}
 	}
 }
diff --git a/src/Encryption.cs b/src/Encryption.cs
index 581e6d3..1fae77c 100644
--- a/src/Encryption.cs
+++ b/src/Encryption.cs
@@ -109,7 +109,7 @@ namespace WTelegram
 			var (tmp_aes_key, tmp_aes_iv) = ConstructTmpAESKeyIV(resPQ.server_nonce, pqInnerData.new_nonce);
 			var answer = AES_IGE_EncryptDecrypt(serverDHparamsOk.encrypted_answer, tmp_aes_key, tmp_aes_iv, false);
 
-			using var encryptedReader = new BinaryReader(new MemoryStream(answer));
+			using var encryptedReader = new TL.BinaryReader(new MemoryStream(answer), client);
 			var answerHash = encryptedReader.ReadBytes(20);
 			var answerObj = encryptedReader.ReadTLObject();
 			if (answerObj is not ServerDHInnerData serverDHinnerData) throw new ApplicationException("not server_DH_inner_data");
diff --git a/src/TL.cs b/src/TL.cs
index db532f0..4a3e698 100644
--- a/src/TL.cs
+++ b/src/TL.cs
@@ -24,7 +24,7 @@ namespace TL
 		internal static T Deserialize<T>(byte[] bytes) where T : ITLObject
 		{
 			using var memStream = new MemoryStream(bytes);
-			using var reader = new BinaryReader(memStream);
+			using var reader = new BinaryReader(memStream, null);
 			return (T)reader.ReadTLObject();
 		}
 
@@ -56,7 +56,7 @@ namespace TL
 			if (!Table.TryGetValue(ctorNb, out var type))
 				throw new ApplicationException($"Cannot find type for ctor #{ctorNb:x}");
 			var obj = Activator.CreateInstance(type);
-			var fields = obj.GetType().GetFields().GroupBy(f => f.DeclaringType).Reverse().SelectMany(g => g);
+			var fields = type.GetFields().GroupBy(f => f.DeclaringType).Reverse().SelectMany(g => g);
 			int flags = 0;
 			IfFlagAttribute ifFlag;
 			foreach (var field in fields)
@@ -64,9 +64,10 @@ namespace TL
 				if (((ifFlag = field.GetCustomAttribute<IfFlagAttribute>()) != null) && (flags & (1 << ifFlag.Bit)) == 0) continue;
 				object value = reader.ReadTLValue(field.FieldType);
 				field.SetValue(obj, value);
-				if (field.Name.Equals("Flags", StringComparison.OrdinalIgnoreCase)) flags = (int)value;
+				if (field.Name == "flags") flags = (int)value;
+				else if (field.Name == "access_hash") reader.Client?.UpdateAccessHash(obj, type, value);
 			}
-			return type == typeof(GzipPacked) ? UnzipPacket((GzipPacked)obj) : (ITLObject)obj;
+			return type == typeof(GzipPacked) ? UnzipPacket((GzipPacked)obj, reader.Client) : (ITLObject)obj;
 		}
 
 		internal static void WriteTLValue(this BinaryWriter writer, object value)
@@ -220,9 +221,9 @@ namespace TL
 			writer.Write(0);    // null arrays are serialized as empty
 		}
 
-		internal static ITLObject UnzipPacket(GzipPacked obj)
+		internal static ITLObject UnzipPacket(GzipPacked obj, WTelegram.Client client)
 		{
-			using var reader = new BinaryReader(new GZipStream(new MemoryStream(obj.packed_data), CompressionMode.Decompress));
+			using var reader = new BinaryReader(new GZipStream(new MemoryStream(obj.packed_data), CompressionMode.Decompress), client);
 			var result = reader.ReadTLObject();
 			return result;
 		}
@@ -234,6 +235,12 @@ namespace TL
 #endif
 	}
 
+	public class BinaryReader : System.IO.BinaryReader
+	{
+		public readonly WTelegram.Client Client;
+		public BinaryReader(Stream stream, WTelegram.Client client) : base(stream) => Client = client;
+	}
+
 	[AttributeUsage(AttributeTargets.Class)]
 	public class TLDefAttribute : Attribute
 	{