diff --git a/td/telegram/net/NetQuery.cpp b/td/telegram/net/NetQuery.cpp
index 71e8814a9..c35ac6de9 100644
--- a/td/telegram/net/NetQuery.cpp
+++ b/td/telegram/net/NetQuery.cpp
@@ -171,4 +171,15 @@ StringBuilder &operator<<(StringBuilder &stream, const NetQueryPtr &net_query_pt
   return stream << *net_query_ptr;
 }
 
+void NetQuery::add_verification_prefix(const string &prefix) {
+  CHECK(is_ready());
+  CHECK(is_error());
+  CHECK(!query_.empty());
+  BufferSlice query(prefix.size() + query_.size() - verification_prefix_length_);
+  query.as_mutable_slice().copy_from(prefix);
+  query.as_mutable_slice().substr(prefix.size()).copy_from(query_.as_slice().substr(verification_prefix_length_));
+  verification_prefix_length_ = narrow_cast<int32>(prefix.size());
+  query_ = std::move(query);
+}
+
 }  // namespace td
diff --git a/td/telegram/net/NetQuery.h b/td/telegram/net/NetQuery.h
index 4ea22ff80..8ab74acac 100644
--- a/td/telegram/net/NetQuery.h
+++ b/td/telegram/net/NetQuery.h
@@ -238,6 +238,8 @@ class NetQuery final : public TsListNode<NetQueryDebug> {
     return in_sequence_dispacher_;
   }
 
+  void add_verification_prefix(const string &prefix);
+
  private:
   State state_ = State::Empty;
   Type type_ = Type::Common;
@@ -251,6 +253,7 @@ class NetQuery final : public TsListNode<NetQueryDebug> {
   BufferSlice query_;
   BufferSlice answer_;
   int32 tl_constructor_ = 0;
+  int32 verification_prefix_length_ = 0;
 
   vector<NetQueryRef> invoke_after_;
   vector<uint64> chain_ids_;