diff --git a/src/detail/ca-request-state.hpp b/src/detail/ca-request-state.hpp
index 2c4a1d6..5fbb8f7 100644
--- a/src/detail/ca-request-state.hpp
+++ b/src/detail/ca-request-state.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2017-2021, Regents of the University of California.
+ * Copyright (c) 2017-2022, Regents of the University of California.
  *
  * This file is part of ndncert, a certificate management system based on NDN.
  *
@@ -27,7 +27,7 @@
 
 namespace ndncert {
 
-typedef std::array<uint8_t, 8> RequestId;
+using RequestId = std::array<uint8_t, 8>;
 
 enum class Status : uint16_t {
   BEFORE_CHALLENGE = 0,
@@ -127,7 +127,7 @@
   /**
    * @brief The challenge state.
    */
-  optional<ChallengeState> challengeState;
+  std::optional<ChallengeState> challengeState;
 };
 
 std::ostream&