diff --git a/.gitignore b/.gitignore
index f4aa3fd..57f86eb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 data/*.pem
 data/*.txt
+data/user/*.json
 log/*.txt
 config/config.php
 public/static/*.pem
diff --git a/config/config.sample.php b/config/config.sample.php
index 2ab84b6..114c6a9 100644
--- a/config/config.sample.php
+++ b/config/config.sample.php
@@ -37,4 +37,6 @@ define('RSS_ENABLED', false);
 define('ACTIVITYPUB_ENABLED', false);
 define('MYSQL_ENABLED', false);
 define('CSV_ENABLED', false);
+define('AUTH_ENABLED', false);
+define('AUTH_REGISTER_ENABLED', false);
 define('COPYRIGHT_YEAR', '2018-'.date('Y'));
diff --git a/data/user/.kara b/data/user/.kara
new file mode 100644
index 0000000..e69de29
diff --git a/public/static/style.css b/public/static/style.css
index 9dac7eb..5370408 100644
--- a/public/static/style.css
+++ b/public/static/style.css
@@ -55,6 +55,14 @@ header, main, footer {
   padding-bottom: 10px;
 }
 
+.errormes {
+  background: #ee4030;
+  color: #fcfcfc;
+  border: 4px inset #b61729;
+  border-radius: 2px;
+  padding: 2px;
+}
+
 h1.paragraph, p.paragraph {
   text-align: left;
 }
diff --git a/route.php b/route.php
index a871fcf..3fde0bf 100644
--- a/route.php
+++ b/route.php
@@ -11,6 +11,7 @@ use Site\Controller\Fediverse;
 use Site\Controller\Home;
 use Site\Controller\Notfound;
 use Site\Controller\Page;
+use Site\Controller\User;
 
 define('ROOT', realpath(__DIR__));
 
@@ -35,6 +36,12 @@ if (ACTIVITYPUB_ENABLED) {
   $routes[] = Route::add('GET', 'ap/actor', Fediverse::class.'@apactor');
 }
 
+if (AUTH_ENABLED) {
+  $routes[] = Route::add('POST', 'login', User::class.'@login');
+  $routes[] = Route::add('GET', 'login', User::class.'@login');
+  $routes[] = Route::add('GET', 'logout', User::class.'@logout');
+}
+
 /* if (RSS_ENABLED) {} */
 
 if (ATOM_ENABLED) {
diff --git a/src/Site/Controller/Page.php b/src/Site/Controller/Page.php
index a785922..6d83354 100644
--- a/src/Site/Controller/Page.php
+++ b/src/Site/Controller/Page.php
@@ -61,5 +61,4 @@ class Page {
       throw new \Exception($e->getMessage());
     }
   }
-}
-?>
+}
\ No newline at end of file
diff --git a/src/Site/Controller/User.php b/src/Site/Controller/User.php
new file mode 100644
index 0000000..0e5f0e0
--- /dev/null
+++ b/src/Site/Controller/User.php
@@ -0,0 +1,76 @@
+<?php
+namespace Site\Controller;
+
+use Site\Controller\Mods;
+use Site\Lib\Auth;
+use Site\Lib\Template;
+
+class User {
+  use Mods;
+
+  public function login(array $params): void {
+    try {
+      $auth = new Auth;
+      $user = $auth->getLoggedInUser();
+      if ($user) {
+        header('Location: /');
+        exit();
+      }
+
+      $doLogin = count($_POST) > 0;
+      $error = '';
+
+      if ($doLogin) {
+        $a = [];
+        if (count($_POST) === 2) {
+          $i = 0;
+          foreach ($_POST as $p) {
+            $a[(int)$i] = $p;
+            $i++;
+          }
+        }
+        $auth = new Auth($a[0]);
+        $res = $auth->isUserExist($a[0]);
+        kys($res);
+        if (!$res->isSuccess) {
+          $error = $res->message;
+        } else {
+          $auth->setToken($a[0], $a[1]);
+          header('Location: /');
+          exit();
+        }
+      }
+      $tmpl = new Template('/');
+      $pagetit = 'サインイン';
+      $description = 'サイトにサインイン';
+
+      $tmpl->assign('pagetit', $pagetit);
+      $tmpl->assign('curPage', 'about');
+      $tmpl->assign('custCss', false);
+      $tmpl->assign('menu', $this->getMenu());
+      $tmpl->assign('description', $description);
+      $tmpl->assign('error', $error);
+
+      $tmpl->render('login');
+    } catch (\Exception $e) {
+      throw new \Exception($e->getMessage());
+    }
+  }
+
+  public function logout(array $params): void {
+    try {
+      $auth = new Auth();
+      $user = $auth->getLoggedInUser();
+      if (!$user) {
+        header('Location: /');
+        exit();
+      }
+
+      $auth->logout();
+      header('Location: /');
+      exit();
+    } catch (\Exception $e) {
+      throw new \Exception($e->getMessage());
+    }
+  }
+}
\ No newline at end of file
diff --git a/src/Site/Lib/Auth.php b/src/Site/Lib/Auth.php
new file mode 100644
index 0000000..4e6b12f
--- /dev/null
+++ b/src/Site/Lib/Auth.php
@@ -0,0 +1,280 @@
+<?php
+namespace Site\Lib;
+
+class Auth {
+  private int $id;
+
+  /**
+   * 性別： -1 = 不明, 0 = 男性, 1 = 女性
+   * ロール： -1 = BAN, 0 = ユーザー, 1 = スタッフ
+   * トークン： string token, int expDate
+   * 
+   * パスワードとメールアドレスはArgon2IDでハッシュする
+   */
+  private \stdClass $user;
+  private \stdClass $pubUser;
+  private ?string $token;
+
+  protected string $dataDir = ROOT."/data/user/";
+  protected int $minPassLen = 20;
+  protected int $tokenDuration = 31536000; // 1年間
+  protected string|int|null $algo = PASSWORD_ARGON2ID;
+
+  public function __construct(?string $username = null) {
+    if (!AUTH_ENABLED) return;
+    $secure = (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off');
+    $this->token = \getcookie('kerozen');
+    $this->id = $this->getUserId($username, $this->token);
+    if ($this->id > 0) $this->user = $this->getUserData();
+  }
+
+  public function getLoggedInUser(): \stdClass|null {
+    if ($this->id === 0) return null;
+    $this->pubUser = $this->user;
+    unset($this->pubUser->password);
+    unset($this->pubUser->tokens);
+    return $this->pubUser;
+  }
+
+  private function verifyLogin(string $username, string $password): \Result {
+    $userData = $this->getUserData();
+    if ($username !== $userData->username || !password_verify($password, $userData->password)) {
+      return \Result::error('エラー：ユーザー名又はパスワードが一致していません。');
+    }
+
+    if (password_needs_rehash($userData->password, $algo)) {
+      $userData->password = password_hash($password, $algo);
+      $path = $this->dataDir.$this->id.'.'.$userData->username.'.json';
+      $json = json_encode($userData, JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES);
+
+      if (file_put_contents($path, $json) === false) {
+        return \Result::error('エラー：ユーザーデータの保存に失敗。');
+      }
+    }
+
+    return \Result::success();
+  }
+
+  public function setToken(string $username, string $password): \Result {
+    $userData = $this->getUserData();
+    if (!isset($userData->tokens) || !is_array($userData->tokens)) $userData->tokens = [];
+
+    $verify = $this->verifyLogin($username, $password);
+    if (!$verify->isSuccess) {
+      return \Result::error($verify->message);
+    }
+
+    $ip = $_SERVER['REMOTE_ADDR'];
+    if (!empty($_SERVER['HTTP_X_FORMARDED_FOR'])) {
+      $ipList = explode(',', $_SERVER['HTTP_X_FORMARDED_FOR']);
+      $ip = trim($ipList[0]);
+    }
+
+    $userData = $this->purgeOldTokens($userData);
+    $expire = 0;
+    $tokenExist = false;
+
+    foreach ($userData->tokens as $t) {
+      if ($t->ip === $ip) {
+        $tokenExist = true;
+        break;
+      }
+    }
+
+    if (!$tokenExist) {
+      $token = bin2hex(random_bytes(64));
+      $expire = time() + $this->tokenDuration;
+      $newToken = [
+        'token' => password_hash($token, $algo),
+        'ip' => $ip,
+        'createDate' => time(),
+        'expDate' => $expire,
+        'lastDate' => time(),
+        'userAgent' => $_SERVER['HTTP_USER_AGENT'] ?? '不明'
+      ];
+
+      $userData->tokens[] = $newToken;
+    }
+
+    $path = $this->dataDir.$this->id.'.'.$userData->username.'.json';
+    $json = json_encode($userData, JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES);
+
+    if (file_put_contents($path, $json) === false) {
+      return \Result::error('エラー：ユーザーデータの保存に失敗。');
+    }
+
+    if (!$tokenExist) {
+      $secure = (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off');
+      $domain = $_SERVER['SERVER_NAME'];
+
+      if (!setcookie('kerozen', $token, [
+        'expires' => $expire,
+        'path' => '/',
+        'domain' => $domain,
+        'secure' => $secure,
+        'httponly' => true,
+        'samesite' => 'Strict'
+      ])) {
+        return \Result::error('エラー：クッキーを設定に失敗。');
+      }
+    }
+
+    return \Result::success('ログイン成功');
+  }
+
+  public function getToken(): string {
+    $userData = $this->getUserData();
+    if (!isset($userData->tokens) || !is_array($userData->tokens)) $userData->tokens = [];
+    $userData = $this->purgeOldTokens($userData);
+  }
+
+  public function logout(?string $token = null): \Result {
+    $secure = (!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] !== 'off');
+    $domain = $_SERVER['SERVER_NAME'];
+    $userData = $this->getUserData();
+
+    if (!$token) {
+      $userData->tokens = array_filter($userData->tokens, function($t): bool {
+        return ($t->expires ?? 0) > time();
+      });
+    }
+
+    setcookie('kerozen', null, [
+      'expires' => 0,
+      'path' => '/',
+      'domain' => $domain,
+      'secure' => $secure,
+      'httponly' => true,
+      'samesite' => 'Strict'
+    ]);
+
+    return \Result::success();
+  }
+
+  public function isUserExist(?string $username): \Result {
+    if (null === $username) return \Result::Error('エラー：ユーザー名をご入力下さい。');
+    $userList = scandir($this->dataDir);
+
+    foreach ($userList as $list) {
+      if ($list === '.' || $list === '..') continue;
+      $file = str_replace('.json', '', $list);
+      $user = explode('.', $file)[1];
+      if ($username === $user) return \Result::Success("ユーザー「{$username}」は既に存在します。");
+    }
+
+    return \Result::Error("エラー：ユーザー「{$username}」は存在しません。");
+  }
+
+  ////////////////////
+
+  private function purgeOldTokens(\stdClass $userData): \stdClass {
+    // 有効期限切れたトークンの削除
+    $out = $userData;
+    $out->tokens = array_filter($userData->tokens, function($t): bool {
+      return ($t->expDate ?? 0) > time();
+    });
+
+    $path = $this->dataDir.$userData->id.'.'.$userData->username.'.json';
+    $json = json_encode($userData, JSON_PRETTY_PRINT | JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES);
+
+    if (file_put_contents($path, $json) === false) {
+      return \Result::error('エラー：ユーザーデータの保存に失敗。');
+    }
+
+    return $out;
+  }
+
+  private function getUserId(?string $username = null, ?string $token): int {
+    if (!$username && !$token) return 0;
+    $file = scandir($this->dataDir);
+    $matches = [];
+    $id = 0;
+
+    foreach ($file as $f) {
+      if ($f === '.' || $f === '..') continue;
+
+      $path = "{$this->dataDir}{$f}";
+      if (!is_file($path)) continue;
+      $content = file_get_contents($path);
+      if (str_contains($content, $token)) {
+        $matches[] = $path;
+      }
+    }
+    $id = (int)preg_replace('/^(.*?)\.(.*?)$/', "$1", str_replace($this->dataDir, '', $matches[0]));
+
+    return $id;
+  }
+
+  private function getUserData(): \stdClass {
+    $file = scandir($this->dataDir);
+    $userFile = "";
+    $matches = [];
+
+    foreach ($file as $f) {
+      if ($f === '.' || $f === '..') continue;
+      if (preg_match('/^'.$this->id.'\.(.*?)\.json$/', $f, $matches)) {
+        $userFile = $matches[0];
+        break;
+      }
+    }
+    $path = $this->dataDir.$userFile;
+    unset($file, $userFile);
+
+    if (!file_exists($path) || !is_readable($path)) {
+      kys('エラー');
+    }
+
+    $fp = fopen($path, 'r');
+    assert_not_null($fp);
+    if (!$fp) kys('ユーザーが存在しない。');
+    $lines = "";
+    while (($buf = fgets($fp, 4096)) !== false) $lines .= $buf.PHP_EOL;
+    if (!feof($fp)) kys("エラー：不明なエラー");
+    fclose($fp);
+    unset($path);
+    return json_decode($lines);
+  }
+
+  private function isEmailExist(?string $email): \Result {
+    if (null === $password) return \Result::Error('エラー：パスワードをご入力下さい。');
+  }
+
+  private function verifyUsername(?string $username): \Result {
+    if (null === $username) return \Result::Error('エラー：ユーザー名をご入力下さい。');
+    if (strlen($username) < 6) return \Result::Error('エラー：ユーザー名は6文字以上をご入力下さい。');
+
+    $accepted = 'A-Za-z0-9';
+
+    if (preg_match('/[^'.preg_quote($accepted, '/').']/', $password)) {
+      return \Result::Error('エラー：ユーザー名に不正な文字が含みます。');
+    }
+    return \Result::Success();
+  }
+
+  private function verifyPassword(?string $password): \Result {
+    return \Result::Success();
+  }
+
+  private function checkPasswordStandards(?string $password): \Result {
+    if (null === $password) return \Result::Error('エラー：パスワードをご入力下さい。');
+    if (strlen($password) < $this->minPassLen) return \Result::Error("エラー：パスワード数は{$this->minPassLen}以上をご入力下さい。");
+
+    $specChar = '!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~]/';
+    $accepted = 'A-Za-z0-9'.$specChar;
+
+    if (preg_match('/[^'.preg_quote($accepted, '/').']/', $password)) {
+      return \Result::Error('エラー：パスワードに不正な文字が含みます。');
+    }
+
+    $countUpper = preg_match_all('/[A-Z]/', $password) < 2;
+    $countLower = preg_match_all('/[a-z]/', $password) < 2;
+    $countDigit = preg_match_all('/[0-9]/', $password) < 2;
+    $countSymbol = preg_match_all('/['.$specChar.']/', $password) < 2;
+
+    if ($countUpper || $countLower || $countDigit || $countSymbol) {
+      return \Result::Error('エラー：パスワードは2つ以上の大文字、2つ以上の小文字、2つ以上の数字、及び2つ以上の特別文字を含む事が必須です。');
+    }
+
+    return \Result::Success();
+  }
+};
\ No newline at end of file
diff --git a/util.php b/util.php
index 9f24308..6ee4e98 100644
--- a/util.php
+++ b/util.php
@@ -6,6 +6,24 @@ enum LogType {
   case Csv;
 }
 
+class Result {
+  public bool $isSuccess;
+  public ?string $message;
+
+  public function __construct(bool $isSuccess, ?string $message = null) {
+    $this->isSuccess = $isSuccess;
+    $this->message = $message;
+  }
+
+  public static function Success(?string $message = null): self {
+    return new self(true, $message);
+  }
+
+  public static function Error(string $message): self {
+    return new self(false, $message);
+  }
+}
+
 function uuid(): string {
   $data = random_bytes(16);
   $data[6] = chr(ord($data[6]) & 0x0f | 0x40);
@@ -90,4 +108,38 @@ function to_money($amount, $lang) {
     default:
       return '¥ '.number_format($amount, 0);
   }
+}
+
+function randstr(): string {
+  srand(floor(time() / (60*60*24)));
+  $len = rand() % 20;
+  return bin2hex(random_bytes($len / 2));
+}
+
+function assert_null(mixed $assertion, Throwable|string|null $description = null): bool {
+  if (ini_get('zend.assertions') < 1) trigger_error('assert機能性が無効です。有効するには、php.iniで「zend.assertions = 1」に設定して下さい。', E_USER_WARNING);
+  if (is_null($assertion)) return true;
+  assert(false, $description ?? 'Assertion failed: value is not null');
+  return false;
+}
+
+function assert_not_null(mixed $assertion, Throwable|string|null $description = null): bool {
+  if (ini_get('zend.assertions') < 1) trigger_error('assert機能性が無効です。有効するには、php.iniで「zend.assertions = 1」に設定して下さい。', E_USER_WARNING);
+  if (!is_null($assertion)) return true;
+  assert(false, $description ?? 'Assertion failed: value is null');
+  return false;
+}
+
+function assert_unless_success(Result $assertion, Throwable|string|null $description = null): bool {
+  if (ini_get('zend.assertions') < 1) trigger_error('assert機能性が無効です。有効するには、php.iniで「zend.assertions = 1」に設定して下さい。', E_USER_WARNING);
+  if ($assertion->isSuccess) return true;
+  assert(false, $description ?? $assertion->message ?? 'Assertion failed: Result is not successful');
+  return false;
+}
+
+if (AUTH_ENABLED) {
+  function getcookie(string $name): string|null {
+    if (!$_COOKIE[$name]) return null;
+    return $_COOKIE[$name];
+  }
 }
\ No newline at end of file
diff --git a/view/common/header.maron b/view/common/header.maron
index 616742a..bcee022 100644
--- a/view/common/header.maron
+++ b/view/common/header.maron
@@ -56,6 +56,18 @@
         <a class="{{ $m['class'] }}{{ $curPage == $m['page'] ? ' active' : '' }}" href="{{ $m['href'] }}">{{ $m['text'] }}</a>
 {@ endif @}
 {@ endforeach @}
+{@ if (AUTH_ENABLED) @}
+      <div>
+{@ if (isset($user) && isset($user->id) && $user->id > 0) @}
+        お帰りなしゃ～い、{{ $user->displayname ?? $user->username }}さん！ (<a href="/logout">ログアウト</a>)
+{@ else @}
+        <a href="/login">ログイン</a>
+{@ if (AUTH_REGISTER_ENABLED) @}
+        | <a href="/register">登録</a>
+{@ endif @}
+{@ endif @}
+      </div>
+{@ endif @}
       </nav>
     </header>
     <main>
\ No newline at end of file
diff --git a/view/login.maron b/view/login.maron
new file mode 100644
index 0000000..4be3ba7
--- /dev/null
+++ b/view/login.maron
@@ -0,0 +1,37 @@
+{@ include(common/header) @}
+      <h1 class="paragraph">ログイン</h1>
+      <p class="paragraph">
+        ログインして下さい。
+      </p>
+
+{@ if ($error) @}
+      <p>
+        <div class="errormes">
+          {{ $error }}
+        </div>
+      </p>
+{@ endif @}
+
+      <p>
+        <form action="/login" method="POST">
+          {$ $username = randstr() $}
+          {$ $password = randstr() $}
+          <table>
+            <tbody>
+              <tr>
+                <td><label for="{{ $username }}">ユーザー名：</label></td>
+                <td><input type="text" id="{{ $username }}" name="{{ $username }}" /></td>
+              </tr>
+              <tr>
+                <td><label for="{{ $password }}">パスワード：</label></td>
+                <td><input type="password" id="{{ $password }}" name="{{ $password }}" /></td>
+              </tr>
+              <tr>
+                <td></td>
+                <td><button>サインイン</button></td>
+              </tr>
+            </tbody>
+          </table>
+        </form>
+      </p>
+{@ include(common/footer) @}
\ No newline at end of file