diff --git a/.gitignore b/.gitignore
index a589b70..920d449 100644
--- a/.gitignore
+++ b/.gitignore
@@ -119,7 +119,7 @@ dist
 
 ## Composer
 composer.phar
-/vendor/
+vendor/
 
 # Commit your application's lock file https://getcomposer.org/doc/01-basic-usage.md#commit-your-composer-lock-file-to-version-control
 # You may choose to ignore a library lock file http://getcomposer.org/doc/02-libraries.md#lock-file
diff --git a/Gruntfile.js b/Gruntfile.js
index d9d7f6d..f05f367 100644
--- a/Gruntfile.js
+++ b/Gruntfile.js
@@ -8,7 +8,7 @@ module.exports = grunt => {
         },
         copy: {
             composer: {
-                files: [{expand: true, cwd: "./", src: "vendor/**", dest: "dist/", flatten: false}]
+                files: [{expand: true, cwd: "./vendor/", src: "**", dest: "dist/.vendor/", flatten: false}]
             },
             css: {
                 files: [{expand: true, cwd: "src/main/", src: "**/*.css", dest: "dist/", flatten: true}]
diff --git a/composer.json b/composer.json
index e2aacee..f303587 100644
--- a/composer.json
+++ b/composer.json
@@ -1,7 +1,7 @@
 {
     "name": "fwdekker/death-notifier",
     "description": "Get notified when a famous person dies.",
-    "version": "0.0.23",
+    "version": "0.0.24",
     "type": "project",
     "license": "MIT",
     "homepage": "https://git.fwdekker.com/tools/death-notifier",
diff --git a/composer.lock b/composer.lock
index 254d81d..f337106 100644
Binary files a/composer.lock and b/composer.lock differ
diff --git a/package-lock.json b/package-lock.json
index 7f5b6e6..7f1220d 100644
Binary files a/package-lock.json and b/package-lock.json differ
diff --git a/package.json b/package.json
index 94b6f66..b1cb839 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
     "name": "death-notifier",
-    "version": "0.0.23",
+    "version": "0.0.24",
     "description": "Get notified when a famous person dies.",
     "author": "Florine W. Dekker",
     "browser": "dist/bundle.js",
diff --git a/src/main/api.php b/src/main/api.php
index f7dfa1c..d524056 100644
--- a/src/main/api.php
+++ b/src/main/api.php
@@ -1,191 +1,137 @@
 <?php
 
-use Monolog\ErrorHandler;
-use Monolog\Handler\StreamHandler;
-use Monolog\Logger;
 use php\Database;
+use php\EmailRule;
+use php\EqualsRule;
+use php\IsNotBlankRule;
+use php\IsNotSetRule;
+use php\IsSetRule;
+use php\LengthRule;
 use php\Mailer;
 use php\Mediawiki;
 use php\Response;
 use php\TrackingManager;
 use php\UserManager;
+use php\Util;
+use php\Validator;
 
 /** @noinspection PhpIncludeInspection Exists after `npm run deploy` */
-require_once __DIR__ . "/vendor/autoload.php";
+require_once __DIR__ . "/.vendor/autoload.php";
 
 
-/// Preparations
-// Load config
-// TODO: Check permissions, exit if too permissive
-$config = parse_ini_file("config.default.ini.php", process_sections: true, scanner_mode: INI_SCANNER_TYPED) or exit(1);
-if (file_exists("config.ini.php")) {
-    $config_custom =
-        parse_ini_file("config.ini.php", process_sections: true, scanner_mode: INI_SCANNER_TYPED) or exit(1);
-    $config = array_merge($config, $config_custom);
-}
-
-// Create logger
-$logger = new Logger("main");
-$logger->pushHandler(new StreamHandler($config["logger"]["file"], $config["logger"]["level"]));
-ErrorHandler::register($logger);
-
-// Connect to database
-$db_exists = file_exists($config["database"]["filename"]);
+// Preamble
+$_POST = Util::parse_post();
+$config = Util::read_config() ?? Util::http_exit(500);
+$logger = Util::create_logger($config["logger"]);
 $conn = Database::connect($config["database"]["filename"]);
 
-// Instantiate utility classes
-$mediawiki = new Mediawiki($logger->withName("Mediawiki"));
-$user_manager = new UserManager($conn);
-$tracking_manager = new TrackingManager($mediawiki, $conn);
 $mailer = new Mailer($logger->withName("Mailer"), $config, $conn);
+$mediawiki = new Mediawiki($logger->withName("Mediawiki"));
+$user_manager = new UserManager($conn, $mailer);
+$tracking_manager = new TrackingManager($conn, $mailer, $mediawiki);
 
-// Create db if it does not exist
-if (!$db_exists) {
-    $logger->warning("Database does not exist. Creating new database at '{$config["database"]["filename"]}'.");
-
+if (Database::is_empty($conn)) {
+    $logger->info("Database does not exist. Creating new database at '{$config["database"]["filename"]}'.");
+    $mailer->install();
     $user_manager->install();
     $tracking_manager->install();
-    $mailer->install();
-}
-
-// Start session
-/**
- * Generates a CSRF token.
- *
- * @return string the generated CSRF token
- */
-function generate_csrf_token(Logger $logger): string
-{
-    try {
-        return bin2hex(random_bytes(32));
-    } catch (Exception $exception) {
-        $logger->emergency("Failed to generate token.", ["cause" => $exception]);
-        http_response_code(500);
-        exit();
-    }
 }
 
 session_start();
-if (!isset($_SESSION["token"]))
-    $_SESSION["token"] = generate_csrf_token($logger);
-
-// Read JSON from POST
-$post_input = file_get_contents("php://input");
-if ($post_input !== false) $_POST = json_decode($post_input, associative: true);
+$_SESSION["token"] = $_SESSION["token"] ?? Util::generate_csrf_token($logger);
 
 
-/// Define validation helpers
-/**
- * Validates the user's CSRF token by comparing `$_POST["token"]` against `$_SESSION["token"]`.
- *
- * @return Response|null `null` if the user's CSRF token is valid, or an error response if either value is not set or if
- * the two are not equal
- */
-function validate_csrf(): ?Response
-{
-    if (!(isset($_POST["token"]) && isset($_SESSION["token"]) && hash_equals($_POST["token"], $_SESSION["token"])))
-        return new Response(
-            payload: ["target" => null, "message" => "Invalid CSRF token. Try refreshing the page."],
-            satisfied: false
-        );
-
-    return null;
-}
-
-/**
- * Validates that the user is logged out by checking that `$_SESSION["uuid"]` is not set.
- *
- * @return Response|null `null` if the user is logged out, or an error response otherwise
- */
-function validate_logged_out(): ?Response
-{
-    if (isset($_SESSION["uuid"]))
-        return new Response(payload: ["target" => null, "message" => "User is already logged in."], satisfied: false);
-
-    return null;
-}
-
-/**
- * Validates that the user is logged in by checking that `$_SESSION["uuid"]` is set.
- *
- * @return Response|null `null` if the user is logged in, or an error response otherwise
- */
-function validate_logged_in(): ?Response
-{
-    if (!isset($_SESSION["uuid"]))
-        return new Response(payload: ["target" => null, "message" => "User is not logged in."], satisfied: false);
-
-    return null;
-}
-
-/**
- * Validates that each of `arguments` is set.
- *
- * @param mixed ...$arguments the arguments to check
- * @return Response|null `null` if all `arguments` are set, or an error response otherwise
- */
-function validate_has_arguments(mixed ...$arguments): ?Response
-{
-    foreach (func_get_args() as $argument) {
-        if (!isset($argument))
-            return new Response(payload: ["target" => null, "message" => "Missing argument."], satisfied: false);
-        if (!is_string($argument))
-            return new Response(payload: ["target" => null, "message" => "Invalid argument type."], satisfied: false);
-    }
-
-    return null;
-}
-
-
-/// Process request
+// Process request
 $response = null;
 
 if (isset($_POST["action"])) {
     // POST requests; alter state
     switch ($_POST["action"]) {
         case "register":
-            $response = validate_csrf()
-                ?? validate_logged_out()
-                ?? validate_has_arguments($_POST["email"], $_POST["password"], $_POST["password_confirm"])
-                ?? $user_manager->register($mailer, $_POST["email"], $_POST["password"], $_POST["password_confirm"]);
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsNotSetRule()]])
+                ?? Validator::validate_inputs($_POST,
+                [
+                    "token" => [new EqualsRule($_SESSION["token"])],
+                    "email" => [new IsSetRule(), new EmailRule()],
+                    "password" => [
+                        new IsSetRule(),
+                        new LengthRule(UserManager::MIN_PASSWORD_LENGTH, UserManager::MAX_PASSWORD_LENGTH)
+                    ],
+                    "password_confirm" => [new IsSetRule()],
+                ])
+                ?? $user_manager->register($_POST["email"], $_POST["password"], $_POST["password_confirm"]);
             break;
         case "login":
-            $response = validate_csrf()
-                ?? validate_logged_out()
-                ?? validate_has_arguments($_POST["email"], $_POST["password"]);
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsNotSetRule()]])
+                ?? Validator::validate_inputs($_POST,
+                [
+                    "token" => [new EqualsRule($_SESSION["token"])],
+                    "email" => [new IsSetRule(), new EmailRule()],
+                    "password" => [
+                        new IsSetRule(),
+                        new LengthRule(UserManager::MIN_PASSWORD_LENGTH, UserManager::MAX_PASSWORD_LENGTH)
+                    ],
+                ]);
             if ($response !== null) break;
 
             [$response, $uuid] = $user_manager->check_login($_POST["email"], $_POST["password"]);
             if ($response->satisfied) $_SESSION["uuid"] = $uuid;
             break;
         case "logout":
-            $response = validate_csrf() ?? validate_logged_in();
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? Validator::validate_inputs($_POST, ["token" => [new EqualsRule($_SESSION["token"])]]);
             if ($response !== null) break;
 
             session_destroy();
             session_start();
-            $_SESSION["token"] = generate_csrf_token($logger);
+            $_SESSION["token"] = Util::generate_csrf_token($logger) ?? Util::http_exit(500);
             $response = new Response(payload: null, satisfied: true);
             break;
         case "update-email":
-            $response = validate_csrf()
-                ?? validate_logged_in()
-                ?? validate_has_arguments($_POST["email"])
-                ?? $user_manager->set_email($_SESSION["uuid"], $_POST["email"], $mailer);
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? Validator::validate_inputs($_POST,
+                [
+                    "token" => [new EqualsRule($_SESSION["token"])],
+                    "email" => [new IsSetRule(), new EmailRule()],
+                ])
+                ?? $user_manager->set_email($_SESSION["uuid"], $_POST["email"]);
             break;
         case "verify-email":
-            $response = validate_has_arguments($_POST["email"], $_POST["token"])
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? Validator::validate_inputs($_POST,
+                [
+                    "token" => [new IsSetRule()],
+                    "email" => [new IsSetRule(), new EmailRule()],
+                ])
                 ?? $user_manager->verify_email($_POST["email"], $_POST["token"]);
             break;
         case "resend-verify-email":
-            $response = validate_csrf()
-                ?? validate_logged_in()
-                ?? $user_manager->resend_verify_email($_SESSION["uuid"], $mailer);
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? Validator::validate_inputs($_POST, ["token" => [new EqualsRule($_SESSION["token"])]])
+                ?? $user_manager->resend_verify_email($_SESSION["uuid"]);
             break;
         case "update-password":
-            $response = validate_csrf()
-                ?? validate_logged_in()
-                ?? validate_has_arguments($_POST["password_old"], $_POST["password_new"], $_POST["password_confirm"])
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? Validator::validate_inputs($_POST,
+                [
+                    "token" => [new EqualsRule($_SESSION["token"])],
+                    "password_old" => [
+                        new IsSetRule(),
+                        new LengthRule(UserManager::MIN_PASSWORD_LENGTH, UserManager::MAX_PASSWORD_LENGTH)
+                    ],
+                    "password_new" => [
+                        new IsSetRule(),
+                        new LengthRule(UserManager::MIN_PASSWORD_LENGTH, UserManager::MAX_PASSWORD_LENGTH)
+                    ],
+                    "password_confirm" => [new IsSetRule()],
+                ])
                 ?? $user_manager->set_password(
                     $_SESSION["uuid"],
                     $_POST["password_old"],
@@ -194,20 +140,37 @@ if (isset($_POST["action"])) {
                 );
             break;
         case "user-delete":
-            $response = validate_csrf()
-                ?? validate_logged_in()
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? Validator::validate_inputs($_POST, ["token" => [new EqualsRule($_SESSION["token"])]])
                 ?? $user_manager->delete($_SESSION["uuid"]);
             break;
         case "add-tracking":
-            $response = validate_csrf()
-                ?? validate_logged_in()
-                ?? validate_has_arguments($_POST["person_name"])
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? Validator::validate_inputs($_POST,
+                [
+                    "token" => [new EqualsRule($_SESSION["token"])],
+                    "person_name" => [
+                        new IsSetRule(),
+                        new LengthRule(TrackingManager::MIN_TITLE_LENGTH, TrackingManager::MAX_TITLE_LENGTH),
+                        new IsNotBlankRule()
+                    ],
+                ])
                 ?? $tracking_manager->add_tracking($_SESSION["uuid"], $_POST["person_name"]);
             break;
         case "remove-tracking":
-            $response = validate_csrf()
-                ?? validate_logged_in()
-                ?? validate_has_arguments($_POST["person_name"])
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? Validator::validate_inputs($_POST,
+                [
+                    "token" => [new EqualsRule($_SESSION["token"])],
+                    "person_name" => [
+                        new IsSetRule(),
+                        new LengthRule(TrackingManager::MIN_TITLE_LENGTH, TrackingManager::MAX_TITLE_LENGTH),
+                        new IsNotBlankRule()
+                    ],
+                ])
                 ?? $tracking_manager->remove_tracking($_SESSION["uuid"], $_POST["person_name"]);
             break;
         default:
@@ -230,14 +193,18 @@ if (isset($_POST["action"])) {
             if (!$response->satisfied) {
                 session_destroy();
                 session_start();
-                $_SESSION["token"] = generate_csrf_token($logger);
+                $_SESSION["token"] = Util::generate_csrf_token($logger) ?? Util::http_exit(500);
             }
             break;
         case "get-user-data":
-            $response = validate_logged_in() ?? $user_manager->get_user_data($_SESSION["uuid"]);
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? $user_manager->get_user_data($_SESSION["uuid"]);
             break;
         case "list-trackings":
-            $response = validate_logged_in() ?? $tracking_manager->list_trackings($_SESSION["uuid"]);
+            $response =
+                Validator::validate_inputs($_SESSION, ["uuid" => [new IsSetRule()]])
+                ?? $tracking_manager->list_trackings($_SESSION["uuid"]);
             break;
         default:
             $response = new Response(
@@ -255,7 +222,7 @@ if (isset($_POST["action"])) {
     switch ($argv[1]) {
         case "update-all-trackings":
             $logger->info("Updating all trackings.");
-            $tracking_manager->update_trackings($mailer, $tracking_manager->list_all_unique_person_names());
+            $tracking_manager->update_trackings($tracking_manager->list_all_unique_person_names());
             exit("Successfully updated all trackings.");
         case "process-email-queue":
             $logger->info("Processing email queue.");
diff --git a/src/main/index.html b/src/main/index.html
index 3cbefd1..39fdc03 100644
--- a/src/main/index.html
+++ b/src/main/index.html
@@ -91,7 +91,7 @@
                         </label>
                         <label for="registerPasswordConfirm">
                             Confirm password
-                            <input id="registerPasswordConfirm" type="password" name="passwordConfirm" />
+                            <input id="registerPasswordConfirm" type="password" name="password_confirm" />
                             <span class="validationInfo"></span>
                         </label>
                         <!-- TODO: Display loading indicator while waiting for form to complete -->
@@ -119,7 +119,7 @@
                                         <span class="validationInfo"></span>
                                     </p>
                                     <label for="addTrackingPersonName">
-                                        <input id="addTrackingPersonName" type="text" name="personName"
+                                        <input id="addTrackingPersonName" type="text" name="person_name"
                                                autocomplete="on" />
                                         <button id="addTrackingButton">Add</button>
                                         <span class="validationInfo"></span>
@@ -179,17 +179,17 @@
                         </p>
                         <label for="updatePasswordPasswordOld">
                             Old password
-                            <input id="updatePasswordPasswordOld" type="password" name="passwordOld" />
+                            <input id="updatePasswordPasswordOld" type="password" name="password_old" />
                             <span class="validationInfo"></span>
                         </label>
                         <label for="updatePasswordPasswordNew">
                             New password
-                            <input id="updatePasswordPasswordNew" type="password" name="passwordNew" />
+                            <input id="updatePasswordPasswordNew" type="password" name="password_new" />
                             <span class="validationInfo"></span>
                         </label>
                         <label for="updatePasswordPasswordConfirm">
                             Confirm new password
-                            <input id="updatePasswordPasswordConfirm" type="password" name="passwordConfirm" />
+                            <input id="updatePasswordPasswordConfirm" type="password" name="password_confirm" />
                             <span class="validationInfo"></span>
                         </label>
                         <button id="updatePasswordButton">Change password</button>
diff --git a/src/main/php/Database.php b/src/main/php/Database.php
index d035764..45ab3a6 100644
--- a/src/main/php/Database.php
+++ b/src/main/php/Database.php
@@ -24,5 +24,19 @@ class Database
     }
 
 
+    /**
+     * Returns `true` if no tables exist in the database.
+     *
+     * @param PDO $conn the database connection to use to check
+     * @return bool `true` if no tables exist in the database
+     */
+    public static function is_empty(PDO $conn): bool
+    {
+        $stmt = $conn->prepare("SELECT count(*) FROM sqlite_master WHERE type = 'table';");
+        $stmt->execute();
+        return $stmt->fetch()[0] === 0;
+    }
+
+
     // TODO: Add version number, etc., and auto-migration
 }
diff --git a/src/main/php/Mailer.php b/src/main/php/Mailer.php
index ebdaced..1580eec 100644
--- a/src/main/php/Mailer.php
+++ b/src/main/php/Mailer.php
@@ -55,6 +55,7 @@ class Mailer
                                                     PRIMARY KEY (type, arg1, arg2));");
     }
 
+
     /**
      * Queues an email to be sent for a newly registered user.
      *
@@ -174,6 +175,7 @@ class Mailer
         ];
     }
 
+
     /**
      * Sends all emails in the queue.
      *
diff --git a/src/main/php/Mediawiki.php b/src/main/php/Mediawiki.php
index b45b18b..a73c488 100644
--- a/src/main/php/Mediawiki.php
+++ b/src/main/php/Mediawiki.php
@@ -26,7 +26,7 @@ class Mediawiki
      *
      * Since the record for a single page is 252 categories, setting to the maximum of 500 is more than sufficient.
      */
-    private const CATS_PER_QUERY = "500";
+    private const CATS_PER_QUERY = 500;
 
     /**
      * @var Logger The logger to use for logging.
@@ -171,7 +171,7 @@ class Mediawiki
      */
     public function people_statuses(array $people_names): QueryOutput
     {
-        $output = $this->api_query(["prop" => "categories", "cllimit" => self::CATS_PER_QUERY], $people_names);
+        $output = $this->api_query(["prop" => "categories", "cllimit" => strval(self::CATS_PER_QUERY)], $people_names);
 
         $pages =
             array_combine(
diff --git a/src/main/php/Response.php b/src/main/php/Response.php
index 3d2bf91..8228f34 100644
--- a/src/main/php/Response.php
+++ b/src/main/php/Response.php
@@ -21,6 +21,9 @@ class Response
     /**
      * Constructs a new response.
      *
+     * If `satisfied` is `false`, then `payload` must be an array containing at least the strings `target` and
+     * `message`. Otherwise, `payload` may be anything.
+     *
      * @param mixed $payload the payload corresponding to the client's query
      * @param bool $satisfied `true` if and only if the request was fully completed
      */
diff --git a/src/main/php/TrackingManager.php b/src/main/php/TrackingManager.php
index 0b166b3..8c1d9c3 100644
--- a/src/main/php/TrackingManager.php
+++ b/src/main/php/TrackingManager.php
@@ -10,30 +10,41 @@ use PDO;
  */
 class TrackingManager
 {
+    /**
+     * The minimum length of a Wikipedia page title.
+     */
+    public const MIN_TITLE_LENGTH = 1;
     /**
      * The maximum length of a Wikipedia page title.
      */
-    private const MAX_TITLE_LENGTH = 255;
+    public const MAX_TITLE_LENGTH = 255;
 
-    /**
-     * @var Mediawiki The Mediawiki instance to use for interacting with Wikipedia.
-     */
-    private Mediawiki $mediawiki;
     /**
      * @var PDO The database connection to interact with.
      */
     private PDO $conn;
+    /**
+     * @var Mailer The mailer to send emails with.
+     */
+    private Mailer $mailer;
+    /**
+     * @var Mediawiki The Mediawiki instance to use for interacting with Wikipedia.
+     */
+    private Mediawiki $mediawiki;
 
 
     /**
      * Constructs a new tracking manager.
      *
      * @param PDO $conn the database connection to interact with
+     * @param Mailer $mailer the mailer to send emails with
+     * @param Mediawiki $mediawiki the Mediawiki instance to use for interacting with Wikipedia
      */
-    public function __construct(Mediawiki $mediawiki, PDO $conn)
+    public function __construct(PDO $conn, Mailer $mailer, Mediawiki $mediawiki)
     {
-        $this->mediawiki = $mediawiki;
         $this->conn = $conn;
+        $this->mailer = $mailer;
+        $this->mediawiki = $mediawiki;
     }
 
 
@@ -44,8 +55,8 @@ class TrackingManager
      */
     public function install(): void
     {
-        $this->conn->exec("CREATE TABLE trackings(user_uuid text not null,
-                                                  person_name text not null,
+        $this->conn->exec("CREATE TABLE trackings(user_uuid TEXT NOT NULL,
+                                                  person_name TEXT NOT NULL,
                                                   PRIMARY KEY (user_uuid, person_name),
                                                   FOREIGN KEY (user_uuid) REFERENCES users (uuid)
                                                       ON DELETE CASCADE
@@ -53,9 +64,9 @@ class TrackingManager
                                                   FOREIGN KEY (person_name) REFERENCES people (name)
                                                       ON DELETE CASCADE
                                                       ON UPDATE CASCADE);");
-        $this->conn->exec("CREATE TABLE people(name text not null unique primary key,
-                                               status text not null,
-                                               is_deleted int not null default 0);");
+        $this->conn->exec("CREATE TABLE people(name TEXT NOT NULL UNIQUE PRIMARY KEY,
+                                               status TEXT NOT NULL DEFAULT '',
+                                               is_deleted INT NOT NULL DEFAULT 0);");
         $this->conn->exec("CREATE TRIGGER people_cull_orphans
                            AFTER DELETE ON trackings
                            FOR EACH ROW
@@ -65,26 +76,17 @@ class TrackingManager
                            END;");
     }
 
+
     /**
      * Adds a tracking to the database.
      *
      * @param string $user_uuid the user to whom the tracking belongs
      * @param string $person_name the name of the person to track
-     * @return Response a response with no message, and a status indicating whether the insertion succeeded
+     * @return Response a satisfied `Response` with payload `null` if the tracking was added, or an unsatisfied
+     * `Response` otherwise
      */
     public function add_tracking(string $user_uuid, string $person_name): Response
     {
-        if (trim($person_name) === "")
-            return new Response(
-                payload: ["target" => "personName", "message" => "Invalid page name: empty."],
-                satisfied: false
-            );
-        if (strlen($person_name) > self::MAX_TITLE_LENGTH)
-            return new Response(
-                payload: ["target" => "personName", "message" => "Invalid page name: too long."],
-                satisfied: false
-            );
-
         $pages_exist = $this->mediawiki->pages_exist([$person_name]);
         $normalized_name = $pages_exist->redirects[$person_name];
         if (in_array($normalized_name, $pages_exist->missing))
@@ -104,15 +106,13 @@ class TrackingManager
         // Insert person and tracking
         $this->conn->beginTransaction();
 
-        $stmt = $this->conn->prepare("INSERT OR IGNORE INTO people (name, status)
-                                      VALUES (:name, :status);");
+        $stmt = $this->conn->prepare("INSERT OR IGNORE INTO people (name) VALUES (:name);");
         $stmt->bindValue(":name", $normalized_name);
-        $stmt->bindValue(":status", $status->value);
         $stmt->execute();
 
         $stmt = $this->conn->prepare("UPDATE people SET status=:status WHERE name=:name;");
-        $stmt->bindValue(":status", $status->value);
         $stmt->bindValue(":name", $normalized_name);
+        $stmt->bindValue(":status", $status->value);
         $stmt->execute();
 
         $stmt = $this->conn->prepare("INSERT OR IGNORE INTO trackings (user_uuid, person_name)
@@ -130,16 +130,10 @@ class TrackingManager
      *
      * @param string $user_uuid the user to whom the tracking belongs
      * @param string $person_name the name of the tracked person to remove
-     * @return Response a response with no message, and a status indicating whether the removal succeeded
+     * @return Response a satisfied `Response` with payload `null`
      */
     public function remove_tracking(string $user_uuid, string $person_name): Response
     {
-        if (strlen($person_name) > self::MAX_TITLE_LENGTH)
-            return new Response(
-                payload: ["target" => "personName", "message" => "Invalid page name: too long."],
-                satisfied: false
-            );
-
         $stmt = $this->conn->prepare("DELETE FROM trackings
                                       WHERE user_uuid=:user_uuid AND person_name=:person_name;");
         $stmt->bindValue(":user_uuid", $user_uuid);
@@ -153,7 +147,7 @@ class TrackingManager
      * Lists all trackings of the indicated user.
      *
      * @param string $user_uuid the user to return the trackings of
-     * @return Response a response with all trackings of the indicated user
+     * @return Response a satisfied `Response` with the trackings as its payload
      */
     public function list_trackings(string $user_uuid): Response
     {
@@ -169,10 +163,11 @@ class TrackingManager
         return new Response(payload: $results, satisfied: true);
     }
 
+
     /**
      * Lists all unique names being tracked in the database.
      *
-     * @return array<string> the unique records in the database
+     * @return array<string> all unique names in the database
      */
     public function list_all_unique_person_names(): array
     {
@@ -184,23 +179,21 @@ class TrackingManager
     /**
      * Updates trackings for the given names.
      *
-     * @param Mailer $mailer the mailer to notify the user with
      * @param string[] $people_names the names of the pages to update the tracking of
      * @return void
      */
-    public function update_trackings(Mailer $mailer, array $people_names): void
+    public function update_trackings(array $people_names): void
     {
         if (empty($people_names)) return;
 
+        // Fetch statuses from Mediawiki
         $people_statuses = $this->mediawiki->people_statuses($people_names);
 
         // Begin transaction
         $this->conn->beginTransaction();
 
         // Process redirects
-        $stmt = $this->conn->prepare("UPDATE people
-                                      SET name=:new_name
-                                      WHERE name=:old_name;");
+        $stmt = $this->conn->prepare("UPDATE people SET name=:new_name WHERE name=:old_name;");
         $stmt->bindParam(":old_name", $from);
         $stmt->bindParam(":new_name", $to);
         foreach ($people_statuses->redirects as $from => $to)
@@ -208,9 +201,7 @@ class TrackingManager
                 $stmt->execute();
 
         // Process deletions
-        $stmt = $this->conn->prepare("UPDATE people
-                                      SET is_deleted=1
-                                      WHERE name=:name;");
+        $stmt = $this->conn->prepare("UPDATE people SET is_deleted=1 WHERE name=:name;");
         $stmt->bindParam(":name", $missing_title);
         foreach ($people_statuses->missing as $missing_title)
             // TODO: Inform user that page has been deleted
@@ -242,7 +233,7 @@ class TrackingManager
             if (sizeof($stmt_status->fetchAll(PDO::FETCH_ASSOC)) > 0 && $person_status === PersonStatus::Alive) {
                 $stmt_emails->execute();
                 foreach (array_column($stmt_emails->fetchAll(PDO::FETCH_ASSOC), "email") as $user_email)
-                    $mailer->queue_death_notification($user_email, $person_name);
+                    $this->mailer->queue_death_notification($user_email, $person_name);
             }
         }
 
diff --git a/src/main/php/UserManager.php b/src/main/php/UserManager.php
index 4dbfda1..8b42677 100644
--- a/src/main/php/UserManager.php
+++ b/src/main/php/UserManager.php
@@ -12,18 +12,14 @@ use PDO;
 // TODO: Remove duplication in this class
 class UserManager
 {
-    /**
-     * The maximum length of an email address.
-     */
-    private const MAX_EMAIL_LENGTH = 254;
     /**
      * The minimum length of a password;
      */
-    private const MIN_PASSWORD_LENGTH = 8;
+    public const MIN_PASSWORD_LENGTH = 8;
     /**
      * The maximum length of a password.
      */
-    private const MAX_PASSWORD_LENGTH = 64;
+    public const MAX_PASSWORD_LENGTH = 64;
     /**
      * The minimum number of minutes between two verification emails.
      */
@@ -33,16 +29,22 @@ class UserManager
      * @var PDO The database connection to interact with.
      */
     private PDO $conn;
+    /**
+     * @var Mailer The mailer to send emails with.
+     */
+    private Mailer $mailer;
 
 
     /**
      * Constructs a new user manager.
      *
      * @param PDO $conn the database connection to interact with
+     * @param Mailer $mailer the mailer to send emails with
      */
-    public function __construct(PDO $conn)
+    public function __construct(PDO $conn, Mailer $mailer)
     {
         $this->conn = $conn;
+        $this->mailer = $mailer;
     }
 
 
@@ -53,43 +55,29 @@ class UserManager
      */
     public function install(): void
     {
-        $this->conn->exec("CREATE TABLE users(uuid text not null unique primary key default(lower(hex(randomblob(16)))),
-                                              email text not null unique,
-                                              email_verification_token text,
-                                              email_verification_token_timestamp int not null,
-                                              password text not null,
-                                              password_update_time int not null);");
+        $this->conn->exec("CREATE TABLE users(uuid TEXT NOT NULL UNIQUE PRIMARY KEY DEFAULT(lower(hex(randomblob(16)))),
+                                              email TEXT NOT NULL UNIQUE,
+                                              email_verification_token TEXT,
+                                              email_verification_token_timestamp INT NOT NULL,
+                                              password TEXT NOT NULL,
+                                              password_update_time INT NOT NULL);");
     }
 
+
     /**
      * Registers a new user.
      *
-     * @param Mailer $mailer the mailer to notify the user with
      * @param string $email the user-submitted email address
      * @param string $password the user-submitted password
      * @param string $password_confirm the user-submitted password confirmation
-     * @return Response a response with message `null` if the user was registered, or a response with a message
-     * explaining what went wrong otherwise
+     * @return Response a satisfied `Response` with payload `null` if the registration was successful, or an unsatisfied
+     * `Response` otherwise
      */
-    public function register(Mailer $mailer, string $email, string $password, string $password_confirm): Response
+    public function register(string $email, string $password, string $password_confirm): Response
     {
-        // Validate
-        if (!filter_var($email, FILTER_VALIDATE_EMAIL) || strlen($email) > self::MAX_EMAIL_LENGTH)
-            return new Response(
-                payload: ["target" => "email", "message" => "Invalid email address."],
-                satisfied: false
-            );
-        if (strlen($password) < self::MIN_PASSWORD_LENGTH || strlen($password) > self::MAX_PASSWORD_LENGTH)
-            return new Response(
-                payload: [
-                    "target" => "password",
-                    "message" => "Your password should be at least 8 and at most 64 characters long."
-                ],
-                satisfied: false
-            );
         if ($password !== $password_confirm)
             return new Response(
-                payload: ["target" => "passwordConfirm", "message" => "Passwords do not match."],
+                payload: ["target" => "password_confirm", "message" => "Passwords do not match."],
                 satisfied: false
             );
 
@@ -97,13 +85,11 @@ class UserManager
         $this->conn->beginTransaction();
 
         // Check if email address is already in use
-        $stmt = $this->conn->prepare("SELECT COUNT(*) AS count
-                                      FROM users
-                                      WHERE email=:email;");
+        $stmt = $this->conn->prepare("SELECT EXISTS(SELECT 1 FROM users WHERE email=:email);");
         $stmt->bindValue(":email", $email);
         $stmt->execute();
-        $result = $stmt->fetch(PDO::FETCH_ASSOC);
-        if ($result["count"] > 0) {
+        $result = $stmt->fetch();
+        if ($result[0] === 1) {
             $this->conn->rollBack();
             return new Response(
                 payload: ["target" => "email", "message" => "Email address already in use."],
@@ -124,13 +110,14 @@ class UserManager
                                               unixepoch())
                                       RETURNING email_verification_token;");
         $stmt->bindValue(":email", $email);
+        // TODO: Specify password hash function, for forwards compatibility
         $stmt->bindValue(":password", password_hash($password, PASSWORD_DEFAULT));
         $stmt->execute();
         $email_verification_token = $stmt->fetchAll(PDO::FETCH_ASSOC)[0]["email_verification_token"];
+        $this->mailer->queue_registration($email, $email_verification_token);
 
         // Respond
         $this->conn->commit();
-        $mailer->queue_registration($email, $email_verification_token);
         return new Response(payload: null, satisfied: true);
     }
 
@@ -138,19 +125,13 @@ class UserManager
      * Deletes the user with the given UUID.
      *
      * @param string $uuid the UUID of the user to delete
-     * @return Response a response with message `null` if the user was deleted, or a response with a message explaining
-     * what went wrong otherwise
+     * @return Response a satisfied `Response` with payload `null`
      */
     public function delete(string $uuid): Response
     {
         $stmt = $this->conn->prepare("DELETE FROM users WHERE uuid=:uuid;");
         $stmt->bindValue(":uuid", $uuid);
         $stmt->execute();
-
-        $stmt = $this->conn->prepare("DELETE FROM trackings WHERE user_uuid=:uuid");
-        $stmt->bindValue(":uuid", $uuid);
-        $stmt->execute();
-
         return new Response(payload: null, satisfied: true);
     }
 
@@ -159,27 +140,11 @@ class UserManager
      *
      * @param string $email the email address of the user whose password should be checked
      * @param string $password the password to check against the specified user
-     * @return array{0: Response, 1: string|null} a response and the user's UUID
+     * @return array{Response, string|null} a satisfied `Response` with payload `null` and the user's UUID if the
+     * password is correct, or an unsatisfied `Response` and `null` otherwise
      */
     public function check_login(string $email, string $password): array
     {
-        if (!filter_var($email, FILTER_VALIDATE_EMAIL) || strlen($email) > self::MAX_EMAIL_LENGTH)
-            return [
-                new Response(
-                    payload: ["target" => "email", "message" => "Invalid email address."],
-                    satisfied: false
-                ),
-                null
-            ];
-        if (strlen($password) > self::MAX_PASSWORD_LENGTH)
-            return [
-                new Response(
-                    payload: ["target" => "password", "message" => "Incorrect combination of email and password."],
-                    satisfied: false
-                ),
-                null
-            ];
-
         $stmt = $this->conn->prepare("SELECT uuid, password FROM users WHERE email=:email;");
         $stmt->bindValue(":email", $email);
         $stmt->execute();
@@ -202,7 +167,8 @@ class UserManager
      * Returns a satisfied response if a user with the given UUID exists, or an unsatisfied response otherwise.
      *
      * @param string $uuid the UUID of the user to check
-     * @return Response a satisfied response if a user with the given UUID exists, or an unsatisfied response otherwise
+     * @return Response a satisfied `Response` with payload `null` if a user with the given UUID exists, or an
+     * unsatisfied `Response` otherwise
      */
     public function user_exists(string $uuid): Response
     {
@@ -220,7 +186,8 @@ class UserManager
      * Returns the user with the given UUID.
      *
      * @param string $uuid the UUID of the user to return
-     * @return Response the user with the given UUID, or a response with an explanation what went wrong otherwise
+     * @return Response a satisfied `Response` with the user's data if the user exists, or an unsatisfied `Response`
+     * otherwise
      */
     public function get_user_data(string $uuid): Response
     {
@@ -245,19 +212,11 @@ class UserManager
      *
      * @param string $uuid the UUID of the user whose email address should be updated
      * @param string $email the new email address
-     * @param Mailer $mailer the mailer to send a notification with to the user
-     * @return Response a response with message `null` if the email address was updated, or a response with a message
-     * explaining what went wrong otherwise
+     * @return Response a satisfied `Response` with payload `null` if the email address was updated, or an unsatisfied
+     * `Response` otherwise
      */
-    public function set_email(string $uuid, string $email, Mailer $mailer): Response
+    public function set_email(string $uuid, string $email): Response
     {
-        // Validate
-        if (!filter_var($email, FILTER_VALIDATE_EMAIL) || strlen($email) > self::MAX_EMAIL_LENGTH)
-            return new Response(
-                payload: ["target" => "email", "message" => "Invalid email address."],
-                satisfied: false
-            );
-
         // Begin transaction
         $this->conn->beginTransaction();
 
@@ -266,21 +225,22 @@ class UserManager
         $stmt->bindValue(":uuid", $uuid);
         $stmt->execute();
         $email_old = $stmt->fetch(PDO::FETCH_ASSOC)["email"];
-        if (hash_equals($email_old, $email))
+        if (hash_equals($email_old, $email)) {
+            $this->conn->rollBack();
             return new Response(
                 payload: ["target" => "email", "message" => "That is already your email address."],
                 satisfied: false
             );
+        }
 
         // TODO: Reject update if email address was changed too recently (within, say, 5 minutes)
 
         // Check if email address is already in use
-        $stmt = $this->conn->prepare("SELECT COUNT(*) AS count
-                                      FROM users
-                                      WHERE email=:email;");
+        $stmt = $this->conn->prepare("SELECT EXISTS(SELECT 1 FROM users WHERE email=:email);");
         $stmt->bindValue(":email", $email);
         $stmt->execute();
-        if ($stmt->fetch(PDO::FETCH_ASSOC)["count"] > 0) {
+        $result = $stmt->fetch();
+        if ($result[0] === 1) {
             $this->conn->rollBack();
             return new Response(
                 payload: ["target" => "email", "message" => "Email address already in use."],
@@ -302,7 +262,7 @@ class UserManager
 
         // Respond
         $this->conn->commit();
-        $mailer->queue_verification($email, $email_verification_token);
+        $this->mailer->queue_verification($email, $email_verification_token);
         return new Response(payload: null, satisfied: true);
     }
 
@@ -311,45 +271,35 @@ class UserManager
      *
      * @param string $email the email address to verify
      * @param string $email_verification_token the token to verify the email address with
-     * @return Response the server's response to the request
+     * @return Response a satisfied `Response` if the email address was newly verified, or an unsatisfied response if
+     * the email address is unknown, already verified, or the token is incorrect
      */
     public function verify_email(string $email, string $email_verification_token): Response
     {
-        if (!filter_var($email, FILTER_VALIDATE_EMAIL) || strlen($email) > self::MAX_EMAIL_LENGTH)
-            return new Response(
-                payload: ["target" => "email", "message" => "Invalid email address."],
-                satisfied: false
-            );
-
         $this->conn->beginTransaction();
 
-        $stmt = $this->conn->prepare("SELECT email_verification_token
-                                      FROM users
-                                      WHERE email=:email;");
+        $stmt = $this->conn->prepare("SELECT EXISTS(SELECT 1
+                                                    FROM users
+                                                    WHERE email=:email
+                                                        AND email_verification_token=:email_verification_token);");
         $stmt->bindValue(":email", $email);
+        $stmt->bindValue(":email_verification_token", $email_verification_token);
         $stmt->execute();
-        $result = $stmt->fetch(PDO::FETCH_ASSOC);
-
-        // TODO: Reformat the following line
-        if ($result === false ||
-            ($result["email_verification_token"] !== null
-                && !hash_equals($result["email_verification_token"], $email_verification_token))) {
+        $result = $stmt->fetch();
+        if ($result[0] !== 1) {
             $this->conn->rollBack();
             return new Response(
-                payload: ["target" => "email", "message" => "Failed to verify email address."],
+                payload: [
+                    "target" => "email",
+                    "message" =>
+                        "Failed to verify email address. " .
+                        "Perhaps this email address has already been verified."
+                ],
                 satisfied: false
             );
         }
 
-        if ($result["email_verification_token"] === null) {
-            // Email already verified, returning
-            $this->conn->rollBack();
-            return new Response(payload: null, satisfied: true);
-        }
-
-        $stmt = $this->conn->prepare("UPDATE users
-                                      SET email_verification_token=null
-                                      WHERE email=:email;");
+        $stmt = $this->conn->prepare("UPDATE users SET email_verification_token=null WHERE email=:email;");
         $stmt->bindValue(":email", $email);
         $stmt->execute();
 
@@ -361,11 +311,10 @@ class UserManager
      * Resends the email verification email.
      *
      * @param string $uuid the UUID of the user to resend the email for
-     * @param Mailer $mailer the mailer to send the email with to the user
-     * @return Response a response with message `null` if the email was sent, or a response with a message explaining
-     * what went wrong otherwise
+     * @return Response a satisfied `Response` with payload `null` if the email was sent, or an unsatisfied `Response`
+     * otherwise
      */
-    public function resend_verify_email(string $uuid, Mailer $mailer): Response
+    public function resend_verify_email(string $uuid): Response
     {
         $this->conn->beginTransaction();
 
@@ -375,7 +324,6 @@ class UserManager
         $stmt->bindValue(":uuid", $uuid);
         $stmt->execute();
         $user = $stmt->fetch(PDO::FETCH_ASSOC);
-
         if (!isset($user["email_verification_token"])) {
             $this->conn->rollBack();
             return new Response(
@@ -400,13 +348,12 @@ class UserManager
             );
         }
 
-        $mailer->queue_verification($user["email"], $user["email_verification_token"]);
-
         $stmt = $this->conn->prepare("UPDATE users
                                       SET email_verification_token_timestamp=unixepoch()
                                       WHERE uuid=:uuid;");
         $stmt->bindValue(":uuid", $uuid);
         $stmt->execute();
+        $this->mailer->queue_verification($user["email"], $user["email_verification_token"]);
 
         $this->conn->commit();
         return new Response(payload: null, satisfied: true);
@@ -419,26 +366,12 @@ class UserManager
      * @param string $password_old the old password
      * @param string $password_new the new password
      * @param string $password_confirm the confirmation of the new password
-     * @return Response a response with message `null` if the password was updated, or a response with a message
-     * explaining what went wrong otherwise
+     * @return Response a satisfied `Response` with payload `null` if the password was updated, or an unsatisfied
+     * `Response` otherwise
      */
     public function set_password(string $uuid, string $password_old, string $password_new,
                                  string $password_confirm): Response
     {
-        // Validate
-        if (strlen($password_old) > self::MAX_PASSWORD_LENGTH)
-            return new Response(
-                payload: ["target" => "passwordOld", "message" => "Incorrect old password."],
-                satisfied: false
-            );
-        if (strlen($password_new) < self::MIN_PASSWORD_LENGTH || strlen($password_new) > self::MAX_PASSWORD_LENGTH)
-            return new Response(
-                payload: [
-                    "target" => "passwordNew",
-                    "message" => "Your password should be at least 8 and at most 64 characters long."
-                ],
-                satisfied: false
-            );
         if ($password_new !== $password_confirm)
             return new Response(
                 payload: ["target" => "passwordConfirm", "message" => "New passwords do not match."],
diff --git a/src/main/php/Util.php b/src/main/php/Util.php
new file mode 100644
index 0000000..d7af47a
--- /dev/null
+++ b/src/main/php/Util.php
@@ -0,0 +1,96 @@
+<?php
+
+namespace php;
+
+use Exception;
+use Monolog\ErrorHandler;
+use Monolog\Handler\StreamHandler;
+use Monolog\Logger;
+
+
+/**
+ * Helper functions.
+ */
+class Util
+{
+    /**
+     * Sets HTTP status code and exits the script.
+     *
+     * @param int $status the HTTP status code to set
+     * @return never-returns
+     */
+    static function http_exit(int $status): void
+    {
+        http_response_code($status);
+        exit(1);
+    }
+
+    /**
+     * Reads the configuration file and overrides it with the user's custom values.
+     *
+     * @return array<string, array<string, mixed>>|null the configuration
+     */
+    static function read_config(): ?array
+    {
+        // TODO: Check permissions, return `null` if too permissive
+        $config = parse_ini_file("config.default.ini.php", process_sections: true, scanner_mode: INI_SCANNER_TYPED);
+        if ($config === false) return null;
+
+        if (file_exists("config.ini.php")) {
+            $config_custom = parse_ini_file("config.ini.php", process_sections: true, scanner_mode: INI_SCANNER_TYPED);
+            if ($config_custom === false) return null;
+
+            $config = array_merge($config, $config_custom);
+        }
+
+        return $config;
+    }
+
+    /**
+     * Creates the main logger instance.
+     *
+     * @param array<string, mixed> $logger_config the logger section of the configuration
+     * @return Logger the main logger instance
+     */
+    static function create_logger(array $logger_config): Logger
+    {
+        $logger = new Logger("main");
+        $logger->pushHandler(new StreamHandler($logger_config["file"], $logger_config["level"]));
+        ErrorHandler::register($logger);
+
+        return $logger;
+    }
+
+    /**
+     * Generates an appropriate CSRF token.
+     *
+     * @param Logger $logger the logger to use if something goes wrong
+     * @return string|null the CSRF token, or `null` if no CSRF token could be created
+     */
+    static function generate_csrf_token(Logger $logger): ?string
+    {
+        try {
+            return bin2hex(random_bytes(32));
+        } catch (Exception $exception) {
+            $logger->emergency("Failed to generate token.", ["cause" => $exception]);
+            return null;
+        }
+    }
+
+    /**
+     * Parses POST values from JSON-based inputs.
+     *
+     * @return ?array<string, mixed> the parsed POST-ed values
+     */
+    static function parse_post(): ?array
+    {
+        $output = $_POST;
+
+        $post_input = file_get_contents("php://input");
+        if ($post_input !== false)
+            $output = json_decode($post_input, associative: true);
+
+        return $output;
+    }
+
+}
diff --git a/src/main/php/Validator.php b/src/main/php/Validator.php
new file mode 100644
index 0000000..d06c0f9
--- /dev/null
+++ b/src/main/php/Validator.php
@@ -0,0 +1,280 @@
+<?php
+
+namespace php;
+
+
+/**
+ * Validates arrays of inputs such as `$_POST` or `$_SESSION` using `Rule`s.
+ */
+class Validator
+{
+    /**
+     * Checks whether values in `inputs` match the rules specified in `rule_sets`.
+     *
+     * @param array<string, string> $inputs the array of inputs in which to check the values
+     * @param array<string, Rule[]> $rule_sets maps keys in `inputs` to an array of `Rule`s to be checked
+     * @return Response|null `null` if all rules are satisfied, or an unsatisfied `Response` otherwise
+     */
+    static function validate_inputs(array $inputs, array $rule_sets): Response|null
+    {
+        foreach ($rule_sets as $key => $rules) {
+            foreach ($rules as $rule) {
+                $is_valid = $rule->check($inputs, $key);
+                if ($is_valid !== null)
+                    return $is_valid;
+            }
+        }
+
+        return null;
+    }
+}
+
+
+/**
+ * A rule/constraint/assertion that should hold over an input.
+ */
+abstract class Rule
+{
+    /**
+     * @var string|null The message to return if the rule does not apply to some input. If `null`, the rule
+     * implementation can choose an appropriate message.
+     */
+    public ?string $override_message;
+
+
+    /**
+     * Instantiates a new rule.
+     *
+     * @param string|null $override_message the message to return if the rule does not apply to some input. If `null`,
+     * the rule implementation can choose an appropriate message
+     */
+    public function __construct(?string $override_message = null)
+    {
+        $this->override_message = $override_message;
+    }
+
+
+    /**
+     * Checks whether the rule holds for `$inputs[$key]`.
+     *
+     * @param array<string, mixed> $inputs the list of inputs in which the value at `key` should be checked
+     * @param string $key the key in `inputs` of the input to check
+     * @return Response|null `null` if the rule holds, or an unsatisfied `Response` otherwise
+     */
+    public abstract function check(array $inputs, string $key): Response|null;
+}
+
+/**
+ * Requires the input to be a valid email address.
+ */
+class EmailRule extends Rule
+{
+    /**
+     * Checks whether the input is a valid email address.
+     *
+     * @param array<string, mixed> $inputs the list of inputs in which the value at `key` should be checked
+     * @param string $key the key in `inputs` of the input to check
+     * @return Response|null `null` if `$inputs[$key]` is an email address, or an unsatisfied `Response` otherwise
+     */
+    public function check(array $inputs, string $key): Response|null
+    {
+        if (!isset($inputs[$key]) || !filter_var($inputs[$key], FILTER_VALIDATE_EMAIL))
+            return new Response(
+                payload: ["target" => $key, "message" => $this->override_message ?? "Invalid email address."],
+                satisfied: false
+            );
+
+        return null;
+    }
+}
+
+/**
+ * Requires the input to be equal to a pre-known value.
+ */
+class EqualsRule extends Rule
+{
+    /**
+     * @var string|null The known value to check against, or `null` if the check should always fail.
+     */
+    private string|null $known;
+
+
+    /**
+     * Instantiates a new rule.
+     *
+     * @param string|null $known the known value to check against, or `null` if the check should always fail
+     * @param string|null $override_message the message to return if the rule does not apply to some input. If `null`,
+     * the rule implementation can choose an appropriate message
+     */
+    public function __construct(string|null $known, ?string $override_message = null)
+    {
+        parent::__construct($override_message);
+
+        $this->known = $known;
+    }
+
+
+    /**
+     * Checks whether the input equals the known value.
+     *
+     * @param array<string, mixed> $inputs the list of inputs in which the value at `key` should be checked
+     * @param string $key the key in `inputs` of the input to check
+     * @return Response|null `null` if `$this->known` is not `null` and `$inputs[$key] === $this->known`, or an
+     * unsatisfied `Response` otherwise
+     */
+    public function check(array $inputs, string $key): Response|null
+    {
+        if ($this->known === null || !isset($inputs[$key]) || !hash_equals($this->known, $inputs[$key]))
+            return new Response(
+                payload: [
+                    "target" => $key,
+                    "message" => $this->override_message ?? "Input '$key' does not equal known string."
+                ],
+                satisfied: false
+            );
+
+        return null;
+    }
+}
+
+/**
+ * Requires the input to not be blank.
+ */
+class IsNotBlankRule extends Rule
+{
+    /**
+     * Checks whether the input is not blank.
+     *
+     * @param array<string, mixed> $inputs the list of inputs in which the value at `key` should be checked
+     * @param string $key the key in `inputs` of the input to check
+     * @return Response|null `null` if `trim($inputs[$key])` is not an empty string, or an unsatisfied `Response`
+     * otherwise
+     */
+    public function check(array $inputs, string $key): Response|null
+    {
+        if (!isset($inputs[$key]) || trim($inputs[$key]) === "")
+            return new Response(
+                payload: ["target" => $key, "message" => $this->override_message ?? "'$key' should not be blank."],
+                satisfied: false
+            );
+
+        return null;
+    }
+}
+
+/**
+ * Requires the input to be unset.
+ */
+class IsNotSetRule extends Rule
+{
+    /**
+     * Checks whether the input is unset.
+     *
+     * @param array<string, mixed> $inputs the list of inputs in which the value at `key` should be checked
+     * @param string $key the key in `inputs` of the input to check
+     * @return Response|null `null` if `!isset($inputs[$key])`, or an unsatisfied `Response` otherwise
+     */
+    public function check(array $inputs, string $key): Response|null
+    {
+        if (isset($inputs[$key]))
+            return new Response(
+                payload: ["target" => $key, "message" => $this->override_message ?? "Unexpected input '$key'."],
+                satisfied: false
+            );
+
+        return null;
+    }
+}
+
+/**
+ * Requires the input to be set.
+ */
+class IsSetRule extends Rule
+{
+    /**
+     * Checks whether the input is set.
+     *
+     * @param array<string, mixed> $inputs the list of inputs in which the value at `key` should be checked
+     * @param string $key the key in `inputs` of the input to check
+     * @return Response|null `null` if `isset($inputs[$key])`, or an unsatisfied `Response` otherwise
+     */
+    public function check(array $inputs, string $key): Response|null
+    {
+        if (!isset($inputs[$key]))
+            return new Response(
+                payload: ["target" => $key, "message" => $this->override_message ?? "Missing input '$key'."],
+                satisfied: false
+            );
+
+        return null;
+    }
+}
+
+/**
+ * Requires the input to be of a specific length.
+ */
+class LengthRule extends Rule
+{
+    /**
+     * @var int|null The minimum length (inclusive), or `null` if there is no minimum length.
+     */
+    private readonly ?int $min_length;
+    /**
+     * @var int|null The maximum length (inclusive), or `null` if there is no maximum length.
+     */
+    private readonly ?int $max_length;
+
+
+    /**
+     * Instantiates a new rule.
+     *
+     * @param int|null $min_length the minimum length (inclusive), or `null` if there is no minimum length
+     * @param int|null $max_length the maximum length (inclusive), or `null` if there is no maximum length
+     * @param string|null $override_message the message to return if the rule does not apply to some input. If `null`,
+     * the rule implementation can choose an appropriate message
+     */
+    public function __construct(?int $min_length = null, ?int $max_length = null, ?string $override_message = null)
+    {
+        parent::__construct($override_message);
+
+        $this->min_length = $min_length;
+        $this->max_length = $max_length;
+    }
+
+
+    /**
+     * Checks whether the input is of the specified length.
+     *
+     * @param array<string, mixed> $inputs the list of inputs in which the value at `key` should be checked
+     * @param string $key the key in `inputs` of the input to check
+     * @return Response|null `null` if the input is of the specified length, or an unsatisfied `Response` otherwise
+     */
+    public function check(array $inputs, string $key): Response|null
+    {
+        if (!isset($inputs[$key]))
+            return new Response(
+                payload: ["target" => $key, "message" => $this->override_message ?? "Missing input '$key'."],
+                satisfied: false
+            );
+
+        if ($this->min_length !== null && strlen($inputs[$key]) < $this->min_length)
+            return new Response(
+                payload: [
+                    "target" => $key,
+                    "message" => $this->override_message ?? "'$key' should be at least $this->min_length characters."
+                ],
+                satisfied: false
+            );
+
+        if ($this->max_length !== null && strlen($inputs[$key]) > $this->max_length)
+            return new Response(
+                payload: [
+                    "target" => $key,
+                    "message" => $this->override_message ?? "'$key' should be at most $this->max_length characters."
+                ],
+                satisfied: false
+            );
+
+        return null;
+    }
+}