diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index 9b7269d..6312c47 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         operating-system: ['ubuntu-latest', 'macos-latest']
-        php-versions: ['5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1']
+        php-versions: ['8.1', '8.2']
         phpunit-versions: ['latest']
 
     steps:
diff --git a/composer.json b/composer.json
index f4f0eb1..8a4a96c 100644
--- a/composer.json
+++ b/composer.json
@@ -17,12 +17,11 @@
         }
     ],
     "require": {
-        "php": "~5.6|~7.0|~8.0",
-        "paragonie/random_compat": ">=2.0"
+        "php": ">=8.1"
     },
     "require-dev": {
-        "phpunit/phpunit" : ">=5.6",
-        "squizlabs/php_codesniffer": "^2.3"
+        "phpunit/phpunit" : ">=10.0",
+        "squizlabs/php_codesniffer": "^3.7"
     },
     "archive": {
         "exclude": [
diff --git a/src/Client.php b/src/Client.php
index cb6aea2..261e070 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 namespace Hidehalo\Nanoid;
 
 class Client
@@ -16,27 +16,25 @@ class Client
      * @param string $alphabet Symbols to be used in ID.
      * @param integer $size number of symbols in ID.
      */
-    protected $alphabet;
-    protected $size;
+    protected string $alphabet;
+    protected int $size;
 
     /**
      * @var CoreInterface $core Core dynamic random
      */
-    private $core;
+    private CoreInterface $core;
 
     /**
      * @var GeneratorInterface $generator Random Btyes Generator
      */
-    protected $generator;
+    protected GeneratorInterface $generator;
 
     /**
      * Constructor of Client
      *
      * @codeCoverageIgnore
-     * @param integer $size
-     * @param GeneratorInterface $generator
      */
-    public function __construct($size = 21, GeneratorInterface $generator = null)
+    public function __construct(int $size = 21, GeneratorInterface $generator = null)
     {
         $this->size = $size > 0 ? $size : 21;
         $this->generator = $generator ?: new Generator();
@@ -47,11 +45,9 @@ public function __construct($size = 21, GeneratorInterface $generator = null)
     /**
      * Generate nanoid via optional modes
      *
-     * @param integer $size
      * @param integer $mode Client::MODE_NORMAL|Client::MODE_DYNAMIC
-     * @return string
      */
-    public function generateId($size = 0, $mode = self::MODE_NORMAL)
+    public function generateId(int $size = 0, int $mode = self::MODE_NORMAL): string
     {
         $size = $size > 0 ? $size : $this->size;
         switch ($mode) {
@@ -67,12 +63,9 @@ public function generateId($size = 0, $mode = self::MODE_NORMAL)
      * you have been implements your custom GeneratorInterface as correctly.
      * Otherwise use the build-in default random bytes generator
      *
-     * @param GeneratorInterface $generator
-     * @param integer $size
      * @param string $alphabet default CoreInterface::SAFE_SYMBOLS
-     * @return string
      */
-    public function formattedId($alphabet, $size = 0, GeneratorInterface $generator = null)
+    public function formattedId(string $alphabet, int $size = 0, GeneratorInterface $generator = null): string
     {
         $alphabet = $alphabet ?: CoreInterface::SAFE_SYMBOLS;
         $size = $size > 0 ? $size : $this->size;
@@ -84,14 +77,9 @@ public function formattedId($alphabet, $size = 0, GeneratorInterface $generator
     /**
      * Backwards-compatible method name.
      *
-     * @param string $alphabet
-     * @param integer $size
-     * @param GeneratorInterface $generator
-     *
-     * @return string
      * @since 1.0.0
      */
-    public function formatedId($alphabet, $size = 0, GeneratorInterface $generator = null)
+    public function formatedId(string $alphabet, int $size = 0, GeneratorInterface $generator = null): string
     {
         $size = $size > 0 ? $size : $this->size;
 
@@ -104,10 +92,8 @@ public function formatedId($alphabet, $size = 0, GeneratorInterface $generator =
      * as UUID v4.
      *
      * @see https://github.com/ai/nanoid/blob/master/non-secure/index.js#L19
-     * @param integer $size
-     * @return string
      */
-    private function normalRandom($size)
+    private function normalRandom(int $size): string
     {
         $id = '';
         while (1 <= $size--) {
diff --git a/src/Core.php b/src/Core.php
index c0a4891..52f094d 100644
--- a/src/Core.php
+++ b/src/Core.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 namespace Hidehalo\Nanoid;
 
 class Core implements CoreInterface
@@ -7,8 +7,11 @@ class Core implements CoreInterface
      * @inheritDoc
      * @see https://github.com/ai/nanoid/blob/master/async/index.browser.js#L4
      */
-    public function random(GeneratorInterface $generator, $size, $alphabet = CoreInterface::SAFE_SYMBOLS)
-    {
+    public function random(
+        GeneratorInterface $generator,
+        int $size,
+        string $alphabet = CoreInterface::SAFE_SYMBOLS
+    ): string {
         $len = strlen($alphabet);
         $mask = (2 << (int) (log($len - 1) / M_LN2)) - 1;
         $step = (int) ceil(1.6 * $mask * $size / $len);
diff --git a/src/CoreInterface.php b/src/CoreInterface.php
index 6c6c5db..8241ece 100644
--- a/src/CoreInterface.php
+++ b/src/CoreInterface.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 namespace Hidehalo\Nanoid;
 
 interface CoreInterface
@@ -10,10 +10,10 @@ interface CoreInterface
      * Secure random string generator with custom alphabet.
      * Alphabet must contain 256 symbols or less. Otherwise, the generator
      * will not be secure.
-     *
-     * @param GeneratorInterface $generator
-     * @param string $alphabet
-     * @param integer $size
      */
-    public function random(GeneratorInterface $generator, $size, $alphabet = CoreInterface::SAFE_SYMBOLS);
+    public function random(
+        GeneratorInterface $generator,
+        int $size,
+        string $alphabet = CoreInterface::SAFE_SYMBOLS
+    ): string;
 }
diff --git a/src/Generator.php b/src/Generator.php
index efc3233..5659e75 100644
--- a/src/Generator.php
+++ b/src/Generator.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 namespace Hidehalo\Nanoid;
 
 class Generator implements GeneratorInterface
@@ -6,7 +6,7 @@ class Generator implements GeneratorInterface
     /**
      * @inheritDoc
      */
-    public function random($size)
+    public function random(int $size): array
     {
         return unpack('C*', \random_bytes($size));
     }
diff --git a/src/GeneratorInterface.php b/src/GeneratorInterface.php
index c0fa52a..071f350 100644
--- a/src/GeneratorInterface.php
+++ b/src/GeneratorInterface.php
@@ -1,13 +1,10 @@
-<?php
+<?php declare(strict_types=1);
 namespace Hidehalo\Nanoid;
 
 interface GeneratorInterface
 {
     /**
      * Return random bytes array
-     *
-     * @param integer $size
-     * @return array
      */
-    public function random($size);
+    public function random(int $size): array;
 }
diff --git a/tests/ClientTest.php b/tests/ClientTest.php
index 3adfb62..08224b4 100644
--- a/tests/ClientTest.php
+++ b/tests/ClientTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 namespace Hidehalo\Nanoid\Test;
 
 use Hidehalo\Nanoid\Client;
@@ -10,9 +10,8 @@ class ClientTest extends TestCase
     /**
      * @group passed
      * @dataProvider clientProvider
-     * @param Client $client
      */
-    public function testGenerateId(Client $client)
+    public function testGenerateId(Client $client): void
     {
         $size = 7;
         $normalRandom = $client->generateId($size);
@@ -29,9 +28,8 @@ public function testGenerateId(Client $client)
     /**
      * @group passed
      * @dataProvider clientProvider
-     * @param Client $client
      */
-    public function testFormattedId(Client $client)
+    public function testFormattedId(Client $client): void
     {
         $size = 10;
         $alphabet = '0123456789abcdefghi';
@@ -47,9 +45,8 @@ public function testFormattedId(Client $client)
     /**
      * @group passed
      * @dataProvider clientProvider
-     * @param Client $client
      */
-    public function testFormatedId(Client $client)
+    public function testFormatedId(Client $client): void
     {
         $size = 10;
         $alphabet = '0123456789abcdefghi';
@@ -64,10 +61,8 @@ public function testFormatedId(Client $client)
 
     /**
      * Client Provider
-     *
-     * @return mixed
      */
-    public function clientProvider()
+    public static function clientProvider(): array
     {
         return [
             [new Client()]
diff --git a/tests/CoreTest.php b/tests/CoreTest.php
index 8bba9d6..bc85134 100644
--- a/tests/CoreTest.php
+++ b/tests/CoreTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 namespace Hidehalo\Nanoid\Test;
 
 use Hidehalo\Nanoid\Core;
@@ -12,10 +12,8 @@ class CoreTest extends TestCase
     /**
      * @group passed
      * @dataProvider coreProvider
-     * @param CoreInterface $core
-     * @param GeneratorInterface $generator
      */
-    public function testRandom(CoreInterface $core, GeneratorInterface $generator)
+    public function testRandom(CoreInterface $core, GeneratorInterface $generator): void
     {
         $alph = 'abcdefghijk0123456789';
         $size = 5;
@@ -25,10 +23,8 @@ public function testRandom(CoreInterface $core, GeneratorInterface $generator)
 
     /**
      * Core Provider
-     *
-     * @return mixed
      */
-    public function coreProvider()
+    public static function coreProvider(): array
     {
         return [
             [new Core(), new Generator()]
diff --git a/tests/GeneratorTest.php b/tests/GeneratorTest.php
index fbcc121..a3d32cd 100644
--- a/tests/GeneratorTest.php
+++ b/tests/GeneratorTest.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 namespace Hidehalo\Nanoid\Test;
 
 use PHPUnit\Framework\TestCase;
@@ -11,9 +11,8 @@ class GeneratorTest extends TestCase
     /**
      * @group passed
      * @dataProvider generatorProvider
-     * @param CoreInterface $core
      */
-    public function testRandom(GeneratorInterface $generator)
+    public function testRandom(GeneratorInterface $generator): void
     {
         $size = 5;
         $ret = $generator->random($size);
@@ -22,10 +21,8 @@ public function testRandom(GeneratorInterface $generator)
 
     /**
      * Generator Provider
-     *
-     * @return mixed
      */
-    public function generatorProvider()
+    public static function generatorProvider(): array
     {
         return [
             [new Generator()]
diff --git a/tests/Support/DummyGenerator.php b/tests/Support/DummyGenerator.php
index 512a1d2..c33f57f 100644
--- a/tests/Support/DummyGenerator.php
+++ b/tests/Support/DummyGenerator.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 namespace Hidehalo\Nanoid\Test\Support;
 
 use Hidehalo\Nanoid\GeneratorInterface;
@@ -8,7 +8,7 @@ class DummyGenerator implements GeneratorInterface
     /**
      * @inheritDoc
      */
-    public function random($size)
+    public function random(int $size): array
     {
         $ret = [];
         while ($size--) {