diff --git a/README.md b/README.md
index 9670279..747ce8c 100644
--- a/README.md
+++ b/README.md
@@ -1,40 +1,30 @@
-Google Authenticator (TOTP)
-===========================
+# Google Authenticator (TOTP)
 
 ![Build Status](https://github.com/Vectorface/GoogleAuthenticator/workflows/Test/badge.svg)
 
 **English** | [中文](./README.zh-CN.md)
 
-This is a fork of https://github.com/PHPGangsta/GoogleAuthenticator with the following changes:
+A PHP library for two-factor authentication (2FA) compatible with the Google Authenticator mobile app.
+It can generate secrets, generate and verify one-time codes, and render the secret as a scannable
+QR code — implementing TOTP as specified in [RFC 6238](https://tools.ietf.org/html/rfc6238).
 
-- Uses https://github.com/endroid/qr-code to generate QR code data URIs
-- No longer generates Google's Chart API to make QR code links
-- Uses namespacing
-- Augmented test coverage to 100%
-- Bumped minimum PHP version to 8.2
-
-Original License:
------------------
+This is a fork of [PHPGangsta/GoogleAuthenticator](https://github.com/PHPGangsta/GoogleAuthenticator) with the following changes:
 
-* Copyright (c) 2012-2016, [http://www.phpgangsta.de](http://www.phpgangsta.de)
-* Author: Michael Kliewe, [@PHPGangsta](http://twitter.com/PHPGangsta) and [contributors](https://github.com/PHPGangsta/GoogleAuthenticator/graphs/contributors)
-* Licensed under the BSD License.
-
-Description:
-------------
+- Uses [endroid/qr-code](https://github.com/endroid/qr-code) to generate QR code data URIs
+- No longer relies on Google's Chart API for QR code links
+- Uses namespacing
+- Test coverage raised to 100%
+- Minimum PHP version bumped to 8.2
 
-This PHP class can be used to interact with the Google Authenticator mobile app for 2-factor-authentication. This class
-can generate secrets, generate codes, validate codes and present a QR-Code for scanning the secret. It implements TOTP 
-according to [RFC6238](https://tools.ietf.org/html/rfc6238)
+## Installation
 
-For a secure installation you have to make sure that used codes cannot be reused (replay-attack). You also need to
-limit the number of verifications, to fight against brute-force attacks. For example you could limit the amount of
-verifications to 10 tries within 10 minutes for one IP address (or IPv6 block). It depends on your environment.
+Install via [Composer](https://getcomposer.org/doc/01-basic-usage.md):
 
-Usage:
-------
+```bash
+composer require vectorface/googleauthenticator
+```
 
-See following example:
+## Quick Start
 
 ```php
 <?php
@@ -43,24 +33,26 @@ require_once 'vendor/autoload.php';
 use Vectorface\GoogleAuthenticator;
 
 $ga = new GoogleAuthenticator();
+
+// 1. Create a secret and share it with the user
 $secret = $ga->createSecret();
 echo "Secret is: {$secret}\n\n";
 
+// 2. Render the secret as a QR code (PNG data URI) for the user to scan
 $qrCodeUrl = $ga->getQRCodeUrl('Admin', $secret, 'Blog');
 echo "PNG Data URI for the QR-Code: {$qrCodeUrl}\n\n";
 
+// 3. Verify the one-time code entered by the user
 $oneCode = $ga->getCode($secret);
 echo "Checking Code '$oneCode' and Secret '$secret':\n";
 
-// 2 = 2*30sec clock tolerance
+// discrepancy = 2 allows a clock tolerance of ±2 × 30 seconds
 $checkResult = $ga->verifyCode($secret, $oneCode, 2);
-if ($checkResult) {
-    echo 'OK';
-} else {
-    echo 'FAILED';
-}
+echo $checkResult ? 'OK' : 'FAILED';
 ```
-Running the script provides output similar to:
+
+Running the script produces output similar to:
+
 ```
 Secret is: OQB6ZZGYHCPSX4AK
 
@@ -70,17 +62,71 @@ Checking Code '848634' and Secret 'OQB6ZZGYHCPSX4AK':
 OK
 ```
 
-Installation:
--------------
+## Security Considerations
+
+### Preventing replay attacks
+
+A TOTP code must never be accepted twice. `verifyCode()` accepts an optional by-reference
+parameter that receives the time slice which matched. As required by
+[RFC 6238 section 5.2](https://tools.ietf.org/html/rfc6238#section-5.2), persist the last
+accepted time slice per user and refuse any code that matches a slice less than or equal to
+the stored value:
+
+```php
+$matchedTimeSlice = null;
+if ($ga->verifyCode($secret, $oneCode, 2, $matchedTimeSlice)) {
+    if ($matchedTimeSlice <= $user->lastUsedTimeSlice) {
+        // Code already used: reject to prevent a replay attack
+    } else {
+        $user->lastUsedTimeSlice = $matchedTimeSlice; // persist this value
+        // Code accepted
+    }
+}
+```
+
+### Rate limiting
+
+To defend against brute-force attacks, limit the number of verification attempts. For example,
+allow at most 10 tries within 10 minutes per IP address (or IPv6 block) — adjust to fit your
+environment.
+
+## Advanced Usage
+
+### Custom code length
+
+Codes are 6 digits by default. Lengths of 6 to 8 digits are supported (RFC 4226):
+
+```php
+$ga = (new GoogleAuthenticator())->setCodeLength(8);
+```
+
+### Building otpauth:// URIs
+
+For full control over the provisioning URI (algorithm, digits, period, HOTP counter, etc.),
+use the fluent `UriBuilder`:
+
+```php
+$uri = $ga->getUriBuilder()
+    ->issuer('Blog')
+    ->account('Admin')
+    ->secret($secret)
+    ->getUri(); // otpauth://totp/Blog:%20Admin?secret=...&issuer=Blog
+```
+
+## Running Tests
+
+All tests live in the `tests` folder.
+
+```bash
+composer install
+composer test
+```
 
-- Use [Composer](https://getcomposer.org/doc/01-basic-usage.md) to
-  install the package
+## License
 
-```composer require vectorface/googleauthenticator```
+Licensed under the [BSD License](./LICENSE.md).
 
-Run Tests:
-----------
+Original work:
 
-- All tests are inside `tests` folder.
-- Execute `composer install` to prepare your environment.
-- Run `composer test` from the project root directory.
+- Copyright (c) 2012-2016, [http://www.phpgangsta.de](http://www.phpgangsta.de)
+- Author: Michael Kliewe, [@PHPGangsta](http://twitter.com/PHPGangsta) and [contributors](https://github.com/PHPGangsta/GoogleAuthenticator/graphs/contributors)
diff --git a/README.zh-CN.md b/README.zh-CN.md
index 976ce1d..14c1e83 100644
--- a/README.zh-CN.md
+++ b/README.zh-CN.md
@@ -1,36 +1,30 @@
-Google Authenticator (TOTP)
-===========================
+# Google Authenticator (TOTP)
 
 ![构建状态](https://github.com/Vectorface/GoogleAuthenticator/workflows/Test/badge.svg)
 
 **中文** | [English](./README.md)
 
-这是 https://github.com/PHPGangsta/GoogleAuthenticator 的一个分支，包含以下更改：
+一个与 Google Authenticator 移动应用兼容的 PHP 双因素认证（2FA）库。
+它可以生成密钥、生成并校验一次性验证码，并将密钥渲染为可扫描的二维码 —— 按照
+[RFC 6238](https://tools.ietf.org/html/rfc6238) 规范实现 TOTP。
 
-- 使用 https://github.com/endroid/qr-code 生成二维码数据 URIs
-- 不再生成 Google 的 Chart API 来制作二维码链接
-- 使用命名空间
-- 将测试覆盖率增加到 100%
-- 将最低 PHP 版本提升到 8.2
-
-原始许可证：
------------------
+本项目是 [PHPGangsta/GoogleAuthenticator](https://github.com/PHPGangsta/GoogleAuthenticator) 的分支，包含以下改动：
 
-* 版权所有 (c) 2012-2016, [http://www.phpgangsta.de](http://www.phpgangsta.de)
-* 作者：Michael Kliewe, [@PHPGangsta](http://twitter.com/PHPGangsta) 和 [贡献者](https://github.com/PHPGangsta/GoogleAuthenticator/graphs/contributors)
-* 根据 BSD 许可证授权。
-
-描述：
-------------
+- 使用 [endroid/qr-code](https://github.com/endroid/qr-code) 生成二维码数据 URI
+- 不再依赖 Google Chart API 生成二维码链接
+- 使用命名空间
+- 测试覆盖率提升至 100%
+- 最低 PHP 版本提升至 8.2
 
-这个 PHP 类可以用来与 Google Authenticator 移动应用进行双重因素认证交互。该类可以生成密钥、生成代码、验证代码并提供用于扫描密钥的二维码。它实现了根据 [RFC6238](https://tools.ietf.org/html/rfc6238) 的 TOTP。
+## 安装
 
-为了您能安全安装，您必须确保使用的代码不能被重复使用（重放攻击）。您还需要限制验证次数，以对抗暴力攻击。例如，您可以将一个 IP 地址（或 IPv6 块）的验证次数限制为 10 分钟内 10 次尝试。这取决于您的环境。
+通过 [Composer](https://getcomposer.org/doc/01-basic-usage.md) 安装：
 
-用法：
-------
+```bash
+composer require vectorface/googleauthenticator
+```
 
-请参见以下示例：
+## 快速开始
 
 ```php
 <?php
@@ -39,43 +33,96 @@ require_once 'vendor/autoload.php';
 use Vectorface\GoogleAuthenticator;
 
 $ga = new GoogleAuthenticator();
+
+// 1. 创建密钥并分发给用户
 $secret = $ga->createSecret();
 echo "密钥是: {$secret}\n\n";
 
+// 2. 将密钥渲染为二维码（PNG 数据 URI）供用户扫描
 $qrCodeUrl = $ga->getQRCodeUrl('Admin', $secret, 'Blog');
 echo "二维码的 PNG 数据 URI: {$qrCodeUrl}\n\n";
 
+// 3. 校验用户输入的一次性验证码
 $oneCode = $ga->getCode($secret);
-echo "检查代码 '$oneCode' 和密钥 '$secret':\n";
+echo "检查验证码 '$oneCode' 和密钥 '$secret':\n";
 
-// 2 = 2*30秒的时钟容差
+// discrepancy = 2 表示允许 ±2 × 30 秒的时钟偏差
 $checkResult = $ga->verifyCode($secret, $oneCode, 2);
-if ($checkResult) {
-    echo 'OK';
-} else {
-    echo 'FAILED';
-}
+echo $checkResult ? 'OK' : 'FAILED';
 ```
-运行脚本会提供类似以下的输出：
+
+运行脚本会得到类似以下的输出：
+
 ```
 密钥是: OQB6ZZGYHCPSX4AK
 
 二维码的 PNG 数据 URI: data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAARgAAAEYCAIAAAAI[已截断]
 
-检查代码 '848634' 和密钥 'OQB6ZZGYHCPSX4AK':
+检查验证码 '848634' 和密钥 'OQB6ZZGYHCPSX4AK':
 OK
 ```
 
-安装：
--------------
+## 安全注意事项
+
+### 防止重放攻击
+
+同一个 TOTP 验证码绝不能被接受两次。`verifyCode()` 接受一个可选的引用参数，
+用于接收匹配成功的时间片。按照 [RFC 6238 第 5.2 节](https://tools.ietf.org/html/rfc6238#section-5.2)
+的要求，请为每个用户持久化最后一次接受的时间片，并拒绝任何匹配时间片小于或等于该存储值的验证码：
+
+```php
+$matchedTimeSlice = null;
+if ($ga->verifyCode($secret, $oneCode, 2, $matchedTimeSlice)) {
+    if ($matchedTimeSlice <= $user->lastUsedTimeSlice) {
+        // 验证码已被使用：拒绝以防止重放攻击
+    } else {
+        $user->lastUsedTimeSlice = $matchedTimeSlice; // 持久化该值
+        // 验证码通过
+    }
+}
+```
+
+### 限制验证频率
+
+为防御暴力破解攻击，请限制验证尝试次数。例如，可以将单个 IP 地址（或 IPv6 网段）
+的验证限制为 10 分钟内最多 10 次尝试 —— 请根据实际环境调整。
+
+## 进阶用法
+
+### 自定义验证码长度
+
+验证码默认为 6 位，支持 6 到 8 位（RFC 4226）：
+
+```php
+$ga = (new GoogleAuthenticator())->setCodeLength(8);
+```
+
+### 构建 otpauth:// URI
+
+如需完全控制配置 URI（算法、位数、周期、HOTP 计数器等），可使用链式的 `UriBuilder`：
+
+```php
+$uri = $ga->getUriBuilder()
+    ->issuer('Blog')
+    ->account('Admin')
+    ->secret($secret)
+    ->getUri(); // otpauth://totp/Blog:%20Admin?secret=...&issuer=Blog
+```
+
+## 运行测试
+
+所有测试位于 `tests` 文件夹内。
+
+```bash
+composer install
+composer test
+```
 
-- 使用 [Composer](https://getcomposer.org/doc/01-basic-usage.md) 安装包
+## 许可证
 
-```composer require vectorface/googleauthenticator```
+基于 [BSD 许可证](./LICENSE.md)授权。
 
-运行测试：
-----------
+原始项目：
 
-- 所有测试都在 `tests` 文件夹内。
-- 执行 `composer install` 来准备您的环境。
-- 从项目根目录运行 `composer test`。
\ No newline at end of file
+- 版权所有 (c) 2012-2016, [http://www.phpgangsta.de](http://www.phpgangsta.de)
+- 作者：Michael Kliewe, [@PHPGangsta](http://twitter.com/PHPGangsta) 及[贡献者](https://github.com/PHPGangsta/GoogleAuthenticator/graphs/contributors)
diff --git a/src/GoogleAuthenticator.php b/src/GoogleAuthenticator.php
index 59b5eda..9b33243 100644
--- a/src/GoogleAuthenticator.php
+++ b/src/GoogleAuthenticator.php
@@ -5,6 +5,7 @@
 use Endroid\QrCode\Builder\Builder;
 use Endroid\QrCode\Writer\PngWriter;
 use Exception;
+use InvalidArgumentException;
 use Vectorface\OtpAuth\Base32;
 use Vectorface\OtpAuth\UriBuilder;
 
@@ -138,12 +139,19 @@ protected function getQRCodeDataUri(string $uri) : string
      * Check if the code is correct. This will accept codes starting from $discrepancy*30sec ago to $discrepancy*30sec from now
      *
      * @param int $discrepancy This is the allowed time drift in 30 second units (8 means 4 minutes before or after)
+     * @param int|null $matchedTimeSlice Set to the time slice that matched on success; persist the last accepted
+     *                                   slice and reject codes matching a slice <= that value to prevent replay
+     * @throws InvalidArgumentException if $discrepancy is out of the 0-60 range
      */
-    public function verifyCode(string $secret, string $code, int $discrepancy = 1) : bool
+    public function verifyCode(string $secret, string $code, int $discrepancy = 1, ?int &$matchedTimeSlice = null) : bool
     {
+        if ($discrepancy < 0 || $discrepancy > 60) {
+            throw new InvalidArgumentException('Discrepancy must be between 0 and 60 time slices');
+        }
+
         $currentTimeSlice = floor(time() / 30);
 
-        if (strlen($code) != 6) {
+        if (strlen($code) !== $this->_codeLength || !ctype_digit($code)) {
             return false;
         }
 
@@ -155,6 +163,7 @@ public function verifyCode(string $secret, string $code, int $discrepancy = 1) :
             }
 
             if (hash_equals($calculatedCode, $code)) {
+                $matchedTimeSlice = (int) ($currentTimeSlice + $i);
                 return true;
             }
         }
@@ -163,10 +172,15 @@ public function verifyCode(string $secret, string $code, int $discrepancy = 1) :
     }
 
     /**
-     * Set the code length, should be >=6
+     * Set the code length, must be between 6 and 8 (RFC 4226)
+     *
+     * @throws InvalidArgumentException
      */
     public function setCodeLength(int $length) : self
     {
+        if ($length < 6 || $length > 8) {
+            throw new InvalidArgumentException('Code length must be between 6 and 8');
+        }
         $this->_codeLength = $length;
         return $this;
     }
diff --git a/tests/GoogleAuthenticatorTest.php b/tests/GoogleAuthenticatorTest.php
index f642650..177fc67 100644
--- a/tests/GoogleAuthenticatorTest.php
+++ b/tests/GoogleAuthenticatorTest.php
@@ -3,6 +3,7 @@
 namespace Tests\Vectorface;
 
 use Exception;
+use InvalidArgumentException;
 use PHPUnit\Framework\TestCase;
 use Vectorface\GoogleAuthenticator;
 use Vectorface\OtpAuth\Parameters\Algorithm;
@@ -163,6 +164,77 @@ public function testVerifyCodeWithLeadingZero()
         $this->assertEquals(false, $result);
     }
 
+    /**
+     * @throws Exception
+     */
+    public function testVerifyCodeWithEightDigits()
+    {
+        $secret = 'SECRET';
+        $ga = $this->googleAuthenticator->setCodeLength(8);
+
+        $code = $ga->getCode($secret);
+        $this->assertEquals(8, strlen($code));
+        $this->assertTrue($ga->verifyCode($secret, $code));
+
+        // A 6-digit code must not verify when 8 digits are configured
+        $this->assertFalse($ga->verifyCode($secret, substr($code, 0, 6)));
+    }
+
+    /**
+     * @throws Exception
+     */
+    public function testVerifyCodeRejectsNonNumericCode()
+    {
+        $secret = 'SECRET';
+
+        $this->assertFalse($this->googleAuthenticator->verifyCode($secret, 'abcdef'));
+        $this->assertFalse($this->googleAuthenticator->verifyCode($secret, '12345x'));
+        $this->assertFalse($this->googleAuthenticator->verifyCode($secret, "12345\n"));
+    }
+
+    /**
+     * @throws Exception
+     */
+    public function testVerifyCodeReportsMatchedTimeSlice()
+    {
+        $secret = 'SECRET';
+        $currentTimeSlice = (int) floor(time() / 30);
+
+        // A code from the previous time slice should match at $currentTimeSlice - 1
+        $code = $this->googleAuthenticator->getCode($secret, $currentTimeSlice - 1);
+        $matchedTimeSlice = null;
+        $result = $this->googleAuthenticator->verifyCode($secret, $code, 1, $matchedTimeSlice);
+
+        $this->assertTrue($result);
+        $this->assertSame($currentTimeSlice - 1, $matchedTimeSlice);
+
+        // On failure, the matched time slice must remain untouched
+        $matchedTimeSlice = null;
+        $this->assertFalse($this->googleAuthenticator->verifyCode($secret, '000000', 0, $matchedTimeSlice));
+        $this->assertNull($matchedTimeSlice);
+    }
+
+    public function invalidDiscrepancyProvider()
+    {
+        return [
+            'Negative' => [-1],
+            'Too large' => [61],
+        ];
+    }
+
+    /**
+     * @dataProvider invalidDiscrepancyProvider
+     * @param int $discrepancy
+     * @throws Exception
+     */
+    public function testVerifyCodeRejectsOutOfRangeDiscrepancy(int $discrepancy)
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Discrepancy must be between 0 and 60 time slices');
+
+        $this->googleAuthenticator->verifyCode('SECRET', '123456', $discrepancy);
+    }
+
     public function testSetCodeLength()
     {
         $result = $this->googleAuthenticator->setCodeLength(6);
@@ -170,6 +242,28 @@ public function testSetCodeLength()
         $this->assertInstanceOf(GoogleAuthenticator::class, $result);
     }
 
+    public function invalidCodeLengthProvider()
+    {
+        return [
+            'Too short' => [5],
+            'Too long' => [9],
+            'Zero' => [0],
+            'Negative' => [-1],
+        ];
+    }
+
+    /**
+     * @dataProvider invalidCodeLengthProvider
+     * @param int $length
+     */
+    public function testSetCodeLengthRejectsOutOfRangeValues(int $length)
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Code length must be between 6 and 8');
+
+        $this->googleAuthenticator->setCodeLength($length);
+    }
+
     public function badSecretProvider()
     {
         return [