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<%@ Page Language="C#" AutoEventWireup="true" %>
<!DOCTYPE html>
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<title>Add Google Authenticator 2FA to C# | AuthenticatorAPI.com</title>
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<div class="lang-badge">C# / .NET</div>
<h1>Add 2FA to Your C# Application</h1>
<p>A complete guide to integrating Google Authenticator using AuthenticatorAPI.com</p>
</div>
<div class="lang-nav">
<span style="line-height:36px;font-size:13px;font-weight:700;color:#888;">Also available in:</span>
<a href="/integrate-2fa-php.aspx">PHP</a>
<a href="/integrate-2fa-python.aspx">Python</a>
<a href="/integrate-2fa-javascript.aspx">JavaScript</a>
<a href="/integrate-2fa-java.aspx">Java</a>
</div>
<p class="lead text-muted">This guide shows how to integrate Google Authenticator-compatible 2FA into C# applications — covering ASP.NET Core MVC, ASP.NET Web Forms, and standalone console/service applications. Uses only <code>HttpClient</code> from the BCL, with no NuGet packages required.</p>
<div class="info-box">👉 <strong>Compatibility:</strong> .NET 6, 7, 8 (modern), and .NET Framework 4.6.1+ (legacy). All examples use <code>HttpClient</code> with async/await.</div>
<!-- Tabs for .NET vs Framework -->
<div class="tab-switcher" style="margin-top:30px;">
<button class="active" onclick="switchTab('modern', this)">.NET 6/7/8 (Modern)</button>
<button onclick="switchTab('legacy', this)">.NET Framework 4.x</button>
</div>
<div class="section-heading">Step-by-Step Integration</div>
<div id="tab-modern" class="tab-content-block active">
<div class="step-block">
<div class="step-num">1</div>
<h3>Create a TotpService Helper Class</h3>
<p>Encapsulate all calls to the API in a single service class. Register it with the DI container as a singleton (sharing a single <code>HttpClient</code> instance is the .NET best practice).</p>
<pre class="code-block"><span class="kw">using</span> System.Net.Http;
<span class="kw">using</span> System.Security.Cryptography;
<span class="kw">using</span> System.Web;
<span class="kw">public class</span> <span class="dc">TotpService</span>
{
<span class="kw">private const string</span> Base32Alphabet = <span class="st">"ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"</span>;
<span class="kw">private const string</span> ApiBase = <span class="st">"https://www.authenticatorApi.com"</span>;
<span class="kw">private readonly</span> HttpClient _http;
<span class="kw">public</span> <span class="fn">TotpService</span>(HttpClient http) => _http = http;
<span class="cm">/// <summary>Generates a cryptographically random 32-character Base32 secret.</summary></span>
<span class="kw">public string</span> <span class="fn">GenerateSecret</span>(<span class="kw">int</span> length = <span class="st">32</span>)
{
<span class="kw">var</span> bytes = RandomNumberGenerator.<span class="fn">GetBytes</span>(length);
<span class="kw">return new string</span>(bytes.<span class="fn">Select</span>(b => Base32Alphabet[b % <span class="st">32</span>]).<span class="fn">ToArray</span>());
}
<span class="cm">/// <summary>Returns the HTML img tag QR code for the setup page.</summary></span>
<span class="kw">public async</span> Task<<span class="kw">string</span>> <span class="fn">GetQrCodeHtmlAsync</span>(<span class="kw">string</span> appName, <span class="kw">string</span> userInfo, <span class="kw">string</span> secret)
{
<span class="kw">var</span> url = <span class="vr">$"{ApiBase}/pair.aspx"</span>
+ <span class="vr">$"?AppName={HttpUtility.UrlEncode(appName)}"</span>
+ <span class="vr">$"&AppInfo={HttpUtility.UrlEncode(userInfo)}"</span>
+ <span class="vr">$"&SecretCode={HttpUtility.UrlEncode(secret)}"</span>;
<span class="kw">return await</span> _http.<span class="fn">GetStringAsync</span>(url);
}
<span class="cm">/// <summary>Returns true if the PIN is valid for the given secret.</summary></span>
<span class="kw">public async</span> Task<<span class="kw">bool</span>> <span class="fn">ValidatePinAsync</span>(<span class="kw">string</span> pin, <span class="kw">string</span> secret)
{
<span class="kw">if</span> (!pin.<span class="fn">All</span>(char.IsDigit) || pin.Length != <span class="st">6</span>) <span class="kw">return false</span>;
<span class="kw">var</span> url = <span class="vr">$"{ApiBase}/Validate.aspx"</span>
+ <span class="vr">$"?Pin={pin}"</span>
+ <span class="vr">$"&SecretCode={HttpUtility.UrlEncode(secret)}"</span>;
<span class="kw">var</span> response = <span class="kw">await</span> _http.<span class="fn">GetStringAsync</span>(url);
<span class="kw">return string</span>.<span class="fn">Equals</span>(response.<span class="fn">Trim</span>(), <span class="st">"True"</span>, StringComparison.OrdinalIgnoreCase);
}
}</pre>
</div>
<div class="step-block">
<div class="step-num">2</div>
<h3>Register with Dependency Injection</h3>
<pre class="code-block"><span class="cm">// Program.cs</span>
builder.Services.<span class="fn">AddHttpClient</span><<span class="dc">TotpService</span>>(client =>
{
client.Timeout = TimeSpan.<span class="fn">FromSeconds</span>(<span class="st">10</span>);
});
<span class="cm">// Or as a plain singleton if not using typed HttpClient:</span>
builder.Services.<span class="fn">AddSingleton</span><<span class="dc">TotpService</span>>(sp =>
<span class="kw">new</span> <span class="fn">TotpService</span>(sp.<span class="fn">GetRequiredService</span><IHttpClientFactory>().<span class="fn">CreateClient</span>()));</pre>
</div>
<div class="step-block">
<div class="step-num">3</div>
<h3>Enrolment Controller (ASP.NET Core MVC)</h3>
<pre class="code-block"><span class="kw">public class</span> <span class="dc">TwoFactorController</span> : Controller
{
<span class="kw">private readonly</span> TotpService _totp;
<span class="kw">private readonly</span> IUserRepository _users;
<span class="kw">public</span> <span class="fn">TwoFactorController</span>(TotpService totp, IUserRepository users)
{
_totp = totp;
_users = users;
}
[HttpGet]
<span class="kw">public async</span> Task<IActionResult> <span class="fn">Setup</span>()
{
<span class="kw">var</span> secret = _totp.<span class="fn">GenerateSecret</span>();
HttpContext.Session.<span class="fn">SetString</span>(<span class="st">"Pending2FASecret"</span>, secret);
<span class="kw">var</span> qrHtml = <span class="kw">await</span> _totp.<span class="fn">GetQrCodeHtmlAsync</span>(
<span class="st">"MyApp"</span>,
User.Identity!.Name!,
secret);
ViewBag.QrHtml = qrHtml;
ViewBag.Secret = secret;
<span class="kw">return</span> <span class="fn">View</span>();
}
[HttpPost]
<span class="kw">public async</span> Task<IActionResult> <span class="fn">ConfirmSetup</span>(<span class="kw">string</span> pin)
{
<span class="kw">var</span> secret = HttpContext.Session.<span class="fn">GetString</span>(<span class="st">"Pending2FASecret"</span>);
<span class="kw">if</span> (<span class="kw">await</span> _totp.<span class="fn">ValidatePinAsync</span>(pin, secret!))
{
<span class="kw">await</span> _users.<span class="fn">SaveTotpSecretAsync</span>(User.Identity!.Name!, secret!);
<span class="kw">return</span> <span class="fn">RedirectToAction</span>(<span class="st">"Index"</span>, <span class="st">"Home"</span>);
}
ModelState.<span class="fn">AddModelError</span>(<span class="st">""</span>, <span class="st">"Invalid code — please try again."</span>);
<span class="kw">return</span> <span class="fn">View</span>(<span class="st">"Setup"</span>);
}
[HttpPost]
<span class="kw">public async</span> Task<IActionResult> <span class="fn">Verify</span>(<span class="kw">string</span> pin)
{
<span class="kw">var</span> secret = <span class="kw">await</span> _users.<span class="fn">GetTotpSecretAsync</span>(User.Identity!.Name!);
<span class="kw">if</span> (<span class="kw">await</span> _totp.<span class="fn">ValidatePinAsync</span>(pin, secret))
{
HttpContext.Session.<span class="fn">SetString</span>(<span class="st">"2FAVerified"</span>, <span class="st">"true"</span>);
<span class="kw">return</span> <span class="fn">RedirectToAction</span>(<span class="st">"Dashboard"</span>);
}
ModelState.<span class="fn">AddModelError</span>(<span class="st">""</span>, <span class="st">"Invalid code — please try again."</span>);
<span class="kw">return</span> <span class="fn">View</span>();
}
}</pre>
</div>
</div><!-- /tab-modern -->
<div id="tab-legacy" class="tab-content-block">
<div class="step-block">
<div class="step-num">1</div>
<h3>.NET Framework 4.x — Helper Class</h3>
<pre class="code-block"><span class="kw">using</span> System;
<span class="kw">using</span> System.Net.Http;
<span class="kw">using</span> System.Security.Cryptography;
<span class="kw">using</span> System.Web;
<span class="kw">public static class</span> <span class="dc">TotpHelper</span>
{
<span class="kw">private static readonly</span> HttpClient _http = <span class="kw">new</span> <span class="fn">HttpClient</span>();
<span class="kw">private const string</span> Base32Alphabet = <span class="st">"ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"</span>;
<span class="kw">public static string</span> <span class="fn">GenerateSecret</span>()
{
<span class="kw">var</span> rng = <span class="kw">new</span> <span class="fn">RNGCryptoServiceProvider</span>();
<span class="kw">var</span> bytes = <span class="kw">new byte</span>[<span class="st">32</span>];
rng.<span class="fn">GetBytes</span>(bytes);
<span class="kw">var</span> sb = <span class="kw">new</span> System.Text.<span class="fn">StringBuilder</span>();
<span class="kw">foreach</span> (<span class="kw">var</span> b <span class="kw">in</span> bytes) sb.<span class="fn">Append</span>(Base32Alphabet[b % <span class="st">32</span>]);
<span class="kw">return</span> sb.<span class="fn">ToString</span>();
}
<span class="kw">public static string</span> <span class="fn">GetQrCodeHtml</span>(<span class="kw">string</span> appName, <span class="kw">string</span> userInfo, <span class="kw">string</span> secret)
{
<span class="kw">var</span> url = <span class="vr">$"https://www.authenticatorApi.com/pair.aspx"</span>
+ <span class="vr">$"?AppName={HttpUtility.UrlEncode(appName)}"</span>
+ <span class="vr">$"&AppInfo={HttpUtility.UrlEncode(userInfo)}"</span>
+ <span class="vr">$"&SecretCode={HttpUtility.UrlEncode(secret)}"</span>;
<span class="kw">return</span> _http.<span class="fn">GetStringAsync</span>(url).<span class="fn">Result</span>;
}
<span class="kw">public static bool</span> <span class="fn">ValidatePin</span>(<span class="kw">string</span> pin, <span class="kw">string</span> secret)
{
<span class="kw">var</span> url = <span class="vr">$"https://www.authenticatorApi.com/Validate.aspx?Pin={pin}&SecretCode={HttpUtility.UrlEncode(secret)}"</span>;
<span class="kw">var</span> result = _http.<span class="fn">GetStringAsync</span>(url).<span class="fn">Result</span>.<span class="fn">Trim</span>();
<span class="kw">return string</span>.<span class="fn">Equals</span>(result, <span class="st">"True"</span>, StringComparison.OrdinalIgnoreCase);
}
}</pre>
</div>
<div class="step-block">
<div class="step-num">2</div>
<h3>ASP.NET Web Forms Usage</h3>
<pre class="code-block"><span class="cm">// Setup.aspx.cs</span>
<span class="kw">protected void</span> <span class="fn">Page_Load</span>(<span class="kw">object</span> sender, EventArgs e)
{
<span class="kw">if</span> (!IsPostBack)
{
<span class="kw">var</span> secret = TotpHelper.<span class="fn">GenerateSecret</span>();
Session[<span class="st">"Pending2FASecret"</span>] = secret;
QrCodeLiteral.Text = TotpHelper.<span class="fn">GetQrCodeHtml</span>(<span class="st">"MyApp"</span>, User.Identity.Name, secret);
SecretLabel.Text = secret;
}
}
<span class="kw">protected void</span> <span class="fn">btnVerify_Click</span>(<span class="kw">object</span> sender, EventArgs e)
{
<span class="kw">var</span> secret = Session[<span class="st">"Pending2FASecret"</span>] <span class="kw">as string</span>;
<span class="kw">if</span> (TotpHelper.<span class="fn">ValidatePin</span>(PinTextBox.Text.<span class="fn">Trim</span>(), secret))
{
<span class="cm">// Save secret to user's profile and redirect</span>
SaveSecretToProfile(secret);
Response.<span class="fn">Redirect</span>(<span class="st">"~/Default.aspx"</span>);
}
<span class="kw">else</span>
{
ErrorLabel.Text = <span class="st">"Invalid code — please try again."</span>;
ErrorLabel.Visible = <span class="kw">true</span>;
}
}</pre>
</div>
</div><!-- /tab-legacy -->
<div class="section-heading">Security Checklist for C#</div>
<ul style="font-size:15px;line-height:2;color:#444;">
<li>✅ Use <code>RandomNumberGenerator.GetBytes()</code> (.NET 6+) or <code>RNGCryptoServiceProvider</code> (.NET Framework)</li>
<li>✅ Use the <code>IHttpClientFactory</code> pattern — never instantiate <code>HttpClient</code> directly in a loop</li>
<li>✅ Store secrets encrypted with <code>IDataProtector</code> (ASP.NET Core) or AES-256</li>
<li>✅ Always call the API over HTTPS</li>
<li>✅ Apply rate limiting to the verification endpoint (<code>AspNetCoreRateLimit</code> NuGet)</li>
<li>✅ Regenerate the authentication cookie/session after successful 2FA</li>
<li>✅ Provide recovery codes and a documented admin reset path</li>
</ul>
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