Authentication Patterns

Comprehensive guidance for implementing secure authentication systems, covering JWT, OAuth 2.0, OIDC, Passkeys, MFA, and session management.

When to Use This Skill

Use this skill when:

• Implementing JWT-based authentication
• Setting up OAuth 2.0 or OpenID Connect flows
• Implementing passwordless authentication (Passkeys/FIDO2)
• Adding multi-factor authentication (MFA/2FA)
• Designing session management and secure cookies
• Implementing SSO (Single Sign-On)
• Reviewing authentication security
• Choosing between authentication approaches

Authentication Method Selection

Recommendation: Prefer Passkeys for new applications. Use OAuth 2.0 + PKCE for SPAs. Always add MFA as a second factor.

JWT Best Practices Quick Reference

Algorithm Selection

Token Structure

// Header
{
  "alg": "RS256",
  "typ": "JWT",
  "kid": "key-id-for-rotation"  // Key ID for key rotation
}

// Payload (Claims)
{
  "iss": "https://auth.example.com",  // Issuer
  "sub": "user-123",                   // Subject (user ID)
  "aud": "https://api.example.com",   // Audience
  "exp": 1735300000,                   // Expiration (short-lived)
  "iat": 1735296400,                   // Issued at
  "jti": "unique-token-id",            // JWT ID (for revocation)
  "scope": "read write"                // Permissions
}

Token Lifetimes

For detailed JWT security: See JWT Security Reference

OAuth 2.0 Flow Selection

Authorization Code + PKCE Flow

┌──────────┐                              ┌───────────────┐
│  Client  │                              │ Auth Server   │
└────┬─────┘                              └───────┬───────┘
     │                                            │
     │ 1. Generate code_verifier (random)         │
     │    code_challenge = SHA256(code_verifier)  │
     │                                            │
     │ 2. Authorization Request ─────────────────>│
     │    (response_type=code, code_challenge)    │
     │                                            │
     │ 3. User authenticates & consents           │
     │                                            │
     │ 4. <────────── Authorization Code ─────────│
     │                                            │
     │ 5. Token Request ─────────────────────────>│
     │    (code, code_verifier)                   │
     │                                            │
     │ 6. <────────── Access + Refresh Tokens ────│
     └────────────────────────────────────────────┘

For detailed OAuth flows: See OAuth Flows Reference

Passkeys/WebAuthn Quick Start

Passkeys provide phishing-resistant, passwordless authentication using public key cryptography.

Registration Flow

// 1. Get challenge from server
const options = await fetch('/api/webauthn/register/options').then(r => r.json());

// 2. Create credential
const credential = await navigator.credentials.create({
  publicKey: {
    challenge: base64ToBuffer(options.challenge),
    rp: { name: "Example App", id: "example.com" },
    user: {
      id: base64ToBuffer(options.userId),
      name: options.username,
      displayName: options.displayName
    },
    pubKeyCredParams: [
      { type: "public-key", alg: -7 },   // ES256
      { type: "public-key", alg: -257 }  // RS256
    ],
    authenticatorSelection: {
      authenticatorAttachment: "platform",  // or "cross-platform"
      residentKey: "required",              // Discoverable credential
      userVerification: "required"          // Biometric/PIN required
    },
    timeout: 60000
  }
});

// 3. Send credential to server for storage
await fetch('/api/webauthn/register/verify', {
  method: 'POST',
  body: JSON.stringify({
    id: credential.id,
    rawId: bufferToBase64(credential.rawId),
    response: {
      clientDataJSON: bufferToBase64(credential.response.clientDataJSON),
      attestationObject: bufferToBase64(credential.response.attestationObject)
    }
  })
});

For complete Passkeys implementation: See Passkeys Implementation Guide

MFA Implementation Patterns

MFA Methods (by Security)

TOTP Implementation

using System.Security.Cryptography;
using OtpNet;  // Install: Otp.NET package

/// <summary>
/// TOTP (Time-based One-Time Password) service for MFA.
/// </summary>
public sealed class TotpService
{
    private const int SecretLength = 20;  // 160 bits

    /// <summary>
    /// Generate a new TOTP secret for user enrollment.
    /// </summary>
    public static string GenerateSecret()
    {
        var secretBytes = RandomNumberGenerator.GetBytes(SecretLength);
        return Base32Encoding.ToString(secretBytes);
    }

    /// <summary>
    /// Generate provisioning URI for authenticator apps (Google Authenticator, etc.)
    /// </summary>
    public static string GetProvisioningUri(string secret, string email, string issuer)
    {
        return $"otpauth://totp/{Uri.EscapeDataString(issuer)}:{Uri.EscapeDataString(email)}" +
               $"?secret={secret}&issuer={Uri.EscapeDataString(issuer)}&algorithm=SHA1&digits=6&period=30";
    }

    /// <summary>
    /// Verify TOTP code during login. Allows 1-step time drift.
    /// </summary>
    public static bool VerifyTotp(string secret, string otp)
    {
        var secretBytes = Base32Encoding.ToBytes(secret);
        var totp = new Totp(secretBytes, step: 30, totpSize: 6);

        // VerificationWindow allows for clock drift (1 step = 30 seconds each direction)
        return totp.VerifyTotp(otp, out _, VerificationWindow.RfcSpecifiedNetworkDelay);
    }
}

Session Management

Secure Cookie Configuration

// ASP.NET Core cookie configuration
app.UseCookiePolicy(new CookiePolicyOptions
{
    HttpOnly = HttpOnlyPolicy.Always,        // Prevent JavaScript access (XSS protection)
    Secure = CookieSecurePolicy.Always,      // HTTPS only
    MinimumSameSitePolicy = SameSiteMode.Lax // CSRF protection (or Strict for more security)
});

// Per-cookie configuration
Response.Cookies.Append("session_id", sessionId, new CookieOptions
{
    HttpOnly = true,             // Prevent JavaScript access
    Secure = true,               // HTTPS only
    SameSite = SameSiteMode.Lax, // CSRF protection
    MaxAge = TimeSpan.FromHours(1),
    Domain = ".example.com",
    Path = "/",
    IsEssential = true           // Required for GDPR essential cookies
});

Session Security Checklist

• Generate cryptographically random session IDs (128+ bits)
• Regenerate session ID after authentication
• Set HttpOnly flag on session cookies
• Set Secure flag (HTTPS only)
• Set SameSite attribute (Lax or Strict)
• Implement session timeout (idle and absolute)
• Invalidate session on logout (server-side)
• Bind session to user fingerprint (optional, consider privacy)

Password Security (When Required)

Password Hashing

using System.Security.Cryptography;
using Konscious.Security.Cryptography;  // Install: Konscious.Security.Cryptography.Argon2

/// <summary>
/// Argon2id password hashing service (recommended by OWASP).
/// </summary>
public sealed class PasswordHasher
{
    private const int SaltSize = 16;
    private const int HashSize = 32;
    private const int Iterations = 3;      // time_cost
    private const int MemorySize = 65536;  // 64 MB
    private const int Parallelism = 4;     // threads

    /// <summary>
    /// Hash a password using Argon2id.
    /// </summary>
    public static string HashPassword(string password)
    {
        var salt = RandomNumberGenerator.GetBytes(SaltSize);

        using var argon2 = new Argon2id(System.Text.Encoding.UTF8.GetBytes(password))
        {
            Salt = salt,
            DegreeOfParallelism = Parallelism,
            MemorySize = MemorySize,
            Iterations = Iterations
        };

        var hash = argon2.GetBytes(HashSize);

        // Combine salt + hash for storage
        var combined = new byte[SaltSize + HashSize];
        Buffer.BlockCopy(salt, 0, combined, 0, SaltSize);
        Buffer.BlockCopy(hash, 0, combined, SaltSize, HashSize);

        return Convert.ToBase64String(combined);
    }

    /// <summary>
    /// Verify a password against stored hash.
    /// </summary>
    public static bool VerifyPassword(string password, string storedHash)
    {
        var combined = Convert.FromBase64String(storedHash);
        if (combined.Length != SaltSize + HashSize) return false;

        var salt = combined[..SaltSize];
        var expectedHash = combined[SaltSize..];

        using var argon2 = new Argon2id(System.Text.Encoding.UTF8.GetBytes(password))
        {
            Salt = salt,
            DegreeOfParallelism = Parallelism,
            MemorySize = MemorySize,
            Iterations = Iterations
        };

        var actualHash = argon2.GetBytes(HashSize);

        // Timing-safe comparison
        return CryptographicOperations.FixedTimeEquals(actualHash, expectedHash);
    }
}

Password Policy

Quick Decision Tree

What authentication are you implementing?

1. New web/mobile app → Passkeys + OAuth 2.0 fallback
2. SPA with API backend → OAuth 2.0 + PKCE with JWT access tokens
3. Service-to-service → Client Credentials flow or mTLS
4. Adding MFA to existing → TOTP authenticator app (minimum), Passkeys (ideal)
5. Traditional web app → Session cookies + CSRF tokens
6. CLI/device with no browser → Device Authorization flow

Security Checklist

Token Security

• Short-lived access tokens (5-15 minutes)
• Secure refresh token storage
• Token revocation mechanism
• Proper token validation (signature, claims, expiry)

OAuth/OIDC Security

• Use PKCE for all public clients
• Validate redirect URIs strictly
• Validate state parameter
• Validate nonce for OIDC
• Use exact redirect URI matching

Session Security

• HttpOnly, Secure, SameSite cookies
• Session regeneration after auth
• Proper session invalidation
• Idle and absolute timeouts

MFA Security

• MFA on all accounts (enforce or encourage)
• Secure recovery codes
• Rate limit MFA attempts
• Prefer phishing-resistant methods

References

• JWT Security Deep Dive - Complete JWT implementation guide
• OAuth 2.0 Flows - OAuth/OIDC flow diagrams and implementation
• Passkeys Implementation - WebAuthn/FIDO2 complete guide

Related Skills

authorization-models

cryptography

api-security

secure-coding

Version History

• v1.0.0 (2025-12-26): Initial release with JWT, OAuth 2.0, Passkeys, MFA, sessions

Last Updated: 2025-12-26