sequenceDiagram
    participant User
    participant Browser (Next.js Frontend)
    participant NextAuth Middleware
    participant NextAuth Callbacks (Server)
    participant Database (Prisma)
    participant Plivo API

    User->>Browser (Next.js Frontend): Enters Email/Password on /login
    Browser (Next.js Frontend)->>NextAuth Callbacks (Server): Submits Credentials via Server Action
    NextAuth Callbacks (Server)->>Database (Prisma): Verify User & Password
    alt User Found & Password Correct & 2FA Disabled
        NextAuth Callbacks (Server)->>Database (Prisma): Fetch User Data
        NextAuth Callbacks (Server)-->>Browser (Next.js Frontend): Create Session Cookie, Redirect to Dashboard
    else User Found & Password Correct & 2FA Enabled
        NextAuth Callbacks (Server)->>NextAuth Callbacks (Server): Generate OTP
        NextAuth Callbacks (Server)->>Database (Prisma): Store OTP Hash & Expiry for User
        NextAuth Callbacks (Server)->>Plivo API: Send OTP SMS to User's Phone
        Plivo API-->>User: Delivers SMS with OTP
        NextAuth Callbacks (Server)-->>Browser (Next.js Frontend): Redirect to /verify-otp page (with partial/pending state)
        User->>Browser (Next.js Frontend): Enters OTP on /verify-otp
        Browser (Next.js Frontend)->>NextAuth Callbacks (Server): Submits OTP via Server Action
        NextAuth Callbacks (Server)->>Database (Prisma): Fetch Stored OTP Hash for User
        NextAuth Callbacks (Server)->>NextAuth Callbacks (Server): Verify Submitted OTP against Stored Hash & Expiry
        alt OTP Correct
            NextAuth Callbacks (Server)->>Database (Prisma): Clear OTP Hash
            NextAuth Callbacks (Server)-->>Browser (Next.js Frontend): Establish Full Session, Redirect to Dashboard
        else OTP Incorrect/Expired
            NextAuth Callbacks (Server)-->>Browser (Next.js Frontend): Show Error on /verify-otp
        end
    else Invalid Credentials
         NextAuth Callbacks (Server)-->>Browser (Next.js Frontend): Show Error on /login
    end

    %% Subsequent Requests
    Browser (Next.js Frontend)->>NextAuth Middleware: Request Protected Route (e.g., /dashboard)
    NextAuth Middleware->>NextAuth Middleware: Check Session Cookie
    alt Valid Full Session
        NextAuth Middleware-->>Browser (Next.js Frontend): Allow Access
    else No/Invalid/Partial Session
        NextAuth Middleware-->>Browser (Next.js Frontend): Redirect to /login (or /verify-otp if pending)
    end

    %% Enable/Disable 2FA (Example on /profile page)
    User->>Browser (Next.js Frontend): Clicks Enable/Disable 2FA on /profile
    Browser (Next.js Frontend)->>NextAuth Callbacks (Server): Submits Request via Server Action
    NextAuth Callbacks (Server)->>Database (Prisma): Update user.twoFactorEnabled flag
    NextAuth Callbacks (Server)-->>Browser (Next.js Frontend): Confirm Success/Failure

npx create-next-app@latest next-plivo-2fa --typescript --tailwind --eslint --app --src-dir --import-alias "@/*"
cd next-plivo-2fa

npm install next-auth@beta @auth/prisma-adapter prisma
npm install plivo-node bcrypt zod otp-generator
npm install -D @types/bcrypt @types/otp-generator
# Optional: If using shadcn/ui for UI components
# npx shadcn-ui@latest init
# npx shadcn-ui@latest add button input label

npx prisma init --datasource-provider postgresql

# .env

# Database (Prisma will add this during init - update with your actual connection string)
DATABASE_URL="postgresql://user:password@host:port/database?sslmode=require" # Example for PostgreSQL

# NextAuth Configuration
# Generate a strong secret. This is crucial for security.
# Any sufficiently long, random string will work.
# Example generation command (Linux/macOS with OpenSSL): openssl rand -base64 32
NEXTAUTH_SECRET="YOUR_NEXTAUTH_SECRET"
NEXTAUTH_URL="http://localhost:3000" # Use your production URL in deployment

# Plivo Credentials
PLIVO_AUTH_ID="YOUR_PLIVO_AUTH_ID"
PLIVO_AUTH_TOKEN="YOUR_PLIVO_AUTH_TOKEN"
PLIVO_PHONE_NUMBER="+1##########" # Your Plivo SMS-enabled number in E.164 format

# OTP Settings (Optional - Defaults can be set in code)
OTP_EXPIRY_MINUTES=5 # How long an OTP is valid

// prisma/schema.prisma

generator client {
  provider = "prisma-client-js"
}

datasource db {
  provider = "postgresql" // Or your chosen provider
  url      = env("DATABASE_URL")
}

model Account {
  id                String  @id @default(cuid())
  userId            String
  type              String
  provider          String
  providerAccountId String
  refresh_token     String? @db.Text
  access_token      String? @db.Text
  expires_at        Int?
  token_type        String?
  scope             String?
  id_token          String? @db.Text
  session_state     String?

  user User @relation(fields: [userId], references: [id], onDelete: Cascade)

  @@unique([provider, providerAccountId])
}

model Session {
  id           String   @id @default(cuid())
  sessionToken String   @unique
  userId       String
  expires      DateTime
  user         User     @relation(fields: [userId], references: [id], onDelete: Cascade)

  // Custom field to track 2FA verification status within a session
  twoFactorVerified Boolean? @default(false)
}

model User {
  id            String    @id @default(cuid())
  name          String?
  email         String?   @unique
  emailVerified DateTime?
  image         String?
  password      String? // For Credentials provider
  accounts      Account[]
  sessions      Session[]

  // Custom Fields for 2FA
  phoneNumber       String?   @unique // Store user's verified phone number for OTP
  twoFactorEnabled  Boolean   @default(false)
  twoFactorSecret   String?   // Store hashed OTP or related secret
  twoFactorExpiry   DateTime? // Store OTP expiry timestamp
}

model VerificationToken {
  identifier String
  token      String   @unique
  expires    DateTime

  @@unique([identifier, token])
}

npx prisma migrate dev --name init

src/
├── app/                 # Next.js App Router pages and layouts
│   ├── api/             # API routes (including NextAuth catch-all)
│   ├── (auth)/          # Route group for auth pages (login, register, verify-otp)
│   │   ├── login/
│   │   ├── register/
│   │   └── verify-otp/
│   ├── (protected)/     # Route group for authenticated pages
│   │   ├── dashboard/
│   │   └── layout.tsx   # Layout protecting routes in this group
│   ├── layout.tsx       # Root layout
│   └── page.tsx         # Home page
├── components/          # Reusable UI components
│   ├── ui/              # (If using shadcn/ui) Generated components
│   └── auth/            # Authentication specific components
├── lib/                 # Utility functions, constants, Prisma client, etc.
│   ├── prisma.ts        # Prisma client instance
│   ├── actions.ts       # Server Actions
│   ├── auth.ts          # NextAuth configuration (main)
│   ├── auth.config.ts   # NextAuth configuration (callbacks, pages - used by middleware)
│   ├── plivo.ts         # Plivo client setup and functions
│   └── utils.ts         # General utility functions (e.g., OTP generation)
└── middleware.ts        # Next.js Middleware for route protection

// src/lib/prisma.ts
import { PrismaClient } from '@prisma/client';

declare global {
  // eslint-disable-next-line no-var
  var prisma: PrismaClient | undefined;
}

export const prisma = global.prisma || new PrismaClient();

if (process.env.NODE_ENV !== 'production') {
  global.prisma = prisma;
}

// src/lib/plivo.ts
import * as plivo from 'plivo';

const authId = process.env.PLIVO_AUTH_ID;
const authToken = process.env.PLIVO_AUTH_TOKEN;
const plivoPhoneNumber = process.env.PLIVO_PHONE_NUMBER;

let plivoClient: plivo.Client | null = null;

if (!authId || !authToken || !plivoPhoneNumber) {
  // In production, log a clear error. In development, a warning is acceptable
  // as SMS might not be needed for all dev tasks.
  if (process.env.NODE_ENV === 'production') {
     console.error('Plivo credentials or phone number missing in environment variables. SMS sending will fail.');
     // Consider throwing an error here in production if SMS is absolutely critical
     // throw new Error('Plivo configuration is incomplete.');
  } else {
     console.warn('Plivo credentials or phone number missing. SMS functionality will be disabled/simulated.');
  }
} else {
  // Initialize client only if credentials are available
  plivoClient = new plivo.Client(authId, authToken);
}

export const sourcePhoneNumber = plivoPhoneNumber;

/**
 * Sends an SMS message using Plivo.
 * @param to The recipient's phone number in E.164 format.
 * @param text The message body.
 * @returns Promise resolving with the Plivo API response or rejecting with an error.
 */
export async function sendSms(to: string, text: string) {
  if (!plivoClient || !sourcePhoneNumber) {
    // Log error or simulate success in development
    if (process.env.NODE_ENV !== 'production') {
        console.log(`SIMULATING SMS to ${to}: ${text}`);
        // Return a shape similar to Plivo's success response for consistency
        return Promise.resolve({
             message: 'SMS simulated',
             messageUuid: ['simulated-uuid-' + Math.random().toString(36).substring(7)],
             apiId: 'simulated-api-id'
         });
    }
    // In production, this should ideally not be reached if the check above is strict,
    // but throw an error just in case.
    console.error('Plivo client not initialized or source number missing. Cannot send SMS.');
    throw new Error('SMS service is not configured.');
  }

  try {
    const response = await plivoClient.messages.create(
      sourcePhoneNumber, // src
      to,                // dst
      text               // text
    );
    // Avoid logging the entire response object in production if it contains sensitive info
    console.log('Message sent successfully via Plivo. Message UUID:', response.messageUuid);
    return response;
  } catch (error) {
    console.error('Error sending SMS via Plivo:', error);
    throw error; // Re-throw the error to be handled by the caller
  }
}

// src/lib/auth.ts (Add these declarations at the top or in a separate d.ts file)
import { DefaultSession } from 'next-auth';
import { JWT as NextAuthJWT } from 'next-auth/jwt';

// Extend Session type
declare module 'next-auth' {
  interface Session {
    twoFactorVerified?: boolean; // Add custom property
    user: {
      id: string; // Ensure id is always present
    } & DefaultSession['user']; // Keep default user properties
  }
}

// Extend JWT type
declare module 'next-auth/jwt' {
  interface JWT {
    twoFactorVerified?: boolean; // Add custom property
    id?: string; // Ensure id is present
  }
}

// src/lib/auth.config.ts
import type { NextAuthConfig } from 'next-auth';
import { Session } from 'next-auth'; // Import base Session type
import { JWT } from 'next-auth/jwt'; // Import base JWT type

export const authConfig = {
  pages: {
    signIn: '/login', // Redirect users to `/login` if required
    // error: '/auth/error', // Optional: Custom error page
  },
  callbacks: {
    // This callback is crucial for protecting routes via Middleware
    authorized({ auth, request: { nextUrl } }) {
      const isLoggedIn = !!auth?.user;
      // Use the augmented Session type implicitly now
      const session = auth;
      const isTwoFactorVerified = session?.twoFactorVerified ?? false;
      const isOnDashboard = nextUrl.pathname.startsWith('/dashboard');
      const isOnVerifyOtp = nextUrl.pathname === '/verify-otp';

      if (isOnDashboard) {
        if (isLoggedIn) {
          // If user is logged in but hasn't completed 2FA for this session, redirect to verify
          if (session.user && !isTwoFactorVerified) {
             // console.log(""Middleware: User logged in but 2FA not verified. Redirecting to /verify-otp."");
             return Response.redirect(new URL('/verify-otp', nextUrl.origin));
          }
          // If logged in and 2FA verified (or not needed), allow access
          return true;
        }
        return false; // Redirect unauthenticated users to login page
      } else if (isOnVerifyOtp) {
        // Allow access to /verify-otp only if the user is logged in but hasn't verified 2FA yet
        return isLoggedIn && !isTwoFactorVerified;
      } else if (isLoggedIn) {
        // If logged in user tries to access login/register page, redirect appropriately
        if (nextUrl.pathname === '/login' || nextUrl.pathname === '/register') {
           if(!isTwoFactorVerified) {
              // If trying to access login/register while needing OTP verification, redirect to verify page
              // console.log(""Middleware: Logged in user needing 2FA tried accessing auth page. Redirecting to /verify-otp."");
              return Response.redirect(new URL('/verify-otp', nextUrl.origin));
           }
           // Otherwise (logged in and verified), redirect to dashboard
           // console.log(""Middleware: Logged in user tried accessing auth page. Redirecting to /dashboard."");
           return Response.redirect(new URL('/dashboard', nextUrl.origin));
        }
      }
      // Allow access to all other pages (like login, register, home) by default
      return true;
    },

    // JWT Callback: Called whenever a JWT is created or updated.
    async jwt({ token, user, trigger, session: updateSessionData }) {
        // Initial sign in
        if (user) {
            token.id = user.id;
            // IMPORTANT: Initially mark 2FA as NOT verified upon login,
            // even if the user has it enabled. Verification happens per-session.
            token.twoFactorVerified = false;
        }

        // If the session is updated (e.g., after successful OTP verification via unstable_update)
        if (trigger === ""update"" && updateSessionData?.twoFactorVerified) {
            token.twoFactorVerified = true;
             // console.log(""JWT Updated: 2FA verified status set to true."");
        }

        return token;
    },

    // Session Callback: Called whenever a session is checked.
    async session({ session, token }: { session: Session, token?: JWT }) {
      // Ensure the user ID and 2FA status from the token are added to the session object
      if (token?.id && session.user) {
        session.user.id = token.id;
      }
      if (token?.twoFactorVerified !== undefined) {
         session.twoFactorVerified = token.twoFactorVerified;
      } else {
         // Default to false if not present in token (shouldn't happen with jwt callback setup)
         session.twoFactorVerified = false;
      }
       // console.log(`Session Callback: User ID: ${session.user?.id}, 2FA Verified: ${session.twoFactorVerified}`);
      return session; // Return the augmented session object
    }
  },
  providers: [], // Add providers in auth.ts, keep empty here for middleware compatibility
  session: { strategy: 'jwt' }, // Use JWT strategy for session management
} satisfies NextAuthConfig;

// src/lib/auth.ts
import NextAuth from 'next-auth';
import Credentials from 'next-auth/providers/credentials';
import { PrismaAdapter } from '@auth/prisma-adapter';
import bcrypt from 'bcrypt';
import { z } from 'zod';

import { prisma } from '@/lib/prisma';
import { authConfig } from './auth.config'; // Import base config
import { sendSms } from './plivo';
import { generateOtp, verifyOtp } from './utils'; // OTP utilities defined in Sec 2.5
import { User } from '@prisma/client'; // Import User type

// Module augmentation for Session/JWT types (already shown above)
// --- Add the declare module blocks here if not in a separate file ---

// Schema for the initial login form (Email/Password only)
const LoginSchema = z.object({
  email: z.string().email({ message: 'Invalid email format.' }),
  password: z.string().min(1, { message: 'Password is required.' }), // Min 1, actual length check is via bcrypt
});

export const {
  handlers: { GET, POST },
  auth,
  signIn,
  signOut,
  unstable_update // Use unstable_update for modifying session state
} = NextAuth({
  ...authConfig, // Spread the base config (pages, session strategy, basic callbacks)
  adapter: PrismaAdapter(prisma),
  providers: [
    Credentials({
      async authorize(credentials) {
        try {
          const parsedCredentials = LoginSchema.safeParse(credentials);

          if (!parsedCredentials.success) {
            console.warn(""Credential parsing failed:"", parsedCredentials.error.flatten());
            return null; // Invalid input format
          }

          const { email, password } = parsedCredentials.data;

          // console.log(`Attempting login for email: ${email}`);
          const user = await prisma.user.findUnique({ where: { email } });

          if (!user || !user.password) {
            // console.log(""User not found or password not set."");
            return null; // User not found or doesn't use password auth
          }

          const passwordsMatch = await bcrypt.compare(password, user.password);

          if (!passwordsMatch) {
            // console.log(""Password mismatch for user:"", email);
            return null; // Incorrect password
          }

          // console.log(`User ${email} authenticated successfully (primary).`);

          // --- 2FA Check ---
          if (user.twoFactorEnabled && user.phoneNumber) {
            // console.log(`2FA enabled for user ${email}. Generating OTP.`);
            const otp = generateOtp(); // Generate OTP (defined in Sec 2.5)
            const otpExpiry = new Date(Date.now() + (parseInt(process.env.OTP_EXPIRY_MINUTES || '5') * 60 * 1000));
            const otpHash = await bcrypt.hash(otp, 10); // Hash the OTP before storing

            // Store hashed OTP and expiry in DB
            await prisma.user.update({
              where: { id: user.id },
              data: {
                twoFactorSecret: otpHash,
                twoFactorExpiry: otpExpiry,
              },
            });
             // console.log(`Stored OTP hash for user ${user.id}. Expiry: ${otpExpiry}`);

            // Send OTP via Plivo
            try {
               await sendSms(user.phoneNumber, `Your verification code is: ${otp}. It expires in ${process.env.OTP_EXPIRY_MINUTES || 5} minutes.`);
               // console.log(`OTP SMS potentially sent to ${user.phoneNumber}`); // Use 'potentially' due to simulation possibility
            } catch (smsError: any) {
               console.error(`Failed to send OTP SMS to ${user.phoneNumber}:`, smsError.message || smsError);
               // Throw a specific error that can be caught by the authenticate action
               // This will result in a CallbackRouteError in NextAuth
               throw new Error(""Failed to send verification code. Please try again later."");
            }

            // IMPORTANT: Return the user object here.
            // The JWT callback will set `twoFactorVerified = false` initially.
            // The `authorized` callback in middleware will then force the redirect to /verify-otp.
            // console.log(""Returning user object to signal pending 2FA verification."");
            return user;

          } else {
            // 2FA not enabled, proceed with normal login
            // console.log(`2FA not enabled for user ${email}. Proceeding with login.`);
            // Clear any potentially stale OTP data
            if(user.twoFactorSecret || user.twoFactorExpiry) {
               await prisma.user.update({
                  where: { id: user.id },
                  data: { twoFactorSecret: null, twoFactorExpiry: null },
               });
            }
            // The JWT callback will run, but the middleware won't redirect to /verify-otp
            // because the user object doesn't require it (or 2FA is disabled).
            // The session callback will set twoFactorVerified based on the token (which defaults based on this path).
            return user; // Return full user object
          }

        } catch (error) {
           // Catch errors from within authorize (e.g., the SMS sending error)
           console.error(""Error during authorization:"", error);
           // Re-throw the error if it's one we want to surface (like the SMS failure)
           if (error instanceof Error && error.message.startsWith(""Failed to send"")) {
               throw error; // Ensure this specific error propagates to AuthError
           }
           // For other errors (Zod, DB issues during OTP storage), return null to indicate failure
           return null;
        }
      },
    }),
    // ... other providers like Google, GitHub, etc. if needed
  ],
  // Callbacks are already defined in authConfig and spread above
});

// src/middleware.ts
import NextAuth from 'next-auth';
import { authConfig } from '@/lib/auth.config'; // Use the specific config for middleware

// Initialize NextAuth with the specific config for middleware checks
const { auth: middleware } = NextAuth(authConfig);

export default middleware; // Export the auth handler

// Matcher configuration: Apply middleware to relevant paths
export const config = {
  // Apply middleware logic to these paths.
  // The `authorized` callback determines access/redirects.
  matcher: [
     '/dashboard/:path*', // Protect all dashboard routes
     '/verify-otp',      // Manage access to the OTP verification page
     '/login',           // Apply logic to redirect logged-in users
     '/register',        // Apply logic to redirect logged-in users
     // Exclude API routes, Next.js static files, and image optimization files
     // Ensure this doesn't accidentally exclude your auth pages if they aren't listed above
     '/((?!api|_next/static|_next/image|favicon.ico).*)',
  ],
};

// Explanation of Matcher:
// - Explicitly lists key routes (/dashboard*, /verify-otp, /login, /register) where auth logic is important.
// - Uses a negative lookahead `(?!...)` to exclude common static/API paths.
// - The final broad matcher `.*` ensures the middleware runs on other pages not explicitly excluded,
//   allowing the `authorized` callback to handle redirects (e.g., logged-in user visiting `/`).
// Adjust the matcher based on your specific public/private route structure if needed.

// src/lib/utils.ts
import otpGenerator from 'otp-generator';
import bcrypt from 'bcrypt';
import { prisma } from './prisma';

/**
 * Generates a secure numeric OTP.
 * @param length The desired length of the OTP (default: 6).
 * @returns A string containing the generated OTP.
 */
export function generateOtp(length: number = 6): string {
  return otpGenerator.generate(length, {
    upperCaseAlphabets: false,
    lowerCaseAlphabets: false,
    specialChars: false,
    digits: true,
  });
}

/**
 * Verifies a submitted OTP against the stored hash for a user.
 * Also clears the OTP data on successful verification or if expired.
 * @param userId The ID of the user attempting verification.
 * @param submittedOtp The OTP code entered by the user.
 * @returns Promise resolving to true if OTP is valid and not expired, false otherwise.
 */
export async function verifyOtp(userId: string, submittedOtp: string): Promise<boolean&gt; {
  try {
    const user = await prisma.user.findUnique({
      where: { id: userId },
      select: { twoFactorSecret: true, twoFactorExpiry: true },
    });

    if (!user || !user.twoFactorSecret || !user.twoFactorExpiry) {
      // console.log(`Verification failed: No OTP data found for user ${userId}.`);
      return false; // No OTP data stored
    }

    // Check expiry first
    const isExpired = new Date() &gt; user.twoFactorExpiry;

    if (isExpired) {
      // console.log(`Verification failed: OTP expired for user ${userId}. Expiry: ${user.twoFactorExpiry}`);
      // Clear the expired OTP data for hygiene
      await prisma.user.update({
          where: { id: userId },
          data: { twoFactorSecret: null, twoFactorExpiry: null },
      });
      return false; // OTP expired
    }

    // Compare submitted OTP with the stored hash
    const isOtpValid = await bcrypt.compare(submittedOtp, user.twoFactorSecret);

    if (isOtpValid) {
       // console.log(`Verification successful for user ${userId}.`);
       // OTP is valid, clear it immediately after successful verification
       await prisma.user.update({
          where: { id: userId },
          data: { twoFactorSecret: null, twoFactorExpiry: null },
       });
       // console.log(`Cleared OTP data for user ${userId} after successful verification.`);
       return true; // OTP is valid
    } else {
       // console.log(`Verification failed: Submitted OTP does not match stored hash for user ${userId}.`);
       // Do not clear the OTP here, allow user to retry within expiry window.
       return false; // OTP is invalid
    }

  } catch (error) {
    console.error(`Error during OTP verification for user ${userId}:`, error);
    return false; // Return false on any unexpected error
  }
}