diff --git a/ballotiq/src/app/api/gemini/assess/route.ts b/ballotiq/src/app/api/gemini/assess/route.ts
index 8680d98..ebb8e1f 100644
--- a/ballotiq/src/app/api/gemini/assess/route.ts
+++ b/ballotiq/src/app/api/gemini/assess/route.ts
@@ -5,10 +5,11 @@
  */
 
 import { NextRequest, NextResponse } from 'next/server';
+import { withAuth } from '@/lib/security/auth';
 import { analyzeAssessment } from '@/lib/gemini/assessment';
 import type { AssessmentAnswer } from '@/types';
 
-export async function POST(req: NextRequest) {
+export const POST = withAuth(async (req: NextRequest) => {
   try {
     const body = await req.json() as {
       answers: AssessmentAnswer;
@@ -41,4 +42,4 @@ export async function POST(req: NextRequest) {
       { status: 500 }
     );
   }
-}
+});
diff --git a/ballotiq/src/app/api/gemini/assistant/route.ts b/ballotiq/src/app/api/gemini/assistant/route.ts
index 28c9e91..089bfb1 100644
--- a/ballotiq/src/app/api/gemini/assistant/route.ts
+++ b/ballotiq/src/app/api/gemini/assistant/route.ts
@@ -7,8 +7,9 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { getAssistantResponse } from '@/lib/assistant/hybridAssistant';
 import type { ChatMessage, ElectionStep, UserContext } from '@/types';
+import { withAuth } from '@/lib/security/auth';
 
-export async function POST(req: NextRequest) {
+export const POST = withAuth(async (req: NextRequest) => {
   try {
     const body = await req.json() as {
       question: string;
@@ -38,4 +39,4 @@ export async function POST(req: NextRequest) {
       { status: 500 }
     );
   }
-}
+});
diff --git a/ballotiq/src/app/api/gemini/guide/route.ts b/ballotiq/src/app/api/gemini/guide/route.ts
index 609828d..2808888 100644
--- a/ballotiq/src/app/api/gemini/guide/route.ts
+++ b/ballotiq/src/app/api/gemini/guide/route.ts
@@ -5,10 +5,11 @@
  */
 
 import { NextRequest, NextResponse } from 'next/server';
+import { withAuth } from '@/lib/security/auth';
 import { generatePersonalizedGuide, generatePersonalizedGuideStream } from '@/lib/gemini/guide';
 import type { KnowledgeLevel } from '@/types';
 
-export async function POST(req: NextRequest) {
+export const POST = withAuth(async (req: NextRequest) => {
   try {
     const body = await req.json() as {
       countryCode: string;
@@ -75,4 +76,4 @@ export async function POST(req: NextRequest) {
       { status: 500 }
     );
   }
-}
+});
diff --git a/ballotiq/src/app/api/gemini/insight/route.ts b/ballotiq/src/app/api/gemini/insight/route.ts
index 2520fd8..5e23982 100644
--- a/ballotiq/src/app/api/gemini/insight/route.ts
+++ b/ballotiq/src/app/api/gemini/insight/route.ts
@@ -5,10 +5,11 @@
  */
 
 import { NextRequest, NextResponse } from 'next/server';
+import { withAuth } from '@/lib/security/auth';
 import { generatePerformanceInsight } from '@/lib/gemini/quiz';
 import type { KnowledgeLevel, QuizQuestion, QuizResult } from '@/types';
 
-export async function POST(req: NextRequest) {
+export const POST = withAuth(async (req: NextRequest) => {
   try {
     const body = await req.json() as {
       results: QuizResult[];
@@ -45,4 +46,4 @@ export async function POST(req: NextRequest) {
       { status: 500 }
     );
   }
-}
+});
diff --git a/ballotiq/src/app/api/gemini/micro-quiz/route.ts b/ballotiq/src/app/api/gemini/micro-quiz/route.ts
index 9688842..8d43fcf 100644
--- a/ballotiq/src/app/api/gemini/micro-quiz/route.ts
+++ b/ballotiq/src/app/api/gemini/micro-quiz/route.ts
@@ -5,10 +5,11 @@
  */
 
 import { NextRequest, NextResponse } from 'next/server';
+import { withAuth } from '@/lib/security/auth';
 import { generateMicroQuiz } from '@/lib/gemini/quiz';
 import type { ElectionStep, KnowledgeLevel } from '@/types';
 
-export async function POST(req: NextRequest) {
+export const POST = withAuth(async (req: NextRequest) => {
   try {
     const body = await req.json() as {
       step: ElectionStep;
@@ -41,4 +42,4 @@ export async function POST(req: NextRequest) {
       { status: 500 }
     );
   }
-}
+});
diff --git a/ballotiq/src/app/api/gemini/quiz/route.ts b/ballotiq/src/app/api/gemini/quiz/route.ts
index 6858bbf..f7c21f2 100644
--- a/ballotiq/src/app/api/gemini/quiz/route.ts
+++ b/ballotiq/src/app/api/gemini/quiz/route.ts
@@ -5,10 +5,11 @@
  */
 
 import { NextRequest, NextResponse } from 'next/server';
+import { withAuth } from '@/lib/security/auth';
 import { generatePersonalizedQuiz } from '@/lib/gemini/quiz';
 import type { ElectionStep, KnowledgeLevel } from '@/types';
 
-export async function POST(req: NextRequest) {
+export const POST = withAuth(async (req: NextRequest) => {
   try {
     const body = await req.json() as {
       completedSteps: ElectionStep[];
@@ -44,4 +45,4 @@ export async function POST(req: NextRequest) {
       { status: 500 }
     );
   }
-}
+});
diff --git a/ballotiq/src/app/api/gemini/re-explain/route.ts b/ballotiq/src/app/api/gemini/re-explain/route.ts
index 18c8e9c..a1baa78 100644
--- a/ballotiq/src/app/api/gemini/re-explain/route.ts
+++ b/ballotiq/src/app/api/gemini/re-explain/route.ts
@@ -5,10 +5,11 @@
  */
 
 import { NextRequest, NextResponse } from 'next/server';
+import { withAuth } from '@/lib/security/auth';
 import { reExplainConcept } from '@/lib/gemini/assistant';
 import type { ElectionStep, KnowledgeLevel } from '@/types';
 
-export async function POST(req: NextRequest) {
+export const POST = withAuth(async (req: NextRequest) => {
   try {
     const body = await req.json() as {
       step: ElectionStep;
@@ -45,4 +46,4 @@ export async function POST(req: NextRequest) {
       { status: 500 }
     );
   }
-}
+});
diff --git a/ballotiq/src/lib/security/auth.ts b/ballotiq/src/lib/security/auth.ts
new file mode 100644
index 0000000..4556032
--- /dev/null
+++ b/ballotiq/src/lib/security/auth.ts
@@ -0,0 +1,63 @@
+import { NextRequest, NextResponse } from 'next/server';
+import * as admin from 'firebase-admin';
+
+// Initialize Firebase Admin if not already initialized
+if (!admin.apps.length) {
+  try {
+    admin.initializeApp({
+      credential: admin.credential.applicationDefault(),
+    });
+  } catch (error) {
+    console.error('Firebase admin initialization error', error);
+  }
+}
+
+/**
+ * Validates the Authorization header and returns the decoded Firebase ID token.
+ * 
+ * @param req NextRequest
+ * @returns DecodedIdToken if valid, null if invalid or missing
+ */
+export async function verifyAuthToken(req: NextRequest) {
+  try {
+    const authHeader = req.headers.get('Authorization');
+    if (!authHeader || !authHeader.startsWith('Bearer ')) {
+      return null;
+    }
+
+    const token = authHeader.split('Bearer ')[1];
+    const decodedToken = await admin.auth().verifyIdToken(token);
+    return decodedToken;
+  } catch (error) {
+    console.error('Error verifying auth token', error);
+    return null;
+  }
+}
+
+/**
+ * Middleware wrapper for Next.js API routes to enforce authentication.
+ * If the request is a state-mutating method (POST, PUT, PATCH, DELETE) and lacks a valid token,
+ * it returns a 401 Unauthorized response.
+ * 
+ * @param handler The API route handler function
+ * @returns A wrapped API route handler
+ */
+export function withAuth(handler: (req: NextRequest, ...args: any[]) => Promise<NextResponse>) {
+  return async (req: NextRequest, ...args: any[]) => {
+    // Check if the request is state-mutating
+    if (['POST', 'PUT', 'PATCH', 'DELETE'].includes(req.method)) {
+      const decodedToken = await verifyAuthToken(req);
+      if (!decodedToken) {
+        return NextResponse.json(
+          { error: 'Unauthorized: Missing or invalid authentication token.' },
+          { status: 401 }
+        );
+      }
+      
+      // Inject user info into headers or pass to handler if needed
+      // (Next.js App router doesn't allow mutating req easily, but we know it's verified)
+    }
+
+    return handler(req, ...args);
+  };
+}