diff --git a/backend/shared/cache/nonceCache.ts b/backend/shared/cache/nonceCache.ts
new file mode 100644
index 00000000..1fe68e8b
--- /dev/null
+++ b/backend/shared/cache/nonceCache.ts
@@ -0,0 +1,52 @@
+import { createClient, RedisClientType } from 'redis';
+
+type RedisType = RedisClientType | null;
+
+export default class NonceCache {
+  private redis: RedisType = null;
+  private memory: Map<string, number> = new Map();
+
+  constructor() {
+    const url = process.env.REDIS_URL;
+    if (url) {
+      try {
+        const client = createClient({ url });
+        client.connect().catch(() => {});
+        this.redis = client;
+      } catch (e) {
+        this.redis = null;
+      }
+    }
+  }
+
+  async has(nonce: string): Promise<boolean> {
+    if (this.redis) {
+      try {
+        const v = await this.redis.get(nonce);
+        return v !== null;
+      } catch {
+        // fallback to memory
+      }
+    }
+    const ts = this.memory.get(nonce);
+    if (!ts) return false;
+    if (Date.now() > ts) {
+      this.memory.delete(nonce);
+      return false;
+    }
+    return true;
+  }
+
+  async set(nonce: string, ttlSeconds = 600): Promise<void> {
+    if (this.redis) {
+      try {
+        await this.redis.set(nonce, '1', { EX: ttlSeconds });
+        return;
+      } catch {
+        // fallback
+      }
+    }
+    const expires = Date.now() + ttlSeconds * 1000;
+    this.memory.set(nonce, expires);
+  }
+}
diff --git a/backend/shared/middleware/signature.ts b/backend/shared/middleware/signature.ts
new file mode 100644
index 00000000..d42ebb5f
--- /dev/null
+++ b/backend/shared/middleware/signature.ts
@@ -0,0 +1,22 @@
+import { Request, Response, NextFunction } from 'express';
+import SignatureService from '../webhook/SignatureService';
+
+export function signatureMiddleware(signatureService: SignatureService) {
+  return async (req: Request, res: Response, next: NextFunction) => {
+    try {
+      // Attempt to obtain raw body; if not available, stringify body
+      let raw: string;
+      // Some apps attach rawBody earlier; prefer that.
+      // @ts-ignore
+      if (req.rawBody && typeof req.rawBody === 'string') raw = req.rawBody;
+      else if (typeof req.body === 'string') raw = req.body;
+      else raw = JSON.stringify(req.body || '');
+
+      const header = (req.get('X-Signature') || req.get('x-signature') || '') as string;
+      await signatureService.verify(raw, header);
+      return next();
+    } catch (err) {
+      return res.status(401).json({ error: 'invalid_signature', message: String(err) });
+    }
+  };
+}
diff --git a/backend/shared/webhook/SignatureService.ts b/backend/shared/webhook/SignatureService.ts
new file mode 100644
index 00000000..31e5c726
--- /dev/null
+++ b/backend/shared/webhook/SignatureService.ts
@@ -0,0 +1,81 @@
+import crypto from 'crypto';
+import KeyStore from './keyStore';
+import NonceCache from '../cache/nonceCache';
+
+export interface SignatureOptions {
+  timestampTolerance?: number; // seconds
+  clockSkewTolerance?: number; // seconds
+  nonceTtl?: number; // seconds
+}
+
+export default class SignatureService {
+  private keys: KeyStore;
+  private nonceCache: NonceCache;
+  private opts: Required<SignatureOptions>;
+
+  constructor(keyStore: KeyStore, opts?: SignatureOptions) {
+    this.keys = keyStore;
+    this.nonceCache = new NonceCache();
+    this.opts = {
+      timestampTolerance: opts?.timestampTolerance ?? 300,
+      clockSkewTolerance: opts?.clockSkewTolerance ?? 30,
+      nonceTtl: opts?.nonceTtl ?? 600,
+    };
+  }
+
+  generate(body: string, secret?: string, timestamp?: number, nonce?: string) {
+    const ts = timestamp ?? Math.floor(Date.now() / 1000);
+    const n = nonce ?? crypto.randomBytes(12).toString('hex');
+    const key = secret ?? this.keys.getCurrent();
+    const hmac = crypto.createHmac('sha256', key).update(`${ts}.${body}`).digest();
+    const sig = hmac.toString('base64');
+    const header = `t=${ts},s=${sig},v=1,n=${n}`;
+    return { header, sig, ts, nonce: n };
+  }
+
+  parseHeader(header: string) {
+    const parts = header.split(',').map(p => p.trim());
+    const map: Record<string, string> = {};
+    for (const part of parts) {
+      const [k, v] = part.split('=');
+      if (k && v) map[k] = v;
+    }
+    return map;
+  }
+
+  async verify(rawBody: string, header: string) {
+    if (!header) throw new Error('missing signature header');
+    const parsed = this.parseHeader(header);
+    const ts = parseInt(parsed.t, 10);
+    const sig = parsed.s;
+    const ver = parsed.v;
+    const nonce = parsed.n;
+    if (!ts || !sig || !ver || !nonce) throw new Error('invalid signature header');
+    if (ver !== '1') throw new Error('unsupported signature version');
+
+    const now = Math.floor(Date.now() / 1000);
+    const allowed = this.opts.timestampTolerance + this.opts.clockSkewTolerance;
+    if (Math.abs(now - ts) > allowed) throw new Error('timestamp outside tolerance');
+
+    // nonce replay check
+    if (await this.nonceCache.has(nonce)) {
+      throw new Error('replay detected');
+    }
+
+    // compute hmac against active keys
+    const keys = this.keys.getActiveKeys();
+    let match = false;
+    for (const k of keys) {
+      const h = crypto.createHmac('sha256', k).update(`${ts}.${rawBody}`).digest().toString('base64');
+      if (h === sig) {
+        match = true;
+        break;
+      }
+    }
+    if (!match) throw new Error('signature mismatch');
+
+    // store nonce
+    await this.nonceCache.set(nonce, this.opts.nonceTtl);
+    return true;
+  }
+}
diff --git a/backend/shared/webhook/keyStore.ts b/backend/shared/webhook/keyStore.ts
new file mode 100644
index 00000000..68bd3fee
--- /dev/null
+++ b/backend/shared/webhook/keyStore.ts
@@ -0,0 +1,28 @@
+export default class KeyStore {
+  private current: string;
+  private previous: string | null;
+
+  constructor(initialKey: string) {
+    this.current = initialKey;
+    this.previous = null;
+  }
+
+  getActiveKeys(): string[] {
+    if (this.previous) return [this.current, this.previous];
+    return [this.current];
+  }
+
+  rotate(newKey: string) {
+    this.previous = this.current;
+    this.current = newKey;
+  }
+
+  setKeys(current: string, previous: string | null) {
+    this.current = current;
+    this.previous = previous;
+  }
+
+  getCurrent() {
+    return this.current;
+  }
+}
diff --git a/backend/tests/signature.test.ts b/backend/tests/signature.test.ts
new file mode 100644
index 00000000..b2d79b94
--- /dev/null
+++ b/backend/tests/signature.test.ts
@@ -0,0 +1,43 @@
+import KeyStore from '../../backend/shared/webhook/keyStore';
+import SignatureService from '../../backend/shared/webhook/SignatureService';
+
+describe('SignatureService', () => {
+  const initialKey = 'test-secret-1';
+  let ks: KeyStore;
+  let svc: SignatureService;
+
+  beforeEach(() => {
+    ks = new KeyStore(initialKey);
+    svc = new SignatureService(ks, { timestampTolerance: 300, clockSkewTolerance: 30, nonceTtl: 2 });
+  });
+
+  test('generates and verifies a signature', async () => {
+    const body = JSON.stringify({ hi: 'there' });
+    const { header } = svc.generate(body);
+    await expect(svc.verify(body, header)).resolves.toBe(true);
+  });
+
+  test('rejects replayed nonce', async () => {
+    const body = 'payload';
+    const { header } = svc.generate(body, undefined, undefined, 'fixednonce');
+    await expect(svc.verify(body, header)).resolves.toBe(true);
+    await expect(svc.verify(body, header)).rejects.toThrow(/replay/);
+  });
+
+  test('rejects old timestamp beyond tolerance', async () => {
+    const body = 'payload';
+    const oldTs = Math.floor(Date.now() / 1000) - 10000; // far in past
+    const { header } = svc.generate(body, undefined, oldTs, 'nonce2');
+    await expect(svc.verify(body, header)).rejects.toThrow(/timestamp/);
+  });
+
+  test('accepts signature with rotated previous key', async () => {
+    const body = 'payload2';
+    // sign with current key
+    const s1 = svc.generate(body);
+    // rotate keys
+    ks.rotate('new-key');
+    // previous key (initial) should still verify
+    await expect(svc.verify(body, s1.header)).resolves.toBe(true);
+  });
+});
diff --git a/backend/webhook/controller/signatureController.ts b/backend/webhook/controller/signatureController.ts
new file mode 100644
index 00000000..51e63966
--- /dev/null
+++ b/backend/webhook/controller/signatureController.ts
@@ -0,0 +1,17 @@
+import { Request, Response } from 'express';
+import KeyStore from '../../shared/webhook/keyStore';
+
+export default function signatureController(keyStore: KeyStore) {
+  return {
+    getKeys: (req: Request, res: Response) => {
+      // For admin use only; do not expose in production without auth
+      res.json({ current: keyStore.getCurrent(), active: keyStore.getActiveKeys() });
+    },
+    rotate: (req: Request, res: Response) => {
+      const { newKey } = req.body || {};
+      if (!newKey) return res.status(400).json({ error: 'newKey required' });
+      keyStore.rotate(newKey);
+      res.json({ ok: true });
+    },
+  };
+}
diff --git a/package.json b/package.json
index ac3e5334..174a8b68 100644
--- a/package.json
+++ b/package.json
@@ -79,6 +79,8 @@
     "react-native-svg": "15.15.4",
     "zod": "^3.23.8",
     "zustand": "^4.5.2"
+    ,
+    "redis": "^4.6.7"
   },
   "devDependencies": {
     "@babel/core": "^7.29.0",