diff --git a/packages/app/src/chat/nip24.ts b/packages/app/src/chat/nip24.ts
index d65ce9e7..ac25a359 100644
--- a/packages/app/src/chat/nip24.ts
+++ b/packages/app/src/chat/nip24.ts
@@ -1,12 +1,13 @@
 import { ExternalStore, dedupe } from "@snort/shared";
 import {
   EventKind,
-  SystemInterface,
   NostrPrefix,
   encodeTLVEntries,
   TLVEntryType,
   TLVEntry,
   decodeTLV,
+  PowWorker,
+  NostrEvent,
 } from "@snort/system";
 import { GiftWrapCache } from "Cache/GiftWrapCache";
 import { UnwrappedGift } from "Db";
@@ -103,16 +104,23 @@ export class Nip24ChatSystem extends ExternalStore<Array<Chat>> implements ChatS
           }
           return eb;
         });
-        const messages = [];
+        const messages: Array<Promise<NostrEvent>> = [];
+        const powTarget = 4 * 4; // 4-char zero
         for (const pt of participants) {
-          const recvSealedN = await pub.giftWrap(await pub.sealRumor(gossip, pt.id), pt.id);
+          const recvSealedN = pub.giftWrap(
+            await pub.sealRumor(gossip, pt.id),
+            pt.id,
+            powTarget,
+            new PowWorker("/pow.js")
+          );
           messages.push(recvSealedN);
         }
-        const sendSealed = await pub.giftWrap(await pub.sealRumor(gossip, pub.pubKey), pub.pubKey);
-        return [...messages, sendSealed];
+        messages.push(
+          pub.giftWrap(await pub.sealRumor(gossip, pub.pubKey), pub.pubKey, powTarget, new PowWorker("/pow.js"))
+        );
+        return await Promise.all(messages);
       },
-      sendMessage: (ev, system: SystemInterface) => {
-        console.debug(ev);
+      sendMessage: (ev, system) => {
         ev.forEach(a => system.BroadcastEvent(a));
       },
     } as Chat;
diff --git a/packages/app/webpack.config.js b/packages/app/webpack.config.js
index 42b9f793..dd94e14a 100644
--- a/packages/app/webpack.config.js
+++ b/packages/app/webpack.config.js
@@ -17,6 +17,10 @@ const config = {
       import: "./src/service-worker.ts",
       filename: "service-worker.js",
     },
+    pow: {
+      import: require.resolve("@snort/system/dist/pow-worker.js"),
+      filename: "pow.js",
+    },
   },
   target: "browserslist",
   mode: isProduction ? "production" : "development",
@@ -24,12 +28,7 @@ const config = {
   output: {
     publicPath: "/",
     path: path.resolve(__dirname, "build"),
-    filename: ({ runtime }) => {
-      if (runtime === "sw") {
-        return "[name].js";
-      }
-      return isProduction ? "[name].[chunkhash].js" : "[name].js";
-    },
+    filename: isProduction ? "[name].[chunkhash].js" : "[name].js",
     clean: isProduction,
   },
   devServer: {
@@ -50,7 +49,7 @@ const config = {
     new HtmlWebpackPlugin({
       template: "public/index.html",
       favicon: "public/favicon.ico",
-      excludeChunks: ["sw"],
+      excludeChunks: ["sw", "pow"],
     }),
     new ESLintPlugin({
       extensions: ["js", "mjs", "jsx", "ts", "tsx"],
diff --git a/packages/system/src/event-builder.ts b/packages/system/src/event-builder.ts
index c9beb2b3..cfe88bb6 100644
--- a/packages/system/src/event-builder.ts
+++ b/packages/system/src/event-builder.ts
@@ -1,4 +1,4 @@
-import { EventKind, HexKey, NostrPrefix, NostrEvent, EventSigner } from ".";
+import { EventKind, HexKey, NostrPrefix, NostrEvent, EventSigner, PowMiner } from ".";
 import { HashtagRegex, MentionNostrEntityRegex } from "./const";
 import { getPublicKey, unixNow } from "@snort/shared";
 import { EventExt } from "./event-ext";
@@ -10,6 +10,8 @@ export class EventBuilder {
   #createdAt?: number;
   #pubkey?: string;
   #tags: Array<Array<string>> = [];
+  #pow?: number;
+  #powMiner?: PowMiner;
 
   kind(k: EventKind) {
     this.#kind = k;
@@ -38,6 +40,17 @@ export class EventBuilder {
     return this;
   }
 
+  pow(target: number, miner?: PowMiner) {
+    this.#pow = target;
+    this.#powMiner = miner ?? {
+      minePow: (ev, target) => {
+        EventExt.minePow(ev, target);
+        return Promise.resolve(ev);
+      },
+    };
+    return this;
+  }
+
   /**
    * Extract mentions
    */
@@ -74,14 +87,21 @@ export class EventBuilder {
   async buildAndSign(pk: HexKey | EventSigner) {
     if (typeof pk === "string") {
       const ev = this.pubKey(getPublicKey(pk)).build();
-      EventExt.sign(ev, pk);
-      return ev;
+      return EventExt.sign(await this.#mine(ev), pk);
     } else {
       const ev = this.pubKey(await pk.getPubKey()).build();
-      return await pk.sign(ev);
+      return await pk.sign(await this.#mine(ev));
     }
   }
 
+  async #mine(ev: NostrEvent) {
+    if (this.#pow && this.#powMiner) {
+      const ret = await this.#powMiner.minePow(ev, this.#pow);
+      return ret;
+    }
+    return ev;
+  }
+
   #validate() {
     if (this.#kind === undefined) {
       throw new Error("Kind must be set");
diff --git a/packages/system/src/event-ext.ts b/packages/system/src/event-ext.ts
index c8d6bacc..9a048387 100644
--- a/packages/system/src/event-ext.ts
+++ b/packages/system/src/event-ext.ts
@@ -3,7 +3,7 @@ import * as utils from "@noble/curves/abstract/utils";
 import { getPublicKey, sha256, unixNow } from "@snort/shared";
 
 import { EventKind, HexKey, NostrEvent, NotSignedNostrEvent } from ".";
-import { Nip4WebCryptoEncryptor } from "./impl/nip4";
+import { minePow } from "./pow-util";
 
 export interface Tag {
   key: string;
@@ -44,6 +44,7 @@ export abstract class EventExt {
     if (!secp.schnorr.verify(e.sig, e.id, e.pubkey)) {
       throw new Error("Signing failed");
     }
+    return e;
   }
 
   /**
@@ -58,13 +59,14 @@ export abstract class EventExt {
 
   static createId(e: NostrEvent | NotSignedNostrEvent) {
     const payload = [0, e.pubkey, e.created_at, e.kind, e.tags, e.content];
+    return sha256(JSON.stringify(payload));
+  }
 
-    const hash = sha256(JSON.stringify(payload));
-    if (e.id !== "" && hash !== e.id) {
-      console.debug(payload);
-      throw new Error("ID doesnt match!");
-    }
-    return hash;
+  /**
+   * Mine POW for an event (NIP-13)
+   */
+  static minePow(e: NostrEvent, target: number) {
+    return minePow(e, target);
   }
 
   /**
diff --git a/packages/system/src/event-publisher.ts b/packages/system/src/event-publisher.ts
index 0a652e10..af2c605f 100644
--- a/packages/system/src/event-publisher.ts
+++ b/packages/system/src/event-publisher.ts
@@ -10,6 +10,7 @@ import {
   Lists,
   NostrEvent,
   NotSignedNostrEvent,
+  PowMiner,
   PrivateKeySigner,
   RelaySettings,
   TaggedNostrEvent,
@@ -285,7 +286,7 @@ export class EventPublisher {
   /**
    * NIP-59 Gift Wrap event with ephemeral key
    */
-  async giftWrap(inner: NostrEvent, explicitP?: string) {
+  async giftWrap(inner: NostrEvent, explicitP?: string, powTarget?: number, powMiner?: PowMiner) {
     const secret = utils.bytesToHex(secp.secp256k1.utils.randomPrivateKey());
     const signer = new PrivateKeySigner(secret);
 
@@ -296,6 +297,9 @@ export class EventPublisher {
     eb.pubKey(signer.getPubKey());
     eb.kind(EventKind.GiftWrap);
     eb.tag(["p", pTag]);
+    if (powTarget) {
+      eb.pow(powTarget, powMiner);
+    }
     eb.content(await signer.nip44Encrypt(JSON.stringify(inner), pTag));
 
     return await eb.buildAndSign(secret);
diff --git a/packages/system/src/impl/nip4.ts b/packages/system/src/impl/nip4.ts
index c775dca5..146e8342 100644
--- a/packages/system/src/impl/nip4.ts
+++ b/packages/system/src/impl/nip4.ts
@@ -23,10 +23,10 @@ export class Nip4WebCryptoEncryptor implements MessageEncryptor {
     return {
       ciphertext: new Uint8Array(result),
       nonce: iv,
-      v: MessageEncryptorVersion.Nip4
+      v: MessageEncryptorVersion.Nip4,
     } as MessageEncryptorPayload;
   }
-  
+
   async decryptData(payload: MessageEncryptorPayload, sharedSecet: Uint8Array) {
     const key = await this.#importKey(sharedSecet);
     const result = await window.crypto.subtle.decrypt(
@@ -35,7 +35,7 @@ export class Nip4WebCryptoEncryptor implements MessageEncryptor {
         iv: payload.nonce,
       },
       key,
-      payload.ciphertext
+      payload.ciphertext,
     );
     return new TextDecoder().decode(result);
   }
diff --git a/packages/system/src/index.ts b/packages/system/src/index.ts
index e437d565..86763ea3 100644
--- a/packages/system/src/index.ts
+++ b/packages/system/src/index.ts
@@ -21,6 +21,7 @@ export * from "./profile-cache";
 export * from "./zaps";
 export * from "./signer";
 export * from "./text";
+export * from "./pow";
 
 export * from "./impl/nip4";
 export * from "./impl/nip44";
@@ -60,9 +61,9 @@ export const enum MessageEncryptorVersion {
 }
 
 export interface MessageEncryptorPayload {
-  ciphertext: Uint8Array,
-  nonce: Uint8Array,
-  v: MessageEncryptorVersion
+  ciphertext: Uint8Array;
+  nonce: Uint8Array;
+  v: MessageEncryptorVersion;
 }
 
 export interface MessageEncryptor {
diff --git a/packages/system/src/nostr-link.ts b/packages/system/src/nostr-link.ts
index c89aa231..54e9ec22 100644
--- a/packages/system/src/nostr-link.ts
+++ b/packages/system/src/nostr-link.ts
@@ -15,16 +15,17 @@ export function createNostrLink(prefix: NostrPrefix, id: string, relays?: string
     type: prefix,
     id,
     relays,
-    kind, author,
+    kind,
+    author,
     encode: () => {
-      if(prefix === NostrPrefix.Note || prefix === NostrPrefix.PublicKey) {
+      if (prefix === NostrPrefix.Note || prefix === NostrPrefix.PublicKey) {
         return hexToBech32(prefix, id);
-      } 
-      if(prefix === NostrPrefix.Address || prefix === NostrPrefix.Event || prefix === NostrPrefix.Profile) {
+      }
+      if (prefix === NostrPrefix.Address || prefix === NostrPrefix.Event || prefix === NostrPrefix.Profile) {
         return encodeTLV(prefix, id, relays, kind, author);
       }
       return "";
-    }
+    },
   } as NostrLink;
 }
 
diff --git a/packages/system/src/nostr.ts b/packages/system/src/nostr.ts
index 7ea45b80..50fd1934 100644
--- a/packages/system/src/nostr.ts
+++ b/packages/system/src/nostr.ts
@@ -86,4 +86,4 @@ export interface FullRelaySettings {
   settings: RelaySettings;
 }
 
-export type NotSignedNostrEvent = Omit<NostrEvent, "sig">;
\ No newline at end of file
+export type NotSignedNostrEvent = Omit<NostrEvent, "sig">;
diff --git a/packages/system/src/pow-util.ts b/packages/system/src/pow-util.ts
new file mode 100644
index 00000000..952e60af
--- /dev/null
+++ b/packages/system/src/pow-util.ts
@@ -0,0 +1,57 @@
+import { sha256 } from "@noble/hashes/sha256";
+import { bytesToHex } from "@noble/hashes/utils";
+
+export interface NostrPowEvent {
+  id: string;
+  pubkey: string;
+  created_at: number;
+  kind: number;
+  tags: Array<Array<string>>;
+  content: string;
+  sig: string;
+}
+
+export function minePow(e: NostrPowEvent, target: number) {
+  let ctr = 0;
+
+  let nonceTagIdx = e.tags.findIndex(a => a[0] === "nonce");
+  if (nonceTagIdx === -1) {
+    nonceTagIdx = e.tags.length;
+    e.tags.push(["nonce", ctr.toString(), target.toString()]);
+  }
+  do {
+    //roll ctr and compute id
+    const now = Math.floor(new Date().getTime() / 1000);
+    // reset ctr if timestamp changed, this is not really needed but makes the ctr value smaller
+    if (now !== e.created_at) {
+      ctr = 0;
+      e.created_at = now;
+    }
+    e.tags[nonceTagIdx][1] = (++ctr).toString();
+
+    e.id = createId(e);
+  } while (countLeadingZeroes(e.id) < target);
+
+  return e;
+}
+
+function createId(e: NostrPowEvent) {
+  const payload = [0, e.pubkey, e.created_at, e.kind, e.tags, e.content];
+  return bytesToHex(sha256(JSON.stringify(payload)));
+}
+
+function countLeadingZeroes(hex: string) {
+  let count = 0;
+
+  for (let i = 0; i < hex.length; i++) {
+    const nibble = parseInt(hex[i], 16);
+    if (nibble === 0) {
+      count += 4;
+    } else {
+      count += Math.clz32(nibble) - 28;
+      break;
+    }
+  }
+
+  return count;
+}
diff --git a/packages/system/src/pow-worker.ts b/packages/system/src/pow-worker.ts
new file mode 100644
index 00000000..195e268f
--- /dev/null
+++ b/packages/system/src/pow-worker.ts
@@ -0,0 +1,21 @@
+/// <reference lib="webworker" />
+
+import { minePow, NostrPowEvent } from "./pow-util";
+
+export interface PowWorkerMessage {
+  id: string;
+  cmd: "req" | "rsp";
+  event: NostrPowEvent;
+  target: number;
+}
+
+globalThis.onmessage = ev => {
+  const data = ev.data as PowWorkerMessage;
+  if (data.cmd === "req") {
+    queueMicrotask(() => {
+      minePow(data.event, data.target);
+      data.cmd = "rsp";
+      globalThis.postMessage(data);
+    });
+  }
+};
diff --git a/packages/system/src/pow.ts b/packages/system/src/pow.ts
new file mode 100644
index 00000000..543ed3ce
--- /dev/null
+++ b/packages/system/src/pow.ts
@@ -0,0 +1,56 @@
+import { v4 as uuid } from "uuid";
+import { NostrEvent } from "./nostr";
+import { PowWorkerMessage } from "./pow-worker";
+
+export interface PowMiner {
+  minePow(ev: NostrEvent, target: number): Promise<NostrEvent>;
+}
+
+interface PowQueue {
+  resolve: (ev: NostrEvent) => void;
+  reject: () => void;
+  timeout: ReturnType<typeof setTimeout>;
+}
+
+export class PowWorker implements PowMiner {
+  #worker: Worker;
+  #queue: Map<string, PowQueue> = new Map();
+
+  constructor(script: string) {
+    this.#worker = new Worker(script, {
+      name: "POW",
+    });
+    this.#worker.onerror = ev => {
+      console.error(ev);
+    };
+    this.#worker.onmessage = ev => {
+      const data = ev.data as PowWorkerMessage;
+      const job = this.#queue.get(data.id);
+      if (job) {
+        clearTimeout(job.timeout);
+        this.#queue.delete(data.id);
+        job.resolve(data.event);
+      }
+    };
+  }
+
+  minePow(ev: NostrEvent, target: number) {
+    return new Promise<NostrEvent>((resolve, reject) => {
+      const req = {
+        id: uuid(),
+        cmd: "req",
+        event: ev,
+        target,
+      } as PowWorkerMessage;
+      this.#queue.set(req.id, {
+        resolve: ex => resolve(ex),
+        reject,
+        timeout: setTimeout(() => {
+          this.#queue.delete(req.id);
+          reject();
+        }, 600_000),
+      });
+      this.#worker.postMessage(req);
+    });
+  }
+}
diff --git a/packages/system/src/signer.ts b/packages/system/src/signer.ts
index 19651828..d2d25c81 100644
--- a/packages/system/src/signer.ts
+++ b/packages/system/src/signer.ts
@@ -59,7 +59,7 @@ export class PrivateKeySigner implements EventSigner {
         nonce: base64.decode(iv),
         v: MessageEncryptorVersion.Nip4,
       },
-      secret
+      secret,
     );
   }
 
diff --git a/packages/system/src/utils.ts b/packages/system/src/utils.ts
index 3553ece2..a38fd70a 100644
--- a/packages/system/src/utils.ts
+++ b/packages/system/src/utils.ts
@@ -49,4 +49,4 @@ export function splitByUrl(str: string) {
     /((?:http|ftp|https|nostr|web\+nostr|magnet):\/?\/?(?:[\w+?.\w+])+(?:[a-zA-Z0-9~!@#$%^&*()_\-=+\\/?.:;',]*)?(?:[-A-Za-z0-9+&@#/%=~()_|]))/i;
 
   return str.split(urlRegex);
-}
+}
\ No newline at end of file