diff --git a/esbuildconf.js b/esbuildconf.js
index 8062f34..59e5fe4 100644
--- a/esbuildconf.js
+++ b/esbuildconf.js
@@ -20,6 +20,7 @@ export const options = {
     'src/main.css',
     'src/main.js',
     'src/manifest.json',
+    'src/worker.js',
   ],
   outdir: 'dist',
   //entryNames: '[name]-[hash]', TODO: replace urls in index.html with hashed paths
diff --git a/src/cryptoutils.js b/src/cryptoutils.js
new file mode 100644
index 0000000..2c2b265
--- /dev/null
+++ b/src/cryptoutils.js
@@ -0,0 +1,24 @@
+/**
+  * evaluate the difficulty of hex32 according to nip-13.
+  * @param hex32 a string of 64 chars - 32 bytes in hex representation
+  */
+export const zeroLeadingBitsCount = (hex32) => {
+  let count = 0;
+  for (let i = 0; i < 64; i += 2) {
+    const hexbyte = hex32.slice(i, i + 2); // grab next byte
+    if (hexbyte == '00') {
+      count += 8;
+      continue;
+    }
+    // reached non-zero byte; count number of 0 bits in hexbyte
+    const bits = parseInt(hexbyte, 16).toString(2).padStart(8, '0');
+    for (let b = 0; b < 8; b++) {
+      if (bits[b] == '1' ) {
+        break; // reached non-zero bit; stop
+      }
+      count += 1;
+    }
+    break;
+  }
+  return count;
+};
diff --git a/src/main.js b/src/main.js
index cdb2b44..5bb97e0 100644
--- a/src/main.js
+++ b/src/main.js
@@ -1,4 +1,5 @@
-import {relayPool, generatePrivateKey, getEventHash, getPublicKey, signEvent} from 'nostr-tools';
+import {relayPool, generatePrivateKey, getPublicKey, signEvent} from 'nostr-tools';
+import {zeroLeadingBitsCount} from './cryptoutils';
 import {elem, parseTextContent} from './domutil.js';
 import {dateTime, formatTime} from './timeutil.js';
 // curl -H 'accept: application/nostr+json' https://relay.nostr.ch/
@@ -40,6 +41,8 @@ let pubkey = localStorage.getItem('pub_key') || (() => {
   return pubkey;
 })();
 
+// arbitrary difficulty, still experimenting.
+// measured empirically, takes N sec on average to mine a text note event.
 const difficulty = 16;
 
 const subList = [];
@@ -763,14 +766,11 @@ function hideNewMessage(hide) {
 
 async function upvote(eventId, relay) {
   const privatekey = localStorage.getItem('private_key');
-  const newReaction = powEvent({
+  const newReaction = await powEvent({
     kind: 7,
     pubkey, // TODO: lib could check that this is the pubkey of the key to sign with
     content: '+',
-    tags: [
-      ['nonce', '0', `${difficulty}`],
-      ['e', eventId, relay, 'reply'],
-    ],
+    tags: [['e', eventId, relay, 'reply']],
     created_at: Math.floor(Date.now() * 0.001),
   }, difficulty);
   const sig = await signEvent(newReaction, privatekey).catch(console.error);
@@ -803,11 +803,11 @@ writeForm.addEventListener('submit', async (e) => {
   }
   const replyTo = localStorage.getItem('reply_to');
   const tags = replyTo ? [['e', replyTo, eventRelayMap[replyTo][0]]] : [];
-  const newEvent = powEvent({
+  const newEvent = await powEvent({
     kind: 1,
     content,
     pubkey,
-    tags: [['nonce', '0', `${difficulty}`], ...tags],
+    tags,
     created_at: Math.floor(Date.now() * 0.001),
   }, difficulty);
   const sig = await signEvent(newEvent, privatekey).catch(onSendError);
@@ -945,11 +945,11 @@ profileForm.addEventListener('submit', async (e) => {
   const form = new FormData(profileForm);
   const privatekey = localStorage.getItem('private_key');
 
-  const newProfile = powEvent({
+  const newProfile = await powEvent({
     kind: 0,
     pubkey,
     content: JSON.stringify(Object.fromEntries(form)),
-    tags: [['nonce', '0', `${difficulty}`]],
+    tags: [],
     created_at: Math.floor(Date.now() * 0.001),
   }, difficulty);
   const sig = await signEvent(newProfile, privatekey).catch(console.error);
@@ -967,29 +967,46 @@ profileForm.addEventListener('submit', async (e) => {
   }
 });
 
+/**
+ * check that the event has the id has the desired number of leading zero bits
+ * @param {EventObj} evt to validate
+ * @returns boolean
+ */
 function validatePow(evt) {
   const tag = evt.tags.find(tag => tag[0] === 'nonce');
   if (!tag) {
     return false;
   }
-  const [, , difficulty2] = tag;
-  if (difficulty2 < 16) {
-    return false;
-  }
-  return evt.id.substring(0, difficulty2 / 4) === '00'.repeat(difficulty2 / 8);
-}
+  const [, , difficultyCommitment] = tag;
+  return zeroLeadingBitsCount(evt.id) === difficultyCommitment;
+}
+
+/**
+ * run proof of work in a worker until at least the specified difficulty.
+ * if succcessful, the returned event contains the 'nonce' tag
+ * and the updated created_at timestamp.
+ *
+ * powEvent returns a rejected promise if the funtion runs for longer than timeout.
+ * a zero timeout makes mineEvent run without a time limit.
+ */
+function powEvent(evt, difficulty, timeout) {
+  return new Promise((resolve, reject) => {
+    const worker = new Worker('./worker.js');
+
+    worker.onmessage = (msg) => {
+      worker.terminate();
+      if (msg.data.error) {
+        reject(msg.data.error);
+      } else {
+        resolve(msg.data.event);
+      }
+    };
 
-function powEvent(newEvent, difficulty) {
-  const chars = difficulty / 8;
-  let n = Number(newEvent.tags[0][1]) + 1;
-  // const until = Date.now() + 15000;
-  console.time('pow');
-  while (true/*Date.now() < until*/) {
-    newEvent.tags[0][1] = `${n++}`;
-    const id = getEventHash(newEvent, privatekey);
-    if (id.substring(0, chars * 2) === '00'.repeat(chars)) {
-      console.timeEnd('pow');
-      return newEvent;
-    }
-  }
-}
\ No newline at end of file
+    worker.onerror = (err) => {
+      worker.terminate();
+      reject(err);
+    };
+
+    worker.postMessage({event: evt, difficulty, timeout});
+  });
+}
diff --git a/src/worker.js b/src/worker.js
new file mode 100644
index 0000000..ca1e9f0
--- /dev/null
+++ b/src/worker.js
@@ -0,0 +1,47 @@
+import {getEventHash} from 'nostr-tools';
+import {zeroLeadingBitsCount} from './cryptoutils.js';
+
+function mine(event, difficulty, timeout) {
+  const max = 256; // arbitrary
+  if (!Number.isInteger(difficulty) || difficulty < 0 || difficulty > max) {
+    throw new Error(`difficulty must be an integer between 0 and ${max}`);
+  }
+  // continue with mining
+  let n = BigInt(0);
+  event.tags.unshift(['nonce', n.toString(), `${difficulty}`]);
+
+  const start = Math.floor(Date.now() * 0.001);
+  console.time('pow');
+  while (true) {
+    const now = Math.floor(Date.now() * 0.001);
+    // if (now > start + 15) {
+    //   console.timeEnd('pow');
+    //   return false;
+    // }
+    if (now !== event.created_at) {
+      event.created_at = now;
+      // n = BigInt(0); // could reset nonce as we have a new timestamp
+    }
+    event.tags[0][1] = (++n).toString();
+    const id = getEventHash(event);
+    if (zeroLeadingBitsCount(id) === difficulty) {
+      console.log(event.tags[0][1], id);
+      console.timeEnd('pow');
+      return event;
+    }
+  }
+}
+
+addEventListener('message', async (msg) => {
+  const {
+    difficulty,
+    event,
+    timeout,
+  } = msg.data;
+  try {
+    const minedEvent = mine(event, difficulty, timeout);
+    postMessage({event: minedEvent});
+  } catch (err) {
+    postMessage({error: err});
+  }
+});
\ No newline at end of file