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/cards.css b/src/cards.css
index 2a407b4..c3a7a58 100644
--- a/src/cards.css
+++ b/src/cards.css
@@ -158,6 +158,4 @@
   max-width: 48rem;
   padding: 1.5rem 1.8rem;
   width: 100%;
-  /* TODO: revert when things calm down or we find an alternative */
-  display: none;
 }
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 e223e7c..8fbecb3 100644
--- a/src/main.js
+++ b/src/main.js
@@ -1,4 +1,5 @@
 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 +41,10 @@ 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 = [];
 const unSubAll = () => {
   subList.forEach(sub => sub.unsub());
@@ -161,9 +166,9 @@ function renderProfile(evt, relay) {
   if (content) {
     profileAbout.textContent = content.about;
     profileName.textContent = content.name;
-    const noxyImg = getNoxyUrl('data', content.picture, evt.id, relay);
+    const noxyImg = validatePow(evt) && getNoxyUrl('data', content.picture, evt.id, relay);
     if (noxyImg) {
-      profileImage.setAttribute('src', getNoxyUrl('data', noxyImg, evt.id, relay));
+      profileImage.setAttribute('src', noxyImg);
       profileImage.hidden = false;
     }
   }
@@ -450,7 +455,7 @@ function createTextNote(evt, relay) {
     ]),
     elem('div', {/* data: isLongContent ? {append: evt.content.slice(280)} : null*/}, [
       ...content,
-      firstLink ? linkPreview(firstLink, evt.id, relay) : ''
+      (firstLink && validatePow(evt)) ? linkPreview(firstLink, evt.id, relay) : '',
     ]),
     elem('button', {
       className: 'btn-inline', name: 'star', type: 'button',
@@ -626,7 +631,7 @@ function setMetadata(evt, relay, content) {
     }
   }
   // update profile images
-  if (user.picture) {
+  if (user.picture && validatePow(evt)) {
     document.body
       .querySelectorAll(`canvas[data-pubkey="${evt.pubkey}"]`)
       .forEach(canvas => (canvas.parentNode.replaceChild(elem('img', {src: user.picture}), canvas)));
@@ -686,7 +691,7 @@ function getMetadata(evt, relay) {
   const name = user?.metadata[relay]?.name;
   const userName = name || evt.pubkey.slice(0, 8);
   const userAbout = user?.metadata[relay]?.about || '';
-  const img = userImg ? elem('img', {
+  const img = (userImg && validatePow(evt)) ? elem('img', {
     alt: `${userName} ${host}`,
     loading: 'lazy',
     src: userImg,
@@ -761,13 +766,13 @@ function hideNewMessage(hide) {
 
 async function upvote(eventId, relay) {
   const privatekey = localStorage.getItem('private_key');
-  const newReaction = {
+  const newReaction = await powEvent({
     kind: 7,
     pubkey, // TODO: lib could check that this is the pubkey of the key to sign with
     content: '+',
     tags: [['e', eventId, relay, 'reply']],
     created_at: Math.floor(Date.now() * 0.001),
-  };
+  }, difficulty);
   const sig = await signEvent(newReaction, privatekey).catch(console.error);
   if (sig) {
     const ev = await pool.publish({...newReaction, sig}, (status, url) => {
@@ -798,13 +803,13 @@ writeForm.addEventListener('submit', async (e) => {
   }
   const replyTo = localStorage.getItem('reply_to');
   const tags = replyTo ? [['e', replyTo, eventRelayMap[replyTo][0]]] : [];
-  const newEvent = {
+  const newEvent = await powEvent({
     kind: 1,
-    pubkey,
     content,
+    pubkey,
     tags,
     created_at: Math.floor(Date.now() * 0.001),
-  };
+  }, difficulty);
   const sig = await signEvent(newEvent, privatekey).catch(onSendError);
   if (sig) {
     const ev = await pool.publish({...newEvent, sig}, (status, url) => {
@@ -940,13 +945,13 @@ profileForm.addEventListener('submit', async (e) => {
   const form = new FormData(profileForm);
   const privatekey = localStorage.getItem('private_key');
 
-  const newProfile = {
+  const newProfile = await powEvent({
     kind: 0,
     pubkey,
     content: JSON.stringify(Object.fromEntries(form)),
-    created_at: Math.floor(Date.now() * 0.001),
     tags: [],
-  };
+    created_at: Math.floor(Date.now() * 0.001),
+  }, difficulty);
   const sig = await signEvent(newProfile, privatekey).catch(console.error);
   if (sig) {
     const ev = await pool.publish({...newProfile, sig}, (status, url) => {
@@ -961,3 +966,47 @@ profileForm.addEventListener('submit', async (e) => {
     }).catch(console.error);
   }
 });
+
+/**
+ * validate the difficulty target matches the 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 [, , 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);
+      }
+    };
+
+    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..c72967c
--- /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 = 5) {
+  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