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chromium/third_party/blink/web_tests/external/wpt/webmessaging/resources/post-cryptokey-to-opener.html 

<!DOCTYPE html>
<script>
crypto.subtle.generateKey({name: 'AES-CBC', length: 128}, false, ['encrypt', 'decrypt'])
.then(key => opener.postMessage({key}, '*'))
.catch(error => opener.postMessage({error: error.toString()}, '*'));
</script>