Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<form action="demo_keygen.asp" method="get">
  Username: <input type="text" name="usr_name">
  Encryption: <keygen id="myKeygen" name="security">
  <input type="submit">
</form><br>

<button onclick="disableKey()">Disable Keygen field</button>
<button onclick="undisableKey()">Undisable Keygen field</button>

<script>
function disableKey() {
    document.getElementById("myKeygen").disabled = true;
}

function undisableKey() {
    document.getElementById("myKeygen").disabled = false;
}
</script>

</body>
</html>