https://andreacarriero.com/2018/05/21/encrypted-and-password-protected-pages-on-jekyll

protected.html

---
layout: default
---

{{ page.title | escape}}

<div id="content">
  Insert password for decryption
  <input id="password" type="password">
  <button onclick="decrypt()">Decrypt</button>
  <p id="errmsg" style="color: red;"></p>
</div>


<script src="https://cdnjs.cloudflare.com/ajax/libs/crypto-js/3.1.9-1/crypto-js.min.js" integrity="sha256-u6BamZiW5tCemje2nrteKC2KoLIKX9lKPSpvCkOhamw=" crossorigin="anonymous"></script>
<script>
  function decrypt() {
    var protectedContent = "{{ page.protected_content }}";
    var password = document.getElementById('password').value;

    var payload = protectedContent.split("|");
    var iv = payload[0];
    var hmac = payload[1];
    var cipherText = payload[2];

    var passphraseDgst = CryptoJS.SHA256(password).toString();
    var decryptedhmac = CryptoJS.HmacSHA256(cipherText, CryptoJS.enc.Hex.parse(passphraseDgst)).toString().trim();
    if(CryptoJS.enc.Base64.parse(hmac).toString() === decryptedhmac){
      var decrypted = CryptoJS.AES.decrypt(
        {ciphertext:CryptoJS.enc.Base64.parse(cipherText)},
        CryptoJS.enc.Hex.parse(passphraseDgst),
        {iv:CryptoJS.enc.Base64.parse(iv)}
      );
      var content = CryptoJS.enc.Utf8.stringify(decrypted);
      document.getElementById('content').innerHTML = content;
    } else {
      document.getElementById('errmsg').innerHTML = "Wrong password";
    }
  }

  var passwordInput = document.getElementById('password');
  passwordInput.addEventListener("keyup", function(event) {
    event.preventDefault();
    if (event.keyCode === 13) {
      decrypt();
    }
  })
</script>

This is only working in Jekyll, not GitHub Pages.