function validateDigistoreSha() {
  const THANKYOU_PAGE_KEY = "XXXXXXXXXXXXXX";

  const data = new URLSearchParams(window.location.search);

  function hexString(buffer) {
    const byteArray = new Uint8Array(buffer);

    const hexCodes = [...byteArray].map(value => {
      const hexCode = value.toString(16);
      const paddedHexCode = hexCode.padStart(2, "0");
      return paddedHexCode;
    });

    return hexCodes.join("");
  }

  function digestMessage(message) {
    const encoder = new TextEncoder();
    const data = encoder.encode(message);
    return window.crypto.subtle.digest("SHA-512", data);
  }

  function digistore_signature(thankyouPageKey, input) {
    input.delete("sha_sign");

    input.sort();

    let shaString = "";

    input.forEach((value, key) => {
      const empty =
        typeof value === "undefined" || value === "" || value === false;

      if (!empty) {
        shaString += `${key}=${value}${thankyouPageKey}`;
      }
    });

    const shaSign = digestMessage(shaString).then(digestValue =>
      hexString(digestValue).toUpperCase()
    );

    return shaSign;
  }

  async function shaSignValid() {
    const receivedSignature = data.get("sha_sign");
    const expectedSignature = await digistore_signature(
      THANKYOU_PAGE_KEY,
      data
    );
    return receivedSignature == expectedSignature;
  }

  return shaSignValid();
}