Accesstoken

<?php

$private_key = "-----BEGIN PRIVATE KEY-----\n###-----END PRIVATE KEY-----\n"; // private_key of JSON file retrieved by creating Service Account
$client_email = "###"; // client_email of JSON file retrieved by creating Service Account
$scopes = ["https://www.googleapis.com/auth/drive.readonly"]; // Sample scope

$url = "https://www.googleapis.com/oauth2/v4/token";
$header = array("alg" => "RS256", "typ" => "JWT");
$now = floor(time());
$claim = array(
	"iss" => $client_email,
	"sub" => $client_email,
	"scope" => implode(" ", $scopes),
	"aud" => $url,
	"exp" => (string)($now + 3600),
	"iat" => (string)$now,
);
$signature = base64_encode(json_encode($header, JSON_UNESCAPED_SLASHES)) . "." . base64_encode(json_encode($claim, JSON_UNESCAPED_SLASHES));
$b = "";
openssl_sign($signature, $b, $private_key, "SHA256");
$jwt = $signature . "." . base64_encode($b);
$curl_handle = curl_init();
curl_setopt_array($curl_handle, [
    CURLOPT_URL => $url,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_POST => true,
    CURLOPT_POSTFIELDS => array(
        "assertion" => $jwt,
        "grant_type" => "urn:ietf:params:oauth:grant-type:jwt-bearer"
    ),
]);
$res = curl_exec($curl_handle);
curl_close($curl_handle);
$obj = json_decode($res);
$accessToken = $obj -> {'access_token'};
print($accessToken . "\n");

// This is the script for testing the access token. The file list of the service account's Drive is retrieved using Drive API.
$curl_test = curl_init();
curl_setopt($curl_test, CURLOPT_URL, 'https://www.googleapis.com/drive/v3/files?access_token=' . $accessToken);
curl_setopt($curl_test, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($curl_test);
print($response);

?>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jsencrypt/3.0.0-rc.1/jsencrypt.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/crypto-js/4.0.0/crypto-js.min.js"></script>

<script>
  async function sample() {
    const private_key = "###"; // private_key of JSON file retrieved by creating Service Account
    const client_email = "###"; // client_email of JSON file retrieved by creating Service Account
    const scopes = ["https://www.googleapis.com/auth/drive.readonly"]; // Scopes

    const url = "https://www.googleapis.com/oauth2/v4/token";
    const header = { alg: "RS256", typ: "JWT" };
    const now = Math.floor(Date.now() / 1000);
    const claim = {
      iss: client_email,
      scope: scopes.join(" "),
      aud: url,
      exp: (now + 3600).toString(),
      iat: now.toString(),
    };
    const signature =
      btoa(JSON.stringify(header)) + "." + btoa(JSON.stringify(claim));
    const sign = new JSEncrypt();
    sign.setPrivateKey(private_key);
    const jwt =
      signature + "." + sign.sign(signature, CryptoJS.SHA256, "sha256");
    const params = {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify({
        assertion: jwt,
        grant_type: "urn:ietf:params:oauth:grant-type:jwt-bearer",
      }),
    };
    const obj = await fetch(url, params)
      .then((res) => res.json())
      .catch((err) => console.log(err));
    console.log(obj);
  }

  sample();
</script>

const cryptor = require('crypto');
const request = require('request');

const privateKey = "###"; // private_key of JSON file retrieved by creating Service Account
const clientEmail = "###"; // client_email of JSON file retrieved by creating Service Account
const scopes = ["https://www.googleapis.com/auth/drive.readonly"]; // Sample scope

const url = "https://www.googleapis.com/oauth2/v4/token";
const header = {
  alg: "RS256",
  typ: "JWT",
};
const now = Math.floor(Date.now() / 1000);
const claim = {
  iss: clientEmail,
  scope: scopes.join(" "),
  aud: url,
  exp: (now + 3600).toString(),
  iat: now.toString(),
};

const signature = Buffer.from(JSON.stringify(header)).toString('base64') + "." + Buffer.from(JSON.stringify(claim)).toString('base64');

var sign = cryptor.createSign('RSA-SHA256');
sign.update(signature);
const jwt = signature + "." + sign.sign(privateKey, 'base64');

request({
  method: "post",
  url: url,
  body: JSON.stringify({
    assertion: jwt,
    grant_type: "urn:ietf:params:oauth:grant-type:jwt-bearer",
  }),
}, (err, res, body) => {
  if (err) {
    console.log(err);
    return;
  }
  console.log(body);
});

/**
 * ### Description
 * Get access token from service account.
 * ref: https://tanaikech.github.io/2018/12/07/retrieving-access-token-for-service-account-using-google-apps-script/
 *
 * @param {Object} object Object including private_key, client_email, impersonate_email.
 * @param {String} object.private_key
 * @param {String} object.client_email
 * @param {String} object.impersonate_email
 * @param {Array} object.scopes
 * @returns {String} Access token.
 */
function getAccessTokenFromServiceAccount_(object) {
  const { private_key, client_email, impersonate_email = "", scopes = [] } = object;
  const url = "https://www.googleapis.com/oauth2/v4/token";
  const header = { alg: "RS256", typ: "JWT" };
  const now = Math.floor(Date.now() / 1000);
  const claim = { iss: client_email, scope: scopes.join(" "), aud: url, exp: (now + 3600).toString(), iat: now.toString() };
  if (impersonate_email != "") {
    claim.sub = impersonate_email;
  }
  const signature = Utilities.base64Encode(JSON.stringify(header)) + "." + Utilities.base64Encode(JSON.stringify(claim));
  const jwt = signature + "." + Utilities.base64Encode(Utilities.computeRsaSha256Signature(signature, private_key));
  const params = { payload: { assertion: jwt, grant_type: "urn:ietf:params:oauth:grant-type:jwt-bearer" } };
  const res = UrlFetchApp.fetch(url, params);
  const { access_token } = JSON.parse(res.getContentText());
  return access_token;
}


// Please run this script.
function main() {
  const object = {
    private_key: "-----BEGIN PRIVATE KEY-----\n###-----END PRIVATE KEY-----\n",
    client_email: "###",
    // impersonate_email: "###", // If you want to use an impersonate email, please set this.
    scopes: ["###", "###",,,],
  };
  const accessToken = getAccessTokenFromServiceAccount_(object);
  console.log(accessToken);
}