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app.js

const uuidv4 = require("uuid/v4");
const fs = require('fs');
const crypto = require('crypto');

var key = uuidv4();
var expire = new Date;
expire.setHours(expire.getHours() + 1);
var private_key = fs.readFileSync("service_account.pem", "utf8");
var policy = '{"expiration": "' + expire.toISOString() + '",' +
 '"conditions": ['  +
  '["starts-with", "$key", "' + key + '" ],' +
  '["eq", "$Content-Type", "application/zip" ],' +
  '{"bucket": "renderer_storage"},'  +
  '["content-length-range", 5242880, 419430400]'  +
  ']' +
'}';

var policy_64 = Buffer(policy, "utf8").toString('base64');
var signature_64 = crypto.createSign('sha256').update(policy_64).sign(private_key, 'base64');

var result = {"key" : key, "policy" : policy_64, "signature" : signature_64};
res.status(200).json(result);