policy.js

var crypto = require("crypto");

var SECRET_KEY = 'MY_AWS_SECRET_KEY';
var POLICY_JSON = {
  "expiration": "2030-12-01T12:00:00.000Z",
  "conditions": [
    {"bucket": "my-bucket-name"},
    ["starts-with", "$key", ""],
    {"acl": "public-read"},
    ["starts-with", "$Content-Type", ""],
    ["content-length-range", 0, 524288000]
  ]
};

// No need to edit anything below
var encodedPolicy = new Buffer(JSON.stringify(POLICY_JSON)).toString('base64');
console.log("Encoded policy:");
console.log("-------------------------");
console.log(encodedPolicy, "\n");

var signature = crypto.createHmac("sha1", SECRET_KEY).update(encodedPolicy).digest("base64");
console.log("Signature:");
console.log("-------------------------");
console.log(signature, "\n");

Simple Node.js AWS Policy + Signature generator. Save this file as policy.js, modify the two variables according to your needs and settings and finally run this file with Node: $ node policy.js. All done!