Better Generation of S3 Policy with Tests (From: https://github.com/javiersuweijie/s3policy)

S3Policy.js

// Better https://github.com/javiersuweijie/s3policy

var crypto = require('crypto');
var path = require('path');

var _ = require('lodash');
var uuid = require('node-uuid');
var mimetype = require('mimetype');
var AWS = require('aws-sdk');
var debug = require('debug')('s3policy');

function getAwsConfig() {
  if (!AWS.config.credentials) {
    console.error('AWS Credentials are not configured correctly. Missing IAM Role or ENV variables.');
    var e = new Error('Internal Error: Missing authentication information to generate policy.');
    e.status = 500;
    throw e;
  }
  return {
    accessKey: AWS.config.credentials.accessKeyId,
    secretKey: AWS.config.credentials.secretAccessKey
  };
}

// Makes it easy to follow http://docs.aws.amazon.com/AmazonS3/latest/dev/request-rate-perf-considerations.html
function buildKey(context, filename) {
  var environment = process.env.NODE_ENV || 'development';
  var contextPath = _.isEmpty(context) ? '' : context + '/';

  return path.join(environment + '/' + contextPath + uuid.v1() + '/' + filename);
}

var DEFAULT_WRITE_OPTIONS = {
  expireInSec: 120,
  maxSizeInMb: 10,
  context: undefined,
  acl: 'public-read'
};

module.exports = {
  getWritePolicy: function(filename, bucket, redirectUrl, options, cb) {
    try {
      if (_.isFunction(options)) {
        cb = options;
        options = DEFAULT_WRITE_OPTIONS;
      } else {
        _.defaults(options, DEFAULT_WRITE_OPTIONS);
      }
      var awsConfig = getAwsConfig();

      debug('Generating write policy:', options);

      var key = buildKey(options.context, filename);
      var duration = options.expireInSec;
      var filesize = options.maxSizeInMb;
      var acl = options.acl;

      var dateObj = new Date();
      var dateExp = new Date(dateObj.getTime() + duration * 1000);
      var policy = {
        'expiration': dateExp.toISOString(),
        'conditions': [
          {
            bucket: bucket
          },
          {
            key: key,
          },
          {
            'success_action_redirect': redirectUrl
          },
          {
            acl: acl
          },
          ['content-length-range', 0, filesize * 1024 * 1024],
          ['starts-with', '$Content-Type', 'image']
        ]
      };
      var policyString = JSON.stringify(policy);
      var policyBase64 = new Buffer(policyString).toString('base64');
      var signature = crypto.createHmac('sha1', awsConfig.secretKey).update(policyBase64);

      var s3Credentials = {
        url: 'http://' + bucket + '.s3.amazonaws.com',
        redirectUrl: redirectUrl,
        key: key,
        policy: policyBase64,
        signature: signature.digest('base64'),
        accessKey: awsConfig.accessKey,
        acl: acl,
        mimetype: mimetype.lookup(filename)
      };

      debug('Generated write policy:', s3Credentials);

      cb(undefined, s3Credentials);
    } catch (e) {
      cb(e);
    }
  }
};

test.js

'use strict';

var path = require('path');
var fs = require('fs');
var request = require('supertest');
var FormData = require('form-data');
var s3policy = require('s3policy');

var expect = require('chai').expect;

var filename = 'avatar_1.jpg';
var pathToMedia = path.resolve(__dirname, './fixtures/media/', filename);

describe('Upload an image using UploadPolicy', function() {

  it('upload', function(done) {
    s3policy.getUploadPolicy(filename, function(err, policy) {
      expect(err).to.not.exist();
      expect(policy, 'policy required').to.exist();
      var form = new FormData();
      form.append('AWSAccessKeyId', policy.accessKey);
      form.append('key', policy.key);
      form.append('policy', policy.policy);
      form.append('signature', policy.signature);
      form.append('acl', policy.acl);
      form.append('success_action_redirect', policy.redirectUrl);
      form.append('Content-Type', policy.mimetype);
      form.append('file', fs.createReadStream(pathToMedia));
      form.submit(policy.url, function(err, res) {
        if (err) {
          return done(err);
        }

        expect(res.statusCode).to.equal(303);
        if (res.statusCode === 303 && res.headers.location) {
          expect(res.headers.location).to.include(helper.config.targetServer);
          request(res.headers.location)
            .get('').end(function(err, res) {
              if (err) {
                return done(err);
              }

              expect(res.body).to.be.an.instanceOf(Object);
              expect(res.body).to.have.property('bucket');
              done();
            });
          return;
        }

        // debugging information only! When failing!
        var data = '';
        res.on('data', function(chunk) {
          data += chunk;
        });
        res.on('end', function() {
          expect(data).to.not.be.empty();
          expect(false).to.be.true();
          done();
        });
        res.resume();
      });
    });
  });
});