s3.js

var crypto = require("crypto");
var moment = require("moment")

var s3 = {
    generateS3Policy: function (fileName) {
        var s3Policy = {
            'conditions': [
                {'bucket': CONF.s3.bucket},
                ['starts-with', '$key', 'uploads/' + fileName], 
                {'acl': 'public-read'}, 
                {'success_action_redirect': 'http://' + CONF.host + '/upload_done'},
                ["content-length-range", 0, CONF.s3.policy.maxSize],
                ['starts-with', '$Content-Type', 'image']
            ], 
            'expiration': moment().add('minutes', CONF.s3.uploadWindow).format("YYYY-MM-DDTHH:MM:ss\\Z")
        };
        return s3Policy;
    },
    generateS3Credentials: function (s3Policy) {
        var encodedPolicy = new Buffer(JSON.stringify(s3Policy)).toString('base64'),
        credentials = {
            policy: encodedPolicy,
            signature: crypto.createHmac("sha1", CONF.s3.secretKey).update(encodedPolicy).digest("base64"),
            key: CONF.s3.key,
            redirect: 'http://' + CONF.host + '/upload_done',
            plain_policy: s3Policy
        };
        return credentials;
    }
};

module.exports = s3;

upload.js

var S3 = require('./s3');

var policy = S3.generateS3Policy("user_" + req.user.id);
var credentials = S3.generateS3Credentials(policy);