getUrl.js

var AWS = require('aws-sdk');
var s3 = new AWS.S3();

var getUrl = function(bucket, path, callback) {
    var params = {Bucket: bucket, Key: path};
    s3.getSignedUrl('getObject', params, callback);
}

instructions.md

to make this work, first:

npm install aws-sdk

then export your s3 key to environment variable

export AWS_ACCESS_KEY_ID=YOUR_ACCESS_KEY_HERE
export AWS_SECRET_ACCESS_KEY=YOUR_SECRET_KEY_HERE

and then just call the above function like this:

your_keystone_app.js

getUrl("help-center", "files/1x1_wallpaper_bundle.zip", function (err, url) {
    // make a response here and set: STATUS=302, LOCATION=url
});

a tutorial to use pre-signed url to get s3 object