Uploading Client-side Files Directly to S3

April 18, 2016

This post originally appeared on the site of the now defunct Vuse Media. It has been reproduced here, with modifications as necessary to remove/correct dead links.

The typical approach to uploading files from a client sees them transit the files through your server, either to reside there, or to transit on to somewhere else. This makes sense because the client is already talking to your server, and it’s easy to manage auth issues.

This model breaks down, however, not only when the files get large but also when working with a more modern workflow where files won’t be stored locally on the server but somewhere else. Today we’ll take a look at how Vuse leveraged Amazon S3 to allow for direct uploads to S3 from the users’s browser without compromising security.


The architecture for our customer control center, which allows users to upload video to our service, revolves around a simple static JavaScript client-side application which communicates with a backend API server written in Python.

Incoming media files are stored on an Amazon S3 bucket before they’re picked up for transcoding and delivery to our wider content distribution network.

Given how large customer files can be, it’s a huge win both for us and the customer if they can bypass having to transit the upload through our API servers and go directly to our incoming S3 bucket. It results in a quicker upload, higher reliability, and lower costs.

Of course you can’t just let anyone upload anything to your S3 bucket, and you don’t want to be giving away your credentials, so it’s important to do this securely. The way this is accomplished is having the client contact the API server to announce its intention of creating a new media file. The API server will respond with a policy that can be used to securely upload the file to the S3 server - this policy can be used to upload to a particular location within a specific bucket in a specific time window, without needing to share your secret key.

It’s important to understand that securing access to the API endpoint that will give the client this policy as well as the security of the data uploaded to S3 is not touched upon in this post.

Additionally, recognize that while we use a variant of this approach for uploading very large video files, that this code as is can stall during large uploads. We’ll have a follow up post discussing how to resolve these issues.

Below we’ll show you how to set things up on both the server and the client.

Generating the Policy on Your Server

Like with many things with AWS, the power of the API is only matched by how complicated it can be. Getting this to work correctly was a rather long effort, reading unclear and sometimes seemingly conflicting documentation and examples on the internet. Once the correct values and order were determined, it was all cake.

The basis of this is a series of HMAC signed messages built on top of each other to encode important information in a secure way. Given that, we found it worthwhile to ever so slightly shorten the repetitive calls to HMAC.new by creating a simple signing function:

import hmac
import hashlib

def sign(key, msg):
  return hmac.new(key, msg.encode('utf-8'), hashlib.sha256).digest()

You’ll also need to ensure that your server has access to the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY of a user with the ability to place files on your S3 bucket. We’ll also use two other things you’ll want to configure on your server: the AWS_DEFAULT_REGION and CONTENT_UPLOAD_BUCKET. We’ll use the Python library Yaep to load these from a .env file into our environment, and then into variables. You can use whatever you need to get them in there, Yaep isn’t required.

Also, we’ll go ahead and generate a UUID to act as our key on S3. To clarify - we don’t upload with the user’s given filename. This is to avoid collisions and wonky encoding issues that can crop up when using the user provided filename. For us this is part of a model instance describing the content, but for the purposes of this post, we’ll just generate one:

key = uuid.uuid4()

Finally, lets load in some key values from the environment that we’ll need to use to communicate with S3:

# Here we'll load several values we need in from environment
# variables.
AWS_ACCESS_KEY_ID = yaep.env(




With that out of the way, lets step through what’s necessary to generate the policy to return to the client.

First, we set up some date related values that we’ll use:

import datetime

current_date_YYYYMMDD = datetime.datetime.utcnow().strftime('%Y%m%d')
current_date = datetime.datetime.utcnow().strftime('%Y%m%dT%H%M%SZ')
expires = (
  datetime.datetime.utcnow() + datetime.timedelta(seconds=60)

These values are largely self explanatory. One note on the expires value - it’s worth noting that this is for when you start uploading the file. Once uploading, you can continue past the expiration time. The value here of sixty seconds can be changed as you see fit.

Next we’ll want to create a credential, which ties your policy to your AWS_ACCESS_KEY_ID, in a specific region, on a specific date:

# Note that as mentioned, "app" is the Flask app.

credential = '{}/{}/{}/s3/aws4_request'.format(

Next we’ll generate the conditions, which allow us to specify the circumstances under which this policy will apply:

conditions = [
  {'x-amz-date': current_date},
  {'x-amz-credential': credential},
  {'x-amz-algorithm': 'AWS4-HMAC-SHA256'},
  ['starts-with', '$key', str(key)]

As you can see, this ties it to the current date, using the previously constructed credential, as well as tying it to the bucket and file. You can read the S3 documentation for more options in configuring these conditions.

Now we set up the policy:

from base64 import b64encode

policy = b64encode(json.dumps({
  'conditions': conditions,
  'expiration': expires,
}).replace('\n', '').replace('\r', '').encode('utf-8'))

Here we dump our conditions and expiration as JSON, removing line breaks before we base64 encode it. This is our policy. We just need to do a few things in order to sign it to prove it was generated by us.

Now we’ll finally get to use our handy sign function in order to generate a signing key to sign our policy.

date_key = sign(

date_region_key = sign(

date_region_service_key = sign(date_region_key, 's3')
signing_key = sign(date_region_service_key, 'aws4_request')

As you can see, we basically are just continuously signing pieces of information. We then use this signing_key to create a signing key:

signature = hmac.new(signing_key, policy, hashlib.sha256).hexdigest()

Note here that we don’t use our handy sign function, as we want a hex digest of this signature, because why be consistent?

Finally we have what we need, and we can return it to the client:

from flask import jsonify

return jsonify({
  'uuid': key,
  'policy': {
    'Policy': policy,
    'key': str(key),
    'X-Amz-Date': current_date,
    'X-Amz-Signature': signature,
    'X-Amz-Credential': credential,
    'X-Amz-Algorithm': 'AWS4-HMAC-SHA256',
    'action': 'https://{}.s3.amazonaws.com/'.format(

This will get returned to the client as JSON. Next we’ll see how the client can use this to upload directly to S3.

Client-side Uploading to S3

On the client, you’ll want to make a call to your API, and create a function to handle the returned data. We’ll take a look at what the various parts of that function should do:

// data is the input to our function, which is
// the data we returned from our server.

var file = $('#upload input[type=file]'),
  ext = file.val().split('.').pop(),
  s3URL = data.policy.action,
  formData = data.policy;

First we set up some variables. We assume that you have an element with an id of “upload” which contains a file input elment. We then grab the extension of the file, get the url from the policy we got from the server. We then create a formData variable from our policy returned from the server, and this will end up being the basis for what we send to S3.

Next, we do some housekeeping on our formData variable:

delete formData.action;
formData.key += '.' + ext;

First we remove the action - we’ve saved that in our s3URL variable. We also append the file’s extension to the key that was set serverside (the UUID if you recall). This means that if you were uploading foo.avi, it would end up on the bucket as UUID.avi, where the UUID is whatever the server set it to.

The next bit is a little tricky:

formData = Object.keys(data.policy).reduce(function(p, c) {
  p.append(c, data.policy[c]);
  return p;
}, new FormData());

Essentially, we’re looking to change our formData dictionary into a FormData JavaScript object.

Next we add our file to our new FormData-ified formData variable:

formData.append('file', file[0].files[0]);

Next we make a pretty standard AJAX call to initiate our upload:

  type: 'POST',
  url: s3URL,
  data: formData,
  contentType: false,
  dataType: undefined,
  xhr: function() {
    var xhr = $.ajaxSettings.xhr();
    if (xhr.upload) xhr.upload.addEventListener('progress', progress);
    return xhr;
  }, // xhr
  success: success,
  error: error,
  complete: complete,
  processData: false,

This should all be fairly straight forward. The progress, success, error, and complete variables are all references to functions you should define to handle these as you see fit, as you normally would on any AJAX call.

Now you should have a setup that allows you to securely upload files to S3 from client-side JavaScript.

Concluding Thoughts

This technique is one that works especially well in a workflow where you need to push large data to a server other than the one hosting your API. There are some areas you’ll likely want to tweak this setup - pay special attention to the expiration and conditions on the server side.

Of course it’s important to do things with the files when they arrive on S3. One method might be to have the client notify the server that the upload completed. We took a different approach, and leveraged AWS Lambda, initiated by S3 itself on the arrival of new files, but we’ll save that discussion for another blog post.

Jim Kelly

Written by Jim Kelly, an American software professional living in Germany, specializing in Data Engineering and Full Stack Web Development.