Short version: How do I make signed URLs "on-demand" to mimic Nginx's X-Accel-Redirect behavior (i.e. protecting downloads) with Amazon CloudFront/S3 using Python.
I've got a Django server up and running with an Nginx front-end. I've been getting hammered with requests to it and recently had to install it as a Tornado WSGI application to prevent it from crashing in FastCGI mode.
Now I'm having an issue with my server getting bogged down (i.e. most of its bandwidth is being used up) due to too many requests for media being made to it, I've been looking into CDNs and I believe Amazon CloudFront/S3 would be the proper solution for me.
I've been using Nginx's X-Accel-Redirect header to protect the files from unauthorized downloading, but I don't have that ability with CloudFront/S3--however they do offer signed URLs. I'm no Python expert by far and definitely don't know how to create a Signed URL properly, so I was hoping someone would have a link for how to make these URLs "on-demand" or would be willing to explain how to here, it would be greatly appreciated.
Also, is this the proper solution, even? I'm not too familiar with CDNs, is there a CDN that would be better suited for this?
Sign in to the AWS Management Console and open the Amazon S3 console at https://console.aws.amazon.com/s3/ . In the Buckets list, choose the name of the bucket that contains the object that you want a presigned URL for. In the Objects list, select the object that you want to create a presigned URL for.
Both S3 and CloudFront have URL signing features that work differently. However, only S3 refers to them as Pre-signed URLs; CloudFront refers to them as Signed URLs and Signed Cookies. Note the service names in the URLs, in the documentation below.
Amazon CloudFront Signed URLs work differently than Amazon S3 signed URLs. CloudFront uses RSA signatures based on a separate CloudFront keypair which you have to set up in your Amazon Account Credentials page. Here's some code to actually generate a time-limited URL in Python using the M2Crypto library:
Create a keypair for CloudFront
I think the only way to do this is through Amazon's web site. Go into your AWS "Account" page and click on the "Security Credentials" link. Click on the "Key Pairs" tab then click "Create a New Key Pair". This will generate a new key pair for you and automatically download a private key file (pk-xxxxxxxxx.pem). Keep the key file safe and private. Also note down the "Key Pair ID" from amazon as we will need it in the next step.
Generate some URLs in Python
As of boto version 2.0 there does not seem to be any support for generating signed CloudFront URLs. Python does not include RSA encryption routines in the standard library so we will have to use an additional library. I've used M2Crypto in this example.
For a non-streaming distribution, you must use the full cloudfront URL as the resource, however for streaming we only use the object name of the video file. See the code below for a full example of generating a URL which only lasts for 5 minutes.
This code is based loosely on the PHP example code provided by Amazon in the CloudFront documentation.
from M2Crypto import EVP import base64 import time def aws_url_base64_encode(msg): msg_base64 = base64.b64encode(msg) msg_base64 = msg_base64.replace('+', '-') msg_base64 = msg_base64.replace('=', '_') msg_base64 = msg_base64.replace('/', '~') return msg_base64 def sign_string(message, priv_key_string): key = EVP.load_key_string(priv_key_string) key.reset_context(md='sha1') key.sign_init() key.sign_update(message) signature = key.sign_final() return signature def create_url(url, encoded_signature, key_pair_id, expires): signed_url = "%(url)s?Expires=%(expires)s&Signature=%(encoded_signature)s&Key-Pair-Id=%(key_pair_id)s" % { 'url':url, 'expires':expires, 'encoded_signature':encoded_signature, 'key_pair_id':key_pair_id, } return signed_url def get_canned_policy_url(url, priv_key_string, key_pair_id, expires): #we manually construct this policy string to ensure formatting matches signature canned_policy = '{"Statement":[{"Resource":"%(url)s","Condition":{"DateLessThan":{"AWS:EpochTime":%(expires)s}}}]}' % {'url':url, 'expires':expires} #sign the non-encoded policy signature = sign_string(canned_policy, priv_key_string) #now base64 encode the signature (URL safe as well) encoded_signature = aws_url_base64_encode(signature) #combine these into a full url signed_url = create_url(url, encoded_signature, key_pair_id, expires); return signed_url def encode_query_param(resource): enc = resource enc = enc.replace('?', '%3F') enc = enc.replace('=', '%3D') enc = enc.replace('&', '%26') return enc #Set parameters for URL key_pair_id = "APKAIAZVIO4BQ" #from the AWS accounts CloudFront tab priv_key_file = "cloudfront-pk.pem" #your private keypair file # Use the FULL URL for non-streaming: resource = "http://34254534.cloudfront.net/video.mp4" #resource = 'video.mp4' #your resource (just object name for streaming videos) expires = int(time.time()) + 300 #5 min #Create the signed URL priv_key_string = open(priv_key_file).read() signed_url = get_canned_policy_url(resource, priv_key_string, key_pair_id, expires) print(signed_url) #Flash player doesn't like query params so encode them if you're using a streaming distribution #enc_url = encode_query_param(signed_url) #print(enc_url)
Make sure that you set up your distribution with a TrustedSigners parameter set to the account holding your keypair (or "Self" if it's your own account)
See Getting started with secure AWS CloudFront streaming with Python for a fully worked example on setting this up for streaming with Python
This feature is now already supported in Botocore, which is the underlying library of Boto3, the latest official AWS SDK for Python. (The following sample requires the installation of the rsa package, but you can use other RSA package too, just define your own "normalized RSA signer".)
The usage looks like this:
from botocore.signers import CloudFrontSigner # First you create a cloudfront signer based on a normalized RSA signer:: import rsa def rsa_signer(message): private_key = open('private_key.pem', 'r').read() return rsa.sign( message, rsa.PrivateKey.load_pkcs1(private_key.encode('utf8')), 'SHA-1') # CloudFront requires SHA-1 hash cf_signer = CloudFrontSigner(key_id, rsa_signer) # To sign with a canned policy:: signed_url = cf_signer.generate_presigned_url( url, date_less_than=datetime(2015, 12, 1)) # To sign with a custom policy:: signed_url = cf_signer.generate_presigned_url(url, policy=my_policy)
Disclaimer: I am the author of that PR.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With