Python script that generates signed cookies to control access to CloudFront content

generate_cloudfront_signed_cookies.py

#!/usr/bin/env python

"""
Copyright (C) 2017 Matt Sullivan

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.


Creates and signs cookies that provide access to CloudFront objects
Based on: https://aws.amazon.com/premiumsupport/knowledge-center/cf-signed-cookies-s3-origin/

requirements:
>=Python3.5
cryptography>=1.7.1
"""

import time
import json
import base64

from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import padding


def _replace_unsupported_chars(some_str):
    """Replace unsupported chars: '+=/' with '-_~'"""
    return some_str.replace("+", "-") \
        .replace("=", "_") \
        .replace("/", "~")


def _in_an_hour():
    """Returns a UTC POSIX timestamp for one hour in the future"""
    return int(time.time()) + (60*60)


def rsa_signer(message, key):
    """
    Based on https://boto3.readthedocs.io/en/latest/reference/services/cloudfront.html#examples
    """
    private_key = serialization.load_pem_private_key(
        key,
        password=None,
        backend=default_backend()
    )
    signer = private_key.signer(padding.PKCS1v15(), hashes.SHA1())
    signer.update(message)
    return signer.finalize()


def generate_policy_cookie(url):
    """Returns a tuple: (policy json, policy base64)"""

    policy_dict = {
        "Statement": [
            {
                "Resource": url,
                "Condition": {
                    "DateLessThan": {
                        "AWS:EpochTime": _in_an_hour()
                    }
                }
            }
        ]
    }

    # Using separators=(',', ':') removes seperator whitespace
    policy_json = json.dumps(policy_dict, separators=(",", ":"))

    policy_64 = str(base64.b64encode(policy_json.encode("utf-8")), "utf-8")
    policy_64 = _replace_unsupported_chars(policy_64)
    return policy_json, policy_64


def generate_signature(policy, key):
    """Creates a signature for the policy from the key, returning a string"""
    sig_bytes = rsa_signer(policy.encode("utf-8"), key)
    sig_64 = _replace_unsupported_chars(str(base64.b64encode(sig_bytes), "utf-8"))
    return sig_64


def generate_cookies(policy, signature, cloudfront_id):
    """Returns a dictionary for cookie values in the form 'COOKIE NAME': 'COOKIE VALUE'"""
    return {
        "CloudFront-Policy": policy,
        "CloudFront-Signature": signature,
        "CloudFront-Key-Pair-Id": cloudfront_id
    }


def generate_curl_cmd(url, cookies):
    """Generates a cURL command (use for testing)"""
    curl_cmd = "curl -v"
    for k, v in cookies.items():
        curl_cmd += " -H 'Cookie: {}={}'".format(k, v)
    curl_cmd += " {}".format(url)
    return curl_cmd


def generate_signed_cookies(url, cloudfront_id, key):
    policy_json, policy_64 = generate_policy_cookie(url)
    signature = generate_signature(policy_json, key)
    return generate_cookies(policy_64, signature, cloudfront_id)


if __name__ == "__main__":
    import argparse

    # Parse the command line args
    parser = argparse.ArgumentParser(description="Generates signed cookies for Amazon CloudFront")
    parser.add_argument("url", type=str, help="URL to sign")
    parser.add_argument("key", type=str, help="private key to use to sign the cookie")
    parser.add_argument("id", type=str, help="CloudFront id")
    args = parser.parse_args()

    with open(args.key, "rb") as f:
        cookies = generate_signed_cookies(args.url, args.id, f.read())

    print(generate_curl_cmd(args.url, cookies))