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Signing requests to Amazon Web Services using HMAC in Python involves utilizing libraries like pycrypto for generating custom policies and signatures.

# Python HMAC Usage

In the context provided, the user is inquiring about how to sign requests to the Amazon Web Services within a Python application running on the Google App Engine. The user mentions that Amazon has stated they will only accept signed requests since August 15, 2009. While the user is not well-versed in Python libraries for SHA256, they are looking for guidance on how to implement this signing process. The user mentions that the GAE Python Signature Service Web App uses a native Python library and supports pycrypto. Additionally, the user has implemented signing requests directly but is interested in whether there are existing solutions utilizing urlfetch/httplib before resorting to custom implementations.

## Approach Using Python Libraries

To sign requests to the Amazon Web Services within a Python application, particularly on the Google App Engine, several Python libraries and methods can be utilized. The user mentions the use of the pycrypto library for custom policy generation, which involves importing necessary modules such as SHA, RSA, and PKCS1_v1_5 from the Crypto package. The user provides a code snippet demonstrating how to generate a custom policy using pycrypto for signing requests with AWS.

## Example Code Snippet

import json
import time
from Crypto.Hash import SHA
from Crypto.PublicKey import RSA
from Crypto.Signature import PKCS1_v1_5
from base64 import b64encode

url = “http://*”
expires = int(time.time() + 3600)



key_pair_id = ‘APK…..’

policy = {}
policy[‘Statement’] = [{}]
policy[‘Statement’][0][‘Resource’] = url
policy[‘Statement’][0][‘Condition’] = {}
policy[‘Statement’][0][‘Condition’][‘DateLessThan’] = {}
policy[‘Statement’][0][‘Condition’][‘DateLessThan’][‘AWS:EpochTime’] = expires

policy = json.dumps(policy)

private_key = RSA.importKey(pem)
policy_hash = SHA.new(policy)
signer = PKCS1_v1_5.new(private_key)
signature = b64encode(signer.sign(policy_hash))

print ‘?Policy=%s&Signature=%s&Key-Pair-Id=%s’ % (b64encode(policy), signature, key_pair_id)

## Additional Resources

In addition to the provided code snippet, the user mentions references to external sources such as blogs and websites that offer further insights and examples on how to sign AWS requests using Python. These resources can provide additional guidance and examples for implementing HMAC signing in Python applications interacting with Amazon Web Services.

Overall, the process of signing requests to Amazon Web Services using HMAC in Python involves utilizing libraries such as pycrypto for generating custom policies and signatures, as demonstrated in the code snippet provided. By following the outlined steps and leveraging available resources, developers can effectively sign requests to AWS within their Python applications.