How to sign amazon web service requests from the python app engine?

Question

I use Amazon web service api from within my Google app engine application. Amazon have said that they will only accept signed requests from Aug 15, 2009. While they have given simple instructions for signing, I am not so knowledgeable of Python libraries for SHA256. The app engine documentation says it supports pycrypto but I was just wondering (read being lazy) if anyone has already done this. Any code snippets you could share? Any issues I might be missing here?

Answer 1

Here is an example of a REST request based on lower level (then boto) libraries. Solution was taken from http://cloudcarpenters.com/blog/amazon_products_api_request_signing.

All you need is valid entries for AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY

def amazon_test_url():
    import base64, hashlib, hmac, time
    from urllib import urlencode, quote_plus

    AWS_ACCESS_KEY_ID = 'YOUR_KEY'
    AWS_SECRET_ACCESS_KEY = 'YOUR_SECRET_KEY'  
    TEST_ISBN = '9780735619678' #http://stackoverflow.com/questions/1711/what-is-the-single-most-influential-book-every-programmer-should-read

    base_url = "http://ecs.amazonaws.com/onca/xml"
    url_params = dict(
        Service='AWSECommerceService', 
        Operation='ItemLookup', 
        IdType='ISBN', 
        ItemId=TEST_ISBN,
        SearchIndex='Books',
        AWSAccessKeyId=AWS_ACCESS_KEY_ID,  
        ResponseGroup='Images,ItemAttributes,EditorialReview,SalesRank')

    #Can add Version='2009-01-06'. What is it BTW? API version?


    # Add a ISO 8601 compliant timestamp (in GMT)
    url_params['Timestamp'] = time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())

    # Sort the URL parameters by key
    keys = url_params.keys()
    keys.sort()
    # Get the values in the same order of the sorted keys
    values = map(url_params.get, keys)

    # Reconstruct the URL parameters and encode them
    url_string = urlencode(zip(keys,values))

    #Construct the string to sign
    string_to_sign = "GET\necs.amazonaws.com\n/onca/xml\n%s" % url_string

    # Sign the request
    signature = hmac.new(
        key=AWS_SECRET_ACCESS_KEY,
        msg=string_to_sign,
        digestmod=hashlib.sha256).digest()

    # Base64 encode the signature
    signature = base64.encodestring(signature).strip()

    # Make the signature URL safe
    urlencoded_signature = quote_plus(signature)
    url_string += "&Signature=%s" % urlencoded_signature

    print "%s?%s\n\n%s\n\n%s" % (base_url, url_string, urlencoded_signature, signature)

Answer 2

Pycrypto will work fine - it's supported on App Engine, though the public ciphers are implemented in Python rather than C. You also ought to be able to use one of the existing AWS libraries, now that urlfetch/httplib are supported on App Engine.

I have an app that uploads images to S3, and I've implemented the request signing myself, but mostly because I wrote it before urlfetch/httplib were available. It works just fine, however.