use AWS APIs with Python to use Polly Services

Question

As I do not succeed in integrating boto3 to Google App Engine, I'm trying to use APIs directly using the docs : as Polly (text to speech API) uses Signature Version 4 process, I refered to :

http://docs.aws.amazon.com/general/latest/gr/sigv4-signed-request-examples.html

Here is the given script :

#http://docs.aws.amazon.com/general/latest/gr/sigv4-signed-request-examples.html

# AWS Version 4 signing example

# EC2 API (DescribeRegions)

# See: http://docs.aws.amazon.com/general/latest/gr/sigv4_signing.html
# This version makes a GET request and passes the signature
# in the Authorization header.
import sys, os, base64, datetime, hashlib, hmac
sys.path.append('../')
import requests # pip install requests

# Read AWS access key from env. variables or configuration file. Best practice is NOT
# to embed credentials in code.
access_key = os.environ.get('AWS_ACCESS_KEY_ID')
secret_key = os.environ.get('AWS_SECRET_ACCESS_KEY')
if access_key is None or secret_key is None:
    print 'No access key is available.'
    sys.exit()

# ************* REQUEST VALUES *************
method = 'GET'
##service = 'ec2'
##host = 'ec2.amazonaws.com'
##region = 'us-east-1'
##endpoint = 'https://ec2.amazonaws.com'
##request_parameters = 'Action=DescribeRegions&Version=2013-10-15'
##### START MODIFIED VALUES #######
service='polly'
host='polly.eu-west-1.amazonaws.com'
region='eu-west-1'
endpoint='https://'+host+'/v1/voices'
request_parameters='LanguageCode=en-US'
####### END MODIFIED VALUES #######

# Key derivation functions. See:
# http://docs.aws.amazon.com/general/latest/gr/signature-v4-examples.html#signature-v4-examples-python
def sign(key, msg):
    return hmac.new(key, msg.encode('utf-8'), hashlib.sha256).digest()

def getSignatureKey(key, dateStamp, regionName, serviceName):
    kDate = sign(('AWS4' + key).encode('utf-8'), dateStamp)
    kRegion = sign(kDate, regionName)
    kService = sign(kRegion, serviceName)
    kSigning = sign(kService, 'aws4_request')
    return kSigning

# Create a date for headers and the credential string
t = datetime.datetime.utcnow()
amzdate = t.strftime('%Y%m%dT%H%M%SZ')
datestamp = t.strftime('%Y%m%d') # Date w/o time, used in credential scope


# ************* TASK 1: CREATE A CANONICAL REQUEST *************
# http://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html

# Step 1 is to define the verb (GET, POST, etc.)--already done.

# Step 2: Create canonical URI--the part of the URI from domain to query 
# string (use '/' if no path)
canonical_uri = '/' 

# Step 3: Create the canonical query string. In this example (a GET request),
# request parameters are in the query string. Query string values must
# be URL-encoded (space=%20). The parameters must be sorted by name.
# For this example, the query string is pre-formatted in the request_parameters variable.
canonical_querystring = request_parameters

# Step 4: Create the canonical headers and signed headers. Header names
# must be trimmed and lowercase, and sorted in code point order from
# low to high. Note that there is a trailing \n.
canonical_headers = 'host:' + host + '\n' + 'x-amz-date:' + amzdate + '\n'

# Step 5: Create the list of signed headers. This lists the headers
# in the canonical_headers list, delimited with ";" and in alpha order.
# Note: The request can include any headers; canonical_headers and
# signed_headers lists those that you want to be included in the 
# hash of the request. "Host" and "x-amz-date" are always required.
signed_headers = 'host;x-amz-date'

# Step 6: Create payload hash (hash of the request body content). For GET
# requests, the payload is an empty string ("").
payload_hash = hashlib.sha256('').hexdigest()

# Step 7: Combine elements to create create canonical request
canonical_request = method + '\n' + canonical_uri + '\n' + canonical_querystring + '\n' + canonical_headers + '\n' + signed_headers + '\n' + payload_hash


# ************* TASK 2: CREATE THE STRING TO SIGN*************
# Match the algorithm to the hashing algorithm you use, either SHA-1 or
# SHA-256 (recommended)
algorithm = 'AWS4-HMAC-SHA256'
credential_scope = datestamp + '/' + region + '/' + service + '/' + 'aws4_request'
string_to_sign = algorithm + '\n' +  amzdate + '\n' +  credential_scope + '\n' +  hashlib.sha256(canonical_request).hexdigest()


# ************* TASK 3: CALCULATE THE SIGNATURE *************
# Create the signing key using the function defined above.
signing_key = getSignatureKey(secret_key, datestamp, region, service)

# Sign the string_to_sign using the signing_key
signature = hmac.new(signing_key, (string_to_sign).encode('utf-8'), hashlib.sha256).hexdigest()


# ************* TASK 4: ADD SIGNING INFORMATION TO THE REQUEST *************
# The signing information can be either in a query string value or in 
# a header named Authorization. This code shows how to use a header.
# Create authorization header and add to request headers
authorization_header = algorithm + ' ' + 'Credential=' + access_key + '/' + credential_scope + ', ' +  'SignedHeaders=' + signed_headers + ', ' + 'Signature=' + signature

# The request can include any headers, but MUST include "host", "x-amz-date", 
# and (for this scenario) "Authorization". "host" and "x-amz-date" must
# be included in the canonical_headers and signed_headers, as noted
# earlier. Order here is not significant.
# Python note: The 'host' header is added automatically by the Python 'requests' library.
headers = {'x-amz-date':amzdate, 'Authorization':authorization_header}


# ************* SEND THE REQUEST *************
request_url = endpoint + '?' + canonical_querystring

print '\nBEGIN REQUEST++++++++++++++++++++++++++++++++++++'
print 'Request URL = ' + request_url
print 'Header'
print headers

r = requests.get(request_url, headers=headers)

print '\nRESPONSE++++++++++++++++++++++++++++++++++++'
print 'Response code: %d\n' % r.status_code
print r.text

And I get the error message :

BEGIN REQUEST++++++++++++++++++++++++++++++++++++
Request URL = https://polly.eu-west-1.amazonaws.com/v1/voices?LanguageCode=en-US

RESPONSE++++++++++++++++++++++++++++++++++++
Response code: 403

{"message":"The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.\n\nThe Canonical String for this request should have been\n'GET\n/v1/voices\nLanguageCode=en-US\nhost:polly.eu-west-1.amazonaws.com\nx-amz-date:20170122T160127Z\n\nhost;x-amz-date\ne3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855'\n\nThe String-to-Sign should have been\n'AWS4-HMAC-SHA256\n20170122T160127Z\n20170122/eu-west-1/polly/aws4_request\n6e1004fbcc11a73baa0c7fe9ff8d5629bada5061fdc8b03fbf307696ea41728d'\n"}

How can I debug that? I suspect that it is not a secret keys problem as they work when in boto3

Answer 1

I strongly discourage you to build your own HTTP requests manually to interact with AWS services. Constructing the AWS signature yourself, you will enter an unending world of 403 errors, pain and misery.

Using boto3 really is the only recommendable way to go in a Python environment.

(by the way, this might help you getting boto3 to work inside your local dev server)