sign AWS Lambda HTTP API request - using AWS Version 4 signature

aws_signature_v4.py

# AWS Version 4 signing example
# taken from:  
#    http://docs.aws.amazon.com/general/latest/gr/sigv4-signed-request-examples.html

# Lambda API (InvokeAsync)
# http://docs.aws.amazon.com/lambda/latest/dg/API_InvokeAsync.html

# See: http://docs.aws.amazon.com/general/latest/gr/sigv4_signing.html
# This version makes a POST request and passes request parameters
# in the body (payload) of the request. Auth information is passed in
# an Authorization header.
import sys, os, base64, datetime, hashlib, hmac 
import requests # pip install requests


#  curl -XPOST https://lambda.us-east-1.amazonaws.com/2014-11-13/functions/hello-reinvent/invoke-async/ --data {}
# {"message":"Missing Authentication Token"}

# {"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\n
# The Canonical String for this request should have been\n


# ************* REQUEST VALUES *************
method = 'POST'
service = 'lambda'
host = 'lambda.us-east-1.amazonaws.com'
region = 'us-east-1'
endpoint = 'https://lambda.us-east-1.amazonaws.com'
path = '/2014-11-13/functions/hello-reinvent/invoke-async/'
request_url = endpoint + path

# POST requests use a content type header. For DynamoDB,
# the content is JSON.
content_type = 'application/x-amz-json-1.0'
# DynamoDB requires an x-amz-target header that has this format:
#     DynamoDB_<API version>.<operationName>
#amz_target = 'DynamoDB_20120810.CreateTable'

# Request parameters  passed in a JSON
request_parameters =  '{'
request_parameters +=  '}'

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

# 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')

access_key = os.environ.get('AWS_ACCESS_KEY')
secret_key = os.environ.get('AWS_SECRET_KEY')
if access_key is None or secret_key is None:
    print 'No access key is available.'
    sys.exit()

# Create a date for headers and the credential string
t = datetime.datetime.utcnow()
amz_date = t.strftime('%Y%m%dT%H%M%SZ')
date_stamp = 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 = '/'
canonical_uri = path

## Step 3: Create the canonical query string. In this example, request
# parameters are passed in the body of the request and the query string
# is blank.
canonical_querystring = ''

# Step 4: Create the canonical headers. Header names and values
# must be trimmed and lowercase, and sorted in ASCII order.
# Note that there is a trailing \n.
#canonical_headers = 'content-type:' + content_type + '\n' + 'host:' + host + '\n' + 'x-amz-date:' + amz_date + '\n' + 'x-amz-target:' + amz_target + '\n'
canonical_headers = 'content-type:' + content_type + '\n' + 'host:' + host + '\n' + 'x-amz-date:' + amz_date + '\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 include those that you want to be included in the
# hash of the request. "Host" and "x-amz-date" are always required.
# For DynamoDB, content-type and x-amz-target are also required.
#signed_headers = 'content-type;host;x-amz-date;x-amz-target'
signed_headers = 'content-type;host;x-amz-date'


# Step 6: Create payload hash. In this example, the payload (body of
# the request) contains the request parameters.
payload_hash = hashlib.sha256(request_parameters).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
print canonical_request

'''
POST\n
/2014-11-13/functions/hello-reinvent/invoke-async/\n
\n
content-type:application/x-amz-json-1.0\n
host:lambda.us-east-1.amazonaws.com\n
x-amz-date:20141215T104432Z\n
\n
content-type;host;x-amz-date\n
99887766554433221100ffeeddccbbaa99887766554433221100ffeeddccbbaa
'''

# ************* 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 = date_stamp + '/' + region + '/' + service + '/' + 'aws4_request'
string_to_sign = algorithm + '\n' +  amz_date + '\n' +  credential_scope + '\n' +  hashlib.sha256(canonical_request).hexdigest()
print string_to_sign

'''
AWS4-HMAC-SHA256\n
20141215T104432Z\n
20141215/us-east-1/lambda/aws4_request\n
99887766554433221100ffeeddccbbaa99887766554433221100ffeeddccbbaa
'''

# ************* TASK 3: CALCULATE THE SIGNATURE *************
# Create the signing key using the function defined above.
signing_key = getSignatureKey(secret_key, date_stamp, 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 *************
# Put the signature information in a header named Authorization.
authorization_header = algorithm + ' ' + 'Credential=' + access_key + '/' + credential_scope + ', ' +  'SignedHeaders=' + signed_headers + ', ' + 'Signature=' + signature

# For DynamoDB, the request can include any headers, but MUST include "host", "x-amz-date",
# "x-amz-target", "content-type", and "Authorization". Except for the authorization
# header, the headers must be included in the canonical_headers and signed_headers values, as
# noted earlier. Order here is not significant.
# # Python note: The 'host' header is added automatically by the Python 'requests' library.
headers = {'Content-Type':content_type,
           'Host':host,
           'X-Amz-Date':amz_date,
           'Authorization':authorization_header}

# ************* SEND THE REQUEST *************
print '\nBEGIN REQUEST++++++++++++++++++++++++++++++++++++'
print 'Request URL = ' + endpoint

r = requests.post(request_url, data=request_parameters, headers=headers)

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