
import datetime, hashlib, hmac


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


def sign_request(method, access_key, secret_key, token, requset_url, scope_url, request_parameters='', body=''):

    service = 'execute-api'
    host = scope_url
    region = 'us-east-1'
    endpoint = 'https://'+scope_url
    accept_header = 'application/json'

    print host
    print endpoint

    # 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 = requset_url

    # 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 = 'accept:' + accept_header + '\n' + '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 = 'accept;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(body).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,
               'Accept': accept_header,
               'x-amz-security-token': token,
               }


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

    return {
        'headers': headers,
        'request_url': request_url,
    }
