import hmac
from datetime import datetime
from hashlib import sha256
import requests


class SigV4Auth(object):
    def __init__(self, access_key, secret_key, session_token=None, region='us-east-1'):
        self.access_key = access_key
        self.secret_key = secret_key
        self.session_token = session_token
        self.region = region

    def add_auth(self, request):
        timestamp = datetime.utcnow().strftime('%Y%m%dT%H%M%SZ')
        request.headers['X-Amz-Date'] = timestamp

        if self.session_token:
            request.headers['X-Amz-Security-Token'] = self.session_token

        # https://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html
        canonical_headers = ''.join('{0}:{1}\n'.format(k.lower(), request.headers[k]) for k in sorted(request.headers))
        signed_headers = ';'.join(k.lower() for k in sorted(request.headers))
        payload_hash = sha256(request.body.encode('utf-8')).hexdigest()
        canonical_request = '\n'.join([request.method, '/', '', canonical_headers, signed_headers, payload_hash])

        # https://docs.aws.amazon.com/general/latest/gr/sigv4-create-string-to-sign.html
        algorithm = 'AWS4-HMAC-SHA256'
        credential_scope = '/'.join([timestamp[0:8], self.region, 'sts', 'aws4_request'])
        canonical_request_hash = sha256(canonical_request.encode('utf-8')).hexdigest()
        string_to_sign = '\n'.join([algorithm, timestamp, credential_scope, canonical_request_hash])

        # https://docs.aws.amazon.com/general/latest/gr/sigv4-calculate-signature.html
        key = 'AWS4{0}'.format(self.secret_key).encode('utf-8')
        key = hmac.new(key, timestamp[0:8].encode('utf-8'), sha256).digest()
        key = hmac.new(key, self.region.encode('utf-8'), sha256).digest()
        key = hmac.new(key, 'sts'.encode('utf-8'), sha256).digest()
        key = hmac.new(key, 'aws4_request'.encode('utf-8'), sha256).digest()
        signature = hmac.new(key, string_to_sign.encode('utf-8'), sha256).hexdigest()

        # https://docs.aws.amazon.com/general/latest/gr/sigv4-add-signature-to-request.html
        authorization = '{0} Credential={1}/{2}, SignedHeaders={3}, Signature={4}'.format(
            algorithm, self.access_key, credential_scope, signed_headers, signature)
        request.headers['Authorization'] = authorization


def generate_sigv4_auth_request(header_value=None):
    """Helper function to prepare a AWS API request to subsequently generate a "AWS Signature Version 4" header.

    :param header_value: Vault allows you to require an additional header, X-Vault-AWS-IAM-Server-ID, to be present
        to mitigate against different types of replay attacks. Depending on the configuration of the AWS auth
        backend, providing a argument to this optional parameter may be required.
    :type header_value: str
    :return: A PreparedRequest instance, optionally containing the provided header value under a
        'X-Vault-AWS-IAM-Server-ID' header name pointed to AWS's simple token service with action "GetCallerIdentity"
    :rtype: requests.PreparedRequest
    """
    request = requests.Request(
        method='POST',
        url='https://sts.amazonaws.com/',
        headers={'Content-Type': 'application/x-www-form-urlencoded; charset=utf-8', 'Host': 'sts.amazonaws.com'},
        data='Action=GetCallerIdentity&Version=2011-06-15',
    )

    if header_value:
        request.headers['X-Vault-AWS-IAM-Server-ID'] = header_value

    prepared_request = request.prepare()
    return prepared_request