Function that calculates AWS signature version 4 headers

aws_sig_v4_headers.py

import datetime
import hashlib
import hmac
import urllib.parse


def aws_sig_v4_headers(access_key_id, secret_access_key, pre_auth_headers,
                       service, region, host, method, path, query, payload):
    algorithm = 'AWS4-HMAC-SHA256'

    now = datetime.datetime.utcnow()
    amzdate = now.strftime('%Y%m%dT%H%M%SZ')
    datestamp = now.strftime('%Y%m%d')
    payload_hash = hashlib.sha256(payload).hexdigest()
    credential_scope = f'{datestamp}/{region}/{service}/aws4_request'

    pre_auth_headers_lower = {
        header_key.lower(): ' '.join(header_value.split())
        for header_key, header_value in pre_auth_headers.items()
    }
    required_headers = {
        'host': host,
        'x-amz-content-sha256': payload_hash,
        'x-amz-date': amzdate,
    }
    headers = {**pre_auth_headers_lower, **required_headers}
    header_keys = sorted(headers.keys())
    signed_headers = ';'.join(header_keys)

    def signature():
        def canonical_request():
            canonical_uri = urllib.parse.quote(path, safe='/~')
            quoted_query = sorted(
                (urllib.parse.quote(key, safe='~'), urllib.parse.quote(value, safe='~'))
                for key, value in query.items()
            )
            canonical_querystring = '&'.join(f'{key}={value}' for key, value in quoted_query)
            canonical_headers = ''.join(f'{key}:{headers[key]}\n' for key in header_keys)

            return f'{method}\n{canonical_uri}\n{canonical_querystring}\n' + \
                   f'{canonical_headers}\n{signed_headers}\n{payload_hash}'

        def sign(key, msg):
            return hmac.new(key, msg.encode('utf-8'), hashlib.sha256).digest()

        string_to_sign = f'{algorithm}\n{amzdate}\n{credential_scope}\n' + \
                         hashlib.sha256(canonical_request().encode('utf-8')).hexdigest()

        date_key = sign(('AWS4' + secret_access_key).encode('utf-8'), datestamp)
        region_key = sign(date_key, region)
        service_key = sign(region_key, service)
        request_key = sign(service_key, 'aws4_request')
        return sign(request_key, string_to_sign).hex()

    return {
        **pre_auth_headers,
        'x-amz-date': amzdate,
        'x-amz-content-sha256': payload_hash,
        'Authorization': f'{algorithm} Credential={access_key_id}/{credential_scope}, '
                         f'SignedHeaders={signed_headers}, Signature=' + signature(),
    }