# File lib/aws/core/authorize_v4.rb, line 55
      def signature signer, datetime
        k_secret = signer.secret_access_key
        k_date = hmac("AWS4" + k_secret, datetime[0,8])
        k_region = hmac(k_date, region)
        k_service = hmac(k_region, service)
        k_credentials = hmac(k_service, 'aws4_request')
        hmac(k_credentials, string_to_sign(datetime))
      end