# File lib/aws/core/signers/version_4.rb, line 91
        def derive_key(datetime)
          k_secret = credentials.secret_access_key
          k_date = hmac("AWS4" + k_secret, datetime[0,8])
          k_region = hmac(k_date, region)
          k_service = hmac(k_region, service_name)
          k_credentials = hmac(k_service, 'aws4_request')
        end