Using acronyms as paper keywords
The intent of keywords is to aid in indexing so that your article can be easier to the reach of people who search for specific topic (DoS in your example). Hence, bear in mind the people whom you are trying to reach when selecting the keywords.

Certain journal publishers, such as IEEE, have a complete list of keywords you ought to follow. These lists are often updated to catch up with the technological trends of the time. It would be better to check for such a list with your publisher.

If you cannot find such a list, check out the papers that are published in the journal you wish to submit the manuscript in. Search for papers in your topic and see what sort of keyword they used that is similar to yours to get a better idea of what to include in your keywords section.