Is my contribution enough to ask for co-authorship?
In my opinion, it does not deserve authorship credit. Some university have specific requirements about authorship. For example, one university where I was previously studying had this requirement: an author should at least have done two of the following three things : (1) participate in finding the main idea of the paper, (2) participate in designing and carrying the experiments, and (3) participate in writting the paper.  According to what you said, you would only qualify for (2), and it would be indirectly, since you just implemented something that was already published. Thus, in my opinion, they could put your name in the "acknowledgements" section instead of as a co-author but if they don't it may also be ok depending on the context. Moreover, if you got paid to do what you did, then perhaps that they don't even need to put your name at all.