Right now, we have three PKI-related tags: certificates, public-key-infrastructure and certificate-authority. There is considerable overlap between these three tags. Existing questions which use them seem to have used the tags somewhat at random. Could we simply merge them all together ? I suggest public-key-infrastructure as the common name (the others becoming synonyms to that one).
Note that there are sub-tags for specific kinds of PKI, namely x.509 and pgp. I do not suggest to merge those (however, a lot of PKI-related questions could be tagged x.509 as well).
(Also, "certificate authority" should be "certification authority".)