Sounds like this is not an authentication but an identification protocol applied for a very limited API calls rate-limiting use case. While there are some quest

Sounds like this is not an authentication but an identification protocol applied for a very limited API calls rate-limiting use case. While there are some question about this use case as well (e.g. it there is a DOS attack possible against legitimate clients if the secured_api_key is leaked), it might be practical and acceptable in some limited set of cases. The danger here is that the same protocol will be used for access control use cases for which it is not suitable (for a variety of reasons). A very good documentation (including thread modeling with attack vectors analysis) should be added to make sure this kind of mis-use does not occur. I would also change the title since it is too ambitious and misleading.