Is it on-topic to ask whether a given cryptographic function I’m describing in my question already exists (and how such a process would be called)?
The currently most-upvoted, related question seems to be “Time Capsule cryptography?” with a total of 111 upvotes at the time of writing this.
As you can see when searching the site accordingly, such questions tend to be on-topic… assuming you keep in mind that Crypto.SE is not a replacement for your favorite search engine, and assuming you do your research.
It might help to check some related questions to get a feel for “how to formulate such questions”. Especially, since such questions really tend to attract a lot of comments along the lines of “What research have you done?” with according down-votes. To avoid such pitfalls, you should definitely describe research efforts, etc. Doing so also adds more quality to the question – which never hurts.