Hi,

So I would like to revisit this thread if I may... If I had a publicly advertised bitcoin address which was the "pay point" for a service and I wanted to return to any entity paying to this address some encrypted data (in TX_OUT?) would I be able to do it so that they could just use their (bitcoin) private key to decrypt?

In this situation the sender would have by default had to already reveal (to the network) their public key, so i could encrypt the data (likely a hash) I want to return to them, and store it in the tx (somewhere) which is returned to the sending address (I know I know, bad bitcoin practice and LukeJR will probably have a field day with my terminology if he sees this), but hopefully you can understand some light on what I am thinking of doing here and let me know if it might be possible?

A possible application might be a tokenised authorisation/access system. Bitcoin user can send any amount of coin to a publicised pay point and recieve back a small dust (or other size) tx along with prunable, encrypted data. If this data was a hash which acts as, or corresponds to an access token (where the period/size of access was likely related to the size of initial deposit to the pay point). In this situation you could trustlessly give access to paying entities using a distributed ledger to exchange information, automagically.

Perhaps someone already though of this before and it just has a name I am not familiar with, or perhaps it won't work for technical reasons. Either way, perhaps it is enough of an idea for someone to use...?