Encryption could be quiet easy, public key cryptography. You can't "trade" the keys like that with PKI.
If I sent you my public key then you can encrypt a message which only I can read on my end. If you sent me your public key then I can encrypt a message which only you can read on your end. Because each of us exclusively have our private keys, only we can decrypt the messages even if it's stored on a blockchain.
I have received a private message over Steemit this way and no one can read it but me.
I was assuming the post is blockchained only once, otherwise (I'm assuming) it won't scale. I mean you'd need to have the same post encrypted as many times as there are readers. A 10kb post served x1000 would become 10mb.
Steemit can handle that kind of scaling. Is it a high value post though? A transaction fee would be required.
There are other ways too, like break the private key up so it's owned by a group and then let them vote to decrypt it in some fashion and then it's posted in a self destructing note page. There are ways if you really want to do it, or just look at Bitmessage which managed to do a decentralized encrypted 4chan type thing.
You can create a single key for the post and then encrypt that key using the public keys of the authorized readers. With 256 bit keys this is 32 KB of keying.
But still the problem remains that the posts after already being decrypted will be reposted or leaked.
Yep, I don't see how it can work. The more followers, the bigger the risk of weak "links" that could leak the info.
Plus all encrypted blockchained info are just running on a ticking clock till their decryption. I think Satoshi first said that you don't want any kind of encrypted messages in the blockchain - it would be an accident waiting to happen...