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Is in-site messaging really possible? Does it require a lot of effort?

What do you mean by in-site messaging?

Private messaging (DM) between users on social media

My initial thinking is that it would be difficult. But... it could be possible using the Memo Key (which I don't really understand) and perhaps possible using it's capability via the wallet (i.e. each message you send has a "transaction fee" of 0.001 Steem and I'm simply providing an interface for the encryption and decryption. It's beyond my thinking (and current understanding) at the moment but it's possible that all of the technology currently exists on the blockchain.

@starlord28 - what do you think?

Ahhh.. I already had the same thought of using the memo key for encryption and decryption. However, this was previously attempted and there was a lot of spam which resulted in the increase in the log size of SteemD and was not something the community liked. There is something that steemchiller and I had a chat about called https://matrix.org/ This is a decentralized solution that would be great to have. Might give this a try sometime this year.

How did the previous attempt work? Did messages appear like replies which only the recipient could decode? Or was it a separate interface.