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That will fail the moment the first person accidentally sends a message unencrypted with the correct answer. I guarantee that there will be vultures looking for a chance to cash in with a copypasta off of that.

As far as I understand it, it is encrypted unless the first symbol is not #. Seems to me way more likely that someone answers in a reply. Especially since a PM costs Steem Dollar. Up to @ksolymosi if this method is accepted. One upside is that the Steemit name cannot be mistaken.

Yes you cannot send a memo without sending anything (sbd or steem). We can create a simple bot, what send the money back with a memo: "you are right / wrong".

Also they can encrypt the solution and just post here. But it is a way easier to just login to steem.chat or discord and send the solution.

Btw talking about mistakes, I'm sure a bunch of people won't read the rules and just simply post the solution here.

Yes, that is my concern that people don't read the rules, but let's test it... :-) This is part of the evolution of the game...
I am thinking about the encrypted messages. Now it seems easier to send it via Discord or in e-mail but let's see how it will work out... :-)

I understand. But I looked at Discord and signing up there requires an e-mail address. As I want to stay fully anonymous, that is a no-go for me. Also it turns out that the solution provided by @spiz0r provides no accurate time stamp, which you need.

Would it be possible to use PGP? This can be achieved by exchanging public keys (once) and then posting encrypted answers using your public key signed with mine. Public keys can be freely posted without anyone else being able to decrypt (for that you need the receiver's private key).

A clear advantage this way would be that the answer is in one central place (as a reply on your Steamit post) but a disadvantage is that it takes some doing in order to read it (copy/paste into PGP software).

here is the timestamp:
https://steemd.com/tx/c32b04fa5ed16e5965e115d302101febdfda4e0c

But yes it is more complicated.

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