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Steemrollers likes Steemit, and we don't control of the txid's. Any mistakes by the bot will be corrected. If you have an issue please report it a steem.chat #steemrollers.

My point is - the players control their txids, and thereby the dice roll.
It is a fatal flaw to rely on the randomness of txids.

How do they control their txid? Isn't it assign by the blockchain?

The tx is is basically a hash of the tx data, including the signature.
Players can create a transaction, look at the id, and then choose to publish the transaction or not.

I guess I will add a secret key then and I will have to publish it each day.

That's no longer provably fair. You could choose the secret key afterwards.

If I chose the key afterwards it wouldn't match up with every transaction. They can match the roll number to the sha256(txid+key). I will get the hex value of the hash returned. If I choose a key afterwards it would change the hash and the rolls wouldn't match up.