How can I securely generate a random number in my smart contract?

There are a few trade-offs and key points to keep in mind in this area. Any decision that a user makes which affects the outcome gives that user an unfair advantage. Examples include: Using a blockhash, timestamp, or other miner-defined value. Keep in mind that the miner has a choice of whether to publish a block or not, so they could conceivably have one chance at the prize per block they mine. A

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