Why is 6 the number of confirms that is considered secure?

66

I believe this is addressed in page 8 of Satoshi's Bitcoin paper when showing the probability of an attacker catching up.

The wiki states:

"Only 6 blocks or 1 hour is enough to make reversal computationally impractical."

The key word is "impractical".

The important sentence in Satoshi's paper is:

"Assuming the honest blocks took the average expected time per block, the attacker's potential progress will be a Poisson distribution".

So, it is not that someone couldn't create the worlds first and own a $50 million ASIC farm, so as to have the ability to overtake the blockchain from a fork six blocks prior but that doing so would be a.) a massive undertaking, b.) obvious to all and c.) terrifically unprofitable to the attacker.

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