24 August 2009

Coin Flips

Schneier on Security: Non-Randomness in Coin Flipping

It turns out that flipping a coin has all sorts of non-randomness:

Here are the broad strokes of their research:
  1. If the coin is tossed and caught, it has about a 51% chance of landing on the same face it was launched. (If it starts out as heads, there's a 51% chance it will end as heads).
  2. If the coin is spun, rather than tossed, it can have a much-larger-than-50% chance of ending with the heavier side down. Spun coins can exhibit "huge bias" (some spun coins will fall tails-up 80% of the time).
  3. If the coin is tossed and allowed to clatter to the floor, this probably adds randomness.
  4. If the coin is tossed and allowed to clatter to the floor where it spins, as will sometimes happen, the above spinning bias probably comes into play.
  5. A coin will land on its edge around 1 in 6000 throws, creating a flipistic singularity.
  6. The same initial coin-flipping conditions produce the same coin flip result. That is, there's a certain amount of determinism to the coin flip.
  7. A more robust coin toss (more revolutions) decreases the bias.
I'm reminded of the coin-flipping in Friday Night Lights, in which a coin had to strike the ceiling, and then come to rest on the floor before it was read. That's always struck me as a the gold standard of coin-flipping entropy-generation.

No comments:

Post a Comment