Why It's Hard to Cheat

You hash the same sentence on your phone today and on a laptop next year. What do you get?

• Two different hashes, because hashing adds randomness
• The same hash both times
• The same hash only if both devices use the same app

An attacker controls 20% of a proof-of-work network's hashing power. What happens to their chance of rewriting your payment as more blocks are added on top of it?

• It shrinks with every block
• It stays the same no matter how long you wait
• It grows, because they have more time to work