Bitcoin Puzzle #78

What puzzle #78 is

Puzzle #78 is one address in a set of 160 created in January 2015, when someone funded a sequence of Bitcoin addresses whose private keys were deliberately confined to ever-larger ranges. Puzzle #78's key is an integer somewhere in 2^77 … 2^78 − 1 — in hexadecimal, from 20000000000000000000 to 3fffffffffffffffffff. Whoever finds that integer controls the 7.8 BTC held at 15qF6X51huDjqTmF9BJgxXdt1xcj46Jmhb.

How hard is it?

The range holds about 2^77 candidate keys. A fast multi-core browser search covers a few million keys per second; at that pace, sweeping the entire range would take on the order of 9.6×10^8 years. In practice nobody sweeps the whole range — a solver simply hopes to land on the key by chance, so raw speed buys lottery tickets rather than a deadline.

For puzzle #78 only the address is public, not the public key. That rules out fast discrete-log methods like Pollard's kangaroo, so the only approach is to generate candidate keys and check each one's address — exactly what the scanner below does.

Why the key is safe to search for

Searching is legal and harmless: these funds were placed deliberately as a public challenge by their owner. The scanner runs entirely in your browser, generates candidate keys locally, and never transmits anything. The practical catch is not the search but the claim — recent solvers of nearby puzzles had winning transactions snatched from the public mempool by bots, so anyone who did find a key would need to broadcast with great care.