Crypto glossary
Brain wallet
A brain wallet derives a private key from a phrase the owner memorises; human-chosen phrases are easy to guess, so attackers routinely drain them.
A brain wallet turns a passphrase into a private key, typically by hashing it. The idea is appealing: nothing is written down, and the owner carries the wallet in their head. Anyone who types the same phrase into the same tool gets the same key and can spend the coins.
That last point is the problem. People pick song lyrics, quotes, names and patterns, which are far less random than a key generated by a computer. Attackers hash huge dictionaries, leaked password lists and book texts, then watch the resulting addresses around the clock, sweeping any deposit within seconds. Researchers presented brain-wallet cracking at DEF CON in 2015 and found many such wallets already emptied. The episode is a textbook lesson on entropy in key generation.
Seed phrases are sometimes confused with brain wallets, but they differ: the words of a standard seed phrase are chosen randomly by the wallet, giving enough entropy to resist guessing. Memorising a randomly generated seed phrase is a different practice, though it still risks being forgotten. Adding a strong passphrase to a seed is safer than any self-chosen brain wallet.
Example
A user creates a brain wallet from a famous film quote and sends bitcoin to it; a bot that already had that phrase in its list moves the funds out within a minute.