Brain Wallet

A brain wallet derives a private key from a passphrase the owner memorises, so that nothing needs to be written down or stored anywhere. The appeal is obvious and the flaw is arithmetic. A key derived from a chosen phrase is only as unguessable as the phrase, and an attacker does not have to guess it against a live system: they can derive keys from enormous lists of phrases offline — song lyrics, scripture, quotations, passwords from old breaches, and mechanical variations on all of them — check each resulting address against the public ledger, and sweep anything that holds a balance. Because addresses are public, this can be done continuously and in advance, so a funded brain wallet can be emptied moments after first use. Human-chosen phrases carry far less entropy than the random material a wallet generates, which is why generated recovery phrases replaced the technique rather than improving it.