“Brain wallets” were designed to be so safe you could store them in plain sight on the Internet. Wrong!
It’s always a pain to memorize a password. This is why we so often choose weak ones. A technique used for protecting bitcoin wallets (called a “brain wallet”) seemed to offer a workaround. You use a strong form of cryptography to convert a password that you only keep in your mind—thus reducing vulnerability to malware and other attacks—into something that resists brute force. Brain wallets could thus be stored in the clear (effectively unencrypted) in the bitcoin blockchain, making them always available to an owner without weaknesses that would expose their value to others.