Bip39. The passphrase will not be chosen from your predefined list of 2048 words – it could be any word or phrase you select. Anyone else who discovers the phrase can steal the bitcoins. It have to be retained solution, such as it must not be typed into any Web-site. https://indexedbookmarks.com/story19501817/bip-39-https-linktr-ee-bip39-a-simple-key-for-bip39-unveiled