A master password in a password manager is a memorized secret that is used to generate other passwords for other purposes. Or it is a password that is used to unlock a database of other passwords, usually the passwords contained in the databased are not generated from the master password but randomly created, then stored in the database.
Master passwords are mostly used in password managers. It is very important that users have a strong master password because if it is corrupted, all the other passwords will need to be changed.
The goal of a such password is to be the only one the user will need to remember.
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