A unique character string held by each user, a copy of which is stored within the system. During login an authentication process takes place; the password entered by the intending user must correspond with the stored value before the user is accepted by the system. A good password should contain at least eight apparently random characters. Personal details, such as vehicle license numbers or relatives' names, are too easily guessed to be secure when used as passwords; even dictionary words are susceptible to exhaustive search. See also irreversible encryption.