Many breaches of computer security involve guessing another user's password. While the passwd command enforces some criteria for making sure the password is hard to guess, a clever person can sometimes figure out a password just by knowing something about the user. Thus, a good password is one that is easy for you to remember but hard for someone else to guess. A bad password is one that is so hard for you to remember that you have to write it down (which you are not supposed to do), or that is easy for someone who knows about you to guess.
A password must meet the following requirements:
Length. By default, a password must have at least six characters. Only the first eight characters are significant. (In other words, you can have a password that is longer than eight characters, but the system only checks the first eight.) Because the minimum length of a password can be changed by a system administrator, it may be different on your system.
Characters. A password must contain at least two letters (either upper- or lower-case) and at least one numeral or symbol such as @,#,%. For example, you can use dog#food or dog2food as a password, but you cannot use dogfood.
Not your login ID. A password cannot be the same as your login ID, nor can it be a rearrangement of the letters and characters of your login ID. (For the purpose of this criteria, upper and lower case letters are considered to be the same.) For example, if your login ID is Claire2 you cannot have e2clair as your password.
Different from old password. Your new password must differ from your old one by at least three characters. (For the purpose of this criterion, upper- and lower-case letters are considered to be the same.) For example, if your current password is Dog#fooD you can change it to dog#Meat but you cannot change it to daT#Food.
Bad choices for passwords include:
Any password based on your name
Names of family members or pets
Car license numbers
Social Security numbers
Names related to a hobby or interest
Seasonal themes, such as Santa in December
Any word that is in a standard dictionary
Good choices for passwords include:
Phrases plus numbers or symbols (beam#meup)
Nonsense words made up of the first letters of every word in a phrase plus a number or symbol (swotrb7 for SomeWhere Over The RainBow)
Words with numbers, or symbols substituted for letters (sn00py for snoopy)