The password strength calculation in F-Secure KEY is based on password entropy.
Password entropy indicates how easy it is for a password cracking software to predict a password. The more passwords the software has to try before guessing the password the higher the entropy is.
To get the maximum entropy score, it is important to include at least one upper case letter, one lower case letter, one number and one special character (or symbol) in the password. Also, avoid all character sequences, such as "12345678" or "abcdefgh", and do not use character repetition (for example "aaaaaa"). Avoid using common male or female names, common words and typical passwords. Finally, also the password length has an effect on the password entropy score.