Which one of the following rules for selecting a password is the best?

10 digits: 0123456789 26
lower case letters:
26 upper case letters:
33 special characters: `[email protected]#$%^&*()--=+[ ]\{}\;':", ./<>?

a) 8 characters: 4 Numbers, 2 Lowercase Letters, 2 Uppercase letters
b) 7 characters: 4 Numbers, 1 Lowercase Letters, 1 Uppercase letter, 1 Symbol
c) 12 characters: 12 Numbers
d) 9 characters: 4 Numbers, 5 Lowercase Letters

(b) 7 characters: 4 Numbers, 1 Lowercase Letter, 1 Uppercase letter, 1 Symbol

Explanation:

The best rule that makes a password great is that rule that increases the entropy of the password. The entropy of a password is the measure of how easy it is for the password to be cracked e.g by guessing, by brute force cracking, by dictionary attacks and other related cracking methods.

A rule that makes use of a logical combination of the four types of characters - digits, lowercase letters, uppercase letters and special symbols - gives a greater strength to the password than the rules that just make use of the some of the four types of characters.

Increasing the length of the password too is a great way of increasing the entropy of the password. But then, if there is a trade-off between length and combination, the latter is better.