Quote from Wikipedia (https://en.wikipedia.org/wiki/Password_strength):
"Forcing the inclusion of lowercase letters, uppercase letters, numbers, and symbols in passwords was a common policy but has been found to decrease security, by making it easier to crack. Research has shown how predictable the common use of such symbols are, and the US[36] and UK[37] government cyber security departments advise against forcing their inclusion in password policy."
A simple way to calculate something like 'entropy' is to calculate the lowest and highest ascii value in the password, the difference between them is the range. Then calculate (range+1)^pos for each position in the password, and add them together. This gives an indication of the number of trials needed to bruteforce guess the password. A minimum number should be calculated with current high end cpu's in mind. See http://www.davidstang.org/app/password.html for an example.
Quote from Wikipedia (https://en.wikipedia.org/wiki/Password_strength):
"Forcing the inclusion of lowercase letters, uppercase letters, numbers, and symbols in passwords was a common policy but has been found to decrease security, by making it easier to crack. Research has shown how predictable the common use of such symbols are, and the US[36] and UK[37] government cyber security departments advise against forcing their inclusion in password policy."
A simple way to calculate something like 'entropy' is to calculate the lowest and highest ascii value in the password, the difference between them is the range. Then calculate (range+1)^pos for each position in the password, and add them together. This gives an indication of the number of trials needed to bruteforce guess the password. A minimum number should be calculated with current high end cpu's in mind. See http://www.davidstang.org/app/password.html for an example.