You want to ensure that passwords are not easily guessable or crackable.
Use CrackLib, which is available from http://www.crypticide.org/users/alecm/.
When users are allowed to choose their own passwords, a large number of people will inevitably choose passwords that are relatively simple, making them either easy to guess or easy to crack. Secure passwords are often difficult for people to remember, so they tend to choose passwords that are easy to remember, but not very secure. Some of the more common choices are simple words, dates, names, or some variation of these things.
Recognizing this tendency, Alec Muffett developed a program named Crack that takes an encrypted password from the system password file and attempts to guess?or crack?the password. It works by trying words found in a dictionary, combinations of the user's login name and real name, and simple patterns and combinations of words.
CrackLib is the core functionality of Crack, extracted into a library for the intended purpose of including it in password-setting and -changing programs to prevent users from choosing insecure passwords. It exports a simple API, consisting of a single function, FascistCheck( ), which has the following signature:
char *FascistCheck(char *pw, char *dictpath);
This function has the following arguments:
Buffer containing the password that the user is attempting to use.
Buffer containing the name of a file that contains a list of dictionary words for CrackLib to use in its checks.
The dictionary file used by CrackLib is a binary data file (actually, several of them) that is normally built as part of building CrackLib itself. A small utility built as part of CrackLib (but not normally installed) reads in a text file containing a list of words one per line, and builds the binary dictionary files that can be used by CrackLib.
If the FascistCheck( ) function is unable to match the password against the words in the dictionary and its other tests, it will return NULL to indicate that the password is secure and may be used safely. Otherwise, an error message (rather than an error code) is returned; it is suitable for display to the user as a reason why the password could not be accepted.
CrackLib is intended to be used on Unix systems. It relies on certain Unix-specific functions to obtain information about users. In addition, it requires a list of words (a dictionary). Porting CrackLib to Windows should not be too difficult, but we are not aware of any efforts to do so.
CrackLib by Alec Muffett: http://www.crypticide.org/users/alecm/