Please recommend a Password or passphrase strength tester. It should preferably:
- be used off-line (but on-line answers are also fine)
- display the entropy
- be open-source
- be available on Linux (although cross-platform solutions are more than welcome)
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You seem to be under the delusion that password strength checkers actually exist. That's wrong: it is impossible to check the strength of a password, only the strength of a password generation method.
A password strength checker can only test how long it would take to a particular cracker to find the password. A different cracker using a different strategy would take a different amount of time.
Or, in the words of Security Stack Exchange acclaimed expert Thomas Pornin:
Password strength checkers are good at telling you how robust your password is against incompetent attackers. This has some value, if only because there are so many incompetent wannabe hackers. But it would be a mistake to rely too much on such tools.
See also other Sec.SE questions on the topic, including
If you want to know about your own passwords, then you know how they were generated. Calculate the entropy, i.e. the amount of randomness that goes into the password generation process (How can I create a secure password? may help).
If you're an administrator and you want to assess your users' passwords, there may be some value in rejecting passwords that are too weak.
pam_cracklib is commonly used for that.