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I know that the formula for the entropy of a message is: entropy formula (I believe b can be set to 2)

but when calculating the entropy of a password websites usually only compute log2(p-1), ignoring the first term. Is there a difference in these types of entropy?

Note: The website I am referring to in this instance is http://passwordstrengthcalculator.org/index.php. Thank you for the help.

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chs
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    That's because password "entropy" is calculated **wrong**. See http://security.stackexchange.com/questions/87535/how-many-combinations-does-a-specific-password-policy-allow/87540#87540 and http://security.stackexchange.com/questions/61931/are-efforts-being-made-to-bring-about-password-standards/61942#61942 – Steve Dodier-Lazaro May 22 '15 at 16:06
  • And http://security.stackexchange.com/questions/21050/calculating-complex-password-entropy ! – Steve Dodier-Lazaro May 22 '15 at 16:07
  • Yikes, missed all of those. Thanks for the resources! – chs May 31 '15 at 04:20

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