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When I login after boot, or maybe after a idle time, I am presented with a login box. If I enter the incorrect password, it takes a longer time to verify, and give the error message, than entering the correct password, and getting logged in. Why is that?

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up vote 7 down vote accepted

Probably because this makes brute-force guessing passwords more time-consuming and therefore less likely. See also 1

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This is correct. The login screen (GDM) communicates with Linux-PAM to check user credentials. If the user fails to authenticate, Linux-PAM waits a random amount of time before sending its response back to GDM. –  ændrük Mar 1 '11 at 19:59
    
@ændrük The link is not working. I have the following revised link from web.archive.org: web.archive.org/web/20081008011541/http://www.kernel.org/pub/… –  hexafraction Oct 7 '12 at 15:06
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