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Date: Sat, 11 Mar 2006 12:04:13 +0300 From: Solar Designer <solar@...nwall.com> To: john-users@...ts.openwall.com Subject: Re: checking only first 5 characters of a md5 hash On Sat, Mar 11, 2006 at 08:49:08AM +0100, Turko wrote: [38 lines of context removed] > OK Please try to not quote irrelevant context in your responses. You did not need to quote all those many lines just to say "OK". I wrote: > > <?php > > $mycode = "ZTNlM"; > > $mykey = "a"; > > while (1) { > > $code = substr(base64_encode(md5($mykey)), 0, 5); > > if ($code == $mycode) break; > > $mykey++; > > } > > print "$mykey\n"; > > ?> > > > > Please note that this loop may run forever in case $mycode is set to a > > string not produced by the $code = ... line from your message. > > Thanks a lot. I m trying this (easy) script. Of course I had to setup > php.ini in "Maximum execution time" (30 seconds by default) . > Now is 10 hours it works, but nothing. There's no point in running this for more than a few seconds. If it does not crack a password, then $mycode is not set to a string produced by the $code = ... line. > I m sure that $mycode is an existing part of the hash. ...existing "part of the hash" or "the first 5 characters of base64 encoding of hex encoding of the MD5 hash"? Obviously, the script will only crack the latter. > Ho can I calculate the amunt of time I need (theory) ? It should be under 1 second. As I have mentioned, the $code = ... line can only produce a little over 20,000 different values[1]. If you search this many candidate passwords, you have a 63% probability[2] of finding a suitable one. If you search even further, the probability quickly approaches 100%. On a 1 GHz x86 system with PHP 5, the above script can search around 40,000 candidate passwords per second - so the probability of finding a password is around 85% after 1 second and around 98% after 2 seconds[3]. [1,2,3] I've omitted the detail behind these calculations. I think you want to get the overall picture before you likely get confused with the detail. Besides, this has little to do with John the Ripper. -- Alexander Peslyak <solar at openwall.com> GPG key ID: B35D3598 fp: 6429 0D7E F130 C13E C929 6447 73C3 A290 B35D 3598 http://www.openwall.com - bringing security into open computing environments Was I helpful? Please give your feedback here: http://rate.affero.net/solar
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