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Re: password patterns
From: Anders Thulin <anders.thulin () sentor se>
Date: Mon, 20 Sep 2010 09:07:07 +0200

On 2010-09-13 14:56, Emin İslam Tatlı wrote:

I have recently written an article about password patterns. What do
you think about the topic? Can it be the next generation attack
replacing dictionary attacks? Any comments are welcome.

  Though the article reports some interesting observations, it does not
go far enough to allow any significant results to be extracted. For one thing:
how many Alpha strings were plain dictionary words?  If a majority, then it
seems possible to take a dictionary and create a set of rewrite rules. But
there is no report that it was.

  There also seems to be at least one serious flaw: Section 1.3 does not take
patterns such as Alpha + Digit + Alpha into account, but there is no
explanation why that is.  This alone makes that part of the study suspect.
Nor is it clear that the character sets you used for your study cover all
the characters found in the passwords ... so there seems to be a possibility
that a number of passwords may simply have not matched those patterns.

  Here are some of the question I noted as I was reading it.

  What character set was used for passwords? ASCII (i.e. 7-bit), some
8-bit code or Unicode? Or perhaps some even more  restricted? Without knowing what
characters were permissible in passwords, it's difficult to evaluate if the statistics
you report are generally applicable, or if they only apply to this particular
system. Is space allowed, for example?  And what about characters outside those
in [:alpha:] [:digit:] [:punct:] ?  (Did the password alphabet include characters
such as carriage return or line feed or horizontal tab, for example?)

  Another interesting question is if there were any password recommendations
associated with this set. It often happens that such recommendations are
followed very closely -- if it said, for example that users can make a password
from a plain word, prefixed by '#1' or ending with '.,' , I would expect to find a
large number of such passwords in the data set. And that, of course, is a bias in
any statistics based on the set. Are there any such biases to take into account?

  How did you select the patterns you examined?  Was it based on what you
found in the file, or on your own assumptions or the tools that you used for the analysis?
Was examination systematical (i.e. all combinations were examined), or ad hoc?  (Part of this
question may be because it's not quite clear to a reader what you mean by, say, Alpha+Digit.  Is that
<a single alphabetical character> followed by <a single digit> ? Or is it a <a sequence of
alphabetical characters> followed by <a sequence of digits> ?  As you use the term 'character set',
I initially assumed that you meant single characters, but I suspect that may not be what you
intended.)

  The password 'mekster11' fit both models.  Would 'foo12bar' also be an Alpha+Digit pattern? Or
is that an Alpha + Digit + Alpha pattern? For some unexplained reason that particular pattern
seems to be missing from section 1.3 -- it really needs to be explained why it was left out, I think.
As it is, the impression is that you did not do a systematic examination of this type of pattern.

  For this question, I'm assuming you mean 'sequence of'. So: I'm sorry you stopped there.  It also makes
sense to examine the characteristics of each of the strings.  In this particular case, a useful
question is: what digits *are* used when the string contains only one digit? Only two? Only three?
(In the case of three digits, is '007' more common than other '00x' strings, for example?)
I see that section 3 contains 'ending with 1', but it's not at all clear from the context if
that pattern (covering only 3047 passwords) was applied to the entire set of passwords, or only to some subset
of passwords, and in that case, what subset. Alternatively, what single digits do appear?
The patern you mention (Ending with 1) 'dark1' seems to contain Alpha + Digit pattern to me. If that is right, the
last digit must have very strange statistics, as there are almost 10 millions Alpha+Digit passwords, and so
'ending with 1' should produce at least one million 'ending with 1', assuming even distribution of the last digit.
But you report 3047. That's odd ... you probably need to explain why.


  An interesting comparison could also have been made by taking the rewriting rules of a password
cracker such as John the Ripper, and compared them with your findings.


-- 
Anders Thulin      anders.thulin () sentor se      070-757 36 10 / Intl. +46 70 757 36 10

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