[cryptography] Password non-similarity?

Steven Bellovin smb at cs.columbia.edu
Tue Dec 27 18:12:30 EST 2011

On Dec 27, 2011, at 5:48 PM, Solar Designer wrote:

> On Tue, Dec 27, 2011 at 03:54:35PM -0500, Jeffrey Walton wrote:
>> We're bouncing around ways to enforce non-similarity in passwords over
>> time: password1 is too similar too password2 (and similar to
>> password3, etc).
>> I'm not sure its possible with one way functions and block cipher residues.
>> Has anyone ever implemented a system to enforce non-similarity business rules?
> In passwdqc, we opted to only do it for the current vs. previous
> password, not maintaining a password history.  (The previous password is
> normally entered by the user at password change time.)
> Password histories are controversial.  They do not obviously improve
> security; they may as well make things a lot worse (even if you're just
> storing hashes).
> Also, you shouldn't declare two passwords too similar just because they
> contain e.g. an N-character substring in common; rather, you should see
> if the remainder of the new password (with the too-similar portion
> removed or partially discounted) would still meet the policy.  This is
> what passwdqc does.
> KoreLogic ran a password hash cracking contest at DEFCON 2010 (with many
> remote participants as well) focused on effects of password histories -
> that is, they tried to simulate users' behavior patterns that they
> observed in corporate environments with password histories.  After the
> contest, they released John the Ripper rules that try to match users'
> typical approaches at bypassing password histories - appending the
> current year, month name, etc.  Apparently, this is what actually
> happens when there's a password history and regular password changes are
> enforced.
> The DEFCON 2010 contest (including related data files):
> http://contest-2010.korelogic.com
> passwdqc tested on the contest passwords:
> http://www.openwall.com/lists/john-users/2011/02/20/2

Also see http://www.cs.unc.edu/~reiter/papers/2010/CCS.pdf -- they
describe an algorithm to guess new passwords from old.

Here's a heretical thought: require people to change their passwords --
and publish the old ones.  That might even be a good idea...

		--Steve Bellovin, https://www.cs.columbia.edu/~smb

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