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Date: Sat, 16 Feb 2013 22:51:25 +0000
From: Marsh Ray <>
To: Jens Christian Hillerup <>, Jens Steube
CC: "" <>
Subject: RE: [PHC] Different cost settings and optional input

> -----Original Message-----
> From: [] On Behalf
> However, this attack is not that important I'd argue, because if two equal
> passwords *exist* it is likely that that password is in some dictionary anyway,
> or that they originate from the same person.

Without disagreeing with your point, I thought I'd take the opportunity to dig up some anecdotes:

>From :
> Per Thorsheim wrote:
> > I would like to see a comparison of Twitters 370 banned passwords
> > against the top 370 or so passwords stolen from rockyou (
> A grand total of 38 of the top 100 passwords did not appear in the Twitter blacklist.

Solar would probably have a better sense of things, but my feeling is that probably of all the passwords that have been chosen by exactly two users independently, most of them are not in a dictionary.

I've tried to convince people of the merits of the following scheme:
Use a fixed salt or otherwise allow the comparison of hashes.
Maintain a dictionary or other database of 'seen' hashes across all users. Incorporate the latest breaches into your dictionary.
If new user B selects a password (or it appears in a new breach) that is the same as existing user A, force B to select another one and immediately *disable* user A until A successfully performs a password reset operation.

No one seems to care much for this proposal though :-)

- Marsh

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