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gmane.comp.security.passwordsafe.devel |
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<[email protected]> |
Rony,
The only way I can think of to deal with "if there will be something else, such
as a weakness in Twofish or SHA-256" is to allow more than one hash and
encryption algorithm - i.e. add 2 more 1-byte fields:
Hash Algorithm:
0 = SHA-1
1 = SHA-256
... future algorithms
Encryption Algorithm:
0 = Blowfish
1 = Twofish
2 = AES ;-)
... future algorithms
Each new version/release would (potentially) add a new algorithm as necessary.
As long as the current release knows its limits, it can tell the user that it
doesn't understand values greater than it knows. PWS V1 & V2 can only can
handle (0,0); V3 can initially handle (0,0) and (1,1) - although eventually
only (1,1) with a separate conversion utility to first convert V1 or V2 to V3
format. There is no need for support to be (n,n) - it could be (n,m) where n !
= m.
If this is taken up, I would hope that future versions support only the highest
n and m i.e. they do not still have obsolescent code for prior algorithms that
have been superseded because of some issue/weakness.
Regards,
David
Quoting Rony Shapiro <[email protected]>:
> Er, what I meant about "the weak link" was: I wonder if in the future we'll
> find ourselves increasing the number of iterations as Frank has anticipated,
> or if there will be something else, such as a weakness in Twofish or
> SHA-256, that will force us to make an incompatible change in the format...
>
> So, there'll be another 32-bit (little-endian) value with the number of
> iterations in the database, not less than 2048, after the SALT.
>
> Anyway, since the cost/benefit of this looks very good, I've implemented it
> and updated the format document as well - Thanks, Frank!
>
> I think we're getting close to a first beta for 3.0.
>
> Cheers,
>
> Rony
>
> > -----Original Message-----
> > From: Frank Pilhofer [mailto:[email protected]]
> > Sent: Monday, February 20, 2006 7:09 PM
> > To: Rony Shapiro; 'Password Safe Development'
> > Subject: Re: [Passwordsafe-devel] Variable Key Stretching for v3
> >
> > On Mon, 20 Feb 2006 02:14:07 -0500, Rony Shapiro
> > <[email protected]> wrote:
> > >
> > > Interesting idea. I wonder if this will be the weak link of
> > the scheme or
> > > not.
> > >
> >
> > It doesn't weaken the scheme at all, as long as
> > implementations of Password
> > Safe enforce a certain minimum number of iterations.
> >
> > >
> > > A nice property of your suggestion is that it allows a time/security
> > > trade-off. PDA versions of PasswordSafe might, for example,
> > set the
> > > number of iterations to a lower value.
> > >
> >
> > Right. You could imagine tuning the number of iterations so
> > that testing
> > the password takes a certain amount of time.
> >
> > >
> > > The number of iterations used should of course be in
> > plaintext, since
> > > without it the key-stretch algorithm doesn't "know" when to stop.
> > >
> >
> > It could be a 32-bit integer between the SALT and H(P').
> >
> > >
> > > Hm. Another option might be to not put the number in the
> > database at all,
> > > and let the aplication test all values after a minimum
> > threshold up to an
> > > insane maximum. This has the nice property that wrong
> > passphrases would
> > > take *much* longer to be rejected than for correct passphrases to be
> > > accepted.
> > >
> >
> > I agree with Philip that this is probably not the best idea. The
> > implementation
> > has no way of stopping at any threshold of n, because the
> > file may have
> > been
> > generated by a future version of Password Safe with a
> > threshold of n+1.
> > And it
> > is a bit cruel to the user to block indefinitely, or to tell
> > the user that
> > the
> > passphrase is wrong when it fact it is not.
> >
> > Frank
> >
> > --
> > Frank Pilhofer, [email protected]
> >
> >
>
>
>
>
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