RE: Variable Key Stretching for v3

Hm,

Interesting idea. I wonder if this will be the weak link of the scheme or
not.

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.

The number of iterations used should of course be in plaintext, since
without it the key-stretch algorithm doesn't "know" when to stop.

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.

What say?

	Rony

> -----Original Message-----
> From: [email protected] 
> [mailto:[email protected]] On 
> Behalf Of Frank Pilhofer
> Sent: Saturday, February 18, 2006 7:34 PM
> To: Password Safe Development
> Subject: [Passwordsafe-devel] Variable Key Stretching for v3
> 
> Hi,
> 
> the v3 database format uses key stretching to make brute force attacks
> against the password harder. The current proposal stretches the key
> using 2048 iterations of SHA-256.
> 
> The other day, I was reading Linux Weekly News article on OpenBSD's
> crypt_blowfish module, see http://lwn.net/Articles/170248/
> 
> The major point of the article is,
> 
>      "[I]n order to have a future-proof password hashing algorithm,
>      one must be able to dial up the computational cost of that
>      algorithm over time. If the cost can be provided as a parameter
>      - and stored with the hashed password - then password hashing
>      can be made more expensive [...] while maintaining compatibility
>      with currently-hashed passwords."
> 
> So I was wondering if we wanted to apply the same idea to Password
> Safe.
> 
> We could easily add the number of iterations for the key stretching
> algorithm to the file header, instead of using the constant 2048.
> Even if that value is not exposed to the user, future versions of
> Password Safe could crank up the iterations in a both forward and
> backward compatible manner.
> 
> Frank
> 
> -- 
> Frank Pilhofer, [email protected]
> 
> 
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