Re: [chrony-dev] Support for another crypto hash?

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Fri, Nov 04, 2011 at 09:57:02AM -0700, Bill Unruh wrote:
> On Fri, 4 Nov 2011, Miroslav Lichvar wrote:
> >I'm not sure I understand how this works. The same hash function is
> >used multiple times on the same data?
> 
> Both the old des based hash and the new MD5 based hashes do a huge amount of
> rerunning a similar hash function over the data, with mixing of the data
> between the hashes to try to prevent condensation of the string of hashes (
> Ie, Hash(Hash(hash(hash(....(hash(data))....)))) almost certainly cycles into
> t fixed point eventually. The multiple hashes are there solely to slow down
> the hash function, so that 100million per second. is brought down to 10 per
> second or so. By the way, that 100million persecond is not for the password
> hashes, but for simple hashes like sha or md whose whole design principle was
> to be cryptographically secure AND be as fast as possible. Password hashes you
> want to be very slow. )

[...]

> I am not at all sure that this is all necessary for
> chrony. On the other hand, the software (crypt) is there on all linux/bsd/...
> machines to generate and recognize all of the variants. The question is how
> critical is it that that hash used in chrony not be broken in say 10 sec (or
> 10 years?) Passwords have to be safe for months. Do the chrony hashes need to
> be? The password hash comes at a cost-- slowness. And a program whose purpose
> is fast efficient time transfer, slowness is not an advantage.

Ok, so the purpose of the password hash is to make dictionary attacks
impractical. While it could be useful for the command key where the
passwords are usually typed in manually by operators and so probably
are not very long/high quality, I think it would make easier to do a
DoS attack.

-- 
Miroslav Lichvar

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