Re: a challenge
On Thu, Oct 18, 2001 at 09:11:05AM -0700, Greg Wiley wrote:
> This is just a general idea:
>
> I'm assuming a 32-bit timestamp.
>
> You want to encode that value somewhat securely
> with an invertable hash into 16 * 7 = 112 bits
> (less if you can't use control chars). It has to be
> somewhat resistant to tampering (you gave no
> indication of the degree to which it must resist).
>
> So you can multiply your timestamp by
> any number < 2^(112-32) = 2^80 =
> 1208925819614629174706176 .
> and still fit in 16 7-bit chars.
>
> I would pick a prime value < 2^80 and multiply the time
> by it, then format it into 7-bit characters.
>
> To extract, pack it and divide.
>
> Does that work?
>
> Note that to break this, the attacker will have to
> search a fairly large space but not an impossible
> one. Some of the effectiveness of this will depend
> on how often you change the prime factor. Also,
> if it is fairly expensive for an attacker to check
> a single value, the search could take a while.
> This also doesn't take into account the value
> of breaking it, i.e. how big is the "prize" for
> finding your prime factor?
If the attacker knows the algorithm (although not the prime number) this is
unfortunately trivial to crack: they just have to guess the time that is
encoded by the timestamp. :(
--
David Roundy
http://civet.berkeley.edu/droundy/
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