# [RC5] RC5 question

Elektron elektron_rc5 at yahoo.ca
Sat Oct 4 15:29:02 EDT 2003

```> More interesting information (the stats were updated since the
> original e- mail):
>
> K = 2^64 using the RC5-64 (all) stats produces t = 5.084313307 years
> (5 years, during the first month into the year). RC5-64 finished just
> under 5 years at 89% of the keyspace,

Set K=2^64*0.89, then use the initial RC5-64 rate (but the graphs seem
to be down).

> so this is a good formula.

It should be t = ln(2^128/(k ln(a)) + 1)/ln(a). Of course, 2^128/(k
ln(a)) is around 1.93*10^20, so the +1 makes almost no difference (calc
gives no difference in the calculation for t anyway).

> K = 2^72 using the RC5-72 stats produces t = 17.20178906 years (17
> years, during the third month into the year)
>
> Up to 17 years on this project. Now I can see why you don't want to go
> any further in RSA's secret key contests. Do you think anyone else
> will try at their next contest?

Each 8 bits is 8*1.5=12 years of moore's law (i.e. in 12 years, RC5-80
will be just as feasible as RC5-72 is now). So if we complete RC5-72 in
17 years, RC5-80 will be more feasible then than RC5-72 is now.

<snip>
> useless before this project finishes.

Another quote,
> ...Moore noted that such a shift would double the size of the
> processor and shoot the power consumption up to 40 watts, a power
> consumption rating that would generate untenable amounts of heat.

40 watts is typical, now. Pretty soon, computers may be
superconducting, or we may move onto quantum computing (I'm not sure
how that's supposed to work, but that's what everyone else seems to be
saying).

Just that moore's law holding is the best estimate, without any better
data to work with.

- Purr

```