# key length/size RSA discussion/recommendations in the wiki

Ingo Klöcker kloecker at kde.org
Thu Oct 30 00:38:24 CET 2014

```On Wednesday 29 October 2014 22:18:13 Peter Lebbing wrote:
> On 2014-10-29 21:49, vedaal at nym.hush.com wrote:
> > Surely Peter knows this too ;-)
> >
> > More likely 128 was a typo for the more common older RSA key of 1028
> > ...
>
> No, I'm using a strict definition of brute force.
>
> For p = 2^63 to 2^64-1
>    For q = 2^63 to 2^64-1
>      If p * q == n:
>        Break
>    Next
> Next

If anything then I'd do

For p = 2^63 to 2^64-1
If n modulo p == 0:
Break
Next
q = n / p

which is O(n^(1/2)), but IMO still brute force (even in your strict
definition), while yours is O((n^(1/2)^2) = O(n). "brute force" doesn't
mean that you have to use the most naïve algorithm.

> I don't feel the method outlined by Rob is still brute force. That
> brute actually is using his brain. Possibly his brain resembles a
> sieve, but still :). Am I too strict?

Actually, that brute doesn't seem to be using his brain. If he'd use his
brain then he'd use he fists to brute force the secret out of you. ;-p

Regards,
Ingo
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